Strength of Hot Isostatically Pressed Si3N4 After Heat Treatment in Ar-O2 and H2-H2O

The effects of heat treatment in Ar-O₂ and H₂-H₂O atmospheres on the flexural strength of hot isostatically pressed Si₃N₄ were investigated. Increases in room-temperature strength, to values significantly above that of the aspolished material, were observed when the Si₃N₄ was exposed at 1400°C to (1) H₂ with water vapor pressure ( P _H2O) greater than 1 × 10⁻⁴ MPa or (2) Ar with oxygen partial pressure ( P _O2) of between 7 × 10⁻⁶ and 1.5 × 10⁻⁵ MPa. However, the strength of the material was degraded when the P _H2O in H₂ was lower than 1 × 10⁻⁴ MPa, and essentially unaffected when the P _O2 in Ar was higher than 1.5 × 10⁻⁵ MPa. We suggest that the observed strength increases are the result of strength-limiting surface flaws being healed by a Y₂Si₂O₇ layer formed during exposure.     

Oxygen Non-Stoichiometry and Thermal–Chemical Expansion of Ce0.8Y0.2O1.9−δ Electrolytes by Neutron Diffraction

A dense tubular solid electrolyte with the composition Ce_0.8Y_0.2O_1.9−δ (CY20) was prepared. In situ time-of-flight neutron powder diffraction (TOF-ND) was performed at 900°C in the oxygen partial pressure p O₂ range from 10⁻¹–10⁻¹⁸ atm, and TOF-ND data were analyzed by the Rietveld method. Diffraction data showed that the lattice parameter moderately increased with decreasing p O₂ in the range of p O₂>10⁻¹⁴ atm, while a dramatic expansion (∼0.6%) of the fluorite structure occurred at a p O₂ of 10⁻¹⁸ atm. By refining all reasonable structural parameters, an approximately linear relationship between lattice parameter and oxygen vacancy δ was observed, resulting in ɛ_c/δ=0.08 and corresponding to δ=0.10 at a p O₂ of 10⁻¹⁸ atm, all in agreement with the data published in the literature. The relative change in lattice parameter Δ a / a followed a −1/4 power relation with p O₂ in a low- p O₂ regime. As several (often strongly correlated) structural parameters can affect the intensities in ND profiles, care was taken to select refinement variables. It was found that O atom thermal factors for CY20 increased as the oxygen vacancy concentration and lattice expansion increased.     

Structural Behavior of Oxygen Permeable SrFe0.2Co0.8Ox Ceramic Membranes with and Without pO2 Gradients

The oxygen partial pressure ( p O₂)-dependent structural behaviors of two dense tubular ceramic membranes in composition SrFe_0.2Co_0.8O _x with cubic perovskite structure have been investigated by high-temperature neutron powder diffraction: one in "static" mode and one in simulated-operation mode in which one side of the membrane was exposed to air and the other side to reducing gases with variable p O₂ levels. Rietveld analysis on data collected for the membrane without p O₂ gradients showed that the perovskite is stable in p O₂ down to ∼10⁻¹² atm, and at ∼10⁻¹⁴ atm it starts to decompose into a three-phase mixture containing layered intergrowth Ruddlesden–Popper phases Sr _{n +1}(Fe,Co) _n O _x with n =2 and 3, along with CoO with rocksalt structure. Similar phase evolution was observed when insufficient air flowed on the air side of the membrane exposed to a p O₂ gradient. The data support a nonlinear model of oxygen content in perovskite across the membrane thickness, corresponding to a p O₂ profile that is shallow inside and steep near the reducing side surface. Gas compositions measured with mass spectrometry indicated that oxygen is permeated from the air side to the reducing side of the membrane. The oxygen permeation fluxes at 900°C were estimated to be 0.4–0.9 sccm/cm² for the ∼1 mm thick membrane containing perovskite, depending upon p O₂ gradient.     

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 Oxide Defect Structure on the Electrical Properties of ZrO2

The defect structure of monoclinic ZrO₂ was studied by measuring the transfer numbers and electrical conductivity as functions of O₂ pressure and temperature. The data suggest a defect structure of doubly ionized oxygen vacancies at low pressures, i.e. <10⁻¹⁹ atm, and singly ionized oxygen interstitials at pressures >10⁻⁹ atm. Zirconia is primarily an ionic conductor below #700°C and an electronic conductor at 700° to 1000°C for 10⁻²²≤P_o2≤1 atm.     

