High-Resolution Electron Microscopy of Chemically Vapor-Deposited β-Si3N4-TiN Composites

Microstructures of Si₃N₄-TiN composites prepared by chemical vapor deposition (CVD) were investigated by the multibeam imaging technique using a 1 MV electron microscope. High-resolution images showed a number of fibrous TIN crystallites dispersed in the matrix of CVD β-Si₃N₄. Crystallographic orientation relations between β-Si₃N₄ and TiN were determined directly from the observed images in the subcell scale. The fibrous axis of TiN is parallel to the (110) direction of the NaCl structure and lies along the c axis of the hexagonal β-Si₃N₄ crystal. Domain boundaries, planar faults, nonplanar faults, and dislocations were found in the CVD β-Si₃N₄ matrix near the TiN crystallites. The origin of the structure defects is briefly discussed in connection with the formation of TiN crystallites in the matrix.     

Chemical Vapor Deposition of B13C2 from BCl3-CH4-H2-Argon Mixtures

The deposition of boron carbide (B₁₃C₂) onto graphite substrates was accomplished by using a hot-wall chemical vapor deposition (CVD) reactor at a pressure of 10.1 kPa in the temperature range of 1000°–1400°C. A modified impinging-jet geometry was used to simplify the mass-transfer analysis. Coatings were characterized using X-ray diffractometry (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The surface morphology was composed of well-defined facets, the size of which was dependent on the growth rate and deposition time, as would be expected from a competitive growth mechanism. TEM micrographs of the coating showed long, columnar grains that emanated from a narrow nucleation zone. The growth rate could be adequately described by a first-order kinetic expression, with respect to the bulk gas phase boron chloride (BCl₃) concentration. The activation energy of the kinetic expression was estimated to be 93.1 kJ/mol. It was proposed that the deposition was limited by the adsorption of (BCl₃) onto the substrate surface.     

Oxidation of CVD Si3N4 at 1550° to 1650°C

A thermo gravimetric study of the oxidation behavior of chemically vapor-deposited amorphous and crystalline Si₃N₄ (CVD Si₃N₄) was made in dry oxygen (0.1 MPa) at 1550° to 1650°C. The specimens were prepared under various deposition conditions using a mixture of SiCl₄, NH₃, and H₂ gases. The crystalline CVD Si₃N₄ indicated a parabolic oxidation kinetics over the whole temperature range, whereas the amorphous CVD Si₃N₄ changed from a parabolic to a linear law with increased temperature. The oxidation mechanism is discussed in terms of the activation energy for the oxidation and the microstructure of the formed oxide films.     

11B and 27Al Nuclear Quadrupole Interactions in SiO2-B2O3-Al2O3-R2O Glass Systems

Quadrupole interactions of ¹¹B and ²⁷Al in SiO₂-B₂O₃-Al₂O₃-R₂O glass systems were investigated to determine the structure of these glasses, which should be amenable to chemical strengthening. The ratio of BO₄ units to BO₃ units approached unity as the R₂O/Al₂O₃ ratio for compounds having fixed B₂O₃ contents approached unity. Nuclear quadrupole coupling constants ( e²Qq/h =2.73 to 2.93 MHz) were measured for the NMR spectra of ¹¹B triangles. The line shapes of ²⁷Al spectra varied with chemical composition, but a few glasses exhibited ²⁷Al line shapes similar to those of the AlO₄ triclusters in SiO₂-Al₂O₃-Na₂O glasses. Compositional trends in the formation of BO₄ and AlO₄ were deduced from the NMR spectra.     

Ag-Bi1.5Y0.5O3 Composite Cathode Materials for BaCe0.8Gd0.2O3-Based Solid Oxide Fuel Cells

Ag-Bi_1.5Y_0.5O₃ composites exhibit high electronic and ionic conductivities when the volume fraction of the silver phase is properly chosen, indicating that these composites are candidate cathode materials for solid-state ionic devices. In this study, the chemical and thermal compatibilities between Ag-Bi_1.5Y_0.5O₃ composites and BaCe_0.8Gd_0.2O₃ electrolyte are studied at intermediate temperatures. Furthermore, the electrochemical properties of the interfaces between the composites and the electrolyte are characterized under various conditions using impedance spectroscopy. The stability of the electrodes is estimated from the time dependence of the interfacial resistances. Results indicate that Ag-Bi_1.5Y_0.5O₃ composites are promising cathode materials for solid oxide fuel cells based on BaCe_0.8Gd_0.2O₃ electrolyte at intermediate temperatures.     

