ALBERT

Notizen: The effective Young's moduli of co-continuous Al–Al2O3 composites over the 5–97 vol% Al2O3 composition range were experimentally measured and compared with theoretical composite modulus values predicted using the methods of Ravichandran, Tuchinskii, Hashin-Shtrikman, and the effective medium approximation (EMA). The influence of phase morphology and the modulus ratio (E1/E2) of the constituent phases on the resulting experimental and calculated Young's modulus is discussed. For two-phased co-continuous composites with a modulus ratio greater than 5, the EMA, with an appropriate microstructural shape factor, was the most consistent method for approximating the composite Young's modulus over the entire composition range.

Notizen: Three different series of porous silicon nitride ceramics with volume fraction porosities in the range 0–0.5 were fabricated using different preparation routes: (i) partial sintering, (ii) the addition of fugitive inclusions, and (iii) partial hot pressing. The use of different sintering additives and firing conditions, depending on the preparation route, gives rise to different materials within a certain porosity range with variations in terms of microstructure and grain boundary phase. Mechanical properties, elastic moduli, and strength have been evaluated separately for each series of materials. Porosity dependences of Young's modulus, shear modulus, Poisson's ratio, and fracture strength have been assessed and a comparison of the different materials is made and discussed in relation to their microstructural features.

Notizen: Monodisperse, spherical, and nonporous zirconia particles ranging from submicrons to microns for use as supports for capillary electrophoresis and chromatography were synthesized by the controlled hydrolysis of zirconium tetra-alkoxides in alcohol solutions in the presence of long-chain organic acids. Particle characteristics varied as a function of water concentration, the chain length of the organic acid, aging time, temperature, and extent of stirring. Changing these experimental parameters affected particle size, particle size distribution, and the degree of particle aggregation. Submicron particles were made by increasing the water concentration and decreasing aging time. Particle size increases as temperature during the aging period increases. Most importantly, large monodisperse particles (4 μm) could be made by extended stirring. In general, we found that particle size increases as the chain length of the organic acid increases. We also found that gentle rotation during the aging period prevented both particle settling and secondary nucleation. This allowed the production of large particles. BET nitrogen adsorption uptake measurements demonstrate that the particles could be sintered, without aggregating them, to remove virtually all internal porosity.

Notizen: Fabrication of microstructures has been the subject of considerable attention in recent years. Techniques such as surface silicon micromachining and bulk silicon etch have emerged as practical methods for thin microstructures and processes such as LIGA (an acronym for the German words for lithography, electrodeposition, and molding) are being used to produce high aspect ratio structures from a limited set of electroformable metals and polymers. As microsystem technologies and application prospects continue to grow, it is of interest and much practical value to expand the material choices for LIGA-scale microstructures to ceramics and a broader class of metals. To this end, this paper investigates a new technique for the fabrication of high aspect ratio ceramic or metal microparts. This technique is based on capillary-driven microcasting and curing of an epoxy-based metallic or ceramic nanoparticulate slurry into a sacrificial plastic mold produced by microinjection molding. The cured preform is subsequently heated to remove the organic phase and to sinter the particulate ceramic or metallic phase. The fabrication process is discussed in the paper along with illustration of example microparts produced using the process. A theoretical model of the fluid flow during the microcasting process is developed and validated with experimental data from the fabrication of rectangular rib sections.

Notizen: Compression molding of glass aspherical lenses has become a viable manufacturing process for precision optics. The widespread use of this process has been hampered by the lack of its fundamental understanding. This research is a part of the ongoing effort to understand some of the issues related to the process. Simple lens molding experiments were performed on a commercial precision lens molding machine. A finite element method (FEM) program was used to create a simple numerical model and analyze the molding process. Experimental results show that this process is capable of producing precision optical components. A comparison of the experimental results with the predicted results indicates that with a more sophisticated numerical model, it is possible to use FEM as a tool for process analysis.

Notizen: A model and numerical framework is developed for piezoelectric materials. The model treats the piezoelectric and electrostrictive effects by incorporating orientation-dependent, single-crystal properties. The method is implemented in Object Oriented Finite Element program, a public domain finite element code, so it can be applied to arbitrary two-dimensional microstructures with crystallographic anisotropy. The model is validated against analytic solutions. Consistency of the method for known cases permits application of the technique to more complicated two-dimensional systems. The piezoelectric and electrostrictive response is determined for a few simple device geometries and provides insight for design and convergence criteria.

Notizen: Molecular dynamic simulations of glasses belonging to the anorthite–diopside system have been performed in order to obtain an atomistic description of the material's structure. The structural parameters obtained by the simulations allow to confirm that the glass materials are characterized by a very similar short-range environment. The main differences have been observed in the intermediate-range order of the structure that describe the distribution and the packing of the tetrahedra constituting the three-dimensional networks. It is shown that the glass materials with composition close to the two extremes, corresponding to the pure glass anorthite or pure glass diopside, display the typical structural features of the tectosilicates and inosilicates subclass of minerals, respectively, to which anorthite and diopside crystals belong.

Notizen: 3-D ordered macroporous sol–gel bioactive glass particles (3DOM-BG, pore size: 345 nm; molar composition: 75 SiO2–21 CaO–4 P2O5) were converted into nano-crystalline calcium-deficient carbonate apatite after soaking at 37°C in simulated body fluid. The as-formed, flake-like apatite transformed into a coral-like, highly crystalline hydroxyapatite with a minor tri-calcium phosphate phase after heating for 1 h at a temperature of 700°C. Fourier transform infrared results show the loss of carbonate and isolation of hydroxyl during heating. The heat treatment improved the apatite's resistance to dissolution in a buffer solution (pH=5).

Notizen: Engineering tests and full-field strain measurements are used to assess the accuracy of predictions made by the Binary Model, a computational tool for textile composites. The test case is a carbon fiber/SiC matrix composite, in which the reinforcement is a three-dimensional angle-interlock weave. The test composites are thin, having been designed for heat exchanger applications. The thinness leads to strong variations in local strains and strong effects of tow waviness upon macroscopic elasticity. The model performs well in predicting both local variations in strain and macroscopic elasticity. The effect of averaging local strains over variable gauge lengths is explored. Strains averaged over an appropriate gauge length have recently been proposed as the preferred measures of strain for use in local failure criteria.

Notizen: A temperature-induced gelation method has been developed for the preparation of sialon green bodies composed of Si3N4, AlN, Y2O3, and Al2O3. Using a polyester/polyamine condensation polymer as dispersant, stable suspension containing sialon, precursor powders could be prepared in methyl ethyl keton (MEK)/ethanol (E) solvent mixture with solids loading as high as 60 vol%. The solvency of the dispersant in MEK/E decreased dramatically on cooling. The sedimentation tests showed that the settling behavior changed from slow accumulation at ∼20°C to fast deposition of clusters at −15°C, because of the collapse of dispersant chains adsorbed at the surface of the particles. Steady shear viscosity and oscillatory measurements performed for 60 vol% suspensions as functions of temperature and added amounts of dispersant proved that suspensions were of low viscosity and high stability at room temperature, becoming predominantly very highly viscous and elastic as temperature decreased. The gelation mechanism was mainly based on the coiling up of dissolved dispersant molecules, inducing in situ gelation and the formation of a rigid network bridging the suspended particles. The green bodies showed negligible shrinkage during gelation, a small shrinkage during drying, homogeneous microstructures, narrow pore size distributions, and high relative density. It was possible to achieve dense α-sialon ceramics through pressureless sintering at 1750°C for 2 h.

Notizen: Mechanical stress–strain behavior of LaCoO3, La0.8Ca0.2CoO3, and La0.7Ca0.3CoO3 was studied under compression at 25° and 300°C. A hysteresis in the stress–strain relationship due to reorientation of ferroelastic domains (deformation twins) was observed, and a remanent strain is measured after unloading. The cohersive stress, defined as the maximum in effective elastic compliance during first loading, increases with substitution of Ca for La and decreases with increasing temperature. Domain reorientation was confirmed by X-ray diffraction of surfaces parallel and perpendicular to the loading direction. LaCoO3 can be regarded as a soft ferroelastic material while the 30% Ca-substituted material is a hard ferroelastic. The hysteresis of the stress–strain relationship was clearly dependent on both composition and temperature.

Notizen: Ceramic laminates exhibiting a threshold strength have been fabricated by dip-coating thick tape-cast Al2O3 layers into slurries containing mixtures of Al2O3 and either unstabilized zirconia (MZ-ZrO2) or mullite to produce thin compressive layers via both a molar volume change and a differential thermal contraction. Porosity was introduced into the thin compressive layers by adding rice starch to the dip-coating slurries, which decomposed during densification of the laminate. As the volume fraction of porosity is increased, the residual compressive stress (σC), as measured by piezospectroscopy, is reduced and approaches zero at approximately 0.65 volume fraction of porosity. The elastic modulus mismatch (E1/E2) between the thin and thick laminate layers accounted for approximately one-half of the threshold strength for volume fractions of porosity ≤0.30 (E1/E2〈0.4). Above 0.40 volume fraction of porosity, the strength significantly increased as did the scatter in strength values, and it was observed that the highly porous layers completely arrested crack extension; these materials no longer exhibited a threshold strength. For these laminates, failure occurred by the independent, sequential failure of one layer after another, followed by catastrophic failure due to delamination.

Notizen: Single-phase silicon carbide (SiC) nanotubes were successfully synthesized by the reaction of carbon nanotubes with silicon powder at 1200°C for 100 h. X-ray diffraction patterns indicated that most of the carbon from the carbon nanotubes that were reacted with silicon at 1200°C for 100 h was transformed to SiC. Transmission electron microscopy observations revealed that both single-phase SiC nanotubes and C–SiC coaxial nanotubes, which are carbon nanotubes sheathed with a SiC layer, were synthesized after 100 h of reaction. The ratio of single-phase SiC nanotubes to C–SiC nanotubes increased with heat treatment at 600°C in air for 1 h because the remaining carbon was removed.

