Low-surface Emission Research Articles

Femtosecond laser-textured superhydrophilic coral-like structures spread AgNWs enable strong thermal camouflage and anti-counterfeiting

Modulating the thermal emission of a material in the infrared (IR) range can be essential for various practical applications such as smart textiles, camouflage, and anti-counterfeiting. Although many different materials or structures have been proposed, the complex manufacturing processes are still hindering their widespread use. Herein, a facile femtosecond laser processing technology and a drop-coating method are introduced to form a patternable low emissivity film. Laser-treated polyimide films resulted in superhydrophilic structured surfaces that are uniformly coated with silver-nanowires (AgNWs) in aqueous solutions for low emissivity surfaces. Furthermore, the emissivity of the samples is as low as ∼0.2 without deterioration over 800 bending-releasing cycles. The as-prepared films also display good thermal camouflage properties, namely, the films reduced the thermal radiation temperature of an object by 35.8 °C when the object temperature was ∼69.1 °C. Additionally, this IR camouflage effect of the AgNWs coated samples shows excellent stability even in harsh environments such as immersion in water, acid, alkali, and salt solution and applied voltage. We also show that information encryption was possible by adjusting the amount of AgNWs. The design of this programmable patterned low emissivity film indicates an idea for the thermal camouflage and anti-counterfeiting technology, which can carry more abundant application scenario and disguise them more complex and sophisticated.

Improvement of heating and cooling performance for thermoelectric devices in medical storage application

Thermoelectric devices provide several advantages over typical refrigeration systems, including compact size, quiet operation, minimal energy use, and environmentally friendly. By harnessing these benefits, we have developed a blood storage system that has the potential to transform the way blood products are preserved and safeguarded. Thermal performance was evaluated using a variety of scenarios, including thermal fin material and emissivity, voltage operation, heat sink type (I, U, O type), and tube load. This research used thermoelectric devices, Styrofoam materials, thermal fin and the heatsink to design the thermoelectric storage cabinet with heat insulation function. The aims to improve the performance of thermoelectric devices by optimizing the thermal fin design during heating/cooling storage mechanisms. Our findings emphasize the importance of thermal radiation, particularly in circumstances with low natural convection and high surface emissivity. Thermal performance is optimized when heating storage elements are incorporated into aluminum finned tubes and run at an input voltage of 8V in a 25 °C ambient temperature. In addition, using U-shaped cooling fins in cooling conditions results in superior cooling performance. The outcomes of this study highlight the possibility and potential benefits of using thermoelectric devices to heat and cool medical storage, notably blood storage applications. The insights gained here contribute to advancing the efficiency and sustainability of medical storage systems, ultimately benefiting healthcare and the environment.

Long‐Term Infrared Stealth by Sandwich‐Like Phase‐Change Composites at Elevated Temperatures via Synergistic Emissivity and Thermal Regulation

AbstractThe rapid development of infrared surveillance technologies has attracted great attention for scientists to design advanced functional materials with prominent infrared stealth and thermal camouflage effectiveness. In the current study, a sandwich‐like functional composite based on a crosslinked polyimide aerogel, a meso‐erythritol (mE)‐based phase‐change composite, and an MXene film has been developed to achieve long‐term thermal camouflage at elevated temperatures. In this composite system, the lower aerogel layer can act as a barrier to insulate heat transfer through its layer‐stacking structure under ultralow directional thermal conduction. The introduction of the middle phase‐change composite layer ensures that the composite system obtains a dynamical temperature‐regulation capability through sensible and latent heat absorption of mE as a phase change material, while the upper MXene layer provides a very low emissivity surface for the system. As a result, the developed composite achieves a significant reduction in the thermal radiation temperature of a high‐temperature target. Moreover, the MXene film exhibits good electromagnetic interference shielding effectiveness, making the sandwich‐like composite obtain a thermal camouflage capability in various complicated scenarios. This work provides a promising approach for the design of advanced functional materials to realize long‐term infrared stealth and thermal camouflage of high‐temperature targets in security protection and counter‐surveillance.

