Intrinsic spectral selectivity in ultra-high temperature ceramics for solar applications
Different concepts for solar receiver systems have been developed, and each collector architecture (linear parabolic systems, solar dishes, solar tower plants, etc) demands peculiar system solutions [1]. It is a general rule that the efficiency of solar thermal systems rapidly increases with increasing working temperature. In solar tower plants, a critical parameter for temperature increasing is the receiver, where the whole heliostat field concentrates the collected sunlight. The family of Ultra High Temperature Ceramics (UHTCs) can be interesting for this application because of its unique combination of properties. In fact UHTCs are characterized by some of the highest melting points of any known material (>3900°C for the monocarbides of Ta and Hf), high hardness, good wear resistance, good chemical stability and mechanical strength at high temperatures and high thermal conductivities. Up to now, UHTCs are employed mainly in the aerospace industry for hypersonic vehicles, rocket motor nozzles or atmospheric entry probes capable of the most extreme entry conditions. The ultra-high melting point of UHTCs, together with the unique combination of good thermal conductivity and chemical stability appear intriguing for employing them in high temperature solar furnaces [2].
Processing, mechanical properties and oxidation behavior of TaC and HfC composites containing 15 vol%TaSi2 or MoSi2
Fully dense HfC and TaC-based composites containing 15 vol% TaSi 2 or MoSi 2 were produced by hot pressing at 1750–1900 °C. TaSi 2 enhanced the sinterability of the composites and nearly fully dense materials were obtained at lower temperatures than in the case of MoSi 2 -containing ones. The TaC-based composites performed better than HfC composites at room temperature, showing values of mechanical strength up to 900 MPa and a fracture toughness of 4.7 MPa·m 1/2 . However, preliminary oxidation tests carried out in air at 1600 °C revealed that HfC-based composites have a superior high temperature stability compared to TaC-based materials.
Ion beam texturing
Wehner and Hajicek have reported that a microscopic surface texture could be created by sputter etching a surface while simultaneously sputter depositing a lower sputter yield material onto the surface. A xenon ion beam source has been used to perform this texturing process on samples as large as 3 cm diam. Ion‐beam‐textured surface structures have been characterized with SEM photomicrographs for a large number of materials, including Cu, Al, Si, Ti, Ni, Fe, stainless steel, Au, and Ag. Surfaces have been textured using a variety of low sputter yield materials—Ta, Mo, Nb, and Ti. The initial stages of the texture creation have been documented, and the technique of ion beam sputter removal of any remaining deposited material has been studied. A number of other texturing parameters have been studied such as the variation of the texture with ion beam power, surface temperature, and the rate of texture growth with sputter etching time.