With the development of miniaturization, integration and increasing high-power density of electronic devices, efficient heat dissipation from electronic packaging materials has become a crucial issue when designing new electronic devices, such as high powered IGBT [[1], [2], [3]]. Among metals, Cu has an excellent thermal conductivity (TC) with 400 W·m^-1·K^-1. However, the coefficient of thermal expansion (16.24 ppm/K) of Cu induces mismatch between them and those semiconductors or PCB substrates which are approximately in the magnitude of 4-9 ppm/K [4,5]. In order to counteract this limitation and simultaneously improve TC of Cu, carbon materials, such as diamond [[6], [7], [8]], carbon fibers [9,10], carbon nanotubes [11,12] and graphite flakes (GF) [13,14], were preferentially taken into account as reinforcement in Cu phase owing to their intrinsic excellent thermal properties. The diamond/Cu composites reportedly exhibit extraordinary TC and tailorable coefficient of thermal expansion, but the poor machinability restrict their commercial application [[15], [16], [17]]. Carbon fibers/Cu and carbon nanotubes/Cu composites have a better machinability in comparison to diamond/Cu composites, while their ultrahigh TC perform in only one direction (axial direction) [13,14,18]. In contrast, the GF could be a promising selection for reinforcement phase of Cu due to its remarkable attributes with low density, low cost, ease of machining, an excellent TC and low coefficient of thermal expansion in the basic plane [19,20].

In the fabrication process of GF/Cu composites, the highly oriented alignment (i.e. along the basal plane) of the GF in Cu phase plays a critical role in determining the thermo-physical properties of the composites since two-dimensional GF have the in-plane TC of 1000 W·m^-1·K^-1 and through-plane TC of 38 W·m^-1·K^-1 [21,22]. With regard to this issue, Prieto et al. and Zhou et al. tried to mix uniformly the reinforcement particles (e.g. SiC or Si particles) and GF by mechanical stirring for at least 30 min and then press into a preform with the help of a hand-pressing machine, which contributed to ascertain alignment of GF and alternation of layers of reinforcement particles and of the oriented GF in liquid metal infiltration [[23], [24], [25], [26]]. The as-manufactured GF/SiC(Si)/Al composite exhibited alternation layers of GF but its thermal performance was not well due to the presence of spacers, which hindered heat transfer and reduced the TC of GF/SiC(Si)/Al composite [27]. Given the above factors, together with the great density difference between GF and Cu, liquid metal infiltration is not suitable for the preparation of GF/Cu composites [28].

Apart from the infiltration method, a simple and economical hot pressing method has also been extensively adopted to prepare the GF/Cu composites, wherein two types of powder, such as physically mixed powder and coated powder, are commonly used in sintering process, which is similar to the fabrication of diamond/Cu composites [29]. For example, Ren et al. adopted mechanically mixed method to mix GF and Cu powder in absolute ethyl alcohol to prevent both powders from layering and uneven distribution, which was pre-compacted under certain pressure to ascertain alignment of GF followed by drying of the pre-molded compact at temperature of 80 °C for 12 h. And the 20-60 vol% GF/Cu composites with a TC of 435-609 W·m^-1·K^-1 were fabricated by hot pressing [30]. Zhang et al. employed mechanically mixed to mix the GF and Cu in absolute ethanol environment, and a portion of mixture was positioned in graphite mold under a teeny pressing to guarantee the GFs well aligned that was repeated to fabricate a orientation precursor. Ultimately, they reported that GF/Cu composites containing 30-60 vol% of GF exhibited TC values from 477 to 596 W·m^-1·K^-1 [31]. Sohn et al. also applied the mechanically mixed to premix the GF and pure Cu powder in a nitrogen atmosphere for 6 h to improve its homogeneity and dispersion in the subsequent sintering process. It is found that the GF particles of the 59 vol% GF/Cu composite with the highest TC value of 456.9 W·m^-1·K^-1 were showed to directly contact with each other at the local level, indicating unsatisfactory dispersion of the GF [32]. Compared with mixed powders, the advantage of coated powders is that the distribution of GF in the Cu phase is homogeneous. And diverse approaches have been employed to prepare core-shell coated powders, including sol-gel [33,34], physical vapor deposition [35,36], chemical vapor deposition [37,38] and electroless plating [[39], [40], [41]]. Among these strategies, the classical and general electroless plating has attracted tremendous attention. Jang et al. used electroless Cu plating and stacking-pressing process to improve the homogeneous distribution and orientation of GF in the composites, respectively, and the 70 vol% GF/Cu composite with TC of 640 W·m^-1·K^-1 was prepared by the pulsed current activated sintering [42]. Liu et al. adopted an electroless plating to fabricate Cu-coated GF composite powders and the result showed that the TC values were higher as 455-565 W·m^-1·K^-1 in the 44-71 vol% GF/Cu composites, wherein, the GF was distributed homogeneously in the Cu phase [43]. Sohn et al. further analyzed the arrangement of GF in GF/Cu composites fabricated by sintering Cu-coated GF composite produced by electroless plating and it is found that GFs were relatively well aligned in the in-plane direction [32]. Accordingly, the Cu-coated GF composite powders are beneficial to the uniform dispersion and directional distribution of GF within Cu phase in sintered body. Nevertheless, electroless plating is susceptible to a two-step process pretreatment of sensitization and activation and it takes longer to achieve a given thickness of coating deposition. Additionally, the solution of electroless plating is inferior in cost effectiveness due to complicated composition and limited lifetime of the plating solution with accompanying environmental pollution, which hinders its large-scale uses in Cu plating [[44], [45], [46]].

