- Email: [email protected]
W core–shell structured powders by one-step activation and chemical reduction process
Journal of Alloys and Compounds 619 (2015) 704–708
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Preparation of TiC/W core–shell structured powders by one-step activation and chemical reduction process Xiao-Yu Ding a, Lai-Ma Luo a,c,⇑, Li-Mei Huang a, Guang-Nan Luo b, Xiao-Yong Zhu a,c, Ji-Gui Cheng a,c, Yu-Cheng Wu a,c,⇑ a b c
College of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, China Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, China Engineering Research Center of Powder Metallurgy of Anhui Province, Hefei 230009, China
a r t i c l e
i n f o
Article history: Received 19 December 2013 Received in revised form 21 July 2014 Accepted 28 August 2014 Available online 16 September 2014 Keywords: One-step activation Chemical reduction TiC/W Core–shell
a b s t r a c t In the present study, one-step activation and chemical reduction process as a novel wet-chemical route was performed for the preparation of TiC/W core–shell structured ultra-fine powders. The XRD, FE-SEM, TEM and EDS results demonstrated that the as-synthesized powders are of high purity and uniform with a diameter of approximately 500 nm. It is also found that the TiC nanoparticles were well-encapsulated by W shells. Such a unique process suggests a new method for preparing X/W (X refers the water-insoluble nanoparticles) core–shell nanoparticles with different cores. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Plasma facing material (PFM) is one of the most crucial issues for fusion power plant [1,2]. So far, tungsten (W) is considered to be the primary candidate for use in plasma facing materials/components (PFMs/PFCs) owning to its superiority to other materials in many respects, including reduced radio activation, high melting point and thermal conductivity, low vapor pressure, low sputtering erosion rate, low tritium inventory and excellent compatibility with liquid metals, etc. [3–6]. However, tungsten metal used as PFM is also facing serious brittleness in several aspects i.e. lowtemperature brittleness, high-temperature or recrystallization brittleness and radiation induced brittleness [7–9]. Therefore there is a great need for the development of novel tungsten materials with improved ductility and stability against high temperatures and neutron radiation. In bulk tungsten, nano-sized dispersoids are considered to be very effective in mitigating these problems in tungsten. Generally, Y2O3, La2O3, TiC, and similar dispersoids are the most commonly used, because these well-dispersed nanoparticles could inhibit the grain growth during sintering, hinder the grain boundary sliding and stabilize the microstructure, and it could act as point of annihilation for radiation induced defects ⇑ Corresponding authors at: College of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, China. Tel./fax: +86 551 62901012. E-mail address: [email protected] (L.-M. Luo). http://dx.doi.org/10.1016/j.jallcom.2014.08.242 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
[10,11]. TiC dispersed tungsten materials are demonstrated to be effective to improve the fracture strength, room-temperature ductility, and resistance to irradiation [12–14]. There are many reports on the preparation of W–TiC composite materials by mechanical alloying (MA) and powder metallurgy [15–19]. However, the nanoparticles still tend to be agglomerated to some extent because of the high surface energies introduced, while the milling process itself could produce detrimental phase due to the wear of the milling equipment and media. Core–shell structure nanoparticles that the core and the shell are composed of different phases have better stability, being able to protect the core materials from the surrounding environment, and for improved physical and chemical properties [20–22]. The most common techniques used in the preparation of core–shell structure nanoparticles are sol–gel method, wet chemical method, co-precipitation method, etc. [23– 25]. The synthesis of core/shell nanoparticles is to split these particles into nanosized ones and spread them into the grain interior, rather than having them coarsened and concentrated at the grain boundaries. This strategy not only alleviates the problem discussed above, but also encourages the dislocation trapping by the particles in the grain interior, which would help sustain work hardening and uniform elongation [26]. In this paper, TiC/W nanoparticles with core–shell structure were produced by one-step activation and chemical reduction process. TiC/W nanoparticles with core–shell structure are supposed and expected to be effective to overcome the above mentioned
X.-Y. Ding et al. / Journal of Alloys and Compounds 619 (2015) 704–708
problems, because it could inhibit the aggregation of TiC nanoparticles on the surface of tungsten powders. And during the sintering process, TiC cores are expected to precipitate partially at the grain boundaries, resulting in homogeneous distribution of TiC nanoparticles in the matrix and grain boundaries. FE-SEM, TEM and EDS images demonstrate that TiC nanoparticles were homogenously distributed in or around the tungsten grain.
