Effect of boron on the crystallization of amorphous Si-(B-)C-N polymer-derived ceramics

Amorphous Si-(B-)C-N polymer-derived ceramics (PDCs) with a boron content ranging from 0 to 8.3 at.%, were synthesized by thermolysis of boron-modified poly(methylvinylsilazane). Correlation of the boron content and the thermal stability of these materials in the course of annealing were investigated using high temperature thermal gravimetric analysis (HT-TGA). Furthermore, the initial crystallization of the as-thermolyzed amorphous ceramics was studied by X-ray diffraction (XRD) measurements. The increase of boron content promotes the crystallization of SiC, and inhibits the crystallization of Si3N4. Moreover, the ratio of alpha-Si3N4/beta-Si3N4 in crystalline ceramic decreases with increasing boron content. Thermodynamic modeling proves the influence of boron content on driving force for crystallization. The available thermodynamic model of amorphous Si-C-N domains, nano-crystalline silicon nitride, and nano-crystalline silicon carbide treated as the separated phases has been used to interpret these results. (C) 2009 Elsevier B.V. All rights reserved.