High Emissivity Coatings Research Articles

Surface phonon activation for broadband high emissivity via textured structure on TiO2 coating

In the pursuit of developing broadband high emissivity coatings for metallic radiation thermal protection, conventional approaches often involve complex photonic structures or the incorporation of transition metals and rare earth additives, which pose significant technological challenges and environmental concerns. Here, we present a straightforward and eco-friendly surface structuring strategy for creating a high emissivity TiO2 coating on titanium, eliminating the need for environmentally hazardous additives. By controlling surface micropore diameter to ∼6 um through soft-sparking discharge, we successfully engineered a textured surface that effectively mitigates the intrinsic emissivity dip of planar TiO2 at wavelengths with weak polarization extinction (3–8 µm) and strong polarization resonance (>12 µm). Finite-difference time-domain (FDTD) simulations highlighted the crucial role of micropores in confining the incident electric field, maximizing the absorption potential of surface phonon polaritons. This improvement yields an impressive emissivity of 0.83 across the thermal infrared spectrum. The coating also demonstrated robust resistance to oxidation and retained its high emissivity at elevated temperatures, presenting a promising solution for sustainability in spacecraft and electronic applications.

High reflectivity and high emissivity integrated double layer coating on the flexible alumina fiber fabric with enhanced heat-dissipation efficiency

High emissivity coatings on hypersonic vehicle surfaces dissipate heat by emitting thermal radiation. While the difficulty of increasing emissivity limits the performance of coatings to improve heat-dissipation of the thermal insulation felt. Herein, a novel high-reflectivity and high-emissivity integrated double-layer coating was developed to enhance the thermal insulation performance of the alumina fiber fabric. The coating comprised of TiO2@SiO2 (T@S) as reflective agent and ZrO2/SiCw (ZSC) as emittance agents, was applied on the surface of alumina fiber fabric (AFF). The reflecting effect of radiation heat from the high reflective agent enhanced the thermal insulation properties of the double-layer coating. In the wavelength range of 0.4–2.5 μm, the average emissivity of the ZSC coating was 0.83 and the average reflectivity of the T@S coating was 0.63. With continuous heating at 1400 °C for 10 min with a butane torch, the temperature on the backside of the T@S–ZSC-coated AFF remained relatively stable at 225 °C, which is notably decreased by 80 °C compared to that of the single-layer ZSC-coated AFF. The tensile strength of AFF with T@S–ZSC coating reached 35 MPa after being heat treated at 1200 °C. Moreover, the bonding strength between the coating and the AFF substrate was measured to be 0.15 MPa. The double-layer coating with high emissivity and reflectivity, along with a flexible AFF substrate, offers a promising solution for addressing the challenging of surface heat-dissipation in hypersonic vehicles.

High Emissivity MoSi2-SiC-Al2O3 Coating on Rigid Insulation Tiles with Enhanced Thermal Protection Performance.

High emissivity coatings with sol as the binder have the advantages of room temperature curing, good thermal shock resistance, and high emissivity; however, only silica sol has been used in the current systems. In this study, aluminum sol was used as the binder for the first time, and MoSi2 and SiC were used as emittance agents to prepare a high emissivity MoSi2-SiC-Al2O3 coating on mullite insulation tiles. The evolution of structure and composition at 1000-1400 °C, the spectral emissivity from 200 nm to 25 μm, and the insulation performance were studied. Compared with the coating with silica sol as a binder, the MoSi2-SiC-Al2O3 coating has better structural uniformity and greater surface roughness and can generate mullite whiskers at lower temperatures. The total emissivity is 0.922 and 0.897, respectively, at the wavelength range of 200-2500 nm and 2.5-25 μm, and the superior emissivity at a low wavelength (<10 μm) is related to a higher surface roughness and reduced feature absorption. The emissivity reduction related to the oxidation of emittance agents at a high temperature (-10.2%) is smaller than that of the silica-sol-bonded coating (-18.6%). The cold surface temperature of the coated substrate is 215 °C lower than the bare substrate, suggesting excellent thermal insulation performance of the coating.

High-temperature oxidation resistance and high emissivity of a novel NbSi2/SiO2-Nb2O5/MoSi2-Yb2O3 multilayer coating on Nb substrate for thermal protection

To develop anti-oxidation and high emissivity coatings on Nb alloys for thermal protection systems, the NbSi2/SiO2-Nb2O5/MoSi2-xYb2O3 (0, 10, 20, and 30 wt%) multilayer coatings are prepared by a two-step method. The NbSi2/SiO2-Nb2O5/MoSi2-20Yb2O3 coating possesses the smallest oxidation mass gain (4.86 mg/cm2) at 1200 °C for 100 h and is the best oxidation resistance of the four coatings. The appropriate Yb3+ filles the SiO2 network voids, enhancing the Si-O bond and raising the viscosity of the SiO2 scale, thus further blocking oxygen diffusion. The coatings with Yb2O3 below 20 wt% achieve an emissivity of ∼0.9 at room temperature and still reach ∼0.7 at 1200 °C, showing excellent radiant thermal protection.