2. Experimental procedure In the present study, high-purity TiC powder (99.9%) with an average size of 50 nm was used. The other chemicals used in this experiment were all of analytical reagent grade. The as-received TiC powder was pretreated prior to compacting, which is to form a uniform distribution of defects on TiC powder surface which could increase the roughness and hydrophilicity of the TiC surface. TiC powders (10 g/L) were immersed into an aqueous solution of 40–80 ml/L hydrofluoric acid (40%) and 2–3 g/L ammonium fluoride. The immersion process was accelerated by an ultrasonic wave for 20–30 min at room temperature. After the pretreatment, the powders were cleaned with deionized water for three times. Then the powder was dried in a vacuum oven at 50 °C. A field emission electron microscope (FE-SEM)
705
was utilized to analyze the surface modification of the TiC powder under pretreatment process by comparing the results of the original and the pretreated TiC powders. Chemical reduction process was performed in a beaker that was placed in a thermostatically controlled bath. The diagram of experimental device was shown in Fig. 1. The plating solution chemistry and the plating conditions were 10 g/L tungsten hexachloride (WCl6), 30 ml/L lactic acid (C3H6O3), 60 ml/L hydrazine hydrate (H4N2H2O) and 0.2 g/L 2,2-Dipyridyl (C10H6N2). The initial pH of the bath was adjusted by adding ammonia water (NH3H2O) to 10.0. Then the core–shell TiC/W nanoparticles with 1.0 wt% TiC content were prepared with ultrasonically dispersed TiC powders added into the obtained solution. The temperature of the plating bath was maintained at 80 °C under stirring for 3 h. Then the powder was collected by filtration, washed with ethanol and dried at 60 °C overnight. Crystal structure of the powders was studied by X-ray diffraction (XRD). Morphologies of the as-synthesized powders were characterized by FE-SEM and transmission electron microscope (TEM) equipped with an energy-dispersive spectroscope (EDS).
Fig. 1. The diagram of experimental device.
Fig. 3. FE-SEM micrograph of the as-synthesized powder after chemical reduction. (a) Low-magnification image and (b) high-magnification image.
Fig. 2. FE-SEM morphologies of original and the pretreated TiC powders; inset: high-magnification morphology (a) original TiC powders and (b) the pretreated TiC powders.
Fig. 4. XRD patterns of the as-synthesized powders showing the purity of the tungsten.
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Fig. 5. TEM images of the as-synthesized powders. (a) and (b) Typical low-magnification TEM images, reveal that TiC particles were coated by W; (c) EDS spectrum of the selected square region in (b) and (d) EDS mapping image of (b) shows TiC particles were encapsulated by W.
3. Results and discussion Fig. 2a and b shows the FE-SEM surface morphologies of original and the pretreated TiC powders. As shown in Fig. 2a, no obvious steps on the surface of the original TiC powder before pretreatment
the surface was flat and smooth and there were no surface defects. Fig. 2b is the FE-SEM images of TiC powder after simple pretreatment. As can be seen, step-like defects formed on the surface of as-pretreated TiC powder. The aim of pretreatment is to form a surface with a good catalytic activity for chemical reduction process.
