ALBERT

Notes: Composite (biphasic) mixtures of two of the most important inorganic phases of synthetic bone applications-namely, calcium hydroxyapatite (Ca10(PO4)6(OH)2 (HA)) and tricalcium phosphate (Ca3(PO4)2 (TCP))-were prepared as submicrometer-sized, chemically homogeneous, and high-purity ceramic powders by using a novel, one-step chemical precipitation technique. Starting materials of calcium nitrate tetrahydrate and diammonium hydrogen phosphate salts that were dissolved in appropriate amounts in distilled water were used during powder precipitation runs. The composite bioceramic powders were prepared with compositions of 20%-90% HA (the balance being the TCP phase) with increments of 10%. The pellets prepared from the composite powders were sintered to almost full density in a dry air atmosphere at a temperature of ~1200°C. Phase-evolution characteristics of the composite powders were studied via X-ray diffractometry as a function of temperature in the range of 1000°-1300°C. The sintering behavior of the composite bioceramics were observed by using scanning electron microscopy. Chemical analysis of the composite samples was performed by using the inductively coupled plasma-atomic emission spectroscopy technique.

Notes: Monodisperse, spherical Si3N4 powder composed of fine particulates was synthesized by pyrolyzing spherical organo-silica powder under nitrogen. The organo-silica powder was prepared by hydrolyzing a mixture of phenyltrimethoxysilane (PTMS) and tetraethoxysilane (TEOS) in a methanol solution of water and ammonia. The organo-silica powder consisted of 81.3 at.% silicon units derived from PTMS and 18.7 at.% silicon units derived from TEOS. During the pyrolysis under nitrogen, the organo-silica powder decomposed to a mixture of free carbon and silica, with an increase of the surface area, at 500°-600°C, followed by the formation of alpha-Si3N4, with ß-Si3N4 as a minor phase, at 1450° and 1500°C and ß-SiC at 1550°C. The pyrolyzed powders, which retained the spherical shape and monodispersity of the organo-silica powders, with a reduction in mean particle diameter, were composed of fine particulates that were ~40 nm in size.

Notes: A new approach to tensile creep testing and analysis based on stress relaxation is described for sintered silicon nitride. Creep rate data covering up to 5 orders of magnitude are generated in tests lasting less than 1 day. Tests from various initial stresses at temperatures up to 1300°C are analyzed and compared with creep rates measured during conventional constant load testing. It is shown that at least 40% of the creep strain accumulated under all test conditions is recoverable, and that the deformation may properly be described as viscoelastic. A regime which approximated as Newtonian viscous behavior (creep rate directly proportional to stress) was observed during decreasing stress at temperatures between 1200° and 1300°C. This resulted in anomalous behavior at low strains in pseudo stress-strain curves generated from the stress relaxation data. However, the otherwise systematic rate dependence provides a possible basis for design in terms of a secant modulus analysis. The anelastic, recoverable component of creep may lead to complex deformation history-dependent phenomena.

Notes: Continuous fiber ceramic composites (CFCCs) based on oxides are of interest for high-temperature applications owing to their inherent oxidative stability. An enabling element is a matrix with an optimum combination of toughness and strength, which may be achieved by incorporating a controlled amount of fine, well-distributed porosity. Implementation of this concept by vacuum infiltration of aqueous mullite-alumina slurries into two-dimensional woven preforms of alumina fibers has been investigated. Evaluation of these materials shows stress-strain characteristics similar to other CFCCs, especially carbon-matrix composites. Moreover, promising notch and creep properties have been found. Microstructural and processing issues relevant to the attainment of these behaviors are discussed.

Notes: BaTiO3 is widely used as the dielectric in ceramic chip capacitors and multilayer capacitors, because of its high dielectric constant and ferroelectric properties. Multilayer capacitors provide fairly high capacitance per unit volume (volumetric efficiency); however, processing difficulties in the preparation of ultrathin layers limit further enhancement. Tantalum solid electrolytic capacitors, on the other hand, provide very high volumetric efficiencies, because of the large surface area of the sintered, porous tantalum anode on which the dielectric Ta2O5 is electrochemically deposited. Recent developments in electrochemical methods to deposit BaTiO3 on titanium substrates provide an opportunity to fabricate barium titanate electrolytic capacitors using sintered, porous titanium anodes. The high dielectric constant of BaTiO3 and the high surface area of the sintered, porous anode provide a good combination to achieve larger volumetric efficiencies. Current work involves the fabrication and characterization of barium titanate electrolytic capacitors. Effects of electrochemical processing parameters on the formation of BaTiO3 on the surface of sintered titanium anodes are described. Influence of the purity of titanium powder, the porosity of the sintered anode, and the post-deposition heat treatment on the dielectric properties of the fabricated capacitors is discussed. Complete penetration of the electrolyte solution and a thin uniform coating of TaTiO3 over the entire titanium surface was achieved using high-porosity (35%-40% of theoretical density) sintered titanium anodes. Samples treated for 8 h in 0.5M Ba(OH)28H2O electrolyte solutions at 100°C with an applied cell voltage of 12 V show the formation of a dense, uniform BaTiO3 coating on the surface of the titanium anode. High-purity, chloride-free titanium powder provides smaller dissipation factors at low frequencies. Heat treatment at 400°C significantly increases the capacitance at all frequencies, whereas the heat treatment lowers the dissipation factors at low frequencies. Calculated volumetric efficiencies are comparable to those typically obtained for tantalum solid electrolytic capacitors but are not as high as expected for barium titanate electrolytic capacitors. Penetration of the colloidal-carbon (external) electrode was limited to a depth of ~300 µm, which might have caused the lower volumetric efficiencies.

Notes: Sintering of In2O3 has been carried out in air to full density. Because of the difference in densification between the agglomerates and the matrix, large interagglomerate pores were observed to form at the initial stage of sintering. Such pore formation could be prevented by applying a small external pressure which resulted in the beneficial rearrangement of agglomerates.

Notes: We propose a flow method to produce barium hexaferrite (BaO6Fe2O3) particles with hydrothermal crystallization in supercritical water. Aqueous iron(III) and barium nitrate solution at room temperature was pressurized to 30 MPa and then mixed with potassium hydroxide solution (OH:NO3 = 4) at the same conditions to generate metal hydroxides. This mixture was then rapidly heated to 400°C by mixing with supercritical water and then fed into a tubular reactor. Residence time was ~1 min. The reaction was terminated by cooling at the exit of the reactor. The Ba:Fe mole ratio was varied over a range of 0.1-2. When the Ba:Fe ratio was ~1/12, which is the stoichiometric ratio for BaO6Fe2O3, the main products were alpha-Fe2O3. However, for the case of Ba:Fe 〉 0.5, fine particles of single-phase BaO6Fe2O3 were produced. Batch experiments (380°C, 30 MPa) at Ba:Fe = 0.5 in supercritical water at a reaction time of 10 min produced a mixture of alpha-Fe2O3 and BaO6Fe2O3. This product transformed to the equilibrium phase, BaO2Fe2O3, in 4 h as the reaction time increased, which suggests that the BaO6Fe2O3 that formed in supercritical water with our proposed flow method under nonstoichiometric conditions was an intermediate but stable product. Furthermore, the nonstoichiometric and nonequilibrium (dynamic) conditions are important for producing single-phase BaO6Fe2O3 particles. The single-phase particles are highly stable and can be produced continuously in a reaction time of 〈1 min.

Notes: The piezoelectric properties of Pb[Zr0.45Ti0.5-xLux(Mn1/3-Sb2/3)0.05]O3 ceramics, with 0 lessthan equal to x lessthan equal to 0.03, have been investigated. The partial substitution of Ti4+ with Lu3+ permitted improvement of the electromechanical coupling factor (kp), the dielectric constant (epsilonT33), and the piezoelectric constant (d33), while the dielectric loss (tan delta) increased and the mechanical quality factor (Qm) decreased with an increase of x. A pertinent piezoelectric material for actuator applications was Pb[Zr0.45Ti0.48Lu0.02(Mn1/3Sb2/3)0.05]O3, and the piezoelectric properties were kp = (58.5 ± 0.5)%, epsilonT33 = 32 ± 25, d33 = (373 ± 6) 10-12 C/N, Qm = 714 ± 22, and tan delta = (0.98 ± 0.03)%.

Notes: In the soda-lime-silica glass family, the effect of each constituent of the composition on the brittleness was first investigated. Vickers indentation was employed to estimate the brittleness (ratio of harness (H) to fracture toughness (Kc)) by measuring the C/a ratios (where C and a are the characteristic crack and indentation diagonal lengths, respectively). It was observed that a higher silica content and a lower lime content helped to lower the brittleness. Substitution of potash and magnesia for soda and calcia, respectively, was effective in lowering the brittleness. From these results, a higher molar volume was found to be a key factor for reducing the brittleness. A new low-brittleness glass was then developed with a brittleness as low as 5.1 µm-1/2 as compared with the brittleness of 7.1 µm-1/2 for commercial soda-lime-silica glass. The crack initiation load (P*), measured by the Vickers indentation method, for this new low-brittleness glass was almost 10 times as high as P* of commercial soda-lime-silica glass. The new glass shows lower hardness and higher fracture toughness than the commercial soda-lime-silica glass.

Notes: Intermetallic/ceramic composites represent an interesting class of materials for high-temperature structural and functional applications. These materials can be prepared via high-energy milling of pure metals with Al2O3 as well as of aluminum with metal oxides. During subsequent compaction via pressureless sintering, the components react to form dense composites that consist of interpenetrating networks of the ceramic and intermetallic phases. Microstructural investigations, mechanical properties, and resistivity and wear resistance measurements of selected composites are presented. Improved fracture toughness and bending strength, with respect to monolithic Al2O3, have been achieved.

