Normal Spectral Emissivity Measurements Research Articles

Normal spectral emissivity measurement of thermal management materials for electronic components in the 2–14 μm range

Reducing the operating temperature of electronic components is an effective method for enhancing their performance. Selecting suitable thermal management materials is crucial for optimizing cooling efficiency. Normal spectral emissivity, a dimensionless physical quantity, is key in assessing a material’s capability for radiative cooling. This paper outlines the construction of a reflective infrared emissivity measurement apparatus using the IS-50 Fourier Transform Infrared Spectrometer. A gold-coated diffuse integrating sphere was strategically placed in the spectrometer’s sample chamber. By ingeniously leveraging the spectrometer’s original optical path, a dual-sided gold-coated mirror was employed to capture both incident and reflected light, enabling accurate measurements of the normal spectral emissivity of materials used for cooling electronic components. The measurement results for typical materials were consistent with those reported in the literature, satisfyingly so. The uncertainty of the measurement setup was thoroughly evaluated, achieving a combined uncertainty of better than 1 %. This experimental study measured the normal spectral emissivity of various thermal management materials and analyzed the influence of temperature on normal spectral emissivity. These results provide crucial data support for thermal design, simulation analysis, and temperature monitoring in the development of thermal structures for electronic components.

In-situ measurement method for surface temperature and normal spectral emissivity of TC-4 high-temperature alloy based on thin film interference mechanism model

This paper proposes a novel in-situ measurement method for surface temperature and normal spectral emissivity of thermally oxidized TC-4 high-temperature alloy, and establish a model for solving temperature and normal spectral emissivity based on the thin film interference mechanism. The results indicate that the temperature calculations are in good agreement with the measurements from surface thermocouples, with a maximum error of less than 3 K within the temperature range of 473 K to 973 K. The maximum error in the total directional emissivity within the wavelength range of 2 μm to 14 μm is 0.014. The measurement method and apparatus exhibit excellent repeatability, with the standard deviation of multiple measurements of normal spectral emissivity at temperatures of 473 K and 973 K is 0.035, 0.019, respectively. This method provides an effective solution for addressing the issue of fluctuating emissivity on the thermally oxidized surface of TC-4 high-temperature alloys over service time.

Normal spectral emissivity measurement study of high-temperature alloy (316L) at 673–1273 K with a thermal oxidation surface

One effective approach to better understanding the performance characteristics of high-temperature alloys is to study their thermal oxidation behavior, and to accurately determine their spectral emissivity is helpful for increasing the accuracy of temperature measurements. Using a self-developed high-temperature emissivity testing system, the normal spectral emissivity across the range 673–1273 K and a spectral band of 1–14 μm was measured relative to this background radiation. The normal spectral emissivity of 316L high-temperature alloy was investigated during the thermal oxidation process. The change in emissivity of 316L high-temperature alloy during thermal oxidation was observed. The emissivity of the 316L high-temperature alloy undergoes great changes during thermal oxidation; for example, at 1273 K it was 0.055 in its un-oxidized state but after 50 h’ thermal oxidation went up to 0.829. Below 1073 K the emissivity of 316L high-temperature alloy in 8–14 μm range showed little change, making it ideal for radiation thermometry. By fitting the measurement data, we derived that the emissivity of 316L high-temperature alloy has an exponential relationship with thermal oxidation temperature, and is a power function relationship with thermal oxidation time.

Melting spectral emissivity measurement of metal during the melting and solidification process

In the field of energy-intensive industrial processes, investigating the spectral emissivity of molten metals is crucial for improving energy utilization and efficiency. However, previous research efforts have not fully explored the measurement of melting emissivity. This study presents an effective method for measuring the spectral emissivity of molten metals. A new experimental apparatus was developed to measure the melting spectral emissivity within a range of 2–8 μm and temperatures ranging from 873 K to 2373 K. The sample was rapidly melted using a specially designed electromagnetic cold crucible, while its spectral radiance was detected using a Fourier transform infrared (FTIR) spectrometer. To enhance energy utilization rate, numerical simulation of the cold crucible improved induction heating performance. Additionally, calculating radiation within the cylindrical chamber enhanced measurement accuracy of emissivity. To validate the reliability of this apparatus, normal spectral emissivity measurements were conducted on liquid nickel. The results demonstrate that comprehensive uncertainty associated with liquid nickel's spectral emissivity does not exceed 1.82 %. Furthermore, verification based on the Drude-Roberts model confirms normal spectral emissivity values for liquid nickel as well.

