Lanthanum hexaboride (LaB6) is a known thermionic material. Its ability in emitting electrons has been widely exploited in last 60 years to build hot cathodes for several applications, including electron guns in cathode tubes, cathodes for plasma production and electron microscopes. While at present, mainly single crystalline emitters are used, sintered ceramics show thermionic properties as well, and historically they have been the first to be studied.
Renewable energies, and in particular solar energy exploitation issues, are a key topic for the future. At present, the main technologies for concentrating solar systems are solar thermal power and concentrating photovoltaics. However, advanced thermionic-based approaches have recently been introduced for electrical power generation from the conversion of concentrated solar radiation, as combined thermionic-thermoelectric generators [1], hybrid thermionic-photovoltaic converters [2], etc. For this kind of devices, electron emitting thin layers are deposited on the back of solar absorbers, which commonly are taken from the family of UHTCs [3], [4]. The consequent twofold function is sunlight capturing on one face and electron emission for successive conversion into electricity on the opposite face. This kind of system allows maximizing both radiation absorption and electron emission properties by engineering independently the separate functionalities. The choice of a specific thin film and the corresponding absorber as substrate must be performed according to some physical matching constraints (thermal and electrical conductivity, thermal and mechanical stability, etc.) and considering the operating temperature.  Anyway, the thin-emitter approach is an overly complex matter, since the electron emission properties and the performance stability can be affected by several aspects, such as interface instability, electrons’ reflections, formation of undesired layers, etc.
Polycrystalline LaB6 ceramics prepared by sintering of commercial powders can potentially simultaneously play the role of high-performance thermionic cathode and solar receiver for concentrated systems, being its properties (optical, thermal, electric, and thermo-electronic) competitive for a highly efficient solar-to-electrical conversion. Therefore, the present work shows the characterization of optical and electronic properties of LaB6 ceramic samples as a function of sample porosity and surface roughness, in order to evaluate the feasibility of LaB6 discs in practical solar-enabled thermionic energy converters. Moreover, femtosecond laser texturing has been applied on the solar absorbing face to optimize solar absorptance and thermal emittance. Depending on the laser processing parameters, the optical properties can be remarkably improved or, on the contrary, degraded. This result opens promising perspectives towards the practical development of a double-function thermionic-thermal converter to be used in novel hybrid CSP plants [5]–[7].
[1]      D. M. Trucchi et al., doi: 10.1002/aenm.201802310.
[2]      A. Bellucci et al., “doi: 10.1021/acsenergylett.0c00022.
[3]      E. Sani et al, doi: 10.1016/j.scriptamat.2011.07.033.
[4]      E. Sani et al, doi: 10.1063/1.4717515.
[5]      E. Sani et al, doi: 10.1038/s41598-017-00749-w.
[6]      A. Bellucci et al., doi: 10.1016/j.ceramint.2021.04.079.
[7]      E. Sani et al., “Femtosecond-laser enhanced spectral selectivity on sintered lanthanum hexaboride ceramics for thermal-thermionic solar absorbers,” submitted, 2023.