Subambient daytime cooling enabled by hierarchically architected all-inorganic metapaper with enhanced thermal dissipation

Abstract

A hierarchical hydroxyapatite inorganic radiative cooling (HIRC) metapaper achieves efficient radiative cooling and enhanced thermal dissipation to accelerate heat release.

1 . A method of fabricating a hierarchical hydroxyapatite (HAP) inorganic radiative cooling (HIRC) metapaper, comprising the steps of:
 synthesizing HAP fibers using a calcium oleate precursor in a solvothermal process; and   transforming the HAP fibers into a metapaper sheet comprising hierarchically structured HAP fibers.   
     
     
         2 . The method of  claim 1 , further comprising precipitating a stable wool-like HAP fiber suspension using ethanol after the solvothermal process. 
     
     
         3 . The method of  claim 1 , wherein transforming the HAP fibers into a metapaper sheet comprises using a vacuum-assisted suction filtration process to facilitate self-assembly of the HAP fibers into the metapaper sheet. 
     
     
         4 . The method of  claim 3 , wherein the HAP fibers self-assemble in parallel fashion into bundles to form ultralong fibers. 
     
     
         5 . The method of  claim 1 , further comprising baking the metapaper sheet to increase evaporation of ethanol and water. 
     
     
         6 . The method of  claim 1 , wherein the thickness of the sheet can be varied by adjusting the concentration or the mass of the HAP fibers. 
     
     
         7 . The method of  claim 1 , wherein the calcium oleate precursor comprises CaCl 2 ), NaOH, oleic acid, NaH 2 PO 4 ·2H 2 O, ethanol, and deionized (DI) water. 
     
     
         8 . The method of  claim 1 , wherein the solvothermal process comprises a solvothermal reaction carried out at about 180° C. for about 24 hours. 
     
     
         9 . A hierarchical hydroxyapatite inorganic radiative cooling (HIRC) metapaper for radiative cooling and heat dissipation comprising hierarchically structured HAP fibers fabricated using a calcium oleate precursor solvothermal process. 
     
     
         10 . The metapaper of  claim 9 , wherein the metapaper comprises a flexible paper-like sheet. 
     
     
         11 . The metapaper of  claim 9 , wherein the metapaper has a solar reflectance of about 0.99. 
     
     
         12 . The metapaper of  claim 9 , wherein the metapaper has a mid-infrared thermal emittance of about 0.90. 
     
     
         13 . The metapaper of  claim 9 , wherein the metapaper yields a subambient temperature drop of 5.1° C. under solar irradiance of 950 W m −2 . 
     
     
         14 . The metapaper of  claim 9 , wherein the metapaper has a peak radiative cooling power of 104 W m −2  under the solar intensity of 910 W m −2  without polyethylene windshields. 
     
     
         15 . The metapaper of  claim 9 , wherein the metapaper has radiative cooling power of 80 W m −2  under average solar irradiation of 860 W m −2 . 
     
     
         16 . The metapaper of  claim 9 , wherein the metapaper has a thickness of about 500 μm. 
     
     
         17 . The metapaper of  claim 9 , wherein the metapaper is flexible. 
     
     
         18 . The metapaper of  claim 9 , wherein the metapaper comprises nanoscale HAP fibers self-assembled into bundles forming ultralong fibers having a length of about 200 μm with diameters of 1 μm. 
     
     
         19 . The metapaper of  claim 9 , wherein the metapaper has a layer-staggered structure. 
     
     
         20 . The metapaper of  claim 9 , wherein the metapaper has HAP fibers that are self-assembled into bundles and intertwined to form a porous network that backscatters sunlight to provide a high solar reflectance and features strong molecular vibrations of phosphate radicals to provide high thermal emittance.