Characterization of selective solar absorber microstructures: Electron microscope studies

Abstract

Thin films (0.01 μm thick) of vapor-deposited nickel on cleaved NaCl crystal substrates were coated with a stop-off lacquer, leaving a nickel window (0.3 cm 2 in area) which was exposed to a Chromonyx bath for short periods of time in order to electrodeposit black chrome nuclei. The nickel films with the attached nuclei were then observed by transmission electron microscopy where the nuclei were identified as chromium and Cr 2O 3 crystals and polycrystalline aggregates not greater than 0.1 μm in diameter. Black chrome coatings electrodeposited onto nickel, copper and zinc substrates were also observed (by selective electrochemical removal of the metal substrates followed by ion milling) by transmission electron microscopy and were compared with observations of the coating surfaces by scanning electron microscopy. Utilizing selected area electron diffraction patterns and dark field electron microscopy, the coating structures and compositions were examined and determined to be composites of chromium and Cr 2O 3. The chromium and Cr 2O 3 size distributions remained somewhat constant throughout the coating thickness and the mean chromium and Cr 2O 3 particle sizes were measured to be 0.024 μm and 0.13 μm respectively. The optically desirable coatings were characterized by a chromium concentration gradient which decreased with distance from the substrate-coating interface. The Cr 2O 3 particles contained chromium particles and aggregates, and these Cr 2O 3 particles were aggregates of large crystals and small grains which in some cases were less than 0.01 μm in diameter.