Development and characterization of a high-emissivity reference blackbody radiation source with V-grooves and large aperture at low-temperature

A high-emissivity blackbody radiation source can improve the measurement level of infrared radiation temperature, which is critical for applications in high-value industries and human health monitoring. In this study, a new high-emissivity blackbody source with V-grooves and large aperture was redesigned and developed, which can be used as a radiation temperature reference source for the calibration of secondary blackbody sources and precision radiation thermometers in the temperature range from −10 ℃ to 100 ℃, including the temperature range of human body infrared radiation considered in the study of people exhibiting fever symptoms. The wavelengths of the infrared spectral radiation temperatures involved in the study were in the range of 8–14 μm. The blackbody source consisted of a blackbody cavity, a jet-stirred precision constant-temperature bath, a standard platinum resistance thermometer, a high-precision digital multimeter, a frost-proof cover, and an auxiliary test frame. The cavity was cylindrical and conical with a diameter of 60 mm, a conical top angle of 120°, and a total length of 300 mm. The V-grooves with an angle of 50° on the inner wall were coated with high-emissivity coatings. The cavity was then completely immersed in a thermostatic bath. The radiation temperature of the blackbody source was traced to the International Temperature Scale of 1990 using a standard platinum resistance thermometer, which was calibrated at fixed-points of the Ar, Hg, H2O, Sn, and Zn. The experimental results revealed that the normal effective emissivity of the blackbody measured by the thermal cavity reflectometer method was 0.999 751, which is consistent with the theoretical calculation results based on the Monte Carlo method using optical ray tracing technology. In addition, the accuracy of the radiation temperature was verified by comparing it with a reference blackbody source from the National Institute of Metrology (NIM), China. The temperature stability of the blackbody source did not exceed 0.005 ℃ within 30 min, the axial temperature uniformity of the blackbody cavity did not exceed 0.048 ℃, and the temperature uniformity at the bottom of the cavity was better than 0.053 ℃. The combined standard uncertainty of the radiation temperature of the blackbody radiation source was evaluated from 0.010 ℃ to 0.032 ℃. The novel design with 50° V-grooves allowed for the blackbody source to provide the combination of a large aperture and high emissivity, which improved radiance temperature measurements. The reliability of the new design was experimentally verified. This design provides a novel method and experimental basis for the future design of compact light-weight blackbody sources, which is useful for the development of compact spaceborne blackbody sources.

Coating Ca2+–Fe3+ co-doped LaAlO3 on high-temperature electric furnace for energy-saving

The energy-saving effect of high-emissivity coatings on high-temperature electric furnaces has not been fully explored. In this study, high infrared emissivity coatings (ε1–2.5μm, 1200 °C = 0.81) are successfully prepared using La0.9Ca0.1Al0.9Fe0.1O3 powder as radiation agent and aluminium sol as binder. The experimental results on a high-temperature electric furnace reveal that the power consumption for heating air and heating refractory bricks before and after lining coating increase by 4.7% and 5.4%, respectively. This energy consumption increase is primarily concentrated in the heating stage because the high absorption of the coating causes an increase in the heat loss of the lining. Moreover, the scheme of the high-emissivity coating application on the outer surface of a crucible is proposed. Experimental results indicate that this technique can effectively improve thermal efficiency and, therefore, can be successfully implemented in future industrial applications.

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.

Study on Sintering Behavior of Reaction-Cured Glass Coating

High-emissivity coatings constitute an essential component of reusable thermal protection systems, determining the success or failure of hypersonic spacecraft. Reaction-cured glass coating is the basis for all current high-emissivity coatings, and the study of its sintering behavior is of great scientific significance for the development and performance enhancement of the coating. Microstructures and phase compositions of the samples before and after the sintering process were determined using SEM, XRD, and EDS. The sintering temperature, inserting temperature, and heating rate were systematically investigated. The results show that the effects of the sintering temperature, inserting temperature, and heating rate on the coating occur in decreasing order. The optimum condition for coating sintering in this study is an insertion temperature of 1100 °C, a heating rate of 10 °C/min, and a sintering temperature of 1200 °C, and a crack-free and containing SiB4 borosilicate glass coating was successfully prepared.

