Normal Spectral Emissivity Research Articles

Density of Liquid Tantalum and Estimation of Critical Point Data

In order to determine the density of tantalum over the entire liquid phase (at the pressure applied) and several hundred K into the super-heated region, the method of ohmic pulse-heating was applied. For this purpose, images of the thermal radial expansion of the resistively heated sample wires were taken with an adapted CCD system. A newly integrated high-power photoflash and improved triggering of the experiment allowed the acquisition of high-contrast shadow images of the expanding wires. To reduce the uncertainty arising from simultaneous pyrometric temperature measurement, the change in normal spectral emissivity as a function of temperature was additionally taken into account. In this work, the density versus temperature relationship of tantalum is reported and compared to existing literature data. From the newly obtained liquid-phase density, critical point data of tantalum, such as critical temperature and critical density, were estimated via an extrapolation procedure. Furthermore, an estimate of the phase diagram in the density versus temperature plane is given. The work is concluded by a rigorous density uncertainty estimation according to the guide to the expression of uncertainty in measurement (GUM).

Study on Infrared Radiation Coating and its Application in Glass Furnace

A new type of infrared radiation coating for glass furnace without pollution was studied and prepared, and sprayed the coating onto the inner surface of the siliceous refractory in the melting end of glass furnace. The phase composition and microstructure were characterized by XRD and SEM, and the normal spectral emittance of coatings and thermal data of the glass furnace were measured. The results showed that the main phase of the coating was SiO2, and the coating firmly combined with the siliceous refractory at high temperature. The infrared spectral emissivity of the coating can reach as high as 0.92 at 1600°C. When the melting end of glass furnace was sprayed by infrared radiation coating, resulting in 7.75% energy efficiency.

Effects of environmental and operational variability on the spectrally selective properties of W/WAlN/WAlON/Al2O3-based solar absorber coating

The stability of solar selective absorber coatings in hostile environments (e.g., humid condition, corrosive medium, longer exposure at elevated temperature) needs to be critically assessed to ensure durability of such coatings. In the present work, magnetron sputtered W/WAlN/WAlON/Al2O3-based absorber coating with a high absorptance (0.958) and low emittance (82 °C) has been tested in humid and corrosive environments. The selectivity (absorptance/emittance) of the coating did not change after keeping it in 95% humidity at 37 °C for 400 h. Corrosion study in 3.5% NaCl solution reveals that this novel multilayer coating has a better corrosion resistance than that of uncoated stainless steel (SS) substrate. The nanoindentation test on the coating indicates that it has a hardness of 9.6 ± 0.5 GPa. The performance of the coating did not degrade after heat treatment at 350 °C in air for 1000 h. Additionally, the activation energy for degradation has been determined to predict the stability of the coating at high temperature. Further, the normal spectral emissivity of the tandem solar absorber was measured in the full angular range from 200 to 500 °C. The results of emissivity measurements by varying observation angles are analysed using numerical integration to obtain the total hemispherical emissivity. In summary, W/WAlN/WAlON/Al2O3 stack is an attractive candidate absorber coating for photo-thermal conversion systems.

A new non-contact optical method for determining the thermal conductivity of metals and carbides at their melting/freezing temperatures

It is now recognized that the contact methods to measure a thermal conductivity of metals and carbides in liquid/solid phase transition are facing some difficulties. On the other side, the normal spectral emissivity for these materials at melting/freezing points can be measured with high degree of accuracy. Therefore, it is of considerable interest to develop a method that makes it possible to determine the thermal conductivity through the measured value of the normal spectral emissivity. A new optical non-contact method to determine the thermal conductivity of metals and carbides at their melting/freezing points is proposed. This method is based on the use of the Drude model, the Hagen–Rubens relation, and the Wiedemann–Franz law. To define the thermal conductivity of materials at melting/freezing points, the experimental measurements of the normal spectral emissivity in a specific far-infrared range are needed. The proposed method does not require to model the power balance of heat transport for calculating the thermal conductivity. The applicability of the proposed method was demonstrated on cobalt, nickel, and zirconium carbide. A good agreement with experimental data published in the literature is obtained. The gap between the thermal conductivity of the materials under study in the solid and liquid phases at their melting/freezing temperatures is calculated. The temperature dependence of the thermal conductivity of the “ideal” solar power emitter is obtained.

