SOLID-STATE NMR STUDIES ON PRECURSOR-DERIVED SI-B-C-N CERAMICS

Precursor-derived Si-B-C-N ceramics are known for their exceptional good thermal stability up to temperatures of about 2000 degrees C. It is anticipated that this property is strongly related to the unique structural composition, comprising nanocrystalline SiC and Si3N4 domains along with a turbostratic BNCx phase. In this contribution, solid-state NMR studies are performed in order to get a deeper insight into the structural composition of these ceramic materials. Particular emphasis is given to the BNCx phase and its alteration during the thermolytic preparation process. In this context, suitable NMR experiments are presented, from which internuclear boron-nitrogen and boron-boron distances can be derived. Generally, these internuclear distances are found to be longer than those in pure h-BN. These longer distances are in agreement with other experimental NMR data which also point to materials with some local disorder, which holds even for the highest annealing temperature. The longer internuclear distances within the BNCx component are related to some stress caused by the sp(2)-carbon layers within the BNCx phase. Hence, it is claimed that at 1050 degrees C and above, distorted BN layers and sp(2)-carbon sheets form the BNCx phase. Whether they occur as intercalated, but separate layers or whether they exist as interdigitated nanodomains of h-BN and sp(2)-carbon cannot be conclusively answered.