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A novel preparation method for W–Cu composite powders
Journal of Alloys and Compounds 661 (2016) 471e475
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
A novel preparation method for WeCu composite powders Xiaoxiao Wei, Jiancheng Tang*, Nan Ye, Haiou Zhuo School of Materials Science and Engineering, Nanchang University, Nanchang 330031, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 October 2015 Received in revised form 20 November 2015 Accepted 21 November 2015 Available online 26 November 2015
A novel method of nitridation-denitridation was developed to prepare WeCu composite powders. The preparation process consists of calcination, nitridation and denitridation, and the performance of WeCu composite powders mainly depends on nitridationedenitridation process. When the nitridation is carried out in ammonia at 450 C for 3 h, the phases of CuWO4X, WO3 and CuO transform to WO3-X and Cu. While the temperature reaches 550 C, the phase of W2N appears, and it's further increase to 650 C or even higher leads to only W2N and Cu exist in products. A small quantity of W2N is observed during the process of denitridation occurs in hydrogen at 825 C for 2 h. When the denitridation temperature is above 850 C, the patterns only contain peaks corresponding to W and Cu phases. The mean particle size of composite powders decreases from 109.23 nm to 90.19 nm with the increase of denitridation temperature from 825 C to 875 C, and it's further increase to 900 C results in the increase of the mean particle size of composite powders. The spherical WeCu composite powders with the mean particle size of 90.19 nm are obtained when the nitridation temperature and the denitridation temperature are 650 C and 875 C, respectively. WeCu alloys are prepared by sintering such composite powders in hydrogen at 1200 C for 90 min, and the relative density and hardness reach 98.2% and 258.7 HV, respectively. © 2015 Elsevier B.V. All rights reserved.
Keywords: WeCu composite powders Nitridationedenitridation Microstructure
1. Introduction WeCu alloys have been widely used in the manufacture of electrical contact materials, high voltage interrupters, electrodes and heat sinks materials [1e3], because of their excellent properties including high thermal conductivity (TC), low coefficient of thermal expansion (CET), nonmagnetic and well high-temperature behavior [4e7]. However, preparing WeCu alloys with highdensity and uniform microstructure are difficult due to the absence of mutual solubility in the entire concentration range, large differences between their melting points and densities, and large wetting angle of liquid copper on tungsten [3,7e9]. It has been known that using ultrafine or nano-level WeCu composite powders can obtain WeCu alloys with high-density and uniform microstructure [8]. Therefore, preparing ultrafine or nano-level powders with uniform distribution is the key issue during the process of densification. Ultrafine or nano-level WeCu composite powders can be fabricated by co-precipitation [10], freeze-drying technique [7], mechanical alloying (MA) [11], spray drying [12], sol-gel method [13], synthetic chemical procedure [14,15] and a
* Corresponding author. E-mail address: [email protected] (J. Tang). http://dx.doi.org/10.1016/j.jallcom.2015.11.158 0925-8388/© 2015 Elsevier B.V. All rights reserved.
mechano-chemical process [8]. The disadvantages of these methods are abnormal grain growth of W particles and grain coarsening, which are caused by volatilization of tungsten oxides during the hydrogen reduction process. In this study, a novel method of nitridationedenitridation was developed to prepare WeCu composite powders, and the preparation mechanism was investigated. 2. Experimental procedure Ammonium metatungstate and copper nitrate were dissolved in deionized water to obtain mixed salt solution, and the solution was heated at 80 C until the blue WeCu salt mixture was formed. The mixture was dried at 120 C for 8 h, and subsequently calcined at 650 C in air. The calcined powders were nitrified in ammonia atmosphere at 450 C, 550 C, 650 C and 750 C for 3 h. Denitridation was carried out in the hydrogen atmosphere at 825 C, 850 C, 875 C and 900 C for 2 h. WeCu samples were sintered in hydrogen atmosphere at 1200 C for 90 min. The flowing rate of ammonia and hydrogen during the nitridation and denitridation were 600 and 100 mL min1, respectively. To avoid the influence of oxygen on nitrified powders and denitrified powders, the samples were cooled in ammonia and hydrogen atmosphere, respectively. At last, the samples were passivated in nitrogen for 10 min at room
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Fig. 1. XRD patterns of the calcined powders at 650 C.