Oxidation of HIPed TiC Ceramics in Dry O2, Wet O2, and H2O Atmospheres

Isothermal oxidation of dense TiC ceramics, fabricated by hot-isostatic pressing at 1630°C and 195 MPa, was performed in Ar/O₂ (dry oxidation), Ar/O₂/H₂O (wet oxidation), and Ar/H₂O (H₂O oxidation) at 900°–1200°C. The weight change measurements of the TiC specimen showed that the dry, wet, and H₂O oxidation at 850°–1000°C is represented by a one-dimensional parabolic rate equation, while the oxidation in the three atmospheres at 1100° and 1200°C proceeds linearly. Cross-sectional observation showed that the dry oxidation produces a lamellar TiO₂ scale consisting of many thin layers, about 5 μm thick, containing many pores and large cracks, while H₂O-containing oxidation decreases pores in number and diminishes cracks in scales. Gas evolution of CO₂ and H₂ with weight change measurement was simultaneously followed by heating the TiC to 1400°C in the three atmospheres. Cracking in the TiO₂ scale accompanied CO₂ evolution, and the H₂O-containing oxidation produced a small amount of H₂. A piece of single crystal TiC was oxidized in ¹⁶O₂/H₂¹⁸O to reveal the contribution of O from H₂O to the oxidation of TiC by secondary ion mass spectrometry.     

Use of Stabilized ZrO2 to Measure O2 Permeation

The rate of permeation of CaO-stabilized ZrO₂ (CSZ) by O₂ gas was measured from 640° to 1200°C with the CSZ tubing used simultaneously as the sample and the O₂ pressure detector. The apparent permeation rate depended significantly on the O₂ pressure at the low-pressure side. The rate measured by this method was orders of magnitude smaller than that measured under steady-state conditions, except when the O₂ partial pressure was high (>10⁻⁴ atm), in which case the agreement was good. The difference between steady-state permeability and non-steady-state permeability is related to the deviation in stoichiometry in a sample or detector. The transient response (measured under variable pressure difference) may be very different from steady-state permeation (measured under constant differential pressures across the membranes). To apply CSZ to typical O₂ gas permeability measurements, the O₂ pressure must be kept above ∼ 10^−3.5 atm. In this range, the permeability of CSZ may be regarded as a temperature-dependent material property which is governed by the electron-hole mobility. At lower O₂ pressures the permeation rate is a more complex function of the pressure difference and level.     

Studies on 4CaOAl2.O3.13H2O and the Related Natural Mineral Hydrocalumite

Single-crystal X-ray and electron-diffraction studies show the existence in one polymorph of 4CaO.Al₂O₃. 13H₂O of a hexagonal structural element with α= 5.74 a.u., c = 7.92 a. u. and atomic contents Ca₂(OH)₇- 3H₂O. These structural elements are stacked in a complex way and there are probably two or more poly-types as in SiC or ZnS. Hydrocalumite is closely related to 4CaO.A1₂O₃.13H₂O, from which it is derived by substitution of CO₃^2-for 20H^-+ 3H₂O once in every eight structural elements; similar substitutions explain the existence of compounds of the types 3CaO Al₂O₃.Ca Y ₂- xH₂O and 3CaO Al₂O₃ Ca Y xH₂O. On dehydration, 4CaO.Al₂O₃.13H₂O first loses molecular water and undergoes stacking changes and shrinkage along c. At 150° to 250°C., Ca(OH)₂ and 4CaO.3Al₂O₃.3H₂O are formed and, by 1000°C., CaO and 12CaO.7Al₂O₈. The dehydration of hydrocalumite follows a similar course, but no 4CaO.3Al₂O₃.3H₂O is formed.     

Effect of Fe4+ in the System SrFeO3-SrTiO3

Perovskites of the system SrFeO₃-SrTiO₃ were prepared, and measurements were made of their magnetic and electrical behavior. Chemical analysis showed that the percentage of Fe⁴⁺ varied from 72.5% for SrFeO_2.86 to about zero for Sr(Fe_0.1Ti_0.9)O_2.95; the remainder of the iron was in the Fe³⁺ state and electrical balance was achieved by oxygen loss. Sr(Fe_{1- x} Ti _x )O₃ was antiferromagnetic between x = 0 and x = 0.9, with a Néel temperature below 60°K. A parasitic ferromagnetic component developed when these compounds were cooled in a magnetic field, the magnitude of this component being dependent on the cooling field. The conductivity of these perovskites ranged from 10⁻⁸ ohm⁻¹ cm⁻¹ for x = 1.0 to 10⁻² for x = 0.0 and showed a marked change at x = 0.8. The break corresponded to a change in slope of the lattice parameter and the disappearance of Fe⁴⁺. The Fe⁴⁺ content depended on the heat treatment and atmosphere during formation.     

