Direct current reactive sputtering Cr–Cr2O3 cermet solar selective surfaces for solar hot water applications

Replacing conventional electroplating method in manufacture of black chrome solar selective surfaces with reactive sputtering technique is desirable due to the environmental concern in order to meet rapid expansion of utilization of solar hot water systems. Experimental investigation revealed that the rapid transition region in reactive sputtering from a pure chromium metal to oxygen-rich materials resulted in the difficulty of optimization. We proposed a low-high-low (or with further repetition if needed) metal volume fraction profile for the cermet layer and a method using the target current as a control parameter in optimization. The solar absorptance of the sputtered black chrome selective surfaces was in the range 0.92–0.96 and the thermal emittance was between 0.05 and 0.08. Accelerated aging test showed that the surfaces were thermally stable to approximately 300–400 °C. Aging continuously at 170 °C in ambient environment for a period equivalent to 17 years in extreme stagnation condition, the surfaces did not show noticeable degradation. The developed method offers additional freedoms for designing new solar selective surfaces using non-conventional materials.