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  • Articles  (8,275)
  • Wiley  (8,275)
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  • Articles  (8,275)
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  • 2015-2019  (8,275)
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  • 1
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    A low temperature approach for photo/cathodoluminescent Gd2O2S:Tb (GOS:Tb) nanophosphors (2018)
    Wiley
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    Publication Date: 2018
    Description: Gd2O2S:Tb nanophosphor with favorable dispersion and high chemical/phase purity was obtained via a low temperature precipitation approach. Abstract Titrating the aqueous solution of equimolar RE(NO3)3 and (NH4)2SO4 with NH4OH to pH~9 at ~4°C produced an amorphous precursor that yielded phase‐pure and well‐dispersed RE2O2S nanopowder (RE = Gd0.99Tb0.01; GOS:Tb) via a RE2O2SO4 intermediate upon annealing in H2. The powders calcined at the typical temperatures of 700/1200°C exhibited unimodal size distributions and have the average crystallize sizes of ~17/55 nm, average particle sizes of ~284/420 nm, and specific surface areas of ~14.62/4.53 m2/g (equivalent particle sizes: ~56/180 nm). The 1200°C product exhibited sharp green luminescence at ~544 nm (FWHM = 2.3 nm; λex = 275 nm), with an absolute quantum yield of ~24.8% and a fluorescence lifetime of ~1.34 ms at room temperature. It was also shown that the powder possesses favorable thermal stability (the activation energy for thermal quenching of luminescence ~0.305 eV) and is stable under electron beam irradiation up to 7 kV and 50 μA. The synthetic technique has the advantages of scalability and favorable dispersion and high chemical/phase purity for GOS powder, which may allow the sintering of scintillation ceramics at lower temperatures.
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  • 2
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    Toward temperature‐dependent Bi3+‐related tunable emission in the YVO4:Bi3+ phosphor (2018)
    Wiley
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    Publication Date: 2018
    Description: Abstract Up until now, many previous works have indicated us that the photoluminescence (PL) properties of phosphors sometimes can be changed with the change in the external temperature, resulting in the anomalous PL phenomena and correlated new applications that are difficult to achieve at room temperature. In this work, we report the temperature‐dependent Bi3+‐related PL properties in the YVO4:Bi3+ phosphor. Our findings show that increasing the temperature from 10 to 300 K enables manipulating the energy interaction from groups to Bi3+, thereby leading to the temperature‐induced color tuning from blue (0.183, 0.212) to yellow (0.418, 0.490). Upon this heating process, we further reveal that the dynamic Bi3+ luminescence has experienced a regular transition from double‐exponential to single‐exponential decay, which results in the decrease in the average Bi3+ lifetime from 122.606 to 0.376 μs. Discussions on the PL results imply that the tunable PL observations are due to the interplay of temperature‐dependent energy transfer from groups to Bi3+ and redistribution of the excited 3P0 and 3P1 states of Bi3+ upon the thermal stimulation. This work not only presents the temperature‐triggered Bi3+ tunable properties in the well‐studied YVO4 host lattice but also can provide new insights into revealing Bi3+‐related PL mechanism in other Bi3+‐doped photonic materials in the future and, in the meanwhile, gives some directive ideas for us to explore previously unnoticed applications for rare‐earth (RE; eg, Eu3+, Pr3+, Tb3+, Eu2+, Er3+, etc) and other non‐RE (eg, Bi3+, Mn4+, Mn2+, Cr3+, etc) doped phosphors.
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  • 3
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    Composition‐driven broad phase boundary for optimizing properties and stability in lead‐free barium titanate ceramics (2018)
    Wiley
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    Publication Date: 2018
    Description: Broad phase boundary with successive R‐O and O‐T to optimize the piezoelectric/strain properties and enhance the temperature stability in lead‐free barium titanate ceramics. Abstract A new piezoelectric system of (1−x−y)BaTiO3‐yCaZrO3‐xBaSnO3 (BT‐yCZ‐xBS) was designed to achieve enhanced piezoelectric/strain properties and temperature stability. First, the relationships between composition, phase, and electrical properties are systematically investigated. The broad phase boundary with successive rhombohedral‐orthorhombic (R‐O) and orthorhombic‐tetragonal (O‐T) was obtained in 0.04 ≤ x ≤ 0.05 and 0.04 ≤ y ≤ 0.07 by tailoring the relationship of composition and phase structure, confirmed by X‐ray diffraction, temperature‐dependent dielectric constants, and Raman spectra. The optimized piezoelectric coefficient of d33 = 560 pC/N, high strain of 〉0.20%, and large converse piezoelectric coefficient of d33* = 1170 pm/V were realized. Second, the optimized piezoelectricity both demonstrate a stable performance with fluctuation 〈8% for d33* and 20% for d33 between 22 and 60°C since the broad phase boundary is exhibited in this temperature range. We believe that this work is a successful example to optimize piezoelectric properties and enhance the stability for piezoceramics.
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  • 4
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    Anomalous photovoltaic effect in Bi(Ni2/3Ta1/3)O3‐PbTiO3 ferroelectric solid solutions (2018)
    Wiley
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    Publication Date: 2018
    Description: Abstract Dense Bi(Ni2/3Ta1/3)O3‐PbTiO3 (BNT‐PT) ceramics were prepared by solid‐state reaction method. Morphotropic phase boundary between tetragonal and rhombohedral phase was observed around the composition 0.38BNT‐0.62PT, at which large photovoltages of 13.2V were obtained under 405 nm laser illumination with power density of 200 mW/cm2. By B‐site Ni2+ ions doping, the bandgap values of BNT‐PT solid solutions were reduced to 2.25~1.85 eV, and the anomalous photovoltaic response was extended from the ultraviolet region to a wavelength of 550 nm at the visible light region.
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  • 5
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    Mesoscale pore structure of a high‐performance concrete by coupling focused ion beam/scanning electron microscopy and small angle X‐ray scattering (2018)
    Wiley
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    Publication Date: 2018
    Description: Abstract This contribution couples (a) Small angle X‐ray scattering (SAXS) experiments of a high‐performance concrete (HPC) at the millimetric scale, and (b) Focused ion beam/scanning electron microscopy (FIB/SEM) of the cement paste of the HPC, with 10‐20 nm voxel size. The aim is to improve the understanding of the 3D pore network of the HPC at the mesoscale (tens of nm), which is relevant for fluid transport. The mature HPC is an industrial concrete, based on pure Portland CEMI cement, and planned for use as structural elements for deep underground nuclear waste storage. Small angle X‐ray scattering patterns are computed from the 3D pore images given by FIB/SEM (volumes of 61‐118 μm3). They are positively correlated with SAXS measurements (volumes of 5 mm3). Aside from correlations with FIB/SEM data, experimental SAXS allows to investigate a wider range of effects on the pore structure. These are mainly the HPC drying state, the presence of aggregates (by analyzing data on cement paste alone), and the use of Poly Methyl MethAcrylate resin impregnation.
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  • 6
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    Synthesis and characterization of (Pr, Ce)‐ZrSiO4 ceramic pigments: The properties of the pigments and the effect of Ce (2018)
    Wiley
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    Publication Date: 2018
    Description: Abstract A series of (Pr, Ce)‐ZrSiO4 ceramic pigments were synthesized and characterized using XRD, SEM, EDS, XPS, XRF, colorimeter, and a UV‐VIS‐NIR spectrometer. The prepared pigments were mainly composed of the zircon phase and were well crystallized. Only a specific amount of Pr was incorporated into the ZrSiO4 lattice. Compared with Pr, Ce was almost completely incorporated into the ZrSiO4 lattice and was homogeneously distributed within the pigment particles. The dopant Ce reduced the amount of Pr dissolved in the ZrSiO4 lattice and thus caused the b* values of the samples to decrease slightly. Meanwhile, the presence of Ce induced an apparent increase in red tone in the samples. The enhanced red tone resulted primarily from an increase in the absorption of light with wavelengths between 500 and 565 nm. High‐temperature stability analysis demonstrated that it is feasible to improve the tone of Pr‐ZrSiO4 pigment by doping with Ce.
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  • 7
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    Microwave heating controlled reactive melt infiltration for graphite‐Si‐SiC ceramics manufacturing (2018)
    Wiley
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    Publication Date: 2018
    Description: Abstract In this work, we investigated a procedure which exploits microwave ovens to produce SiC‐ based components by reactive melt infiltration of silicon into graphite preforms. The employed oven is designed to grant optical access to the sample surface, which allows to measure its temperature evolution though a noncontact pyrometer. This signal was used as a feedback to control the power provided to the preform and as an experimental output whose analysis provides insight into the reaction mechanism. Specifically, it is found that complete infiltration is achieved much before the end of the reaction. The latter is not fully self‐sustained as the global reaction rate continuously decreases with time until it is no more able to keep the temperature above the silicon solidification value. At that point, the reaction stops. The analysis of the processed samples proved that this procedure allows producing fully infiltrated samples without material failure by adjusting the heat provided during the infiltration stage rather than by tuning the preform structure and composition, which is the usual approach. The proposed method is less time and energy consuming than the standard one.
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  • 8
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    Stress‐induced anisotropy during sintering of hierarchical porosity ceramics (2018)
    Wiley
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    Publication Date: 2018
    Description: Intrinsic pores orient, preferentially, parallel to the applied compressive stress during sinter‐forging. Abstract There is significant interest in the design and processing of porous ceramics due to their use in a variety of applications including energy storage, catalysis, adsorption, separation, and life science applications. For many of these applications, it is desirable to have a hierarchical porous structure in which there is a distinct difference between sizes of pores. Our previous study has shown that microstructure and properties of porous materials become anisotropic after sinter‐forging. In particular, the small interparticle pores (intrinsic pores) orient parallel to the applied compressive stress, in contrast to large pores from pore formers (extrinsic), which orient perpendicular to the applied stress. However, the pore size, for transition from extrinsic to intrinsic behavior, (transient pore size) has not been quantified. In this study, we report on the effect of applied stresses during sinter‐forging on the morphology (shape and size) of pores of different size. Based on these results, we propose a two‐step approach to predict transient pore size for hierarchically porous ceramics. We use this approach to quantify the effect of applied stresses on the transient pore size. Finally, we postulate that the stress dependence of the transient pore size may be related to sintering stress—a fundamental quantity in continuum models of sintering. In addition, it can be used to calculate the effective surface energy of complex sintering systems.
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  • 9
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    A statistical model for the failure of glass plates due to nickel sulfide inclusions (2018)
    Wiley
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    Publication Date: 2018
    Description: Abstract Nickel Sulfide (NiS) inclusions can provoke the rupture of thermally treated glass due to a phase transformation with volume increase that stresses the surrounding glass. Starting from a Pareto statistics for the population of inclusion sizes, from an assumed kinetics of the phase transformation, a micro‐mechanically motivated model provides the statistical characterization of the probability of spontaneous failure of glass during lifetime. A distinction based upon the composition of NiS is used to discuss the effects of the heat soak test (HST), where glass remains at high temperature for a certain time to speed‐up the phase transformation and destroy those elements with critical inclusions. Three functions à la Weibull for the probability of spontaneous rupture during lifetime are theoretically derived for the case of no HST, short HST, and long HST. In particular, the probability of collapse for long HSTs depends upon the holding time in the oven. An explanatory example shows the potentiality of the model for optimizing the HST parameters toward a target probability of failure, but experimental campaigns are needed for a proper calibration.
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  • 10
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    Investigation of mechanical and thermal properties of rare earth pyrochlore oxides by first‐principles calculations (2018)
    Wiley
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    Publication Date: 2018
    Description: Abstract The pyrochlore‐type rare‐earth oxides attract considerable attentions due to their outstanding properties and extensive applications. In this work, contour maps of mechanical/thermal properties as a function of A and B cation radii across a wide variety of A2B2O7 (A = La‐Lu and Y; B = Ti, Sn, Hf, Zr, Pr and Ce) pyrochlore oxides are studied using the first‐principles calculations. The mechanical/thermal properties vary dramatically with increasing of the B cation sizes but do not show a strong systematic dependence on the A cation sizes. Furthermore, the machine learning algorithm is performed for the large family of pyrochlores and the parameters playing key role on mechanical/thermal properties are clarified. Besides, the expressions of focused mechanical and thermal properties are constructed. These results are expected to guide the future material design through composition tailoring.
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  • 11
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    High carriers transmission efficiency ZnS/SnS2 heterojunction channel toward excellent photoelectrochemical activity (2018)
    Wiley
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    Publication Date: 2018
    Description: Abstract ZnS has been found superiority in photoelectrochemistry for the fast response of photo‐inducing and its high conductor band position (~0.8 eV) results in strong reduction ability for hydrogen production. However, the solar absorbance of ZnS is much low for the wide band gap (~3.2 eV) and the carriers’ migration efficiency also need to be improved. Here, nano‐ZnS were coupled with ultrathin SnS2 nanosheets as heterojunction composites. This heterojunction composite demonstrated largely increase in specific surface area (from 4 to 12‐25 m2/g), obvious improvement of UV‐vis absorbance and narrower band gap. Furthermore, the carriers’ migration efficiency of ZnS/SnS2 heterojunction has been confirmed to be much higher by photocurrent response and electrochemical impedance spectroscopy. Due to the improvement in structure, compared with pristine ZnS, this ZnS/SnS2 heterojunction exhibited vast enhancement in photoelectrochemical performance. The composite with best activity exhibited 12.8 times enhancement in photocurrent density. The conduction band and valence band of ZnS are both more negative than those of SnS2, the photo‐induced electrons at the conduction band of ZnS will transfer into the conduction band of SnS2 while the photo‐induced holes at the valence band of SnS2 will transfer into the valence band of ZnS. In this way, the photo‐produced carriers will flow into different semiconductors and the carriers’ migration efficiency is enhanced. The work improves a new structure to develop the heterojunction property for photoelectrochemical application.
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  • 12
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    Low sintering temperature for lead‐free BiFeO3‐BaTiO3 ceramics with high piezoelectric performance (2018)
    Wiley
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    Publication Date: 2018
    Description: Lead‐free piezoelectric performance; relationship among sintering temperature, piezoelectric coefficient, and Curie temperature. Abstract Through modification of the heat‐treatment process using a higher heating rate and a lower binder burnout temperature, the piezoelectric performance of water‐quenched 0.67Bi1.05FeO3‐0.33BaTiO3 (BF33BT) lead‐free piezoelectric ceramics was improved. The observed physical properties of BF33BT ceramics were very sensitive to the process temperatures. The sintering temperature (TS) was changed within a narrow temperature range, and its effects were investigated. The largest rhombohedral distortion (90°‐αR = 0.14°) and tetragonality (cT/aT = 1.022) were observed for the ceramic sintered at 980°C, and its Curie temperature was 476°C. This ceramic showed good piezoelectric properties and large grains; the piezoelectric sensor charge coefficient (d33) was 352 pC/N, and the piezoelectric actuator charge coefficient () was 270 pm/V. The high piezoelectric performance and low TS of BF33BT ceramics indicate their potential as new low‐cost eco‐friendly lead‐free piezoceramics.
