Preparation of copper-diamond composites with ...

Applied Thermal Engineering 60 (2013) 423e429

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Preparation of copperediamond composites with chromium carbide coatings on diamond particles for heat sink applicationsq Qiping Kang, Xinbo He*, Shubin Ren, Lin Zhang, Mao Wu, Caiyu Guo, Wei Cui, Xuanhui Qu School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China

h i g h l i g h t s Cr7C3 coatings were successfully formed on diamond particles in molten salt. Cr7C3 coatings obviously promoted the wettability of diamond and copper matrix. Cr7C3 coatings greatly enhanced thermal conductivity of Cuediamond composites. The composites are suitable candidates for being heat sink applications.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 1 March 2013 Accepted 21 May 2013 Available online 14 June 2013

Cr7C3 coatings were formed on diamond particles for improving the wettability between diamond particles and copper. The coatings were formed with a reaction medium of chromium in mixed molten salt. Copperediamond composites with Cr7C3 coatings on diamond particles have been fabricated by vacuum pressure infiltration method. The microstructure of interfacial bonding between diamond and copper was discussed. Thermal conductivity and thermal expansion behavior of the obtained copper ediamond composites with various fractions of diamond particles were investigated. The as-fabricated composite exhibit a thermal conductivity of 562 W m1 K1 associated with coefficient of thermal expansion of 7.8 106 K1 with a Cr7C3-coated diamond volume fraction of 65%. These results indicate that the obtained composites are suitable for being heat sink applications. Ó 2013 The Authors. Published by Elsevier Ltd. All rights reserved.

Keywords: Metal matrix composite Coatings Thermal properties Interfacial thermal resistance

1. Introduction Thermal considerations in power electronic devices have become extremely important with the continuing miniaturization and integration of microelectronics, requiring ever more efficient heat dissipation. Metal matrix composites (MMCs) offer the possibility to tailor the properties of a metal by adding an appropriate reinforcement phase and to meet the demand for thermal management. The ideal thermal management materials working as heat sink applications should have thermal conductivity above 300 W m1 K1 with a low and tailorable coefficient of thermal expansion (CTE) of 4e9 106 K1 for future electronic component, such as microprocessors, LED, laser diodes or high power [1,2]. Traditional heat sink materials such as Mo/Cu, W/Cu, SiC/Al or

q This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-No Derivative Works License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited. * Corresponding author. Tel.: þ86 10 62332727; fax: þ86 10 62334311. E-mail address: [email protected] (X. He).

SiC/Cu, however, despite featuring matched CTEs, have thermal conductivities lower than 300 W m1 K1. Diamond has the highest thermal conductivity in nature, w2200 W m1 K1, as well as an appropriate coefficient of thermal expansion (CTE) of 2.3 106 K1. These properties make diamond an ideal reinforcement in metal matrix composites for heat sink applications. Copper is widely used as heat-spreading material because it exhibits high thermal conductivity (400 W m1 K1). However, it presents a large CTE (17.0 106 K1) which is too high as heat sink applications. Therefore, in recently, the copper matrix composites reinforced with diamond particles have been extensively investigated [3e6]. Unfortunately, copper is known to be naturally non-wetting with diamond due to chemical incompatibility, leading to weak interfacial bonding and high thermal resistance. The interface plays a crucial role in determining thermal conductivity and CTE of copper/diamond composites [7e9]. At present, the ways to improve the interfacial bonding include metal matrix alloying and modification of diamond particles. The addition of Zr to copper matrix has a thermal conductivity of 533 W m1 K1 in CueZr/diamond composites [10]. Zhang et al. [9] have reported a thermal

1359-4311/$ e see front matter Ó 2013 The Authors. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.applthermaleng.2013.05.038

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Q. Kang et al. / Applied Thermal Engineering 60 (2013) 423e429

