Spectral Emissivity Research Articles

Effects of dust attachment on the spectral emittance of spacecraft radiators

In recent years, the method used to collect planetary samples has entailed launching a projectile from a spacecraft onto the surface of a planet at touchdown and collecting the sample raised by the impact. However, there is concern that the dust that is raised at touchdown would adhere to the radiator and affect radiative emissions from the radiator into deep space and radiative reflectance against irradiance from the sun and planets into the radiator. In other words, the dust could affect the thermal control of the spacecraft. Using electromagnetic field analysis, a new evaluation parameter was introduced in this study, and the effect of dust particles on the radiation heat transfer of a SiO2 substrate surface, which is commonly used in spacecraft radiators, was calculated. The electromagnetic field analysis was validated experimentally by spectroscopy. The effects of dust particle size, adhesion, and material on the changes in radiative properties were quantified and analyzed. It was suggested that dust particles could change the radiative properties of the radiator and adversely affect thermal control. This effect increased with increasing particle adhesion.

Multiple Inversion Techniques with Multispectral Pyrometry for the Estimation of Temperature and Emissivity of Liquid Niobium and 100c6 Steel

The article presents the estimation of temperature and emissivities of niobium and 100c6 steel around their melting points using techniques based on the minimization of the least squares norm or on Bayesian inference. Two models were considered for the spectral emissivity, including a linear variation with respect to the wavelength and independent spectral values. The measured data used for the solution of the parameter estimation problem were radiative fluxes collected from a five-wavelength pyrometer. The experimental apparatus used in this work was designed for the characterization of metal samples with sizes of few millimeters at high temperatures, combining aerodynamic levitation and laser heating. The use of the linear spectral emissivity model provided quite consistent results with the ordinary least squares estimator and with stochastic simulations using the Metropolis-Hastings algorithm. Similarly, the model of independent spectral emissivities resulted in accurate emissivities, but it required an informative prior for temperature, such as the known melting point of the metal sample. Therefore, whatever the inverse method used, a priori supplementary information in terms of emissivities or temperatures are needed due to the temperature-emissivity correlation and the unknow emissivity behavior.

Study of the roughness effect on the normal spectral emissivity of GH3044

The normal spectral emissivity is a significant parameter characterizing the radiative capacity of the object's surface. However, the normal spectral emissivity of superalloys varies with the rough state of the surface, causing unacceptable errors in radiation thermometry and heat transfer calculation for aero-engines and gas turbines. A computational method for estimating the effect of roughness on the normal spectral emissivity of superalloys was established based on constructing randomly rough surfaces. Through this method, the effect of roughness on four emissivity models commonly used in multispectral radiometric thermometry (MRT) was computationally evaluated, and the relationship between roughness and spectral emissivity was quantitatively analyzed. In addition, the normal spectral emissivity of GH3044 superalloy with different rough surfaces at 300 K and high temperatures was experimentally measured and compared with the result calculated by this computational method. Their maximum relative error was close to ± 5 % for the rough surface without emissivity oscillation. This study promotes understanding the mechanism of normal spectral emissivity varying with roughness.

Deep learning-based inverse design optimization of efficient multilayer thermal emitters in the near-infrared broad spectrum.

This study proposes a deep learning architecture for automatic modeling and optimization of multilayer thin film structures to address the need for specific spectral emitters and achieve rapid design of geometric parameters for an ideal spectral response. Multilayer film structures are ideal thermal emitter structures for thermophotovoltaic application systems because they combine the advantages of large area preparation and controllable costs. However, achieving good spectral response performance requires stacking more layers, which makes it more difficult to achieve fine spectral inverse design using forward calculation of the dimensional parameters of each layer of the structure. Deep learning is the main method for solving complex data-driven problems in artificial intelligence and provides an efficient solution for the inverse design of structural parameters for a target waveband. In this study, an eight-layer thin film structure composed of SiO2/Ti and SiO2/W is rapidly reverse engineered using a deep learning method to achieve a structural design with an emissivity better than 0.8 in the near-infrared band. Additionally, an eight-layer thin film structure composed of 3 × 3 cm SiO2/Ti is experimentally measured using magnetron sputtering, and the emissivity in the 1-4 µm band was better than 0.68. This research provides implications for the design and application of micro-nano structures, can be widely used in the fields of thermal imaging and thermal regulation, and will contribute to developing a new paradigm for optical nanophotonic structures with a fast target-oriented inverse design of structural parameters, such as required spectral emissivity, phase, and polarization.