X.-Y. Ding et al. / Journal of Alloys and Compounds 619 (2015) 704–708
TiC ceramic substrate does not have catalytically active surfaces. Therefore, the present paper focused on obtaining surface defects on TiC powders for an efficient catalytic activation. Hence, compared with the original TiC powder, the existence of surface steps in TiC powders obviously increased their specific surface areas and surface catalytic activities. The chemical reduction process on the surface of the powder is related to the catalytic capability of the plated surface. Catalytic capability is exhibited during activation (activation refers to the mass of catalytic products on the catalytic agent of the unit surface area within a unit of time). The activation on the surface of the solid is associated with the number of active centers on the solid surface, which pertains to surface defects such as margin, step, margin of adsorbate, and certain surface atoms or atom groups with space unsaturation. These positions easily adsorb foreign matter bonding. The simplified pretreatment process formed a mass of defects or step projections on the surface of the powder to adsorb foreign matter bonding, without the use of palladium (Pd) and other precious metals. Fig. 3a and b shows the SEM images of the as-synthesized powder after chemical deposition. As can be seen, no obvious second phase was found on the particle surface, which further confirmed that the TiC nanoparticles were well-coated with tungsten shells. The powders display well dispersibility with a particle diameter distribution range of about 0.1–1 lm. The composition of the assynthesized powder was examined by XRD, as shown in Fig. 4. Clearly, no other impurities are detected, revealing a pure as-synthesized cubic tungsten phase. The strong intensity and narrow width of the peaks also demonstrate well crystal crystallinity with a lattice constant of a = 0.31648 nm, which agrees well with the standard cell for body centered cubic structure pure tungsten (JCPDS#04-0806) with hexagonal structure. Clearly, no other impurities are detected, revealing a pure as-synthesized cubic tungsten phase. The strong intensity and narrow width of the peaks also demonstrate well crystal crystallinity. It should be noted that no peak of TiC phase was detected in the XRD spectrum, which is probably because of the low proportion of
707
TiC in the composite materials. In addition, no TiC particles were observed on the surface of the tungsten particles from the SEM image, which further validates the XRD results and confirms the absence of TiC on the powder surface. Where did they go? It was deduced that the dispersive TiC phase may exist in the interior of tungsten grain. To check that the TiC was successful doped into the tungsten grains, and analyze the intrinsic structure of the assynthesized particles, the as-synthesized particles were studied by TEM. Fig. 5 is typical TEM images of as-synthesized powder. As can be seen in Fig. 5a and b, there exist obvious core-coating structure in an individual W particle. Fig. 5c is the EDS spectrum of the selected circle region in Fig. 5a, which indicates that the selected particle was composed of Ti, C and W. Cu was excluded because of the copper grid. Fig. 5d is the EDS mapping image of Fig. 5b, which also demonstrates that TiC nanoparticles were well-coated by W shells. According to the HRTEM shown in Fig. 6, there are two phases corresponding to cubic tungsten (lattice parameter a 0.316 nm) cubic TiC (lattice parameter a 0.436 nm), respectively, again validate the core–shell structure of the coated nanoparticles. It was therefore to conclude that the dispersive TiC phase exist in the interior of tungsten grain and form core–shell nano-structured powders. 4. Conclusions In this work, one-step activation and chemical reduction process was developed as a novel wet-chemical route for the preparation of TiC/W core–shell structured ultra-fine powders. The assynthesized coated powders were uniform with the diameters about 500 nm. The TiC nanoparticles were well-encapsulated by W shells and the TiC phase was present in the interior of tungsten grains, which can prevent the accumulation of the doped phase in tungsten matrix. The current work suggests a new method for preparing X/W (X refers the water-insoluble nanoparticles) core–shell nanoparticles with different cores. Acknowledgements This paper was supported by National Magnetic Confinement Fusion Program with Grant Nos. 2014GB121001 and 2010GB109004, National Natural Science Foundation of China No. 51204064. References
Fig. 6. HRTEM image of the as-synthesized powders. (a) HRTEM image of the selected square region in Fig. 5b and (b) enlarged HRTEM image of (a).
