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copper composites
Composites: Part B 55 (2013) 1–4
Contents lists available at SciVerse ScienceDirect
Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
Effect of tungsten addition on thermal conductivity of graphite/copper composites Wulin Yang, Lingping Zhou ⇑, Kun Peng, Jiajun Zhu, Long Wan College of Materials Science and Engineering, Hunan University, Changsha 410082, China
a r t i c l e
i n f o
Article history: Received 12 April 2012 Received in revised form 17 September 2012 Accepted 20 May 2013 Available online 31 May 2013 Keywords: A. Metal–matrix composites (MMCs) E. Surface treatments B. Interface B. Thermal properties
a b s t r a c t Graphite/copper composites with high thermal conductivity were fabricated by tungsten addition, which formed a thin tungsten carbide layer at the interface. The microstructure and thermal conductivity of the composite material were studied. The results indicated that the insertion of tungsten carbide layer obviously suppressed spheroidization of copper coating on the graphite particles during the sintering process, and decreased the interfacial thermal resistance of the composites. Compared with the graphite/copper composites without tungsten, the thermal conductivity of the obtained composites was increased by 43.6%. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction With the rapid development of microelectronic device and industry, the power density increase rapidly, the ability to dissipate heat becomes a very important factor for microelectronic and optoelectronic devices [1]. Therefore, development of materials with high thermal conductivity and optimal thermal expansion coefficient similar to that of silicon is imperative for heat sinks and heat spreaders. Many materials such as CuMo, CuW, Al/SiCp, Cu/diamond are usually used for heat sink with high thermal conductivity and low coefficient of thermal expansion. However, high density of CuMo and CuW, high price and poor machinability of Al/SiCp and Cu/diamond limited their application more or less. Graphite/Cu metal–matrix composites would be candidate materials for thermal management application for their acceptable thermal and electrical conductivity, low coefficient of thermal expansion, low density and good machinability [2–3]. But high interfacial energy of the C–Cu system lead to de-wetting between the graphite and Cu metal matrix, which causes phase segregation and pore during the sintering stage at elevated temperatures [4]. In order to avoid this phenomenon, copper alloying with strong carbide forming elements was used to prepare the carbon/copper composites [5–9], it was reported that Ti or Cr addition could effectively reduce the contact angle between liquid Cu and graphite. However the addition of alloying element signifi⇑ Corresponding author. Tel./fax: +86 731 88822663. E-mail address: [email protected] (L. Zhou). 1359-8368/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compositesb.2013.05.023
cantly reduces the thermal conductivity of the copper matrix [10]. Neubauer reported that the thermal conductivity of copper rapidly reduced from 400 W/m K to 175 W/m K with 1 wt% titanium in it [11], and thick chemical bonding interphases between the matrix and particles enhance the interfacial thermal resistance. Therefore, alloy element addition has a significant impact on the comprehensive performance of the composites, should be strictly controlled [10]. Recently researches indicate that surface treatment of particles by coating transition element is an effective method to improve the wettabilities and thermal conductivities of carbon/metal composites. Bonding strength and thermal physical characteristics are both increased by forming a thin carbide interlayer during the manufacturing process or heat treatment [12,13]. It was reported that Ti, Cr, Si, Zr atoms had shown an obvious diffusion into copper [14,15], and almost no solubility of Mo, W in copper could be detected. Taking the effect of carbide former on the effective thermal conductivity of the matrix into consideration, Mo or W is the promising element in surface modification of particles [16,17]. On the other hand, the wetting angle of copper on tungsten carbide is about 17°, lower than that of other carbides. And the thermal conductivities of tungsten and corresponding carbide are higher than other transitional elements. So tungsten may be the preferable carbide former for surface modification on graphite in the fabrication of graphite/copper composites. In this paper, near-net-shaped graphite/copper composites with tungsten carbide interfaces were successfully fabricated by vacuum hot press Cu coated graphite composite powder. The effect of tungsten addition on thermal conductivity and microstructure of graphite/copper composites are studied. Suppressing the de-
2
W. Yang et al. / Composites: Part B 55 (2013) 1–4
wetting or spheroidization of the electroless Cu coating by the element will be also mentioned.
