Epoxy resin-based nanocomposite films with highly oriented BN nanosheets prepared using a nanosecond-pulse electric field Hong-Baek Cho ⁎, Nguyen Chung Tu, Takeshi Fujihara, Shin Endo, Tsuneo Suzuki, Satoshi Tanaka, Weihua Jiang, Hisayuki Suematsu, Koichi Niihara, Tadachika Nakayama ⁎ Extreme Energy-Density Research Institute, Nagaoka University of Technology,1603-1 Kamitomioka, Nagaoka, Niigata 940-2188, Japan abstract article info Article history: Received 19 March 2011 Accepted 2 May 2011 Available online 13 May 2011 Keywords: Anisotropy BN nanosheet Nanocomposites Nanosecond pulse Polyepoxide Facile orientation of boron nitride (BN) with high anisotropy in epoxy resin-based nanocomposite films was performed in a polyepoxide matrix using a nanosecond-pulse electric field to generate a high electric flux. Control of the BN anisotropy was achieved in the polymer without damaging the composite films or requiring surface modification of the BN. The degree of BN orientation perpendicular to the nanocomposite film plane, which was parallel to the electric flux, could be controlled by applying the nanosecond pulse for different lengths of time before cross-linking. The resulting composite films with oriented BN nanosheets manifested improved thermal diffusivity compared to a composite prepared without orientation. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Polymer/nanosheet composites have attracted considerable atten- tion [1–3] because micro- and nano-composites with refined structures can greatly increase performance after proper orientation of nanosheets in the polymer matrix. One-dimensionally aligned nanosheets in a polymer matrix can achieve electrical conductivity at a remarkably low volume fraction, compared to the pristine powder [4,5]. The anisotropic alignment of graphite nanosheets (GN) in a polymer has attracted interest for the fabrication of polymer/GN composites, which can exhibit high thermal and electrical conduc- tivity [3,6,7]. However, because the electrical properties of graphite range from metallic to semiconducting, its application as an electrical insulator is restricted [8]. Boron nitride (BN) is not only an excellent conductor of heat, but is also an electrical insulator, and its thermal conductivity is among the highest of all electrical insulators [9–11]. Hexagonal BN nanosheets have a graphite-like structure, and their thermal conductivity changes with their anisotropy: when the sheets are aligned perpendicular to the c-axis (┴), their thermal conduc- tivity increases by almost 20 times to 6 W cm -1 °C, compared to 0.3 W cm -1 °C for sheets aligned parallel to the c-axis (║ ) [12,13]. An electric field can be applied to form a linear network structure of one- dimensional nanofillers, such as carbon nanotubes, through electro- phoretic and Coulomb attraction in a polymer matrix [14,15]. The orientation of nanosheets in viscous polymers under an applied electric field can be enhanced by modifying the nanosheet surface with a metallic nanopowder, such as iron nanoparticles. For field-induced orientation of BN nanosheets in polysiloxane without surface modifi- cation, a strong electric field is required because of the wide band gap, but the electric field is limited by the low breakdown voltage of the polymer [16]. In this study, a random dispersion of BN nanosheets in an unsaturated polyepoxide prepolymer solution was subjected to nanosecond-pulse electricity to overcome the typical limitation of breakdown voltage. Polyepoxide was used as a polymer matrix instead of polysiloxane because it is favorable to use a solvent in order to control the matrix viscosity and facilitates homogeneous dispersion of fillers. Since only a nanosecond pulse was applied to the polymer, the necessity of modifying the BN surface with metal nanoparticles was eliminated. X-ray diffraction (XRD), scanning electron microscopy (SEM), and thermal diffusivity measurements indicated that the BN nanosheets embedded in the polymer matrix with a high degree of order, resulting in a high thermal diffusivity. 2. Experimental Polyepoxide/BN nanosheet composite films were prepared by introducing BN nanosheets into a bisphenol A diglycidyl ether (BADGE) and curing the suspension with 1,2-ethylenediamine (EDA). Hexagonal BN nanosheets of commercial origin with lateral dimensions of D 90 = 10.6 μm and thicknesses of 2 – 10 nm were used. 10 g of BADGE was diluted with 1.50 mL of acetone. 1.31 g of boron nitride (BN) nanosheets was introduced into the diluted resin as filler and the mixture was ultrasonicated for 1 h at 50 °C. 0.8 g of EDA was added to Materials Letters 65 (2011) 2426–2428 ⁎ Corresponding authors. Tel.: +81 258 47 9893; fax: +81 258 47 9890. E-mail addresses: hong-baekcho@etigo.nagaokaut.ac.jp (H.-B. Cho), nky15@vos.nagaokaut.ac.jp (T. Nakayama). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.05.005 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet