Porous Polymers with Switchable Optical Transmittance for Optical and Thermal Regulation

Authors: J. Mandal, M. Jia, A. Overvig, Y. Fu, E. Che, N. Yu, Y. Yang.

Journal Link: Joule 3, 1-12 (2019)

Download Paper + Supporting Information (accepted version)

Summary: This work shows that porous polymers, which normally scatter light and appear white due to the air voids in them, can be turned transparent or translucent by wetting them with suitable liquids. The idea itself has been known for as long as humans have noticed paper or cloth turn more translucent when wetted. What I try is to push it to a limit and use it for switchable cooling/heating applications.

The key to achieving a white to near-transparent switching is choosing the right liquid. In its porous form, the polymer contains air voids which have a different refractive index (n~1) from that of the polymer (n~1.4-1.5), causing light to scatter off the pores and yield the white colour. But if the pores are filled with a liquid that has the same refractive index as the polymer, then to light the whole system just behaves like one uniform material, so light transmits, as it would through glass.

We achieve this behaviour by using common materials and liquids, and also extend the switching behavior to thermal infrared wavelengths. Promisingly, the switching in the thermal is opposite to that in the solar wavelengths, meaning that porous polymers can switch from icehouse to greenhouse states – something that has not been observed with electrochromic or other switchable designs as far as I know.

With regard to applications, we show that this behavior can be used for switchable heating and cooling of buildings depending on the season, controlling daylight in buildings, thermal camouflage and other uses. Given the low cost and simplicity of the designs we use, they could potentially see large scale uses.

Li4Ti5O12: A Visible-to-Infrared Broadband and Fast Electrochromic Material for Thermal and Optical Management

Authors: J. Mandal, S. Du, M. Dontigny, K. Zaghib, N. Yu, Y. Yang.

Journal Link: Advanced Functional Materials 28 (36), 201802180 (2018).

Download Paper and Supporting Information (Accepted versions)

Summary: In this work, we looked at Lithium Titanium Oxide (Li4Ti5O12), a promising material for battery anodes which  is known to change from white to black when it is charged. We investigated the optical properties of Li4Ti5O12 across the visible to LWIR wavelengths (i.e. 0.4-14 µm), and found that the transition from white to black actually stretches all the way from the visible to the LWIR. We explore the potential physical mechanisms of the switching, and demonstrate that it can be used for radiative cooling and heating, and thermal camouflage.

Note: the original idea of investigating the Li4Ti5O12 was not mine, but that of my doctoral adviser, Dr. Yuan Yang. He had the hunch that the optical switching of the material can extend into the infrared, and asked me to optical characterize samples he had prepared. After confirming his hypothesis, I explored the physics of the optical transition, and the optical design aspects with the goal of achieving radiative cooling and thermal camouflage.