Sintering and Grain-Growth Kinetics of MgAl2O4

The sintering and grain-growth kinetics of finely divided MgAl₂O₄ were determined from 1300° to 1600°C for densities up to 96% of theoretical. These results show that sintering is governed by volume diffusion and that the temperature dependence of the diffusivities is 157 exp [(-118 kcal/mol)/ RT ] cm²/s. Grain growth follows the expression ( G ²- G ₀²)= Kt , where K =51.3 exp [(-110 kcal/mol)/ RT ] cm²/s.     

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.     

Oxidation Kinetics of Single-Crystal SrTiO3

The oxidation kinetics were determined for single-crystal SrTiO₃ by measuring the time and temperature dependence of the weight gain of reduced crystals. The oxidation can be described as a diffusion-controlled process. The calculated diffusion coefficients between 850° and 1460°C are represented by D = 0.33 exp (-22.5 ± 5.0 kcal/ RT ) cm²/sec. Directly measured oxygen ion diffusion coefficients in the same temperature interval reported earlier are interpreted as being extrinsic and can be represented by D = 5.2 × 10⁻⁶ exp (-26.1 ± 5.0 kcal/ RT ) cm²/sec, where the activation energy is for mobility only. Assuming that the calculated diffusion coefficients are for vacancy diffusion and the two activation energies are equivalent within experimental error, a vacancy concentration (fraction of vacant lattice sites), [O□], fixed by impurities in the fully oxidized crystal is calculated to be 1.6 × 10⁻⁵ by virtue of the relation between the oxygen self-diffusion coefficient, D_02-, and the oxygen vacancy diffusion coefficient, D_o□ ; D _o2-= [O□] D _o□ where the oxygen ion concentration [O^2-] is taken as unity.     

Solid-State Storage of Radioactive Krypton in a Silica-Based Matrix

A solid silica-based matrix containing 30 cm³ of Kr (STP)/cm³ of glass was prepared by sintering 96% SiO₂ with 28% porosity under 140 MPa krypton pressure. The glass was heated to 850° or 900°C and held at temperature until the glass density was ∼2 g/cm³. At 420°C, only 0.7% of the krypton would be released after one half-life of ⁸⁵Kr (10.7 years). At T>600°C, release of krypton is accompanied by crack development, comminution, and glass softening. Advantages and disadvantages of this technique for radioactive gas storage and diffusion data are presented.     

Cation Interdiffusion and Phase Stability in Polycrystalline Tetragonal Ceria–Zirconia–Hafnia Solid Solution

Zr–Hf interdiffusions were carried out at 1350° to 1520°C for polycrystalline tetragonal solid solutions of 14CeO₂·86(Zr_{1- x} Hf _x )O₂ with X = 0.02 and 0.10. Lattice and grain-boundary interdiffusion parameters were calculated from the concentration distributions by using Oishi and Ichimura's equation. Lattice interdiffusion coefficients were described by D = 3.0 × 10³ exp[-623 (kJ/mol)/ RT ] cm²/s and grain-boundary interdiffusion parameters by δ D ' = 0.29 exp[-506 (kJ/mol)/ RT ] cm³/s. The cation diffusivity was lower than the anion diffusivity. The results were compared with diffusivities in the fluorite-cubic solid solution. The critical grain radii for stabilization of the tetragonal phase in CeO₂-doped ZrO₂ were 11 and 6 μm for the solutions with 2 and 10 mol% HfO₂ substitution, respectively, both of which are much greater than in the Y₂O₃-doped ZrO₂ solid solution.     

Oxygen Self-Diffusion in Single-Crystal Mn-Zn Ferrite

Self-diffusion coefficients for the oxygen ion in single-crystal Mn-Zn ferrite were determined by the gas-solid isotope exchange technique. The oxygen volume diffusion coefficients can be expressed as D =6.70 × 10⁻⁴ exp (-330 (kJ /mol) /RT)m₂/s (>1350°C), D=3.94 × 10⁻¹⁰ exp (−137 (kJ/mol)/RT)m²/s (1100° to 1350°C), and D=7.82 × 10⁴ exp (−507 (kJ/mol)/RT)m²/s (<1100°C).     

