Kinetics and mechanisms of pyrolysis of polyborosilazanes to thermally stable amorphous and crystalline states by a novel synthesis route

A new type of SiBCN ceramic is being investigated and considered for a variety of high temperature applications. SiBCN ceramics have been produced by various synthesis methods in different monomer systems and subsequent pyrolysis processes at different temperatures. The final ceramic composition and structure are significantly affected by the selection of the polymer precursor material and the pyrolysis conditions. Although detailed studies or discussions of the synthesis of the preceramic materials are rather limited, considerable progress has been achieved by alternative synthesis approaches in recent decades.; Our study introduced a novel synthesis route to produce thermally stable SiBCN-based ceramics. This route also is approximately an order of magnitude less costly, and processing is simpler and faster than other routes investigated to date. Polyborosilazane was synthesized by using three monomers. Polymerization is initiated by substituting chlorines attached to silicon and boron for trimethylsilylamino groups in hexamethyldisilazane (HMDZ). The polyborosilazane becomes an amorphous structure with crosslinked bonds during the elimination of [(CH3) 3SiCl]n. In a second stage, an intermolecular condensation reaction occurred, with the loss of HMDZ in the absence of crosslinking agents. This new route allows the SiBCN preceramic polymers to be produced with a high yield, and without a special filtering step.; The polyborosilazane preceramic polymers were modified by altering the monomers and their respective molar ratios, and by optimizing the reaction temperature during polymerization. Remarkably, the pyrolyzed ceramic products remain amorphous to temperatures up to 1600°C. Results show that SiBCN-containing ceramics have enough thermal stability to retard crystallization.; The chemical structure and composition of the polyborosilazane preceramic polymers were postulated and analyzed via a combination of structural and compositional analysis. By pyrolysis of the preceramic polymers, the mechanisms and kinetics of the structural and compositional changes associated with thermal decomposition were determined.; Hydrothermal stability of the pyrolyzed or partially pyrolyzed products was evaluated by exposing material to 350°C and 3000 psi in water. After hydrothermal exposures, the boron content of the effluent from a dissolved polyborosilazane preceramic polymer in water was determined to be below 200 ppm. Results show that the low-cost polyborosilazane preceramic polymer remains hydrothermally stable, under both severe hydrothermal conditions and elevated temperature heat treatments.