- Email: [email protected]
Synthesis of WC nanopowders from novel precursors
Int. Journal of Refractory Metals and Hard Materials 29 (2011) 372–375
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / I J R M H M
Synthesis of WC nanopowders from novel precursors Yongzhong Jin a,b,⁎, Dongliang Liu a, Xinyue Li a, Ruisong Yang a a b
Department of Materials and Chemistry Engineering, Sichuan University of Science and Engineering, Zigong 643000, China The Key Laboratory of Material Corrosion and Protection of Sichuan colleges and Universities, Zigong 643000, China
a r t i c l e
i n f o
Article history: Received 5 October 2010 Accepted 19 January 2011 Keywords: WC powders Chemical synthesis X-ray diffraction Scanning electron microscopy Transmission electron microscopy X-ray photoelectron spectroscopy
a b s t r a c t Nano-WC powders with granular particle of ~ 20–80 nm were synthesized by a new precursor method, namely carbothermal reduction–carburization of amorphous WO3-C mixture, which was made initially from salt solution containing tungsten and carbon elements by air drying and subsequent calcining at 400 °C for 1 h. The reaction products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) techniques. The results show that the synthesizing temperature of WC powders was reduced greatly by the novel precursor method. Thus, the preparation of the single-phase nano-WC powders is at only 1000 °C for 2 h. The lowering of synthesizing temperature is mainly due to the homogeneous chemical composition of the amorphous oxide-carbon mixture. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction WC was used for many applications such as cutting tools, high temperature furnace crucibles, catalysis materials and aerospace coatings due to its high hardness, high melting temperature, high fracture toughness, high thermal conductivity, and low thermal expansion coefficient [1–4]. WC powder was prepared by a variety of methods such as mechanical alloying [5], thermo-chemical reaction [6], thermal decomposition of metal complexes [7], chemical vapor deposition [8], combustion synthesis [9], solid state metathesis [10]. For the traditional industrial method, WC powder was synthesized commercially by carbonizing W with C at temperature of 1400– 1600 °C in a flowing hydrogen atmosphere for 2–10 h [11]. The process is cumbersome, time consuming and expensive as it proceeds as a twostep process, in which W metal powder was first produced by using very pure tungsten trioxide, tungstic acid (hydrated trioxide), or ammonium paratungstate (APT) and then carbonized to form WC powder. More recently, Kodambaka performed the initial research on reducing the synthesis temperature of WC powder by using carboncoated WO3 powders, in which the synthesizing temperature was as low as 1100 °C, but the particle size of WC powders was sub-micron [12]. In this study, a new precursor method was designed to synthesize nanometer WC powder at only 1000 °C. Ammonium ⁎ Corresponding author at: Department of Materials and Chemistry Engineering, Sichuan University of Science and Engineering, Zigong 643000, China. Tel./fax: +86 813 5505860. E-mail address: [email protected] (Y. Jin). 0263-4368/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2011.01.007
metatungstate (AMT, (NH4)6H2W12O40 ∙ XH2O), Glucose (C6H12O6) were purposely used as tungsten source and carbon source, respectively. This synthesis method has potential for the economical production of WC and other carbide nanopowders for industrial applications. 2. Experimental procedure The purity of all the starting powders was more than 99%. Ammonium metatungstate (Zigong Cemented Carbide Corp., Ltd, China), glucose (Beijing Jingqiu Chemistry Corp. Ltd, China) were put into hot distilled water and mixed them uniformly. After air drying the precursor solution at 30 °C for 36 h, the well-proportioned precursor mixture was obtained, and then calcined in silica tube furnace (model XD-1200NT, China) with flowing argon atmosphere at 400 °C for 1 h to form the complex oxide-carbon mixture. The purity of argon was more than 99.999%. All carbothermal reduction–carburization experiments were carried out with 100 g of the oxide-carbon mixture in a vacuum carbon tube furnace (model HZA2000-140, China). The furnace was evacuated to 3.8 × 10− 2 Pa using a rotary vacuum pump and then heated to the carburization temperature at a rate of 7–10 °C/min. After reaching the carburization temperature, the samples were isothermal treated for 1–2 h. Microstructural examinations of samples were observed by TEM (JEM-100CX, Japan) with accelerating voltage of 100 KV and SEM (JSM-5900LV, Japan). The phase composition analysis of reaction products was investigated by XRD (DX-2000, China) using Cu Kα radiation with a step size of 0.04°/s. In addition, XPS measurements were carried out on the sample surface using a XSAM 800
Y. Jin et al. / Int. Journal of Refractory Metals and Hard Materials 29 (2011) 372–375
373
Fig. 1. XRD patterns of WO3 (reference powder ) (a) and reaction products after different treatments: (b) calcined at 400 °C for 1 h; (c) carburized at 950 °C for 2 h; (d) carburized at 1000 °C for 1 h and (e) carburized at 1000 °C for 2 h.