Notes: A new preparation method for a highly sinterable yttria powder has been developed, the resultant powder characterized, and its sinterability studied. As a precursor of the yttria powder, a fine and needle-shaped yttrium carbonate was prepared by a precipitation and aging method. A fine and low-agglomeration yttria powder was obtained by calcining the carbonate precursor at 1100°C. The primary crystallites measured ∼0.1 μm and weakly agglomerated to a size of ∼0.3 μm. The powder had a very high sinterability, so that the powder compact could be sintered to transparency by normal sintering under vacuum without additives at low temperatures, over 1600°C. The sintered transparent ceramics exhibited a homogeneous microstructure with no abnormal grain growth.

Notes: Glasses of composition (x - 1)PbO(100 - x)GeO21Ln2O3, with x= 30 mol% (Ln = Nd, Eu, Er), 40 mol% (Ln = Pr, Nd, Sm, Eu, Dy, Ho, Er, Tm), and 50 mol% (Ln = Eu, Er), have been prepared by quenching the oxidic melts. From the optical absorption and emission spectra in the ultraviolet-visible-near-infrared (UV-VIS-NIR) region, the intensity parameters, spontaneous emission probabilities, branching ratios, radiative lifetimes, and, for selected NIR transitions, peak stimulated emission cross sections have been obtained. The trends observed in the intensity parameters have been discussed, as a function of the number of f electrons as well as a function of the lead content. As the amount of lead increases, the covalency of the Ln-O bond increases, the symmetry of the rare-earth site increases, and the dopant site distribution narrows. The peak stimulated emission cross sections rank among the highest found for oxide glasses.

Notes: The role of starting powder in the resistance of silicon nitride (Si3 N4) ceramics to strength-impairing contact damage is studied. Si2N4 materials are prepared from three starting powders, at selectively increasing hot-pressing temperatures to coarsen the microstructures: (i) from relatively coarse α-phase powder, essentially equiaxed α-Si3N4 grains, with limited, slow transformation to β-Si3N4 grains toward the latter half of the temperature range; (ii) from relatively fine α-phase powder, a more rapid transfor-mation to β-Si3N4, with attendant grain elongation; (iii) from fine β-phase powder, an essentially equiaxed β-Si3N4 structure over the entire temperature range. The resulting microstructures thereby provide a spectrum of β/α phase ratios and grain sizes and shapes for investigation. Indentations with hard spheres (Hertzian test) are used to induce damage into the Si3N4 specimens. Examination of the indentation sites indicates a competition between brittle and quasi-plastic damage modes: in structures with relatively equiaxed grains, the damage takes the form of classical cone cracking; in structures with large elongate grains, the damage is distributed beneath the contact as grain-localized microfailures within a subsurface “yield” zone. Bend tests on specimens containing single-cycle contact damage reveal those structures with well-developed cone cracks to be highly susceptible to strength degradation. The microstructures with the greatest resistance to strength loss are those formed from fine α-phase starting powder at intermediate firing temperatures (1700°C), with characteristic large, elongate β grains. Implications of the results in the context of other mechanical properties, e.g., toughness, wear, and fatigue resistance, are discussed.

Notes: Quasi-ternary phase diagrams of the NdO1.5-BaO-CuOx system near the CuOx corner have been constructed near the peritectic temperature in air of the Nd1+xBa2−xCu3O6+delta (Nd123) solid-solution phase. Liquidus curves were determined by measuring the temperature dependences of the neodymium, barium, and copper solubilities in Nd-Ba-Cu-O solutions with different BaO:CuO ratios. Solidus line compositions and equilibrium tie lines were determined by analyzing the compositions of the Nd123 solid solution equilibrated with the melt by quenching samples held isothermally. Based on the tie-line features in the Nd123-and- liquid two-phase field, the Nd123 solid solution with the smaller substitution content was observed to be equilibrated with the solution melt with a higher BaO:CuO ratio, even in an air atmosphere. Nd123 crystal with a substitution content of ∼0.02 could be formed from the solution with the BaO:CuO ratio of greaterthan equal to0.75, which resulted in higher critical superconducting transition temperatures.

Notes: The stress rupture characteristics of an enhanced Nicalon/SiC composite at 900°C have been examined. This temperature has been identified as being in the regime wherein oxidation embrittlement is operative. The enhancement of the composite involves the use of a coating around the fiber tows, comprising a C-rich matrix and B-containing particulates. The efficacy of this oxidation protection scheme has been evaluated by comparing the stress rupture characteristics with those of both Nicalon/SiC composites without the enhancement and the fibers alone. Such comparisons indicate that a substantial portion of the strength loss is attributable to a degradation of the fibers, and that the performance of the enhanced material is marginally better than that of the reference (nonenhanced) composite. Moreover, at stress levels greater than the matrix cracking limit, oxidation embrittlement occurs rapidly and the rupture times (several hours) are short in relation to the targeted service lives of most ceramic composite components. The mechanisms associated with the embrittlement have been identified using scanning electron microscopy and Auger spectroscopy.

Notes: Different chem-adsorbed silane molecules have been used to produce weakly attractive silicon nitride particle networks for aqueous colloidal processing. Silanes with diamino and poly(ethylene glycol) hydrophilic heads yielded slurries with the lowest viscosity, longest sedimentation stability, and highest packing density. Chem-adsorbed silane molecules protected silicon nitride and yttrium oxide, a common processing aid, from hydrolysis at pHs between 5.5 and 11. A novel approach was used to produce short-range repulsive potentials necessary to yield the weakly attractive networks. Addition of salt to dispersed silicon nitride slurries with particles coated with poly(ethylene glycol)-silane caused the collapse of the 22-atom-long chains and residual electrical double layer. This produced a weakly attractive network which persisted during consolidation to yield a plastic body with a flow stress that was dependent on the counterion size. When 0.5M tetramethylammonium chloride was used at pH 10, plastic bodies had a flow stress similar to clay, whereas lithium counterions produced bodies with a much higher flow stress.

Notes: Si3N4 ceramics reinforced with SiC platelets were fabricated by hot pressing at 1800°C. The microstructure of the Si3N4 matrix itself was the same with or without the addition of the SiC platelets. However, the mechanical properties of the Si3N4 were changed remarkably by the SiC addition. The fracture toughness and the crack resistance with crack propagation (R-curve behavior) were improved while the fracture strength was decreased slightly by the platelets. Improvement in crack resistance was attributed to the extensive interaction of cracks with the platelets. The reduction in strength, on the other hand, is believed to be due to cracks associated with weak platelet-matrix interfaces.

Notes: This paper focused on the effects of various phases of SiO2 additives on the γ-Al2O3-to-α-Al2O3 phase transition. In the differential thermal analysis, the exothermic peak temperature that corresponded to the theta-to-α phase transition was elevated by adding amorphous SiO2, such as fumed silica and silica gel obtained from the hydrolysis of tetraethyl orthosilicate. In contrast, the peak temperature was reduced by adding crystalline SiO2, such as quartz and cristobalite. Amorphous SiO2 was considered to retard the γ-to-α phase transition by preventing γ-Al2O3 particles from coming into contact and suppressing heterogeneous nucleation on the γ-Al2O3 surface. On the other hand, crystalline SiO2 accelerated the α-Al2O3 transition; thus, this SiO2 may be considered to act as heterogeneous nucleation sites. The structural difference among the various SiO2 additives, especially amorphous and crystalline phases, largely influenced the temperature of γ-Al2O3-to-α-Al2O3 phase transition.

Notes: Xenotime-type RPO4 (R = Y, Er, Yb, or Lu) powder was dry-pressed into disks and bars. The disks and bars could be sintered to a relative density of greaterthan equal to98% in air without cracking at 1300° (R = Yb or Lu) or 1500°C (R = Y or Er), depending on the grain size. The linear thermal expansion coefficient (at 1000°C), thermal conductivity (at 20°C), and bending strength (at 20°C) of the xenotime-type RPO4 ceramics were 6.2 10-6/°C, 12.02 W(mK)-1, and 95 ± 29 MPa for R = Y; 6.0 10-6/°C, 12.01 W(mK)-1, and 100 ± 21 MPa for R = Er; 6.0 10-6/°C, 11.71 W(mK)-1, and 135 ± 34 MPa for R = Yb; and 6.2 10-6/°C, 11.97 W(mK)-1, and 155 ± 25 MPa for R = Lu. The xenotime-type RPO4 ceramics did not react with SiO2, TiO2, Al2O3, ZrO2, or ZrSiO4, even at 1600°C for 3 h in air, and were stable in aqueous solutions of HCl, H2SO4, HNO3, NaOH, and NH4OH at 20°C.

Notes: The effects of Ni3Al and Al2O3 additions on the mechanical properties of hydroxyapatite (HAp) were investigated. The addition of Ni3Al particles increased the strength as well as the fracture toughness of HAp. However, the improvements in the properties were limited because of the formation of microcracks around the metal particles. The microcracks were formed because of the large difference in the coefficients of thermal expansion between HAp and Ni3Al, and because of the relatively large size of Ni3Al particles (∼20 µm). The addition of submicrometer Al2O3 powder was also effective in increasing the mechanical properties. The flexural strength and the fracture toughness were increased from about 100 MPa and 0.7 MPam1/2, respectively, to 200 MPa and 1.5 MPam1/2 by the addition of 20 vol% Al2O3. When Ni3Al and Al2O3 were added together, the fracture toughness was further increased to 2.3 MPam1/2. This increase in the fracture toughness was attributed to the synergistic effect of matrix strengthening and crack interactions with the metal particles.

Notes: The synthesis of a nickel/yttria-stabilized zirconia (Ni/YSZ) cermet with tailored microstructural characteristics has much interest for many solid-state electrochemical applications. These materials are generally obtained through a route based on the reduction, via hydrogen gas, of poorly sintered mixtures of YSZ with nickel oxide. The present paper proposes an alternative route based on a thermite reaction performed in a combustion regime. Such an approach indicates the application of combustion synthesis or a self-propagating high-temperature synthesis (SHS) technique. The more-interesting features of this route include the absence of any external source of energy during the reaction, the possibility to synthesize a porous cermet in a single step, its sintering, and the doping of the ceramic component with yttria. The synthesis of Ni/YSZ cermets through this route is analyzed in some detail, and this analysis shows the influence of the process parameters on the phase composition and microstructure of the products.

