Experimental and numerical study of the emissivity of rolled aluminum

Directional spectral emissivity measurements on a rolled aluminum sheet are reported between 423 and 823 K in vacuum. The results are compared to available literature data and to theoretical predictions, revealing the crucial role of the surface state in explaining the observed scatter of values. In particular, it is argued that the cold-rolling process induces a multi-scale roughness profile that significantly enhances emission at all wavelengths, a phenomenon that can be described using rigorous coupled-wave analysis (RCWA). A small peak in the p-polarized component at oblique angles is formed by the native oxide layer. Aside from the intrinsic value of the emissivity data for the application of thermographic techniques to rolled aluminum materials, the results contained in this work also serve to validate the usefulness of RCWA to simulate the emissivities of randomly rough metal surfaces, highlighting directions of further research.This work was funded by the University of the Basque Country, Spain (GIU19/019) and the Basque Government, Spain (IT-1714-22 and PIBA-2021-1-0022). J. Gabirondo-López and I. González de Arrieta also acknowledge financial support from pre- and post-doctoral fellowships by these institutions (University of the Basque Country, Spain: PIF 21/06; Basque Government, Spain : POS-2021-2-0022). Technical support was provided by the SGIker service of UPV/EHU for the XPS analysis. Finally, the authors thank Jordan Edmunds for assistance with the RCWA code

Emissivity measurements conducted on intermetallic γ-TiAl-based alloys for aeronautical applications

The directional spectral emissivity of a Ti–48Al–2Nb–2Cr alloy (in at.%), 4822 alloy, and a Ti-43.5Al–4Nb–1Mo-0.1B alloy (in at.%),TNM alloy, used in the aeronautical industry, are measured between 150 and 850 °C. The differences in the emissivity values between both alloys at the lowest temperatures, indicates that the βo phase, only present in TNM, exhibit higher emissivity values. By numerical integration of the measured data, the total directional and hemispherical emissivity have been calculated. At 850 °C the total hemispherical emissivity in vacuum are nearly identical with 0.274 ± 0.006 for the 4822 alloy and 0.273 ± 0.007 for the TNM alloy. The lower emissivity change with temperature measured in TNM alloys is related with the deconvolution of βo phase by diffusion processes. Afterwards, near-normal spectral emissivity measurements are performed in both alloys during isothermal oxidation treatments at 750 °C and 850 °C for 120 h. The emissivity data reveal that the TNM alloy exhibits higher oxidation resistance especially at 750 °C. In parallel, microstructural characterization has been performed before the measurements, after the directional emissivity measurements prior to oxidation and after isothermal oxidations. The formed oxide scale is composed of four layers that coincide with those reported in the literature: an outer layer of TiO2 contiguous with a layer of Al2O3, followed by a TiO2/Al2O3 mixed layer and finally a thin layer of Nb-rich nitride. This mixed layer governs the interferential part of the alloys’ emissivity spectra, which, in combination with the background, determines the overall radiative behavior of the alloys under service conditions.This work was supported by the Education Department of Basque Government via PIBA-PUE2021_1_0022 and IT-1714-22 projects as well as a predoctoral grant (M. Sainz-Menchón: PRE-2022-1-0086). The authors thank for technical and human support provided by SGIker of UPV/EHU and European funding (ERDF and ESF) as well as R. Wartbichler for sample preparation and helpful discussions

First spectral emissivity study of a solar selective coating in the 150–600 1C temperature range

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 1C. 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.European Commission Project HITECO FP7-ENERGY-2010-1Ministerio de Ciencia e Innovación CSD2008-00023Ministerio de Ciencia e Innovación RyC2007-002