Solid-state NMR investigations on Si-B-C-N ceramics derived from boron-modified poly(allylmethylsilazane)

Multinuclear (C-13, Si-29, B-11, H-1) solid-state NMR and EPR spectroscopy are employed to study the thermolysis of boron-modified poly(allylmethylsilazane), [B{C3H6-Si(CH3)NH}(3)](n), from which a quaternary Si-C-B-N ceramic is formed. The investigations are primarily focused on the characterization of the amorphous intermediates on the atomic scale, where such spectroscopic techniques have demonstrated their particular suitability. In addition, NMR data are provided for the transformation from the amorphous to the crystalline ceramic. It is shown that the transformation of the polymeric precursor into the (amorphous) preceramic network is completed at 600degreesC. At this temperature BN domains as well as SiCxN4-x units of mixed composition are formed. Above this temperature, a continuous transformation to the final ceramic material takes place. At 1400degreesC the amorphous ceramic consists of three main components: (i) amorphous (graphite-like) carbon, (ii) BN domains (mainly hexagonal boron nitride), and (iii) a Si-C-N matrix (SiCxN4-x units with x = 0, 1, 2 or 4). The EPR measurements reveal the presence of carbon-centred free radicals between 600 and 1400degreesC, which also have some impact on the C-13 NMR data. The NMR investigations further demonstrate that in the temperature range between 1600 and 1800 degreesC a demixing of the amorphous ceramic sets in, along with the formation of crystalline silicon nitride and silicon carbide phases. Likewise, structural changes for BN domains are registered that are attributed to the formation of turbostratic B-N-(C) interface layers. It is interesting to note that a finite amount of hydrogen can be detected in the ceramic samples even after exposure to temperatures near 2000degreesC.