## 5 Induced Aging Mechanisms of STSCs
### Isothermal Aging Mechanism  
Coating degradation, both mechanically and optically, occurs due to prolonged exposure to concentrated sunlight at high temperatures and fluctuations in solar intensity. A common testing method for this is isothermal aging, where the adapted STSC is exposed to a fixed temperature higher than its typical operational conditions [95]. Prolonged exposure to elevated temperatures induces morphological changes, speeds up elemental diffusion, and intensifies chemical reactions, all of which lead to the deterioration of the STSC [95]. [PERSON] et al. [93] examined the lifetime degradation of Pyromark STSC using thermally cyclic tests. These samples were exposed to square-wave heat flux cycles with rapid cooling and heating rates to mimic the fluctuations in solar intensity. In the most rigorous tests, the heat flux ranged from 250 kW/m2 to 700 kW/m2, ranging from 400 \({}^{\circ}\)C to 800 \({}^{\circ}\)C over a period of up to 200 cycles. [PERSON] et al. [93] demonstrated that thermal stress is a significant factor contributing to coating failure, especially when combined with the reduction in the layers bonded strength that happens within aging. However, this study was based on isothermal testing with only periodic cycling to facilitate optical characterization of the samples. The coating spalled due to two key factors: reduced bonding strength from micro-defects and oxide layers and stress from thermal expansion mismatch between layers, as schematically seen in Figure 8.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Country**} & \multirow{2}{*}{**Product Company**} & \multirow{2}{*}{**Material Structure**} & **Operating** & **Optical Properties** & **Ref.** \\ \cline{3-6}  & & & **Temperature** (\({}^{\circ}\)C) & \(\alpha_{s}\) (\%) & \(\varepsilon_{\text{th}}\) (\%) & \\ \hline \multirow{4}{*}{Germany} & ALANOD Solar & Mirotherm® (Al) & - & 95 & 5.0 & [94] \\  & Suneelect (CrN–CrxOy) & 100 & 95 & 5.0 & [94] \\ \cline{2-6}  & Spezialglas & ALUXID® & 280 & 95 & 5.0 & [94] \\ \cline{2-6}  & IKARUS & a-C:H/Cr & 250 & 92 & 8.0 & [30] \\ \cline{2-6}  & ALMECO & TiNOX/Cu & 400 & 95 & 4.0 & [94] \\ \cline{2-6}  & SCOTT & PTR®70 Premium & 500 & 95 & 9.5 & [30] \\ \hline \multirow{2}{*}{Japan} & SIEMENS & Al2O3 cermet W–Al2O3 & 350–500 & 96 & 10 & [30] \\ \hline \multirow{2}{*}{China} & \multirow{2}{*}{TurboKuhki} & SS–C & 300 & 93 & 4.0 & [94] \\  & & SS–AlN & 300–350 & 95 & 12 & [94] \\ \cline{1-1}  & & W–AlN & 330–500 & 94 & 8.0 & [94] \\ \hline Sweden & S-Solar & Ni–NiO & 300 & 96 & 10 & [30] \\ \hline Italy & Angelantoni-ENEA & Mo–SiO2 & 580 & 94 & 13 & [30] \\ \hline \hline \end{tabular}
\end{table}
Table 3: Commercially available solar selective coatings.  
[PERSON] et al. [96] studied the formation of oxidation and thermal efficiency of Pyromark STSC through isothermal aging at different cooling rates. Following a 24 h aging period at 750 \({}^{\circ}\)C, thermally cyclic tests were conducted, where the sample temperature fluctuated between 750 \({}^{\circ}\)C and 300 \({}^{\circ}\)C, with a 2 h hold at the peak temperature, over a duration of up to 300 cycles. N\(\rm\ddot{o}\)ca et al. [97] performed extended thermally cyclic testings on a porous coating applied to an Inconel substrate to replicate day/night temperature variations. They conducted over 1600 cycles (approximately 4000 h) and compared the degradation in absorptance with samples that were subjected to isothermal testings at 770 \({}^{\circ}\)C, 750 \({}^{\circ}\)C, and 720 \({}^{\circ}\)C--temperatures equivalent to the peak temperatures.