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Fabrication of Mo–Cu composite powders by heterogeneous precipitation and the sintering properties of the composite compacts
Journal of Alloys and Compounds 674 (2016) 347e352
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Fabrication of MoeCu composite powders by heterogeneous precipitation and the sintering properties of the composite compacts Dezhi Wang, Bangzhu Yin, Aokui Sun, Xiulin Li, Chengkang Qi, Bohua Duan* Key Laboratory of Ministry of Education for Noneferrous Materials Science and Engineering, School of Materials Science and Engineering, Central South University, Changsha 410083, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 January 2016 Accepted 5 March 2016 Available online 7 March 2016
MoeCu composite powders with a coreeshell structure were fabricated by heterogeneous precipitation and reduction process. The sintering behavior of the composite powders and the sintering properties of the composite compacts were investigated. It was found that the MoeCu powders exhibit good sinterability. Relative density of 97.02% was achieved for the specimen sintered at 1150 C for 2 h. Furthermore, the hardness, electrical conductivity and thermal conductivity of the MoeCu specimens sintered at 1150 C are 191.1 HV, 27.00 MS/m and 188.64 Wm 1 K 1, respectively. The excellent properties are attributed to the microstructure that almost every Mo particle is capsulated by continuous network structure of Cu. © 2016 Elsevier B.V. All rights reserved.
Keywords: MoeCu Heterogeneous precipitation Reduction Sintering Microstructure
1. Introduction Due to many excellent physical and mechanical properties, such as high thermal and electrical conductivity, low coefficient of thermal expansion, low weight as well as highetemperature behavior, MoeCu alloys have been widely used in a variety of industrial and military applications [1e7]. MoeCu alloys are usually fabricated by liquid phase sintering of MoeCu powders compacts. However, high densification is difficult to be achieved [8]. Moreover, copper infiltration process during the liquid phase sintering results in inhomogeneous microstructure, which is detrimental to physical properties of the materials. Many researchers think that the Moecoated Cu composite powders with the uniform coreeshell structure would not only prevent the seepage of liquid copper, but also can inhibit the growth and coalescence of Cu grains effectively [9,10]. A few methods were used to prepare the coated powders, such as heterogeneous precipitation method [11e14], mechanical alloying process [10], microwaveeassisted hydrothermal method [15], spray drying method [16] and electroless plating method [17] etc., among which, the first mentioned one is most widely used. Some researches suggested that heterogeneous precipitation method was advantageous over other methods, because it could not only reduce sintering time and temperature, but also
* Corresponding author. E-mail address: [email protected] (B. Duan). http://dx.doi.org/10.1016/j.jallcom.2016.03.027 0925-8388/© 2016 Elsevier B.V. All rights reserved.
optimize the material microstructures. The result of Zheng's [18] has shown that LaPO4ecoated aeAl2O3 powders were successfully synthesized through the heterogeneous precipitation method. Y.J. Yao et al. [19] prepared nanoscale Al2O3ecoated ZrO2 composite powders via a heterogeneous precipitation process, which could prevent agglomeration of particles and obtain fineegrain ceramics. But heterogeneous precipitation method has not been applied for the fabrication of MoeCu composite powders. In this paper, Moecoated Cu composite powders were produced by heterogeneous precipitationehydrogen reduction method. The aim of the present work is to investigate the influence of the powders of Cu coreeMo shell structure on the sintering properties and to determine the optimum sintering temperature. 2. Experimental The raw materials used in the present study were ammonium heptamolybdate [(NH4)6Mo7O24$4H2O], hexamethylenetetramine (C6H12N4), pure copper powders with average particle size of 1.5 mm, polyethylene glycol 20000 (PEG 20000) and deionized water. In all experiments, weight proportion of each material corresponded to the final composition of Moe30 wt.%Cu. Fig. 1 describes the preparation of Moecoated Cu composite powders by heterogeneous precipitation and hydrogen reduction. (NH4)6Mo7O24$4H2O aqueous solution (0.25 mol/L) and C6H12N4 aqueous solution (0.25 mol/L) with the molar ratio 1:2 were prepared. Meanwhile, suitable amount of PEG 20000 accompanied
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Fig. 2. XRD pattern of MoeCu composite powders reduced at 800 C.
