Three dimensional AlN skeleton-reinforced highly oriented graphite flake composites with excellent mechanical and thermophysical properties

Accepted Manuscript Three dimensional AlN skeleton-reinforced highly oriented graphite flake composites with excellent mechanical and thermophysical properties Xiaoyu Zhang, Zhongqi Shi, Xia Zhang, Ke Wang, Yingying Zhao, Hongyan Xia, Jiping Wang PII:

S0008-6223(18)30100-3

DOI:

10.1016/j.carbon.2018.01.091

Reference:

CARBON 12832

To appear in:

Carbon

Received Date: 7 December 2017 Revised Date:

23 January 2018

Accepted Date: 25 January 2018

Please cite this article as: X. Zhang, Z. Shi, X. Zhang, K. Wang, Y. Zhao, H. Xia, J. Wang, Three dimensional AlN skeleton-reinforced highly oriented graphite flake composites with excellent mechanical and thermophysical properties, Carbon (2018), doi: 10.1016/j.carbon.2018.01.091. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT Three Dimensional AlN Skeleton-reinforced Highly Oriented Graphite Flake Composites with Excellent Mechanical and Thermophysical Properties Xiaoyu Zhang, Zhongqi Shi*, Xia Zhang, Ke Wang, Yingying Zhao, Hongyan Xia,

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Jiping Wang State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University,

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Xi’an 710049, China

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ABSTRACT

To obtain light, strong materials with high thermal conductivity and low coefficient of thermal expansion (CTE), three-dimensional (3D) AlN ceramic skeleton reinforced highly oriented graphite flake (AlN/GF) composites were successfully

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prepared by combining vacuum filtration and spark plasma sintering. The effects of AlN content on the microstructure, mechanical and thermophysical properties of the composites were investigated. It was found that the introduction of AlN together with

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Y2O3 significantly promoted the densification and mechanical properties of the

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composites. The GF grains in the composites were highly oriented with the Lotgering factor > 98%, and a 3D continuous ceramic skeleton was formed when the AlN content increased to 20 wt.%. The formed ceramic skeleton tailors the thermophysical properties of the composites effectively. Particularly, the unfavourable high through-plane CTE (~28×10-6 K-1) of highly oriented graphite matrix remarkably decreased to a low value (~7×10-6 K-1) due to the thermal expansion constraint of the formed ceramic skeleton architecture. The 20 wt.% AlN/GF composite possessed the

*

Corresponding author. Tel: +86-29-82667942. E-mail: [email protected] (Zhongqi Shi)

ACCEPTED MANUSCRIPT best comprehensive properties with the bending strength >80 MPa, high in-plane thermal conductivity of 442 W·m-1·K-1 and low through-plane CTE value of 7.3×10-6 K-1. Our strategy would facilitate the practical application of graphite-based

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composites in current demanding thermal management.

1. Introduction

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With modern electronics becoming smaller, lighter and more powerful, there is

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increasing demand for advanced thermal management materials to meet the thermal dissipation requirements of electronic components [1–3]. The advanced thermal management materials are supposed not only to dissipate heat quickly, but also exhibit lower density and good thermal shock resistance, as well as a similar coefficient of

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thermal expansion (CTE) with semiconductor devices (4-7×10-6 K-1) [4–6]. Specifically, in thermal management materials application, effective heat dissipation in the normal direction of thermal interface plane is highly desired and the thermal

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expansion in the direction parallel to the interface shall be as close as possible to that

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of semiconductor. These special physical and structural properties could achieve the efficient heat dissipation and extending the lifetime of high-power electric chips by reducing thermal strain in the interface at the same time [7]. Recently, graphite materials are becoming attractive in thermal management

applications because of its light weight (2.25 g·cm-3), excellent thermal conductivity in basal plane, good machinability and low costs [8]. The thermal conductivity of natural crystalline graphite flake along basal plane has been reported as high as 2200

ACCEPTED MANUSCRIPT W·m-1·K-1 at room temperature and the maximum thermal conductivity in (002) crystal plane of a graphite crystal can reach 2800 W·m-1·K-1 at 80K [9,10]. However, the thermal conductivity of commercial poly-crystalline graphite is still low (70~150