Fig. 4. SEM images of the nitrified powders at different temperatures: (a) 450 C; (b) 550 C; (c) 650 C; (d) 750 C.
temperature before exposure to air. The phases were characterized by X-ray diffraction (XRD, Bruker D8 X-ray diffractometer). The morphology was observed by scanning electron microscopy (Nova Nano SEM450). The components of WeCu composite powders were analyzed by energy dispersive spectrometer (EDS, INCA 250 X-Max 50). The density and hardness of sintered sample were measured by the Archimedes method and Vickers hardness tester (HVD-1000), respectively.
3. Results and discussion The XRD patterns of the calcined powders at 650 C are shown in Fig. 1. As can be seen in Fig. 1, the calcined powders consist of Fig. 2. SEM image of the calcined powders at 650 C.
Fig. 3. XRD patterns of the nitrified powders at different temperatures: (a) 450 C; (b) 550 C; (c) 650 C; (d) 750 C.
Fig. 5. XRD patterns of denitrified powders at different temperature for 2 h: (a) 825 C; (b) 850 C; (c) 875 C; (d) 900 C.
X. Wei et al. / Journal of Alloys and Compounds 661 (2016) 471e475
CuWO4X, WO3 and CuO phases. The phases of CuWO4X appear in the calcined powders, indicating that CuO and WO3 have solutionized in some content. The morphology of the calcined powders is shown in Fig. 2. As can be seen in Fig. 2, the calcined powders are of irregular polyhedron, and small particles agglomerate together or attach to the polyhedron surface. Fig. 3 shows the XRD patterns of the nitrified powders at different temperatures. As can be seen in Fig. 3(a), the nitrified powders at 450 C are composed of WO3-x and Cu phases. While the nitridation temperature reaches 550 C, the phase of W2N appears (Fig. 3(b)). With the nitridation temperature further increases, the patterns only contain peaks corresponding to W2N and Cu are observed in Fig. 3(ced), indicating that CuWO4eX, WO3 and CuO phases have been transformed to W2N and Cu phases completely. Fig. 4 shows the morphology of the nitrified powders at different temperatures. As can be seen from Fig. 4(a), the shape of nitrified powders still maintain that of the calcined powders, which means the phases of CuWO4x, WO3 and CuO have been transformed to W2N and Cu incompletely. This result is consistent with the results of XRD patterns in Fig. 3(a). But similar result is not
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observed in (Fig. 4(b)) when the nitridation occurred at 550 C. With the temperature further increases, the particles are of irregular shape and have a slight aggregation (Fig. 4(ced)). According to the mentioned above, the calcined powders can not converted to W2N and Cu phases incompletely in low temperature, and high temperature leads to grain growth and slight aggregation which are bad for denitridation. Therefore, it can be concluded that the optimum nitridation temperature is 650 C. Fig. 5 shows the XRD patterns of denitrified powders at different temperatures. As can be seen in Fig. 4(a), relatively weak peaks corresponding to W2N phase are observed, indicating that W2N and Cu phases have been transformed to W and Cu phases incompletely. The patterns only contain peaks corresponding to W and Cu phases Fig. 4(bed) when the denitridation temperature is above 850 C. The morphology of the denitrified powders and the corresponding map analyses of each image are shown in Fig. 6. It can be seen that the denitrified powders are of regular shape, and the distribution of W and Cu phases have low homogeneous when the denitridation occurred at 825 C (Fig. 6(aec)). While denitrified at 850 C, the morphology changes to nearly spherical shape and
Fig. 6. SEM image and elemental distribution of denitrified powders at different temperatures: SEM image (a), copper map analysis (b) and tungsten map analysis (c) at 825 C; SEM image (d), copper map analysis (e) and tungsten map analysis (f) at 850 C; SEM image (g), copper map analysis (h) and tungsten map analysis (i) at 875 C; SEM image (j), cooper map analysis (k) and tungsten map analysis (l) at 900 C.