2. Experimental 2.1. Pre-coated tungsten Synthetic graphite particles with isotropic thermal properties were used for synthesis graphite/copper composites. The particle size of graphite is about 30–150 lm. The thermal conductivity of the particles is about 45 W/m K. To improve the interfacial bonding, graphite particles were pre-coated W by magnetron sputtering. The target-substrate distance was 3.5 cm. Sputtering from a tungsten target was performed at 150 W DC power in Ar working gas with 40 min, and the working pressure was 1 Pa. The particles were continually moved by vibration and translation motions during the deposition process to obtain homogenous coating.
2.2. Electroless plating and fabrication processes The electroless copper plating process without sensitization and activation was used to deposit copper on the surface of graphite. In consideration of the coefficient of thermal expansion of the composites used for thermal management must be adapted to those of semi-conductors or ceramic insulators, the volume fraction of graphite particle amounted to approximately 70% of all samples. Composition of the electrolysis bath solution was shown in Table 1. After plating, the composite powder was washed with distilled water and absolute ethyl alcohol in turn, and then quickly dried in an infrared furnace at 80 °C for 20 min. Purified Ar and H2 gas (10:1) was introduced into the tube furnace to remove the possible existence of oxide in the composite powder at 400 °C for 1 h. Subsequently, the composite powder was sintered by vacuum hot pressing at 950 °C, 40 MPa with the soaking time of 30 min.
3. Results and discussion The morphology of the composite powder with electroless plating is shown in Fig. 1. And from the observation of X-ray diffraction pattern, the oxides such as CuO and Cu2O were not formed in the process of composite powders. The copper microlite were formed on the surface of the graphite particles, but did not form a complete coating, still part of the graphite exposed as shown in Fig. 1a. The copper coating on the graphite particles with precoated tungsten was continuous and compact, and the copper microlite was much smaller (Fig. 1b), indicating that the precoated tungsten was conducive to the electroless copper plating. Electroless copper deposition can be described as nucleation and grain growth processes, once the nucleation of copper is initiated on the graphite particles, the Cu atoms themselves act as a catalyst for further Cu deposition. The nucleation sites were limited on the without pretreatment graphite particles, and the growth mechanism was favored. Compared with raw graphite particles, the uniformly distributed tungsten atoms which were used to nucleation sites for the copper atoms deposited favored the nucleation process. For this reason, two different coating behaviors were presented. Micrographs of sintered graphite/copper composites are shown in Fig. 2. Fig. 2a is the microstructure of Cu coated graphite composite without interfacial modification, the copper phases appear obvious segregation. Compared with the former, as seen in
2.3. Characterizations and testing The morphology, surface composition of composite powders and microstructure of bulk composites were characterized by Scanning Electron Microscope (SEM, FEI Quanta 200) equipped with Energy-dispersive X-ray spectroscopy (EDS). X-ray diffraction (XRD, SIEMENS D5000) was employed to analyze the constituent of the composite powder. The mass of the copper coating was measured by weighing method. Sintered density of the sample was determined by Archimedes’ principle. The thermal conductivity in the pressure direction was calculated by means of the equation that relates this magnitude with the thermal diffusivity (a) as:
j ¼ a Cp q
ð1Þ
where q is the bulk density, and Cp is the specific heat at constant pressure of the composite. In this work, the specific heat was calculated by means of the linear rule of mixture using the values of Cp for the different components. Table 1 Composition of electroless bath solution. Agent
Formula
Role in bath solution
Concentration
Cupric sulphate EDTA-2Na Sodium hydroxide Formaldehyde
CuSO45H2O C10H14N2Na2O8 NaOH HCHO
Coating ions (Cu2+) Complexing agent Buffering solution Reducing agent
15 g/L 30 g/L 16 g/L 15 ml/L
Fig. 1. Surface morphology of composite powder (a) Cu coated graphite with none interlayer and (b) Cu coated graphite with tungsten interlayer.
