Optical characterization of hafnium boride and hafnium carbide-based ceramics for solar energy receivers

The availability of spectrally-selective, thermally stable and more efficient sunlight absorbers represents a crucial aspect for the development of solar energy technologies able to operate at elevated temperature. In this regard, the optical properties of dense HfB2, HfC, HfB2-SiC and HfB2-HfC-SiC products are assessed, compared and discussed in the present work, in view of the possible utilization of such Ultra-High-Temperature Ceramics (UHTCs) for thermodynamic solar energy conversion. The materials above are first produced in bulk form by Spark Plasma Sintering (SPS), starting from ceramic powders preliminarily prepared by Self-propagating High temperature Synthesis (SHS), or by reactive-SPS, using elemental reactants. All sintered specimens displayed relative density above 95%, with the composite systems approaching the theoretical density values, due to the beneficial role played by SiC as sintering aid, which also improves the oxidation resistance of the resulting UHTCs. The material composition is found to largely affect the related optical properties, as the SiC addition typically increases solar absorbance and decreases spectral selectivity with respect to pure UHTC boride and carbide phases. A final discussion is given about the best tradeoff material.