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higher homogeneous state of W and Cu phases is obtained (Fig. 6(def)). With the denitridation temperature further increases, the morphology changes to spherical shape completely and the distribution of W and Cu phases has the highest homogeneous state (Fig. 6(gel)). However, when the denitridation temperature reaches to 900 C, the distribution has no significant change (Fig. 6(kel)). The effect of denitridation temperature on the mean particle size of composite powders is shown in Fig. 7. As can be seen in Fig. 7, the mean particle size of composite powders decreases from 109.23 nm to 90.19 nm with the increase of denitridation temperature from 825 C to 875 C, and it's further increase to 900 C results in the increase of the mean particle size of composite powders. Therefore, it can be concluded that the mean particle size of composite powders mainly depends on the denitridation temperature, and the optimum denitridation temperature for preparing WeCu composite powders is 875 C in the hydrogen atmosphere for 2 h. The morphology of WeCu composite powders prepared by different process are shown in Fig. 8. As can be seen from Fig. 8(a), the morphology of WeCu composite powders prepared by hydrogen-reduction process has a polygonal morphology and agglomeration. Whereas the morphology of WeCu composite powders prepared by nitridationedenitridation process is of spherical shape. Meanwhile, there is no obvious agglomeration in Fig. 8(b). The mean particle sizes of W-Cu composite powders prepared by hydrogen-reduction process and nitridationedenitridation process are 150 nm and 90.19 nm, respectively. This phenomenon can be explained as follows: Hydrogenreduction process is complicated and the approach to reduce tungsten oxides takes place under the following reactions:
WO3 þ 0:1H2 ¼ WO2:90 þ 0:1H2 O
WO2:90 þ 0:18H2 ¼ WO2:72 þ 0:18H2 O
Fig. 8. SEM images of W-Cu composite powders prepared by different process: (a) hydrogen-reduction process; (b) nitridation-denitridation process.
WO2:72 þ 0:72H2 ¼ WO2 þ 0:72H2 O
(3)
WO2 þ 2H2 ¼ W þ 2H2 O
(4)
The step of WO2/W is the critical step which determined the grain size of W powders [16]. During the process, the colorless transparent liquid was seen in outlet side when the temperature is lower than 400 C. With the increase of temperature, some volatile substance appears. Furthermore, the higher the temperature, the more volatile substance. The phenomena are consistent with the literature report [17e20]. The formation of volatile substance (WO2(OH)2) describes as below [18]:
WO3 þ H2 O ¼ WO2 ðOHÞ2
(5)
WO2:9 þ 1:1H2 O ¼ WO2 ðOHÞ2 þ 0:1H2
(6)
WO2:72 þ 1:28H2 O ¼ WO2 ðOHÞ2 þ 0:28H2
(7)
WO2 þ 2H2 O ¼ WO2 ðOHÞ2 þ H2
(8)
W þ 4H2 O ¼ WO2 ðOHÞ2 þ 3H2
(9)
(1)
(2)
The partial pressure PH2 o =pH2 of H2O during the reduction process shows strong effect on the nucleation and grain growth of W, and the H2O in hydrogen mainly comes from the reaction of WO3 with hydrogen. Once the reaction starts, water is formed and can participate in the following steps of reduction. If the PH2 o =pH2 in reduction process isn't effectively controlled, it will leads to high PH2 o =pH2 . The high PH2 o =pH2 not only decreases the nucleation rate of W particles but also significantly promotes the growth of W
Fig. 7. Effect of denitridation temperature on the mean particle size of composite powders.
Fig. 9. SEM image of fracture surface of W-Cu composites sintered at 1200 C.
X. Wei et al. / Journal of Alloys and Compounds 661 (2016) 471e475
particles via chemical vapor transport (CVT) through reactions Eqs. (5)e(8). In order to obtain ultrafine or nano-level powders, the transition must proceed under relatively slow PH2 o =pH2 in hydrogen atmosphere throughout. The nitridationedenitridation process is quite different from hydrogen-reduction process. Sun et al. [21] have shown that the nitridation reaction is a one-step reaction between WO3 and NH3. The reaction equation describes as follows:
4WO3 þ 8NH3 ¼ 2W2 N þ 12H2 O þ 3N2
(10)
The W powders are formed by the decomposition of W2N powders (Eq. (11)) due to its low thermal stability at high temperature [21,22].
2W2 N ¼ 4W þ N2
(11)
The Gibbs' free energy changes (DGT in Eq. (12)) of the reaction was calculated based on HSC thermodynamic database [22].
