Preparation And Thermal Stability Of SiBONC Ceramics

The synthesis and pyrolysis of the polymeric precursors, morphologies and chemical bonds in polymeric precursor and ceramic powders derived from precursors, mechanical properties of sintered ceramics, microstructures, thermal stability and crystallization mechanism were studied by means of TG-DTA, XRD, XPS, FT-IR, SEM, TEM and HRTEM.Suitable SiBONC polymeric precursors were successfully synthesized from the starting materials, silicon tetrachloride, HPSO and BCl3. The polymeric precursors have dissolvability, polymerization and structural stability. At the same time, the synthesis can be conducted in a mild condition. Toluene was found to be a effective solvent, which can control the extent and speed of the reaction. The precursors with different Si/B ratio can be synthesized through control the reaction temperature and the dosage of cross-linking agent. The main cross-linking reaction is occurred between≡Si-Cl(H) groups and H-NH- groups, the product has a loop or network polymer structure where Si-O-Si bonds with phenyl amine or ethylene diamine groups, and boron element is fixed into the Si-O-N-C network through B-N bonds.The ceramic yield of the precursor is very high, with a maximum value of 57%. The ceramic powder made from pyrolysis of the polymeric precursor shows an amouphous structure, consists of Si, B, O, N, C elements, forming Si-N, Si-O-N, Si-O-Si, O-Si-N-B, B-N, Si-C groups. The process of the pyrolysis can be divided into two steps, the low temperature step and high temperature step. In the first step, the radical of low binding energy were first discomposed, generated some Si?,C?,O?,N? electrically charged radicals. In the second step, Si-O/Si-O,Si-O/Si-C,Si-H/Si-O,Si-O/Si-N,B-Cl, N-H radicals occurred rearrangement reaction, generated C-Si-O and O-Si-N-B radicals. The particles of the ceramic powder present spheroidal morphorlogy with diameters of 50-150nm, and self-assemle into chains and balls.Dense SiBONC ceramics were prepared by hot-pressing the SiBONC ceramic powders. The ceramics have low densies ranging in 1.8-2.2g/cm3, but...