Constraining Thermal Emission of Pluto’s Haze from Infrared Rotational Lightcurves

The rotational lightcurves of the Pluto-Charon system were previously believed to be solely attributed to their surfaces. However, a proposed scenario of haze cooling suggests that the atmospheric haze of Pluto could significantly contribute to mid-infrared emission, which calls for a revisit of previous analyses. In this study, we employ a Bayesian retrieval approach to constrain the haze emission from the rotational lightcurves of the Pluto-Charon system. The lightcurves were observed by the Spitzer and Herschel telescopes at 24 and 70 μm, and were combined with the latest surface albedo maps of Pluto and Charon from the New Horizons spacecraft. Our results show that including the haze emission is consistent with all current observations, with the best-fit haze flux around 1.63 mJy. This is in agreement with the composition of Titan-like tholins. However, the “surface only” scenario, which excludes the haze contribution, can still explain the observations. We conclude that the current data at 24 μm cannot constrain Pluto’s haze emission due to the degeneracy with Charon’s surface emission. Regardless, some surface properties of Pluto are well constrained by the shape of the lightcurves, with a thermal inertia of approximately 8–10 MKS and a relatively low CH4 emissivity of 0.3–0.5. We suggest that observations by the JWST telescope at 18 μm, which can resolve Pluto from Charon, could directly probe the haze emission of Pluto due to the low surface emission at that wavelength.

Pulsed infrared thermal imaging as inline quality assessment tool

Pulsed infrared thermography (PIRT) is a very fast and non-destructive technique for testing various types of specimens. This makes PIRT interesting for 100 %-inspections in industrial production lines. Especially for the die attach interface inspection of sintered and soldered electronic components and layer-to-layer interfaces in fiber-reinforced plastic (FRP), the industry is looking for alternatives to cost intensive X-ray methods and the time-consuming scanning acoustic microscopy (C-SAM). A big challenge to use the PIRT technology for inspections in industrial production lines is the low emissivity of the surface of the device under test (DUT). This work will present a PIRT system including a vacuum foil lamination setup to avoid the necessary spray coating of DUTs with low emissivity surfaces in failure analyses. This system can be used as inline quality assessment tool with an integrated technique to perform reliable component inspections on a wide range of different specimens.

Study on the Effect of Radiant Insulation Panel in Cavity on the Thermal Performance of Broken-Bridge Aluminum Window Frame

Windows have a great impact on building energy consumption, and the thermal performance of window frames directly affects its energy-saving potential. In this paper, a novel method is proposed to optimize the thermal performance of commercially available broken-bridge aluminum window frames, by incorporating radiant insulation panels (RIPs) into the window frame cavity. A typical aluminum alloy window frame heat transfer model is theoretically analyzed and validated, and the effects of key design parameters on the equivalent thermal conductivity (ETC) of the cavity radiation heat transfer and the heat transfer coefficient (U-factor) of window frames are quantitatively analyzed by a finite element simulation method using the THERM software. Moreover, the RIP, the insulation material filling, and low surface emissivity on the thermal performance of the window frame are compared and analyzed. The results show that the RIP is better placed in the middle, the width and quantity of RIPs are negatively correlated with the U-factor, while the surface emissivity of RIPs is positively correlated with the U-factor. Adding RIPs in the cavity can reduce the U-factor of the window frame by more than 7.43%, slightly lower than 8.97% for the filling type, but significantly higher than 0.81% for the low-emissivity type. Inserting RIPs is a simple and effective way to reduce the U-factor of the window frame and have a great potential of use.

Exploring Step-Heating and Lock-In Thermography NDT Using One-Sided Inspection on Low-Emissivity Composite Structures for New Rail Carbodies.

This paper aims to explore the qualification of step- and lock-in heating thermography as techniques capable of inspecting new composite rail carbodies following input and inspection requirements set by the rail manufacturing industry. Specifically, we studied (a) a monolithic CFRP sample (20 mm thickness) and (b) a CFRP-PET foam-CFRP sandwich (40 mm total thickness) component, that were manufactured with artificial defects, to replicate the side wall sections of a carbody. The samples proved to be very challenging to test using only one-sided inspection due to (1) exhibiting significant thickness compared to existing literature, (2) low surface emissivity and (3) that the foam core of the sandwich sample was a thermal insulating material. In addition, the sandwich sample was designed with defects on both skins. Both thermography techniques provided similar defect detection results, although step heating offered faster detection. In the case of the monolithic panel, defects up to 10 mm depth were detected, with minor detection of defects at 15 mm depth with a step-heating protocol between 90 s and 120 s overall acquisition, which was faster than the 140 s used with the lock-in technique. For the sandwich component only the front skin defects were detected, with both techniques using heating protocols between 70-120 s.