W. Yang et al. / Composites: Part B 55 (2013) 1–4
3
in the composites. In order to confirm this suppose, the evolution of de-wetting or spheroidization of the copper coating of particles was studied at 950 °C, soaking 30 min in the same vacuum condition. Fig. 3 shows the de-wetting or spheroidization of the copper coating on particles. The formation of these isolated spherical particles of copper is linked to the nucleation process and wettability. This nucleation process is different from that of electroless plating, but is closely related. During the heat treatment, the limited small neighboring particles which were deposited the condition far from equilibrium are urgent to contracted together to reduce their surface energy [12], and the wettability of C–Cu system is poor. So the bigger isolated spherical particles of copper were formed on the low nucleus density area. In the case of graphite with tungsten interlayer, there is also the same process with the formation of spherical particles of copper (see in the bottom left part of Fig. 3b). But the interlayer improved the wettability of graphite by copper, the most copper particles were contacted with each other and the nucleation was much more homogenous, these were all conducive to spread the copper coating and only sparsely scattered holes appeared on copper coating. The discontinuous copper coating and exposed graphite particles would become to obstacles to the densification of composite. Furthermore, an interfacial gap which sharply reduced the interfacial conductance would be also developed between the raw graphite particles and copper coating. In order to meet the high performance of the composites, the insertion of tungsten interlayer
Fig. 2. Microstructure of graphite/copper composite made from graphite with surface modification (a) raw graphite and (b) W-coated.
Fig. 2b, the tungsten pre-coated graphite particles distributed relatively homogeneous, and a near-net-shaped copper matrix was formed. Refer to the property of the composites, the thermal conductivity of the composites was calculated by Eq. (1) and the results were presented in Table 2. The thermal conductance of the composites with the intermediate layer was higher than that of without interlayer one, increased by almost 43.6%. The addition of alloy element improves the wettability of graphite by copper, and reduces the interface thermal resistance. A similar phenomenon was reported in the diamond/copper system with Cr or B alloying element in the metal matrix [18]. Comparing the two different composite microstructures, the segregations of copper in Fig. 2a indicate that the effect of tungsten on the evolution of the electroless copper coating on the graphite particles during the sintering could not be ignored. The surface modification plays a significant role in organizing the homogeneous microstructure of graphite/copper composites. Otherwise, the copper coating would be separated from the graphite particles during the sintering stage, and formatted the segregation of copper Table 2 Physical properties of graphite/copper composites. Properties
Cu@C
Cu–W@C
Density (g/cm3) Specific heat capacity (J/g K) Thermal diffusion (mm2/s) Thermal conductivity (W/m K)
4.05 0.502 54 110
4.08 About 0.502 77 158
Fig. 3. De-wetting or spheroidization of Cu coating on graphite (a) raw graphite and (b) W pre-coating.
4
W. Yang et al. / Composites: Part B 55 (2013) 1–4
combined effects of the near-net-shaped copper matrix and good bonding interface of the composites. Cu
4. Conclusions
W
Graphite/copper composites were fabricated by surface modification, electroless copper coating and hot pressing route. A good dispersion of graphite particles in the copper matrix could be achieved. With the insertion of alloying element interlayer, continuous and compact copper coating was obtained by electroless plating. The alloying element layer obviously suppressed the dewetting or spheroidization of copper coating. Compared with conventional powder metallurgy, higher thermal conductivity was obtained through this approach. W pre-coating by physical vapor deposition is an available method to fabricate graphite/copper composites with high thermal conductivity. The thermal conductivity of the obtained graphite/copper composites reached 158 W/m K.
C
Fig. 4. SEM of graphite/copper composites made from graphite particles with tungsten interlayer.
Acknowledgements
Intensity (CPS)
at the Cu/C interface may be an essential part of suppressing dewetting or spheroidization and preventing from the formation discontinuous coating and interfacial gaps. It is beneficial for copper distribution by a network during the sintering and improving the interfacial contact of the composites. According to the result of the thermal conductivity test, the thermal diffusion coefficient of the composite with a W intermediate layer was higher than other samples. By analyzing cross-sections of the composites (Fig. 4), except for a little of tungsten distribute in the copper matrix, the element is almost at the interface. Considering the mutual insolubility of Cu and W, the adverse effect of alloying element on thermal conductivity of the matrix can be ignored. In order to confirm the interphase, XRD analysis was carried out, the result in Fig. 5 shows that tungsten element on the graphite generated corresponding carbides at 950 °C. It was deduced that carbide formation taken place, spheroidization of the copper coating was suppressed by the WC, W2C interlayer. In the hot press process, the continuous copper coating formed a near-net-shaped copper matrix, which increased the heat transfer ability of the composites. On the other hand, tungsten carbide do not react with copper and mutual insolubility of Cu and W, the interface in the composites was a mixed interface which has a characteristic of both chemical reaction bonding and wetting, improving the weak interface bonding between graphite and copper. So the increasing thermal conductivity can be explained by the
-graphite -W -WC -W2C
(a)
(b) 20
30
40
50
60
70
80
2θ (°) Fig. 5. XRD of surface modified graphite particles before and after vacuum thermal treatment (a) tungsten pre-coating and (b) after thermal treatment at 950 °C, soaking 30 min.
