Quasi-Isotropically Thermal Conductive, Highly Transparent, Insulating and Super-Flexible Polymer Films Achieved by Cross Linked 2D Hexagonal Boron Nitride Nanosheets.

Polymer-based thermal management materials (TIMs) show great potentials as TIMs due to their excellent properties, such as high insulation, easy processing, and good flexibility. However, the limited thermal conductivity seriously hinders their practical applications in high heat generation devices. Herein, highly transparent, insulating, and super-flexible cellulose reinforced polyvinyl alcohol/nylon12 modified hexagonal boron nitride nanosheet (PVA/(CNC/PA-BNNS)) films with quasi-isotropic thermal conductivity are successfully fabricated through a vacuum filtration and subsequent self-assembly process. A special structure composed of horizontal stacked hexagonal boron nitride nanosheets (h-BNNSs) connected by their warping edges in longitudinal direction, which is strengthened by cellulose nanocrystals, is formed in PVA matrix during self-assembly process. This special structure makes the PVA/(CNC/PA-BNNS) films show excellent thermal conductivity with an in-plane thermal conductivity of 14.21 W m-1 K-1 and a through-plane thermal conductivity of 7.29 W m-1 K-1 . Additionally, the thermal conductive anisotropic constants of the as-obtained PVA/(CNC/PA-BNNS) films are in the range of 1 to 4 when the h-BNNS contents change from 0 to 60 wt%, exhibiting quasi-isotropic thermal conductivity. More importantly, the PVA/(CNC/PA-BNNS) films exhibit excellent transparency, super flexibility, outstanding mechanical strength, and electric insulation, making them very promising as TIMs for highly efficient heat dissipation of diverse electronic devices.

Boron nitride nanosheets as improved and reusable substrates for gold nanoparticles enabled surface enhanced Raman spectroscopy.

Atomically thin boron nitride (BN) nanosheets have been found to be excellent substrates for noble metal particles enabled surface enhanced Raman spectroscopy (SERS), thanks to their good adsorption of aromatic molecules, high thermal stability and weak Raman scattering. Faceted gold (Au) nanoparticles have been synthesized on BN nanosheets using a simple but controllable and reproducible sputtering and annealing method. The size and density of the Au particles can be controlled by sputtering time, current and annealing temperature etc. Under the same sputtering and annealing conditions, the Au particles on BN of different thicknesses show various sizes because the surface diffusion coefficients of Au depend on the thickness of BN. Intriguingly, decorated with similar morphology and distribution of Au particles, BN nanosheets exhibit better Raman enhancements than silicon substrates as well as bulk BN crystals. Additionally, BN nanosheets show no noticeable SERS signal and hence cause no interference to the Raman signal of the analyte. The Au/BN substrates can be reused by heating in air to remove the adsorbed analyte without loss of SERS enhancement.

Quasi-isotropically thermoconductive, antiwear and insulating hierarchically assembled hexagonal boron nitride nanosheet/epoxy composites for efficient microelectronic cooling.

Herein, Pebax functionalized h-BNNSs (P-BNNSs) fabricated by a mechanical exfoliation and in-situ modification process are employed to improve the thermal conductivity and antiwear performance of epoxy resin (EP). Pebax can effectively improve the dispersibility of P-BNNSs, achieving hierarchical assembly of P-BNNSs in EP matrix during EP curing process to form a multinetwork structure only at a low P-BNNS filling contents (≤6 wt%). This multinetwork structure can act as excellent heat conductive pathways to realize simultaneously vertical and horizontal heat diffusion, obtaining quasi-isotropical thermal conductive P-BNNS/EP composites. Fascinatingly, a through-plane thermal conductivity of 3.9 W/(m·K) and an in-plane thermal conductivity of 2.9 W/(m·K) are obtained. More importantly, this special structure can simultaneously improve the antiwear, mechanical and electrically insulating performances of pure EP. The friction coefficients and wear rates of P-BNNS/EP composites (P-BNNS contents ≤ 6 wt%) are dramatically decreased to less than 0.2 and 1 × 10-5 mm3/(N·m), comparing with those of pure EP which are over 0.6 and 2 × 10-5 mm3/(N·m), respectively. The enhanced tensile stress of over 110 MPa and electric volume resistivity of over 1.50 × 1013 Ω·cm are also observed for P-BNNS/EP composite films. These improved properties make the P-BNNS/EP composites very promising as packaging or heat dissipation materials in the high density integration systems and high frequency printed circuit boards.