spectrometer (Kratos, England) with the Mg Kα X-ray source in order to deduce the element composition and binding state. 3. Results and discussion Fig. 1 shows the XRD patterns of reaction products. According to French's study [13], ammonium metatungstate can transform into WO 3 during calcining at 400 °C, lastly accompanied by the decomposition of all the crystalline H2O, NH3 and condensed H2O. The reduction process of WO3 complies with step reduction theory [11] and usually meets the reaction sequence of WO3 → WO2.9 → WO2.72 → WO2 → W. Though the trasformation temperature of WO3 → WO2.9 is the lowest during reducing WO3, the thermodynamical reduction temperatures of WO2.9 is about 415 °C, as shown in Eq. (1). 10WO3 ðsÞ + CðsÞ = 10WO2:9 ðsÞ + COðgÞ ΔGT = 109720–159:401 TðJ = molÞ
ð1Þ
Obviously, this reaction cannot thermodynamically occur during calcining the precursor mixture at 400 °C for 1 h. Therefore, the calcined products at 400 °C shown in Fig. 1b are just oxide (WO3)carbon mixtures. Note that the thermodynamic data of individual reactants, used for the calculation of standard free energy of reactions (1)–(3), were taken from the literature [14]. In addition, it may be
Fig. 3. Secondary electron image (a) and TEM image (b) of the final product obtained at 1000 °C for 2 h.
presumed that the oxygen reacts with carbon completely and produces carbon monoxide in a closed reaction atmosphere [15]. Fig. 1a shows the diffraction peaks of single-phase WO3 powders as reference phase whose purity is more than 99%. There are more than two peaks for single-phase WO3 in Fig. 1a, and yet only two apparent and smooth “peaks” are shown in the diffraction patterns of the WO3-C mixtures obtained at 400 °C for 1 h (in Fig. 1b). Additionally, these peaks in Fig. 1b are so broad that the pattern appears more like that from an amorphous material than a crystalline material. It indicates that the WO3-C mixtures are of amorphous structure. The crystalline size of WO3 calculated by Scherrer equation [16] is estimated to be only ~1.9 nm in Fig. 1b. From the discussion above, it can be deduced that C and W elements have been mixed homogeneously at molecular level in the oxide-carbon mixture.
Fig. 2. The illustration of three different structure characteristics of reactant systems (WO3 and C) before carbothermal reduction–carburization: (a) amorphous state in the new precursor method, (b) carbon-coated WO3 state in Ref. [12] and (c) ball-milled state in the traditional method.
374
Y. Jin et al. / Int. Journal of Refractory Metals and Hard Materials 29 (2011) 372–375
Fig. 4. XPS spectra of nano-WC powders obtained at 1000 °C for 2 h.