Thermal infrared spectral characteristics of martian dust deposits and evidence for atmosphere-regolith interactions

It has been hypothesized that the dust component of the Martian surface is globally homogeneous, based on chemical similarity between landing sites and spectral similarity from select areas within bright regions. We tested that hypothesis by producing the first near-global data set of surface spectral emissivity (excluding polar regions) across the ∼233–508 and 825–1650 cm−1 (∼20–50 and 6–12 μm) spectral ranges from Mars Global Surveyor Thermal Emission Spectrometer data and using various data reduction techniques to search for any spectral heterogeneity in bright regions that might be present. We found no unequivocal evidence for spectral heterogeneity, supporting the hypothesis that dust is globally homogenized. The global emissivity product permits new spectral parameter maps and preliminary assessments of atmosphere-regolith interactions. We produced the first map of the Christiansen feature (CF) and show that, unlike on the Moon, where CF is a proxy for bulk silica content, CF position on Mars is primarily associated with dust cover. We produced an updated map of the 1630 cm−1 emissivity peak that arises from bound H2O in fine-particulate material and show that the peak is nearly ubiquitous across the Martian surface, including in dark regions with relatively low dust cover. This is attributed to minor amounts of dust disproportionally contributing to the spectral signal in the ∼1630 cm−1 region. We show that regions within the equatorial dust deposits with higher annual modeled frequency of nighttime CO2 frosts are more likely to have lower emissivity in the ∼1350-1400 cm−1 region, consistent with a higher fraction of unconsolidated dust. This provides the first spectral evidence for a previously hypothesized regolith gardening process via a diurnal CO2 cycle, representing an important surface-atmosphere interaction that may contribute to near-surface porosity and affect diffusive exchange of H2O between atmosphere and hydrated solids (ice, minerals) in the regolith.

Spectral Emissivity Profiles for Radiative Cooling

Spectral Emissivity Profiles for Radiative Cooling

Strong plasmon − phonon coupling for graphene/ hBN thermal emitter atomic system

The surface plasmon polaritons of graphene coupled with hyperbolic phonon polaritons of hexagonal boron nitride (hBN) have been considered as the ideal platform to enhance thermal emitter, which provides a powerful tool to implement energy conversion and thermal management. The conventional approaches for the strong plasmon − phonon coupling rely on the bulk hBN materials, which usually have a large footprint and display little tunability. Recently, hBN atomic emitter has been demonstrated that its peak emissivity is hopefully as high as the bulk hBN emitters. Here, we propose a graphene/hBN thermal emitter atomic system to flexibly modify the spectral emissivity by the strong plasmon − phonon coupling effect, which can be separately manipulated by the graphene width, chemical potential of graphene, number of hBN atomic layers and hBN width. Our scheme is hopefully realizing a comparable function with the bulk graphene/hBN system. Furthermore, we employ coupled oscillator model involving three oscillators to investigate the energy exchange and mode evolution between three plasmon − phonon hybridized modes, providing an excellent agreement with simulated and theoretical results. Consequently, optically manipulating plasmon − phonon interaction opens a broad design space for the spectral emissivity control, which explores the mid-infrared applications of advanced graphene thermo-optoelectronics and electrically driven thermal emitters.

Emission spectra of fullerenes: Computational evidence for blackbody-like radiation due to structural diversity and electronic similarity

The spectral emission of hot ${\mathrm{C}}_{60}$ has been experimentally shown to be broad and continuous, in apparent contradiction with the discrete and narrow absorption spectrum associated with the high symmetry of buckminsterfullerene. In the present work we computationally model the emission spectrum of isolated carbon clusters, assuming a broad distribution of isomers that are likely populated under the experimental conditions. The contributions of individual structures to the global spectrum correspond to the relaxation via recurrent fluorescence and vibrational emission, electronic and vibrational structures being described by a simple but efficient density-functional-based tight-binding scheme. The model predicts a blackbody-like emission spectrum that is naturally broad and correctly accounts for the experimental measurements, except for a maximum that is quantitatively shifted with respect to Wien's displacement law. To quantify such differences, we introduce an emissivity parameter $\ensuremath{\varepsilon}$ as the ratio between the spectral emittance and the corresponding exact blackbody spectrum; $\ensuremath{\varepsilon}$ is numerically found to scale as ${(\ensuremath{\lambda}T)}^{\ensuremath{-}2}$ at leading order with increasing temperature $T$ and for wavelengths $\ensuremath{\lambda}>350$ nm, and we provide a theoretical justification for this behavior. Our results are discussed in the light of the astrophysical detection of interstellar fullerenes, as well as in combustion environments where carbon clusters are relevant in the context of nascent soot particle formation.