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W·m-1·K-1) at room temperature due to its isotropic microstructure. Therefore, it is an efficient way to enhance the in-plane thermal conductivity by controlling the preferred orientation of graphite flakes. Many studies have reported that the highly oriented

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graphite blocks can be prepared successfully by hot-pressing of graphite flake

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powders, and a high in-plane thermal conductivity (550~700 W·m-1·K-1) was obtained as the result. However the strength was still unsatisfied (< 30MPa) and the CTE of graphite blocks across the basal plane (28×10-6 K-1) was much higher than that of semiconductor materials [11–14]. Although oriented graphite blocks with high

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in-plane thermal conductivity have been reported, the combination of low through-plane CTE and sufficient mechanical properties in one material for the practical application is still challenging.

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It is noteworthy that the CTE of graphite is dependent on the state of stress and it

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can be reduced when graphite is under tensile stress[15]. Based on this theory, we propose that the CTE of graphite composites could be decreased by putting the graphite under tensile stress. Generally, the CTE of ceramics with high thermal conductivity, such as SiC and AlN, is one magnitude lower than that of graphite in through-plane direction. Therefore, if graphite flakes are bonded by ceramic network, the graphite flakes will endure tensile stress across the basal plane in the composites, which is beneficial to the reduction of the CTE of the integrated composites along

ACCEPTED MANUSCRIPT through-plane direction. Simultaneously, the mechanical properties of graphite composites should be enhanced by the ceramic network. Inspired by these principles, we propose that the mechanical and thermophysical properties of the high oriented

three-dimensional (3D) continuous ceramic network.

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graphite blocks could be enhanced synergistically by the design of a

To realize the preferred structure that graphite flakes strengthened by a 3D

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continuous ceramic skeleton structure, a process combining vacuum filtration and

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spark plasma sintering (SPS) in sequence was used. Vacuum filtration is an universal technique to prepare paper-like materials with significantly anisotropic microstructure by using the slurry containing well-dispersed layered materials [16]. On the other hand, SPS is an efficient hot-pressing method achieving ultrafast densification of

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ceramics and composites at relatively low temperatures [17–19]. In our preparation process, vacuum filtration firstly provides an effective way to form the green body. The architecture of ceramic skeleton-reinforced orientated graphite flakes is

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accomplished by embracing the graphite flakes with ceramic particles. And the

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following SPS with uniaxial pressure in through-plane direction further enhances densification step as well as the orientation. Aluminum nitride (AlN) ceramic was chosen as the reinforcement based on the consideration of its high bending strength (400 MPa), high thermal conductivity (about 320 W·m-1·K-1 for single crystal, 30~270 W·m-1·K-1 for polycrystal) and especially the low CTE value (~4×10-6 K-1) [20,21]. In this study, 3D AlN ceramic skeleton reinforced highly oriented graphite flake (AlN/GF) composites were prepared by combining vacuum filtration and SPS method

ACCEPTED MANUSCRIPT in sequence. The effects of AlN content on the microstructure, mechanical and thermophysical properties of the composites were investigated. Particularly, the thermal expansion constraint mechanism for the obtained graphite-based composites

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was proposed and further discussed in detail.

2. Experimental

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2.1 Sample preparation

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The raw materials were graphite flake (GF) powder (Alfa Aesar, America, purity >99.8 %) and AlN powder (Tokuyama, Japan, purity >99.9 %). 5 wt.% of Y2O3 powder (Shanghai Yuelong, China, purity >99.99 %) was used as the sintering additive. The morphologies of GF and AlN powders are shown in Fig.1 (a) and (b),

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respectively. The GF powders exhibit 15 µm in lateral size and 1 µm in thickness, and

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AlN powders show an equiaxial shape with an average particle size of 0.5 µm.