With the increase of reaction temperature, the carbon thermal reduction of WO3 can proceed according to the total potential reduction reaction (2), and its carburization complies with reaction (3): WO3 ðsÞ + 3CðsÞ = WðsÞ + 3COðgÞ ΔGT = 492053–516:851TðJ = molÞ
ð2Þ
WðsÞ + CðsÞ = WCðsÞ ΔGT = –38756 + 2:818TðJ = molÞ
ð3Þ
Obviously, reaction (2) can thermodynamically occur at about 679 °C. In addition, reaction (3) can thermodynamically take place in the presence of W because of ΔGT b0 above room temperature. In other words, the formation of WC can thermodynamically form at about 679 °C. But we can see the existence of large numbers of W, apart from some WC at 950 °C (in Fig. 1c), indicating that the formation of WC is closely related to the reaction kinetics. Generally, the reaction rate can be estimated by Arrhenius Equation [17]. The kinetics would be slower at lower temperatures. Likewise, the incomplete carburization reaction also occurs in the case of short carburization time, as shown in Fig.1d. Here, larger numbers of W and W2C phases can be observed in reaction products at 1000 °C for only 1 h, showing that these phases have not been transformed completely to single phase of WC. As seen in Fig.1e, no other phase is present, except WC at 1000 °C for 2 h. Comparing with 1400–1600 °C for 2–10 h required to form single-phase WC in the traditional method [11], this study permits the lowering of synthesis temperature by as much as several hundreds centigrade and shorting of reaction time. This is a lower temperature for synthesizing WC nanopowders than 1100 °C using the carboncoated WO3 method to obtain sub-micron WC powders reported by Koc [12]. The reason for low synthesizing temperature is mainly that the novel precursor method can provide homogeneous chemical composition (discussed above in Fig. 1b) for the complex WO3-C mixture. Compared with the ball-milled state of WO3 + C (in Fig. 2c) in the traditional method, the surface contact between WO3 and C is partly improved (seen in Fig. 2b) when using carbon precursor to coat WO3 particles, and thus WO3 can be relatively easily carburized, leading to the reduction of the synthesizing temperature of sub-micro WC from 1400–1600 °C to 1100 °C. However, in this case WO3 particles still remain in micro scale, which inhibits the diffusion of carbon atoms into the internal of WO3 particles. In this study, tungsten precursor and carbon precursor were simultaneously used as starting materials.
Fig. 5. XPS spectra of W 4f (a), O 1s (b) and C (c) energy peaks for nano-WC powders obtained at 1000 °C for 2 h.
Subsequently, the mixing precursors were calcined to obtain amorphous oxide-carbon mixtures with homogeneous composition which allows intimate contact of between WO3 and C. Therefore, the high-surface contact between WO3 and C (seen in Fig. 2a) shortens significantly the diffusion path of carbon, oxygen and tungsten atoms, avoids the
Y. Jin et al. / Int. Journal of Refractory Metals and Hard Materials 29 (2011) 372–375
undersupply of carbon in the local reaction regions and appearance of W2C, and thus contributes to the formation of nano-WC powders at only 1000 °C for 2 h. Fig. 3 shows the SEM and TEM micrograph of WC powders produced by the new precursor method. It is seen that WC powders consist of granular particle of ~ 20–80 nm with average crystalline size of ~7.8 nm (calculated by Scherrer equation). It can be also seen that some particles are partially agglomerated with a size of ~100 nm. Just because of low synthesizing temperature, it is difficult for WC grains to grow. In order to determine the element composition and binding state, simultaneous XPS was carried out on the sample obtained at 1000 °C for 2 h as shown in Figs. 4 and 5. From Fig. 4, it can be seen that the sample surface mainly consist of W, C and O elements. The W 4f peak in Fig. 5a shows contributions from WC at 32.05 and 34.05 eV and WO3 at 35.5 and 37.85 eV. Here, W2C cannot be observed, which confirms the XRD result in Fig. 1e and hints the complete transformation of W → WC. It is well known that WC, especially nano-WC powders are thermodynamically unstable and will oxidize in the presence of air at room temperature. Warren's study [18] suggests that WO3 in WC powders is the oxidation