Atmospheric Window Research Articles

Controllable-gradient-porous cooling materials driven by multistage solvent displacement method

Passive radiative cooling holds significant potential for efficiently cooling terrestrial objects by simultaneously reflecting sunlight and radiating heat to the cold outer space. Numerous studies have designed high-performance radiative cooling structures using polymethyl methacrylate (PMMA) due to its inherent extinction coefficient.However, obtaining highly favorable spectral characteristics for reflecting sunlight using PMMA, often done through the fabrication of micro/nanoscale structures, remains a central barrier for widespread application. Herein, we report a facile, economical, and scalable multistage solvent displacement-based method for fabricating hierarchically gradient porous PMMA metafilms with highly efficient daytime and nighttime passive radiative cooling performance. Here, the “ouzo effect” works as the driving force for micro/nanopore formation by solvent displacement, which also controls the size of the pores based on different solution ratios. The PMMA metafilm features an ultrahigh solar reflectance of 0.99 and superior mid-infrared thermal emittance of 0.97, which allows for the average and peak subambient temperature drops of 4.6 °C and 8.2 °C, respectively, along with an average radiative cooling power of 90 W/m2 during a 24-h uninterrupted thermal measurement. The hierarchically gradient micro/nanopore distribution of the PMMA metafilm significantly enhances the solar reflectance and thermal emittance within the atmospheric transparency window. Moreover,the multistage solvent displacement method is highly versatile and promising for fabricating porous structures, further offering a cost-efficient, eco-friendly, and sustainable manufacturing path for high-performance radiative cooling application.

Facilely fabricated polyethylene film composed of directional microfibrils for passive radiative cooling

Nowadays, passive radiative cooling (PRC) has being attracted intensive attention because it can realize cooling effect without consuming any energy. Unfortunately, fabricating polymer film with radiative cooling effect by a facile, continuous and environmentally friendly method is still a large challenge. In this study, high density polyethylene (HDPE)/polyvinyl oxide (PEO) film were first fabricated by melt extrusion casting method, and polyethylene film composed of directional microfibrils (PFCDM) were finally obtained after selective dissolution of PEO phase in deionized water at 70 °C. The as-prepared PFCDM shows higher average solar reflectivity of 85.17% in the visible range and stronger average mid-infrared emissivity of 82.04% within the atmospheric window. It shows a sub-ambient cooling of 23.28 °C during the day and 3.1 °C at night, suggesting that it has a remarkable PRC performance. Moreover, the as-prepared PFCDM has outstanding wearability and self-cleaning property, showing promising potential of personal thermal management. This study proposes a facile but environmentally friendly method for fabricating PFCDM, paving a new way to develop wearable PRC film following the concept of thermoplastic polymers “functionalized” processing.

A 3D porous polymer film designed for efficient passive radiative cooling

Passive daytime radiative cooling (PDRC) technique can cool objects without energy consumption by reflecting sunlight and radiating heat into the cold outer space through atmospheric transparent windows (ATW). Here, 3D porous polydimethylsiloxane (P-PDMS) films with excellent radiative cooling properties were fabricated using citric acid monohydrate (CAM) templates. Integrating with high sunlight reflectance (78.6%) and great thermal emissivity (99.4%) in ATW, the test was performed in an outdoor environment with an average solar radiation of 715.09 W/m2. The maximum temperature of P-PDMS film is reduced to 15.4℃, and in the actual roof application test, P-PDMS film also show excellent passive radiant cooling performance of 8.3℃ cooling in the house. The 3D-porous structure plays an important role in improving solar reflectivity and IR emissivity. Combined with efficient DRC capabilities and mass production compatibility, this work brings a new paradigm for the preparation of cooling devices with potential commercial applications in sustainable energy-free cooling.

