B(N) redor and B spin echo studies for structural charactreization of Si-B-N precursor ceramics

Abstract

A Solid-state NMR spectroscopy is employed for the structural characterization oj precursorderived Si-B-C-N ceramics. Particular emphasis is given to the structural composition of the BNCx phase which plays a key role for the unusual high temperature stability of these materials. In the present work nB('5N) REDOR and B spin echo experiments are presented for two /SN enriched precursor systems, made from substituted polysilazanes and polvsilvlcarbodiimides. which provide interatomic boron-boron and boron-nitrogen distances. The obtained results are compatible with the presence of layered structures as reported for hexagonal boron nitride [h-BJ. The derived boron-nitrogen and boron-boron distances, however, are larger than in h-BN, reflecting some layer distortions. The boron-boron distances are found to decrease with increasing pyrolysis temperature, whereas the boron-nitrogen distances remain practically unaltered at elevated pyrolysis temperatures. On the basis of the present results it is concluded that intercalated BN.and sp2-carbon layers most likely constitute the BNCX. phase. The graphite-like carbon layers are assumed to create some internal pressure, which in turn is responsible for the observed interatomic distance increase in the BN layers. However, other scenarios, like the direct incorporation of small sp1-carbon domains into the BN-sheets, cannot be ruled out completely. Further work along this line appears to be necessary to develop a comprehensive structural model for the BNCxphase in such quarternarv ceramic systems.