Polymer composite with enhanced thermal conductivity and mechanical strength through orientation manipulating of BN

Abstract

Herein, we report a fabrication of epoxy resin/oriented BN composites via a facile hot-pressing strategy. Benefitting from the force exerted vertically on the BN platelets, a well-ordered BN microstructure in the composites has been achieved, which renders the prepared composites an elevated thermal conductivity up to 6.09 W m⁻¹K⁻¹ at 50 wt% loading, while it is only 2.44 W m⁻¹K⁻¹ for epoxy resin/random BN composites. In addition, the composites show an enhanced tensile strength (31.79 MPa) and Young's modulus (4.63 GPa) when comparing with epoxy resin/random BN composites at the same 50 wt% loading (21.21 MPa and 1.48 GPa). We attribute the improved thermal conductivity to the well-aligned BN platelets in higher heat conduction direction based on the anisotropic thermal conduction ability of BN. The enhanced mechanical properties are ascribed to the increased friction area along the tensile in-plane direction and the decreased defects which are easily incorporated during the fabrication process. This study sheds a light for exploring future thermal management materials to meet the increasing heat dissipation demands.

Introduction

More and more attention has increasingly been focused on heat dissipation issues with the near end of Moore's law for electronic devices [1,2]. Polymers have been used widely as electronic packaging materials, due to their plentiful merits, including excellent mechanical properties, easy-processing and low-cost [3,4]. However, most polymer resins with amorphous arrangement of molecular chains possess an undesired thermal conductivity lower than 0.2 W m⁻¹K⁻¹ [5]. Normally, adding ceramic fillers into polymer resins to obtain highly thermally conductive polymer composites is a good solution [[6], [7], [8], [9]]. However, the intrinsic phonon spectrum mismatch between organic polymer resins and inorganic ceramic phases heavily deteriorates the efficiency of phonon transferring [10,11].

To fully improve the compatibility between resin and filler interfaces and decrease the thermal interface resistance, surface modification seems to be a good choice [[12], [13], [14]]. However, most of the surface modification process is complex and the functionalization efficiency is not sufficient enough as-expected, which is restricted by the surface inertness of most ceramic fillers. Therefore, some more intrinsic factors, e.g., filler types, size, shape and orientation, have gained tremendous attention [[15], [16], [17], [18], [19], [20], [21], [22]]. Among them, orientation is a vital and easy-processing factor, especially for one-dimensional tubes or wires and two-dimensional platelet fillers that possess an anisotropic thermal conductivity property—the thermal conductivity along the in-plane direction is dozen times higher than that along the through-plane direction [[23], [24], [25], [26]]. Orienting fillers in the heat flow direction is widely adopted to achieve a higher thermal conductivity of polymer composites [[27], [28], [29]]. Furthermore, for handheld electronic devices, e.g., cellphones, effective heat dissipation along the in-plane direction has attracted more and more attentions, because much through-plane heat dissipation would cause terrible touch feeling and user experience [30]. Therefore, a well orientation along in-plane direction not only contributes to obtain enhanced thermal dissipation ability, but also possess favorable prospects in practical applications.

Varieties of means were adopted to form a well orientation of fillers along either in-plane or through-plane direction in composites. For instance, Lin et al. aligned BN platelets via adding an external magnetic field to obtain an enhanced thermal conductivity of polymer composites along the alignment direction [23]. However, the incorporated Fe₃O₄ brings negative influences on total mass and thermal expansion match. Shen et al. successfully fabricated aligned BN@PDA composite films through doctor-blading method [31]. Although the prepared films exhibited a higher thermal conductivity when compared with the films fabricated by simple casting, the alignment was coarse and not proper for standard scalable manufacturing. In our previous work, a well-aligned and interconnected 3D-BN network in polymer composites was prepared via combining ice-templating assembly and infusing strategy [32]. An enhanced thermal conductivity along the BN orientation direction was successfully obtained benefitting from the pre-formed thermally conductive scaffolds. However, the fabricated process was relatively complicated and time-consuming. Other methods including injection molding, electrospinning, gravitational force, etc were also performed to form orientation of filler [[33], [34], [35], [36]]. However, most of them are limited by the specific equipment and not suitable for scalable production.

On the other hand, increasing filler loadings elevate the enhancement of thermal conductivity, but overloading fillers deteriorate the mechanical strength [37]. It is hard to achieve an enhanced thermal conductivity and mechanical properties simultaneously in a relatively high loading level [3,38]. Nacre, possessing a favorable strength and toughness mainly benefitting from the unique highly ordered “brick-and-mortar” structure, seems to provide an excellent instruction for it [39]. The well-ordered assembling strategy can be imitated for polymer composites to achieve an enhanced thermal conductivity and mechanical strength simultaneously [33,40,41].

Herein, we chose BN as filler and epoxy resin as polymer matrix to assembly polymer composite films via facile hot pressing method. Hot-pressing strategy, which can form well-aligned orientation along the in-plane direction under a facile and easy-manipulating condition, has been proved to be a promising method for practically scalable manufacturing to construct layer-by-layer assembling structure with fewer defects incorporated during the fabrication process [27,39]. Benefitting from the force exerted vertically on the BN platelets in epoxy resin under the heating condition, a well-ordered BN microstructure has been achieved, which renders the prepared epoxy resin/oriented BN composites an elevated thermal conductivity up to 6.09 W m⁻¹K⁻¹ at 50 wt% loading, while it is 2.44 W m⁻¹K⁻¹ for epoxy resin/random BN composites. The composites also exhibit an enhanced tensile strength (31.79 MPa), Young's modulus (4.63 GPa) and toughness (0.29 MJ m⁻³) when comparing with epoxy resin/random BN composites at the same 50 wt% loading (21.21 MPa, 1.48 GPa and 0.22 MJ m⁻³). Moreover, the influences of BN content, BN size, hot-pressing temperature and pressure on the morphology and thermal conductivity of composites were further investigated, respectively. The interface mechanisms of thermal conduction and mechanical enhancement for oriented composites were also discussed in this work.