Determination of core thermal resistance errors of reflective multifoil insulation products measured in a hot box depending on evaluation accuracy of its low emissivity surfaces

The multilayer reflective insulation is relatively new product on a construction market, used as efficient insulation and continuously improved. The thermal resistance of this product has to be determined according to the standard EN 16012+A1 [1], which not defines product's sample installation procedure into a hot box and indicates only rough requirement for emissivity values of surfaces of a tested sample and of adhesive tape for fixing the thermocouples must be similar, but it does not specify allowed differences and its effect on the accuracy of the measurement result. These standard's shortcomings do not ensure reliable and comparable results of the thermal resistance of multilayer thermal insulation. Therefore, a study was conducted and the method for installation and fixing of these products into hot box using frame was created. The influence of the frame on the measured thermal resistance was evaluated as a linear thermal bridge. The research of the influence of the difference between emissivity values of the sample and adhesive tape surfaces on the measured thermal resistance was performed using a calibration standard with known R-value. The research results showed that the linear thermal bridge value of frame was increasing with the increment of emissivity value of adhesive tape. Significant difference between sample and adhesive tape emissivity values can create more than 20% relative difference between both measured and calculated R-values from the known real R-value of the calibration standard. In order to avoid more than 3% relative errors, the limits of difference between emissivity values were set.

On surface temperature measurement of low emittance artefact coating by active infrared laser radiation thermometry

Accurate measurement of the material surface temperature plays a vital role in many scientific and technological advances. With the application of the active infrared laser radiation thermometry, the surface temperature can be obtained independently of the target emissivity. This method has been developed gradually, but there is still lacking of the experimental result about low emissivity surfaces, and the determination of the system core parameter has not been sufficiently discussed. Therefore, a new active infrared laser radiation thermometry system is reported in this paper. The temperature change radiance signals emitted from the target surface at the same frequency as the heating laser are accurately screened and collected by using a lock-in amplifier, and the calibration method of the instrument constant is discussed. The experimental results show that, for high emissivity sample, in the range of 873–1173 K, the relative deviation between the temperature measurement results of the experimental device and the reference value is within 0.5%. For the low emissivity sample, also in the range of 873–1173 K the relative deviation between the measurement results of the experimental device and the reference temperature is within 0.8%, and the average absolute deviation is 3.3 K. The measurement accuracy is improved and compared with previous literature.

Buoyancy-driven ventilation of an enclosure containing a convective area heat source

Buoyancy-driven ventilation of an enclosure containing a convective area heat source with low surface emissivity is studied in this paper. Based on the dynamics of turbulent plumes generated from an area heat source, the mathematical models for dimensionless thermal stratification height are deduced and integrated into a unified formula in this study. And a mathematical model governing the transition zone between these two zones is derived and developed. In order to test the validity of the theoretical model, a series of simulated models were conducted to compare theoretical results with simulated results. It was found that the transition zone will disappear directly when virtual origin distance zv/Ds≥−1.14, which results in that the thermal stratification height can only be obtained in the zones close to the area source and far away from the area source. And the thermal stratification height varies widely when the A∗/H2 values are from 0 to 0.1. In addition, thermal stratification height is negatively correlated with the dimensionless source diameter ξs. This study can be used as a basis for follow-up study of buoyancy-driven flows by area heat source in naturally ventilated building.

