Rod-like boron nitride based-coating for subambient and above-ambient passive radiative cooling

Passive radiative cooling (PRC) has the potential to reduce energy demand and mitigate global warming. Polymer-dielectric radiative cooling coatings have been widely studied because of their strong emittance as well as simplicity and low cost of preparation, while the low thermal transmission has restricted their capability of radiative cooling under elevated temperature conditions. In this paper, PRC coatings consisting of highly thermally conductive boron nitride (BN) particles with different morphologies were developed by a simple drop- coating method with acrylic matrix for subambient and above-ambient passive radiative cooling. Comparing plate-like BN based-coating (PBNC) and flower-like BN based-coating (FBNC), the rod-like BN based-coating (RBNC) exhibited the optimum cooling performance, with high solar reflectance (90.7 %) and superior mid- infrared emittance (90.9 %), achieving subambient cooling of 9.71 degrees C. More importantly, RBNC with high thermal conductivity demonstrated superior above-ambient cooling performance, by reducing the substrate heater temperature in sunlight of approximately 13.76 degrees C and 8.61 degrees C, respectively, compared to polymethyl methacrylate and acrylic coatings. This work offers an idea to regulate the morphology of BN particles for the current material design in the field of passive radiative cooling, and opens up a new way for the cooling in subambient and above-ambient applications.