ALBERT

Description: New, commensurate members of the fluorite-related Bi 3 Nb 1− x Ta x O 7 family were synthesized and their crystal structures, microstructures, and microwave (MW) dielectric properties were characterized. The incorporation of Ta into the tetragonal Bi 3 Nb 1− x Ta x O 7 solid solution was found to gradually affect the density and the MW dielectric properties. The materials sintered at 870°C exhibited relative permittivities in the range k ′ = 86–72, Q  ×  f values from 793 to 1189 GHz and a positive temperature coefficient of resonant frequency from 88 to 12 ppm/K. The formation of the members of the fluorite-related solid solution along the Bi 3 Nb 1− x Ta x O 7 composition depends on a phase transition, and thus their properties are compared within the compositional range. The correlations between their MW dielectric properties, compositions, crystal structures, and processing parameters were discussed in detail. Optimization of MW properties can be achieved by utilizing the ability of the Bi 3 Nb 1− x Ta x O 7 solid solution that it undergoes a phase transformation from cubic to tetragonal structure which are both characterized by unique properties, under certain synthesis conditions.

Description: Transparent novel glass-ceramics containing Sr 2 YbF 7 :Er 3+ nanocrystals were successfully fabricated by melt-quenching technique. Their structural and up-conversion luminescent properties were systemically investigated by XRD, HRTEM, and a series of spectroscopy methods. The temperature-dependent up-conversion spectra prove that 2 H 11/2 and 4 S 3/2 levels of Er 3+ are thermally coupled energy levels (TCEL). Consequently, the 2 H 11/2 4 I 15/2 and 4 S 3/2 4 I 15/2 emissions of Er 3+ in Sr 2 YbF 7 :Er 3+ glass-ceramics can be used as optical thermometry based on fluorescence intensity ratio (FIR) technique. Combined with low phonon energy and high thermal stability, Er 3+ ions in Sr 2 YbF 7 glass-ceramics present broad operating temperature range (300–500 K), large energy gap of TCEL (786 cm −1 ) and high theoretical maximum value of relative sensitivity (62.14 × 10 −4  K −1 at 560 K), which suggests that Sr 2 YbF 7 :Er 3+ glass-ceramics may be excellent candidates for optical temperature sensors.

Description: Accurate information on the interactions between water and silica is critical to the understanding of its properties including mechanical strength under stress and long-term chemical durability of silica and silicate glasses. In this study, interactions between water and nanoporous amorphous silica models were investigated using density functional theory (DFT) based ab initio molecular dynamics (AIMD) simulations which accurately describe bond breakage and formation as well as chemical reactions. AIMD simulations up to 30 ps were performed for systems containing water and nanoporous silica with a wide range of porosities (31%–67%). Partial removal of defects, such as two-membered rings, was observed during the AIMD runs whereas more reactive coordination defects were removed during the initial geometry optimization. The limited two-membered ring removal can be attributed to restricted water-defect movement or the increased stability of rings located on concave surfaces. Two-membered ring removal mechanisms included the formation of an overcoordinated silicon (Si 5 ) intermediate defect from the dynamic simulations. Si 5 defects continued to develop throughout the simulations, indicating a thermodynamic drive for two-membered ring removal which is kinetically limited. Changes in the electronic structures, such as atomic charges, and bond length-bond angle correlation functions were monitored during the hydroxylation process.

Description: Amorphous calcium polyphosphate (ACPP), an inorganic polymer ceramic, has shown promise as a drug delivery matrix following a repeat gelling protocol. This study described a simple method of preparing ACPP hydrogel in the presence of an excess volume of water. The increased water availability accelerates water molecule ingress and microstructural transformation of ACPP hydrogels. The impact of some experimental settings (soaking time, temperature, stirring, and ACPP particle size) on the physiochemical and rheological natures of ACPP hydrogel were investigated and from which possible hydrogel formation mechanisms were inferred. We believe that the formation of ACPP hydrogel is through the mechanisms of intermolecular ionic interaction and entanglement of polyphosphate chains. The potential application of ACPP hydrogel as a ceramic matrix for sustained drug release warrants further investigation.

Description: Melting gels are hybrid gels that have the ability to soften and flow at around 100°C for some combinations of mono- and di-substituted alkoxysiloxanes, where substitutions are either all aromatic or all aliphatic. In this study, melting gels were prepared using phenyltriethoxysilane (PhTES) and dimethyldiethoxysilane (DMDES), meaning both an aromatic and aliphatic substitution. Differential scanning calorimetry was performed to identify glass-transition temperatures, and thermal gravimetric analysis coupled with differential thermal analysis (TGA-DTA) was performed to measure weight loss. The glass-transition temperatures ( T g ) ranged from −61°C to +5.6°C, which are between the values in the methyl only system, where all T g values are less than 0°C, and those values in the phenyl only system, where T g values are greater than 0°C. The T g decreased with an increase in the DMDES fraction. Below 450°C, the gels lost little weight, but around 600°C there was a drop in weight. This temperature is lower than the temperature for gels prepared with only aromatic substitutions, but higher than that for gels prepared with only aliphatic substitutions. Final heat treatment was carried out at 150°C for the gel with 80%PhTES-20%DMDES (in mol%), and the consolidation temperature increased with increasing DMDES content to 205°C for the gel with 50%PhTES-50%DMDES. After this heat treatment, the melting gels no longer soften.

Description: Direct integration of nanostructures into macroscopic substrates is very important for their practical applications. In this work, we report a simple method that can be introduced for the Sn-catalyzed growth of alumina nanowires on ceramic substrates such as porous disk, monolith, and foam. Our study focuses on the role of the Sn catalysts in the formation mechanisms governing nanowire growth. Using the proposed approach, hair- or grass-like tufts of 20 nm diameter nanowires grow on the surface of the ~3 μm diameter Sn particles, in a tip growth mechanism. The nanowires of α-phased polycrystalline structure grow and are packed via a complex process involving batch-by-batch, branching, and amalgamation growth. The detailed observations reveal that the Sn catalyst is key to tailoring the growth patterns of the nanowires. In addition, cathodoluminescence studies highlight the potential optical applications of the alumina nanowires.

Description: Flash sintering is a nonlinear phenomenon characterized by a sharp increase in the conductivity of the sample and concomitant rapid densification under an electric field in low temperatures in a matter of seconds. Since it is a transient phenomenon, the power dissipation on the sample is not uniform during the process. Thus, a transient analysis is needed to estimate the temperature of the sample during flash sintering due to Joule heating. In this work, the Finite Element Method on a coupled electrothermal nonlinear analysis was used in order to obtain the specimen temperature of 8YSZ after 5 s of flashing. The results agree with the experimental data obtained by the flashing of dense samples and with previous literature.

Description: Temperature-dependent in-situ Raman spectroscopy is used to investigate the phase transformation of zinc metastannate (ZnSnO 3 ) to zinc orthostannate (Zn 2 SnO 4 ) induced upon annealing in the ambient. ZnSnO 3 microcubes (MCs) were synthesized at room temperature using a simple aqueous synthesis process, followed by characterization using electron microscopy, X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and thermogravimetric analysis (TGA). Annealing of the ZnSnO 3 MCs was carried out up to 1000°C, while recording the Raman spectra in-situ at regular intervals. Phase transformation from metastannate to orthostannate was found to begin around 500°C with an activation energy of ~0.965 eV followed by the recrystallization into the inverse spinel orthostannate phase at ~750°C. Results from this study provide detailed understanding of the phase transformation behavior of perovskite ZnSnO 3 to inverse spinel Zn 2 SnO 4 upon thermal annealing.

Description: Here, we demonstrate the relationship between glass network topological structure and the chemical state of embedded lanthanide ions. It is revealed that a more dispersed state of lanthanide ions is shown in more constrained 3D rigid network, which delivers valuable information toward homogeneous doping in glasses from the perspective of glass topological structure. The results are believed to be of great significances in the development of advanced optoelectronic devices like high-power laser, efficient fiber amplifier, smaller integrated photonic circuit, etc.

Description: The mechanofusion process, a dry particle coating route, has been successfully applied to coat micrometric SiC particles with submicrometric Ni filaments. In a first step, the mechanofusion parameters were optimized to form a continuous Ni coating onto SiC particles. In a second step, the Ni-coated SiC particles were sintered by hot isostatic pressing. The temperature and pressure cycles were determined to ensure a good densification of the material. Such a densification process leads to the formation of a δ-Ni 2 Si bilayer at the SiC/Ni interface; the inner δ-Ni 2 Si layer in contact with SiC being more rich in carbon than the one in contact with the matrix. From X-ray diffraction, wavelength-dispersive X-ray spectrometry and scanning electron microscopy characterizations, a mechanism is proposed to explain the microstructure of the end-product.

Description: Multiple doping is widely used to improve the performance of a material, including its electrical transport, mechanical, and photovoltaic properties. In this paper, Sn–Se dual-doped Li 10 GeP 2 S 12 (LGPS, thio-LISICON II analogue) electrolytes were synthesized via ball milling and sintering and compared with those Sn or Se single-doped. Successful Sn and/or Se substitution expanded the unit cell and formed units, which were verified by X-ray powder diffraction, energy-dispersive X-ray spectroscopy, and Raman spectroscopy. In contrast to the limited benefits of Se single doping and the negative effects of Sn single doping, Sn–Se dual doping demonstrated up to 53% enhancement in ionic conductivity. More importantly, Sn–Se dual-doped LGPS showed an extremely low activation energy of 16 kJ/mol, which is one of the lowest known values for lithium ion conductors; as well as one of the widest electrochemical windows of 8 V. Sn–Se dual-doped LGPS is a promising electrolyte for advanced all-solid-state batteries.

Description: Polycrystalline Cd 1− x Ba x O (0 ≤  x  ≤ 0.08) ceramics were synthesized via conventional solid-state reaction method, and the effect of Ba 2+ doping on the microstructure as well as the thermoelectric transport properties of the samples were investigated. It was found that doping of Ba 2+ can inhibit the grain growth of CdO, resulting in a considerable reduction in grain size. Moreover, with the increase in Ba 2+ doping content, both the electrical conductivity and the thermal conductivity of Cd 1− x Ba x O decreased, whereas the Seebeck coefficient increased. A high ZT value of 0.47 was achieved for Cd 0.99 Ba 0.01 O at 1000 K, 38% higher than the undoped CdO, mostly due to reduction of the thermal conductivity.

Description: Crystalline argon oxygen decarburization slag, in powdery form, was investigated for its hydration potential by alkali activation and curing at 80°C. Na-silicate and K-silicate of the same modulus were used as activators. Isothermal calorimetry at 80°C indicated exothermic reactions in the slag pastes. When the slag mortars were cured under steam at 80°C appreciable gain in compressive strength was measured. This was attributed to C–S–H which was detected in TG, FTIR, and 29 Si NMR analyses. Upon hydration at 90 d, the amount of crystalline phases decreased, whereas the XRD amorphous content in the slag increased. Electron microscopy showed the formation of different morphologies of reaction products depending on the alkaline activator employed. Presence of reaction rims around the crystalline phases with a major presence of Ca, Si, and O in the reacted matrix was observed in elemental maps.

