No abstract

Silicon carbide whiskers (SiC(w)) are synthesized by the pyrolysis of rayon fibers impregnated with sol-gel-derived SiO 2 . The influence of boric acid on whisker growth and the effect of varying the TEOS:rayon weight ratio are studied and reported. Evolution of SiC(w) from impregnated rayon fibers in the carbonized and in the carbothermal stages is investigated.

X-ray studies of the stoichiometrically prepared a-Ga2Se3 and Ga~Te3 are reported after various stages of air-oxidation in the temperature ranges 250 to 825°C and 250 to 650°C respectively. Diffractometric powder data of Ga~Te8 are also reported over the complete 20 range with remarkable difference in the relative intensities of the (444) and (642) reflections. In a-Ga2Se3 the oxidation proceeds by formation of the most stable phase, beta-gallium sesquioxide, complete oxidation occurring at 650 ° C. For Ga2Te3 a mixture of Ga~TeO8 and TeO~ is obtained as the intermediate oxidation products in the range 500 to 600 ° C, while at 450 ° C some extra lines which could be indexed on the super-lattice cell of GaaTe3, along with To and unchanged Ga~Te3 lines, are observed. Oxidation at the higher temperature of 650 ° C led to the disappearance of TeO~ lines leaving Ga~TeO6 as the final well-crystallised phase.

ZnGa2Te4 was found to crystallize in a defect tetrahedral structure with possible space group I4(82) with Z=2. Complete X-ray powder diffraction data were obtained and the unit cell parameters a and c and X-ray density were calculated. These were a=0.5930(1) nm, c=1.1859(3) nm, and Dx=5.7×103 kg/m3.

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