conductivity of 493 W m1 K1 using Ti-coated diamond particles to reinforce copper matrix. The present study focuses on copper matrix composites reinforced with Cr7C3-coated diamond particles, and the composites are fabricated by vacuum pressure infiltration method. The Cr7C3 coatings were successfully synthesized on diamond particles surfaces using molten salt method for the first time. It is revealed that the Cr7C3 coatings on diamond particles have good wettability with copper, and metallurgical interfacial bonding was formed to decrease the interfacial thermal resistance of composites. The intrinsic contribution of the Cr7C3 coatings to enhance the thermal conductivity of copperediamond composites has been discussed. Microstructure, interfaces, thermal conductivity and thermal expansion behavior of the obtained composites were further investigated. 2. Experimental The infiltrated oxygen free copper with purity more than 99.99% was used as matrix. Diamond particles used in this study are synthetic MBD8 grade diamond with nitrogen content of 230 ppm, purchased from Polaris Diamond Powder Co. Ltd., China. The thermal conductivity is estimated to be about 1500 W m1 K1 according to the level of nitrogen content. The diamond particles are faceted with hexagonal or octahedral shapes and diameters around 70 mm (Fig. 1(a)). The chromium powders of w30 mm with purity more than 99.8% were used as coating materials (Fig. 1(b)). The chromium carbide coatings were synthesized on diamond particles surfaces using the molten salt method. Diamond particles and chromium powders were mixed uniformly with molar ratio of 10:1 and then embedded in an alumina crucible with a mixture of chloride salt (mol ratio of NaCl:KCl ¼ 1:1), heat-treated at 900 C for 60 min in a tube furnace under high purity argon atmosphere and then cooled to room temperature. The processed powders were separated by ultrasonic wave with boiling distill water, and then the coated diamond particles were dried in the vacuum drying oven at 120 C for 60 min. The diamond porous preforms used for the fabrication of copperediamond composites were prepared by two-step procedure: that is, pressing the mixture of coated diamond particles with various fractions (50e70 vol.%) and organic binder into a cylindrical mold; then degreasing the binder in high purity hydrogen atmosphere at 400 C for 1 h. Copperediamond composites with chromium carbide coating on diamond particles were fabricated by vacuum pressure infiltration. The diamond preform and copper were positioned in graphite die, and then heated at 1150 C for 10 min. After that, a pressure of 20 MPa was applied to infiltrate the molten copper into the diamond preform for 5 min. The heating was switched off and the system was cooled down in the furnace. The copperediamond composites with uncoated diamond particles were also fabricated for comparison.

The phase structures and microstructure of coated diamond particles were characterized by Siemens D5000 X-ray diffraction (XRD) using Cu radiation and LEO1450 scanning electron microscopy (SEM). The interface area on the composites fracture surfaces and energy disperse spectroscopy (EDS) element line scanning across the interface was characterized by ZEISS ULTRA 55 SEM. The bulk densities of the composites were measured using Archimedes’ method and compared with the theoretical densities. The theoretical density (rt) of Cuediamond (carbide-coated) was calculated using the rule of mixture:

rt ¼ rd $Vd þ ri $Vi þ rm $Vm

(1)

where r is density, and V is volume fraction. The subscript “d”, “i” and “m” refer to diamond, interfacial coating and copper matrix, respectively. Samples were machined by laser processing. The specific heat capacity was derived from the theoretical value calculated by rule of mixture: a) Composites with uncoated diamonds

Cc ¼

Cd $Vd $rd þ Cm $Vm $rm

rc

(2)

b) Composites with carbide-coated diamonds

Cc ¼

Cd $Vd $rd þ Ci $Vi $ri þ Cm $Vm $rm

rc

(3)

where C is specific heat capacity. The subscript “c” refers to composite. Thermal diffusivity (a) of the sample (sample size: F10 mm 3 mm) was measured by Netzsch LFA427 Laser Flash machine at room temperature. Thermal conductivity, l, was calculated from thermal diffusivity measurement using the following formula:

lc ¼ a$Cc $rc

(4)

The CTEs of composites were measured by a dilatometer (DIL 402 C NETZSCH, sample size: 3 mm 4 mm 25 mm), and the device was calibrated using an alumina sample. The test temperature ranged from 25 to 100 C at a heating rate of 3 C/min. The CTE was evaluated according to the relationship of CTE ¼ Dl/(lDT), where Dl, land DT are the thermal expansion displacement, the original dimension of sample, and the temperature change, respectively. The uncertainty in the thermal measurements was 5%.

Fig. 1. SEM images of (a) the diamond particles and (b) the chromium powders.