Description: The (1− x )BiFeO 3 - x BaTiO 3 (with x  = 0.1, 0.2, 0.3, and 0.4) ceramics were fabricated successfully by solid-state reaction method. Single-phase perovskite was obtained in all ceramics, as confirmed by XRD technique. It was observed that 0.7BiFeO 3 –0.3BaTiO 3 was the morphotropic phase boundary (MPB) between rhombohedral and cubic phases, as also revealed from ferroelectric and magnetic properties. The simulated and experimental X-Ray Absorption Spectroscopy (XAS) study revealed that BT in 0.75BF-0.25BT is possibly taken a rhombohedral structure. Furthermore, the rounded ferroelectric hysteresis loops observed for 0.9BiFeO 3 –0.1BaTiO 3 and 0.8BiFeO 3 –0.2BaTiO 3 compositions could be attributed to their microstructure and surface charge effects and electron transfer between Fe 3+ and Fe 2+ ions. It was also found that high dielectric constant of 0.9BiFeO 3 –0.1BaTiO 3 composition was a result of grain and grain-boundary effects, as observed in SEM micrographs. In addition, a strong signature of dielectric relaxation behavior was observed in this ceramic system with the activation energy 0.467 eV obtained from the Arrhenius' law. Finally, the local structure investigation with XAS technique provided additional information to better understand the electric and magnetic properties in the BF-BT ceramic system.

Description: In this study, ZnS powders with homogeneous morphology were synthesized using a colloidal processing method. Vacuum hot pressing was subsequently applied to consolidate the ZnS powders into infrared transparent ceramics (77.3% transmittance at wavelengths of 6.74 and 9.29 μm). The phase composition of the sintered ZnS suggests the presence of wurtzite as a minor phase in addition to the primary sphalerite phase, and microstructural analysis indicates that the ceramics are highly densified. It has been found that the VHP-sintered ZnS ceramics exhibit blue (450 nm) and green (530 nm) luminescence, which is due to the formation of zinc vacancies and sulfur interstitials, respectively, during the sintering process.

Description: In this study, tribological investigations were carried out on ZTA ceramics with 17 vol% Y-TZP and different stabilizer contents (1, 1.5, and 2 mol% yttria) to analyze the influence of zirconia transformation on wear properties. Samples were tested in a linearly reciprocating ball on flat setup with different applied loads. Raising the fracture toughness by transformation toughening, microcracking, and residual stresses improves the wear resistance only at transition load but increases the wear at high loads. Higher yttria content of 2 mol% and lower zirconia grain size and thus low transformability, decreases fracture toughness but increases the wear resistance at high loads. Therefore the adjustment of stabilizer content on zirconia volume fraction in ZTA plays a decisive role in tribological applications.

Description: Highly (100)-oriented 0.38Bi(Ni 1/2 Hf 1/2 )O 3 -0.62PbTiO 3 relaxor-ferroelectric films were fabricated on Pt(111)/Ti/SiO 2 /Si(111) substrates by introducing a lead oxide seeding layer. A moderate relative permittivity , a low dissipation factor (tan δ 〈 5%), and strong relaxor-like behavior (γ = 0.74) over a broad temperature region were observed. The energy storage density of approximately 45.1 ± 2.3 J/cm 3 was achieved for films with (100) preferential orientation, which is much higher than the value ~33.5 ± 1.7 J/cm 3 obtained from films with random orientation. Furthermore, the PbO-seeded films are more capable of providing larger piezoelectric response (~113 ± 10 pm/V) compared to the films without seeds (~85 ± 8 pm/V). These excellent features indicate that the highly (100)-oriented 0.38Bi(Ni 1/2 Hf 1/2 )O 3 -0.62PbTiO 3 films could be promising candidates for applications in high-energy storage capacitors, high-performance MEMS devices, and particularly for potential applications in the next-generation integrated multifunctional piezoelectric energy harvesting and storage system.

Description: Uniformly dispersed TiC nanoparticle strengthened In 4 Se 2.65 composites have been fabricated by a combined process of mechanical alloying (MA) and hot pressing (HP) successfully. Due to the good electrical conductivity and the extra phonon scattering effect of the TiC nanoinclusions, the electrical resistivity and thermal conductivity decrease with the TiC content up to 0.8 wt%, and a maximum ZT of 0.98 at 723 K was achieved in the sample with 0.8 wt% TiC. Taking account of the measurement uncertainly, the enhancement of ZT value by TiC nanoinclusions is less obvious. On the other hand, the mechanical performance of In 4 Se 2.65 can be effectively improved by TiC nanoinclusions due to the dispersive strengthening effect of the nanoinclusions , and the flexural strength of the sample with 0.8 wt% TiC is improved to 73 MPa, which is over 40% higher than that of the pristine sample.

Description: The radiation damage response of Ti 3 SiC 2 heated from 120°C to 850°C during 700 keV Si + irradiation has been investigated. The samples were analyzed using glancing incidence X-ray diffraction, Rutherford backscattering spectrometry, Raman spectroscopy, and scanning electron microscopy. For the sample at 120°C, irradiation results in a buildup of a heterogeneous surface and the formation of TiC x . Irradiation at 200°C results in maximum microstrain, a maximum in the c lattice parameter, and the appearance of a β phase in addition to the normal α phase of Ti 3 SiC 2. A minimum in the observed damage level near the surface was seen for irradiation at a sample temperature of 300°C but the damaged phase increases at higher temperatures. Differences between the present work and a previous C irradiation study have been ascribed to the enhanced Si defect transport at low temperatures.

Description: The temperature distribution in copper and martensitic steel spheres has been investigated for the initial stage of field-activated sintering (FAST)/spark plasma sintering (SPS) using capacitor discharges (CD) with applied voltages from one to 15 V as model experiments. At first, the evolution of the contact resistance between the spheres has been studied. The results show the reduction in the contact resistance after discharge with increasing electrical load, yet no significant dependence on the length or number of the discharge pulses. Thereby the initial resistance is only decreased distinctly if at least a certain minimal voltage was applied. Subsequently, the melting of thin coatings of different metals on copper spheres has been studied and the occurrence of molten phase and its melting point were assigned to the corresponding discharge current. Extrapolation from the currents necessary to melt the coating layers in the CD experiments to lower values typical for FAST was used to estimate the contact overtemperature in the latter case. Resulting values for copper range from 0.05 K for normal heating with 100 K/min to 5 K for maximum current output.

Description: In this study, we report on the microstructure of SiO 2 -coated Al-doped ZnO nanoparticles densified by spark plasma sintering(SPS), using a multiscale approach. Our observations show that it is possible to successfully prepare dense pellets while keeping the nanostructure with well-defined Si-rich grain boundaries. Although a very limited partial solubility of Si in the ZnO matrix has been observed, Si is mostly concentrated at the grain boundaries. More surprisingly, we evidenced some areas with nanoscale inhomogeneity of the Al concentration, which can locally strongly exceed the average composition of the matrix. It could explain the apparent discrepancy observed in the literature between the simultaneous presence of ZnAl 2 O 4 in Al-doped ZnO, which should be the signature of the doping level exceeding the solubility limit, and the concentration of carriers that still depends on the nominal Al concentration in ZnO even in the presence of ZnAl 2 O 4 .

Description: Pressureless sintering is a well-established powder metallurgical route for processing and consolidation of mixed materials. Especially materials exhibiting a high melting point could be densified without tool abrasion by this sintering technique. As the sintering temperatures are often higher compared to pressure-assisted techniques care must be taken by means of grain growth. In our studies we used a ternary compound mixture to obtain Mo-based alloys. Consolidation applying pressure-assisted methods (hot pressing, spark plasma sintering) and pressureless sintering were used, respectively. The densities reached and the microstructures obtained were compared. These Mo–Si–B alloys were processed using a nitride-powder-based route offering lower impurity contents due to short processing times by avoiding time consuming mixing / milling steps. The sintering conditions depending on the powder particle size as well as the sample shape will be presented in detail. The composition investigated in this article offered a continuous α-Mo matrix with intermetallic islands consisting of Mo 3 Si and Mo 5 SiB 2 (T2) phases. The combination of a ductile α-Mo matrix and intermetallic phases embedded within offered an enhanced mechanical behavior at room temperature compared to MoSi 2 or other intermetallic alloys. Moreover, the intermetallic compounds as well as Mo are candidates for high-temperature applications. As the high-temperature behavior could be strongly influenced by the respective microstructure we present here the processing and the microstructure obtained.

Description: As the most promising candidate of the solid electrolyte materials for future lithium batteries, oxide electrolytes with high–lithium-ion conductivity have experienced a rapid development in the past few decades. Existing oxide electrolytes are divided into two groups, i.e., crystalline group including NASICON, perovskite, garnet, and some newly developing structures, and amorphous/glass group including Li 2 O–MO x (M = Si, B, P, etc.) and LiPON-related materials. After a historical perspective on the general development of oxide electrolytes, we try to give a comprehensive review on the oxide electrolytes with high–lithium-ion conductivity, with special emphasis on the aspect of materials selection and design for applications as solid electrolytes in lithium batteries. Some successful examples and meaningful attempts on the incorporation of oxide electrolytes in lithium batteries are also presented. In the conclusion part, an outlook for the future direction of oxide electrolytes development is given.

Description: Dual-phase oxygen transport membranes are fast-growing research interest for application in oxyfuel combustion process. One such potential candidate is CGO-FCO (60 wt% Ce 0.8 Gd 0.2 O 2−δ –40 wt% FeCo 2 O 4 ) identified to provide good oxygen permeation flux with substantial stability in harsh atmosphere. Dense CGO-FCO membranes of 1 mm thickness were fabricated by sintering dry pellets pressed from powders synthesized by one-pot method (modified Pechini process) at 1200°C for 10 h. Microstructure analysis indicates presence of a third orthorhombic perovskite phase in the sintered composite. It was also identified that the spinel phase tends to form an oxygen deficient phase at the grain boundary of spinel and CGO phases. Surface exchange limitation of the membranes was overcome by La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3−δ (LSCF) porous layer coating over the composite. The oxygen permeation flux of the CGO-FCO screen printed with a porous layer of 10 μm thick LSCF is 0.11 mL/cm 2 per minute at 850°C with argon as sweep and air as feed gas at the rates of 50 and 250 mL/min.

Description: Two-step sintering (TSS) in a reducing atmosphere has been employed to obtain fine-grain BaTiO 3 ceramics with a core-shell microstructure, a more uniform grain-size distribution, and superior reliability for multilayer ceramic capacitor applications. Compared to ceramics of the same composition conventionally sintered for about the same time, TSS ceramics feature a thinner shell thickness thus a stronger dopant localization, which leads to a lower concentration, higher internal resistance and more dopant- association. Improved reliability is manifest in a 50% higher breakdown strength at ambient temperature and a 400% longer endurance time to withstand DC stress at 185°C, in addition to a less field-and-temperature-dependent capacitance. A scaling analysis of the redistribution and endurance dynamics identifies transmission across the shell-grain-boundary region as the critical element beneficially impacted by core-shell structure and two-step sintering.

Description: (Ba, Ca)(Ti, Zr)O 3 ceramics have been considered as a potential lead-free alternative to commonly used lead-based piezoelectric ceramics due to their high piezoelectric performance at room temperature. In this study, the bipolar fatigue behavior of this material is investigated at room temperature. Two compositions were cycled with a bipolar electric field signal at 10 Hz with a maximum of three times the coercive field for up to approximately 10 7 cycles. Both investigated compositions exhibited high bipolar fatigue resistance compared to other ceramics reported in the literatures. The high fatigue resistance originates from the lack of mechanical damage and a weak domain wall pinning effect due to their location in the phase transition region. It was also found that pore morphology affected bipolar fatigue behavior.

