Optical and structural properties of black molybdenum photothermal converter layers deposited by the pyrolysis of Mo(CO)6

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Abstract
Black molybdenum thin films, combining high infrared reflectance with significant solar absorptance, have been prepared by pyrolyzing Mo(CO)6 under oxidizing conditions and subsequent annealing under a hydrogen atmosphere. X-ray and electron diffraction, electron microprobe, and Auger analysis show that the films, as deposited, are fine grained (300 A), stress-free, monoclinic MoO2 with [010] preferred orientation. As the annealing progresses the reflectance in the infrared rises faster than in the visible. Consequently, the spectral selectivity can be optimized in a partially annealed state characterized by a complex mixture of Mo and MoO2. Like black chrome, black molybdenum is a composite material, consisting of metal particles suspended in an oxide matrix. However, black molybdenum is capable of prolonged operation at temperatures in excess of the decomposition temperature of black chrome. After passivation with Si3N4, very simple photothermal solar converter coatings with a respectable spectral selectivity and significant temperature durability at 500 C in vacuum are obtained.