Stabilization and hyperfine characterization of metastable tetragonal ZrO 2
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Hydrolyzed ZrCl4 and ZrO2 • nH 2 O have been used as starting compounds in a time-differential perturbed-angular correlation (TDPAC) investigation on the stabilization and thermal evolution of the metastable tetragonal form of ZrO2. This phase, of quadrupole parameters very similar to those reported for the high temperature tetragonal form, emerges at moderate temperatures previous to the monoclinic phase, when starting from hydrolyzed ZrCl4 and from ZrO2 • 2H2O treated previously at 673 K. Though in all cases zirconia appears initially as an amorphous compound characterized by unique hyperfine parameters, two different precursors have been observed to exist immediately previous to the occurrence of either the monoclinic or the metastable tetragonal crystal phases. Each of them exhibits a quadrupole frequency identical with and an asymmetry parameter higher than the ones characterizing the forthcoming corresponding crystal phases. A crystallization enthalpy of (33 ± 5) kJ/mol has been determined for the formation of the metastable tetragonal phase out of its precursor.
I. INTRODUCTION
The aim of studying in detail zirconia polymorphism is based on the increasing interest in the stabilization of metastable phases in ZrO2 ceramics because of their contribution to high mechanical strength, fracture toughness, and thermal shock resistance,1 achieved through the stress-induced metastable-to-monoclinic transformation. It is known that the monoclinic room temperature (RT) phase of ZrO 2 (baddeleyite) is subject to a reversible transition to the tetragonal modification around 1273 K,2 the hyperfine quadrupole parameters of which have been already determined in this laboratory.3 Nevertheless, a tetragonal modification probably identical with the high temperature tetragonal form4'5 may be occasionally stabilized at temperatures well below, provided certain conditions are fulfilled. In fact, it has been reported that though the end product in the thermolysis of some zirconium salts and also of ZrO 2 -nH 2 O is monoclinic ZrO2, the stabilization of the metastable tetragonal phase has been observed previous to the occurrence of baddeleyite, perhaps due to the presence of (OH)" or other ions as a result of incomplete decomposition of the parent compound.4'6'7 Recently, Srinivasan et al.8 have discussed the precipitation conditions under which either the metastable tea)Member
of Carrera del Investigador Cientifico, CICPBA, Argentina. b)Fellow of CONICET, Argentina. c)Member of Carrera del Investigador Cientifico, CONICET, Argentina. 1940
http://journals.cambridge.org
J. Mater. Res., Vol. 5, No. 9, Sep 1990
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tragonal or the monoclinic phases can be obtained by calcination of hydrous zirconium oxide. In this work hydrolyzed zirconium chloride, hydrous zirconia, and anhydrous amorphous zirconia have been chosen as starting compounds for carrying out a time-differential perturbed-angular correlation (TDPAC) investigation of the hyperfine interactions appearing upon heating, in order to investigate the met
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