Simultaneous measurement of normal spectral emissivity and temperature of materials with low thermal conductivity

This study develops a measurement system and a method for measuring the normal spectral emissivity and temperature of low thermal conductivity materials at high temperatures. A self-design heating device using a synergistic heating treatment including a high-power laser and a radiation heating cavity is introduced, and a Fourier transform infrared spectrometer is utilized to measure the radiation signal of sample. Based on the temperature hypothesis model, which considers normal spectral emissivity keeps constant within small temperature difference less than 10.0 K, a method is proposed for realizing the inversion of normal spectral emissivity and temperature with the multispectral radiation signal. A genetic algorithm-based technique with big mutations is used to estimate the normal spectral emissivity and temperature of sample at high temperatures. The variation of blackbody radiation signals with the blackbody radiance is analyzed to verify the linearity for the detector of FTIR spectrometer. The apparent normal spectral emissivity of an alumina ceramic fiber is determined in the spectral range of 2.5–25.0 μm at 673–1273 K. Furthermore, the uniformity of temperature distribution on the sample surface and inside the sample is numerically evaluated. The standard uncertainty in the experiment at 1273 K is less than 4.0 % in the investigated bands.

Measurement of normal spectral emissivity of materials at high temperature

为满足隐身材料、热防材料和隔热涂层等高温材料涂层的光谱发射率的高精度测量需求，研究了在1 273 K～3 100 K条件下准确测量材料法向光谱发射率的方法。基于发射率定义，建立了材料法向光谱发射率测量模型，并在该基础上研建了光谱范围为0.7 μm～12 μm的材料法向光谱发射率测量装置。为克服测量装置中样品高精度加热时伴随腔体效应的技术难点，研制了具备可移动石墨坩埚的样品加热炉，取得了良好的实验效果。使用发射率测量装置对SiC与低发射率涂层2种样品的法向光谱发射率进行实验测量。结果表明：2种样品的法向光谱发射率均随波长增加而降低，随温度的升高而升高。最后对高温状态下材料法向光谱发射率测量不确定度进行了评定，相对扩展不确定度为3.6%。

High temperature oxidation of SiC-coated Fe-Cr-Al-Mo alloys in muffle and concentrated solar furnaces

The maximal temperature of a solar receiver is limited by its resistance to oxidation, thermal cycling and mechanical stresses as it is a key component of solar power plant with a central tower. Currently used Ni-based alloys cannot heat the air coolant at temperatures beyond 1000 K due to the severe oxidation that may affect the surfaces exposed to air at high temperatures, affecting the solar absorptivity of the receiver. A SiC coating should protect a high temperature metallic alloy from oxidation and improve its solar absorptivity, only if the accommodation between the metallic substrate and the ceramic coating is optimized to prevent the coating from cracking and peeling due to the thermal expansion of the assemblage. To reduce the interfacial stresses, SiC was deposited using High Temperature Chemical Vapor Deposition on a pre-oxidized Fe-Cr-Al-Mo alloy, so that a 2µm-thick alumina layer could work as an accommodation layer. The oxidations performed in muffle and solar furnaces (associated to ex situ characterizations using scanning electron microscopy and energy-dispersive X-ray spectroscopy mapping, X-ray diffraction, and room-temperature normal spectral emissivity measurements) have demonstrated that this multilayer structure could support long-term oxidation and thermal stresses up to1400 K without significant changes regarding the spectral emissivity, but the coating peels if the sample is treated at 1600 K. The multilayer structure can therefore work beyond the current working point of solar receivers, nevertheless the accommodation still has to be improved in case of accidental exposures to higher solar fluxes.

Normal spectral emissivity measurement of graphite in the temperature range between 200°C and 500°C

Graphite is an important material constituting the first wall of the Experimental Advanced Superconducting Tokamak (EAST), and the temperature distribution of the first wall is a key parameter which is used to reflect the operating state of the EAST. Therefore, the experimental investigation of emissivity characteristics of the first wall material is necessary for achieving high-precision temperature measurement. In this work, an emissivity measurement model based on the Fourier transform infrared (FTIR) spectrometer was constructed. On this basis, the normal spectral emissivity within 3 μm to 5 μm of graphite in the temperature range between 200 °C and 500 °C is measured, and two nonlinear models are also given to describe the dependence of emissivity on the wavelength and the temperature. In addition, based on these emissivity measurement results, the temperature measurement performance of the FTIR-based radiation thermometer after emissivity correction is also evaluated in this work. Experimental results show the normal spectral emissivity of graphite increases when temperature rises and increases first and then decreases with the increase of the wavelength. And after the emissivity correction, the FTIR-based radiation thermometer can achieve better performance of temperature measurement, where the maximum absolute error can be reduced from 21.1624 °C to 2.4779 °C after emissivity correction. The method and experimental results proposed in this work may provide a valuable guidance on temperature measurement of the first wall of the EAST, and also provide a reference for the normal spectral emissivity measurement of other materials in the mid-infrared region.

