Swelling of Hot-Pressed A12O3

Fully dense aluminas, prepared by hot-pressing, were found to swell during annealing at 1600°C in air, but not during annealing in a reducing atmosphere (po₂= 10^-7 Pa). The reaction followed the relation p - po = -K log t, where p_o and p are the initial and final densities, respectively, t is the time, and AT is a constant. The rate of swelling was enhanced by MgO solute. The reduction in density resulted from the nucleation and growth of grain-boundary pores. Pore formation was attributed to the reaction of carbon and sulfur impurities at the boundaries with oxygen, which had diffused down the grain boundaries from the ambient, to form CO/CO₂ and SO₂ gas at high pressures. Preliminary results indicate that this reaction can be avoided by preannealing powders in flowing oxygen prior to hot-pressing. The consequences of internal gas-forming reactions to other processes such as high-temperature creep and sintering are also discussed.     

Suspension Processing of Al2O3/SiC Whisker Composites

Homogeneous Al₂O₃ powder/SiC whisker compacts were prepared by suspension processing. By optimizing the conditions for particle/whisker codispersion, castable suspensions could be prepared at total-solids concentrations 50 vol%. Green bodies with high relative density (∼66% to 70%) were obtained with SiC whisker contents in the range of 5 to 30 vol%. Although densification was severely inhibited by the SiC whiskers, significantly higher sintered densities were obtained by suspension processing compared to dry processing.     

Tritium Diffusion in A12O3 and BeO

Tritium diffusion in aluminum and beryllium oxides was studied by recoiling tritium into specimens and measuring the time rate of tritium release during postirradiation heating. Results were consistent with classical diffusion solutions and gave single values for the diffusion activation energy over the temperature range of measurement for single-crystal, sintered, and powdered specimens. Varying amounts of tritium were released as tritiated water even after attempts to remove most of the water absorbed by the specimens. Condensable amounts of tritium ranged from <20% for single-crystal specimens to as high as 90% for some powders. Alumina containing 0.2% MgO gave tritium diffusion coefficients 4 to 5 orders of magnitude higher than those for undoped A1₂O₃, implying a significant impurity effect on tritium diffusion. Measured diffusion coefficients were many orders of magnitude lower than hydrogen diffusion coefficients in metals. When a simple equilibrium permeation model was used which neglected tritium partition effects at interfaces and enhanced diffusion along grain boundaries, it was shown that thin coatings of beryllium or aluminum oxides on metal substrates could markedly reduce tritium permeation rates through metals. The present results may be applied in the future to fusion reactors where tritium inventories will be high and tritium diffusion within the blanket region undesirable.     

Preparation and Sintering Behavior of Fine-Grained A12O3-SiO2 Composite

A fine, uniform A1₂O₃-SiO₂ powder was prepared by heterocoagulation of narrow Al₂O₃ and SiO₂ powders. This composite powder was dispersed, compacted, and fired in air at 900° to 1580°C for 1 to 13 h. Full density was achieved at 1550°C with the formation of a mullite phase. Relative densities of 83% and 98% (0.3 μm grain size) were measured for samples sintered at 1200°C for 13 h and at 1400°C for 1 h, respectively.     

Behavior of Chromium in the System MgA12O4– A12O3

Single crystals of chromium-doped magnesium aluminate spinels with varying amounts of excess alumina have been grown by the flame fusion process. Presumably these crystals contain stoichiometrically significant amounts of defects. When these chromium-doped spinels are annealed at temperatures below their melting point, the spinel structure cannot tolerate such a high concentration of defects and at least some of the excess alumina is exsolved. The chromium ion in magnesium aluminate spinel is pink, whereas in magnesium aluminate spinel containing excess alumina it is green. When exsolution of the alumina occurs during annealing, the aggregate changes to pink. The behavior of the chromium ion is monitored by observing the significant changes in the optical absorption as well as the emission or fluorescence spectra for both single crystals and polycrystalline materials in the system [MgA1₂O₄– A1₂O₃] :Cr.     

