Carl G. Ribbing

Optical constants and spectral selectivity of stainless steel and its oxides

Reflectance measurements on eleven grades of stainless steel have been performed. It is found that the metallurgical phase is a more important parameter for the reflectance behavior than the detailed content of the alloying elements. The optical constants for one austenitic and one ferritic grade have been calculated by the Kramers‐Kronig analysis. For the extrapolation, a fitting procedure to the results from measurements with polarized light at 10° and 60° incidence was used. The austenitic steels have a higher reflectance than the ferritic and martensitic, but their integrated solar reflectance of 68% is considered too low for solar reflectors. Two of the natural oxides on stainless steel, Fe2O3 and Cr2O3, have been studied as separate films, and their optical constants determined by combined transmission and reflectance measurements. The metal and oxide results are combined in an evaluation of the optical selectivity that can be obtained with oxidized stainless steel surfaces. It is concluded that a f...

Titanium–aluminum–nitride coatings for satellite temperature control

Abstract Intense solar irradiation, radiative cooling to outer space, and internal heat generation determine the equilibrium temperature of a spacecraft. The balance between the solar absorption and thermal emittance of the surface is therefore crucial, in particular for autonomous parts directly exposed to the solar radiation and thermally insulated from the main thermal mass of the spacecraft. The material composition but also the coating thickness are found to influence the equilibrium temperature of an object in space. In this paper we report on a systematic search for a suitable composition and thickness of TixAlyNz alloy coatings prepared by reactive, unbalanced magnetron sputtering from targets consisting of differently sized titanium and aluminum sectors. The films were deposited on glass, glassy carbon, aluminum sheet metal, and on sputtered aluminum and TixAl(1−x) films on glass. The stoichiometry and sheet resistance of the films was determined with Rutherford backscattering and four-point probe measurements respectively. Reflectance spectra for the visible and infrared spectral ranges were used to obtain average solar absorptance and thermal emittance values used in model calculations of the equilibrium temperature. Neglecting internal heat contributions, the lowest calculated equilibrium temperature in orbit around the Earth, 32.5°C, was obtained for a 505-nm-thick Ti0.14Al0.47N0.40-film.

Infrared properties of β-sialon as a function of composition

The infrared properties of β-sialons, Si6−zAlzOzN8−z, prepared by hot, isostatic pressing for nine z-values, as well as a corresponding set of samples containing 1 wt% of yttria have been studied. The samples all have densities over 99% of their theoretical value and were found to be single-phased for z<3.8. Polished and cleaned sialon samples and one fully dense β-Si3N4 sample were used for infrared reflectance measurements. The lattice bands in the infrared reflectance spectra were analyzed for the different compositions. Numerical fits, using the parameters for a maximum of five classical Lorentz oscillators as free variables, were carried out. A strong Si–N resonance at 900 cm−1 was identified, the oscillator strength of which scaled linearly with z. The IR reflectance bands represent a potential for selectively low emittance applications on radomes in the 8–12 μm atmospheric window. The evaluation of the appropriate average window emittance showed a close to linear increase from about 0.5 to 0.9 for 0.35

Anomalies in integrating sphere measurements on structured samples

Anomalies are reported in total and diffuse reflectance values obtained from measurements on structured samples with a double-beam integrating sphere. It is demonstrated that these anomalies are caused by an interplay between the sample structure and geometric sphere imperfections. The sample structure causes a confinement of the diffuse reflectance which may suffer port losses as well as erroneously high signal levels. This can lead to deviations in the recorded reflectance values as high as 50%.

Interpretation of integrating sphere signal output for non-Lambertian samples

A new formalism is derived for the analysis of signal output from double-beam integrating spheres. The analysis explicitly considers the effects of port losses and a non-Lambertian sample surface and introduces a separation of the diffusely reflected light into two parts: one which should be analyzed as a specular component and one which is fully diffuse. An experimental procedure to determine the two parameters in the formalism is described for two cases, a brushed copper and a rolled aluminum surface, and it is experimentally verified that the formalism eliminates spurious structure from the barium sulfate reference. A criterion is also given for the selection of barium sulfate or polytetrafluorethylene powder as a reference material.