Raindrop Signature from Microwave Radiometer Over Deserts

AbstractRainfall estimates from spaceborne microwave radiometers form the foundation of global precipitation data sets. Since the beginning of the satellite microwave rainfall estimation era in the 1980s, the primary signature leveraged over land for these estimates has been the brightness temperature (TB) depression due to ice particle scattering. Contrary to this practice, time series analyses based on observations from two spaceborne radars and two spaceborne radiometers reveal a TB increase at H19 due to raindrop emission as the primary cloud particle signature over desert terrain. Low surface emissivity supports the use of liquid raindrop emission as the primary signature over desert surfaces. In these regions, the surface rain rate better correlates with the liquid raindrop emission signal than with the scattering induced by ice further aloft, suggesting a new potential for improving rainfall estimation over deserts by exploiting the liquid raindrop emission signature.

Steel-steel composite metal foam in simulated pool fire testing

A comprehensive experimental and numerical simulated pool fire test is conducted on stainless steel composite metal foam (S–S CMF) panels and is reported in this study. The uncertainty assessments for the measured and calculated unexposed surface temperatures in the calibration and simulated pool fire tests on the S–S CMF specimens are also conducted and reported. This test procedure is designed to measure the thermal performance of new or untried thermal protection systems, such as S–S CMF, and to test for its survivability when exposed to a 100-min pool fire condition. The assembly was tested in triplicate in three consecutive simulated pool-fire exposures as specified in 49 CFR Part 179, Appendix B and achieved successful results. Based on the experimental and modeling results as well as the uncertainty studies, the 15.9 mm thick steel-steel composite metal foams tested as a novel insulation system met the acceptance criteria for the simulated pool fire test specified in 49 CFR 179 Appendix B by a large margin and is expected to pass with near certainty if the test were to be reproduced in a different laboratory. The main reason for successful performance of S–S CMF is attributed to the large air content in the material. The numerical studies reported in this study indicated that the low surface emissivity of the material is also contributing to the superior performance of the material to some extent. To complete the full test requirements of CFR Part 179 App. B, the material will need to be tested against the torch-fire exposure in duplicate. This research indicates that one of the potential applications of lightweight S–S CMF can be in tank cars carrying hazardous materials and replacing conventional structural steel with demonstrated benefits of excellent thermal insulation, fire resistance, low weight along with its established energy absorption capabilities.

Infra-red thermography estimate of deposited heat load dynamics on the lower tungsten divertor of WEST

A thermography inversion algorithm (TEDDY) was developed for the physical exploitation of WEST infra-red thermography (at 3.9 μm) of uncooled graphite tiles coated with 12 μm of tungsten, located at the lower divertor. The numerical scheme features a conservative finite volume approach with the full temperature non linearity of thermal properties of material composing the tile. The inversion algorithm is validated against synthetic test cases built from standardized software. A first in situ quantification of tungsten surface emissivity and reflected luminance along an outer target coated tile is then detailed. This method relies on a comparison of thermography data with absolute temperature measurements from embedded thermal sensors. Low surface emissivity (ϵ ≈ 0.05) is estimated in the common outer strike point area, while increasing to about 0.1–0.15 far from it, possibly due to deposited impurity layers. The heat flux inversion from corrected thermography temperature is illustrated on an L-mode plasma heated with 4 MW of input power, featuring a strike point sweeping of 30 mm along the divertor tile. Power balance and sweeping dynamics are consistently recovered by the heat flux inversion, while the radial profile of parallel heat flux remapped to outer midplane features a Gaussian-exponential shape common to diverted configurations in low dissipation divertor regime.

Thermal performance of thermal paint and surface coatings in buildings in heating dominated climates

A purported approach to reducing heating energy in solid wall “hard to heat” housing is the simple application of a thin layer (<1 mm) of thermal paint containing insulating additives. The objective of this study was to test the energy saving claims by a systematic study of the material characteristics and thermal performance of internal coatings using accepted international standard test methods. The coatings have been compared with conventional internal coverings such as emulsion paint, wallpapers and expanded polystyrene liner. A dynamic model of the Energy House research facility has been used to evaluate energy savings, costs, and payback times.The thermal resistance of the thermal paint coatings was generally found to be not much better than that of conventional vinyl textured wallpapers with a lining paper. When all building heat losses are considered, modelling predictions for thermal paint coatings indicate an unfavourable payback period of several hundred years, and energy savings of between 0.4% and 2.9% depending on coating thickness and type. The evidence from the results and models, as well as scanning electron microscopy, do not support the claims that the additive powder particles are effectively nano-porous, evacuated, or that the coatings have low emissivity surfaces.