Verification of parameterizations for clear sky downwelling longwave irradiance in the Arctic

Abstract. Ground-based high resolution observations of downward longwave irradiance (DLI), surface air temperature, water vapor surface partial pressure and column amount, zenith sky infrared (IR) radiance in the atmospheric window, and all-sky camera images are regularly obtained at the Thule High Arctic Atmospheric Observatory (THAAO, 76.5° N, 68.8° W), northwestern Greenland. The datasets for the years 2017 and 2018 have been used to assess the performance of different empirical formulas used to infer clear sky DLI. An algorithm to identify clear sky observations has been developed, based on value, variability, and persistence of zenith sky IR radiance. Seventeen different formulas to estimate DLI have been tested against the THAAO dataset, using the originally determined coefficients. The formulas that combine information on total column water vapor and surface air temperature appear to perform better than others, with a mean bias with respect to the measured DLI smaller than 1 W m−2 and a root mean squared error (RMSE) around 6 W m−2. Unexpectedly, some formulas specifically developed for the Arctic are found to produce poor statistical results. This is attributed partly to limitations in the originally used dataset, which does not cover a whole year or is relative to very specific condition (i.e., the presence of an ice sheet). As expected, the bias displays a significant improvement when the coefficients of the different formulas are calculated using the THAAO dataset. The presence of 2 full years of data allows the determination and the applicability of the coefficients for singular years and the evaluation of results. The smallest values of the bias and RMSE reach 0.1 and 5 W m−2, respectively. Overall, the best results are found for formulas that use both surface parameters and total water vapor column content, and have been developed from global datasets. Conversely, formulas that express the atmospheric emissivity as a linear function of the logarithm of the column integrated water vapor appear to reproduce poorly the observations at THAAO.

Calcium-Salt-Enhanced Fiber Membrane with High Infrared Emission and Hydrophilicity for Efficient Passive Cooling.

Radiative cooling fabrics have gained significant attention for their ability to enhance comfort without consuming extra energy. Nevertheless, sweat accumulation on the skin and diminishing cooling efficiency usually exist in the reported polymer cooling membranes. Herein, we report a universal method to obtain a calcium (Ca)-salt-enhanced fiber membrane with high infrared emission and hydrophilicity for efficient passive cooling and flame retardancy. The modification by Ca salts (including CaSiO3, CaSO3, and CaHPO4) with strong infrared emission results in an improvement in hygrothermal management ability, especially for moisture absorption and perspiration regulation in hot and humid environments. As an example, the CaSiO3@PMMA fiber membrane exhibits exceptional reflectivity in the solar spectrum (∼94.5%), high emittance in the atmospheric window (∼96.7%), and superhydrophilicity with a contact angle of 31°. Under direct sunlight, the CaSiO3@PMMA membrane exhibits an obvious temperature drop of 11.7 °C and moisture management achieves an additional cooling of 8.9 °C, as further confirmed by the ability to reduce the rate of ice melting. Additionally, the composite membrane provides notable flame retardancy and UV resistance. This work paves a new path in developing new materials with perspiration management and flame retardancy for zero energy consumption cooling in hot and humid environments.

Thermochromic Nanocellulose Films for Temperature-Adaptive Passive Cooling.

Energy efficiency in habitation spaces is a pivotal topic for maintaining energy sufficiency, cutting climate impact, and facilitating economic savings; thus, there is a critical need for solutions aimed at tackling this problem. One viable approach involves complementing active cooling methods with powerless or passive cooling ones. Moreover, considerable scope remains for the development of passive radiative cooling solutions based on sustainable materials. Cellulose, characterized by its abundance, renewability, and biodegradability, emerges as a promising material for this purpose due to its notable radiative cooling potential exploiting the mid-infrared (MIR) atmospheric transmission window (8-13 μm). In this work, we propose the utilization of thermochromic (TC) materials in conjunction with cellulose nanofibrils (CNF) to confer temperature-dependent adaptivity to hybrid CNF films. We employ a concept where high reflection, coupled with MIR emission in the heated state, facilitates cooling, while high visible light absorption in the cold state allows heating, thus enabling adaptive thermal regulation. CNF films were doped with black-to-leuco TC particles, and a thin silver layer was optionally applied to the films. The films exhibited a rapid transition (within 1 s) in their optical properties at ∼22 °C, becoming transparent above the transition temperature. Visible range transmittance of all samples ranged from 60 to 90%, with pronounced absorption in the 8-13 μm range. The cooling potential of the films was measured at 1-4 °C without any Ag layer and ∼10 °C with a Ag layer. In outdoor field testing, a peak cooling value of 12 °C was achieved during bright sunshine, which is comparable to a commercial solar film. A simulation model was also built based on the experimental results. The concept presented in this study extends beyond applications as standalone films but has applicability also in glass coatings. Overall, this work opens the door for a novel application opportunity for green cellulose-based materials.

