C. G. Granqvist

Optical properties of sputter‐deposited ZnO:Al thin films

ZnO:Al coatings were prepared by rf magnetron sputtering of ZnO together with dc magnetron sputtering of Al onto rapidly revolving unheated substrates under weakly oxidizing conditions. Optimized films had ∼1% luminous absorptance, ∼85% thermal infrared reflectance, and ∼5×10−4 Ω cm electrical resistivity at a thickness of ∼0.3 μm. The Al content was ≲2 at. %, as determined by Rutherford backscattering spectrometry. Transmission electron microscopy and electron diffraction showed ∼50‐nm average crystallite size and a hexagonal wurtzite structure. Spectrophotometric transmittance and reflectance were recorded in the 0.2–50‐μm wavelength interval, and the complex dielectric function was evaluated by computation. The optical data were explained from an effective mass model for n‐doped semiconductors. The Al atoms are singly ionized, and the associated electrons occupy the bottom of the conduction band as free‐electron gas. The Al ions act as pointlike Coulomb scatterers and are screened by the electrons acco...

Band-gap widening in heavily Sn-dopedIn2O3

In this paper we investigate the optical properties of evaporated films of doped semiconducting In2O3 in the 2-6-eV range, i.e., around the fundamental bandgap. The study serves two main purposes: to elucidate basic properties of a heavily n-doped semiconductor, and to improve our understanding of a technologically important material which is widely used when transmittance of visible or solar radiation needs to be combined with good electrical conduction or low thermal emittance.

Effective medium models for the optical properties of inhomogeneous materials.

The Maxwell Garnett and Bruggeman effective medium theories are derived for the average dielectric permeability of heterogeneous materials from a unified theoretical approach. It starts by specifying two random unit cells which represent different microstructures. Requiring that these cells should not be detectable by electromagnetic radiation when embedded in an effective medium, we show from an extended optical theorem that the forward scattering amplitude must vanish. Setting the leading term in the expansion series of this quantity equal to zero yields the effective medium theories pertaining to the two microstructures. The remaining terms provide estimates of the accuracy of the approximations. This approach is then used in numerical computations for Co-AI(2)O(3) cermets.

Optical properties and solar selectivity of coevaporated Co‐Al2O3 composite films

Co‐Al2O3 composite films were produced by electron‐beam coevaporation in a system with elaborate process control. The deposits were analyzed by transmission electron microscopy, electron diffraction, Auger electron spectroscopy, secondary ion mass spectroscopy, Rutherford backscattering, field ion microscopy, mechanical stylus measurements, and electrical dc and ac measurements. A uniform separated‐grain structure with regular hcp Co particles embedded in Al2O3 was found for Co contents (fCos’) up to ∼30 vol %. The complex dielectric permeability e was evaluated in the 0.3≲λ≲40‐μm wavelength range for samples with 0.11≲ fCo ≲0.60 by carefully selected combinations of spectrophotometric transmittance and reflectance data. Numerical accuracy and internal consistency were investigated. Effective medium theories for e were derived by applying classical scattering theory to spherical random unit cells defined so as to properly represent a number of typical microgeometries. The formulations due to Maxwell Gar...

Radiative heating and cooling with spectrally selective surfaces

Matter continuously exchanges energy with its surroundings. This exchange can be dominated by radiation, conduction, or convection. In this brief review we discuss how proper design of radiative surface properties can be used for heating and cooling purposes. The desired properties can be understood once it is realized that solar and terrestrial radiation take place in different wavelength ranges and that only part of the solar spectrum is useful for vision and for photosynthesis in plants. These facts allow the possibility of tailoring the spectral absorptance, emittance, reflectance, and transmittance of a surface to meet different demands in different wavelength intervals, i.e., to take advantage of spectral selectivity. One example is the selective surface for efficient photothermal conversion of solar energy, which has high absorptance over the solar spectrum but low emittance for the longer wavelengths relevant to thermal reradiation. Below we discuss the pertinent spectral radiative properties of our ambience. These data are then used as background to the subsequent sections treating four examples of spectrally selective surfaces. The first example is the previously mentioned selective surface for converting solar radiation to useful heat. The second example considers surfaces capable of reaching low temperatures by benefiting from the spectral emittance of the clear night sky. The third example concerns two related types of transparent heat mirror. The fourth example, finally, treats radiative cooling of green leaves; this part is included since it gives a nice example of how nature solves a difficult problem in an elegant and efficient way. This example hence provides an interesting background to the other cruder types of artificial selective surfaces. Throughout our discussion we treat the ideal spectral properties, give an illustrative experimental example of how well this goal can be realized, and-where this is possible-show a corresponding theoretical curve indicating to what extent the measured results can be theoretically understood.

