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Hydrogen-reduction behavior and microstructural characteristics of WO3–CuO powder mixtures with various milling time
Journal of Alloys and Compounds 354 (2003) 239–242
L
www.elsevier.com / locate / jallcom
Hydrogen-reduction behavior and microstructural characteristics of WO 3 –CuO powder mixtures with various milling time a b a a a, Dae-Gun Kim , Sung-Tag Oh , Hyeongtag Jeon , Chang-Hee Lee , Young Do Kim * a
b
Division of Materials Science and Engineering, Hanyang University, Seoul 133 -791, South Korea Department of Metallurgy and Materials Science, Hanyang University, Ansan 425 -791, South Korea Received 3 September 2002; accepted 5 November 2002
Abstract Microstructure and hydrogen reduction behavior of WO 3 –CuO with different milling time are discussed in terms of characteristic of hygrometry curve and W particle size of reduced W–Cu composite powders. With increased milling time, the peak temperatures for the reduction of CuO and WO 3 in hygrometry curve are shifted to low temperatures and the peaks are changed to sharp shape. Microstructural analysis revealed that the hydrogen-reduced powder with milling time of 20 h showed smaller particle size than that of 1 h. The change of reduction behavior and particle size is discussed based on the refinement of oxide powders and resultant increased surface area as well as reduction process of WO 2 by CVT. 2003 Elsevier Science B.V. All rights reserved. Keywords: Nanostructures; Powder metallurgy; Nanofabrications
1. Introduction The tungsten–copper (W–Cu) composites are mainly used for micro-electronic applications like blocking materials for microwave package, high voltage contact materials and heat sink materials for high density integrated circuit [1,2]. Generally, these materials are produced by conventional Cu-infiltration process or sintering method. However, such processes have some restriction for use in practical applications; Cu-infiltration technique can be only applied for the fabrication of W–Cu composites with low W content. Also, the W–Cu composite is difficult for full densification by the liquid phase sintering without the addition of activators such as Ni, Co and Fe due to mutual insolubility and high contact angle between W and Cu [3–5]. However, the sintering aids induce the deterioration of physical properties such as electrical and thermal conductivity of composites [1]. To obtain the fully densified W–Cu composites with desired properties, there-
*Corresponding author. Tel.: 182-2-2290-0408; fax: 182-2-22821976. E-mail address: [email protected] (Y.D. Kim).
fore, new processing without the use of sintering aids is strongly required. Recently, a mechanochemical process that consists of ball milling of metal oxide powders and a subsequent hydrogen reduction has been suggested to prepare composite powders [6,7]. This process makes the synthesis of nano-sized composite powders with homogenous mixing possible. In addition, such powders show an improved sinterability compared with micro-sized powder mixtures, because of decreased particle size. In this regard, many investigations for mechanochemical process have been performed to prepare and consolidate nano-sized W–Cu composite powders [7–9]. However, there has been reported few details of the effects of processing factors such as milling time and hydrogen-reduction condition on the final microstructure. Thus, the understanding of microstructural development during ball milling and reduction process is essential to synthesis of the nano-sized W–Cu powders with required microstructure and properties. In the present work, the effect of the ball-milling time on powder characteristics of WO 3 –CuO is described. Also, the hydrogen reduction behavior of the ball-milled powders is analyzed in terms of characteristic of hygrometry curve and W particle size of reduced W–Cu composite powders depending on the ball-milling time.
0925-8388 / 03 / $ – see front matter 2003 Elsevier Science B.V. All rights reserved. doi:10.1016 / S0925-8388(03)00007-0
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tometer (XRD) to evaluate the grain size and the phase of powder. The microstructures of powders were observed through transmission electron microscope (TEM).
3. Results and discussion
Fig. 1. XRD patterns of elemental powders and ball-milled WO 3 –CuO mixtures.
2. Experimental procedure To fabricate W–25 wt% Cu composite powder, WO 3 powder with mean particle size of 20 mm and purity of 99.95% (TaeguTec, South Korea) and CuO powder with mean particle size of 10 mm and purity of 99.9% (Kojundo Chemical Laboratory, Japan) were used. The WO 3 –CuO mixtures, premixed on 62 rpm for 0.5 h with tubular mixer, were ball-milled on 400 rpm for 1 to 20 h in air atmosphere with Simoloyer (Zoz, Germany, CM01) that is the horizontal-typed attritor with the chamber of 2000 cc. To prevent the contamination, the milling media made of stainless steel were coated with the WO 3 –CuO mixture by the pre-milling on 400 rpm for 100 h. The ball-milled WO 3 –CuO mixtures were reduced at the heating rate of 10 8C min 21 to 1000 8C in the hydrogen atmosphere (276 8C). The content of water vapor in outlet gas was measured by in situ humidity measuring system. The ball-milled WO 3 –CuO mixtures and the reduced W–Cu nanocomposite powders were analyzed by X-ray diffrac-
Fig. 2. Grain size of WO 3 –CuO powder mixtures as a function of milling time.