In this paper, an intermittently electroplated method, which processes simple controlling condition, controllable deposition rate, stable and recyclable electroplating solution avoiding circumstance pollution, is proposed to perpare uniform Cu-coated GF composite powders. The presence of a Cu shell contributed to the homogeneous distribution and preferred orientation of GF in the Cu, promoting the heat transfer of GF phase in composites. And the content of Cu can be precisely controlled by adjusting the intermittent electrodeposition time. Subsequently, above powders were hot pressing into a dense GF/Cu bulks. In addition, the surface morphology, fracture morphology, phase composition, thermal properties and mechanical properties of the as-received GF/Cu composite materials were analyzed in detail. The resulting GF/Cu composite bulks with distinguished thermo-physical properties may be a promising candidate as heat sink materials.

The natural graphite flakes were reinforcements (density: 2.26 g/cm³, purity >99.9 %, -10 meshes, purchased from Alfa Aesar Co., Shanghai, China). The CuSO₄·5H₂O and H₂SO₄ were analytically pure and purchased from Sinopharm Chemical Reagent Co., Ltd and Beijing Chemical Industry Group Co., Ltd, respectively.

The homemade intermittently electroplated device was composed of an anode and cathode with a 20 cm diameter pure Cu plate equipped with a stirring rod. The structural diagram of the device is presented in Fig. 1(a). The current was supplied by a DC power supply. More details can be found in Ref. [47]. The electroplating solution consisted of 40 g/L CuSO₄ and 80 mL/L H₂SO₄. During a trial, a certain amount of GF was firstly placed into the electroplating device. Under the condition of a current density of 10 A/dm ², the desired (40 vol%, 50 vol%, 60 vol%, 70 vol%, 80 vol% and 90 vol%) GF/Cu composite powders were obtained by adjusting the electroplating time to 75 min, 70 min, 63 min, 55 min, 43 min and 28 min, with stirring every 5 min, respectively. Then, Cu-coated GF composite powders were removed from the bath, and washed several times with deionized water and alcohol, followed by drying in a vacuum chamber at 60 °C for 2 h. Thirdly, the Cu-coated GF composite powders were uniformly placed into a graphite die with an inner diameter of 30 mm to guarantee the well alignment for the GF. The hot-pressing sintering was heated to 950 °C and held for 60 min. A pressure of 40 MPa was adopted and held until the furnace was cooled to 200 °C. Ultimately, the sintered composites were labeled as x GF/Cu composite, where x is the volume fraction of GF, ranging from 40 vol% to 90 vol%. The process of the fabrication of the GF/Cu composite bulks is illustrated in Fig. 1(a).

