Structural Evolution of Polymer-Derived Amorphous SiBCN Ceramics at High Temperature

Abstract

Polymer-derived amorphous SiBCN ceramics are synthesized through a simple dehydrocoupling and hydroboration reaction of an oligosilazane containing amine and vinyl groups and BH3·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 (<i>h</i>-BN), (ii) turbostratic boron nitride (<i>t</i>-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 °C, whereas Si₂B₁ ceramics crystallized at 1600 °C.