Total Hemispherical Emissivity Research Articles

A hybrid experimental configuration for emissivity measurement at cryogenic temperatures

Abstract In cryogenic and high vacuum systems, radiation mode of heat transfer is the primary source of energy interaction owing to the wide temperature differentials and very low temperatures and pressures. Measurement of radiative properties such as emissivity is crucial in the design of cryogenic systems. Emissivity measurement at cryogenic temperatures requires careful consideration of the sample preparation and measurement conditions to ensure accuracy and reliability. Most experimental techniques developed so far are complex in their design and equipment, and they work only on a single measuring method. To address these challenges, this research has proposed a simpler and hybrid experimental setup that could employ both calorimetric and heat flux methods to directly measure the total hemispherical emissivity in the range 90 K–180 K. Experiments were conducted with five types of materials (black paint, stainless steel, aluminum foil, aluminized mylar foil, and copper sheet) by using both methods, and the results were validated with the literature data. The data obtained by calorimetric and heat flux methods for each sample showed a high level of consistency with each other, with a variation of only ±1.5%. Moreover, the discrepancy between the measured and literature data was within the acceptable range of 0.3%–10%. The total hemispherical emissivity of mechanically treated copper was higher than that of chemically polished copper by 11%. This simplified setup has incorporated all possible means to minimize heat leakages into and from the cryostat while maintaining accuracy. The obtained data can be used to accurately estimate the radiation heat loads in cryopumps and other cryogenic systems that use similar components.

A coupling method for rapid, continuous and extremely wide-temperature measurement of total hemispherical emissivity and conversion efficiency of the kinetic energy of an electron beam to the thermal energy of a material based on a scanning electron beam

Precise measurements of a material's total hemispherical emissivity and conversion efficiency of the Kinetic energy of an Electron beam to the Thermal energy of a Material (KETM) are crucial for the advancement of new materials and electron beam applications. We propose a coupling method for the rapid and continuous measurements of the above parameters across an extremely broad temperature range, the basic idea of which is that a thin plate sample is uniformly heated to a high-temperature state by a scanning electron beam and naturally cooled, and equations of the transient heat transfer process in the cooling section and the heating section are solved separately based on the assumption of the lumped heat capacity, and the above parameters varying with temperature are obtained in turn. The characterization experiments of the wire-drawn and polished 304 stainless steel are carried out. The emissivities of two surface states increase monotonically in the ranges of [0.17(400K), 0.26(954K)] and [0.12(400K), 0.20(983K)], respectively, aligning well with trends and values reported in the literature. Oxidation on both surfaces worsens as the preset temperature rises, resulting in a corresponding increase in the emissivity. Considering the impact of oxidation on emissivity, KETM is calculated in various regions within the considered temperature ranges. The results indicate a slight variation around 0.819 in KETMs of wire-drawn and polished 304 stainless steel within the temperature range of 400∼1000 K, aligning well with the value reported in the literature. This efficient measurement method can provide potential support for the application of the above two parameters.

Data reduction and uncertainty assessment of the direct pulse heating technique with both contact and radiance temperature measurements

This work presents a data reduction procedure of the direct pulse heating technique with both contact and radiance temperature measurements. The technique is applied for the measurement of thermophysical properties of electroconductive solids over a wide temperature range, i.e., from room temperature up to about 2300°C. Absolute and radiance temperatures of a tested material are measured by thermocouple and radiation thermometer, respectively and these and other experimental data are then reduced to the values of specific heat, specific electrical resistivity, hemispherical total emissivity and normal spectral emissivity of the tested material. The related data reduction and corresponding uncertainty assessment for each property is described in detail. An example of uncertainty assessment is given for the recent measurement of specific heat, specific electrical resistivity, hemispherical total emissivity and normal spectral emissivity at 900 nm of molybdenum alloy TZM specimens.

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.