Modified two-temperature calibration method for emissivity measurements at high temperatures

In this study, an experimental apparatus for normal spectral emissivity measurements in the temperature range from 673 to 1473 K and spectral range from 2 to 25 μm was constructed. To eliminate disturbances caused by the drift of the background radiation and the zero-point deviation of the FTIR spectrometer at high temperatures, an improved algorithm based on the modified two-temperature calibration method for enhancing the measurement accuracy of emissivity was proposed. The spectral response function was computed with the modified two-temperature calibration method. The excellent linearity of detector and the good stability of the spectral response function were verified. The necessity of the compensation offset for the drift of the background radiation and the zero-point deviation of the FTIR spectrometer were systematically analyzed. The potential reference material (silicon carbide) was used to validate the reliability of the emissivity results obtained by the experimental apparatus. The excellent agreements with the literature data around the Christiansen point demonstrate that the improved algorithm can provide reliable emissivity measurements. The calculated combined uncertainty of the spectral emissivity for the silicon carbide sample at 1473 K is estimated to be less than 2.6% except for the spectral bands of atmospheric absorption.

An improved algorithm for spectral emissivity measurements at low temperatures based on the multi-temperature calibration method

An experimental apparatus for normal spectral emissivity measurements at low temperatures was constructed by using the Fourier transform infrared (FTIR) spectrometer. The FTIR spectrometer was calibrated against a high quality reference blackbody based on the multi-temperature method. The spectral response function R (λ) was computed by the least-square method. To improve the precision of emissivity measurement, an improved algorithm to eliminate disturbances by background radiation was presented. Emissivity uncertainty caused by the spectral response function R (λ) was analyzed. A comparison between the results obtained by the multi-temperature calibration method and the two-temperature method was presented. The linearity of the FTIR spectrometer response in the studied spectral range is better than 1% except spectral bands due to atmospheric absorptions. A high-purity (99wt%) alumina sample was used to validate emissivity results obtained by the improved algorithm. The excellent agreement with the literature data around Christiansen wavelength demonstrates that our experimental apparatus and the improved algorithm can provide reliable measurements. The calculated relative combined uncertainty of the spectral emissivity for the alumina sample is estimated to be less than 5.1% for the spectral range considered.

Normal spectral emissivity measurement on five aeronautical alloys

Aeronautical alloys have a good performance on resisting oxidization, corrosion and fatigue in high temperature environments. Therefore, they are not only widely employed in aero-engine but also in nuclear and petroleum industries. The normal spectral emissivity of five alloys in the wavelength ranging 1–15 μm at moderate and high temperature (400–1200 K) is measured. It is found that the emissivity decreases with the increasing of wavelength and increases when temperature rises. The effects of oxidation and flame treatment process are also studied. The results of the oxidized samples indicate that oxidation can enhance the emissivity significantly and alter the wavelength dependent trend. The results of the flame treatment processed samples show that the impurities and defects caused by the flame treatment process moderately increase the emissivity. The experimental data presented in this paper can be used directly to improve the accuracy of radiation computation and other relevant applications.

Normal Spectral Emissivity Measurement of Molten Cu–Co Alloy Using an Electromagnetic Levitator Superimposed with a Static Magnetic Field

The normal spectral emissivity of molten Cu–Co alloy with different compositions was measured in the wavelength range of 780 nm to 920 nm and in the temperature range of 1430 K to 1770 K including the undercooled condition by an electromagnetic levitator superimposed with a static magnetic field. The emissivity was determined as the ratio of the radiance from a levitated molten Cu–Co droplet measured by a spectrometer to the radiance from a blackbody calculated by Planck’s law at a given temperature, where a static magnetic field of 2.5 T to 4.5 T was applied to the levitated droplet to suppress the surface oscillation and translational motion of the sample. We found little temperature dependence of the normal spectral emissivity of molten Cu–Co alloy. Concerning the composition dependence, the emissivity decreased markedly above 80 at%Cu and reached that of pure Cu, although its dependence was low between 20 at%Cu and 80 at%Cu. In addition, this composition dependence of the emissivity of molten Cu–Co alloy can be explained well by the Drude free-electron model.

