Effect Of Dopants on the Crystallization of Zircon From Zirconium Silicate Gels
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EFFECT OF DOPANTS ON THE CRYSTALLIZATION OF ZIRCON FROM ZIRCONIUM SILICATE GELS J.D. BARRIE AND M.J. MESHISHNEK Materials Sciences Laboratory, The Aerospace Corporation, Los Angeles, CA 90009-2957 ABSTRACT Zirconium silicate gels containing small amounts of foreign cations were found to exhibit different crystallization behavior as compared to undoped gels. The reaction to form the zircon phase was enhanced by the presence of these dopants, with zinc oxide additions causing the largest decrease in crystallization temperature. The crystallization behavior of these gels is compared to that of zircon prepared from alternative processing techniques. INTRODUCTION The development of spacecraft thermal control coatings requires the production of pigments which resist color center formation in the natural space radiation (UV, electron, proton) environment. One compound which has been suggested as a possible candidate for this application is tetragonal ZrSiO 4 (zircon). Unfortunately, this material is difficult to prepare in high purity and fine particle size by traditional ceramic processing techniques [1,2]. The presence of transition metal impurities significantly reduces the suitability of materials for optical applications due to undesired absorption bands in the solar spectral region. Recent studies have shown that sol-gel processing offers distinct advantages towards the synthesis of high purity materials, often at substantially lower temperatures than is required for traditionally processed ceramics. Zirconium silicate gels have received considerable attention of late as precursors for alkali resistant glasses and glass ceramics [3,4]. Studies have been performed to ascertain the devitrification behavior of glasses containing zirconia-silica ratios as high as 60-40 [4,5]. The crystallization behavior of zirconium silicate gels prepared with and without addition of crystalline zircon seeds has been explored to demonstrate the possibility of solid-state epitaxy effects [6]. This novel approach has demonstrated that the nucleation of the zircon phase is indeed a rate limiting step at temperatures below 1300°C in 50-50 zirconia-silica gels, and that the addition of seeds drastically lowers the temperature at which crystallization to form zircon proceeds at measurable rates. This paper examines the applicability of sol-gel processing to the formation of zircon pigments for thermal control paints, and addresses the effects of small amounts of dopants (incorporated to alter the optical properties and radiation stability of the pigments [7]) on the formation of crystalline tetragonal ZrSiO 4 . EXPERIMENTAL PROCEDURES Zirconium silicate gels were prepared using a base catalyzed reaction of alcoholic solutions of the precursors for each component of the gel. A solution containing tetraethoxysilane (TEOS), zirconium-n-propoxide [Zr(OPrrn) 4 ] and ethanol (1:1:8 mole ratio) was prepared under flowing nitrogen and stirred until homogeneous. No attempt was made to prehydrolyze any of the components. The resultant solution was sta
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