Raman Spectrometric Determination of the Oxygen Self-Diffusion Coefficients in Oxides

New methods of determining the oxygen self-diffusion coefficients (D*_o) in oxides have been developed using Raman spectroscopy combined with the ¹⁶O–¹⁸O exchange technique. From the depth-profiles of the ¹⁸O concentration in the ¹⁶O–¹⁸O exchanged oxides, which was measured by Raman microscope with a spatial resolution of 5 μm, D *_o was determined for 2.8 mol% Y₂O₃-containing tetragonal zirconia polycrystall (the depth-profile method). Thus-obtained results are expressed as D *_O,D-P= 1.82(^+0.41_−0.40) × 10⁻¹·exp{−(139.3 ± 0.2) [kJ/mol]/ RT } [cm²/s] in the temperature range of 700–950^°C. We also determined D *_o for the same sample from the Raman spectrometric monitoring of the ambient gas during the ¹⁶O–¹⁸O exchange reaction (the gas-monitoring method). Thus-obtained results are expressed as D *_O,G-M= 1.14(^+0.05_−0.04) × 10⁻² exp{−(117.5 ± 0.4) [kJ/mol]/ RT } [cm²/s] in the temperature range of 700–1165°C. The results obtained from the above two different methods virtually agree with each other, indicating that reliable D *_o can be obtained by either of these two methods. We demonstrate that Raman spectroscopy is a useful tool for determining D *_o in oxides.     

Wetting of Ceramic Oxides by Molten Metals Under Ultrahigh Vacuum

Contact angles, surface free energies, and work of adhesion were determined by a sessile drop technique for the wetting of MgO, Al₂O₃, and SiO₂ by In, Ga, and Sn at 10^-10 torr. The surface free energies of In, Ga, and Sn were 540, 632, and 573 ergs/cm² (±5%), respectively, at their melting points. Works of adhesion and equilibrium contact angles for wetting of MgO by In are 172 ergs/cm² and 133° by Ga, 356 ergs/cm² and 116° by Sn, 278 ergs/cm² and 121°. For wetting of Al₂O₃ by In, they are 237 ergs/cm² and 124° by Ga, 226 ergs/cm² and 130° by Sn, 257 ergs/cm² and 123°. For wetting of SiO₂ by In, they are 208 ergs/cm² and 128° by Ga, 260 ergs/cm² and 126° by Sn, 252 ergs/cm² and 124°.     

Concurrent Measurement of Volume, Grain-Boundary, and Surface Diffusion Coefficients in the System NiO-Al2O3

Surface, grain-boundary, and volume inter diffusion coefficients for the NiO-Al₂O₃ system were measured concurrently by using a diffusion couple consisting of an A1₂O₃ bicrystal and an NiO single crystal. The A1₂O₃ bicrystals having various tilt angles were fabricated by firing 2 single crystals to be joined in an H₂ atmosphere at 1800°C for 30 h. Diffusion profiles over the surface, along the grain boundary, and in the bulk of the bicrystal were determined with an electron probe microanalyzer. Mathematical analysis of the diffusion profiles gives D _s = 7.41×10^-2 exp (-35,200/ RT ), D _gb = 2.14×10^-1 exp (-63,100/ RT ) (tilt angle =30°), and D _v = 1.26×10⁴ exp (-104,000/ RT ). The grain-boundary diffusion coefficient increases with the mismatch at the boundary.     

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).     

Oxygen Self-Diffusion in Magnesium- or Titanium-Doped Alumina Single Crystals

Oxygen Setf-diffusion coefficients have been measured in single crystals of N₂O₃ doped with Mg or Ti under an oxygen partial pressure of 20 kPa in the temperature range 1400° to 1700°C. Diffusion coefficients in Mg-doped crystals obey the equation D_o (cm²/s) = 1 × 10¹¹∼ (1×10¹³) exp[−915 ± 50(kJ/mol)/ RT ]. The diffusivity of oxygen in Ti-doped A1₂O₃ is lower than Mg-doped A1₂O₃. A vacancy mechanism explains these results.     

