Influence of anion on the synthesis of gel-derived zircon

Abstract

The sol-gel technique, the main advantage of which is to avoid melting and the problems associated with it [1], has been more widely applied for making glass than for making ceramic products. Although the preparation of ceramic powders from colloidal dispersions has sometimes been termed a sol-gel process [2], it was Yoldas who first used the sol-gel technique to produce transparent alumina at a temperature as low as 500 °C [3]. Later Yoldas used an all-alkoxide method to produce ceramic monoliths in the entire range of the AlaO3-SiO2 binary system. The all-alkoxide route, although ideal, is not essential in making binary silicate gels simply because it is silica that forms a network in a glassy silicate structure. Modifier or intermediate cations such as A1, Zr, Ti, Zn and Sr can be introduced either in the form of nitrate, halides, etc., or in the form of salts of weak (organic) acids. It has been shown in earlier work [1] that although mineral acid salts have poor compatibility with the gel structure, the addition of weak acid salts causes no such problems of segregation and inhomogeneity. We have further found that weak acid salts behave significantly differently from their mineral acid counterparts in influencing crystallization in binary silicate xerogels. To illustrate this point, gels of zircon (ZrSiO4) composition were produced with two precursor systems, namely zirconium oxychloride-tetraethyl orthosilicate (TEOS) and zirconium acetate-TEOS mixtures hydrolysed in ethanolic medium. Zirconium acetate was obtained by reacting a stoichiometric mixture of acetic acid and freshly precipitated hydroxide of zirconium in a water bath (90-100 °C) for 10-12 h. Since the addition of vanadium is necessary for zircon synthesis, 5 wt % vanadium in the form of ammonium vanadate (NH4VO3) was added to each of the zirconia-silica sols. Gelation was allowed to take place in covered containers and, after it was over, the samples were exposed to the atmosphere for drying. As drying proceeded, separation of salt from the gel structure was observed in chloride-containing gel (chloridegel), whereas the acetate-containing gel (acetategel) retained its chemical homogeneity throughout the entire course of drying. In the chloride-gel, salt was carried out by pore liquid from the core of the gel to the surface in the form of fine fluffy powder. Chemical analyses confirmed the salt to be that of zirconium. Chloride-gels were intimately ground after being dried to avoid large-scale compositional variation.