Spectral Emissivity Research Articles

Optimization of stacked Fabry-Perot cavities for VO2-based broadband adaptive thermal radiators

The performance of an adaptive thermal radiator (ATR) for temperature regulation depends on its ability to modulate spectral emissivity across a broad wavelength range. For a single cavity, we found that the tunable, thermal emissivity from 2-30 µm is maximized using a spacer material with low n and k, such as BaF2 . However, a single cavity produces a narrowband peak in the spectral emissivity. Stacking multiple cavities introduces additional resonances that create high-temperature spectral emissivity peaks. Here, we designed cavities composed of VO2 and BaF2 to have different resonant wavelengths to demonstrate a broadband response across the infrared spectrum out to ∼30 µm. In this work, we find that up to three cavities increases the tunable thermal emissivity with negligible changes from additional cavities.

A Measurement Device for the Normal Spectral Emissivity of Materials Under Low-Temperature and Vacuum Conditions

Based on parallel light, a device for measuring sample emissivity under vacuum and low temperature was established. In this paper, a new emissivity measurement formula was designed, which replaces the derivation of Kirchhoff’s law of thermal radiation. The device is designed with a light-weight blackbody, an improved cooling speed, and an improved PID temperature control system to achieve a good temperature stability, close to 1 mK. The device is capable of measuring within a wavelength range of 4 μm to 16 μm. The results of the uncertainty assessment show that the uncertainty of the normal emissivity is better than 1.6% in the range of 4 μm to 7 μm, and better than 0.6% in the range of 7 μm to 14 μm (k = 1) at 0 °C.

In situ high-temperature emissivity measurements of heat-treated, silicon coated stainless steel for solar thermal applications

Understanding thermal emissivity at high temperatures is crucial for developing efficient materials for solar thermal applications. We present a new approach for creating an efficient material for solar absorber by developing a nano structured surface on stainless steel substrate through Si deposition and annealing. We prepare five samples by annealing them at five temperatures between 700 °C and 1100 °C. Afterwards, we perform a systematic study of the spectral emissivity at elevated temperatures, focusing on different parameters: angle dependence, wavelength dependence, and temperature dependence. The spectral directional emissivity experiments performed in the mid-infrared range reveal a dielectric behavior of the samples in the short wavelength region (λ < 6 μm) and metallic behavior in the long wavelength region (λ > 12 μm). The results indicate an increase in hemispherical and total normal emissivity with measurement temperature (from 200 °C to 700 °C), influenced by oxide/silicide formation due to interdiffusion, and by surface roughness. Notably, samples annealed at 900 °C and 1000 °C demonstrate enhanced thermal stability at 700 °C, showcasing promising characteristics for high-temperature applications. Consequently, this study presents a viable method for developing cost-effective silicon-based solar absorber coatings on stainless steel with tailored properties for solar thermal applications along with its real time high temperature emissivity details.

Spectral and global emissivity assessment by means of a novel infrared methodology

Spectral and global emissivity assessment by means of a novel infrared methodology

Experimental setup for the determination of spectral normal emissivity of conductive materials in the 1–18 μm wavelength range at 300–1700 °C in air

Experimental setup for the determination of spectral normal emissivity of conductive materials in the 1–18 μm wavelength range at 300–1700 °C in air

Determination of emissivity profiles using a Bayesian data-driven approach

In this paper, we explore the determination of a spectral emissivity profile that closely matches real data, intended for use as an initial guess and/or a priori information in a retrieval code. Our approach employs a Bayesian method that integrates the CAMEL (Combined ASTER MODIS Emissivity over Land) emissivity database with the MODIS/Terra＋Aqua Yearly Land Cover Type database. The solution is derived as a convex combination of high-resolution Huang profiles using the Bayesian framework. We test our method on IASI (Infrared Atmospheric Sounding Interferometer) data and find that it outperforms the linear spline interpolation of the CAMEL data and the Huang emissivity database itself.

