Effects of Chemical Species on the Crystallization Behavior of a Sol-D erived Zirconia Precursor Chiau Ling Ong, † John Wang, * ,† Ser Choon Ng, ‡ and Leong Ming Gan § Department of Materials Science, Department of Physics, and Department of Chemistry/IMRE, Faculty of Science, National University of Singapore, Singapore 119260 The thermal and crystallization behaviors of a sol-derived zirconia precursor are affected by the chemical species (such as chloride, acetate, and hydroxyl; i.e., Cl - , CH 3 COO - , and OH - ) present in the solution in which they are aged. The crystallization temperature of zirconia pre- cipitate increases steadily with decreasing pH when hydro- chloric acid is added to the aging mother liquor. A more dramatic increase in crystallization temperature is ob- served when acetic acid is used to reduce the pH of the aging solution. The precursors aged in the solutions of in- termediate pH range demonstrate a typical two-step crys- tallization behavior, due to the transition from a OH - - dominated surface structure to a CH 3 COO - -dominated one. These results show that a higher crystallization tem- perature results when the OH - groups attached to the sur- face of precursor particles are replaced by the CH 3 COO - groups. Only a slight decrease in the crystallization tem- perature of aged precipitates occurs when free species, such as CH 3 COO - and OH - , are being removed from the aging solution. I. Introduction C ONSIDERABLE attention has recently been focused on the processing of advanced ceramic materials of high density and controlled microstructure. 1–3 The microstructural features of a sintered ceramic material are largely dependent on the characteristics of the starting powder, such as the particle size and size distribution, particle shape and shape distribution, the degree of particle agglomeration, chemical composition, and homogeneity. Therefore, the preparation of a desirable ceramic powder is of significant importance to the success of fabricat- ing a ceramic material of high performance. Zirconia (ZrO 2 ) powders prepared via (co)precipitation routes exhibit many of the desirable features required for fab- ricating a zirconia ceramic of high sintered density and refined microstructure. They are sintered to a high density at a rela- tively lower sintering temperature than that required for a zir- conia powder prepared via the conventional powder processing route. Unfortunately, particle agglomeration occurs when the precipitated precursors are calcined at temperatures in the range of 400° to 700°C. This leads to a significant reduction in the sinterability of the resulting powder and often results in the formation of large structural defects in the sintered ceramic, due to the differential sintering between agglomerates of dif- fering density. Therefore, it is desirable to calcine a precipi- tated precursor at a temperature as low as possible, in order to avoid the occurrence of large and hard agglomerates/ag- gregates in the calcined powder. However, the calcination tem- perature required is often dictated by the crystallization tem- perature of the precursor. It is thus necessary to reduce the crystallization temperature of the precursor in order to be able to calcine it at a lower temperature. This has been attempted by adding nanosized zirconia crystallites as seedings in the pre- cipitated zirconia precursors 4 and by adjusting the surface structure of precipitates. 5 The crystallization temperature of zirconia precipitates is dependent on the processing parameters, although many of them exhibit a ‘‘glow-exotherm’’ at temperatures close to 450°C. 6–9 For example, Wang and Lin, 10 who prepared an ultrafine zirconia powder via the alkoxide hydrolysis route, showed that the crystallization temperature of an alkoxide- derived precursor varies with the base concentration during hydrolysis. They observed a crystallization temperature of 463° and 546°C for the hydrolyzed precursors formed in water and 0.1M sodium hydroxide (NaOH), respectively. The crystalliza- tion-led exothermal reaction disappears in the precursor hydro- lyzed with a 0.5M NaOH solution. On the other hand, Wu and Yu, 11 who investigated the effects of adding various amounts of sulfuric acid (H 2 SO 4 ) on the crystallization and phase trans- formation in precipitated zirconia precursors, reported that the crystallization-led exotherm disappeared when the concentra- tion of H 2 SO 4 was high enough, although the crystallization temperature (at around 440°C) was not significantly affected by the concentration of acid. They attributed this phenomenon to the replacement of zirconia hydroxide by sulfate in the pre- cursor. Mamott et al. 12,13 have experimentally shown that the pH of the precipitating solution affects both the temperature and rate of crystallization in precipitated zirconium hydroxides when calcined at a specific heating rate. There are other species which have been shown to affect both the crystallization behavior of precipitated zirconia pre- cursors and the characteristics of resulting zirconia powders. According to Liu and Xue, 14 residual ammonium chloride (NH 4 Cl) in precipitated zirconia precursors lowers the crystal- lization temperature. They observed a crystallization tempera- ture of 376°C for the precursor containing NH 4 Cl, in contrast to 445°C for the NH 4 Cl-free precursor. Osendi et al., 15 who carried out a systematic study of thermal evolution on two ZrO 2 precursors obtained by two different processing methods, also observed differences in the crystallization temperature and intensity of the corresponding thermal peaks. They attributed the differences observed to the influence of impurities left over from the two preparation methods. In a previous study, we reported that a sol-derived zirconia precursor precipitated at pH 10.5 exhibited a lower crystallization temperature (448°C) than that of the precursor prepared by directly drying the zirconia sol at pH 3.4 (533°C). This was believed to be due to the difference between the two precursors in the type of species attached to their surfaces. 16 The aim of the present work is to study the effects of pH and species present in aging/washing solution on the thermal and crystallization behaviors of a sol- derived zirconia precursor. C. F. Zukoski—contributing editor Manuscript No. 190895. Received June 24, 1997; approved January 28, 1998. * Member, American Ceramic Society. ² Department of Materials Science. ‡ Department of Physics. § Department of Chemistry/IMRE. J. Am. Ceram. Soc., 81 [10] 2624–28 (1998) J ournal 2624