[1] R.A. Pitts, S. Carpentier, F. Escourbiac, T. Hirai, V. Komarov, A.S. Kukushkin, S. Lisgo, A. Loarte, M. Merola, R. Mitteau, A.R. Raffray, M. Shimada, P.C. Stangeby, J. Nucl. Mater. 415 (2011) 957–964. [2] X.Y. Shu, H.X. Qiu, B. Huang, Z.X. Gu, J.J. Yang, J.L. Liao, Y.Y. Yang, N. Liu, J. Tang, J. Nucl. Mater. 440 (2013) 414–419. [3] G.M. Song, Y. Zhou, Y.J. Wang, J. Mater Sci. 37 (2002) 3541–3548. [4] G.M. Song, Y.J. Wang, Y. Zhou, Mater. Sci. Eng. A 334 (2002) 223–232. [5] Z.C. Chen, M.L. Zhou, T.Y. Zuo, Scr. Mater. 43 (2000) 291–297. [6] S. Matsuo, H. Kurishita, H. Arakawa, T. Ogawa, A. Hasegawa, K. Abe, H. Arakawa, S. Matsuo, T. Takida, K. Takebe, M. Kawai, N. Yoshida, Mater. Sci. Eng. A 492 (2008) 475–480. [7] H. Kurishita, S. Matsuo, H. Arakawa, S. Kobayashi, K. Nakai, T. Takida, K. Takebe, M. Kawai, Mater. Sci. Eng. A 477 (2008) 162–167. [8] H. Kurishita, S. Matsuo, H. Arakawa, S. Matsuo, H. Arakawa, T. Sakamoto, S. Kobayashi, K. Nakai, T. Takida, M. Kato, M. Kawai, N. Yoshida, J. Nucl. Mater. 398 (2010) 87–92. [9] H. Kurishita, S. Kobayashi, K. Nakai, T. Ogawa, A. Hasegawa, K. Abe, H. Arakawa, S. Matsuo, T. Takida, K. Takebe, M. Kawai, N. Yoshida, J. Nucl. Mater. 377 (2008) 34–40. [10] M. Xia, Q.Z. Yan, L. Xu, H.Y. Guo, L.X. Zhu, C.C. Ge, J. Nucl. Mater. 434 (2013) 85– 89. [11] M. Xia, Q.Z. Yan, L. Xu, L.X. Zhu, H.Y. Guo, C.C. Ge, J. Nucl. Mater. 430 (2012) 216–220. [12] H. Kurishita, S. Matsuo, H. Arakawa, S. Kobayashi, K. Nakai, T. Takida, K. Takebe, M. Kawai, Mater. Sci. Eng. A 477 (2008) 162–167. [13] H. Kurishita, Y. Amano, S. Kobayashi, K. Nakai, H. Arakawa, Y. Hiraoka, T. Takida, K. Takebe, H. Matsui, J. Nucl. Mater. 367–370 (2007) 1453–1457.
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[14] G.M. Song, Y.J. Wang, Y. Zhou, Int. J. Refract. Met. Hard Mater. 21 (2003) 1–12. [15] G.Q. Zhang, D.D. Gu, Appl. Surf. Sci. 273 (2013) 364–371. [16] M. Kajioka, T. Sakamoto, K. Nakai, S. Kobayashi, H. Kurishita, S. Matsuo, H. Arakawa, J. Nucl. Mater. 417 (2011) 512–515. [17] A. Genç, S. Coskun, M.L. Öveçog˘lu, J. Alloys Comp. 497 (2010) 80–89. [18] H. Kurishita, S. Matsuo, H. Arakawa, M. Narui, M. Yamazaki, T. Sakamoto, S. Kobayashi, K. Nakai, T. Takida, K. Takebe, M. Kawai, N. Yoshida, J. Nucl. Mater. 386–388 (2009) 579–582. [19] A. Genç, S. Coskun, M.L. Öveçog˘lu, Int. J. Refract. Met. Hard Mater. 28 (2010) 451–458. [20] N. Wang, L. Zhang, Y.P. Zheng, W.J. Yao, J. Alloys Comp. 538 (2012) 224–229.
