The cermet-based solar selective absorbing coatings consisting of metal nanoparticles and ceramic matrices are the most widely used photothermal conversion coatings. However, these coatings cannot function at high temperatures for a long time due to the insufficient thermal stability of metal nanoparticles. Various methods, including element alloying, the method of covering the metal nanoparticles with a passivation layer, and the method of substituting metal nanoparticles with oxide/nitride nanoparticles, have been proposed to improve the thermal stability of nanoparticles in cermet coatings. As the method of covering the metal nanoparticles with a passivation layer is not suitable for large-scale production, the approaches by alloying or substituting the metal nanoparticles with oxide/nitride nanoparticles as light absorption components offer huge development potential. It is worth noting that the detailed spectral selective absorption mechanisms for different particles, such as bimetallic, oxide, and nitride nanoparticles, are still lacking sufficient research. Therefore, we employ the first principle calculation and finite-difference time-domain (FDTD) simulation to investigate the effect of band structure, binding, and distribution of nanoparticles on the solar selective absorbing characteristics of cermet coatings for photothermal conversion. Furthermore, we propose a new strategy to enhance the thermal stability of cermet coatings while retaining the selective absorption performance.

The light absorption principle of metal nanoparticles is localized surface plasmon resonance. To be specific, the free electrons on the metal surface turn into heat energy via non-radiative dissipation under the influence of external electromagnetic fields. The substantial orbital hybridization of bimetals alters the electrical structure significantly. Meanwhile, oxides and nitrides contain metallic bonds, ionic bonds, and covalent bonds. The energy band structure is significantly different from that of metal nanoparticles, which can show metallicity and dielectric properties. Moreover, the size and distribution of nanoparticles can influence the performance of the photothermal conversion coating as the particle size of the nanoparticles in the cermet is much smaller than the incoming wavelength. The electronic structures and energy bands of WTi, Cr₂O₃, and TiN are estimated and analyzed by the first principle in this study. The electron redistribution inside WTi, Cr₂O₃, and TiN is calculated with conventional electronic properties such as differential charge density and electronic density of states, as well as the bonding state between constituent atoms. In this way, we reveal the photothermal conversion mechanism of cermet coatings with various types of nanoparticles as light-absorbing components. On this basis, the FDTD simulation is used to simulate the effect of nanoparticle distribution characteristics on the selective absorption characteristics. Given the above results, a microstructure containing multi-scale layered nanoparticles is designed and fabricated by ion-source-assisted cathodic-arc plating. Then, the thermal stability and optical properties of the as-deposited and annealed coatings are investigated.

This paper theoretically investigates the band structures and electron binding characteristics of bimetals, oxides, and nitrides and analyzes the effect of nanoparticle type on the absorbing behavior of cermet-based solar selective coatings. The results show that the doping of Ti in the bimetallic nanoparticle WTi can produce strong atomic orbital hybridization, which leads to the band upshift and narrow bandwidth. This can strengthen the local confinement of electrons and help enhance the inter-band coupling and the surface plasmon resonance effect. There is a narrow band gap in the oxide nanoparticle Cr₂O₃, and the bond length is easy to change. Therefore, the light absorption mechanism of Cr₂O₃ will change from the electron transition mode to the surface plasmon resonance effect under high temperatures. There is no forbidden band in the nitride nanoparticle TiN, which reflects a wider range of light absorption wavelengths. Meanwhile, the stable bond length and high carrier concentration between particles enhance the stability of the system and the light absorption effect of free carriers. The FDTD simulation demonstrates that in the band range of 0.3-1.5 μm, small-sized nanoparticles have a high absorption coefficient while large-sized nanoparticles are more dependent on scattering. Thus, the layered distribution of nanoparticles shows superiority to the conventional layered structure and the randomly dispersed structure (Fig. 8). Thus, a Cr/AlCrN/AlCrON/AlCrO multilayer coating consisting of lamellarly distributed nanoparticles in the absorbing sublayer is designed and fabricated by ion-source-assisted cathodic arc plating (Fig. 12). More importantly, the multilayer coating shows an outstanding selectivity (0.901/0.184) (Fig. 9) and excellent thermal stability even after annealing at 500 ℃ for 1000 h in the air (Table 2). This suggests that the deposited coating is a potential candidate for photothermal conversion under high temperatures.

In this paper, the effects of band structure, electron binding properties, and distribution characteristics of WTi, Cr₂O₃, and TiN on selective absorption characteristics are studied by the first principle calculation and the FDTD method. Intense electron localization of WTi prevents free carrier absorption, which makes its absorption mechanism more susceptible to surface plasmon resonance and intrinsic absorption. Optical absorption effect of Cr₂O₃ is largely in the form of an electronic transition at room temperature, with exciton absorption in the low energy photon region. However, at high temperatures, the transition absorption effect is decreased owing to the crystal volume expansion, atomic spacing increase, and additional band gap rise while thermal movement boosts lattice vibration and surface plasmon resonance. The strong electron delocalization in TiN encourages carrier absorption, and as it has both covalence and metallicity, TiN contains optical absorption modes such as intrinsic absorption. Nanoparticles with varying diameters have distinct absorption benefits. Small-sized nanoparticles of 0.3-1.5 μm have a high absorption coefficient, whereas large-sized nanoparticles have a great scattering coefficient but not a high absorption coefficient. In addition, the layered distribution of nanoparticles in the absorbing sublayer can enhance the interaction between the solar light and nanoparticles, which will increase the absorptivity. This unique microstructure can retard the agglomeration of nanoparticles during long-term operation at high temperatures, and thus boost the thermal stability of the multilayer coating. Moreover, a Cr/AlCrN/AlCrON/AlCrO tandem multilayer coating is prepared by ion-source-assisted cathodic arc plating. The coating exhibits a relatively high absorptance of 0.901, a relatively low emittance of 0.184, as well as outstanding thermal stability with a selectivity of 0.914/0.198 even after annealing at 500 °C for 1000 h in the air.