Evolution of crystalline zirconia structure in heat-treated ceria stabilized zirconia gels
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Transformation characteristics of CeO 2 stabilized ZrO 2 gels have been studied during heating in the temperature region 1273-1473 K. The nature of the major phase present changes drastically in this temperature region. The crystallite sizes of tetragonal and monoclinic ZrO 2 crystallites have been calculated by x-ray line broadening. The change in the nature of the major phase observed during heat treatment seems to be related to the presence of a barrier for transformation from a metastable to a stable regime, which in turn appears to be related to the diffusion barriers of Ce4+ in ZrO 2 .
I. INTRODUCTION The stable structure of pure ZrO 2 at laboratory temperature is monoclinic. It transforms to tetragonal symmetry at ^-1373 K and transforms back to monoclinic phase with large hysteresis and in a manner characteristic of martensitic transformations.1 This transformation is unique, since the monoclinic phase has a lower density and has been the key factor2-3 in the application of stabilized ZrO2s and ZrO 2 containing ceramic composites as tough materials. The tetragonal phase is stabilized by making solid solutions with a number of oxides such as MgO, CaO, Y 2 O 3 , CeO 2 , and other rare earth oxides. 45 Phase stability is also controlled by particle size since extremely fine particles of ZrO 2 are stable in their tetragonal or even cubic structure at laboratory temperatures.6 This stability is simply a consequence of the trade-off of unfavorable bulk free energy differences for a favorable surface free energy difference between the two phases,7'8 although recent studies of this transformation in other matrices show that the stability of the phases depends upon the strain energies9'10 and kinetic factors.11-12 For a purely sol-gel derived powder the absence of a rigid matrix makes the strain terms less likely to influence the transformation. Several studies of the evolution of tetragonal and monoclinic structures in particulates of ZrO 2 obtained by different methods including sol-gel techniques have been reported.13-14 Crystallite size studies15"18 have been done both as a function of temperature and isothermal holding times. In brief, such studies have indicated that at higher temperatures the crystallite sizes of the tetragonal phase increase while their numbers decrease and above certain critical sizes, typically >-300 A in linear dimension, tetragonal ZrO 2 transforms to the monoclinic.15 Such studies have not been made in sol-gel derived CeO 2 partially stabilized zirconia. Since control of the initial a
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J. Mater. Res., Vol. 6, No. 12, Dec 1991
http://journals.cambridge.org
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particle sizes can be achieved by the sol-gel route, we consider it both interesting and important to examine the evolution of structures in partially stabilized zirconia compositions. In this communication, we report our findings on the evolution of structures in ZrO 2 gels containing 2.5 and 5 mol% CeO 2 . We have followed the kinetics by up-quenching the xero
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