[21] R.G. Chaudhuri, S. Paria, Chem. Rev. 112 (2012) 2373–2433. [22] N.V. Longa, T.D. Hien, T. Asaka, M. Ohtaki, M. Nogami, J. Alloy, J. Alloys Comp. 509 (2011) 7702–7709. [23] S. Wahlberg, M.A. Yar, M.O. Abuelnaga, H.G. Salem, M. Johnsson, M. Muhammed, J Mater Chem. (2012). http://dx.doi.org/10.1039/C2JM30652B. [24] M.A. Yar, S. Wahlberg, H. Bergqvist, H.G. Salem, M. Johnsson, M. Muhammed, J. Nucl. Mater. 412 (2011) 227–232. [25] M. Xia, Q.Z. Yan, L. Xu, L. Xu, H.Y. Guo, L.X. Zhu, C.C. Ge, J. Nucl. Mater. 434 (2013) 85–89. [26] G. Liu, G.J. Zhang, F. Jiang, X.D. Ding, Y.J. Sun, J. Sun, E. Ma, Nat. Mater. PUBLISHED ONLINE (2013). http://dx.doi.org/10.1038/NMAT3544.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Preparation of TiC/W core–shell structured powders by one-step activation and chemical reduction process Xiao-Yu Ding a, Lai-Ma Luo a,c,⇑, Li-Mei Huang a, Guang-Nan Luo b, Xiao-Yong Zhu a,c, Ji-Gui Cheng a,c, Yu-Cheng Wu a,c,⇑ a b c
College of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, China Institute of Plasma Physics, Chinese Academy of Sciences, Hefei 230031, China Engineering Research Center of Powder Metallurgy of Anhui Province, Hefei 230009, China
a r t i c l e
i n f o
Article history: Received 19 December 2013 Received in revised form 21 July 2014 Accepted 28 August 2014 Available online 16 September 2014 Keywords: One-step activation Chemical reduction TiC/W Core–shell
a b s t r a c t In the present study, one-step activation and chemical reduction process as a novel wet-chemical route was performed for the preparation of TiC/W core–shell structured ultra-fine powders. The XRD, FE-SEM, TEM and EDS results demonstrated that the as-synthesized powders are of high purity and uniform with a diameter of approximately 500 nm. It is also found that the TiC nanoparticles were well-encapsulated by W shells. Such a unique process suggests a new method for preparing X/W (X refers the water-insoluble nanoparticles) core–shell nanoparticles with different cores. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Plasma facing material (PFM) is one of the most crucial issues for fusion power plant [1,2]. So far, tungsten (W) is considered to be the primary candidate for use in plasma facing materials/components (PFMs/PFCs) owning to its superiority to other materials in many respects, including reduced radio activation, high melting point and thermal conductivity, low vapor pressure, low sputtering erosion rate, low tritium inventory and excellent compatibility with liquid metals, etc. [3–6]. However, tungsten metal used as PFM is also facing serious brittleness in several aspects i.e. lowtemperature brittleness, high-temperature or recrystallization brittleness and radiation induced brittleness [7–9]. Therefore there is a great need for the development of novel tungsten materials with improved ductility and stability against high temperatures and neutron radiation. In bulk tungsten, nano-sized dispersoids are considered to be very effective in mitigating these problems in tungsten. Generally, Y2O3, La2O3, TiC, and similar dispersoids are the most commonly used, because these well-dispersed nanoparticles could inhibit the grain growth during sintering, hinder the grain boundary sliding and stabilize the microstructure, and it could act as point of annihilation for radiation induced defects ⇑ Corresponding authors at: College of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, China. Tel./fax: +86 551 62901012. E-mail address: [email protected] (L.-M. Luo). http://dx.doi.org/10.1016/j.jallcom.2014.08.242 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
[10,11]. TiC dispersed tungsten materials are demonstrated to be effective to improve the fracture strength, room-temperature ductility, and resistance to irradiation [12–14]. There are many reports on the preparation of W–TiC composite materials by mechanical alloying (MA) and powder metallurgy [15–19]. However, the nanoparticles still tend to be agglomerated to some extent because of the high surface energies introduced, while the milling process itself could produce detrimental phase due to the wear of the milling equipment and media. Core–shell structure nanoparticles that the core and the shell are composed of different phases have better stability, being able to protect the core materials from the surrounding environment, and for improved physical and chemical properties [20–22]. The most common techniques used in the preparation of core–shell structure nanoparticles are sol–gel method, wet chemical method, co-precipitation method, etc. [23– 25]. The synthesis of core/shell nanoparticles is to split these particles into nanosized ones and spread them into the grain interior, rather than having them coarsened and concentrated at the grain boundaries. This strategy not only alleviates the problem discussed above, but also encourages the dislocation trapping by the particles in the grain interior, which would help sustain work hardening and uniform elongation [26]. In this paper, TiC/W nanoparticles with core–shell structure were produced by one-step activation and chemical reduction process. TiC/W nanoparticles with core–shell structure are supposed and expected to be effective to overcome the above mentioned
X.-Y. Ding et al. / Journal of Alloys and Compounds 619 (2015) 704–708
problems, because it could inhibit the aggregation of TiC nanoparticles on the surface of tungsten powders. And during the sintering process, TiC cores are expected to precipitate partially at the grain boundaries, resulting in homogeneous distribution of TiC nanoparticles in the matrix and grain boundaries. FE-SEM, TEM and EDS images demonstrate that TiC nanoparticles were homogenously distributed in or around the tungsten grain.
