ELECTRICAL-PROPERTIES OF ELECTRODEPOSITED MOLYBDENUM BLACK COATINGS

The electrical properties of molybdenum black coatings electrodeposited on aluminium have been investigated. These coatings are semiconductors and exhibit weak solar selectivity. (They are closely related to dip coatings which are usually thinner than electrodeposited coatings and show good solar selectivity). Coatings were annealed prior to measurement leading to an almost fifty-fold increase in conductivity. The average activation energy associated with conduction at temperatures above about 150°C is 0.45±0.02eV and the activation energy decreases with decreasing temperature. This is consistent with the presence of localised electronic states within the energy gap (the Poole-Frenkel conduction mechanism being applicable at higher applied voltages). The coatings dielectric constant which is 4.5 (at 500 Hz and ∼20°C) decreases slowly with increasing frequency and is somewhat temperature dependent. There is evidence that the electrical conductivity and the optical energy gap together with the coating composition depend somewhat on the electrodeposition rate used in coating preparation. The solar selectivity of relatively thick coatings shows little dependence on electrodeposition rate. © 1990.