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  • 13
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    New insights into tricalcium silicate hydration in paste (2018)
    Wiley
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    Publication Date: 2018
    Description: Abstract The knowledge of the aqueous phase composition during the hydration of tricalcium silicate (C3S) is a key issue for the understanding of cement hydration. A new in situ method of computing calcium ion concentration from the measurement of the electrical conductivity on paste was coupled to isothermal calorimetry and BET measurements to get new insights on the early hydration of C3S. Ion concentrations of the aqueous phase are mainly dependent on the degree of hydration and the water to C3S ratio. In the case of C3S paste, the calcium and silicon concentrations determined at low degrees of hydration can be related to the equilibrium curve of C‐S‐H having C/S = 1.27 and named C1.27SHy. It is expected that C1.27SHy thermodynamically controls the aqueous phase composition at this early stage. Indeed, the formation of C1.27SHy is quasi‐immediate when C3S is in contact with water inducing a very rapid increase of the specific surface area that remains constant during the induction period. At higher degrees of hydration, the aqueous phase composition departs from the C1.27SHy equilibrium curve. C1.27SHy appears to be a metastable C‐S‐H that could be related to an intermediate phase previously reported. The quasi‐immediate precipitation of C1.27SHy on C3S surface explains why calcium and silicon concentrations remain low during early hydration even though C3S is strongly undersaturated. This also agrees with the control of the end of the induction period by the nucleation and growth of more stable C‐S‐H.
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  • 14
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    Electrocaloric behavior and piezoelectric effect in relaxor NaNbO3‐based ceramics (2018)
    Wiley
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    Publication Date: 2018
    Description: Abstract We firstly reported the electrocaloric properties in relaxor (1−x−y)NaNbO3–yBaTiO3–xCaZrO3 ceramics, and high electrocaloric effect (∆T ~0.451 K and∣∆T/∆E∣~0.282 Km/MV) can be realized in the ceramics (x = 0.04 and y = 0.10) under low temperature and low electric field. Relaxor behavior of NaNbO3 ceramics can be found by doping both BaTiO3 and CaZrO3. In addition, optimized piezoelectric effects (d33 ~235 pC/N and d33* ~230 pm/V) can be observed in the ceramics (x = 0.04 and y = 0.10) due to the involved morphotropic phase boundary (MPB). Excellent piezoelectric effect (ie, d33~330 pm/V at 41°C, and d33*~332 pm/V at 60°C) can be found because of the characteristics of MPB. Good temperature reliability of piezoelectric effect can be shown because of both MPB and relaxor behavior. We believe that the ceramics with high electrocaloric effect and good piezoelectric effect can be considered as one of the most promising lead‐free materials for piezoelectric devices.
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  • 15
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    A single‐phase full‐color emitting phosphor Na3Sc2(PO4)3:Eu2+/Tb3+/Mn2+ with near‐zero thermal quenching and high quantum yield for near‐UV converted warm w‐LEDs (2018)
    Wiley
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    Publication Date: 2018
    Description: Abstract A single‐phase full‐color emitting phosphor Na3Sc2(PO4)3:Eu2+/Tb3+/Mn2+ has been synthesized by high‐temperature solid‐state method. The crystal structure is measured by X‐ray diffraction. The emission can be tuned from blue to green/red/white through reasonable adjustment of doping ratio among Eu2+/Tb3+/Mn2+ ions. The photoluminescence, energy‐transfer efficiency and concentration quenching mechanisms in Eu2+‐Tb3+/Eu2+‐Mn2+ co‐doped samples were studied in detail. All as‐obtained samples show high quantum yield and robust resistance to thermal quenching at evaluated temperature from 30 to 200°C. Notably, the wide‐gamut emission covering the full visible range of Na3Sc2(PO4)3:Eu2+/Tb3+/Mn2+ gives an outstanding thermal quenching behavior near‐zero thermal quenching at 150°C/less than 20% emission intensity loss at 200°C, and high quantum yield‐66.0% at 150°C/56.9% at 200°C. Moreover, the chromaticity coordinates of Na3Sc2(PO4)3:Eu2+/Tb3+/Mn2+ keep stable through the whole evaluated temperature range. Finally, near‐UV w‐LED devices were fabricated, the white LED device (CCT = 4740.4 K, Ra = 80.9) indicates that Na3Sc2(PO4)3:Eu2+/Tb3+/Mn2+ may be a promising candidate for phosphor‐converted near‐UV w‐LEDs.
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  • 16
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    Issue Information (2018)
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    Publication Date: 2018
    Description: Journal of the American Ceramic Society, Volume 101, Issue 12, Page 5289-5292, December 2018. 〈br/〉
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  • 17
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    Cover Image, Volume 101, Issue 12 (2018)
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    Publication Date: 2018
    Description: Cover Photograph: Top‐left: SEM‐EDS analysis in line‐scan mode across the cross section at BZCY‐containing anode|GDC electrolyte interface. Top‐middle: Schematic diagram of the metal ion diffusion mechanism at BZCY‐containing anode|GDC electrolyte interface. Top‐right: OCV values of some typical DCO‐based SOFCs with or without electron‐blocking layers. Bottom‐left: TEM image of the grain from NiO‐BZCY|GDC foam|NiO‐BZCY. Bottom‐middle: EDS results in line‐scan mode of the grain from NiO‐BZCY|GDC foam|NiO‐BZCY. Bottom‐right: HRTEM image of the grain obtained from NiO‐BZCY|GDC foam|NiO‐BZCY.
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  • 18
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    Novel device for in situ process characterization of oxide/oxide ceramic matrix composites fabricated by flexible injection (2018)
    Wiley
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    Publication Date: 2018
    Description: Abstract In this work, a new manufacturing process of CMC by liquid molding was studied. An instrumented device has been developed to characterize the through‐thickness impregnation of ceramic fibers by a slurry charged with submicron ceramic particles. This instrument was used to characterize the permeability of the fibrous reinforcement and the formation of the ceramic cake by filtration of a ceramic particle suspension. Slurries containing different concentrations (10, 25, 33, and 40 vol%) of mono‐dispersed alumina particles were filtered under different pressure conditions (345, 415, 485 kPa) to optimize the cake formation and filling of fibrous reinforcements while controlling the porosity level. Ceramic cakes exhibited an average permeability of 1.0 × 10−17 m2 while the manufactured all‐oxide composites resulted in a permeability of 0.6 × 10−17 m2. Furthermore, a mathematical model based on Darcy's law was developed in this study to predict the rate of filtering and cake formation during injection using the permeability and filtration data measured with the experimental device. This mathematical model allows to determine the filtration time to produce a dense ceramic composite with an accuracy of ±15%, which corresponds to an error of less than 0.1 mm on the thickness of formed CMC.
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  • 19
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    Luminomagnetic Nd3+ doped fluorapatite coated Fe3O4 nanostructures for biomedical applications (2018)
    Wiley
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    Publication Date: 2018
    Description: Abstract Luminomagnetic nanostructured Nd3+ doped fluorapatite (FAP) coated Fe3O4 nanoparticles were produced by hydrothermal method. X‐ray diffraction analysis indicates that the prepared nanoparticles contain both FAP and Fe3O4 phases. Electron microscope analysis shows the formation of nanoparticles of Fe3O4 encased in rod shaped FAP nanoparticles of average length 40 nm. Magnetic measurements confirm the room temperature superparamagnetic nature of the nanoparticles with saturation magnetization value up to 7.8 emu/g. The prepared nanoparticles display strong near infrared (NIR) emission at 1060 nm under 800 nm excitation. Cell viability studies for 72 hour demonstrate the survival rate of over 84% with 500 μg/mL concentration indicating the good cytocompatibility of the prepared materials. The present Nd3+ doped FAP coated Fe3O4 nanostructure provides an excellent multifunctional platform for diagnostics and therapeutic applications.
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  • 20
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    Effects of water immersion and humid weathering on the near‐surface mechanical properties of phosphate laser glass (2018)
    Wiley
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    Publication Date: 2018
    Description: Abstract In this paper, the effect of water immersion and humid weathering on the near‐surface mechanical properties of phosphate laser (PL) glass was investigated using nanoindentation. The results indicate that, in the water immersion condition, the reduced modulus and nano‐hardness of PL glass decrease first, then increase and finally keeps unchanged with the increase in immersion duration; however, in the humid weathering condition, they decrease monotonously. The reaction mechanism occurring between water molecules and the glass network, especially during the later stage of the reaction process, determines the near‐surface mechanical properties and their differences when exposed to water and/or humid air. The results in this paper provide additional insight into the nano‐mechanics of glass surfaces, which also help understand the surface alteration process of phosphate laser glass during machining, storage, and serving in wet environments.
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    Effect of heating rate and process atmosphere on the thermodynamics and kinetics of the sintering of pre‐alloyed water‐atomized powder metallurgy steels (2018)
    Wiley
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    Publication Date: 2018
    Description: Abstract During the sintering of powder metallurgy steels the full removal of the iron oxide layer is required in order to develop strong inter‐particle necks. Although this iron oxide layer has low thermodynamic stability, its removal from the powder compact is a very complex process that is determined by a number of parameters such as temperature profile, sintering atmosphere, compact properties, powder properties, and additives. This paper summarizes these sintering parameters in correlation with the powder properties through the use of thermogravimetry analysis. In this work, hydrogen additions were identified as the most effective agent for the removal of the surface iron oxide during the early stages of sintering (at temperature range between ~300 and 600°C). The process depends on the heating rate and a rather low activation energy of 48 kJ/mol was determined for this reaction. Carbothermal reduction plays the largest role in the oxide reduction at high temperatures where two main reactions can be distinguished. The first was the reduction of the surface oxide residue and particulates, which occurred at temperatures between 950 and 1150°C. This reaction is characterized by an activation energy of 253 kJ/mol. Second was believed to be associated with the reduction of the internal oxides, occurred at temperatures above 1150°C. This reaction is characterized by rather high activation energy of 422 kJ/mol.
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    Multi‐component yttrium aluminosilicate (YAS) fiber prepared by melt‐in‐tube method for stable single‐frequency laser (2018)
    Wiley
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    Publication Date: 2018
    Description: Abstract The multi‐component glass fibers have demonstrated their unique advantages in the application of single‐frequency lasers due to their higher solubility of rare‐earth ions and thus a higher gain per unit length in a compact fiber laser cavity. In this study, multi‐component yttrium aluminosilicate (YAS) fiber with high doping concentration of Yb3+ was prepared by the “melt‐in‐tube” (MIT) method. A unit‐length gain of 3 dB/cm was obtained in a 4.4 cm‐long YAS fiber, the laser output slope efficiency reached 23.8% in a 10 cm‐long Yb:YAS fiber. Single‐frequency laser operation was achieved in a 1.7‐cm‐long Yb:YAS active fiber. To the best of our knowledge, this is the first demonstration of single‐frequency laser with this YAS glass fiber as gain medium. The novel multi‐component YAS fiber can be applied as a new gain material to realize single‐frequency fiber laser.
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    Colored amorphous silica‐based powder with TiN nanocrystals precipitated by ammonolysis of Ti–Si–O ternary glass (2018)
    Wiley
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    Publication Date: 2018
    Description: Ammonolysis changes the white color of amorphous titania‐silica to greenish brown to bluish color depending the preparation condition. Abstract Coloration of amorphous silica powder containing titania was investigated by nitridation in an ammonia flow. The oxide precursors were obtained by the hydrolysis of a mixture of tetraethyl orthosilicate (TEOS) and tetrabutoxy titanium (TBT). The color changed with the amount of TBT in the mixture, the hydrolysis pH and the ammonolysis temperature. The original white color of the 8 mol% TBT powder hydrolyzed under basic pH conditions changed to pale goldenrod at 700°C, then to dark olive green at 800°C, and further darkened with increasing ammonolysis temperature. A steel‐blue color appeared at 900°C for the powder obtained with 3 mol% TBT, and increased in darkness at 1000°C. A similar bluish color was observed for powders obtained by acidic hydrolysis after ammonolysis above 900°C, and this was independent of the amount of titania, although the chroma decreased with increasing firing temperature for the powder with 3 mol% TBT. The ammonolysis powder products were characterized using X‐ray diffraction (XRD), electron probe micro analysis (EPMA), transmission electron microscopy‐electron energy‐loss spectroscopy (TEM‐EELS), scanning transmission electron microscopy‐high‐angle annular dark‐field imaging (STEM‐HAADF) and Ti–K edge X‐ray absorption fine structure (XAFS). The color change was related to both precipitated TiN nanocrystals and residual titanium in the amorphous silica matrix. The TiN exhibited a goldish reflection and also plasmonic absorption from light blue to gray depending on the TiN crystallite size. The plasmonic absorption and resonance of nanocrystalline TiN will be useful similarly to that of gold in nanotechnology for various kinds of energy application.
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    ZnO‐based single crystal‐polycrystal structures for piezotronic applications (2018)
    Wiley
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    Publication Date: 2018
    Description: Abstract Electrostatic potential barriers at doped ZnO‐ZnO interfaces can be modified by stress‐induced polarization charges. This concept was enhanced by preparing ZnO‐based single crystal‐polycrystal‐single crystal structures by diffusion bonding. Increasing time for epitaxial solid‐state transformation results in structures with a decreasing thickness of residual polycrystalline material in between two well‐oriented single crystals. Microstructural and electrical analysis quantifies the influence of high‐temperature treatment during epitaxial growth on the stress sensitivity of the prepared structures. The orientation of the single crystals is defined to maximize the interaction between stress‐induced polarization charges and the potential barriers at doped ZnO‐ZnO interfaces. With decreasing thickness of residual polycrystalline material, the percentage of grain boundaries with favorably aligned polarization vectors is increased resulting in a higher stress sensitivity. This effect is compensated by an adverse effect of the high‐temperature treatment on the initial potential barrier height. Hence, a maximum in stress sensitivity can be observed for intermediate times of epitaxial growth. The prepared structures close the gap between the varistor piezotronics based on bulk ceramics with random orientation of the polarization vector and the bicrystal piezotronics with perfect orientation of the polarization vector, demonstrating the capability of microstructural engineering for varistor‐based piezotronic devices.
