Solar selective black cobalt: preparation, structure, and thermal stability

Abstract

In the quest for an electroplated selective black coating stable to 500 °C in air, black cobalts have been prepared by three techniques to yield (a) plated cobalt sulphides, (b) plated cobalt oxide-hydroxide, and (c) cobalt oxide prepared by thermal oxidation of electropolated cobalt metal. The optical properties of the various coatings are analyzed before and after exposure to air for extended periods of time at temperatures in the 300 °–500 °C range. The sulfide black cobalt is not acceptable as a high-temperature selective absorber due to severe thermal degradation. The plated oxide is a good selective absorber to about 400 °C, and the thermally oxidized black to a slightly higher temperature, but degrades at 500 °C. Structure studies via scanning electron microscopy (SEM), Auger electron spectroscopy (AES), and x-ray photoemission spectroscopy (XPS) are reported which yield a full account of the coating chemistry before and after heating. The studies reveal that the high solar absorptance of the acceptable black cobalt coatings is due to a continuation of a porous outer layer grading into nondense oxides of cobalt; either CoO or Co3O4, depending on the film. Absorption is intrinsic but not due to metal particles as in black chrome. A limited amount of optical degradation occurs upon heating the oxide black cobalt in air due to oxidation of hydroxide. However, the major degradation problem is shown to be substrate oxidation in contrast to black chrome where film oxidation is the principal problem.

XPS characterization of Co and Cr pigmented copper solar absorbers

“…Our results reveal that only oxidized or oxy-hydrated forms of cobalt or chromium are present on the selective surfaces as opposed to the previously reported observation of metallic Co and Cr as the electrochemically deposited composites of Co 5,6,21]. From the thermodynamic point of view the presence of metallic cobalt is possible since it is a slightly noble metal and can electrochemically be obtained in the metallic state by the action of hydrogen at low potential values [22].…”

“…Equally important is the nature and chemical state of the pigments used. Transition metal salts of Cr, Co and Ni are the most common pigments and Al, Cu or stainless steel are used as substrates [1][2][3][4][5][6][7][8][9][10][11][12].…”

Complex–Surfactant‐Assisted Hydrothermal Synthesis and Properties of Hierarchical Worm‐Like Cobalt Sulfide Microtubes Assembled by Hexagonal Nanoplates

“…[25][26][27] In addition, cobalt sulfides have also been used in solar cells because of their high solar absorption, which is due to the strong intrinsic absorption coupled with surface morphology effects. [28] Due to these important properties, many recent efforts have been dedicated to the synthesis of micro-and nanocrystals of cobalt sulfides with various morphologies, such as Co 9 S 8 and CoS 2 nanoparticles, [29] CoS nanowires, [30,31] thin films, [32] octahedrons of Co 3 S 4 , [25] and sea urchin-like Co 9 S 8 . [33] It is worth noting that Co 3 S 4 nanotubes [34] and cobalt sulfide hollow nanocrystals [35] have been fabricated based on the Kirkendall effect.…”

Low-temperature chemical vapor deposition of cobalt oxide thin films from a dicobaltatetrahedrane precursor

“…9,10 Various processes are available for the deposition of cobalt oxides such as molecular beam epitaxy, 4 sputtering, 11 spray pyrolysis, 12 electrochemical deposition, 13,14 thermal oxidation 14 and chemical vapor deposition. 15 Thermal oxidation of pre-deposited metallic cobalt is not feasible in terms of temperature and rate requirements, since temperatures above 375 C are required to accelerate diffusion for a sufficiently fast oxidation.…”