T.S. Eriksson

Thermochromic VO2 films for energy-efficient windows

Abstract VO 2 films were produced by reactive e-beam evaporation followed by annealing post-treatment. Electrical measurements demonstrated a semiconductor-metal transition at τ c ∼ 60° C . Spectrophotometry showed that the near-infrared solar transmittance was reduced when τ c was exceeded while the luminous transmittance remained relatively unchanged. This thermochromism may be utilized for regulating the energy throughput of windows. Practical application hinges on improved transmittance and on τ c - depression . These goals can be accomplished to some extent by dielectric overlayers, as verified by measurements on SiO 2 -coated VO 2 films.

Infrared optical properties of evaporated alumina films

The dielectric function epsilon identical with epsilon(1) + iepsilon(2) has been determined for Al(2)O(3) films prepared by electron beam evaporation, in the 5-50-microm wavelength range. The data were extracted from spectrophotometric recordings of transmittance and reflectance by use of a novel technique. Supplementary measurements were made of the refractive index for visible and near-infrared wavelengths and of the dielectric constant at 1 MHz. Kramers-Kronig analysis was employed to check the consistency of our results for epsilon(1) and epsilon(2).

Radiative cooling computed for model atmospheres.

Calculations of spectral radiance are reported from several model atmospheres appropriate to different climatic conditions by use of the LOWTRAN 5 computer code. From these data we evaluate the radiative cooling power and the temperature drop below ambient temperature for horizontal surfaces that radiate toward the sky. The surfaces are ideal blackbodies or have ideal infrared-selective properties with zero reflectance in the 8-13-microm range and unity reflectance elsewhere. For freely radiating surfaces, the cooling power at ambient temperature lies between 58 Wm(-2) and 113 Wm(-2) for the different surfaces and model atmospheres. The maximum temperature difference for a device with a nonradiative heat transfer coefficient of 1 Wm(-2) K(-1) is between 11 and 21 degrees C for a blackbody and between 18 and 33 degrees C for an infrared-selective surface. For radiators arranged so that they intercept only the atmospheric zenith radiance, the cooling powers and temperature differences are higher than for freely radiating surfaces, the increase being largest for humid atmospheres. The influence of altered contents of water vapor was found to affect strongly the radiative cooling, whereas changes in ozone and aerosol abundance were less important. The significance of these results to different cooling applications is briefly discussed.

Radiative cooling and frost formation on surfaces with different thermal emittance: theoretical analysis and practical experience

Radiative cooling power was computed as a function of the emittance c, of an exposed surface, air temperature, humidity, etc. from the LOWTRAN 5 code. Meteorological data were then used to make semiquantitative estimates on how often frost will form on a surface with given epsilon(s). Practical tests, using SnO(2)-covered glass with epsilon(s) approximately 0.2, demonstrated that a low-emittance coating can prevent frost formation and maintain transparency of a window exposed to the clear sky.

Surface coatings for radiative cooling applications: Silicon dioxide and silicon nitride made by reactive rf-sputtering

Abstract Coatings of silicon dioxide and silicon nitride were produced by rf-sputtering of Si in the presence of O2 or N2. The sputter unit, for coating surfaces up to 0.5 × 0.5 m2 in size, is described. Spectrophotometric data were used to evaluate the complex dielectric function in the range 5–50 μm. The parameters specifying the radiative cooling performance were studied by computation for oxide/nitride bilayers on A1.

Materials for radiative cooling to low temperature

Abstract The clear sky can act as a heat sink. Cooling to low temperatures is possible with materials which are strongly emitting in the 8–13 μm band and non-absorbing elsewhere. In this paper we discuss the resource for radiative cooling and its implementation with thin solid films (SiO 0.6 N 0.2 coatings on Al) and with slabs of certain gases (C 2 H 4 , C 2 H 4 O and NH 3 backed by Al). Results are given on spectrophotometric infrared reflectance and transmittance, computed parameters which govern the predicted cooling performance, and some preliminary field tests.

Infrared optical properties of electron-beam evaporated silicon oxynitride films.

The complex dielectric function of SiO0.6N0.2 coatings, produced by reactive electron-beam deposition, was evaluated in the 5–50-μm range. The composition was determined by Rutherford backscattering spectrometry. The IR optical properties of the films make them well suited for radiative cooling applications.

Thermochromic VO[sub]2[/sub] Films for Energy-Efficient Windows

VO2 films were produced by reactive e-beam evaporation followed by annealing post-treatment. Electrical measurements demonstrated a semiconductor-metal transition at Tc ~ 60°C. Spectrophotometry showed that the near-infrared solar transmittance was reduced when Tc was exceeded while the luminous transmittance remained relatively unchanged. This thermochromism may be utilized for regulating the energy throughput of windows. Practical application hinges on improved transmittance and on Tc-depression. These goals can be accomplished to some extent by dielectric overlayers, as verified by measurements on SiO2-coated VO2 films.

Infrared-transparent convection shields for radiative cooling: Initial results on corrugated polyethylene foils

Abstract Radiative cooling to a temperature far below that of the ambience requires a new type of convection shield which combines high transmittance in the 8–13 μm wavelength range with high non-radiative thermal resistance. We investigated a design with crossed layers of vee-corrugated high-density polyethylene foils. Typical results were infrared transmittance up to 73% (measured on an infrared-imaging instrument) together with thermal resistance of 1.1 m 2 K W −1 (determined with a modified guarded hot-plate technique).

Radiative cooling to low temperatures with selectivity IR-emitting surfaces☆

Abstract Radiative cooling to low temperatures is possible with surfaces which radiate primarily in the 8–13 μm atmospheric “window” range. We outline a theretical analysis of the required radiative properties and report some small-scale experiments using evaporated SiO and “Si 3 N 4 ” films.