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    Glass‐forming region and enhanced Bi NIR emission in sodium tantalum silicate laser glass (2018)
    Wiley
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    Publication Date: 2018
    Description: Abstract Bismuth (Bi)‐doped glasses and fibers are of current interest as promising active media for new fiber lasers and amplifiers due to their 800‐1700 nm near‐infrared (NIR) emission. However, the optically active Bi centers in silica are easily volatilized during high‐temperature fiber drawing, which results in low Bi doping concentration and low gain NIR luminescence. Here, we explored the glass‐forming region in a model glass system of sodium tantalum silicate (Na2O‐Ta2O5‐SiO2) glass and attained suitable glass host for enhancing Bi NIR emission, right followed by detailed analysis on optical and structural characterization. Glass‐forming region roughly lies in where Ta2O5 ≤ 30 mol%, SiO2 ≥ 40 mol%, and Na2O ≤ 40 mol%. Not only is glass‐forming ability improved but also Bi NIR emission is enhanced (~60 times) by the introduction of Ta into glass network. Dissociated Na cations are restricted beside Ta, the high‐field‐strength element, so that the negative impacts of Na cations on glass formation and Bi NIR emission are weakened, which is responsible for the highly enhanced Bi NIR emission. This work helps us understand the glass‐forming of tantalum silicate glass systems and luminescent behaviors of Bi. Hopefully, it could contribute to designing the Bi‐doped laser glasses and high gain fibers with stable luminescent properties in future.
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    Pressure‐enhanced densification of TaC ceramics during flash spark plasma sintering (2018)
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    Publication Date: 2018
    Description: Abstract Flash spark plasma sintering (FSPS) offers extremely high heating rates to consolidate ceramics at a short time. However, significant grain growth sometimes occurs accompanied by rapid densification. In this work, a FSPS apparatus available for applying pressure was used to sinter TaC ceramics from powder compacts without preheating. It is found that the use of a higher pressure can efficiently promote densification and retard significant grain growth. Dense bulk TaC ceramics (95.18%) with average grain size of 4.09 μm were obtained in 90 seconds under 80 MPa. Such a process should facilitate the fast preparation of refractory ceramics with fine‐grained microstructure.
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    The effects of external fields in ceramic sintering (2018)
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    Publication Date: 2018
    Description: Abstract Field‐assisted processing techniques can enhance the kinetics of powder synthesis, accelerate sintering processes, and drive phase transformations at significantly lower temperatures compared to conventional methods. However, the exact nature of this nonthermal interaction between field and matter remains vastly speculative. A 2‐day workshop on “Electromagnetic Effects in Materials Synthesis” was organized at Carnegie Mellon University (Pittsburgh, USA) in June 2017, jointly sponsored by the U.S. National Science Foundation and the U.S. Office of Naval Research. This workshop gathered the scientific community working on field‐assisted techniques of materials processing. Inspired by the discussions held at the workshop, this paper summarizes the advancements to date and opens scientific questions and research opportunities in the three major field‐assisted sintering techniques (laser, microwave, and flash sintering). Significant challenges remain in (a) experimental design, measurements, and computational simulations to distinguish the nonthermal effects of the externally applied fields from conventional thermal phenomena; and (b) identifying fundamental mechanisms behind low temperature, nonthermal effects that produce phase transitions and microstructural evolution in materials under externally applied fields. We also present the recent developments in multiscale characterization techniques and the theory and modeling efforts, which aim to tackle the aforementioned grand multidisciplinary challenges facing researchers.
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    Low‐temperature synthesis of ultrafine TiB2 nanopowders by molten‐salt assisted borothermal reduction (2018)
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    Publication Date: 2018
    Description: Abstract The synthesis of TiB2 nanopowders arouses considerable interests due to its importance for implementing the extensive applications of TiB2 ceramic. Herein, the high‐purity ultrafine TiB2 nanopowders were successfully synthesized via a molten salt assisted borothermal reduction technique at a relatively low temperature of 1173 K using TiO2 and B powders as precursors within a KCl/NaCl salt. The results showed that the as‐obtained TiB2 nanopowders possessed a polycrystallinity structure, and their specific surface area and equivalent average particle size were 33.18 m2/g and 40 nm, respectively. This study provides a new low temperature synthesis technique of TiB2 nanopowders.
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    MnO2 thin film electrodes for enhanced reliability of thin glass capacitors (2018)
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    Publication Date: 2018
    Description: Journal of the American Ceramic Society, Volume 101, Issue 12, Page 5870-5870, December 2018. 〈br/〉
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    Synthesis, densification, and microstructure of TaC‐TaB2‐SiC ceramics (2018)
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    Publication Date: 2018
    Description: Abstract The usual way to prepare TaC‐TaB2 ceramics by adding B4C to TaC leads to formation of residual C, which degrades samples’ mechanical properties. To eliminate the residual C, we suggest incorporating Si together with B4C into TaC ceramics, resulting in new ultrahigh‐temperature ceramics (TaC‐TaB2‐SiC). Dense ceramics (〉99%) with SiC volume fraction ranging from 15.86% to 41.04% were fabricated by reactive spark plasma sintering at 1900°C for 5 minutes. The formation of SiO2‐based transient liquid phase was evidenced by the “film” in intermediate products, which can promote densification. The fine‐grained microstructure in final products was found to be associated with the in situ formed SiC, which impeded TaC and TaB2 grains from coarsening by the pinning effect. Besides, ultrafine TaB2 grains (~100 nm) produced during the reaction and then rearranged in liquid also contributed to grain refinement. Compared to TaC‐TaB2(‐C) ceramics prepared from TaC and B4C, the acquired composites exhibit better mechanical properties, due to their fine‐grained microstructures and the elimination of residual C.
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    Strong magnetoelectric effect in pulse laser deposited [Ba(Zr0.2Ti0.8)O3‐0.5(Ba0.7Ca0.3)TiO3]/CoFe2O4 bilayer thin film (2018)
    Wiley
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    Publication Date: 2018
    Description: Abstract Polycrystalline bilayer thin film of multiferroic [Ba(Zr0.2Ti0.8)O3‐0.5(Ba0.7Ca0.3)TiO3]/CoFe2O4([BZT‐0.5BCT]/CFO) has been deposited on Pt/Si (100) substrate using a pulsed laser deposition technique. The dielectric analysis reveals a significant change in the dielectric constant (~39% at a typical frequency of 100 Hz) at room temperature when a magnetic field is applied, in addition to a substantial improvement in the saturation polarization. A low leakage current density (~ 5 × 10−7 A/cm2) and a high magnetoelectric coupling coefficient (αE) both in the transverse (~2.085 V/Oe cm) as well as in the longitudinal (~0.708 V/cm Oe) directions, indicate in‐principle usability of this system for multifunctional device applications in thin film form.
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    Effect of additive structure and size on SiO2 formation in polymer‐derived SiOC ceramics (2018)
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    Publication Date: 2018
    Description: Abstract Silicon oxycarbide (SiOC) ceramics with highly adjustable properties and microstructures have many promising applications in batteries, catalysis, gas separation, and supercapacitors. In this study, additive structures on the nucleation and growth of SiO2 within SiOC ceramics are investigated by adding cyclic tetramethyl‐tetravinylcyclotetrasiloxane (TMTVS) or caged octavinyl‐polyhedral oligomeric silsesquioxane (POSS) to a base polysiloxane (PSO) precursor. The effects of the 2 additives on the polymer‐to‐ceramic transformation and the phase formation within the SiOC are discussed. POSS encourages SiO2 nucleation and leads to more SiO2 formation with significantly increased ceramic yield, which subsequently leads to higher specific surface of 1557 m2/g with a larger pore size of ∼1.8 nm for the porous SiOC. High TMTVS content decreases both the specific surface area and pore volume of the resulting porous SiOCs. This study demonstrates a new approach of using Si‐rich additive POSS to increase the SiOC yield while maintaining or even increasing the specific surface area.
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    Long‐term ceramic reliability analysis including the crack‐velocity threshold and the “bathtub” curve (2018)
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    Publication Date: 2018
    Description: Abstract A thermally activated crack‐velocity formulation that includes a threshold at thermodynamic equilibrium is used in the prediction of long‐term time‐to‐failure for brittle materials. A new closed‐form time‐to‐failure solution is derived for straight cracks propagating under the influence of constant stress. Explicit connections are made between the macroscopic crack‐velocity parameters and the underlying bond‐rupture parameters. A feature of the solution is the divergence of time‐to‐failure for applied loading approaching the thermodynamic threshold. A new reliability framework is developed and long‐term reliability and hazard predictions made using the time‐to‐failure solution. A bathtub hazard curve is shown to be generated by a single crack‐velocity failure mechanism.
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    Correlation between IR peak position and bond parameter of silica glass: Molecular dynamics study on fictive temperature (cooling rate) effect (2018)
    Wiley
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    Publication Date: 2018
    Description: Abstract The fictive temperature of glass is a consequence of its thermal history (cooling rate, primarily) and has a direct effect on physical and chemical properties of the glass. But, it is not easy to measure. The ability to nondestructively and spectroscopically measure it at room temperature would be of great benefit. Although empirical correlations have been established between fictive temperature and selected absorption peaks in the infrared spectra of silica glass, the fundamental understanding for this correlation has not been reported. Here, we use molecular dynamics simulations to show that the blue shift in the Si–O–Si asymmetric stretching peak of pure silica glass, which is known to correlate with a decrease in fictive temperature, can be attributed to a decrease in the average length of the Si–O bond in the silica network, not changes in the density or the Si–O–Si bond angle. The decrease in density at higher fictive temperatures of silica is associated with a decreased population of 5‐ and 6‐membered rings and broadening of the ring‐size distribution, and an increase in the average Si–O–Si bond angle.
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    Influence of carbon on stability, mechanical property, electronic structure, and lattice dynamics of silicon carbonitride (2018)
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    Publication Date: 2018
    Description: Abstract In this study, first‐principles calculations were performed to study the stability, mechanical property, electronic structure and lattice dynamics of β‐Si3(Cx,N1−x)4 silicon carbonitride. The solubility of carbon in β‐Si3(Cx,N1−x)4 having a stable structure is shown to be about 15 at.%. Within the limit of solubility, an increase in carbon concentration in β‐Si3(Cx,N1−x)4 will lead to a decrease in the Young's modulus and density and an increase in the Poisson's ratio. The study of deformation behavior shows that the most likely slip system of is on prismatic plane rather than on basal plane. This feature of β‐Si3(Cx,N1−x)4 is similar to WC. In addition, the ductility and fracture toughness of β‐Si3(Cx,N1−x)4 can be optimized by controlling the carbon concentration. The improvement in ductility and fracture toughness can be attributed to the formation of metallic bonds by the incorporation of carbon atoms. The lattice dynamics study shows that the structural stability of β‐Si3(Cx,N1−x)4 is controlled by energy stability criteria under stress‐free condition. In the stressed state, the structural stability of β‐Si3(Cx,N1−x)4 is controlled by the elastic stability criteria. Subsequently, the β‐Si3(Cx,N1−x)4 solid solution was prepared by self‐propagating high‐temperature synthesis (SHS), and the carbon‐concentration‐dependent mechanical properties were consistent with the first‐principles calculations. The maximum fracture toughness of 10.4 MPa·m0.5 was obtained in β‐Si3(Cx,N1−x)4 at carbon concentration of 5 wt%, which means that the solid solution toughening can be used as a supplement to crack bridging toughening and phase transition toughening for ceramic toughening. The results obtained in this study reveal that the β‐Si3(Cx,N1−x)4 solid solution is a promising candidate for high‐speed ceramic bearings.
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    CaO‐containing LaCO3OH nanogears and their luminescence and de‐NOx properties (2018)
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    Publication Date: 2018
    Description: Abstract Large‐scale, uniform, monodisperse LaCO3OH cherry‐blossom‐like nanogears and/or nanocubes have been synthesized under hydrothermal reaction conditions. Upon the addition of only 5 mol% Ca2+ ions into a La nitrate salts solution with pH 8.5, LaCO3OH crystals with novel cubic or nanogear structures are formed in the hexagonal phase. The hydrothermal reactions were carried out without the addition of a template or catalysts. Both 24 hour and 48 hour hydrothermal reactions yield 100% pure LaCO3OH with no irregular particles. We examined the photoluminescence properties of the as‐synthesized powders of the pure LaCO3OH nanogears and found one broad emission band centered at 394 nm after excitation at λ  =  280 nm. The NO reduction activity was also examined over highly dispersed CaO‐containing La2O3 obtained after calcination the LaCO3OH at 800○C for 2 hours. The CaO‐containing La2O3 catalysts showed good stability for NO reduction with CH4 in the presence of O2 and H2O vapor.
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    Magnetization reversal induced by Mn substitution in spinel chromite NiCr2O4 (2018)
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    Publication Date: 2018
    Description: Abstract Polycrystalline Ni(Cr1−xMnx)2O4 (0.1 ≤ x ≤ 0.325) ceramic samples were studied through different protocols of dc magnetization measurements. The samples exhibit 2 kinds of magnetic compensation effects below the ferrimagnetic transition temperature TC. Remarkable magnetization reversal is observed between the 2 compensation temperatures Tcomp1 and Tcomp2, which is regarded as arising from the negative exchange coupling between the 2 magnetic sublattices at different crystallographic sites. The magnetization is reversed at TSR due to spin‐reorientation caused by magnetostructural coupling. The spin‐reorientation is supported by Mn substitution and TSR is increased to 96 K when x reaches 0.325. However, it is suppressed due to the strong ionic site preference and thus the magnetization is slightly increased in the negative direction of the magnetic field. Near the 2 compensation temperatures, tunable magnetic switching effects can be obtained just by changing the magnitude of the applied magnetic field. Moreover, both normal and inverse magnetocaloric effects were also demonstrated.
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    Temperature‐stable dielectric and energy storage properties of La(Ti0.5Mg0.5)O3‐doped (Bi0.5Na0.5)TiO3‐(Sr0.7Bi0.2)TiO3 lead‐free ceramics (2018)
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    Publication Date: 2018
    Description: Abstract A new type of (0.7−x)Bi0.5Na0.5TiO3‐0.3Sr0.7Bi0.2TiO3‐xLaTi0.5Mg0.5O3 (LTM1000x, x = 0.0, 0.005, 0.01, 0.03, 0.05 wt%) lead‐free energy storage ceramic material was prepared by a combining ternary perovskite compounds, and the phase transition, dielectric, and energy storage characteristics were analyzed. It was found that the ceramic materials can achieve a stable dielectric property with a large dielectric constant in a wide temperature range with proper doping. The dielectric constant was stable at 2170 ± 15% in the temperature range of 35‐363°C at LTM05. In addition, the storage energy density was greatly improved to 1.32 J/cm3 with a high‐energy storage efficiency of 75% at the composition. More importantly, the energy storage density exhibited good temperature stability in the measurement range, which was maintained within 5% in the temperature range of 30‐110°C. Particularly, LTM05 show excellent fatigue resistance within 106 fatigue cycles. The results show that the ceramic material is a promising material for temperature‐stable energy storage.
