Ultra-High Temperature Ceramics (UHTCs) include borides, carbides and nitrides of group IV and V and are characterized by the highest melting points of known materials, elevated temperature strength, oxidation resistance and other peculiar properties such as high thermal and electrical conductivity. This combination of excellent properties makes them the election materials for extreme applications. They were mainly investigated for military and aerospace applications so far, but our group proposed and extensively studied their use for solar applications from more than a decade. We proved their promising properties such as low thermal emittance and high spectral selectivity, identifying the relevant parameters and best strategies to optimize them [1]–[6]. In the present work we describe our multi-scale optimization approach to tailor the properties of UHTC ceramics towards their application in novel high-temperature solar thermal receivers. Using an integrated approach from macro- to micro- to nano-scale, we show that acting on primary and secondary phases, density, porosity and surface finishing, we can modify the optical properties of ceramics. The most investigated systems are ZrB2, HfB2, TaB2 with MoSi2 as secondary phase.
 
Furthermore, using a similarly integrated approach from macro- to micro-scale, other intrinsic weaknesses (e.g. brittleness, damage tolerance, heavy weight) can be overcome by the creation of composite materials, namely new carbon fibre-reinforced UHTCs [7]. This new class of developed materials, called UHTCMCs  has the ability to combine the oxidation resistance of UHTCs phases with the damage tolerance of fibres and, in a recent preliminary work, has been shown to possess dramatically favourable optical properties as well [8].
 
[1]      E. Sani, L. Mercatelli, F. Francini, J.-L. Sans, D. Sciti, “Ultra-refractory ceramics for high-temperature solar absorbers,” Scr. Mater., 65, 775–778, 2011, doi: 10.1016/j.scriptamat.2011.07.033.
[2]      L. Silvestroni et al., “An overview of ultra-refractory ceramics for thermodynamic solar energy generation at high temperature,” Renew. Energy, 133, 1257–1267, 2019, doi: 10.1016/j.renene.2018.08.036.
[3]      E. Sani, L. Mercatelli, M. Meucci, L. Silvestroni, A. Balbo, D. Sciti, “Process and composition dependence of optical properties of zirconium, hafnium and tantalum borides for solar receiver applications,” Sol. Energy Mater. Sol. Cells, 155, 368–377, 2016, doi: 10.1016/j.solmat.2016.06.028.
[4]      E. Sani, E. Landi, D. Sciti, V. Medri, “Optical properties of ZrB2 porous architectures,” Sol. Energy Mater. Sol. Cells, 144, 608–615, 2016, doi: 10.1016/j.solmat.2015.09.068.
[5]      E. Sani, S. Failla, D. Sciti, “Dark alumina for novel solar receivers,” Scr. Mater., 176, 58–62, 2020, doi: 10.1016/j.scriptamat.2019.09.038.
[6]      E. Sani, L. Mercatelli, M. Meucci, A. Balbo, L. Silvestroni, D. Sciti, “Compositional dependence of optical properties of zirconium, hafnium and tantalum carbides for solar absorber applications,” Sol. Energy, 131, 199–207, 2016, doi: 10.1016/j.solener.2016.02.045.
[7]      D. Sciti et al., “Properties of large scale ultra-high temperature ceramic matrix composites made by filament winding and spark plasma sintering,” Compos. Part B Eng., 216, 108839, 2021.
[8]      L. Zoli, S. Failla, E. Sani, D. Sciti, “Novel ceramic fibre-Zirconium diboride composites for solar receivers in concentrating solar power systems,” Compos. Part B Eng., 242, 110081, 2022, doi: 10.1016/j.compositesb.2022.110081.