Notes: The environmental stability of uncoated and boron nitride-coated (BN-coated) Nicalon fiber has been investigated by studying the effect of annealing in air at 1000°C (2 h) on the strength of the fibers. The results imply that uncoated and BN-coated fibers both degrade in strength, with the BN-coated fiber suffering a higher strength loss. The degradation is significantly enhanced if the fibers are exposed to salt (NaCl) water prior to the air anneal, if the concentration of salt is 〉0.5 wt%. The BN-coated Nicalon fibers also have been studied at 800° and 900°C; the degradation in strength due to salt water exposure is greater at 800°C than that at 900° or 1000°C.

Notes: Quantitative texture analysis, which included calculation of the orientation distribution function, was used to demonstrate textures in hot-pressed and subsequently annealed silicon carbide (SiC). The results indicated that the hot pressing and annealing could produce strong textures in SiC. Grain rotation during hot pressing and preferred grain growth of plate-shaped α-SiC grains during annealing both apparently contributed to texture development in the SiC materials. The {111} pole figure in hot-pressed material (mostly ß-SiC) and the (004) pole figure in annealed material (mostly α-SiC) were consistent with the microstructural observations. The fracture toughness of hot-pressed and annealed material measured parallel to the hot-pressing direction (5.7 MPam1/2) was higher than that measured perpendicular to the hot-pressing direction (4.4 MPam1/2), because of the texture and the microstructural anisotropy.

Notes: Niobium monocarbide (NbC) whiskers were successfully synthesized by heating mixtures of niobium oxide (Nb2O5) powder and carbon black in an argon atmosphere to temperatures in excess of 1100°C. The whiskers were mainly cuboid or cycloid needles with a square-pyramid-shaped tip and were 0.1-2.0 µm in width and 5-100 µm in length. The particular shape of the whiskers and the very low impurity levels detected suggest that the NbC whiskers grew via a solid-vapor growth process.

Notes: The fabrication of dense Si3N4/SiC nanocomposite materials that contained 2.5-30 wt% SiC via gas-pressure sintering and hot pressing was investigated. The SiC particles originated from admixed commercial SiC powders, SiCN powders produced by plasma synthesis, in situ reaction pyrolysis of carbon-coated Si3N4 particles, and pyrolysis of a polycarbosilazane-based SiCN precursor. Based on thermodynamic calculations, criteria for minimum liquid-phase decomposition during sintering were developed. The best sintering results were obtained for sintering cycles that observed this criteria. Materials that contained plasma-synthesized SiCN exhibited high strengths (835-995 MPa) and fracture toughness values (7.4-7.8 MPam1/2) at room temperature. Post-sintering thermal treatments led to a strength reduction.

Notes: This investigation is devoted to grain boundaries, since, for high-frequency applications, eddy-current losses can be minimized by selectively increasing the resistivity of grain boundaries. Two specific commercial Mn-Zn ferrites were considered. The first one, selected to be a reference sample, exhibited good permeability, whereas the second one was chosen to promote a strong segregation of calcium on the grain boundaries via the addition of CaO and SiO2. To explain the behavior of this material chemical, crystallographical and electrical properties were collated. Initially, maps of potential barriers were recorded on a scale representative of the bulk microstructure by voltage contrast via scanning electron microscopy at 1 keV. In addition to the potential maps, the local electrical potential was measured quantitatively via local probing. It was evidenced that there was no percolation path through weak barriers at the grain boundaries. To gain information on very small voltage drops, in particular within grains, the microelectrode technique was used on the same areas. It showed a very different behavior between the two samples for the intragranular resistivity, as well as for interfacial barriers. Then, the crystallochemistry of the grain boundaries was investigated via analytical conventional transmission electron microscopy and scanning transmission electron microscopy techniques, and the specific segregation of calcium and the depletion of Fe2+ versus Fe3+ ions at general grain boundaries (GBs) was evidenced. The chemistry and microstructure of the grain boundaries were related to their crystallography. By local electrical probing via transmission electron microscopy, the interfacial barriers were directly related to the type of grain boundary. It was shown that high electrical barriers correspond to high-energy GBs, where there are no orientation relationships between adjacent grains; on the other hand, low-energy GBs, which present orientation relationships and/or dense interfacial planes, are characterized by low barriers.

Notes: Densification of alumina coated with a MgO-Al2O3-SiO2 (MAS) glass was investigated from 1400° to 1460°C. Spinel was observed to form at the liquid-alumina interface, whereas mullite crystallized uniformly in the liquid. Spinel and mullite crystallization kinetics were accelerated by smaller alumina particle size. Mullite and spinel crystallization retarded densification by forming a percolating network. Boron doping suppressed spinel formation, and thus alumina sintered to higher densities at low temperature. The concept of glass basicity is proposed as a useful guide for selecting dopants for low-temperature sintering.

Notes: Low-temperature-sinterable (Zr0.8Sn0.2)TiO4 powders were prepared using 3 mol% Zn(NO3)2 additive and then compared with powders prepared using 3 mol% ZnO additive and no additives. Sintering at 1200°C for 2 h produced a dielectric ceramic with ρ= 98.6% of theoretical density (TD), ɛr= 38.4, Q×f (GHz) = 42000, and τf=−1 ppm/°C. Sintering at 1250°C resulted in an excellent dielectric with ρ= 99% of TD, epsilonr= 40.9, Q×f (GHz) = 49000, and τf=−2 ppm/°C. Scanning electron microscopy showed a microstructure with grains measuring 0.5 to 1 μm, and transmission electron microscopy revealed secondary phase in the grain boundaries.

Notes: The room-temperature tensile strengths of chemically-vapor-deposited SCS-6 silicon carbide fibers were measured after 1 to 400 h heat treatments in 0.1 MPa of argon at temperatures up to 2100°C. The fibers heat-treated for 1 h above 1400°C and those heat-treated for 400 h above 1300°C showed strength degradation. Scanning and transmission electron microscopic examination of the degraded fibers showed formation of a recrystallization region within the outer zone of the SiC sheath and the growth of SiC particles in the carbon-rich surface coating. The activation energy for the growth of the recrystallization region was ∼370 kJ/mol. The tensile strength of the fibers was found to vary as an inverse function of the recrystallized zone thickness.

Notes: As a step toward creating a chemical vapor deposition (CVD) process for PbTiO3 thin films, lead oxide films were deposited and then examined. The reaction was oxidation controlled, with an apparent activation energy of 97 kJ/mol in this low-temperature, low-pressure metalorganic CVD (MOCVD) process. Across the deposition parameters examined, several distinct types of morphology were observed. Growth occurred as a combination of layer-on-layer and island formation. The structural and chemical properties of the lead oxide were examined by Auger electron spectroscopy, Raman spectroscopy, X-ray diffraction, scanning electron microscopy, and electron diffraction. Various forms of lead oxide were produced (litharge, massicot, and scrutinyite, singly or in combination with each other). The deposition parameters used in this work showed a tendency to maintain the same crystalline form from the initial nucleation stages through post-deposition annealing. Lead oxide formed readily on SiO2 surfaces (contrary to studies by other researchers) and, indeed, reacted with the underlying SiO2 layer.

Notes: SrNb2O6 and BaNb2O6 were selected as the precursor powders for the reaction sintering of strontium barium niobate of the composition SrxBa1−xNb2O6 (x= 0.5 and 0.6) (SBN). In comparing the densification behavior of the reaction sintering and the normal sintering of SBN, it was observed that the densification rate of the former was slower during reaction but became faster after reaction. It was also found that the chemical reaction occurred prior to densification, but a high-density and uniform microstructure could be obtained for the reaction-sintered sample. The densification rate of the reaction-sintered sample was pronouncedly enhanced when the reaction was almost completed. We also proposed the reason for the enhancement of the densification rate of the reaction-sintered sample when the reaction was almost completed. A high-density (〉98% of the theoretical density), uniform, and fine-grained (3-4 µm) SBN microstructure could be achieved by using reaction sintering.

Notes: The working zone of a sea urchin tooth is a ceramic-fiber-reinforced ceramic-matrix composite. It is composed of reinforcing calcitic fibers, a matrix of high-magnesium-containing calcite crystals, and organic material. It is much harder and tougher than most other calcitic materials. Fracture toughness and microhardness have been studied using microindentation. A comparison of fracture morphologies between intact teeth and teeth from which the organic component has been removed reveals that the thin organic sheath at the fiber/matrix boundary has an important contribution to the fiber pull-out fracture behavior and, hence, to the toughness of the teeth.

Notes: Mullite/Y-TZP composites in the mullite-rich region, using powders of mullite prepared via the hydrazine method and a commercially available zirconia compound (ZrO2(2Y)), have been fabricated by sintering for 3 h at 1450°C in air. The grain sizes of mullite and ZrO2(2Y) are changed with increased ZrO2(2Y) content; the former decreases from 0.25 µm to 0.13 µm, and the latter increases from 0.21 µm to 0.35 µm. High strength (780 MPa) and fracture toughness (5.4 MPam1/2) are obtained in the 50/50 (vol%) mullite/ZrO2(2Y) composite with 99.6% of theoretical density.

Notes: Single-phase perovskites were formed in the (1−x)Ba(Zn1/3Nb2/3)O3-(x)La(Zn2/3Nb1/3)O3 system for compositions with 0.0≤x≤0.6. Although the stability of the trigonal “1:2” ordered structure of the Ba(Zn1/3Nb2/3)O3 end member is very limited (0.0≤x≤0.05), low levels of lanthanum induce a transformation to a cubic, “1:1” ordered structure that has a broad range of homogeneity (0.05≤x≤0.6). Samples with x 〉 0.6 were comprised of La3NbO7, ZnO, and a perovskite with x = 0.6. The cubic 1:1 phases were fully ordered and no evidence was found for a compositionally segregated microstructure. These observations could not be reconciled in terms of a “space-charge” model; rather, they supported a charge-balanced, “random-site” structure for the 1:1 cation-ordered Ba(β1/2′β1/2″)O3 phases.

