Ultrahigh-Temperature Ceramic Aerogels

We demonstrate the synthesis of high-surface-area, low-density refractory aerogels. The monolithic hafnium boride (HfB2) and zirconium boride (ZrB2) 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–MO2) 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 HfB2 and ZrB2 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 ZrB2 monoliths ranged from 0.18 to 0.33 W/(m K). The surface areas of the HfB2 and ZrB2 aerogels were 10 and 19 m2/g, respectively. Successful reduction of the aerogels to the diboride phase was confirmed by X-ray diffraction.