A transparent insulation façade enhanced with a selective absorber: A cooling energy load and validated building energy performance prediction model

Solar thermal façade concepts for building integration have a relevant contribution in the field of renewable solar energy exploitation. This study is focused on an experimental analysis of a transparent insulation façade (TIF) concept and the options for the prediction of its performance using building energy simulation (BES) modelling. A TIF prototype based on a transparent insulation material (TIM) and a selective absorber is tested and compared with a nonselective type of absorber. An experimental study was conducted involving full-scale climatic chamber and dynamic outdoor tests. The proposed TIF concept with a selective absorber and its solar effective layers provides the same thermal performance as conventional types of thermal insulation. As the TIM integration is potentially limited by overheating phenomena, a maximum cooling energy load obtained by dynamic outdoor conditions provides its real outdoor performance. The results show that a significant difference is measured when compared with the nonselective absorber type. Finally, a detailed comparative investigation of the thermal performance of both TIFs was conducted using the BES method. When applying the low-e functionality of the solar absorber, significant limitations were found specifically concerning the surface radiant heat exchange with the low emissivity surface affecting the air cavity enclosed zone.

Nonlinear Split-Window Algorithms for Estimating Land and Sea Surface Temperatures From Simulated Chinese Gaofen-5 Satellite Data

This paper proposes a different thermal channel combination split-window (DTCC-SW) method to estimate the land surface temperature (LST) and sea ST (SST) from the Chinese Gaofen-5 (GF-5) satellite thermal infrared (TIR) data. A nonlinear combination of two adjacent channels CH8.20(centered at $8.20~\mu \text{m}$ ) and CH8.63 (centered at $8.63~\mu \text{m}$ ) was proposed to estimate LST for low-emissivity surfaces. A nonlinear combination of two adjacent channels, CH10.80 (centered at $10.80~\mu \text{m}$ ) and CH11.95 (centered at $11.92~\mu \text{m}$ ), was developed to estimate LST and SST for high-emissivity surfaces under dry atmospheric conditions, and a nonlinear combination of two channels, CH8.63 and CH11.95, was used to estimate LST and SST for high-emissivity surfaces under wet atmospheric conditions. The numerical values of the DTCC-SW coefficients were obtained using a statistical regression method from synthetic data simulated with an accurate atmospheric radiative transfer model moderate spectral resolution atmospheric transmittance mode 5 over a wide range of atmospheric and surface conditions. The LST (SST), mean emissivity, and atmospheric water vapor content were divided into several tractable subranges to improve the fitting accuracy. The experimental results and the preliminary evaluation results showed that the root-mean-square error between the actual and estimated LSTs (SSTs) is less than 0.7 K (0.3 K), provided that the land surface emissivities are known, which indicates that the proposed DTCC-SW method can accurately estimate the LST and SST from the GF-5 TIR data.

Low-emissivity coating coupled with aerogel-based plaster for walls' internal surface application in buildings: Energy saving potential based on thermal comfort assessment

Building envelope reflective finishing can play a major role in the energy saving of buildings. Extensive research demonstrated the influence of such a finishing on the external surface of horizontal and vertical walls; however, research on low-emissivity (low-e) finishing at the interior surface of vertical envelopes is hardly found in the literature. Accordingly, this study starts with a state-of-the-art on low-e applications in buildings. Dynamic simulations are then carried out to investigate the thermal and hygro-thermal behavior of envelopes with interior surface low-e coating, during heating and cooling season in Lyon, France. A new methodology to simulate the operative temperature of the indoor environment, considering the emissivity of the indoor surfaces, is proposed. Results showed that the energy-saving potential of low-emissivity finishing is major when applied to low/non-insulated buildings, particularly when considering thermal comfort. A foreseen application is to couple this technology with a system that controls the indoor operative temperature, reaching energy saving potentials of between 7% and 13%. If an operative-temperature-based control is implemented at a position near a low-emissivity surface, energy savings of up to 22% could be achieved. Beneficial contribution is obtained when simulating low-e coating coupled with interior aerogel plaster, where energy savings of 21% and 30% are obtained for 0.5 cm and 1 cm of aerogel plaster, respectively. This thin layer of aerogel plaster demonstrated also to be effective in retrofitting existing walls to reduce the moisture risks. It is worth mentioning that from an architectural point of view, a low- emissivity coating means a metal appearance wall finishing, which needs to be accepted by the occupants.