Constraint of Oxygen Fugacity During Field-Assisted Sintering: TiO2 as a Test Case

Field-assisted sintering exposes samples in a graphite die to reducing conditions. Using TiO₂ as a test case, this work shows that internal redox equlibria in the sample, rather than the graphite–CO–O₂ equilibrium, appear to control the oxygen fugacity. Samples sintered at 1160°C for 20 min are homogeneous in oxygen content and have an average composition of TiO_1.983±0.001. The oxygen fugacity during these sintering experiments is calculated to be about 10⁻¹⁶ atm, which is higher than the value obtained from thermodynamic equilibrium of graphite–CO–O₂ at the given temperature. The oxygen fugacity is similar to that for the quasi-two-phase region, or hysteresis loop, representing the coexistence of reduced rutile with random crystallographic shear (CS) planes and the first ordered CS phase.     

Electrical Properties and Defect Structure of Y2O3

Guarded measurements of the electrical conductivity of high-purity, polycrystalline Y₂O₃ in thermodynamic equilibrium with the gas phase were made under controlled temperature and oxygen partial pressure conditions. Data are presented as isobars from 1200° to 1600°C, and as isotherms from oxygen partial pressures of 10⁻¹ to 10⁻¹⁷ atm. The ionic contribution to the total conductivity, determined by the blocking electrode polarization technique, was less than 1% over the entire range of temperatures and oxygen partial pressures studied. Yttria is shown to be an amphoteric semiconductor with the region of predominant hole conduction shifting to higher pressures at higher temperatures. In the region of p -type conduction, the conductivity is represented by the expression σ= 1.3 × 10³ p O₂^3/16 exp (-1.94/kT). The observed pressure dependence is attributed to the predominance of fully ionized yttrium vacancies. Yttria is shown to be a mixed conductor below 900°C.     

Oxidation/Vaporization Kinetics of Cr2O3

The weight loss of Cr₂O₃ in oxidizing environments (Po₂= 1 to 10⁻³ atm) at 1200°C was measured. Both hot-pressed and sintered Cr₂O₃ pellets were investigated in O₂/Ar gas mixtures, and the dependence of the weight loss on the O₂ partial pressure, the gas flow rate, and the total pressure was determined independently. The experimentally determined O₂ partial pressure dependence (rate ∝ PO₂^3/4) corresponds to that expected for the reaction Cr₂O₃(s)+3/2O₂⇌2CrO₃(g). The flow rate and total pressure dependencies show that mass transport through a gaseous boundary layer is the rate-controlling step in the oxidation/vaporization of Cr₂O₃. Evaporation coefficients for the loss of CrO₃(g) under the experimental conditions were <0.01.     

Thermal Stability of Barium Ferrite (BaFe12O19)

Compositions in the system Fe₂O3-FeO-BaO in the vicinity of the compound BaFe₁₂O₁₉ were studied at temperatures from 1300° to 1550°C and oxygen pressures from 10⁻² to 10² atm. Equilibrium relations involving several barium ferrous ferrites are described. Barium ferrite can be crystallized congruently from the melt at 40 atm oxygen pressure and 15400°C.     

Effect of Hydrogen-Water Atmospheres on Corrosion and Flexural Strength of Sintered α-Silicon Carbide

Sintered α-SiC was exposed for 10 h to H₂ containing various partial pressures of H₂O ( P _H2O from 5×10⁻⁶ to 2×10⁻² atm; 1 atm≅10⁵ Pa) at 1300° and 1400°C. Weight loss, surface morphology, and room-temperature flexural strength were strongly dependent on P _H2O. The strength of the SiC was not significantly affected by exposure to dry H₂ at a P _H2O of 5×10⁻⁶ atm; and following exposure at P _H2O >5×10⁻³ atm, the strength was even higher than that of the as-received material. The increase in strength is thought to be the result of crack blunting associated with SiO₂ formation at crack tips. However, after exposure in an intermediate range of water vapor pressures (1×10⁻⁵< P _H2O <1×10⁻³ atm), significant decreases in strength were observed. At a P _H2O of about 1×10⁻⁴ atm, the flexural strength decreased approximately 30% and 50% after exposure at 1300° and 1400°C, respectively. The decrease in strength is attributed to surface defects caused by corrosion in the form of grain-boundary attack and the formation of pits. The rates of weight loss and microstructural changes on the exposed surfaces correlated well with the observed strength changes.     