Fig. 1. Flow chart of preparing MoeCu composite powders.
with Cu powders were thoroughly dispersed in deionized water, then ultrasonic vibration was conducted for 30 min after mechanically stirring to remove the agglomerates. Finally, (NH4)6Mo7O24$4H2O solution and C6H12N4 solution were simultaneously added into Cu suspension with vigorous stirring, then the precipitates [(C6H12N4)2(NH4)4Mo7O24$2H2O] formed [20]. The precipitates were filtered and dried to form the precursor. The precursor was calcined in air at 400 C for 90 min, and then the asecalcined powders were reduced at 800 C for 2 h in hydrogen. The dew point of H2 was 30 to 40 C, the flow rate was 0.8 L/min, and the height of powder bed was 8 mm. After being cooled to room temperature, grinded and sieved, the MoeCu composite powders were obtained. The resultant MoeCu composite powders were compacted in a steel die under 200 MPa pressure to produce green parts. Green parts were sintered at different temperatures of 1050 C, 1100 C, 1150 C and 1200 C for 2 h in the hydrogen atmosphere. Phase composition was analyzed by X-ray diffraction (XRD) at a scan rate of 8 /min from 20 to 80 to record the diffraction patterns. Microstructures of the specimens were observed by scanning electron microscopy (SEM). Transmission electron microscope (TEM) coupled with high resolution transmission electron microscope (HRTEM) was applied to investigate the particle shape and corresponding phase composition. The densities of the sintered compacts were measured according to Archimedes' principle. The hardness of the specimens was determined by a Vickers hardness tester. Electrical conductivity was examined using a four wire method by a microeohmmeter. Thermal conductivity was measured by the thermal diffusivity analyzer.
Fig. 3. SEM of MoeCu composite powders.
3. Results and discussion Fig. 2 shows the XRD pattern of the MoeCu composite powders obtained by reducing at 800 C for 2 h, diffraction peaks attributed to Mo and Cu phases can be identified and no other peaks can be found, indicating that the powders were reduced completely. Fig. 3 and Fig. 4 show SEM and TEM images of the MoeCu composite powders reduced at 800 C. It is found that the MoeCu composite powders consist of basically spherical nanoparticles with uniform size distribution. Meanwhile, the particles obtained have a smooth surface (Fig. 3). Fig. 4(a) shows TEM micrograph of composite coating particles. It has been found that there is a homogeneous layer on the Cu cores, suggesting that predominantly heterogeneous precipitation took place. Fig. 4(b) presents HRTEM micrograph of composite coating particles, corresponding to Fig. 4(a). The interplanar crystal spacing of the area A and B is 0.2092 nm and 0.2227 nm, respectively. Referring to the PDF cards, the interplanar crystal spacing of Cu in the (1$1$1) crystal planes is 0.20871 nm and Mo in the (1$1$0) crystal planes is 0.22247 nm. Therefore, area A corresponds to Cu with the faceecentered cubic (fcc) structure, and area B corresponds to Mo with the bodyecentered cubic (bcc) structure, namely, Cu coreeMo shell
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Fig. 4. TEM (a) and HRTEM (b) of MoeCu composite powders.
Fig. 5. Crossesection microstructure of the sintered compacts at different sintering temperatures: (a) 1050 C; (b) 1100 C; (c) 1150 C; (d) 1200 C.
structure. This structure can prevent the growth and agglomeration of Cu grains, which will play a beneficial role in the subsequent sintering process. Fig. 5 shows the crossesection microstructure of the compacts
sintered for 2 h at the temperature of 1050 C, 1100 C, 1150 C and 1200 C. It can be clearly seen that quantities of interparticle pores are distributed over the section of the compacts sintered at 1050 C. Moreover, there are a large amount of Cu rich areas (Fig. 5(a)). With
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Fig. 6. Fractograph of the sintered compacts at different sintering temperatures: (a) 1050 C; (b) 1100 C; (c) 1150 C; (d) 1200 C.
Fig. 7. Effect of sintering temperature on relative density of the sintered compacts.