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3. Results and discussion 3.1. Formation of Cr7C3 coatings on diamond particles Fig. 2 displays the phase composition of the diamond particles after coating. As can be seen in Fig. 2, only Cr7C3 and diamond could be identified, and the lattice parameters of Cr7C3 (a ¼ 7.02 A, b ¼ 12.15 A, c ¼ 4.53 A) were in good agreement with that of Cr7C3 in the literature [11]. Fig. 3 illustrates the morphology of the coatings on diamond particles. It is very clear that the white coatings were uniformly and compactly coated on the surfaces of diamond particles, and the coated diamond particles maintained the original shape. Combined with the XRD patterns analysis, these white coatings on diamond particles were Cr7C3. The dissolution of metals in molten salt is not well understood, but it has been suggested that transport reactions occur because metals dissociated to mobile cations and delocalized electrons, a state which is considered to be intermediate between ionic and metallic [12,13]. The molten salt mixture is thus believed to facilitate the dissolution and transport of the chromium, and hence the formation of the Cr7C3, through diffusion of chromium cations from the molten salt to the surface of the diamond particles with subsequent reaction. Prior research showed that the chemical reaction in molten salt system was easier and more uniform [13]. 3.2. Microstructure of copperediamond composites Fig. 4 shows the SEM images of fracture of copperediamond composites with uncoated and Cr7C3-coated diamond particles, respectively. As can be seen from Fig. 4(a), poor interfaces exist between the copper matrix and diamond particles, and gaps show obviously at the interfaces. This is most likely due to the lack of wettability between the copper matrix and diamond particles. However, in Fig. 4(b), the Cr7C3-coated diamond particles uniformly dispersed in the copper matrix and had intimated contact with copper, which indicate the Cr7C3-coated diamond particles and copper matrix have strong interface bonding. The Cr7C3 coatings promoted the wettability and interface bonding between diamond particles and copper matrix, and compact copperediamond composites were obtained.

Fig. 3. SEM images of Cr7C3-coated diamond particles.

Fig. 5 displays the morphology of polished copperediamond composites. As can be seen from Fig. 5(a), the Cr7C3 intermediate coatings are shown clearly on the margin of diamond particles, and the Cr7C3-coated diamond particles filled in copper matrix homogeneously. Fig. 5(b) and (c) shows EDS line-scan analysis of the intermediate coatings across the interface between diamond particles and copper. The Cr7C3 coatings were about 1 mm, and there was copper infiltrated into the intermediate coatings. 3.3. Density and bending strength of copperediamond composites Copperediamond composites with diamond volume fractions ranging from 50% to 70% were fabricated. As shown in Fig. 6, the relative densities of composites with Cr7C3 coatings on diamond particles ranged from 99.3% to 97% for diamond volume fractions ranging from 50% to 70%, respectively. We observed an obvious decrease of the composites density with Cr7C3-coated diamond volume fractions ranging from 65% to 70%. This result indicates that the percolation threshold (i.e. the reinforcement volume fraction at which it exists a continuous path between reinforcements into the composite) has been reached. The percolation threshold is thus located between 65% and 70% in diamond volume fraction. However, the relative densities of composites with uncoated diamond particles were much lower than composites with Cr7C3-coated diamond particles, and the relative densities decrease regularly with the diamond volume fractions increasing. As can be seen in Fig. 7, the bending strength presents the same phenomenon as the relative density of copperediamond composites. These results confirm that the Cr7C3 coatings have a positive effect on improving the wettability and interface bonding between diamond and copper. 3.4. Thermal conductivity of copperediamond composites

Fig. 2. XRD patterns of Cr7C3-coated diamond particles.

In order to profoundly understand the thermal conductivity behavior of copperediamond composites, it is necessary to compare the experimental results with theoretical predictions. Many researchers have constructed theoretical models to describe the impact of interface on the thermal conductivities of composites. The HasselmaneJohnson (HeJ) model [14] is proved to be the most accurate because it takes into account the combined effects of particles size, volume fraction and interfacial thermal resistance. The model can be represented by:

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Fig. 4. SEM images of fracture surfaces of copperediamond composites with diamond particles (a) uncoated and (b) Cr7C3-coated.

lc ¼ lm

½ld ð1 þ 2Rlm =aÞ þ 2lm þ 2Vd ½ld ð1 Rlm =aÞ lm ½ld ð1 þ 2Rlm =aÞ þ 2lm Vd ½ld ð1 Rlm =aÞ lm

h ¼ p$q (5)

where l stands for thermal conductivity, a is the radius of spherical reinforcement and R is the interfacial thermal resistance, respectively. The interfacial thermal resistance R can be theoretically estimated by using the acoustic mismatch model (AMM) [15,16]:

R ¼

4

rm Cm vm h

(6)

where rm is the density, Cm is the specific heat, vm is the Debye velocity of the matrix, and h is the average probability for the transmission of the phonons across the interface into the particles. The value of h can be estimated by:

(7)

where p is the transmitted probability of a phonon incident within the critical angle qc, which can be calculated by Refs. [15,16]:

p ¼

4Zm Zd ðZm þ Zd Þ2

(8)

where Zm, Zd is the acoustic impedance of the matrix and reinforcement, which can be calculated by Z ¼ r$v. Where q is the fraction of the phonons incident on the interface within the critical angle qc from the matrix side, because only these phonons have the opportunity of being transmitted:

q ¼

1 2 1 nm 2 sin qc ¼ 2 2 nd

Fig. 5. SEM images of composites with Cr7C3 coating on diamond particles (a) polished and (b, c) EDS line-scan analysis across the interface.

(9)


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Table 1 Parameters for interfacial thermal resistance calculation of copperediamond composites [15,17].

Fig. 6. The relative density of copperediamond composites with uncoated and Cr7C3coated diamond particles.

Thus, Eqs. (7)e(9) can be substituted into Eq. (6), and the interfacial thermal resistance can be expressed by:

R ¼

2ðrm vm þ rd vd Þ2 vd 2 2 2 Cm $rm $vm $rd $vd vm

(10)

In this experiment, the interfacial thermal resistance accounting for copperediamond composites consists of three terms: namely, the resistance at the diamond/Cr7C3 interface, the resistance of the molybdenum carbide itself, and the resistance of the Cr7C3/copper interface. The total interfacial thermal resistance of copperediamond composites is signed by RT, RT ¼ RdiamondjCr7C3 þ RCr7C3 þ RCr7C3jcopper. When RdiamondjCr7C3 is calculated, we can assume that the Cr7C3 is the matrix and diamond is the reinforcement. However, when RCr7C3jcopper is calculated, copper is the matrix and Cr7C3 is the reinforcement. RCr7C3 can be estimated by using RCr7C3 ¼ L/l, where L is the thickness and l is thermal conductivity of the Cr7C3 coatings, respectively. Table 1 lists all values of parameters in the above equation, and the thickness of Cr7C3 coating is 1 mm. We obtained the

Fig. 7. The bending strength of copperediamond composites with uncoated and Cr7C3-coated diamond particles.

Material

r (g/cm3)

C (J/kg K)

l (W m1 K1)

v (ms1)

Diamond Copper Cr7C3

3.52 8.96 6.92

510 385 543

1500 400 27

13,430 2801 4631

RdiamondjCr7C3 ¼ 0.4 108 m2 KW1, RCr7C3 ¼ 3.7 108 m2 KW1 and RCr7C3jcopper ¼ 0.23 108 m2 KW1. Therefore, the total interfacial thermal resistance of the composites RT ¼ 4.33 108 m2 KW1. The calculation results indicate the thermal resistance of Cr7C3 coatings is the major factor in the interfacial thermal resistance of copperediamond composites. Substituting the theoretical calculation of interfacial thermal resistance into HeJ model Eq. (5), the comparison of thermal conductivities between our experiment results and theoretical predictions are shown in Fig. 8. As seen, the prediction of thermal conductivities were some lower than experimental data. Meanwhile, Fig. 8 shows that the thermal conductivity of copperediamond composites increases regularly to 562 W m1 K1 with respect to Cr7C3-coated diamond volume fraction increasing to 65%. We observed a drop of thermal conductivity to 522 W m1 K1 with the Cr7C3-coated diamond volume fraction increasing to 70%, which maybe due to the diamond volume fraction exceeds the percolation threshold and some coated diamonds contact each other. However, the thermal conductivity is only 230 W m1 K1 with 50 vol.% uncoated diamonds and decreases regularly with uncoated diamond volume fractions increasing. The composites with uncoated diamond particles have rather low thermal conductivities, which indicate high interfacial thermal resistance between copper matrix and diamonds. As seen in Fig. 9, the theoretical prediction of interfacial thermal resistance using AMM is some higher than that experimentally derived measurements using HeJ model, which maybe due to the fact that the copper infiltrates into the Cr7C3 intermediate layers and leads to the thermal conductivity of the coatings more than 27 W m1 K1. Thus, the theoretical prediction using AMM overestimates interfacial thermal resistance of composite. The experimentally derived interfacial thermal resistance using HeJ model is w2.9 108 m2 KW1, which is on the same order of magnitude as

Fig. 8. Experimental thermal conductivity of copperediamond composites with compared with theoretical prediction.

