AVS 64th International Symposium & Exhibition
    Thin Films Division Thursday Sessions
       Session TF+MI-ThA

Paper TF+MI-ThA9
Microsphere-Based Disordered Coatings for Effective Radiative Cooling

Thursday, November 2, 2017, 5:00 pm, Room 20

Session: Control, Characterization, and Modeling of Thin Films II
Presenter: Sarun Atiganyanun, University of New Mexico
Authors: S. Atiganyanun, University of New Mexico
J. Plumley, University of New Mexico
K. Hsu, University of New Mexico
J. Cytrynbaum, Williams College
T. Peng, Air Force Research Laboratory
S.M. Han, University of New Mexico
S.E. Han, University of New Mexico
Correspondent: Click to Email
Being able to cool the buildings below the ambient temperature under the sun in the middle of a summer without having to use air conditioning would result in tremendous energy savings. As a step towards this goal, we have investigated a facile application of coatings made of silica microspheres in disordered structures, using evaporation as well as spray-coating. For the evaporation coating, silica microspheres are dispersed in water, and the colloidal stability is disrupted by dissolving ionic salt into the solution. The colloidal solution is confined onto a substrate and is allowed to evaporate. For the spray-coating, much like commercial painting, the aqueous colloidal solution is forced through a spray nozzle and deposited onto a substrate. Scanning electron microscopy images and autocorrelation analyses show that the resulting structures are disordered without short- or long-range order. Optical measurements also indicate that the coatings produced under optimal conditions have a short transport photon mean free path of approximately 4-8 μm in the solar spectral region. These coatings exhibit high emissivity above 95% in the atmospheric transparency window. These results suggest strong photon scattering properties in the visible region, while providing a strong thermal emission. Such films would enable effective radiative cooling. To estimate the theoretical limit, a computational model is first used to calculate the cooling power of the coatings under direct sunlight. The model predicts that the disordered coating with 200 μm thickness has a cooling power of ~250 W/m2 at 27ºC and could reduce the temperature of the sample under the direct sunlight by approximately 37ºC below the ambient temperature. Our experimental measurements under direct sunlight show that our coatings perform better than commercial sunlight and heat reflective paints. We will further discuss how coatings of disordered, random, inverse structures can enhance the durability of our coating in a paint format, while maintaining radiative cooling properties.