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 fr om a coal tar pitch and a zirconia gel are combined, were prepared as precursors for carbothermal reduction with and without simultaneous ni tridation to form Zr(C,O,N) solid solutions. Zirconium(IV) propoxide a s the starting material for zirconia was hydrolyzed in the presence of acetylacetone in order to avoid undesirable precipitation. After pyro lysis 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. Microporo sity was generated by crystallization of the zirconia constituent in t he temperature range 800-1100 degrees C. The carbothermal conversion i nto 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 soluti ons 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-micro meter crystallites may be explained by the high homogeneity and reacti vity of the gel-derived carbon reactant. Nitrogen adsorption is shown to be a useful technique to follow the reduction reactions in gel prec ursors.