Lowering the synthesis temperature of zircon powder by yttria addition

Abstract

Zircon (ZrSiO4) is an important material with many unique properties. It has a very low coefficient of heat conductivity (5.1 W/m ◦C at room temperature and 3.5 W/m ◦C at 1000 ◦C) and low coefficient of thermal expansion (about 4.1 × 10−6/◦C between 25 and 1400 ◦C) [1, 2]. Its mechanical strength does not degrade even at a temperature higher than 1400 ◦C, and sintered zircon has a thermal shock resistance superior to those of mullite and alumina sintered bodies. These properties make zircon a potential candidate as a useful structural ceramic for high-temperature applications. The purity of natural zircon sand is often low with many kinds of impurities such as Al2O3, Fe2O3, TiO2 and so on, which will decompose zircon at a low temperature. Normally pure zircon decomposes into ZrO2 and SiO2 at a temperature of about 1700 ◦C according to the ZrO2-SiO2 phase diagram. So it is meaningful to synthesize zircon powder with high purity for its applications, especially at high temperatures. However, it is well known that preparation of high purity zircon powder is very difficult [3]. One strategy that has been adopted by some investigators [4, 5] is to add zircon seeds into the precursors to lower the synthesis temperature, but the problem still has not been well solved. In this paper, the authors propose a new processing technique to synthesize zircon, which has a higher yield at a relatively low temperature. The zircon powders have been prepared using zirconium oxychloride (ZrOCl2) as the ZrO2 source and tetrethoxysilane (Si(OC2H5)4) as the SiO2 source. ZrOCl2 · 8H2O was dissolved in distilled water first and 2.5 mol % Y2O3 (taking the amount of ZrO2 as 100%) powder was added to the solution. The mixture was vigorously stirred at 40 ◦C to obtain an even solution. The solutions of ZrO2 and ZrO2 (Y2O3) were both mixed with an equimolar solution of Si (OC2H5)4 respectively using a magnetic stirrer to obtain a homogeneous slurry. Both slurries were brought to pH = 9.0 by adding aqueous ammonia solution while under vigorous stirring. The homogeneous gels obtained were filted and washed with deionized water and ethanol, and subsequently dried at 90 ◦C. 3 wt % zircon seeds were added to each of the dried powders and the mixtures were wet-milled for 6 h with zirconia balls in polypropylene jars. The dried powders with and without Y2O3 (being abbreviated to YSZ and SZ for convenience, respectively) were both calcined at different temperatures and the holding time was 4 h. The calcined powders obtained were analyzed by the XRD method. The formation rate of zircon in the powders obtained was calculated by the following equation [3, 4]: