Thermal oxidation of stainless steel substrate with tunable spectral selectivity: Transition from a reflecting to a highly absorbing Cr–Fe spinel surface

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The conventional methods of obtaining solar selective surfaces for high temperature solar thermal applications involve coating of the substrate by various methods such as physical vapor deposition, plasma spraying, anodization, etc. The present work is an attempt to enhance the optical properties of metals by heat treatment. The oxide layers formed by annealing of stainless steel 304 (SS 304) enhance the absorptance in the solar spectrum region. Influences of oxidation temperature and oxidation time span on the values of solar absorptance and thermal emittance have been studied. The annealing of SS 304 substrate was carried out in air at 600–900 °C. The time period of annealing plays a crucial role in the amount of oxides formed and thus is a variable parameter. The absorptance values obtained with isothermal oxidation at shorter and longer durations have been compared at a temperature of 900 °C. A cyclic loading approach is also employed to arrive at the optimal absorptance of the samples. It is further used to study the dependency of solar absorptance on the reaction kinetics with respect to varying oxidation time. Under the optimized annealing conditions, heat-treated SS 304 sample exhibited an absorptance of 0.920 and an emittance of 0.37. A plausible model for the high optical absorption in these oxidized surfaces with relatively low thermal emittance is rationalized. High temperature materials such as Inconel and Nimonic have also been subjected to isothermal annealing and the absorptance values were found to be 0.887 and 0.880, respectively.