Refractive index and extinction coefficient of hollow microspheres for solar reflection

While hollow microspheres and various porous structures have received much attention for solar reflection in the recent literature, their fundamental determinants of optical properties and material selection criteria are relatively little known. Here, we study hollow microspheres with varying refractive index and extinction coefficient and identify their role in determining the solar reflectivity. Our simulations based on finite-difference time-domain method show the effects of refractive index between 1.5 and 100 and extinction coefficient between 10−6–100 in the wavelength region of 0.2–2.4 μm and explain how the reflectivity of hollow microspheres is attributed to a combination of strong backscattering and limited absorption. Our analysis indicates that ceramic materials with a high refractive index and a low extinction coefficient such as Y2O3 are promising. When Y2O3 hollow microspheres are randomly distributed with the diameter ranging from 0.5 to 1 μm, our simulation shows the solar reflectivity reaches 0.97 even at 300 μm thickness, and a diffusion theory-based model predicts the solar reflectivity to exceed 0.98 at 500 μm or 0.99 at 1 mm thickness. Our findings can guide optimal designs of hollow microspheres and related porous structures toward complete solar reflection and enable breakthroughs in thermal management and deep-space applications.