Mixed Oxide Powders Research Articles

Phases in the System NaAlO2‐Al2O3

Mixed oxide powders prepared by ashing filter paper soaked in aqueous salt solutions were heat‐treated in the range 800° to 1150°C. Compositions spanning the range NaAlO2 to pure Al2O3 were examined. A phase with the mullite structure, λ‐Na2O ·xAl2O3 with x varying from 3 to ≊ 12, and a new hydrated form of sodium aluminate, ρ‐Na2O · Al2O3· 6H2O, were found. The stability ranges and decomposition behavior of these materials were determined. A K analog to the A phase was also found.

Properties, blending and homogenization of (U, Th)O 2-UO 2 powder

(U, Th)O 2 mixed-oxide powder was prepared by coprecipitation from mixed-nitrate solutions using NH 4OH, and the physical properties of material calcined at different temperatures obtained. Properties measured included particle morphology and size distribution, surface area, O/M ratio and bulk and tap density. Blending of calcined powder at a 6 wt% level with UO 2 was studied for different time periods and the distribution of Th particles in the powder and in sintered pellets evaluated statistically; little increase in homogeneity was observed after about 1 hour. The sintered-pellet densities were not influenced by the mixed-oxide addition, which appeared to act as an inert diluent.

Synthesis and Mechanical Properties of Stoichiometric Aluminum Silicate (Mullite)

Highly dense translucent polycrystalline bodies of stoichiometric aluminum silicate (mullite) were obtained by vacuum‐hot‐pressing of high‐purity submicron mixed oxide powders. The powders were prepared by the hydrolytic decomposition of mixed metal alkoxides. X‐ray diffraction and electron microscopy indicated that the initially amorphous needlelike fine particulates transform into highly crystalline orthorhombic mullite at ∼1200°C. Optimum hot‐pressing conditions were 5 kpsi and 1500°C for 30 min. Densities within experimental error of the theoretical value of 3.19 g/cm3 were obtained. A typical microstructure consisted of fine interlocking needlelike grains arranged in an overall mosaic or “jigsaw” pattern. Microprobe traverses across the samples indicated homogeneous dispersion of SiO2 in the AlO3 matrix. Room‐temperature mechanical properties were measured and correlated with the microstructure and crystal structure of the ceramic compact. A slightly higher melting temperature than has been previously reported was observed for the 3Al2O3·2SiO2 studied. All specimens exhibited a smooth surface finish and excellent thermal‐shock resistance from 1200°C to room temperature.

Reduction of mixed nickel and iron oxide powders

1. By reducing mechanical mixtures of the oxides NiO and Fe2O3 at 500 and 800°C, it is possible to produce virtually pure nickel and iron, with practically no solid solution formation. 2. The reduction of mixed oxides prepared by the simultaneous evaporation and roasting of nickel and iron nitrates is accompanied by the formation of solid solutions having the average powder concentration at 800°C and, in the case of equal component concentrations, even at 500°C. 3. The product yielded by the evaporation and roasting of a mixture of nickel and iron nitrates is a finely divided mixture of the oxides NiO and Fe2O3, which is conducive to intense diffusional homogenization of the alloy powder during the reduction process.

Discussion of “Detection of Thermal Reaction in Mixed Oxide Powders by Electrical Resistance Measurements” [A. N. Copp, M. Nakohira, and G. W. Brindley (pp. 237–239, Vol. 113, No. 3)

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Closure to “Discussion of ‘Detection of Thermal Reaction in Mixed Oxide Powders by Electrical Resistance Measurements’ [A. N. Copp, M. Nakohira, and G. W. Brindley (pp. 237–239, Vol. 113, No. 3)

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Detection of Thermal Reaction in Mixed Oxide Powders by Electrical Resistance Measurements

Resistivity measurements are made on 1:1M mixtures of , , , and powders heated in oxygen, and in nitrogen atmospheres at 2.5°C/min. Sharp inflections are found in the curves; two inflections are found for , and , heated in oxygen. X‐ray diffraction measurements are made of similar powders heated under identical conditions and quenched from various temperatures, and curves are obtained of α (fraction reacted) vs. T, maximum heating temperature. The temperatures of the inflections in the curves (the lower inflection when two are observed) correspond with reasonable extrapolations of the α vs. T curves to , i.e., these inflections appear to correspond with, or to be closely related to, the onset of reaction in the powder mixtures. The method has potential application for studying the onset of reactions under conditions where more bulky equipment cannot be used and particularly in closed systems.

Observations on phase equilibria and sintering behaviour in the PuO 2-UO 2 system

The phases present in UO 2 and PuO 2 mixed powder compacts of varying composition have been determined after sintering in hydrogen and argon, after argon-arc melting and after subsequent oxidation. Phase equilibria in the Pu-U-O system are discussed in terms of these findings. The sintering behaviour of the mechanically mixed oxide powders is found to vary appreciably with composition and sintering atmosphere.