The research was financially supported by Program for New Century Excellent Talent in University (NCET-11-0127) and The Fundamental Research Funds for the Central University. References [1] Zweben C. Thermal materials solve power electronics challenges. Power Electron Technol 2006;32:40–7. [2] Prieto R, Molina JM, Narciso J, Louis E. Fabrication and properties of graphite flakes/metal composites for thermal management applications. Scripta Mater 2008;59:11–4. [3] Ueno T, Yoshioka T, Ogawa J, Ozoe N, Sato K, Yoshino K. Highly thermal conductive metal/carbon composites by pulsed electric current sintering. Synth Met 2009;159:2170–2. [4] DeVincent SM, Michal GM. Improvement of thermal and mechanical properties of graphite/copper composites through interfacial modification. J Mater Eng Perform 1993;2:323–32. [5] Veillère A, Sundaramurthy A, Heintz JM, Douin J, Lahaye M, Chandra N, et al. Relationship between interphase chemistry and mechanical properties at the scale of micron in Cu–Cr/CF composite. Acta Mater 2011;59:1445–55. [6] Yang L, Shen P, Lin Q, Qiu F, Jiang Q. Wetting of porous graphite by Cu–Ti alloys at 1373 K. Mater Chem Phys 2010;124:499–503. [7] Liu ZG, Mang XB, Chai LH, Chen YY. Interface study of carbon fibre reinforced Al–Cu composites. J Alloys Compd 2010;504:S512–4. [8] Liu YW, Zhang CY, Qiao SR, Yang ZM. Fabrication and microstructure of C/Cu composites. Adv Eng Mater 2010;12:493–6. [9] Oku T, Oku T. Effects of titanium addition on the microstructure of carbon/ copper composite materials. Solid State Commun 2007;141:132–5. [10] Lloyd JC, Neubauer E, Barcena J, Clegg WJ. Effect of titanium on copper– titanium/carbon nanofibre composite materials. Compos Sci Technol 2010;70:2284–9. [11] E. Neubauer. Interface optimisation in copper–carbon metal matrix composites. Ph.D. thesis. Austria: Austrian Institute of Technology; 2003. [12] Eisenmenger-Sittner C, Neubauer E, Schrank C, Brenner J, Tomastik C. Solid state de-wetting of vapor deposited films on planar and fiber-shaped carbon substrates. Surf Coat Technol 2004;180–181:413–20. [13] Shen X-Y, He X-B, Ren S-B, Zhang H-M, Qu X-H. Effect of molybdenum as interfacial element on the thermal conductivity of diamond/Cu composites. J Alloys Compd 2012;529:134–9. [14] Xia Y, Song Y-q, Lin C-g, Cui S, Fang Z-z. Effect of carbide formers on microstructure and thermal conductivity of diamond–Cu composites for heat sink materials. Transactions of Nonferrous Metals Society of China 2009;19:1161–6. [15] Lee S, Matsunaga K, Ikuhara Y. Effect of alloying elements on the interfacial bonding strength and electric conductivity of carbon nano-fiber reinforced Cu matrix composites. Mater Sci Eng, A 2007;449–451:778–81. [16] Abyzov AM, Kidalov SV, Shakhov FM. High thermal conductivity composites consisting of diamond filler with tungsten coating and copper (silver) matrix. J Mater Sci 2011;46:1424–38. [17] Schrank C, Eisenmenger-Sittner C, Neubauer E, Bangert H, Bergauer A. Solid state de-wetting observed for vapor deposited copper films on carbon substrates. Thin Solid Films 2004;459:276–81. [18] Schubert T, Trindade B, Weissgarber T, Kieback B. Interfacial design of Cubased composites prepared by powder metallurgy for heat sink applications. Mater Sci Eng A-Struct 2008;475:39–44.