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.

Relationship between infrared spectral emissivity and temperature distribution of thermophotovoltaic systems

Matching the thermal radiation power spectrum to the band gap of photovoltaic cells is an important factor governing the efficiency of thermophotovoltaic systems. The ability of the emitter to achieve low radiation outside the operating spectrum of the photovoltaic cells effectively minimizes energy waste. In this study, we designed and developed a thermophotovoltaic system with a hexahedral structure and measured the effect of surface emissivity on the temperature distribution of the system. Comparing different thermal emitters at a fixed heat-source power of 200 W, the system core temperature reached 839 K and 900 K under standard atmospheric and vacuum conditions, respectively. The trend of the simulation was consistent with that of the experiment. Thus, we established a pattern of emissivity on the system temperature field and created a basis for a more efficient design of thermophotovoltaic systems.

Numerical and experimental investigation of electromagnetic cold crucible used for emissivity measurement of molten material

The electromagnetic cold crucible levitation heating technology has tremendous potential in the spectral emissivity measurement of molten metal for its high melting temperature and no contamination. In this work, we designed a dedicated cold crucible with a hemispherical bottom to establish the spectral emissivity measurement apparatus at high temperature. The effects of different numbers of segments, slit width, wall thickness and height-diameter ratio on the magnetic permeability of crucible were studied by the finite element method. The results indicate that increasing the number of crucible segments and the slit width can effectively improve the magnetic permeability of cold crucible, while increasing the wall thickness and the ratio of height to diameter will reduce its magnetic permeability. Based on the simulation result, the cold crucible with optimal parameters was made and used for studying the spectral emissivity of molten metal. The spectral emissivity of iron, cobalt and nickel was measured successively to prove the availability of cold crucible in emissivity measurement. The results agree well with the literature data, and the combined uncertainty of the molten iron is 2%. This work demonstrates that electromagnetic cold crucibles have a promising prospect in emissivity measurement of molten metal.

Multi-Scale Femtosecond-Laser Texturing for Photothermal Efficiency Enhancement on Solar Absorbers Based on TaB2 Ceramics.

Tantalum boride is an ultra-refractory and ultra-hard ceramic known so far for its favorable high-temperature thermo-mechanical properties and also characterized by a low spectral emittance, making it interesting for novel high-temperature solar absorbers for Concentrating Solar Power. In this work, we investigated two types of TaB2 sintered products with different porosities, and on each of them, we realized four femtosecond laser treatments differing in the accumulated laser fluence. The treated surfaces were then characterized by SEM-EDS, roughness analysis, and optical spectrometry. We show that, depending on laser processing parameters, the multi-scale surface textures produced by femtosecond laser machining can greatly increase the solar absorptance, while the spectral emittance increase is significantly lower. These combined effects result in increased photothermal efficiency of the absorber, with interesting perspectives for the application of these ceramics in Concentrating Solar Power and Concentrating Solar Thermal. To the best of our knowledge, this is the first demonstration of successful photothermal efficiency enhancement of ultra-hard ceramics using laser machining.

Optical reflection characteristic-based emissivity analysis of a pyramid array flat-plate blackbody for remote sensor calibration.

The flat-plate blackbody (FPB) is the core device in infrared remote sensing radiometric calibration for providing accurate infrared radiation energy. The emissivity of an FPB is an important parameter that directly affects calibration accuracy. This paper uses a pyramid array structure based on the regulated optical reflection characteristics to analyze the FPB's emissivity quantitatively. The analysis is accomplished by performing emissivity simulations based on the Monte Carlo method. The effects of specular reflection (SR), near-specular reflection (NSR), and diffuse reflection (DR) on the emissivity of an FPB with pyramid arrays are analyzed. In addition, various patterns of normal emissivity, small-angle directional emissivity, and emissivity uniformity are examined under different reflection characteristics. Further, the blackbodies with the NSR and DR are fabricated and tested experimentally. The experimental results show a good agreement with the corresponding simulation results. The emissivity of the FPB with the NSR can reach 0.996 in the 8-14 µm waveband. Finally, the emissivity uniformity of FPB samples at all tested positions and angles is better than 0.005 and 0.002, respectively. The standard uncertainty of experimental measurement of waveband emissivity and spectral emissivity are 0.47% and 0.38% respectively, and the simulation uncertainty is 0.10%.

