Enhancing infrared emission behavior of polymer coatings for radiative cooling applications
Meijie Chen, Dan Pang, Xingyu Chen, Hongjie Yan
Abstract
Abstract Radiative cooling can cool objects on Earth without an energy input, which has drawn significant attention recently. While polymers have been widely studied for radiative cooling applications due to their promising infrared emission behaviors, ease of fabrication and low cost, the cooling performance of polymer coatings still needs to be enhanced through structure design. In this work, the effects of polymer materials, coating thicknesses and surface microstructures on the cooling performance of coatings were studied. Results showed that the PDMS coating can achieve a great radiative cooling ability with less materials consumption and the polymer coating can reach a relative high saturation value of thermal emittance ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mover> <mml:mi>ε</mml:mi> <mml:mo>ˉ</mml:mo> </mml:mover> <mml:mrow> <mml:mrow> <mml:mtext>LWIR</mml:mtext> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> ) in the atmosphere’s long-wave infrared transmission window (8–13 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>μ</mml:mi> <mml:mrow> <mml:mtext>m</mml:mtext> </mml:mrow> </mml:math> ) with a large thickness, but interface reflectance in the wavelength range from 8 to 13 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>μ</mml:mi> <mml:mrow> <mml:mtext>m</mml:mtext> </mml:mrow> </mml:math> limits the further enhancement of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mover> <mml:mi>ε</mml:mi> <mml:mo>ˉ</mml:mo> </mml:mover> <mml:mrow> <mml:mrow> <mml:mtext>LWIR</mml:mtext> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> . By introducing a grating-pattern microstructure, the minimal reflectance at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>λ</mml:mi> </mml:math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo>=</mml:mo> </mml:math> 8–13 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>μ</mml:mi> <mml:mrow> <mml:mtext>m</mml:mtext> </mml:mrow> </mml:math> was nearly zero and the low emittance region at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>λ</mml:mi> </mml:math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo>=</mml:mo> </mml:math> 9–11 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>μ</mml:mi> <mml:mrow> <mml:mtext>m</mml:mtext> </mml:mrow> </mml:math> of the bare PDMS coating was greatly enhanced. At the coating thickness of t <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo>=</mml:mo> </mml:math> 200 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mtext>m</mml:mtext> </mml:mrow> </mml:math> , <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mover> <mml:mi>ε</mml:mi> <mml:mo>ˉ</mml:mo> </mml:mover> <mml:mrow> <mml:mrow> <mml:mtext>LWIR</mml:mtext> </mml:mrow> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> and the net nighttime cooling power of the PDMS coating can reach 0.996 and 110.5 W m −2 , respectively, when the ambient temperature is 298.15 K. In addition, the solar reflectance of the PDMS coating can reach 0.946 using the Ag substrate, making it attractive for radiative cooling applications.