17O NMR Spectroscopy of α-TeO2 and Na2TeO3

The synthesis of ¹⁷O-enriched α-tellurite (α-TeO₂) is described. TeO₂ is obtained stochiometrically by the reaction of tellurium isopropoxide with ¹⁷O-enriched H₂O. The reaction is mild and simple and opens the field of tellurite-based ceramics to ¹⁷O nuclear magnetic resonance spectroscopy. We also report the first ¹⁷O nuclear magnetic resonance spectra of α-TeO₂ and Na₂TeO₃. Their isotropic chemical shifts, δ_iso, are 180 ppm and 158 ppm, respectively, and their quadrupolar coupling parameters are C_Q=7.48 MHz and η=0.43 for α-TeO₂, and C_Q=6.63 MHz and η=0.33. By using extended Hückel tight binding calculations, we interpret the chemical shifts of tetravalent inorganic oxide network formers (SiO₂, GeO₂, and TeO₂) in terms of the local paramagnetic shielding effect. We also show that this is the predominant effect in determining the chemical shifts of non-bridging oxide sites in this type of network.     

Microstructural Characterization of ZrO2 Particles Prepared by Reaction of Gaseous ZrCl4 with Fe2O3

The microstructure of ZrO₂ fine particles produced by a novel synthesis method at 450° and 950°C has been studied. The fundamentals of the synthesis method, which involves both chemical and diffusion phenomena, are presented. The method is based on mass transport through the gaseous phase between metallic zirconium and Fe₂O₃ powder. The mass-transporting chemical species are zirconium and iron chlorides. This article focuses on the microstructure and structure of ZrO₂ particles formed by the reaction between gaseous ZrCl₄ and solid Fe₂O₃, which is a relevant reaction step that occurs during the synthesis process. The resulting ZrO₂ crystals grown on Fe₂O₃ particles have been analyzed using transmission electron microscopy. Microstructural characterization has been complemented by X-ray diffractometry analysis. Tetragonal-ZrO₂ is produced at 450°C and monoclinic-ZrO₂ single crystals are produced at 950°C.     

Tentative Phase Relationships in the System CaHfTi2O7-Gd2Ti2O7 with up to 15 mol% Additions of Al2TiO5 and MgTi2O5

The phase relationships in the CaHfTi₂O₇-Gd₂Ti₂O₇ (zirconolite-pyrochlore) pseudobinary system were investigated, after heating at 1500°C, because of their importance in the design of pyrochlore-rich titanate ceramics for immobilization of impure surplus plutonium. Up to 15 mol% of MgTi₂O₅ and Al₂TiO₅ were added to CaHfTi₂O₇-Gd₂Ti₂O₇ compositions to elucidate the effects of divalent and trivalent impurities on the phase stability within these systems. From X-ray diffractometry analysis, scanning electron microscopy, and energy dispersive X-ray spectrometry, phase formation and compositional stability limits were evaluated. The main phases observed in these systems were pyrochlore, perovskite, and polytypes of zirconolite. The formation of the 2 M -, 4 M -, and 3 O -zirconolite polytypes was dependent on the amount of aluminum or magnesium present. In the magnesium system, a large area of pyrochlore-only was observed, which indicated that divalent impurities of appropriate ionic size could be readily incorporated in the eightfold site of the pyrochlore. The locations of the tentative phase boundaries are discussed with respect to the chemical composition.     

Thermodynamic Modeling of the Isomorphous Phase Diagrams in the Al2O3–Cr2O3 and V2O3–Cr2O3 Systems

The phase diagrams in the Al₂O₃–Cr₂O₃ and V₂O₃–Cr₂O₃ systems have been assessed by thermodynamic modeling with existing data from the literature. While the regular and subregular solution models were used in the Al₂O₃–Cr₂O₃ system to represent the Gibbs free energies of the liquid and solid phases, respectively, the regular solution model was applied to both phases in the V₂O₃–Cr₂O₃ system. By using the liquidus, solidus, and/or miscibility gap data, the interaction parameters of the liquid and solid phases were optimized through a multiple linear regression method. The phase diagrams calculated from these models are in good agreement with experimental data. Also, the solid miscibility gap and chemical spinodal in the V₂O₃–Cr₂O₃ system were estimated.     

