CHEMICAL SYNTHESIS AND STRUCTURAL EVOLUTION OF ZIRCONIUM TITANATE

Chemical polymerization of (a) titanium alkoxide-acetate and zirconium acetylacetonate ([HOAc]/[Ti] = 17.5; [H2O]/[Ti + Zr] = 27.8), and (b) titanium and zirconium alkoxides in the presence of acetic acid ([HOAc]/[Ti + Zr] = 8.75; [H2O]/[Ti + Zr] = 15.3) was used to prepare ZrTiO4 pre-ceramic gels. The effects of the organozirconium precursor structure on the gelation kinetics and the structural evolution of crystalline ZrTiO4 were investigated. Owing to its stability, room temperature gelation of sols prepared from zirconium acetylacetonate occurred after several days. In contrast, the sols prepared with modified zirconium n-propoxide underwent relatively rapid gelation (t(gel) < 10 min). It was possible to expedite gelation of mixed-cation sols derived using zirconium acetylacetonate, by employing higher temperatures (t(gel) = 2 h at 70-degrees-C). Crystallization of ZrTiO4 from pre-ceramic gels was investigated using differential thermal analysis, thermogravimetic analysis, X-ray diffraction and electron microscopy. The results of these analyses indicated that gel pyrolysis (200-500-degrees-C) led to the formation of crystalline single-phase ZrTiO4 powders at temperatures (about 700-degrees-C) significantly lower than those encountered in traditional processing (1000-1350-degrees-C). Particles of gel powders calcined at 700-degrees-C exhibited a smooth surface morphology; however, those calcined at and above 900-degrees-C showed the formation of submicron crystallites throughout the volume of the original gel particles. Gel powders calcined at 700-degrees-C were uniaxially pressed (about 50 000 lbf in-2) and sintered (1550-degrees-C for 4 h) to form single-phase ZrTiO4 ceramics with rho(th) = 62-65%, without any sintering additives. Preliminary measurements indicated that the dielectric constant values of ZrTiO4 ceramics were about 10 and 14-21 (at 1 kHz) for acetylacetonate- and alkoxide-derived ceramics, respectively. The loss factor (tan-delta) was lower than 1 x 10(-4) for ceramics derived from either of the organozirconium precursors.