Thermal decomposition and crystallization of aqueous sol-gel derived zirconium acetate gels occurring during the preparation of ceramic coatings.

摘要

A sol-gel procedure using an aqueous zirconium acetate precursor was used to form continuous, crack- and bridge-free zirconia coatings on graphite fibers. A comprehensive understanding of the precursor's structure and structural evolution during the sol to gel and gel to solid transitions is required to control the properties of the coating.; Decomposition of the gel prepared by desiccation of the solution of zirconium acetate and pyrolyzed to form crystalline zirconia powders in N2, Air and O2 and the transformations occurring during pyrolysis to 800°C was investigated. Three transition temperatures were detected: the amorphous-to-cubic (am-c), cubic-to-tetragonal (c-t) and tetragonal-to-monoclinic (t-m) transition. The presence of molecular oxygen decreased the crystallization temperature, increased the grain size of the tetragonal crystals formed and decreased the temperature at which the t-m transformation was initiated.; Decomposition and transition temperatures occurring in the pyrolysis and the polymorph distribution were dependent on the pH of the precursor solution and on the presence of additive anions such as chloride, nitrate or sulfate. Lowering the pH of the precursor solution produced changes in the relative concentration of zirconium complexes present in the solution and, hence, reduced the carbon content in the crystalline zirconia obtained by pyrolysis at 555°C and reduced transition temperatures. The presence of counterions altered the ratio of m/t polymorph in the zirconia powders because of the structural and compositional changes in the precursor gel. Controlling the chemistry involved in crystal formation results in controlling the polymorph composition, consequently gaining control of the properties of the coating of graphite fibers.