DGT ¼ DG0T þ RT,ln ðPN2 =P0 Þ
(12)
Where DG0T represents the change of Gibbs' free energy under standard state; R and T are the ideal gas constant and temperature; P0 and PN2 are the standard atmospheric pressure and the partial pressure of N2, respectively. The higher temperature and lower nitrogen partial pressure lead to lower DGT which indicates W2N decomposes into tungsten and nitrogen more easily. Most importantly, abnormal grain growth of W particles and grain coarsening caused by volatilization of tungsten oxides during the hydrogen reduction process do not happen due to the absence of H2O in the reaction system. Fig. 9 presents the morphology of fracture surface of W-Cu composites sintered at 1200 C for 2 h. It can be seen that the W particles are capsulated by network structure of Cu, and the mean particle size of W is 400 nm. The relative density and hardness of W-Cu composites prepared use such powders reach 98.2% and 258.7 HV, respectively. 4. Conclusions In this study, a novel method of nitridation-denitridation was developed to prepare W-Cu composite powders. The preparation process includes calcination, nitridation and denitridation, and the performance of W-Cu composite powders mainly depends on nitridation-denitridation process. The spherical W-Cu composite powders with the mean particle size of 90.19 nm are obtained when
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the nitridation temperature and the denitridation temperature are 650 C and 875 C, respectively. The relative density and hardness of W-Cu composites used such powders reach 98.2% and 258.7, respectively. Acknowledgment This work was supported by National Natural Science Foundation of China (Nos. 51471083, 51271090 and 51364036). References [1] E. Ahmadi, M. Malekzadeh, S.K. Sadrnezhaad, Int. J. Refract. Met. Hard Mater. 28 (2010) 441e445. [2] Y.P. Li, X.H. Qu, Z.U. Zheng, C.M. Lei, Z.Q. Zou, S. Yu, Int. J. Refract. Met. Hard Mater. 21 (2003) 259e264. [3] 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. [4] L.H. Duan, W.S. Lin, J.L. Wang, G.L. Yang, Int. J. Refract. Met. Hard Mater 46 (2014) 96e100. [5] P.G. Chen, G.Q. Luo, Q. Shen, M.J. Li, L.M. Zhang, Mater. Des. 46 (2013) 101e105. [6] B.Z. Sun, J.P. Song, Y. Yu, Z.G. Zhuang, M.J. Niu, Y. Liu, T.H. Zhang, Y. Qi, Int. J. Refract. Met. Hard Mater. 45 (2014) 76e79. [7] X.L. Xi, X.Y. Xu, Z.R. Nie, S. He, W. Wang, J. Yi, T.Y. Zuo, Int. J. Refract. Met. Hard Mater 28 (2010) 301e304. [8] J.G. Cheng, P. Song, Y.F. Gong, Y.B. Cai, Y.H. Xia, Mater. Sci. Eng. A 488 (2008) 453e457. [9] H. Abbaszadeh, A. Masoudi, H. Safabinesh, M. Takestani, Int. J. Refract. Met. Hard Mater 30 (2012) 145e151. [10] X.L. Shi, H. Yang, S. Wang, G.Q. Shao, X.L. Duan, Z. Xiong, T.G. Wang, Mater. Chem. Phys. 104 (2007) 235e239. [11] Ata Dolatmoradi, Shahram Raygan, Hossein Abdizadeh, Powder Technol. 233 (2013) 208e214. [12] T. Raghu, R. Sundaresan, P. Ramakrishnan, T.R. Rama Mohan, Mater. Sci. Eng. A 304e306 (2001) 438e441. [13] Y. Zhou, Q.X. Sun, R. Liu, X.P. Wang, C.S. Liu, Q.F. Fang, J. Alloys Comp. 547 (2013) 18e22. [14] P.K. Sahoo, S.S.K. Kamal, M. Premkumar, B. Sreedhar, S.K. Srivastava, L. Durai, Int. J. Refract. Met. Hard Mater. 29 (2011) 547e554. [15] P.K. Sahoo, S.S.K. Kamal, A.K. Singh, B. Sreedhar, L. Durai, S.K. Srivastava, J. Nanosci. Nanotechnol. 11 (2011) 2506e2513. [16] X.W. Wu, J.S. Luo, B.Z. Lu, C.H. Xie, Z.M. Pi, M.Z. Hu, X. Tao, G.G. Wu, Z.M. Yu, D.Q. Yi, Trans. Nonferrous Met. Soc. China 19 (2009) 785e789. [17] Dae-Gun Kim, Kyung Ho Min, Si-Yong Chang, Sung-Tag Oh, Chang-Hee Lee, Young Do Kim, Mater. Sci. Eng. A 399 (2005) 326e331. [18] M. Hashempour, H. Razavizadeh, H.R. Rezaie, M.T. Salehi, Mater. Charact. 60 (2009) 1232e1240. [19] W.D. Schubert, Int. J. Refract. Met. Hard Mater 9 (1990) 178e191. [20] W.D. Schubert, Int. J. Refract. Met. Hard Mater 10 (1991) 133e141. [21] S.K. Sun, Y.M. Kan, G.J. Zhang, P.L. Wang, J. Am, Ceram. Soc. 93 (2010) 3565e3568. [22] S.K. Sun, Y.M. Kan, D.W. Ni, J. Zou, G.J. Zhang, Int. J. Refract. Met. Hard Mater 35 (2012) 202e206.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
A novel preparation method for WeCu composite powders Xiaoxiao Wei, Jiancheng Tang*, Nan Ye, Haiou Zhuo School of Materials Science and Engineering, Nanchang University, Nanchang 330031, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 October 2015 Received in revised form 20 November 2015 Accepted 21 November 2015 Available online 26 November 2015
