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A selective solar photothermal absorber can be created by the formation of suitably spaced metallic dendrites. Spacings of around a micron achieve absorption of visible light through multiple internal reflections in a geometric maze effect while retaining the low emissivity properties of the metal at longer wavelengths. Recent experiments elsewhere on the chemical vapor deposition (CVD) of W dendrites on various substrates demonstrated the feasibility of this approach to produce a selective solar photothermal absorber. W dendrite coatings, however, may be too expensive to find a practical application in solar collectors. Ni dendrites, on the other hand, appear to be a potential low-cost selective solar photothermal absorber. Dendrites are formed by the CVD of Ni from Ni carbonyl, Ni (CO)4. This paper presents the physical and optical results of experiments with CVD Ni dendrites on Al substrates. The preparation of the aluminum substrates, the CVD experimental apparatus and methods, and the optical measurement techniques of solar absorptivity (αs) and thermal infrared emissivity (ε) are discussed. The results are described in terms of scanning electron microscope (SEM) photographs of dendritic coatings, and the measured optical properties.

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Finely divided calcium for laboratory pnrposes can also be prepared by recrystallizing bulk calcium from a sodium solution. The bulk calcium dissolves in sodium at elevated temperatures and, upon cooling, crystallizes out in small dendritic platelets that become trapped in a sodium matrix when the sodium solidifies. As in the previous synthesis, the sodium matrix may be removed by preferential reaction with a hydrous lower alcohol.