Porous and dense hafnium and zirconium ultra-high temperature ceramics for solar receivers

•Dense and porous Hf and Zr-based UHTCs are sintered and structurally characterized.•They are spectrally selective, according to RT optical characterization.•Thermal emittance in the 1100–1800 K temperature range shows promising properties.•Thus they appear promising for novel solar absorbers operating at high temperature.

The present work reports on the comparative characterization of optical properties of hafnium and zirconium-based ultra-refractory ceramics at room and high temperature, in view of their possible use in novel solar receivers for thermal solar plants operating at higher temperatures. We show how porosity and surface finishing affect both the spectral reflectance and the high-temperature emittance. Moreover, structural and chemical changes occurring at high temperatures are assessed.