The microstructures of the composite powders and composite bulks (fracture morphology and surface morphology) were investigated by field-emission scanning electron microscopy (FE-SEM, JEM-7001 F, JEOL, Japan). X-ray diffraction (XRD, SmartLab, Japan) was used to characterize the phase structure of the GF/Cu composite bulks in the XY and Z directions. The TC value of the composites was calculated by multiplying the thermal diffusivity, specific heat capacities and density. The thermal diffusivity of the composite materials was measured at room temperature using a laser flash apparatus (NETZSCH LFA427, Germany). The dimensions of the sample in the XY and Z directions were 10 mm × 2.5 mm and 10 mm × 1.8 mm, respectively. The specific heat capacities ${{C}_{p}}$ were calculated using the following expression: ${{C}_{P}}=\left( {{C}^{f}}{{V}^{f}}{{\rho }^{f}}+{{C}^{m}}{{V}^{m}}{{\rho }^{m}} \right)/\rho $, where V, ρ and C are the volume content, density and specific heat of each component, respectively, and the superscripts f and m stand for the properties of the GF and Cu. The bulk density was measured by Archimedes’ principle. The bending strength of the GF/Cu composites (25×3×4 mm) was measured by a three-point bending method. Only the mechanical properties in the XY direction were tested owing to a limitation of sample size.

The morphologies of Cu-coated GF composite powders with a varying amount of Cu prepared by the intermittently electroplated method are shown in Fig. 1. Fig. 1(b) shows that the dense Cu coating with ‘cauliflower’ shape are obviously formed on the surface of GF in 40 vol% GF/Cu (similar to 50 vol% GF/Cu) composite powders. This result could be explained by the fact that Cu is firstly deposited on the GF surface and with electroplating time prolonging, then accumulated on the predeposited Cu surface to reach the designed Cu content, thus forming a ‘cauliflower’-like Cu shape. In comparison with Fig. 1(b), the morphologies of the 60 vol% GF/Cu (like 70 vol% GF/Cu) composite powders retain the shape of the GF itself owing to the relatively low content of Cu in composite powders, as illustrated in Fig. 1(c). Fig. 1(d) presents the surface morphologies of the 90 vol% GF/Cu (equal with 80 vol% GF/Cu) composite powders, in which a small amount of Cu is uniformly deposited on the surface of GF. All the samples present even and dense core-shell Cu-coated GF composite structure due to the intermittently stirred characteristics with the intermittently electroplating. It is noteworthy that the core-shell structure powder could effectively overcome accumulation of GF and contribute to homogeneous distribution of GF in Cu phase in the subsequent sintering process.

Fig. 2 displays the phase composition of GF/Cu composite bulks prepared by sintering the above powders. There are the peaks of GF and Cu in the perpendicular direction to the pressure direction (XY plane) and parallel to the pressure direction (Z plane). No impurity peaks, such as CuO and Cu₂O, are found on the surface of the GF/Cu composite, which indicates that high-purity Cu have been successfully formed on matrix, as shown in Fig. 2(a) and (b). Herein, evident diffraction peaks (2θ) at 43.3°, 50.4° and 74.1°, are well indexed to the characteristic (111), (200) and (220) crystal planes of cubic Cu, respectively. With regard to GF, its peak intensity increases with raising volume fraction of GF and the peak patterns of GF in the XY and Z plane are different due to the GF anisotropy. For example, the (002) d-spacing of GF is consistent with that of single crystal graphite, indicating an extremely high TC (~1000 W·m^-1·K^-1) in the XY plane [48]. Compared with the XY plane, there are new peaks (2θ) at 42.5°, 44.6°, 77.4° and 83.2°, belonging to (100), (101), (110) and (112) crystal planes in the Z plane, respectively. Additionally, the (002) crystal plane peaks decreases and the (004) crystal plane peaks almost disappears in the Z plane, implying preferred orientation of GF crystallites in composites [49,50]. Obviously, the core-shell structure composite powders fabricated by intermittently electroplated is an effective strategy for enhancing GFs alignment.

In order to further deeper understanding the alignment of the anisotropic GF in Cu phase, the GF distribution of GF/Cu composites with different GF volume fractions are further observed by SEM in the XY plane, as shown in Fig. 3. The elongated black and gray region in the GF/Cu composites represent the GF and Cu phase, respectively. The GF is evenly distributed in the Cu phase in a parallel direction, wherein the parallel direction is well perpendicular to the direction of pressure. There results indicate that GFs are preferred-oriented along the in-plane direction, which is well consistent with the above findings on XRD pattern. Moreover, the alignment of GF in the GF/Cu composites shows no difference with varying volume fraction of GF, because the Cu shell layer of the core-shell structure facilitates a uniform and orderly arrangement of GF. For example, when the volume fraction of GF ranges from 40 % to 90 %, the GF is still uniformly distributed in Cu phase. The well-controlled alignment of GF within Cu is beneficial to enhance significantly TC properties of composites in XY plane, thus the intermittently electroplated method is a promising way to prepare the core-shell structure composite powder in GF/Cu composites.

