In the development of hexagonal boron nitride (h-BN)-based polymeric composites with high thermal conductivity, it is always challenging to achieve a dense filling of h-BN fillers to form a desired high-density thermal transfer network. Here, a series of boron nitride nanosheets (BNNSs)/epoxy resin (EP) bulk composites filled with ultrahigh BNNSs content (65−95 wt %) is successfully constructed through a well-designed mechanical-balling prereaction combined with a general pressure molding method. By means of this method, the highly filled BNNSs fillers are uniformly dispersed and strongly bonded with EP within the composites. As a result, the densely BNNSs-filled composites can exhibit multiple performances. They have excellent mechanical properties, and their maximum compression strength is 30−97 MPa. For a BNNSs/EP composite with filling ultrahigh BNNSs fraction up to 90 wt %, its highly in-plane thermal conductivities (TC) are 6.7 ± 0.1 W m −1 K −1 (at 25 °C) to 8.7 ± 0.2 W m −1 K −1 (200 °C), respectively. In addition, the minimum coefficient of thermal expansion of BNNSs/EP composites is 4.5 ± 1.3 ppm/°C (only ∼4% of that of the neat EP), while their dielectric constants are basically located between 3−4 along with their dielectric loss tangent values exceptionally <0.3 in the ultrahigh frequency range of 12−40 GHz. Additionally, these BNNSs/EP composites exhibit remarkable cycle stability in heat transfer during heating and cooling processes because of their structural robustness. Thus, this type of densely BNNSs-filled BNNSs/ EP composite would have great potential for further practical thermal management fields.

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Advanced heat dissipation materials are necessary for powerful and miniaturized electronics. Hexagonal boron nitride (h-BN) is an ideal material for thermal management due to its electrical insulation but thermal conductivity. However, its utilization is seriously restricted by the pulverulence, and multilayer and chemical inertness. Herein, highly functionalized BN nanosheets (BNNSs) are exfoliated by a boric acid-assisted chemo-mechanical method and show stable dispersion in various solvents. An ingenious chemical etch-and mechanical forceinduced exfoliation and functionalization mechanism is proposed. Due to the existence of strong interfacial interaction between BNNSs and poly(vinyl alcohol) (PVA), a series of composite films with excellent thermal stability, low dielectric constant (2−4), negligible dissipation factor (<0.2), good machinability, and flexible bending and folding performances are prepared. The BNNSs' filling content can reach ∼90 wt %, and the BNNSs/PVA films' maximum tensile strength and modulus can be as high as ∼85.5 MPa and ∼7.6 GPa, respectively. Importantly, their in-plane thermal conductivity increases monotonously with the elevated BNNS content and temperature. The maximum thermal conductivity can reach up to 27.3 and 39.3 W•m −1 •K −1 at 25 and 100 °C, respectively. Noteworthily, the BNNSs/ PVA films show rapid thermal diffusion and can be readily designed as efficient heat dissipation components for integrated and intelligent electronic devices.

Turbostratic and oxygen doping (3.7 atom %) hexagonal boron nitride nanosheets (TO-BNNSs) with abundant defect sites, were synthesized by pyrolyzing the mixture of melamine cyanurate and boric acid. Systematic analyses reveal a highly disordered structures and covalent oxygen-doping in the TO-BNNSs. These features endow the product with increased unpaired electrons, localized charge asymmetry and spin polarization. While compared with bulk h-BN, the optical bandgap of TO-BNNSs drop down to ∼5.2 from ∼5.7 eV, dielectric constant raised from ∼2.1 to ∼2.4, the saturation magnetic moment increased from ∼0.011 to ∼0.033 emu g−1, and the coercivity enlarged from ∼73.56 to ∼367.39 Oe. These results suggest that h-BN materials with turbostratic structure and heteroatom-doping have extensive application prospect in the fields of nanoscale optics, electronics and magnetics.

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