Notizen: Long crack R-curve of a porous Si3N4 with aligned fibrous grains was investigated, using a chevron-notched beam technique. A crack was constrained to propagate normal to the grain alignment. The crack growth resistance of aligned porous Si3N4 was much larger compared with that of dense Si3N4 ceramics. Microstructure observations showed that pullouts of fibrous grains in aligned porous Si3N4 markedly increased during crack propagation relative to those of dense Si3N4, due to the existence of pores. The efficient grain pullouts in porous Si3N4 increased the bridging stress at the crack wake.

Notizen: Hydroxyapatite (HA) is an interesting ceramic material for orthopedic applications, in particular for implant operations and bone regeneration, owing to its bioactivity and biocompatibility with the surrounding tissues. Even if different shaping processes can be used in order to obtain porous ceramic bodies with optimal final properties, the sponge impregnation method with aqueous HA dispersions is particularly suitable to achieve an appropriate macroporosity to bone regeneration. As for conventional slip or tape casting, the rheological behavior of HA suspensions must be properly fitted to process conditions and controlled through a satisfactory stabilization of the disperse phase, i.e., through an appropriate dispersant dosage and an accurate homogenization of dispersed powder. In the present work, HA powders with different crystallinity degree were used to prepare aqueous dispersions with different contents of solids and dispersant (ammonium polyacrylate). The specific surface area of HA powders is very high, and then special attention must be dedicated to the dispersant selection and the dispersion process since the rheology of HA suspensions is strongly influenced by the structural conditions of the disperse phase. Even small differences in dispersant concentration can lead to dramatic changes in the rheological properties also at relatively low values of solids volume concentration. Above a critical concentration, the viscosity drop associated with the apparently plastic behavior is confined within a very narrow stress range, a neat transition is observed in the linear viscoelastic properties, and the time-dependent effects induced by the shear history become quite important and crucial for the experimental characterization.

Notizen: Pristine, Si-doped, and Si/Nd-codoped yttrium aluminum garnet (YAG) nanoparticles were synthesized by pyrolysis of complex compounds of aluminum and yttrium with triethanolamine. It was found that the coexistence of Si4+ and Nd3+ increased the solubility of both ions and promoted the formation of YAG phase. Single-phase, nanocrystalline Si/Nd:YAG powders were obtained at calcination temperatures as low as 920°C. The optical behavior of the Si/Nd:YAG nanopowders was similar to that of single-crystal Nd:YAG.

Notizen: Alumina bilayers of different relative thicknesses and densities were produced with a strong interface using a modified gel-casting technique. Tolerance to surface damage is examined using biaxial flexure of disks damaged with a single Vickers indentation at various loads. The greatest surface flaw tolerance is seen in bilayers consisting of a thin porous layer on the tensile surface coupled to a thick dense layer. Here, the modulus mismatch causes redistribution of the applied stress, and fracture initiates at the internal porous-dense interface rather than at the surface from the introduced indentation flaw.

Notizen: An extensive X-ray study of CeO2–Nd2O3 solid solutions was performed, and the densities of solid solutions containing various concentrations of NdO1.5 were measured using several techniques. Solid solutions containing 0–80 mol% NdO1.5 were synthesized by coprecipitation from Ce(NO3)3 and Nd(NO3)3 aqueous solutions, and the coprecipitated samples were sintered at 1400°C. A fluorite structure was observed for CeO2–NdO1.5 solid solutions with 0–40 mol% NdO1.5, which changed to a rare earth C-type structure at 45–75 mol% NdO1.5. The change in the lattice parameters of CeO2–NdO1.5 solid solutions, when plotted with respect to the NdO1.5 concentration, showed that the lattice parameters followed Vegard's law in both the fluorite and rare earth C-type regions. The maximum solubility limit for NdO1.5 in CeO2 solid solution was approximately 75 mol%. The relationship between the density and the Nd concentration indicated that the defect structure followed the anion vacancy model over the entire range (0–70 mol% NdO1.5) of solid solution.

Notizen: Yttrium silicates are promising materials for improved oxidation and erosion protection for carbon fiber-reinforced composites. A two-layer coating system of low-pressure plasma-sprayed yttrium silicate on chemical vapor deposition-SiC-precoated C/C–SiC was tested under atmospheric re-entry conditions simulated within a plasma wind tunnel test facility. The thermal expansion behavior of Y2SiO5 and Y2Si2O7 was investigated. The chemical compatibility with and without increasing oxygen partial pressure at the interface of the two-layer system was calculated by the CALPHAD method. The calculations were compared with experimental results. Furthermore, a thermodynamic explanation is presented to understand and predict the observed coating failure mechanism, identified as blister formation.

Notizen: The early stage of phase separation in aluminoborosilicate glass for liquid crystal display substrates was investigated using a scanning electron microscope (SEM) and small-angle X-ray scattering measurement device (SAXS). The phase-separated particles were detected with SAXS in samples in which no particle was observed with SEM. The particle diameter in an as-formed sample was 2 nm. The relationship between the diameter and heat-treatment time indicates that the phase separation proceeds by a diffusion-controlled process. Based on the results of Fourier-transform infrared spectroscopy, fast compositional change may occur in the early stage of phase separation.

Notizen: Tin (Sn) substitution for titanium (Ti) was investigated in Ba6−3xNd8+2xTi18O54 (x=1/2, 2/3, and 3/4) ceramics. A small amount (z〈0.1) of Sn substitution resulted in Ba6−3xNd8+2x(Ti1−zSnz)18O54 solid solutions, and some secondary phases were observed with increasing Sn content. A small amount of Sn substitution improved the Qf value significantly, while, due to the formation of secondary phases, the Qf value degraded sharply for larger Sn content. The relative dielectric constant (ɛr) decreased with increasing Sn-content, while the temperature coefficient of resonant frequency (τf) generally decreased, although an obvious fluctuation was observed for x=3/4.

Notizen: In this paper we report the synthesis of polygonal single-crystal aluminum borate microtubes by the direct calcination of alumina and boron nitride powders in air. The diameters of the obtained tubes vary between 1 and 15 μm; and the lengths are in the range of tens to hundreds of micrometers. The cross-sections of the microtubes are in the shapes of rectangular, hexagonal, and quasi-circle. A new solid–liquid–solid growth mechanism was proposed to account for the formation of microtubes instead of whiskers.

Notizen: A modified Pechini method was used to prepare undoped and doped La0.67Ca0.33MnO3 (LCMO) with homogeneous compositions and uniform fine powders. The variations of the lattice parameters and cell volumes of doped LCMO are attributed to the effect of ionic size and the creation of the oxygen vacancy. The effect of Ba2+ on the magnetic and conduction properties of LCMO is mainly related to the enhancement of the Mn3+–O–Mn4+ bond angle. The occupation of Al ions in Mn sites and the creation of the oxygen vacancies lead to an increase of the resistivity and a decrease of Tc and Ms. The variable-range hopping model with p=1/4 and ρ0 independent of temperature was found to be the best fit for undoped and doped LCMO, shown in 〈link href="#f4"/〉. It is suggested that T0 is mainly the measure of the extent of the disorder. For Al3+-doped LCMO, the small-polaron mechanism was found to be dominant in the temperature range from 300 to 350 K, and the evaluated values of the activation energy are about 0.22 eV and independent of the Al concentration. Low-field magnetoresistance (LFMR) was found to be enhanced by Ba doping but diminished by Al doping. The segregation of Al ions to the grain-boundary region may play a significant role in the intent to form a one-slope field-dependent MR response. The mechanisms of MR response of doped LCMO were suggested. 〈figure xml:id="f4"〉4〈mediaResource alt="image" href="urn:x-wiley:00027820:JACE00610:JACE_00610_f4"/〉 Resistivity as a function of temperature showing that the best fit was the variable-range hopping model with p=1/4 and ρ0 independent of temperature for (a) La0.67(Ca1−xBax)0.33MnO3 in the temperature range from 300 to 430 K, (b) La0.67Ca0.33(Mn1−yAly)O3 in the temperature range from 260 K to Tc, and the small-polaron mechanism (ρ=CT exp (Ea/kT) for (c) La0.67Ca0.33(Mn1−yAly)O3, in the temperature range from 300 to 350 K.

Notizen: Transparent glass ceramics containing CaF2 nano-crystals co-doped with Er–Yb were developed by heat treatment of glasses in the system SiO2–Al2O3–CaF2–LnF3 (Ln=Er, Yb). The crystal size of CaF2 increased with increasing heat-treatment temperature. Upconversion emission intensities at 660 and 540 nm increased dramatically with increasing heat-treatment temperature. The cathode luminescence mapping indicates that the visible Er-luminescence arises mainly from the precipitated CaF2 nano-crystals with a phonon energy lower than that of a silicate matrix. It is concluded that both Er and Yb were concentrated in the CaF2 nano-crystals, and the quantum efficiency of Er3+-luminescence and the energy transfer efficiency from Yb3+ were considerably improved after ceramization.

Notizen: Titania microtube arrays were fabricated by replication of human hair. The microtube is nanoporous in its wall and has nanoporous platelets on its outer surface, which were derived from cuticle cells on hair surface. The wall thickness of titania microtubes could be controlled by adjusting the number of sol–gel cycles applied. The nanopores acted as an effective nanoreactor where Au nanoparticles were in situ synthesized. The microchannels, nanopores, and noble metal nanoparticles may provide a unique combination that would be attractive in such applications as catalysis, adsorption, and separation.

Notizen: Tantalum(V) nitride, prepared by nitridation of nanosized Ta2O5 at 800°C for 8 h under ammonia flow, was thermally decomposed to cubic nanocrystalline TaN at a temperature of 1000°C for 3 h under argon atmosphere. The resulting powders have been characterized using X-ray diffraction (XRD) and transmission electron microscopy. XRD-pure cubic TaN nanoparticles with a diameter of 50–100 nm can be obtained by the process. The decomposition process was found to depend on the temperature. Mechanisms that account for the decomposition of Tantalum(V) nitride are discussed. The results indicate that the method can permit formation of cubic-phase Tantalum(III) nitride under ambient pressure and moderate temperatures.