Fig. 8. Hardness of the sintered compacts at different temperatures.
sintering temperature rising up to 1100 C, the diffusion and flowability of Cu is enhanced and the distribution of Cu tends to be more homogenous. The amount and size of the pores are both reduced, but, there are still some Cu enrichment regions and pores (Fig. 5(b)). The distribution of Cu reaches the maximum uniformity
and pores tend to decline further with the sintering temperature up to 1150 C (Fig. 5(c)). Increasing the sintering temperature to 1200 C results in enrichment of Cu and the growth of the Mo particles, as is shown in Fig. 5(d). Fig. 6 presents the fractograph of the compacts sintered at the
D. Wang et al. / Journal of Alloys and Compounds 674 (2016) 347e352
temperature of 1050 C, 1100 C, 1150 C and 1200 C for 2 h Fig. 6(a) reveals that large pores exist in MoeCu compacts sintered at 1050 C. With sintering temperature increasing to 1100 C, the distribution of Cu tends to be more uniform and there are also some pores (Fig. 6(b)). A dense and homogenous distribution of Mo and Cu particles can be seen apparently from Fig. 6(c). On the whole, every Mo particle is capsulated by continuous network structure of Cu. This network structure that benefits the uniform distribution of Mo and Cu is favorable to mechanical properties, electrical conductivity and thermal conductivity of MoeCu alloys, and the grain size is about 1e2.5 mm. When the temperature is further increased to 1200 C, significant grain growth takes place and pores is caused by exudation of Cu in the compact, shown in Fig. 6(d). Fig. 7 presents the effect of sintering temperature on relative density of the sintered compacts. It can be seen that the relative density of the MoeCu compacts increase with increasing sintering temperature from 1050 C to 1200 C. When sintered at 1050 C, which is below the melting point of copper (1083 C), the sintering process is known as solid phase sintering. During solid phase sintering, densification of composite powders is determined only by internal atomic diffusion ability. Limited atomic diffusion brings about the pores and the low relative density. Relative density of MoeCu compacts increases sharply with sintering temperature rising from 1050 C to 1100 C. As the sintering temperature is up to 1200 C, relative density of the compacts further increases slowly. Reasons for the phenomenon are concluded as follows: When sintering temperature increases to 1100 C and 1150 C, the densification is related to liquid phase sintering, which is considered as the dominant sintering mechanism for the compacts. In this case, Cu melts and flows through the structure, the movement of liquid phase provides a capillary force, which causes rearrangement of the Mo particles and induces mass transport. Besides, uniform MoeCu composite powders with the coreeshell structure can prevent the growth and agglomeration of Cu grains, above all, preventing the seepage of liquid copper during liquid phase sintering. As the temperature increases to 1200 C, relative density continues to increase. However, because Cu content is as high as 30%, the softened Mo skeleton could not sustain the excessive Cu at the high temperature, which leads to a small amount of Cu exuding to the surface. Thus the ratio of the Mo and Cu is changed, some pores occur in the impact, as is shown in Fig. 6(d), which is harmful to properties of compacts. Therefore, the optimum sintering temperature was considered as 1150 C, corresponding to the relative density of 97.02%. Fig. 8 shows the hardness of the compacts sintered at different temperatures. As seen, the hardness of specimens increases when sintering temperature increases from 1050 C to 1150 C. Obviously, the high relative density of the sintered compacts is the main factor for high hardness. However, the hardness of the sintered samples decreases as the temperature increases to 1200 C. The reason is that the hardness of the composites also depends on volume fraction of harder phase, in which the hardness of copper is lower than molybdenum. At 1200 C, small amounts of Cu exude to the surface layer, resulting in the decrease of testing hardness. Hence the maximum value of hardness (191.1 HV) is obtained at the sintering temperature of 1150 C. Fig. 9 and Fig. 10 present the electrical conductivity and thermal conductivity of the sintered compacts at different temperatures, respectively. The results show that the electrical conductivity increases gradually from 1050 C to 1150 C and decreases when sintered at 1200 C, the maximum value of electrical conductivity (27.00 MS/m) is achieved at 1150 C. The reason for this phenomenon is that high densification of the sintered compacts and homogenous distribution of Cu are beneficial to the electron transfer. Meanwhile, the copper phase inherently has a much higher
351
Fig. 9. Electrical conductivity of the sintered compacts at different temperatures.
electrical conductivity than molybdenum, the uneven distribution and loss of Cu result in the decrease of electrical conductivity when sintered at 1200 C. Thermal conductivity exhibits a similar trend with the electrical conductivity. From the fractograph Fig. 6(c), we can clearly find out that Cu particles are connected together and almost every Mo particle is capsulated in continuous network structure of Cu. Consequently, the heat can be fast transferred between Mo and Cu. Maximum value of thermal conductivity is 188.64 Wm 1 K 1 for the compacts sintered at 1150 C. 4. Conclusions In this work, we have fabricated MoeCu composite powders with a Cu coreeMo shell structure by heterogeneous precipitation and hydrogen reduction method, which can inhibit the growth and coalescence of Cu grains effectively and benefit homogenous distribution of Mo and Cu components. The MoeCu composite powders exhibit remarkable sinterability, corresponding to relative density of 97.02% at 1150 C. Furthermore, the hardness, electrical conductivity and thermal conductivity of the MoeCu specimens are 191.1 HV, 27.00 MS/m and 188.64 Wm 1 K 1, respectively, which is attributed to the microstructure that almost every Mo particle is
Fig. 10. Thermal conductivity of the sintered compacts at different temperatures.