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    Continuously tunable broadband emission of Mn2+‐doped low‐melting point Sn–F–P–O glasses for warm white light‐emitting diodes (2018)
    Wiley
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    Publication Date: 2018
    Description: Abstract Tin fluorophosphate (TFP) glass, which can be used to manufacture a phosphor‐in‐glass (PiG) for achieving high‐power white light‐emitting diodes (w‐LEDs), has attracted a great deal of attention because of its low‐melting point. Mn2+‐doped ultralow glass transition temperature (~122°C) Sn–F–P–O glasses were prepared to achieve broadband visible light emission from 390 to 720 nm. By controlling the concentration of MnO, the emission color of the TFP glass can be adjusted from blue/cool white to warm white/red. In particular, 0.2 mol% MnO‐doped TFP glass, which yields bright and warm white light and has ultralow glass transition temperature and thermal stability, has a promising application prospect in the field of high‐power w‐LEDs.
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    Thermochemistry of BaSm2O4 and thermodynamic assessment of the BaO–Sm2O3 system (2018)
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    Publication Date: 2018
    Description: Abstract The BaO–Sm2O3 system is of interest for the optimization of synthesis of electroceramics. The only systematic experimental study of phase equilibria in the system was performed more than 40 years ago. The reported experimental values of the enthalpy of formation of BaSm2O4 are in conflict, and the reported compound Ba3Sm4O9 has never been confirmed. In this work we synthesized BaSm2O4 by solid‐state reaction and determined its heat capacity, enthalpy of formation, and phase transitions by differential scanning calorimetry, high‐temperature oxide melt solution calorimetry and ultra‐high‐temperature differential thermal analysis, respectively. We confirmed the existence of Ba3Sm4O9 and its apparent stability from 1873 to 2273 K by X‐ray diffraction on quenched laser‐melted samples but were not able to obtain single‐phase material for calorimetric measurements. The CALPHAD method was used to assess phase equilibria in the BaO–Sm2O3 system, using both available literature data and our new measurements. A self‐consistent thermodynamic database and the calculated phase diagram of the BaO–Sm2O3 system are provided. This work can be used to model and thus to understand the relationships among composition, temperature, and microstructure for multicomponent systems with BaO and Sm2O3.
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    Multiscale designed SiCf/Si3N4 composite for low and high frequency cooperative electromagnetic absorption (2018)
    Wiley
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    Publication Date: 2018
    Description: Abstract With the aim to design a particular material for low and high frequency cooperative electromagnetic absorption at high temperature, a multiscale design is proposed by combining the microstructure and meta‐structure in one material. The SiCf/Si3N4 composite is prepared via the chemical vapor infiltration technique with SiCf as the EM wave absorbing phase and Si3N4 as the wave‐transparent ceramic matrix. The crossing grooved meta‐structure is designed and fabricated to further improve its absorbing properties and to guarantee its absorbing capacity stability at high temperature. A minimum reflection loss of −15.3 dB and −14.8 dB can be reached at 8 and 18 GHz with a total thickness of 5 mm. The temperature‐dependent reflection loss of the designed meta‐structure keeps relative reliable high temperature absorbing performances from room temperature to 500°C. This effective enhanced EM wave absorbing property is believed to be a consequence of multiscale effect induced by combining the traditional EM absorbing materials with metamaterial structure.
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    Synthesis of CuInTe2 nanowires: A polycrystalline‐to‐single‐crystalline transformation process (2018)
    Wiley
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    Publication Date: 2018
    Description: Abstract A facile solvothermal synthetic route has been developed to prepare CuInTe2 nanowires with the template aiding of anodic aluminum oxide. Microstructure analysis reveals that the as‐prepared single crystalline CuInTe2 nanowires have a [112] direction preferential growth. Oriented attachment mechanism has been proposed to explain the anisotropic growth of CuInTe2 nanowires during a polycrystalline‐to‐single‐crystalline transformation process. CuInTe2 nanowires have strong absorption in the visible region based on UV‐Vis absorption spectra measurement, confirming its suitability as a light absorbing material in solar cells.
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    Silicon carbide fiber oxidation behavior in the presence of boron nitride (2018)
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    Publication Date: 2018
    Description: Abstract The oxidation behavior of Sylramic SiC fibers without a boron nitride surface layer was compared to Sylramic iBN SiC fibers with a boron nitride surface layer by conducting thermogravimetric analysis in dry O2 at temperatures ranging from 800 to 1300°C for times up to 100 hours. Sylramic fibers followed the Deal and Grove oxidation kinetic model. A transient period of accelerated oxidation kinetics was observed with Sylramic iBN fibers. Raman spectroscopic analysis of oxidized fibers provided evidence for a borosilicate glass structure. The boron concentrations in the oxides, quantified by inductively coupled plasma‐optical emission spectrometry, were correlated with the weight change behavior, oxide thickness, and fiber recession of the oxidized fibers. Oxides formed from Sylramic iBN fibers were typically higher in boron concentration, which led to initial rapid oxidation rates that were 3‐10 times faster than observed for pure SiC. Slower oxidation rates followed as the oxide surface became increasingly enriched with SiO2 due to boria volatilization, thus limiting boria effects on SiC fiber oxidation kinetics. The accelerated high‐temperature oxidation of SiC fibers due to the presence of BN are discussed in terms of the borosilicate glass structure and composition.
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    Tailoring the nanostructure of anodic aluminum oxide cladding on optical fiber (2018)
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    Publication Date: 2018
    Description: Abstract We report a comprehensive investigation of fabricating nanostructured anodic aluminum oxide (AAO) cladding on optical fiber. We show that the pore size and interpore distance in the AAO cladding with pore channels vertically aligned to fiber surface can be readily controlled by applied voltage, the type, and concentration of electrolytic acid during anodization of aluminum‐coated optical fiber. The structural characteristics of the AAO cladding were examined by scanning electron microscopy (SEM) and analyzed using ImageJ software. Processing maps correlating AAO growth and anodization parameters were established. Compared to planar AAO growth on aluminum foil, higher growth rate as well as larger pore diameter and interpore distance were observed for AAO cladding formation on optical fiber under identical anodization conditions due to circumferential tensile stress in the AAO growth front at the convex AAO/aluminum interface. This tensile stress also contributed to radial cracking of the AAO cladding upon exceeding some threshold thickness.
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    Potassium sodium niobate (KNN)‐based lead‐free piezoelectric ceramic coatings on steel structure by thermal spray method (2018)
    Wiley
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    Publication Date: 2018
    Description: Abstract For the first time, potassium sodium niobate (KNN)‐based lead‐free piezoelectric ceramic coating with strong piezoelectric response was fabricated on stainless steel substrates by thermal spray process, after introducing NiCrAlY and yttria‐stabilized zirconia (YSZ) intermediate layers. A large effective piezoelectric coefficient (d33) of 125 pm/V was obtained with the thermal‐sprayed KNN‐based ceramic coating on the steel substrates. The mechanisms of improving the structure and enhancing the properties of the KNN‐based piezoelectric ceramic coatings by introducing the intermediate layers were analyzed. Ultrasonic transducers were designed and fabricated from the KNN‐based coatings directly formed on a steel plate structure, and the feasibility for generation and detection of ultrasonic waves for structural health monitoring using the thermal‐sprayed lead‐free piezoelectric ceramic coating was demonstrated.
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    Copper‐alumina nanocomposites derived from CuAlO2: Phase transformation and microstructural coarsening (2018)
    Wiley
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    Publication Date: 2018
    Description: Abstract Partial reduction of bulk CuAlO2 results in hierarchical structures wherein there are copper‐alumina regions with widely differing morphologies and scale. At the finest level, the distribution of the 2 phases is at the nanoscale. By means of atomic resolution STEM and electron diffraction, the nanocomposite regions were shown to consist of a dense array of metallic copper platelets dispersed in a matrix of θ‐Al2O3. The copper nanoplatelets were single crystal, and they all exhibited the same orientation relationship with the matrix, namely [110]Cu//[010]θ‐Al2O3, (111)Cu//()θ‐Al2O3. It was shown that the 2‐phase regions where the copper exhibited a significantly coarser, globular morphology, resulted from discontinuous coarsening. Interestingly, a change in the matrix phase from θ‐ to δ‐alumina was also observed as a result of the coarsening reaction. It is believed that in the nanocomposite regions, the θ‐alumina phase was stabilized by the lower interfacial energy between the copper (110) platelets and the matrix.
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    Fabrication of β‐tricalcium phosphate composite ceramic scaffolds based on spheres prepared by extrusion‐spheronization (2018)
    Wiley
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    Publication Date: 2018
    Description: Abstract In this study, β‐tricalcium phosphate/phosphate‐based glass (β‐TCP/PG) composite spheres were prepared by an extrusion‐spheronization method featuring high production and fine control of sphere size. Subsequently, fully interconnected β‐TCP composite ceramic sphere‐based (TCCS) scaffolds were fabricated by sintering the randomly packed β‐TCP/PG composite spheres. The results manifested that at least 20% microcrystalline cellulose (MCC) was required to obtain β‐TCP/PG composite spheres in good spherical shape. The prepared TCCS scaffolds showed hierarchical pore architecture, which consisted of interconnected macropores among the spheres, a hollow core in the sphere, plentiful medium‐sized pores in the sphere shell and micropores among the grains. The pore architecture and mechanical strength of the TCCS scaffolds could be tailored by adjusting the sintering temperature, sphere size, and amounts of PG and MCC in the β‐TCP/PG composite spheres. This work is believed to open up new paths for the design and fabrication of interconnected bioceramic scaffolds for application in bone regeneration.
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    Molecular structure of CaO–FeOx–SiO2 glassy slags and resultant inorganic polymer binders (2018)
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    Publication Date: 2018
    Description: Abstract The molecular structures of CaO–FeOx–SiO2 slags and their inorganic polymer counterparts were determined using neutron and X‐ray scattering with subsequent pair distribution function (PDF) analysis. The slags were synthesized with approximate molar compositions: 0.17CaO–0.83FeO–SiO2 and 0.33CaO–0.67FeO–SiO2 (referred to as low‐Ca and high‐Ca, respectively). The PDF data on the slags reasserted the predominantly glassy nature of this iron‐rich industrial byproduct. The dominant metal‐metal correlation was Fe–Si (3.20‐3.25 Å), with smaller contributions from Fe–Ca (3.45‐3.50 Å) and Fe–Fe (2.95‐3.00 Å). After inorganic polymer synthesis, a rise in the amount of Fe3+ was observed via the shift of the Fe–O bond length to shorter distances. This shortening of the Fe–O distance in the binder is also evidenced by the apparent rise of the Fe–Fe correlation at 2.95‐3.00 Å, although this feature may also suggest a potential aggregation of FeOx clusters. In general, the atomic arrangements of the reaction product was shown to be very similar to the precursor structure and the dominance of the Fe–Si correlation suggests the participation of Fe in the silicate network. The binder was shown to be glassy, as no distinct atom‐atom correlations were observed beyond 8 Å.
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    Amorphous zirconia at high pressure (2018)
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    Publication Date: 2018
    Description: Abstract We show, by means of ab initio calculations, that amorphous zirconia progressively transforms to a high‐density amorphous phase with the application of pressure. The average coordination number of Zr and O atoms under pressure rises gradually to 8 and 4, respectively. The main building unit of the dense noncrystalline state is the eightfold‐coordinated Zr atoms (62.5%). When the coordinated modification of Zr atoms in the zirconia crystal at high pressure and temperature conditions is considered, it can be perceived that amorphous zirconia follows a transformation mechanism similar to the one observed at high temperature but different than the one detected at high pressure. The dense disordered phase is indeed found to be locally comparable with the high‐temperature tetragonal crystal. Upon decompression, some high‐pressure arrangements are persevered in the model and a transformation into another amorphous state whose structure is intermediate between uncompressed and dense amorphous phases is observed in the simulations. The high‐pressure amorphous structures are found to be semiconductors with a band gap smaller than that of the original model.
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    Temperature‐insensitive piezoelectricity in lead‐free NaNbO3‐based ceramics (2018)
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    Publication Date: 2018
    Description: Abstract In this work, we report a lead‐free piezoelectric ceramic of (0.9‐x)NaNbO3‐0.1BaTiO3‐xBaZrO3, and the effects of BaZrO3 on the phase structure, microstructure, electrical properties and temperature stability are investigated. A morphotropic phase boundary‐like region consisting of rhombohedral (R) and tetragonal (T) phases is constructed in the compositions with x = 0.035‐0.04. More importantly, in situ temperature independence of the piezoelectric effect {piezoelectric constant (d33) and strain} can be achieved below the Curie temperature (Tc). Intriguingly, the electric field‐induced strain is still observed at T ≥ Tc due to the combined actions of the electrostrictive effect and the electric field‐induced phase transition. We believe that NaNbO3‐based ceramics of this type have potential for applications in actuators and sensors.
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    Dy and Eu activated Ca3B2O6 phosphors for near ultraviolet‐based light‐emitting diodes (2018)
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    Publication Date: 2018
    Description: Abstract The Dy‐ and Eu‐activated Ca3B2O6 phosphors were synthesized by a high‐temperature solid‐state reaction technique and their structural and luminescent properties were investigated. The phosphors are characterized by X‐ray diffraction, photoluminescence spectra, and Commission International de I'Eclairage (CIE) chromaticity coordinates. It is found that the charge compensator Na+ plays an important role in modifying the emission spectral profiles of Dy and Eu ions in the phosphors. The ratio of the emission located at the yellow wavelength portion to that located at the blue wavelength region of the Dy3+ ions can be apparently tuned by changing the Na+ content. The luminescence intensity of the phosphors can be enhanced with introducing Na+ ions as well. The emission colors of Dy/Eu codoped phosphors change from blue to white and successfully acquire the superior white light emission (x = 0.330, y = 0.329) by appropriately tuning the Na+/Dy3+ content and the excitation wavelength. The energy transfer process from Eu2+ to Dy3+ and Eu3+ occurs in the Dy/Eu codoped phosphors, providing a further approach to modify the emission spectral profile of the examined phosphors. The phosphors presented here have promising applications in the fields of light‐emitting diodes.
