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Cu composites
Journal of Alloys and Compounds 529 (2012) 134–139
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Effect of molybdenum as interfacial element on the thermal conductivity of diamond/Cu composites Xiao-Yu Shen a , Xin-Bo He a,∗ , Shu-Bin Ren a , Hao-Ming Zhang a , Xuan-Hui Qu a,b a b
School of Material Science and Engineering, University of Science and Technology Beijing, 30 Xue Yuan Road, Beijing 100083, People’s Republic of China State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, 30 Xue Yuan Road, Beijing 100083, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 8 January 2012 Received in revised form 4 March 2012 Accepted 10 March 2012 Available online 17 March 2012 Keywords: Composite materials Liquid–solid reactions Heat conduction Microstructure
a b s t r a c t As a carbide forming additive, molybdenum gets maximum benefit of high thermal conductivities of diamond and copper. Coating Mo on the diamond surface can promote the interfacial bonding in diamond/Cu composites. A Mo coated diamond/Cu composite with high thermal conductivity of 726 W/mK is obtained, which is achieved by a thin nano-sized Mo2 C layer. Mo coated diamond or un-coated diamond reinforced copper composites have been made by pressure assisted liquid copper infiltration method. The interfacial configuration, the bulk thermal conductivity and the evolution of interfacial thermal resistance are presented, indicating that the good adhesion at the interface can decrease the thermal boundary resistance and thus increase the thermal conductivity of the composites. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Heat sink materials with ultrahigh thermal conductivities, low coefficient of thermal expansions (CTEs) and low densities developed in the last few years can solve key packaging problems [1], including reducing the CTE and increasing thermal conductivity of modern power electronics [2,3]. In particular diamond/Cu composite has become the focus of research and development [4,5]. Since the thermal conductivity of a composite is mainly governed by the thermal conductivity of each component, the volume fraction, as well as the interfacial condition, the main hurdle that has to be overcome is a reduction of the thermal contact resistance [6]. The understanding of influence of the interface formation on the thermal conductivity of the composite provides useful insight into active elements adding, which allows overall thermal conductivities in excess of 400 W/mK to be obtained from reasonably small diamond particles [7,8]. Hence, surface coated diamonds [9] or copper alloys [10] have been used separately to improve the interface between matrix and diamond. Strong carbide formers as Ti, Cr or B added as alloying elements to the copper matrix has been investigated previously. Then superior thermal conductivities were achieved in diamond reinforced CuCr or CuB matrix composites [11,12], which are about 300–600 W/mK. The composite microstructure is reported to be inhomogeneous, consisting of both well-dispersed and
∗ Corresponding author. Tel.: +86 10 62332727; fax: +86 10 62334311. E-mail addresses: xb [email protected], [email protected] (X.-B. He). 0925-8388/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2012.03.045
agglomerated active elements. Besides, the direct combination of diamond and Cu at the interface is observed. However, the amount of active element must be limited in order to maintain high thermal conductivity in the matrix. Diamond surface modification of depositing Ti, Cr, Mo or W on surface of diamond particles can promote dramatically the adhesion between diamond and matrix. In addition, the coating also protects the diamond powder from the oxidization and reduces the degree of diamond graphitization at high temperature. The thermal conductivities of the Ti-coated diamond/Cu composites and Cr-coated diamond/Cu composites are 350 W/mK and 550 W/mK [13] respectively [14]. Confronting the choice of active elements, it must be regarded that the coating element is dissolved into Cu with a simultaneous decrease of the thermal conductivity of the matrix. Experimental data have suggested that the thermal conductivity of copper rapidly reduced from 400 W/mK to 175 W/mK after alloying with 1 wt.% titanium and reduced from 400 W/mK to 290 W/mK after alloying with 1 wt.% chromium. But Mo and Cu are mutually insoluble and can form intimate composites best-known as thermal management material. And the process of coating Mo on diamond particles is efficient, speedy and practical to produce in bulk. Therefore our approach to reduce the thermal contact resistance is to coat the diamonds with appropriate molybdenum film to improve the thermal contact between copper and diamond. The thermal conductivity, microstructure of the coating on diamond surface, fracture and interface in the composite, as well as the calculation of the thermal conductivity of composites with interfacial resistance are investigated. As a carbide forming additive,
X.-Y. Shen et al. / Journal of Alloys and Compounds 529 (2012) 134–139
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Fig. 1. Optical micrographs of the diamond crystals: (a) average diameter 100 m as received, (b) average diameter 100 m Mo coated, (c) average diameter 40 m as received, and (d) average diameter 40 m Mo coated.