Multi-stepwise pulse calorimetry for enthalpy and emissivity measurements on tantalum, considering gas–metal reactions at elevated temperatures

Refractory metals such as tantalum are widely used for high-temperature applications; however, the thermophysical properties of tantalum are not accurately determined at temperatures approaching the melting point. Combining dynamic and static calorimetric methods, the enthalpy and total hemispherical emissivity of tantalum were determined within the 1000–3230 K temperature range. The combined approach considers the influence of reactions between the metal, minor impurities, and residual gas within a high-vacuum environment. The tantalum sample consistently absorbs residual water vapor within the vacuum chamber. At temperatures near the tantalum melting point, the absorbed hydrogen and oxygen atoms are desorbed. The kinetic nature of the gas–metal reactions determines the appropriate conditions for enthalpy and emissivity measurements via dynamic and static calorimetry, respectively. The enthalpy measured under dynamic conditions favorably agrees with the literature data derived from levitation drop calorimetry, rather than those from classical pulse calorimetry, especially near the melting point.

A cryogenic method for testing thermal radiative properties of surfaces for space probes

We present a cryogenic method for testing of thermal radiative properties of materials commonly used in ground-based systems as well as for space probes. It is possible to measure total hemispherical emissivity or absorptivity of metallic or dielectric surfaces (0.1% - 100%) in the range of temperatures from 10 K up to 320 K of the source of thermal radiation. Emissivity of thermal control coatings for the space probe JUICE are presented together with other materials used for space exploration or cryogenics.

Daytime radiative cooling multilayer films designed by a machine learning method and genetic algorithm.

Recently, there has been growing interest and attention towards daytime radiative cooling. This cooling technology is considered a potentially significant alternative to traditional cooling methods because of its neither energy consumption nor harmful gas emission during operation. In this paper, a daytime radiative cooling emitter (DRCE) consisting of polydimethylsiloxane, silicon dioxide, and aluminum nitride from top to bottom on a silver-silicon substrate was designed by a machine learning method (MLM) and genetic algorithm to achieve daytime radiative cooling. The optimal DRCE had 94.43% average total hemispherical emissivity in the atmospheric window wavelength band and 98.25% average total hemispherical reflectivity in the solar radiation wavelength band. When the ambient temperature was 30°C, and the power of solar radiation was about 900W/m 2, the net cooling power of the optimal DRCE could achieve 140.38W/m 2. The steady-state temperature of that could be approximately 9.08°C lower than the ambient temperature. This paper provides a general research strategy for MLM-driven design of DRCE.

High-toughness/resistivity ruthenium-based refractory alloy wires

High-resistivity Ru–Mo–W-alloy wires, exhibiting excellent strength and ductility, were fabricated using the micro-pulling-down melt-growth method under vacuum for application as resistance heating wires. Furthermore, their mechanical and electrical properties were investigated. The fabricated Ru0.6Mo0.15W0.25 wires (⌀0.8 mm × 10.8 m long) showed a room-temperature tensile strength and nominal rupture strain of 1287 MPa and 21.2%, respectively. Transmission electron microscopy of Ru0.6Mo0.15W0.25 revealed the presence of active 112¯2<112¯3> pyramidal slip systems. Ru0.6Mo0.15W0.25 demonstrated an electrical resistivity of ∼0.936 μΩ∙m at 1873 K, 1.77 and 1.24 times higher than those of W and Ta, respectively. The dependence of the hemispherical total emissivity on temperature was also measured. The temperature dependence of the electrical resistivity of Ru0.6Mo0.15W0.25 was lower than those of Ta and W. Furthermore, the hemispherical total emissivities of Ru0.6Mo0.15W0.25 and Ta were comparable at ∼1800 K. When tested at 1873 K for approximately 3000 h, the durability of a Ru0.6Mo0.2W0.2 wire was equal or superior to that of a Ta wire. The current (power) consumption required to deposit LiF and tris(8-hydroxyquinolinato)aluminum(III) (Alq3) at the same deposition rate using Ru0.6Mo0.2W0.2 was 21% (11%) and 34% (12%) lower, respectively, than that when using conventional Ta heating wires. The results reveal that the Ru0.6Mo0.15W0.25 heat-stabilization time was reduced by approximately 44%. Moreover, its deposition rate recovered more rapidly from errors such as a sudden boiling during heating.