Notes: An analysis of transverse cracks induced in brittle coatings on soft substrates by spherical indenters is developed. The transverse cracks are essentially axisymmetric and geometrically conelike, with variant forms dependent on the location of initiation: outer cracks that initiate at the top surface outside the contact and propagate downward; inner cracks that initiate at the coating/substrate interface beneath the contact and propagate upward; intermediate cracks that initiate within the coating and propagate in both directions. Bilayers consisting of hard silicon nitride (coating) on a composite underlayer of silicon nitride with boron nitride platelets (substrate), with strong interfacial bonding to minimize delamination, are used as a model test system for Hertzian testing. Test variables investigated are contact load, coating/substrate elastic-plastic mismatch (controlled by substrate boron nitride content), and coating thickness. Initiation of the transverse coating cracks occurs at lower critical loads, and shifts from the surface to the interface, with increasing elastic-plastic mismatch and decreasing coating thickness. This shift is accompanied by increasing quasi-plasticity in the substrate. Once initiated, the cracks pop in and arrest within the coating, becoming highly stabilized and insensitive to further increases in contact load, or even to coating toughness. A finite element analysis of the stress fields in the loaded layer systems enables a direct correlation between the damage patterns and the stress distributions: between the transverse cracks and the tensile (and compressive) stresses; and between the subsurface yield zones and the shear stresses. Implications of these conclusions concerning the design of coating systems for damage tolerance are discussed.

Notes: Thermal reactions of magnesium compounds and kaolinite were investigated to obtain dense cordierite ceramics without additives. Magnesium hydroxide was precipitated from aqueous solution in the form of ultrafine hexagonal tabular particles of about 0.1 µm, and heating this mixture with kaolinite in a mole ratio of MgOAl2O32SiO2 resulted in formation of an amorphous state at about 900°C following thermal decomposition. µ-Cordierite was then crystallized from the amorphous phase at about 950°C, and gradually transformed into alpha-cordierite. Firing the pressed specimens yielded a dense alpha-cordierite ceramic with a relative density of 97.7% at 1350°C.

Notes: Using microwave electron-cyclotron-resonance plasma-enhanced chemical vapor deposition, diamond-like carbon films were directly grown at low temperatures (lessthan equal to400°C) on Fe-based alloy substrates without diamond seeding or use of a template layer. A single, broad line in the Raman spectra was observed in the region of 1328-1335 cm-1 for films grown in gas mixtures with a ratio of CH4:H2 greaterthan equal to 2%. In contrast, disordered carbon and graphite phases appeared in the spectra for film grown with a concentration of 20% CH4 in hydrogen. Diamond nucleation with an amorphous carbon layer was observed in the initial growth stage, while many diamond particles with irregular morphological features were observed on the surface of thicker films. These growth features are a consequence of the catalytic nature of the Fe-based substrate.

Notes: Microfabrication by coextrusion is a novel ceramic processing technique capable of creating complex ceramic-metal structures. These structures are fabricated by multiple pass coextrusion of a feedrod comprised of several powder-filled thermoplastic compounds. The compounds contain either ceramic, metal, or fugitive powders. To illustrate the capabilities of microfabrication, a demonstration part containing lead manganese niobate-lead titanate ceramic and silver palladium was fabricated. The final part was microconfigured, with a fenestrated structure containing 3110 repeat units per square centimeter. The repeat unit feature sizes were 15 and 5 µm for the ceramic and electrode, respectively. Microfabrication by coextrusion is proposed as a fabrication technique for the production of smart structures and materials.

Notes: Real and imaginary permittivity (ɛ′ and ɛ″, respectively) values were determined on mullite-silicon carbide whisker (mullite-SiCw) composites, mullite-ZrO2 composites, and spinel-SiCw composites, in the frequency range of 2-18 GHz, using a 14 mm coaxial air-line measurement system. Depending on the frequency, the addition of 30 vol% of SiCw to mullite increases ɛ′ from 6.6-6.9 to 16-22 and increases ɛ′ from 0 to 16-22. The addition of 32 vol% of SiCw to spinel increases ɛ′ from 7.5-7.8 to 37-44 and increases ɛ″ from 0 to 13-20. The addition of 21 vol% of ZrO2 to mullite increases ɛ″ to ∼9 but does not change ɛ″. Real and imaginary permeability values were 1 and 0, which are within experimental error.

Notes: Spin-polarized first-principles calculations based on density functional theory were performed for 23- and 101-atom embedded clusters representing annite. Hyperfine interactions and magnetic moments were obtained and compared with experimental Mössbauer spectra and SQUID magnetometry measurements. It was found that the electric field gradients and the magnetic fields are profoundly affected by the position of the hydroxyls. The calculated magnetic moments are close to 4 μB in both octahedral sites.

Notes: The Potts Monte Carlo simulation was used to simulate microstructural evolution in uranium dioxide fuel rods. During service, grain growth, pore migration, and thermal segregation of the pores and UO2 occur in the rods in a thermal gradient. In this investigation, we developed a model which simulates simultaneous grain growth, pore migration, and thermal segregation of the pores and UO2 in a temperature gradient. Grain growth in a thermal gradient was simulated using the Monte Carlo Potts model technique developed by Anderson, Srolovitz, and co-workers. Pore migration was simulated using conserved dynamics with minimum-energy exchanges at a finite temperature. A temperature gradient was introduced into the model via interfacial mobility gradient. Finally, thermal segregation of the pores and UO2 was achieved by introducing a heat of migration term, ΔEt, which biased the motion of porosity to the high-temperature region. The development of this model is described and the incorporation of the proper physics of pore migration and thermal segregation is discussed.

Notes: The macroscopic fracture response of real materials originates from the competition and interplay of several atomic-scale mechanisms of decohesion and shear, such as inter-planar cleavage and dislocation nucleation and motion. These phenomena involve processes over a wide range of length scales, from the atomic to the macroscopic. We briefly review the attempts to span these length scales in dislocation and fracture modeling by (1) fully atomistic large-scale simulations of millions of atoms or more, approaching the continuum limit from the “bottom-up”; (2) directly coupling atomic-scale simulations and continuum mechanics, in a “top-down” approach; and (3) by defining a set of variables common to atomistic simulations and continuum mechanics and feeding the results of atomistic simulations into continuum-mechanics models in the form of constitutive relations. For this latter approach we discuss in detail the issues crucial to ensuring the consistency of the atomistic results and continuum mechanics. A case study of the constitutive-relation approach is presented for the problem of dislocation nucleation from a crack tip in a crystal under stress; a comparison of the results of atom-istic simulations to the Peierls–Nabarro continuum model is made.

Notes: The Zener pinning effect with growing second-phase particles in Al2O3-ZrO2 composite systems were studied by two-dimensional (2-D) computer simulations using a diffuse-interface field model. In these systems, all second-phase particles are distributed at grain corners and boundaries. The second-phase particles grow continuously, and the motion of grain boundaries of the matrix phase is pinned by the second-phase particles which coarsen through the Ostwald ripening mechanism, i.e., long-range diffusion. It is shown that both matrix grains and second-phase particles grow following the power-growth law, Rtm - R0m = kt with m = 3. It is found that the mean size of the matrix phase (D) depends linearly on the mean size of the second-phase particles (r) for all volume fractions of second phase from 10% to 40%, which agrees well with experimental results. It is shown that D/r is proportional to the volume fraction of the second phase (f) as f−1/2 for a volume fraction less than 30%, which agrees with Hillert and Srolovitz's predictions for 2-D systems, while experimental results from 2-D cross sections of three-dimensional (3-D) Al2O3-rich systems showed that either a f−1/2 or a f−1/3 relation might be possible. It is also found that D/r is not proportional to f−1/3 and f−1 in 2-D simulations, which suggests that the Zener pinning effect can be very different in 2-D and 3-D systems.

Notes: The influence of adsorption coverage and free polymer on the rheological properties of aqueous alumina suspensions with polyelectrolyte was studied. The flow curves of the suspensions followed the Casson model very well in the observed range of shear rate. The Casson yield value, τc, was used to evaluate the stability. The coverage exerted a profound influence on the rheological properties by affecting the interaction between particles. The zeta potential increased and the value of τc decreased as the coverage increased. The free polymer could cause weak flocculation as its concentration was increased to a certain level. The opposite effects of the adsorbed polymer and free polymer on stability resulted in the presence of optimum coverage for stabilization. The concentration of free polymer increased rapidly as the adsorption coverage approached the saturation limit, and then the free polymer started to dominate the stability. Stabilization could be achieved at a condition of unsaturated adsorption and was related to the non-high-affinity adsorption at alkaline pH (9.2). The impact of adsorption affinity on stability was discussed.

Notes: A series of low-toxicity gelcasting systems has been developed. The reagents used in these systems have very low acute toxicity. The new systems perform at least as well as, and in some cases better than, the original acrylamide-based system. The development of these systems is described herein, including the search for new gel compositions, the study of suspensions made with the new gel precursor solutions, and pyrolysis of the dried gels and gelcast parts. Applications of the new gelcasting systems include complex silicon nitride parts, large-diameter rings, rapid prototyping by green machining, and metal-powder gel casting.

Notes: The chemical equilibrium between the C-S-H gel phase and the aqueous phase in the CaO-SiO2-H2O system was modeled using the free-energy-based Thermo-Calc commercial software system. Unlike programs that are based on solubility products, the minimization of free energy allows the composition and solubility of the C-S-H phase to vary, as in real cementitious systems. The C-S-H gel is modeled as a defect tobermorite structure, based on recent compositional data gathered using 29Si NMR. The composition is varied by removal of some of the bridging silica tetrahedra in the SiO layers and by incorporating different amounts of Ca in the interlayer spaces.

