Cermet solar thermal selective absorber coatings are an important component of high-efficiency concentrated solar power (CSP) receivers. The oxidation of the metal nanoparticles in cermet solar absorbers is a great challenge for vacuum-free operation. Recently, we have demonstrated that oxidation is kinetically retarded in solution processed, high-optical-performance Ni nanochain-SiOxcermet system compared to conventional Ni-Al2O3 system when annealed in air at 450–600 °C for several hours. However, for long-term, high-temperature applications in CSP systems, thermodynamically stable antioxidation behavior is highly desirable, which requires new mechanisms beyond kinetically reducing the oxidation rate. Towards this goal, in this paper, we demonstrate that pre-operation annealing of Ni nanochain-SiOxcermets at 900 °C in N2 forms the thermodynamically stable orthorhombic phase of NiSi at the Ni/SiOxinterfaces, leading to self-terminated oxidation at 550 °C in air due to this interfacial engineering. In contrast, pre-operation annealing at a lower temperature of 750 °C in N2 (as conducted in our previous work) cannot achieve interfacial NiSi formation directly, and further annealing in air at 450–600 °C for >4 h only leads to the formation of the less stable (metastable) hexagonal phase of NiSi. Therefore, the high-temperature pre-operation annealing is critical to form the desirable orthorhombic phase of NiSi at Ni/SiOxinterfaces towards thermodynamically stable antioxidation behavior. Remarkably, with this improved interfacial engineering, the oxidation of 80-nm-diameter Ni nanochain-SiOx saturates after annealing at 550 °C in air for 12 h. Additional annealing at 550 °C in air for as long as 20 h (i.e., 32 h air annealing at >550 °C in total) has almost no further impact on the structural or optical properties of the coatings, the latter being very sensitive to any interfacial changes due to the localized surface plasmon resonances of the metal nanostructures. This phenomenon holds true for Ni nanoparticle diameter down to 40 nm in Ni-SiOx system, where the optical response remains stable for 53 h at 550 °C in air. The oxidation vs. time curve also shows saturation behavior deviating from the kinetic Deal-Grove oxidation model. These results strongly suggest a promising approach to thermodynamically stable, anti-oxidation Ni/SiOxcermet absorbers via interfacial engineering.
Part of this work had previously been supported by the National Science Foundation (NSF) Small Business Innovation Research (SBIR) Program under Contract No. 1315245 via the subcontract from Norwich Technologies, Inc. Currently, the work was supported by the Department of Energy, Office of Energy Efficiency and Renewable Energy (EERE), under Award No. DE-EE0007112. We would also like to thank Dr. Charles P. Daghlian at Dartmouth College for his help with the transmission electron microscopy.
I. INTRODUCTION II. EXPERIMENTAL III. RESULTS AND DISCUSSION A. Chemical bonding analysis B. TEM analysis C. Towards thermodynamically stable, long-term anti-oxidation behavior 1. Saturation behavior in oxidation vs. time curve upon air annealing 2. Evolution of reflectance spectra upon air annealing 3. Mechanism of the self-terminated oxidation process in Ni-SiOxcermets IV. CONCLUSIONS