Subambient passive cooling enabled by hydroxyapatite powder formulated into a paint

As concerns towards climate change grow, more sustainable ways to help preserve our standard of living have become an increasingly important topic of conversation in our society. One topic of conversation which has grown in recent years is over diurnal and nocturnal radiative cooling, since the ability to passively cool down structures without added energy input has some obvious benefits towards bringing about a more sustainable world. Previous research into this field has yielded numerous potential candidates, such as dielectric nanophotonic structures, photonic polymers, and delignified wood. Unfortunately, many of these materials have serious issues which hamper their ability to be used on a commercial scale, ranging from high build costs to degradation under direct sunlight. One material that has shown promise, not just in passive cooling, but also in solving some of the issues referenced above is hydroxyapatite (??10(??)2(??4)6). Hydroxyapatite (HAP) is an abundant, inexpensive, biodegradable, and environmentally friendly inorganic compound derived from the calcium phosphate family. It has previously been shown to exhibit strong radiative cooling properties when manufactured into a paper-like material, but little research has been conducted into the effects of using HAP to formulate a paint with radiative cooling properties. This thesis changes that by reporting on the spectral and physical properties of formulating a paint from HAP. This thesis demonstrates that HAP powder when mixed with mineral spirits can produce a paint-like substance which has an average solar reflectivity of 0.99 in the visible light spectrum, an average solar reflectivity of 0.85 between a wavelength range of 300 to 2500 nm, and an average emissivity of 0.91 within the atmospheric window. All of which suggests that a paint with strong radiative cooling properties can indeed be formulated using HAP as a white pigment.--Author's abstract