Description: 0.94(Na 0.5 Bi 0.5+ x )TiO 3 –0.06BaTiO 3 ( x  = −0.04, 0, 0.02; named NB 0.46 T-6BT, NB 0.50 T-6BT, NB 0.52 T-6BT, respectively) lead-free piezoelectric ceramics were prepared via the solid-state reaction method. Effects of Bi 3+ nonstoichiometry on microstructure, dielectric, ferroelectric, and piezoelectric properties were studied. All ceramics show typical X-ray diffraction peaks of ABO 3 perovskite structure. The lattice parameters increase with the increase in the Bi 3+ content. The electron probe microanalysis demonstrates that the excess Bi 2 O 3 in the starting composition can compensate the Bi 2 O 3 loss induced during sample processing. The size and shape of grains are closely related to the Bi 3+ content. For the unpoled NB 0.50 T-6BT and NB 0.52 T-6BT, there are two dielectric anomalies in the dielectric constant–temperature curves. The unpoled NB 0.46 T-6BT shows one dielectric anomaly accompanied by high dielectric constant and dielectric loss at low frequencies. After poling, a new dielectric anomaly appears around depolarization temperature ( T d ) for all ceramics and the T d values increase with the Bi 3+ amount decreasing from excess to deficiency. The diffuse phase transition character was studied via the Curie–Weiss law and modified Curie–Weiss law. The activation energy values obtained via the impedance analysis are 0.69, 1.05, and 1.16 eV for NB 0.46 T-6BT, NB 0.50 T-6BT and NB 0.52 T-6BT, respectively, implying the change in oxygen vacancy concentration in the ceramics. The piezoelectric constant, polarization, and coercive field of the ceramics change with the variation in the Bi 3+ content. The Rayleigh analysis suggests that the change in electrical properties of the ceramics with the variation in the Bi 3+ amount is related to the effect of oxygen vacancies.

Description: Despite technological improvements in its production process, the sanitary ware industry inevitably generates a certain volume of discards, products whose quality is not up to standard. The present paper is the first to scientifically explore clay-based sanitary ware waste (SW) with a view to its valorization as an addition in the design of new, more environmentally friendly cements. The focus is on characterization of the waste and its pozzolanicity, as well as the structural and microstructural changes taking place in the pozzolan/Ca(OH) 2 system in the first 90 d of reaction. The findings show that pozzolanicity in clay-based waste is comparable to the activity observed in silica fume (SF) and higher than that found in other clay-based materials and fly ash (FA). The microstructural study of the clay-based waste/Ca(OH) 2 system, in turn, reveals that the proportion of C–S–H gels rises with hydration time. These gels are characterized by long mean chain lengths (MCL) and low Ca/Si ratios. The intrinsic characteristics of this thermally activated clay-based waste qualify it as a type Q pozzolans as defined in the European cement standards, making it apt for use in the manufacture of CEM II, IV, and V cements.

Description: High-Energy Ball Milling (HEBM) is proposed as a cost effective and environmental friendly technique to produce Co- and Mn- based oxides suitable for application as protective coating. Mixtures of manganese and cobalt oxides in different molar ratio (Co:Mn = 1:1 and Co:Mn = 2:1) were subjected to mechanochemical treatment up to 100 h and morpho-structural evolution was evaluated. XRD analysis results show that the HEBM treatment promotes the solid-state reaction of the starting compounds, with the formation of different crystalline phases when compared to high-temperature solid-state synthesis. SEM obs erv ations and N 2 adsorption measurements suggest that all processed powders are composed by aggregates of nanometric particles. While long milling time is required to complete the reaction, 10 hours are enough to activate the powders to obtain the desired phases after a mild thermal treatment, as evidenced by in situ thermal XRD analysis. Electrical conductivity measures performed with the Van der Pauw method on sintered pellets evidence a significant difference between the two compositions, related to the dual-phase nature of Co:Mn = 1:1 material at intermediate temperatures (i.e., T  〈 700°C), Co:Mn = 2:1 sample showing higher conductivity values in the whole tested range (500°C–800°C).

Description: The Na–SrSiO 3 as a potential high-conductivity ionic conductor for intermediate temperature solid oxide electrochemical cells (SOECs) has drawn much attention recently. Some of these studies questioned the feasibility of Na doping and therefore the creation of oxygen vacancies, while others suggested an alternative phase responsible for the ionic conduction. In this study, a systematic investigation was carried out to understand the ionic conduction in Na–SrSiO 3 . Through in situ high-temperature X-ray diffraction, thermal analysis, microstructural characterization, and electrical conductivity measurement, Na–SrSiO 3 was shown as a two-phase material, one being slightly Na-doped SrSiO 3 and another being amorphous Na 2 Si 2 O 5 . The former was an electrical insulator whereas the latter was a good ionic conductor. It was also found that the amorphous Na 2 Si 2 O 5 phase was unstable at the temperature ≥500°C, crystallizing into the insulating polycrystalline Na 2 Si 2 O 5 which causes conductivity to “bend-over” at higher temperatures. A preliminary Ab Initio Molecular Dynamics (AIMD) simulation suggested that the amorphous Na 2 Si 2 O 5 be predominantly a Na + conductor.

Description: This study investigated the influence of codoping with Ge and GeO 2 on the nonlinear coefficient α and the breakdown voltage E B of TiO 2 –Nb 2 O 5 –SrCO 3 varistor ceramics. Ge–GeO 2 codoping notably changed the microstructure of the TiO 2 –Nb 2 O 5 –SrCO 3 ceramics, increased α, and decreased E B . When the doping contents of Nb 2 O 5 and SrCO 3 were 0.1 and 0.2 mol%, respectively, the optimum doping content of 0.25 mol% Ge and 0.75 mol% GeO 2 exhibited high α (11.6), low E B (13.8 V/mm), and high grain-boundary barrier Φ B (0.96 eV). These results are superior to previous findings. In addition, Ge and GeO 2 , which function as sintering aids, reduced the sintering temperature caused by the low melting point. The optimal sintering temperature was 1260°C for the TiO 2 –Nb 2 O 5 –SrCO 3 ceramics doped with Ge and GeO 2 .

Description: Pure perovskite K 0.5 Na 0.5 NbO 3 – x SrTiO 3 ( x  =   0.16, 0.17, 0.18, and 0.19) ceramics were prepared by using a solid-state reaction process. The ceramics were optically transparent for visible and near-infrared wavelengths. Then, high tunability (24.1%) and low dielectric loss (0.016) for the x  =   0.18 sample indicated the transparent ceramics could be used in tunable devices. The Lorentz-type relation fitting for the temperature dependence of dielectric permittivity showed that these ceramics had a typical relaxor behavior, and the polar nanoregions were related to the tunable dielectric properties. The nonlinear dielectric behavior was further explored by the Johnson model combined with Langevin terms, which revealed that the polar nanoregions contributed to the nonlinear ε( E ) dependencies with contributions of 12.3%, 11.6%, 5.9%, and 3.6% for x  =   0.16, 0.17, 0.18, and 0.19, respectively.

Description: The effect of Cr 3+ on the electrical properties of SnO 2 -based films deposited by electrophoresis on Si/Pt substrate was considered. The films were sintered in a microwave oven at 1000°C/40 min and then the surface was modified with deposition of Cr 3+ ions by electrophoresis. The diffusion of Cr 3+ contributes to the modification of the potential barrier formed on the grain boundary improving the electrical properties due to electron acceptor species adsorption on the grain boundary. The influence on the properties of grain boundary was verified by I versus V characterization in as a function of temperature. The films showed nonlinear coefficient over 9, potential barrier height over 0.5 eV and resistivity greater than 10 7  Ω·cm. 4 samples were prepared at same conditions and presented similar electrical behavior, showing the efficiency of technique on reproducibility to varistor properties control. Thereby the nonlinear coefficient increases while decreasing the conductivity of the system is noticed.

Description: The plasmonic effects of infiltrated silver (Ag) nanoparticles, with different contents, inside a nanostructured TiO 2 film on the photovoltaic performance of dye-sensitized solar cells (DSSCs) are explored. The synthesized Ag nanoparticles are immobilized onto deposited TiO 2 nanoparticles by a new strategy using 3-mercaptopropionic acid (MPA), a bifunctional linker molecule. Transmission electron microscope (TEM) images show that monodispersed Ag and polydispersed TiO 2 nanoparticles have an average diameter of 12 ± 3 nm and 5 ± 1 nm, respectively. Moreover, Fourier transform infrared spectroscopy (FTIR) analysis reveals that Ag nanoparticles were successfully functionalized and capped with MPA. Optical studies on the MPA-capped Ag nanoparticles inside TiO 2 film show an increase in the total absorbance of the electrode. Moreover, EIS measurements confirm that MPA-capped Ag nanoparticles inhibit the charge recombination and improve the stability of nanoparticles in I 3 − / I − electrolyte. The DSSC assembled with optimal content of MPA-capped Ag nanoparticles demonstrated an enhanced power conversion efficiency (8.82% ± 0.07%) compared with the pure TiO 2 (7.30% ± 0.05%). The increase in cell efficiency was attributed to the enhanced dye light absorption in strength and spectral range due to the surface plasmon resonance of MPA-capped Ag nanoparticles in the photoanode.

Description: A large and stable shape memory effect has been observed in 0.6 wt% Mn-doped (Pb 0.99 Nb 0.02 )[(Zr 0.70 Sn 0.30 ) 0.52 Ti 0.48 ] 0.98 O 3 (Mn:PNZST) ceramics after being poled at high temperature. A maximum shape memory of 0.41% was achieved. This effect is related to the preferentially oriented defect dipoles along the poling direction after poling at high temperature and electric field. Furthermore, the shape memory effect is stable after 10 4 electric cycles at 30 kV/cm. The shape memory piezoelectric actuator may be fabricated using this kind of material.

Description: Glassmelting efficiency largely depends on heat transfer to reacting glass batch (melter feed), which in turn is influenced by the bulk density (ρ b ) and porosity (ϕ) of the reacting feed as functions of temperature ( T ). Neither ρ b ( T ) nor ϕ( T ) functions are readily accessible from direct measurements. For the determination of ρ b , we monitored the profile area of heated feed pellets and calculated the pellet volume using numerical integration. For the determination of ϕ, we measured the material density of feeds quenched at various stages of conversion via pycnometry and then computed the feed density at heat-treatment temperature using thermal expansion values of basic feed constituents.

Description: Spray-drying is an effective method for producing powder aggregates with controlled size and morphology. Here, we report on a systematic study aimed at determining how spray-drying parameters such as nozzle temperature, gas flow, salt concentration and solution feed rate, influence the characteristics of BaCl 2 granules prepared from aqueous solutions. We correlate the granule characteristics to these conditions through the use of processing maps and modeling. It is found that well-dispersed, high density and spherical aggregates, which are favorable for subsequent powder compaction and sintering, can be obtained within a limited range of processing conditions.

Description: When the sample temperature is warm enough so that the Joule heating takes over the environment's radiation heating as the dominant heating means, thermal runaway follows and flash sintering is triggered. This condition accurately predicts the reported onset temperature T on of all the constant-field ( E ) experiments on flash sintering, performed under a constant heating rate. The predicted linear ln( E 2 / T on 4 ) versus T on −1 relation determines the activation energy and the relative ranking of the electrical conductivity of the tested materials.