A novel method of nitridation-denitridation was developed to prepare WeCu composite powders. The preparation process consists of calcination, nitridation and denitridation, and the performance of WeCu composite powders mainly depends on nitridationedenitridation process. When the nitridation is carried out in ammonia at 450 C for 3 h, the phases of CuWO4X, WO3 and CuO transform to WO3-X and Cu. While the temperature reaches 550 C, the phase of W2N appears, and it's further increase to 650 C or even higher leads to only W2N and Cu exist in products. A small quantity of W2N is observed during the process of denitridation occurs in hydrogen at 825 C for 2 h. When the denitridation temperature is above 850 C, the patterns only contain peaks corresponding to W and Cu phases. The mean particle size of composite powders decreases from 109.23 nm to 90.19 nm with the increase of denitridation temperature from 825 C to 875 C, and it's further increase to 900 C results in the increase of the mean particle size of composite powders. The spherical WeCu composite powders with the mean particle size of 90.19 nm are obtained when the nitridation temperature and the denitridation temperature are 650 C and 875 C, respectively. WeCu alloys are prepared by sintering such composite powders in hydrogen at 1200 C for 90 min, and the relative density and hardness reach 98.2% and 258.7 HV, respectively. © 2015 Elsevier B.V. All rights reserved.
Keywords: WeCu composite powders Nitridationedenitridation Microstructure
1. Introduction WeCu alloys have been widely used in the manufacture of electrical contact materials, high voltage interrupters, electrodes and heat sinks materials [1e3], because of their excellent properties including high thermal conductivity (TC), low coefficient of thermal expansion (CET), nonmagnetic and well high-temperature behavior [4e7]. However, preparing WeCu alloys with highdensity and uniform microstructure are difficult due to the absence of mutual solubility in the entire concentration range, large differences between their melting points and densities, and large wetting angle of liquid copper on tungsten [3,7e9]. It has been known that using ultrafine or nano-level WeCu composite powders can obtain WeCu alloys with high-density and uniform microstructure [8]. Therefore, preparing ultrafine or nano-level powders with uniform distribution is the key issue during the process of densification. Ultrafine or nano-level WeCu composite powders can be fabricated by co-precipitation [10], freeze-drying technique [7], mechanical alloying (MA) [11], spray drying [12], sol-gel method [13], synthetic chemical procedure [14,15] and a
* Corresponding author. E-mail address: [email protected] (J. Tang). http://dx.doi.org/10.1016/j.jallcom.2015.11.158 0925-8388/© 2015 Elsevier B.V. All rights reserved.
mechano-chemical process [8]. The disadvantages of these methods are abnormal grain growth of W particles and grain coarsening, which are caused by volatilization of tungsten oxides during the hydrogen reduction process. In this study, a novel method of nitridationedenitridation was developed to prepare WeCu composite powders, and the preparation mechanism was investigated. 2. Experimental procedure Ammonium metatungstate and copper nitrate were dissolved in deionized water to obtain mixed salt solution, and the solution was heated at 80 C until the blue WeCu salt mixture was formed. The mixture was dried at 120 C for 8 h, and subsequently calcined at 650 C in air. The calcined powders were nitrified in ammonia atmosphere at 450 C, 550 C, 650 C and 750 C for 3 h. Denitridation was carried out in the hydrogen atmosphere at 825 C, 850 C, 875 C and 900 C for 2 h. WeCu samples were sintered in hydrogen atmosphere at 1200 C for 90 min. The flowing rate of ammonia and hydrogen during the nitridation and denitridation were 600 and 100 mL min1, respectively. To avoid the influence of oxygen on nitrified powders and denitrified powders, the samples were cooled in ammonia and hydrogen atmosphere, respectively. At last, the samples were passivated in nitrogen for 10 min at room
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Fig. 1. XRD patterns of the calcined powders at 650 C.
