Polyimide-assisted fabrication of highly oriented graphene-based all-carbon foams for increasing the thermal conductivity of polymer composites

Abstract

Graphene and its derivatives are often preferentially oriented horizontally during processing because of their two-dimensional (2D) layer structure. As a result, thermal interface materials (TIMs) composed of a polymer matrix and graphene-derived fillers often have a high in-plane (IP) thermal conductivity (K), however, the low through-plane (TP) K makes them unsuitable for practical use. We report the development of high-quality polyimide/graphite nanosheets (PG) perpendicular to the plane using a directional freezing technique that increase the TP K of polymer-based composites. Graphene-derived nanosheets (GNs) were obtained by the crushing of scraps of highly thermally conductive graphene films. A water-soluble polyamic acid salt solution was used to disperse the hydrophobic GNs filler to achieve directional freezing. The polyimide, which facilitated the directional alignment of the GNs, was then graphitized. The introduction of the GNs increases the order and density of the PG, thus improving the strength and heat transfer performance of its polydimethylsiloxane (PDMS) composite. The obtained PG/PDMS composite (21.1% PG, mass fraction) has an impressive TP K of 14.56 W·m−1·K−1, 81 times that of pure PDMS. This simple polyimide-assisted 2D hydrophobic fillers alignment method provides ideas for the widespread fabrication of anisotropic TIMs and enables the reuse of scraps of graphene films.

1. Introduction

The performance of integrated circuits often degrades significantly or fails to operate due to overheating. Consequently, effective thermal management materials are vital for the optimal operation of high-power electronic devices[1–3]. Polymer-based thermal interface materials (TIMs) are widely investigated for the heat dissipation of electronic devices due to their multiple merits of lightweight, chemical stability, easy processability, and cost-effectiveness[4–6]. However, polymers generally exhibit low thermal conductivity (K) (～0.5 W·m−1·K−1) due to the disordered structures and entanglements which cause considerable phonon scattering[7–8]. To improve the K of composites, thermally conductive fillers are often compounded with polymers for the goal of increasing the phonon transport path[9].

Graphene and its derivatives are considered to be the most promising candidates for thermal fillers because of their high in-plane (IP) K[10–14]. Lin et al. obtained polymer/graphene composites with K of 1.43 − 1.84 W·m−1·K−1 by first coating the polymer powder with graphene and then hot pressing, in which the graphene content was 10% (mass fraction)[15]. Fang et al. prepared polyvinyl butyral (PVB)/graphene composites by solution blending, and the K of the sample reached 4.52 W·m−1·K−1 with 30% (mass fraction) graphene nanoplates[16]. Nevertheless, even with high graphene concentration, the K of composites formed by directly mixing graphene sheets with polymer matrix did not rise as significantly as predicted. It is mostly owing to the disconnected phonon transport channel between the filler and polymer matrix and the large filler-filler interfacial thermal resistance, which can be mitigated by pre-establishing three-dimensional (3D) filler networks in the polymer matrix[17–22]. Bai et al. fabricated graphene foams with 3D interconnected networks by chemical vapor deposition, and the K of its polydimethylsiloxane (PDMS) composite was 1.2 times higher than that of non-3D structured graphene sheets with the same mass loading[23]. The successive filler networks act as high-speed channels, thus effectively reducing phonon scattering while ensuring that most of the heat can be transported through the fillers[24]. However, current polymer/graphene composites typically exhibit heat transfer isotropy, or high IP K, due to the tendency of graphene to orientate horizontally during processing[25–26]. Notably, TIMs are also expected to have high through-plane (TP) K, to minimize the temperature difference between the heat spreaders and the electronics in practical applications[5,27–28]. To achieve this goal, designing anisotropic polymer/graphene composites with high TP K is of great importance.

Directional freezing is used extensively to vertically align the fillers with a high aspect ratio of two-dimensional (2D) lamellar structures, whi