Notizen: E-glass fibers were coated with a 15CaO–15BaO–20SiO2–50TiO2 thin film by the sol–gel method. Mechanical and chemical tests were performed on coated and uncoated fibers in cement and cement extract solutions to investigate the interactions between cement and gel-glass film. The results show that the resistance of E-glass fibers to the alkali cement medium is enhanced by the 15CaO–15BaO–20SiO2–50TiO2 coating. The significant roles of TiO2, CaO, and BaO in the protection fibers from the alkaline attack of cement are described. Some evidence is presented that the alkali corrosion of the coated fibers results in the formation of a thick and compact Ti film that suppresses further corrosion reaction.

Notizen: The sinterabilities of fine zirconia powders including 5 mass% Y2O3 were investigated, with emphasis on the effect of Al2O3 at the initial sintering stage. The shrinkage of powder compact was measured under constant rates of heating (CRH). The powder compact including a small amount of Al2O3 increased the densification rate with elevating temperature. The activation energies at the initial stage of sintering were determined by analyzing the densification curves. The activation energy of powder compact including Al2O3 was lower than that of a powder compact without Al2O3. The diffusion mechanisms at the initial sintering stage were determined using the new analytical equation applied for CRH techniques. This analysis exhibited that Al2O3 included in a powder compact changed the diffusion mechanism from grain boundary to volume diffusions (VD). Therefore, it is concluded that the effect of Al2O3 enhanced the densification rate because of decrease in the activation energy of VD at the initial sintering stage.

Notizen: Crack-healing behavior of silicon carbide ceramics sintered with AlN and Sc2O3 has been studied as a function of heat-treatment temperature and applied stress. Results showed that heat treatment in air could significantly increase the indentation strength whether a stress is applied or not. After heat treatment with no applied stress at 1300°C for 1 h in air, the indentation strength of the specimen with an indentation crack of ∼100 μm (≈2c) recovered its strength fully at room temperature. In addition, a simple heat treatment at 1200°C for 5 h under an applied stress of 200 MPa in air resulted in a complete recovery of the unindented strength at the healing temperature. However, higher applied stress led to fracture of the specimens during heat treatment. The static fatigue limit of the specimens crack healed at 1200°C for 5 h under 200 MPa was ∼450 MPa at the healing temperature. The ratio of the static fatigue limit of the crack-healed specimen to the unindented strength was ∼80%.

Notizen: Tetracalcium phosphate (TTCP) Ca4(PO4)2O is one of the major powder components of self-setting orthopedic and dental cements. Traditionally, TTCP powders are produced by a solid-state process by soaking Ca- and P-containing precursors between 1350° and 1500°C. Such procedures require expensive high-temperature furnaces and subsequent grinding of sintered particulates. Grinding not only introduces contamination but alters the structure of TTCP, thereby reducing its bioactivity. The present paper offers a lower temperature synthesis process for TTCP with several interesting features. First, the synthesis procedure used Ca(CH3COO)2·H2O and NH4H2PO4 as separate sources for Ca and P, respectively. Second, the reactants underwent multiple melting and decomposition stages, thus increasing the reactivity of the synthesis process. NH4H2PO4 melted at 190°C and engulfed the calcium acetate particles. The Ca-acetate component decomposed into CaCO3 at around 400°C while still surrounded by the molten phosphate liquid and an amorphous Ca-metaphosphate phase. Hydroxyapatite, Ca10(PO4)6(OH)2 (HA), and β-Ca3(PO4)2 crystallized upon heating the powder mixture to 700°C. Slightly above 1200°C, the TTCP phase was formed. This sequence of reactions led to a process temperature of 1230°C, the lowest temperature ever reported for the synthesis of TTCP. Third, the resulting powders required much less grinding, which itself is advantageous. Fourth, the resulting powders were in situ seeded with HA. HA-seeded TTCP powders were tested for their apatite-inducing ability by soaking them in synthetic body fluid at 37°C. TTCP powders of this study were readily covered with carbonated apatitic calcium phosphates within the first 72 h.

Notizen: The effect of microstructure on the sliding-wear properties has been studied in two types of pressureless liquid-phase-sintered (LPS) SiC ceramics, one with coarse, in situ-grown elongated grains and the other with fine, equiaxed grains. Elongated-grain LPS SiC has been found to have dramatically improved sliding-wear properties over equiaxed-grain LPS SiC, which has been attributed to the interlocking network of elongated SiC grains. The introduction of an interlocking network of elongated grains represents a new guideline for the microstructural design of polycrystalline ceramics that are both sliding-wear resistant and toughened.

Notizen: Microwave dielectric properties of polycrystalline Ca5Ta2TiO12 ceramics have been tailored using different glass additives. Several glasses such as B2O3, SiO2, B2O3–SiO2, ZnO–B2O3, Al2O3–SiO2, Al2O3–B2O3–SiO2, BaO–B2O3–SiO2, MgO–B2O3–SiO2, ZnO–B2O3–SiO2, PbO–B2O3–SiO2, and 2MgO–Al2O3–5SiO2 were added to calcined Ca5Ta2TiO12 powder in different weight percentages. Among these glasses, addition of 0.1 wt% of SiO2, Al2O3–SiO2, Al2O3–B2O3–SiO2, MgO–B2O3–SiO2, and 2MgO–Al2O3–5SiO2 improved the microwave dielectric properties. Fluxing of Ca5Ta2TiO12 with 0.1 wt% of 2MgO–Al2O3–5SiO2 glass has ɛr=39, Qu×f〉40 000 at 5 GHz and τf=+8 ppm/°C. Near-zero τf was obtained with the addition of glasses like Al2O3–SiO2, Al2O3–B2O3–SiO2, MgO–B2O3–SiO2, and 2MgO–Al2O3–5SiO2. Addition of all the glasses except BaO–B2O3–SiO2 reduced the τf of Ca5Ta2TiO12 ceramics. Two weight percent glass addition to Ca5Ta2TiO12 ceramics reduced their sintering temperature down to 1450°C from 1625°C and τf close to zero but deteriorated the ɛr and unloaded quality factor. The effects of glass content on the structure and microstructure of Ca5Ta2TiO12 ceramics were studied by X-ray diffraction and scanning electron microscopic techniques.

Notizen: Neutron diffraction data of α-sialon and complementary phases were collected on the neutron powder diffraction (NPD) diffractometer installed in the NFL Studsvik, at a wavelength of 1.470 Å. Calculations were carried out by using the FullProf 2000 utilizing the crystal structure of the yttrium α-sialon phase. Selected profile and structure parameters were refined in the calculations. The calculated data showed that either La or Nd were also present in the α-sialon crystal structure in the presence of Y. The comparison of the present phases' weight contents, which were determined by the qualitative phase analysis between NPD and X-ray diffraction data, was carried out.

Notizen: A series of [0.8Pb(Ni1/3Nb2/3)O3–0.2PbTiO3]/[(Ni0.2Cu0.2 Zn0.6)Fe2O4] (PNNT/NiCuZn) ferroelectric–ferromagnetic composites were prepared by the conventional solid-state route. The composites show good co-firing behaviors. X-ray diffraction and scanning electron microscope studies confirmed the coexistence of ferroelectric PNNT phase and ferromagnetic NiCuZn phase in the composites. No significant chemical interaction has occurred between PNNT phase and NiCuZn phase. All sintered samples exhibit typical magnetic hysteresis loops at room temperature. The saturation magnetization of composites rises linearly with the increase in NiCuZn content. Frequency dependence of initial permeability was also measured. With an increase in NiCuZn, the initial permeability increases and the cutoff frequency tends to decrease. The Maxwell–Garnett (MG) effective medium theory was used to model the magnetic properties of composites. Although the MG equation cannot give an accurate prediction for the initial permeability of composites because of the oversimplified assumption, it gives upper and lower limits for the initial permeability of compositions.

Notizen: Bioresorbable and functionally graded apatites (fg-HAp) ceramics, which are characterized by gradations in crystallinity and the grain size of hydroxyapatite (HAp:Ca10(PO4)6(OH)2), were designed using bovine bone by the calcination and partial dissolution–precipitation method. The fg-HAp ceramics had macropores of 100–600 μm originated from spongy bone, and micropores of 10–160 nm. Fg-HAp ceramics loaded with recombinant human bone morphogenetic protein-2 (rhBMP-2/fg-HAp) were implanted into the subcutaneous tissue of rats. Eight weeks after the implantation, the surface and bulk degradations of the fg-HAp-containing body fluid occured, and small pieces of fg-HAp were incorporated into the induced bone and fatty marrow, suggesting that osteoinduction occurred in conjunction with bone remodeling. The rhBMP-2/fg-HAp ceramics developed could become a resorbable biomimetic material with fast bioresorption and osteoinduction characteristics.

Notizen: A hydrothermally treated zirconia colloid was introduced during the precipitation of hydroxyapatite (HAP) to achieve an HAP–zirconia nanocomposite. High dispersion of zirconia was achieved at zirconia loadings below 8 wt%. Vickers hardness was optimized with a 1.5 wt% loading of zirconia. A low loading (1.5 wt%) of tetragonal zirconia was able to increase the bending strength of nanocrystalline HAP from 183 to 243 MPa. This HAP–zirconia nanocomposite was sintered to full density by 1000°C under an applied load of 50 MPa. The average grain sizes of HAP and zirconia were maintained at ≤100 nm. With the high strength and low loading of secondary phase, these HAP-based nanocomposites should be attractive for orthopedic and dental implant applications.

