Large-surface-area BN nanosheets and their utilization in polymeric composites with improved thermal and dielectric properties
High-throughput few-layered BN nanosheets have been synthesized through a facile chemical blowing route. They possess large lateral dimensions and high surface area, which are beneficial to fabricate effectively reinforced polymeric composites. The demonstrated composites made of polymethyl methacrylate and BN nanosheets revealed excellent thermal stability, 2.5-fold improved dielectric constant, and 17-fold enhanced thermal conductivity. The results indicate multifunctional practical applications of such polymeric composites in many specific fields, such as thermoconductive insulating long-lifetime packaging for electrical circuits.
KeywordsBN nanosheet Polymeric composite Thermal conductivity Dielectric constant
Honeycomb-like mono-/few-layered hexagonal boron nitride (h-BN; also called ‘white’ graphene) is a structural analogue of graphene, which may serve as one of the outstanding representatives of 2D crystals due to its unique physics and diverse functionalities [1, 2]. The robust B-N bonding within a BN layer, even stronger than C-C bonding in graphene, makes mono-/few-layered BN nanosheets highly thermoconductive (ca. thermal conductivity of 100 to 1,000 W/mK), mechanically strong and elastic, and thermally and chemically stable [3, 4]. Partially ionic B-N bonds, different from pure covalent bonds in graphene, make BN nanosheets an intrinsic insulator with a wide band gap (ca. 5.5 eV) valuable for dielectric applications and deep ultraviolet luminescence. Besides, the weak van der Waals bonds out of planes of few-layered BN nanosheets are advantageous for good solid-state lubricants.
Quick heat-releasing and good electrical insulation are required in the packaging materials in high-speed electronics, and polymer materials with a high dielectric constant are attractive for large capacitors and high-k gate in flexible electronics [5, 6, 7]. The standard polymer materials normally have low thermal conductivity. One approach toward preparing highly thermal conductive polymeric materials is to embed fillers with high thermal conductivity, such as traditional silicon nitride, aluminum nitride, and boron nitride microparticles. Nanomaterials are more effective fillers for the so-called nanocomposites due to their developed surfaces and high aspect ratios. The polymers embedded with graphenes or other conductive fillers may exhibit high thermal conductivities and dielectric constants before the percolation threshold ; however, the possible electrical current leakage is undesired. BN materials exhibit good electrical insulation to deal with these drawbacks. Comparing with 0D BN nanoparticles  and 1D BN nanofibers/tubes [10, 11, 12, 13, 14], 2D BN nanosheets maximally expose their basal (002) crystal planes; therefore, the regarded excellent in-plane properties become dominant because both directions parallel to the (002) planes substantially work for phonon transport in a BN sheet. Intrinsic thermal conductivity of BN nanosheets is notably higher than the reported values of AlN powders and BN powders/nanotubes . BN nanosheets are thus envisaged to be one of the best fillers in composites owing to the highly insulating and thermoconductive properties.
Filling of BN nanosheets into polymeric or ceramic composites requires a sufficient mass of BN nanosheets. The current methods, such as mechanical cleavage, solution exfoliation [16, 17, 18], high-energy electron beam irradiation, reaction of boric acid and urea , unwrapping nanotubes , and chemical vapor deposition [20, 21, 22, 23, 24], have been utilized to successfully fabricate BN nanosheets; however, mass quantities of BN nanosheets via those methods are still not available on the market due to many problems in reliable chemical intercalations and exfoliations. Recently, a new strategy, the so-called ‘chemical blowing’, has been developed by us [25, 26], which relies on making large bubbles with atomically thin B-N-H polymer walls starting from a precursor ammonia borane compound (AB, which is relatively cheap) and then annealing polymer bubbles into BN ones having ultra-thin BN walls, i.e., BN nanosheets. Here, we use the regarded chemical blowing technology to prepare large amount of BN nanosheets (gram-level) and reveal their high surface area. Based on the high throughput, high surface area, and unique 2D-crystal properties, the produced BN nanosheets effectively perform as excellent fillers in polymeric composites for improving thermal stability, thermal conductivity, and dielectric properties.
