Liquid Metal-Assisted High-Efficiency Exfoliation of Boron Nitride for Electrically Insulating Heat-Spreader Film

Abstract

Boron nitride nanosheets (BNNSs) are regarded as promising two-dimensional materials in thermally conductive yet electrically insulating applications. Attributed to the strong interlayer “lip–lip” interactions in bulk hexagonal boron nitride (h-BN), high-efficiency exfoliation and scalable fabrication of BNNSs via the top-down strategies remain formidable challenges. Herein, an interesting observation is manifested that gallium-based liquid metal (LM) forming robust coordination interactions with h-BN helps reduce the lip–lip interlayer interactions and thus facilitates successful exfoliation under intense shearing force. For example, employing the ball-milling technique, the BNNS yield can increase to 41.21% with the assistance of LM at only 2 h milling time. Its exfoliation efficiency (yield/time) reaches as high as 26.72%/h, more than 2-fold that of other previously reported methods, including sonication and other ball-milling methods. Moreover, the exfoliated BNNSs are still found to be highly electrically insulating with a band gap of 4.65 eV, showing prospective potential in thermally conductive yet electrical insulating applications. As a proof of concept, a microwave-transparent heat spreader (cellulose nanofiber/BNNSs) is fabricated and verified for applications in high-frequency thermal-management fields.

Methods for Preparation of Hexagonal Boron Nitride Nanomaterials

“…219 To date, the types of liquid media have been expanded to include sodium hypochlorite, 220 urea/ water solution, 221 isopropanol and ethylene glycol, 222 tannic acid aqueous, 223,224 polylactide, 225 and liquid metal. 226 In addition to liquid media, dry ball milling, where bulk h-BN is milled with solid media such as NH 4 Cl, 227 red phosphorus, 228 NaOH, 229 dry ice, 230 xylitol crystals, 231 polyethylenimine, 232 H 3 BO 3 , 233 and sugar, 234 has also been developed for the exfoliation production of BNNSs. Among various additives considered, H 3 BO 3 and sugar are noncorrosive, easily recyclable, and nontoxic choices, exhibiting high yields of 89% and 87.3%, respectively.…”

“…Zhou et al developed a ball milling method assisted by a viscous solvent (glucose syrup) to prepare BNNSs (Figure g), which increased the exfoliation energy and reduced the impact energy, minimizing the damage caused by two-dimensional crystal impact forces and achieving a high exfoliation yield of 74% for producing BNNSs with high aspect ratios in large-scale production of high-quality two-dimensional materials (Figure h–i) . To date, the types of liquid media have been expanded to include sodium hypochlorite, urea/water solution, isopropanol and ethylene glycol, tannic acid aqueous, , polylactide, and liquid metal . In addition to liquid media, dry ball milling, where bulk h -BN is milled with solid media such as NH 4 Cl, red phosphorus, NaOH, dry ice, xylitol crystals, polyethylenimine, H 3 BO 3 , and sugar, has also been developed for the exfoliation production of BNNSs.…”

Self-standing boron nitride bulks enabled by liquid metals for thermal management

“…This phenomenon indicated that the bonding interactions between LM and BN platelets might be a coordination bond, where the empty orbital of LM was the electron acceptor and the lone pairs of electrons of the N atom were the electron donors. 46,49 More specifically, the formation energy (E f ) of the BN@LM mixture was calculated using the formula E f = E BN@LM À (E BN + E LM ), where E BN@LM , E BN , and E LM denote the energies of the BN@LM, BN and LM, respectively. Formation energy is the energy released by the synthesis of compounds from the corresponding monomers, and can be used to characterize the ease of compound formation.…”

“…As shown in Fig.3(i), only one fitted peak (397.7 eV) was observed from the N1s XPS spectra of BN platelets, which was assigned to the N-B bond. In comparison to BN platelets, two newly fitted peaks at 397.4 eV and 396.1 eV appeared in the N1s spectra of the LM@BN bulks, which implied the existence of bonding interaction between LM and N atoms (Fig.3(j)) 46.…”

Thermal conductivity and closed-loop recycling of bulk biphenyl epoxy composites with directional controllable thermal pathways

“…18 Among nonspherical llers, due to the superior stability, chemical resistance, and electrical insulation, BN has stimulated the interest of researchers. 19,20 However, because of the anisotropy and poor compatibility with the matrix, the reported isotropic thermal conductivity of polymer-based composites embedded with BN by simple blending is in the range from 0.5 W m −1 K −1 to 3 W m −1 K −1 . [21][22][23] For instance, Gu et al prepared polyphenylene sulde composites embedded with functionalized hexagonal nanometer boron nitride llers (f-nBN).…”