Synthesis and characterization of Al2O3/SiC powders from natural aluminosilicates

Carbothermal reduction of natural aluminosilicates by carbon black was used to obtain Al2O3/SiC powders. Three different precursors, namely, kaolin, kyanite and illite were independently investigated. The powders were mixed in water, in stoichiometric proportion. The dried blend (precursor + carbon black) was broken in mortar and heated at 1500 degrees C for 2 to 3h in reduced atmosphere for the reaction to proceed. The resultant powder was characterized by X-ray diffraction, scanning and transmission microscopy. Results showed that for illite and kaolin only alpha-alumina and alpha-SiC were formed as crystalline phases, but for kyanite residual mullite was also observed. This infers that the reaction temperature/time was not sufficient for the complete reduction of this precursor, The Al2O3 phase was precipitated mainly as equiaxial particles, however, SiC formed whiskers, woolly structures, filaments, spheres and particles. Many whiskers were grown by the VLS process. It was observed that the different morphologies were a consequence of the metallic impurities in the material.