Commercial fresh AB (Sigma-Aldrich Corporation, St. Louis, USA) was first pre-treated at 80°C for 1 h. Using a 8°C/min heating rate, the precursors were heated to 1,300°C to obtain the BN products. The products were characterized by scanning electron microscopy (SEM, Hitachi S-4800, Tokyo, Japan), high-resolution transmission electron microscopy (HRTEM, JEOL JEM-3000F, Tokyo, Japan), atomic force microscopy (AFM, JEOL JSPM-5200), electron energy loss spectroscopy (EELS, attachment in TEM), and nitrogen adsorption-desorption measurements carried out at liquid nitrogen temperature (Quantachrome Autosorb-1, Boynton Beach, FL, USA). Brunauer-Emmett-Teller (BET) surface area was estimated over a relative pressure range of 0.05 to 0.3 P/P0. Pore distributions were analyzed using a Barrett-Joyner-Halenda method.
To fabricate polymer/BN composites, as-grown BN products were dispersed in a polymethyl methacrylate (PMMA)/chloroform solution with a controlled weight ratio. The mixture was then spread on a glass plate; this made the solvent naturally volatilized. The residual solvent was removed by further drying at 50°C. The composites were studied by thermogravimetry (TG; Rigaku Thermo plus TG 8120, Tokyo, Japan) and differential scanning calorimetry (DSC; Rigaku Thermo plus EVO DSC8230), thermoconductivity analysis (thermal constant analyzer of Kyoto Electronics Manufacturing Co., Ltd., Kyoto, Japan) following a hot disk method, and dielectric constant analysis (Wayne Kerr precision component analyzer, West Sussex, UK).
Results and discussion
where λc, λf, and λm are the thermal conductivities of composites, BN fillers, and PMMA matrices, respectively; ϕ is the volume fraction of BN fillers. Parameter C1 relates to structures of polymer matrix; and C2 means the difficulty level in forming conductive chains of fillers. In this fitting, C1 = 0.94; C2 = 3.9. The C2 is generally larger than that of powder fillers, implying an easily constructed thermal conductive path, which results from the large lateral size of BN nanosheets grown using chemical blowing technology (tens of μm lateral size of BN nanosheets dispersed in a solution). Together with the abundant interfaces and strong interfacial interactions, the heat in a polymer matrix can, thus, be easily collected and conducted by BN fillers, resulting in a high thermoconductive of the composites.
To sum up, mass production of meso-/macro-porous large-SSA few-layered BN nanosheets is realized; the sheets have successfully been integrated into PMMA polymeric composites. The outstanding (002)-crystal plane properties and abundant interfaces of BN nanosheets are utilized to increase the thermal stability, thermal conductivity, and dielectric properties of the composites, i.e., 17-fold gained thermal conductivity, 2.5-fold increased dielectric constant, and a 18°C increased glass transition temperature were documented. The fabricated PMMA/BN composite plastics are, thus, envisaged to be valuable for diverse functional applications in many fields, especially for the new-generation thermoconductive insulating long-lifetime packaging materials.
XW is a junior researcher of MANA, NIMS, as well as a Ph.D. candidate of Waseda University (Japan) under the supervision of Prof. YB. He obtained his bachelor and master degrees at Nanjing University. At present, he focuses on the syntheses and applications of 2D sp2 hybrid crystals, such as BN nanosheets and graphenes. CZ is a scientist of MANA, NIMS, and is moving to City University of Hong Kong (China) as an assistant professor. He is an expert in the fields of inorganic nanomaterials and functional polymeric composites. DG and YB are two group leaders and professors in MANA, NIMS.
The authors thank Dr. A. Nukui, Dr. N. Kawamoto, Dr. I. Yamada, and Ms. Y. Hirai for the experimental support, and also the MANA support staff for the technical assistance. Financial support from the WPI-MANA, NIMS, Tsukuba, Japan is gratefully acknowledged.
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