Description: In recent years, kaolinite-based wastes are focusing the attention of researchers to obtain recycled metakaolinite, with consequent environmental and socioeconomic benefits. One of these lines of research is based on coal mining waste, which once activated thermally, it becomes a highly pozzolanic product (ACM). This study reports the influence of activated carbon mining waste on the formation and evolution of the mineralogical phases in the ACM/cement system as well as their influence on the microstructure up to 90 d of reaction. Mineralogical analyses clearly show that the addition of ACM modified mineralogical compounds of blended cements. The C 4 AH 13 and C 4 A H 12 were the predominant phases in this type of cements; while in the ordinary portland cement cements, portlandite, ettringite, and carboaluminate were main hydrated phases. Two differential zones in the pore size distribution of the C–S–H gels at 12 and 4.5 nm were observed, predominating the formation of C–S–H gels at 12 nm when 20% of ACM was added to the cement.

Description: Ce 3+ , Nd 3+ codoped (Sr 0.6 Ca 0.4 ) 3 (Al 0.6 Si 0.4 )O 4.4 F 0.6 phosphors were synthesized through the high-temperature solid-state reaction method. Luminescence spectra, absorption spectra, and decay lifetimes of these samples have been measured to prove the energy-transfer process from Ce 3+ to Nd 3+ . Under UV and blue light excitation, (Sr 0.6 Ca 0.4 ) 3 (Al 0.6 Si 0.4 )O 4.4 F 0.6 :Ce 3+ ,Nd 3+ phosphors exhibit near-infrared (NIR) emission, mainly peaking at 1093 nm and secondarily at 916 nm. The NIR emission matches well with the band gap of c-Si. Results of this work suggest that the (Sr 0.6 Ca 0.4 ) 3 (Al 0.6 Si 0.4 )O 4.4 F 0.6 :Ce 3+ , Nd 3+ phosphors have potential application as down-shifting luminescent convertor for enhancing the photoelectric conversion efficiency of c-Si solar cell.

Description: It was determined that the mean grain boundary radius of curvature in 3 mol% yttria-stabilized zirconia isothermally annealed without and with a DC electric field  = 18 V/cm was uniquely proportional to the mean linear intercept grain size , the proportionality constant α   = 3/2 being in accord with the Rios-Fonseca stereological model.

Description: The electric-field-driven phase transition in (K, Na, Li)(Nb, Ta, Sb)O 3 lead-free piezoelectric ceramics was investigated by X-ray diffraction, Raman spectra, and the temperature dependences of permittivity spectra. After poling under different electric fields, phase of the ceramics transformed gradually from orthorhombic–tetragonal coexisting phase to orthorhombic phase, indicating that the crystal structure of ceramics was strongly sensitive to electric field as an external stimulus. A secondary phase K 3 Li 2 Nb 5 O 15 induced by electric field was detected in the ceramics with Li content of 7 mol%, which was close to the solubility limit of lithium. This field-induced secondary phase resulted from the movement of Li ions and the structural deformation induced by electric field. Moreover, piezoelectric constant d 33 increased with the increasing poling field strength and the enhancement can be attributed to the field-triggered domain switching. This study implied that in addition to temperature and composition, which has been reported in previous researches, electric field might be an effective way for inducing phase transition in lead-free piezoelectric ceramics and improving the electrical performances simultaneously.

Description: In this study, we introduce a simple and effective seed-mediated growth method (SMGM) for the controlled synthesis of boron suboxide powder. By employing starting powders with different concentrations and adding boron suboxide seeds with a star-like morphology, we demonstrate that B 6 O ceramics that exhibit high-level crystallinity can be synthesized using SMGM at ambient pressure conditions. The formation of multilayered star-shaped B 6 O particles via SMGM is reported for the first time.

Description: In this study, thermal radiation was employed for sintering silicon carbide foams that achieved a gradient porous structure. The simultaneous use of graphite and carbon fiber reinforced carbon composite ( C f / C ) radiators resulted in an axial temperature gradient of ~600°C along the cylindrical sample, as confirmed by both numerical simulation and experimental measurement. By sintering the cylinder top at 1600°C for 5 min, the porous SiC body achieved an axial pore size gradient from ~106 ± 36 μm to ~250 ± 84 μm and an open porosity from 41.4 to 79.8 vol%. This work indicates the potential of sintering by intense thermal radiation technique for rapid manufacturing functionally graded materials through asymmetric assembly of thermal radiators.

Description: Y 4 Si 2 O 7 N 2 : Eu 2+ phosphor has been prepared by a pretreatment method. Reduction in Eu 3+ ions into Eu 2+ by the use of hydrogen iodide (HI) is verified by X-ray absorption near-edge structure (XANES) and electrode potential analysis. Y 4 Si 2 O 7 N 2 : Eu 2+ phosphor has a broad emission band in the range of 400–500 nm. Furthermore, the effect of Zr doping on the structure and luminescence properties of Y 4 Si 2 O 7 N 2 : Eu 2+ phosphor is researched. It found that the Zr doping leads to an emission blueshift, and improves the luminescence intensity and thermal quenching behavior of Y 4 Si 2 O 7 N 2 : Eu 2+ phosphors. Prospectively, the pretreatment approach could be extended to develop other Eu 2+ -doped compounds.

Description: Effect of electric current on sintering behavior and microstructure evolution of zirconium diboride (ZrB 2 ) was investigated using three different configurations of Field Assisted Sintering Technology/Spark Plasma Sintering. The current flow through the ZrB 2 compact was controlled by modifying the interface between the graphite punches and the electrical conductive powder. Boron nitride discs, graphite foils or direct contact with the graphite punches were the three different interfaces used in order to deflect, conduct or promote, respectively, the current during the sintering process of the electrically conductive ZrB 2 ceramics. The current flow during the sintering process triggered the elimination/reduction in B 2 O 3 , leading to faster diffusion rates at high temperatures and limiting the formation of B 4 C secondary phase. This allows to control the final density, grain size (from 19.6 to 43.2 μm) and secondary phase formation (from 5.95 to 11.61 vol%) as well as the electrical resistivity (from 7.7 to 9.4 μΩ·cm) of the specimens.

Description: An efficient synthesis is used for the first time to prepare CaCu 3 Ti 4− x W x O 12 ( x  = 0.01, 0.03, and 0.05) electroceramics for energy storage capacitors. CaCu 3 Ti 4− x W x O 12 ceramics are synthesized via flame synthesis of metal nitrates precursors in nonaqueous solution using cheap, stable, and insoluble solid TiO 2 powder. The pathway yielded a CaCu 3 Ti 4 O 12 (CCTO) phase with the traces of CuO and CaTiO 3 sintered at 1050°C for 30 h. The SEM micrograph shows the grains with smooth surfaces associated with cubical appearance and the size range of 1.5–7, 2.0–7.5, and 2.0–8.0 μm for CCTWO01, CCTWO03, and CCTWO05, respectively. The EDX and XPS analyses show the presence of Ca, Cu, Ti, W, and O elements confirming the purity of these ceramics. The complex impedance and modulus ( M ) spectroscopy show that the dielectric constant (ε r ) values of the W-doped CCTO were dominantly affected by the electrical properties of the grain boundary, which is also evident from the SEM micrographs. The grain-boundary resistance decreased with increasing tungsten content. The activation energies for the grain boundaries were calculated from the impedance and modulus data using the slope of the ln τ versus 1/ T and were found to be in the range 0.62–0.67 eV.

Description: Presence of unwanted phase(s) such as yttrium iron perovskite (YIP) in yttrium iron garnet (YIG) ceramics has limited the utilization of YIG in the wireless communication domain. These unwanted phase(s) have been deemed responsible for the high dielectric losses, thus contributing to poor performance. This paper focuses on understanding the phenomenological phase transformation during the conventional solid state synthesis of YIG. This is done in order to monitor conditions which favor formation of unwanted phase(s), which shall later be reduced. The phase changes during YIG formation as a function of reaction times and temperatures were determined through XRD analysis. The amounts of YIG formed at various reaction times were fitted into various kinetic models in order to mathematically link what occurs experimentally to the available theoretical descriptions of reactions. It is found that the Ginstling-Brounstein-Habert (GBH) model exhibited good mathematical correlation to the formation of YIG. Meanwhile the activation energy ( E a ) indicated 490 kJ/mol is required for the formation of YIG. At the end, a reaction model and mechanism between Fe 2 O 3 and Y 2 O 3 were established and illustrated to underline the effect of diffusion controlled environment on the formation of phases in YIG ceramics.

Description: The present work reports a simple, inexpensive method for synthesis of calcium hydroxide [Ca(OH) 2 ] nanoparticles (CHNPs). The method involves chemical precipitation (CP) in aqueous medium at room temperature. Calcium nitrate dihydrate [Ca(NO 3 ) 2 .2H 2 O] and sodium hydroxide were used as precursors. The CHNPs were characterized by Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD), Rietveld analysis, field-emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM), BET surface area evaluation as well as particle size distribution analysis techniques. The results confirmed the synthesis of CHNPs as the major phase. The CHNPs exhibited an average size of about 350 nm. In addition, some calcite phase formed due to the inevitable carbonation process. A very minor amount of aragonite phase was also present. A schematically developed new qualitative model is proposed to explain the genesis and subsequent evolution of the various phases at the nanoscale. The model helps to identify the rate-controlling step. It also highlights the implication of reaction kinetics control in synthesis of predesigned nanophase assembly.

Description: The mechanical properties of dense, hot-pressed ZrB 2 –30 vol% SiC ceramics were characterized from room temperature up to 1600°C in air. Specimens were tested as hot-pressed or after hot-pressing followed by heat treatment at 1400°C, 1500°C, 1600°C, or 1800°C for 10 h. Annealing at 1400°C resulted in the largest increases in flexure strengths at the highest test temperatures, with strengths of 470 MPa at 1400°C, 385 MPa at 1500°C, and 425 MPa at 1600°C, corresponding to increases of 7%, 8%, and 12% compared to as hot-pressed ZrB 2 –SiC tested at the same temperatures. Thermal treatment at 1500°C resulted in the largest increase in elastic modulus, with values of 270 GPa at 1400°C, 240 GPa at 1500°C, and 120 GPa at 1600°C, which were increases of 6%, 12%, and 18% compared to as hot-pressed ZrB 2 –SiC. Neither ZrB 2 grain size nor SiC cluster size changed for these heat-treatment temperatures. Microstructural analysis suggested additional phases may have formed during heat treatment and/or dislocation density may have changed. This study demonstrated that thermal annealing may be a useful method for improving the elevated temperature mechanical properties of ZrB 2 -based ceramics.

Description: Si 3 N 4 -based nanocomposites containing 0–50 wt% TiC 0.37 N 0.63 are directly consolidated at 1700°C by spark plasma sintering, and their reciprocal sliding behavior against a Si 3 N 4 counterbody is investigated under a maximum Hertzian stress of 1.27 GPa in unlubricated conditions. The average grain widths of Si 3 N 4 and TiC 0.37 N 0.63 are about 85 and 90 nm, respectively. The decreasing relative densities of the as-sintered nanocomposites indicate that the nano-TiC 0.37 N 0.63 may introduce pores and reduce the hardness and fracture resistance of the materials. The brittleness index for sliding contacts in all the samples is 25–31, indicating brittle fracture taking place on the wear surface and inducing cavities. When the mean free paths of nano-TiC 0.37 N 0.63 are slightly greater than grain length of Si 3 N 4 , the best wear resistance is achieved in Si 3 N 4 containing 20/30 wt% TiC 0.37 N 0.63 due to the process of surface smoothing by triboproducts. Severe wear response can be observed in Si 3 N 4 nanocomposites containing 0, 10, 40, and 50 wt% of TiC 0.37 N 0.63 . The wear responses are explained by considering the microstructural parameters (like grain characteristics for both phases and mean free path of nano-TiC 0.37 N 0.63 ) and contact-induced fracturing behavior, as well as tribochemical reactions.

