Total Hemispherical Emissivity Research Articles

Thermophysical properties of solid phase ruthenium measured by the pulse calorimetry technique over a wide temperature range

Abstract This paper presents experimental results on four thermophysical properties of pure polycrystalline ruthenium samples over a wide temperature range. Specific heat capacity and specific electrical resistivity were measured from 250 to 2 500 K, while hemispherical total emissivity and normal spectral emissivity at 900 nm were measured from 1 300 to 2 500 K. All the properties were obtained by using the pulse calorimetry technique. The 200 mm long specimens were in the form of a thin rod, of about 3 mm in diameter. For necessary corrections, literature data on thermal linear expansion were applied. The results are compared with available literature data and discussed. The specific heat capacity and specific electrical resistivity measurements did not indicate any allotropic transformation of the samples over the entire temperature range.

Total hemispherical emissivity of Inconel 718

Total hemispherical emissivity for Inconel 718 was measured in anticipation of its application in Very High Temperature Gas Reactors (VHTRs). A majority of current emissivity data for Inconel 718 is in the form of spectral measurements. The data presented here were obtained with an experimental apparatus based on the standard ASTM C835-06 for total hemispherical emittance. Measurements of Inconel 718 were made for four different surface types including: (i) ‘as-received’ from the manufacturer, (ii) oxidized in air and humidified helium, (iii) sandblasted with aluminum oxide powder, and (iv) with a thin coating of nuclear grade graphite powder (grade NGB-18). The emissivity for the ‘as-received’ sample ranged from 0.21 to 0.28 in the temperature interval from 760K to 1275K. Oxidation in air at 1073K resulted in an increase in emissivity into the range from 0.2 at 650K to 0.52 at 1200K. There was no dependence on the oxidation times studied here. Oxidation with humidified helium at 1073K produced less of an increase in emissivity than the oxidation in air but there was an increase up to the range from 0.2 at 600K to 0.35 at 1200K. Sandblasting of Inconel 718 was also observed to increase the emissivity up to the range from 0.43 at 780K to 0.53 at 1270K when 60 grit sized powder was used and up to the range from 0.45 at 683K to 0.57 at 1267K when 120 and 220 grit sized powders were used. Coating of graphite powder onto the ‘as-received’ Inconel 718 showed no increase in the emissivity whereas the coating of graphite powder onto the 220 grit sandblasted Inconel 718 did show an increase in emissivity up to the range from 0.60 at 650K to 0.70 at 1200K. This was apparently due to better adhesion of graphite on the roughened surface.

Thermophysical Properties of High-Thermal-Conductivity Graphite Sheet and Application to Deployable/Stowable Radiator

This study proposes a passively deployed radiator for use as a new thermal control device for small satellites. This radiator can control the amount of heat dissipation by varying its heat rejection area depending on the temperature. This radiator consists of a fin, a baseplate, and an actuator. First, thermal diffusivity, specific heat capacity, total hemispherical emissivity, and solar absorptance of graphite sheets, which are a part of this radiator fin, were measured. From these measurements, it was determined that the thermal conductivity of the graphite sheet varied from 950 to , the total hemispherical emissivity varied from 0.22 to 0.31, and the solar absorptance was 0.66. Second, the performance of the radiator was calculated via thermal analysis as a function of size and thickness, and a half-scaled test model of this radiator was designed. Third, the test model was fabricated and its thermal performance was tested under vacuum conditions. The testing showed that the thermal dissipation of this half-scaled radiator was 54 W at 60°C, the fin efficiency difference between the deployed and stowed positions was 0.35, and the specific heat rejection was .

Evolution of atomic structure in Al75Cu25 liquid from experimental and ab initio molecular dynamics simulation studies

X-ray diffraction and electrostatic levitation measurements, together with the ab initio molecular dynamics simulation of liquid Al75Cu25 alloy have been performed from 800 to 1600 K. Experimental and ab initio molecular dynamics simulation results match well with each other. No abnormal changes were experimentally detected in the specific heat capacity over total hemispheric emissivity and density curves in the studied temperature range for a bulk liquid Al75Cu25 alloy measured by the electrostatic levitation technique. The structure factors gained by the ab initio molecular dynamics simulation precisely coincide with the experimental data. The atomic structure analyzed by the Honeycutt–Andersen index and Voronoi tessellation methods shows that icosahedral-like atomic clusters prevail in the liquid Al75Cu25 alloy and the atomic clusters evolve continuously. All results obtained here suggest that no liquid–liquid transition appears in the bulk liquid Al75Cu25 alloy in the studied temperature range.

