  Although spectrally selective materials play a key role in existing and
emerging solar thermal technologies, temperature-related degradation currently
limits their use to below 700C in vacuum, and even lower temperatures in air.
Here we demonstrate a solar-transparent refractory aerogel that offers stable
performance up to 800C in air, which is significantly greater than its
state-of-the-art silica counterpart. We attribute this improved stability to
the formation of a refractory aluminum silicate phase, which is synthesized
using a conformal single-cycle of atomic layer deposition within the
high-aspect-ratio pores of silica aerogels. The transparent refractory aerogel
achieves a record-high receiver efficiency of 77% at 100 suns and an absorber
temperature of 700C based on direct heat loss measurements at this temperature.
Such performance and stability can enable the use of advanced supercritical CO2
power cycles and lead to a ~10% (absolute) improvement in solar-to-electrical
conversion efficiency. Transparent refractory aerogels may also find widespread
applicability in solar thermal technologies by enabling the use of lower-cost
optical focusing systems and eliminating the need for highly evacuated
receivers.
