Solid-state NMR and FT IR studies of the preparation of Si-B-C-N ceramics from boron-modified polysilazanes

Multinuclear (Si-29, C-13 B-11, N-15, H-1) solid-state NMR and FT IR spectroscopy is employed to investigate the thermolysis of boron-modified polyhydridovinylsilazane, [B(C2H4-SiHNH)(3)](n), from which a high-temperature stable Si-B-CN ceramic can be formed. The study is focused primarily on the characterization of the amorphous intermediates on the atomic scale, where such spectroscopic techniques have demonstrated their particular suitability. In addition, data are provided for the transformation from the amorphous to the crystalline ceramic. It is shown that the transformation of the polymeric precursor to the (amorphous) pre-ceramic network is completed at around 500 degreesC. At this temperature the BN domains and Si-C-N units of mixed composition are formed. Above this temperature a continuous transformation to the ceramic takes place. At 1050 degreesC the amorphous ceramic consists of three main components: (1) amorphous (graphite-like) carbon; (ii) planar BN domains; and (iii) an Si-C-N matrix (SiCxN4-x units with x = 0, 1, 2). In addition, a considerable amount of hydrogen is present even at this temperature. The NMR studies have further shown that above 1700 degreesC the amorphous ceramic demixes, along with the formation of crystalline silicon nitride and silicon carbide. Likewise, structural changes for BN domains have been registered that are attributed to the formation of turbostratic BN(C) interface layers. In summary, the present study has demonstrated that the combination of multinuclear solid-state NMR and FT IR spectroscopy is a powerful method to probe the thermolytic preparation of ternary and quaternary ceramic materials. Copyright (C) 2001 John Wiley & Sons, Ltd.