product. Therefore, the existence of WO3 in Fig. 5a mainly results from the exposure of highly active nano-WC powders to air in storage and during test procedure. The XPS spectrum of O 1s energy region for tungsten carbide contains two peaks (Oh and Od), as shown in Fig. 5b. The peak of Oh (532.55 eV) is considered to be associated with OH−, which mainly the adsorption of air. The peak of Od (530.95 eV) is ascribed to tungsten trioxide (W6+) due to the oxidation of nano-WC in the air. Fig. 5c shows the XPS spectrum of C 1s energy peaks of nano-WC powders, including Cc (283.5 eV) and Cf (284.8 eV). According to Moulder's report [19], the Cc peak is closely related to the combination state of WC. The Cf peak shows the existence of a simple carbon. It mainly came from the polluted carbon used for the correction of the XPS measurement, and possibly caused by the free carbon in the sample due to the provision of excess carbon in the reaction system. 4. Conclusions In conclusion, the present study shows that the single-phase WC nanopowders with 20–80 nm can be synthesized at only 1000 °C for 2 h using a new precursor method, which has potential to prepare other carbide nanopowders for industrial applications. The homogeneous chemical composition of the amorphous WO3-C mixture contributes to
375
reduction of synthesis temperature by shortening atomic diffusion path and thus avoids effectively the local undersupply of carbon which easily produces W2C. Acknowledgements This work was funded by the Key Laboratory of Material Corrosion and Protection of Sichuan Colleges and Universities (2009CL06). References [1] Voevodin AA, O'Neill JP, Zabinski JS. Nanocomposite tribological coatings for aerospace applications. Surf Coat Technol 1999;116–119:36–45. [2] Fang ZZ. Correlation of transverse rupture strength of WC-Co with hardness. Int J Refract Met Hard Mater 2005;23:119–27. [3] Upadhyaya GS. Materials science of cemented carbides—an overview. Mater Des 2001;22:483–9. [4] Pansare SS, Torres W, Goodwin Jr JG. Ammonia decomposition on tungsten carbide. Catal Commun 2007;8(4):649–54. [5] Bolokang S, Banganayia C, Phasha M. Effect of C and milling parameters on the synthesis of WC powders by mechanical alloying. Int J Refract Met Hard Mater 2010;28:211–6. [6] Kumar A, Singh K, Pandey OP. Reduction of WO3 to nano-WC by thermo-chemical reaction route. Phys E 2009;41:677–84. [7] Wanner S, Hilaire L, Wehrer P, Hindermann JP, Maire G. Obtaining tungsten carbides from tungsten bipyridine complexes via low temperature thermal treatment. Appl Catal A 2000;203:55–70. [8] Zheng H, Huang J, Wang W, Ma C. Preparation of nano-crystalline tungsten carbide thin film electrode and its electrocatalytic activity for hydrogen evolution. Electrochem Commun 2005;7:1045–9. [9] Nersisyan HH, Won HI, Won CW. Combustion synthesis of WC powder in the presence of alkali salts. Mater Lett 2005;59:3950–4. [10] Nartowski AM, Parkin IP, MacKenzie M, Craven AJ, MacLeod I. Solid state metathesis routes to transition metal carbides. J Mater Chem 1999;9:1275–81. [11] Huang P. Principles of Powder Metallurgy. Beijing: Metallurgy Industry; 2004. [12] Koc R, Kodambaka SK. Tungsten carbide (WC) synthesis from novel precursors. J Eur Ceram Soc 2000;20:1859–69. [13] French GJ, Sale FR. A re-investigation of the thermal decomposition of ammonium paratungstate. J Mater Sci 1981;16:3427–36. [14] Ye DL, Hu JH. Practical Handbook of Inorganic Thermodynamic Data. Beijing: Metallurgy Industry; 2002. [15] Jha A, Yoon SJ. Formation of titanium carbonitride phases via the reduction of TiO2 with carbon in the presence of nitrogen. J Mater Sci 1999;34:307–22. [16] Matyi RJ, Schwartz LH, Butt JB. Particle size, particle size distribution and related measurements of supported metal catalysts. Catal Rev Sci Eng 1987;29:41–99. [17] Bahgat M, Paek M-K, Pak J-J. Reduction investigation of WO3/NiO/Fe2O3 and synthesis of nanocrystalline ternary W-Ni-Fe alloy. J Alloys Compd 2009;472: 314–8. [18] Warren A, Nylund A, Olefjord I. Oxidation of tungsten and tungsten carbide in dry and humid atmospheres. Int J Refract Met Hard Mater 1996;14:345–53. [19] Moulder JF, Stickle WF, Sobol PE, Bomben KD. Handbook of X-ray photoelectron spectroscopy. Eden Prairie, Minnesota: Perkin Elmer; 1992.
