Ultra-high temperature porous graded ceramics for solar energy applications

Porous graded materials (PGMs) consisting of ZrB2 are obtained by Spark Plasma Sintering (SPS) taking advantage of an asymmetric die configuration with changing cross section. Due to the temperature gradient established along the longitudinal direction, porosity changes up to about 26 vol.% were generated across the volume. The surface roughness of the resulting products increased as the holding temperature was reduced. Superior solar absorbance (alpha) and lower spectral selectivity (alpha/epsilon) properties were correspondingly obtained. PGMs displayed higher a and lower alpha/epsilon values with respect to highly dense samples produced using a constant cross section die. More important, these products exhibit similar or improved optical properties with respect to the reference SiC material. These findings are important in view of the development of a novel class of ceramics for solar energy applications, able to operate under higher temperature conditions while simultaneously satisfying optical, microstructural and mechanical needs.