Scalable anisotropic cooling aerogels by additive freeze-casting

Abstract

Cooling in buildings is vital to human well-being but inevitability consumes significant energy, adding pressure on achieving carbon neutrality. Thermally superinsulating aerogels are promising to isolate the heat for more energy-efficient cooling. However, most aerogels tend to absorb the sunlight for unwanted solar heat gain, and it is challenging to scale up the aerogel fabrication while maintaining consistent properties. Herein, we develop a thermally insulating, solar-reflective anisotropic cooling aerogel panel containing in-plane aligned pores with engineered pore walls using boron nitride nanosheets by an additive freeze-casting technique. The additive freeze-casting offers highly controllable and cumulative freezing dynamics for fabricating decimeter-scale aerogel panels with consistent in-plane pore alignments. The unique anisotropic thermo-optical properties of the nanosheets combined with in-plane pore channels enable the anisotropic cooling aerogel to deliver an ultralow out-of-plane thermal conductivity of 16.9 mW m−1 K−1 and a high solar reflectance of 97%. The excellent dual functionalities allow the anisotropic cooling aerogel to minimize both parasitic and solar heat gains when used as cooling panels under direct sunlight, achieving an up to 7 °C lower interior temperature than commercial silica aerogels. This work offers a new paradigm for the bottom-up fabrication of scalable anisotropic aerogels towards practical energy-efficient cooling applications.

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

“…The strong and stiff BNNS fillers effectively reinforced the WPU cell walls, giving rise to less volume shrinkage during the freeze-drying process which in turn resulted in lower densities of composite aerogels than their WPU counterparts. 8 This ameliorating functional feature came together with the potentially enhanced structural robustness of cell walls arising from the better aligned cell walls and smaller pore sizes with BNNSs (Fig. 2j and k) than the WPU aerogel acting alone (Fig.…”

“…The anisotropic k values predicted at a porosity of 98% agreed well with the experimental trends observed in this work and a previous study. 8 It is noteworthy that the predictions from the isotropic model using eqn (1)–(3) manifested monotonic increases in k with the increasing BNNS loading along both directions (blue dashed lines in Fig. 3d and e), unable to accurately reflect the anisotropic k .…”

“…To minimize the heat exchange between external and internal environments, advanced thermally insulating materials, such as gas-filled panels and aerogels, have been developed to achieve low thermal conductivities ( k ). 2,3 Significant research efforts 4–8 have also been devoted to developing superinsulating, anisotropic aerogels with k lower than that of air ( k air = 24 mW m −1 K −1 ). Thanks to the high k in the pore alignment direction ( k align ) for effective heat dissipation, the heat transfer in the through-thickness direction ( k trans ) of anisotropic aerogels can be significantly reduced, exhibiting a better thermal management capability than their isotropic counterparts.…”

Recent progress in organic-based radiative cooling materials: fabrication methods and thermal management properties

“…The large band gap of h-BN corresponds to the absorption of light with a wavelength less than 250 nm, meaning it is conducive to the low low solar heat gain. 96 Chan et al 97 used 2D boron nitride nanosheets (BNNS) exfoliated from bulk h-BN powders and additive freeze-casting technique with waterborne polyurethane (WPU) as matrix to prepare decimeter-scale aerogel panel with consistent planar inner pore arrangement (Fig. 7e–g).…”

Metal oxide cluster-assisted assembly of anisotropic cellulose nanocrystal aerogels for balanced mechanical and thermal insulation properties

“…9 Moreover, the intrinsic abundant pore walls facilitate heat reflection and refraction, greatly suppressing the thermal radiation effects. 10 The heat transport from gas is usually regarded as the predominant thermal contribution in aerogels. 11 When the pore dimension is below the mean free path of air molecules (50 nm, at room temperature and atmospheric pressure), thermal conduction resulting from the collision of the air molecules could be significantly reduced.…”