Apart from surface morphology, the fracture morphology of the as-received composites are studied in detailed. Herein, the XY and Z directions of 50 vol% GF/Cu composite bulk are selected as representative, because a similar fracture surface could be observed for GF/Cu composites with different volume fractions of GF. Fig. 4(a) presents that the fracture characteristics of the GF/Cu composite in the XY direction are composed of lamellar layered GFs with a highly preferred orientation, that is, cracks will preferentially occur inside the layer of GF, indicating the better interfacial bonding between GF and Cu. On the whole, the fracture mode of the GF/Cu composite in the XY direction is the cracks propagation of GFs, though there are a few voids owing to the pulling of GFs. In the Z direction, Fig. 4(b) illustrates an approximate disc shape morphology, which is similar to the raw materials. This morphology can be attributed to the fact that the fracture mode of the GF/Cu composite in the Z direction is mainly the van der Waals force of the GF inner layers, which is lower than the metallurgical bond between GF and Cu [51]. Based on the above mentioned results, it could be speculated that the mechanical properties of the GF/Cu composite in the XY direction are better than those in the Z direction.

The effects of the GF volume fraction on the TC value of GF/Cu composites in the two directions are exhibited in Fig. 5(a) and (b). Fig. 5(a) depicts that the TC values of the GF/Cu composites in the XY plane increase with raising GF volume fraction. The maximum TC value displays 710 W·m^-1·K^-1 when the volume fraction of the GF reaches 80 %. It is noteworthy that the TC value of the 90 vol% GF/Cu composite decreases slightly in comparison to TC value of the 80 vol% GF/Cu composite. This result may be due to the fact that when the content of GF is close to 100 %, the existence of voids could hinder thermal transfer due to lack of Cu in the stacked GF, which reduce heat property. Furthermore, the TC values of the composites present a converse tendency in two directions. In the Z direction, the TC value of the GF/Cu composite decreases gradually with the increase of graphite content, because through-plane TC value of anisotropic GF is only about 38 W·m^-1·K^-1 and heat transfer capacity is mainly dependent on the Cu phase in the Z direction.

It is of great importance to evaluate the theoretical TC limit for the GF/Cu composites. Generally, a Hatta-Taya (H-T) model is used to estimate the theoretical TC value [52] using the formula shown in the following Eq. (1):

where $K_{C}^{i}$ (W·m^-1·K^-1) and ${{K}_{m}}$ (W·m^-1·K^-1) represent the TC of the composite and matrix, respectively; i displays the XY and Z directions, respectively; ${{V}_{f}}$ is the volume fraction of reinforcement; ${{S}_{i}}$ is the parameters related to the appearance of reinforcement, which can be expressed by the following Eqs. (2) and (3):

where t (m) and D (m) represent the thickness and diameter of the reinforcement, respectively. $K_{re}^{e\left( i \right)}$ represents the effective TC of the GF, which includes the interfacial thermal resistance between the GF and Cu phase, and is expressed by the following Eqs. (4) and (5):

$K_{re}^{X-Y}$ and $K_{re}^{Z}$ (W·m^-1·K^-1) are the intrinsic in-plane and perpendicular direction TC of reinforcement; h is the interfacial thermal conductance, which is expressed by the following Eq. (6):

where ρ (kg/m³) is the density, υ (m/s) represents the Debye phonon velocity, and C (J/(kg K)) is the specific heat; the subscripts “m” and “re” are the Cu phase and reinforcement, respectively.