Fig.1 Morphologies SEM image of (a) GF and (b) AlN powders

The AlN/GF composites were fabricated by vacuum filtration followed by SPS route, and the fabrication process is schematically illustrated in Fig.2. Firstly, the AlN, GF and Y2O3 powders were dispersed in ethanol and stirred for 30 min. Secondly, the

ACCEPTED MANUSCRIPT dispersion was ball-milled for 5 h using agate balls as milling medium and then a wet body was prepared by vacuum filtration. Thirdly, the wet body was dried at 80 oC for 24 h to obtain a green body. Finally, the green body was loaded into a graphite die and

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sintered at 1650 oC (heating rate: 200 oC·min-1) with soaking time for 5 min under an axial pressure of 50 MPa provided by a SPS furnace (Ed-PASIII, Elenix Ltd, Japan). Graphitic paper was wrapped around the green body for easy removal of the

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as-sintered sample. To explore the influence of AlN contents on thermal and

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mechanical properties of the fabricated AlN/GF composites, the samples with 10, 15, 20, 25, 30, 50 wt.% AlN were prepared and A10, A15, A20, A25, A30, A50 were used

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respectively for label in the following discussion.

Fig.2 Schematic of the fabrication process for preparing the AlN/GF composites

2.2 Characterization The bulk density and porosity of the samples were determined by the Archimedes method. The theoretical density of the samples was calculated by the rules of mixtures

ACCEPTED MANUSCRIPT using the density of graphite (2.25 g·cm-3), AlN (3.26 g·cm-3) and Y2O3 (5.01 g·cm-3). The crystalline phase and preferred orientation structure of sintered samples were characterized by X-ray diffraction (XRD, X-Pert Pro, Netherlands). The orientation

(00l) orientation was calculated by the equation:

(

)

=

(

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degree was determined on the base of XRD patterns and Lotgering factor f [22] of )

(see S1 in Support

Information). The microstructure of samples was examined by backscattered electron

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image (BEI) and secondary electron image (SEI) in a field emission scanning electron

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microscope (FE-SEM, SU6600, Hitachi, Japan). Raman spectra of samples were obtained by the laser spectrophotometer (HR800, HORIBA, France). The mechanical and thermal properties were examined both parallel (z-axial) and perpendicular (x-y plane) to the axial pressure directions. The flexural strength of

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samples (size: 3 mm×4 mm×16 mm) was tested by a three-point bending method (span: 16mm, crossed speed 0.5 mm·min-1). The laser flash method was used to measure the thermal diffusivity (α) and specific heat capacity (Cp) of specimens (size:

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Ø12.7×3 mm for z-axial direction, 10 mm×10 mm×3 mm for x-y plane direction)

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with the Netzsch LFA447 NanoFlash at room temperature. The thermal conductivity (λ) of samples was calculated according to the equation: λ = ρ ×

× . Each

specimen was tested in three sections and then the λ value was obtained by the average of the three sections. The CTE was measured by a dilatometer (DIL 402C, Netzsch, Germany) from room temperature up to 300 oC at a ramping rate of 5 o

C·min-1.

ACCEPTED MANUSCRIPT 3. Results and discussion 3.1 Phase composition and anisotropy Fig.3(a) and (b) show the XRD patterns of A30 corresponding to z-axial and x-y

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planes. The graphite, AlN and Y2O3 diffraction peaks in the products can be clearly observed. Besides, Al2Y4O9 phase is also found, which should be the products of the reaction between Al2O3 (the native oxide layer of AlN) and Y2O3 as the eutectic

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compound to promote sintering [4]. Compared with Fig.3(a) and (b), the diffraction

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peaks of different directions exhibit significant difference. The (002) peak of graphite at 2θ=26.5° in x-y plane is very sharp and its intensity is almost 40 times of that in z-axial plane, which demonstrates that the composites possess highly anisotropic feature [23]. Lotgering factors f of samples calculated based on the XRD patterns are

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all higher than 98% (see Fig.3(c)), which indicates the perfect orientation of GF grains. The crystalline quality of GF after sintering was evaluated by the ratio of the D (1320 cm-1) to G peak (1580 cm-1) in Raman spectra. The lower D/G ratio

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demonstrates the higher crystalline quality [14]. It can be seen from Fig.3(d) that the

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D/G-intensity ratio increases slowly from A10 to A20 and then grows abruptly from A20 to A30. Hence, a remarkably increased disorder in the GF crystals is formed when the AlN content is higher than 20 wt.%. The disorder of GF crystals should be attributed to the higher hardness of AlN which results in the deformation and fracture of GF during the hot-pressing.