Contents lists available at SciVerse ScienceDirect
Composites: Part B journal homepage: www.elsevier.com/locate/compositesb
Effect of tungsten addition on thermal conductivity of graphite/copper composites Wulin Yang, Lingping Zhou ⇑, Kun Peng, Jiajun Zhu, Long Wan College of Materials Science and Engineering, Hunan University, Changsha 410082, China
a r t i c l e
i n f o
Article history: Received 12 April 2012 Received in revised form 17 September 2012 Accepted 20 May 2013 Available online 31 May 2013 Keywords: A. Metal–matrix composites (MMCs) E. Surface treatments B. Interface B. Thermal properties
a b s t r a c t Graphite/copper composites with high thermal conductivity were fabricated by tungsten addition, which formed a thin tungsten carbide layer at the interface. The microstructure and thermal conductivity of the composite material were studied. The results indicated that the insertion of tungsten carbide layer obviously suppressed spheroidization of copper coating on the graphite particles during the sintering process, and decreased the interfacial thermal resistance of the composites. Compared with the graphite/copper composites without tungsten, the thermal conductivity of the obtained composites was increased by 43.6%. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction With the rapid development of microelectronic device and industry, the power density increase rapidly, the ability to dissipate heat becomes a very important factor for microelectronic and optoelectronic devices [1]. Therefore, development of materials with high thermal conductivity and optimal thermal expansion coefficient similar to that of silicon is imperative for heat sinks and heat spreaders. Many materials such as CuMo, CuW, Al/SiCp, Cu/diamond are usually used for heat sink with high thermal conductivity and low coefficient of thermal expansion. However, high density of CuMo and CuW, high price and poor machinability of Al/SiCp and Cu/diamond limited their application more or less. Graphite/Cu metal–matrix composites would be candidate materials for thermal management application for their acceptable thermal and electrical conductivity, low coefficient of thermal expansion, low density and good machinability [2–3]. But high interfacial energy of the C–Cu system lead to de-wetting between the graphite and Cu metal matrix, which causes phase segregation and pore during the sintering stage at elevated temperatures [4]. In order to avoid this phenomenon, copper alloying with strong carbide forming elements was used to prepare the carbon/copper composites [5–9], it was reported that Ti or Cr addition could effectively reduce the contact angle between liquid Cu and graphite. However the addition of alloying element signifi⇑ Corresponding author. Tel./fax: +86 731 88822663. E-mail address: [email protected] (L. Zhou). 1359-8368/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compositesb.2013.05.023
cantly reduces the thermal conductivity of the copper matrix [10]. Neubauer reported that the thermal conductivity of copper rapidly reduced from 400 W/m K to 175 W/m K with 1 wt% titanium in it [11], and thick chemical bonding interphases between the matrix and particles enhance the interfacial thermal resistance. Therefore, alloy element addition has a significant impact on the comprehensive performance of the composites, should be strictly controlled [10]. Recently researches indicate that surface treatment of particles by coating transition element is an effective method to improve the wettabilities and thermal conductivities of carbon/metal composites. Bonding strength and thermal physical characteristics are both increased by forming a thin carbide interlayer during the manufacturing process or heat treatment [12,13]. It was reported that Ti, Cr, Si, Zr atoms had shown an obvious diffusion into copper [14,15], and almost no solubility of Mo, W in copper could be detected. Taking the effect of carbide former on the effective thermal conductivity of the matrix into consideration, Mo or W is the promising element in surface modification of particles [16,17]. On the other hand, the wetting angle of copper on tungsten carbide is about 17°, lower than that of other carbides. And the thermal conductivities of tungsten and corresponding carbide are higher than other transitional elements. So tungsten may be the preferable carbide former for surface modification on graphite in the fabrication of graphite/copper composites. In this paper, near-net-shaped graphite/copper composites with tungsten carbide interfaces were successfully fabricated by vacuum hot press Cu coated graphite composite powder. The effect of tungsten addition on thermal conductivity and microstructure of graphite/copper composites are studied. Suppressing the de-
2
W. Yang et al. / Composites: Part B 55 (2013) 1–4
wetting or spheroidization of the electroless Cu coating by the element will be also mentioned.