Notizen: The effect of iron and iron/yttrium codoping on the densification and grain growth of ultra high-purity (99.995%) fine-grained alumina has been studied. The experiments were carried out under both oxidizing (flowing air) and reducing conditions (N2/H2 mixture, pO2∼5.1 × 10−14). For studies carried out in air, relative to undoped alumina, the addition of 1000 ppm Fe was found to reduce the densification rate by a factor of 5 and also retard the grain growth rate. This result, which was consistent with tensile creep data obtained in a separate study, was attributed to the retardation of grain-boundary diffusive processes by segregating Fe(III) ions. In contrast, under reducing conditions the 1000 ppm Fe- doped samples exhibited an increase in the densification rate of 2.5 orders of magnitude over that of the undoped samples. In the case of the codoped compositions (1000 ppm Fe/1000 ppm Y), for heat treatment in air, the densification behavior did not differ significantly from that of samples singly doped with Y (1000 ppm). However, under reducing conditions, the presence of the Fe2+ in the samples appeared to compensate for the retarding effect of the yttrium, such that the densification rate of the codoped samples was comparable with that of the undoped material. A mechanism based on compensating point defects is invoked to rationalize the more rapid kinetics under reducing conditions.

Notizen: The phase composition, microstructure, and electrical resistivity of hot-pressed AlN ceramics with 0–4.8 wt% Sm2O3 additive were investigated. The phase composition was approximately consistent with that estimated from the Sm2O3–Al2O3 phase diagram using the amount of added Sm2O3 and oxygen content of the AlN raw material. When sintered at more than 1800°C, the AlN ceramics with 1.0–2.9 wt% Sm2O3 additive contained an Sm-β-alumina phase wetting the grain boundaries, and their electrical resistivity considerably decreased to 1010–1012Ω·cm. This resistivity decrease was caused by the continuity of the Sm-β-alumina phase with a resistivity lower than that of bulk AlN.

Notizen: Thin fibrils (usually 20–100 nm in diameter) of ferroelectric PbZr0.52Ti0.48O3 have a preferred nucleation and growth in the [001] direction in small particles. X-ray diffraction with the X-ray beam facing the fibril surfaces, which lie along the [001] direction, thus involves reflections of manifested intensity from (100) and (010) planes. No reflection occurs from the perpendicular planes to the [001] fibrils. A transmission electron micrograph with the corresponding electron diffraction from fibrils confers the results of the preferred growth. The fibrils have a modified orthorhombic crystal structure of lattice parameters a=0.4038 nm, b=0.4017 nm, and c=04148 nm, with an enhanced 8.033 g/cm3 density over 8.006 g/cm3 in the usual tetragonal structure of a=0.4036 nm and c=04146 nm. Surface anisotropy in a fibril that imposes (and drives) a shear stress over growing crystallites (within the fibril) along the [001] surface favors this specific crystal structure in them according to the precursor structure. A single-phase sample is obtained in 2 h by heating of an amorphous template of precursor (of the metal cations with a polymer of polyvinyl alcohol and sucrose) at 500°–800°C in air.

Notizen: The reaction-bonded aluminum oxide (RBAO) process utilizes the oxidation of intensely milled aluminum/alumina powder compacts that are heat treated in air to make alumina-based ceramics. RBAO samples are typically oxidized in a furnace which is heated at 1°C/min to 1100°C. Heat-treating samples with a characteristic dimension 〉1 mm, without adjusting the furnace temperature program, usually results in a cracked ceramic. Cracking is caused by the excessive thermal and chemical stresses that result from steep temperature gradients (〉30°C/mm) and compositional gradients (〉5000 mol·(m3·mm)−1), which develop under the deleterious ignition and shrinking core reaction regimes. While adjustments to the furnace temperature program based on continuum models have had some success, the use of feedback-controlled firing is investigated as a means to avoid the furnace temperature program design step and to decrease the firing time. Feedback-controlled firing is shown to improve yields and significantly reduce the time required to completely oxidize the aluminum. For example, a 16 g sample with a characteristic dimension of 7.56 mm, which previously took 〉100 h to oxidize completely, was successfully oxidized crack free in 18.3 h using feedback control. Using the typical heat-treatment cycle, a 1 mm sample was fired in 18 h. With feedback-controlled firing, the same sized sample was fired in only 5 h.

Notizen: Qualitative residual stresses in current environmental barrier coatings (EBCs) were inferred from the curvature of EBC-coated SiC wafers, and the effects of EBC stresses on the durability of EBC-coated SiC were evaluated. The magnitude of substrate curvature correlated fairly well with the EBC–SiC coefficient of thermal expansion (CTE) mismatch, EBC modulus, and thermally induced physical changes in EBC. BSAS (1−xBaO·xSrO·Al2O3·2SiO2, 0≤x≤1) components in the current EBCs, i.e., Si/mullite or mullite+BSAS/BSAS or yttria-stabilized zirconia (YSZ: ZrO2–8 wt% Y2O3), were the most beneficial for reducing the EBC stress in as-sprayed as well as in post-exposure EBCs. The reduced stress was attributed to the low modulus of BSAS. The addition of a YSZ top coat significantly increased the substrate curvature because of its high CTE and sintering in thermal exposures. There were clear correlations between the wafer curvature and the EBC durability. The Si/mullite+20 wt% BSAS/BSAS EBC maintained excellent adherence, protecting the SiC substrate from oxidation, while the Si/mullite+20 wt% BSAS/YSZ EBC suffered delamination, leading to severe oxidation of the SiC substrate, after a 100 h −1300°C exposure in a high-pressure burner rig.

Notizen: This paper reports on the analysis of the microstructure of precursor-derived ceramics by plasma etching and field emission scanning electron microscopy. By applying this technique, the nano-crystals in Si–C–N-based ceramics and composites can be clearly observed without difficulty.

Notizen: Approximately 85 million gallons of high-level waste (HLW) is currently stored in underground tanks at the Hanford Reservation and the Savannah River Site (SRS). The waste consists of a hydroxide-rich precipitate (sludge) and a sodium-rich supernate. The supernate is a NaOH rich solution containing lesser amounts of NaNO3 and NaNO2 and small amounts of soluble fission products, cladding materials, and organics (volatile organics and semi-volatile organics known as VOCs and SVOCs). The Department of Energy (DOE) has chosen glass as its waste form for both sludge and sodium-rich supernate. However, because of the volume of the supernate, alternatives to vitrification are being sought for some of this waste. One alternative is to remove 137Cs and 90Sr from the supernate. Decontaminating the waste in this way allows the waste to be designated as low-activity waste (LAW) and as such the waste now becomes eligible for solidification and disposal on site. SRS is solidifying its LAW with a blended Portland cement forming Saltstone. Hanford has been considering a bulk vitrification process in which the LAW will be mixed with Hanford soil and vitrified in place in a disposable carbon-arc powered glass melter/waste container. Both waste forms can then be buried on site in appropriate vaults or low-level waste land fills. A hydroceramic is an alternative waste form designed to solidify and stabilize LAW that is made from metakaolin plus NaOH and/or NaOH rich LAW supernate. In addition to NaOH, LAW can contain a wide range of sodium nitrate and sodium nitrite concentrations. Although a hydroceramic waste form can be made directly from some types of decontaminated waste, e.g., those that are highly alkaline (8–12M NaOH) and contain less than 25 mol% of NOx (NOx is used as the short-note for nitrates and nitrites in this article.) relative to the total Na in the waste, by simply mixing the LAW with metakaolin and curing the resulting paste at 90°C, the remaining LAW, especially that stored at Hanford must be pretreated in some way before it can be similarly solidified; the relative molar proportion of NOx/Na must be reduced to 25% or less. In this paper calcination is evaluated as a potential pretreatment method for Hanford AN-107 (AN-107 is a waste storage tank on Hanford site) LAW, but in choosing this method it is necessary to divide the preparation of the hydroceramic waste form into two steps: denitration/denitrition of the liquid waste stream to produce a granular calcine followed by solidification using a metakaolin plus 4M NaOH binder. A simulated Hanford AN-107 LAW was calcined at 375°, 450°, 525°, 600°, and 675°C in the presence of sucrose and metakaolin added as a calcination aid. It was shown that the leachability of the calcines decreased as calcination temperature increased, i.e., the waste form became more crystalline. In the second step, each of the granular calcines was mixed with additional metakaolin and just enough 4M NaOH to form a thick paste. The paste was precured at 40°C and then autoclaved at 90°C to form a monolith. X-ray diffraction and scanning electron microscopy characterization showed that the calcines themselves contained an amorphous phase and crystalline hydroxysodalite, and that the hydroceramics made from these calcines plus additional metakaolin/NaOH binder consisted predominantly of zeolite A and hydroxysodalite. The temperature used to prepare the calcines not only affected the properties of the calcines, but those of the monolithic hydroceramics as well. Experimental results demonstrated that 525°C represented the optimal temperature for producing the most suitable calcine for subsequent solidification with metakaolin and 4M NaOH. The resulting hydroceramic nuclear waste form was strong and had the lowest overall leachability. The leachability of the hydroceramic is normally lower than that of the corresponding calcines up to ∼600°C. The product consistency test (PCT) determined normalized release rate NRNa for the hydroceramic (0.14 g/m2·day) was comparable to similar leach rates determined for Hanford's low-activity waste reference material glass (0.08 g/m2·day) and a steam reformed calcine made with Hanford's AN-107 tank waste (0.25 g/m2·day).

Notizen: Electrical conduction properties of undoped and 1 mol% Sr-doped LaP3O9 glasses and glass–ceramics were investigated over the temperature range of 673–1123 K. Both the materials showed relatively low conductivities in the glassy state. However, the conductivity of the Sr-doped LaP3O9 glass significantly increased with a heat treatment above the crystallization temperature, while the conductivity of the undoped LaP3O9 glass did not improve even after the heat treatment. It was concluded that crystallization of the Sr-doped LaP3O9 glass induced protonic conduction and thus enhanced the conductivity. Electrical conduction properties of the Sr-doped LaP3O9 glass–ceramic fundamentally resembled those of the sintered crystalline Sr-doped LaP3O9.

Notizen: The phases formed in the ternary system (Na1/2Bi1/2)TiO3–Bi4Ti3O12–BaTiO3 (NBT–BTO–BT) were studied at 1150°C in air. A very accurate picture of the ternary phase diagram was obtained examining almost 90 different compositions, exploiting low-angle XRD analyses to study the layer compounds. New compounds with five perovskite blocks (m=5) were discovered deep in the phase diagram. No compounds with m〉5 were found. It was also established that pure perovskite compounds can be obtained only at compositions very close to the NBT–BT line. The relationships between the phases is discussed and it is hypothesized that the number of perovskite blocks in the system is determined by charged sites being created by the progressive substitution of Bi3+ in the A site of the perovskite blocks of BTO with the A cations of the perovskite end-member.