Crystallization and Properties of Fe2O3—CaO—SiO2 Glasses

Nucleation and crystal growth rates and properties were studied in a two-stage heat treatment process for Fe₂O₃-CaO-SiO₂ glasses. Glass transition (T_g) and crystallization temperatures (T _c ) for the glasses lay between about 612.0° and 710.0°C, and 858.5° and 905.0°C, respectively, and magnetite was the main crystal phase. For a glass of 40Fe₂O₃. 20CaO·40SiO₂ (in wt%) the maximum nucleation rate was (68.6 ± 7) × 10⁶/mm³·s at 700°C, and the maximum crystal growth rate was 9.0 nm/min^1/2 at 1000°C. The mean crystal size of the magnetite increased from 30 to 140 nm with variation of nucleation and crystal growth conditions. The glass showed the maxima in saturation magnetization and coercive force, 212.1 × Wb/m² and 30.8 × 10³ A/m, when heat-treated for 4 h at 1000°C and 1050°C, respectively. The variation of the saturation magnetization could be quantitatively interpreted well in terms of the volume fraction of the magnetite, whereas that of the coercive forces could be explained only qualitatively in terms of the particle size of the magnetite. Hysteresis losses showed the maximum value of 1493 W/m³ when heat-treated at 1000°C for 4 h prenucleated at 700°C for 60 min, and increased linearly with increasing heat treatment time under a magnetic field up to 800 × 10³ A/m.     

Determination of the Oxygen Self-Diffusion Coefficients in Y2O3-Containing Tetragonal Zirconia Polycrystals by Raman Spectrometric Monitoring of the 16O-18O Exchange Reaction

A new method of determining the oxygen self-diffusion coefficients (D) in oxides has been developed. The method is based on Raman spectroscopy combined with the ¹⁶O–¹⁸O exchange technique. By using the time dependent ¹⁸O2 concentration measured in a quasi in situ manner by Raman spectroscopy, the oxygen self-diffusion coefficients are calculated. The calculation takes into account the influence of the surface exchange reaction and the limited gas volume. The result obtained in 2.8 mo1% Y₂O₃-containing tetragonal zirconia polycrystals is D = 1.55 (^+0.07_-0.07) x 10^-2 exp[(-120.0 0.4) (kJ/mol) / RT ] cm²/sec. It was demonstrated that Raman spectroscopy is useful tool for determining the oxygen self-diffusion coefficients in oxides.     

Oxygen Self-Diffusion in Single-Crystal Y2O3

Self-diffusion coefficients of the oxygen ion in single-crystal Y₂O₃ were determined by the gas-solid isotope exchange technique. The results in the range 1100° to 1500°C are described by D=7.3 X 10^-6 exp [-19l(kJ/mol)/RT] cm²/s. Comparison of the results with those for oxides with the fluorite-type structure indicates that the regularly arranged vacant anion sites in the C-type structure do not contribute eflectively to oxygen ion diffusion .     

Kinetics of NiCr2O4 Formation and Diffusion of Cr3+ Ions in NiO

The reaction kinetics for NiCr₂O₄ formation and the diffusion of Cr³⁺ ions into single-crystal NiO were studied between 1300° and 1600°C in air. The experimental activation energy for NiCr₂O₄ formation was about 83 kcal/mol. After incubation, NiCr₂O₄ formed by a diffusion-controlled process. The origin of pores at the NiO/NiCr₂O₄ interface is discussed. The concentration profiles of Cr³⁺ in NiO were linear because the interdiffusion coefficient was directly proportional to the mol fraction Cr³⁺. Theoretical considerations indicate that the interdiffusion coefficient equals 3/2 the self-diffusion coefficient of Cr³⁺, which is rate-determining. The interdiffusion coefficient at 1 mol% Cr₂O₃ can be expressed as =4×10⁻³ exp (−55,000/RT) cm² s⁻¹.     