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capsulated by continuous network structure of Cu. Acknowledgments This work is supported by National Natural Science Foundation of China (No. 51274246). References [1] P. Song, J.G. Cheng, L. Wan, J.S. Zhao, Y.F. Wang, Y.B. Cai, J. Alloys Compd. 476 (2009) 226e230. [2] J.T. Yao, C.J. Li, Y. Li, B. Chen, H.B. Huo, Mater. Des. 88 (2015) 774e780. ~ ez, F. Castro, Mater. Lett. 61 (2007) [3] V. Martínez, C. Aguilar, J. Marín, S. Ordon 929e933. [4] D. Srinivasan, P.R. Subramanian, Mater. Sci. Eng. A 459 (2007) 145e150. [5] F. Fang, Y.Y. Zhou, W. Yang, Int. J. Refract. Met. Hard Mater. 42 (2014) 120e125. n, F. Cuevas, L. Lozada, N. Vielma, [6] C. Aguilar, F. Castro, V. Martínez, D. Guzma Mater. Sci. Eng. A 548 (2012) 189e194. [7] X.L. Zhou, Y.H. Dong, X.Z. Hua, R.U. Din, Z.G. Ye, Mater. Des. 31 (2010)
1603e1606. [8] J.L. Johnson, Int. J. Refract. Met. Hard Mater. 53 (2015) 80e86. [9] A.K. Sun, D.Z. Wang, Z.Z. Wu, L.H. Li, J.S. Wang, B.H. Duan, Mater. Chem. Phys. 148 (2014) 494e498. [10] A.K. Sun, X.J. Dong, X.Y. Wang, B.H. Duan, D.Z. Wang, J. Alloys Compd. 555 (2013) 6e9. [11] X.Q. Shen, M.X. Jing, W.X. Li, D.H. Li, Powder Technol. 160 (2005) 229e233. [12] X.F. Meng, X.Q. Shen, Particuology 10 (2012) 334e338. anský, [13] V. Tyrpekl, J.P. Vejpravov a, A.G. Roca, N. Murafa, L. Szatmary, D. Ni zn Appl. Surf. Sci. 257 (2011) 4844e4848. [14] D.H. Li, J.X. Zhou, X.Q. Shen, W. Liu, Partcuology 8 (2010) 257e261. [15] K.C. Kim, J.P. Jegal, S.M. Bak, K.C. Roh, K.B. Kim, Electrochem. Commun. 43 (2014) 113e116. [16] K. Kondo, T. Niwa, K. Danjo, Eur. J. Pharm. Sci. 51 (2014) 11e19. [17] L.M. Luo, X.Y. Tan, Z.L. Lu, X.Y. Zhu, X. Zan, G.N. Luo, Y.C. Wu, Int. J. Refract. Met. Hard Mater. 42 (2014) 51e56. [18] Q.J. Zheng, X. Wang, J.T. Tian, Z.F. Liu, C.S. Huang, J. Wang, J.H. Xu, Ceram. Int. 35 (2009) 897e900. [19] Y.J. Yao, B. Liu, J.Z. Xiao, K. Zhou, J. Su, J. paix~ ao, Mater. Today Proc. 2 (2015) 303e308. [20] S. Chouzier, T. Czeri, M.R. Auberger, C. Pichon, C. Geantet, M. Vrinat, P. Afanasiev, J. Solid State Chem. 184 (2011) 2668e2677.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Fabrication of MoeCu composite powders by heterogeneous precipitation and the sintering properties of the composite compacts Dezhi Wang, Bangzhu Yin, Aokui Sun, Xiulin Li, Chengkang Qi, Bohua Duan* Key Laboratory of Ministry of Education for Noneferrous Materials Science and Engineering, School of Materials Science and Engineering, Central South University, Changsha 410083, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 15 January 2016 Accepted 5 March 2016 Available online 7 March 2016
MoeCu composite powders with a coreeshell structure were fabricated by heterogeneous precipitation and reduction process. The sintering behavior of the composite powders and the sintering properties of the composite compacts were investigated. It was found that the MoeCu powders exhibit good sinterability. Relative density of 97.02% was achieved for the specimen sintered at 1150 C for 2 h. Furthermore, the hardness, electrical conductivity and thermal conductivity of the MoeCu specimens sintered at 1150 C are 191.1 HV, 27.00 MS/m and 188.64 Wm 1 K 1, respectively. The excellent properties are attributed to the microstructure that almost every Mo particle is capsulated by continuous network structure of Cu. © 2016 Elsevier B.V. All rights reserved.