Numerical and experimental study of conjugate heat transfer in a horizontal air cavity

The demand for general reduction of the energy consumption in civil engineering leads to more frequent use of insulating materials with air gaps or cavities. Heat transfer through a constructional part can be decreased by adding an air gap and low emissivity reflective foils to the structure. In the first part of this paper, the impacts of cavity thickness and inner surface emissivity on combined conduction, convection and radiation heat transfer was experimentally explored in the case of constructional part with a horizontal cavity subjected to constant downward heat flux. The heat flow meter Netzsch HFM 436 Lambda was used for steady-state measurements. Results suggest that the studied parameters seriously affect the combined heat transfer in the composed structure. In the second part the paper reports the numerical study of two-dimensional conjugate heat transfer in closed horizontal cavity having air as the intervening medium. Numerical models validated by related experimental results were performed to further investigate the effect of radiation heat transfer. It was found that in general, the total heat flux through the composed structure decreases with increasing air cavity thickness, which is significant especially when low emissivity inner surfaces are taking into account. The direction of heat flow (downward or upward heat flow) has a significant impact on the convection heat transfer. An important contribution from the present work is the analysis of the optimal thickness of the cavity at different boundary conditions. The optimal thickness of the enclosure with low emissivity surfaces is 16 mm when subjected to upward heat flux.

Microstructural Dependence of Performance in Potentiostatically Electrodeposited Soft Gold Coatings

The control and stability of microstructure in electrodeposited metals is critical to the performance and reliability of many technologies including electrical contacts, tribological coatings, and low-emissivity surfaces. At present, however, the relationships between growth conditions and microstructural development are understood primarily at a phenomenological level, limiting the ability to predictively engineer these materials. In this study, we tacked a subset of this long-term, foundational problem through an experimental investigation of the elementary mechanisms governing grain evolution in electrodeposited soft Au coatings. We focused on grain structure because of its ubiquitous importance to the properties of electrodeposited materials, including emissivity, contact resistance, electrical conductivity, mechanical strength, crystal structure, and surface and bulk morphologies. Our experiments, which employed state-of-the-art surface and bulk microscopies, quantitatively linked the growth conditions to the surface morphology dynamics, subsurface grain structure, and resulting films' properties.

A thermal model for energy loss through walls behind radiators

A one-dimensional thermal network model is developed for the purpose of predicting the heat transfer behaviour between wall-mounted domestic radiators and outdoor-facing walls. Importantly, the model includes radiation heat transfer between the rear of the radiator and the wall and is able to estimate the resulting heat losses to the outdoors for different thermal loading conditions, such as radiator and outdoor temperatures and wall U-values. To contextualize the problem, a model of a conventional single panel domestic radiator is included so that the energy losses through the wall can be evaluated in terms of the influence on the overall efficiency of the radiator thermal system. Depending on the outdoor condition, the heat losses can be in the region of 5% for poorly insulated walls, highlighting an opportunity for small, yet non-negligible, energy savings if cost effective technologies are deployed to reduce these losses. To this end, the influence of wall-mounted reflector panels, aimed to reduce the heat losses, is evaluated. The reflector panels have two influences: they reduce the overall heat losses thus improving the overall efficiency of the radiator system, and they reduce the total thermal output of the radiator; the magnitude of each depending strongly on the U-value of the wall structure. Finally, two radiation shield concepts are evaluated; one with reflecting surfaces on both sides and one with a highly emissive surface facing the radiator and a low emissivity surface facing the wall. The latter is found to be the best performing radiation barrier as it can increase the total heat transfer into the room whilst significantly reducing the heat losses, whereas the former causes a reduction in the total thermal power of the radiator and lower heat transfer to the room for a commensurate reduction in heat losses.