Realization of an Adaptive Radiative Cooler with a Multilayer-Filter VO2-Based Fabry–Pérot Cavity

A high-performance adaptive radiative cooler comprising a multilayer-filter VO2-based Fabry–Pérot (FP) cavity is proposed. The bottom FP cavity has four layers, VO2/NaCl/PVC/Ag. Based on the phase transition of VO2, the average emissivity in the transparent window can be switched from 3.7% to 96.3%. Additionally, the average emissivity can also be adjusted with external strain to the PVC layer, providing another way to attain the desired cooling effect. An upper filter is included to block most of the solar radiation and provide a transmittance of 96.7% in the atmospheric window. At high temperature, the adaptive emitter automatically activates radiative cooling. The net cooling power is up to 156.4 W⋅m−2 at an ambient temperature of 303 K. Our adaptive emitter still exhibits stable selective emissivity at different incident angles and heat transfer coefficients. At low temperature, the radiative cooling automatically deactivates, and the average emissivity decreases to only 3.8%. Therefore, our work not only provides new insights into the design of high-performance adaptive radiative coolers but also advances the development of intelligent thermal management.

Compatible camouflage for dual-band guided-laser radar and infrared via a metamaterial perfect absorber.

Laser-guided detector and infrared detection have attracted increasing attention in a wide range of research fields, including multispectral detection, radiative cooling, and thermal management. Previously reported absorbers presented shortcomings of lacking either tunability or compatibility. In this study, a metamaterial perfect absorber based on a Helmholtz resonator and fractal structure is proposed, which realizes tunable perfect absorptivity (α 1.06μ m >0.99,α 10.6μ m >0.99) of guided-laser radar dual operating bands (1.06 µm and 10.6 µm) and a low infrared average emissivity (ε¯3-5μ m =0.03,ε¯8-14μ m =0.31) in two atmospheric windows for compatible camouflage. The proposed perfect absorber provides a dynamically tunable absorptivity without structural changes and can be applied to optical communication, military stealth or protection, and electromagnetic detection.

Wide-angle ultra-wideband metamaterial absorber based on complex dielectric layer in long and very long-wave infrared

Long and very long-wave infrared are the most important bands in infrared detection technology because of their high atmospheric window radiation energy. At present, long and very long-wave infrared detectors are widely used in atmospheric monitoring, night reconnaissance, deep space exploration and other fields. In this paper, we first analyze the coupling resonance in the dual-band absorber (Ti–Si–Ti), and the absorption rates are 97.05% and 98.95% at 6.1 μm and 19.2 μm, respectively. Then, a four-layer (Ti–Si–SiO2–Ti) absorber with a complex dielectric structure is obtained by using the surface plasmon resonance and the inherent absorption of the lossy material SiO2. In addition to the traditional electromagnetic field analysis, we also used the layer absorption energy loss theory to study the inherent absorption mechanism of SiO2, and the average absorption of the absorber from 19 to 24.7 μm reached 92.87%. Finally, based on the composite dielectric layer, a thin ultra-wideband absorber with four nanorods of the same size surface structure was designed, and the average absorption was 92.03% from 13.3 to 24.6 μm. The polarization-insensitive ultra-wideband absorber proposed by us is light, simple in structure and easy to manufacture, and has potential application value in atmospheric monitoring, night reconnaissance, deep space exploration and other fields.

Wavelength conversion through stimulated Raman scattering in an oxygen-filled fiber for multi-band LiDAR.

Wavelength conversion afforded by stimulated Raman scattering within a hollow core fiber is potentially useful for multispectral light detection and ranging (LiDAR). Herein, we make use of the ideal 1550 cm-1 vibrational Raman shift of an antiresonant fiber filled with gaseous oxygen so that the first and second Raman orders as well as the transmitted pump are all located in separate atmospheric transmission windows. To the best of our knowledge, this is the first report of stimulated Raman scattering in an oxygen-filled fiber. The host of closely spaced rotational stimulated Raman scattering (SRS) lines (12 cm-1) accompanying the transmitted pump and vibrational Raman orders form continuum bands allowing for much greater spectral coverage of the atmospheric transmission windows. The temporal profiles of the Raman orders can be separated without the use of a grating to potentially achieve a multi-band LiDAR.