Optical Properties Of Transparent Heat Mirrors Based On Thin Films Of TiN, ZrN, And HfN

Calculations of the transmittance and reflectance between 0.35 pm and 10 pm of semitransparent films of TiN, ZrN and HfN have been performed. The calculations are based on recently reported optical constants. They show that these compounds can be used as transparent heat-mirrors. These materials show considerable higher emittance than the noble-metals but comparable or higher visible transmittance. It is also shown that the transmittance can be increased by the technique of induced transmission.

Temperature dependence of the band emittance for nongray bodies

The general problem of obtaining correct emittance values from broadband IR radiometric measurements on nongray samples is discussed. If the spectral emittance has structure in a band, the emittance, averaged over that band, will be temperature dependent, even if the spectral emittance is insensitive to the temperature change. We point out that a widely used expression, with correction for radiance from the surroundings reflected by the sample, is valid only if the spectral emittance is temperature and wavelength independent, i.e., gray. If the spectral emittance is nongray, the conventional emission factor, as determined by a broadband radiometer, is temperature dependent and the numerical value is significantly different from the averaged band emittance sought. Two algorithms are suggested to extract the correct band-averaged emittance from the temperature-dependent radiometric emission factor obtained with the conventional expression. The algorithms are demonstrated with a step model for the spectral emittance, and it is shown that the agreement with the correct average band emittance is significantly improved.

Reststrahlen bands as property indicators for materials in dielectric coatings

The usefulness of reststrahlen band measurements for materials characterization is pointed out. In particular, two new aspects are discussed. The spectral position of the long wavelength edge λT of the reststrahlen band is empirically correlated to the mechanical strength of the bulk material. It is demonstrated that the relation between λT and the Youngs modulus E falls close to E = constant × λT−32 for a group of compounds including alkali halides, III–V semiconductors, oxides and carbides. Secondly, the possibility of using reflectance spectroscopy across the reststrahlen region for thickness determination of a dielectric coating is discussed. If a thin oxide film is grown on a metallic substrate the absorption occuring at the longitudinal phonon wavelength λL is a sensitive measure of the film thickness as long as it is much smaller than the wavelength.

Stainless steel solar mirrors. A material feasibility study

Abstract The possibility to use stainless steel solar mirrors for increased energy collection in thermal energy systems was studied. Various grades of austenitic, ferritic and one martensitic steel were considered. The bulk reflectance was measured and the austenitic grades were selected for further analysis. The integrated solar reflectance, 0.67, was considered too low in comparison with alternative materials. The optical constants were used to calculate the solar reflectance at oblique incidence and with dielectric overlayers. Only with a double overlayer of the high-low index type an increase in R sol to 0.72 is noted. It is concluded that stainless steel will not be competitive as material for solar mirrors without such coatings. Careful consideration must also be given to long term stability properties.

Realization of Selective Low Emittance in Both Thermal Atmospheric Windows

The infrared reflectance and emittance of a double layer of silicon and silicon dioxide have been investigated by optical multilayer calculations and spectral and wavelength-integrated measurements. Low emittance in the interval 0.2 to 0.4 can be obtained simultaneously in both thermal atmospheric windows: 3 to 5 and 8 to 13 µm. These results are relevant for IR signature control. The sample consisted of a 0.9-µm Si and a 2.45-µm SiO2 layer on a Si wafer. The layers were grown by standard microelectronic chemical vapor deposition techniques. The key mechanism for lowering the emittance is the interaction between the SiO2 molecular reflectance band, around 9 µm, and interference effects in the double layer. Interference gives one peak in the 3- to 5-µm window, and a widening and strengthening of the SiO2 molecular reflectance band in the 8- to 13-µm window. The calculated spectra are in very good agreement with measured near-normal incidence reflectance spectra in the range 2.7 to 12.5 µm. The emittance of the samples heated to 61 °C was determined in the atmospheric windows using two heat cameras filtered for the respective intervals and equipped with polarizers. Emittance values for the sample in the two windows and the two main polarizations were determined as a function of emission angle from 10 to 60 deg. Qualitative agreement with values calculated from tabulated optical constants was obtained.