Friction Behavior and Structural Evolution of Hexagonal Boron Nitride: A Relation to Environmental Molecules Containing -OH Functional Group.

Sliding contact experiments and first-principles calculations were performed to elucidate the roles of environmental molecules containing -OH functional groups on the friction behavior and structural evolution of hexagonal boron nitride (h-BN). A significant decrease in the friction coefficient (COF) is established by the physisorption and dissociative adsorption of molecules containing -OH functional groups on h-BN, compared with that in a H2 or N2 atmosphere. A key finding is the existence of two friction mechanisms to reconstruct the sliding interface for h-BN crystallites in humid air and carbon contaminant (CH3OH and C2H5OH) atmospheres, which is verified by the friction behavior and morphologies of the wear track. There is a running-in period in the friction process to induce the formation of defects in h-BN in humid air, which facilitates dissociative adsorption of water molecules on h-BN. The formation of nanostructured water at defect sites will promote lamellar slip of h-BN crystal materials for friction reduction. In carbon contaminant environments, both molecules exhibit strong adsorption on the h-BN surface regardless of the presence of defects, thereby weakening the structural damage rate and enhancing the bearing capacity. C2H5OH molecules are more likely to dissociate and bind onto defect sites, endowing h-BN with high in-plane stress to form a coiled structure. h-BN in the annular or tubular form would exhibit a self-protective effect, facilitate incommensurate contact, and reduce the contact area to enhance lubrication. Our work may establish the fundamental basis for future applications of h-BN in new energy vehicles.

High Loading Capacity and Wear Resistance of Graphene Oxide/Organic Molecule Assembled Multilayer Film.

Taking advantage of the strong charge interactions between negatively charged graphene oxide (GO) sheets and positively charged poly(diallyldimethylammonium chloride) (PDDA), self-assembled multilayer films of (GO/PDDA)n were created on hydroxylated silicon substrates by alternating electrostatic adsorption of GO and PDDA. The formation and structure of the films were analyzed by means of water contact angle measurement, thickness measurement, atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). Meanwhile, tribological behaviors in micro- and macro- scale were investigated by AFM and a ball-on-plate tribometer, respectively. The results showed that (GO/PDDA)n multilayer films exhibited excellent friction-reducing and anti-wear abilities in both micro- and macro-scale, which was ascribed to the special structure in (GO/PDDA)n multilayer films, namely, a well-stacked GO-GO layered structure and an elastic 3D crystal stack in whole. Such a film structure is suitable for design molecular lubricants for MEMS and other microdevices.

Boron Nitride Nanosheet Dispersion at High Concentrations.

There is an increasing demand for boron nitride nanosheets (BNNSs) for a range of applications such as advanced composite materials, ion/gas selective membranes, and energy storage. These applications require stable, high-concentration BNNS dispersions as a precursor, which is a challenge because BNNSs do not disperse easily. We report a simple, yet efficient, mechanochemical exfoliation technique to prepare functionalized BNNSs with excellent dispersibility in water and organic solvents. The resultant amino-modified BNNSs (BNNS-NH2) are stable in ethanol for 3 months at an unprecedented high concentration of 46 ± 2 mg/mL. We provide insights into the dispersibility mechanism for amino- and hydroxyl-functionalized BNNSs. High-concentration BNNS dispersions enable a facile painting method that can coat a uniform, insulating, and antioxidant BNNS layer on arbitrary surfaces. In addition, different functional groups enhance the selectivity of different ions of the functionalized BNNS membranes for water purification and other ion separation applications. These stable, high-concentration BNNS dispersions make many exciting applications possible.

Three-dimensional network-like structure formed by silicon coated carbon nanotubes for enhanced microwave absorption.

