Silicon–boron–carbon–nitrogen monoliths with high, interconnected and hierarchical porosity

Abstract

Silicon–boron–carbon–nitrogen (Si–B–C–N) monoliths with high, interconnected and hierarchical porosity have been prepared by spark plasma sintering (SPS) of ordered mesoporous powders with a P6mm hexagonal symmetry at low temperature without any sintering additives. The ordered mesoporous Si–B–C–N powders derived from boron-modified polycarbosilazanes displayed a mesopore population centred on 4.6 nm, a total pore volume of 0.78 cm³ g⁻¹ and a specific surface area of 544 m² g⁻¹. They have been partially sintered in the temperature range 800–1000 °C under nitrogen to lead to robust meso-/macroporous Si–B–C–N monoliths with surface areas of 123–171 m² g⁻¹, mesopore diameters centred on 6.2–6.5 nm and total pore volumes varying from 0.25 to 0.35 cm³ g⁻¹ measured by nitrogen adsorption experiments. As-obtained crack-free Si–B–C–N monoliths displayed porosities from 59 to 69% and a relatively large pore size distribution as determined by helium pycnometry and confirmed by mercury porosimetry. TEM observations and low angle X-ray diffraction (LA-XRD) confirmed the formation of monoliths that maintained a mesoporosity organization in comparison to starting powders while SEM experiments showed a microstructure in which necks are formed around the area of contact between particles. With a thermal stability extending up to 1400 °C in flowing nitrogen and a heat conductivity of 0.647 W m⁻¹ K⁻¹ for the most porous component, these new materials display the necessary intrinsic properties required as porous supports working in a harsh environment.