Amendments to the Discussion on the Paper Entitled “Normal Spectral Emissivity Measurement of Liquid Iron and Nickel Using Electromagnetic Levitation in Direct Current Magnetic Field”

We recalculated the normal spectral emissivity of liquid Fe and Ni using the correct electrical and thermophysical parameters based on the Drude model. The newly calculated emissivity values of liquid Fe and Ni were lower by 15 to 20 pct when compared with the experimentally determined values within the wavelength range of 780 to 920 nm.

Discussion of “Normal Spectral Emissivity Measurement of Liquid Iron and Nickel Using Electromagnetic Levitation in Direct Current Magnetic Field.”

This note corrects nonphysical Drude model parameters presented for molten iron and nickel. A comparison of the corrected Drude models with data published in the literature shows that a simple Drude form is inadequate to describe the optical properties of molten iron and nickel.

Laser scanning heating method for high-temperature spectral emissivity analyses

A new non-contact laser heating method for the normal spectral emissivity measurements of high-temperature coatings is introduced. The method utilizes laser source in combination with scanning optics. General and specific requirements for the method development are discussed. Works on the development of the optimal heating strategy to ensure homogeneous temperature field of the area for the emissivity analyses on the sample surface are summarized. The final experimental set-up and heating procedure are described in detail. Advantages and disadvantages of the method and the method applicability are also discussed. The experimental set-up with 400 W laser is capable to heat steel substrate samples in the range of 300–1200 °C. Temperature transition of about 100 °C takes about 300 s. Temperature homogeneity in the area measured by spectrometer is within 2.5 °C in the whole temperature range.

New experimental device for high-temperature normal spectral emissivity measurements of coatings

A new experimental device for normal spectral emissivity measurements of coatings in the infrared spectral range from 1.38 μm to 26 μm and in the temperature range from 550 K to 1250 K is presented. A Fourier transform infrared spectrometer (FTIR) is used for the detection of sample and blackbody spectral radiation. Sample heating is achieved by a fiber laser with a scanning head. Surface temperature is measured by two methods. The first method uses an infrared camera and a reference coating with known effective emissivity, the second method is based on the combination of Christiansen wavelength with contact and noncontact surface temperature measurement. Application of the method is shown on the example of a high-temperature high-emissivity coating. Experimental results obtained with this apparatus are compared with the results performed by a direct method of Laboratoire National d’Essais (LNE) in France. The differences in the spectra are analyzed.

Metrology for New Generation Nuclear Power Plants–MetroFission

Metrology for New Generation Nuclear Power Plants–MetroFission

Preparation and performance evaluation of Er2O3 coating-type selective emitter

An Er2O3 coating-type selective emitter for themophotovoltaic application was prepared by plasma spray technology. The test results show that plasma spray technology could be used to prepare the Er2O3 coating-type selective emitter with good stability at 1400°C. Based on the measurements of the high temperature normal spectral emissivity and the spectral hemispherical emissivity of the samples at room temperature, the influence of the coating thickness was discussed, and the selective emission performance of the sample was evaluated using radiative efficiency as the criterion. The results demonstrate that the emission of substrate could not be neglected unless the coating thickness would be larger than the penetration depth, which is around 100 μm. The selective emission peak of the Er2O3 coating occurs at 1550 nm, matching well with the GaSb cells. However, the radiative efficiency is not larger than that of the SiC emitter, because the non-convertible emission of 1.725–5 μm accounts for a large proportion of the total radiation power, especially at high temperature. Effective suppression of this band emission is essential to the improvement of the radiation efficiency of the emitter.

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.

Normal spectral emissivity measurement of molten copper using an electromagnetic levitator superimposed with a static magnetic field

The normal spectral emissivity of molten copper was determined in the wavelength range of 780–920 nm and in the temperature range of 1288–1678 K, by directly measuring the radiance emitted by an electromagnetically levitated molten copper droplet under a static magnetic field of 1.5 T. The spectrometer for radiance measurement was calibrated using the relation between the theoretical blackbody radiance from Planck's law and the light intensity of a quasi-blackbody radiation source measured using a spectrometer at a given temperature. As a result, the normal spectral emissivity of molten copper was determined as 0.075 ± 0.011 at a wavelength of 807 nm, and it was found that its temperature dependence is negligible in the entire measurement temperature range tested. In addition, the results of the normal spectral emissivity and its wavelength dependence were discussed, in comparison with those obtained using the Drude free-electron model.