Notizen: Channeled yttria-stabilized cubic phase of the zirconia (Ni–YSZ) cermets are produced by reduction of laser-assisted directionally solidified NiO–YSZ lamellar eutectics. The material is formed by ∼400 nm wide alternating lamellae of 40% porous Ni and YSZ, which serve as parallel channels for gas flow and electronic transport and for oxygen ion diffusion. The low-energy interfaces formed between the metallic Ni particles and the YSZ prevent particle coarsening and impart long-term stability to the anode at operating temperatures. The electrical conductivity and the pore size distribution present no degradation after 300 h at 900°C under a H2/N2 atmosphere. This stability is indicative of an improvement in comparison with conventional Ni–YSZ anodes for solid oxide fuel cells.

Notizen: A sol–gel synthesis method based on the diol route is reported here for the fabrication of strontium bismuth tantalate (SBT, SrBi2Ta2O9) ferroelectric thin films. A tantalum penta-glycolate (TPG)-based sol is synthesized, which is air stable and has low toxicity. The structure of the TPG compound is studied by nuclear magnetic resonance (NMR) spectrometry, and the results show that TPG contains a tantalum cation bonded to five diol groups. The molecular association of the TPG derivatives in the sol, through the terminal hydroxyl groups of the diol ligands, is responsible for its transformation into a gel. Other metal compounds can be easily added to the TPG-based sols for the preparation of air-stable and precipitate-free multicomponent solutions. These solutions are precursors of complex oxide materials. In this paper, SBT thin films have been prepared by this sol–gel method. These films develop a layered perovskite structure after crystallization. The films have a remanent polarization of Pr∼10 μC/cm2, a coercive field of Ec∼65 kV/cm, and are fatigue free up to ∼1010 cycles.

Notizen: Dy-α-sialon and β-Si3N4 materials containing Dy-oxynitride glass were hot pressed at 1800°C for 1 h. The luminescence spectra of Dy3+ in these samples were compared when excited at 350 nm. The results showed that two strong emission bands in the region 470–500 nm and 570-600 nm, associated with the 4F9/2→6H15/2 and 4F9/2→6H13/2 transitions of Dy3+ ions, were observed in Dy-α-sialon. However, no emission peak was detected from the β-Si3N4 sample, despite it containing the same amount of Dy3+ cations. This proved that only the Dy3+ cations in the α-sialon structure, not those in the oxynitride glass, produce the luminescence spectrum.

Notizen: By lamination of silicon nitride tapes, components with complex geometries can be produced. Unstructured tapes can be laminated by common thermal compression. Structured tapes, however, have to be joined by pressureless processes using e.g. pastes as lamination aids because deformation of the structures would occur. These pastes usually contain a binder for maintaining the mechanical contact between the tapes during processing. To prevent the high mass loss of typical organic binders during burnout, pre-ceramic polymers were used in this work. These ceramic precursors convert partly into an inorganic material during heat treatment with a significant reduced mass loss compared with common organic binders. Thus, the porosity in the interlayer of a laminated stack is strongly decreased, which should be favorable for the mechanical and thermal properties. This work discusses the resulting microstructure, strength, and thermal diffusivity data of stacks laminated with pastes containing various precursor contents. These results are compared with those obtained by samples prepared by compression of green tapes. It is found that except for some large pores, the microstructure of the precursor-derived interlayers is qualitatively the same as in the tape material. For stacks made by both lamination methods, strength measurements reveal that the properties parallel and perpendicular to the layers are different. It is shown that the same strength level can be obtained both by using the pressureless route and by the compression method. Unlike the strength, the thermal conductivity does not change with the direction of measurement.

Notizen: A method based on the use of four piezoelectric resonances for three sample geometries that allows obtaining the full set of linear electric, mechanical, and electromechanical coefficients, and all related losses of a piezoelectric ceramic has been applied to Mn-doped 0.655Pb(Mg1/3Nb2/3)O3–0.345PbTiO3 at the morphotropic phase boundary (MPB PMN–PT). Length-poled MPB PMN–PT ceramic plates presented piezoelectric shear double resonances associated with a thickness gradient of tetragonal and rhombohedral (or monoclinic) phases that originated during poling. The versatility of the method still allowed addressing these double resonances and obtaining all the linear coefficients and losses of the well-poled material. These are given for MPB PMN–PT and compared with those of a Navy type II Pb(Zr,Ti)O3 (PZT) ceramic. MPB PMN–PT presents piezoelectric coefficients as high as soft PZT but significantly lower losses, and so less overheating and hysteresis under high driving fields. Its thermal stability has been studied up to 100°C, and the temperature dependence of a number of linear coefficients and of the thickness and planar coupling factors and frequency constants of disks has been obtained. The latter thickness parameters hardly changed with temperature, while planar ones showed a relative variation of 10%.

Notizen: This study describes the isothermal cyclic loading dilatometry (ICLD) technique to measure the viscosity of glass-based materials. We demonstrate its merit relative to constant-load techniques in minimizing the stress history effects (changes in shrinkage anisotropy and sample microstructure) that arise due to the application of an external load. A constant-load test overestimates the viscosity by an order of magnitude compared with a cyclic load test. To obtain accurate viscosity data, maximum loading rates and longer unloading periods are desirable as they reduce effects of shrinkage anisotropy on viscosity values. Representative data for a low-temperature cofired ceramic (LTCC) material are reported. Nonparametric statistical tests revealed insignificant differences between the viscosity data sets at 5% significance level and thus indicate good reproducibility of the testing methodology.

Notizen: Synthesis of zirconia nanoparticles by flame spray pyrolysis (FSP) at high production rates is investigated. Product powder is collected continuously in a baghouse filter unit that is cleaned periodically by air-pressure shocks. Nitrogen adsorption (BET), X-ray diffractometry (XRD), transmission electron microscopy (TEM), and thermogravimetric analysis (TGA) are used to characterize the product powder. The effect of powder production rate (up to 600 g/h), dispersion gas flow rate, and precursor concentration on product particle size, crystallinity, morphology, and purity is investigated. The primary particle size of zirconia is controlled from 6 to 35 nm, while the crystal structure consists of mostly tetragonal phase (80–95 wt%), with the balance monoclinic phase at all process conditions. The tetragonal crystal size is close to the primary particle size, which indicates weak agglomeration of single crystals.

Notizen: A method based on the use of four piezoelectric resonances for three sample geometries is presented that allows one to obtain all the dielectric permittivities, compliances, and piezoelectric coefficients of a piezoelectric ceramic in complex form and, therefore, all related losses. Piezoelectric losses are responsible for heat generation and hysteresis in actuators. The method is applied to a Navy type II PZT-based piezoelectric ceramic (PZT = lead zirconate titanate), for which the full set of linear electric, mechanical, and electromechanical coefficients is given in complex form. Full sets of coefficients for the available piezoceramics are required for exploiting all the possibilities of finite element analysis, both in fundamental research (mechanisms of degradation) and in development (element design). This numerical technique is necessary to explore arbitrary shapes provided by solid free-form-fabrication technologies.

Notizen: Low-frequency dielectric response of air- and oxygen-sintered ceramics with the composition 0.9BaTiO3–0.1La(Mg1/2Ti1/2) O3 (0.9BT–0.1LMT) has been studied in the temperature range of 12–550 K. In comparison with pure BT, in 0.9BT–0.1LMT the dielectric permittivity maximum is shifted by almost 300 K toward lower temperatures. Both real and imaginary parts of dielectric permittivity of the solid solution, in the range 12–150 K, show a strong frequency-dependent behavior, which is typical of relaxors. On the basis of the model of exponential cluster size distribution and the Cole–Cole equation, the degree of interaction between the polar clusters was estimated. It was shown that the oxygen vacancies arising during sintering at high temperatures did not affect noticeably the relaxor properties of the material. The role of heterovalent La3+/Ba2+ and Mg2+/Ti4+ substitutions in the relaxor behavior formation is discussed.

Notizen: Integrated lead zirconate titanate thin films deposited on Pt/Ti/SiO2/Si substrates using a novel triol-based route were characterized using X-ray diffraction and transmission electron microscopy. Crack-free single-layer PZT films of up to 200 nm thick were prepared by triol-based sol–gel processing onto Pt/Ti/SiO2/Si substrates. Films ∼75 nm thick exhibited a microstructure free of pores and second phase. As film thickness increased, film texture changed from {100} to {111} perovskite. Essentially, single-phase multilayer films could be prepared by the deposition and pyrolysis of several 75 nm layers, followed by a single crystallization step. The influence of heat-treatment schedule on the microstructure and orientation of the multilayer films is discussed. Comparison has been made between multilayer films prepared using the triol-based sol and an inverted mixing order/acetic acid-based sol.

Notizen: It has been shown that polydimethylsiloxane (PDMS)–CaO–SiO2–TiO2 and poly(tetramethylene oxide) (PTMO)–CaO–TiO2 hybrids form apatite on their surfaces in a simulated body fluid (SBF) and show mechanical properties similar to those of human cancellous bones. In the present study, changes, caused by soaking in SBF, were measured in the mechanical properties of PDMS–CaO–SiO2–TiO2 hybrids with different CaO and TiO2 contents and PTMO–CaO–TiO2 hybrids with different CaO contents. Significant decreases in the strength and strain at failure of the hybrids were observed for the PDMS–CaO–SiO2–TiO2 hybrids with high CaO or TiO2 contents and PTMO–CaO–TiO2 hybrids with a high CaO content after soaking in SBF for 4 w. This indicates that incorporation of a large amount of CaO component into the hybrids should result in the deterioration of the hybrids in the body environment.

