Polymeric Precursors for Yttria
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ABSTRACT Polymers that can be easily processed into fibers, films, or bodies are excellent precursors for ceramics with useful morphologies. The ideal preceramic polymer not only has proper physical properties (e.g. solubility or fusibility) for processibility but also decomposes at low temperature with high ceramic yield. One possibility for such precursors for yttria are yttrium oxycarboxylates. The preparation of such polymers from YOCI has been investigated and several compounds with useful physical properties have been discovered. These include a methanol-soluble yttrium oxycarboxylate, (CH 3OCH2CH 2 OCH 2 CO 2 YO)n, a liquid yttrium tricarboxylate, (CH 3 OCH 2 CH 2OCH 2 CO 2 )3 Y, and a method for preparing very concentrated yttrium mixed acetate and formate sols.
INTRODUCTION One promising method for the synthesis of ceramic coatings and fibers is polymer pyrolysis, whereby the film or fiber is formed from a processible polymer
which is converted, upon heating, to the ceramic material. This process is a natural extension of the tremendous success demonstrated by the conversion of organic polymers into carbon fibers and bodies [1]. The application of such technology to the
production of ceramic fibers has produced excellent results and many oxide and non-oxide ceramic fibers have been produced, often with superior properties to graphite fibers [2]. Seyferth [31 has delineated several features required for the preparation of ceramics by polymer pyrolysis; (1) the monomer must be inexpensive and readily available, (2) the polymerization reaction is simple and proceeds with high yield (3) the polymer is stable in air at room temperature (4) the polymer is processible, ie.
fusible or soluble, and (5) the pyrolysis proceeds smoothly without violent gas evolution, producing the ceramic in high yield with a minimum of shrinkage. One type of polymeric ceramic oxide precursor which satisfies the latter three requirements is macromolecules containing a metal-oxygen backbone and pendant carboxylate side-groups. One example of such polymers is carboxylate-substituted alumoxane polymers, (RCO2AlO)x, that are successful preceramics for alumina [4]. The synthesis of metaloxane polymers that might be useful preceramics requires a different tact than that employed in the preparation of organic polymers. The latter materials are generally prepared via polymerization of monomers, a route that is, for the most part, unfeasible for inorganic macromolecules due to the 679 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
paucity of unsaturated inorganic monomers. The main alternatives for the preparation of inorganic polymers is the use of condensation processes, ringopening polymerization, and derivitization of chemically-reactive polymeric intermediates. The latter method is the most promising of the three, since the ultimate properties of a polymer often depend dramatically on the nature of the pendant groups. Thus, derivitization of a polymer with many different side groups allows extensive tailoring
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