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 Me(2)S, followed by pyrolysis. Two types of ceramics, denoted as Si(2)B(1) and Si(4)B(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(2)C groups. Increasing pyrolysis temperature leads to the transformation of BN(2)C 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(4)B(1) contains more free carbon cluster than Si(2)B(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 BN(2)C groups to the BN(3) groups and the free carbon affects the crystallization of SiBCN, which leads to Si(4)B(1) ceramics crystallized at 1500 degrees C, whereas Si(2)B(1) ceramics crystallized at 1600 degrees C