Ultrafine nickel particles for photothermal conversion of solar energy

Ni particles with mean diameters ≲10 nm were produced by inert-gas evaporation onto KBr plates. Heating in air to 400 °C for a few minutes led to the electrical decoupling of the grains by formation of NiO pellicles. Transmittance measurements in the interval 0.3–30 μm showed excellent spectral selectivity with high absorption over the solar spectrum and low absorption in the thermal infrared range. Heat treatments up to 600 °C in air did not affect the selectivity significantly. These properties prove that coatings with Ni grains are very useful for the photothermal conversion of solar energy. A theoretical analysis of the optical properties of aggregated spherical particles with dielectric shells was carried out by use of an extended effective-medium theory founded on the Maxwell Garnett approach. We obtained satisfactory agreement between computations and experiments.