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MATERIALS RESEARCH

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Sol-gel synthesis of phosphate ceramic composites II Zhicheng Cao and Burtrand I. Leea) Department of Ceramic and Materials Engineering, Clemson University, Clemson, South Carolina 29634-0907

William D. Samuels, Li-Qiong Wang, and Gregory J. Exarhos Battelle Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352 (Received 13 January 1997; accepted 12 January 1998)

Phosphate ceramics were synthesized using sol-gel technique of direct reaction of P2 O5 with tetraethoxy silane (TEOS) or titanium tetraethoxide (Ti(OEt)4 ). The reaction mechanism of P2 O5 and TEOS was deduced using liquid and solid-state NMR. Hexacoordinated silicon in phosphosilicate gels was observed. A specially structured titanium phosphate-layered Ti(HPO4 )2 ? 2H2 O was synthesized for the first time through the sol-gel method. The gelation process and phase transformations were investigated.

I. INTRODUCTION

Phosphate ceramic materials have potential applications in many advanced technology areas, such as new laser sources and prosthetic materials. However, due to processing difficulties, the potential of phosphate ceramics has not been fully realized. In our previous paper,1 we reported the sol-gel synthesis of the phosphate ceramics using PO(OH)32x (OR)x and alkoxides of silicon and titanium. The alkyl phosphate PO(OH)32x (OR)x was prepared from anhydrously reacting P2 O5 and an alcohol. Phosphate ceramics have also been studied by several other research groups2–6 for applications as optical fibers7,8 and as host materials for fast ionic conductors.9,10 All of them used either phosphoric acid or PO(OH)32x (OR)x or P(OH)3 as the precursor of phosphorus. Schrotter et al.5 used various phosphorous alkoxides including P(OEt)3 , (OEt)2 POP(OEt)2 , or PO(OEt)3 , and could not synthesize homogeneous phosphosilicate compounds successfully due to the big difference in hydrolysis rate of the silicon precursor and phosphorus precursors. A more direct method is to react P2 O5 and a silicon alkoxide to obtain the phosphosilicate sols and gels. By using P2 O5 directly instead of other available phosphorus precursors, our study shows that not only was the reaction process simplified, but also the gelation process and the problem of phosphorus loss upon firing were greatly improved. II. EXPERIMENTAL A. Materials

The chemicals used in this study were obtained from Aldrich Chemical Company, Milwaukee, WI as reagent grade or better. Solvent purities were either that with the a)

Author to whom correspondence should be addressed. J. Mater. Res., Vol. 13, No. 6, Jun 1998

http://journals.cambridge.org

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lowest water concentration or were dried by distilling with sodium metal prior to use. All reaction processes were either carried out in glove box or in an inert gas atmosphere to maintain anhydrous condition. All glassware used in the preparations was dried overnight at 250 ±C to remove surface water. The glassware was cooled under a blanket of nitrogen to prevent rehyd