Refractory borides are well-known for their high melting temperatures and as such interest is growing for their application as a thermal barrier coating for extreme environments such as spacecraft reentry and hypersonic flight. One shortcoming of these materials is their higher than desirable thermal conductivity, which may limit their applications where active cooling is not possible. To combat this issue, we have developed diboride aerogels and xerogels with high porosity and sub-micron particle size.
Aerogels are known to have some of the lowest recorded thermal conductivities but are typically composed of oxide materials with melting temperatures too low to be suitable for thermal protection. Boride aerogels hold the promise of merging the material advantages of ultra-high temperature ceramics with the structural advantages of aerogels.
We present the synthesis of monolithic ZrB2 and HfB2 aerogels by reduction of boron-zirconia and boron-hafnia aerogel precursors. 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. Borothermal reduction of this composite aerogel results in diborides with 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 (i.e. xerogels). 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 and samples were analyzed for thermal stability in oxidizing atmospheres.