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Fig.3 XRD patterns of A30 corresponding to (a) z-axial and (b) x-y planes, respectively; (c) the calculated Lotgering factor f of (002) orientation based on XRD

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patterns of AlN/GF composites; (d) the D/G-intensity ratio of the AlN/GF composites tested by Raman spectra.

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3.2 Microstructure and phase distribution

Fig.4 shows the polished surface microstructures of the AlN/GF composites with

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different AlN contents. In the BSI of the composites (Fig.4(a)-(e)), AlN phase (bright area) disperses uniformly with good arrangement (highlight by red dot line) in the graphite matrix (dark area) and no obvious gaps are observed. The bright area increases with the AlN content increasing, leading to the connection of AlN phase. Therefore, more and more 3D ceramic skeletons are formed in the composites. From Fig.4 (e), it can be noticed that almost all the graphite clusters are surrounded by AlN skeleton in A30. Meanwhile, as seen from Fig.4, the orientation of AlN phase (red dot

ACCEPTED MANUSCRIPT line) drops slightly as its content arises, which means the disorder of graphite slightly increases. This phenomenon is in accordance with the Raman results shown in Fig.3(d). During the fabrication process, AlN arranges synergistically with the

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orientation of GF powders by the vacuum filtration and sintering pressure. In the vacuum filtration process, the deflection resistance of GF increases with the AlN content, which decreases the orientation degree of GF (see Fig.S1). In the SPS process,

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some GF powders around AlN slightly deformed by the axial sintering pressure due to

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the discrepancy of hardness between AlN and GF, which can be clearly observed in the insert image of Fig.4(c). After the fabrication process, most of the graphite layers stack parallel to each other with a tiny amount arranged disorderly (see Fig.S2). Hence, it can be easily comprehended that the orientation degree of GF is still high

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AlN content increases.

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(see Fig.3(c)) while the disorder of the graphite crystal increases (see Fig.3(d)) as the

Fig.4 Polished surface (parallel to z-axial direction) microstructures of (a)A10; (b)A15; (c)A20; (d)A25; (e)A30; (f) after removal graphite of A30.

ACCEPTED MANUSCRIPT To observe the AlN skeleton structure clearly, the A30 sample was decarburized by calcining in air at 700 oC for 5 h. The microstructure of the decarburized A30 (parallel to the z-axial direction) is shown in Fig.4(f). It can be seen that a continuous

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AlN skeleton structure (the red dot rectangles) is successfully formed and the directional holes distribute in the AlN network after the GF burning out. Meanwhile, the length of the AlN skeleton unit is similar to the lateral size of GF powders, and its

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height is about 1-5 µm, which means several GF particles stack together in a skeleton

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unit. Therefore, by using vacuum filtration combined with SPS, a continuous AlN skeleton bonded highly oriented GF composites can be prepared successfully. 3.3 Density and mechanical property

Fig.5(a) shows the density and relative density of the composites with different

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AlN contents. It can be found that both of the density and relative density increase gradually with the AlN content. The density increases from 2.29 g·cm-3 for A10 to 2.47 g·cm-3 for A30, a little higher than that of bulk graphite. The relative density of

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all the composites is higher than 96 %, indicating that the addition of AlN together

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with Y2O3 is helpful to densify the graphite-based composites.