Description: In this study, a novel chalcogenide glass system, Ga–Sb–S, is reported. The glass-forming ability, the physical properties, and the structure are investigated; and the potential applications of the glasses are evaluated. The compositions consisting of ~3%–10% Ga, ~29%–37% Sb, and ~57%–63% S can form glasses. The glasses have a wide transparent window of ~0.8–14 μm, high linear refractive indices of ~2.62–2.70 (@10 μm), high third-order nonlinear refractive indices of ~12.4 × 10 −14 cm 2 /W (@ 1.55 μm), low phonon energy, and large rare earth solubility. These favorable properties make them promising materials for mid-infrared applications such as thermal imaging, nonlinear optics, and lasers.

Description: We demonstrate the resistive switching behavior on SrTiO 3 (STO) (100) single-crystalline substrate with graphene nanoribbon (GNR) electrodes. The planar GNR was fabricated on STO substrate by means of dip-pen nanolithography and polystyrene-etching techniques. The nanogap for the GNR electrodes was induced by electroburning, which was performed by applying a voltage across the GNR. The nanoscale GNR/STO/GNR RRAM device showed bipolar resistive switching behavior with low set/reset voltages and current as well as good retention characteristics. The bipolar resistive switching behavior might be attributed to roles of oxygen vacancies, originating from intrinsic characteristics of dislocation of STO single crystal.

Description: A model of reactive ion-exchange diffusion in glassy materials (glasses and glass-ceramics) has been developed. The model can be used to simulate the processes of ion-exchange-induced decrystallization of glass-ceramics and ion-exchange-induced glass crystallization. Analysis of the model in dimensionless form resulted in determining a set of dimensionless complexes comprising dimensional parameters of the ion exchange system (diffusivity, rate constant of grain dissolution, initial size and composition of the crystalline grains, etc.). Numerical values of the complexes enable one, not solving the problem, to predict the composition and structure of glass-ceramic subsurface layers produced in a certain glassy material and under given ion-exchange conditions. It was shown that the developed model correctly describes experimental data on ion-exchange-induced glass-ceramics decrystallization. It appears that the model can be used in developing a new technology of optical glass-ceramics for photonic applications.

Description: Yb:(LaY) 2 O 3 ceramic samples have been sintered to almost full density in a fast microwave heating process with zero hold time. Rapid densification is observed at heating rates ranging from 50°C to 2400°C/min. The estimated value of the power absorbed in the materials per unit volume is from 10 to 400 W/cm 3 , which is similar to the processes of flash sintering under an applied dc/ac voltage. The microstructure of the sintered samples exhibits traces of a liquid-like intergranular phase. The observed flash microwave sintering effect is associated with both preferential microwave absorption and enhanced mass transport within the softened grain boundaries arising due to an elevated concentration of defects and impurities therein. The volumetric nature of microwave heating gives rise to thermal stresses which can act as an additional driving force for sintering. The advantage of the microwave flash sintering process is that no electrodes are needed to supply the power to the articles undergoing sintering.

Description: High-temperature aero-thermal heating in a 30 kW inductively coupled plasma torch was used to replicate the effects of harsh oxidizing environments during hypersonic atmospheric entry on fracture behavior and microstructure of two-dimensional woven SiC fibers. Hi-Nicalon SiC woven cloths were exposed to surface temperatures over 1400°C with different high-enthalpy dissociated oxygen and nitrogen plasma flows, and were subsequently deformed in pure tension at room temperature. Changes in fiber microstructure and surface chemistry after thermal exposure were examined by scanning electron microscopy. Pure nitrogen plasmas resulted in a 50% decrease of strength in woven SiC fibers with minimal effects on the fiber structure, except for highly localized surface pitting caused by partial decomposition of silicon oxycarbonitride phase at high temperature. In contrast, exposure to dissociated oxygen and air plasmas led to severe strength reduction and embrittlement over significantly short time scales, corresponding to degradation rates up to 200 times higher than those reported with static heating at equivalent temperatures. The origin of accelerated embrittlement at microscopic scale was found related to complex gas-surface interactions and high-temperature oxidizing processes involving the formation of SiO 2 bubbles and microcracks on the surface. These findings are important for the development of outer fabric materials for new flexible thermal protection systems in space applications.

Description: Pressure-less spark plasma sintering (SPS) is a new approach during which rapid densification of ceramic nanopowder green bodies is accompanied by rapid grain growth. Although the origin of this phenomenon has not yet been fully understood significant, difference in grain growth between pressure-less and pressure-assisted SPS was expected. In this work 3Y-TZP nanopowder with average particle size of 12 nm was consolidated using two-step approach: (1) at an intermediate temperature (600°C to 1000°C) SPS warm pressing followed by (2) high temperature (1400°C to 1600°C) pressure-less SPS. The standard one step pressure-assisted SPS experiments were quoted as references. Rapid grain growth was observed during both pressure-less and standard SPS. The samples prepared by both approaches at the same sintering temperature (1400°C–1600°C) achieved identical grain size and grain size distribution, if large pores were eliminated in early stage by SPS warm pressing. The electric current, electromagnetic field, and mechanical pressure is proven to have a negligible direct influence on grain growth in 3Y-TZP ceramics at temperatures above 1000°C under standard SPS conditions.

Description: Bismuth ferrite, BiFeO 3 , is an important multiferroic material that has attracted remarkable attention for potential applications in functional devices. While thin films of BiFeO 3 are attractive for applications in nanoelectronics, bulk polycrystalline BiFeO 3 has great potential as a lead-free and/or high-temperature actuator material. However, the actuation mechanisms in bulk BiFeO 3 are still to be resolved. Here we report the microscopic origin of electric-field-induced strain in bulk BiFeO 3 ceramic by means of in situ high-energy X-ray diffraction. Quantification of intrinsic lattice strain and extrinsic domain switching strain from diffraction data showed that the strain response in rhombohedral bulk BiFeO 3 is primarily due to non-180° ferroelectric domain switching, with no observable change in the phase symmetry, up to the maximum field used in the study. The origin of strain thus differs from the strain mechanism previously shown in thin film BiFeO 3 , which gives a similar strain/field ratio as rhombohedral bulk BiFeO 3 . A strong post-poling relaxation of switched non-180° ferroelectric domains has been observed and hypothesized to be due to intergranular residual stresses with a possible contribution from the conductive nature of the domain walls in BiFeO 3 ceramics.

Description: Morphological details of calcium silicate hydrate (C–S–H) stemming from the hydration process of Portland cement (PC) phases are crucial for understanding the PC-based systems but are still only partially known. Here we introduce the first soft X-ray ptychographic imaging of tricalcium silicate (C 3 S) hydration products. The results are compared using both scanning transmission X-ray and electron transmission microscopy data. The evidence shows that ptychography is a powerful method to visualize the details of outer and inner product C–S–H of fully hydrated C 3 S, which have fibrillar and an interglobular structure with average void sizes of 20 nm, respectively. The high-resolution ptychrography image enables us to perform morphological quantification of C–S–H, and, for the first time, to possibly distinguish the contributions of inner and outer product C–S–H to the small angle scattering of cement paste. The results indicate that the outer product C–S–H is mainly responsible for the q −3 regime, whereas the inner product C–S–H transitions to a q −2 regime. Various hypotheses are discussed to explain these regimes.

Description: Rare-earth containing phosphors have been widely applied in lighting and display fields in the past century. Lower cost rare-earth free phosphors with high performance are highly desired driven by the exhaustion of rare earth resources and the requirement of cheaper production. Herein, Cu + ions doped tetracalcium phosphate (TTCP) yellow emitting phosphors with quantum yield of 21% are exploited. Particularly, ultra-broad band emission with a full width at half maximum (FWHM) about 200 nm throughout almost entire visible light region is observed for TTCP: Cu + phosphors, evidencing its promising application in high color rendering index (CRI) lighting source. White light emission with CRI value about 94.3 is generated by combining this TTCP: Cu + phosphor with commercial BaMgAl 10 O 17 : Eu 2+ blue phosphor, exhibiting superiority over the traditional trichromatic phosphors. Therefore, we predict great potential application for this cheaper rare-earth free TTCP: Cu + phosphor in high CRI lighting sources.

Description: Eu 3+ -doped tungstate Ba 2 La 2 ZnW 2 O 12 phosphors with perovskite-structure were prepared by the high temperature solid-state reaction. The X-ray powder diffraction (XRD) patterns and structure refinements indicate that the phosphors crystalized in the trigonal layer-perovskite. The luminescence properties of the phosphors were investigated such as photoluminescence (PL) excitation and emission spectra, decay lifetimes, and color coordinates. It was found that the pure host shows self-activated emission excited by the UV light. Moreover, Ba 2 La 2 ZnW 2 O 12 also shows scintillation characteristics under the X-ray irradiation. The near-UV and blue light can efficiently excite Eu 3+ -doped Ba 2 La 2 ZnW 2 O 12 phosphors inducing the strong orange–red luminescence. The optimal Eu 3+ doping concentration in this host is 40 mol%. The luminescence spectra and the luminescence color of the phosphors strongly depend on the doping levels and excitation wavelength. The different luminescence features were discussed on the base of crystal structure. Eu 3+ ions have two possible substitutions on A or B sites in this trigonal layered perovskite. The phosphor could act as a candidate for the potential application in near-UV excited white-LEDs lighting.

Description: The solubility of Co 3 O 4 , Cu 2 O, CuO, NiO, and Mn 2 O 3 in molten B 2 O 3 and Na 2 O–2B 2 O 3 has been studied at a temperature of 900°C under static conditions. The concentration of the dissolved metal oxides was determined by X-EDS and XPS elemental analysis. Uniformity of metal distribution has been confirmed using X-EDS and backscatter electron image mapping. It was found that the solubility of all metal oxides increased significantly with Na 2 O content in the B 2 O 3 solvent. The impact of a temperature increase of 150°C and the influence of K 2 O doping were evaluated and found to not cause any significant change.

Description: An original catalytic method has been proposed to synthesize carbon nanotubes (CNTs)–ZrB 2 –ZrO 2 heterostructures using ZrB 2 polymeric precursor. The pyrolysized gases from the ZrB 2 polymeric precursor are identified to be the carbon sources for CNTs growth. A parametric study is conducted to control the CNTs growth by optimizing parameters such as synthesis temperature and catalyst content. Observations show that the in situ grown CNTs are homogeneously dispersed in the powders, and the structure and the amount of CNTs are significantly dependent on the synthesis parameters. There are two kinds of grown CNTs existed in the produced hybrid heterostructures: (i) the kinking structured CNTs that are disordered and incomplete graphitization; (ii) the improved and graphitized CNTs. The ZrB 2 polymeric precursor during thermal pyrolysis provides capable of supplying substantial carbon source for CNTs nucleation and growth by homogeneous vapor–liquid–solid reactions.

Description: The setting processes in the commercial glass ionomer cement (Fuji IX Fast) and resin modified glass ionomer cement (Fuji II LC) were investigated by the dielectric spectroscopy. The changes in the real and imaginary parts of the dielectric permittivity [ε*(ω)] as well as dielectric loss (tan δ) with time for Fuji IX Fast show several distinct regions which are related to the different stages of the acid–base reaction. Interestingly, the first stage that lasts for approximately 12 min terminates with a sharp decrease in dielectric parameters which is most probably related to the instantaneous (micro)fracturing of the sample due to a rapid build–up of the shrinkage stress. On the other hand, evolution of the dielectric properties during the setting of Fuji II LC indicates fast reaction in the initial stage (within few minutes) followed by the slow gradual change as a result of the competing nature of the acid–base reaction and light-activated polymerization.

