Chemical Durability of Silicon Oxycarbide Glasses

Silicon oxycarbide (SiOC) glasses with controlled amounts of Si—C bonds and free carbon have been produced via the pyrolysis of suitable preceramic networks. Their chemical durability in alkaline and hydrofluoric solutions has been studied and related to the network structure and microstructure of the glasses. SiOC glasses, because of the character of the Si—C bonds, exhibit greater chemical durability in both environments, compared with silica glass. Microphase separation into silicon carbide (SiC), silica (SiO₂), and carbon, which usually occurs in this system at pyrolysis temperatures of >1000°–1200°C, exerts great influence on the durability of these glasses. The chemical durability decreases as the amount of phase separation increases, because the silica/silicate species (without any carbon substituents) are interconnected and can be easily leached out, in comparison with the SiOC phase, which is resistant to attack by OH⁻ or F⁻ ions.     

High-Resolution Electron Microscopy of Silicon Carbide-Whisker-Reinforced Alumina Composite Interfaces in Specimens Subjected to Elevated Temperatures

Interfaces of silicon carbide-whisker-reinforced alumina (SiC( w )/Al₂O₃) composites were examined using high-resolution electron microscopy (HREM). HREM specimens were prepared from the bulk of samples that were previously tested for fracture toughness at 25°, 1000°, 1200°, or 1400°C, in ambient air. The test temperature history served as an independent variable. It was found that the as-received material did not possess a distinct interfacial layer and that the test temperature history (which included a 30°C/min heating and cooling rate, a 30-min soak prior to specimen loading, and a typical test duration of 5–10 min) did not appreciably change the interface thickness at any of the elevated test temperatures.     

Phase Separation in an SiCO Glass Studied by Transmission Electron Microscopy and Electron Energy-Loss Spectroscopy

SiCO glasses prepared from sol–gel precursors via pyrolysis in argon at temperatures ranging from 1000° to 1400°C were studied by transmission electron microscopy (TEM), in conjunction with electron energy-loss spectroscopy (EELS). EELS analysis showed that stoichiometric SiCO glass underwent phase separation, forming SiO₂- and SiC-based environments. This process started at ∼1200°C. However, at temperatures >1300°C, precipitation of nanometer-sized SiC particles embedded in vitreous SiO₂ was monitored by high-resolution TEM.     

Preparation of Aluminum Nitride–Silicon Carbide Nanocomposite Powder by the Nitridation of Aluminum Silicon Carbide

Aluminum nitride (AlN)–silicon carbide (SiC) nanocomposite powders were prepared by the nitridation of aluminum-silicon carbide (Al₄SiC₄) with the specific surface area of 15.5 m²·g⁻¹. The powders nitrided at and above 1400°C for 3 h contained the 2H-phases which consisted of AlN-rich and SiC-rich phases. The formation of homogeneous solid solution proceeded with increasing nitridation temperature from 1400° up to 1500°C. The specific surface area of the AlN–SiC powder nitrided at 1500°C for 3 h was 19.5 m²·g⁻¹, whereas the primary particle size (assuming spherical particles) was estimated to be ∼100 nm.     

Suppression of Rhombohedral-Phase Appearance and Low-Temperature Sintering of Scandia-Doped Cubic-Zirconia

Inhibition of cubic-rhombohedral phase transformation and low-temperature sintering at 1000°C were achieved for 10-mol%-Sc₂O₃-doped cubic-ZrO₂ by the presence of 1 mol% Bi₂O₃. The powders of 1-mol%-Bi₂O₃–10-mol%-Sc₂O₃-doped ZrO₂ were prepared using a hydrolysis and homogeneous precipitation technique. No trace of rhombohedral-ZrO₂ phase could be detected, even after sintering at 1000°–1400°C. The average grain size of the ZrO₂ sintered at 1200°C was >2 μm because of grain growth in the presence of Bi³⁺. Cubic, stabilized Bi-Sc-doped ZrO₂ sintered at 1200°C had sufficient conductivity at 1000°C (0.33 S/cm) to be used as an electrolyte for a solid-oxide fuel cell (SOFC) and at 800°C (0.12 S/cm) for an intermediate-temperature SOFC.     