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Fig. 9. The comparison of interfacial thermal resistance experimentally derived measurements using HeJ model with theoretical prediction using AMM.

Fig. 10. The experimentally measured CTEs of copperediamond composites (uncoated and Cr7C3-coated diamonds) compared with the predictions of Turner and Kerner models.

theoretical prediction using AMM. Meanwhile, we observed the interfacial thermal resistance increases abruptly with the Cr7C3coated diamond volume fraction increasing to 70%, which agrees with the thermal conductivity of composites. However, in term of the HeJ model, we can evaluate the interfacial thermal resistance of copperediamond composites with uncoated diamond particles is more than 3.0 107 m2 KW1, which is one order of magnitude higher than with Cr7C3 coatings on diamond particles. These results confirm that the Cr7C3 intermediate coatings greatly decrease the interfacial thermal resistance and enhance the thermal conductivities of copperediamond composites.

occur with a change of temperature in these composites. This effect can be associated with the metallurgical bonding between the copper matrix and Cr7C3-coated diamond particles. The Cr7C3 intermediate coatings on the surfaces of diamond particles contribute more restriction of copper matrix expansion than those of other places, and the CTE of composite is 7.8 106 K1 when the Cr7C3coated diamond volume fraction add to 65%. However, unlike thermal conductivity, the CTE continuously decline with the Cr7C3coated diamond volume fraction increasing to 70% because the more interfaces bonding means the more restriction of diamond particles and copper matrix. On the other hand, the CTEs of composites with uncoated diamond particles are much higher than Turner and Kerner model prediction. Even the uncoated diamond volume fraction reach 65%, the CTE of composite is still more than 13 106 K1. These suggest weaker interfacial bonding between copper and diamond in composites with uncoated diamond than in composites with Cr7C3-coated diamond. Thus, the Cr7C3 intermediate coatings played a positive role to limit the thermal expansion of copper because the strong interfacial bonding between the copper matrix and diamond. The Cr7C3 coatings effectively adjust the expansion of diamond particles and copper matrix.

3.5. CTEs of copperediamond composites There are two originally models were proposed for prediction of composite CTE. The fist one was the Turner model [18], which assumes the strain in the composite is homogeneous and the shear deformation is negligible. The expression for the composite CTE (ac) is obtained as:

ac ¼

am Vm Km þ ad Vd Kd Vm Km þ Vd Kd

(11)

4. Conclusions

where a,V and K are the CTE, volume fraction and bulk modulus, respectively. The second one was the Kerner model [19], which assumes the shape of particle is spherical and the shear deformation is also taken into account. The composite CTE (ac) can be expressed as:

ac ¼ ad Vd þ am Vm þ ðad am Þ Vd Vm

Kd Km Vd Kd þ Vm Km þ ð3Kd Km =4Gm Þ

(12)

where Gm is the shear modulus of matrix. The model predictions of the CTE for copperediamond composites are obtained with the following matrix and particle properties [20]: aCu ¼ 17 106 K1, KCu ¼ 140 GPa, GCu ¼ 49 GPa, adiamond ¼ 2.3 106 K1 and Kdiamond ¼ 580 GPa. The experimental data of copperediamond composites are compared with Turner and Kerner model prediction in Fig. 10. The CTEs of composites with Cr7C3 coatings on diamond particles follow the Kerner line and these suggest that shear deformation does

Cr7C3 coatings were successfully formed on diamond particles by molten salt method. The compact copperediamond composites were fabricated through infiltration of pure copper into preforms of Cr7C3-coated diamond particles by vacuum pressure infiltration. The as-fabricated composites were investigated from both aspects of microstructure and thermal properties. The obtained composites exhibit promising thermal performances by combining thermal conductivity of 562 W m1 K1 and CTE of 7.8 106 K1 with diamond volume fraction of 65%. Compared to the composites with uncoated diamond particles, the Cr7C3 intermediate coatings greatly improve the interface bonding and decrease the interfacial thermal resistance of copperediamond composites, which make the composites suitable for being heat sink applications. Acknowledgements This work was financially supported by “the Fundamental Research Funds for the Central Universities (FRF-TP-10-003B)” and


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