2. Experimental 2.1. Pre-coated tungsten Synthetic graphite particles with isotropic thermal properties were used for synthesis graphite/copper composites. The particle size of graphite is about 30–150 lm. The thermal conductivity of the particles is about 45 W/m K. To improve the interfacial bonding, graphite particles were pre-coated W by magnetron sputtering. The target-substrate distance was 3.5 cm. Sputtering from a tungsten target was performed at 150 W DC power in Ar working gas with 40 min, and the working pressure was 1 Pa. The particles were continually moved by vibration and translation motions during the deposition process to obtain homogenous coating.
2.2. Electroless plating and fabrication processes The electroless copper plating process without sensitization and activation was used to deposit copper on the surface of graphite. In consideration of the coefficient of thermal expansion of the composites used for thermal management must be adapted to those of semi-conductors or ceramic insulators, the volume fraction of graphite particle amounted to approximately 70% of all samples. Composition of the electrolysis bath solution was shown in Table 1. After plating, the composite powder was washed with distilled water and absolute ethyl alcohol in turn, and then quickly dried in an infrared furnace at 80 °C for 20 min. Purified Ar and H2 gas (10:1) was introduced into the tube furnace to remove the possible existence of oxide in the composite powder at 400 °C for 1 h. Subsequently, the composite powder was sintered by vacuum hot pressing at 950 °C, 40 MPa with the soaking time of 30 min.
3. Results and discussion The morphology of the composite powder with electroless plating is shown in Fig. 1. And from the observation of X-ray diffraction pattern, the oxides such as CuO and Cu2O were not formed in the process of composite powders. The copper microlite were formed on the surface of the graphite particles, but did not form a complete coating, still part of the graphite exposed as shown in Fig. 1a. The copper coating on the graphite particles with precoated tungsten was continuous and compact, and the copper microlite was much smaller (Fig. 1b), indicating that the precoated tungsten was conducive to the electroless copper plating. Electroless copper deposition can be described as nucleation and grain growth processes, once the nucleation of copper is initiated on the graphite particles, the Cu atoms themselves act as a catalyst for further Cu deposition. The nucleation sites were limited on the without pretreatment graphite particles, and the growth mechanism was favored. Compared with raw graphite particles, the uniformly distributed tungsten atoms which were used to nucleation sites for the copper atoms deposited favored the nucleation process. For this reason, two different coating behaviors were presented. Micrographs of sintered graphite/copper composites are shown in Fig. 2. Fig. 2a is the microstructure of Cu coated graphite composite without interfacial modification, the copper phases appear obvious segregation. Compared with the former, as seen in
2.3. Characterizations and testing The morphology, surface composition of composite powders and microstructure of bulk composites were characterized by Scanning Electron Microscope (SEM, FEI Quanta 200) equipped with Energy-dispersive X-ray spectroscopy (EDS). X-ray diffraction (XRD, SIEMENS D5000) was employed to analyze the constituent of the composite powder. The mass of the copper coating was measured by weighing method. Sintered density of the sample was determined by Archimedes’ principle. The thermal conductivity in the pressure direction was calculated by means of the equation that relates this magnitude with the thermal diffusivity (a) as:
j ¼ a Cp q
ð1Þ
where q is the bulk density, and Cp is the specific heat at constant pressure of the composite. In this work, the specific heat was calculated by means of the linear rule of mixture using the values of Cp for the different components. Table 1 Composition of electroless bath solution. Agent
Formula
Role in bath solution
Concentration
Cupric sulphate EDTA-2Na Sodium hydroxide Formaldehyde
CuSO45H2O C10H14N2Na2O8 NaOH HCHO
Coating ions (Cu2+) Complexing agent Buffering solution Reducing agent
15 g/L 30 g/L 16 g/L 15 ml/L
Fig. 1. Surface morphology of composite powder (a) Cu coated graphite with none interlayer and (b) Cu coated graphite with tungsten interlayer.