Fig.5 (a) Density and relative density of AlN/GF composites with different AlN

ACCEPTED MANUSCRIPT content; (b) the bending strength of two principle directions of the composites The bending strength of the sintered samples tested along the two principle directions is plotted in Fig.5(b). The bending strength tested along x axis gradually

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rises from 73 to 89 MPa as the AlN content increases, and the strength tested along z axis also increases slightly from 83 MPa to 87 MPa. The strength of composites improved strength with the higher AlN content is not as significant as expected. The

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reason is that the AlN/GF composites have similar critical crack length (equal to the

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lateral size of GF) due to the weak mechanical behavior of graphite. In addition, it can be seen from Fig. 5(b) that the strength of the samples tested along z axis is a little higher than that tested along x axis. On one hand, more striped concave and convex areas can be seen from the fracture surface shown in Fig.6(a) when the loading is

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along z-axial direction, so the cracks consume more energy in the rupture process. On the other hand, similar crack propagation paths (schematically illustrated in Fig.6(c) and (d)) in these two directions result in the similar bending strength [7]. In any case,

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the strength of the as-fabricated AlN/GF composites is about 3 times in comparison

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with that of the preferred orientation graphite block reinforced by in-situ grown carbon nanotubes (27.1 MPa) [13], indicating the excellent reinforcement of 3D AlN skeleton. The formation of AlN skeleton not only benefits the densification of the graphite matrix, but also bears most of loading during the fracture due to its high mechanical properties.

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Fig.6 Fracture morphologies of A10 sample tested along (a) z axis and (b) x axis directions; schematic diagrams of crack propagation paths tested along (c) z axis and

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(d) x axis.

3.4 Thermal conductivity

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Fig.7 shows the thermal conductivity of AlN/GF composites in the z-axial and x-y plane directions with different AlN contents. Obviously, the thermal conductivity

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in z-axial direction is one magnitude lower than that of x-y plane direction, reflecting a remarkable anisotropy. Meanwhile, the z-axial thermal conductivity monotonously increases from 17 W·m-1·K-1 to 30 W·m-1·K-1 when the AlN content arises from 10 wt.% to 30 wt.%, which is caused by the higher thermal conductivity of AlN than that of graphite layers in perpendicular direction (15 W·m-1·K-1) [24].

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Fig.7 Thermal conductivity of the composites in the two principal directions

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It can also be seen from Fig.7 that the x-y plane thermal conductivity first

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increases and then decreases with the rising of AlN content, and achieving a maximum value of 442 W·m-1·K-1 when the AlN content reaches 20 wt.%. Generally, phonon propagation is largely degraded by scatterings from pores, defects and interfaces in ceramic composites, leading to the decrease of thermal conductivity [6].

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In our case of AlN/GF composites, GF grains deform and fracture under the sintering pressure, and the amount of the defects increases as the AlN content increasing, which has been verified in the above analyses. Additionally, more interface areas are

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produced in the composites with higher AlN content. Therefore, in theory, thermal

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conductivity of the AlN/GF composites in x-y plane direction should decrease gradually. But the experimental thermal conductivity of the composites in this direction increases when the AlN content is less than 20 wt.%. The crossover phenomenon could be explained in terms of different dominating factors before and after the successful formation of continuous AlN skeleton. Garret et al. [25] reported that the conductive barrier decreases as the particle size increases. In the AlN/GF composites, the AlN particles aggregate and sintered to be a skeleton when the AlN

ACCEPTED MANUSCRIPT content increases up to 20 wt.%. Thus, the phonon scattering of the composites can be reduced and the thermal conductivity is promoted with the higher AlN content. Furthermore, the rise in density and heat capacity with the addition of AlN into

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graphite matrix also promotes the thermal conductivity. After the AlN content exceeds 20 wt.%, the integral AlN skeleton can be formed which separates the GF grains, leading to the decrease of heat conduction paths in-plane of GF. Meanwhile, the

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increase of graphite crystal disorder and AlN/graphite interfaces with the AlN content

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increasing also induces high interface resistance. Therefore, the thermal conductivity of AlN/GF composites declines with AlN content increasing from 20 wt.% to 30 wt.%.