Keywords: MoeCu Heterogeneous precipitation Reduction Sintering Microstructure
1. Introduction Due to many excellent physical and mechanical properties, such as high thermal and electrical conductivity, low coefficient of thermal expansion, low weight as well as highetemperature behavior, MoeCu alloys have been widely used in a variety of industrial and military applications [1e7]. MoeCu alloys are usually fabricated by liquid phase sintering of MoeCu powders compacts. However, high densification is difficult to be achieved [8]. Moreover, copper infiltration process during the liquid phase sintering results in inhomogeneous microstructure, which is detrimental to physical properties of the materials. Many researchers think that the Moecoated Cu composite powders with the uniform coreeshell structure would not only prevent the seepage of liquid copper, but also can inhibit the growth and coalescence of Cu grains effectively [9,10]. A few methods were used to prepare the coated powders, such as heterogeneous precipitation method [11e14], mechanical alloying process [10], microwaveeassisted hydrothermal method [15], spray drying method [16] and electroless plating method [17] etc., among which, the first mentioned one is most widely used. Some researches suggested that heterogeneous precipitation method was advantageous over other methods, because it could not only reduce sintering time and temperature, but also
* Corresponding author. E-mail address: [email protected] (B. Duan). http://dx.doi.org/10.1016/j.jallcom.2016.03.027 0925-8388/© 2016 Elsevier B.V. All rights reserved.
optimize the material microstructures. The result of Zheng's [18] has shown that LaPO4ecoated aeAl2O3 powders were successfully synthesized through the heterogeneous precipitation method. Y.J. Yao et al. [19] prepared nanoscale Al2O3ecoated ZrO2 composite powders via a heterogeneous precipitation process, which could prevent agglomeration of particles and obtain fineegrain ceramics. But heterogeneous precipitation method has not been applied for the fabrication of MoeCu composite powders. In this paper, Moecoated Cu composite powders were produced by heterogeneous precipitationehydrogen reduction method. The aim of the present work is to investigate the influence of the powders of Cu coreeMo shell structure on the sintering properties and to determine the optimum sintering temperature. 2. Experimental The raw materials used in the present study were ammonium heptamolybdate [(NH4)6Mo7O24$4H2O], hexamethylenetetramine (C6H12N4), pure copper powders with average particle size of 1.5 mm, polyethylene glycol 20000 (PEG 20000) and deionized water. In all experiments, weight proportion of each material corresponded to the final composition of Moe30 wt.%Cu. Fig. 1 describes the preparation of Moecoated Cu composite powders by heterogeneous precipitation and hydrogen reduction. (NH4)6Mo7O24$4H2O aqueous solution (0.25 mol/L) and C6H12N4 aqueous solution (0.25 mol/L) with the molar ratio 1:2 were prepared. Meanwhile, suitable amount of PEG 20000 accompanied
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Fig. 2. XRD pattern of MoeCu composite powders reduced at 800 C.
Fig. 1. Flow chart of preparing MoeCu composite powders.
