Si(Al)BCN Ceramics from the Pyrolsysis of Aluminum-Modified Poly(boro)silazanes

 

            The addition of Al to silicon-based ceramics advantageously contributes to the improvement of

the hydrothermal stability of the derived materials.  As an illustration, the addition of Al to silicon oxynitrides forming SiAlON27 results in an improved thermal and chemical stability in oxidizing atmosphere, in the presence of water vapour and corrosion species which is not the case of SiON ceramics.  Moreover, the addition of Al to SiCN in flowing air has been shown to result in a non-parabolic oxidation curve (at T = 1000°C) which decreases more rapidly with time, down to a negligible level. It was suggested that the remarkably low oxidation rates of these materials can be

attributed to the lower permeability of the formed oxide layer to molecular oxygen, due to the incorporation of Al in the silica network. The passivation layer hinders diffusion-controlled oxidation in the bulk which protects the material from oxidation. 

 

            More reports have shown that SiAlCN ceramics own a superior creep resistance, improved oxidation and corrosion resistance and also a better thermal conductivity in comparison to Al-free Si/C and Si/C/N systems. These materials also exhibit an enhanced AC conductivity with the increase of the temperature and frequency. It has been shown that adding Al to SiCN improves its electrical conductivity and it has been demonstrated that SiAlCN/CarbonNanotubes shell-core composites can be successfully used as battery electrodes.

 

            Polymer-Derived SilicoAluminum CarboNitride (SiAlCN) ceramics are also considered as promising candidates for high-temperature and harsh environment applications.  Si(Al)BCN, in particular, is an excellent candidate for nose cones and wing leading edges in hypersonic vehicles and reentry systems due to its high heat resistance and exceptional microstructural stability. Si(Al)BCN can also be used in gas turbine engines for various components, including ceramic matrix composites, ceramic fibers, and high-temperature sensors.