The System MgO─P2O5─H2O at 25°C

The phase diagram for the ternary system MgO─P₂O₅─H₂O at 25°C has been constructed. The magnesium phosphates represented are Mg(H₂PO₄)₂· n H₂O ( n = 4, 2, 0), MgHPO₄·3H₂O, and Mg₃(PO₄)₂· m H₂O ( m = 8, 22). Because of the large differences in the solubilities of these compounds, the technique which involves plotting the mole fractions of MgO and P₂O₅ as their 10th roots has been employed. With the exception of MgHPO₄·3H₂O, the magnesium phosphates are incongruently soluble. Because incongruency is associated with a peritectic-like reaction, the phase Mg₂(PO₄)₃· 8H₂O persists metastably for an extended period.     

Effect of Oxygen on the Reaction Between Copper and Sapphire

The effect of oxygen potential on the wetting behavior and interfacial energy between Cu and sapphire was studied using the sessile drop technique in a CO-CO₂ atmosphere. A linear relation was found between γ_SL and log p O₂ (atm) from 10⁻¹⁶ to 10⁻⁵. Beyond 10⁻⁵ atm γ_SL approached a constant value asymptotically. A barrier surface layer was proposed to explain this change. The Gibbs adsorption equation was used to evaluate the characteristics of the interfaces. Formation of a Cu₂O film at the liquid-vapor interface and a CuAlO₂ film at the solid-liquid interface is suggested. The work of adhesion reached a maximum at ∼ 0.01 at.% oxygen, corresponding to p O₂∼ 10⁻⁹atm. Measurements of the basal radius as a function of oxygen content were used to evaluate the role of oxygen in promoting spreading. Spreading on sapphire is directly proportional to the logarithm of oxygen present in the molten Cu drops.     

Kinetics of the UO2-C-N2 Reaction at 1700°C

The mechanism of the reaction of UO₂ with carbon in the presence of N₂ at 1700°C and the rate of formation of the carbonitride product were determined. Uranium carbonitride forms at specific O₂ and N₂ chemical potentials by reactions such as (1) UO₂( s ) + 0.67HCN( g )→UO_1.33N_0.45( s ) + 0.67CO( g ) + 0.11N₂( g ) + 0.335H₂( g ) and (2) UO_1.33N_0.45( s ) + 1.58HCN( g )→UO_0.25N_0.75( s ) + 1.33CO( g ) + 0.79H₂( g ) + 0.64N₂( g ). At P _H2=2×10^-5 atm, HCN formed, permitting a gas-phase transport of reactions not observed in the UO₂-C reaction. Reaction (1) is completed in 0.01 to 0.1 of the time for complete conversion to carbonitride; reaction (2), which proceeds as soon as oxynitride is available, is controlled by solid-state diffusion across the carbonitride layers after they become continuous on the entire specimen. The reaction rates and product compositions depend on the P _N2 and P_CO in the system.     

Final Sintering of Cr2O3

The effect of oxygen activity on the sintering of high-purity Cr₂O₃ is shown. Theoretical density was approached at the equilibrium O₂ partial pressure needed to maintain the Cr₂O₃ phase ( P _o2=2×10⁻¹² atm). The presence of N₂ in the atmosphere during sintering did not prevent final sintering. The addition of 0.1 wt% MgO at this equilibrium pressure effectively controlled the grain growth and further increased the sintered density to very near the theoretical value. The solute segregation of MgO at the grain boundaries, followed by nucleation of spherulites of magnesium chromite spinel on the boundaries, accounted for the grain-growth control. It is speculated that these isolated spherulites locked the grain boundaries together, changing the fracture mode of the sintered oxide from inter-to intragranular and also that larger MgO additions produced a more continuous spinel formation at the boundaries, resulting in decreased sintered density. Weight loss, which was also monitored as a function of O₂ activity, correlated with the changing predominant volatile species in the Cr-O system.     

Hot Isostatic Pressing of Reactive SnO2 Powder

Tin(IV) oxide (SnO₂) crystallizes at room temperature by adding hydrazine monohydrate ((NH₂)₂· H₂O) to a hydrochloric acid solution of tin, followed by washing and drying. Well-densified SnO₂ ceramics (99.8% of theoretical) with an average grain size of 0.9 μm have been fabricated by hot isostatic pressing for 2 h at 900°C and 196 MPa. Their Vickers hardness and bending strength are 14.4 GPa and 200 MPa, respectively. They exhibit an electrical conductivity of 2 × 10⁻³−9 × 10⁻³ S·cm⁻¹ at room temperature.