The film thickness dependent thermal stability of Al2O3:Ag thin films as high-temperature solar selective absorbers

Abstract

The monolayer Al 2 O 3 :Ag thin films were prepared by magnetron sputtering. The microstructure and optical properties of thin film after annealing at 700°C in air were characterized by transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and spectrophotometer. It revealed that the particle shape, size, and distribution across the film were greatly changed before and after annealing. The surface plasmon resonance absorption and thermal stability of the film were found to be strongly dependent on the film thickness, which was believed to be associated with the evolution process of particle diffusion, agglomeration, and evaporation during annealing at high temperature. When the film thickness was smaller than 90 nm, the film SPR absorption can be attenuated until extinct with increasing annealing time due to the evaporation of Ag particles. While the film thickness was larger than 120 nm, the absorption can keep constant even after annealing for 64 h due to the agglomeration of Ag particles. On the base of film thickness results, the multilayer Al 2 O 3 :Ag solar selective thin films were prepared and the thermal stability test illustrated that the solar selectivity of multilayer films with absorbing layer thickness larger than 120 nm did not degrade after annealing at 500°C for 70 h in air. It can be concluded that film thickness is an important factor to control the thermal stability of Al 2 O 3 :Ag thin films as high-temperature solar selective absorbers.

Alloyed nanoparticle-embedded alumina nanocermet film: A new attempt to improve the thermotolerance

“…The annealed sample was pretreated in turn at 200 and 300 • C in air for 2 h. After data acquisition at the topmost surface, the XPS measurements were recorded after each step with Ar plasma etching for 45 s. Compared with the as-deposited case, the distribution profile of Ag in the annealed sample does not show any obvious changes, although a certain amount of Ag distribution is detected on the top face of the sandwich structure. This phenomenon has also been observed by Xiao et al [12].…”

“…However, the intensity of the peak at 38.1 • begins to decrease monotonically with extending the annealing time. Only one weak peak of (1 1 1) plane can be observed after annealing at 500 • C for 62 h. The time evolution of diffraction peak suggests that the volume fraction of Ag is gradually reduced, which in turn is stemmed from the outwards diffusion and even evaporation of the Ag elements [12,31,32]. The loss of Ag would aggravate the optical degradation of the Ag-Al 2 O 3 films during the long-time annealing, as shown in Fig.…”

“…Unfortunately, however, as a metal with low melting temperature (about 960 • C), Ag nanoparticles are easily subjected to agglomeration and outwards diffusion when working at elevated temperatures, which would limit their high-temperature applications [12][13][14][15][16]. Barshilia et al [7] have clearly pointed out that Ag-Al 2 O 3 coatings have dramatic degradation at higher temperatures (>400 • C) due to the defragmentation of the Ag nanoparticles in the dielectric matrix.…”

“…Since Al 2 O 3 exhibits excellent stability at high temperatures, the thermal stability of the nanocermet mainly depends on the thermal tolerance of Ag nanoparticles. Xiao et al [12] believed that the agglomeration of Ag nanoparticles was favorable to improving the thermal resistance of Ag-Al 2 O 3 nanocermets due to the low surface free energy of the agglomerates. However, simply increasing the size of the Ag nanoparticles would become invalid eventually after long-term annealing, so it is not an appropriate solution.…”

Inverted nanotaper-based Ag film for optical absorption and SERS applications

“…It can be seen that the intensity of the peak at 38.23°ascribed to Ag (1 1 1) plane becomes stronger after the annealing treatment, indicating a higher crystallinity of Ag film. In addition, for the thicker Ag coating (80 nm), the peaks of Ag (2 2 0) and (3 1 1) appear and the peak of Al (3 1 1) disappears after annealing due to the preferential orientation [26]. There are some unknown peaks marked by ''⁄'' induced by some impurities which may be introduced during fabrication and measurement processes.…”