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    Full‐visible spectrum emitting Zn–In–S:Cu/ZnS nanocrystals based on varying Cu concentrations and its application in white LEDs (2018)
    Wiley
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    Publication Date: 2018
    Description: Abstract A series of high‐quality Cu‐doped Zn–In–S nanocrystals (d‐NCs) were prepared by a conventional hot injection process. The full‐visible spectrum emission from 480 to 648 nm can be easily achieved by adjusting the Cu doping concentration in the Zn–In–S system, but not by varying the ratio of In/Zn in the alloyed host material. After wrapping the ZnS shell around the Zn–In–S:Cu d‐NCs core, the resultant Zn–In–S:Cu/ZnS core/shell d‐NCs not only exhibited an enhanced prominent photoluminescent quantum yield (PLQY) up to 65% but also possessed the excellent thermal, photochemical stability, and longer PL lifetime. Furthermore, high color rendition white light was generated from a single color converter Zn–In–S:Cu/ZnS core/shell NCs‐assisted white light‐emitting diodes (LEDs). Under operation of 38 mA forward bias current, the fabricated white LEDs emitted bright natural white light with a luminous efficiency of 62 lm/W, and the correlated color temperature of 5658 K. Simultaneously, the good color stability was accompanied by the CIE color coordinates of (0.3287, 0.3527) under different forward bias currents.
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    Development of a lead‐free copper co‐fired BNT‐based piezoceramic sintered at low temperature (2018)
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    Publication Date: 2018
    Description: Abstract Compatibility of Bi‐based piezoelectric ceramic and copper electrodes is demonstrated by co‐firing 0.88Bi1/2Na1/2TiO3–0.08Bi1/2K1/2TiO3–0.04BaTiO3 (BNKBT88) with copper. A combination of Bi2O3, CuO, ZnO, Li2CO3, and B2O3 are used as additives to reduce firing temperature to 900°C with minimal effect on the electromechanical properties compared to sintering at 1150°C without additives. Co‐firing with copper electrodes requires controlled oxygen sintering at low temperature. The atmosphere is controlled using carbon dioxide and hydrogen gas to maintain an oxygen partial pressure of 6.1 × 10−8 atm, which is necessary for the coexistence of Cu metal and Bi2O3. The thermodynamic activity of bismuth oxide in BNKBT88 is calculated to be 0.38. BNKBT88 ceramics were successfully co‐fired with internal as well as surface Cu metal electrodes. The copper co‐fired ceramics were successfully polarized and the dielectric and piezoelectric properties are evaluated.
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    Chemomechanical analysis for interfacial reactions between γ‐TiAl alloy and glass‐ceramic coating in micro/nano scale (2018)
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    Publication Date: 2018
    Description: Abstract To simulate interfacial reactions process and deformation in γ‐TiAl and glass‐ceramic coating system at high temperature, we developed a coupled chemomechanical model with chemical reactions, diffusion, and mechanical deformation incorporated. Tensile radial and circumferential stresses develop in interfacial zone and coatings as the interfacial reactions occur. Tensile radial stress could tear coatings off alloys while the tensile circumferential stress could result in crack initiation and propagation, even spalling of coatings. The bulk radial stress can be markedly reduced by taking the surface effect into account. Consequently, tensile radial stress vanishes in alloy‐coating system on nano scale.
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    Lithium aluminum‐layered double hydroxide chlorides (LDH): Formation enthalpies and energetics for lithium ion capture (2018)
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    Publication Date: 2018
    Description: Layer double hydroxide (LDH) structure (left) and formation enthalpies vs various compositions (right). Abstract Layered aluminum double hydroxide chloride sorbents, LiCl∙Al2(OH)6.nH2O, Li‐LDH, have shown promising application in selective Li extraction from geothermal brines. Maintaining LiCl uptake capacity and retaining a long cycle life are critical to widespread application of sorbent materials. To elucidate the energetics of Li capture, enthalpies of LDH with different Li content have been measured by acid solution calorimetry. The formation enthalpies generally become less exothermic as the Li content increases, which indicates that Li intercalation destabilizes the structure, and the enthalpies seem to approach a limit after the Li content x = 2Li/Al exceeds 1. To improve stability, metal doping of the aluminum LDH structure with iron was performed. Introduction of a metal with greater electron density but a similar ionic radius was postulated to improve the stability of the LDH crystal structure. The calorimetric results from Fe‐doped LDH samples corroborate this as they are more exothermic than LDH‐lacking Fe. This suggests that Fe doping is an effective way to stabilize the LDH phase.
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    The anomalous increase in tunability in Ba0.55Sr0.45TiO3‐MgO‐Mg2SiO4 composite ceramics (2018)
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    Publication Date: 2018
    Description: Abstract The Ba0.55Sr0.45TiO3–Mg2SiO4–MgO composite ceramics were prepared via solid‐stated method and the dielectric tunable properties of composite ceramics were investigated. With the increase in dielectrics content, the tunability of composites ceramics increased abnormally first and then decreased, while the anharmonic coefficient increased monotonously. A simple 3D Finite Element model was developed to simulate the dielectric response and an anomalous increase in tunability when increasing the dielectrics content in composites was obtained. The simulation indicated that adding dielectrics into ferroelectrics causes the redistribution of the electric field. Increasing dielectrics content, the average electric field strength in the enhanced area of ferroelectrics increases, which lead to the increase in the tunability. The abnormal increase in tunability originates from the enhancement of the electric field on a fraction of the ferroelectrics around dielectrics.
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    Planetary ball‐milling of AlON powder for highly transparent ceramics (2018)
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    Publication Date: 2018
    Description: Abstract A detailed investigation of planetary ball‐milling for coarsened AlON powder was carried out. Our results showed that the weight ratios of milling ball‐to‐powder, the revolution rate and the planetary ball‐milling time have significant impacts on the microscopic morphology, particle size distribution and average particle size of powder. The process and mechanism were analyzed, and the outcome of our study can be used to optimize the complicated planetary ball‐milling method by controlling the planetary ball‐milling time or adjusting the revolution rate at the final stage of planetary ball‐milling. Sequentially, using fine and uniform AlON powder by optimized planetary ball‐milling with an average particle size below 300 nm and excellent sintering properties, highly transparent AlON ceramic with an in‐line transmittance of 84% at 2000 nm was successfully prepared through pressureless sintering at 1880°C for 6 hours using the elaborative treated powder synthesized from carbothermal nitridation method.
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    Tailoring particle alignment and grain orientation during tape casting and templated grain growth (2018)
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    Publication Date: 2018
    Description: Abstract The quality of crystallographic alignment in textured ceramics produced by tape casting and templated grain growth (TGG) has been little studied despite its demonstrated impact on magnetic, piezoelectric, and optical properties. Physical and crystallographic alignment of anisotropic template particles is shown to be directly linked to the casting rate, gap height, and casting viscosity during tape casting. These parameters are shown to affect the shape and magnitude of the shear rate profile under the doctor blade during casting which in turn causes a gradient in the torque acting on anisotropic particles. The magnitude of the torque, the time the slurry is exposed to torque during casting, and the ratio of casting height to template diameter are demonstrated to enable the particle alignment process to be tailored to produce well‐aligned template particles. Crystallographic alignment of the textured ceramic was quantified by grain misalignment angle (full width at half maximum, FWHM) and degree of orientation (r) and is directly correlated with the degree of torque during casting. High‐quality alignment (FWHM = 4.5°; r = 0.13) was demonstrated in the model TGG system consisting of submicrometer alumina and 5 vol% 11 μm diameter template platelet particles.
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    Residual stress determination of silicon containing boron dopants in ceramic matrix composites (2018)
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    Publication Date: 2018
    Description: Abstract Raman spectroscopy was utilized to investigate residual stresses found within a SiC/SiC ceramic matrix composite containing Hi‐Nicalon™ fibers, a slurry melt‐infiltrated matrix of silicon carbide particles, and silicon matrix. Large gradients of electrically active boron are found throughout various regions within the crystalline lattice of the silicon matrix. The regions were identified by the varying degrees of asymmetry and peak width measured in the resonant Fano profile of the doped silicon. A methodology to determine the residual stress state of silicon exhibiting varying degrees of electrically active boron is presented by utilizing the changes in the Raman profile parameters. Previous works on similar SiC/SiC CMCs have attributed spatial gradients in the wavenumber to large fluctuations in stress. By applying the proposed methodology, we show that these observations are related to active boron that is segregated in various matrix areas. Utilizing this methodology, mean compressive stresses in various silicon regions were found to be approximately 300 MPa, with complementary tensile silicon carbide particle stresses of approximately 300 MPa.
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    Development of boron oxide potentials for computer simulations of multicomponent oxide glasses (2018)
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    Publication Date: 2018
    Description: Abstract Molecular dynamics and related atomistic computer simulations are effective ways in studying the structures and structure–property relations of glass materials. However, simulations of boron oxide (B2O3)‐containing oxide glasses pose a challenge due to the lack of reliable empirical potentials. This paper reports development of a set of partial charge pairwise composition‐dependent potentials for boron‐related interactions that enable simulations of multicomponent borosilicate glasses, together with some of the existing parameters. This set of potentials was tested in sodium borate glasses and sodium borosilicate glasses and it is shown capable to describe boron coordination change with glass composition in wide composition ranges. Structure features such as boron N4 value, density, Qn species distribution, fraction of non‐bridging oxygen around boron and silicon, total correlation function, and bond angle distribution function were calculated and compared with available experimental data. Mechanical properties of the simulated glasses calculated with the new potential also show good agreement with experiments. Therefore, this new set of potential can be used to simulate boron oxide‐containing multicomponent glasses including those with wide industrial and technology applications.
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    Reactive flash sintering: MgO and α‐Al2O3 transform and sinter into single‐phase polycrystals of MgAl2O4 (2018)
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    Publication Date: 2018
    Description: Abstract We show that flash experiments with three phase mixed‐powders of yttria‐stabilized zirconia (8YSZ), MgO, and α‐Al2O3 not only produce polycrystals of high density, but also the transformation of magnesia and alumina into single‐phase spinel. The presence of zirconia facilitates the onset of the flash. The sintering experiments in the laboratory were extended to live experiments at the National Synchrotron Light Source II at Brookhaven National Laboratory in order to measure the time‐dependent evolution of single‐phase spinel. The phase transformation occurred in 〈3 seconds during Stage II. Later, during Stage III the cubic zirconia transformed partly into the monoclinic phase, which reverted back to the cubic phase when the flash was extinguished by turning off the current to the specimen. The results underpin a recent report on the synthesis of single‐phase bismuth ferrite from constituent oxides in reactive flash experiments, raising the specter of flash as a method for synthesis as well as sintering of complex oxide ceramics. The role of zirconia in catalyzing the flash in the present study is discussed.
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    Building submicron crystalline piezoceramics: One‐step pressureless sintering partially amorphized nanopowder (2018)
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    Publication Date: 2018
    Description: Abstract The key to miniaturization of piezoelectric devices is to build high‐performance fine‐grained piezoceramics. Although the preparation of fine‐grained ceramics can be achieved by hot press sintering or spark plasma sintering, complex processes, and high costs hinder the popularization of such technologies. In this work, a one‐step conventional pressureless sintering technique based on partially amorphized nano‐precursors has been proposed for low‐cost preparation of submicron‐structured ceramics. Using this technology, 0.36BiScO3–0.64PbTiO3 ceramics with an average particle size of 170 nm and a relative density of 95% can be prepared at temperature as low as 900°C, while the samples still have excellent piezoelectric properties (d33 = 220 pC/N, g33 = 40 × 10−3 Vm/N). In the process of sintering partially amorphized nano‐precursors, the initial densification rate is faster than the grain growth rate, which can be attributed to two effects promoting low temperature densification, one is the liquid phase sintering mechanism associated with the amorphous phase, and the other is the filling effect of small particles deposited at the grain boundaries. This work is simple and easy to implement, and it is expected to be extended to the preparation of other types of fine‐grained ceramics.
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    Three‐dimensional (3D) flexible nanofibrous network knitting on hierarchical porous architecture (2018)
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    Publication Date: 2018
    Description: Abstract Hierarchical structured porous ceramics have attracted tremendous research interests because of their numerous excellent properties including robust mechanical strength and large surface area. In this work, silicon carbide (SiC)‐based porous ceramics with three levels of pore hierarchy are fabricated from silicon particle‐stabilized foams and a subsequent one‐step calcination after they were embedded with coke. Three‐dimensional (3D) flexible nanofibrous network is adhered and wrapped on cell walls of porous ceramics, which is readily fine‐tuned and tailored by the temperature to provide optimized pore structure. The resultant SiC‐based porous ceramics present a density of 1.03 g/cm3 at a porosity of 72% with a large quantity of hierarchical micro‐ and macropores. This hierarchical structure leads to robust compressive strength (23.52 MPa) and large surface area (64.32 m2/g). The fabrication method is straightforward and sought‐after, providing a facile technical route for advanced hierarchical porous ceramics used in filtration and catalysis fields.
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    Structural evolution of thermal sprayed bismuth sodium titanate piezoelectric ceramic coatings (2018)
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    Publication Date: 2018
    Description: Abstract The structural evolution of lead‐free piezoelectric bismuth sodium titanate (BNT) coatings with excess Bi derived from thermal spray process was investigated with transmission electron microscopy (TEM). Bi‐rich composition was identified as the nucleation agent of the BNT perovskite phase, and fine‐grained microstructure was obtained with more excess Bi. The Bi‐rich composition of the perovskite phase crystallized from melt was BixNayTiO3 (0.5 ≤ X ≤ 0.55, 0.46 ≤ y ≤ 0.5). The crystallization of the Bi‐rich BNT perovskite phase first from liquid precursor phase would result in continuous depletion of Bi composition in the residual amorphous phase, and thus Ti‐rich amorphous phase in the as‐deposited coating and sodium titanate secondary phase in the heat‐treated coating formed in the composition without enough access Bi. The results and analyses suggest only appropriate compensation with adequate amount of excess Bi can realize single perovskite phase in thermal sprayed BNT‐based coatings and hence the superior piezoelectric performance property.
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    Design on improving piezoelectric strain and temperature stability of KNN‐based ceramics (2018)
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    Publication Date: 2018
    Description: Abstract Lead‐free 0.99(0.96K0.46Na0.54Nb1‐xTaxO3‐0.04Bi0.5(Na0.82K0.18)0.5ZrO3)‐0.01CaZrO3 (0.99(0.96KNNTax‐0.04BNZ)‐0.01CZ) ceramics were prepared by a solid‐state sintering method. Ta2O5 doped in the 0.99(0.96KNNTax‐0.04BNZ)‐0.01CZ ceramics results in a phase structure transition from the orthorhombic (O)/tetragonal (T) phase to the rhombohedral (R)/T phase. The Ta2O5 dopant induces a decrease in the average grain size from ~1.70 to ~0.69 μm. At x = 0.02 and 0.04, the ceramics have a high reverse piezoelectric coefficient (~500 pm/V under 25 kV/cm). The ceramics with x = 0.04 show an optimal level of unipolar strain, reaching 0.17% under 35 kV/cm at room temperature, and their field‐induced strain varies 〈10% in the temperature range from 25 to 135°C. The presence of the O phase in the polymorphic phase boundary (PPB) improves the temperature stability the reverse piezoelectric coefficient (). Obtaining KNN‐based ceramics with good piezoelectric properties and weak temperature sensitivity by designing a R/O/T phase boundary and controlling the average grain size to the submicrometer level is highly feasible.
