Reflectivity of solid and hollow microsphere composites and the effects of uniform and varying diameters

Abstract

While solid and hollow microsphere composites have received significant attention as solar reflectors or selective emitters, the driving mechanisms for their optical properties remain relatively unclear. Here, we study the solar reflectivity in the 0.4-2.4 μm wavelength range of solid and hollow microspheres with the diameter varying from 0.125 μm to 8 μm. SiO2 and TiO2 are considered as low- and high-refractive-index microsphere materials, respectively, and polydimethylsiloxane is considered as a polymer matrix. Based on the Mie theory and finite-difference time-domain simulations, our analysis shows that hollow microspheres with a thinner shell are more effective in scattering the light, compared to solid microspheres, and lead to a higher solar reflectivity. The high scattering efficiency, owing to the refractive-index contrast and large interface density, in hollow microspheres allows low-refractive-index materials to have a high solar reflectivity. When the diameter is uniform, 0.75 μm SiO2 hollow microspheres provide the largest solar reflectivity of 0.81. When the diameter is varying, the randomly distributed 0.5-1 μm SiO2 hollow microspheres provide the largest solar reflectivity of 0.84. The effect of varying diameter is characterized by strong backscattering in the electric field. These findings will guide optimal designs of microsphere composites and hierarchical materials for optical and thermal management systems.