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Comparison of spectral reflectivity of the radiative cooling coating with different TiO2 402 particle diameters. 403
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Radiative cooling can achieve cooling effect without consuming any energy by 14 delivering energy into outer space (3K) through "atmospheric window" (8-13 μm). 15 Conventional radiative cooling coating with multi-layer structure was severely 16 restricted during application due to its complex preparation process, high cost. In this 17 study, a sing...
Citations
... Remarkable progress has been made recently to achieve PDRC by the normal PDRC coatings (Zhao et al., 2022;Cheng et al., 2021;Weng et al., 2021;Zhang et al., 2020;Wang et al., 2021). The spectrum manipulations for normal PDRC coatings were realized by various structures (Zhai et al., 2017;Chen et al., 2016;Yu et al., 2020;Fuqiang, 2020). Normal PDRC coatings can efficiently cool the object in direct contact Huang and Ruan, 2017;Atiganyanun et al., 2018;Li et al., 2020b), while in non-contact heat dissipations, the encapsulated air with a low thermal conductivity between coatings and objects would weaken the cooling performance. ...
Passive daytime radiative cooling (PDRC) can dissipate heat to outer space with high solar reflectance (R¯solar) and thermal emittance (ε¯LWIR) in the atmospheric transmission window. However, for the non-contact heat dissipation, besides the high R¯solar, a high infrared transmittance (τ¯LWIR) is needed to directly emit thermal radiation through the IR-transparent coating to outer space. In this work, An IR-transparent porous PE (P-PE) coating with R¯solar = 0.96 and τ¯LWIR = 0.88 was prepared for non-contact heat dissipations. Under the direct sunlight of 860 W m⁻², the IR-transparent coating obtained a 4 oC lower heater temperature than the normal PDRC coating under the same condition. In addition, the spectral reflectance of the P-PE coating after immersing in air or water changed little, which showed excellent durability for long-term outdoor applications. These results indicate the P-PE coating can be a potential IR-transparent coating for non-contact heat dissipations under direct sunlight
... Over the past few years, various structures have been proposed to realize PDRC, mainly including metal-polymer layer coatings [14]- [16] , dielectric microsphere stacking coatings [5], [17]- [19] , dielectric microsphere-polymer composite coatings [20], [21] , and porous polymer coatings [6], [22]- [24] . Due to the large differences in the refractive index of air and polymer in porous polymer coatings, high ...
Passive daytime radiative cooling (PDRC) without additional energy input to cool objects by directly radiating energy into outer space and reflecting sunlight provides a promising pathway to replace current compression-based cooling systems. However, the state-of-art PDRC coating still suffers from contamination during long-term operation to maintain the high solar reflectance (R ̅_solar) and thermal emittance (ε ̅_LWIR) in the long-wavelength infrared (LWIR) atmospheric transmission window. Here, to balance the super-hydrophobicity with the micro-pores and high-efficient PDRC performance with the nano-pores, we report a sustainable and scalable method to prepare a self-cleaning bilayer porous coating to achieve a static water contact angle (CA) ~ 163°, R ̅_solar= 0.97 and ε ̅_LWIR= 0.96. Results show that R ̅_solar changes little after being rubbed and exposed to air. And R ̅_solar only decreases slightly from 0.97 to 0.92 in the muddy water, illustrating the excellent contamination resistance of the super-hydrophobic bilayer coating. Under the direct sunlight of 774 W/m2, the bilayer coating achieves an average temperature of 4.0 °C below the ambient temperature and 4.7 °C below the commercial coating temperature. Such a remarkable cooling performance can be achieved even after contamination, which can be 3.1 °C below the ambient temperature after muddy water treatment. The super-hydrophobic bilayer coating remains clean and dry outdoor since the dust or contaminant can be easily carried away by rainwater, indicating that energy-free and labor-free cleaning of the coatings by rain favors long-term practical applications.
Monodispersed microspheres play a major role in optical science and engineering, providing ideal building blocks for structural color materials. However, the method toward high solid content (HSC) monodispersed microspheres has remained a key hurdle. Herein, a facile access to harvest monodispersed microspheres based on the emulsion polymerization mechanism is demonstrated, where anionic and nonionic surfactants are employed to achieve the electrostatic and steric dual‐stabilization balance in a synergistic manner. Monodispersed poly(styrene‐butyl acrylate‐methacrylic acid) colloidal latex with 55 wt% HSC is achieved, which shows an enhanced self‐assembly efficiency of 280% compared with the low solid content (10 wt%) latex. In addition, Ag‐coated colloidal photonic crystal (Ag@CPC) coating with near‐zero refractive index is achieved, presenting the characteristics of metamaterials. And an 11‐fold photoluminescence emission enhancement of CdSe@ZnS quantum dots is realized by the Ag@CPC metamaterial coating. Taking advantage of high assembly efficiency, easily large‐scale film‐forming of the 55 wt% HSC microspheres latex, robust Ag@CPC metamaterial coatings could be easily produced for passive cooling. The coating demonstrates excellent thermal insulation performance with theoretical cooling power of 30.4 W m⁻², providing practical significance for scalable CPC architecture coatings in passive cooling.
