부산대학교 도서관

The structure and surface properties of thorium dioxide were studied from the viewpoints of electron microscopy, electron diffraction, x-ray diffraction, and water vapor adsorption isotherms in an attempt to obtain a better understanding of the caking tendencies exhibited by some aqueous thoria slurries when circulated in loop experiments at elevated temperatures. In water vapor adsorption isotherms, there is observed chemisorption, as evidenced by relatively high heats of adsorption, slow rates of adsorption, and zero point drift. Kelvic pore size distributions computed from desorptioc isotherms exhibit a relatively sharp pore distribution at a radius of about 10 A, as well as a very broad spectrum of pores in the range of 10 to 200 A. Water surface areas are approximately the same as, or slightly larger than, nitrogen surface areas, suggesting that the predominant pores are able to accommodate both nitrogen and water molecules. X-ray- and electron-diffraction studies demonstrate that all samples are identical with standard thorium oxide, with no detectable amounts of polymorphs or crystalline impurities. Certain "extra" x-ray-diffraction lines observed have been traced in the use of x radiation not monochromatized by crystal reflection methods. X-ray line broadening decreases as the temperatare of heat-treatment of the sample is increased. High-resolution electron micrographs show that the particles, ordinarily observed, actually consist of aggregates of minute crystals whose predominant morphology is that of octahedrons, with no indication of a plate -like structure. Electron micrographs show no definite correlation between caking and the morphology of the aggregates of crystallites. Thoria crystals heated to 1600 deg C exhibit rounded edges, suggesting the beginning of sintering. Such spheroidal particles should re sult in le ss erosion in circulation experiments. An electron micrograph of a sample of thoria, peptlzed by the addition of a small amount of palladium nitrate, exhibited excellent dispersion of individual crystallites. It is suggested that in future studies every effort be made to ensure complete dispersion of the samples, in the event high resolution and high magnification electron microsccpy is contemplated. (auth)