molybdenum diffuses to the diamond surface then reacts to form thin, continuous carbide interlayer. Meanwhile, it possesses hardly any mutual diffusion with copper [15]. The relationship between composite thermal conductivity and the bonding nature of the interface was probed. 2. Experimental techniques For this approach, synthetic diamond single crystals of the MBD-6 grade with two designated particle diameters (d): between 91 and 106 m (averages d = 100 m) or between 38 and 44 m (averages d = 40 m), and cubo-octahedral morphology were purchased from Henan Huanghe Whirlwind Co., Ltd. Crystals were coated with molybdenum. The experiment was carried out using technique as follows: the mixture of the diamond powder with the molybdenum oxides was placed in an alundum crucible in a layer 5 mm thick and annealed for 1 h at 1123 K by the addition of hydrogen under atmospheric pressure. It was shown that the molybdenum coatings on diamond form as a result of contact interaction of diamond with molybdenum oxides and of transfer of molybdenum and carbon through the gas phase [16]. And the coating thickness depended on the modification of the processing parameters. When the weight difference of the diamond particles before and after coating is measured, the thickness of Mo coating is found. With this method, a molybdenum coating with average thickness of 2 m was obtained. The processes of formation of molybdenum coatings on diamond during metalizing of the latter in a mixture with oxidized molybdenum powder have been studied in [17–19]. Fig. 1(a) and (c) are the scanning electron microscopy (SEM) pictures of original cubo-octahedral synthetic diamond crystals, as used in this study. And Fig. 1(b) and (d) are Mo coated diamond particles. The Mo coating is successfully deposited on the surface of the diamonds forming a continuous coverage and it was seen to be granular and homogeneous. We investigated diamond/Cu composites with the Mo coated diamonds by pressure assisted liquid metal infiltration method, which is proved to be feasible in fabricating high thermal conductivity composites. Pure copper with commercial purity of more than 99.999% was used as a matrix, which introduced on top of the densely packed diamond powder bed in a graphite cylinder. A powder bed of MBD6 diamond with average d = 100 m or d = 40 m, with or without Mo coating was found to have a diamond density of approximately 65% by volume with identical size. The infiltration process is shown in Fig. 2. As in Fig. 2(a), after loading diamond power, grille, copper, and piston in sequence from bottom up into graphite die, the system was highly evacuated to a level of 0.01 Pa. And as shown in Fig. 2(b) and (c), the system was heated subsequently at a rate of 300 K h−1 . When the infiltration temperature of 1360 K had been reached, the piston moved downwards to extrude the excess molten copper and repack the diamond powder tightly. Then a
certain pressure of 10 × 106 Pa was added with Argon gas (Fig. 2(d)). The pressure maintained simultaneously until the solidification was completed. Composite density was determined using the Archimedes technique. Composites thermal conductivity, were determined from measurements of composite density, thermal diffusivity, ˛, and specific heat capacity, C using the relationship = ˛C [20]. Specific heat capacity was measured using differential scanning calorimetry (TAInstruments Q100) on cylindrical discs 4 mm in diameter and 0.5 mm thick. The room temperature thermal diffusivity, ˛, of these composites were measured in the transverse direction, using a transient thermal flash technique (Netzsch LFA 457). When measuring the samples with high thermal diffusivity, the temperature difference from top to bottom surface of sample is small. The energy of laser pulse as well as the thickness of samples should be adjusted to get relatively standard pulse length correction curve. The experimental result indicates that the measurements of the composites should be made on cylindrical discs 12.7 mm in diameter and 1.5 mm thick. To prove reliability of the measurements, the samples of high thermal diffusivity metals of pure copper (purity of 99.999%) and silver (purity of 99.9999%) were prepared. And measurements were taken under conditions exactly the same with that of the composite samples. The thermal diffusivity values measured were well consistent with the theoretical value. Therefore this technique provides precise measurement of high thermal diffusivity samples. X-ray diffraction (XRD) patterns recorded by a D5000 Siemens diffraktometer were used to study the interfacial phases. Because the extreme hardness of the diamond particles prevents polishing the exterior surface of the composites, the interface area on the composite fracture surfaces and energy dispersive spectroscopy (EDS) element line scanning cross the interface were studied by LEO JSM-7001F field emission SEM. In addition, transmission electron microscopy (TEM) observations of the interface structure and phase identification using electron diffraction patterns were also performed on the thin foils prepared by focused ion beam (FIB) milling.
3. Results and discussion 3.1. Thermal conductivity measurements The composites with 65% volume function of diamond particles of average d = 100 m or d = 40 m, with or without Mo coating have been prepared. It was found that the relative densities of the composites are all up to 99%, under the optimum infiltration process. As shown in Table 1, without Mo coating rather low thermal conductivities (<200 W/mK) are obtained from both No-40
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Fig. 2. Procedure of infiltration process: (a) loading and evacuating the system, (b) heating the system, (c) lowering the piston, and (d) adding pressure with Ar gas. Table 1 Comparison of the relative densities and thermal conductivities of different composites and denotation of samples. Sample ID
Diamond surface modification
Average diamond size d (m)/particle density (g/cm3 )
Theoretical density the (g/cm3 )
Measured density (g/cm3 )
/the (%)
Thermal diffusivity (mm2 /s)
Specific heat (J/g K)
Thermal conductivity (W/mK)
No-40 Mo-40 No-100 Mo-100
None Mo coating None Mo coating
40/3.50 40/4.21 100/3.50 100/3.80
5.39 6.47 5.39 5.87
5.35 6.45 5.35 5.86
99.26 99.85 99.07 99.83
49.342 148.250 78.205 281.006
0.447 0.438 0.445 0.441
118 386 186 726
and No-100, which indicating a high thermal contact resistance in the composite is the main cause of this degradation. Depending on the diamond surface modification thermal conductivities up to 726 W/mK have been achieved by using diamonds with average d = 100 m. When using diamond with fine size, Mo-40 has the thermal conductivity of 386 W/mK. The effect of the higher interfacial area results in a decrease of thermal conductivity [21]. As can be seen clearly, the influence of diamond surface modification on thermal conductivities of the composites is great.
The Mo2 C bonds to square (with {1 0 0} orientations) surfaces of the diamonds, as well as the {1 1 1} hexagonal diamond faces. This is different from the selective bonding of Cr or B onto the {1 0 0} surfaces of diamonds observed by Weber et al. [11], selective bonding
3.2. Study of phase-identification and composite microstructure XRD analyses were performed for diamonds as received (Fig. 3 pattern a), Mo coated diamonds (Fig. 3 pattern b) and diamonds released from the composites by simple chemical etching with nitric acid (Fig. 3 pattern c). As shown in Fig. 3, the peaks of Mo2 C were both obtained form pattern b and c, indicating that the carbide layer was obtained during the coating process and Mo2 C is produced between Mo coating and diamond, which is consistent with the results reported in [18]. To examine the morphology of the interfacial phase, the diamond particles are extracted out of the composite Mo-100 as shown in Fig. 4. As can be seen in Fig. 4, the surface of diamond is uniformly covered by a rough layer of discrete pits.
Fig. 3. XRD analysis for: (pattern a) diamonds as received, (pattern b) Mo coated diamonds, and (pattern c) diamonds released from the composites.
