Water‐Dispersible Boron Nitride Nanospheres with High Thermal Conductivity for Heat‐Transfer Nanofluids

The low dispersion stability of hexagonal boron nitride ( h ‐BN) materials in aqueous solutions restricts their further application. Functionalization has been demonstrated as a promising way to solve this problem. However, functionalized BN is difficult to disperse and stabilize in water at high concentrations. Herein, we have developed water‐dispersible BN nanospheres without requiring modification or the addition of surfactants. The lower‐temperature heat treatment of the HTC@H 3 BO 3 · C 3 N 6 H 6 (boric acid–melamine‐wrapped carboxyl‐rich hydrothermal carbon sphere) precursors during the pyrolysis process resulted in a high oxygen content on the surface of the BN, which prominently increased the charge on the surface of the BN nanospheres and thus improved the dispersion stability of BN in water. The as‐prepared BN nanospheres, with a diameter of 550 nm, have high yield, high surface area (316.77 m 2 g –1 ), and large pore volume (0.65 cm 3 g –1 ). In particular, the thermal conductivity of the water‐based fluid was significantly enhanced by 55 % at 0.10 vol.‐% of BN nanospheres. Moreover, BN–water nanofluids exhibited excellent long‐term stability. These results suggest that the as‐obtained BN nanospheres have broad prospects in improving the thermal conductivity of water‐based fluids.