Three-Dimensionally Printable Ultrawhite Radiative Cooler Using Hollow Structured Nanoparticles

Owing to the global consensus on energy-saving, radiative cooling has attracted much interest as a passive thermal management technology. Technically, to cool down objects below ambient temperature, radiative coolers must exhibit low absorption under solar illumination (e.g. l = 0.3 – 2.5 mm ) and high emission in the mid-infrared spectral range (e.g. l = 8 – 13 mm ). It can be achieved by stacking several materials or using a nanoscale photonic structure, which bring an increase in cost and fabrication difficulty.<br/>We use dielectric nanoparticles as an alternative while maintaining the optical properties for sub-ambient radiative cooling. We demonstrated near-perfect reflectance in ultraviolet to near-infrared spectral range using hollow structured silica nanoparticles (HSNPs), owing to the enhanced scattering characteristics of the hollow structure. Note that low absorption and strong reflection in ultraviolet and near-infrared wavelengths are difficult to achieve with conventional silica nanoparticles. HSNPs coated Si shows an average of 0.96 reflectance in the solar spectral range and an average of 0.95 emissivity in the mid-infrared spectral range. We conducted an outdoor cooling experiment on Si wafers. The HSNPs coated Si, covered 8.89% of surface area of full Si wafer, show 4.4°C temperature drop in comparison to bare one.<br/>But one critical issue remains for printability; hollow structured nanoparticles aggregate each other during the solvent evaporation (the final step of printing sequences) and leads to a crack on its surface that diminishes the overall reflectance and durability. We used PVP (Polyvinylpyrrolidone) to prevent aggregation. The PVP molecules are attached to the surface of the HSNPs and impede free motion to fix their coordinates rigidly. The crack-resistant property of the PVP assisted HSNPs paste holds the promise of implementing three-dimensionally printed devices with high precision, such as serpentine-shaped electrodes. In addition, spray coater also can be used to sprinkle HSNPs on a large area due to its great uniformity. We believe that the printable HSNPs developed herein can be extended to a variety of components on arbitrarily designable platforms such as curved or flexible optoelectronic devices.