Sol-gel synthesis of phosphate ceramic composites I

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Sol-gel synthesis of phosphate ceramic composites I Burtrand I. Lee Department of Ceramic Engineering, Clemson University, Clemson, South Carolina 29634-0907

William D. Samuels, Li-Qiong Wang, and Gregory J. Exarhos Battelle Pacific Northwest Laboratory, P.O. Box 999, Richland, Washington 99352 (Received 20 March 1995; accepted 11 September 1995)

Monolithic gels of phosphate ceramics were synthesized using PO(OH)32x (OR)x and alkoxides of silicon and titanium. The PO(OH)32x (OR)x species were synthesized from the reaction of P2 O5 and ethanol or n-butanol, and the products consisted of approximately equal molar amounts of mono- and dialkyl phosphate. The phosphate gels containing titanium lost less phosphorus than from the gels of silicon/phosphorus upon firing of gels in air. At phosphorus contents above 60 mole %, the gels were completely crystallized upon firing at temperatures above 700 ±C, while the gels containing zinc and alkali metals remained amorphous after firing at 850 ±C. Solid state nuclear magnetic resonance spectroscopy showed that all of the silicon is hexacoordinated in the phosphate gels containing silicon and titanium upon firing at temperatures above 520 ±C I. INTRODUCTION

Corner-sharing tetrahedra of phosphates, (PO4 )32 , resemble silicates (SiO4 )42 . Phosphates possess a number of interesting physical properties, such as nonlinear optical second harmonic generation, superionic conductivity, and luminescence. Applications of phosphate ceramics1–8 vary from lasers, ceramic filters, ion exchangers, diffusion barrier coating, low thermal expansion materials, bioceramics, to others. Livage et al.9 have synthesized alkyl phosphate by reacting P2 O5 with an alcohol to be used as the precursors to phosphate ceramics. Woignier et al.10 synthesized monolithic aerogels of SiO2 –P2 O5 and SiO2 –B2 O3 –P2 O5 using trimethyl phosphate and the respective alkoxides. Szu et al.11 synthesized phosphosilicate gels using trimethyl phosphite, trietyl phosphate, and phosphoric acid. They found that devitrification began at 200 ±C for trimethyl phosphite, and the loss of phosphorous during firing is significant for P2 O5 content above 30 mole %. The formation of the Si –O–P and P–O –P networks was evident for the gels fired at temperatures above 200 ±C. Although the loss of P was most severe for triethyl phosphate, the tendency for crystallization was the least among the different P precursors. Kim and Tressler12 examined the microstructural evolution in phosphosilicate gels prepared by hydrolyzing tetraethoxy silane with H3 PO4 . Crystallization began at 300 ±C for 56 mole % P2 O5 . Beall and Quinn13 formed zinc alkali phosphate glass-polymer composites with a number of desired properties. The composites were formed by the conventional metal oxide melt-quench method to obtain the phosphate glass. This glass was blended with high temperature organic polymers. This method of synthesizing the composite is energy intensive. 134

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