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    Transparent red ‐emitting silicone resin for color conversion and encapsulation of NUV light‐emitting diodes (2018)
    Wiley
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    Publication Date: 2018
    Description: Schematic illustration of the processes involved in the synthesis of the EFHSN and the application of the EFHSN. Abstract Herein, a novel transparent phenyl vinyl silicone resin nanocomposite containing exfoliated fluorescent Eu‐containing hydrotalcite‐like compound (EFHSN) is developed to serve as a dual role of color conversion and encapsulation material. Excited by near‐NUV light, the EFHSN emits a strong characteristic red light (614 nm). Meanwhile, the EFHSN shows not only excellent transparency (T 〉 95%) in the UV‐Vis region and high refractive index (1.46) but also superior thermal stability, which could meet the need of commercial encapsulation materials and optical application. Therefore, the EFHSN is expected to be used in InGaN‐based NUV light‐emitting diodes and flexible optoelectronic device.
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    Submicron‐grained Yb:Lu2O3 transparent ceramics with lasing quality (2018)
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    Publication Date: 2018
    Description: Abstract We report on our recent progress of fabricating Yb3+‐doped Lu2O3 transparent ceramics for 1 μm solid‐state laser application. Well‐dispersed 3.3 at.% Yb:Lu2O3 nanopowders were synthesized using a co‐precipitation method. Without using any sintering aids, the Yb:Lu2O3 nanopowders could be densified by vacuum sintering at 1500°C/10 hours followed by HIPing at 1480°C/4 hours. Such obtained Yb:Lu2O3 ceramics had not only dense microstructure and submicron grain size of about 0.6 μm, but also in‐line transmission of 80.0% at 600 nm. Preliminary continuous wave (CW) laser experiments with an uncoated Yb:Lu2O3 ceramic slab have demonstrated highly efficient CW laser oscillation at 1079.8 nm.
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    Diffused and successive phase transitions of (K, Na)NbO3‐based ceramics with high strain and temperature insensitivity (2018)
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    Publication Date: 2018
    Description: Part I: Schematic diagram of temperature‐dependent permittivity for three kinds of phase boundary. Part II: In situ temperature dependence of XRD (2θ=44.5–46o) for the sample owing the diffused and successive phase transitions. Part III: Temperature‐dependent d33* and d33 in this work and other representative works including CZ5 (diffused O‐T, Adv. Func. Mater., 2013), non‐textured LF4 (O‐T, Nature, 2004), KNNL‐BNT‐BZ (diffused R‐T, J. Mater. Chem. C, 2018) and KNN‐BLT‐BZ (R‐T, J. Am. Ceram. Soc., 2015). Abstract Phase boundaries realize enhanced piezoelectricity in lead‐free (K, Na)NbO3‐based ceramics but suffer from the weakness of undesirable temperature sensitivity. Here, an effective method is designed to develop temperature‐insensitive piezoelectricity (small signal piezo‐coefficient [d33] and large signal piezo‐coefficient []) in KNN‐based piezoceramics by constructing the diffused and successive phase transitions, which results in a broadness of the optimal temperature range of the electrical properties. The room‐temperature value in KNN‐based ceramics modified with BaZrO3 and (Bi0.5Na0.5)HfO3 reaches as high as 540 (±10) pm V−1, which is higher than PZT‐5H and most reported KNN‐based systems. Notably, superior temperature insensitivity of the and Pr values is also observed among the diffused and successive phase transitions region (20‐100°C), with 〈5% fluctuation. In addition, the in situ temperature‐dependent d33 measurement shows a high‐temperature reliability and less fluctuation (〈15%) in a wide temperature range (20‐120°C). These results open a new window for further development of highly temperature‐insensitive lead‐free piezoceramics.
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    Multi‐phase glass‐ceramics containing CaF2: Er3+ and ZnAl2O4:Cr3+ nanocrystals for optical temperature sensing (2018)
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    Publication Date: 2018
    Description: Abstract Oxyfluoride transparent glass‐ceramics (GC) containing CaF2 and ZnAl2O4 nanocrystals have been fabricated with melt‐quenching method. By carrying out the heat treatment of the precursor glass (PG), Er3+ and Cr3+ were selectively partitioned into CaF2 and ZnAl2O4 nanocrystals, respectively. The obtained multi‐phase GC exhibited strong upconversion (UC) fluorescence of Er3+ as well as intense down‐conversion (DC) fluorescence of Cr3+. Under 980 nm excitation, the green UC fluorescence of Er3+ due to 2H11/2,4S3/2 → 4I15/2 transition and the red DC fluorescence lifetime of Cr3+ due to 2E, 4T2 → 4A2 transition were found to be highly dependent on the temperature and makes them possibly suitable for Optical Thermometry. With least‐square fitting methods, the FIR of Er3+ from thermally coupled energy states (2H11/2 and 4S3/2) produced maximum temperature sensing sensitivity values of 0.33% K−1 at 437 K and 0.36% K−1 at 267 K, respectively. Similarly, fluorescence lifetime of Cr3+ attributed to the parity forbidden (2E → 4A2) and spin allowed (4T2 → 4A2) produced the maximum temperature sensor sensitivity value equal to 0.67% K−1 at 535 K.
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    NMR study on reaction processes from aluminum chloride hydroxides to alpha alumina powders (2018)
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    Publication Date: 2018
    Description: Abstract Starting from gelatinous aluminum chloride hydroxide, the transformation process toward α‐Al2O3 was examined using 27Al NMR, both in the liquid and solid states, as a main analytical tool. By increasing the hydrolysis ratio (h, defined as [OH−]/[Al3+]) of the starting aqueous precursor up to h = 2.5, the transition temperature to the final product, α‐Al2O3, decreased to as low as 500°C. In this case, the structural change from amorphous alumina to α‐Al2O3 took place without intermediate transition Al2O3 phases. Examining the process of networking during the transition from aqueous sol–through the state of xerogel–to final anhydrous oxide by nuclear magnetic resonance (NMR) revealed the presence of highly polymeric species mainly ascribed to δ‐[Al2O8Al28(OH)56(H2O)24]18+ (δ‐Al30). δ‐Al30 species were found in the solution phase and became predominant after drying. We conclude that the lower temperature synthesis of α‐Al2O3 became possible due to preformation of polymerized AlO6 construction units in the precursor, reducing the energy barrier for the nucleation of the final α‐Al2O3 phase.
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    Phenomenological modeling of phase transitions and electrocaloric effect in Ba(Zr0.2Ti0.8)O3‐(Ba0.7Ca0.3)TiO3 (2018)
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    Publication Date: 2018
    Description: Abstract Ba(Zr0.2Ti0.8)O3‐x(Ba0.7Ca0.3)TiO3 (BZT‐xBCT) is a promising lead‐free ferroelectric system. In this paper, we present two sets of free energy coefficients and carry out phenomenological modeling to study the phase transition and electrocaloric effect. The calculated phase diagram is in excellent quantitative agreement with experiments. Furthermore, we propose a new method based on effective internal electric field to simulate polarization in the macroscopic paraelectric state of ferroelectric relaxor. The computed composition and temperature‐dependent entropy and temperature change induced by electrocaloric effect are in good agreement with the measured data available for single crystal.
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    Pressure‐induced ferroelectric‐relaxor phase transition in (Bi0.5Na0.5)TiO3‐based ceramics (2018)
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    Publication Date: 2018
    Description: Abstract In this work, the effects of composition, temperature, and poling on the phase transition behavior under hydrostatic pressure of lead‐free (1‐x)[(0.98Bi0.5Na0.5Ti0.995Mn0.005O3‐0.02BiAlO3)]‐xNaNbO3 (BNT‐BA‐xNN) ceramics were investigated. It was found that hydrostatic pressure can induce a phase transformation from ferroelectric (FE) to relaxor (RE). Meanwhile, the FE‐RE phase transition pressure (PFR) tends to decrease as the NN addition increases. Moreover, increasing temperature exhibits a similar effect that is, reducing PFR. The reduced PFR were considered to result from the reduced FE stability and increasing proportion of RE phase. However, PFR was obviously enhanced after poling, which can be attributed to the induced FE phase and the formation of macrodomains with application of an electric field. These results will aid in understanding the phase transition behavior of BNT‐based relaxor ferroelectrics for applications under hydrostatic pressure.
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    Improved ZT in ball‐milled and spark plasma sintered Cu (2018)
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    Publication Date: 2018
    Description: Ball‐.milling Cu15As30Te55 glass favors the appearance of the β‐As2Te3 crystalline phase in the sintered glass‐ceramics and improves the thermoelectric figure of merit ZT. Abstract The influence of ball milling followed by Spark Plasma Sintering (SPS) on the thermoelectric properties of the Cu15As30Te55 glass composition has been investigated by means of X‐ray diffraction, scanning electron microscopy, and differential scanning calorimetry. The microstructure, composition, and thermal stability of the ball‐milled samples before and after SPS have been correlated with the electrical and thermal transport properties. Upon ball milling, a glass‐to‐glass transformation is evidenced for short ball‐milling times, followed by crystallization of the metastable β‐As2Te3 phase within the glassy matrix. This glass‐to‐glass transformation favors the occurrence of the β‐As2Te3 phase during the SPS process. A maximum figure of merit ZT of 0.29 at 400 K is obtained in the sample exhibiting the largest β‐As2Te3 crystalline fraction. This ZT value is twice as high as the value obtained for the glass‐ceramic sintered from non‐ball‐milled powder.
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    Chemical hardening of glazed porcelain tiles (2018)
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    Publication Date: 2018
    Description: Abstract Glazed ceramic tiles are used for wall and floor covering thanks to their high resistance to chemicals attacks, although big efforts should be done to increase their surfaces resistance to mechanical stresses. This study investigates the applicability of a well‐known glass hardening process to glazed ceramic tiles following a rational design based on Design of Experiments technique. A Mixture Design has been carried out to formulate new frits compositions to enhance the ion‐exchange process, starting from a commercial product. Vickers Hardness and SEM‐EDS techniques have been employed to evaluate the frits and elaborate the model. Results suggest that frits for ceramic tiles are positively affected by ion‐exchange process only if an appropriate combination of ions in the starting composition is present, establishing a new category of frit formulations suitable for that purpose. The results have been confirmed employing the optimized frit for the glazing of porcelain stoneware.
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    Fabrication of ZrB2 ceramics by reactive hot pressing of ZrB and B (2018)
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    Publication Date: 2018
    Description: Abstract Zirconium diboride (ZrB2) ceramics were prepared by reactive hot pressing of ZrB+B powder mixture. Formation of a transient liquid due to eutectic reaction of ZrB2+Zr→Leu(ZrB2+Zr) at 1661°C following peritectic decomposition of 2ZrB=ZrB2+Zr at 1250°C during heating up of the ZrB+B mixture facilitated densification. The liquid phase was subsequently eliminated via reaction of B with Zr in the eutectic liquid Leu(ZrB2+Zr) to result in a dense ZrB2 ceramic. Full density was reached after reactive hot pressing at 1900°C under 30 MPa for 1 h. The ZrB2 ceramic had a refined microstructure consisting of grains of 〈1.5 μm in size and relatively good Vickers hardness (21 ± 2 GPa) and flexural strength (595 ± 63 MPa).
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    Electronically conductive porous TiN ceramics with enhanced strength by aqueous gel‐casting (2018)
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    Publication Date: 2018
    Description: Abstract In this paper, we report on a study of electronically conductive porous TiN ceramics prepared by aqueous gel‐casting. The effects of solid loading, sintering temperature, and sintering aids on the phase composition, microstructure, and volume fraction of porosity of the prepared porous TiN ceramics are studied. The SEM results show that porosity is uniformly distributed in all of the samples studied. With increasing solid loading and sintering temperature, the volume fraction of porosity decreases slowly. Moreover, the relationship between volume fraction of porosity and mechanical and electrical properties has also been investigated. Our results show that adding Y2O3‐TiO2 as combined sintering aids results in a sharp decrease in the volume fraction of porosity, and the volume fraction range changes from 42%‐60% to 28%‐52%. Moreover, adding sintering aids results in an increase in flexural strength and electrical conductivity with a change in maximum value from 34.6 MPa and 2.3 × 104 S∙m−1 to 101.6 MPa and 5.1 × 104 S∙m−1, respectively.
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    Low temperature synthesis and characterization of nano‐crystalline Mg(Zr0.05Ti0.95)O3 ceramics (2018)
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    Publication Date: 2018
    Description: Abstract Mg(Zr0.05Ti0.95)O3 (MZrT) ceramics nanoparticles have been synthesized by polyol method for the first time. The phase evaluation of the MZrT nanoparticles was confirmed using thermo gravimetric analysis and the phase purity of the samples were analyzed using X‐ray diffraction and Raman spectroscopy. The transmission electron microscopy (TEM) images revealed the average particle size between 30 and 40 nm. The optical bandgap is in the range of 3.66‐3.82 eV and is attributed to the quantum confinement effect. Interestingly, the nanopowders sintered at 950°C for 3 hours exhibit the maximum density of 97.52% of the theoretical density which is attributed to the higher sintering velocity of the smaller particles. The obtained microstructure of the ceramics reveals porous free uniform microstructure with prominent grain boundaries. A best combination of microwave dielectric properties (εr ~18.04, Q × fo ~175 THz at 9.5 GHz) are obtained for MZrT ceramics sintered at 950°C for 3 hours. The non‐Debye‐like relaxation process is found to exist inside the sample confirmed by impedance spectroscopy. The AC conduction mechanism is explained on the basis of Correlated Barrier Hopping model. Thermal conductivity of the MZrT ceramics is found to be 10 W/mK. The obtained properties of MZrT ceramics are suitable for resonator, microwave integrated circuit and LTCC applications.
