Influence of atmosphere on crystallization of zirconia from a zirconium alkoxide
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Influence of atmosphere on crystallization of zirconia from a zirconium alkoxide David E. Collins and Keith J. Bowman School of Materials Engineering, Purdue University, West Lafayette, Indiana 47907 (Received 9 January 1997; accepted 27 June 1997)
Dibutoxybis (acetylacetonato) zirconium, a difunctional zirconium alkoxide, was polymerized at 130 ±C for 5 h in vacuo to produce oligomers that could be pyrolyzed to form a tetragonal zirconia (t-ZrO2 ), metastable at room temperature. This metastable phase was retained considerably below the equilibrium transformation temperature (,1200 ±C) without the use of dopants. Comparative pyrolysis of the oligomers between 600 and 900 ±C in either flowing O2 or N2 for processing times under 12 h indicated t-ZrO2 nucleated first. Pyrolysis in oxygen facilitated transformation to the monoclinic symmetry, whereas pyrolysis in nitrogen demonstrated retention of the tetragonal phase. The formation of oxygen vacancies during pyrolysis, their role in stabilizing the metastable tetragonal phase, and contributions of O2 and crystallite size in the polymorphic transformation are discussed.
I. INTRODUCTION
The pyrolysis of metal-organics and salts is a common method to produce uniform, fine-grain ceramic materials. The formation of zirconia is no exception. However, the pyrolysis of common zirconia precursors such as zirconium acetates, alkoxides, and nitrates forms a metastable tetragonal phase well below the equilibrium transformation temperature (,1200 ±C), below which the monoclinic phase is stable. One argument, proposed by Garvie, is that tetragonal metastability is dictated by an “intrinsic” size effect.1 Garvie produced crystallites of metastable tetragonal ˚ from the calcination ZrO2 (t-ZrO2 ) no larger than 300 A of hydrated zirconium hydroxide [Zr(OH)4 ? xH2 O] precipitated from ZrOCl2 and ZrO(NO3 )2 . The calcination of these materials was conducted in air at temperatures up to 1000 ±C. Assuming nucleation was homogeneous and the transformation criteria governed primarily by the surface energy of the crystallites, Garvie claimed the tetragonal phase is thermodynamically favored be˚ transforming to the monoclinic phase for low 300 A, ˚ 1 crystallites above 300 A. However, results from other investigations contradict this “intrinsic” size effect. Regarding the limitations of the monoclinic stability, Morgan2 produced ˚ monoclinic ZrO2 (m-ZrO2 ) crystallites by aging 60 A ZrO(NO3 )2 with 6 N HNO3 at 150 ±C for 14 days. The crystallite size was considerably smaller than the bounds imposed by Garvie, suggesting that the monoclinic phase stability is not restricted by size.2 Srinivasan et al. made a similar observation in an investigation of supernatant pH 1230
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J. Mater. Res., Vol. 13, No. 5, May 1998
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effects on ZrO2 prepared from ZrO(NO3 )2 .3,4 They found m-ZrO2 transformed from t-ZrO2 had a smaller grain size than its parent t-ZrO2 . Thus, Srinivasan et al. argued that factor
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