3.5 Thermal expansion and thermal residual stress

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Fig.8(a) and (b) displays the measured and the calculated CTE values in two principle directions of the composites. It can be seen that there is an obvious discrepancy of measured CTE in the two principle directions, which further

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demonstrates the high anisotropy of the composites. The measured CTE in x-y plane

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increases linearly from 1.18×10-6 K-1 to 3.67×10-6 K-1 with AlN content increasing from 10 wt.% to 50 wt.%. But the measured CTE in z-axial direction first drops quickly from 16×10-6 K-1 for A10 to 7.3×10-6 K-1 for A20 and then decreases slightly when the AlN content higher than 20 wt.%. Therefore, the AlN skeleton plays a vital role in tailoring the CTE of the composites. In order to understand the effect of AlN on the thermal expansion, three classical models were employed to calculate the theoretical values of CTE [26,27]:

ACCEPTED MANUSCRIPT The rule of mixture (ROM) model: =

+

(1)

Turner model: = Kerner model: =

+

+

(

)(

)

(3)

!

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(2)

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Where α, V, K and G represent the CTE, volume fraction, bulk modulus and shear

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modulus, and the subscripts c, r, m refer to the composite, reinforcement and matrix, respectively. It can be seen from Fig.8 (a) that the measured CTE values of composites in x-y plane are more consistent with the values estimated by the Kerner model (graphite matrix), confirming the presence of shear effects in the composites

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[28]. However, the predicted CTE values by the three models are much higher than the measured values in z-axial direction, especially when the AlN content exceeds 20 wt.%. This indicates that the formation of continuous AlN skeleton constrains the

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thermal expansion of the graphite matrix effectively, which is similar to the 3D

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ceramic network reinforced metal matrix with low CTE [29]. In addition, there exists a thermal residual stress between graphite and AlN phase when the composites were cooled from high temperature (1650 oC) to room temperature (25 oC) [30]. The thermal residual stress is calculated using Hsueh’s formula [31]: "=

( ! # $ )∆& '(+)# $ '()! ('()# $ )*! *# $

(4)

where σ0, α, ν, E, ∆T are the thermal residual stress, thermal expansion coefficient, Poisson’s ratio, Young’s modulus and the temperature change (negative for cooling),

ACCEPTED MANUSCRIPT respectively; subscripts G, AlN denote the graphite matrix and AlN reinforcement. Table 1 lists the above physical properties of AlN and graphite [32]. The thermal residual stress between AlN and graphite is calculated to be about -1.46 GPa based on

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Hsueh’s formula. According to elasticity theory, the stress induced strain restrains the

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thermal expansion of graphite matrix.

Fig.8 Measured and calculated CTE values in two principle directions of the

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composites: (a) x-y plane direction; (b) z-axial direction. Table 1. The physical properties of AlN and graphite CTE/×10-6·K-1 4.5 -1.5 28

E/GPa 320 1109 38.7

G/GPa 133 485 5

B/GPa 178 36.4

ν 0.2 0.12 0.01

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Physical property AlN x-y plane Graphite z axial

We now derive how the presence of thermally induced stress transforms the

thermal expansion of the composites. The strain ε in the composites with the temperature T and the thermal stress σ are connected via Hooke’s law [33]: , = -" + .

(6)

where S is the compliance and m is the thermal strain under zero external stress. In A20 sample, the difference of CTE in x-y plane between composites and intrinsic

ACCEPTED MANUSCRIPT CTE of graphite will cause the strain in x-y plane of graphite. The CTE difference in x-y plane is ∆

/

=

/

−

8

= 1.55 − (−1.5) = 3.05 × 10

1,

∙:

, which is

connected to the thermal derivation of strain by the following Equation: ;&

=∆

Moreover, the x and y-axial stresses " ,

(7)

/

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;<''

= "== = " are related to the strains by

= ,== = (-

+ - = )"

(8)

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Because graphite is a hexagonal crystal, the strain along z-axial direction is obtained according to elasticity theory: =@'

=A'

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,>> = 2- > " = @

'' @++

,

=−

A

,

(9)

where Cij is the elastic stiffness constant of graphite. Inserting Equation (9) into Equation (6), the strain along z-axial direction can be rewritten as: =A' A

, (B) + .>> (B)

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,>> (B) = −

(10)

here m33(T) is the thermal strain under ambient condition. The temperature derivative of Equation (10) is the CTE of graphite along z-axial direction: =

