Structural Evolution Of Polymer-Derived Amorphous Sibcn Ceramics At High Temperature

Polymer-derived amorphous SiBCN ceramics are synthesized through a simple dehydrocoupling and hydroboration reaction of an oligosilazane containing amine and vinyl groups and BH3 · Me 2S, followed by pyrolysis. Two types of ceramics, denoted as Si 2B 1 and Si 4B 1, are produced from preceramic polymers with Si/B ratios of 2/1 and 4/1, respectively. The structural evolution of these ceramics with respect to the pyrolysis temperature and boron concentration is investigated using solid-state NMR, Raman, and EPR spectroscopy. Solid-state NMR suggests the presence of three major components in the ceramics: (i) hexagonal boron nitride (h-BN), (ii) turbostratic boron nitride (t-BN), and (iii) BN 2C groups. Increasing pyrolysis temperature leads to the transformation of BN 2C groups into BN 3 and free carbon. A thermodynamic model is proposed to explain such transformation. Raman spectroscopy measurements reveal that the concentration of the free carbon cluster decreases with increasing pyrolysis temperature, and Si 4B 1 contains more free carbon cluster than Si 2B 1. EPR studies reveal that the carbon (C)-dangling bond content also decreases with increasing pyrolysis temperature. It appears that the complete decomposition of the metastable BN2C groups to the BN3 groups and the free carbon affects the crystallization of SiBCN, which leads to Si 4B 1 ceramics crystallized at 1500 °C, whereas Si2B1 ceramics crystallized at 1600 °C. © 2011 American Chemical Society