X.-Y. Shen et al. / Journal of Alloys and Compounds 529 (2012) 134–139
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Fig. 4. Diamonds released from the composite Mo-100 by simple chemical etching with nitric acid.
was no longer observed in this work. Therefore, the thermal resistance at both interfaces between the metal matrix and either the square or the hexagonal diamond faces are the same. From the inspection on SEM fracture image of No-100 and Mo100 in Fig. 5(a) and (b), an excellent distribution of the diamonds were both observed which is independent of the coating condition. And as is exemplarily shown in Fig. 5(b), it is worth noting that nearly all the diamond particles are fractured. This transcrystalline fracture only happens when the interfacial bonding strength in the composite is higher than the fracture stress of diamond particle. Otherwise, intercrystalline failure happens as in Fig. 5(a).
Fig. 6. Optical microscopy of the interface between the Mo layer and diamond in composite Mo-100 (a) and the distribution of elements C, Cu and Mo was checked by EDS line scanning (b).
According to Griffith’s theory, a particle is considered to be broken if the stress in the particle exceeds the Griffith criterion. p
p c = √c d
(1)
p
where c is a constant related to the fracture toughness of the parp ticle together with geometrical factors, c is the fracture stress of p the particle with diameter of d [22]. High c of diamond particle p results in large c , and that makes diamond particles hard to broken. In this situation, the transcrystalline fracture indicates high interfacial bonding strength, which could be explained by the good adhesions between Cu, Mo coating and diamond particles. 3.3. Interfacial structure
Fig. 5. SEM fracture micrograph of (a) composite No-100 and (b) composite Mo-100.
In order to further research about the thermal conduction and bonding effects of Mo coating at the interface, the interfacial structure of composite Mo-100 is investigated by field emission SEM and TEM. Fig. 6(a) shows the interfacial microstructure of the diamond and Mo coating, where the diamond is tightly adhered to the coating layer and the results show that after the infiltration process, the coating layer on the diamond surface still remains. On the other side, internal flaws such as the pores in the Mo layer or on the diamond/Mo interface are observed, which mainly caused by the high rate of deposition during the Mo coating. The distribution of the composite elements was checked by EDS line scanning and the results are presented in Fig. 6(b). Following the scan path, first a Mo grain is crossed, followed by a very low Cu
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Fig. 7. TEM micrograph of the interface between Mo and diamond (a), and the interface between Mo and Cu matrix (b), as well as diffraction patterns at (d1) Mo2 C Z = [0 1 1 0], (d1) Cu Z = [0 1 1], (d1) Mo Z = [0 0 1].
signal, indicating low diffusion of Cu atoms into Mo layer. When entering the interface layer, it reveals distinct signal for carbon (C) and a sharply decreasing signal for Mo, and the atomic ratio of Mo to C is about 2–1, which corresponds with the result of XRD analysis. Such a layer was always found between the Mo coating and the diamond surface for all samples investigated in this study, regardless of process history. Taken over numbers of observations, the average thickness of this interface layer is about 50 nm. Following the interface layer, the scan enters the diamond bulk where the signals from Mo and Cu drop to the detection limit while the C signal sharply increases. Moreover, the detailed observation of the in-depth nature and distribution of intermediate crystalline structure of the Mo coating were determined by TEM in Fig. 7(a). Further TEM study also identifies the reaction layer between Mo and diamond as Mo2 C by electron diffraction analysis. This chemical reaction involves the rupture and rearrangement of the crystal structure of diamond. C atoms bond Mo atoms together, leading to a good chemical contact between these two phases while the ability to conduct heat from the diamond bulk crystal to the metal layer obviously improved. Fig. 7(b) reveals a good bonding between Cu and Mo besides their respective crystal forms. The interface between Mo and Cu are always thin and straight. This could be explained by the low solubility between Mo and Cu. The Mo layer reacting with the diamond to form the Mo2 C rather than diffused into the Cu.
be exploited. The obtained values are compared to the theoretical predictions using the model that for the thermal conductivity of a composite with an interfacial thermal barrier resistance proposed by Hasselmann and Johnson (H–J) [23]. c = 1
[2 (1 + 2˛) + 21 ] + 2V2 [2 (1 − ˛) − 1 ] [2 (1 + 2˛) + 21 ] − V2 [2 (1 − ˛) − 1 ]
(2)
where c is the thermal conductivity of composite (W/mK), 1 is the thermal conductivity of matrix (W/mK), 2 is the thermal conductivity of dispersed particles(W/mK), V2 is the volume fraction of dispersed particles (dimensionless), and ˛ is the interfacial thermal barrier resistance factor(m2 K/W). The calculations were made under the assumption of the thermal conductivities of 1500 W/mK and 400 W/mK for diamond and the copper matrix, respectively. And the thermal barrier resistances of all composite samples presented in work can be calculated. The comparison between our experiments and theoretical calculations based on this model are shown in Fig. 8. The interfacial
3.4. Relationship between interfacial microstructure and thermal conductivity After having discussed the interfacial microstructure and its effect on the bonding between Cu and diamond in composites, it is of interest to investigate how these findings correlate with the thermal conductivity. Interfaces in composites are crucial to thermal conductivity, as they determine to which extent the properties of a highly conductive reinforcement phase such as diamond can
Fig. 8. H–J plot of the thermal conductivities of diamond/Cu composites against the interfacial thermal barrier resistance.