Description: The crystal structure and microwave dielectric properties of a novel low-firing compound Li 2 Mg 2 W 2 O 9 were investigated in this study. The phase purity and crystal structure were investigated using X-ray diffraction analyses and Rietveld refinement. The best microwave dielectric properties of the ceramic with a low permittivity (ε r ) ~11.5, a quality factor ( Q  ×  f) ~31 900 GHz (at 10.76 GHz) and a temperature coefficient of the resonant frequency (τ f ) ~ −66.0 ppm/°C were obtained at the optimum sintering temperature (920°C). CaTiO 3 was added into the Li 2 Mg 2 W 2 O 9 ceramic to obtain a near zero τ f , and 0.93Li 2 Mg 2 W 2 O 9 –0.07CaTiO 3 ceramic exhibited improved microwave dielectric properties with a near-zero τ f ~ −1.3 ppm/°C, a ε r ~ 21.6, a high Q u   ×  f value ~20 657 GHz. The low sintering temperature and favorable microwave dielectric properties make it a promising candidate for LTCC applications.

Description: The Al 2 O 3 –CaO–SiO 2 system provides the basis for describing many important chemical processes. Although the system has previously been extensively studied, recent advances in experimental technique have provided the opportunity to obtain accurate liquidus measurements in the low-silica region at fixed temperatures. The experimental procedures involve equilibration of high-purity oxide powder mixtures at selected temperatures, rapid quenching, and accurate measurement of phase compositions using electron probe X-ray microanalysis. The liquidus isotherms have been determined at selected temperatures between 1503 and 1873 K in the anorthite, gehlenite, pseudowollastonite, corundum, CaAl 12 O 19 , CaAl 2 O 6 , lime, Ca 3 SiO 3 , and Ca 2 SiO 4 primary phase fields. The results are compared with currently available thermodynamic model predictions of the phase chemistry.

Description: The mechanical properties of a ZrB 2 -10 vol% ZrC ceramic were measured up to 2300°C in an argon atmosphere. Dense billets of ZrB 2 -9.5 vol% ZrC-0.1 vol% C were produced by hot-pressing at 1900°C. The ZrB 2 grain size was 4.9 μm and ZrC cluster size was 1.8 μm. Flexure strength was 695 MPa at ambient, decreasing to 300 MPa at 1600°C, increasing to 345 MPa at 1800°C and 2000°C, and then decreasing to 290 MPa at 2200°C and 2300°C. Fracture toughness was 4.8 MPa·m ½ at room temperature, decreasing to 3.4 MPa·m ½ at 1400°C, increasing to 4.5 MPa·m ½ at 1800°C, and decreasing to 3.6 MPa·m ½ at 2300°C. Elastic modulus calculated from the crosshead displacement was estimated to be 505 GPa at ambient, relatively unchanging to 1200°C, then decreasing linearly to 385 GPa at 1600°C, more slowly to 345 GPa at 2000°C, and then more rapidly to 260 GPa at 2300°C. Surface flaws resulting from machining damage were the critical flaw up to 1400°C. Above 1400°C, plasticity reduced the stress at the crack tip and the surface flaws experienced subcritical crack growth. Above 2000°C, microvoid coalescence ahead of the crack tip caused failure.

Description: In this study, a simple thick-film humidity sensor was fabricated by coating wet-synthesized ZnO nanorods on screen-printing interdigitated electrodes. We investigated the influence of the coating procedure on the microstructure of ZnO nanorod films and thereby on humidity sensing. The experimental results revealed that the specific surface area (SSA) decreased and the average pore size (APS) increased with increasing the sintering time and the number of coating layer. The humidity response depended significantly on the pore properties of the ZnO nanorod films. By virtue of the incipient wetness analysis, it was found that the adsorption of water molecules on the ZnO surface led to the decrease in electrical resistance even though the ZnO was rod like, n -type semiconductor. While tuning the pore structure of the ZnO nanorod film, the thick-film humidity sensor might display near-linear response in the full range of 0%–100% relative humidity (RH).

Description: Microelectromechanical Systems (MEMS) devices typically need to be designed against a very low failure probability, which is on the order of or lower. Experimental determination of the target strength for such a low failure probability requires testing of tens of thousands of specimens, which can be cost prohibitive for the design process. Therefore, understanding the probabilistic failure of MEMS devices is of paramount importance for design. Currently available probabilistic models for predicting the strength statistics of MEMS structures are based on classical Weibull statistics. Significant advances in experimental techniques for measuring the strength of MEMS devices have produced data that have unambiguously demonstrated that the strength distributions consistently deviate from the Weibull distribution. Such deviations can be explained by the fact that the Weibull distributions are derived based on extreme value statistics, which is inapplicable to MEMS devices where the dimensions of the material microstructure are not negligible compared to the characteristic structural dimensions. This paper presents a robust probabilistic model for strength distribution of polycrystalline silicon (poly-Si) MEMS structures that could be extended to other brittle materials at the microscale. The overall failure probability of the structure is related to the failure probability of each material element along its sidewalls through a weakest-link statistical model. The failure statistics of the material element is determined by both the intrinsic random material strength as well as the random stress field induced by the sidewall geometry. Different from the classical Weibull statistics, the present model is designed to account for structures consisting of a finite number of material elements, and it predicts a scale effect on their failure statistics. It is shown that the model agrees well with the measured strength distributions of poly-Si MEMS specimens of different sizes, and the calibrated mean strength of the material element is consistent with the theoretical strength of silicon. Meanwhile, it is shown that the two-parameter and three-parameter Weibull distributions cannot provide optimum and consistent fits of the observed size-dependent strength distributions, and thus have very limited prediction capability. The present model explicitly relates the strength statistics to the size effect on the mean structural strength, and therefore provides an efficient means of determining the failure statistics of MEMS structures.

Description: The conduction parameters related to dc conductivity and electric modulus formulation in the temperature range 313–533 K for x Li 2 O (100 −  x ) (50V 2 O 5. 20 Bi 2 O 3 . 30 ZnO) glass system have been studied. The temperature-dependant dc conductivity is analyzed using the Mott's model for transition metals and modified Mott's VRH model. It is observed that Mott's model is in good agreement with the experimental data in high- and intermediate-temperature region as a strong electron–phonon interaction seems to be prevalent. The conduction in these glasses has been attributed to small polaron hopping at the vanadium sites in nonadiabatic regime. The ion-polaron effect (coupling between ions and polarons) leads to drop of the effective mobility and it can manifest itself as decrease in the dc conductivity with the addition of lithium content. The composition and temperature dependence of imaginary part of electric modulus ( ) collapsed onto a single master curve, confirms the presence of same type of relaxation phenomenon in all the studied glass compositions.

Description: By means of dielectric permittivity, electric modulus and impedance, the dielectric properties of LiF single crystals were investigated in the temperature range of 30°C–800°C and frequency range of 50 Hz–10 MHz. Two thermally activated relaxations, R1 and R2, were observed. The relaxation R1 showing activation energy around 0.8 eV was found to be related to the Li-ion diffusion in the crystal. The relaxation R2 contains three Arrhenius segments, the low-, mid-, and high-T segments, separated by boundary temperatures of 325°C and 425°C. These segments in the order of ascending temperature were found to be associated with F 3 , F 3 + centers, F 2 centers, and F centers, respectively.

Description: The delicate tuning of TiO 2 electronic structure is essential to achieve the maximized photoresponse of light harvesting energy devices such as photocatalysis and photovoltaic cells. In this study, the photocurrent response of TiO 2 -based diodes with and without Fluorine doping method was investigated. The differences in electronic structure between undoped and F-doped TiO 2 thin films are associated to the Fermi energy level which determines the available empty state energy against the valence band maximum. The incident photoconversion efficiency results indicate that the visible photoabsorption process and charge transport can be enhanced by energy matching between subgap states in TiO 2 and the interference light and provides an effective strategy for light harvesting using TiO 2 .

Description: 3 at.% Eu 3+ -doped La 2− x Gd x Zr 2 O 7 ( x  = 0–2.0) transparent ceramics were fabricated by vacuum sintering. The effect of Gd content on crystal structure, in-line transmittance, and luminescence property of the ceramics were investigated. The ceramics are all cubic pyrochlore structure with high transparency. The cut-off edge of the transmittance curve of the ceramics varied with Gd content and was also affected by the annealing process. The luminescence intensity became stronger for the ceramics annealed in air. As Gd content increased, the energy band structure as well as the luminescence behavior of the ceramics was changed; in addition, the symmetry of the crystal lattice reduced, resulting in the shift of the strongest luminescence peak from 585 nm to around 630 nm.

Description: Polycrystalline Bi 2− x O 2 Se ceramics were synthesized by spark plasma sintering process. Their thermoelectric properties were evaluated from 300 to 773 K. All the samples are layered structure with a tetragonal phase. The introduction of Bi deficiencies will cause the orientation alignment and change of effective mass. As a result, a significant enhancement of thermoelectric performance was achieved. The maximum of Seebeck coefficient is −568.8 μV/K for Bi 1.9 O 2 Se at 773 K, much larger than −445.6 μV/K for pristine Bi 2 O 2 Se. Featured with very low thermal conductivity [~0.6 W·(m·K) −1 ] and an optimized electrical conductivity, ZT at 773 K is significantly increased from 0.05 for pristine Bi 2 O 2 Se to 0.12 for Bi 1.9 O 2 Se by introducing Bi deficiencies, which makes it a promising candidate for medium temperature thermoelectric applications.

Description: An experimental investigation on electric contact resistance in the tooling utilized in spark plasma sintering (SPS) is conducted. Two different SPS regimes are operated without the presence of processed powder to obtain the punch–punch (horizontal) and punch–die (vertical) contact resistance assessments separately. The processing temperature and applied pressure are considered to be the main factors that affect the contact resistance. These effects are analyzed by performing experiments at a maximum temperature of 1600°C with constant heating rate of 100°C/min. The applied pressure is ranging from 20 to 60 MPa and a 1 min holding time-step is used to ensure stable measurements. It is shown that the contact resistance goes down as the temperature ramps up. An increase in the pressure leads to a decrease in the contact resistance. The mechanisms that cause these variations in the contact resistance are discussed. The dependence of assessed contact resistance of the utilized SPS system on the processing temperature and applied pressure is analytically described based on the regression analysis of the collected experimental data. A finite-element simulation based on the obtained experimental results evidences that the electric contact resistance dominates the temperature evolution. The obtained results are of significant importance for modeling and optimization of SPS processes.

Description: Thermodynamic evaluation of the reducing and carburizing activities of N 2 -based lean sintering atmospheres containing CO, H 2 , and small amounts of hydrocarbons with their total content not exceeding 5 vol% was performed with regard to sintering of chromium alloyed PM steels. Based on the thermodynamic evaluation, three sintering atmosphere compositions were chosen for experimental studies. For the chosen sintering atmospheres, the effect of graphite addition, sintering temperature, holding time and cooling rate on the microstructure, and final oxygen and carbon contents was studied. The compacts without admixed graphite exhibited a microstructure with pearlitic, and in some cases bainitic regions, with their content varying according to the temperature profile and increasing with increase in CO concentration in the atmosphere. These results are indicative of the carburizing ability of the atmospheres which was also supported by the results from chemical analysis. Pearlitic–bainitic microstructures were observed in the compacts admixed with graphite after sintering which suggests that the proposed atmospheres provide conditions sufficient for at least partial reduction of the surface iron oxide during sintering. The final oxygen content obtained after sintering in the proposed lean atmospheres was higher than that obtained in the case of nitrogen/hydrogen mix but still was within acceptable limits. The results obtained suggest that the proposed lean CO and H 2 containing sintering atmosphere compositions are promising candidates for robust sintering of chromium alloyed PM steels.