Lava Worlds Surface Measurements at High Temperatures

First measurements of the emission of lava worlds with the James Webb Space Telescope (JWST) probe the conditions on worlds so hot that their surfaces are likely molten or partially molten. These observations provide a unique opportunity to explore rocky planets’ evolution. Surfaces of lava world exoplanets can give insights into their composition and their interior workings. However, data of spectral emissivity of a wide range of potential exoplanet surface compositions and temperatures is required to understand JWST data. Here, we chose eight synthetic, potential exoplanet surfaces that span a wide range of chemical compositions to provide observers with a tool for the interpretation of JWST data for the exploration of lava worlds. We present the measured infrared emissivity spectra (2.5–20 μm) of these materials for temperatures between 800° C and 1350° C. Our data comprise the first spectral library of possible high-temperature exoplanet surfaces. From these measurements, we establish the link between composition and a strong spectral feature at around 9 μm, the Christiansen frequency (CF) for different temperatures. Additionally, we report that the shift in the CF associated with the bulk composition of the material is significantly more important than its temperature. This provides a critical tool to aid in interpreting future spectra of lava worlds that will be collected by the JWST and future missions.

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.

Accurately Measuring the Infrared Spectral Emissivity of Inconel 601, Inconel 625, and Inconel 718 Alloys during the Oxidation Process.

Accurate measurement of the infrared spectral emissivity of nickel-based alloys is significant for applications in aerospace. The low thermal conductivity of these alloys limits the accuracy of direct emissivity measurement, especially during the oxidation process. To improve measurement accuracy, a surface temperature correction method based on two thermocouples was proposed to eliminate the effect of thermal conductivity changes on emissivity measurement. By using this method, the infrared spectral emissivity of Inconel 601, Inconel 625, and Inconel 718 alloys was accurately measured during the oxidation process, with a temperature range of 673-873 K, a wavelength range of 3-20 μm, and a zenith angle range of 0-80°. The results show that the emissivity of the three alloys is similar in value and variation law; the emissivity of Inconel 718 is slightly less than that of Inconel 601 and Inconel 625; and the spectral emissivity of the three alloys strongly increases in the first hour, whereafter it grows gradually with the increase in oxidation time. Finally, Inconel 601 has a lower emissivity growth rate, which illustrates that it possesses stronger oxidation resistance and thermal stability. The maximum relative uncertainty of the emissivity measurement of the three alloys does not exceed 2.6%, except for the atmospheric absorption wavebands.

Optically transparent and infrared tunable flexible camouflage device

Dynamic thermal camouflage is an emerging technology that adjusts the thermal radiation emitted by the surface of an object to match that emitted by the surroundings. Although multilayered graphene effectively modulates the surface emissivity with low power consumption, its opacity hinders its application in independent camouflage across the visible and infrared (IR) spectra. In this study, we demonstrate a novel flexible graphene-based camouflage device that dynamically controls emissivity while preserving optical transparency in the visible range. By introducing a composite structure comprising 12 layers of graphene on a flexible dielectric substrate, we achieved over 70% optical transparency with spectral and average emissivity modulation ranges of 0.42 and 0.27, respectively, while using minimal power (∼120μW/cm2). Furthermore, the ease of patterning the graphene layers enables the creation of various IR patterns that respond to changes in the applied voltage while maintaining optical transparency. This study paves the way for advanced camouflage technology and information distortion by enabling the transmission of distinct signals across visible and IR spectra.

All-dielectric spectral selective emitter for infrared-microwave compatible stealth

The advancement of infrared detection technology has imposed increasingly stringent requirements on the infrared radiation regulation materials. Spectral selective emitters can effectively mitigate infrared signals within the detected bands, while simultaneously leveraging undetected band for efficient radiative cooling, thereby enhancing the infrared stealth effectiveness. Herein, we design an all-dielectric selective emitter consisting of only five layers by incorporating the SiO2 reststrahlen layer with a Ge/MgF2 photonic multilayers. We quantified the infrared stealth performance of the selective emitter that the emissivity in the undetected band (5−8 μm) is 0.81 and the emissivity values in detected bands are 0.20 (3−5 μm) and 0.17 (8−13 μm), respectively. The simulation results demonstrate that the selective emitter exhibits superior radiative cooling performance and infrared stealth capabilities, as evidenced by significantly lower physical temperature and radiation intensity compared to traditional broadband low emissivity emitters. Furthermore, the all-dielectric structure ensures nearly perfect microwave transmissivity for selective emitters. When combined with microwave absorbers, they exhibit a remarkable microwave absorptivity exceeding 0.9 within the 8−18 GHz range, effectively achieving infrared-microwave compatible stealth. The all-dielectric emitter presents a novel structure for spectral selective infrared radiation control materials, featuring reduced layer complexity and the absence of metallic materials, thereby enhancing its potential for temperature resistance. This work holds significant implications for advancing selective emitters in high-temperature infrared-microwave compatible stealth applications.

