## 5 Induced Aging Mechanisms of STSCs
### Aging Mechanism Induced by Grain Size Variation  
The optical properties of the coating are heavily influenced by its grain size, meaning that alterations in the coating's microstructure can negatively impact its performance. Specifically, a reduction in grain size leads to increased scattering of ultraviolet light, which in turn results in a decrease in reflectance as simply shown in Figure 10 [102; 109].  
[PERSON] et al. [110] numerically analyzed the grain size variation of a Mo-SiO\({}_{2}\) cermet STSC at different inclination angles. Their findings showed that a clear refinement of Mo grains and the formation of a thick cermet film occurred in vacuum annealing at 600 \({}^{\circ}\)C, which enhanced solar conversion efficiency. However, when annealing was performed at 800 \({}^{\circ}\)C, the Mo grains exhibited abnormal growth, resulting in a decrease in thermal conversion efficiency. This phenomenon is primarily attributed to the additional energy present at the grain boundaries following deposition. Reducing this energy promotes a stable state by providing the driving force needed for changes in grain size. Furthermore, extending the strengthening time decreases residual stresses and facilitates atom expatriation, which also contributes to altering the grain size. [PERSON] et al. [111] investigated the painting propertiesof the AlCrNO/AlCrN/AlCrO under the effects of induced aging and thermal shock. Their findings indicated that annealing at 500 \({}^{\circ}\)C enhanced the absorptance of the coating by reducing the grain size. Additionally, thermal shock increased the nitride content in the coating, which resulted in a reduction in reflectance in a sequential attempt on the same STSC [112]. It was observed that the \(\alpha/\varepsilon\) values of the sample after the thermal shock test changed from 0.93/0.20 to 0.94/0.21, demonstrating the coating's excellent resistance to thermal shock. Similar to the sample after thermal aging testing, the nitride content in the coating increased. However, the reflectance pattern of the STSC subjected to thermal shock differed from that of the thermally aged sample due to the appearance of micro defects on the coating's surface after the thermal shock test.  
In summary, the surveyed results indicate that diffusion and oxidation aging-induced mechanisms are the most extensively studied for STSCs, with efforts focused on enhancing their long-term stability. Furthermore, it mostly indicated that the oxidation of STSCs at high temperatures includes the oxidation of metals, metal oxides, and alloys. In contrast, these oxides exhibit diverse behaviors at high temperatures. While metal oxidation has been extensively studied, the oxidation behavior of metallic and alloy oxides remains complex due to the multitude of factors influencing their performance, particularly at elevated temperatures. The behavior of these oxides is influenced by various factors, including operational temperature, coating structure, reaction resistance, material type, preparation pressure, diffusion pathways of metastable phases, thermodynamic properties, and elemental composition. To date, there is no widely accepted model that accurately represents the oxidation of solar absorber coatings. Consequently, further analysis is required to better understand these coatings. Regarding the mechanism of grain size changes, the degradation process primarily hinges on defects arising from alterations in the optimized surface morphology, composition distribution, and microstructure of the STSCs. Changes in thickness, grain size, and density are crucial factors influencing the degradation of STSCs.