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / I J R M H M
Synthesis of WC nanopowders from novel precursors Yongzhong Jin a,b,⁎, Dongliang Liu a, Xinyue Li a, Ruisong Yang a a b
Department of Materials and Chemistry Engineering, Sichuan University of Science and Engineering, Zigong 643000, China The Key Laboratory of Material Corrosion and Protection of Sichuan colleges and Universities, Zigong 643000, China
a r t i c l e
i n f o
Article history: Received 5 October 2010 Accepted 19 January 2011 Keywords: WC powders Chemical synthesis X-ray diffraction Scanning electron microscopy Transmission electron microscopy X-ray photoelectron spectroscopy
a b s t r a c t Nano-WC powders with granular particle of ~ 20–80 nm were synthesized by a new precursor method, namely carbothermal reduction–carburization of amorphous WO3-C mixture, which was made initially from salt solution containing tungsten and carbon elements by air drying and subsequent calcining at 400 °C for 1 h. The reaction products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) techniques. The results show that the synthesizing temperature of WC powders was reduced greatly by the novel precursor method. Thus, the preparation of the single-phase nano-WC powders is at only 1000 °C for 2 h. The lowering of synthesizing temperature is mainly due to the homogeneous chemical composition of the amorphous oxide-carbon mixture. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction WC was used for many applications such as cutting tools, high temperature furnace crucibles, catalysis materials and aerospace coatings due to its high hardness, high melting temperature, high fracture toughness, high thermal conductivity, and low thermal expansion coefficient [1–4]. WC powder was prepared by a variety of methods such as mechanical alloying [5], thermo-chemical reaction [6], thermal decomposition of metal complexes [7], chemical vapor deposition [8], combustion synthesis [9], solid state metathesis [10]. For the traditional industrial method, WC powder was synthesized commercially by carbonizing W with C at temperature of 1400– 1600 °C in a flowing hydrogen atmosphere for 2–10 h [11]. The process is cumbersome, time consuming and expensive as it proceeds as a twostep process, in which W metal powder was first produced by using very pure tungsten trioxide, tungstic acid (hydrated trioxide), or ammonium paratungstate (APT) and then carbonized to form WC powder. More recently, Kodambaka performed the initial research on reducing the synthesis temperature of WC powder by using carboncoated WO3 powders, in which the synthesizing temperature was as low as 1100 °C, but the particle size of WC powders was sub-micron [12]. In this study, a new precursor method was designed to synthesize nanometer WC powder at only 1000 °C. Ammonium ⁎ Corresponding author at: Department of Materials and Chemistry Engineering, Sichuan University of Science and Engineering, Zigong 643000, China. Tel./fax: +86 813 5505860. E-mail address: [email protected] (Y. Jin). 0263-4368/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2011.01.007
metatungstate (AMT, (NH4)6H2W12O40 ∙ XH2O), Glucose (C6H12O6) were purposely used as tungsten source and carbon source, respectively. This synthesis method has potential for the economical production of WC and other carbide nanopowders for industrial applications. 