Particle dispersed coatings with gradient distributions, resulting from either gravity or artificial control, are frequently encountered in practical applications. However, most current studies investigating the optical properties of coatings use the uniform model (uniform single layer assumption), overlooking the gradient distribution effects. Given the pervasiveness of gradient distributions and the widespread use of the uniform model, it is imperative to evaluate applicability conditions of the uniform model in practical applications. In this work, we comprehensively investigate the quantitative performance of the uniform model in predicting the infrared optical properties of coatings with gradient distributions of particle volume fraction using the superposition T-matrix method. The results show that the gradient distribution of particle volume fraction has a limited impact on the emissivity properties of ${{\rm TiO}_2}$ T i O 2 -PDMS coatings in the midwavelength-infrared (MWIR) and long-wavelength-infrared (LWIR) bands, which validates the uniform model for the gradient coatings with weakly scattering dielectric particles. However, the uniform model can yield significant inaccuracies in estimating the emissivity properties of Al-PDMS coatings with gradient distributions in the MWIR and LWIR bands. To accurately estimate the emissivity of such gradient coatings with the scattering metallic particles, meticulous modeling of the particle volume fraction distribution is essential.
The highly reflective solar radiation of passive daytime radiative cooling (PDRC) increases heating energy consumption in the cold winter. Inspired by the temperature-adaptive skin color of chameleon, we efficiently combine temperature-adaptive solar absorption and PDRC technology to achieve "warm in winter and cool in summer". The temperature-adaptive radiative cooling coating (TARCC) with color variability is designed and fabricated, achieving 41% visible light regulation capability. Comprehensive seasonal outdoor tests confirm the reliability of the TARCC: in summer, the TARCC exhibits high solar reflectance (∼93%) and atmospheric transmission window emittance (∼94%), resulting in a 6.5 K subambient temperature. In the winter, the TARCC's dark color strongly absorbs solar radiation, resulting in a 4.3 K temperature rise. Compared with PDRC coatings, the TARCC can save up to 20% of annual energy in midlatitude regions and increase suitable human hours by 55%. With its low cost, easy preparation, and simple construction, the TARCC shows promise for achieving sustainable and comfortable indoor environments.
Extinction and attenuation by particles in an absorbing host have suffered a long-lasting controversy, which has impeded the physical insights on the radiative transfer in the voids dispersed composite. In this paper, we outline the existing extinction definitions, including an equivalence theorem neglecting the host absorption, the near-field analytical definition neglecting the far-field effects, and the operational way which simulates the actual detector readings. It is shown that, under the independent scattering approximation, the generalized operational definition is equivalent to a recent effective medium method according to the rigorous theory of multiple scattering. Using this generalized extinction, we show the important influences of the host absorption on the void extinction. Specifically, at the void resonance, the extinction cross sections of the small voids can be positive, zero, and even negative, which is regulated quantitively by host absorption. Considering the voids in SiC or Ag, the intriguing properties are verified through the attenuation coefficient calculated by the Maxwell-Garnett effective medium theory. In contrast, the equivalent theorem cannot describe any void resonance structures in the absorbing media. Also, the near-field definition fails to generate negative extinction and cannot thus describe the diminished total absorption by the voids. Our results might provide a better understanding of complex scattering theory in absorbing media.
Surfaces with efficient passive daytime radiative cooling (PDRC) are underpinned by maximizing both solar reflection and thermal radiation to the outer space at no additional energy cost. Despite notable progress, their practical applications are of great challenge due to their complicated fabrication processes, easy contamination and damage, and high costs. Herein, we fabricate a hierarchically designed passive daytime radiative cooling film (HPRF) comprising cost-effective Al2O3 particles and poly(dimethylsiloxane) (PDMS) via a simple phase separation method. The designed film possesses a high solar spectrum reflectance of ∼0.96 and a mid-infrared emittance of ∼0.95, achieving a ∼12.4 °C subambient cooling under direct solar irradiation. This excellent PDRC is due to the efficient Mie scattering of sunlight by hierarchical micro-/nanostructures and selected molecular vibrations of PDMS combined with the phonon polariton resonance of Al2O3 particles, respectively. Moreover, the designed HPRF is accompanied with robust durability endowed by superior self-cleaning, flexibility, and anti-ultraviolet radiation that can present substantial application promises of thermal management in various electronic devices and wearable products.
Recently, there has been growing interest and attention towards daytime radiative cooling. This cooling technology is considered a potentially significant alternative to traditional cooling methods because of its neither energy consumption nor harmful gas emission during operation. In this paper, a daytime radiative cooling emitter (DRCE) consisting of polydimethylsiloxane, silicon dioxide, and aluminum nitride from top to bottom on a silver-silicon substrate was designed by a machine learning method (MLM) and genetic algorithm to achieve daytime radiative cooling. The optimal DRCE had 94.43% average total hemispherical emissivity in the atmospheric window wavelength band and 98.25% average total hemispherical reflectivity in the solar radiation wavelength band. When the ambient temperature was 30°C, and the power of solar radiation was about ${900}\;{{\rm W/m}^2}$ 900 W / m 2 , the net cooling power of the optimal DRCE could achieve ${140.38}\;{{\rm W/m}^2}$ 140.38 W / m 2 . The steady-state temperature of that could be approximately 9.08°C lower than the ambient temperature. This paper provides a general research strategy for MLM-driven design of DRCE.