Notizen: The stresses of laminate structures obtained by joining single layers of pure alumina (A), pure yttria-stabilized tetragonal zirconia, 3Y-TZP (Z), and an intimate mixture of alumina and zirconia (AZ) have been determined by fluorescence (in alumina) and Raman (in zirconia) piezospectroscopy. Three symmetrical stacking sequences were examined, namely, A/Z/A, A/AZ/A, and AZ/Z/AZ, with the aim of designing structures where the higher coefficient of thermal expansion (CTE) of zirconia could be used to induce compressive stress in the external layers (and ensuing tensile stress in the central layer). Two experimental sessions, 6 years apart, were conducted on the same samples, also taking care to record the spectra from the same locations; during the time elapsed between the two sessions, the samples were kept at room temperature and humidity. The stress values in alumina obtained during the more recent session were markedly different from those observed in the first session. Monoclinic zirconia (m-zirconia) was absent in all samples in the first session, whereas up to 25 vol% zirconia could be observed during the second session. m-Zirconia could only be observed in AZ layers and not in Z layers, irrespective of the position in the stacking sequence. It was concluded that 3Y-TZP underwent spontaneous tetragonal-to-monoclinic (t–m) transformation, that is, “aging,” when mixed with alumina at the grain-size level. Aging occurred only where pristine t-zirconia was subject to tensile stresses larger than ∼400 MPa.

Notizen: Various glass-ceramics were prepared based on the BaO–Al2O3–B2O3–SiO2 system with the addition of La2O3, ZrO2, or NiO in an attempt to develop a suitable sealant for planar solid-oxide fuel cells operating at 800°C. To estimate the applicability of these glasses as suitable sealants, their thermal and chemical stabilities as well as the crystallization behavior and bonding characteristics of their parent glasses were investigated. The thermal properties and crystallization behavior of the parent glasses were dependent on the type of additive used, and the bonding characteristics were strongly influenced by the B2O3/SiO2 value in the glass composition. Observation of microstructural change and chemical reaction at the glass-ceramic/electrolyte interface confirmed that the prepared glass-ceramics possessed long-term stability during heat treatment at the operation temperature for up to 1000 h.

Notizen: Nanocrystalline titanium nitride (TiN) particles were directly synthesized in an atmospheric microwave plasma torch, using gas-phase titanium tetrachloride (TiCl4), with N2 and H2 as reactants. The gas-phase TiCl4 was injected with H2 gas into the microwave plasma-torch flame generated by N2 swirl gas, and then the dark blue powders were deposited on the inner wall of the quartz tube. The synthesized powder samples were analyzed by scanning electron microscopy (SEM), SEM-EDX, X-ray diffractometer (XRD), transmission electron microscopy, and FT-Raman. The average crystallite sizes of TiN measured from XRD patterns decreased from 15.5 to 11.6 nm, as the flow rate of H2 injected into the microwave plasma torch increased from 0.03 to 0.3 liters per minute.

Notizen: The changes in porosity and elastic moduli of YSZ-containing nickel-based anode materials for solid oxide fuel cells were studied as a function of the fraction of reduced NiO. Anode samples were reduced in a gas mixture of 4% hydrogen and 96% argon for different periods of time at 800°C and their Young's and shear moduli were determined afterward at room temperature using resonant ultrasound spectroscopy and impulse excitation. It was found that the magnitude of Young's and shear moduli decreased significantly with increasing fraction of reduced NiO and that the magnitude of the elastic moduli of a fully reduced Ni–YSZ anode was ∼45% lower than that of unreduced NiO–YSZ. Because the elastic moduli of NiO are close to those of Ni, the observed decrease in the magnitude of the elastic moduli was found to be caused mainly by the significant increase in the porosity of the sample as a result of NiO reduction. Expressions are presented for the amount of porosity and the magnitude of the elastic moduli as a function of the fraction of reduced NiO.

Notizen: Addition of fructose has been shown to reduce the viscosity of aqueous suspensions of nanometric alumina powders. Highly concentrated and flowable aqueous nanometric alumina suspensions were achieved by fructose addition. Oxygen-17 nuclear magnetic resonance spectroscopy was used to investigate the relaxation time and the molecular mobility of water in the suspensions with and without fructose. It was found that a significant fraction of water is bound to the surface with lower mobility. Average water mobility increases and the viscosity of the suspension decreases with fructose addition, both of which are because of displacement of water from the alumina surface by fructose molecules. The results were related to rheological behavior of the suspensions.

Notizen: Cracks situated parallel to, and very near, the interface in layered, ductile-brittle composite specimens were investigated with finite-element analysis. Elastic, plastic and thermal properties previously obtained from experiments were utilized in the model. A routine was employed for automatic crack extension and remeshing, enabling simulation of incremental crack propagation. The elastic, thermal and plastic contributions to crack propagation behavior were investigated, along with the variation of these with crack length and crack-tip position. Thermal residual stresses are shown to have a large influence on crack path, although this is mitigated to some extent by plasticity. The implications on the inherent reliability of joints and layered materials containing brittle constituents are discussed.

Notizen: Processing effects on the dielectric properties of sol–gel-derived PbZrO3–PbTiO3 (PZT) films integrated onto Pt/Ti/SiO2//Si substrates are reported. Sol–gel synthesis and deposition conditions were designed to produce films of varying thickness (95–500 nm) with consistent chemical composition (Pb (Zr0.53Ti0.47)O3), phase content (perovskite), grain size (∼110 nm), crystallographic orientation (nominally (111) fiber textured), and measured residual stress. The Stoney method, using laser reflectance to determine wafer curvature, derived biaxial tensile stress values of 150 and 180 MPa for PZT films after a baseline correction for electrode interactions during thermal processing was employed. The PZT films were of high dielectric quality, with low losses and negligible dispersion. Calculated values of dielectric constant (K̄′) were found to decrease from 960 to 600 with decreasing film thickness. A series-capacitor model successfully recovered a room-temperature K1′ for the PZT (1,170) in good agreement with bulk reports but was unable to reproduce the expected dielectric anomaly near 380°C. This discrepancy and the resulting diffuse phase transformation were attributed to the biaxial tensile stress present in the PZT films.

Notizen: The piezoelectric effect in ferroelectric single crystals and ceramics is investigated considering intrinsic (lattice), and extrinsic (originating mainly from displacement of domain walls) contributions. The focus of the study of intrinsic properties is on piezoelectric anisotropy, which was examined using the Landau–Ginsburg–Devonshire phenomenological theory. It is shown that the enhanced piezoelectric response along nonpolar directions, observed in many perovskite systems, is a consequence of the flattening of the Gibbs free energy profile. This flattening is common for temperature-, composition-, and external field-induced enhancement of the piezoelectric properties along nonpolar axes. A brief review of recent advances in understanding the origins of the piezoelectric nonlinearity, hysteresis, and frequency dispersion is also given.

Notizen: Carbon nanotube (CNT)-dispersed Si3N4 ceramics with electrical conductivity were developed based on the lower temperature densification technique, in which the key point is the addition of both TiO2 and AlN as well as Y2O3 and Al2O3 as sintering aids. This new ceramic with a small amount of CNTs exhibits very high electrical conductivity in addition to high strength and toughness. Since Si3N4 ceramics with Y2O3–Al2O3–TiO2–AlN were originally used as a wear material, electrically conductive Si3N4 ceramics are expected to be applied for high-performance static-electricity-free bearings for aerospace and other high-performance components.

Notizen: Monotonic and cyclic fatigue behavior of single fibers or fiber fabrics are of significant interest, since fiber assemblies or fiber-reinforced composite materials in structural applications are often subjected to cyclic loading. Studying the cyclic fatigue behavior of fibers is particularly difficult because of their small diameter (∼10 μm) and high aspect ratio. In this paper, we report results of monotonic tension and tension–tension fatigue behavior of two sol–gel-derived ceramic fibers: Al2O3–SiO2–B2O3 (Nextel 312) and Al2O3 (Nextel 610). Nextel 312 exhibited a great deal of variability in tensile strength, reflected by a Weibull modulus of 4.6, versus Nextel 610, which had a Weibull modulus of 10.5. Our experiments showed clearly that cyclic loading was more damaging than static loading and, thus, resulted in a lower cyclic fatigue life compared with static loading. The fracture behavior under fatigue loading was distinctly different from that under monotonic loading. It is believed that processing-induced flaws acted as crack initiation sites, and that the cyclic loading induced subcritical cracking, followed by coalescence of cracks immediately prior to failure.

Notizen: This paper presents the applicability of a simple and novel technique based on capillary suction time (CST) to characterize the dispersibility of suspension. The method is simple, rapid, and requires little experience. It has been shown to be extremely useful for comparing the dispersibility during preparation of a stable suspension of ceramic powder with high solids loading. The method has also been found to be suitable for obtaining homogeneous suspensions of maximum stability. The dispersibility of particles in any suspension has been assessed with respect to the condition at point of zero charge (pHpzc). A quantitative measure of the dispersibility has been proposed in term of a dispersion ratio (DR). According to this proposition, the suspension is in a state of dispersion when the value of DR is more than unity, whereas it is in a flocculated state when the value of DR is less than unity.

Notizen: In this paper, the spectroscopic properties of bismuth borate glasses doped with different Nd3+ concentrations have been investigated. The intensity parameters Ωt (t=2, 4, 6), spontaneous emission probability, fluorescence lifetime, radiative quantum efficiency, fluorescence branching ratio, and stimulated emission cross section were determined. The highest radiative quantum efficiency of the glasses is 54%. The optimal Nd3+ concentration in the glasses is proposed for high quantum efficiency. The good spectroscopic properties show the possible utilization of the Nd3+ doped bismuth borate glasses as laser materials.