Thermal Diffusivity of Be4B, Be2B, and BeB6

The thermal diffusivities of polycrystalline Be₄B, Be₂B, and BeB₆ were measured by the flash method. Generally, the thermal diffusivities at a given temperature decrease with increasing boron content. The thermal diffusivities of Be₄B, Be₂B, and BeB₆ varied from 0.13 to 0.072 to 0.031 cm²/s, respectively, at 200°C and from 0.068 to 0.038 to 0.007 cm²/s at 1000°C. Heat transport in BeB₆ is expected to occur almost entirely by phonon conduction, whereas electronic conduction probably plays a major role in Be₄B and Be₂B. Analytical expressions for the thermal diffusivities (α) of Be₄B and Be₂B at 200° to 1000°C and of BeB₆ at 25° to 1500°C are: α(Be₄B)=1/(5.83+9.05×10 3 T ), α(Be₂B)=1/(10.92+1.40×10 2 T ), and α(BeB₆)=5.60×10 4+5.72/ T +77.3/T²-4.09×10⁴/T³ cm²/s.     

Annealing of Irradiation-Induced Thermal Conductivity Changes in ThO2-1.3 wt% UO2

The thermal conductivities of sintered pellets of ThO₂-1.3 wt% U0₂ were measured at 60°C before and after irradiation. The irradiation temperature was below 156°C, and the exposures varied from 3.1 × 10¹⁴ to 4.7 × lo^L7 fissions/cm³. Each fission fragment damaged a region of 2.2 × 10^-16 cm³ with the reduction in conductivity saturating by about 10¹⁷ fissions/cm³. Samples having exposures from 10¹⁵ to 10¹⁶ fissions/cm³ were annealed isothermally at 651 °C or isochronally from 300° to 1200° C to study the annealing of damage. Most of the annealing occurred between 500° and 900°C. The width of this interval plus the slow isothermal annealing suggest that the damage is annealed by a number of single order processes with a spectrum of activation energies from 1.8 to 3.9 eV or, less probably, by a high order process with an activation energy of 3.55 ± 0.4 eV.     

Anion Diffusion in α-Cr2O3

Oxygen-18 exchange between gaseous oxygen, held at a pressure of 125 mm Hg in a Pt10Rh chamber, and spheres of α-Cr₂O₃ containing three or less grains was determined from 1100° to 1450°C. Isotope equilibrium on crystal surfaces appears to be quickly established, and the rate-determining factor is self-diffusion conforming to the relation D = 15.9 exp(-101,000/ RT) cm²sec⁻¹. Changing sphere diameters caused no detectable variation in diffusion coefficients. Anions are the much slower diffusing species in this oxide.     

Formation of ZnAl2O4 in the Presence of Cl2

The formation of ZnAl₂O₄ spinel in diffusion couples of Al₂O₃ and ZnO was investigated between 1000° and 1390°C in air and in air containing 4.8 vol% Cl₂ by X-ray diffraction, electron probe microanalysis, and scanning electron microscopy. The rate of formation of a spinel layer obeyed a parabolic rate law and was accelerated remarkably by the presence of Cl₂. The interdiffusion coefficient, , and the activation energy, E, were calculated to be 10⁻⁸ to 10⁻⁹ cm²/s and 123 kcal/mol (514 kJ/mol) in air and 10⁻⁷ cm²/s and 31 kcal/mol (130 kJ/mol) in air containing 4.8 vol% Cl₂, respectively.     

Oxygen Self-Diffusion in Lithium-Doped Single-Crystal Magnesium Oxide

Oxygen self-diffusion coefficients for a single-crystal MgO doped with 400 ppm Li were determined at 848° to 1300°C by isotope exchange technique using ¹⁸O as the tracer. The diffusion coefficient increased with increasing temperature in the lower temperature range, as described by D = 7.8 × 10⁻⁴ exp[−279 (kJ/mol)/RT] cm²/s, which was interpreted to be an extrinsic diffusion due to the oxygen vacancies introduced by substitutional Li ions. The oxygen diffusivity tended to decrease above 1050°C, presumably becauuse of evaporation of Li₂O.