;<

= C=D ∆

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>>

;&

/

+

;E+F ;&

,

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with C=D = − and

;E+F ;&

+

=A' A

1,>

= 13 × 10 8 K

= −0.83

(12)

=

(13)

= 4.3 × 10

K

(11)

as the two-dimensional equivalent of Poisson’s ratio and its temperature derivative, respectively. The elastic constants are C13=15 GPa, C33=36 GPa, and their temperature derivatives are dC13/dT=-0.8 GPa·K-1 and dC33/dT=-0.05 GPa·K-1 [34]. The residual strain in x-y plane was estimated as ,

≈ −0.3 × 10 > . These result in a CTE in

z-axial direction of α33=13×10-6 K-1. We replaced αG,3 =28×10-6 K-1 by α33=13×10-6

ACCEPTED MANUSCRIPT K-1 to modulate Kerner model for the thermal expansion of AlN/GF composites. The αz of the composites fits well with the modulated model except for the sample A10. It can be understood that the AlN addition in A10 distributes dispersedly and does not

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form a continuous skeleton in the graphite matrix. The thermal stress in the GF induced by the formation of 3D AlN skeleton architecture strongly reduces the CTE of graphite matrix in z-axial direction from 28×10-6 K-1 to about 7×10-6 K-1.

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Fig.9 exhibits the correlation between thermal conductivity and expansion. Light

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purple area represents the acceptable CTE band to match that of semiconductor. The fabricated AlN/GF composites display outstanding thermal properties compared with most of primary thermal management materials. The thermophysical properties of the AlN/GF composites can be tailored effectively by the 3D AlN network in the matrix.

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The sample A20 has the most excellent thermophysical properties (λ=442 W·m-1·K-1, CTE=7.3×10-6 K-1) compared with other AlN/GF composites with different AlN contents. The materials comparable in their thermal properties to our samples are

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diamond/Cu-Ti, GF/Al-Si and GF/Cu composites [14]. However, the diamond/Cu-Ti

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composite is hard to manufacture due to the high price of diamond and the poor machinability [28], while the flexural strength of GF/Al-Si composites is only 16.9 MPa which limited its application [11]. Hence, highly oriented graphite-based composites restrained by the 3D AlN skeleton have a prominent application potential as advanced thermal management materials.

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Fig.9 Ashby plot of AlN/GF composites and several key thermal management

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materials. Light purple area represents the acceptable CTE band to match with that of

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semiconductor materials.

4. Conclusions

3D AlN ceramic skeleton reinforced high orientated GF (AlN/GF) composites

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were proposed and prepared by combing vacuum filtration and SPS method in sequence to obtain a light and strong thermal management material. The results can be summarized as follows:

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(1) The addition of AlN together with Y2O3 effectively promoted the densification

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and mechanical properties of the composites. The relative densities for all the samples were higher than 96%, and bending strength for which was about 80 MPa. (2) The orientation of GF grains in all the composites was high (Lotgering factor

f > 98%). The increase of AlN content did not influence the orientation significantly, but enhanced the disorder of GF crystals in the composites, especially when the AlN content was higher than 20 wt.%. (3) A continuous ceramic skeleton was formed when the AlN content increased to

ACCEPTED MANUSCRIPT 20 wt.%, which can tailor the thermal expansion effectively by introducing the thermal stress in the composites, and hence strongly reduce the through-plane thermal expansion of graphite.

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(4) The 20wt.% AlN/GF composite possessed the optimal comprehensive properties with good bending strength of >80 MPa, high in-plane thermal

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conductivity of 442 W·m-1·K-1 and low through-plane CTE value of 7.3×10-6 K-1.

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Acknowledgments

This work was supported by the National Key Research and Development Program of China (2017YFB0310400), the Natural Science Basic Research Plan in Shaanxi Province of China (2017JM5033) and the Fundamental Research Funds for

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the Central University (XJJ2015104).

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Luedtke A. Thermal management materials for high-performance applications. Adv Eng Mater 2004;6(3):142–4.

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