Low-Temperature Synthesis of Calcium Hexa-Aluminate

Calcium hexa-aluminate (CaO·6Al₂O₃) has been prepared from calcium nitrate and aluminum sulfate solutions in the temperature range of 1000°–1400°C. A 0.3 mol/L solution of aluminum sulfate was prepared, and calcium nitrate was dissolved in it in a ratio that produced 6 mol of Al₂(SO₄)₃·16H₂O for each mole of Ca(NO₃)₂·4H₂O. It was dried over a hot magnetic stirrer at ∼70°C and fired at 1000°–1400°C for 30–360 min. The phases formed were determined by XRD. It was observed that CaO·Al₂O₃ and CaO·2Al₂O₃ were also formed as reaction intermediates in the reaction mix of CaO·6Al₂O₃. The kinetics of the formation of CaO·6Al₂O₃ have been studied using the phase-boundary-controlled equation 1 − (1 − x )^1/3= K log t and the Arrhenius plot. The activation energy for the low-temperature synthesis of CaO·6Al₂O₃ was 40 kJ/mol.     

Oxidation–Reduction Equilibrium in Copper Phosphate Glass Melted in Air

Copper phosphate glasses with 40, 50, and 60 mol% CuO in batch were melted in air at 1000°, 1100°, and 1200°C using quartz or alumina crucibles, and the [Cu²⁺]/[Cu_total] ratio variations with melting time were measured. Glasses were oxidized during melting and reached equilibrium [Cu²⁺]/[Cu_total] ratios which were independent of melting temperature and identical for the 40 and 50 mol% CuO content glasses. Structural considerations seemed to have determined oxidation-reduction equilibrium rather than an equilibrium redox reaction. Also, the effects of crucible type on the oxidation-reduction balance were examined. It was found that a quartz crucible is more inert and has less effect on the oxidation-reduction equilibrium of glass than an alumina crucible. Crucible contamination and phosphorus vaporization were found to diminish as the CuO content in the batch was increased.     

Phase Evolution in Silicon Carbide–Whisker-Reinforced Mullite/Zirconia Composite during Long-Term Oxidation at 1000° to 1350°C

A composite consisting of 30 wt% SiC whiskers and a mullite-based matrix (mullite–32.4 wt% ZrO₂–2.2 wt% MgO) was isothermally exposed in air at 1000°–1350°C, for up to 1000 h. Microstructural evolution in the oxidized samples was investigated using X-ray diffractometry and analytical transmission electron microscopy. Amorphous SiO₂, formed through the oxidation of SiC whiskers, was devitrified into cristobalite at T ≥ 1200°C and into quartz at 1000°C. At T ≥ 1200°C, the reaction between ZrO₂ and SiO₂ resulted in zircon, and prismatic secondary mullite grains were formed via a solution–reprecipitation mechanism in severely oxidized regions. Ternary compounds, such as sapphirine and cordierite, also were found after long-term exposure at T ≥ 1200°C.     

High-Temperature Chemistry and Oxidation of ZrB2 Ceramics Containing SiC, Si3N4, Ta5Si3, and TaSi2

The effect of Si₃N₄, Ta₅Si₃, and TaSi₂ additions on the oxidation behavior of ZrB₂ was characterized at 1200°–1500°C and compared with both ZrB₂ and ZrB₂/SiC. Significantly improved oxidation resistance of all Si-containing compositions relative to ZrB₂ was a result of the formation of a protective layer of borosilicate glass during exposure to the oxidizing environment. Oxidation resistance of the Si₃N₄-modified ceramics increased with increasing Si₃N₄ content and was further improved by the addition of Cr and Ta diborides. Chromium and tantalum oxides induced phase separation in the borosilicate glass, which lead to an increase in liquidus temperature and viscosity and to a decrease in oxygen diffusivity and of boria evaporation from the glass. All tantalum silicide-containing compositions demonstrated phase separation in the borosilicate glass and higher oxidation resistance than pure ZrB₂, with the effect increasing with temperature. The most oxidation-resistant ceramics contained 15 vol% Ta₅Si₃, 30 vol% TaSi₂, 35 vol% Si₃N₄, or 20 vol% Si₃N₄ with 10 mol% CrB₂. These materials exceeded the oxidation resistance of the ZrB₂/SiC ceramics below 1300°–1400°C. However, the ZrB₂/SiC ceramics showed slightly superior oxidation resistance at 1500°C.     