W. Yang et al. / Composites: Part B 55 (2013) 1–4
3
in the composites. In order to confirm this suppose, the evolution of de-wetting or spheroidization of the copper coating of particles was studied at 950 °C, soaking 30 min in the same vacuum condition. Fig. 3 shows the de-wetting or spheroidization of the copper coating on particles. The formation of these isolated spherical particles of copper is linked to the nucleation process and wettability. This nucleation process is different from that of electroless plating, but is closely related. During the heat treatment, the limited small neighboring particles which were deposited the condition far from equilibrium are urgent to contracted together to reduce their surface energy [12], and the wettability of C–Cu system is poor. So the bigger isolated spherical particles of copper were formed on the low nucleus density area. In the case of graphite with tungsten interlayer, there is also the same process with the formation of spherical particles of copper (see in the bottom left part of Fig. 3b). But the interlayer improved the wettability of graphite by copper, the most copper particles were contacted with each other and the nucleation was much more homogenous, these were all conducive to spread the copper coating and only sparsely scattered holes appeared on copper coating. The discontinuous copper coating and exposed graphite particles would become to obstacles to the densification of composite. Furthermore, an interfacial gap which sharply reduced the interfacial conductance would be also developed between the raw graphite particles and copper coating. In order to meet the high performance of the composites, the insertion of tungsten interlayer
Fig. 2. Microstructure of graphite/copper composite made from graphite with surface modification (a) raw graphite and (b) W-coated.
Fig. 2b, the tungsten pre-coated graphite particles distributed relatively homogeneous, and a near-net-shaped copper matrix was formed. Refer to the property of the composites, the thermal conductivity of the composites was calculated by Eq. (1) and the results were presented in Table 2. The thermal conductance of the composites with the intermediate layer was higher than that of without interlayer one, increased by almost 43.6%. The addition of alloy element improves the wettability of graphite by copper, and reduces the interface thermal resistance. A similar phenomenon was reported in the diamond/copper system with Cr or B alloying element in the metal matrix [18]. Comparing the two different composite microstructures, the segregations of copper in Fig. 2a indicate that the effect of tungsten on the evolution of the electroless copper coating on the graphite particles during the sintering could not be ignored. The surface modification plays a significant role in organizing the homogeneous microstructure of graphite/copper composites. Otherwise, the copper coating would be separated from the graphite particles during the sintering stage, and formatted the segregation of copper Table 2 Physical properties of graphite/copper composites. Properties
Cu@C
Cu–W@C
Density (g/cm3) Specific heat capacity (J/g K) Thermal diffusion (mm2/s) Thermal conductivity (W/m K)
4.05 0.502 54 110
4.08 About 0.502 77 158
Fig. 3. De-wetting or spheroidization of Cu coating on graphite (a) raw graphite and (b) W pre-coating.
4
W. Yang et al. / Composites: Part B 55 (2013) 1–4
combined effects of the near-net-shaped copper matrix and good bonding interface of the composites. Cu
4. Conclusions
W
Graphite/copper composites were fabricated by surface modification, electroless copper coating and hot pressing route. A good dispersion of graphite particles in the copper matrix could be achieved. With the insertion of alloying element interlayer, continuous and compact copper coating was obtained by electroless plating. The alloying element layer obviously suppressed the dewetting or spheroidization of copper coating. Compared with conventional powder metallurgy, higher thermal conductivity was obtained through this approach. W pre-coating by physical vapor deposition is an available method to fabricate graphite/copper composites with high thermal conductivity. The thermal conductivity of the obtained graphite/copper composites reached 158 W/m K.
C
Fig. 4. SEM of graphite/copper composites made from graphite particles with tungsten interlayer.
Acknowledgements
Intensity (CPS)
at the Cu/C interface may be an essential part of suppressing dewetting or spheroidization and preventing from the formation discontinuous coating and interfacial gaps. It is beneficial for copper distribution by a network during the sintering and improving the interfacial contact of the composites. According to the result of the thermal conductivity test, the thermal diffusion coefficient of the composite with a W intermediate layer was higher than other samples. By analyzing cross-sections of the composites (Fig. 4), except for a little of tungsten distribute in the copper matrix, the element is almost at the interface. Considering the mutual insolubility of Cu and W, the adverse effect of alloying element on thermal conductivity of the matrix can be ignored. In order to confirm the interphase, XRD analysis was carried out, the result in Fig. 5 shows that tungsten element on the graphite generated corresponding carbides at 950 °C. It was deduced that carbide formation taken place, spheroidization of the copper coating was suppressed by the WC, W2C interlayer. In the hot press process, the continuous copper coating formed a near-net-shaped copper matrix, which increased the heat transfer ability of the composites. On the other hand, tungsten carbide do not react with copper and mutual insolubility of Cu and W, the interface in the composites was a mixed interface which has a characteristic of both chemical reaction bonding and wetting, improving the weak interface bonding between graphite and copper. So the increasing thermal conductivity can be explained by the
-graphite -W -WC -W2C
(a)
(b) 20
30
40
50
60
70
80
2θ (°) Fig. 5. XRD of surface modified graphite particles before and after vacuum thermal treatment (a) tungsten pre-coating and (b) after thermal treatment at 950 °C, soaking 30 min.
