METASTABLE PHASE SELECTION AND PARTITIONING FOR ZR(1-X)ALXO(2-X/2) MATERIALS SYNTHESIZED WITH LIQUID PRECURSORS

Aqueous solutions of zirconium acetate and aluminum nitrate were spray pyrolyzed at 250-degrees-C and upquenched to different temperatures to yield metastable solid solutions of composition Zr(1-x)AlxO(2-x/2). An amorphous oxide forms first during pyrolysis which subsequently crystallizes as a single phase for x less-than-or-equal-to 0.57 (less-than-or-equal-to 40 mol% Al2O3). The crystallization temperature increased with Al2O3 content. Electron diffraction, supported by Raman spectroscopy, indicates that the initial phase is tetragonal. At higher temperatures, the initial solid solution partitions to other metastable phases, viz., t-ZrO2 + gamma-Al2O3, prior to achieving their equilibrium phase assemblage, m-Zro2 + alpha-Al2O3. Partitioning yields a nanocomposite microstructure with grain sizes of 20-100 nm, compared to the 3 to 5 nm in the initial, single phase. Compositions containing 45 to 50 mol% Al2O3 concurrently crystallize and partition. The structure selected during crystallization and the partitioning phenomena are discussed in terms of diffusional constraints during crystallization, which are conceptually similar to those operating during rapid solidification.