Showing papers by "Claes-Göran Granqvist published in 1981"

Effective medium models for the optical properties of inhomogeneous materials

Abstract: 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.

Radiative heating and cooling with spectrally selective surfaces.

Abstract: 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.

Infrared optical properties of evaporated alumina films

Abstract: The dielectric function ∊ = ∊1 + i∊2 has been determined for Al2O3 films prepared by electron beam evaporation, in the 5–50-μm 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 ∊1 and ∊2.

Optical properties of cermet materials

Abstract: Cermet films can show a spectral selectivity which makes them well suited for efficient photothermal conversion of solar energy. Sufficieritly fine-grained materials can be theoretically understood from effective medium theories. Two formulations apply depending on the microstructure topology ; the Maxwell Garnett theory is valid for metallic particles embedded in an insulating host, whereas the Bruggeman theory holds for a random mixture of metallic and insulating particles. We discuss extensions to treat size dependent electron scattering, shape and orientation effects, and the role of local aggregation. The theories are applied to several types of cermets : discontinuous metal films, gas evaporated coatings, coevaporated and cosputtered layers, electrodeposited films, and electrolytically and integrally coloured anodic aluminium oxide coatings. Successes and failures of the theoretical models are discussed and remaining uncertainties are pointed out. One general conclusion is that effective medium theories are very useful for understanding the optical properties both qualitatively and quantitatively. Another general result is that lack of sufficiently accurate sample characterization is still hampering further progress for several types of technologically interesting cermets.