KCA/Na2SiO3/PNIPAm hydrogel with highly robust and strong solar modulation capability for thermochromic smart window

Thermochromic smart windows have strong competitive advantages in the realm of building energy conservation. However, achieving thermochromic smart window with a lower critical solution temperature (LCST), high solar modulation (ΔTsol), and excellent visible light transmittance (Tlum) remains a great challenge. Here, a novel composite hydrogel based on polydiisopropylacrylamide (PNIPAm) doped with carrageenan (KCA) and Na2SiO3 was developed for high-performance smart window. The lower LCST of PNIPAm was precisely tuned in a range from 25 °C to 28 °C by controlling the content of added Na2SiO3. The inclusion of KCA diminishes agglomeration within the gel system and enhances both Tlum and ΔTsol. As a result, the KCA/Na2SiO3/PNIPAm exhibits a high Tlum (87.37 %), ΔTsol (69.65 %), a lower LCST (27.2 °C), and a high atmospheric window emissivity (0.962). The smart window achieves temperature drop of 12.3 °C under direct sunlight, and keeps highly stable even after 100 cycle tests high and low-temperature durability due to its robust solar modulation capability. This work offers valuable insights for developing high-performance and robust thermochromic smart windows and holds promising applications in building energy conservation.

Facile preparation of infrared-transparent, superhydrophobic coatings for infrared detection and personal thermal management

Capturing thermal radiation using highly infrared (IR)-transparent materials have drawn great attention for diverse applications, such as IR guidance, IR detection, thermal radiation imaging, radiative cooling etc. However, the previously reported IR-transparent materials have the bottleneck of lacking superhydrophobicity, resulting in the degradation due to the dust, fog, rain, frost outdoors. Here, a durable IR-transparent superhydrophobic coating is reported via the spraying of polyester (PE) microspheres and fluorinated ZnS nanoparticles (NPs). Due to the high transmittance of ZnS and PE, the coating shows excellent IR transparency (around 90 %) in 8 ∼ 14 μm thermal IR band. Benefitting from the robustness of 3D connected PE network, the coating maintains superhydrophobicity after 180 cycles of linear wearing under 6 kPa. Moreover, the water jetting/atomizing/dust tests indicate that this coating effectively ensures the high accuracy of IR imaging lens under rainy/foggy/dusty environments. This coating on fabrics also achieves a remarkable radiative cooling effect for outdoor conditions to keep human body comfortable. With its facile preparation and versatile features, this coating has great potential on IR window, IR imaging and personal thermal management.

Measurements of infrared absorption cross-sections for n-C3F8, c-C4F8, n-C4F10, and n-C5F12 from 298 to 350 K

Perfluoro-n-propane, perfluorocyclobutane, perfluoro-n-butane, and perfluoro-n-pentane are non-ozone-depleting industrial alternatives to chlorofluorocarbons and hydrochlorofluorocarbons. However, these perfluoroalkanes have significant band strength in the atmospheric window from 800 to 1200 cm−1. Coupled with their millennial-scale atmospheric lifetimes, they can lead to significant long-term global warming. Infrared spectra are required to quantify the climate impacts. This work provides a set of high-temperature infrared absorption cross-sections in the range 298–350 K at 0.1 cm−1 resolution from 515 to 1500 cm−1 for each compound. Our cross-sections generally agree with literature measurements except for perfluoro-n-pentane. We use density functional theory to calculate the absorption cross-sections from 0 to 515 cm−1 using the B3LYP functional and several basis sets. The 6-31G(d,p) basis set provides the best results for linear perfluoroalkanes, while the def2-TZVP basis set provides the best results for cyclic perfluoroalkanes. Using experimental cross-sections, we calculate the radiative efficiency and global warming potential for each compound, utilizing the Pinnock curve from Shine and Myhre (2020) and atmospheric lifetimes from Hodnebrog et al. (2020). These quantities are found to be independent of temperature. The average 100-year global warming potential derived from all cross-sections is 9,610±1,260, 10,800±1,420, 10,100±1,330, and 9,380±1,230 for perfluoro-n-propane, perfluorocyclobutane, perfluoro-n-butane, and perfluoro-n-pentane, respectively. Combining the data in this work with our previous measurements reveals that the global warming potential for perfluoroalkanes with an increasing number of CF bonds depends on the ratio of radiative efficiency to molecular weight.