Notes: The electrodynamic balance (EDB) is demonstrated to be useful for the in situ study of aqueous droplet solidification. We have used an EDB to study the solidification of solution droplets used for ceramic powder synthesis by spray pyrolysis. The mass change and the elastic light scattering of the drying 20 μm solution droplets were monitored. We found that aqueous MgCl2 droplets crystallize but Mg(COOCH3)2, MgSO4, ZrO(OH)Cl, and Zr(COOCH3)4 droplets appear to form gels at high concentrations. As a result of the gel formation, the rate of loss of water from the droplets can be reduced by approximately 150 times and drying is limited by solute diffusion in the gel.

Notes: The structures of compositions across the Ba(Mg1/3Nb2/3)-O3-BaZrO3 (BMN–BZ) system have been examined using X-ray diffractometry and transmission electron microscopy, and their dielectric properties have been characterized in the microwave range. Although pure BMN adopts a 1:2 ordered structure, of space group Pm〈inlineGraphic alt="inline image" href="urn:x-wiley:00027820:JACE670:JACE_670_mu1" location="equation/JACE_670_mu1.gif"/〉1, additions of 5–15 mol% BZ stabilize a cubic (Fm〈inlineGraphic alt="inline image" href="urn:x-wiley:00027820:JACE670:JACE_670_mu1" location="equation/JACE_670_mu1.gif"/〉m), 1:1 ordered phase with a doubled perovskite repeat. At higher levels of substitution (〉25 mol% BZ), the B-site cations are disordered. After normal sintering, the niobates in the 1:1 phase field are comprised of nanometer-sized ordered domains that are dispersed in a disordered matrix. However, by reducing the cooling rate to 10°C/h, a fully ordered microstructure is formed with domain sizes 〉100 nm in size. The structure of the 1:1 phases has been interpreted using a “random-layer” model, in which one site is occupied by niobium, and the second is occupied by a random distribution of the remaining cations. The addition of small concentrations of BZ produces a 100% improvement in the dielectric-loss properties of BMN, and a Q·f value of 82000 is obtained for a 5 mol% substitution.

Notes: Electrical properties of semiconducting ZnO(n)-CuO(p) ceramic composites with varying composition were investigated. The electrical conductivity increased with increasing CuO volume fraction between 1 to 95 mol%. Impedance response showed three semicircles, indicating three resistive elements contributing to the total resistance of the composite. A new model based on the equivalent circuits was developed to explain the contribution of grain boundaries to the resistance of the composite. The change of electrical conductivity was explained by the probability change of two equivalent circuits.

Notes: Mechanical properties of silicon nitride which was superplastically deformed in plane-strain compression were investigated. Superplastically deformed silicon nitride exhibited a highly anisotropic microstructure, where rod-shaped grains tended to be aligned along the extruding direction. The bending strength and fracture toughness were increased substantially by the deformation process when a stress was applied in the extruding direction. It appears that these improvements were mainly due to effective operation of grain bridging and pull-out by the grain alignment.

Notes: Dimensionless parameters have been developed to study microstructural pathways for the sintering of powders. These parameters are designed to facilitate the comparison of microstructural paths for any sintering experiments, independent of the characteristic length scales in the microstructures. Microstructural pathways constructed with the dimensionless parameters are found to be similar for three different alumina ceramics. The systematic differences between the experimental results and the predictions of models based on simplifying geometric assumptions are explained in terms of the packing disruptions in the green microstructures.

Notes: PbO-BiO1.5-GaO1.5-based glasses are good candidates for optical applications, because of some of their interesting characteristics, such as high refraction indices and high transmission in the ultraviolet (UV), visible (VIS), and infrared (IR) regions. A limited stage in the processing of these glasses is the corrosion that is caused by the melt in all currently used conventional crucibles, such as noble metals (platinum or gold) and Al2O3. The absorption of crucible material by the glass composition may reduce the transmission level, the cutoff in the UV-VIS, and IR regions, and the thermal stability. In this study, a SnO2 crucible has been tested for PbO-BiO1.5-GaO1.5 molten glass. Optical and thermal analyses show, in some cases, advantages over the use of platinum and Al2O3 crucibles. A visible cutoff value of 474 nm has been measured, and a longer melting time (850°C for 4 h) results in a significant reduction of the O-H absorption band at 3.2 μm.

Notes: The electrical and mechanical properties of piezoelectric lead zirconate titanate (PZT) ceramics are strongly influenced by domain-wall motion, which can be tailored via the substitution of ions within the perovskite structure. Different domain mobilities are achieved by lead and/or oxygen vacancies, according to the valency and ionic radius of the dopants. To quantify the surface near domain mobility, hard (silver-doped), soft (lanthanum-doped), and undoped PZT ceramics have been prepared. An applied mechanical stress after sintering causes texturing near the surface, because of the ferroelastic behavior of the 90° domains. The texture is quantified via X-ray diffractometry (XRD) analysis of the tetragonal FT(002)- and FT(200)-peak intensities, using least-squares refinement with Gaussian profile functions. The samples are subsequently annealed to remove the surface texture and again characterized via XRD measurements. However, annealed samples still reveal a preferred domain orientation that can be removed only by a second annealing of the pulverized samples. A comparison of the tetragonal additive systems clearly reveals the greatest domain mobility for lanthanum-doped PZT ceramics, whereas the silver-doped and undoped samples have similar ferroelastic behavior. Furthermore, the surface texture of all the compositions is dependent on the applied mechanical stress and cannot be removed completely by heat treatment.

Notes: Hydroxyapatite (HAp) whiskers have been used to prepare fibrous, porous HAp ceramics. The fibrous microstructure was retained in all cases after hot-pressing the HAp whiskers at 800-900°C (1 h, 30 MPa). The fracture path in the fibrous, porous HAp ceramics was partially intergranular, indicating the occurrence of crack deflection, bridging, and pull-out effects. When the whiskers were hot-pressed at 1000-1100°C (1 h, 30 MPa), only large equiaxed grains were present in the HAp ceramics; thus no toughening occurred. When nonstoichiometric HAp whiskers were used to fabricate a porous body, β-TCP precipitated on the whisker surface without destroying the fibrous microstructure. Sintering the HAp whiskers is thus an easy way for in situ fabrication of HAp/β-TCP fibrous, porous materials with a controlled biodegradation rate.

Notes: The microstructures of Al2O3–SiO2–SiC–C refractory matrices with aluminum, silicon, Si3N4, BN, B2O3, and B4C additives are characterized before and after a crucible slag test, and the phases present are compared to those expected at thermodynamic equilibrium. The carbon content dominates the resistance to CaO–MgO–Al2O3–SiO2 slag penetration, while the viscosity of liquid phases present has a significant influence when the matrix carbon contents are similar. Silicon and Si3N4 additives reduce slag penetration resistance because of indirect oxidation of carbon to form SiC. B4C, in particular, and B2O3 also reduce slag penetration resistance because of formation of a more fluid boron-containing liquid, while aluminum and BN addition have no significant effect. Carbon and BN hardly react with the slag, while SiC partially reacts with it, leading to deposition of carbon as a dense layer. Corundum present in the refractories also readily dissolves in the slag. Microstructurally, slag penetration resistance is associated with the dense carbon layer located at the slag-refractory interface.

Notes: The saturation surface of cassiterite, SnO2, was determined for liquids in the system K2O–Al2O3–SiO2 as a function of bulk composition and temperature. At fixed K2O/Al2O3 cassiterite solubility varies weakly with SiO2 concentration (76 to 84 mol%), temperature (1350° to 1550°C), and log (fO2) (−0.7 to −5.3). Cassiterite solubility is also approximately independent of composition in liquids with molar ratios of K2O/Al2O3 lessthan equal to 1 (peraluminous liquids). As K2O/Al2O3 increases from 1 (peralkaline liquids), however, cassiterite solubility increases steeply and approximately linearly with K2O in excess of Al2O3. It is proposed that potassium in excess of aluminum combines with Sn4+ to form quasi-molecular complexes with an effective stoichiometry of K4SnO4.

Notes: Titanium carbide and composites and functionally graded materials (FGMs) of TiC–xCu were synthesized by an electrothermal combustion (ETC) method. TiC synthesized by ETC showed small amounts of porosity relative to those synthesized by ignition using radiative heating. Composites and FGMs with higher copper content can be synthesized by ETC. In the FGM products a nearly linear change in composition in the graded region was observed in samples with 0 ≤x (wt%) ≤ 75.

Notes: Conventional transmission electron microscopy (TEM), coupled with energy-dispersive X-ray (EDX) analysis, has been used to examine the intermediate phases produced during the transformation of Ba-Al-alpha-Al2O3-SiO2 (cristobalite) precursors into celsian, BaAl2Si2O8. Analyses were conducted on samples that had been exposed to pure, flowing oxygen at peak temperatures of 300°C for 24 h, 650°C for 72 h, or 900°C for 24 h. Particles of Al2O3 retained after the 300-900°C heat treatments were coated with a layer of BaAl2O4. An amorphous, alumina-poor Ba-Al-Si-O (G1) phase was observed in contact with residual SiO2 particles after the 650°C heat treatment. Although not a starting component of the precursor, elemental silicon was also detected after the 300-900°C heat treatments. Silicon particles produced during the 300°C heat treatment were surrounded by an amorphous Ba-Al-Si-O (G2) phase that contained more alumina than the G1 phase. Fine-grained barium orthosilicate (Ba2SiO4) and sanbornite (BaSi2O5) were observed after the 650°C treatment, along with thin, plate-shaped grains of BaAl2Si2O8. The latter two phases were surrounded by an amorphous Ba-Al-Si-O (G3) phase that possessed less alumina than the G2 phase. Smaller platelets of BaAl2Si2O8 were also detected after the 300°C treatment. After annealing at a peak temperature of 1650°C, BaAl2Si2O8 was the only silicate compound detected by TEM. Novel reaction paths to BaAl2Si2O8, which are consistent with the present TEM observations and prior XRD and SEM/EDX analyses (Part I), are discussed.