Fig. 4. SEM images of the nitrified powders at different temperatures: (a) 450 C; (b) 550 C; (c) 650 C; (d) 750 C.
temperature before exposure to air. The phases were characterized by X-ray diffraction (XRD, Bruker D8 X-ray diffractometer). The morphology was observed by scanning electron microscopy (Nova Nano SEM450). The components of WeCu composite powders were analyzed by energy dispersive spectrometer (EDS, INCA 250 X-Max 50). The density and hardness of sintered sample were measured by the Archimedes method and Vickers hardness tester (HVD-1000), respectively.
3. Results and discussion The XRD patterns of the calcined powders at 650 C are shown in Fig. 1. As can be seen in Fig. 1, the calcined powders consist of Fig. 2. SEM image of the calcined powders at 650 C.
Fig. 3. XRD patterns of the nitrified powders at different temperatures: (a) 450 C; (b) 550 C; (c) 650 C; (d) 750 C.
Fig. 5. XRD patterns of denitrified powders at different temperature for 2 h: (a) 825 C; (b) 850 C; (c) 875 C; (d) 900 C.
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CuWO4X, WO3 and CuO phases. The phases of CuWO4X appear in the calcined powders, indicating that CuO and WO3 have solutionized in some content. The morphology of the calcined powders is shown in Fig. 2. As can be seen in Fig. 2, the calcined powders are of irregular polyhedron, and small particles agglomerate together or attach to the polyhedron surface. Fig. 3 shows the XRD patterns of the nitrified powders at different temperatures. As can be seen in Fig. 3(a), the nitrified powders at 450 C are composed of WO3-x and Cu phases. While the nitridation temperature reaches 550 C, the phase of W2N appears (Fig. 3(b)). With the nitridation temperature further increases, the patterns only contain peaks corresponding to W2N and Cu are observed in Fig. 3(ced), indicating that CuWO4eX, WO3 and CuO phases have been transformed to W2N and Cu phases completely. Fig. 4 shows the morphology of the nitrified powders at different temperatures. As can be seen from Fig. 4(a), the shape of nitrified powders still maintain that of the calcined powders, which means the phases of CuWO4x, WO3 and CuO have been transformed to W2N and Cu incompletely. This result is consistent with the results of XRD patterns in Fig. 3(a). But similar result is not
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observed in (Fig. 4(b)) when the nitridation occurred at 550 C. With the temperature further increases, the particles are of irregular shape and have a slight aggregation (Fig. 4(ced)). According to the mentioned above, the calcined powders can not converted to W2N and Cu phases incompletely in low temperature, and high temperature leads to grain growth and slight aggregation which are bad for denitridation. Therefore, it can be concluded that the optimum nitridation temperature is 650 C. Fig. 5 shows the XRD patterns of denitrified powders at different temperatures. As can be seen in Fig. 4(a), relatively weak peaks corresponding to W2N phase are observed, indicating that W2N and Cu phases have been transformed to W and Cu phases incompletely. The patterns only contain peaks corresponding to W and Cu phases Fig. 4(bed) when the denitridation temperature is above 850 C. The morphology of the denitrified powders and the corresponding map analyses of each image are shown in Fig. 6. It can be seen that the denitrified powders are of regular shape, and the distribution of W and Cu phases have low homogeneous when the denitridation occurred at 825 C (Fig. 6(aec)). While denitrified at 850 C, the morphology changes to nearly spherical shape and
Fig. 6. SEM image and elemental distribution of denitrified powders at different temperatures: SEM image (a), copper map analysis (b) and tungsten map analysis (c) at 825 C; SEM image (d), copper map analysis (e) and tungsten map analysis (f) at 850 C; SEM image (g), copper map analysis (h) and tungsten map analysis (i) at 875 C; SEM image (j), cooper map analysis (k) and tungsten map analysis (l) at 900 C.