Emissivity measurements of W/TiAlN/TiAlSiN/TiAlSiON/TiAlSiO -based multilayer spectrally selective absorbers at high temperature

Solar absorber with low infrared emissivity is an important component of the solar thermal system. The major problem of these absorbers has been the high emissivity at high temperatures causing heat loss from the system. One key solution for this issue is to study the emissive characteristics of these absorbers at high temperatures. This paper describes the infrared emissivity of W/TiAlN/TiAlSiN/TiAlSiON/TiAlSiO at elevated temperatures of 100 – 600 °C. The absorber exhibits excellent spectral selectivity with a high absorptance (α) in the solar spectrum (0.25 – 2.5 µm) and low thermal emissivity (ε) in the infrared range (2.5 – 25 µm). Directional spectral emissivity measurements performed between 10 and 80° show an emissivity decrease with the angle for wavelengths below 3.5 µm, followed by an increase in emissivity at higher wavelengths that reaches a maximum at an emission angle of 70 – 80°. The total normal emissivity shows an increase from 0.064 ± 0.016 to 0.216 ± 0.003 when the measured temperature increases from 100 to 600 °C. On the other hand, the total hemispherical emissivity does not show a significant temperature dependence. Microstructural characterizations including grazing incident X-ray diffractogram (GIXRD), scanning electron microscopy (SEM), and energy dispersive X-ray analysis (EDS) confirmed the thermal stability of the sample even after high temperature exposure up to 600 °C during emissivity measurements.

Analytical expressions for thermophysical properties of solid and liquid beryllium relevant for fusion applications

The status of the literature is reviewed for thermophysical properties of pure polycrystalline solid and liquid beryllium which constitute input for the modeling of intense plasma–surface interaction phenomena that are important for fusion applications (thermal analysis, vapor shielding, melt motion, arcing, dust generation, dust transport). Reliable experimental data are analyzed for the latent heats, specific isobaric heat capacity, electrical resistivity, thermal conductivity, mass density, vapor pressure, work function, total hemispherical emissivity and absolute thermoelectric power from the room temperature up to the normal boiling point of beryllium as well as for the surface tension and the dynamic viscosity across the liquid state. Analytical expressions are recommended for the temperature dependence of these thermophysical properties, which involve high temperature extrapolations given the absence of extended liquid beryllium measurements.

The experimental measurement of the thermal emissivity of a black coating from 50 K to 300 K

High-emissivity coatings are commonly applied in satellite detectors and large-scale cryogenic systems. Accurate and quick measurement of emissivity is in high demand for practical applications at cryogenic temperatures. In this paper, a method for the measurement of the total hemispherical emissivity from 50 K to 300 K was presented based on the static calorimetric method. A two-stage GM cryocooler was used as a cooling source in the measuring device without assumption of cryogenic liquids. The thermal emissivity of a black carbon coating was measured by monitoring temperatures timely in the device. The tested results exhibited good accordance with previous reports. This cryogenic measurement device has made it a strong candidate for the radiative heat dissipation at cryogenic temperatures. Design details of the experiment and measurement results will be presented.

Spectral directional and total hemispherical emissivity of virgin and oxidized 316L stainless steel from 1000 to 1650 K

The 316L stainless steel is used in spacecraft. In order to mitigate debris in the Low-Earth Orbit and to avoid dramatic collisions on Earth after atmospheric re-entry at its end-of-life, the spacecraft mission has to take into account the influence of reentry on the spacecraft survivability. In this way, the emissivity of 316L stainless steel samples: as-received (virgin), pre-oxidized in air plasma and oxidized in situ in air was measured from 1000 K up to 1650 K – as no data can be found in the literature at such high temperatures – and in different atmospheres: high vacuum and for two low total pressures of 300 and 2000 Pa concerning the aerospace domain. Moreover, as it could be interesting for other applications, emissivity measurements at atmospheric pressure were also performed. Experimental results obtained for this material at high temperatures and different pressure conditions together with microstructural characterization using SEM, XRD, Raman spectroscopy and 3D profilometry are presented. As expected, the emissivity of the oxidized 316L samples is much higher than the one measured in high vacuum and there are also some differences between air plasma and standard air conditions – in reduced atmospheres of 300 and 2000 Pa – with respective emissivity increase by a factor 3 and 2.5 in comparison with the virgin material, due to the different levels of oxidation. For the measurements carried out at atmospheric pressure, the increase of the emissivity is also 3 times higher compared to the virgin alloy.