Design and Scalable Fabrication of Liquid Metal and Nano-Sheet Graphene Hybrid Phase Change Materials for Thermal Management.

Here, a paraffin/liquid metal (LM)/graphene hybrid thermal composite material with a high thermal-conductivity as well as high latent heat is developed. The paraffin is encapsulated in calcium alginate, which produces leakage-free phase change material (PCM) capsules. LM is filled amongthe gaps of PCM capsules to enhance overall heat conduction. Graphene nano-sheets coating attains efficient heat dissipation because of its high spectral emissivity (>91%) in the spectrum of the mid-infrared region. The developed material is verified to have strong compatibility and durable stability. The composite is utilized as a thermal buffer (TB) for central processing unit thermal management to demonstrate the synergy of these superior thermal properties. In certain cases, active cooling normally used could be replaced by the developed TB without any energy consumption for thermal management, demonstrating a completely passive cooling strategy. Compared to traditional heat sink active cooling, general energy savings of 10.4-26.3% could thus be achieved by the developed composite in wider operating conditions, proving itspotential for more efficient and sustainable data center cooling alongside thermal management of other ground-based electrical/electronic equipment.

A Review on Measurement of Spectral Directional Emissivity of Microstructured Surfaces in Infrared Wavelength Range

A survey of researches in the field of spectral directional emissivity measurements of microstructured surfaces by using direct and indirect radiometric methods in infrared wavelength range is presented. Nowadays, new concepts of periodic surface microstructures with geometrical dimensions of the same order of magnitude as the thermal radiation wavelengths are becoming one of the attractive methods for controlling spectral radiative properties of a surface like emissivity and reflectivity. Since the resonance effect between the periodic microstructures and the electromagnetic field has become one of the most promising ways to vary optical or radiative properties of the radiating surfaces artificially, in this review paper emphasis is given to the measurement of directional spectral emissivity of microstructured surfaces. A review of many systems is presented that are capable of measuring the spectral directional emissivities of solid surfaces at moderate temperatures. An overview of experimental setups is provided including the reference sources of radiation, the sample clamping and heating, detection systems, methods for the determination of surface temperature and procedures for emissivity evaluation. The paper is focused on methods of finding the emissivity of microstructures. Selected experimental data in the form of graph are presented for various microstructured surfaces.

Normal spectral emissivity characteristics of tungsten in the mid-infrared region

Tungsten is an important material constituting the first wall of the Experimental Advanced Superconducting Tokamak due to its excellent corrosion resistance. The temperature distribution of the first wall during the discharge process are the key to ensuring the safe and stable operation of the EAST. Therefore, it is necessary to explore the dependence of material emissivity on temperature and wavelength for improving temperature measurement accuracy. This work quantitatively explores the normal spectral emissivity of tungsten within the 3 μm to 5 μm in the temperature range between 100°C and 500°C based on the Fourier transform infrared spectrometer. The dependence of emissivity on wavelength and temperature are experimentally analyzed, and the possible functional relationship between the emissivity of tungsten and the surface temperature and the wavelength are established respectively. This work could lay the foundation for the application of multi-wavelength thermometry on temperature measurement of the first wall of the EAST.

Predicting the directional spectral emissivity for rough surfaces polished by sandpaper

A new method was proposed to accurately predict the emissivity of rough surface polished by sandpaper. The complex rough surface was numerically reestablished based on the surface characteristic parameters extracted from the statistical data of 3D optical profiler measurement. Directional spectral emissivity of the established surface was simulated by the finite element method in the wavelength range of 2–20 µm and in the emission angle range of 0–80°. The influence of roughness on spectral emissivity was carefully studied. The calculated emissivity was compared to the data measured by a self-made emissivity measurement apparatus. The predicted results are in good agreement with the experimental data, which well proves the reliability of the proposed method.