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.

Optical-thermal modeling and geographic analysis of dusty radiative cooling surfaces

Radiative cooling for energy conservation and harvesting has gained considerable attention worldwide in recent years. The optical and thermal characteristics of radiative cooling surface at outdoors is susceptible to dust soiling. Currently, limited research has been devoted to this problem. This work presents a model utilizing the beam envelope method and effective medium theory to investigate the optical characteristics of radiative cooling surfaces impacted by dust soiling. Subsequently, an optical-thermal coupling model is developed to investigate the spectral emissivity, average absorption/emission rates, and net heat gain of radiative cooling surfaces affected by dust soiling. The effect of dust soiling on the absorption of solar radiation is evident, with a substantial effect observed, but its impact on emissivity in the atmospheric window bands is comparatively minor. Meanwhile, it could be noticed that the mechanisms by which various material parameters of dust layers impact optical characteristics are different. In addition, the integration of the optical-thermal coupling model with EnergyPlus meteorological data and MERRA-2 facilitates an exploration of the geographical distribution of radiative cooling power variations induced by dust soiling across different regions in China. It is concluded that regions in China with a higher potential for radiative cooling often characterized by elevated dust deposition rates and low precipitation, rendering them more susceptible to the adverse effects of dust soiling. Effective cleaning significantly enhances both the optical and thermal characteristics of radiative cooling surfaces. The findings of this study offer a theoretical foundation for the enduring application of radiative cooling technology.

Tailoring the Spectral and Directional Emissivity of Functionalized Laser Processed Surfaces

Tailoring the Spectral and Directional Emissivity of Functionalized Laser Processed Surfaces

Effect of Dewpoint on the Evolving Spectral Emissivity of Advanced High-Strength Steel During Intercritical Annealing

Effect of Dewpoint on the Evolving Spectral Emissivity of Advanced High-Strength Steel During Intercritical Annealing

An Extremely Sparse Tomography Reconstruction of a Multispectral Temperature Field without Any a Priori Knowledge.

When undertaking optical sparse projection reconstruction, the reconstruction of the tested field often requires the utilization of a priori knowledge to compensate for the lack of information due to the sparse projection angle. In order to reconstruct the radiation field of unknown materials or in situations where a priori knowledge cannot be obtained, this paper proposes an extremely sparse tomography multispectral temperature field reconstruction algorithm that analyzes the similarity (the similarity here compares and calculates the Euclidean distance of the spectral emissivity values at various wavelengths between different spectral curves) of radiation characteristics of materials under the same pressure and concentration but different temperature, describes the similarity between the radiation information of the tested field using the dynamic time warping (DTW) algorithm, and uses the similarity sum of the radiation information among the subregions of the temperature field as the optimization objective. This is combined with the equation-constrained optimization algorithm and multispectral thermometry to establish the statistical law between the missing information and finally realize the reconstruction of the temperature field. Simulation experiments show that, without any a priori knowledge, the method in this paper can realize reconstruction of the temperature field with an accuracy of 1.53-12.05% under two projection angles and has fewer projection angles and stronger robustness than other methods.

