Synthesis, characterization and thermal properties of boron and silicon containing preceramic oligomers

Abstract

In recent years, considerable interest has been shown towards developing boron and silicon containing polymers as they possess certain technological advantages when compared to polymers containing only boron or silicon [1, 2]. These polymers on pyrolysis in inert atmosphere give mixed non-oxide ceramics such as B4CSiC, B4C-Si3N4 and BN-SiC. These mixed non-oxide ceramics are reported to possess superior high temperature properties, oxidation resistance and thermal shock resistance when compared to boron and silicon based simple non-oxide ceramics [1, 3, 4]. Boron and silicon containing polymers have also gained importance as protective coatings for materials which are constantly exposed to oxidizing environment [5–8]. When these coatings are exposed to oxidizing environment at elevated temperatures, a protective passive borosilicate glassy layer is formed on the surface which prevents further oxidation of the protective coating and the substrate. Boric acid is considered to be the cheapest source of boron for making boron and silicon containing polymers. Yajima and coworkers [9] synthesized poly (borodiphenylsiloxane) through the reaction of boric acid with diphenyldichlorosilane in n-butylether at 300 ◦C. Hoshii et al. [10–12] prepared borodiphenylsiloxane oligomer by reacting boric acid and diphenylsilanediol. However, these polymers give relatively low ceramic residue on pyrolysis (<50%) and also require high temperature (∼300◦) for their synthesis. In this communication, we report the synthesis of boron and silicon containing oligomers, capable of giving a high ceramic yield, through the condensation of boric acid with phenyltrimethoxysilane and vinyltriethoxysilane at a relatively low temperature (150–160 ◦C). A mixture containing 57 g (0.3 mol) of vinyltriethoxy-silane, 9.27 g (0.15 mol) of boric acid, 50 ml of diglyme and 1 ml of concentrated hydrochloric acid was taken in a three necked round bottom flask equipped with a mechanical stirrer, an inlet and outlet for nitrogen. The contents of the flask were heated under the flow of nitrogen in an oil bath maintained at 150–160 ◦C. The reaction mixture was stirred for 3 h at this temperature, after which it was allowed to cool to room temperature. Ethanol which was formed as a by product was distilled off. Diglyme was distilled off