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.

Nickel pigmented anodic aluminum oxide for selective absorption of solar energy

Spectrally selective surfaces on Al metal sheets were prepared by dc anodization in dilute phosphoric acid followed by black coloration via ac electrolysis in a bath containing NiSO4. These coatings had good durability, as found from several accelerated tests. The optical performance was studied by the recording of hemispherical reflectance or specular reflectance in conjunction with diffuse light scattering. From these results we extracted a solar absorptance of 0.93–0.96 and a corresponding thermal emittance of 0.10–0.20. The structure of the coatings was investigated by scanning electron microscopy on fractured specimens, Auger electron spectroscopy combined with depth profiling by sputtering, and atomic absorption analysis. Based on this information, a multilayer model was formulated, its most conspicuous feature being a sheath near to the Al interface comprised of metallic Ni particles in an Al2O3 matrix. The optical properties of this layer were described either by the Maxwell Garnett theory or the ...

Infrared optical properties of silicon oxynitride films: Experimental data and theoretical interpretation

Films of SiOxNy, with 0.25≲x≲2 and 0<y≲1.52, were prepared by reactive rf‐magnetron sputtering. The composition was determined by Rutherford backscattering spectrometry. Spectrophotometric measurements were conducted in the 2.5–50 μm range, and the complex dielectric function e was extracted by computation. We interpreted e by considering the vibrational properties of five basic Si‐centered tetrahedra, whose relative occurrence was given by the stoichiometry, and using the Bruggeman effective medium theory to average over the different tetrahedra. Theoretical and experimental data on e were found to agree very well.

Radiative cooling to low temperatures: General considerations and application to selectively emitting SiO films

Radiative cooling occurs because the atmospheric emittance is low in the wavelength interval 8–13 μm particularly if the air is dry. We derive expressions which specify the optical properties demanded for a surface capable of being cooled to low temperatures. The key factor is infrared selectivity with low reflectance in the 8–13 μm ’’window’’ but high reflectance elsewhere. Considering only radiation balance, ideal surfaces of this type can yield temperature differences of ∼50 °C while the cooling power at near‐ambient temperatures is ∼100 W/m2. However, nonradiative exchange limits the practically achievable temperature difference. SiO films on Al were investigated as an example of an infrared‐selective surface. The infrared optical properties of SiO were determined by a novel and accurate technique. These data were used to compute the spectral radiative properties of Al coated with SiO films of different thicknesses. The spectral selectivity was largest for 1.0‐μm‐thick films. This kind of surface was produced by evaporation of SiO onto smooth Al. The measured reflectance agreed with computations. Practical tests of radiative cooling were performed using a SiO‐coated Al plate placed under transparent polyethylene films in a polystyrene box. An identical panel containing a blackbody radiator was used for comparison. The performance of the panels was tested during clear nights. It was in good qualitative agreement with theoretical expectations.

Electrochromic coatings for “smart windows”

We introduce electrochromic coatings for use on “smart windows” with dynamic control of radiant energy. Applications, operating principles, materials options, and device configurations are briefly reviewed. We then focus on electrochromic WO3 and investigate crystalline films by computations aimed at assessing performance limits, and amorphous films with regard to practical performance in contact with liquid or solid electrolytes. It appears that there are several approaches to achieving a gradual and reversible modulation of the radiative performance in a manner consistent with uses on “smart windows”.