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higher homogeneous state of W and Cu phases is obtained (Fig. 6(def)). With the denitridation temperature further increases, the morphology changes to spherical shape completely and the distribution of W and Cu phases has the highest homogeneous state (Fig. 6(gel)). However, when the denitridation temperature reaches to 900 C, the distribution has no significant change (Fig. 6(kel)). The effect of denitridation temperature on the mean particle size of composite powders is shown in Fig. 7. As can be seen in Fig. 7, the mean particle size of composite powders decreases from 109.23 nm to 90.19 nm with the increase of denitridation temperature from 825 C to 875 C, and it's further increase to 900 C results in the increase of the mean particle size of composite powders. Therefore, it can be concluded that the mean particle size of composite powders mainly depends on the denitridation temperature, and the optimum denitridation temperature for preparing WeCu composite powders is 875 C in the hydrogen atmosphere for 2 h. The morphology of WeCu composite powders prepared by different process are shown in Fig. 8. As can be seen from Fig. 8(a), the morphology of WeCu composite powders prepared by hydrogen-reduction process has a polygonal morphology and agglomeration. Whereas the morphology of WeCu composite powders prepared by nitridationedenitridation process is of spherical shape. Meanwhile, there is no obvious agglomeration in Fig. 8(b). The mean particle sizes of W-Cu composite powders prepared by hydrogen-reduction process and nitridationedenitridation process are 150 nm and 90.19 nm, respectively. This phenomenon can be explained as follows: Hydrogenreduction process is complicated and the approach to reduce tungsten oxides takes place under the following reactions:
WO3 þ 0:1H2 ¼ WO2:90 þ 0:1H2 O
WO2:90 þ 0:18H2 ¼ WO2:72 þ 0:18H2 O
Fig. 8. SEM images of W-Cu composite powders prepared by different process: (a) hydrogen-reduction process; (b) nitridation-denitridation process.
WO2:72 þ 0:72H2 ¼ WO2 þ 0:72H2 O
(3)
WO2 þ 2H2 ¼ W þ 2H2 O
(4)
The step of WO2/W is the critical step which determined the grain size of W powders [16]. During the process, the colorless transparent liquid was seen in outlet side when the temperature is lower than 400 C. With the increase of temperature, some volatile substance appears. Furthermore, the higher the temperature, the more volatile substance. The phenomena are consistent with the literature report [17e20]. The formation of volatile substance (WO2(OH)2) describes as below [18]:
WO3 þ H2 O ¼ WO2 ðOHÞ2
(5)
WO2:9 þ 1:1H2 O ¼ WO2 ðOHÞ2 þ 0:1H2
(6)
WO2:72 þ 1:28H2 O ¼ WO2 ðOHÞ2 þ 0:28H2
(7)
WO2 þ 2H2 O ¼ WO2 ðOHÞ2 þ H2
(8)
W þ 4H2 O ¼ WO2 ðOHÞ2 þ 3H2
(9)
(1)
(2)
The partial pressure PH2 o =pH2 of H2O during the reduction process shows strong effect on the nucleation and grain growth of W, and the H2O in hydrogen mainly comes from the reaction of WO3 with hydrogen. Once the reaction starts, water is formed and can participate in the following steps of reduction. If the PH2 o =pH2 in reduction process isn't effectively controlled, it will leads to high PH2 o =pH2 . The high PH2 o =pH2 not only decreases the nucleation rate of W particles but also significantly promotes the growth of W
Fig. 7. Effect of denitridation temperature on the mean particle size of composite powders.
Fig. 9. SEM image of fracture surface of W-Cu composites sintered at 1200 C.
X. Wei et al. / Journal of Alloys and Compounds 661 (2016) 471e475
particles via chemical vapor transport (CVT) through reactions Eqs. (5)e(8). In order to obtain ultrafine or nano-level powders, the transition must proceed under relatively slow PH2 o =pH2 in hydrogen atmosphere throughout. The nitridationedenitridation process is quite different from hydrogen-reduction process. Sun et al. [21] have shown that the nitridation reaction is a one-step reaction between WO3 and NH3. The reaction equation describes as follows:
4WO3 þ 8NH3 ¼ 2W2 N þ 12H2 O þ 3N2
(10)
The W powders are formed by the decomposition of W2N powders (Eq. (11)) due to its low thermal stability at high temperature [21,22].
2W2 N ¼ 4W þ N2
(11)
The Gibbs' free energy changes (DGT in Eq. (12)) of the reaction was calculated based on HSC thermodynamic database [22].
