Engineering anisotropic structures of thermally insulating aerogels with high solar reflectance for energy-efficient cooling applications

Abstract

Aerogel materials with an anisotropic nature have attracted increasing interest because of fascinating, potential applications arising from their novel functional capabilities. However, the strategy to achieve a highly anisotropic structure of aerogel materials for thermal superinsulation has not been fully exploited. In this work, the microstructure and thermal conductivity (k) of waterborne polyurethane (WPU) aerogels are tailored by using different freezing temperatures ranging from −196 to −20 °C in unidirectional freeze-casting. A more anisotropic porous aerogel is obtained at a lower freezing temperature, yielding highly different k in two orthogonal directions. The anisotropy of WPU aerogels is further intensified with the addition of two-dimensional boron nitride nanosheets (BNNSs) possessing anisotropic k values and a high reflectance. Surprisingly, an ultralow density of 20.2 mg cm−3 is achieved by the composite aerogel freeze-cast at −196 °C, much lower than the WPU matrix acting alone, by creating more pores of smaller sizes. The unique thermo-optical properties of BNNSs in highly aligned cell walls of the BNNS/WPU composite aerogels provide fast heat dissipation in the alignment direction while largely diminishing heat transfer through the thickness direction, achieving an ultralow k of 16.2 mW m−1 K−1 for thermal superinsulation. A new theoretical approach is also proposed to estimate the anisotropic k of porous materials, verifying the positive roles played by BNNSs in efficient thermal management for directional insulation. In addition, the presence of abundant pores and reflective BNNSs is responsible for the excellent solar reflectance (∼97%) of the aerogels. The coupling effect of highly anisotropic k and high sunlight reflectance offers better thermal management under direct sunlight with up to 6 °C lower internal temperature than a commercial SiO2 blanket and expanded polystyrene foam coated with commercial reflective paint in the outdoor test. This work can provide general guidelines for designing and producing highly anisotropic aerogels for more energy efficient cooling applications.