Metastability of tetragonal ZrO 2 derived from Zr- n -propoxide-acetylacetone-water-isopropyl alcohol
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Metastability of tetragonal ZrO2 derived from Zr-n-propoxide-acetylacetone-water-isopropyl alcohol Zhaoqi Zhan and Hua C. Zenga) Department of Chemical Engineering, Faculty of Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 (Received 2 May 1997; accepted 30 December 1997)
ZrO2 nanopowders derived from zirconium n-propoxide [Zr(OC3 H7 )4 ]-acetylacetonewater-isopropanol have been investigated with respect to their tetragonal metastability on heating-cooling processes. The transformation temperature of metastable tetragonal to monoclinic (t 0 ! m) phase is found to be governed by ultimate firing temperature, time, and atmospheres employed. Crystallite growth is fastened with increase in calcination temperatures over 1000–1400 ±C, and the t 0 ! m transformation temperature is correlated linearly with crystallite size in the studied range of 12–20 nm. Heating in an oxygen environment increases the size of the final crystallites and hence the rate of the t 0 ! m transformation. It is revealed that the t 0 ! m transformation temperature depends largely on the heating atmosphere, but only weakly on the cooling one. Based on the findings of this work, surface oxygen deficiencies are attributed to be responsible for low-temperature tetragonal metastability. A crystallite growth model to explain the decline of t 0 -ZrO2 phase is proposed. Kinetic and thermodynamic factors are also discussed in connection with the existing theories of tetragonal metastability.
I. INTRODUCTION
In recent years, zirconium-dioxide (ZrO2 ) based materials have received increasing attention for a wide variety of applications ranging from catalytic carriers to engineering ceramics.1–5 In particular, there has been an active field of research for the material polymorphism of ZrO2 , concerning its structural properties in these applications.6–8 The metastable low-temperature tetragonal phase (t 0 -ZrO2 ), which is structurally similar to its normal high-temperature form (t-ZrO2 ), is technologically useful,7–10 owing to its excellent mechanical strength and other superior physical/chemical properties. Various methods and technologies to increase the population of the t 0 -ZrO2 in material matrices have been advanced, and the theoretical explanations/models for the tetragonal stability at low temperatures have been postulated and verified accordingly.7–15 To control final t 0 -phase content for a specific application, the ability to alter the transformation temperature between the monoclinic, a thermodynamically stable form of zirconia at low temperature (m-ZrO2 ),6 and tetragonal phases is extremely important. To this end, a number of factors have been recognized to be responsible for the stabilizing low-temperature t 0 -ZrO2 .7,8,13–19 For example, metal dopants,4,15 crystallite size,7,8 surface/ lattice defects,8 adsorbed species on the surface,16–19 and domain boundary stresses8,13,14 have been identified and undergone extensive investigation with respect to the
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