Paper published on: Optical Materials, Volume 36, Issue 2 , December 2013, Pages 163–168 DOI 10.1016/j.optmat.2013.08.020 http://www.sciencedirect.com/science/article/pii/S092534671300462X Porous and dense Hafnium and Zirconium Ultra-High Temperature Ceramics for solar receivers Elisa Sani* a , Luca Mercatelli a , Jean-Louis Sans b , Laura Silvestroni c and Diletta Sciti c . a CNR-INO National Institute of Optics, Largo E. Fermi, 6, I-50125 Firenze, Italy b PROMES-CNRS Processes, Materials and Solar Energy Laboratory,7 rue du Four Solaire, 66120 Font Romeu, France c CNR-ISTEC,Institute of Science and Technology for Ceramics, Via Granarolo 64, I-48018 Faenza (Italy) * corresponding author: Tel: +39(0)55.23081 Fax:+39(0)55.233775 email elisa.sani@ino.it Abstract: 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. Keywords: borides; carbides; optical properties; emittance; solar plants; concentrating solar power; porosity; roughness. 1. Introduction Finding an environmental-friendly, efficient and sustainable answer to the energy problem is one of the main challenges the mankind has to face now and in the next future. Concentrating Solar Power (CSP) is considered to be one of the most promising and sustainable technologies both for electricity [1] and solar fuels production [2]. For both applications, the increase of the plant operating temperature is a critical requirement for increasing the overall system efficiency. The key parameter is the absorber material constituting the solar receiver, which is devoted to collect and efficiently transfer to the thermal exchange medium the energy concentrated by the whole mirror field. The ideal sunlight absorber for high temperatures must possess a series of favourable properties: a good mechanical and chemical