The rapid development of electronic technology generates a great deal of electromagnetic wave (EMW) that is tremendously hazardous to environment and human health. Correspondingly, the high efficient EMW absorption materials with lightweight, high capacity and broad bandwidth are highly required. Herein, a series of three-dimensional (3D) network-like structure formed by silicon coated carbon nanotubes (NW-CNT@SiO2) are massively prepared through an improved sol-gel process. The as-obtained 3D NW-CNT@SiO2 exhibit low densities of about 1.6 ± 0.2 g/cm3. The formation of this special 3D structure can provide high dielectric loss and good impedance matching for EMW absorption. As expected, a minimum reflection loss (RL) of -54.076 dB is obtained when uses the sample prepared by 0.1 g of CNTs and 0.2 mL of tetraethoxysilane as absorbent with a low loading rate of 10 wt% and thin absorber thickness of 1.08 mm. This specific minimum RL value exceeds many other CNT based EMW absorbers reported in previous literature. These findings featured with a green and scalable preparation process provides a facile strategy to design and fabricate high-performance EMW absorption materials, which can be applied to other materials such as carbon fibers and graphene.

Facile fabrication of Hildewintera-colademonis-like hexagonal boron nitride/carbon nanotube composite having light weight and enhanced microwave absorption.

Hildewintera-colademononis-like hexagonal boron nitride carbon nanotubes (BN@CNT) composites can be fabricated via two steps: a composite structure predesign in a solvent and a subsequent thermal treatment process at high temperature. The as-obtained hildewintera-colademononis-like BN@CNT composites contain porous h-BN microrods as stems and CNTs as spines. The densities and specific surface area of these BN@CNT composites can be tuned by adjusting the relative amounts of CNTs in the composites, which can reach 0.072 ± 0.0046 g/cm3 and 583.63 m2/g, respectively. These BN@CNT composites based absorbers show excellent microwave absorption (MA) properties which have effective frequency absorption width (≤-10 dB) from 2.8 to 18 GHz when the absorber thicknesses are in the range of 1.0-6.0 mm, and the minimum RL values can reach up to -48.45 dB for BN@CNTs-3 based absorber with an absorber thickness only of 1.4 mm. Moreover, the widest absorption bandwidth of 4.24 GHz (12.96-17.20 GHz) can be obtained for BN@CNTs-2 based absorber when the absorber thickness is 1.6 mm. Therefore, these hildewintera-colademononis-like BN@CNT composites are expected to be used as microwave absorption materials as they are lightweight and have broad absorption bands and strong absorption with thin thickness. This facile and controllable fabrication process offers a new strategy for designing and fabricating diverse h-BN/carbon based composites for different applications.

Facile fabrication of carbon microspheres decorated with B(OH)3 and α-Fe2O3 nanoparticles: Superior microwave absorption.

We demonstrate that novel three-dimensional (3D) B(OH)3 and α-Fe2O3 nanoparticles decorated carbon microspheres (B(OH)3/α-Fe2O3-CMSs) can be fabricated via a facile thermal treatment process. The carbon microspheres with diameter of 1-3µm and decorated B(OH)3 and α-Fe2O3 nanoparticles with diameters of several to tens of nanometers are successfully fabricated. These novel 3D B(OH)3/α-Fe2O3-CMS composites exhibit enhanced microwave absorption with tunable strong absorption wavebands in the frequency range of 2-18GHz. They have a minimum reflection loss (RL) value of -52.69dB at a thickness of 3.0mm, and the effective absorption bandwidth for RL less than -10dB is as large as 5.64GHz. The enhanced microwave absorption performance arises from the synergy of the impedance matching caused by the B(OH)3 nanoparticles, dielectric loss as well as the enhancement of multiple reflection among 3D α-Fe2O3 nanocrystals. These results provide a new strategy to tune electromagnetic properties and enhance the capacity of high-efficient microwave absorbers.

Selective separation of oil and water with mesh membranes by capillarity.

The separation of oil and water from wastewater generated in the oil-production industries, as well as in frequent oil spillage events, is important in mitigating severe environmental and ecological damage. Additionally, a wide arrange of industrial processes require oils or fats to be removed from aqueous systems. The immiscibility of oil and water allows for the wettability of solid surfaces to be engineered to achieve the separation of oil and water through capillarity. Mesh membranes with extreme, selective wettability can efficiently remove oil or water from oil/water mixtures through a simple filtration process using gravity. A wide range of different types of mesh membranes have been successfully rendered with extreme wettability and applied to oil/water separation in the laboratory. These mesh materials have typically shown good durability, stability as well as reusability, which makes them promising candidates for an ever widening range of practical applications.