G. Zajac

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

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 accep...

Refinement of solar absorbing black chrome microstructure and its relationship to optical degradation mechanisms

An in depth characterization of the electrodeposited solar absorber black chrome resulting in refinement of the microstructural model of the coating is presented in light of current studies in valence band Auger electron spectroscopy, x‐ray photoelectron spectroscopy, thermal desorption spectroscopy, secondary ion mass spectroscopy, and the chromium electrodeposition mechanism. Through the presented model, the structural and chemical parameters of the coating are linked to its optical response with the degradation of the optical response at elevated temperatures related to changes in these parameters. The degradation process is divided into two distinct phases. First, upon heating to low temperatures (≲300 °C) the chromium hydroxide localized on the surface and within the microstructure of the film decomposes producing H2O and chromium oxide leaving an expected microvoid‐metallic particle structure. Heating to temperatures in excess of 300 °C leads to the second degradation phase. The fine chromium crysta...

The surface microstructure optical properties relationship in solar absorbers: black chrome

Abstract The relationship between the surface microstructure and the optical properties of solar absorbing electrodeposited black chrome films has been studied by scanning electron microscopy (SEM), X-ray photoemission spectroscopy (XPS) and sputter depth-profiling techniques. The black chrome films have been determined to consist of a top layer of small (≈400 A) Cr 2 O 3 particles with one or two sublayers of larger (≈1000 A) closely packed chromium particles. The optical properties of the solar absorping films have been measured and analyzed via the spheroid model and it has been shown that the optical response of the particulate black chrome films is significantly determined by their microstructure.

Microstructure dependence of the optical properties of solar‐absorbing black chrome

The surface microstructure and chemical constituency of solar‐absorbing black chrome films have been studied by scanning electron microscopy (SEM), x‐ray photoemission spectroscopy (XPS), and sputter depth‐profiling techniques. The films have been determined to consist of a top layer of small (∼400 A) Cr2O3 particles with one or two sublayers of larger (∼1000 A) closely packed chromium particles. In addition, it has been shown that the optical response of the particulate black chrome films is significantly determined by the microstructure of the films.

Relationship of optical degradation to surface morphology changes in solar absorbers

The physical basis for high‐temperature optical degradation of electrodeposited black chrome coatings has been investigated via scanning electron microscopy and Auger electron spectroscopy coupled with inert gas depth profiling. Severe oxidation of the metallic chromium in the coatings has been observed and it is believed to result in shape changes of the particles comprising the film thereby decreasing their near‐ir absorptance as is observed for air heat‐treated coatings.

Surface composition of solar selective black chrome films as determined by SIMS

Abstract The composition of electro-deposited black chrome solar absorbing films has been investigated via secondary ion mass spectrometry (SIMS). Positive and negative SIMS show a number of (OH) − bearing ions which have been related to a near-surface region dominated by CrO(OH) and Cr(OH) 3 compounds. Depth concentration profiles indicate that the hydroxide content decreases and the metallic chromium fraction increases with depth into the film. The studies further refine the chemical and microstructural model for the black chrome coating from which its solar selective optical properties can be derived.

High temperature optical and structural degradation of black chrome coatings

Abstract The microstructural basis for the optical degradation of electrodeposited black chrome coatings heated in high vacuum has been investigated via SEM and AES depth profiling. Compaction and agglomeration of the fine chromium particles comprising the film has been observed. The effect is to degrade the optical selectivity of the thin films.

High flux photochemical changes in Black Chrome solar absorbing coatings

Abstract Black Chrome samples have been exposed to concentrated solar radiation (350kW m−2–2.5 MW m−2) at elevated temperatures (250–500°C) over limited periods of time with the resultant observation that the coatings degrade less optically than equivalent coatings heated by IR radiation. The decreased degradation is a result of decreased oxidation of the Black Chrome coating under solar heating as compared to oven heating. This is proposed to be due to photo-desorption of gaseous oxygen bearing species from the surface of the film.

Photodesorption studies of CO and CO2 from the solar absorber black chrome

Photodesorption of CO and CO2 has been studied on black chrome solar absorbing coatings. It is shown that the desorption is a quantum effect with possible basis in the photo‐hole model applicable to semiconductor surfaces, but with yields similar to that reported for photodesorption from metals.

Percolation‐type behavior in black chrome selective solar films

The optical constants (n and k) and dc resistivity have been measured for black chrome particulate solar absorbing coatings for the purpose of defining the applicability of various inhomogeneous medium theories in the description of the black chrome optical response. It has been shown that percolation‐type behavior does exist in the films and hence of the various theories, only those exhibiting percolation at the 0.3 packing fraction value determined for the black chrome films are applicable. The two valid approaches are an effective medium theory first proposed by Bruggeman and a modified Maxwell Garnett theory which includes a particle shape distribution.