Assessing the CO2 emission reduction potential of steam cracking furnace innovations via computational fluid dynamics: From high-emissivity coatings, over coil modifications to firing control

Accurate 3D modeling of steam cracking fireboxes is instrumental in the identification of the strengths and weaknesses of a specific firebox. A 3D computational fluid dynamics model for the firebox, combined with a 1D reactor model, is validated using industrial data of a naphtha steam cracker. The framework is used to assess the impact of a high-emissivity coating, geometrical changes and operational conditions on the CO2 emissions per ton ethene produced. A high-emissivity coating reduces the CO2 emissions by 4% and increases the radiation efficiency by 1.9%. A geometry with a higher number of inlets per reactor coil reduces the CO2 emissions by 3%. When the excess oxygen concentration increases at the bridgewall, the firebox efficiency decreases while CO2 emissions increase. The operational changes and technologies studied in this work can be combined to further minimize the CO2 emissions, but controlling and monitoring the excess bridgewall oxygen proved to be the most crucial parameter to realize a reduction in CO2 emissions.

TaSi2–SiC high-emissivity coating modified by SiB6 on flexible alumina fibre fabric with enhanced oxidation resistance and high interfacial bonding strength

To improve the oxidation inhibition of TaSi2-based high-emissivity coatings at high temperatures, TaSi2–SiC coating modified by SiB6 was prepared on the surface of alumina fibre fabrics. The effects of the SiB6 content on the surface appearance and emissivity of the coating were investigated, and the mechanical properties of the coated fabrics were compared. When the SiB6 content in the coating was 2.5%, the borosilicate glass liquid phase generated by SiB6 oxidation effectively prevented the oxidation of TaSi2. The bond strength between the coatings and fibre fabric was 207 kPa after calcination at 1200 °C, which was 39% higher than that of the coated fabric without SiB6. The emissivity of the TaSi2–SiC coating, modified by a SiB6 content of 2.5%, reached above 0.92 after calcination at 1200 °C for 5 h. Therefore, the TaSi2–SiC high-emissivity coating modified by SiB6 has good application prospects in the field of thermal protection.

Design and thermal characteristic test of a temperature control system for spacecraft precision instrument

The temperature control precision is crucial for the sensitive elements in a spacecraft. In the present study, a temperature control system to improve the performance of spacecraft precision instruments was designed and experimentally investigated. In the temperature control system, heat pipes, high emissivity coatings, and semiconductor coolers are applied to dissipate the internal heat and control the temperature of a sensitive element. Heat pipes are arranged on the sensitive element and the innermost shell, high radiation coatings are sprayed on each shell surface to transfer the heat to the outer layer, and semiconductor coolers are mounted on the outermost shell and dissipate heat to the external environment. The temperature of the sensitive element is compensated by an electric heating device to reduce the temperature fluctuations. A digital-analog control scheme was designed to maintain the temperature of the outer shell and the sensitive component of the temperature control system. The control accuracy of the temperature control system is within ±0.1 ℃ under different working conditions. When the frequency point is at 0.089928 Hz, the signal amplitude of temperature is taken as the maximum value of 5.05 mK; the amplitude spectral density is 0.006 ∼ 9 mK/Hz1/2 in the frequency range 0.001 ∼ 0.25 Hz.

Effect of Doping Content of MgO on Solar Absorptivity to IR Emissivity Ratio of Al2O3 Coatings

High-emissivity coatings are often used on the surfaces of solar probes to block heat from the sun. Therefore, improving the thermal control ability is the standard for determining the quality of the coatings. In this study, MgO was doped into Al2O3 to improve the thermal control ability of the coatings and to analyse the effect of different MgO doping contents. The solar absorptivity to infrared (IR) emissivity ratio (α/ε) was used to evaluate the thermal control ability of the coatings. The results showed that 5 wt.% MgO doping content is the best choice. The main reason for the change in α/ε is related to the doping of MgO, which affects the grain size of Al2O3.