X.-Y. Shen et al. / Journal of Alloys and Compounds 529 (2012) 134–139
thermal barrier resistances of samples No-40 and No-100 are approximately the same, which are 1 × 10−6 m2 K/W about and the resultant composite thermal conductivity lies below that of copper. The diamond particles do not contribute to the composite conductivity in this case. This can be explained by no chemical affinity between copper and diamond, and therefore no interfacial bonding and no electron–phonon coupling for an optimum thermal transfer at the interface [24], indicating that the influence of the poor interfacial adhesion is prominent over any effect that might be introduced through the addition of Mo coating of diamond. In the case of a Mo coated diamond/Cu composite, the samples Mo-40 and Mo-100 have significantly higher thermal conductivities than samples without surface modification of diamond. A value of more than 200% enhancement in thermal conductivity has been observed. And by calculation they have much lower interfacial thermal barrier resistances. The interfacial thermal resistance is within the range from 0.5 × 10−7 m2 K/W to 5 × 10−7 m2 K/W. This can be explained by the strong interfacial chemical bonding between Mo coating and the diamond surface in the composites due to the formation of Mo2 C. Obviously, the use of suitable carbide former results in a better interface design in this material system. Besides, the result reveals some potential for further improvement of the Mo coated diamond/Cu composites. An obvious restraint in the interface thermal conduction results from the thickness and flaws of Mo layer which limited by the coating techniques of Mo on diamond particles. Compared with the Ti coating or Cr coating of diamond particles [14], the Mo coating has more internal flaws. As we all know, pores are harmful to the thermal conductivity. And thermal conductivities of Mo and Mo2 C (135 W/mK and 80 W/mK respectively) are lower than those of diamond particles and Cu matrix. Compared with Mo-100 (2 m Mo layer and 726 W/mK thermal conductivity), if the thickness of Mo layer is 1 m in the composite, it can be calculated that the thermal conductivity of composite should be 900 W/mK. Therefore thick layers of Mo is undesirable, An interfacial thermal resistance of 5 × 10−8 m2 K/W can be calculated for this Mo coated diamond/Cu composite with the measured bulk thermal conductivity of 726 W/mK. The estimated bulk thermal conductivity of 900 W/mK of this composite can be achieved if the interfacial thermal resistance is better than 1 × 10−8 m2 K/W corresponding to a perfect interface design. This suggests that coating flaws should be avoided and the thickness of the coating should be as low as possible to maintain a maximum thermal interface conductance. 4. Conclusion (1) Weak interface and high thermal boundary resistance between diamond and Cu caused low thermal conductivity of the diamond/Cu composite. Therefore, the method of coating Mo on the surface of diamond particles and then forming the compound with Cu has been used to obtain good adhesion at the interface. It can decrease the thermal boundary resistance effectively and thus increase the thermal conductivity of the composites.
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(2) The carbide former Mo appears to be the best candidate for diamond/copper composites. It s has a low solubility in copper at processing temperature so the thermal conductivity of the Cu matrix is insignificantly affected. Meanwhile the Mo coating on the surface of the diamond able to reacts with diamond can form nano-sized Mo2 C, which provides an optimum thermal transfer at the interface. And the achieved thermal conductivity of the composite can be doubled. (3) A further reduction of interfacial thermal resistance of Mo coated diamond/Cu composite shall be realized with both the reduction of coating thickness and avoiding of pores in the Mo coating. This indicates broad prospects of the increase of thermal conductivity for this electronic substrate material. Acknowledgment This work was financially supported by the National Natural Science Foundation of China (NSFC) under Grant No. 50774005. References [1] K.A. Weidenmann, R. Tavangar, L. Weber, Compos. Sci. Technol. 69 (2000) 1660–1666. [2] R. Prieto, J.M. Molina, J. Narciso, E. Louis, Scripta Mater. 59 (2008) 11–14. [3] A. Kelly, Key Eng. Mater. 334–335 (2007) 1017–1020. [4] N. Bresson, S. Cristoloveanu, C. Mazuré, F. Letertre, H. Iwai, Solid State Electron. 49 (2005) 1522–1528. [5] K. Yoshida, H. Morigami, Microelectron. Reliab. 44 (2004) 303–308. [6] J. Flaquer, A. Ríos, A. Martín-Meizoso, S. Nogales, H. Böhm, Compos. Mater. Sci. 47 (2007) 156–163. [7] M. Leers, C. Scholz, K. Boucke, M. Oudart, Next generation heat sinks for high-power diode laser bars, in: Semiconductor Thermal Measurement and Management Symposium, 2007. SEMI-THERM 2007, Twenty Third Annual IEEE, San Jose, CA, 2007, pp. 105–111. [8] E. Neubauer, P. Angerer, Advanced composite materials with tailored thermal properties for heat sink applications, in: European Conference on Power Electronics and Applications, 2007, Aalborg, 2008, pp. 1–8. [9] Y. Xia, Y. Song, C. Lin, S. Cui, Z. Fang, Trans. Nonferr. Met. Soc. 19 (2009) 1161–1166. ´ ´ W. Zielinski, A. Michalski, T. Weißgärber, B. Kieback, [10] T. Schubert, Ł. Ciupinski, Scripta Mater. 58 (2008) 263–266. [11] L. Weber, R. Tavangar, Scripta Mater. 57 (2007) 988–991. [12] Th. Schubert, B. Trindade, T. Weißgärber, B. Kieback, J. Mater. Sci. Eng.: A. 475 (2008) 39–44. [13] K. Chu, Z.F. Liu, C.C. Jia, H. Chen, X.B. Liang, W.J. Gao, W.H. Tian, H. Guo, J. Alloys Compd. 490 (2010) 453–458. [14] S.B. Ren, X.Y. Shen, C.Y. Guo, Nan Liu, J.B. Zang, X.B. He, X.H. Qu, J. Compos. Sci. Technol. 71 (2011) 1550–1555. [15] A.M. Abyzov, S.V. Kidalov, F.M. Shakhov, Mater. Sci. 46 (2010) 1424–1438. [16] V.G. Chuprina, V.V. Shurkhal, Powder Metall. Met. Ceram. 