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    Optoelectronic performances on different structures of Al‐doped ZnO (2018)
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    Description: Abstract ZnO films and Al‐doped ZnO (AZO) films were deposited on p‐Si substrate by magnetron sputtering to investigate its chemical composition, structural and photoelectric properties. XRD and FTIR show that Al ions can enter into the substitutional and interstitial site of ZnO crystal, and O atoms in AZO films are more abundant. Three different structures of Al‐doped ZnO (substitutional Al, interstitial Al, and O‐rich Al‐doped ZnO) were built using first‐principles method based on experimental results, charge density difference, and density of States (DOS) illustrate that there are strong ionic interactions between Al and O atoms in substitutional Al‐doped ZnO, moreover, substitutional and interstitial Al doping both are beneficial to N type, but oxygen‐enriched ZnO is not conducive to N type. Furthermore, the optical properties of 3 different Al‐doped ZnO structures were investigated respectively. Compared with pure ZnO, the real and imaginary part of dielectric function of O‐rich and interstitial Al have a significant increase and move to lower energy (red shift), the reflectivity of O‐rich is 3 times of pure ZnO and substitutional Al‐doped ZnO. The results are hoped to be helpful to study AZO thin film and predict the properties of Al‐doped ZnO.
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    Local phenomena in bismuth sodium titanate perovskite studied by Raman spectroscopy (2018)
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    Publication Date: 2018
    Description: Abstract In this work, 0.8Na0.5Bi0.5TiO3‐0.2K0.5Bi0.5TiO3 (BNKT) is studied by electrical characterizations, in situ Raman spectra in conjunction with the phenomenological Landau predictions to reveal the local phenomena. An electric field‐induced nonergodic relaxor‐to‐ferroelectric phase transition and domain switching start to take place at a critical field of 3 kV/mm. The polar phase was disrupted into the disordered relaxor phase at ferroelectric‐relaxor transition temperature (TFR) along with the discontinuous phonon anomalies caused by the decoupling between the off‐centered Bi and Ti cations. The R3c and P4bm polar nanoregions (PNRs) can coexist in a wide temperature range and can be transformed reversibly below the temperature of dielectric maximum (Tm). The broad Raman bands were found to persist in high‐temperature paraelectric state induced by the existence of off‐centered Ti cation and ferroically deformed TiO6 octahedra. A phenomenological Landau model and local structural evolution sequence at different thermal history were proposed to further elucidate the underlying mechanisms. This work will be conducive to understanding the local structure‐physical property correlations of high‐performance lead‐free piezoelectrics.
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    Enhanced piezoelectric properties and electrocaloric effect in novel lead‐free (Bi0.5K0.5)TiO3‐La(Mg0.5Ti0.5)O3 ceramics (2018)
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    Publication Date: 2018
    Description: Abstract The crystal structure, electromechanical properties, and electrocaloric effect (ECE) in novel lead‐free (Bi0.5K0.5)TiO3‐La(Mg0.5Ti0.5)O3 ceramics were investigated. A morphotropic phase boundary (MPB) between the tetragonal and pseudocubic phase was found at x = 0.01‐0.02. In addition, the relaxor properties were enhanced with increasing the La(Mg0.5Ti0.5)O3 content. In situ high‐temperature X‐ray diffraction patterns and Raman spectra were characterized to elucidate the phase transition behavior. The enhanced ECE (ΔT = 1.19 K) and piezoelectric coefficient (d33 = 103 pC/N) were obtained for x = 0.01 at room temperature. Meanwhile, the temperature stability of the ECE was considered to be related to the high depolarization temperature and relaxor characteristics of the Bi0.5K0.5TiO3‐based ceramics. The above results suggest that the piezoelectric and ECE properties can be simultaneously enhanced by establishing an MPB. These results also demonstrate the great potential of the studied systems for solid‐state cooling applications and piezoelectric‐based devices.
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    Porosity effect on microstructure, mechanical, and fluid dynamic properties of Ti2AlC by direct foaming and gel‐casting (2018)
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    Publication Date: 2018
    Description: Abstract In this study, Ti2AlC foams were fabricated by direct foaming and gel‐casting using agarose as gelling agent. Slurry viscosity, determined by the agarose content (at a fixed solids loading), as well as surfactant concentration and foaming time were the key parameters employed for controlling the foaming yield, and hence the foam porosity after sintering process. Fabricated foams having total porosity in the 62.5‐84.4 vol% range were systematically characterized to determine their pore size and morphology. The effect of the foam porosity on the room‐temperature compression strength and elastic modulus was also determined. Depending on the amount of porosity, the compression strength and Young's modulus were found to be in the range of 9‐91 MPa and 7‐52 GPa, respectively. Permeability to air flow at temperatures up to 700°C was investigated. Darcian (k1) and non‐Darcian (k2) permeability coefficients displayed values in the range 0.30‐93.44 × 10−11 m2 and 0.39‐345.54 × 10−7 m, respectively. The amount of porosity is therefore a very useful microstructural parameter for tuning the mechanical and fluid dynamic properties of Ti2AlC foams.
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    Preparation and characterization of Pb(Lu1/2Nb1/2)O3–Pb(In1/2Nb1/2)O3–PbTiO3 ternary ferroelectric ceramics with high phase transition temperatures (2018)
    Wiley
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    Publication Date: 2018
    Description: Abstract To explore new relaxor‐PbTiO3 systems for high‐power and high‐temperature electromechanical applications, a ternary ferroelectric ceramic system of Pb(Lu1/2Nb1/2)O3–Pb(In1/2Nb1/2)O3–PbTiO3 (PLN–PIN–PT) have been investigated. The phase structure, dielectric, piezoelectric, and ferroelectric properties of the as‐prepared PLN–PIN–PT ceramics near the morphotropic phase boundary (MPB) were characterized. A high rhombohedral‐tetragonal phase transition temperature TR‐T of 165°C and a high Curie temperature TC of 345°C, together with a good piezoelectric coefficient d33 of 420 pC/N, were obtained in 0.38PLN–0.20PIN–0.42PT ceramics. Furthermore, for (0.8−x)PLN–0.2PIN–xPT ceramics, the temperature‐dependent piezoelectric coefficients, coercive fields and electric‐field‐induced strains were further studied. At 175°C, their coercive fields were found to be above 9.5 kV/cm, which is higher than that of PMN–PT and soft P5H ceramics at room temperature, indicating PLN–PIN–PT ceramics to be one of the promising candidates in piezoelectric applications under high‐driven fields. The results presented here could benefit the development of relaxor‐PbTiO3 with enhanced phase transition temperatures and coercive fields.
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    An investigation of metal ion diffusion in ceria‐based solid oxide fuel cell with barium‐containing anode (2018)
    Wiley
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    Publication Date: 2018
    Description: Abstract Metal ion diffusion is an effective strategy to suppress the internal electronic short circuit in ceria‐based solid oxide fuel cells (SOFCs). This could be achieved by fabricating an electron‐blocking layer between the barium‐containing anode and ceria‐based electrolyte. In this paper, a 0.6NiO‐0.4BaZr0.1Ce0.7Y0.2O3‐δ (NiO‐BZCY) anode‐supported cell based on Gd0.1Ce0.9O2‐δ (GDC) electrolyte was employed to evaluate the internal metal ion diffusion behavior. The high open circuit voltages of about 1 V obtained at 550‐700°C can be attributed to in situ formation of an electron‐blocking interlayer between NiO‐BZCY and GDC. Microstructural analyses of the interlayer grains obtained by traditional solid‐state reaction were carried out. Phase identification demonstrated that the electron‐blocking interlayer had a perovskite structure. SEM and TEM analyses indicated formation of a new compound in the interlayer, of which the composition was determined as Zr, Y, and Ni co‐doped BaCe0.9Gd0.1O3 with orthorhombic structure.
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    Enhanced photoresponse and photocatalytic activities of graphene quantum dots sensitized Ag/TiO2 thin film (2018)
    Wiley
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    Publication Date: 2018
    Description: Abstract TiO2 thin films were fabricated through hydrothermal method. Silver nanoparticles were loaded on TiO2 thin films via photoreduction technique. Subsequently, the graphene quantum dots (GQDs) were spin‐coated on the Ag/TiO2 nanocomposites thin films. The crystal structure, surface morphology and UV‐vis absorbance were tested by XRD, SEM and ultraviolet‐visible spectrophotometer. These results indicated that Ag nanoparticles and GQDs are anchored on the TiO2 nanorods. Absorbance of Ag/TiO2 and GQDs/Ag/TiO2 nanocomposite thin films have been extended into the visible region. Visible‐light response of the samples were investigated by electrochemical workstation. The photoresponse of the sample can be enhanced by sensitization of the Ag nanoparticles and GQDs. The enhanced visible‐light response may be due to the surface plasmon resonance of silver nanoparticles and visible absorbance of GQDs. The highest photocatalytic activity has been observed in the 9‐GQDs/Ag/TiO2 composite thin film. The efficient charge separation and transportation can be achieved by introducing the Ag nanoparticles and GQDs in the TiO2 thin film.
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    Behaviors of helium in Cr2AlC from first principles (2018)
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    Publication Date: 2018
    Description: Abstract The properties of helium incorporation and transport in Cr2AlC have been investigated using first principles method. The results of calculation show that a single helium atom is preferred to reside at an interstitial position near the Al plane in perfect Cr2AlC crystal, attributing to the low predicted formation energy. Helium atoms are expected to aggregate in Al layers. The interaction between helium and lattice atoms is primarily elastic due to the closed‐shell electronic structure of helium. The Doping of helium leads to a weakening of the Cr–Al bonds. Furthermore, an interstitial helium atom is likely to migrate along an indirect migration pathway from the hexahedral interstitial position on Al plane (IAlC), passing by the nearest tetrahedral interstitial position near Al plane (IAlCr), and finally reach to another hexahedral interstitial position (IAlC) with an activation energy of 1.21 eV. The high activation energy suggests a relatively low migration for helium atoms in Cr2AlC as well as a slow growth rate of helium bubbles for the early stage of helium irradiation.
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    Rapid mass production of novel 3D Cu@CuI core‐shell mesh as highly flexible and efficient photocatalyst (2018)
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    Publication Date: 2018
    Description: Abstract We report a relatively simple and economical approach for mass production of Cu@CuI mesh. This novel core‐shell structure is CuI‐coated Cu mesh, fabricated by one‐step in situ iodination of commercial copper mesh at an ambient temperature. Such a Cu@CuI mesh is low‐cost and can be successfully used for the degradation of soluble organic pollutants in water under UV light. The novel 3D Cu@CuI core‐shell structure allows for high electron/hole separation and thus leads to high catalytic efficiency. Rhodamine B (RhB) is fully degraded in even just 6 minutes using a Cu@CuI mesh as the photocatalyst. Unlike the currently reported photocatalysts mostly in the form of powders, nanoparticles, and/or nanowires, the 3D Cu@CuI mesh is freestanding and flexible, and therefore is easily separated from water after photocatalysis without causing secondary pollution. This is a significant advance toward tackling the expansive separation issue of the conventional catalysts, because the ultra‐simple separation process of 3D Cu@CuI mesh can facilitate its industry application. With a fantastic combination of low cost, facile and green fabrication, high catalytic efficiency and easy separation 3D architecture, the Cu@CuI mesh may serve as a promising candidate for water purification.
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    The effect of cooling rate on structural and electrical properties of multiferroic BLF‐PT ceramics (2018)
    Wiley
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    Publication Date: 2018
    Description: Abstract The solid solutions of 0.57(Bi0.8La0.2)FeO3‐0.43PbTiO3 (BLF‐PT) were prepared by the solid‐state reaction process. The as‐sintered ceramics were annealed at 350°C and then cooled in different media. BLF‐PT ceramics cooled in boiling water (BW) and NaCl solutions exhibit the rhombohedral structure with higher domain density, while the tetragonal structure was observed in the specimen cooled in the furnace. Moreover, BLF‐PT ceramics cooled in BW with cooling rate of about 10°C/s exhibit the enhanced electrical properties with dielectric constant εr, remnant polarization Pr, piezoelectric constant d33 of 940, 28 μC/cm2 and 210 pC/N, respectively. Rayleigh analysis was utilized to study of domain wall motion of BLF‐PT ceramics cooled in different media.
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    Understanding the effect of surface flaws on the strength distribution of brittle single crystals (2018)
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    Publication Date: 2018
    Description: Abstract The dependence of strength on the surface quality of brittle single crystals is highly relevant in microelectronic applications, where a certain roughening of the crystal surface is required to guarantee or enhance the functional properties of the final component. In this work, the effect of surface conditioning on the strength distribution of single crystals is assessed on LiTaO3 and LiNbO3 samples with distinct surfaces (i.e., polished, grinded, and scratched). Artificial surface cracks (scratches) were introduced using a Berkovich nanoindenter tip oriented under various angles with respect to the most critical {012} cleavage plane. Biaxial tensile tests were performed using the ball‐on‐three‐balls test, supported by fractographic analyses to interpret the strength results. A direct correlation between sub‐surface damage and strength was observed, associated with the hardness and elastic constants of the material, being strongly dependent on the crystal orientation with respect to the loading axis. Furthermore, we provide an explanation for the usually low Weibull modulus in single crystals and discuss the validity of the biaxial tests on scratched specimens for fracture toughness evaluation.
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    Autonomous high‐temperature healing of surface cracks in Al2O3 containing Ti2AlC particles (2018)
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    Publication Date: 2018
    Description: Abstract In this work, the oxidation‐induced crack healing of Al2O3 containing 20 vol.% of Ti2AlC MAX phase inclusions as healing particles was studied. The oxidation kinetics of the Ti2AlC particles having an average diameter of about 10 μm was studied via thermogravimetry and/or differential thermal analysis. Surface cracks of about 80 μm long and 0.5 μm wide were introduced into the composite by Vickers indentation. After annealing in air at high temperatures, the cracks were filled with stable oxides of Ti and Al as a result of the decomposition of the Ti2AlC particles. Crack healing was studied at 800, 900, and 1000°C for 0.25, 1, 4, and 16 hours, and the strength recovery was measured by 4‐point bending. Upon indentation, the bending strength of the samples dropped by about 50% from 402 ± 35 to 229 ± 14 MPa. This bending strength increased to about 90% of the undamaged material after annealing at 1000°C for just 15 minutes, while full strength was recovered after annealing for 1 hour. As the healing temperature was reduced to 900 and 800°C, the time required for full‐strength recovery increased to 4 and 16 hours, respectively. The initial bending strength and the fracture toughness of the composite material were found to be about 19% lower and 20% higher than monolithic alumina, respectively, making this material an attractive substitute for monolithic alumina used in high‐temperature applications.
