Ultrahigh-Temperature Ceramic Aerogels

We demonstrate the synthesis of high-surface-area, low-density refractory aerogels. The monolithic hafnium boride (HfB 2 ) and zirconium boride (ZrB 2 ) aerogels are prepared via borothermal reduction of precursor hafnia and zirconia aerogels, respectively, consisting of a fine mixture of boron nanoparticles and the metal oxide. This precursor boron–metal oxide (B–MO 2 ) composite aerogel was synthesized by modifying the pure ethanol solvent typically used in the epoxide-initiated sol–gel synthesis of metal oxide aerogels with an ethanolic boron nanoparticle suspension. After reduction, precursor aerogels are converted to metal boride aerogels containing primary particles in the sub-100 nm regime. The relative densities of the HfB 2 and ZrB 2 aerogels are 3 and 7%, respectively, and could be tailored by simply changing the density of the precursor aerogels via modifying the reagent concentrations or the drying conditions. Thermal conductivities of the ZrB 2 monoliths ranged from 0.18 to 0.33 W/(m K). The surface areas of the HfB 2 and ZrB 2 aerogels were 10 and 19 m 2 /g, respectively. Successful reduction of the aerogels to the diboride phase was confirmed by X-ray diffraction.