Description: Sintering mechanisms and kinetics were investigated for ZrB 2 ceramics produced using reaction hot pressing. Specimens were sintered at temperatures ranging from 1800°C to 2100°C for times up to 120 min. ZrB 2 was the primary phase, although trace amounts of ZrO 2 and C were also detected. Below 2000°C, the densification mechanism was grain-boundary diffusion with an activation energy of 241 ± 41 kJ/mol. At higher temperatures, the densification mechanism was lattice diffusion with an activation energy of 695 ± 62 kJ/mol. Grain growth exponents were determined to be ~4.5, which indicated that a grain pinning mechanism was active in both temperature regimes. The diffusion coefficients for grain growth were 1.5 × 10 −16  cm 4 /s at 1900°C and 2.1 × 10 −15  cm 4 /s at 2100°C. This study revealed that dense ZrB 2 ceramics can be produced by reactive hot pressing in shorter times and at lower temperatures than conventional hot pressing of commercial powders.

Description: The effect of the phosphor curvature in the range 0.1766–0.2589 mm −1 on the luminous efficacy of Y 2.95 Al 5 O 12 :0.05Ce 3+ (YAG)-based white-light-emitting diodes (WLEDs) was investigated at the similar correlated color temperature (CCT) of ~6300 K by tuning the concentration of YAG phosphors in the phosphor layer ranging from 7.5 to 15 wt%. It was found that both the luminous efficacy and luminous power increased monotonically with the increasing curvature. The luminous efficacy (=82.4 lm/W) and luminous power (=297.85 mW) of the YAG-based WLED at the preferable phosphor curvature of 0.2589 mm −1 were 19.44% and 17.36%, respectively, higher than those at the curvature of 0.1766 mm −1 under 350 mA. This finding reveals that the surface curvature of phosphor layers is a critical factor which cannot be ignored for the investigation of the light output of phosphor-converted WLEDs. Moreover, the color rendering index (CRI) enhancement of YAG-based WLED with substitution of Y 2.94 Al 5 O 12 :0.05Ce 3+ , 0.01Pr 3+ (YPrAG), Y 2.45 Gd 0.5 Al 5 O 12 :0.05Ce 3+ (YGdAG), and Y 2.95 Al 4.8 Si 0.2 O 11.8 N 0.2 :0.05Ce 3+ (YAlSiON) for YAG were assessed under the same phosphor curvature of 0.2589 mm −1 and the similar CCT ~6350 K. Taking the luminous efficacy, preparation cost of phosphors, and CRI into consideration, we suggest that the YGdAG is a preferable candidate for replacing the YAG for use in WLEDs among the four kinds of phosphors. Compared with the YAG (7.5 wt%)-based WLED, the YGdAG (7 wt%)-based WLED exhibited an improved CRI, less preparation cost of phosphors, and the acceptable reduction in luminous efficacy under 350 mA.

Description: The microstructure of polycrystalline barium titanate (BaTiO 3 ) thin films processed with a liquid-phase can be controlled by the crystallographic orientation of the underlying sapphire substrate. During postdeposition crystallization, the tendency for {111} twin nucleation, which drives subsequent abnormal grain growth, depends upon the specific sapphire facet. Specifically, tilting away from the close-packed c- plane modifies the orientation, morphology, and relative amount of an interfacial BaAl 2 O 4 second phase. These factors control the density of twin formation, and thus overall grain size of the crystallized BaTiO 3 . As the substrate orientation transitions from c -plane, to r -plane, to a -plane, the twin density is reduced, the average grain size decreases systematically from 270 to 130 nm, and the grain structure becomes overall more homogeneous. This twinning mechanism and abnormal grain growth occur by 900°C, several hundred degrees lower than reported previously.

Description: The preparation of Ba 0.85 Ca 0.15 Zr 0.1 Ti 0.9 O 3 (BCZT) powders by wet chemical methods has been investigated, and the powders used to explore relationships between the microstructure and piezoelectric properties ( d 33 coefficient) of sintered BCZT ceramics. Sol–gel synthesis has been shown to be a successful method for the preparation of BCZT nanopowders with a pure tetragonal perovskite phase structure, specific surface area up to 21.8 m 2 /g and a mean particle size of 48 nm. These powders were suitable for the fabrication of dense BCZT ceramics with fine-grain microstructures. The ceramics with the highest density of 95% theoretical density (TD) and grain size of 1.3 μm were prepared by uniaxial pressing followed by a two-step sintering approach which contributed to the refinement of the BCTZ microstructure. A decrease in the grain size to 0.8–0.9 μm was achieved when samples were prepared using cold isostatic pressing. Using various sintering schedules, BCZT ceramics with broad range of grain sizes (0.8–60.5 μm) were prepared. The highest d 33  = 410.8 ± 13.2 pC/N was exhibited by ceramics prepared from sol–gel powder sintered at 1425°C, with the relative density of 89.6%TD and grain size of 36 μm.

Description: A class of Yb 3+ /Er 3+ co-doped NaY(MoO 4 ) 2 upconversion (UC) phosphors have been successfully synthesized by a facile hydrothermal route with further calcination. The structural properties and the phase composition of the samples were characterized by X-ray diffraction (XRD). The UC luminescence properties of Yb 3+ /Er 3+ co-doped NaY(MoO 4 ) 2 were investigated in detail. Concentration-dependent studies revealed that the optimal composition was realized for a 2% Er 3+ and 10% Yb 3+ -doping concentration. Two-photon excitation UC mechanism further illustrated that the green enhancement arised from a novel energy-transfer (ET) pathway which entailed a strong ground-state absorption of Yb 3+ ions and the excited state absorption of Yb 3+ –MoO 4 2− dimers, followed by an effective energy transfer to the high-energy state of Er 3+ ions. We have also studied the thermal properties of UC emissions between 303 and 523 K for the optical thermometry behavior under a 980 nm laser diode excitation for the first time. The higher sensitivity for temperature measurement could be obtained compared to the previous reported rare-earth ions fluorescence based optical temperature sensors. These results indicated that the present sample was a promising candidate for optical temperature sensors with high sensitivity.

Description: Li 1.4 Al 0.4 Ti 1.6 (PO 4 ) 3 (LATP) was synthesized using a glass-ceramics approach through crystallization in a conventional box furnace and a modified microwave furnace. The microstructure of samples that were microwave processed at 1000°C showed a larger average grain size (0.87 μm) when compared with the grain size of conventionally processed samples (0.30 μm) at the same temperature. Microwave processing led to significant enhancement of the conductivity when compared with conventional processing for all crystallization temperatures investigated. The highest total conductivity achieved was of glass microwave processed at 1000°C, with a conductivity of 5.33 × 10 −4  S/cm. This conductivity was five times higher than that of LATP crystallized conventionally at the same temperature.

Description: Micro/mesoporous SiOC bulk ceramics with the highest surface area and the narrowest pore size distribution were prepared by water-assisted pyrolysis of polysiloxane in argon atmosphere at controlled temperatures (1100°C–1400°C) followed by etching in hydrofluoric acid (HF) solution. Their pyrolysis behaviors, phase compositions, and microstructures were investigated by DSC, FTIR, XRD, and BET. The Si–O–Si bonds, SiO 2 -rich clusters, and SiO 2 nanocrystals in the pyrolyzed products act as pore-forming species and could be etched away by HF. Water injection time and pyrolysis temperature have important effects on phase compositions and microstructures of the porous SiOC bulk ceramics, which have a maximum-specific surface area of 2391.60 m 2 /g and an average pore size of 2.87 nm. The porous SiOC ceramics consist of free carbon phase, silicon carbide, and silicon oxycarbide.

Description: The microstructures and mechanical properties of tantalum carbides containing predominantly the ζ-Ta 4 C 3− x phase are compared with the properties of the monocarbide (γ-TaC) and the hemicarbide (α-Ta 2 C) and two-phase composites. It is shown that a Ta and γ-TaC powder mixture corresponding to a C/Ta at. ratio of 0.66 can be hot-pressed (1800°C, 2 h) to obtain ~95 wt% of ζ-Ta 4 C 3− x with a density of 98% of theoretical. This material has an attractive combination of high fracture toughness (13.8 ± 0.2 MPa√m) and fracture strength (759 ± 24 MPa) with modest hardness (5.6 ± 0.5 GPa). The fracture toughness and strength measured for this material were the highest among all the materials with C/Ta ratio ranging from 0.5 (hemicarbide) to 1.0 (monocarbide). It is also shown that a material containing 86 wt% ζ-Ta 4 C 3− x can be consolidated by pressureless sintering of a hydrogenated Ta and γ-TaC powder mixture without significant drop in density (97% of theoretical) or mechanical properties (13.4 ± 0.2 MPa√m, 700 ± 20 MPa, 6.0 ± 0.4 GPa). Materials containing high weight fraction of the ζ-Ta 4 C 3− x phase exhibited rising crack-growth-resistance ( R -curve) behavior. Optical and scanning electron microscope observations suggested crack-face bridging was the dominant toughening mechanism. The crack-bridging ligaments were lamellae of the basal planes of the ζ-Ta 4 C 3− x phase produced by their easy cleavage. The thickness of the lamellae ranged from 40 to 2000 nm, significantly less than the grain size.

Description: A red-emitting phosphor MgY 4 Si 3 O 13 :Eu 3+ was synthesized by conventional solid-state reaction for the first time. Photoluminescence spectra show that the phosphor can be efficiently excited by ultraviolet and blue light and exhibit an intense emission at 615 nm due to the dominant 5 D 0 7 F 2 electric dipole transition of Eu 3+ . The decay times of Eu 3+ monitored at the different wavelengths present an interesting change trend. According to the results of XRD Rietveld refinement, in a unit cell, the number of Eu 3+ is obtained to be 0.064 for 4 f site and 0.294 for 6 h site. The remarkable preference occupancy for one site could be the leading reason of the abnormal change in decay times. The testing of thermal quenching indicates that the photoluminescence intensity of the phosphor is very sensitive to temperature and furthermore, the emission intensities from 5 D 1 7 F J and 5 D 0 7 F J show the different decay ratios with the increase in temperature. Based on the configurational coordinate diagram, an underlying mechanism is proposed and can explain the phenomenon reasonably. The mechanism could be helpful for the understanding of the energy transfer phenomenon among the various energy levels in the process of temperature change as reference.

Description: Fracture of silicon wafers is responsible for lower than desirable manufacturing yields in the photovoltaic industry. This study investigates the fracture response of polycrystalline silicon wafers under sliding contacts at different length scales, by means of macro and microscratch tests which simulate cutting processes. The dominant fracture modes were found to be partial cone cracking (macro) and radial cracking (micro). Statistical analysis of the critical loads for crack initiation showed that polycrystalline wafers are weaker than their single-crystal counterparts, that is, they crack at lower applied loads under comparable conditions. Moreover, the Weibull modulus of polycrystalline silicon was found to be the average of the relevant single-crystal directions. Subsequent microscopic observations and flexure tests reveal that the lower resistance of polycrystalline silicon to scratch fracture is due mainly to the presence of relatively large polishing defects, and not to the weakness of its grain boundaries. Alternatives are proposed to minimize damage during ingot cutting, with a view to minimizing wafer breakages during wafer handling and machining.

