Kinetics of the Graphite-Uranium Dioxide Reaction from 1400° to 1756°C

The reaction of microspheres of UO_z with graphite was studied from 1400° to 1756°C. When a spherically symmetrical layer of carbide was produced around the UO₂ core, only UC₂ was formed, and the diffusion of oxygen through this layer was rate-controlling. The Arrhenius relation for this system is k_D= 21.0 exp(−90, 000/ RT ) cm²/s
The reaction of a geometrically nonsymmetrical configuration of UO₂ and UC₂ was also studied. Comparison of the reactions in the symmetrical and nonsymmetrical systems demonstrated that the kinetic behavior of the two systems is quite different and that the conversion in the nonsymmetrical system was 2 to 5 times faster. The importance of these observations to kinetic results reported in the literature for analogous systems is discussed.     

Linear Thermal Expansion of Aluminum Oxide and Thorium Oxide from 100° to 1100°K

The linear thermal expansion of single-crystal and polycrystalline aluminum oxide and poly-crystalline thorium oxide was measured from 100° to 1100° K with an interferometric technique. For each substance the results are well described by Gruneisen's equation using a Nernst-Linde-mann energy function.     

Thermal Conductivity of Uranium Dioxide from -57° to 1100°C by a Radial Heat Flow Technique

A radial heat flow technique was used to measure the thermal conductivity, k , of polycrystalline UO₂in the range -57° to 1100°C. The technique yielded results with a probable accuracy of ±1.5% and a precision of ±0.1% in the range 50° to 1100°C. Meaningful measurements were limited to 1100°C by Pt-90 Pt10Rh thermocouple instability, although the apparatus was structurally sound to 1400°C. The thermal conductivity data up to 1000°C could be explained on the basis of heat transport by phonons. The thermal resistance, l/ k , exhibits a linear temperature dependence from 200° to 100°C, which is expected for an insulator well above the Debye temperature. The slope of the l/ k -temperature plot is 0.0223 cm w⁻¹ which is independent of impurity content and is associated with three phonon umklapp processes. The intercept is sensitive to impurity content as indicated by the fact that it was decreased by a decrease in the oxygen/uranium ratio. Between 1000° and 1100°C, there is a slight departure of l/ k from linearity which may be due to the onset of an electronic contribution. Near room temperature, UO₂ has a maximum in k which is apparently caused by the rapid decrease in specific heat below this temperature.     

Transitions in Vapor-Deposited Alumina from 300° to 1200°C

The transition of amorphous alumina to α-alumina was studied by X-ray diffraction, electron diffraction, DTA, TGA, and microscopic observation. The amorphous alumina was prepared by condensing vapor from evaporating molten alumina in vacuo onto the glass envelope of the vacuum chamber. The amorphous alumina was transformed to a poorly crystalline material by heating for 16 hr between 570° and 670°C. Between 670° and 1200°C, the poorly crystalline alumina was converted to α-alumina via two parallel series of transition aluminas. The principal series was γ-alumina to δ-alumina to α-alumina. A minor amount of θ-alumina developed from the initial crystallization and persisted throughout the duration of the principal series as a parallel path. Some conversion of δ- to θ-alumina was detected above 900°C. DTA produced an unexplained exothermic peak at 320°C and a second exothermic peak at 860°C which corresponded to formation of metastable aluminas.     

Reaction of UC with Nitrogen from 1475° to 1700°C

The reaction of nitrogen with arc-melted UC was studied from 1475° to 1700°C at an N₂ pressure of about 400 torr. The reaction appeared to proceed toward equilibrium in distinct stages: (1) UC and N₂ reacted to form UC₂ and UC_0.8N_0.2; (2) UC₂ reacted with N₂ to produce more UC_0.3N_0.2 and free C; and (3) this U(C,N) reacted with N₂ to produce more free C and the equilibrium product, a high-N U(C,N). These results are consistent with known thermodynamic and phase behavior in the U-C-N system.     

Decomposition of Carbon Dioxide to Carbon with Active Wustite at 300°C

Active wustite (FeδO, with a δ value of 0.98) was prepared by keeping normal wustite (δ value of 0.94) in a N₂ atmosphere at 300°C for 10 min. This reaction is given by (4δ₂–3)Fe_δ1O→(4δ₁–3)Fe_δ2O + (δ₂–δ₁)Fe₃O₄ where δ₁= 0.94 and δ₂= 0.98. The equation indicates that the normal wustite undergoes eutectoid decomposition into active wustite and stoichiometric magnetite (Fe₃O₄). Carbon dioxide (1.013 × 10⁵ Pa) was almost completely (100%) decomposed into carbon (zero valence) by the active wustite at the low temperature of 300°C, which was associated with the transformation of the active wustite into the stoichiometric magnetite. The internal pressure of the reaction cell eventually became a vacuum.     

