Diffraction Study on the Atomic Structure and Phase Separation of Amorphous Ceramics in the Si-(B)-C-N System. 1. Si-C-N Ceramics

Amorphous Si-C-N ceramics were produced over a wide range of compositions by thermolysis of structurally different polysilazanes. The structure of the asthermolyzed amorphous ceramics and of ceramic samples after additional heat treatments was investigated by diffraction experiments in the wide-angle and the small-angle scattering regimes. Adopting contrast variation by combination of X-rays and neutrons and isotopic substitution, the observed correlations were assigned to individual atomic pairs. The atomic distances found in the ceramics correspond well with the distances in graphite and -Si3N4. Using the concentration contrast method it could be shown quantitatively that the amorphous Si-C-N ceramics consist of an amorphous graphite-like phase and of amorphous(alpha)Si3+(1\4)xN4-xCx (x = 0-4). The partial pair correlation functions of the two phases could be evaluated. The size of the phase-separated regions, of the order of 10 A in the as-thermolyzed ceramics, as determined from small-angle scattering, increases upon annealing up to some 10 A. The partial structure factor of the amorphous carbon phase was simulated by a model. The simulation shows that the atomic arrangement of the amorphous carbon phase in Si-C-N ceramics in the layers is similar to that of glassy carbon, but the order between the layers is less pronounced. With increasing temperature the arrangement of the layers approaches the order in glassy carbon. A sputter-deposited Si-C-N ceramic shows similar short-range order, but no phase separation on a medium-range scale, as compared with precursor-derived ceramics