27 (1988) 917–921. [17] V.G. Chuprina, G.P. Volk, I.A. Lavrinenko, Poroshk Metall. 11 (1986) 56–60. [18] V.G. Chuprina, I.M. Shalya, Powder Metall. Met. Ceram. 47 (2011) 712–716. [19] V.G. Chuprina, G.P. Volk, Powder Metall. Met. Ceram. 27 (1988) 311–314. [20] I. Khorunzhii, H. Gabor, R. Job, W.R. Fahrner, A. Denisenko, D. Brunner, U. Peschek, Measurement 32 (2002) 163–172. [21] K. Hanada, K. Matsuzaki, T. Sano, J. Mater. Process. Technol. 153–154 (2004) 514–518. [22] H. Feng, J.K. Yu, W. Tan, Mater. Chem. Phys. 124 (2010) 851–855. [23] J. Wang, X. Yi, Compos. Sci. Technol. 64 (2004) 1623–1628. [24] X. Zhang, H. Guo, F. Yin, Y. Fan, Y. Zhang, Rare Met. 30 (2011) 94.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Effect of molybdenum as interfacial element on the thermal conductivity of diamond/Cu composites Xiao-Yu Shen a , Xin-Bo He a,∗ , Shu-Bin Ren a , Hao-Ming Zhang a , Xuan-Hui Qu a,b a b
School of Material Science and Engineering, University of Science and Technology Beijing, 30 Xue Yuan Road, Beijing 100083, People’s Republic of China State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, 30 Xue Yuan Road, Beijing 100083, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 8 January 2012 Received in revised form 4 March 2012 Accepted 10 March 2012 Available online 17 March 2012 Keywords: Composite materials Liquid–solid reactions Heat conduction Microstructure
a b s t r a c t As a carbide forming additive, molybdenum gets maximum benefit of high thermal conductivities of diamond and copper. Coating Mo on the diamond surface can promote the interfacial bonding in diamond/Cu composites. A Mo coated diamond/Cu composite with high thermal conductivity of 726 W/mK is obtained, which is achieved by a thin nano-sized Mo2 C layer. Mo coated diamond or un-coated diamond reinforced copper composites have been made by pressure assisted liquid copper infiltration method. The interfacial configuration, the bulk thermal conductivity and the evolution of interfacial thermal resistance are presented, indicating that the good adhesion at the interface can decrease the thermal boundary resistance and thus increase the thermal conductivity of the composites. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Heat sink materials with ultrahigh thermal conductivities, low coefficient of thermal expansions (CTEs) and low densities developed in the last few years can solve key packaging problems [1], including reducing the CTE and increasing thermal conductivity of modern power electronics [2,3]. In particular diamond/Cu composite has become the focus of research and development [4,5]. Since the thermal conductivity of a composite is mainly governed by the thermal conductivity of each component, the volume fraction, as well as the interfacial condition, the main hurdle that has to be overcome is a reduction of the thermal contact resistance [6]. The understanding of influence of the interface formation on the thermal conductivity of the composite provides useful insight into active elements adding, which allows overall thermal conductivities in excess of 400 W/mK to be obtained from reasonably small diamond particles [7,8]. Hence, surface coated diamonds [9] or copper alloys [10] have been used separately to improve the interface between matrix and diamond. Strong carbide formers as Ti, Cr or B added as alloying elements to the copper matrix has been investigated previously. Then superior thermal conductivities were achieved in diamond reinforced CuCr or CuB matrix composites [11,12], which are about 300–600 W/mK. The composite microstructure is reported to be inhomogeneous, consisting of both well-dispersed and
∗ Corresponding author. Tel.: +86 10 62332727; fax: +86 10 62334311. E-mail addresses: xb [email protected], [email protected] (X.-B. He). 0925-8388/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2012.03.045
agglomerated active elements. Besides, the direct combination of diamond and Cu at the interface is observed. However, the amount of active element must be limited in order to maintain high thermal conductivity in the matrix. Diamond surface modification of depositing Ti, Cr, Mo or W on surface of diamond particles can promote dramatically the adhesion between diamond and matrix. In addition, the coating also protects the diamond powder from the oxidization and reduces the degree of diamond graphitization at high temperature. The thermal conductivities of the Ti-coated diamond/Cu composites and Cr-coated diamond/Cu composites are 350 W/mK and 550 W/mK [13] respectively [14]. Confronting the choice of active elements, it must be regarded that the coating element is dissolved into Cu with a simultaneous decrease of the thermal conductivity of the matrix. Experimental data have suggested that the thermal conductivity of copper rapidly reduced from 400 W/mK to 175 W/mK after alloying with 1 wt.% titanium and reduced from 400 W/mK to 290 W/mK after alloying with 1 wt.% chromium. But Mo and Cu are mutually insoluble and can form intimate composites best-known as thermal management material. And the process of coating Mo on diamond particles is efficient, speedy and practical to produce in bulk. Therefore our approach to reduce the thermal contact resistance is to coat the diamonds with appropriate molybdenum film to improve the thermal contact between copper and diamond. The thermal conductivity, microstructure of the coating on diamond surface, fracture and interface in the composite, as well as the calculation of the thermal conductivity of composites with interfacial resistance are investigated. As a carbide forming additive,
X.-Y. Shen et al. / Journal of Alloys and Compounds 529 (2012) 134–139
135
Fig. 1. Optical micrographs of the diamond crystals: (a) average diameter 100 m as received, (b) average diameter 100 m Mo coated, (c) average diameter 40 m as received, and (d) average diameter 40 m Mo coated.