Passive-Oxidation Kinetics of High-Purity Silicon Carbide from 800° to 1100°C

Highly textured chemically vapor-deposited silicon carbide (CVD-SiC) thick films were oxidized and compared to single-crystal SiC and single-crystal silicon. The oxidation rates of the (111) face of the cubic CVD-SiC were the same as those of the (0001) face of the single-crystal SiC. Similarly, the opposite faces of the two materials, (     ) and (000     ), also oxidized at nominally the same rates. The (     ) and (000     ) faces oxidized much faster than their opposite (lll)/(0001) faces. Ellipsometry measurements and kinetic results implied that differences existed between the oxides that grew on the opposite faces. A regression method was developed to analyze the oxide thickness versus time versus temperature behavior of the specimens simultaneously. This technique was compared to typical methods for analyzing temperature-dependent processes and estimated temperature- dependent parameters (e.g., activation energy) and their errors more accurately.     

Ionic Conductivity of the System Si-Al-O-N from 850° to 1400°C

Electrical conductivity was measured from 850° to 1400°C for β-sialon and pure X phase as well as for the sintered system Si₃N₄-Al₂O₃, containing β-sialon, X phase, β-Si₃N₄, and glassy phase. Ionic conductivity was measured at >1000°C. The charge carriers were identified by electrolysis. The results showed that pure β-sialon is ionically conducting because of Si⁴⁺ migration for the temperature range studied. Pure X phase shows ionic conduction by Si⁴⁺ above 1000°; below 1000°C, it shows electronic conduction because of impurities. The conductivity of the sintered system Si₃N₄-Al₂O₃ containing β-sialon, β-Si₃N₄ X phase, and glassy phase changes as the relative quantities of β -sialon and X phase change. The apparent activation energies for the ionic and electronic conductivities are 45 and 20 kcal/mol, respectively.     

Vapor Pressure of Gd2O3 from 2350° to 2590°K

Knudsen evaporation experiments were made on gadolinium sesquioxide from 2350° to 2590°K. Two sets of Knudsen cell data yielded average values of 468.7 ± 0.8 and 471.0 ± 0.5 kcal/mole for the assumed evaporation reaction

The results indicate that the evaporation coefficient may be much less than one.     

Microstructure of Chromite-Periclase at 1650° to 2310°C

Progressive solid solution of chromite in periclase occurs with increasing temperature. Solid solution is virtually complete except for a few coarse remnants at 1980° C and these are gone at 2090° C. Specimens cooled slowly exhibit exsolution structures, whereas specimens quenched from the higher temperatures consist essentially of a single solid-solution phase. Two periclase structure phases are indicated by X-ray patterns in some quenched specimens. Silicate alters the relations between chromite and periclase in specimens cooled slowly and facilitates segregation of spinel from the solid solution in quenched specimens. Textural features observed in electrically fused specimens are similar to those resulting from predominantly solid-state reactions in specimens fired to 2200°C.     

Oxidation Behavior of Chemically-Vapor-Deposited Silicon Carbide and Silicon Nitride from 1200° to 1600°C

The isothermal oxidation of pure CVD SiC and Si₃N₄ has been studied for 100 h in dry, flowing oxygen from 1200° to 1600°C in an alumina tube furnace. Adherent oxide formed at temperatures to 1550°C. The major crystalline phase in the resulting silica scales was alpha-cristobalite. Parabolic rate constants for SiC were within an order of magnitude of literature values. The oxidation kinetics of Si₃N₄ in this study were not statistically different from that of SiC. Measured activation energies were 190 kJ/mol for SiC and 186 kJ/mol for Si₃N₄. Silicon oxynitride did not appear to play a role in the oxidation of Si₃N₄ under the conditions herein. This is thought to be derived from the presence of ppm levels of sodium impurities in the alumina furnace tube. It is proposed that sodium modifies the silicon oxynitride, rendering it ineffective as a diffusion barrier. Material recession as a function of oxide thickness was calculated and found to be low. Oxidation behavior at 1600°C differed from the lower temperatures in that silica spallation occurred after exposure.     

Magnetostrictive Ferrites with Small Change in Coupling Coefficient from 0° to 20°C

Ferrite materials which possess electromechanical coupling coefficients of 30% or more over the temperature interval 0° to 20°C were developed by the control of the various anisotropies. A high order of temperature stability of the coupling coefficient was achieved by a new technique in which a number of ferrites are combined in a composite core structure. Composite transducers made by the technique emerge as single bodies adn not as a composite made up of discrete parts.