Simultaneous measurements of high-temperature total hemispherical emissivity and thermal conductivity using a steady-state calorimetric technique

A method was developed to simultaneously measure the total hemispherical emissivity and the thermal conductivity of samples at high temperatures. The inverse problem to determine the emissivity and thermal conductivity from steady-state high-temperature calorimetric experiments was established based on models for these two quantities. The accuracy of the inverse solution was numerically analyzed for various noise levels for samples with various thermophysical properties. The simulation results illustrate that the calculation accuracies for the emissivity and thermal conductivity strongly depend on the proportions of the radiation and conduction heat fluxes in the strip sample arising from the temperature distributions in the sample. Steady-state high-temperature experiments with nickel samples were used to experimentally verify the method. The inverse solution results for the emissivity and thermal conductivity calculated from the measured data agree well with reported data in the literature. This research provides a useful reference for measuring the total hemispherical emissivity and thermal conductivity of conductive samples at high temperatures.

Evolution of the emissivity of tungsten at high temperature with and without proton bombardment

The Solar Probe Plus mission (NASA) will be the first mission to enter the solar corona. The spacecraft will orbit the Sun at 8.5 solar radii from the Sun’s surface at closest approach. Some metallic parts of the two on-board instruments, SWEAP (a Faraday cup) and FIELDS (antennas), will directly face the Sun, while the rest of the payload will be protected by a heat shield. For application to these instruments, a candidate refractory material, tungsten, was studied, confronting conditions similar to the ones expected close to the Sun: high radiative flux leading to high temperatures (1100–2500K) and proton bombardment (1 and 4keV; 1016, 1017 and 1018ionsm−2s−1) to simulate the solar wind in high vacuum (10−4Pa). Total directional and hemispherical emissivities in the two wavelength ranges 0.6–2.8 and 0.6–40μm were recorded in situ during treatments. Material characterization was performed before and after each high temperature and bombardment experiment to correlate a possible emissivity evolution to other material properties: mainly the microstructure and the surface topography. This paper reports some results on the evolution of the emissivity at high temperature for two different tungsten materials elaborated by two manufacturers – having thus different initial surface states, impurity contents and microstructures – and also with the addition of proton bombardment to the high temperature. However, the proton bombardment showed no effect on the surface topography or the emissivity, despite the fact that the ion fluxes used in our experiments were up to three orders of magnitude higher than the one expected from the solar winds.

Accurate determination of the total hemispherical emittance and solar absorptance of opaque surfaces at elevated temperatures

In many applications such as solar power conversion technologies, radiative heat transfer plays a significant role in the energy balance. For accurate performance predictions of solar power conversion devices the total hemispherical emittance and solar absorptance of surfaces need to be known with high accuracy. Often times the emittance of a surface is calculated from indirect spectral bidirectional or directional-hemispherical reflection measurements at room temperature which can significantly underestimate the total hemispherical emittance. Here, we report a simple steady-state calorimetric method to directly measure the total hemispherical emittance of opaque surfaces at elevated temperatures with a maximum experimental uncertainty of 5%. The method is further expanded to directly measure the solar absorptance of the surface at elevated temperature using a solar simulator. We show experimental total hemispherical emittance and solar absoprtance results for various surfaces such as machine polished copper and stainless steel and several spectrally selective solar absorbers. We describe a methodology to characterize solar absorbers and apply it to a cermet-based spectrally selective solar absorber.

Enhanced Thermal Stability of W‐Ni‐Al2O3 Cermet‐Based Spectrally Selective Solar Absorbers with Tungsten Infrared Reflectors

Solar thermal technologies such as solar hot water and concentrated solar power trough systems rely on spectrally selective solar absorbers. These solar absorbers are designed to efficiently absorb the sunlight while suppressing re‐emission of infrared radiation at elevated temperatures. Efforts for the development of such solar absorbers must not only be devoted to their spectral selectivity but also to their thermal stability for high temperature applications. Here, selective solar absorbers based on two cermet layers are fabricated on mechanically polished stainless steel substrates using a magnetron sputtering technique. The targeted operating temperature is 500–600 °C. A detrimental change in the morphology, phase, and optical properties is observed if the cermet layers are deposited on a stainless steel substrate with a thin nickel adhesion layer, which is due to the diffusion of iron atoms from the stainless steel into the cermet layer forming a FeWO4 phase. In order to improve thermal stability and reduce the infrared emittance, tungsten is found to be a good candidate for the infrared reflector layer due to its excellent thermal stability and low infrared emittance. A stable solar absorptance of ≈0.90 is demonstrated, with a total hemispherical emittance of 0.15 at 500 °C.