Study of the normal emissivity of molybdenum during thermal oxidation process

The infrared normal spectral emissivity of the oxidized molybdenum was measured during thermal oxidation process, and the integral emissivity was calculated from the data of spectral emissivity. It is found that the surface oxidation has a remarkable effect on the spectral emissivity of molybdenum, and the spectral emissivity curves become more fluctuant with the increase in oxidation time. The integral emissivity grows exponentially with the oxidation time at 773 K, remains almost constant at 823 K, and fluctuates at 873 and 923 K. The X-ray fluorescence spectrometer, the X-ray diffraction, and the scanning electron microscopy were employed to analyze the changes in surface composition and surface morphology. The results show that the most probable reason for the variation of integral emissivity is the change in surface roughness caused by the variation in the size and shape of oxide particle on specimen surface.

Thermophysical properties of liquid Co measured by electromagnetic levitation technique in a static magnetic field

Density, normal spectral emissivity, heat capacity, and thermal conductivity of liquid Co were measured by an electromagnetic levitation technique in a static magnetic field. High-purity Co (99.9995 mass%) prepared by an anion exchange method was used for the measurements. Uncertainty analysis was conducted for all experimental data. The emissivity data at 807 nm deviated from the Drude model, owing to interband transitions of electrons. The heat capacity was successfully measured at a low magnetic field of 3 T with some residual convection within the droplet. However, for thermal conductivity measurements, a larger magnetic field was required to suppress convection, which made the levitation unstable owing to the magnetic force. Although the thermal conductivity data showed a relatively large scatter, the data agreed with the Wiedemann–Franz law at low temperatures. At higher temperatures, the experimental data deviated from the Wiedemann–Franz law. For heat capacity measurements, translational motion of the Co droplet should be suppressed, while thermal transportation by convection flow in the droplet should be preserved. Conversely, for thermal conductivity measurements, the convection flow should also be suppressed. Therefore, in this study, the heat capacity and thermal conductivity of Co were measured under static magnetic fields of 3 and 9 T, respectively. Moreover, our experimental results were compared with free electron models, namely, the Drude model and Wiedemann–Franz law.

Low emissivity double sides antireflection coatings for silicon wafer at infrared region

Silicon wafer, when operating from the band edge to the far infrared, inherently possess nearly polarization-independent low intrinsic loss - an appropriate infrared transmission window sheet for infrared devices. However, the incident light is mostly reflected at the silicon-air interface due to large admittance mismatch. We show that the reflection of silicon wafer may be sufficiently suppressed by utilizing a double sides non-quarter wave anti-reflective coatings (ARCs). The underlying mechanism is that the interfaces of the ARCs with the silicon wafer and the air structure are selected such that the matched admittance has a real value. For an optimized double sides ARCs, we achieve a lowest light reflectance of ∼4% over a broad infrared spectral range at various light incident angles, which is superior to a single side admittance-matched ARCs. We further demonstrate that, compared with bare silicon wafer, the observed infrared normal spectral emissivity of the silicon wafer with the double sides ARCs increased by only ∼0.02. As the advantages above are not at a cost of surface modification, this structure is promising to be applied in low emissivity infrared window for radiation thermometry, sensing, and so on.

Study on IR Properties of Reduced Graphene Oxide

Firstly, the reduced graphene oxide was prepared by modified hummer method and characterized. Then, the complex refractive index of reduced graphene oxide in IR band was tested and its IR absorption and radiation properties were researched by correlated calculation. The results show that reduced graphene oxide prepared by hummer method are multilayered graphene with defects and functional groups on its surface. Its absorption in near and far IR bands is strong, but it’s weaker in middle IR band. At the IR atmosphere Window, its normal spectral emissivity decreases with wavelength increasing, and its total normal spectral emissivity in 3 ∼ 5μm and 8 ∼ 14μm are 0.75 and 0.625, respectively. Therefore, reduced graphene oxide can be used as IR absorption and coating materials and have a great potential in microwave and infrared compatible materials.

Investigation of the normal spectral band emissivity characteristic within 7.5 to 13 μm for Molybdenum between 100 and 500 °C

Molybdenum (Mo) is an important material to construct the first wall for the Experimental Advanced Superconducting Tokamak (EAST). The real-time monitoring of temperature distribution of the first wall based on radiation thermometry is essential to guarantee the stable operation of EAST. So, it is especially important for the acquisition of emissivity property of Mo. In this work, a self-designed emissivity measurement apparatus is developed, and the relationship between the normal infrared spectral band (7.5–13 μm) emissivity of Mo against the temperature and surface roughness of material samples is experimentally investigated under the vacuum condition over the temperature ranging from 100 °C to 500 °C. Moreover, the dependence of spectral band emissivity of Mo exposed to air on the heating-duration time at a given elevated temperature is also studied. The emissivity measurement apparatus is mainly composed of a high temperature furnace and a radiation thermometer as well as a benchmark blackbody furnace. The radiation thermometer is firstly calibrated against the blackbody furnace by means of the multi-temperature methods. And then the temperature of the sample is simultaneously measured by the two highly accurate S-type thermocouples and the radiation thermometer. Finally the emissivity value of the sample is calculated based on the direct radiometric method. The developed emissivity measurement method and experimental results obtained in this work may be helpful to understand the work state the EAST and to use of Mo as an emissivity reference.