Identification of BaO. A12O3.5TiO2 in the Ternary System BaO. A12O3-TiO2

A new identification and indexing for the phase BaAl₂Ti₅O₁₄ were accomplished using an X-ray diffraction technique. The new lattice parameters for the tetragonal lattice structure are: a₀=9.990 × 10^-10 m and c₀=12.264 × 10^-10 m, with a corresponding volume 1.224 × 10^-27 m³. The data provided by the Joint Committee on Powder Diffraction Standards are inconsistent both in lattice parameter values and Miller indices. The X-ray powder diffraction pattern of BaAl₂Ti₅O₁₄ was indexed using the LSUCR (least-squares unit cell refinement) computer program.     

Elastic Properties of Al2O3 and Si3N4 Matrix Composites with SiC Whisker Reinforcement

A study of the elastic moduli of Al₂O₃ and Si₃N₄ ceramics reinforced with 0 to 25 wt% SiC whiskers has been performed. The Young's moduli, shear moduli, and longitudinal modulus are compared with calculated predictions for aligned fiber composites by Hill and Hashin and Rosen, and for fibers randomly oriented in three dimensions by Christensen and Waal. The measured values are in excellent quantitative agreement with those derived for the random orientation of the SiC whiskers.     

Strength Measurements of Monolithic SiC and A12O3/SiC Composites after Exposure to Coal Slag

Four Si-based ceramics—siliconized SiC (Si-SiC), sintered α-SiC, and two grades of SiC-particulate-reinforced A1₂O₃—were exposed to a typical western coal ash with high calcium content (Wyodak) and a typical eastern coal ash rich in iron (Illinois #6) at two temperatures for 300 h in a muffle furnace. After exposure, the coupons were cut into flexure bars and remaining strength was measured in four-point flexure at room temperature. The residual strength was compared to the as-received strength, and the fracture originating flaws were identified. The standard grade of the particulate-reinforced A1₂O₃ showed no corrosive attack under any of the exposure conditions, and a slight strength reduction was attributed to aging effects. The three other materials showed varying degrees of strength reduction, with corresponding fracture-initiating flaws being pits produced during the exposure. A general trend was that the eastern coal seemed to cause more corrosion than the western, and that 1260°C was a harsher exposure condition than 1093°C.     

Surface Tension and Density of Molten A12O3

An X-radiographic technique was developed to obtain profiles of molten ceramic menisci and pendant drops sealed in molybdenum or tungsten capsules. The surface tension for Al₂O₃ was calculated from the shape of the drops or menisci. Molybdenum capsules lowered the surface tension value. The density was 3.01 g/cm⁸ at the melting point.     

Creep Behavior and Grain-Boundary Sliding in Polycrystalline A12O3

The creep properties of polycrystalline A1₂O₃ (grain size 14 to 65 μm) were examined under compressive stresses of between 4,000 and 18,000 psi (27.6 and 124 MPa) in the range 1600° to 1700°C. Two distinct types of behavior were observed. The creep rate of medium-grained specimens (14 to 30 μm) could be described by ασ^1.2 / d² where σ is the applied stress and d is the grain size. These results are consistent with the Nabarro-Herring creep mechanism. For the coarse-grained (65 μm) specimens, the creep rate was related to the stress by ασ^2.6. This behavior was not related to cracking; instead, a dislocation mechanism was thought to be rate-controlling. Considerable evidence for grain-boundary sliding was seen, and measurements showed that grain-boundary sliding contributed between 46 and 77% of the total strain in the 3 medium-grained specimens examined and between 38 and 50% in the 3 coarsegrained specimens examined.     

Effects of Temperature and Whisker Volume Fraction on Average Residual Thermal Strains in a SiC/Al2O3 Composite

Residual thermal strains in a SiC-whisker-reinforced/Al₂O₃-matrix composite were measured as functions of temperature and volume fraction of whiskers by means of a neutron diffraction technique. The residual strains in both phases decrease with increasing temperature. At room temperature, the residual strains in the whiskers decrease with increasing whisker volume fraction, but the tensile residual strains in the matrix increase. The results agree well with those predicted by an analytical method.     

