Scalable-manufactured randomized glass-polymer hybrid metamaterial for daytime radiative cooling

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Science  10 Mar 2017:
Vol. 355, Issue 6329, pp. 1062-1066
DOI: 10.1126/science.aai7899
  • Fig. 1 Glass-polymer hybrid metamaterial.

    (A) A schematic of the polymer-based hybrid metamaterial with randomly distributed SiO2 microsphere inclusions for large-scale radiative cooling. The polarizable microspheres interact strongly with infrared light, making the metamaterial extremely emissive across the full atmospheric transmission window while remaining transparent to the solar spectrum. (B) Normalized absorption (blue), scattering (red), and extinction (black) cross sections of individual microspheres as functions of size parameter (k0a). The extinction—the sum of the scattering and absorption—peaks at a size parameter of 2.5, which corresponds to a microsphere radius of 4 μm. (Inset) The electric field distributions of two microspheres with 1- and 8-μm diameters, illuminated at a 10-μm wavelength. Scale bar, 4 μm. The smaller microsphere resonates at the electric dipolar resonance, whereas higher-order electric and magnetic modes are excited in the larger microsphere. (C) Angular diagram for the scattering far-field irradiance of an 8-μm-diameter microsphere with 10-μm wavelength illumination. The incident field is polarized along the y direction and propagating along the z direction.

  • Fig. 2 Fröhlich resonance and broadband infrared absorbance of the hybrid metamaterial.

    (A and B) The real (A) and imaginary (B) part of the effective index of refraction for the glass-polymer hybrid metamaterials. The metamaterial with 1-μm-diameter SiO2 microspheres (black curves) shows a strong Fröhlich resonance at its phonon-polariton frequency of 9.7 μm, whereas the metamaterial with 8-μm-diameter microspheres (red curves) shows significantly more broadband absorption across infrared wavelengths. The strong Fröhlich resonance not only limits the bandwidth of strong emissivity but also introduces strong reflectance of incident infrared radiation. In both cases, the metamaterial contains 6% SiO2 by volume. (C) The attenuation lengths of the two hybrid metamaterials, with the 8-μm-diameter SiO2 microsphere case showing an average attenuation length of ~50 μm from λ = 7 to 13 μm.

  • Fig. 3 Spectroscopic response of the hybrid metamaterial.

    (A) Schematic of the hybrid metamaterial backed with a thin silver film. The silver film diffusively reflects most of the incident solar irradiance, whereas the hybrid material absorbs all incident infrared irradiance and is highly infrared emissive. (B) Three-dimensional confocal microscope image of the hybrid metamaterial. The microspheres are visible because of the autofluorescence of SiO2. (C) Power density of spectral solar irradiance [air mass (AM) 1.5] and thermal radiation of a blackbody at room temperature. The sharply varying features of both spectra are due to the absorbance of the atmosphere. The radiative cooling process relies on strong emission between 8 and 13 μm, the atmospheric transmission window. (D) The measured emissivity/absorptivity (black curve) of the 50-μm-thick hybrid metamaterial from 300 nm to 25 μm. Integrating spheres are used for the measurement of both solar (300 nm to 2.5 μm) and infrared (2.5 to 25 μm) spectra. Theoretical results for the same hybrid metamaterial structure (red curves) are plotted for comparison. Two different numerical techniques, RCWA and incoherent transfer matrix methods, are used for the solar and infrared spectral ranges, respectively.

  • Fig. 4 Performance of scalable hybrid metamaterial for effective radiative cooling.

    (A) A photo showing the 300-mm-wide hybrid metamaterial thin film that was produced in a roll-to-roll manner, at a speed of 5 m/min. The film is 50 μm in thickness and not yet coated with silver. (B) A 72-hour continuous measurement of the ambient temperature (black) and the surface temperature (red) of an 8-in-diameter hybrid metamaterial under direct thermal testing. A feedback-controlled electric heater keeps the difference between ambient and metamaterial surface temperatures less than 0.2°C over the consecutive 3 days. The heating power generated by the electric heater offsets the radiative cooling power from the hybrid metamaterial. When the metamaterial has the same temperature as the ambient air, the electric heating power precisely measures the radiative cooling power of the metamaterial. The shaded regions represent nighttime hours. (C) The continuous measurement of radiative cooling power over 3 days shows an average cooling power of >110 W/m2 and a noontime cooling power of 93 W/m2 between 11 a.m. and 2 p.m. The average nighttime cooling power is higher than that of the daytime, and the cooling power peaks after sunrise and before sunset. The measurement error of the radiative cooling power is well within 10 W/m2 (32).

Supplementary Materials

  • Scalable-manufactured randomized glass-polymer hybrid metamaterial for daytime radiative cooling

    Yao Zhai, Yaoguang Ma, Sabrina N. David, Dongliang Zhao, Runnan Lou, Gang Tan, Ronggui Yang, Xiaobo Yin

    Materials/Methods, Supplementary Text, Tables, Figures, and/or References

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    • Materials and Methods
    • Supplementary Text
    • Figs. S1 to S6