Fig. 1 shows the XRD patterns of the WO 3 –CuO mixtures with the various ball-milling time. With increasing the ball-milling time, the peaks for WO 3 and CuO phases were broadened, and after milling time for 20 h, CuO peaks nearly disappeared due to the refinement of grain sizes [10]. Variation of the grain size of WO 3 and CuO with ball-milling time, calculated from the full-width half maximum of the XRD patterns using the Hall–Williamson equation, is shown in Fig. 2. The grain size of each oxide significantly decreased with the increase in milling time. At the milling time of 20 h, the measured grain sizes of the WO 3 and CuO powders were 17 and 11 nm, respectively. In order to further characterize the microstructure, TEM observation for the powder mixtures with different milling time was employed. As clearly seen from Fig. 3a, WO 3 – CuO particles with a size of about 40 nm was observed in the powder mixture with milling time of 1 h. However, the oxides mixture, ball-milled for 20 h, exhibited an aggregate of very fine particles of about 15 nm in size (Fig. 3b) which is consistent with the particle size measured from XRD results. Humidity curves for the hydrogen reduction process of the WO 3 –CuO powder mixtures with milling time of 1 and 20 h during heat-up to 1000 8C with a heating rate of 10 8C min 21 are presented in Fig. 4. As reported in the literature [7,11], the humidity peaks of both mixtures are largely divided into two parts: The first one (designated I in Fig. 4) at low temperature is related to the reduction process of CuO to Cu. The second part at high temperature is assigned to the reduction of WO 3 , in which peak II, III and IV belong to the reduction of WO 3 →WO 2.9 – 2.72 , WO 2.9 – 2.72 →WO 2 and WO 2 →W, respectively. In this result, it is noted that the peak positions of I–III in the powder mixture with milling time of 20 h are shifted to low temperatures compared with that of 1 h, whereas peak IV shows the same position. In addition, humidity peaks of II, III and IV showed different shape dependent on milling time; humidity peaks for the WO 3 –CuO mixture of 20 h have a sharp shape, i.e. rapid increase and decrease of humidity compared with that of 1 h. As shown in Fig. 2, increased milling time induced the refinement of particles and resultant increased surface area. Such large surface on which remarkable reduction took place promotes the reaction such as the rapid formation and removal of H 2 O at the heating stage [7]. Thus, the decrease in reduction temperature and the presence of
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241
Fig. 3. TEM micrographs of WO 3 –CuO powder mixtures, ball-milled for (a) 1 and (b) 20 h.
sharp peak in powder mixture of 20 h can be understood as the consequence of fine particle size. On the other hand, the same position of peak IV (reaction of WO 2 →W) was registered in the humidity curves, regardless of milling time (Fig. 4). Generally, the reduction of WO 2 is combined with a chemical vapor transportation (CVT) reaction, in which W is transported via the gas phase of WO 2 (OH) 2 . This CVT reaction consists of three steps [11,12]: an oxidative dissolution step by WO 2 (s)12H 2 O (g)→WO 2 (OH) 2 (g)1H 2 (g), the transportation of WO 2 (OH) 2 and a reductive deposition step by WO 2 (OH) 2 (g)13H 2 (g)→W (s)14H 2 O (g). Thus, the reduction of WO 2 is strongly affected by the humidity in the H 2 gas (PH 2 O /PH 2 ). For example, the reduction of WO 2 does not proceed until the PH 2 O /PH 2 at reaction zone
Fig. 4. Non-isothermal humidity curves obtained on heating WO 3 –CuO powders at 10 8C min 21 in hydrogen atmosphere.
