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The mid-infrared (3–22 μm) emissivity of high-purity Ni has been studied in its pure state, during an isothermal oxidation in air at 730 °C and in the fully oxidized state. Measurements in pure Ni were performed in Ar between 200 and 800 °C and a change of slope in the temperature dependence of the total normal emissivity around its Curie temperature (354 °C) was observed. An oxidation process was carried out at 730 °C for 33 days, when the emissivity stopped evolving and the results were representative of NiO. During the first stages, the emissivity evolved forming the usual interference patterns of semi-transparent films. A mixture of oscillatory and monotonic behaviours of the emissivity as a function of wavelength and oxide layer thickness was found, which manifests as a non-trivial evolution of the total normal emissivity, different than that reported in previous studies. Finally, the emissivity of NiO was measured from below its Neel temperature (252 °C) to 850 °C. It showed the typical shape of a ceramic material with an extra vibrational mode due to two-phonon processes and an additional absorption band around 5 μm in the antiferromagnetic phase produced by magnons. The temperature dependence of its total normal emissivity differs significantly from that of a lightly oxidized nickel sample from the literature. Overall, the influence of the surface characteristics on the thermal radiative properties of oxidized Ni is thoroughly discussed and highlights the importance of accounting for all possible sources of infrared emissivity evolution in order to make accurate radiative heat transfer calculations.

High-accuracy thermal emission spectroscopy offers certain advantages over other infrared surface spectroscopic techniques. This paper demonstrates the sensitivity and versatility of this method in the study of structural phase transitions. To that end, the Zn-3%Mg-4%Al (mass %) alloy has been used, as it possesses a first-order structural phase transition with a weak calorimetric signal around 270 °C. Both the infrared spectral emissivity and the integrated total emissivity show the typical first order step, whose value has a strong spectral dependence, with the effect continuously decreasing as the wavelength increases until it can no longer be detected beyond 10 μm. Therefore, this means that this transformation is more easily observed through its effect on interband transitions, whose energies typically lie within the near and mid-infrared regions. Some of the advantages of this technique are also discussed in this work and the results are compared to those of temperature-dependent X-ray diffraction and differential scanning calorimetry.

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° without modifying the spectral emissivity.

The thermal emissivity spectrum in the mid infrared range (3–21 μ m) as well as its dependence on temperature between 225 and 320 °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 oxidation kinetics of Ti-6Al-4V samples nitrided in forming gas (N2+5%H2) between 520 and 805 °C is investigated using infrared emissivity measurements. Several kinetic exponents have been observed depending on temperature and time. The activation energy corresponding to the main parabolic rutile growth stage is 223 ± 10 kJ/mol. This value is significantly higher than that corresponding to TiN oxidation, which suggests that it is not the rate-limiting step. The protective role of the nitridation is attributed to the internal Ti2N layer. The microstructural characterization reveals a reduction in the oxygen-diffusion depth compared to non-nitrided Ti-6Al-4V, and a multi-layered Al2O3-TiO2 oxide.

CSP (Concentrated Solar Power) plants technologies use the concentration of solar energy on a receiver to produce heat and then electricity by a thermodynamical process. A solar absorber material is used to convert the energy carried by light into heat. This type of material works at high temperatures (up to 1000 °C) under a highly concentrated solar flux (up to x1000 or more). Optical properties determine the performance of absorbers and it is thus necessary to measure their spectral absorptance and emittance. Solar absorptance is directly linked to the capacity of the absorber material to convert the solar flux into heat. Emittance drives the radiative thermal losses for the heated absorber and depends on the absorber temperature. The characterization of a material in operational conditions at high temperatures requires advanced apparatuses, and different measurement methods exist for the characterization of these two quantities of relevance regarding an absorber. A Round Robin Test (RRT) was conducted with the objective of comparing different new optical apparatuses and methods for measuring the emittance or luminance of various solar absorbers in air. Measurements were carried out directly at temperatures up to 560 °C while heating the samples, and also indirectly by hemispherical reflectance measurements at room temperature. In this paper, the Round Robin Test procedure to compare apparatuses is described, as well as the corresponding reflectance and emittance results on four types of materials. In addition, a discussion of some factors of influence over high temperature measurements in air and of the observed discrepancies among results from the evaluators is presented. The reliability of reflectance/emittance measurements is also demonstrated and statistics of deviations from the mean value are analysed. These allow us to infer information about measurement reproducibility. The reflectance spectra of all samples after high temperature measurements in air (up to 500 °C) do not show any significant changes.