Anti-oxidation of emissing agents in TaSi2–MoSi2-borosilicate glass high emissivity coating

Glass high emissivity coatings with TaSi 2 as emissing agents faced with the oxidation of emissing agents during coating preparation and sintering. In this work, four kinds of coatings with the same raw materials composition and different preparation processes (using silica sol or water as the solvent for coating slurry, and applying fast heating sintering or normal sintering, respectively) were prepared. The microstructures, phase compositions, coefficients of thermal expansion (CTE), surface roughness and emissivity of the four coatings were compared, and the oxidation processes of TaSi 2 during sintering were investigated. Results showed that using silica sol as solvent or applying fast-heating sintering could effectively reduce the oxidation of TaSi 2 , and the total emissivities at the wavelength range of 1–22 μm were increased by 9.9% and 15.0%, respectively. The anti-oxidation mechanisms involved forming silica gel coverages on the TaSi 2 particles as oxygen diffusion barriers before coating densification and reducing the oxidation time during heating from the initial TaSi 2 oxidation temperature (900 °C) to glassy phase formation temperature (1300 °C), respectively. The oxidation of TaSi 2 was basically avoided when the two anti-oxidation measures were used simultaneously, and the as-prepared coating (Sol-FHS) exhibited the densest structure, the closest CTE to the substrate and the lowest surface roughness. However, Sol-FHS did not present the highest emissivity due to its low surface roughness.

Pulsed Electrophoretic Deposition of Nanocarbons and Their Optical, Thermal, and Electrical Applications

Electrophoretic deposition (EPD) is a well-established, scalable industrial manufacturing technology for depositing a wide variety of coatings across a broad range of substrates, geometries and sizes. Conventional electrophoretic deposition (EPD) is the migration of small, suspended particles in a liquid driven by a constant electrical potential difference. As a subset of electrodeposition, EPD has been used to deposit layers on various conducting and semiconducting surfaces from polar, aqueous, and non-polar suspensions. On the other hand, state of the art developments within nanoscience and nanotechnology has opened a new vista for the field of nanocarbon materials. Nanostructured carbon materials, such as carbon nanotubes, graphene, carbon quantum dots, etc., exhibit excellent physical and chemical properties, and have been gaining attention in both fundamental research and technological applications.In this invited presentation, Faraday Technology will discuss an innovative electrophoretic deposition (EPD) manufacturing process, based on the use of pulsed electric fields, for controlled, reproducible, scalable deposition of a wide variety of nanocarbon based coatings across a broad range of substrates, geometries and sizes (Figure 1). The effects of waveforms, electrolytes, substrates, etc. on the formation of robust and uniform coating will be discussed in this talk. The optical and electrical properties, and environmental survivability of nanocarbon coatings will be demonstrated via a variety of testing or characterizations. Faraday will also introduce their applications ranging from low-reflective coatings, solar absorber, high emissivity coatings, to thin film electrodes for energy storage.In summary, a scalable EPD manufacturing process for fabricating nanocarbon based coatings have been developed at Faraday for various applications ranging from optical, thermal, to electrical fields. Acknowledgements: The financial support from NASA SBIR program through contracts No. 80NNSC18P2062 & 80NSSC19C0177, DOD DMEA STTR program through grant No. HQ0727-21-P-0029, DARPA SBIR program through grant No. W31P4Q-22-C-0014, DOD MDA STTR program through grant No. HQ0147-19-C-7065, DOD Air Force SBIR program through grant No. FA9453-19-P-0573, NASA SBIR program through contracts No. 80NSSC20C0287 are acknowledged. Figure 1

Reducing CO2 emissions of existing ethylene plants: Evaluation of different revamp strategies to reduce global CO2 emission by 100 million tonnes

Steam cracking is the leading technology to produce light olefins and the most energy-consuming process in the chemical industry, using approximately 10% of the sector's total primary energy globally. Six technological improvements, requiring limited hardware changes and implementable today, have been evaluated: high emissivity coatings, 3D reactor designs, advanced coil material, oxy-fuel combustion, carbon capture and storage (CCS), and biogas firing. These technologies have the potential to reduce both energy consumption and raw material demand. Life cycle inventory data are obtained from the first principles-based COILSIM1D model of the furnaces, validated with industrial data, combined with a Petro-SIM model of the overall mass and energy balance of the plant. Cradle-to-gate life cycle assessment (LCA) shows that a major contribution to the overall carbon footprint of light figureolefin production is related to the feedstock supply. Oxy-fuel combustion combined with CCS followed by biofuel combustion has the highest potential to reduce the carbon footprint, and reach a target of over 100 × 106 metric tons/yr of CO2 eq. worldwide. On the other hand a combination of radiant coatings and novel reactor coil design yields in a marginal 0.5% reduction or potentially 3 × 106 metric tons/yr of CO2 eq. worldwide. In order to decrease the carbon footprint more one would have to consider both substantial hardware changes - requiring a large investment - and the use of non-fossil derived feedstocks.

