THERMOGRAVIMETRIC MASS-SPECTROMETRIC INVESTIGATION OF THE THERMAL-CONVERSION OF ORGANOSILICON PRECURSORS INTO CERAMICS UNDER ARGON AND AMMONIA .1. POLY(CARBOSILANE)

The pyrolysis under argon and under ammonia of Yajima's poly(carbosilane) (PCS) was studied by means of thermogravimetric analysis associated with continuous mass spectrometry of the gases evolved. The degradation of the polymer skeleton under argon leads to the evolution of methylsilanes and higher linear oligomers; the escape of hydrogen starts simultaneously. Mechanisms of pyrolysis are proposed, which involve in a first step the homolytic cleavage of Si-H and Si-C bonds in the poly(carbosilane) skeleton. Under ammonia, amination and carbon removal (mainly as methane) start well below the thermal decomposition of ammonia. The first step likely corresponds to the nucleophilic substitution of Si-H bonds by ammonia; Si-C bonds are substituted at higher temperature, and homolytic cleavages are probably involved. However, H-abstraction from N-H bonds hinders the cross-linking via Si-Si and Si-C-Si bridges, thus allowing complete nitridation.