Notes: Vickers microhardness as a function of the indentation load and of grain orientation was studied in individual grains of a gas pressure sintered (GPS) polycrystalline silicon nitride using indentation loads in the range from 1 to 50 g. The microhardness values are lower than the microhardness of ß-Si3N4 single crystals, which is probably caused by the softening of the silicon nitride lattice by the presence of oxygen and aluminum atoms. The indentation load size effect (ISE) was more evident in the prismatic planes of the grains. The threshold load for indent formation is below 1 g for both basal and prismatic planes of Si3N4 grains in the studied GPS silicon nitride.

Notes: Several commercial silver electrode pastes have been evaluated for use with BiNbO4-based low-firing ceramics. After electrode application and subsequent cofiring, the electrode pattern on the ceramic was no longer visible. A bismuth-rich second phase was present at the electrode/BiNbO4 interface, and silver was detected in the second phase. Systematic X-ray diffractometry analysis of the mixtures between different silver pastes and doped/undoped BiNbO4 revealed an interaction between the silver and the BiNbO4. This reaction was responsible for the disappearance of the silver electrode patterns.

Notes: This paper reports the synthesis of technologically important ferrites such as ZnFe2O4, NiFe2O4, MnFe2O4, and CoFe2O4 by using novel microwave-hydrothermal processing. Nanophase ferrites with high surface areas, in the range of 72-247m2/g, have been synthesized in a matter of a few minutes at temperatures as low as 164°C. The rapid synthesis of nanophase ferrites via an acceleration of reaction rates under microwave-hydrothermal conditions is expected to lead to energy savings.

Notes: SrTiO3 thin films have been prepared on titanium substrates using strontium acetate solutions in newly constructed flow-system equipment by the hydrothermal-electrochemical method. The synthesis parameters (temperature of 120°-200°C and flow rate of 1-40 mL/min) allow fabrication of dense, single-phase films with grain sizes in the range of 80-340 nm by controlling nucleation and/or growth rates. The flow can be closed, enabling easy recycling of the solutions used. This processing route may serve as an inexpensive and environmentally friendly way of fabrication of single-phase SrTiO3 thin films as well as functionally graded ferroelectric materials.

Notes: Phase contrast techniques in the transmission electron microscope are used to measure electrostatic potentials at 24° [001] tilt boundaries in nominally undoped and Nb-doped SrTiO3 bicrystals. All of the boundaries are found to have lower scattering potentials than the surrounding bulk crystal. Origins for the measured changes in potential are investigated through the application of high-resolution transmission electron microscopy, diffuse dark-field imaging, energy dispersive X-ray spectroscopy, and parallel electron energy loss spectroscopy. Significantly, we show that space charge is not the dominant contribution to the potential because the magnitudes of the potentials seen cannot be explained on the basis of ionic space charge theory alone.

Notes: The microstructure and nonlinear current-voltage characteristics of Mn3O4-doped ZnO-V2O5 ceramics, microwave-sintered at 800°-1200°C for 10 min, have been investigated. A high density (96% of the theoretical density) has been achieved. The incorporation of Mn3O4 additives does not significantly alter the densification behavior of the ZnO-V2O5 materials, but rather pronouncedly increases the nonlinear coefficient (α= 23.5) and markedly suppresses their leakage current density (JL= 2.4 10-6 A/cm2). On the other hand, the intrinsic properties of the materials, including the Schottky barrier height (Phib) and the donor density (Nd), are only moderately modified; that is, Phib= 1.16 eV and Nd= 5.4 1017/cm3. X-ray diffractometry analyses and energy-dispersive X-ray microanalyses (via scanning electron microscopy) indicate that the V2O5 species facilitate the densification and the development of microstructure via the formation of a liquid phase (Zn3(VO4)2) along the grain boundaries, whereas the Mn3O4 species markedly enhance the nonohmic behavior of the ZnO-V2O5 materials by forming the surface states along the grain boundaries.

Notes: The crystallinity of multicomponent glass-ceramic mixtures has been analyzed via the Rietveld method, using powder X-ray diffractometry data. The Rietveld technique uses an internal standard but requires no calibration data to determine the total amorphous phase content of a mixture. This method of analyzing the crystallinity of synthetic mixtures gives high-precision results, with an error of generally less than ±3.0%, for several glass-ceramics.

Notes: Oversized dopant ions such as yttrium and lanthanum segregate to grain boundaries and reduce the tensile creep rate of α-Al2 O3 by 2-3 orders of magnitude. One explanation for this behavior is that the oversized segregants give rise to a “site-blocking” effect for grain boundary diffusion. It has also been speculated that the dopant ions modify the grain boundary structure in alumina and reduce the creep rate by promoting the formation of special (e.g., coincidence site lattice (CSL)) grain boundaries. In order to test the latter hypothesis, we have used electron backscattered Kikuchi diffraction to characterize the misorientation and special grain boundary distribution for undoped and 1000-ppm-yttrium-doped alumina. The results show that the grain boundary structure in alumina (as characterized by the frequency of selected CSLs and misorientation distribution) was not significantly changed by the addition of yttrium, indicating that creep retardation results mainly from site-blocking.

Notes: A core-shell structure was observed in SrTiO3 doped with 1.2 mol% of Nb2O5, after sintering in a reducing atmosphere (5H2-95N2) and then in an oxidizing atmosphere (air). In undoped and Al2O3-doped SrTiO3 specimens, no core-shell structure formed after the same sintering treatments as those for SrTiO3 doped with 1.2 mol% of Nb2O5. The measured chemical compositions of the core and shell regions of 1.2-mol%-Nb2O5-doped SrTiO3 grains showed that the Sr/(Ti + Nb) ratio of the shell regions grown in air was ~1% less than that of core regions grown in 5H2-95N2, which was in good agreement with a value predicted by available defect equations. Therefore, the observed core-shell structure is thought to result from the formation of strontium vacancies in an oxidizing atmosphere.

Notes: Vanadyl(2+) hydrogen phosphate hemihydrate (VOHPO40.5H2O, VHP) that had high crystallinity and showed a strong (001) peak in its X-ray diffractometry (XRD) pattern was synthesized via thermal treatment of a mixture of vanadyl(2+) acetylacetonate and triethyl phosphate at 200°C in organic solvents that contained a small amount of water. Scanning electron microscopy observations revealed that the product consisted of thin plates (2-4 µm wide and 〈0.5 µm thick). When the reaction was conducted at 250°C, a VHP sample was formed that exhibited stronger XRD peaks and had a plate width of 〉20 µm. The present VHP sample was topotactically transformed to divanadyl(2+) pyrophosphate, which showed a strong (020) XRD peak, via thermal treatment in a nitrogen atmosphere at 500°C.

Notes: This paper describes a novel way to prepare the ternary phase Ti3SiC2 in a single-step procedure that we call electron-beam-ignited solid-state reaction (EBI-SSR). The preparation route is discussed by means of an isothermal section of the Ti-Si-C phase diagram. Properties such as the Vickers hardness and the electrical resistivity of the resulting samples are presented. Our property data compare well to those that have been published. The main advantages of this preparation method are the controllability of process parameters such as heating rates, temperatures, and times, as well as the short duration of the overall sample preparation. However, a disadvantage is the presence of second phases (typically in amounts of 〈8%) that must be reduced via further optimization of the process.

Notes: The coating of a carbonate-containing hydroxyapatite (HAp) on a nonbioactive collagen membrane via a biomimetic method has been investigated. The collagen membranes were soaked in a simulated body fluid (SBF) solution with and without citric acid, and carbonate-containing HAp formed only in the SBF solution that contained citric acid. The results were explained in terms of the strong chelation ability of citric acid with the calcium ion. Practical application may involve the inclusion of citric acid in the SBF solution to promote the formation of HAp on previously nonbioactive collagen membranes.

Notes: A homogeneous and stable amorphous-type cordierite (2MgO2Al2O35SiO2) powder was prepared by a solution-polymerization route employing a Pechini resin or a poly(vinyl alcohol) (PVA) solution as the polymeric carrier. After calcination at 800°C for 1 h under atmospheric conditions, the bulky precursor changed into a very soft and porous powder. A 30 nm size, amorphous-type cordierite powder was prepared by attrition milling the calcined powder, which was made using a PVA precursor solution. The nano-size powder, which had a high specific surface area of 181 m2/g, was obtained after milling for 〈1 h. The sintered cordierite grains did not show the presence of any amorphous SiO2 phase and had a dense microstructure with a relative density of 99% and a thermal expansion coefficient of 2.1 10-6/°C.

Notes: Green and partially sintered compacts of alpha-Al2O3 powder were made by filtration of aqueous suspensions under three conditions: (i) electrostatic stabilization without any organic additive, (ii) strong flocculation near the isoelectric point without any organic additive, and (iii) weak flocculation by the use of maltodextrin or oxalic acid additives. We evaluated relationships between the macroscopic and interparticle mechanical behavior of these compacts using model correlations with measurements of diametral compression, ultrasonic velocity, and ultrasonic attenuation. Although type iii green specimens were less dense than type i, type iii exhibited significant increases in velocity, macroscopic Young's modulus, interparticle-contact stiffness, and diametral compressive strength, suggesting that the mechanism of stiffening/strengthening entailed interparticle bridging of maltodextrin or oxalic acid. These properties were significantly reduced upon heating type iii specimens to 500°C, suggesting that pyrolysis of surface-adsorbed maltodextrin and oxalic acid may have reduced the interparticle stiffness and strength. In contrast, negligible changes in these properties occurred upon heating type i specimens to the same temperature. Despite small increases in packing density, significant decreases in attenuation and significant increases in velocity, interparticle-contact stiffness, and Young's modulus occurred upon heating all specimens to greaterthan equal to700°C, suggesting the formation of interparticle necks by solid-state sintering.

