Solid-State NMR Studies of the Preparation of Si-Al-C -N Ceramics from Aluminum-Modified Polysilazanes and Polysilylcarbodiimides

Multinuclear (13C, 29Si, 27Al, 1H) solid-state NMR spectroscopy and EPR and FT IR experiments are employed to investigate the thermolysis of aluminummodified poly(hydridovinylsilazane), [Al(C2H4-SiHNH) 3]n, from which Si-Al-C-N ceramics can be formed. In addition, a comparative study is provided for different aluminum-modified polysilazanes and polysilylcarbodiimides in the amorphous state at 1400 C. The investigation primarily aims at the structural characterization of the amorphous intermediates, where such spectroscopic techniques have demonstrated their particular suitability. It is shown that the transformation of the polymeric precursor to the (amorphous) preceramic network is completed at around 500 C. At this temperature AlN domains, as well as Si -C-N clusters of mixed composition, are formed. Above this temperature a continuous transformation to the ceramic material takes place. In addition, above 700 C a considerable amount of free carbon-centered radicals is detectable. Eventually, three main components are found for the amorphous ceramic at 1050 C which remain up to 1400 C: (i) amorphous (graphite-like) carbon, (ii) AlN domains (with tetrahedral, pentagonal, and octahedral coordinated aluminum atoms), and (iii) a Si-C-N matrix (SiCxN4-x units with x = 0, 1, 2, 4). It is worthwhile to note that even at such high thermolysis temperatures a considerable amount of hydrogen can be detected as well. Comparative solidstate NMR experiments also have been performed on the amorphous ceramics fired at 1400 C, derived from polysilazanes and polysilylcarbodiimides that differ in their basic structure as well as Si\N and Al\N ratios. For all samples the same basic components could be assigned. The actual composition of the amorphous ceramic, however, differs which can be related to the basic structure of the precursor polymer. In summary, the present spectroscopic study demonstrates that the introduction of aluminum atoms in the ternary Si-C-N matrix shows new aspects in the field of precursor ceramics.