Thermal radiation characteristic parameters of biomass mixed feedstock for concentrating solar gasification reaction

Solar energy and biomass are both important renewable energy, and the studies on them contribute to current goal of global carbon neutral. The solar gasification process directly employs solar radiation to drive the conversion from biomass to syngas via endothermic gasification reactions. This process relies on the biomass feedstock absorbing incident solar irradiation to sustain these reactions. The thermal radiation characteristics of biomass materials, such as spectral emissivity and absorptivity, determine its capacity for solar energy absorption versus losses from self-emission. Thus, these parameters are critical inputs for accurate thermal-balance modeling of the solar reactor. In this study, the reflectance of 9 biomass samples were experimentally measured in the main solar spectrum range of 200–2500 nm. Based on measurement results, the refractive index (n) and extinction coefficient (k) of each biomass materials are obtained via Kramers-Kronig relations. Besides, the dielectric function of the 9 biomass samples are also modeled using a 5-th Lorentz oscillator model and the model parameters are determined. The results indicate that the 5-th Lorentz oscillator model adequately characterizes the thermal radiation properties of diverse biomass feedstocks over the measured spectrum. This work contributes important spectral property data to enable accurate modeling and optimization of solar thermochemical conversion based on biomass gasification processes.

Inverse-design laser-infrared compatible stealth with thermal management enabled by wavelength-selective thermal emitter

With the fast development of multi-band detection technology, the demand for multi-band stealth technology with thermal management in military and civilian fields is increasing. Although the metasurfaces can achieve multi-band stealth and thermal management, their fabrication is complex and costly. In this paper, a simple multilayer structure is used to realize laser-infrared compatible stealth with thermal management. More importantly, the traditional method of optimizing metasurface structures by manually scanning parameters consumes a lot of computational time and resources. In order to speed up design of nanostructures, we propose a wavelength-selective thermal emitter (WSTE) composed of ZnS/Ge3Sb2Te6 (GST) multilayer films and a Ni reflector using inverse design method. The simulation results show that, in the crystalline phase, the WSTE has low spectral emissivity (ε3-5μm = 0.12, ε8-14μm = 0.22) in the mid-wave infrared (MWIR) and long-wave infrared (LWIR), high spectral emissivity (ε5-8μm = 0.72) in the non-atmospheric window and high absorptivity (A1.06μm = 0.90, A1.55μm = 0.97, A10.6μm = 0.91) in the laser wavelengths, which indicates that the WSTE can simultaneously achieve MWIR, LWIR and laser stealth with radiative heat dissipation. The method proposed in this paper overcomes the problems of single-band stealth and inefficient. Moreover, by varying the crystallization fraction of the GST, the WSTE can achieve dynamic control of thermal radiation in the wavelength range of 3–14 µm. The simulated infrared images verify that WSTE has good infrared stealth performance. Finally, the effects of incidence angle and polarization angle on the emission spectrum of WSTE are studied, which can demonstrate the infrared stealth is insensitive to both. In conclusion, the WSTE is attractive in advanced photonics applications, such as radiative cooling and infrared stealth.

Spectral emittance measurements for ytterbium disilicate‐based environmental barrier coatings

AbstractThe spectral reflectance and transmittance of two free‐standing ytterbium disilicate (Yb2Si2O7)—based environmental barrier coatings (EBCs) were measured from 25°C up to 1270°C. The measurements were collected over the spectral range of approximately 2–20 µm. The spectral emittance was then determined through conservation of energy and Kirchhoff's law of thermal radiation. The spectral emittance measurements for the EBCs were then compared to previous measurements for a thermal barrier coating (TBC).

Broadband nonreciprocal thermal emissivity and absorptivity

A body that violates Kirchhoff’s law of thermal radiation exhibits an inequality in its spectral directional absorptivity and emissivity. Achieving such an inequality is of fundamental interest as well as a prerequisite for achieving thermodynamic limits in photonic energy conversion1 and radiative cooling2. Thus far, inequalities in the spectral directional emissivity and absorptivity have been limited to narrow spectral resonances3, or wavelengths well beyond the infrared regime4. Bridging the gap from basic demonstrations to practical applications requires control over a broad spectral range of the unequal spectral directional absorptivity and emissivity. In this work, we demonstrate broadband nonreciprocal thermal emissivity and absorptivity by measuring the thermal emissivity and absorptivity of gradient epsilon-near-zero InAs layers of subwavelength thicknesses (50 nm and 150 nm) with an external magnetic field. The effect occurs in a spectral range (12.5–16 μm) that overlaps with the infrared transparency window and is observed at moderate (1 T) magnetic fields.