In calculation, it is consumed $K_{re}^{X-Y}$ = 1000 W·m^-1·K^-1, ${{K}_{m}}$ = 400 W·m^-1·K^-1, D = 1000 μm, t = 50 μm [53], ${{C}_{m}}$ = 385 J/(kg K), ${{\rho }_{m}}$ = 8960 kg/m³, ${{\rho }_{re}}$ = 2260 kg/m³, ${{\upsilon }_{m}}$= 2881 m/s and ${{\upsilon }_{re}}$ = 14,800 m/s [54]. Through the above formula, the theoretical TC values obtained by the H-T model are shown in Fig. 5. It can be seen from the solid line in Fig. 5 that the variation trend of experimental results are in good accordance with these theoretical calculation values as the GF content increases. Yet, the experimental values slightly lower than the theoretical TC values in the XY direction and it gradually enlarges with the increasing GF volume fraction. This deviation phenomenon may be attributed to the following reasons. On the one hand, some GF bends during the sintering process, which hinders the heat transfer of the GF. On the other hand, the pores are gradually formed owing to the lack of Cu in among neighboring stacking of GF, which forms a thermal insulation barrier and increases the interfacial thermal resistance between GF and Cu. In the Z direction, the TC values of the 40 vol% and 50 vol% GF/Cu composite bulks are 159 W·m^-1·K^-1 and 138 W·m^-1·K^-1, respectively, which are higher than the theoretical calculation data with the same content of GF. This phenomenon could be ascribed to the high purity of Cu in core-shell Cu-coated GF composite powders prepared by intermittent electroplated, which is mainly heat transfer medium in the Z direction.

In addition, the TC values of our GF/Cu composites are compared with those of the GF/Cu composites fabricated in references. To make a reasonable comparison, there literature reports are referenced since their particle size with 1000 μm is consistent with the particle size used in this study. As summarized in Fig. 6, the TC values of our GF/Cu composites are substantially higher than those of the GF/Cu composites reported by others. The reason for this performance is attributed to preferred alignment and homogeneous distribution of GFs in Cu phase, which could be owed to the pre-oriented formation of Cu-coated GF composite powder before the initiation of the hot-pressing process. Consequently, the intermittent electroplated method combined with hot-pressing sintering, which possess a recyclable plating solution and simple and convenient process, may result in broad potential for the preparation of composite materials in electronic packaging materials.

The bending strength in the X-Y plane of the as-obtained GF/Cu composites with a varying volume fraction of GF is systematically investigated by three-point bending tests, as illustrated in Fig. 7. It could be observed that with increase of GF volume fraction, the bending strength of GF/Cu composite follows a decreasing trend, which displays converse tendency with the TC. This phenomenon is attributed to following reasons. On the one hand, the bending strength of as-obtained GF/Cu reduces as the volume fraction of the Cu decreases because Cu possesses higher strength in comparision to GF. On the other hand, high volume fraction of GF gives rise to porosity in the process of dense composite due to lack of Cu.

It could be summarized from the Fig. 7 that at lower GF content, the GF/Cu composite exhibit better mechanical properties. For example, the bending strength of 40 vol% GF/Cu composite is 93 MPa, which meets the required mechanical properties of heat dissipation materials. And this values is higher than other GF/Cu composites fabricated by sintering mechanically mixed powder in the literature [31]. The reason for the phenomenon described may be well interface bonding in GF/Cu composites. The GF/Cu composites with excellent thermo-physical can be a highly potential candidate for electronic packaging materials. As for the GF/Cu composite with high volume fraction of GF, the strength of the composites may be improved once the surface of GF is functionalizated with a high-strength carbide interlayer. And these studies would be conducted in the future to further enhance the mechanical property of GF/Cu composite.

The GF/Cu composites with 40-90 vol% GF have been successfully fabricated by using the intermittently electroplated process, followed by hot-pressing sintering. Although the volume fraction of GF ranged from 40 % to 90 %, the surface of GF was evenly and densely coated with a Cu-shell layer due to the intermittent stirring characteristics of the intermittently electroplated method. XRD analysis and SEM images showed that GF were uniformly distributed with a highly preferred orientation and stacked mostly parallel in GF/Cu composite bulks, which was beneficial to the overall heat transfer of the composites. Ultimately, the TC values of the as-received GF/Cu composites increased from 506 W·m^-1·K^-1 to 710 W·m^-1·K^-1 in the X-Y plane with increasing volume fraction of GF from 40 % to 90 %. The maximum bending strength of GF/Cu composite was 93 MPa when the volume fraction of GF was 40 %, which satisfies the requirements of electronic packaging materials. Taking into account the reported mechanical and thermal properties, together with simple fabrication process, these GF/Cu composites are believed to be promising candidates for electronic packaging materials.

This work was financially supported by the National Key Research and Development Program of China (No. 2016YFC0700905) and the National Natural Science Foundation of China (Grant No. 51674232, No. 51972304 and No. 51702331).