2. Experimental procedure In the present study, high-purity TiC powder (99.9%) with an average size of 50 nm was used. The other chemicals used in this experiment were all of analytical reagent grade. The as-received TiC powder was pretreated prior to compacting, which is to form a uniform distribution of defects on TiC powder surface which could increase the roughness and hydrophilicity of the TiC surface. TiC powders (10 g/L) were immersed into an aqueous solution of 40–80 ml/L hydrofluoric acid (40%) and 2–3 g/L ammonium fluoride. The immersion process was accelerated by an ultrasonic wave for 20–30 min at room temperature. After the pretreatment, the powders were cleaned with deionized water for three times. Then the powder was dried in a vacuum oven at 50 °C. A field emission electron microscope (FE-SEM)
705
was utilized to analyze the surface modification of the TiC powder under pretreatment process by comparing the results of the original and the pretreated TiC powders. Chemical reduction process was performed in a beaker that was placed in a thermostatically controlled bath. The diagram of experimental device was shown in Fig. 1. The plating solution chemistry and the plating conditions were 10 g/L tungsten hexachloride (WCl6), 30 ml/L lactic acid (C3H6O3), 60 ml/L hydrazine hydrate (H4N2H2O) and 0.2 g/L 2,2-Dipyridyl (C10H6N2). The initial pH of the bath was adjusted by adding ammonia water (NH3H2O) to 10.0. Then the core–shell TiC/W nanoparticles with 1.0 wt% TiC content were prepared with ultrasonically dispersed TiC powders added into the obtained solution. The temperature of the plating bath was maintained at 80 °C under stirring for 3 h. Then the powder was collected by filtration, washed with ethanol and dried at 60 °C overnight. Crystal structure of the powders was studied by X-ray diffraction (XRD). Morphologies of the as-synthesized powders were characterized by FE-SEM and transmission electron microscope (TEM) equipped with an energy-dispersive spectroscope (EDS).
Fig. 1. The diagram of experimental device.
Fig. 3. FE-SEM micrograph of the as-synthesized powder after chemical reduction. (a) Low-magnification image and (b) high-magnification image.
Fig. 2. FE-SEM morphologies of original and the pretreated TiC powders; inset: high-magnification morphology (a) original TiC powders and (b) the pretreated TiC powders.
Fig. 4. XRD patterns of the as-synthesized powders showing the purity of the tungsten.
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X.-Y. Ding et al. / Journal of Alloys and Compounds 619 (2015) 704–708
Fig. 5. TEM images of the as-synthesized powders. (a) and (b) Typical low-magnification TEM images, reveal that TiC particles were coated by W; (c) EDS spectrum of the selected square region in (b) and (d) EDS mapping image of (b) shows TiC particles were encapsulated by W.
3. Results and discussion Fig. 2a and b shows the FE-SEM surface morphologies of original and the pretreated TiC powders. As shown in Fig. 2a, no obvious steps on the surface of the original TiC powder before pretreatment
the surface was flat and smooth and there were no surface defects. Fig. 2b is the FE-SEM images of TiC powder after simple pretreatment. As can be seen, step-like defects formed on the surface of as-pretreated TiC powder. The aim of pretreatment is to form a surface with a good catalytic activity for chemical reduction process.