Phase Equilibria and Physical Properties of Oxygen-Deficient Zirconia and Thoria

The compositions of anion-deficient zirconia and thoria in equilibrium with O₂ were measured from 1 to 10⁻⁶ atm and 1400° to 1900°C; for ZrO_{2- x} (po₂ in atm, and T in °K), log x∼−0.890-[(0.400×10⁴)/ T ]-[(log p )/6]; for ThO_{2- x} , log x∼−1.870-[(0.340×10⁴)/ T ]-[(log p )/6]. The ZrO_{2- x} -Zr boundary was located at x=0.014 at 1800°C; thoria was single-phase over the entire range. Consistent results were obtained when O₂/inert gas mixtures were used, but use of H₂/H₂O and CO/CO₂ at 1000° to 1200°C gave abnormal and, in the latter case, erratic data; side reactions in these atmospheres are inferred. The monoclinic-tetragonal phase change of ZrO₂ and the lattice thermal expansion, room-temperature Young's modulus, and strength properties of ZrO₂ and ThO₂ bodies were not appreciably altered by oxygen deficiency. The lattice dimensions decreased slightly with departure from stoichiometry.     

Static Fatigue Limit for Sintered Silicon Carbide at Elevated Temperatures

The modified static loading technique for estimating static fatigue limits was used to study the effects of oxidation and temperature on the static fatigue limit, K ₁₀ for crack growth in sintered silicon carbide. For as-machined, unoxidized sintered silicon carbide with a static load time of 4 h, K ₁₀× 2.25 MPa * m^1/2 at 1200° and ∼1.75 at 1400°C. On oxidation for 10 h at 1200°C, K ₁₀ drops to ∼1.75 MPam^1/2 at 1200° and ∼1.25 at 1400°C when tested in a nonoxidizing ambient. Similar results were obtained at 1200°C for tests performed in air. A tendency for strengthening below the static fatigue limit appears to result from plastic relaxation of stress in the crack-tip region by viscous deformation involving an oxide grain-boundary phase for oxidized material and, possibly, diffusive creep deformation in the case of unoxidized material.     

Oxidation of Chemically-Vapor-Deposited Silicon Carbide in Carbon Dioxide

Chemically-vapor-deposited silicon carbide (CVD SiC) was oxidized in carbon dioxide (CO₂) at temperatures of 1200–1400°C for times between 96 and 500 h at several gas flow rates. Oxidation weight gains were monitored by thermogravimetric analysis (TGA) and were found to be very small and independent of temperature. Possible rate-limiting kinetic mechanisms are discussed. Passive oxidation of SiC by CO₂ is negligible compared to the rates measured for other oxidants that are also found in combustion environments, oxygen and water vapor.     

Threshold Stress Intensity for Crack Growth in Silicon Carbide Ceramics

Measurements of threshold stress intensities for crack growth, K _h, of three polycrystalline SiC materials were attempted using interrupted static fatigue tests at 1200°–1400°C. Weibull statistics were used to calculate conservative K_th values from test results. The K _th of a chemically vapor deposited β-SiC could not be determined, as a result of its wide variations in strength. The K_th ≥ 3.3,2.2, and 1.7 MPa·m^1/2 for an Al-doped sintered α-SiC; and K_th ≥ 3.1, 2.7, and 2.2 MPa·m^1/2 for a hot isostatically pressed α-SiC, both at 1200°, 1300°, and 1400°C, respectively. A damage process concurrent with subcritical crack growth was apparent for the sintered SiC at 1400°C. The larger K_th 's for the HIPed SiC (compared to the sintered SiC) may be a result of enhanced viscous stress relaxation caused by the higher silica content and smaller grain size of this material. Values measured at 1300° and 1400°C were in good agreement with the K_th's predicted by a diffusive crack growth model, while the measured K_th 's were greater than the predicted ones at 1200°C.     