Design and numerical modeling of chalcogenide parabolic‐core waveguide for on‐chip supercontinuum generation extending from near‐IR region to mid‐IR region

AbstractWe present the design and computational examination of a waveguide structure with a parabolic core, implemented in AsSe2 chalcogenide glass. This structure aims to generate a mid‐infrared (mid‐IR) supercontinuum (SC) spectrum, covering the wavelength range of 1.4−7.6 µm. It includes molecular fingerprint region as well as earth's atmospheric transparent window. A pulsed laser providing pulses of peak power of 250 W and pulse width of 200 fs at 2 µm was considered as the pump. The results illustrate that the broadband SC spectrum can be achieved in a short (~8 mm) waveguide pumped with commercially available fiber laser at 2 µm. The novelty of this work lies in the generation of a mid‐IR SC spectrum using a parabolic‐core waveguide pumped in the normal dispersion region, which has not been investigated earlier. This design is crucial as the fabricated rectangular waveguide does not remain rectangular after laser postprocessing but becomes similar to the proposed parabolic design.

Nanofibrous Biomaterial-Based Passive Cooling Paint Structurally Linked by Alkane-Oleate Interactions.

Passive radiative cooling materials, which provide cooling without consuming electricity, are widely recognized as an important technology for reducing greenhouse gas emissions and delivering thermal comfort to less industrialized communities. Optimizing thermal and optical properties is of primary importance for these materials, but for real-world utilization, ease of application and scalability also require significant emphasis. In this work, we embed the biomaterial hydroxyapatite, in the form of nanoscale fibers, within an oil-based medium to achieve passive cooling from an easy-to-apply paint-like solution. The chemical structure and bonding behaviors of this mixture are studied in detail using FTIR, providing transferable conclusions for pigment-like passive cooling solutions. By reflecting 95% of solar energy and emitting 92% of its radiative output through the atmospheric transparency window, this composite material realizes an average subambient cooling performance of 3.7 °C in outdoor conditions under a mean solar irradiance of 800 W m-2. The inflammability of the material provides enhanced durability as well as unique opportunities for recycling which promote circular economic practices. Finally, the surface structure can be easily altered to tune bonding behaviors and hydrophobicity, making it an ideal passive cooling coating candidate for outdoor applications.

Diamond for High-Power, High-Frequency, and Terahertz Plasma Wave Electronics.

High thermal conductivity and a high breakdown field make diamond a promising candidate for high-power and high-temperature semiconductor devices. Diamond also has a higher radiation hardness than silicon. Recent studies show that diamond has exceptionally large electron and hole momentum relaxation times, facilitating compact THz and sub-THz plasmonic sources and detectors working at room temperature and elevated temperatures. The plasmonic resonance quality factor in diamond TeraFETs could be larger than unity for the 240-600 GHz atmospheric window, which could make them viable for 6G communications applications. This paper reviews the potential and challenges of diamond technology, showing that diamond might augment silicon for high-power and high-frequency compact devices with special advantages for extreme environments and high-frequency applications.

Adaptive Thermal Management Radiative Cooling Smart Window with Perfect Near-Infrared Shielding.