The total hemispherical emissivity of materials used in the solar energy industry is a critical parameter in the calculation of the radiative thermal losses and material efficiency, especially in solar thermal collector absorbing surfaces. This is because the radiative heat losses have a significant economic impact on the final cost of the electricity produced in solar plants. Our laboratory, HAIRL, in the University of the Basque Country (UPV/EHU) in Spain [1] is the first to have published infrared spectral emissivity measurements in Solar Absorber Surfaces (SAS) at working temperature [2]. The laboratory allows measuring between 50 and 1000 ºC in the 0.83-25 μm range and is also capable of doing directional measurements at different angles between 0 and 80 degrees. Therefore, it is suitable for measuring solar selective coatings, for studying high temperature stability and for characterizing thermal energy harvesting materials. In this presentation, we show the specifications of our laboratory, the results of spectral emissivity measurements in air-resistant solar selective coatings and in eutectic alloys for thermal storage and we demonstrate the necessity of measuring at working temperature in order to possess reliable data.

The directional spectral emissivities of four new copper-alloyed spinel coatings for concentrated solar power applications were measured up to 800 °C and compared Pyromark 2500®, deposited in the same conditions on Inconel 625. Reproducible results were found for all coatings at all temperatures, with similar spectral features at working temperatures. The temperature and angular dependences are related to the morphology and composition of the samples. The total hemispherical emissivity increases up to 400 °C for all coatings and then stabilizes, with similar values for most materials, except for the porous Cu0.5Cr1.1Mn1.4O4 coating. This coating offers a reduced total hemispherical emissivity due to increased semitransparency at high angles arising from its porosity. This porosity is linked to an increase in both the solar absorptance and the emissivity in the normal direction due to enhanced light trapping, which means that this coating shows signs of directional selectivity. These results, together with the data dispersion reported for Pyromark, suggest that structural properties are key for the high-temperature emissivity of the coatings and highlight the importance of direct emissivity characterization. Combined with absorptance measurements, these emissivity measurements allow for accurate calculations of the high-temperature efficiencies of the coatings, which reach values up to 0.929.

A complete experimental study of temperature dependence of the total spectral emissivity has been performed, for the first time, for absorber-reflector selective coatings used in concentrated solar power (CSP) systems for energy harvesting. The coating consist of double cermet layers of silicon oxide with different amounts of molybdenum over a silver infrared mirror layer. The experimental measurements were carried out by a high accurate radiometer (HAIRL) with controlled atmosphere in the mid-infrared and for temperatures between 150 and 600 C. The spectral emissivity is nearly constant in this temperature range. Therefore, the temperature dependence of the total emissivity is given by Planck function. These results were compared with those obtained with the usual calculus using room temperature reflectance spectrum. Finally, the performance of the coating was analyzed by comparison of coated respect to non-coated stainless steel., This work was financially supported by the European Commission (project HITECO FP7-ENERGY-2010-1Collaborative N. 256830) the Spanish Ministry of Science and Innovation (projects FUNCOAT CSD2008-00023, RyC2007-0026) and program ETORTEK of the Consejería de Industria of the Gobierno Vasco in collaboration with the CIC-Energigune Research Center. L. González-Fernández acknowledges the Basque Government the support through a Ph.D. fellowship.