The volumetric absorbers used in point focus concentrating solar systems act as a heat exchanger, receiving solar radiation and reaching high temperatures. Non-oxide ceramics, especially silicon carbide, are the most used materials in the manufacture of volumetric receivers. However, SiC has some drawbacks, such as its difficulty in forming, its relative ease of oxidation and fracture, and its high cost. In this sense, alumina, which has excellent thermal, mechanical and chemical resistance, can withstand temperatures up to 1400 ?C or higher, remaining stable against detrimental morphological changes, so it could be a suitable and lower cost alternative, if it were not for its low solar absorbance values, due to its white colouring.

In this work, the combination of alumina with different proportions of black ceramic pigments has been studied to improve the absorbance of the material. Two different ways of mixing have been studied: mixing the powdered raw materials or by covering the surface of alumina spray-dried granules with a pigment. The influence of pigment type and proportion, the processing variables (forming and firing variables) and the properties of the materials obtained (apparent density, porosity, thermal emissivity, solar reflectance, thermal conductivity, etc.) have been studied. Accelerated ageing treatments have also been carried out to analyse the possible changes in their mechanical, thermal and optical properties, both in electric and solar ovens (in a solar accelerated ageing test bench).

The results show that the main influencer in the properties and microstructure of the compositions is the way of mixing. Moreover, proportion and type of pigment used determines the forming and firing variables, especially the optimum sintering temperature. The final properties also vary the ratio and type of pigment, with the maximum proportion of pigment being determined by the decrease in the thermal shock resistance of the samples.

The work has been carried out thanks to the project "Ceramic materials for energy applications-CERAMITECH", financed by Generalitat Valenciana, through IVACE (Valencian Institute of Business Competitiveness).

Bibliography
1. T. A. Otitoju et al. Advanced ceramic components: Materials, fabrication, and applications https://doi.org/10.1016/j.jiec.2020.02.002
2. High temperature accelerated ageing of advanced ceramic specimens for solar receivers and other applications under concentrated solar radiation https://www.cencenelec.eu/media/CEN-CENELEC/CWAs/RI/cwa17726_2021.pdf