A Traceable Spectral Radiation Model of Radiation Thermometry

Despite great technical capabilities, the theory of non-contact temperature measurement is usually not fully applicable to the use of measuring instruments in practice. While black body calibrations and black body radiation thermometry (BBRT) are in practice well established and easy to accomplish, this calibration protocol is never fully applicable to measurements of real objects under real conditions. Currently, the best approximation to real-world radiation thermometry is grey body radiation thermometry (GBRT), which is supported by most measuring instruments to date. Nevertheless, the metrological requirements necessitate traceability; therefore, real body radiation thermometry (RBRT) method is required for temperature measurements of real bodies. This article documents the current state of temperature calculation algorithms for radiation thermometers and the creation of a traceable model for radiation thermometry of real bodies that uses an inverse model of the system of measurement to compensate for the loss of data caused by spectral integration, which occurs when thermal radiation is absorbed on the active surface of the sensor. To solve this problem, a hybrid model is proposed in which the spectral input parameters are converted to scalar inputs of a traditional scalar inverse model for GBRT. The method for calculating effective parameters, which corresponds to a system of measurement, is proposed and verified with the theoretical simulation model of non-contact thermometry. The sum of effective instrumental parameters is presented for different temperatures to show that the rule of GBRT, according to which the sum of instrumental emissivity and instrumental reflectivity is equal to 1, does not apply to RBRT. Using the derived models of radiation thermometry, the uncertainty of radiation thermometry due to the uncertainty of spectral emissivity was analysed by simulated worst-case measurements through temperature ranges of various radiation thermometers. This newly developed model for RBRT with known uncertainty of measurement enables traceable measurements using radiation thermometry under any conditions.

Critical assessment and demonstration of high-emissivity coatings for improved infrared signal quality for Taylor–Quinney coefficient experimentation

Over the infrared (IR) spectrum metals have relatively low emissivity. Considering that modern studies focused on determining the Taylor–Quinney coefficient rely on IR-based measurements to quantify temperature rise, there is a need to improve measurement certainty. This is of particular concern during the earliest stages of deformation when small temperature rises result in a low IR-signal to noise ratio. Herein an economical approach to improving signal quality is presented based on the deposition of a thin high emissivity coating on the material substrate (i.e., specimen) of interest. Coating material and coating thickness selection are founded on both heat transfer and electromagnetic performance. A maximum allowable thickness of 1μm was determined for the coating to ensure the recorded temperature of the coating closely matches the temperature of the deforming substrate (i.e., specimen) during the sub-millisecond duration of a split-Hopkinson (or Kolsky) pressure bar experiment. Using the transfer matrix method, the spectral emissivity of coatings made from commonly available metallic deposition sources with increasing thicknesses was calculated. Theoretical predictions identified a 250 nm thick Ti coating as a promising candidate for increasing surface emissivity without the coating acting as a thermal barrier. Confirmation of the coating layer’s ability to substantially improve IR signal amplitudes was examined experimentally for two low emissivity materials, specifically OFHC copper (Cu) and Magnesium (Mg) alloy AZ31B. IR detector calibration experiments showed a four to six times improvement in signal amplitude compared to native Cu and Mg surfaces. Additionally, the calibration curves of coated specimens converge exhibiting the same behavior allowing for a unified calibration curve to be used regardless of substrate material. Split-Hopkinson pressure bar experiments supported by IR thermography were conducted on coated specimens and Taylor–Quinney coefficients are reported. Taylor–Quinney coefficients obtained, are compared to literature values when appropriate, and show positive agreement to previously reported values. Real-time high-speed imaging and post-mortem analysis showed robust adhesion between the substrate to the thin Ti layer up to 16% strain.

Multi-band and wide-angle nonreciprocal thermal radiation

Violating Kirchhoff's radiation law through magneto-optical materials or spatiotemporal (Floquet) metamaterials can open a new door for engineering thermal radiation by breaking the widely-accepted equal constraint of spectral absorptivity (α) and emissivity (ϵ). Most existing work only reports the unequal α and ϵ spectra in one or two bands and within limited angles. This significantly limits the practical applications like the nonreciprocal thermophotovoltaics. In this work, we present a general machine-learning-kernel-based algorithm framework, based on which we achieve four-band nonreciprocal thermal radiation via the magneto-optical (MO) materials. The realization of multi-band nonreciprocity is mainly attributed to the coupling effect of magneto-optical effect and the excitation of cavity modes with different orders, which can be confirmed by investigating the magnetic field distribution. In addition, it is found that dual-band/multi-band strong nonreciprocal thermal radiation can be realized in a wide range of incident angles (15°-85°). The number of bands and range of angle can be further enhanced by modulating the number of layers, structures, materials, and applied magnetic field. The present work offers a general design roadmap for nonreciprocal thermal radiation, and can be extended for designing metamaterials beyond thermal metamaterials.