Solar radiation control by pyrolytic oxide thin films

The growing concern for energy conservation is placing many new demands on materials, especially oxide thin films. A few of these new applications include solar collectors, solar reflectors, and thermal transparent insulation. The engineer must fully understand how oxide thin films optically behave and what mechanisms govern this behavior before utilizing them for these new applications. Only when oxide thin films are fully understood, will they be used to their potential.

This thesis probes into the basic mechanisms governing the optical properties of oxide thin films. Thin films are complex and their behaviors do not always correspond to those of bulk oxides. Absorption in the oxide films studied is due to electronic excitations and it is also found to be dependent upon crystallinity; the more crystalline films being more absorbing. It was determined that absorbance is related to the semiconducting behavior of the oxides. Absorbance increases when conditions are reducing for n-type oxides and when conditions are oxidizing for p-type oxides. A maximum solar reflectance of 35% was obtained with a Co₃O₄ film that was annealed in 95%N₂-5%H₂. Metals such as gold, exhibit higher solar reflectances than both the pure oxides and mixtures of Co₃O₄, Fe₂O₃, SnO₂, TiO₂, and VO_xH_y. However, configurational designs utilizing the oxide film properties to their greatest advantage could selectively screen solar and thermal energy effectively.