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 center dot Me2S, followed by pyrolysis. Two types of ceramics, denoted as Si2B1 and Si4B1, 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) BN2C groups. Increasing pyrolysis temperature leads to the transformation of BN2C groups into BN3 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 Si4B1 contains more free carbon cluster than Si2B1. 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 Si4B1 ceramics crystallized at 1500 degrees C, whereas Si2B1 ceramics crystallized at 1600 degrees C.