Transparent Heat Mirrors Research Articles

Sputtered films for wavelength-selective applications

Spectrally selective coatings have potentially important applications in solar-thermal-electric conversion, solar photovoltaic conversion, solar heating and window insulation. These coatings can be divided into two classes: transparent heat mirrors transmit solar radiation and reflect thermal radiation; selective absorbers absorb solar radiation and have a low IR emissivity. We prepared by r.f. sputtering cermet absorbers (MgO-Au and Cr 2O 3-Cr) and transparent heat mirror films (TiO 2/Ag/TiO 2, tin-doped In 2O 3 and tin-doped In 2O 3 microgrids) that have excellent wavelength-selective properties. We also recently succeeded in preparing excellent tin-doped In 2O 3 films on Mylar by ion beam sputtering. The tin-doped In 2O 3 films also have good electrical conductivities, making them excellent transparent conductors.

Transparent heat mirrors: influence of the materials on the optical characteristics

Transparent heat mirrors of the antireflecting metal type with a dielectric/metal/dielectric structure are studied. It is found that dielectrics of lower refractive index give a higher cutoff wavelength, but the transition becomes more gradual. The angular behavior of these mirrors is also analyzed. The optimum mechanical efficiency obtainable with a plain collector and a carnot engine is given for different concentrations and refractive indices. Moreover it appears that using Al instead of Ag introduces appreciable absorption losses (~25-35%).

Formation of transparent heat mirrors by ion plating onto ambient temperature substrates

Transparent conducting oxide films were produced by the reactive ion plating of indium onto glass and plastic substrates at room temperature. The electrical properties of these films were sufficient, with high values of electrical mobility and carrier density, to give a sharp plasma reflectance edge in the near infrared. These properties are ideally suited for the production of the visibly transmitting, IR-reflecting surfaces needed for heat mirror applications. Optimum properties were achieved with careful control of the gas mixture and the amount of substrate bias acquired by the substrate from the radio frequency used for the discharge. Satisfactory films were made using evaporation or planar magnetron-sputtering sources.

Thin-film conducting microgrids as transparent heat mirrors

A new type of transparent heat mirror for solar-energy applications has been fabricated by chemically etching a Sn-doped In2O3 film to form a transparent conducting microgrid. For square openings 2.5 μm on a side, separated by lines 0.6 μm wide, the solar transmission increases from 0.8 for the original continuous film to 0.9 for the microgrid. Although 65% of the area of the film is removed by etching, the infrared reflectivity decreases by only 9%, from 0.91 to 0.83. A smaller decrease in the infrared reflectivity may be possible if materials with higher optical conductivity are used.

Transparent heat mirrors for solar-energy applications

Transparent heat-mirror films, which transmit solar radiation but reflect ir thermal radiation, have potentially important applications in solar/thermal/electric conversion, solar heating, solar photovoltaic conversion, and window insulation. We have used rf sputtering to prepare two types of films: TiO(2)/Ag/TiO(2) and Sn-doped In(2)O(3). To characterize the properties of heat-mirror films for solar-energy collection, we define the parameters alpha(eff), the effective solar absorptivity, and epsilon(eff), the effective ir emissivity. For our Sn-doped In(2)O(3) films, alpha(eff)/epsilon(eff) is comparable to the values of alpha/epsilon reported for the leading selective absorbers. Even higher values of alpha(eff)/epsilon(eff) are obtained for the TiO(2)/Ag/TiO(2) films.

Properties of Sn‐Doped In2 O 3 Films Prepared by RF Sputtering

An rf sputtering process has been used without postdeposition annealing to prepare Sn‐doped films with low electrical resistivity (down to ), high visible transmission, and high infrared reflectivity (up to 93% at 10 μm) for applications as transparent conductors and heat mirrors. Substrate heating is accomplished entirely by the electron bombardment intrinsic to rf sputtering, rather than by using an auxiliary resistance heater. The film properties improve with increasing substrate temperature up to 650°C, the maximum employed, and are relatively independent of other sputtering parameters. The electrical and optical properties of the films do not depend significantly on the crystallographic orientation, degree of texturing, or substrate material (glass, fused silica, and single crystal and ).