Emissivity model of steel 430 during the growth of oxide layer at 800–1100 K and 1.5 μm

This work studied the variation in spectral emissivity with growth of oxide layer at the different temperatures. For this reason, we measured the normal spectral emissivity during the growth of oxide layer on the sample surface at a wavelength of 1.5 μm over a temperature range 800–1100 K. In the experiment, the temperature was measured by the two thermocouples, which were symmetrically welded onto the front surface of specimens. The average of their readings was regarded as the true temperature. The detector should be perpendicular to the specimen surface as accurately as possible. The variation in spectral emissivity with growth of oxide layer was evaluated at a certain temperature. Altogether 11 emissivity models were evaluated. The conclusion was gained that the more the number of parameters used in the models was, the better the fitting accuracy became. On the whole, all the PEE models, the four-parameter LEE model and the five-parameter PFE, PLE and LEE models could be employed to well fit this kind of variation. The variation in spectral emissivity with temperature was determined at a certain thickness of oxide film. Almost all the models studied in this paper could be used to accurately evaluate this variation. The approximate models of spectral emissivity as a function of temperature and oxide-layer thickness were proposed. The strong oscillations of spectral emissivity were observed, which were affirmed to arise from the interference effect between the two radiations stemming from the oxide layer and from the substrate. The uncertainties in the temperature of steel 430 generated only by the surface oxidization were approximately 4.1–10.7 K in this experiment.

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.

Dependence of the infrared emissivity on SiC content and microstructure of microarc oxidation ceramic coatings formed in Na2SiO3 electrolyte

High emissivity coatings have been widely used in thermal protection systems and high heat efficiency devices in furnaces. In the present work, high emissivity ceramic coatings incorporated with different SiC contents have been prepared on Ti2AlNb alloy by microarc oxidation (MAO) method. The addition of different SiC contents into the Na2SiO3 electrolyte has a distinct influence on the microstructure and normal spectral emissivity of composite coatings. Microstructure of MAO coatings was investigated by using a glancing incidence X-ray diffractometer, a scanning electron microscope, an eddy current thickness meter and a contact surface profiler. A two-step voltage-controlled technology was also used to tailor the microstructure by adjusting the microarc oxidation duration. The composite coatings formed on Ti2AlNb alloy exhibit a porous structure, and consist of R-TiO2, A-TiO2, Al2SiO5, SiC as well as amorphous phase. The normal spectral emissivity of both coated and bared Ti2AlNb samples was measured at 600°C in the infrared wavelength range of 3–20μm. The S-10C composite coating incorporated with a 10g/L of SiC in the Na2SiO3 electrolyte has a much higher emissivity in the wavelength range of 3–20μm than the bared Ti2AlNb alloy. High emissivity of the S-10C coating is attributed to its porous structure and chemical composition, especially SiC content.

Normal spectral emissivity and heat capacity at constant pressure of Fe–Ni melts

The normal spectral emissivity at 807 nm and molar heat capacity at constant pressure of Fe–Ni melts were successfully measured by the combination of an electromagnetic levitation technique and a static magnetic field. The static magnetic field suppressed the surface oscillation and translational motion of the levitated sample droplet to reduce the experimental uncertainty in the measurements. For all compositions of the melts, the normal spectral emissivity values and molar heat capacities showed negligible temperature dependence. The excess heat capacity of the melts was evaluated as a function of composition. This analysis showed a positive deviation from the Neumann–Kopp rule over the entire composition range. Moreover, enthalpy of mixing was calculated from the excess heat capacity up to 2200 K.

Spectral emissivity model of steel 309S during the growth of oxide layer at 800–1100 K

This work strived to determine the analytical relationships between the spectral emissivity and the wavelength at different temperatures during the growth of oxide layer on the specimen surface of steel 309S. In the experiment, the spectral emissivity was measured at eight wavelengths, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, and 2.1μm, at temperatures from 800 to 1100K in increments of 20K by multispectral radiation thermometry. To accurately measure the normal spectral emissivity, the detector employed in the thermometry should be perpendicular to the surface of specimens as accurately as possible. The temperature of specimen surface was measured by the two thermocouples, which were symmetrically welded onto the front surface of specimens. The average of their readings was regarded as the true temperature. With the spectral emissivity measured here, the variation in the spectral emissivity with wavelength was evaluated at different temperatures and different heating times. Ten emissivity models were evaluated. The effect of number of the parameters used in the models on the fitting accuracy was studied. Both the five-parameter LLWE and LWE models were very suitable for fitting the spectral emissivity and could farthest relieve the effect of heating time on the accuracy of temperature prediction. The uncertainties in the temperature prediction of steel 309S specimens were basically within 10K by thermometry using the two models over the present temperature and wavelength ranges.

