Interdiffusion and Association Phenomena in the System NiO-Al2O3

The rate of formation of NiAl₂O₄ by reaction between single crystals of NiO and Al₂O₃ can be described by k = 1.1 × 10⁴ exp (−108,000 ± 5,000/ RT ) cm²/s. In NiO the behavior of D as a function of concentration supports the Lidiard theory of diffusion by impurity-vacancy pairs. A good fit of the theory to the experimental results was obtained by assuming that Al³⁺ ions diffuse as [Al_Ni· V_Ni]'pairs. The diffusion coefficient of pairs, D_p , obeys the equation 6.6 × 10⁻² exp (−54,000 ± 3,000/ RT ) cm²/s. The free energy of association for pairs was calculated to range from 6.5 kcal/mol at 1789°C to 9.0 kcal/mol at 1540°C. The interdiffusion coefficients in the spinel showed a constant small increase with increasing concentration of Al³⁺ dissolved in the spinel.     

Change of Molar Volume and Interdiffusion Coefficients in the System MgO-Cr2O3

The interdiffusion coefficients in the system MgO-Cr₂O₃ were measured in the MgO phase. They are concentration-dependent and increase linearly, below 1600°C, with increasing mole fractions of cation vacancies. Interdiffusion coefficients calculated by the Boltzmann-Matano and Wagner treatments for the system MgO-Cr₂O₃ are compared; some of those obtained by the Boltzmann-Matano treatment are about 70% greater than those obtained by Wagner's analysis. Wagner's treatment gives correct values for the interdiffusion coefficients. The results of the Boltzmann-Matano treatment show the effect of neglecting the molar volume change in the system.     

Grain-Boundary Segregation in MgO-Doped Al2O3

Magnesia-doped alumina was fractured in a high-vacuum Auger electron spectrometer, and the resulting intergranular fracture surface was analyzed for surface composition. Calcium was present on the fracture surface in amounts of ∼2 to 5 at.%. There was no indication of Mg on the fracture surface. The bulk Ca concentration was 5 to 15 ppm, whereas MgO was present at the 1000-ppm level. The presence of excess Ca at the grain boundary can be explained by the fact that its ionic radius is relatively large compared to those of Al and Mg. Sputtering profiles normal to the fracture surface indicate that the segregated Ca is concentrated in a region near tha intergranular fracture surface.     

Grain-Boundary Segregation and Final-Stage Sintering of Y2O3

The addition of ThO₂ to Y₂O₃ inhibits grain growth during sintering and allows the sintering process to proceed to theoretical density by maintaining a high diffusion flux of vacancies from the pores to the grain boundaries. The inhibition of grain growth is accomplished by the segregation of ThO₂ solute at the grain boundaries, causing a decrease in the grain-boundary mobility. The segregation of ThO₂ at the grain boundaries can be inferred from the results of the microhardness and grain-growth studies presented. Further evidence for segregation is provided by quenching experiments and surface activity experiments.     

Measurement of Oxygen Diffusion in Dy2O3 and Gd2O3 by Studying High-Temperature Oxidation of the Metals

Studies of the oxidation of Gd and Dy at P _O2's from 10^−0.3 to 10^−14.5 atm and temperatures from 727° to 1327°C indicate both semiconducting and ionic-conducting domains in the sesquioxides formed. At higher temperatures, where dense coarsegrained oxide layers developed, the rate of oxidation in the high- P ₀₂ semiconducting domain yielded oxygen diffusion coefficients in Dy₂O₃ in excellent agreement with literature values derived from oxidation of partially reduced oxide single crystals. Under the same conditions, the oxidation of Gd yielded oxygen diffusion coefficients in cubic Gd₂O₃ which are considerably below literature values for monoclinic single-crystal Gd₂O₃. At lower temperatures, porous scales were formed, and apparent diffusion coefficients derived from oxidation rates show a smaller temperature dependence than the high-temperature data. At low P _O2, the oxides behave as ionic conductors, and metal oxidation rates result in estimates of the electronic contribution to the electrical conductivity of the order of 10⁻⁶ to 10⁻⁷Ω⁻¹ cm⁻¹.     

Diffusion and Reaction Studies in the System Al2O3-SiO2

High-temperature diffusion kinetics and phase relations between couples of fused SiO₂ or cristobalite and sapphire or mullite were investigated in air and in helium. Subsolidus liquid formation between sapphire and cristobalite indicates the existence of a metastable system without mullite. A liquid phase is considered to be essential for the nucleation of mullite. The growth rate of mullite exceeded its dissolution rate in semi-infinite fused-SiO₂-sapphire couples above 1634°C. The inter-facial liquid compositions provided data for a minor revision of the mullite liquidus curve. Diffusion coefficients calculated from the Al profiles vary greatly with concentration and temperature, resulting in a large range of values for apparent activation energy, which decreases with increasing Al₂O₃ content (∼310 to ∼60 kcal/mol for ∼4 to ∼22 wt% Al₂O₃). The diffusion process in the liquid is considered to be cooperative movement of oxygen-containing Al and Si complexes whose nature changes with composition and temperature; this change in the diffusing species contributes to the range in the values of experimental apparent activation energies.     

