Thermal treatment of binary carbonaceous/zirconia gels and formation of Zr(C,O,N) solid solutions

Binary organic-inorganic gels, in which a carbonaceous gel obtained from a coal tar pitch and a zirconia gel are combined, were prepared as precursors for carbothermal reduction with and without simultaneous nitridation to form Zr(C,O,N) solid solutions. Zirconium(IV) propoxide as the starting material for zirconia was hydrolyzed in the presence of acetylacetone in order to avoid undesirable precipitation. After pyrolysis at 800 degrees C, a precursor consisting of reactive carbon and nanometer-sized ZrO2 was obtained. Its behavior on heat treatment was investigated by TG, X-ray analysis, and nitrogen adsorption. Microporosity was generated by crystallization of the zirconia constituent in the temperature range 800-1100 degrees C. The carbothermal conversion into Zr (C,O,N) begins under nitrogen at about 1100 degrees C and under argon at about 1200 degrees C; mesoporosity is additionally formed in both cases. The final products at 1500 or 1600 degrees C-solid solutions of cubic Zr(C,O) or Zr(C,O,N)-are assemblages of fine crystallites with grain sizes of 1 mu m and less. The formation of these sub-micrometer crystallites may be explained by the high homogeneity and reactivity of the gel-derived carbon reactant. Nitrogen adsorption is shown to be a useful technique to follow the reduction reactions in gel precursors.