Abstract
One type of multimodal (comprised of nanometer and sub-micrometer WC particles) and two types of conventional (comprised of nanometer and sub-micrometer WC particles, respectively) WC/Co powders were deposited on AISI 304L stainless steel substrates by using high velocity oxygen fuel spraying. The use of multimodal WC particles was indicated to have a beneficial effect on the solar absorptance (α) of the WC/Co coatings. The α of the multimodal WC coating reached 0.87, which was much higher than what can be achieved by either fine (0.82) or coarse powders (0.80) alone. By microstructural analysis, the enhancement in solar absorptance of the multimodal WC/Co coating was ascribed to the layer of distributed WC particles. During the thermal spraying, the nanostructured WC particles underwent rapid melting for the large specific surface area while the aggregated powders were heated, but not necessarily melted. The molten nano-WC would fill the available pores between the softened and heated aggregates, providing a layered distribution of WC particles for the spray-deposited coating. In this condition, the light-trapping in the multimodal coating will be enhanced due to the efficient light reflection among the multimodal WC particles, which contributes to the enhancement of solar absorptance.
Highlights
Metal-dielectric coatings that employ highly solar-absorbent and IR-transparent materials deposited onto highly IR-reflective metal substrates, such as Co/WC [1,2], Pt/Al2 O3 [3], Mo/Si3 N4 [4]and W/AlN [5], have been denoted to develop solar selective absorbing coatings for high temperature photo-thermal conversion applications due to their excellent absorptance (α) performance in the wavelength range of solar radiation (0.3~2.5 μm) and low thermal emittance (ε) in the infrared region (2.5~25 μm) [6]
The metal-dielectric selective coatings can offer a high degree of flexibility for tuning the absorption and scattering cutoff wavelengths by particle and matrix constituents, particle sizes and concentrations, coating thickness, etc. [2,7]
It has been well established that the powder morphology, spray parameters and type of feedstock could have a significant effect on the microstructure of the coating [9] and, in turn, the solar selectivity
Summary
W/AlN [5], have been denoted to develop solar selective absorbing coatings for high temperature photo-thermal conversion applications due to their excellent absorptance (α) performance in the wavelength range of solar radiation (0.3~2.5 μm) and low thermal emittance (ε) in the infrared region (2.5~25 μm) [6]. Composition optimization and structure design, various kinds of coatings with absorptance better than 0.90 and emittance lower than. These high solar absorptivity, solar selective coatings either need high cost and vacuum processes, such as chemical vapor deposition and sputtering, or they will exhibit bad spectral selectivity. Enhancements in WC/Co coatings fabricated by nano-WC/Co powders have remained elusive due to the decarburization occurring under thermal spraying conditions
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