Hydrothermal crystallization kinetics of m -ZrO 2 and t- ZrO 2
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The isothermal nucleation and crystallization kinetics of hydrothermally prepared monoclinic and tetragonal ZrO2 have been determined at various pH conditions. It is shown that monoclinic ZrO2 precipitates at low pH whereas at high pH tetragonal ZrO2 crystallizes from an amorphous zirconium (hydrous) oxide, Zr(OH)xO>,, precursor. At intermediate pH conditions mixtures of the polymorphs are formed suggestive of kinetically competing particle formation mechanisms. The data are explained by the proposed existence of three controlling regimes for the formation of crystalline ZrO2: dissolution/precipitation at low pH, a solubility controlled regime at intermediate pH values, and a gel structure controlled regime at high pH. Apparent activation energies for the nucleation and crystallization of monoclinic and tetragonal ZrO2 formed under hydrothermal conditions are presented.
I. INTRODUCTION
Ultrafine zirconia powder is an important raw material for use in transformation toughened ceramics.1 As such, an intense interest has developed in preparing ZrO2 powders with controlled particle size and crystal structure.2"17 Crystalline ZrO2 can exist in three different polymorphic forms: a monoclinic (m-ZrO2), tetragonal (/-ZrO2), or cubic (c-ZrO2) crystal structure. Although m-ZrO2 is thermodynamically the most stable ZrO2 polymorph below =1100 °C,18 £-ZrO2 as well as c-ZrO2 can be prepared by hydrothermal techniques at temperatures as low as 100 °C.19 These phases are found to exist metastably, and transformation to m-ZrO2 occurs irreversibly with prolonged hydrothermal reaction times. The chemical synthesis of ZrO2 usually involves the precipitation of precursor Zr(OH)xO,, particles followed by crystallization to one or more of the polymorphic forms. The precursor Zr(OH)xOy is precipitated from highly acidic solutions of zirconium salts, such as ZrO(NO3)2 or ZrOCl2, by the addition of base. Crystallization can then be carried out by one of two methods: hydrothermal treatment or solid-state reaction. Hydrothermal treatments are accomplished by increasing the temperature of an aqueous suspension of Zr(OH)^O>, particles. In contrast, solid-state crystallization is achieved by removing the amorphous Zr(OH)^Oj, material from the suspension (usually via filtration) followed by high temperature calcination. The sole exception to the above crystallization schemes is found for certain strongly acidic, homogeneous zirconium containing solutions. For these solutions, crystallization of Current address: Research and Development Center, PQ Corporation, Conshohocken, Pennsylvania 19428. 2698
http://journals.cambridge.org
J. Mater. Res., Vol. 5, No. 11, Nov 1990
Downloaded: 30 Sep 2014
the anhydrous oxide proceeds by precipitation from solution under the hydrothermal conditions. Understanding the processing variables (i.e., temperature, time, pH, etc.) needed to achieve desired ratios of the polymorphs has been a technical driving force in the hydrothermal preparation of ZrO2. Many investigators have observed that the hydrothermal proc
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