Inclusions of Nanometer-Sized Al2O3 Particles in a Crystalline (Sc,Lu)2(WO4)3 Matrix

Nanometer-sized Al₂O₃ particles were successfully synthesized as crystalline inclusions by mixing both components to form the nanometer-sized particles and the (Sc,Lu)₂(WO₄)₃ matrices in a crystal lattice by preparing a solid solution of (Sc,Lu)₂(WO₄)₃ and Al₂(MoO₄)₃ and then decomposing the solid solution. The particles were dispersed uniformly and without agglomeration, which is commonly observed with conventional preparation techniques. The average particle size of the Al₂O₃ was 3.5 nm, and the standard deviation was estimated to be 1.1 nm.     

X-Ray Study of the Solid Solution of TiO, Fe2O3, and G, O3 in Mullite (3A12O3a2Si02)

A study of the solid solution of TiO₂, Fe₂O₃, and Cr₂0₃ in mullite was made by measuring the changes in lattice parameters and unit-cell volume. Synthetic mullite (3O₃-2SiO₂) was reacted with up to 12 weight % of the oxides at temperatures ranging from 1000° to 17000C. The approximate minimum temperature required for the formation of solid solution was 12000C. for Fe₂0₃ and 1400°C. for Cr₂O₃ and TiO₃. The maximum amount of solid solution found was 2 to 4% TiO₂ at 1600°C., 10 to 12% Fe₂O_s at 1300°C., and 8 to 10% Cr_ZO₃ at 1600OC. Lattice parameters and unit-cell volumes for each solid solution series increased with increasing amounts of foreign oxide. There was good agreement between the calculated and observed increase in cell dimensions for the iron oxide series. Except in the case of titania, there was good agreement between X-ray data and petrographic observations.     

Electrical Conductivity of Polycrystalline A12O3 Doped with Silicon

Direct current conductivity was measured for polycrystalline Alz03 doped with silicon, which is found to act as a single donor, the donor level lying ∼165 kJ/mol (∼1.7 eV) below the conduction band. Silicon in excess of the solubility limit (∼220 ppm at 1500°C, 300 ppm at 1600°C) is present as a glassy aluminosilicate second phase. Silicon dissolved in Al₂O₃ tends to segregate at grain boundaries.     

High-Temperature Strength of AIN Hot-Pressed with A12O3 Additions

The flexural strength of hot-pressed AIN with alumina was investigated at temperatures up to 1500°C. The strength depended on the crystal phases which formed from raw materials in the compact. The hot-pressed specimens consisted of AIN and a pseudopolytype of AIN and showed considerable flexural strength up to 1500°C.     

Compatibility Relations of A1203 in the System Zr02-Al203-Si02-CaO

Compatibility relations of Al₂0₃ in the quaternary system Zr02-Al₂0₃-Si0₂-CaO were studied by firing and quenching followed by microscopy and energy-dispersive X-ray examination. A projection of the boundary surface of the primary crystallization volume of Al₂0₃ was constructed in terms of the CaO, Si0₂, and Zr0₂ contents of the mixtures recalculated to 100 wt%. Two invariant points, where four solids coexist with a liquid phase, are defined, and the positions of the isotherms were established.     

Electrical Conductivity of A12O3: Fe + Y

High-temperature dc electrical conductivity and emf of oxygen concentration cells with A1₂O₃ as the electrolyte were studied. The defect structure of α-Al₂O₃ doped with Fe and Y was investigated to test an explanation proposed for the favorable effect of Y addition to super alloys (Fe, Cr, Ni, Al) which leads to well-adherent and nonconvoluted A1₂O₃ oxide scales. Results indicate that Y is a singly ionizable donor in Al₂O₃ and Y additions are effective in compensating Fe acceptors in A1₂O₃ at iron concentrations up to the solubility limit of Y. At higher acceptor concentrations leading to a decrease in [ V "'_Al] and an increase in [Al^…_i], incorporation of Y leads to a small increase in the concentration of V'" _Al. This suggests that the mechanism proposed for the prevention of oxide scale spallation based on the Y donor action to suppress the bulk diffusion of aluminum interstitials created by the Fe acceptors cannot explain the effect. Energy level positions of Fe and Y in A1₂O₃ are estimated, and values for electron and hole mobility at 1500°C are derived.