Dielectric and optical evaluation of high-emissivity coatings for temperature measurements in microwave applications

In this work, several commercial high-emissivity coatings have been characterized in terms of emissivity, chemical composition and dielectric properties as a function of temperature, under microwave irradiation. Accurate knowledge of their response under exposure to microwaves provides new and crucial information about their practical usability for non-contact temperature measurements in microwave environments. Due to their high metallic content, some of the studied coatings exhibited unexpected microwave-triggered reactions that hindered their use up to the maximum temperature specified by the manufacturers. Emissivity and chemical analyses before and after the heating cycles confirmed the degradation of some of the samples predicted by dielectric measurements. This work illustrates how a careful characterization of optical and dielectric properties under representative operating conditions (temperature range, microwave exposure) is vital in order to select the appropriate reference coating to obtain reliable temperature measurements in microwave applications.

The experimental measurement of the thermal emissivity of a black coating from 50 K to 300 K

High-emissivity coatings are commonly applied in satellite detectors and large-scale cryogenic systems. Accurate and quick measurement of emissivity is in high demand for practical applications at cryogenic temperatures. In this paper, a method for the measurement of the total hemispherical emissivity from 50 K to 300 K was presented based on the static calorimetric method. A two-stage GM cryocooler was used as a cooling source in the measuring device without assumption of cryogenic liquids. The thermal emissivity of a black carbon coating was measured by monitoring temperatures timely in the device. The tested results exhibited good accordance with previous reports. This cryogenic measurement device has made it a strong candidate for the radiative heat dissipation at cryogenic temperatures. Design details of the experiment and measurement results will be presented.

High infrared emissivity energy-saving coatings based on LaMnO3 perovskite ceramics

In this preliminary work, a series of LaCrxMn1-xO3 (x = 0, 0.25, 0.5, 0.75, 1) ceramic powders were initially synthesized by a facile solid-state reaction method. In the next step, high emissivity coatings were successfully papered by air-spraying using LaMnO3 powder as the filler material (with the highest emissivity ε3–5μm, 600 °C = 0.922) and phosphate as the binder phase. The morphology, structure, and infra-red emissivity of the samples were studied using scanning electron microscopy (SEM), X-ray diffraction (XRD), and infra-red emissometery. High-temperature cyclic heat treatment results showed that the prepared coating has very good adhesion and excellent high-temperature resistance. A simulated furnace heating test revealed that the prepared coating can save input energy as high as 13.2%.

Secrets of high thermal emission of transition metal disilicides TMSi2 (TM = Ta, Mo)

TMSi2 (TM = Ta, Mo) are extensively used as thermal emissivity agents in high emission coatings due to their well-known “high” emissivity in infrared range. However, there is a paucity of the high temperature (HT) emissivity property of these two silicides. Moreover, room temperature (RT) spectrometer measurements have demonstrated that the emittance in infrared range of two silicides was considerably low. Therefore, providing critical HT data and satisfactory elucidation on the emission incompatibility of TMSi2 is eagerly needed. In this contribution, combining first principles calculations and Drude model, the reflectance spectra of TMSi2 were predicted at both RT and HT. Consistent with spectrometer measurements, the intrinsic emittance of silicides was relatively low in the entire investigated temperatures. To explain the incompatible emission behavior, two simplified models including the majority of high emissivity coating, SiO2, were proposed. Intriguingly, with SiO2 considered in simulations, no matter covered on the surface or blended in the composites, the emittance of the TMSi2 enhanced significantly. Our theoretical results demonstrate the non-negligible significance of oxides on the high temperature performance of silicides and provide the guidelines for improving the emission performance of silicides and searching for potential high emissivity agents.

The Effect of Refractory Wall Emissivity on the Energy Efficiency of a Gas-Fired Steam Cracking Pilot Unit.

The effect of high emissivity coatings on the radiative heat transfer in steam cracking furnaces is far from understood. To start, there is a lack of experimental data describing the emissive properties of the materials encountered in steam cracking furnaces. Therefore, spectral normal emissivity measurements are carried out, evaluating the emissive properties of refractory firebricks before and after applying a high emissivity coating at elevated temperatures. The emissive properties are enhanced significantly after applying a high emissivity coating. Pilot unit steam cracking experiments show a 5% reduction in fuel gas firing rate after applying a high emissivity coating on the refractory of the cracking cells. A parametric study, showing the effect of reactor coil and furnace wall emissive properties on the radiative heat transfer inside a tube-in-box geometry, confirms that a non-gray gas model is required to accurately model the behavior of high emissivity coatings. Even though a gray gas model suffices to capture the heat sink behavior of a reactor coil, a non-gray gas model that is able to account for the absorption and re-emission in specific bands is necessary to accurately model the benefits of applying a high emissivity coating on the furnace wall.