Microstructure and Photothermal Conversion Performance of Ti/(Mo-TiAlN)/(Mo-TiAlON)/Al\({}_{2}\)O\({}_{3}\) Selective Absorbing Film for Non-Vacuum High-Temperature Applications  
[PERSON]  
Correspondence: [EMAIL_ADDRESS]; Tel. : +86-591-2286-6793  
[PERSON] and [PERSON] Du  
School of Mechanical Engineering and Automation, Fuzhou University, Fuzhou 350116, China; [EMAIL_ADDRESS] (H.Y. ); [EMAIL_ADDRESS] (X.C. ); [EMAIL_ADDRESS] (S.D.) [EMAIL_ADDRESS] (S.D.) [EMAIL_ADDRESS]. 0.3-2.5 um and low thermal emittance in the infrared region of 2.5-25 um, but they require an outer vacuum glass chamber, or intrinsic absorption will become weakened during the slow oxidation process. To work in air at high temperatures, an oxygen atom is added to the film surface layer to defer oxidation reaction. This film structure has a good effect at first, but for a long time the absorptivity will become lower due to internal diffusion and chemical component homogenization of the oxygen (O) atom. Life span becomes a noticeable problem for non-vacuum high-temperature application, and how to offset the reduction of intrinsic absorption is a pressing problem. Multicomponent TiMoAlON film with Ti/(Mo-TiAlN)/(Mo-TiAlON)/Al\({}_{2}\)O\({}_{3}\) multilayer structure is designed for non-vacuum high temperatures. TiMoAlON photothermal films take (Ti,Al)N and (Ti,Al)(O,N) compounds as an absorbing dielectrics layer, and Mo as a doping element to form a metal ceramic composite layer after O atom internal diffusion. So, the main absorber layer can gradually change from a dielectric layer to a metal-ceramic composite layer. With multiple absorption mechanisms, intrinsic absorption film [5] and cermet film [6] are performed separately or jointly throughout the whole life span. Al is selected as a doping element because of its high chemical affinity to the O atom. Ultrafine Al particles embedded into the film can increase the consumption of the internal-diffused O atom, so as to improve oxidation resistance at high temperatures. In addition, Al doping into the TiN substrate can form an impurity energy level in the TiN bandgap, which can increase intrinsic absorption in the visible spectrum. Mo doping into (Ti,Al)(O,N) substrate can improve light absorption because of inter-band optical transitions and resonance absorption. To obtain high absorptance and low thermal emittance at high operating temperatures, it is essential to ensure an absorber-reflector tandem [7], so it requires to form a multi-layer structure within the micro-nano scale. In such a film structure, the absorber is used to provide high absorptance in the solar spectrum region, while the reflector is designed to provide high reflectance in the thermal infrared region. Therefore, the solar absorber coating, according to this idea, generally has a multi-layer structure, as shown in Figure 1. Based on the multi-layer structure, an interference effect can be designed to promote TiMoAlON films' absorptivity [8]. Gradually varied bimetallic nitride, bimetallic oxynitride, and metallic oxide are important for photothermal conversion. The stoichiometric ratio of each compound decides the final properties. The reactive sputtering process tends to be highly unstable. Commonly, with the increased flow of reactive gases (i.e., N\({}_{2}\) and/or O\({}_{2}\)), there is a sudden drop  
Figure 1: Cross-sectional schematic of graded-index TiMoAlON photothermal film [8]. of sputter yield due to the target surface becoming fully covered by reaction products. Generally, a high-rate reactive deposition process should be operated between metallic state and nonconductive ceramics [9]. To work successfully in such a quick dropped transition region, a rapid reactive gas control system is required. PEM (plasma emission monitor) is the most commonly adopted method and can also be used in combination with other techniques if required [10; 11]. Besides that, to precisely control metal nitrides' gradual change to oxy-nitrides and oxides (i.e., (Ti,Al)N, (Ti,Al)(NO), Al\({}_{2}\)O\({}_{3}\)) is an arduous task.