Low-emittance copper-coating system using atomic-layer-deposited aluminum oxide

• First thermal radiative measurement of copper with thin-film aluminum oxide coating. • Thin-film aluminum oxide coating preserves the low thermal emissivity of copper. • Analysis of dielectric thin-film coating on highly reflective metallic surfaces. Copper, due to its unique properties, has a huge technological importance to our society. However, the oxidation of copper remains an issue in numerous application areas. This is especially the case in visible and IR-band optics, where even minuscule oxide layers degrade the thermo-optical properties of copper surfaces. A solution possibly resides in the application of protective coatings, which can simultaneously impair the low thermal emittance of bare copper surfaces. The present paper examines the use of thin Al 2 O 3 layers as a protective coating for copper. Al 2 O 3 layers with thickness of 4.5, 9.1, 18.5 or 28.3 nm were deposited on polished copper discs using atomic layer deposition (ALD). The total hemispherical emissivity and absorptivity of these coated copper discs were measured from 20 K up to room temperature. The emissivity and absorptivity of the copper with ALD-deposited Al 2 O 3 layers increased with rising temperature and layer thickness. Nonetheless, the observed values stayed below 1.8%, allowing the use of the coated copper in systems where low emission or absorption of thermal radiation is needed. Alongside the experiments, we present a computer-based analysis and interpretation, which may be generally applied for prediction of temperature-dependent emittance of metallic surfaces coated with a thin polar dielectric layer.

Reversible sequin fabrics as variable emittance surfaces

This study demonstrates, for the first time, the use of reversible sequin fabrics as inexpensive, scalable, and durable variable emittance surfaces. First, the spectral normal-hemispherical reflectance's of commercially available gold/black and silver/black reversible sequin fabrics were measured at room temperature for wavelength between 0.25 and 20 μm. The surface roughness and material composition of the sequins were also characterized. The dynamic range of the total hemispherical emittance of the gold/black sequin fabric at room temperature was found to be 0.8, exceeding the performance of other variable emittance surfaces reported to date. The use of reversible sequin fabrics for thermal management was demonstrated experimentally by achieving different equilibrium temperatures of the fabric exposed to simulated solar radiation by simply adjusting the sequin pattern. The experimental measurements were in good agreement with a first order transient thermal model. Finally, thermal infrared images were taken of the sequin fabrics to illustrate their capabilities for thermal camouflage, illusion, and messaging.

Numerical derivation and validation of the angular, hemispherical and normal emissivity and reflectivity of common window glass

The thermal radiative properties of glazing materials are fundamental inputs for evaluating the energetic performance and thermal behaviour of buildings. These properties are required for many practical applications ranging from infrared thermography to building performance simulation and thermal comfort evaluations. Soda–lime–silica glass is the most widely used glass, worldwide, for glazing and the derivation of its thermal and optical properties are the subject of this work. Based on experimentally acquired spectral reference measurement data for soda-lime glass, the complex-valued Fresnel equations are solved and Monte-Carlo integration is carried out over the black-body spectrum, as well as over the hemisphere. These calculations are performed for various radiative temperatures to establish their temperature dependencies, and additionally the temperature-dependent transmittance of soda-lime glass was analysed. By applying a non-linear optimisation algorithm, computationally efficient practical fitting formulae for the angular reflectivity, emissivity and absorptivity are derived. All equations are numerically solved with a high degree of accuracy. The determination of the hemispherical total emissivity for this type of glass confirms the current accepted value (∼0.2%) originally derived by Rubin (1985). A simple yet robust experimental design, based on thermography, is used to validate the results of the numerical reflectivity model. Additionally, we provide angular functions, fitting functions and additional constants that were hitherto unavailable. The practical benefits of the fitting formulae provided are considerable in relation to modelling the reflection of thermal radiation on glazed surfaces, and are demonstrated in two case-study applications in the appendix.

Spectral emissivity, hemispherical total emissivity, and constant pressure heat capacity of liquid vanadium measured by an electrostatic levitator

Vanadium samples were levitated in an electrostatic levitator and their radiation intensities at their melting temperature were measured by spectrometers over a wide wavelength range. By comparing the measured intensities with those of a blackbody, spectral hemispherical emissivity values were calculated. The total hemispherical emissivity of vanadium at its melting temperature was calculated by integrating the spectral hemispherical emissivity and was found to be 0.32. The constant pressure heat capacity was also calculated using time–temperature data and was found to be 48.5 J·mol−1·K−1 at 2200 K.

