Zirconium oxide crystal phase: The role of the p H and time to attain the final p H for precipitation of the hydrous oxi
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MaryB. Harris Kentucky Energy Cabinet Laboratory, University of Louisville, P.O. Box 13015, Lexington, Kentucky40512
Stanley F. Simpson Department of Chemistry, University ofKentucky, Lexington, Kentucky 40506
Robert J. DeAngelis Department of Metallurgical Engineering and Materials Science, University of Kentucky, Lexington, Kentucky 40506
BurtronH. Davis Kentucky Energy Cabinet Laboratory, University of Louisville, P.O. Box 13015, Lexington, Kentucky 40512 and the Department ofMetallurgical Engineering and Materials Science, University ofKentucky, Lexington, Kentucky 40506 (Received 23 March 1987; accepted 28 March 1988) Precipitated hydrous zirconium oxide can be calcined to produce either a monoclinic or tetragonal product. It has been observed that the time taken to attain thefinal/>Hof the solution in contact with the precipitate plays a dominant role in determining the crystal structure of the zirconium oxide after calcination at 500 °C. The dependence of crystal structure on the rate of precipitation is observed only in the/>H range 7-11. Rapid precipitation in this/?H range yields predominately monoclinic zirconia, whereas slow (8 h) precipitation produces the tetragonal phase. AtpK of approximately 13.0, only the tetragonal phase is formed from both slowly and rapidly precipitated hydrous oxide. The present results, together with earlier results, show that both thepH of the supernatant liquid and the time taken to attain this pH play dominant roles in determining the crystal structure of zirconia that is formed after calcination of the hydrous oxide. The factors that determine the crystal phase are therefore imparted in a mechanism of precipitation that depends upon thepK, and it is inferred that it is the hydroxyl concentration that is the dominant factor.
I. INTRODUCTION Zirconia (ZrO 2 ) is an important material that finds many applications in catalysis, tough ceramics, etc. In many materials applications, the volume change for the tetragonal to monoclinic phase transformation is an important factor. Although the monoclinic phase is stable below about ss 1000 "C, the tetragonal phase may, under certain circumstances, be stabilized at lower temperatures. A number of explanations have been advanced to explain this occurrence. Garvie and co-workers l~4 advanced a concept of stabilization of the tetragonal phase that was based upon surface energy arguments. They proposed that, due to their lower surface energy, tetragonal particles smaller than about 30 nm would be stabilized against transformation to the monoclinic form. Osendi et al.5 argued that anion defects serve to initiate the transformation. Murase and Kato 6 proposed that water present during calcination catalyzed the tetragonal to monoclinic transformation. Heuer and Ruhle 7 J. Mater. Res. 3 (4), Jul/Aug 1988
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do not believe that nucleation occurs by a classical heterogeneous process as suggested by Chen and Chiao8; rather, they believe that a "nonclassical" process at regions of high strain within discrete ZrO 2
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