High-temperature solar-selective coatings based on Cr(Al)N. Part 1: Microstructure and optical properties of CrNy and Cr1-xAlxNy films prepared by DC/HiPIMS

Abstract

In order to explore the potentialities of xNy materials in multilayer-based solar selective coatings (SSC) for high temperature applications (T>500 ºC), the optical behavior of Cr1-x(Al)xNy films is studied in this work. Two sets of layers (CrNy and Cr1-xAlxNy) were prepared by direct current (DC) and high-power impulse magnetron sputtering (HiPIMS) technology. The deposition parameters: N2 flux, HiPIMS frequency and aluminum sputtering power, were modified to get a wide variety of stoichiometries. The composition, morphology, phases and electronic structure of the films were characterized in depth. The optical behavior was determined by UV-Vis-NIR and FTIR spectroscopies, and the optical constants were obtained from the measured transmittance and reflectance spectra based on appropriate dielectric function models. Our results indicate that small changes in the layer composition influence the optical constant. In particular, a metallic-like behavior was obtained for CrNy layers with N vacancies (CrN0.95 and CrN0.67 films) while a semiconductor-like behavior was observed for CrN1.08. Thus, the CrNy films can be used as effective absorber layer in multilayerbased SSC, and namely, the CrN0.67 film as an IR reflector/absorber layer too. Likewise, the optical properties of Cr1-xAlxNy layers can also be tuned from metallic to semiconductor-like behavior depending on the chemical composition. Indeed, the absorption coefficients of Cr1-xAlxNy films with optimized Al content and N-vacancies are comparable to those reported for state-of-the-art materials such as TiAlN or TiAlCrN. In addition, a Cr0.96Al0.04N0.89 film was found to be a suitable IR reflector/absorber layer.

High-temperature solar-selective coatings based on Cr(Al)N. Part 2: Design, spectral properties and thermal stability of multilayer stacks

“…5) has been obtained using CODE software [29] considering the following facts: 1) the formation of a thinner Al layer (4 nm) than the nominal one to take into account the formation of disconnected aluminium islands; 2) the growth of a CrN0.67 layer with a lower N/Cr ratio than the nominal CrN0.95; and 3) using two additional CrxAl1-xN layers (Cr0.62Al0.38N1.00 and Cr0.77Al0.23N1.13) as an approximation to the observed gradual Al decrease. The optical constants of the used layers (CrN0.67, Cr0.62Al0.38N1.00 and Cr0.77Al0.23N1.13) were obtained in the companion paper [8]. Values of  = 96.0 % and  = 13.0 % are obtained with this approximation in closer agreement with the experimental ones.…”

“…4c). It is worth noting that, in both stacks, the oxygen content inside the internal Cr(Al)N layers is higher ( ̴ 10 at.%) than the one determined by electron probe microanalysis in the companion paper (<5 at.%) [8]. This can be due to a surface oxidation of the very thin specimen prepared for TEM observation and can, therefore, alter the measured element ratios.…”

“…The n() and k() values of CrNy, Cr1-xAlxNy (see part 1 [8]) and Al2O3 single layers were extracted from the transmittance and reflectance measurements using appropriated dielectric function models for the spectral range (0.25-2.5 μm). The infrared spectral range, which is important for the emittance, is dominated by the highly reflective Al layer (stack #1) and Cr0.96Al0.04N0.89 (stack #2).…”

“…In recent years, transition-metal nitrides, oxides and oxynitrides have received significant attention for use in mid-and high-temperature SSC due to their excellent mechanical, chemical, thermal and optical properties [7] which can be tailored by controlling the stoichiometry ( [8] and references therein). The combination of these materials into multiple configurations as semiconductor-metal tandem, metal-dielectric composite (cermet) in single, gradient or multilayer structures can achieve outstanding performance [2][3][4][9][10][11][12][13][14][15][16][17][18][19].…”

“…Table 1 summarizes some functional multilayer (oxy)nitride coatings reported in the literature for high temperature solar selective applications with information about optical properties and oxidation stability. Based on the results obtained in the companion part 1 of this work [8], which reported the optical and structural properties of CrNy and Cr1-xAlxNy films, we present in this paper the design and characterization of two multilayered SSC, based on these materials, for their potential use at high temperature. The coatings are prepared using high power impulse magnetron sputtering technique (HiPIMS), a relative novel PVD technology where higher plasma densities and ionization degree can be achieved [22,23].…”