Anisotropic thermal conductivity of aluminum matrix composites reinforced by graphene nanoplates and ZrB2 nanoparticles

Abstract

This study investigates the anisotropic thermal conductivity of aluminum matrix composites reinforced with graphene nanoplates (GNPs) and in situ ZrB2 nanoparticles, while simultaneously maintaining high strength and toughness. A discontinuous layered GNPs–ZrB2/AA6111 composite was prepared using in situ melt reactions and semi-solid stirring casting technology, combined with hot rolling deformation processing. Microstructural analysis revealed that the GNPs were aligned parallel to the rolling direction–transverse direction (RD–TD) plane, whereas the ZrB2 nanoparticles aggregated into cluster strips, collectively forming a discontinuous layered structure. This multilayer arrangement maximized the in-plane thermal conductivity of the GNPs. The tightly bonded GNP/Al interfaces with the locking of CuAl2 nanoparticles ensured that the GNPs fully exploited their high thermal conductivity. Therefore, the GNPs–ZrB2/AA6111 composite achieved high in-plane thermal conductivity (230 W/(m·K)), which is higher than that of the matrix (206 W/(m·K)). The improved in-plane thermal conductivity is primarily attributed to the exceptionally high intrinsic in-plane thermal conductivity of the GNPs and their two-dimensional layered structure. However, the composite exhibited pronounced thermal conductivity anisotropy in the in-plane and through-plane directions. The reduced through-plane thermal conductivity is predominantly caused by the intrinsically low through-plane thermal conductivity of the GNPs and the increased interfacial thermal resistance from the additional grain boundaries.

1. Introduction

The rapid development of aerospace, rail transit, electronic communication, and other fields has imposed increasingly stringent demands on the structural and functional performance of Al alloys [1–5]. This is particularly critical for liquid cooling plate materials used in battery thermal management systems in electric vehicles, which require high strength, fracture toughness, and thermal conductivity [6–8]. However, traditional aluminum alloys struggle to achieve these properties [9]. Even for aluminum matrix composites (AMCs) with a single reinforcement, the mechanical properties and functional characteristics cannot be optimized simultaneously [10–11]. Therefore, according to the designability of the composites, different types of high-quality reinforcements were compounded to prepare structural-functional integrated AMCs with high strength, toughness, and thermal conductivity.

In recent years, graphene, as a lightweight material with excellent mechanical and thermal properties, has been considered an ideal reinforcement for AMCs, which has important research value and a broad application background [12–17]. In particular, the tensile strength of graphene reaches 130 GPa, and the in-plane thermal conductivity is as high as 3000–4000 W/(m·K) [18]. Researchers have found that the strength of graphene-reinforced aluminum matrix composites has significantly improved, but their plasticity was greatly reduced, especially the elongation of graphene/aluminum matrix composites with graphene contents below 5% [19]. Hence, breaking through the contradiction of the strength–toughness inversion relationship is the key point of this research. Notably, in situ nanoparticle reinforcement has shown promise in overcoming this strength–toughness dichotomy, enabling the simultaneous enhancement of strength and ductility in AMCs [20–22]. According to our previous research, the AA6111 matrix composites synergistically reinforced with 0.3wt% graphene nanoplates (GNPs) and 0.3wt% in situ ZrB2 nanoparticles achieved a 50% increase in yield strength (349 MPa), and only a 30% compromise on ductility (13.8%), leading to a notable improvement in toughness [23]. Additionally, the strengthening–toughening mechanisms of graphene and particle-reinforced aluminum matrix composites were analyzed [24].

Graphene-reinforced copper matrix composites have demonstrated a 22% enhancement in in-plane thermal conductivity compared to pure copper [6]. This finding strongly suggests that similar improvements in thermal conductivity can be achieved in graphene-reinforced aluminum-matrix composites. Furthermore, the spatial-distribution structure design of heterogeneous, dual-phase, and nanoscale reinforcements must be carefully considered [25–28]. Owing to the special two-dimensional lamellar structure of graphene, its high tensile strength and high thermal conductivity exhibit maxima only in its two-dimensional plane [29–31]. In this study, we successfully fabricated a novel structure-function integrated aluminum matrix composite reinforced with GNPs and in situ ZrB2 nanoparticles. Moreover, this study elucidates the synergistic effects of GNPs and in-situ synthesized ZrB2 nanoparticles on the microstructural characteristics of aluminum matrix composites, correlating its morphological features with anisotropic thermal conductivity properties. A thermal conductivity model of two-phase nanoparticles was established. This provides an important scientific ref