molybdenum diffuses to the diamond surface then reacts to form thin, continuous carbide interlayer. Meanwhile, it possesses hardly any mutual diffusion with copper [15]. The relationship between composite thermal conductivity and the bonding nature of the interface was probed. 2. Experimental techniques For this approach, synthetic diamond single crystals of the MBD-6 grade with two designated particle diameters (d): between 91 and 106 m (averages d = 100 m) or between 38 and 44 m (averages d = 40 m), and cubo-octahedral morphology were purchased from Henan Huanghe Whirlwind Co., Ltd. Crystals were coated with molybdenum. The experiment was carried out using technique as follows: the mixture of the diamond powder with the molybdenum oxides was placed in an alundum crucible in a layer 5 mm thick and annealed for 1 h at 1123 K by the addition of hydrogen under atmospheric pressure. It was shown that the molybdenum coatings on diamond form as a result of contact interaction of diamond with molybdenum oxides and of transfer of molybdenum and carbon through the gas phase [16]. And the coating thickness depended on the modification of the processing parameters. When the weight difference of the diamond particles before and after coating is measured, the thickness of Mo coating is found. With this method, a molybdenum coating with average thickness of 2 m was obtained. The processes of formation of molybdenum coatings on diamond during metalizing of the latter in a mixture with oxidized molybdenum powder have been studied in [17–19]. Fig. 1(a) and (c) are the scanning electron microscopy (SEM) pictures of original cubo-octahedral synthetic diamond crystals, as used in this study. And Fig. 1(b) and (d) are Mo coated diamond particles. The Mo coating is successfully deposited on the surface of the diamonds forming a continuous coverage and it was seen to be granular and homogeneous. We investigated diamond/Cu composites with the Mo coated diamonds by pressure assisted liquid metal infiltration method, which is proved to be feasible in fabricating high thermal conductivity composites. Pure copper with commercial purity of more than 99.999% was used as a matrix, which introduced on top of the densely packed diamond powder bed in a graphite cylinder. A powder bed of MBD6 diamond with average d = 100 m or d = 40 m, with or without Mo coating was found to have a diamond density of approximately 65% by volume with identical size. The infiltration process is shown in Fig. 2. As in Fig. 2(a), after loading diamond power, grille, copper, and piston in sequence from bottom up into graphite die, the system was highly evacuated to a level of 0.01 Pa. And as shown in Fig. 2(b) and (c), the system was heated subsequently at a rate of 300 K h−1 . When the infiltration temperature of 1360 K had been reached, the piston moved downwards to extrude the excess molten copper and repack the diamond powder tightly. Then a
certain pressure of 10 × 106 Pa was added with Argon gas (Fig. 2(d)). The pressure maintained simultaneously until the solidification was completed. Composite density was determined using the Archimedes technique. Composites thermal conductivity, were determined from measurements of composite density, thermal diffusivity, ˛, and specific heat capacity, C using the relationship = ˛C [20]. Specific heat capacity was measured using differential scanning calorimetry (TAInstruments Q100) on cylindrical discs 4 mm in diameter and 0.5 mm thick. The room temperature thermal diffusivity, ˛, of these composites were measured in the transverse direction, using a transient thermal flash technique (Netzsch LFA 457). When measuring the samples with high thermal diffusivity, the temperature difference from top to bottom surface of sample is small. The energy of laser pulse as well as the thickness of samples should be adjusted to get relatively standard pulse length correction curve. The experimental result indicates that the measurements of the composites should be made on cylindrical discs 12.7 mm in diameter and 1.5 mm thick. To prove reliability of the measurements, the samples of high thermal diffusivity metals of pure copper (purity of 99.999%) and silver (purity of 99.9999%) were prepared. And measurements were taken under conditions exactly the same with that of the composite samples. The thermal diffusivity values measured were well consistent with the theoretical value. Therefore this technique provides precise measurement of high thermal diffusivity samples. X-ray diffraction (XRD) patterns recorded by a D5000 Siemens diffraktometer were used to study the interfacial phases. Because the extreme hardness of the diamond particles prevents polishing the exterior surface of the composites, the interface area on the composite fracture surfaces and energy dispersive spectroscopy (EDS) element line scanning cross the interface were studied by LEO JSM-7001F field emission SEM. In addition, transmission electron microscopy (TEM) observations of the interface structure and phase identification using electron diffraction patterns were also performed on the thin foils prepared by focused ion beam (FIB) milling.
3. Results and discussion 3.1. Thermal conductivity measurements The composites with 65% volume function of diamond particles of average d = 100 m or d = 40 m, with or without Mo coating have been prepared. It was found that the relative densities of the composites are all up to 99%, under the optimum infiltration process. As shown in Table 1, without Mo coating rather low thermal conductivities (<200 W/mK) are obtained from both No-40
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Fig. 2. Procedure of infiltration process: (a) loading and evacuating the system, (b) heating the system, (c) lowering the piston, and (d) adding pressure with Ar gas. Table 1 Comparison of the relative densities and thermal conductivities of different composites and denotation of samples. Sample ID
Diamond surface modification
Average diamond size d (m)/particle density (g/cm3 )
Theoretical density the (g/cm3 )
Measured density (g/cm3 )
/the (%)
Thermal diffusivity (mm2 /s)
Specific heat (J/g K)
Thermal conductivity (W/mK)
No-40 Mo-40 No-100 Mo-100
None Mo coating None Mo coating
40/3.50 40/4.21 100/3.50 100/3.80
5.39 6.47 5.39 5.87
5.35 6.45 5.35 5.86
99.26 99.85 99.07 99.83
49.342 148.250 78.205 281.006
0.447 0.438 0.445 0.441
118 386 186 726
and No-100, which indicating a high thermal contact resistance in the composite is the main cause of this degradation. Depending on the diamond surface modification thermal conductivities up to 726 W/mK have been achieved by using diamonds with average d = 100 m. When using diamond with fine size, Mo-40 has the thermal conductivity of 386 W/mK. The effect of the higher interfacial area results in a decrease of thermal conductivity [21]. As can be seen clearly, the influence of diamond surface modification on thermal conductivities of the composites is great.
The Mo2 C bonds to square (with {1 0 0} orientations) surfaces of the diamonds, as well as the {1 1 1} hexagonal diamond faces. This is different from the selective bonding of Cr or B onto the {1 0 0} surfaces of diamonds observed by Weber et al. [11], selective bonding
3.2. Study of phase-identification and composite microstructure XRD analyses were performed for diamonds as received (Fig. 3 pattern a), Mo coated diamonds (Fig. 3 pattern b) and diamonds released from the composites by simple chemical etching with nitric acid (Fig. 3 pattern c). As shown in Fig. 3, the peaks of Mo2 C were both obtained form pattern b and c, indicating that the carbide layer was obtained during the coating process and Mo2 C is produced between Mo coating and diamond, which is consistent with the results reported in [18]. To examine the morphology of the interfacial phase, the diamond particles are extracted out of the composite Mo-100 as shown in Fig. 4. As can be seen in Fig. 4, the surface of diamond is uniformly covered by a rough layer of discrete pits.
Fig. 3. XRD analysis for: (pattern a) diamonds as received, (pattern b) Mo coated diamonds, and (pattern c) diamonds released from the composites.