Notizen: The depth sensitivity of spectroscopic ellipsometry (SE) to the presence of oxygen vacancies (VO••), in perovskite titanates, was examined over the wavelength range from 250 to 750 nm. To determine the effect of oxygen vacancy concentration, reference optical properties of barium titanate (BaTiO3) and Fe-doped SrTiO3 were determined for oxidized and reduced samples. These reference data were used to model the sensitivity of SE to changes due to degradation. Using these models, it was found that SE could detect reduced layers of 4 Å for BaTiO3 and an 8 Å layer for Fe-doped SrTiO3. From the models, concentration gradients were resolvable by SE with an average diffusion depth (√Dt) of 10 Å for BaTiO3 and 50 Å for Fe-doped SrTiO3. These models also predict that SE should be able to discern different degradation mechanisms through the way the optical properties change during degradation.

Notizen: The erosion rates and impact damage of two sintered Si3N4 materials with identical compositions but different microstructures were determined using a gas-blast-type erosion rig. The erodent particles used were SiC grits and the impact angles investigated were 30° and 90°. It was found that the erosion behavior of the two materials could not be related to their mechanical properties, such as hardness and fracture toughness as predicted by the theoretical erosion models. In fact, a close relationship was identified between their microstructure and the erosion mechanism. Microstructures containing evenly dispersed and uniaxially oriented reinforcing whiskers promoted grain-pullout, while the randomly oriented elongated grains hindered it.

Notizen: This paper describes the growth of barium chromate (BaCrO4) nanocrystallites within thermally evaporated thin films of stearic acid (StA) and sodium bis-2-ethylhexyl-sulfosuccinate by a process of Ba2+ ion entrapment followed by in situ reaction with CrO42− ions. Dense spherical assemblies of BaCrO4 nanocrystallites of very uniform size (∼50 nm) were obtained within the two different host matrices. The spherical assemblies were composed of smaller (ca. 5–10 nm size) BaCrO4 crystals indicating that efficient size control over crystal size may be exercised by the matrix. Contact angle measurements of the BaCrO4–StA and BaCrO4–sodium bis-2-ethylhexyl-sulfosuccinate films indicated that they were hydrophobic, thus pointing to the possible role of hydrophobic interaction between the StA and sodium bis-2-ethylhexyl-sulfosuccinate monolayer-covered BaCrO4 crystals in the assembly process.

Notizen: Alumina-rich mixtures of La2O3 and Al2O3 have equilibrium phases that are perovskite (LaAlO3) and β-alumina (LaAl11O18). Annealing initially glassy samples with compositions 3La2O3–5Al2O3 or 11La2O3–14Al2O3 between 860° and 1075°C creates a metastable state that is clearly detected by X-ray diffraction (XRD). While structures have not been determined, the new state appears to have two crystalline phases. This metastable state is favored by low annealing temperatures and by compositions with more rare-earth oxide. Similar behavior is seen with Nd2O3:Al2O3 compositions, although the smaller Nd ion is less effective in promoting the metastable structure. The detection of equilibrium β-alumina by XRD can be difficult, leading to possible misinterpretation of phases that are present.

Notizen: Magnesium aluminate (MgAl2O4) spinel powders of irregular and spherical morphologies were obtained from the bi-component water-based sols following the sol–gel and sol–emulsion–gel methods, respectively. For the synthesis of the oxide microspheres, the surfactant concentration and viscosity of the sols were found to affect the characteristics of the derived microspheres. The gel and calcined powders were investigated by using thermogravimetry analysis, differential thermal analysis, X-ray diffraction (XRD), optical and scanning electron microscopy (SEM), Fourier transformed infrared spectroscopy, and particle size analysis. XRD results indicated crystallization of the only phase MgAl2O4 spinel from 200° to 1000°C. Formation of hollow microspheres with a single cavity was identified by SEM.

Notizen: The increasingly wider application of lime-based materials for the conservation and restoration of historical buildings has led to a revival of interest in lime putty. One of the most well-known and highly regarded properties of lime putty is its plasticity/workability.This paper describes the results of a rheological investigation conducted on putties having identical water content but aged for different times, with the aim of providing an objective and quantitative evaluation of plasticity. Rheological measurements have demonstrated that, all other conditions being equal, shear stress increases with aging. We propose here to consider some parameters derived from Tattersall and Bingham models that can be used as a quantitative measure of plasticity directly correlated with aging.

Notizen: When a BaTiO3 powder compact was sintered in air, the abnormal grain growth (AGG) was observed to occur at the surface of the specimen. The BaO evaporation from the surface and consequent formation of eutectic liquid is suggested to be the cause of AGG. When the evaporation of BaO during sintering was prevented by embedding the specimen in the same powder, no AGG was observed to occur.

Notizen: Nanocrystalline zinc titanate (ZnTiO3) was synthesized at low temperatures through the combination of a sol–gel processing and a polymer binder method. ZnTiO3 powders of ∼5 nm in size were obtained by heating pastes, which were composed of a Zn-Ti methanolic solution containing acetylacetone and an organic polymer binder, at 500°C in air. Thermal decomposition behavior of the pastes was analyzed by thermogravimetry-differential thermal analysis. Crystallinity of ZnTiO3 was examined by transmission electron microscopy. The BET measurement revealed that the powders had a relatively high specific surface area of 106 m2/g.

Notizen: The indirect solid freeform fabrication process is used to fabricate low-loss complex geometry periodic dielectrics for use in the microwave range. Relevant processing details, such as mold fabrication, slurry rheology, curing, and binder burnout, are discussed. Woodpile electromagnetic bandgap structures exhibiting a −40 dB well were shown to be operational at frequencies 〉110 GHz. Novel designs of planar periodic dielectric resonators with Q= 965 at 33 GHz (Q= ratio of the power stored to power lost in the resonator) were realized in alumina. Metallo-dielectric filters with low-insertion loss were also created via ISFF; modeling of these structures using resistor-inductor-capacitor-equivalent circuit analysis was performed to calculate a Q of 645.

Notizen: Two- and three-dimensional SiCr/SiC composites have been prepared starting from Tyranno SA(tm) fiber preforms. Preform densification has been performed by a modified preceramic polymer impregnation and pyrolysis (PIP) process consisting of filling the preform large interbundle voids with SiC powder before the PIP process. This step was accomplished by low-pressure infiltration of a SiC powder dilute slurry through the preform thickness. Specimens were further processed with polymer impregnation and pyrolysis to determine the effects on structural, thermal, and mechanical properties of the obtained composites. High-temperature pyrolysis treatment, which promoted polymer derived SiC matrix crystallization, markedly increased thermal diffusivity.

Notizen: Carbon is commonly added to sulfate-fined silicate-glass batches to enhance the fining process. Reactions between carbon and Na2SO4 modify the SOx emissions from Na2SO4 decomposition. Fourier-transform infrared (FTIR) spectrometry is used to analyze the emission of air pollutants from the isothermal decomposition of Na2SO4 + C undertaken using thermogravimetric analysis (TGA). The FTIR spectrometer is calibrated using standard gas mixtures containing CO, CO2, SO2, NO, and NO2. The collected spectra are quantified using the classical least-squares (CLS) approximation. The TGA-FTIR system provides SOx, and COx, concentrations versus time data from the isothermal decomposition of Na2SO4, in the presence of a carbon black. Mass spectrometry (MS) complements FTIR by being able to detect SO(g).

Notizen: It is generally accepted that MgO additions enhance the sintering of alumina. The distribution of the low concentration required is, however, difficult to attain and critical to the final properties and microstructures. To improve the MgO distributions in ceramic injection-molded (CIM) alumina, Mg-containing precursors, Mg-stearate and Mg-acetate, were added during the kneading step of the CIM process, and the results were compared with those from adding MgO particles. Of the three additives, the Mg-stearate showed the highest sintered density and bending strength. This was attributed to its high molecular weight and it being able to mix with other binder components homogeneously in the liquid state, which leads to a better distribution of the MgO. In contrast, the MgO powder was kneaded in the solid particle form, and its distribution was not uniform. The Mg-acetate also imparted poorer MgO distribution because the liquid Mg-acetate dehydrated in the early stage of the kneading and formed solid lumps before it was well mixed into the binder system. The resulting sintered densities and bending strength using these two additives were, therefore, inferior to those for specimens in which Mg-stearate was used.

Notizen: Different ratios of the precursor phases of SrFeO3–x (SFO) and SrMoO4 (SMO) were used to prepare Sr2FeMoO6 (SFMO) by a solid-state reaction. X-ray diffraction was used to identify the phases. A residual SMO was observed to exist in the sample with an SFO/SMO ratio of 0.9:1. The sample with a residual SMO phase had higher resistivity, lower magnetization, but higher low-field magnetoresistance (LFMR). High-resolution transmission electron microscopy was used to identify the compositions and phases. Nanometer-sized amorphous-like clusters of SMO phase were located inside the grains rather than at grain boundaries; however, some boundaries were rich in the strontium ion. The possible mechanisms for the conduction and the increase of LFMR of SFMO are discussed.

Notizen: The bimodal distribution of grain size is frequently observed in calcium copper titanate (CaCu3Ti4O12 or CCTO) ceramics that exhibit high dielectric constant. In this study, we propose an analytical model composed of grains of two different sizes to estimate the dielectric behavior of CCTO based on microstructural features. Important assumptions are (1) CCTO is a barrier layer dielectric and (2) the insulating layer thickness is the same in both large and small grains. Explicit expressions for the effective dielectric constant of serial, parallel, and logarithmic mixture models are given. When compared with experimental results, the present bimodal model appears to be acceptable for describing the dielectric behavior of CCTO.

Notizen: Er-doped Al2O3–SiO2 (1/9 in mol ratio of Al2O3/SiO2) thin films were prepared by using a modified sol–gel process. The modified process entails the precipitation and digestion of Er(OH)3, obtained from the reaction between Er ions and NH4OH in solution. Thin films were deposited on Si wafers by using a spin coating technique (3000 rpm) and the coated films were heat treated at different temperatures for 1 h in an oxygen-purged furnace. All the films were structurally characterized by the X-ray diffraction technique using CuKα radiation. Refractive indices and the morphologies of the films were studied using a spectroscopic phase modulated ellipsometer and atomic force microscopy, respectively. It was observed that the films were crack free and of about 0.4 μm thickness in a single spin coating and both the lifetime and the photoluminescence intensity of Er ions increased with increasing the annealing temperature. The luminescence properties of the Er-doped Al2O3–SiO2 made by a conventional and our modified doping process were compared and discussed from the stand point of peak intensities and lifetimes as a function of annealing temperatures. It is to be noted here that our modified process was found to be more effective in reducing the clustering of Er ions in Al2O3–SiO2 materials as compared to that of the conventional method.