Notes: Macroporous titania (TiO2) films have been prepared via a sol-gel dip-coating method from a titanium tetraisopropoxide solution that contains poly(ethylene glycol) (PEG). The macroporous morphology-i.e., the size, distribution, and shape of the macropores-is controlled by varying the content and molecular weight of PEG, the withdrawal speed, and the temperature of the dipping solution. The morphology of the TiO2 film is determined by competitive contributions of the following factors: (i) decrease in fluidity, because of the evaporation of solvent; (ii) network formation by polycondensation reactions; and (iii) domain formation during phase separation into gel phases and solvent phases.

Notes: Zinc sulfide (ZnS) powders have been obtained by precipitation from homogeneous solutions of various zinc salt compounds, with S2- as precipitating anion, formed by decomposition of thioacetamide. Spherical particles with a very narrow size distribution can be obtained by controlling the synthesis parameters. The particle sizes are influenced by the nature of the associated anion. For example, nanometer-sized ZnS particles are formed using acetate or acetylacetonate anions under acidic pH conditions, controlled by the addition of acetic acid. Although the nucleation is accelerated by the use of acetic acid, limited particle growth occurs because of the formation of complexes with zinc cations that lowers the concentration of free cations in the solution. Also, the complexing-attachment phenomena of the ZnS particles with acetate and acetylacetonate anions lead to the arrest of particle growth processes. The presence of complexed Zn2+ species in the acidic solution is demonstrated both theoretically, using a model based on the calculations of the solubility isotherms of the soluble species, and by Fourier-transform infrared techniques. The nanostructured ZnS particles may provide a suitable semiconductor nanocluster material for optoelectronic applications as well as a phosphor suitable for application in flat-panel display technology.

Notes: Changes in elastic anisotropy and fracture toughness anisotropy of a lead zirconate titanate (PZT) piezoelectric material as a function of depoling temperature were investigated using ultrasound and Vickers indentation. A gradual transition from anisotropic to isotropic properties occurs with increased depoling temperatures at and above the Curie temperature. Depoling is also evident through changes in domain character revealed by X-ray diffraction. The relationship between the mechanical properties and domain structure variation with respect to different depoling conditions is discussed.

Notes: alpha-Alumina and boehmite particles were synthesized by coprecipitation followed by a hydrothermal treatment. X-ray diffraction (XRD) indicated that alpha-Al2O3 was the major phase and coexisted with 4% of boehmite in the presence of the alpha-Al2O3 seeds. On the other hand, a single boehmite phase was obtained in the absence of the alpha-Al2O3 seed particles. The powder densified in the temperature range from 1050° to 1350°C. High-resolution transmission electron microscopy (HRTEM) showed that the particle size of the synthesized alpha-Al2O3 was 60 nm. The surface area was 245 m2/g.

Notes: A new method to synthesize a hollow structure containing calcium and magnesium oxides is proposed. The method involves three processes: First, magnesium carbonate is formed around needlelike aragonite in an autoclave using urea. Second, the needlelike aragonite is removed with hydrochloric acid, and hollow magnesium carbonate is obtained. Third, the hollow magnesium carbonate is heated to form a product containing magnesium and calcium oxides without its shape being altered.

Notes: The influence of electroless nickel plating on lead-based relaxor ferroelectric multilayer ceramic capacitors (MLCs) was studied by a comparison experiment. It was found that the influence is related to reactions in the electroless plating. It is proposed that adsorbed hydrogen atoms generated in electroless plating can diffuse into the ceramic bodies of MLCs and undergo some reduction reaction with them, resulting in the failure of electroless nickel plating. The implications for the negative influence of electroplating on MLCs and for the degradation in MLCs are also included, in which much attention is paid to the reduction reaction of hydrogen atoms generated by electrolysis of water.

Notes: Nanocomposites with fine, coarse, and bimodal silicon carbide (SiC) particle-size distributions were hot pressed and examined by transmission electron microscopy, scanning electron microscopy, and optical microscopy, as well as by four-point-bend and indentation tests. The finer SiC nanophase was introduced homogeneously by coating a silicon-containing polymer onto the alumina (Al2O3) powder, followed by a pyrolysis procedure; for the coarser SiC, nanophase conventional powder processing was used. Powder- and polymer-processed nanocomposites both had their maximum strengths at 5 vol% of SiC. High-strength nanocomposites that contained a higher volume fraction of SiC could be fabricated when the two methods were combined in a hybrid processing route. The SiC phase in the resulting hybrid materials originated from both the polymer and the SiC powder. The mechanical properties of these materials could be correlated with the fabrication route. Processing-flaw populations and calculated Griffith-flaw sizes were not only smaller, but they were also significantly different in the nanocomposites, in comparison to those in Al2O3 ceramics; this may explain the strength increase in Al2O3/SiC nanocomposite materials.

Notes: The purpose of this investigation was to study the behavior of vacuum-plasma-sprayed hydroxyapatite (VPS-HA) coatings in a defined simulated physiological environment that mimics the conditions that the material experiences after implantation in the human body. Commercially available and clinically used HA coatings on titanium alloy substrates were immersed in an inorganic simulated body fluid (SBF) with ion concentrations that were almost equal to those of human blood plasma and in fetal calf serum (FCS) for time periods of 1, 3, 7, 14, and 28 days. At each time interval, the VPS-HA coatings were analyzed using scanning electron microscopy, X-ray diffractometry, Fourier transform infrared spectroscopy, and inductively coupled plasma-atomic emission spectroscopy. The as-received VPS coatings consisted of HA as the main phase component; in addition, small concentrations of tricalciumphosphate (TCP) were determined to be present. During immersion in SBF, transformation of the peripheral area of the VPS coating to a regular spherical shape was observed. Concentration measurements of the immersion solution indicated that dissolution of the more-soluble TCP started within the first day. At the same time, precipitation of a carbonate-containing, marginally crystalline calcium phosphate with very small coherently scattering domains was observed. In FCS, the dissolution of calcium and phosphorus was observed; however, the precipitation of the new calcium phosphate layer was retarded by the presence of proteins, which indicates their important role in the ion-exchange mechanisms. In both solutions, the coating integrity was not adversely affected, which indicates the high stability of VPS-HA coatings in a simulated physiological environment.

Notes: This paper reports an analytical and experimental investigation of the tensile behavior of a two-dimensional woven carbon-carbon composite with a quasi-isotropic stacking sequence. Specimens in their as-received condition were already saturated with transverse cracks due to thermal stress induced during fabrication. Further cracking under load was not detected. The completely unloaded specimen showed that the slight residual strain tended to increase with applied tensile strain. Damage progression as seen on specimen edges and elastic moduli degradation were recorded. The predicted Young's moduli were in good agreement with the measured data.

Notes: Fine-scale ceramic objects are fabricated by forcing a thermoplastic ceramic extrusion compound through a die with reduction ratio R. Objects with complex shapes are fabricated by assembling an extrusion feedrod from a shaped ceramic compound with space-filling fugitive compound. After each reduction state, R2 extrudates are assembled into a feedrod and extruded again, reducing the size and multiplying the number of shaped objects. Several stages of extrusion produce arrays of objects in the size range of 10 µm.

Notes: Molecular-designed ultrafine mullite precursor powders with a stoichiometric composition were prepared by copolymerization of alkoxides. The precursor powders were calcined in the range from 800° to 1200°C and consolidated by ultra-high-pressure cold isostatic pressing up to 1 GPa. Ultrahigh isostatic pressure of 1 GPa led to a closed packing structure in the green compacts. Interaggregate pores in the green compacts were collapsed by the ultrahigh cold isostatic pressure to reduce the pore size below 6 nm. As a result, the maximum density of the green compacts reached 70% of theoretical. These closely packed green compacts of precursor powders with a stoichiometric composition and calcined at relatively low temperatures could be sintered to 〉95% of theoretical at 1500°C. Relatively low-temperature sintering below the liquid formation temperature resulted in fine microstructure of the resultant mullite ceramic with a grain size below 300 nm.

Notes: The evolution of the phase content in zirconia powders that have been prepared by sol—;gel acid hydrolysis has been investigated using the perturbed-angular-correlation (PAC) technique and X-ray diffractometry. As a consequence of performing annealing treatments at increasing temperatures between room temperature and 1000°C, the amorphous starting material transforms to the tetragonal form and then to the monoclinic form. The metastable tetragonal phase exhibits two hyperfine components, one of which describes very defective zirconium surroundings. The evolution of PAC relative fractions is in agreement with the diffraction results. The durability of the samples in sodium hydroxide seems to increase as the relative amount of the most-defective zirconium surroundings of the tetragonal form increases.

Notes: Morphological changes of bismuth-based cuprate superconductors, such as the 2223 phase, via a small mechanical force were studied mainly by means of high-resolution transmission electron microscopy and atomic force microscopy. Even light mechanical grinding of the sintered 2223 samples led to the formation of superfine particles that had a width of ∼5 nm. Moreover, we found first that the superfine particles had a discrete thickness of odd numbers of a half unit length c/2 of the 2223 phase, e.g. 1.9 nm, 5.6 nm, and so on. The formation of the superfine particles was due to cleavage at Bi—O double layers that were bound weakly to each other in a unit cell.

Notes: Reactions between SiC and mullite in an Ar plasma were investigated using a model composite in which a free-standing CVD SiC coupon was imbedded in mullite cement. After treatment in a radio frequency (RF) plasma, the Si content of the mullite in contact with SiC was found to be less than that in the starting material, and deposits were found on the walls of the plasma chamber due to the reaction of mullite with SiC as follows: Al6Si2O13(s)+ SiC(s)= 3Al2O3(s)+ 3SiO(g)+ CO(g). This reaction, which is endothermic (1405 kJ/mol at 1500 K), absorbs thermal energy and consequently prevents the rapid sintering which is observed for single-phase mullite in similar environments. As a consequence, it is suggested that RF plasma sintering probably cannot be used to densify SiC-reinforced mullite-matrix composites because of the resulting energy consumption and damage to the SiC phase.

