Bio-Inspired CACO³ Nanocomposite for Efficient Radiative Cooling

Passive radiative cooling favors the transfer of energy to the deep space (2.7K) by emitting in the transparent atmosphere region (8-13m) and reflecting incoming solar irradiation. To achieve desired daytime or night time cooling performance, scientists have explored various fine-tuned photonic material combinations and layering techniques. However, the high cost, UV absorption or telecommunication interferences due to the metallic material used. Scalable and low-cost nonmetal materials have been studied, but the absorption in the UV range still remains a limitation. Single crystal CaCO_3was found to be highly reflective in the UV range, but it has not been explored for radiative cooling applications yet. In this work we first studied the reflectance in the solar range of seashells of multi-millimeters thick, and found over 70% reflectance. Inspired by this promising result, we fabricated a bio-inspired material — CaCO_3 acrylic nanocomposite, and optimized the nanoparticle size to most strongly reflect the sunlight. We analyzed its performance using Mie Theory and Monte Carlo Simulation for multiple size distribution with dependent scattering correction. The results are in excellent agreement with the experimental data. With 60% volume concentration, the simulation results showed that the total solar reflectance of CaCO_3 can achieve up to 97% . Insights obtained from this work will aid researchers in selecting economical, scalable, and manufacturable materials for radiative cooling applications.