2. Experimental procedure The purity of all the starting powders was more than 99%. Ammonium metatungstate (Zigong Cemented Carbide Corp., Ltd, China), glucose (Beijing Jingqiu Chemistry Corp. Ltd, China) were put into hot distilled water and mixed them uniformly. After air drying the precursor solution at 30 °C for 36 h, the well-proportioned precursor mixture was obtained, and then calcined in silica tube furnace (model XD-1200NT, China) with flowing argon atmosphere at 400 °C for 1 h to form the complex oxide-carbon mixture. The purity of argon was more than 99.999%. All carbothermal reduction–carburization experiments were carried out with 100 g of the oxide-carbon mixture in a vacuum carbon tube furnace (model HZA2000-140, China). The furnace was evacuated to 3.8 × 10− 2 Pa using a rotary vacuum pump and then heated to the carburization temperature at a rate of 7–10 °C/min. After reaching the carburization temperature, the samples were isothermal treated for 1–2 h. Microstructural examinations of samples were observed by TEM (JEM-100CX, Japan) with accelerating voltage of 100 KV and SEM (JSM-5900LV, Japan). The phase composition analysis of reaction products was investigated by XRD (DX-2000, China) using Cu Kα radiation with a step size of 0.04°/s. In addition, XPS measurements were carried out on the sample surface using a XSAM 800
Y. Jin et al. / Int. Journal of Refractory Metals and Hard Materials 29 (2011) 372–375
373
Fig. 1. XRD patterns of WO3 (reference powder ) (a) and reaction products after different treatments: (b) calcined at 400 °C for 1 h; (c) carburized at 950 °C for 2 h; (d) carburized at 1000 °C for 1 h and (e) carburized at 1000 °C for 2 h.
spectrometer (Kratos, England) with the Mg Kα X-ray source in order to deduce the element composition and binding state. 3. Results and discussion Fig. 1 shows the XRD patterns of reaction products. According to French's study [13], ammonium metatungstate can transform into WO 3 during calcining at 400 °C, lastly accompanied by the decomposition of all the crystalline H2O, NH3 and condensed H2O. The reduction process of WO3 complies with step reduction theory [11] and usually meets the reaction sequence of WO3 → WO2.9 → WO2.72 → WO2 → W. Though the trasformation temperature of WO3 → WO2.9 is the lowest during reducing WO3, the thermodynamical reduction temperatures of WO2.9 is about 415 °C, as shown in Eq. (1). 10WO3 ðsÞ + CðsÞ = 10WO2:9 ðsÞ + COðgÞ ΔGT = 109720–159:401 TðJ = molÞ
ð1Þ
Obviously, this reaction cannot thermodynamically occur during calcining the precursor mixture at 400 °C for 1 h. Therefore, the calcined products at 400 °C shown in Fig. 1b are just oxide (WO3)carbon mixtures. Note that the thermodynamic data of individual reactants, used for the calculation of standard free energy of reactions (1)–(3), were taken from the literature [14]. In addition, it may be
Fig. 3. Secondary electron image (a) and TEM image (b) of the final product obtained at 1000 °C for 2 h.
presumed that the oxygen reacts with carbon completely and produces carbon monoxide in a closed reaction atmosphere [15]. Fig. 1a shows the diffraction peaks of single-phase WO3 powders as reference phase whose purity is more than 99%. There are more than two peaks for single-phase WO3 in Fig. 1a, and yet only two apparent and smooth “peaks” are shown in the diffraction patterns of the WO3-C mixtures obtained at 400 °C for 1 h (in Fig. 1b). Additionally, these peaks in Fig. 1b are so broad that the pattern appears more like that from an amorphous material than a crystalline material. It indicates that the WO3-C mixtures are of amorphous structure. The crystalline size of WO3 calculated by Scherrer equation [16] is estimated to be only ~1.9 nm in Fig. 1b. From the discussion above, it can be deduced that C and W elements have been mixed homogeneously at molecular level in the oxide-carbon mixture.
Fig. 2. The illustration of three different structure characteristics of reactant systems (WO3 and C) before carbothermal reduction–carburization: (a) amorphous state in the new precursor method, (b) carbon-coated WO3 state in Ref. [12] and (c) ball-milled state in the traditional method.
374
Y. Jin et al. / Int. Journal of Refractory Metals and Hard Materials 29 (2011) 372–375
Fig. 4. XPS spectra of nano-WC powders obtained at 1000 °C for 2 h.