becomes lower than the equilibrium condition through an effective removal of H 2 O. In this regard, considering that the powder mixture with milling of 20 h showed rapid reaction for the formation of WO 2 (peak III in Fig. 4), the same temperature for peak IV with powder mixture of 1 h can be explained by the retardation of WO 2 reduction due to the production of large amount of H 2 O in short reaction time. Typical microstructure of the reduced W–Cu powders with the milling time of 1 and 20 h are shown in Fig. 5a and b, respectively. It is observed that the powder mixture with milling time of 20 h seems to have small grain size compared with that of 1 h. The average grain sizes of W particle, calculated from Hall–Williamson equation with the full-width half maximum of the XRD patterns, were measured as 45 nm for the powder of 1 h and 39 nm for 20 h. Generally, it is considered that the particle size of hydrogen-reduced WO 3 –CuO mixture is determined by the initial size of oxide powders and distribution of prereduced Cu phase, in which the homogeneous distribution of Cu particles might effectively lower the contact frequency of W or W-oxide phases, and thus it can considerably reduce the growth of W clusters nucleated from W oxide [13,14]. In addition, the reduction process of WO 2 by CVT can strongly affect the final particle size of W. As reported in the literature [11], the nucleation of W becomes favorable in dry atmosphere (low H 2 O content in H 2 gas) while the growth process is dominant in wet condition. Thus, the formation of fine W powders is possible, when atmosphere condition is optimized to promote nucleation and to suppress the growth of W particles. In this sense, it can be concluded that the powder mixture with milling time of 20 h provides an optimum condition for the
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Fig. 5. TEM micrographs of reduced W–Cu composite powders, ball-milled for (a) 1 and (b) 20 h.
formation of finer W–Cu composite powder by fulfilling the above requirements.
Acknowledgements This work was supported by Korea Research Foundation Grant (KRF-2001-042-E00117).
4. Conclusion References This work demonstrated the effect of the ball-milling time on microstructural characteristics and hydrogen reduction behavior of WO 3 –CuO powders. The grain size of powder mixtures decreased with increase in milling time. In the milling time of 20 h, the grain size of the WO 3 and CuO powders was measured as 17 and 11 nm, respectively. Hydrogen reduction behavior of the WO 3 –CuO mixtures was analyzed in terms of characteristic of hygrometry curve. With increased milling time, the humidity peaks are changed to sharp shape and the peak temperatures for the reduction of CuO and WO 3 are shifted to low temperatures, whereas peak for the reduction of WO 2 shows the same position. The decrease in reduction temperature and the presence of sharp peak are understood as the consequence of fine particle size. Microstructural analysis revealed that the hydrogen-reduced powder with milling time of 20 h showed smaller particle size than that of 1 h. The change of particle size is discussed based on the initial size of oxide powders, distribution of pre-reduced Cu phase and reduction process of WO 2 by CVT.
[1] K.V. Sebastion, Int. J. Powder Metall. Powder Technol. 17 (1981) 297. [2] B.L. Mordike, J. Kaczmar, M. Kielbinski, K.U. Kainer, Powder Metall. Int. 23 (1991) 91. [3] J.L. Johnson, R.M. German, Metall. Trans. A 24A (1993) 2369. [4] I.-H. Moon, J.S. Lee, Powder Metall. Int. 9 (1977) 23. [5] A. Upadhyaya, R.M. German, Int. J. Powder Metall. 34 (1998) 43. [6] Q. Chongliang, W. Enxi, Z. Zhiquiang, Z. Yuhua, in: H. Bildstein, R. Eck (Eds.), Proceedings of the 13th Plansee Seminar, Vol. 1, 1993, p. 461. [7] T.H. Kim, J.H. Yu, J.S. Lee, Nanostruct. Mater. 9 (1997) 213. [8] S. Lee, W.-H. Baek, B.-S. Chun, J. Korean Inst. Metals Mater. 35 (1997) 1710. [9] G.-G. Lee, G.-H. Ha, B.-K. Kim, Powder Metall. 43 (2000) 79. [10] B.D. Cullity, Elements of X-ray Diffraction, 2nd Edition, AddisonWesley, New York, 1978, 281 pp. [11] W.D. Schubert, Int. J. Refract. Metals Hard Mater. (1990) 178. [12] E. Lanssner, W.D. Schubert, Tungsten, Kluwer Academic / Plenum Press, New York, 1999, 85 pp. [13] R. Haubner, W.D. Schubert, E. Lassner, B. Lux, in: H. Bildstein, H. Ortner (Eds.), Proceedings of the 11th Plansee Seminar, Vol. 2, 1993, p. 69. [14] J.S. Lee, T.H. Kim, Solid State Phenom. 25–26 (1992) 143.