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Fig. 4. Diamonds released from the composite Mo-100 by simple chemical etching with nitric acid.
was no longer observed in this work. Therefore, the thermal resistance at both interfaces between the metal matrix and either the square or the hexagonal diamond faces are the same. From the inspection on SEM fracture image of No-100 and Mo100 in Fig. 5(a) and (b), an excellent distribution of the diamonds were both observed which is independent of the coating condition. And as is exemplarily shown in Fig. 5(b), it is worth noting that nearly all the diamond particles are fractured. This transcrystalline fracture only happens when the interfacial bonding strength in the composite is higher than the fracture stress of diamond particle. Otherwise, intercrystalline failure happens as in Fig. 5(a).
Fig. 6. Optical microscopy of the interface between the Mo layer and diamond in composite Mo-100 (a) and the distribution of elements C, Cu and Mo was checked by EDS line scanning (b).
According to Griffith’s theory, a particle is considered to be broken if the stress in the particle exceeds the Griffith criterion. p
p c = √c d
(1)
p
where c is a constant related to the fracture toughness of the parp ticle together with geometrical factors, c is the fracture stress of p the particle with diameter of d [22]. High c of diamond particle p results in large c , and that makes diamond particles hard to broken. In this situation, the transcrystalline fracture indicates high interfacial bonding strength, which could be explained by the good adhesions between Cu, Mo coating and diamond particles. 3.3. Interfacial structure
Fig. 5. SEM fracture micrograph of (a) composite No-100 and (b) composite Mo-100.
In order to further research about the thermal conduction and bonding effects of Mo coating at the interface, the interfacial structure of composite Mo-100 is investigated by field emission SEM and TEM. Fig. 6(a) shows the interfacial microstructure of the diamond and Mo coating, where the diamond is tightly adhered to the coating layer and the results show that after the infiltration process, the coating layer on the diamond surface still remains. On the other side, internal flaws such as the pores in the Mo layer or on the diamond/Mo interface are observed, which mainly caused by the high rate of deposition during the Mo coating. The distribution of the composite elements was checked by EDS line scanning and the results are presented in Fig. 6(b). Following the scan path, first a Mo grain is crossed, followed by a very low Cu
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Fig. 7. TEM micrograph of the interface between Mo and diamond (a), and the interface between Mo and Cu matrix (b), as well as diffraction patterns at (d1) Mo2 C Z = [0 1 1 0], (d1) Cu Z = [0 1 1], (d1) Mo Z = [0 0 1].
signal, indicating low diffusion of Cu atoms into Mo layer. When entering the interface layer, it reveals distinct signal for carbon (C) and a sharply decreasing signal for Mo, and the atomic ratio of Mo to C is about 2–1, which corresponds with the result of XRD analysis. Such a layer was always found between the Mo coating and the diamond surface for all samples investigated in this study, regardless of process history. Taken over numbers of observations, the average thickness of this interface layer is about 50 nm. Following the interface layer, the scan enters the diamond bulk where the signals from Mo and Cu drop to the detection limit while the C signal sharply increases. Moreover, the detailed observation of the in-depth nature and distribution of intermediate crystalline structure of the Mo coating were determined by TEM in Fig. 7(a). Further TEM study also identifies the reaction layer between Mo and diamond as Mo2 C by electron diffraction analysis. This chemical reaction involves the rupture and rearrangement of the crystal structure of diamond. C atoms bond Mo atoms together, leading to a good chemical contact between these two phases while the ability to conduct heat from the diamond bulk crystal to the metal layer obviously improved. Fig. 7(b) reveals a good bonding between Cu and Mo besides their respective crystal forms. The interface between Mo and Cu are always thin and straight. This could be explained by the low solubility between Mo and Cu. The Mo layer reacting with the diamond to form the Mo2 C rather than diffused into the Cu.
be exploited. The obtained values are compared to the theoretical predictions using the model that for the thermal conductivity of a composite with an interfacial thermal barrier resistance proposed by Hasselmann and Johnson (H–J) [23]. c = 1
[2 (1 + 2˛) + 21 ] + 2V2 [2 (1 − ˛) − 1 ] [2 (1 + 2˛) + 21 ] − V2 [2 (1 − ˛) − 1 ]
(2)
where c is the thermal conductivity of composite (W/mK), 1 is the thermal conductivity of matrix (W/mK), 2 is the thermal conductivity of dispersed particles(W/mK), V2 is the volume fraction of dispersed particles (dimensionless), and ˛ is the interfacial thermal barrier resistance factor(m2 K/W). The calculations were made under the assumption of the thermal conductivities of 1500 W/mK and 400 W/mK for diamond and the copper matrix, respectively. And the thermal barrier resistances of all composite samples presented in work can be calculated. The comparison between our experiments and theoretical calculations based on this model are shown in Fig. 8. The interfacial
3.4. Relationship between interfacial microstructure and thermal conductivity After having discussed the interfacial microstructure and its effect on the bonding between Cu and diamond in composites, it is of interest to investigate how these findings correlate with the thermal conductivity. Interfaces in composites are crucial to thermal conductivity, as they determine to which extent the properties of a highly conductive reinforcement phase such as diamond can
Fig. 8. H–J plot of the thermal conductivities of diamond/Cu composites against the interfacial thermal barrier resistance.