Optical and NMR Studies on Interdiffusion of Silver in SiO2-B2O3-AI2O3-R2O Glasses

Interdiffusion of silver ions in SiO₂-B₂O₃-AI₂O₃-R₂O glasses where R=Na or K was investigated, using optical transmission, ESR, and wet chemical methods to determine concentration and the chemical state of silver, and NMR spectra as a probe of the glass structure. The concentration of silver introduced by ion exchange increased monotonically, as the line widths of²⁷AI NMR spectra decreased. The sharp and narrow features of ²⁷Al line shapes were broadened and the amount of colloidal silver produced by ion exchange decreased, as R₂O/B₂O₃ approached unity with fixed AI₂O₃. The BO₄ to BO₃ ratio approached unity and the quadrupole coupling constant of BO₃ units varied from 2.70 to 2.96 MH_Z, as R₂O/AI₂O₃, approached unity for fixed B₂O₃. These diverse data suggest a relation between silver diffusion and glass structure, although the phenomena of phase separation and the mixed-alkali effect could also influence silver-colloid formation in the glasses studied.     

Wet Milling of Fe/Al/Al2O3 and Fe2O3/Al/Al2O3 Powder Mixtures

Wet milling of Al₂O₃-aluminide alloy (3A) precursor powders in acetone has been investigated by milling Fe/Al/Al₂O₃ and Fe₂O₃/Al/Al₂O₃ powder mixtures. The influence of the milling process on the physical and chemical properties of the milled powders has been studied. Particle refinement and homogenization were found not to play a dominant role, whereas plastic deformation of the metal particles leads to the formation of dislocations and a highly disarranged polycrystalline structure. Although no chemical reactions among the powder components in Fe₂O₃/Al/Al₂O₃ powder mixtures were observed, the formation of a nanocrystalline, ordered intermetallic FeAl phase in Fe/Al/Al₂O₃ powder mixtures caused by mechanical alloying was detected. Chemical reactions of Fe and Al particle surfaces with the atmosphere and the milling media lead to the formation of highly porous hydroxides on the particle surfaces. Hence the specific surface area of the powders increases, while the powder density decreases during milling. The fraction of Fe oxidized during milling was determined to be 0.13. The fraction of Al oxidized during milling strongly depends on the metal content of the powder mixture. It ranges between 0.4 and 0.8.     

Determination of 1600° and 1700°C. Liquidus Lines in CaO-2Al2O3 and AI2O3 Stability Fields of the System CaO—Al2O3—SiO2

The 1600° and 1700°C. liquidus lines in the CaO·2Al₂O₃ and A1₂O₃ stability fields of the system CaO-Al₂O₃-SiO₂ are determined from the chemical analyses of saturated slags at these temperatures.     

Internal Phase Changes in Dense Si3N4 Associated with High-Temperature Oxidation

The effects of oxidation at 1400°C for 100 h on both surface and internal composition of commercial and laboratory hot-pressed Si₃N₄ with MgO or ZrO₂ additives as well as chemically vapor deposited (CVD) Si₃N₄ were studied using X-ray diffraction. Samples were also compared to the same temperature treatments in Ar. The results indicate the grain boundaries act as rapid diffusion paths for the transport of oxygen.     

Construction of PbZr0.4Ti0.6O3- and Ba0.9Sr0.1TiO3-Based Optical Microcavities by Chemical Solution Deposition

Herein we report on a simple, low cost, and feasible route for the construction of PbZr_0.4Ti_0.6O₃ (PZT)- or Ba_0.9Sr_0.1TiO₃ (BST)-based optical microcavities using a single chemical solution containing polymer polyvinylpyrrolidone. The obtained multilayer systems not only exhibit good ferroelectric performance, but also display well-defined resonant modes with a quality factor of no <66. Compared with PZT microcavities, the optical properties of the BST microcavities appear to be superior.     