Structure and Properties of Low Phonon Antimony Glasses in the K2O–B2O3–Sb2O3–ZnO System

The monolithic glass-forming region of the low phonon and low softening point antimony glasses containing high Sb₂O₃ (40–75 mol%) in the novel quaternary K₂O–B₂O₃–Sb₂O₃–ZnO system has been found with the help of X-ray diffraction (XRD) analysis. The structure of a series of glasses with the general composition of (mol%) 15K₂O–15B₂O₃–(70− x )Sb₂O₃– x ZnO (where x =5–25) has been evaluated by infrared reflection spectral (FT-IRRS) analyses. All the glasses are found to possess a low phonon energy of around 600 cm⁻¹, as revealed by FT-IRRS. Their softening point ( T _s), glass transition temperature ( T _g), and coefficient of thermal expansion (CTE) have been found to vary in the ranges of 351°–379°C, 252°–273°C, and 195–218 × 10⁻⁷ K⁻¹, respectively. These properties are found to be controlled by their fundamental property, like the covalent character of the glasses, which is found to increase with an increase in Sb₂O₃ content. In addition, the devitrified glasses have been characterized by XRD and field emission scanning electron microscopy, which manifests the presence of nanozinc antimony oxide crystals with sizes of 21–43 nm. The exhibited properties have revealed that they are a new class of versatile materials.     

Electrical Conduction in Single-Crystal and Polycrystalline Al2O3 at High Temperatures

The electrical conductivity and ion/electron transference numbers in Al₃O₃ were determined in a sample configuration designed to eliminate influences of surface and gas-phase conduction on the bulk behavior. With decreasing O₂ partial pressure over single-crystal Al₂O₃ at 1000° to 1650°C, the conductivity decreased, then remained constant, and finally increased when strongly reducing atmospheres were attained. The intermediate flat region became dominant at the lower temperatures. The emf measurements showed predominantly ionic conduction in the flat region; the electronic conduction state is exhibited in the branches of both ends. In pure O₂ (1 atm) the conductivity above 1400°C was σ≃3×10³ exp (–80 kcal/ RT ) Ω⁻¹ cm⁻¹, which corresponds to electronic conductivity. Below 1400°C, the activation energy was <57 kcal, corresponding to an extrinsic ionic condition. Polycrystalline samples of both undoped hot-pressed Al₂O₃ and MgO-doped Al₂O₃ showed significantly higher conductivity because of additional electronic conduction in the grain boundaries. The gas-phase conduction above 1200°C increased drastically with decreasing O₂ partial pressure (below 10⁻¹⁰ atm).     

Effect of Composition on Glass-Metal Interface Reactions and Adherence

Reduction-oxidation reactions and enhanced wetting or spreading of Na₂Fe_xSi₂O_5+x, and Na₂Ni_xSi₂O_5+x glasses on substrates of Fe, Co, Ni, Ni-Fe, and Ni-Co were observed at 1000°C at low partial pressures of O₂ and Na as the O/Si ratio of the glass increased. When the substrate had a higher oxidation potential than the metal of one of the cations in the glass, e.g. CoO-containing glass on Fe, metallic precipitates formed by redox reactions under all conditions. A redox reaction based on reduction of the valence of a cation in the glass, e.g. Fe³⁺ to Fe²⁺, also occurred. Adherence developed between substrates and glasses containing amounts of substrate oxide, either in the starting composition or formed by redox reactions, approaching saturation.     

Reactions between Hot-Pressed Calcium Hexaluminate and Silicon Carbide in the Presence of Oxygen