DGT ¼ DG0T þ RT,ln ðPN2 =P0 Þ
(12)
Where DG0T represents the change of Gibbs' free energy under standard state; R and T are the ideal gas constant and temperature; P0 and PN2 are the standard atmospheric pressure and the partial pressure of N2, respectively. The higher temperature and lower nitrogen partial pressure lead to lower DGT which indicates W2N decomposes into tungsten and nitrogen more easily. Most importantly, abnormal grain growth of W particles and grain coarsening caused by volatilization of tungsten oxides during the hydrogen reduction process do not happen due to the absence of H2O in the reaction system. Fig. 9 presents the morphology of fracture surface of W-Cu composites sintered at 1200 C for 2 h. It can be seen that the W particles are capsulated by network structure of Cu, and the mean particle size of W is 400 nm. The relative density and hardness of W-Cu composites prepared use such powders reach 98.2% and 258.7 HV, respectively. 4. Conclusions In this study, a novel method of nitridation-denitridation was developed to prepare W-Cu composite powders. The preparation process includes calcination, nitridation and denitridation, and the performance of W-Cu composite powders mainly depends on nitridation-denitridation process. The spherical W-Cu composite powders with the mean particle size of 90.19 nm are obtained when
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the nitridation temperature and the denitridation temperature are 650 C and 875 C, respectively. The relative density and hardness of W-Cu composites used such powders reach 98.2% and 258.7, respectively. Acknowledgment This work was supported by National Natural Science Foundation of China (Nos. 51471083, 51271090 and 51364036). References [1] E. Ahmadi, M. Malekzadeh, S.K. Sadrnezhaad, Int. J. Refract. Met. Hard Mater. 28 (2010) 441e445. [2] Y.P. Li, X.H. Qu, Z.U. Zheng, C.M. Lei, Z.Q. Zou, S. Yu, Int. J. Refract. Met. Hard Mater. 21 (2003) 259e264. [3] 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. [4] L.H. Duan, W.S. Lin, J.L. Wang, G.L. Yang, Int. J. Refract. Met. Hard Mater 46 (2014) 96e100. [5] P.G. Chen, G.Q. Luo, Q. Shen, M.J. Li, L.M. Zhang, Mater. Des. 46 (2013) 101e105. [6] B.Z. Sun, J.P. Song, Y. Yu, Z.G. Zhuang, M.J. Niu, Y. Liu, T.H. Zhang, Y. Qi, Int. J. Refract. Met. Hard Mater. 45 (2014) 76e79. [7] X.L. Xi, X.Y. Xu, Z.R. Nie, S. He, W. Wang, J. Yi, T.Y. Zuo, Int. J. Refract. Met. Hard Mater 28 (2010) 301e304. [8] J.G. Cheng, P. Song, Y.F. Gong, Y.B. Cai, Y.H. Xia, Mater. Sci. Eng. A 488 (2008) 453e457. [9] H. Abbaszadeh, A. Masoudi, H. Safabinesh, M. Takestani, Int. J. Refract. Met. Hard Mater 30 (2012) 145e151. [10] X.L. Shi, H. Yang, S. Wang, G.Q. Shao, X.L. Duan, Z. Xiong, T.G. Wang, Mater. Chem. Phys. 104 (2007) 235e239. [11] Ata Dolatmoradi, Shahram Raygan, Hossein Abdizadeh, Powder Technol. 233 (2013) 208e214. [12] T. Raghu, R. Sundaresan, P. Ramakrishnan, T.R. Rama Mohan, Mater. Sci. Eng. A 304e306 (2001) 438e441. [13] Y. Zhou, Q.X. Sun, R. Liu, X.P. Wang, C.S. Liu, Q.F. Fang, J. Alloys Comp. 547 (2013) 18e22. [14] P.K. Sahoo, S.S.K. Kamal, M. Premkumar, B. Sreedhar, S.K. Srivastava, L. Durai, Int. J. Refract. Met. Hard Mater. 29 (2011) 547e554. [15] P.K. Sahoo, S.S.K. Kamal, A.K. Singh, B. Sreedhar, L. Durai, S.K. Srivastava, J. Nanosci. Nanotechnol. 11 (2011) 2506e2513. [16] X.W. Wu, J.S. Luo, B.Z. Lu, C.H. Xie, Z.M. Pi, M.Z. Hu, X. Tao, G.G. Wu, Z.M. Yu, D.Q. Yi, Trans. Nonferrous Met. Soc. China 19 (2009) 785e789. [17] Dae-Gun Kim, Kyung Ho Min, Si-Yong Chang, Sung-Tag Oh, Chang-Hee Lee, Young Do Kim, Mater. Sci. Eng. A 399 (2005) 326e331. [18] M. Hashempour, H. Razavizadeh, H.R. Rezaie, M.T. Salehi, Mater. Charact. 60 (2009) 1232e1240. [19] W.D. Schubert, Int. J. Refract. Met. Hard Mater 9 (1990) 178e191. [20] W.D. Schubert, Int. J. Refract. Met. Hard Mater 10 (1991) 133e141. [21] S.K. Sun, Y.M. Kan, G.J. Zhang, P.L. Wang, J. Am, Ceram. Soc. 93 (2010) 3565e3568. [22] S.K. Sun, Y.M. Kan, D.W. Ni, J. Zou, G.J. Zhang, Int. J. Refract. Met. Hard Mater 35 (2012) 202e206.