With the increase of reaction temperature, the carbon thermal reduction of WO3 can proceed according to the total potential reduction reaction (2), and its carburization complies with reaction (3): WO3 ðsÞ + 3CðsÞ = WðsÞ + 3COðgÞ ΔGT = 492053–516:851TðJ = molÞ
ð2Þ
WðsÞ + CðsÞ = WCðsÞ ΔGT = –38756 + 2:818TðJ = molÞ
ð3Þ
Obviously, reaction (2) can thermodynamically occur at about 679 °C. In addition, reaction (3) can thermodynamically take place in the presence of W because of ΔGT b0 above room temperature. In other words, the formation of WC can thermodynamically form at about 679 °C. But we can see the existence of large numbers of W, apart from some WC at 950 °C (in Fig. 1c), indicating that the formation of WC is closely related to the reaction kinetics. Generally, the reaction rate can be estimated by Arrhenius Equation [17]. The kinetics would be slower at lower temperatures. Likewise, the incomplete carburization reaction also occurs in the case of short carburization time, as shown in Fig.1d. Here, larger numbers of W and W2C phases can be observed in reaction products at 1000 °C for only 1 h, showing that these phases have not been transformed completely to single phase of WC. As seen in Fig.1e, no other phase is present, except WC at 1000 °C for 2 h. Comparing with 1400–1600 °C for 2–10 h required to form single-phase WC in the traditional method [11], this study permits the lowering of synthesis temperature by as much as several hundreds centigrade and shorting of reaction time. This is a lower temperature for synthesizing WC nanopowders than 1100 °C using the carboncoated WO3 method to obtain sub-micron WC powders reported by Koc [12]. The reason for low synthesizing temperature is mainly that the novel precursor method can provide homogeneous chemical composition (discussed above in Fig. 1b) for the complex WO3-C mixture. Compared with the ball-milled state of WO3 + C (in Fig. 2c) in the traditional method, the surface contact between WO3 and C is partly improved (seen in Fig. 2b) when using carbon precursor to coat WO3 particles, and thus WO3 can be relatively easily carburized, leading to the reduction of the synthesizing temperature of sub-micro WC from 1400–1600 °C to 1100 °C. However, in this case WO3 particles still remain in micro scale, which inhibits the diffusion of carbon atoms into the internal of WO3 particles. In this study, tungsten precursor and carbon precursor were simultaneously used as starting materials.
Fig. 5. XPS spectra of W 4f (a), O 1s (b) and C (c) energy peaks for nano-WC powders obtained at 1000 °C for 2 h.
Subsequently, the mixing precursors were calcined to obtain amorphous oxide-carbon mixtures with homogeneous composition which allows intimate contact of between WO3 and C. Therefore, the high-surface contact between WO3 and C (seen in Fig. 2a) shortens significantly the diffusion path of carbon, oxygen and tungsten atoms, avoids the
Y. Jin et al. / Int. Journal of Refractory Metals and Hard Materials 29 (2011) 372–375
undersupply of carbon in the local reaction regions and appearance of W2C, and thus contributes to the formation of nano-WC powders at only 1000 °C for 2 h. Fig. 3 shows the SEM and TEM micrograph of WC powders produced by the new precursor method. It is seen that WC powders consist of granular particle of ~ 20–80 nm with average crystalline size of ~7.8 nm (calculated by Scherrer equation). It can be also seen that some particles are partially agglomerated with a size of ~100 nm. Just because of low synthesizing temperature, it is difficult for WC grains to grow. In order to determine the element composition and binding state, simultaneous XPS was carried out on the sample obtained at 1000 °C for 2 h as shown in Figs. 4 and 5. From Fig. 4, it can be seen that the sample surface mainly consist of W, C and O elements. The W 4f peak in Fig. 5a shows contributions from WC at 