Notizen: Nanosized NiZn ferrite powder is synthesized by a low-temperature method, using a unique combination of citric acid and glycine. An appropriate molar ratio of both citric acid and glycine offers a low-temperature synthetic route by incorporating the complexation behavior of citric acid and the combustion nature of glycine. Thermal decomposition/controlled autocatalytic combustion of the composite gel occurs at a low temperature of around 175°C, with the evolution of a large amount of gases. Transmission electron microscopic studies showed that the average particle size of the ferrite obtained is ∼2.5 nm, with a narrow size distribution. Uniformly distributed fine-grained microstructure with low porosity is obtained for a sample sintered at 1000°C.

Notizen: A novel process for low-density alumina foams has been studied. Aqueous acidic sucrose solution containing aluminum nitrate when concentrated by heating formed a viscous resin. The resin mixed well with aqueous alumina slurry and the resulting powder-filled resin underwent foaming on heating in a Teflon mold. The green foams produced had very good handling strength. Binder removal and sintering of the green foam prepared at sucrose to alumina weight ratio in the range 0.69–1.03 produced alumina foam bodies with porosities 93.5%–96.7%. Microstructure of the foams depends on sucrose to alumina weight ratio such that a clear transition from reticulated structure to cellular foam structure took place at sucrose to alumina weight ratio below 0.89. Average pore size depends on sucrose to alumina weight ratio and was observed in the range 0.48–2.69 mm.

Notizen: Anatase-type TiO2 solid solutions doped with 0–10 mol% scandium were formed by hydrothermal crystallization under weak basic conditions above 180°C for 5 h from amorphous co-precipitates that were obtained from the aqueous precursor solutions of TiOSO4 and Sc(NO3)3 using aqueous ammonia. The anatase particles were spindle-like and consisted of nanosized-crystallites (23–25 nm). The lattice parameter c0 of anatase and the length and width of the spindle-like anatase gradually increased when the scandium content was increased. The diffuse reflectance spectra of the as-prepared TiO2 doped with scandium showed that the onset of absorption slightly shifted to longer wavelengths with increasing scandium content. The band gap of anatase was slightly increased by making solid solutions with scandium oxide.

Notizen: The influence of transition metal oxides additives, especially zinc oxide, on the densification and electrical properties of doped barium zirconate have been examined. With the use of zinc oxide as a sintering aid, BaZr0.85Y0.15O3–δ was readily sintered to above 93% of theoretical density at 1300°C. Scanning electron microscopic investigations showed Zn accumulation in the intergranular regions. Thermogravimetric analysis of the material under flowing CO2 showed ZnO-modified barium zirconate to exhibit excellent chemical stability. The conductivity, as measured by A.C. impedance spectroscopy under H2O saturated nitrogen, was slightly lower than that of unmodified barium zirconate. Electromotive force measurements under fuel cell conditions revealed the total ionic transport number to be ∼0.9 at 600°C. The combination of electrical and chemical properties and good sinterability render ZnO-modified barium zirconate an excellent candidate for reduced temperature solid oxide fuel cell applications.

Notizen: An investigation into the processing of a bismuth ferrite lead titanate (BFPT) solid solution close to the morphotropic phase boundary through mechanical activation was undertaken. High-energy milling of constituent oxides was performed in a shaker mill, which resulted in size reduction of the particles to the nanoscale (40–600 nm). Subsequent calcination indicated a significant decrease in the temperature required for complete formation of the perovskite BFPT phase. It is concluded that the size reduction increases the particle surface area and reduces the diffusion paths for reaction sufficiently to result in a substantial decrease in the temperature required for perovskite formation. However perovskite phase formation as a result of mechanical activation alone was not observed.

Notizen: Yttria-stabilized (8.6 mol% YO1.5) zirconia thermal barrier coatings evolve at high temperatures from the “non-transformable,” metastable tetragonal-prime phase in their as-deposited condition to a mixture of the tetragonal and cubic phases. The kinetics of the transformation at 1200° and 1425°C are reported based on X-ray diffraction measurements. Complementary Raman spectroscopy measurements indicate a sharpening of the tetragonal bands at 263 and 465 cm−1 that is attributed to a systematic decrease in disorder of the Y3+ and oxygen vacancies with annealing. No transformation to the monoclinic form of zirconia is observed immediately after high-temperature treatment. However, partial transformation to monoclinic occurs after a prolonged time (months) at room temperature in those samples treated at 1425°C, indicating the development of isothermal martensite.

Notizen: Barium titanate has been prepared by solid-state reaction of nanocrystalline TiO2 (70 nm) with BaCO3 of different particle size (650, 140, and 50 nm). The results give evidence of a strong effect of the size of BaCO3 in the solid-state synthesis of barium titanate. The use of nanocrystalline BaCO3 already leads to formation of the single-phase BaTiO3 after calcination for 8 h at 800°C. The final powder consists of primary particles of ≈100 nm, has a narrow particle size distribution with d50=270 nm, and no agglomerates larger than 800 nm. For the coarser carbonate, 4 h calcination at 1000°C are required and the final powder is much coarser. Solid-state reaction of nanocrystalline BaCO3 and TiO2 represents an alternative to chemical preparation routes for the production of barium titanate ultrafine powders.

Notizen: Tantalum (V) oxide (Ta2O5) has potential applications as part of an environmental barrier coating system for Si3N4-based turbine components. However, at elevated temperatures, Ta2O5 undergoes a phase transformation from the orthorhombic (β) phase to the tetragonal phase (α), which is undesirable because of the associated volume change. The purpose of the present work was to study the effect of alumina additions (0–5 wt%) on the β to α transformation temperature, and associated modifications to the Ta2O5 microstructure. Sintered microstructures were characterized using SEM (scanning electron microscopy), and XRD (X-ray diffraction) was used to identify the phases present at room temperature. It was found that for undoped Ta2O5, transformation of the low-temperature β-phase begins at ∼1300°C, and leads to extensive microcracking of the sintered sample. For samples containing alumina, an increase in the transformation temperature was observed. The solubility limit of alumina in Ta2O5 was between 1 and 3 wt%; for samples in which this was exceeded, the AlTaO4 second and phase particles were seen to be highly effective at inhibiting grain growth.

Notizen: The impact of different additives on the hydrolysis of AlN powder in aqueous suspensions at room temperature was studied. The results show that citric acid and polyacrylic acid are most effective in chemically protecting AlN against hydrolysis. The protected powder is hydrophilic, which facilitates aqueous processing, and the chemical stability is retained when basic dispersing agents are added. Based on these results, the solid loading of the aqueous slurries was maximized by utilizing bimodal particle size distributions. Combining bimodal powders with the dispersants Dolapix and citric acid, colloidally stable slurries with solid loadings in excess of 50 vol% were obtained.

Notizen: A superconducting MgB2 film with a superconducting transition temperature of ∼36 K was successfully prepared on an MgO substrate by pulsed-laser ablation from a stoichiometric MgB2 target. A multilayer deposition process involving interposed Mg-rich layers was performed at 200°C by controlling the laser energy density in order to maintain the correct stoichiometry, and followed by an in situ annealing process at temperature ranging from 700° to 900°C. It was found that smooth, fine-grained films with superconducting transition temperatures of 24–36 K could be obtained from in situ annealing of the as-deposited multilayer at 900°C without excess Mg addition.

Notizen: The physicomechanical properties of historic bricks in Toledo (Spain) that have undergone a time-dependent self-healing by natural weathering processes have been improved by porous infill with gypsum, ettringite, and, mainly, calcite. Both these bricks and their experimental replica bricks, made from the original calcareous clays fired at the appropriate historical temperatures (700°–900°C), have been analyzed by X-ray diffraction, optical and scanning electron microscopy, mercury intrusion porosimetry, and ultrasound velocities to compare pore structure and strength evolution by mineral cementation. The resultant microstructure and mineral fillings depend on the brick calcareous composition and firing temperature, the brick location environment, burial, indoor or outdoor walls, the lime-based joint mortars and coat plasters, and infiltration waters.

Notizen: Polycarbosilane (PCS) was used as a precursor to prepare porous silicon carbide (SiC) ceramics with in situ growth of β-SiC nanowires. The pore size of the as-prepared porous ceramics was 1.37 μm in average, and had a narrow distribution. The nanowires with diameters ranging from ∼10 to 50 nm existed in the channels of the porous body. Their morphology, microstructure, and composition were characterized by field emission scanning electron microscopy, transmission electron microscopy, and energy-dispersive X-ray spectroscopy, which confirmed that the nanowires had a single-crystal β-SiC structure with the 〈111〉 growth direction. A vapor–liquid–solid process was discussed as a possible growth mechanism of the β-SiC nanowires.

Notizen: We performed a comparative study of electrical and thermal properties of ZnO- and SnO2-based varistor. The electrical properties of commercial ZnO-based varistor are equivalent to that found in SnO2-based varistor system. In spite of this, the SnO2 showed a thermal conductivity higher than commercial samples of ZnO-based varistor, which allied with its simpler microstructure and lower dopant concentration is a remarkable result that point out to the use of this system to compete commercially with ZnO-based varistor devices.

Notizen: The phase constitution during the sintering of pure red mud and red mud–fly ash mixtures was studied in the temperature range of 900°–1250°C. The phases formed at different sintering temperatures were analyzed by X-ray powder diffraction. The phases that are likely to form at equilibrium at any isotherm were predicted using the Gibbs free energy minimization technique and the databases provided in the FactSage software. Although the thermodynamic prediction is in reasonable agreement with the experimental results for the major phases, there is some disagreement regarding the minor phases, especially the more complex phases. The major limitation of the thermodynamic approach presently is the non-availability of thermodynamic data for several complex and multi-component solution phases.