The research was financially supported by Program for New Century Excellent Talent in University (NCET-11-0127) and The Fundamental Research Funds for the Central University. References [1] Zweben C. Thermal materials solve power electronics challenges. Power Electron Technol 2006;32:40–7. [2] Prieto R, Molina JM, Narciso J, Louis E. Fabrication and properties of graphite flakes/metal composites for thermal management applications. Scripta Mater 2008;59:11–4. [3] Ueno T, Yoshioka T, Ogawa J, Ozoe N, Sato K, Yoshino K. Highly thermal conductive metal/carbon composites by pulsed electric current sintering. Synth Met 2009;159:2170–2. [4] DeVincent SM, Michal GM. Improvement of thermal and mechanical properties of graphite/copper composites through interfacial modification. J Mater Eng Perform 1993;2:323–32. [5] Veillère A, Sundaramurthy A, Heintz JM, Douin J, Lahaye M, Chandra N, et al. Relationship between interphase chemistry and mechanical properties at the scale of micron in Cu–Cr/CF composite. Acta Mater 2011;59:1445–55. [6] Yang L, Shen P, Lin Q, Qiu F, Jiang Q. Wetting of porous graphite by Cu–Ti alloys at 1373 K. Mater Chem Phys 2010;124:499–503. [7] Liu ZG, Mang XB, Chai LH, Chen YY. Interface study of carbon fibre reinforced Al–Cu composites. J Alloys Compd 2010;504:S512–4. [8] Liu YW, Zhang CY, Qiao SR, Yang ZM. Fabrication and microstructure of C/Cu composites. Adv Eng Mater 2010;12:493–6. [9] Oku T, Oku T. Effects of titanium addition on the microstructure of carbon/ copper composite materials. Solid State Commun 2007;141:132–5. [10] Lloyd JC, Neubauer E, Barcena J, Clegg WJ. Effect of titanium on copper– titanium/carbon nanofibre composite materials. Compos Sci Technol 2010;70:2284–9. [11] E. Neubauer. Interface optimisation in copper–carbon metal matrix composites. Ph.D. thesis. Austria: Austrian Institute of Technology; 2003. [12] Eisenmenger-Sittner C, Neubauer E, Schrank C, Brenner J, Tomastik C. Solid state de-wetting of vapor deposited films on planar and fiber-shaped carbon substrates. Surf Coat Technol 2004;180–181:413–20. [13] Shen X-Y, He X-B, Ren S-B, Zhang H-M, Qu X-H. Effect of molybdenum as interfacial element on the thermal conductivity of diamond/Cu composites. J Alloys Compd 2012;529:134–9. [14] Xia Y, Song Y-q, Lin C-g, Cui S, Fang Z-z. Effect of carbide formers on microstructure and thermal conductivity of diamond–Cu composites for heat sink materials. Transactions of Nonferrous Metals Society of China 2009;19:1161–6. [15] Lee S, Matsunaga K, Ikuhara Y. Effect of alloying elements on the interfacial bonding strength and electric conductivity of carbon nano-fiber reinforced Cu matrix composites. Mater Sci Eng, A 2007;449–451:778–81. [16] Abyzov AM, Kidalov SV, Shakhov FM. High thermal conductivity composites consisting of diamond filler with tungsten coating and copper (silver) matrix. J Mater Sci 2011;46:1424–38. [17] Schrank C, Eisenmenger-Sittner C, Neubauer E, Bangert H, Bergauer A. Solid state de-wetting observed for vapor deposited copper films on carbon substrates. Thin Solid Films 2004;459:276–81. [18] Schubert T, Trindade B, Weissgarber T, Kieback B. Interfacial design of Cubased composites prepared by powder metallurgy for heat sink applications. Mater Sci Eng A-Struct 2008;475:39–44.