Novel poly(borosiloxane) precursors to amorphous SiBCO ceramics

Novel poly(borosiloxanes), with a B/Si molar ratio of 0.2, were synthesized by a dehydrogen coupling reaction of 1,3,5,7-tetramethyl-1,3,5,7-tetracyclotetrasiloxane (D4H) oligomer and boric acid, B(OH)3, and by hydrolysis/condensation reactions of vinyltriethoxysilane and B(OH)3, followed by a hydrosilylation reaction with D4H, which gave rise to boron and silicon oxycarbide glasses (SiBCO), by pyrolysis. The polymers as well as the polymer-to-ceramic transformation resulting in SiBCO glasses, at 1000 °C, were characterized by thermogravimetric analysis (TGA), X-ray diffraction (XRD), Fourier-transform infrared (FTIR), 29Si and 11B MAS NMR spectroscopies. FTIR spectra of the poly(borosiloxanes) obtained showed an absorption at ∼880 cm−1, assigned to the stretching of triple bond; length of mdashSisingle bondOsingle bondBdouble bond; length as m-dash bonds, which is an evidence for the connectivity between the silicon oligomers and BO3 units. The pyrolysis of these polymers at 1000 °C, in argon atmosphere, produced black and amorphous SiBCO materials. 29Si NMR analyses of these glasses showed different distributions of Si sites, similar to those observed for silicon oxycarbide glasses.