X.-Y. Ding et al. / Journal of Alloys and Compounds 619 (2015) 704–708
TiC ceramic substrate does not have catalytically active surfaces. Therefore, the present paper focused on obtaining surface defects on TiC powders for an efficient catalytic activation. Hence, compared with the original TiC powder, the existence of surface steps in TiC powders obviously increased their specific surface areas and surface catalytic activities. The chemical reduction process on the surface of the powder is related to the catalytic capability of the plated surface. Catalytic capability is exhibited during activation (activation refers to the mass of catalytic products on the catalytic agent of the unit surface area within a unit of time). The activation on the surface of the solid is associated with the number of active centers on the solid surface, which pertains to surface defects such as margin, step, margin of adsorbate, and certain surface atoms or atom groups with space unsaturation. These positions easily adsorb foreign matter bonding. The simplified pretreatment process formed a mass of defects or step projections on the surface of the powder to adsorb foreign matter bonding, without the use of palladium (Pd) and other precious metals. Fig. 3a and b shows the SEM images of the as-synthesized powder after chemical deposition. As can be seen, no obvious second phase was found on the particle surface, which further confirmed that the TiC nanoparticles were well-coated with tungsten shells. The powders display well dispersibility with a particle diameter distribution range of about 0.1–1 lm. The composition of the assynthesized powder was examined by XRD, as shown in Fig. 4. Clearly, no other impurities are detected, revealing a pure as-synthesized cubic tungsten phase. The strong intensity and narrow width of the peaks also demonstrate well crystal crystallinity with a lattice constant of a = 0.31648 nm, which agrees well with the standard cell for body centered cubic structure pure tungsten (JCPDS#04-0806) with hexagonal structure. Clearly, no other impurities are detected, revealing a pure as-synthesized cubic tungsten phase. The strong intensity and narrow width of the peaks also demonstrate well crystal crystallinity. It should be noted that no peak of TiC phase was detected in the XRD spectrum, which is probably because of the low proportion of
707
TiC in the composite materials. In addition, no TiC particles were observed on the surface of the tungsten particles from the SEM image, which further validates the XRD results and confirms the absence of TiC on the powder surface. Where did they go? It was deduced that the dispersive TiC phase may exist in the interior of tungsten grain. To check that the TiC was successful doped into the tungsten grains, and analyze the intrinsic structure of the assynthesized particles, the as-synthesized particles were studied by TEM. Fig. 5 is typical TEM images of as-synthesized powder. As can be seen in Fig. 5a and b, there exist obvious core-coating structure in an individual W particle. Fig. 5c is the EDS spectrum of the selected circle region in Fig. 5a, which indicates that the selected particle was composed of Ti, C and W. Cu was excluded because of the copper grid. Fig. 5d is the EDS mapping image of Fig. 5b, which also demonstrates that TiC nanoparticles were well-coated by W shells. According to the HRTEM shown in Fig. 6, there are two phases corresponding to cubic tungsten (lattice parameter a 0.316 nm) cubic TiC (lattice parameter a 0.436 nm), respectively, again validate the core–shell structure of the coated nanoparticles. It was therefore to conclude that the dispersive TiC phase exist in the interior of tungsten grain and form core–shell nano-structured powders. 4. Conclusions In this work, one-step activation and chemical reduction process was developed as a novel wet-chemical route for the preparation of TiC/W core–shell structured ultra-fine powders. The assynthesized coated powders were uniform with the diameters about 500 nm. The TiC nanoparticles were well-encapsulated by W shells and the TiC phase was present in the interior of tungsten grains, which can prevent the accumulation of the doped phase in tungsten matrix. The current work suggests a new method for preparing X/W (X refers the water-insoluble nanoparticles) core–shell nanoparticles with different cores. Acknowledgements This paper was supported by National Magnetic Confinement Fusion Program with Grant Nos. 2014GB121001 and 2010GB109004, National Natural Science Foundation of China No. 51204064. References
Fig. 6. HRTEM image of the as-synthesized powders. (a) HRTEM image of the selected square region in Fig. 5b and (b) enlarged HRTEM image of (a).
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