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.

A simple expression for the normal spectral emittance of open-cell foams composed of optically thick and smooth struts

A set of 96 open-cell foams with growing porosities (0.35–0.95) and growing pore nominal diameters (0.4–2.6mm) was artificially generated to firmly connect their normal spectral emittances to their textural features. This work is strictly focused on foams that are composed of opaque struts with optically smooth surfaces. To compute the normal spectral emittances, a Monte Carlo Ray Tracing code was carefully used through an indirect method based on Kirchhoff's laws. The Monte Carlo Ray Tracing code considers the complex refractive index of the solid phase constituting the struts foams. Particular attention is therefore paid to performing the calculation with absorption indices (0.5–8) that preserve the opacity of each strut. From a thorough analysis of the ray transport within all the foams, where the Representative Elementary Volumes used for computing the homogenized radiative properties were known beforehand, a general and simple law is established that connects the normal spectral emittance, on the one hand, and the open porosity and the complex index of refraction, on the other hand. In the field of the thermal conversion of solar energy, for example, the new law gives relevant insight on the radiative performance of highly porous foams that are virtually coated with materials that are known for possessing an undeniable spectral selectivity when they are shaped as dense samples.

Normal spectral emissivity of GH536 (HastelloyX) in three surface conditions

The normal spectral emissivity of GH536, a nickel-based superalloy, is measured using a FT-IR spectrometer in the wavelength range 1–15μm at moderate and high temperature (400–1200K), as well as the samples with partially oxidized and flame treatment processed. It is found that the emissivity of GH536 decreases with the increasing of emission wavelength, which is well predicted by the electromagnetic wave theory, and increases linearly when the temperature rises. The emissivity of the other two samples with partially oxidized and flame treatment processed is also investigated in details. The results of the partially oxidized sample indicate that oxidation can enhance the emissivity significantly and alter the wavelength dependence trend. And the results of the flame treatment processed sample shows that the impurities and defects caused by the fire treatment process moderately increase the emissivity. The experimental data of GH536 is compared with the results calculated using Hagen-Rubens equation, which shows an inconsistence at high temperature condition. Therefore, a modified emissivity model is proposed, which has a good agreement in measured wavelength range at moderate and high temperature condition.

Influence of Carbonate Decomposition on Normal Spectral Radiative Emittance in the Context of Oxyfuel Combustion

To investigate whether the radiative properties of carbonate rich ash layers in oxyfuel combustion systems might be influenced by the carbonate decomposition to the corresponding oxide, the emittance of Sr, Mg, and Ca carbonates is examined. In addition “synthetic coal ashes” were produced from mixtures of CaCO3 and SiO2 as well as Fe2O3 and SiO2. The mixture ratios of the minerals were varied. All samples were prepared from powders with known particle size fractions of x < 32 μm and 125 < x < 160 μm. The powders were investigated for their temperature-dependent normal emittance in a radiation test rig by FT-IR spectroscopy in the temperature range from 500 to 1000 °C. The results reveal that the phase transformation from the carbonate to the corresponding oxide has a significant influence on spectral emittance. Whereas the carbonates show characteristic peaks in spectral emittance around 4 μm which stem from the infrared active CO3 group, these peaks vanish after transformation to the oxide. For CaCO3, t...

Investigation of the effects of Ni-based alloy K465 on the normal spectral emissivity during oxidation

The normal spectral emissivity of Ni-based alloy K465 during oxidation is experimentally measured at 810, 914 and 998°C for 12h in air over the wavelength from 1.3 to 2.4μm. The combined standard uncertainty of the normal spectral emissivity is less than 3%. The oscillations of the emissivity and the effects of oxidation temperature, heating time and wavelength on the emissivity are investigated. The oscillations of the emissivity are formed by the interference effect between the radiation from the surfaces of the substrate and the oxidation film. The oscillation extremums of the emissivity shift towards larger wavelengths as the oxidation process proceeds. The results show that the normal spectral emissivity increases as the temperature increases at the initial time. The normal spectral emissivity decreases as wavelength increases except for the occurrence of the oscillations of the emissivity. The normal spectral emissivity increases rapidly at the initial heating time, and the change of emissivity becomes slow when the oxidation tends to be saturated gradually. Besides, the emissivity fitting models versus heating time and wavelength are established, which fit the experimental results very well. The emissivity relative errors of the fitting models are less than 4%.