Bilateral Intercomparison of Spectral Directional Emissivity Measurement Between CENAM and PTB

Both Centro Nacional de Metrologia (CENAM, Mexico) and Physikalisch-Technische Bundesanstalt (PTB, Germany) are national metrology institutes and provide the dissemination of the spectral directional emissivity as a calibration service. CENAM started this service recently. The emissivity measurement capability of PTB took part in two international comparisons performed in the past among other national institutes. The measurement instrumentation and techniques used for emissivity measurements at CENAM and PTB are both based on Fourier transform infrared spectrometers. Both setups are based on the principle of a spectral comparison of the radiances of the sample and blackbody radiator. In detail, the setups differ: CENAM has the capability of measuring the directional spectral emissivity normal to the sample surface, while PTB measures the directional spectral emissivity at angles of observation ranging from \(5^{\circ }\) to \(80^{\circ }\), and provides the hemispherical spectral and total emissivity of samples as well. For this comparison, it was agreed to compare the value of the directional spectral emissivity normal to the sample surface obtained by CENAM with the one determined at an angle of \(5^{\circ }\) by PTB. Four samples of different spectral directional emissivities were measured by the two institutes. For the samples, four copper disks with a diameter of 50 mm and a thickness of 5 mm were used. Three of them were coated with Comex 1402470 (white), 1402471 (aluminum), and 1402474 (black) paints, respectively, and the other one with Nextel 811-21 paint. Measurements were obtained for each sample at a temperature of about \(150\,^{\circ }\)C, and in the spectral range from \(400\,\mathrm{cm}^{-1}\) to \(2500\,\mathrm{cm}^{-1}\) (\(25\,\upmu \mathrm{m}\) to \(4\,\upmu \mathrm{m}\)). The description of the experimental setups used and the comparison results are presented in this paper. It was found that the results obtained at CENAM and at PTB agree well within the declared standard uncertainties.

The area-density-dependence of Ball IR Black’s low-temperature emissivity

Ball Infrared Black™, (BIRB) produced by Ball Aerospace and Technologies Corp, is a proprietary surface coating used in space-flight applications. It has a high total hemispheric emissivity at cryogenic temperatures, making it valuable for radiative cooling of components with very low operating temperatures. BATC is capable of controlling the mass per unit area of their BIRB coating, which impacts its effective thickness. In a previous publication we presented the results of total hemispheric emissivity measurements on a relatively high area-density coating of BIRB at temperatures between 300 and 20K. These measurements were made in a small cryostat using a new technique. We have recently performed similar measurements on two lower area-density BIRB samples. The emissivity values for all samples were at least 0.95 between 30 and 300K.

Total hemispherical radiation properties of oxidized nickel at high temperatures

It is important to quantify the effect of oxide film on the total hemispherical radiation properties. This paper investigated the radiation properties of high-temperature oxidized 99.9% purity nickel. The scale-growth kinetics of nickel oxide film formed on nickel at oxidation temperature of 900°C was analyzed. The apparent total hemispherical emissivities of oxidized nickel with various oxide film thicknesses were measured at high temperatures using a steady-state calorimetric technique. The dependence between the total hemispherical emissivity and oxide scale at various temperatures was determined.

Nanosized Nucleus-Supercooled Liquid Interfacial Free Energy and Thermophysical Properties of Early and Late Transition Liquid Metals

Crystal–liquid interfacial free energy is important to understand in crystal study, for example, nucleation, crystal growth, and vitrification. Here, we report the nanosized nucleus-supercooled liquid interfacial free energy of early and late transition liquid metals using the electrostatic levitation (ESL) technique and classical homogeneous nucleation theory (CNT). For the estimation of the interfacial free energy, we obtained thermophysical parameters of the transition liquid metals (Ti, Fe, Ni, Zr, Nb, Rh, and Hf), such as hypercooling limit (ΔThyp), specific heat (Cp), total hemispherical emissivity (eT), and density (ρ). The estimated interfacial free energies of Ti, Ni, and Zr agreed well with a previous report having similar hypercooling limit and fusion enthalpy, while Fe, Nb, Rh, and Hf show different values from the report. This reflects the importance of accurate measurement of the two quantities. The obtained Turnbull’s coefficients (α) of the liquid metals is higher than 0.45. The interfacia...

