Facile synthesis of monolithic carbon/alumina composite aerogels with high compressive strength using different inorganic aluminium salts

Resorcinol–formaldehyde/alumina composite (RF/Al₂O₃) gels were initially prepared using sol–gel techniques, and then dried to aerogels with supercritical fluid CO₂. RF/Al₂O₃ aerogels were successfully converted to monolithic carbon/alumina composite (C/Al₂O₃) aerogels after carbonization under flowing Ar at 800 °C. The samples were characterized by Brunauer–Emmett–Teller, scanning electron microscopy, transmission electron microscope and X-ray diffraction, and the compressive strengths were also measured. The results indicated that the resulting C/Al₂O₃ aerogels prepared from hydrated AlCl₃ possessed microstructures containing highly reticulated networks of fibers, 2–5 nm in diameter and of varying lengths, whereas the samples prepared from hydrated Al(NO₃)₃ were amorphous with microstructures comprised of interconnected spherical particles with diameters in the 5–15 nm range and the alumina were surrounded by amorphous carbon. The difference in microstructure resulted in each type of aerogels displaying distinct physical and mechanical properties. In particular, the as-prepared C/Al₂O₃ aerogels with the weblike microstructure were far more mechanically robust than those with the colloidal network. Correspondingly, the compressive strengths are 5.6 and 2.8 MPa, respectively.