with Cu powders were thoroughly dispersed in deionized water, then ultrasonic vibration was conducted for 30 min after mechanically stirring to remove the agglomerates. Finally, (NH4)6Mo7O24$4H2O solution and C6H12N4 solution were simultaneously added into Cu suspension with vigorous stirring, then the precipitates [(C6H12N4)2(NH4)4Mo7O24$2H2O] formed [20]. The precipitates were filtered and dried to form the precursor. The precursor was calcined in air at 400 C for 90 min, and then the asecalcined powders were reduced at 800 C for 2 h in hydrogen. The dew point of H2 was 30 to 40 C, the flow rate was 0.8 L/min, and the height of powder bed was 8 mm. After being cooled to room temperature, grinded and sieved, the MoeCu composite powders were obtained. The resultant MoeCu composite powders were compacted in a steel die under 200 MPa pressure to produce green parts. Green parts were sintered at different temperatures of 1050 C, 1100 C, 1150 C and 1200 C for 2 h in the hydrogen atmosphere. Phase composition was analyzed by X-ray diffraction (XRD) at a scan rate of 8 /min from 20 to 80 to record the diffraction patterns. Microstructures of the specimens were observed by scanning electron microscopy (SEM). Transmission electron microscope (TEM) coupled with high resolution transmission electron microscope (HRTEM) was applied to investigate the particle shape and corresponding phase composition. The densities of the sintered compacts were measured according to Archimedes' principle. The hardness of the specimens was determined by a Vickers hardness tester. Electrical conductivity was examined using a four wire method by a microeohmmeter. Thermal conductivity was measured by the thermal diffusivity analyzer.
Fig. 3. SEM of MoeCu composite powders.
3. Results and discussion Fig. 2 shows the XRD pattern of the MoeCu composite powders obtained by reducing at 800 C for 2 h, diffraction peaks attributed to Mo and Cu phases can be identified and no other peaks can be found, indicating that the powders were reduced completely. Fig. 3 and Fig. 4 show SEM and TEM images of the MoeCu composite powders reduced at 800 C. It is found that the MoeCu composite powders consist of basically spherical nanoparticles with uniform size distribution. Meanwhile, the particles obtained have a smooth surface (Fig. 3). Fig. 4(a) shows TEM micrograph of composite coating particles. It has been found that there is a homogeneous layer on the Cu cores, suggesting that predominantly heterogeneous precipitation took place. Fig. 4(b) presents HRTEM micrograph of composite coating particles, corresponding to Fig. 4(a). The interplanar crystal spacing of the area A and B is 0.2092 nm and 0.2227 nm, respectively. Referring to the PDF cards, the interplanar crystal spacing of Cu in the (1$1$1) crystal planes is 0.20871 nm and Mo in the (1$1$0) crystal planes is 0.22247 nm. Therefore, area A corresponds to Cu with the faceecentered cubic (fcc) structure, and area B corresponds to Mo with the bodyecentered cubic (bcc) structure, namely, Cu coreeMo shell
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Fig. 4. TEM (a) and HRTEM (b) of MoeCu composite powders.
Fig. 5. Crossesection microstructure of the sintered compacts at different sintering temperatures: (a) 1050 C; (b) 1100 C; (c) 1150 C; (d) 1200 C.
structure. This structure can prevent the growth and agglomeration of Cu grains, which will play a beneficial role in the subsequent sintering process. Fig. 5 shows the crossesection microstructure of the compacts
sintered for 2 h at the temperature of 1050 C, 1100 C, 1150 C and 1200 C. It can be clearly seen that quantities of interparticle pores are distributed over the section of the compacts sintered at 1050 C. Moreover, there are a large amount of Cu rich areas (Fig. 5(a)). With
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Fig. 6. Fractograph of the sintered compacts at different sintering temperatures: (a) 1050 C; (b) 1100 C; (c) 1150 C; (d) 1200 C.
Fig. 7. Effect of sintering temperature on relative density of the sintered compacts.
Fig. 8. Hardness of the sintered compacts at different temperatures.