High-temperature Solar Selective Absorbers Using Transparent Conductive Oxide Coated Metal

In these days, the application of using thermal energy of sunlight has become widely used in concentrated solar power generation systems (CSP). In CSP, property of the receiver directly contributes to the system efficiency of CSP. The solar selective absorber is used for the receiver to reduce energy loss of thermal radiation. To investigate the mass production technology of fabricating solar selective absorber that can be utilized in high temperature condition, we focused on transparent conductive oxides (TCO) thin film on metal substrate. TCO material has plasma wavelength in infrared region. Therefore the electromagnetic wave with shorter wavelength than plasma wavelength goes through the material, while the electromagnetic wave with longer wavelength is reflected on the surface. It is realized that TCO on metal substrate showed high performance as solar selective absorbers, i.e. high absorptance in visible light range and low emittance in infrared range by numerical simulations. The optical property of fabricated TCO coated metal is well consistent with the simulated property. Consequently, we attain high performance solar selective absorber of which solar absorptance is 0.71 and hemispherical total emittance is 0.11. The thermal stability is also confirmed at 600oC in vacuum for 3 hr.

Influence of roughness and composition on the total emissivity of tungsten, rhenium and tungsten–25% rhenium alloy at high temperature

Abstract Measurements of the total directional and hemispherical emissivity of tungsten, rhenium and tungsten with 25% rhenium samples of various surface states was carried out in two wavelength ranges, 0.6–2.8 and 0.6–40 μm, from 1300 up to 2500 K in high vacuum. A clear trend was identified for the ratio of the 0.6–2.8 μm to the 0.6–40 μm hemispherical emissivity (assumed to be close to the true α/e ratio, which is the ratio of the solar absorptivity α to the total emissivity e) as a function of the RMS roughness. This ratio is decreasing with increasing RMS roughness rapidly up to around 1 μm and then tends to stabilize for higher roughness. It suggests that increasing the roughness, starting from a flat surface, will progressively give more weight to the long wavelength range emissivities in comparison to the short wavelength ones. Beside the RMS roughness, a sharp surface structure on a finer scale found on some samples also led to an additional decrease of the α/e ratio, but these structures are not stable at high temperature and disappear due to a natural high temperature smoothing. Finally our results also show that the α/e ratio of rhenium is lower than the one of tungsten.

Rare-earth modified zirconium diboride high emissivity coatings for hypersonic applications

Sharp features of hypersonic vehicles increases heat transfer to the surface during flight. This thermal energy can be reduced via increasing the radiation and conduction heat transfer away from the surface. In this study, an emissivity modifier was incorporated into an ultra-high-temperature-ceramic coating system (ZrB2/SiC) to increase its surface radiation heat transfer rate by increasing the emissivity of the surface. The rare-earth were incorporated into the coatings via mechanical mixing Sm2O3 or Tm2O3 with ZrB2/SiC or chemically infiltrating Sm(NO3)3/ethanol solution into ZrB2/SiC. Coatings were fabricated using shrouded air plasma spray. Total hemispherical emissivity results show that the Sm(NO3)3 infiltrated ZrB2/SiC coating had a higher emissivity compared to the baseline ZrB2/SiC coatings up to 1200 °C. The thermal conductivity of all coatings presently studied was below 12 W/m/K. The presence of rare-earth in the boria-rich surface glasses formed during oxidation increases the glass evaporation rate of the coatings compared to the ZrB2/SiC coating.

Thermophysical properties of solid phase palladium over a wide temperature range

AbstractThis paper presents experimental results of a range of thermophysical properties of pure polycrystalline palladium samples over a wide temperature range. Specific heat capacity was measured from 250 to 1 770 K, specific electrical resistivity from 250 to 1 800 K, hemispherical total emissivity from 1 100 to 1 800 K, normal spectral emissivity from 1 340 to 1 720 K, and thermal diffusivity in the range from 200 to 1 540 K. From the measured data, the thermal conductivity and Lorentz function were computed in the range from 250 to 1 540 K. Specific heat capacity, electrical resistivity, and both emissivities were measured by means of subsecond pulse calorimetry, while the thermal diffusivity was obtained using the laser flash technique. The first method used specimens in the form of a thin rod, of about 2 mm in diameter and 200 mm in length, and the second disk specimens of about 3 mm in thickness and 10 mm in diameter. For necessary corrections, literature data on thermal linear expansion have been used. All the results are compared with available literature data and discussed.