Formation of SiO2, Al2O3, and 3Al2O3. 2SiO2 Particles in a Counterflow Diffusion Flame

SiO₂, Al₂O₃, and 3Al₂O₃.2SiO₂ powders were synthesized by combustion of SiCl₄ or/and AlCl₃ using a counterflow diffusion flame. The SiO₂ and Al₂O₃ powders produced under various operation conditions were all amorphous and the particles were in the form of agglomerates of small particles (mostly 20 to 30 nm in diameter). The 3Al₂O₃.2SiO₂ powder produced with a low-temperature flame was also amorphous and had a similar morphology. However, those produced with high-temperature flames had poorly crystallized mullite and spinel structure, and the particles, in addition to agglomerates of small particles (20 to 30 nm in diameter), contained larger, spherical particles 150 to 130 nm in diameter). Laser light scattering and extinction measurements of the particle size and number density distributions in the flame suggested that rapid fusion leading to the formation of the larger, spherical particles occurred in a specific region of the flame.     

Stable and Metastable Equilibria in the Systems Y2O2-Al2O3, and Gd2O3-Fe2O3

The phase equilibrium relations in the systems Y₂O₃-Al₂O₃ and Gd₂O₃-Fe₂O₃ were examined. Each system has two stable binary compounds. A 3:s molar ratio garnet-type compound exists in both systems. The 1:1 distorted perovskite structure is stable in the system Gd₂O₃-Fe₂O₃ but only metastable in the system Y₂O₃-AI₂O₃. This interesting example of metastable formation and persistence of a compound with ions of high Z/r values explains the discrepancies in the literature on the structure of the composition YA1O₃. A new 2:1 molar ratio cubic phase has been found in the system Y₂O₃-A1₂O₃. Since silicon can be completely substituted for aluminum in this compound, the aluminum ions are presumably in fourfold coordination.     

Diffusion in the System K2O-SrO-SO2 I, Effective Binary Diffusion Coefficients

Concentration distributions of potassium and strontium were measured with the electron probe after inter diffusion in the system K₂O-SrO-SiO₂ at 750° C for periods of 4 to 14 hr. The effective binary diffusion coefficients for multicomponent diffusion determined for K₂O and SrO behaved in accordance with Fick's laws. The utility and limitation of these coefficients to describe inter diffusion in condensed multicomponent systems are discussed.     

Discontinuous Dissolution and Grain-Boundary Migration in Al2O3-Fe2O3 by Oxygen Partial Pressure Change

When sintered 95Al₂O₃-5Fe₂O₃ (wt%) specimens constituting corundum grains and iron aluminate spinel precipitates were annealed under high oxygen partial pressure (P_o2) where only a corundum phase is stable, fast dissolution of particulate spinel precipitates occurred, together with the migration of corundum grain boundaries. Behind the migrating boundaries, a corundum solid solution enriched with Fe₂O₃ formed. Discontinuous dissolution (DD) of particulate spinel precipitates thus occurred by P_o2 increase. In contrast, when 95Al₂O₃-5Fe₂O₃ specimens constituting only corundum grains were annealed under low P_o2 where both corundum and spinel phases are stable, grain boundaries migrated without spinel precipitation, leaving behind a corundum phase depleted of Fe₂O₃, similar to chemically induced grain-boundary migration (CIGM) observed during solute depletion. The volatilization of Fe₂O₃ appeared to cause the boundary migration without precipitation. The observed CIGM and DD would suggest various possibilities of microstructure control in other oxide systems through oxygen partial pressure change.     

Phases in the Binary Systems PbO—La2O3, Gd2O3, and Sm2O3

In the binary system PbO–La_zO₃ only one compound, 4PbO.La₂O₃, exists; it is flanked by two eutectics. The structure of the compound, although of lower symmetry, is intimately related to the C modification of the rare earths. Below 800° to 1000°C, metastable solid solutions are formed from oxide mixtures coprecipitated from mixed solutions of the nitrates, the cubic parameter a = 5.66 A, if extrapolated to pure La₂O₃, corresponding to half the a parameter of the C form of La₂O₃. The solid solutions existing between the compositions La₂O₃–2Pb0 and pure La₂O₃ have a cubic face–centered lattice and obey Vegard's rule. The systems of PbO with Sm₂O₃ and Gd₂O₈ are quite similar to that with La₂O₃. The compound Sm₂O₃.4Pb0 decomposes at 1000°C with evaporation of PbO; Sm₂O₃ remains in the B modification.     

Free Volume of Network-Forming Oxide Glasses in the Systems P2O5-(GeO2,TeO2,Sb2O3,V2O5)

The free-volume fraction (V_f) defined by Simha and Boyer was measured for network-forming oxide glasses in the systems P₂O₅-(GeO₂, TeO₂,Sb₂O₃.V₂O₅). The V_f values varied from 0.06 to 0.25. The systems P₂O₅-TeO₂: and P₂O₅-Sb₂O₃ have V_f∼0.1, which is near the magnitude of the free-volume fraction for normal metaphosphate glasses and many organic high polymers.