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.