Film Synthesis of Y3Al5O12 and Y3Fe5O12 by the Spray–Inductively Coupled Plasma Technique

Thin films of yttrium aluminum garnet (YAG, Y₃Al₅O₁₂) and yttrium iron garnet (YIG, Y₃Fe₅O₁₂) were synthesized on single-crystal Al₂O₃ substrates by a modification of spray pyrolysis using a high-temperature inductively coupled plasma at atmospheric pressure (spray–ICP technique). Using this technique, films could be grown at faster rates (0.12 μm/min for YAG and 0.10 μm/min for YIG) than using chemical vapor deposition (0.005–0.008 μm/min for YAG) or sputtering (0.003–0.005 μm/min for YIG). The films were dense and revealed a preferred orientation of (211). The growth of YIG was accompanied by coprecipitation of α-Fe₂O₃. The coprecipitation, however, could be largely suppressed by preliminary formation of a Y₂O₃ layer on the substrate.     

Devitrification of High-SiO2 Glasses of the System Al2O3-SiO2

Pertinent works of previous investigators are reviewed briefly. A superior technique for the preparation of experimental glasses of high purity and chemical homogeneity is presented. Devitrification growth-rate data and traction-viscosity data are reported for high-silica glasses of the system Al₂O₃-SiO₂. Glasses with added Al₂O₃ were found to devitrify to cristobalite more rapidly than ultrapure silica glasses at a given temperature, and yet their fluidities (i.e., reciprocal viscosities) were found to be less. The devitrification growth rate, g, was found to be of zero order and to follow the temperature dependence In( g/T ) = -(B/T) + In A over the temperature range 1561°K. T 1730°K.     

Structure, Thermal Behavior, Chemical Durability, and Optical Properties of the Na2O–Al2O3–TiO2–Nb2O5–P2O5 Glass System

The FTIR, Raman, UV-Vis, ³¹P MAS-NMR, DTA, and refractive index measurements have been combined to investigate a series of glasses with the general formula 20Na₂O–5Al₂O₃− x TiO₂–(45− x )Nb₂O₅–30P₂O₅, 15≤ x ≤45. The glass structure, as well as thermal, optical, and chemical durability properties, were then described as functions of the f _Nb/ f _Ti ratio. An increase of the f _Nb/ f _Ti ratio correlates with a decrease in length of the average phosphate chains linked through Nb–O–P and Ti–O–P bonds, with an increase in the glass stability and with increase in the linear refractive indices at 632.8 nm from 1.79 to 1.89. Furthermore, niobium is more effective than titanium in improving chemical durability.     

Chemical Vapor Infiltration Rates of ZrO2 into MoSi2 Porous Bodies and Comparison with Model Calculation

Zirconia (ZrO₂) was chemical vapor infiltrated (CVI) into a partially sintered MoSi₂ body (preform) by using zirconium n -propoxide (Zr(OC₃H₇)₄) as a gas precursor. Infiltration distances at different conditions were compared with the calculated results. Chemical vapor deposition (CVD) film growth rates of ZrO₂ were measured, and the data were incorporated into the model calculations. Two models were used to analyze the observed infiltration distances. Initially a conventional model assuming a pore with constant radius (SP model) was used. With this model, it was possible to predict the approximate infiltration distance. However, the model cannot predict pore closure and the infiltration distances for a variety of CVI conditions. Secondly, a newly proposed model (PC model) from a previous paper was applied to calculate the infiltration distance. Using this model, it was possible to predict the occurrence of pore closure or the formation of the deposition layer on the preform surface.     

Synthesis and Characterization of Ti0.33Ta0.33Nb0.33C and Ti0.33Ta0.33Nb0.33CxN1-x Whiskers

Ta_0.33Ti_0.33Nb_0.33C and Ta_0.33Ti_0.33Nb_0.33C _x N_{1− x} whiskers were synthesized via a carbothermal vapor-liquid-solid growth mechanism in the temperature range 900°-1450°C in Ar or N₂. The optimum temperature was 1250°C. Whiskers were obtained in a yield of 70-90 vol%. The whiskers were 0.5–1 µm in diameter and 10–30 µm in length. The starting materials that produced the highest whisker yield were: TiO₂, Ta₂O₅, Nb₂O₅, C, Ni, and NaCl. C was added to reduce the oxides, and Ni to catalyze whisker growth. NaCl was used as a source of Cl for vapor-phase transportation of Ta and Nb oxochlorides and Ti chlorides to the catalyst. The catalyst metal was recycled several times during the synthesis and was transported as NiCl₂( g ) according to thermodynamic calculations. The rate of formation and the chemical composition of the whiskers depended on the synthesis temperature, the choice of catalyst, and the atmosphere. At low temperatures, the whiskers were enriched in Nb and Ta, whereas the Ti content increased with increased synthesis temperature.