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This paper shows a systematic experimental and theoretical study on the temperature dependence of the infrared optical properties of pyrolytic boron nitride (pBN), from 390 to 1050 $^{\circ}$CC for wavelengths between 4 and 16 {\mu}m. The temperature dependence of these properties has never been analyzed before. The measured emissivity spectra were fitted to a dielectric function model and an effective medium approximation. The phonon frequencies and dielectric constants agreed well with room temperature experimental values from the literature, as well as with ab initio and first principles calculations. In addition, the phonon frequencies of the perpendicular mode and the dielectric constants of the parallel mode showed an appreciable parabolic temperature dependence, which justifies the interest of more theoretical efforts in order to explain this behavior. Finally, the results of this work demonstrate that thermal emission spectroscopy allows obtaining the values of the optical and dielectric parameters of impure ceramic materials in a simple manner as a function of temperature., Comment: 6 pages, 6 figures

This paper indicates that the sensitivity and accuracy of an infrared radiometer for emissivity measurements depends not only on its design and the measurement method, but also on the spurious radiation. This spurious radiation must be taken into account in the calibration processes since it can be of the same order of magnitude as that of the sample in highly reflective surfaces. Its presence may also be the cause of the inability to detect small surface emissivity changes induced by any surface or bulk properties (anomalous skin effect, phase transitions, etc.). In this paper, the analysis of the spurious radiation is performed for a T form radiometer and a measurement method where the surroundings are considered to emit as if they were black bodies. However, the results and conclusions that are obtained can be extended without difficulty to any type of radiometer and to all measurement methods. Our research shows that if the sample is placed normal to the emitted radiation optical path, two different spurious radiation sources are detected. However, for low emitting and highly specular materials, they can be eliminated by tilting the sample between 6 and 20{\deg} without modifying the spectral emissivity.

The thermal emissivity spectrum in the mid infrared range (3 to 21 $\mu$m) as well as its dependence on temperature between 225 and 320 $^{\circ}$C has been obtained for the Mg-51%Zn (weight %) eutectic alloy, a candidate for thermal storage. The spectral curves show the typical behaviour of metals and alloys, with emissivity values between 0.05 and 0.2. It was also found that the emissivity spectrum shows variations in each heating cycle during the first few cycles. These changes are associated with the presence of metastable phases in the solid solid phase transition, present in the alloy below the melting point. The absence of signs of oxidation in air is very favourable for the use of this alloy in thermal energy storage systems. Moreover, the total normal emissivity curves obtained from dynamic spectral measurements have allowed analysing the behaviour phase transition sequence present in this alloy. These experimental results indicate that accurate emissivity measurements can be sensitive enough to account for the structural phase transitions in metals and alloys.

The total emissivity of the absorbing surfaces is a critical parameter in the calculation of the radiative thermal losses in solar thermal collectors. This is because the radiative heat losses have a significant economic impact on the final cost of the electricity produced in a solar thermal plant. This paper demonstrates the need to calculate the total emissivity from spectral emissivity measurements at the working temperature of the solar thermal collector, instead of using extrapolated values from spectral emissivities measured at room temperature. Usual uncertainties produced by the estimation of the total emissivity, in which its temperature dependence is only introduced by the Planck function, are analyzed., Comment: 4 pages, 6 figures

A study on the radiative properties of two pure metals, copper and nickel, using a high accuracy radiometer is carried out. Their spectral emissivity between 3 and 21 {\mu}m and its dependence on emission angle and temperature between 250 and 900 $^{\circ}$C is measured. An evolution of the samples emissivity associated to the surface stress relaxation is observed, which is relieved after two or three heating cycles. Spectral emissivity of metals usually decreases as wavelength increases, but in the case of copper an irregular behaviour has been found. Its spectral emissivity shows a broad plateau around 10 {\mu}m, which can be due to the anomalous skin effect. On the other hand, the emissivity usually increases with temperature, but in the case of nickel the emissivity changes little and even slightly decreases for T > 700 $^{\circ}$C. The exper-imental directional emissivity of both metals shows the dependence on the emission angle predicted by the electromagnetic theory for metallic samples. By increasing the emission angle, the emissivity dependence on the wavelength strongly decreases. Furthermore, in the case of nickel, an emissivity increase with wavelength is observed for k > 20 {\mu}m. The electrical resistivity for both metals is obtained by fitting the experimental emissivity curves with the Hagen Rubens equation. The results agree fairly well with direct electrical resistivity measurements for copper but show a poor agreement in the case of nickel., Comment: 6 pages, 8 figures

Spectral directional emissivity has been measured in copper between 3 and 24 {\mu}m above room temperature. The experimental spectrum shows a weak broad peak between 7 and 14 {\mu}m, which is much more acute for higher emission angles. However, the peak width and position are both independent of the emission angle. The experimental results are in very good agreement with the semiclassical theory of the optical properties of metals in the regime of the anomalous skin effect, in particular with the asymptotic approximation. This comparison suggests that this work shows an optical experimental evidence of the anomalous skin effect., Comment: 4 pages, 4 figures