Synthesis and thermal properties of dense Si─Al─C─N-based polymer-derived ceramics

In the present study, aluminum-modified preceramic silicon polymers were synthesized via chemical modification of a commercially available organopolysilazane using an aluminum amido complex. The incorporation of aluminum into the polymer structure and its effect on ceramization behavior and processability as well as the thermal and mechanical properties of the obtained Si─Al─C─N ceramic materials were systematically investigated. The microstructure and chemical composition of the dense, predominantly amorphous monolithic Si─Al─C─N were characterized using solid-state nuclear magnetic resonance, scanning electron microscopy, and transmission electron microscopy. Aluminum incorporation led to enhanced densification of Si─C─N, resulting in monoliths with a porosity fraction as low as 2 vol% achieved for the composition with the highest aluminum content. Al incorporation was shown to also result in a significant reduction in thermal conductivity, thus, Si─Al─C─N formulations exhibited values as low as 0.6 W m⁻¹ K⁻¹. The hardness and Young's modulus remained nearly unchanged upon aluminum incorporation, with values of ca. 14.5 and 156 GPa, respectively, for the high-aluminum-content sample. To the best of our knowledge, the present study reports for the first time on the thermal and mechanical properties of dense, mainly amorphous Si─Al─C─N ceramics, highlighting the suitability of Si─Al─C─N as excellent material for possible thermal insulation at temperatures beyond 1000°C.