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    Phase control during synthesis of nanocrystalline ultrahigh temperature tantalum‐hafnium diboride powders (2018)
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    Publication Date: 2018
    Description: Abstract Ta1−xHfxB2 material is attractive for various aerospace applications. In this study, 2 low‐cost approaches were adopted to synthesize nanocrystalline Ta0.5Hf0.5B2 solid solution and related composite powders. The first was based on carbothermal reduction reaction (CTR) of intimately mixed tantalum‐hafnium‐boron oxide(s) and carbon obtained from aqueous solution processing of TaCl5, HfCl4, B2O3, and sucrose as precursors. It was found that when using this method, due to the low solubility of each other for Ta2O5 and HfO2 and the difference in reactivity of those 2 oxides with carbon (as well as B2O3), individual TaB2 (‐rich) and HfB2 phases always form separately. Those borides tend to remain phase separated due to the slow inter‐diffusion between them. However, it was observed that addition of copper “catalyst” noticeably accelerates the inter‐diffusion and the solid solution formation. The second approach was based on alkali metal reduction reaction, in which TaCl5 and HfCl4 are directly reacted with sodium borohydride (NaBH4). This method yields a single phase Ta0.5Hf0.5B2 solid solution nanopowders in one step at much lower temperatures (e.g., 700°C) by avoiding the oxides formation and the associated phase separation of individual borides as observed in the CTR‐based process.
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    First principles investigation on mechanical and thermal properties of α‐ and β‐YAlB4 ultra‐high temperature ceramics (2018)
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    Publication Date: 2018
    Description: Abstract Ultra‐high temperature ceramics (UHTCs) exhibit a unique combination of excellent properties that makes them promising candidates for applications in extreme environments. Various UHTCs are needed due to diverse harsh conditions that UHTCs are faced with in different applications. Due to structural similarity to ZrB2, possible high melting point and possible protective oxide scale formed in oxygen rich and water vapor environments, REAlB4 (RE: rare‐earth) is suggested a good candidate for UHTCs. In the present work, temperature‐dependent mechanical and thermal properties of both α‐YAlB4 (YCrB4 type, space group Pbam) and β‐YAlB4 (ThMoB4 type, space group Cmmm) were investigated by first principles calculations in combination with quasi‐harmonic approach. Due to the structural similarity between α‐YAlB4 and β‐YAlB4, their properties are very similar to each other, which are approximately transverse isotropic with properties in (001) plane being almost the same and differing from properties out of (001) plane. The results reveal that resistance to normal strain in (001) plane (~460 GPa) is higher than that along [001] direction (~320 GPa) and thermal expansion in (001) plane (~10 × 10−6 K−1) is lower than that along [001] direction (~17 × 10−6 K−1), which is because the stiff boron networks are parallel to (001) plane. The average thermal expansion coefficient is around 12 × 10−6 K−1, which is fairly high among UHTCs and compatible with metallic frameworks. The combination of high thermal expansion coefficient and protective oxidation scale forming ability suggest that REAlB4 is promising for practical applications not only as high‐temperature structural ceramic but also as oxidation resistant coating for alloys.
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    Antiferroelectric‐ferroelectric phase transition in lead‐free AgNbO3 ceramics for energy storage applications (2018)
    Wiley
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    Publication Date: 2018
    Description: Abstract The high‐energy storage density reported in lead‐free AgNbO3 ceramics makes it a fascinating material for energy storage applications. The phase transition process of AgNbO3 ceramics plays an important role in its properties and dominates the temperature and electric field dependent behavior. In this work, the phase transition behavior of AgNbO3 ceramics was investigated by polarization hysteresis and dielectric tunability measurements. It is revealed that the ferrielectric (FIE) phase at room temperature possesses both ferroelectric (FE)‐like and antiferroelectric (AFE)‐like dielectric responses prior to the critical AFE‐FE transition point. A recoverable energy storage density of 2 J/cm3 was achieved at 150 kV/cm due to the AFE‐FE transition. Based on a modified Laudau phenomenological theory, the stabilities among the AFE, FE and FIE phases are discussed, laying a foundation for further optimization of the dielectric properties of AgNbO3.
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    Effect of particle size on the optical properties of lead zirconate titanate nanopowders (2018)
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    Publication Date: 2018
    Description: Abstract Nanopowder samples of lead zirconate titanate (Pb1.1Zr0.52Ti0.48O3 or PZT) were prepared by the sol‐gel method with controlled pH values. The samples were characterized using FTIR spectroscopy, XRD, FE‐SEM, and TEM techniques. Most of the peaks in the XRD pattern were related to the coexistence of tetragonal‐rhombohedral phases and confirmed the formation of PZT with a perovskite structure. Also, the crystallite size of PZT nanopowders was in a range of 17‐28 nm. FTIR spectroscopy revealed a longitudinal optical (LO) and transverse optical (TO) phonon modes corresponding to the stretching vibration of Ti‐O and Zr‐O bonds. The influence of pH values on the LO and TO phonon modes, LO‐TO splitting, refractive index n(ω), extinction coefficient k(ω), and the real ɛ1(ω) and imaginary ɛ2(ω) parts of dielectric function was discussed. These properties were investigated in the mid‐infrared region (450‐750 cm−1). The energy loss function Im[−1/(ɛ)] of PZT nanopowders was obtained by Kramers‐Kronig dispersion relations. The TO phonon frequency decreases with increasing crystallite size of the PZT samples. This effect does not happen at pH 8 to pH 9. As the crystallite size increased from 17.26 nm (at pH 5) to 27.25 nm (at pH 7), the LO‐TO splitting increased as well. This result showed that the optimum pH for absorption of IR radiation and optical application was at pH 7.
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    High strain (0.4%) Bi(Mg2/3Nb1/3)O3‐BaTiO3‐BiFeO3 lead‐free piezoelectric ceramics and multilayers (2018)
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    Publication Date: 2018
    Description: Abstract The relationship between the piezoelectric properties and the structure/microstructure for 0.05Bi(Mg2/3Nb1/3)O3‐(0.95‐x)BaTiO3‐xBiFeO3 (BBFT, x = 0.55, 0.60, 0.63, 0.65, 0.70, and 0.75) ceramics has been investigated. Scanning electron microscopy revealed a homogeneous microstructure for x 〈 0.75 but there was evidence of a core‐shell cation distribution for x = 0.75 which could be suppressed in part through quenching from the sintering temperature. X‐ray diffraction (XRD) suggested a gradual structural transition from pseudocubic to rhombohedral for 0.63 〈 x 〈 0.70, characterized by the coexistence of phases. The temperature dependence of relative permittivity, polarization‐electric field hysteresis loops, bipolar strain‐electric field curves revealed that BBFT transformed from relaxor‐like to ferroelectric behavior with an increase in x, consistent with changes in the phase assemblage and domain structure. The largest strain was 0.41% for x = 0.63 at 10 kV/mm. The largest effective piezoelectric coefficient (d33*) was 544 pm/V for x = 0.63 at 5 kV/mm but the largest Berlincourt d33 (148 pC/N) was obtained for x = 0.70. We propose that d33* is optimized at the point of crossover from relaxor to ferroelectric which facilitates a macroscopic field induced transition to a ferroelectric state but that d33 is optimized in the ferroelectric, rhombohedral phase. Unipolar strain was measured as a function of temperature for x = 0.63 with strains of 0.30% achieved at 175°C, accompanied by a significant decrease in hysteresis with respect to room temperature measurements. The potential for BBFT compositions to be used as high strain actuators is demonstrated by the fabrication of a prototype multilayer which achieved 3 μm displacement at 150°C.
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    Large‐scale synthesis of hafnium carbide nanowires via a Ni‐assisted polymer infiltration and pyrolysis (2018)
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    Publication Date: 2018
    Description: Abstract Hafnium carbide (HfC) nanowires were successfully synthesized on C/C composites via a Ni‐assisted polymer infiltration and pyrolysis. Before synthesizing HfC nanowires, the composition and microstructure of the organic HfC precursor and its pyrolyzed products were characterized by Fourier transform infrared spectra, X‐ray photoelectron spectroscopy, and X‐ray diffraction. The effect of heat‐treatment temperature on the morphology and microstructure of HfC nanowires were investigated by scanning electron microscopy and transmission electron microscopy. Results show that HfC nanowires exhibits three‐layer core‐shell structure, including HfC core, HfO2 inner shell (~2 nm) and carbon nanosheet outer shell (~1 nm). The obtained HfC nanowire is typically grown along 〈01〉 with the diameters of ~100 nm and the length of several micrometers. The growth of HfC nanowire follows the combination of top‐type vapor‐liquid‐solid and solid‐liquid‐solid mechanism.
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    Sol‐gel processed highly (100)‐textured (K, Na)NbO3‐based lead‐free thin films: Effect of pyrolysis temperature (2018)
    Wiley
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    Publication Date: 2018
    Description: Abstract Recently, lead‐free piezoelectric thin films have received increasing attention due to the growing demands for mircoelectromechanical systems and the significant progress in lead‐free piezoelectric research. Here, potassium sodium niobate [(K, Na)NbO3 (KNN)]‐based thin films were fabricated via a sol‐gel method. The effects of pyrolysis temperature on the resulting microstructure and electrical properties of KNN‐based films were investigated. The KNN‐based film pyrolyzed at 550°C and annealed at 700°C shows a dominant (100) orientation with a high texturing degree of 91.7%. The microstructures, morphologies, piezo‐ and ferroelectric properties of the KNN‐based films were discussed in association with different pyrolysis temperatures. The crystallization mechanism of the (100) textured KNN‐based thin films was elaborated in detail.
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    Ferroelectric aging effects and large recoverable electrostrain in ceria‐doped BaTiO3 ceramics (2018)
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    Publication Date: 2018
    Description: Abstract Ferroelectric aging is a phenomenon that is generally observed in acceptor‐doped ferroelectrics and usually produces a large recoverable electrostrain. In this work, using an oxide ionic conducting material, a Gd‐modified ceria solution (GDC), as a dopant, we find that GDC‐doped BaTiO3 ceramics exhibit ferroelectric aging behaviors at room temperature. These ceramics show double polarization hysteresis (P‐E) loops and sprout‐shaped recoverable strain vs electric field (S‐E) curves. GDC‐doped BT ceramic (3 mol%) produces a large recoverable electrostrain of 0.12% at 3 kV/cm. The large ions of Ce3+ and Gd3+, which have small electronegativity, lead to the high mobility of oxygen vacancies and a rapid symmetry conformation for short range ordering (SC‐SRO) of points defects, which appear as fast aging effects and a large electrostrain in GDC‐doped BT ceramics at room temperature.
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    The comprehensive in vitro evaluation of eight different calcium phosphates: Significant parameters for cell behavior (2018)
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    Publication Date: 2018
    Description: Abstract Eight different calcium phosphate nanoparticles, namely bovine bone bioapatite calcined at 500, 600, and 700°C, Mg‐doped brushite, fluorinated calcium phosphate, Ca‐deficient hydroxyapatite, hydroxyapatite, and tricalcium phosphate, were characterized employing physico‐chemical methods. Their cytocompatibility was evaluated under human osteoblast‐like cell line MG‐63 culture conditions in elution media and via the direct interaction of cells with calcium phosphate nanoparticles. The main objective was to determine the correlation of the cell indices with the differently determined physical and chemical parameters of the calcium phosphates. Chemical composition, which contributes toward pH changes, and the calcium ion concentration in the medium appear to make up particularly significant factors; moreover, it was proved that the number of material types represents a further important aspect. In the case of a large number of material types, almost no correlation was determined between the analyzed parameters; however, in the case of a small number of apatite types, several positive correlations were found. It can be concluded that it is not possible to identify any monitored parameters that had a major impact on cell behavior or, at least, such an effect which can be generalized. It appears more likely that cell behavior is affected by the interplay of various parameters.
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    Phase relations of CaO‐Al2O3‐Sc2O3 ternary system (2018)
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    Publication Date: 2018
    Description: Abstract We investigated the isothermal section of the CaO‐Al2O3‐Sc2O3 ternary system at 1773 and 1873 K for 24 hours in Ar, and quenched in water to determine the operative phase equilibrate. The composition of the phases in equilibrium was determined by electron probe microanalysis. The isothermal section phase diagram of two temperature points (1773 and 1873 K) is obtained. The 1773 K isothermal section consists of one liquid compound (L), six binary compounds (CaO+L, Ca2Sc6Al6O20+L C3A+L, CaO.Sc2O3+L, CA+L, Ca2Sc6Al6O20+Sc2O3) and seven ternary compounds (Ca2Sc6Al6O20+Sc2O3+CA6, Ca2Sc6Al6O20+Sc2O3+L, Ca2Sc6Al6O20+CA+L, Ca2Sc6Al6O20+CA2+CA, Ca2Sc6Al6O20+CA2+CA6, CaO.Sc2O3+L+Sc2O3, C3A+CaO+L). At 1873 K, we found one liquid compound (L), five binary compounds (CaO+L, Ca2Sc6Al6O20+L, CaO.Sc2O3+L, CA+L, Ca2Sc6Al6O20+Sc2O3) and six ternary compounds (Ca2Sc6Al6O20+Sc2O3+CA6, Ca2Sc6Al6O20+Sc2O3+L, Ca2Sc6Al6O20+CA+L, Ca2Sc6Al6O20+CA2+CA, Ca2Sc6Al6O20+CA2+CA6, CaO.Sc2O3+L+Sc2O3) to exist at the isothermal section. The experimental information obtained in the present work not only is essential for the thermodynamic assessment of the CaO‐Al2O3‐Sc2O3 ternary system, but also is important for further investigation on separation of rare earths from metallurgical slags and rare‐earth recovery.
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    Polyhedral distortion control of unusual photoluminescence color tuning in garnet phosphors via cation substitution (2018)
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    Publication Date: 2018
    Description: Abstract Controllable luminescence properties for phosphors can be realized through optimum composition design and structural tuning. The local microstructural evolutions and luminescent properties of Y3Al5O12:Ce3+ via the substitution of the cations Ga3+/Sc3+ for Al3+ are investigated. The emission spectrum of Y3Al2−xGaxAl3O12:Ce3+ has a blueshift from 559 to 531 nm with the substitution Ga3+ for Al3+. This outcome can be attributed to a lengthened Ce–O bond, which leads to a smaller crystal splitting. In the above ending composition Y3Ga2Al3O12:Ce3+, an unusual redshift from 531 to 545 nm can be observed by further replacing Ga3+ with the larger Sc3+. Combining first‐principles calculations and the position of energy level analysis, introduced Sc3+ into Y3Al5O12 lattices can cause a sharp increase in (Ce–O8) polyhedron distortion, leading to a larger additional distorted crystal‐field splitting of eg, levels compared to that of Ga3+ dopant. Moreover, a relatively larger electronegativity of Sc3+ can enhance the nephelauxetic effect and further lower the lowest 5d level. These two factors are responsible for the unusual redshift. A formula was generalized to describe the polyhedral distortion that can explain the unusual redshift very well. This work gives a clear relationship between the composition, structure and photoluminescent properties in garnet‐type phosphors and provides a guide for targeted optimization of photoluminescent properties in other systems.
    Print ISSN: 0002-7820
    Electronic ISSN: 1551-2916
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Published by Wiley
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