Crystal–liquid interfacial free energy and thermophysical properties of pure liquid Ti using electrostatic levitation: Hypercooling limit, specific heat, total hemispherical emissivity, density, and interfacial free energy

Thermophysical properties of liquid Ti are measured by a newly developed electrostatic levitation. In this study, we measure a hypercooling limit (ΔThyp), specific heat (Cp), total hemispherical emissivity (εT), and density (ρ) of liquid Ti. The ΔThyp of the liquid Ti is 341K. The Cp of the liquid Ti shows very weak temperature dependence during supercooling. The εT and ρ of the liquid Ti are given by 0.329 and ρ(T) (g/cm3)=(4.16−2.36)·10−4 (T−Tm). Finally, the interfacial free energy is estimated with the measured thermophysical parameters. The interfacial free energy is 0.164J/m2, and Turnbull’s coefficient is 0.48.

Crystal–Liquid Interfacial Free Energy of Supercooled Liquid Fe Using a Containerless Technique

We report a crystal–liquid interfacial free energy of a supercooled liquid Fe, which is estimated from the classical homogeneous nucleation theory, using a containerless technique, electrostatic levitation (ESL). Thermophysical properties on the supercooled and the stable Fe liquids that are prerequisite for the estimation of interfacial free energy are measured by the ESL, for the first time. A hypercooling limit, which is one of the important parameters to determine the interfacial free energy, is obtained with 357 °C. Specific heat (Cp) and total hemispherical emissivity (εT) are 45.1 ± 3 J/mol·K and 0.314 at melting temperature, respectively. The Cp of the supercooled liquid Fe shows weak temperature dependence, confirmed by a calculated cooling curve. Densities of the stable, the supercooled liquid, and the crystal phases of Fe are successfully measured in a wide temperature range from 650 to 1600 °C. Finally, the crystal–liquid interfacial free energy of Fe is estimated with the measured thermophysical parameters. The value is about 0.22 ± 0.01 J/m2, which is consistent with a molecular dynamic simulation result.

Relevancy between Thermochromic and Magnetic Property of La<sub>1-x</sub>Sr<sub>x</sub>MnO<sub>3</sub> (x=0.2 and 0.33) Smart Radiation Thin Film Materials Prepared by Magnetron Sputtering

La1-xSrxMnO3 thin films with x=0.2 and 0.33 were prepared by magnetron sputtering method for its potential application on thermal control of spacecraft. The materials show a phase transition from ferromagnetic metal phase with a low infrared emittance to paramagnetic insulator phase with a high emittance. Because of the thermochromic property, they can automatically change their infrared emittance greatly in response to environment temperature and thermal load and keep the spacecraft electronic components working normally. A superconduction quantum interference device magnetometer was used to study magnetization over the temperature range 100–315 K. Temperature dependence of total hemispherical emittance was carried out in a liquid nitrogen cooled vacuum chamber by a steady state calorimetric method. Thermal emittance results indicate an obvious tuneability of both La0.8Sr0.2MnO3 and La0.67Sr0.33MnO3 thin films. Compared to La0.8Sr0.2MnO3 thin film, La0.67Sr0.33MnO3 thin film has a higher transition temperature and bigger emittance tuneability. Based on phase separation concept, the thermal emittance was fitted by two-energy-level Boltzmann distribution of metal phase volume fraction fH. The measured magnetization curves were fitted in terms of mean-field approximation theory. The volume fraction of ferromagnetic phase fM was not coinciding with the fH very well. This result showed the fH and the fM plays a different role on thermal emittance property.

Software solution for control and data acquisition in the pulse calorimetry method

This work presents a software solution for adjusting, controlling, displaying, and acquiring parameters and data in the pulse calorimetry experimental technique for specific heat capacity, electrical resistivity, total hemispherical emissivity, and normal spectral emissivity measurements. The software has been developed under the LabVIEW platform, V.7.11, and an example of its application with measurement results is presented in a separate section. The total expanded uncertainty of obtained results for the specific heat capacity and electrical resistivity of palladium was 5% and 1 - 2%, respectively.