The reactions between hot-pressed calcium hexaluminate (CaAl₁₂O₁₉, hibonite) and silicon carbide (SiC) at 1100°-1400°C in air and nominal argon atmospheres were investigated. In inert atmospheres, there was no evidence of reaction at temperatures up to at least 1400°C. In air, the oxidation of SiC produced a layer of silica or a multicomponent amorphous silicate (depending on impurities) that reacted with CaAl₁₂O₁₉. At temperatures below 1300°C, the reaction resulted in the stratification of two distinct interfacial layers: a partially devitrified CaO-Al₂O₃-SiO₂ glass adjacent to SiC and a CaAl₂Si₂O₈ (anorthite) layer adjacent to hibonite. At 1400°C, a large amount of liquid was formed, the majority of which was squeezed out from between the reaction couple. No distinct layer of anorthite was present; instead, the anorthite was replaced by a layer of alumina between the glass-rich layer and hibonite. An activation energy of 290 kJ/mol was determined for the reaction, which is consistant with oxygen diffusion through a calcium aluminosilicate glass. The reaction between rare-earth hexaluminates and SiO₂ was predicted to produce a more-viscous glass than CaAl₁₂O₁₉ and SiO₂ and, therefore, have slower reaction kinetics, because of lower mass transport in the glass.     

In Situ Synthesis of Silicon Carbide Whiskers from Silicon Nitride Powders

Silicon carbide whiskers were synthesized in situ by direct carbothermal reduction of silicon nitride with graphite in an argon atmosphere. Phase evolution study reveals that the formation of β-SiC was initiated at 1400° to 1450°C; above 1650°C silicon was formed when carbon was deficient. Nevertheless, Si₃N₄ could be completely converted to SiC with molar ratio Si₃N₄:C = 1:3 at 1650°C. The morphology of the SiC whiskers is needlelike, with lengths and diameters changing with temperature. SiC fibers were produced on the surface of the sample fired at 1550°C with an average diameter of 0.3 μm. No catalyst was used in the syntheses, which minimizes the amount of impurities in the final products. A reaction mechanism involving the decomposition of silicon nitride has been proposed.     

High-Temperature Proton-Conducting Lanthanum Ortho-Niobate-Based Materials. Part II: Sintering Properties and Solubility of Alkaline Earth Oxides

The sintering properties and microstructure of La_{1− x} A _x NbO₄ powders ( x =0, 0.005, and 0.02 and A=Ca, Sr, and Ba), prepared by spray pyrolysis have been investigated. Dense materials (>97%) were obtained by conventional sintering at 1200°C and by hot pressing (25 MPa) at 1050°C, respectively. Homogeneous materials were obtained and the average grain size obtained by the two densification methods was ∼2.0 and ∼0.4 μm, respectively, for the 2% doped materials. Pure lanthanum ortho-niobate (LaNbO₄) showed a higher degree of grain growth. In the acceptor-doped materials, secondary phases were observed to inhibit grain growth at 1200°C. At 1400°C or higher, molten secondary phases in the Ba-doped materials resulted in severe grain growth, causing microcracking during cooling due to crystallographic anisotropy. A low solubility of AO (A=Ca, Sr, and Ba) in LaNbO₄ is inferred from the presence of secondary phases, and 1 mol% solubility of SrO in LaNbO₄ was found by electron microprobe analysis. The electrical conductivity in wet hydrogen of the materials demonstrated that the main charge carrier was protons up to 1000°C and reached a maximum value of ∼8·10⁻⁴ S/cm at 900°C.     

Effect of Glass Additions on BaO–TiO2–WO3 Microwave Ceramics

The effect of glass addition on the properties of BaO–TiO₂-WO₃ microwave dielectric material N-35, which has Q = 5900 and K = 35 at 7.2 GHz for samples sintered at 1360°C, was investigated. Several glasses including B₂O₃, SiO₂, 5ZnO–2B₂O₃, and nine other commercial glasses were selected for this study. Among these glasses, one with a 5 wt% addition of B₂O₃ to N-35, when sintered at 1200°C, had the best dielectric properties: Q = 8300 and K = 34 at 8.5 GHz. Both Q and K increased with firing temperature as well as with density. The Q of N-35, when sintered with a ZnO–B₂O₃ glass system, showed a sudden drop in the sintering temperature to about 1000°C. The results of XRD, thermal analysis, and scanning electron microscopy indicated that the chemical reaction between the dielectric ceramics and glass had a greater effect on Q than on the density. The effects of the glass content and the mixing process on the densification and microwave dielectric properties are also presented. Ball milling improved the densification and dielectric properties of the N-35 sintered with ZnO–B₂O₃.