Notes: Upconversion fluorescences of the green 4S3/2→4I15/2 and red 4F9/2→4I15/2 transitions of the Er3+ ion are studied for Yb3+- and Er3+-codoped sodium germanate, potassium tantalum gallate, and barium tellurite glasses by InGaAs laser-diode pumping. The phonon energies of the host glasses are determined by infrared-reflection measurements. Compositional effects on the Judd—;Ofelt parameters for the Er3+ ion, the spontaneous emission probability (SPE) of the 2F5/2→2F7/2 transition for the Yb3+ ion, and the phonon energy of the glass network are discussed in terms of glass structure. The factors that affect the upconversion fluorescence intensities of the Er3+ ion are discussed, using the phonon energy of the host glass and the SPE for the Yb3+ ion in the germanate, gallate, and tellurite glasses.

Notes: Consolidating powder compacts to a uniform and high particle packing density is a central aspect of ceramic processing. Experiments on the formation of powder compacts from slurries of differing properties and filter pressed under different conditions are presented. The influence of (1) piston velocity, (2) the solid content of the slurry, and (3) slurry viscosity are discussed. A simplified theory of nonuniform cake growth is developed. This theory is based on the assumption that the cake profile is time independent in a reference plane moving with the cake/slurry interface. Classical theory assumes uniform cake density, while more difficult finite element methods are required for calculating nonuniform cake growth conditions. This theory simplifies nonuniform cake consolidation mechanics and allows simple spread-sheet-type calculations on cake uniformity. Cake uniformity is an important goal in processing. Piston stress-displacement behavior can be reasonably simulated from the fundamental cake properties (permeability and yield curve). The inverse, estimating the fundamental cake properties from piston stress-displacement behavior, appears to provide only rough estimates of these characteristic parameters. The Kozeny-Carman equation appears to reasonably fit the permeability behavior of these filter cakes.

Notes: The oxidation behaviors of monolithic Si3N4 and nanocomposite Si3N4-SiC with Yb2O3 as a sintering aid were investigated. The specimens were exposed to air at temperatures between 1200° and 1500°C for up to 200 h. Parabolic weight gains with respect to exposure time were observed for both specimens. The oxidation products formed on the surface also were similar, i.e., a mixture of crystalline Yb2Si2O7 and SiO2 (cristobalite). However, strength retention after oxidation was much higher for the nanocomposite Si3N4-SiC compared to the monolithic Si3N4. The SiC particles of the nanocomposite at the grain boundary were effective in suppressing the migration of Yb3+ ions from the bulk grain-boundary region to the surface during the oxidation process. As a result, depletion of yttribium ions, which led to the formation of a damaged zone beneath the oxide layer, was prevented.

Notes: Internal-friction data of calcium-doped Si3N4 polycrystalline materials that otherwise contain only SiO2 at grain boundaries were examined and compared with those collected on the same polycrystal in the undoped state or doped with anions (i.e., fluorine and chlorine). Precise microstructural characterizations previously performed on these materials enabled us to quantitatively evaluate the inherent viscosity of the intergranular SiO2 film through the analysis of the anelastic internal-friction-peak components. The intergranular glass viscosity and its scaling with increasing calcium addition followed the same trend as bulk SiO2 glasses with the same chemical composition. Broadening of the internal-friction peak with increased calcium content in the material has also been rationalized according to the reduction of the activation energy for the viscous flow of bulk glasses. The present analysis, which is in agreement with our previous studies on undoped and anion-doped Si3N4, has demonstrated that the overall viscoelastic response of the polycrystal is mainly dictated by the chemistry of the intergranular glass.

Notes: For the present study, 1.1-at.%-neodymium-doped yttrium aluminum garnet (YAG) ceramics with a controlled pore volume (150-930 vol. ppm) were fabricated via a solid-state reaction method using high-purity powders. The scattering coefficients of neodymium-doped YAG (Nd:YAG) ceramics, obtained from Fresnel's equation, increased simply as the pore volume increased. The effective scattering coefficient and the laser-oscillation performance of Nd:YAG ceramics with a pore volume of ∼150 vol. ppm were almost equivalent to those of a 0.9-at.%-Nd:YAG single crystal fabricated via the Czochralski method.

Notes: The microstructure, electrical resistivity, and thermal diffusivity of nickel-zirconia cermets have been studied. Scanning electron microscopy of the fracture surfaces of samples shows that there are some small isolated nickel granules adhering to the surface of the zirconia particles. Considering such a microstructural feature of samples and the law of mixtures, an improved model for electrical and thermal conductivity is established. With this model, the experimental results-that electrical conductivity and thermal diffusivity increase with increasing nickel content-are interpreted. Moreover, the thermal diffusivity is observed to be almost linearly related to temperature.

Notes: This paper discusses the difference between the experimentally observed angle of Hertzian cone cracks and the angle defined by the trajectories of the preexisting stress fields. It is argued that there is no reason why these angles should be the same, as has usually been assumed. A finite element method has been used to model the growth of cracks in the Hertzian stress fields. In this model, the crack is incrementally advanced along the direction of maximum strain energy release, as calculated by the evolving, rather than the preexisting, stress fields. For the modeled Hertzian indentation system, a cone crack is observed to grow, but at an angle which is significantly different from that defined by the normal to the maximum preexisting tensile stress. The angle of the cone crack, as grown in the model, is in excellent agreement with observations on experimentally grown cone cracks in glass, with the same Poisson's ration. It is proposed that, in general, cracks will grow along paths that result in the maximum release of strain energy. For asymmetric, nonuniform preexisting stress fields, such paths do not necessarily coincide with the normal to the maximum preexisting tensile stress.

Notes: Fully dense fine-grained 32.6-vol%-zirconia-toughened alumina composites have been fabricated from nanocrystalline rapidly solidified material. A model considering the thermodynamics of the constrained t-ZrO2m-ZrO2 phase transformation was developed for this percolated two-phase material. This analysis indicated that the grain size at which this phase transformation is thermodynamically favorable was 1.26 µm in a composite that contained 32.6 vol% ZrO2 and was stabilized with 1.50 mol% Y2O3. These results of the model compared favorably with experimental results, showing that grains of this size could be retained after heating to temperatures of as high as 1600°C. The rapidly solidified precursor was ball-milled into submicrometer powder and centrifugally cast into green specimens that were pressureless sintered to full density at temperatures as low as 1500°C. A composite containing nearly 100%t-ZrO2 was produced by pressureless sintering at 1500°C and a composite containing 45 vol%t-ZrO2/55 vol%m-ZrO2 was obtained by sintering at 1600°C. The resulting two-phase microstructures contained uniformly distributed, micrometer-size grains whose sizes are consistent with the facilitation of transformation and microcrack toughening.

Notes: This paper investigates sol-gel processing of three-dimensional (3-D) ceramic-matrix composites, focusing on the effect of addition of particles on drying stresses and composite green density. A 3-D woven carbon fiber/silica composite was used as the model material system for the investigation. Composites were fabricated with silica particle additions to the silica sol using a pressure infiltration technique, and the drying rate and matrix cracking were characterized. The particle size was varied in order to study the role of particle additions in the processing. Ammonium hydroxide (NH4OH) and a particle coating agent, 3-(trimethoxysilyl)propyl methacrylate (TPM), were added in the mixture of particles and sol to improve particle packing in the preform. Sol-gel processing with the addition of particles proves to be an effective route to fabricate 3-D ceramic-matrix composites with higher green density and less matrix cracking.

Notes: Solid solutions of diphosphates of zinc and copper and of zinc and cobalt were synthesized from mixtures of pure diphosphates at temperatures up to 1000°C. Their X-ray diffractometry patterns varied continuously from one end member to the other. Solid solutions of orthophosphates of composition Zn3−xCox(PO4)2, with x= 0.4–1.6, were formed at temperatures up to 950°C; all exhibited the structure of γ-Zn3(PO4)2. Solid solutions of orthophosphates of composition Zn3−xCux(PO4)2 exhibited more-complex behavior. At 1000°C and copper contents of 20–80 mol%, a phase that is related to Cu3(PO4)2, termed here the “ε-phase,” predominated. At 850°–950°C and in the region from 20 mol% to ∼33 mol% of copper, the solid solutions (the “η-phase”) adopted the structure of graftonite. At 800°–900°C and 10–15 mol% of copper, the solid solutions exhibited a new structure (the “δ-phase”), which we found to be related to the mineral sarcopside. At temperatures 〈inlineGraphic alt="geqslant R: gt-or-equal, slanted" extraInfo="nonStandardEntity" href="urn:x-wiley:00027820:JACE252:ges" location="ges.gif"/〉950°C, the solutions that contained 5–15 mol% of copper (the “β-phase”) had the structure of β-Zn3(PO4)2, whereas at 800°–850°C, solutions with 5 mol% of copper (the “-phase”) exhibited the structure of γ-Zn3(PO4)2. Attempts to synthesize Cu+ZnPO4 and Cu+Cu2+Zn3(PO4)3 were unsuccessful.

Notes: Early-stage thermal oxidation (below 1100°C) of carbothermally synthesized β-sialon powder was monitored by X-ray powder diffraction, solid-state 29Si and 27Al MAS NMR spectroscopy, and thermogravimetry. No crystalline oxidation products were detected by XRD but 29Si and 27Al MAS NMR indicated the early formation of amorphous silica, followed by the formation of an amorphous aluminosilicate with an atomic environment similar to that of mullite. The initial oxidation was described by a linear kinetic law with an activation energy of 170 kJmol−1, suggesting the rate-limiting step to be due to dissolution of O2 in an amorphous silica surface layer on the β-sialon particles.