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www.elsevier.com / locate / jallcom
Hydrogen-reduction behavior and microstructural characteristics of WO 3 –CuO powder mixtures with various milling time a b a a a, Dae-Gun Kim , Sung-Tag Oh , Hyeongtag Jeon , Chang-Hee Lee , Young Do Kim * a
b
Division of Materials Science and Engineering, Hanyang University, Seoul 133 -791, South Korea Department of Metallurgy and Materials Science, Hanyang University, Ansan 425 -791, South Korea Received 3 September 2002; accepted 5 November 2002
Abstract Microstructure and hydrogen reduction behavior of WO 3 –CuO with different milling time are discussed in terms of characteristic of hygrometry curve and W particle size of reduced W–Cu composite powders. With increased milling time, the peak temperatures for the reduction of CuO and WO 3 in hygrometry curve are shifted to low temperatures and the peaks are changed to sharp shape. Microstructural analysis revealed that the hydrogen-reduced powder with milling time of 20 h showed smaller particle size than that of 1 h. The change of reduction behavior and particle size is discussed based on the refinement of oxide powders and resultant increased surface area as well as reduction process of WO 2 by CVT. 2003 Elsevier Science B.V. All rights reserved. Keywords: Nanostructures; Powder metallurgy; Nanofabrications
1. Introduction The tungsten–copper (W–Cu) composites are mainly used for micro-electronic applications like blocking materials for microwave package, high voltage contact materials and heat sink materials for high density integrated circuit [1,2]. Generally, these materials are produced by conventional Cu-infiltration process or sintering method. However, such processes have some restriction for use in practical applications; Cu-infiltration technique can be only applied for the fabrication of W–Cu composites with low W content. Also, the W–Cu composite is difficult for full densification by the liquid phase sintering without the addition of activators such as Ni, Co and Fe due to mutual insolubility and high contact angle between W and Cu [3–5]. However, the sintering aids induce the deterioration of physical properties such as electrical and thermal conductivity of composites [1]. To obtain the fully densified W–Cu composites with desired properties, there-
*Corresponding author. Tel.: 182-2-2290-0408; fax: 182-2-22821976. E-mail address: [email protected] (Y.D. Kim).
fore, new processing without the use of sintering aids is strongly required. Recently, a mechanochemical process that consists of ball milling of metal oxide powders and a subsequent hydrogen reduction has been suggested to prepare composite powders [6,7]. This process makes the synthesis of nano-sized composite powders with homogenous mixing possible. In addition, such powders show an improved sinterability compared with micro-sized powder mixtures, because of decreased particle size. In this regard, many investigations for mechanochemical process have been performed to prepare and consolidate nano-sized W–Cu composite powders [7–9]. However, there has been reported few details of the effects of processing factors such as milling time and hydrogen-reduction condition on the final microstructure. Thus, the understanding of microstructural development during ball milling and reduction process is essential to synthesis of the nano-sized W–Cu powders with required microstructure and properties. In the present work, the effect of the ball-milling time on powder characteristics of WO 3 –CuO is described. Also, the hydrogen reduction behavior of the ball-milled powders is analyzed in terms of characteristic of hygrometry curve and W particle size of reduced W–Cu composite powders depending on the ball-milling time.
0925-8388 / 03 / $ – see front matter 2003 Elsevier Science B.V. All rights reserved. doi:10.1016 / S0925-8388(03)00007-0
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tometer (XRD) to evaluate the grain size and the phase of powder. The microstructures of powders were observed through transmission electron microscope (TEM).
3. Results and discussion
Fig. 1. XRD patterns of elemental powders and ball-milled WO 3 –CuO mixtures.
2. Experimental procedure To fabricate W–25 wt% Cu composite powder, WO 3 powder with mean particle size of 20 mm and purity of 99.95% (TaeguTec, South Korea) and CuO powder with mean particle size of 10 mm and purity of 99.9% (Kojundo Chemical Laboratory, Japan) were used. The WO 3 –CuO mixtures, premixed on 62 rpm for 0.5 h with tubular mixer, were ball-milled on 400 rpm for 1 to 20 h in air atmosphere with Simoloyer (Zoz, Germany, CM01) that is the horizontal-typed attritor with the chamber of 2000 cc. To prevent the contamination, the milling media made of stainless steel were coated with the WO 3 –CuO mixture by the pre-milling on 400 rpm for 100 h. The ball-milled WO 3 –CuO mixtures were reduced at the heating rate of 10 8C min 21 to 1000 8C in the hydrogen atmosphere (276 8C). The content of water vapor in outlet gas was measured by in situ humidity measuring system. The ball-milled WO 3 –CuO mixtures and the reduced W–Cu nanocomposite powders were analyzed by X-ray diffrac-
Fig. 2. Grain size of WO 3 –CuO powder mixtures as a function of milling time.