The architectural window with spectrally selective features and radiative cooling is an effective way to save building energy consumption. However, architectural windows that combine both functions are currently based on micro-nano photonic structures, which undoubtedly hinder their commercial application due to the complexity of manufacture. Herein, a novel tunable visible light transmittance radiative cooling smart window (TTRC smart window) with perfect near-infrared (NIR) shielding ability is manufactured via a mass-producible scraping method. TTRC smart window presents high luminous transmittance (Tlum = 56.8%), perfect NIR shielding (TNIR = 3.4%), bidirectional transparency adjustment ability unavailable in other transparent radiative coolers based on photonic structures (ΔTlum = 54.2%), and high emittance in the atmospheric window (over 94%). Outdoor measurements confirm that smart window can reduce 8.2 and 6.6°C, respectively, compared to ordinary glass and indium tin oxide (ITO) glass. Moreover, TTRC smart window can save over 20% of annual energy in the tropics compared to ITO and ordinary glass. The simple preparation method employed in this work and the superior optical properties of the smart window have significantly broadened the scope of application of architectural windows and advanced the commercialization of architectural windows.

Silica-bridged inorganic-organic hybrid membrane for efficient daytime radiative cooling

Passive daytime radiative cooling (PDRC) has been considered as a cost-effective technology for efficient energy consumption and utilization, in which the heat is directly delivered into the outer space (3 K) through reflection and radiation. To achieve the high-performance PDRC, textile-based porous membranes are widely employed as radiators, however, the PDRC efficiency affected by the mechanical robustness. Herein, inspired by natural spider silks, a micro-/nano-sized hierarchical membrane-based radiator, which is rationally constructed with the interconnected polyethylene oxide (PEO) nanofibers bridged by the robust silica (SiO2) microspheres, was fabricated with the assistance of electrospinning. This as-prepared membrane features a hierarchically porous structure and a strong interfacial binding, resulting in the optimal optical and mechanical properties. For example, in the presence of a silane coupling agent, the tensile strength and the toughness of the resultant organic–inorganic hybrid membrane are enhanced by 7.1 and 7.2 times, respectively, compared to the organic PEO-based membrane. Furthermore, the assembled radiator demonstrates a temperature difference between 16.1 and 27.4 °C, accompanied by an average solar reflectance of 95.6 % and an average atmospheric window emittance of 89.9 %. This work provides a promising solution for achieving daytime radiative cooling at a reasonable cost and this technology could be extended into a wide range of application fields, such as smart buildings and aerospace industries.

A strategy for accelerating condensation by radiative cooling with hydrophilic-hydrophobic surface

Using radiative cooling technology for dew collection is a promising solution to the freshwater resource crisis. Environmental factors such as solar radiation intensity, ambient temperature, and humidity can influence both the radiative cooling process and the condensation process. In this study, the influence mechanisms of environmental factors on the radiation condensation effect are studied through simulation. Then, TiO2 nanoparticles are sprayed on the surface of the PDMS film to form a hydrophobic-hydrophilic surface. The experimental research is conducted at an ambient temperature of 25 ℃ and a relative humidity of 70%−90% to analyze the change in condensation water. The experimental results indicate that after TiO2 nanoparticles are added to PDMS film, the emissivity in the atmospheric transparency window and the surface hydrophilicity of the film are increased, which effectively improves the condensation effect. The condensation rate has reached 916.6 g∙m−2∙h−1. At 70% relative humidity, the condensation water of PDMS film with TiO2 nanoparticles increased by 58%, and the improvement effect is most significant at lower relative humidity. It has great application prospects in arid and water-scarce areas.

Single-layer multifunctional metasurface for laser-infrared-microwave compatible stealth.

This paper presents a novel approach for achieving a multifunctional metasurface capable of multiband compatible stealth. The metasurface is designed with a single-layer metallic structure that integrates functions of radar cross-section (RCS) reduction, laser stealth, and infrared shielding simultaneously. The reduction of RCS is achieved by developing two sub-cells that employ the interference cancellation principle, leading to a 10 dB decrease in RCS across a broad frequency range of 13-21 GHz. The laser stealth capability is attained by implementing a chessboard phase distribution in the array, also based on the interference cancellation principle, efficiently cancelling the specular reflection at the laser wavelength of 1.06 µm. The significant difference in wavelength between microwaves and lasers ensures that their operational characteristics do not interfere with each other. Additionally, the metasurface exhibits an infrared shielding property with an extremely low emissivity (less than 0.03) in the infrared atmosphere window of 3-5 µm and 8-14 µm, enabling the infrared stealth capability. The proposed metasurface demonstrates exceptional performance and has an extremely thin single-layer structure, indicating that it has a promising potential for future applications in multiband compatible stealth.