Preparation and Characterization of Silver and Zinc-Oxide Heat Mirrors.

A heat mirror is a spectrally selective coating for windows which transmits solar radiation and highly reflects infrared radiation. When applied to the inside surface of a window the heat mirror maintains the visibility of the window yet suppresses the radiative heat loss. The heat mirror thus functions as a transparent heat insulation. We have developed the techniques for preparing two types of heat mirrors which utilize two different mechanisms for achieving high infrared reflectivity. The first type of heat mirrors consists of a very thin(70A) silver film prepared by resistive evaporation. The highly visible transparent and infrared reflective silver film is deposited onto a water cooled polyester substrate through a helium plasma which acts to provide a uniform film without clustering of the silver layer into isolated islands. An antireflecting dielectric coating of Al _2O_3 enhances the visible transparency and protects the delicate silver film. We have made an excellent silver heat mirror with a visible transmittance of 86% and an emissivity of.13 but these films have poor long term stability. The second type of heat mirrors consists of a thick(2000A) layer of zinc oxide prepared by reactive triode D.C. magnetron sputtering. High infrared reflectance is obtained by making the zinc oxide slightly nonstoichiometric which introduces free carriers into the otherwise empty conduction band of the ZnO. The stoichiometry was controlled by holding the sputtering rate constant and regulating the oxygen flow rate into the system. Films were made with and without a secondary plasma generated near the substrate. The free carrier concentration and the electron mobility of films made with and without the secondary plasma are comparable and the addition of the secondary plasma results in no improvement of the heat mirror qualities of the films. We were able to make good heat mirrors with 80% visible transmittance and.20 emissivity without a secondary plasma or intentional impurity doping or post deposition annealing. These films show excellent stability and durability and are good candidates for commercial development. Samples were characterized through Hall effect, X-ray diffraction, optical transmission, Rutherford backscattering and Scanning Electron Microscope measurements.