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thermal barrier resistances of samples No-40 and No-100 are approximately the same, which are 1 × 10−6 m2 K/W about and the resultant composite thermal conductivity lies below that of copper. The diamond particles do not contribute to the composite conductivity in this case. This can be explained by no chemical affinity between copper and diamond, and therefore no interfacial bonding and no electron–phonon coupling for an optimum thermal transfer at the interface [24], indicating that the influence of the poor interfacial adhesion is prominent over any effect that might be introduced through the addition of Mo coating of diamond. In the case of a Mo coated diamond/Cu composite, the samples Mo-40 and Mo-100 have significantly higher thermal conductivities than samples without surface modification of diamond. A value of more than 200% enhancement in thermal conductivity has been observed. And by calculation they have much lower interfacial thermal barrier resistances. The interfacial thermal resistance is within the range from 0.5 × 10−7 m2 K/W to 5 × 10−7 m2 K/W. This can be explained by the strong interfacial chemical bonding between Mo coating and the diamond surface in the composites due to the formation of Mo2 C. Obviously, the use of suitable carbide former results in a better interface design in this material system. Besides, the result reveals some potential for further improvement of the Mo coated diamond/Cu composites. An obvious restraint in the interface thermal conduction results from the thickness and flaws of Mo layer which limited by the coating techniques of Mo on diamond particles. Compared with the Ti coating or Cr coating of diamond particles [14], the Mo coating has more internal flaws. As we all know, pores are harmful to the thermal conductivity. And thermal conductivities of Mo and Mo2 C (135 W/mK and 80 W/mK respectively) are lower than those of diamond particles and Cu matrix. Compared with Mo-100 (2 m Mo layer and 726 W/mK thermal conductivity), if the thickness of Mo layer is 1 m in the composite, it can be calculated that the thermal conductivity of composite should be 900 W/mK. Therefore thick layers of Mo is undesirable, An interfacial thermal resistance of 5 × 10−8 m2 K/W can be calculated for this Mo coated diamond/Cu composite with the measured bulk thermal conductivity of 726 W/mK. The estimated bulk thermal conductivity of 900 W/mK of this composite can be achieved if the interfacial thermal resistance is better than 1 × 10−8 m2 K/W corresponding to a perfect interface design. This suggests that coating flaws should be avoided and the thickness of the coating should be as low as possible to maintain a maximum thermal interface conductance. 4. Conclusion (1) Weak interface and high thermal boundary resistance between diamond and Cu caused low thermal conductivity of the diamond/Cu composite. Therefore, the method of coating Mo on the surface of diamond particles and then forming the compound with Cu has been used to obtain good adhesion at the interface. It can decrease the thermal boundary resistance effectively and thus increase the thermal conductivity of the composites.
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(2) The carbide former Mo appears to be the best candidate for diamond/copper composites. It s has a low solubility in copper at processing temperature so the thermal conductivity of the Cu matrix is insignificantly affected. Meanwhile the Mo coating on the surface of the diamond able to reacts with diamond can form nano-sized Mo2 C, which provides an optimum thermal transfer at the interface. And the achieved thermal conductivity of the composite can be doubled. (3) A further reduction of interfacial thermal resistance of Mo coated diamond/Cu composite shall be realized with both the reduction of coating thickness and avoiding of pores in the Mo coating. This indicates broad prospects of the increase of thermal conductivity for this electronic substrate material. Acknowledgment This work was financially supported by the National Natural Science Foundation of China (NSFC) under Grant No. 50774005. References [1] K.A. Weidenmann, R. Tavangar, L. Weber, Compos. Sci. Technol. 69 (2000) 1660–1666. [2] R. Prieto, J.M. Molina, J. Narciso, E. Louis, Scripta Mater. 59 (2008) 11–14. [3] A. Kelly, Key Eng. Mater. 334–335 (2007) 1017–1020. [4] N. Bresson, S. Cristoloveanu, C. Mazuré, F. Letertre, H. Iwai, Solid State Electron. 49 (2005) 1522–1528. [5] K. Yoshida, H. Morigami, Microelectron. Reliab. 44 (2004) 303–308. [6] J. Flaquer, A. Ríos, A. Martín-Meizoso, S. Nogales, H. Böhm, Compos. Mater. Sci. 47 (2007) 156–163. [7] M. Leers, C. Scholz, K. Boucke, M. Oudart, Next generation heat sinks for high-power diode laser bars, in: Semiconductor Thermal Measurement and Management Symposium, 2007. SEMI-THERM 2007, Twenty Third Annual IEEE, San Jose, CA, 2007, pp. 105–111. [8] E. Neubauer, P. Angerer, Advanced composite materials with tailored thermal properties for heat sink applications, in: European Conference on Power Electronics and Applications, 2007, Aalborg, 2008, pp. 1–8. [9] Y. Xia, Y. Song, C. Lin, S. Cui, Z. Fang, Trans. Nonferr. Met. Soc. 19 (2009) 1161–1166. ´ ´ W. Zielinski, A. Michalski, T. Weißgärber, B. Kieback, [10] T. Schubert, Ł. Ciupinski, Scripta Mater. 58 (2008) 263–266. [11] L. Weber, R. Tavangar, Scripta Mater. 57 (2007) 988–991. [12] Th. Schubert, B. Trindade, T. Weißgärber, B. Kieback, J. Mater. Sci. Eng.: A. 475 (2008) 39–44. [13] K. Chu, Z.F. Liu, C.C. Jia, H. Chen, X.B. Liang, W.J. Gao, W.H. Tian, H. Guo, J. Alloys Compd. 490 (2010) 453–458. [14] S.B. Ren, X.Y. Shen, C.Y. Guo, Nan Liu, J.B. Zang, X.B. He, X.H. Qu, J. Compos. Sci. Technol. 71 (2011) 1550–1555. [15] A.M. Abyzov, S.V. Kidalov, F.M. Shakhov, Mater. Sci. 46 (2010) 1424–1438. [16] V.G. Chuprina, V.V. Shurkhal, Powder Metall. Met. Ceram. 27 (1988) 917–921. [17] V.G. Chuprina, G.P. Volk, I.A. Lavrinenko, Poroshk Metall. 11 (1986) 56–60. [18] V.G. Chuprina, I.M. Shalya, Powder Metall. Met. Ceram. 47 (2011) 712–716. [19] V.G. Chuprina, G.P. Volk, Powder Metall. Met. Ceram. 27 (1988) 311–314. [20] I. Khorunzhii, H. Gabor, R. Job, W.R. Fahrner, A. Denisenko, D. Brunner, U. Peschek, Measurement 32 (2002) 163–172. [21] K. Hanada, K. Matsuzaki, T. Sano, J. Mater. Process. Technol. 153–154 (2004) 514–518. [22] H. Feng, J.K. Yu, W. Tan, Mater. Chem. Phys. 124 (2010) 851–855. [23] J. Wang, X. Yi, Compos. Sci. Technol. 64 (2004) 1623–1628. [24] X. Zhang, H. Guo, F. Yin, Y. Fan, Y. Zhang, Rare Met. 30 (2011) 94.