Fig. 1 shows the XRD patterns of the WO 3 –CuO mixtures with the various ball-milling time. With increasing the ball-milling time, the peaks for WO 3 and CuO phases were broadened, and after milling time for 20 h, CuO peaks nearly disappeared due to the refinement of grain sizes [10]. Variation of the grain size of WO 3 and CuO with ball-milling time, calculated from the full-width half maximum of the XRD patterns using the Hall–Williamson equation, is shown in Fig. 2. The grain size of each oxide significantly decreased with the increase in milling time. At the milling time of 20 h, the measured grain sizes of the WO 3 and CuO powders were 17 and 11 nm, respectively. In order to further characterize the microstructure, TEM observation for the powder mixtures with different milling time was employed. As clearly seen from Fig. 3a, WO 3 – CuO particles with a size of about 40 nm was observed in the powder mixture with milling time of 1 h. However, the oxides mixture, ball-milled for 20 h, exhibited an aggregate of very fine particles of about 15 nm in size (Fig. 3b) which is consistent with the particle size measured from XRD results. Humidity curves for the hydrogen reduction process of the WO 3 –CuO powder mixtures with milling time of 1 and 20 h during heat-up to 1000 8C with a heating rate of 10 8C min 21 are presented in Fig. 4. As reported in the literature [7,11], the humidity peaks of both mixtures are largely divided into two parts: The first one (designated I in Fig. 4) at low temperature is related to the reduction process of CuO to Cu. The second part at high temperature is assigned to the reduction of WO 3 , in which peak II, III and IV belong to the reduction of WO 3 →WO 2.9 – 2.72 , WO 2.9 – 2.72 →WO 2 and WO 2 →W, respectively. In this result, it is noted that the peak positions of I–III in the powder mixture with milling time of 20 h are shifted to low temperatures compared with that of 1 h, whereas peak IV shows the same position. In addition, humidity peaks of II, III and IV showed different shape dependent on milling time; humidity peaks for the WO 3 –CuO mixture of 20 h have a sharp shape, i.e. rapid increase and decrease of humidity compared with that of 1 h. As shown in Fig. 2, increased milling time induced the refinement of particles and resultant increased surface area. Such large surface on which remarkable reduction took place promotes the reaction such as the rapid formation and removal of H 2 O at the heating stage [7]. Thus, the decrease in reduction temperature and the presence of
D.-G. Kim et al. / Journal of Alloys and Compounds 354 (2003) 239–242
241
Fig. 3. TEM micrographs of WO 3 –CuO powder mixtures, ball-milled for (a) 1 and (b) 20 h.
sharp peak in powder mixture of 20 h can be understood as the consequence of fine particle size. On the other hand, the same position of peak IV (reaction of WO 2 →W) was registered in the humidity curves, regardless of milling time (Fig. 4). Generally, the reduction of WO 2 is combined with a chemical vapor transportation (CVT) reaction, in which W is transported via the gas phase of WO 2 (OH) 2 . This CVT reaction consists of three steps [11,12]: an oxidative dissolution step by WO 2 (s)12H 2 O (g)→WO 2 (OH) 2 (g)1H 2 (g), the transportation of WO 2 (OH) 2 and a reductive deposition step by WO 2 (OH) 2 (g)13H 2 (g)→W (s)14H 2 O (g). Thus, the reduction of WO 2 is strongly affected by the humidity in the H 2 gas (PH 2 O /PH 2 ). For example, the reduction of WO 2 does not proceed until the PH 2 O /PH 2 at reaction zone
Fig. 4. Non-isothermal humidity curves obtained on heating WO 3 –CuO powders at 10 8C min 21 in hydrogen atmosphere.