sintering temperature rising up to 1100 C, the diffusion and flowability of Cu is enhanced and the distribution of Cu tends to be more homogenous. The amount and size of the pores are both reduced, but, there are still some Cu enrichment regions and pores (Fig. 5(b)). The distribution of Cu reaches the maximum uniformity
and pores tend to decline further with the sintering temperature up to 1150 C (Fig. 5(c)). Increasing the sintering temperature to 1200 C results in enrichment of Cu and the growth of the Mo particles, as is shown in Fig. 5(d). Fig. 6 presents the fractograph of the compacts sintered at the
D. Wang et al. / Journal of Alloys and Compounds 674 (2016) 347e352
temperature of 1050 C, 1100 C, 1150 C and 1200 C for 2 h Fig. 6(a) reveals that large pores exist in MoeCu compacts sintered at 1050 C. With sintering temperature increasing to 1100 C, the distribution of Cu tends to be more uniform and there are also some pores (Fig. 6(b)). A dense and homogenous distribution of Mo and Cu particles can be seen apparently from Fig. 6(c). On the whole, every Mo particle is capsulated by continuous network structure of Cu. This network structure that benefits the uniform distribution of Mo and Cu is favorable to mechanical properties, electrical conductivity and thermal conductivity of MoeCu alloys, and the grain size is about 1e2.5 mm. When the temperature is further increased to 1200 C, significant grain growth takes place and pores is caused by exudation of Cu in the compact, shown in Fig. 6(d). Fig. 7 presents the effect of sintering temperature on relative density of the sintered compacts. It can be seen that the relative density of the MoeCu compacts increase with increasing sintering temperature from 1050 C to 1200 C. When sintered at 1050 C, which is below the melting point of copper (1083 C), the sintering process is known as solid phase sintering. During solid phase sintering, densification of composite powders is determined only by internal atomic diffusion ability. Limited atomic diffusion brings about the pores and the low relative density. Relative density of MoeCu compacts increases sharply with sintering temperature rising from 1050 C to 1100 C. As the sintering temperature is up to 1200 C, relative density of the compacts further increases slowly. Reasons for the phenomenon are concluded as follows: When sintering temperature increases to 1100 C and 1150 C, the densification is related to liquid phase sintering, which is considered as the dominant sintering mechanism for the compacts. In this case, Cu melts and flows through the structure, the movement of liquid phase provides a capillary force, which causes rearrangement of the Mo particles and induces mass transport. Besides, uniform MoeCu composite powders with the coreeshell structure can prevent the growth and agglomeration of Cu grains, above all, preventing the seepage of liquid copper during liquid phase sintering. As the temperature increases to 1200 C, relative density continues to increase. However, because Cu content is as high as 30%, the softened Mo skeleton could not sustain the excessive Cu at the high temperature, which leads to a small amount of Cu exuding to the surface. Thus the ratio of the Mo and Cu is changed, some pores occur in the impact, as is shown in Fig. 6(d), which is harmful to properties of compacts. Therefore, the optimum sintering temperature was considered as 1150 C, corresponding to the relative density of 97.02%. Fig. 8 shows the hardness of the compacts sintered at different temperatures. As seen, the hardness of specimens increases when sintering temperature increases from 1050 C to 1150 C. Obviously, the high relative density of the sintered compacts is the main factor for high hardness. However, the hardness of the sintered samples decreases as the temperature increases to 1200 C. The reason is that the hardness of the composites also depends on volume fraction of harder phase, in which the hardness of copper is lower than molybdenum. At 1200 C, small amounts of Cu exude to the surface layer, resulting in the decrease of testing hardness. Hence the maximum value of hardness (191.1 HV) is obtained at the sintering temperature of 1150 C. Fig. 9 and Fig. 10 present the electrical conductivity and thermal conductivity of the sintered compacts at different temperatures, respectively. The results show that the electrical conductivity increases gradually from 1050 C to 1150 C and decreases when sintered at 1200 C, the maximum value of electrical conductivity (27.00 MS/m) is achieved at 1150 C. The reason for this phenomenon is that high densification of the sintered compacts and homogenous distribution of Cu are beneficial to the electron transfer. Meanwhile, the copper phase inherently has a much higher
351
Fig. 9. Electrical conductivity of the sintered compacts at different temperatures.
electrical conductivity than molybdenum, the uneven distribution and loss of Cu result in the decrease of electrical conductivity when sintered at 1200 C. Thermal conductivity exhibits a similar trend with the electrical conductivity. From the fractograph Fig. 6(c), we can clearly find out that Cu particles are connected together and almost every Mo particle is capsulated in continuous network structure of Cu. Consequently, the heat can be fast transferred between Mo and Cu. Maximum value of thermal conductivity is 188.64 Wm 1 K 1 for the compacts sintered at 1150 C. 4. Conclusions In this work, we have fabricated MoeCu composite powders with a Cu coreeMo shell structure by heterogeneous precipitation and hydrogen reduction method, which can inhibit the growth and coalescence of Cu grains effectively and benefit homogenous distribution of Mo and Cu components. The MoeCu composite powders exhibit remarkable sinterability, corresponding to relative density of 97.02% at 1150 C. Furthermore, the hardness, electrical conductivity and thermal conductivity of the MoeCu specimens are 191.1 HV, 27.00 MS/m and 188.64 Wm 1 K 1, respectively, which is attributed to the microstructure that almost every Mo particle is
Fig. 10. Thermal conductivity of the sintered compacts at different temperatures.
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