Physico-Chemical Behavior and Thermo-Optical and Mechanical Properties of Glassy Carbon Up to 2100 K Under Low-Energy Proton and Vacuum Ultraviolet Irradiations

In the frame of mission for the Sun approach, to understand the effects of the heating of the solar corona and the acceleration of the solar winds, research was carried out to study the behavior of glassy carbon that can be used for the heat shield. The physicochemical behavior of glassy carbon from 1600 up to 2100 K with or without proton bombardment and VUV irradiation was studied together with the measurement of the thermal optical properties, the ratio of the solar absorptivity to the total hemispherical emissivity conditioning the thermal equilibrium of the heat shield. Experimental results obtained on massive glassy carbon samples are presented through mass spectrometry, mass loss rate and microstructural characterization (XRD, SEM…), mechanical and thermo-optical properties. Finally, the synergistic effect of high temperature, protons and VUV radiation has a low impact on this material.

Emissivity measurements on reflective insulation materials

Abstract The development and use of new thermal insulation products in many industrial sectors, ranging from building insulations to power generation or satellite applications, requires the accurate knowledge of the radiative properties of the investigated material, i. e. its emissivity. A major objective of the research project “Improvement of emissivity measurements on reflective insulation materials” within the framework of the European Metrology Programme for Innovation and Research was to improve and validate reference techniques for the measurement of the total hemispherical emissivity of low emissivity foils with an absolute measurement uncertainty below 0.03. The calibration and measurement procedures developed within this project shall lead to a significant benefit for industrial manufacturers of reflective foils as well as for the end-users of the industrial instruments used to characterize them.

Application of the subsecond calorimetry technique with both contact and radiance temperature measurements: case study on solid phase tungsten at very high temperatures

This work presents an application of the subsecond calorimetry technique at very high temperatures, which uses both contact and radiance temperature measurements. This technique is normally applied for thermophysical characterization of high temperature solid phase materials in the temperature range from ambient up to about 2600 K, which is the limit of the standard tungsten-rhenium thermocouple use. Simultaneously with contact temperature measurements, noncontact or radiance temperature detection may be performed in the approximate range from 1000 to 2600 K in order to acquire information on spectral normal emissivity of specimen under test. In this study, however, the specimen is heated above 2600 K and, then, the temperature is measured only by the noncontact mean. In the extended temperature range, the obtained values of the spectral normal emissivity are extrapolated for each experimental run, which makes possible a conversion from radiance to absolute specimen temperature. In order to test this application, a pure polycrystalline specimen of tungsten in the form of rod, 3 mm in diameter and 200 mm in length, has been used. The specimen has been heated in vacuum environment of about 10–4 mbar by short pulses of high DC current with a gradual increase of the total heating time from about 0.5–2.5 s. During the specimen heating and the beginning of the cooling period, four sets of experimental data have been recorded and reduced by using the corresponding data reduction procedure. Obtained results of specific heat and specific electrical resistivity from ambient to 3700 K, total hemispherical emissivity from 1000 to 3700 K and spectral normal emissivity from 1000 to 2600 K (extrapolated to 3700 K) are presented, discussed and compared with related literature data.

Emissivity at high temperature of Ni-based superalloys for the design of solar receivers for future tower power plants

In the framework of the H2020 European project POLYPHEM, whose main objective is to improve the flexibility and performance of small-scale solar power plants, studies have been conducted on various nickel-based alloys for the design of the solar receiver. Inconel 600 alloy is used as a reference material in the advanced manufacturing of high temperature solar receivers but other oxidation resistant alloys have been selected such as Inconel 617, Haynes 230, Haynes HR120 and Hastelloy X and the best candidate will be used for making the prototype. Thus, two essential quantities, namely the solar absorptivity α – the higher the better – and the total hemispherical emissivity ε – the lower the better – were measured in high vacuum and under atmospheric air up to 1400 K in the MEDIASE reactor installed at the focus of the 1000 kW solar furnace. The objective is to reach the best solar absorptivity and the α/ε ratio the highest as possible. The emissivity measurement was made by a direct method requiring the measurement of the true temperature of the material using a bi-chromatic pyro-reflectometer and that of its radiance by spectro-radiometry. Total hemispherical emissivity data are presented at the same time as microstructural (SEM, XRD) and topographic (3D profilometry) characterizations allowing the interpretation of the emissivity evolution with temperature and the growth of the oxide layer. Finally, the alloy Haynes 230 was chosen to build the solar receiver.