becomes lower than the equilibrium condition through an effective removal of H 2 O. In this regard, considering that the powder mixture with milling of 20 h showed rapid reaction for the formation of WO 2 (peak III in Fig. 4), the same temperature for peak IV with powder mixture of 1 h can be explained by the retardation of WO 2 reduction due to the production of large amount of H 2 O in short reaction time. Typical microstructure of the reduced W–Cu powders with the milling time of 1 and 20 h are shown in Fig. 5a and b, respectively. It is observed that the powder mixture with milling time of 20 h seems to have small grain size compared with that of 1 h. The average grain sizes of W particle, calculated from Hall–Williamson equation with the full-width half maximum of the XRD patterns, were measured as 45 nm for the powder of 1 h and 39 nm for 20 h. Generally, it is considered that the particle size of hydrogen-reduced WO 3 –CuO mixture is determined by the initial size of oxide powders and distribution of prereduced Cu phase, in which the homogeneous distribution of Cu particles might effectively lower the contact frequency of W or W-oxide phases, and thus it can considerably reduce the growth of W clusters nucleated from W oxide [13,14]. In addition, the reduction process of WO 2 by CVT can strongly affect the final particle size of W. As reported in the literature [11], the nucleation of W becomes favorable in dry atmosphere (low H 2 O content in H 2 gas) while the growth process is dominant in wet condition. Thus, the formation of fine W powders is possible, when atmosphere condition is optimized to promote nucleation and to suppress the growth of W particles. In this sense, it can be concluded that the powder mixture with milling time of 20 h provides an optimum condition for the
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Fig. 5. TEM micrographs of reduced W–Cu composite powders, ball-milled for (a) 1 and (b) 20 h.
formation of finer W–Cu composite powder by fulfilling the above requirements.
Acknowledgements This work was supported by Korea Research Foundation Grant (KRF-2001-042-E00117).
4. Conclusion References This work demonstrated the effect of the ball-milling time on microstructural characteristics and hydrogen reduction behavior of WO 3 –CuO powders. The grain size of powder mixtures decreased with increase in milling time. In the milling time of 20 h, the grain size of the WO 3 and CuO powders was measured as 17 and 11 nm, respectively. Hydrogen reduction behavior of the WO 3 –CuO mixtures was analyzed in terms of characteristic of hygrometry curve. With increased milling time, the humidity peaks are changed to sharp shape and the peak temperatures for the reduction of CuO and WO 3 are shifted to low temperatures, whereas peak for the reduction of WO 2 shows the same position. The decrease in reduction temperature and the presence of sharp peak are understood as the consequence of fine particle size. Microstructural analysis revealed that the hydrogen-reduced powder with milling time of 20 h showed smaller particle size than that of 1 h. The change of particle size is discussed based on the initial size of oxide powders, distribution of pre-reduced Cu phase and reduction process of WO 2 by CVT.
[1] K.V. Sebastion, Int. J. Powder Metall. Powder Technol. 17 (1981) 297. [2] B.L. Mordike, J. Kaczmar, M. Kielbinski, K.U. Kainer, Powder Metall. Int. 23 (1991) 91. [3] J.L. Johnson, R.M. German, Metall. Trans. A 24A (1993) 2369. [4] I.-H. Moon, J.S. Lee, Powder Metall. Int. 9 (1977) 23. [5] A. Upadhyaya, R.M. German, Int. J. Powder Metall. 34 (1998) 43. [6] Q. Chongliang, W. Enxi, Z. Zhiquiang, Z. Yuhua, in: H. Bildstein, R. Eck (Eds.), Proceedings of the 13th Plansee Seminar, Vol. 1, 1993, p. 461. [7] T.H. Kim, J.H. Yu, J.S. Lee, Nanostruct. Mater. 9 (1997) 213. [8] S. Lee, W.-H. Baek, B.-S. Chun, J. Korean Inst. Metals Mater. 35 (1997) 1710. [9] G.-G. Lee, G.-H. Ha, B.-K. Kim, Powder Metall. 43 (2000) 79. [10] B.D. Cullity, Elements of X-ray Diffraction, 2nd Edition, AddisonWesley, New York, 1978, 281 pp. [11] W.D. Schubert, Int. J. Refract. Metals Hard Mater. (1990) 178. [12] E. Lanssner, W.D. Schubert, Tungsten, Kluwer Academic / Plenum Press, New York, 1999, 85 pp. [13] R. Haubner, W.D. Schubert, E. Lassner, B. Lux, in: H. Bildstein, H. Ortner (Eds.), Proceedings of the 11th Plansee Seminar, Vol. 2, 1993, p. 69. [14] J.S. Lee, T.H. Kim, Solid State Phenom. 25–26 (1992) 143.