Description: The 3–4 mol% yttria-stabilized zirconia (YSZ) is widely used as a material for thermal barrier coating; however, the corrosive constituents present in fuel typically result in mechanical disintegration of YSZ coatings. The 3–4 mol% YSZ coatings with respective porosity of ~3% and ~22% have been undertaken with the objective to compare the hot corrosion behavior in air and sulfur-rich atmospheres. The coatings are kept in contact with V 2 O 5  + MgO powder mixture at 750°C for different dwell times of 24 and 76 h. The samples kept in air have shown intact YSZ layer for both the coatings, whereas a delamination of YSZ layer is observed for high porosity sample kept in sulfur-rich atmosphere. XRD patterns of all the samples treated in sulfur-rich atmosphere have indicated a phase transformation in YSZ from tetragonal to monoclinic. However, no such phase transformation has been found for samples treated in air. The V 2 O 5 -induced hot corrosion attack on YSZ coating in air has been successfully inhibited by MgO, which forms a thermally stable Mg 3 V 2 O 8 compound. However, in sulfur-rich atmosphere, MgO is partially consumed to form sulfates, which allows certain fraction of V 2 O 5 to react with Y 2 O 3 causing the degradation of top coat.

Description: Transparent (Ce,Gd) 3 Al 3 Ga 2 O 12 (Ce:GAGG) ceramics are promising scintillators for use in high-energy particle detection application such as nuclear medical imaging. In this study, we report a novel method for the preparation of transparent Ce:GAGG ceramic in oxygen atmosphere without sintering aid. The highest transmittance of as prepared samples with thickness of 1 mm around wavelength of 558 nm reaches 62%, which is close to that value of its comparative single crystal. The average grain size of samples sintered at 1650°C for 10 h is about 11 μm. The spectroscopic properties have also been investigated. The emission peak around 558 nm, which is consistent with that of Ce:GAGG single crystals, matches well with the detection wavelength of photomultiplier.

Description: We investigate the effect of slight Ti substitution (≤0.5%) for Cr in CuCr 1− x Ti x O 2 by measuring the structural, magnetic, and electrical transport properties. Upon Ti doping, the antiferromagnetic transition becomes blurred without a change in Néel temperature. Ti 4+ substituting to Cr 3+ site is found to significantly enhance the resistivity due to the diminishing conducting holes. We find strong time-dependent electrical transport behavior in CuCr 1− x Ti x O 2 induced by slight Ti doping. The key observation is that a metastable behavior occurs in Ti-doped CuCr 1− x Ti x O 2 . A strong resistive relaxation (RR) behavior occurs in Ti-doped CuCr 1− x Ti x O 2 with its magnitude remarkably increasing with increasing Ti content, while it is absent in undoped CuCrO 2 . The RR is found to be described by the combination of an exponential function and a logarithmic dependence at long time. The relaxation behavior in CuCr 1− x Ti x O 2 is found to show a correlation with the local lattice deformation.

Description: In this study, we report highly transparent yttria ceramics fabricated by a facile hot-pressing method with tantalum foil shielding which effectively prevents the ceramic samples from carbon contamination caused by the graphite mold used during the process. The hot-pressed sample was already highly transparent without a post-annealing step or hot isostatic pressing. For a 2-mm-thick specimen doped with 1 at.% ZrO 2 , the in-line transmittance reaches 74.4% at 400 nm and 81.1% at 1100 nm. The sample shows a very fine microstructure with an average grain size of about 1 μm owing to the low sintering temperature of only 1600°C. The study results indicate that it is possible to produce transparent yttria ceramics with excellent optical transparency using the economical and convenient hot-pressing method.

Description: The processing of nanosized powders to produce dense components is a difficult task, mainly due to the strong agglomeration of nanoparticles. In this work, deagglomeration studies were carried out on sol–gel-derived γ-alumina suspensions through the addition of dispersant or dispersant in combination with ethylene glycol as binder and the proper wet ball-milling. For this purpose, zeta-potential, viscosity, and sedimentation studies were carried out, whereas dilatometric studies accompanied by XRD analyses were performed on the powders derived from the most stable suspensions. Sintering studies followed and the as received ceramics were evaluated by SEM. The optimum stabilization was achieved through the synergistic addition of 2.75 mg/g ammonium polymethacrylate with 1 wt% ethylene glycol. The respective powder, although it indicates a shift of the sintering start to higher temperature, leads to the densest ceramic with the finest (500 nm mean grain size) and the most homogeneous microstructure.

Description: Polycrystalline Ba 0.5 Sr 1.5 Co 2 (Fe 1− x Al x ) 12 O 22 ( x  =   0, 0.04, 0.08, and 0.12) hexaferrites were prepared by conventional solid-state reaction method and the magnetic field dependences of magnetic and magnetoelectric properties were investigated. With Al doping ( x  =   0.04, 0.08), much enhanced ferroelectric polarization and abnormal magnetodielectric properties can be observed at a low applied magnetic field and be maintained down to zero field at 200 K, which may result from the occurrence of field-induced spin transverse cone structure. The magnetoelectric coefficient increases from 500 ps/m to 1481 ps/m with increasing x value from 0.04 to 0.08. These results suggest that Al ions play an effective role in modifying the magnetoelectricity of hexaferrites.

Description: The aim of this work was to revisit the two-sphere model of solid-state sintering kinetics to make it consistent with the morphological evolution of the particle contacts during most of the sintering process. Mass transport equations for solid-state sintering are written in a closed-form. The usual simplifications of tangent or secant spheres are avoided and the true evolution of the neck geometry is taken into account, depending on the relative weight of densifying / nondensifying mechanisms. Finite difference methods are used to compute the evolution of geometrical parameters and of the shrinkage rate as a function of time and temperature. The model combines the usual mechanisms of volume, surface, and grain-boundary diffusion as well as vapor transport. The ratio of concave to convex surfaces is deduced from neck growth and from the packing coordination number. A smooth transition from the classical first stage to the intermediate stage of sintering, where curvature gradients have vanished at the particle surface, is obtained. The same model can then be applied to describe sintering kinetics until pore closure. This model providing a realistic evolution of the contact morphology makes it possible to analyze the competition between densifying and nondensifying mechanisms throughout the early and intermediate stages of sintering. The competition between lattice diffusion and vapor transport or surface diffusion is thus analyzed.

Description: Highly transparent (Y 0.95− x Gd x Eu 0.05 ) 2 O 3 ( x  =   0.15–0.55) ceramics have been fabricated by vacuum sintering at the relatively low temperature of 1700°C for 4 h with the in-line transmittances of 73.6%–79.5% at the Eu 3+ emission wavelength of 613 nm (~91.9%–99.3% of the theoretical transmittance of Y 1.34 Gd 0.6 Eu 0.06 O 3 single crystal), whereas the x  =   0.65 ceramic undergoes a phase transformation at 1650°C and has a transparency of 53.4% at the lower sintering temperature of 1625°C. The effects of Gd 3+ substitution for Y 3+ on the particle characteristics, sintering kinetics, and optical performances of the materials were systematically studied. The results show that (1) calcining the layered rare-earth hydroxide precursors of the ternary Y–Gd–Eu system yielded rounded oxide particles with greatly reduced hard agglomeration and the particle/crystallite size slightly decreases along with increasing Gd 3+ incorporation; (2) in the temperature range 1100°C–1480°C, the sintering kinetics of (Y 0.95− x Gd x Eu 0.05 ) 2 O 3 is mainly controlled by grain-boundary diffusion with similar activation energies of ~230 kJ/mol; (3) Gd 3+ addition promotes grain growth and densification in the temperature range 1100°C–1400°C; (4) the bandgap energies of the (Y 0.95− x Gd x Eu 0.05 ) 2 O 3 ceramics generally decrease with increasing x ; however, they are much lower than those of the oxide powders; (5) both the oxide powders and the transparent ceramics exhibit the typical red emission of Eu 3+ at ~613 nm (the 5 D 0 7 F 2 transition) under charge transfer (CT) excitation. Gd 3+ incorporation enhances the photoluminescence and shortens the fluorescence lifetime of Eu 3+ .

Description: Calcium sulfoaluminate (CSA) cements have lower carbon footprint than that of portland cement, which makes them a suitable alternative as a sustainable cementitious binder. Early-age expansion of CSA cements can be exploited to induce compressive stress in restrained concrete which can later counteract tensile stress developed during drying shrinkage, thus enhancing the resistance against shrinkage cracking. However, a proper understanding of the expansion behavior is critical to eliminate any risk related to expansion-induced cracking. This study examines the expansion and hydration characteristics of various ordinary portland cement (OPC)-CSA blends. Early-age expansion of paste samples was monitored. The increase in CSA cement content increased the extent of expansion. Samples having the highest CSA content (30% by mass) exhibited excessive expansion which led to their cracking. Quantitative X-ray diffraction, pore solution extraction, porosity, tensile strength, and dynamic modulus tests were performed to monitor the physico-chemical changes in OPC-CSA blends. It was shown that the ettringite supersaturation in the investigated systems gave rise to the crystallization stress, responsible for the expansion. Thermodynamic models enabled a reasonable prediction of tensile failure, particularly in the blends with the higher CSA content.

Description: We investigate the exchange bias (EB) phenomenon in the layered manganese oxychalcogenides La 2 O 3 Mn 2 Se 2 . The spin canting of the antiferromagnetic (AFM) background leads to the ferromagnetic (FM) contribution and the exchange coupling at the FM/AFM interface is responsible for the EB phenomenon. Due to the subtle equilibrium of the magnetic states, the EB field shows strong temperature and cooling field H FC dependences. These results suggest that the EB effect in La 2 O 3 Mn 2 Se 2 could be tuned by H FC and temperature, which is important for designing related devices.

Description: Oxidation of SiC at 1400°C–1700°C produces carbon monoxide (CO), which can form bubbles at the interface between the substrate and the silica (SiO 2 ) overgrowth. The bubbles add complexity to the mechanistic understanding of the oxidation process. We analyze the kinetics of bubble formation, where the driving force for their nucleation depends on the partial pressure of CO while their growth is determined by the viscosity of silica. The mechanism bears analogy to oxygen migrating from the atmosphere into superalloys, and reacting with carbide precipitates at the grain boundaries to nucleate bubbles of carbon monoxide, or hydrogen migrating into carbon-steel to form methane. The unique feature of the present case is the ability of silica to release the gas pressure within the bubbles by viscous flow, and then, to heal the crater that is left behind. As the bubbles grow out from the interface and escape into the atmosphere, they can expose bare surface to the atmosphere rendering silicon carbide vulnerable to active oxidation. The relative significance of the mechanisms of oxidation of SiC is presented in the form of an oxidation map, where atmospheric and oxidation temperature are the experimental variables. We discover that experiments often lie in the regime of bubble formation.

Description: Crack-free bioactive nanocoatings embedded with uniformly distributed silica-rich bioactive spherical aggregates were successfully prepared in situ by controlling the micellization of a SiO 2 –CaO–P 2 O 5 sol using the tri-block copolymer P123 followed by dip-coating onto a bio-inert glass substrate and calcined. These hierarchically designed nanocoatings embedded with such bioactive glassy nanospheres (BGNS) enabled to induce the deposition of a densely populated, uniform, and well-developed needlelike crystalline carbonated hydroxyapatite coating reminiscence of the mineral phase of natural bone within a short immersion time in simulated body fluid. The BGNS nanocoatings also supported the growth and attachment of human gingival fibroblasts. The results suggest that these newly designed composite nanocoatings are noncytotoxic, capable of supporting rapid and homogeneous calcium phosphate deposition as well as subsequent crystallization, and likely to be promising candidates for inert glass reinforced bone implants.