32.05 and 34.05 eV and WO3 at 35.5 and 37.85 eV. Here, W2C cannot be observed, which confirms the XRD result in Fig. 1e and hints the complete transformation of W → WC. It is well known that WC, especially nano-WC powders are thermodynamically unstable and will oxidize in the presence of air at room temperature. Warren's study [18] suggests that WO3 in WC powders is the oxidation product. Therefore, the existence of WO3 in Fig. 5a mainly results from the exposure of highly active nano-WC powders to air in storage and during test procedure. The XPS spectrum of O 1s energy region for tungsten carbide contains two peaks (Oh and Od), as shown in Fig. 5b. The peak of Oh (532.55 eV) is considered to be associated with OH−, which mainly the adsorption of air. The peak of Od (530.95 eV) is ascribed to tungsten trioxide (W6+) due to the oxidation of nano-WC in the air. Fig. 5c shows the XPS spectrum of C 1s energy peaks of nano-WC powders, including Cc (283.5 eV) and Cf (284.8 eV). According to Moulder's report [19], the Cc peak is closely related to the combination state of WC. The Cf peak shows the existence of a simple carbon. It mainly came from the polluted carbon used for the correction of the XPS measurement, and possibly caused by the free carbon in the sample due to the provision of excess carbon in the reaction system. 4. Conclusions In conclusion, the present study shows that the single-phase WC nanopowders with 20–80 nm can be synthesized at only 1000 °C for 2 h using a new precursor method, which has potential to prepare other carbide nanopowders for industrial applications. The homogeneous chemical composition of the amorphous WO3-C mixture contributes to
375
reduction of synthesis temperature by shortening atomic diffusion path and thus avoids effectively the local undersupply of carbon which easily produces W2C. Acknowledgements This work was funded by the Key Laboratory of Material Corrosion and Protection of Sichuan Colleges and Universities (2009CL06). References [1] Voevodin AA, O'Neill JP, Zabinski JS. Nanocomposite tribological coatings for aerospace applications. Surf Coat Technol 1999;116–119:36–45. [2] Fang ZZ. Correlation of transverse rupture strength of WC-Co with hardness. Int J Refract Met Hard Mater 2005;23:119–27. [3] Upadhyaya GS. Materials science of cemented carbides—an overview. Mater Des 2001;22:483–9. [4] Pansare SS, Torres W, Goodwin Jr JG. Ammonia decomposition on tungsten carbide. Catal Commun 2007;8(4):649–54. [5] Bolokang S, Banganayia C, Phasha M. Effect of C and milling parameters on the synthesis of WC powders by mechanical alloying. Int J Refract Met Hard Mater 2010;28:211–6. [6] Kumar A, Singh K, Pandey OP. Reduction of WO3 to nano-WC by thermo-chemical reaction route. Phys E 2009;41:677–84. [7] Wanner S, Hilaire L, Wehrer P, Hindermann JP, Maire G. Obtaining tungsten carbides from tungsten bipyridine complexes via low temperature thermal treatment. Appl Catal A 2000;203:55–70. [8] Zheng H, Huang J, Wang W, Ma C. Preparation of nano-crystalline tungsten carbide thin film electrode and its electrocatalytic activity for hydrogen evolution. Electrochem Commun 2005;7:1045–9. [9] Nersisyan HH, Won HI, Won CW. Combustion synthesis of WC powder in the presence of alkali salts. Mater Lett 2005;59:3950–4. [10] Nartowski AM, Parkin IP, MacKenzie M, Craven AJ, MacLeod I. Solid state metathesis routes to transition metal carbides. J Mater Chem 1999;9:1275–81. [11] Huang P. Principles of Powder Metallurgy. Beijing: Metallurgy Industry; 2004. [12] Koc R, Kodambaka SK. Tungsten carbide (WC) synthesis from novel precursors. J Eur Ceram Soc 2000;20:1859–69. [13] French GJ, Sale FR. A re-investigation of the thermal decomposition of ammonium paratungstate. J Mater Sci 1981;16:3427–36. [14] Ye DL, Hu JH. Practical Handbook of Inorganic Thermodynamic Data. Beijing: Metallurgy Industry; 2002. [15] Jha A, Yoon SJ. Formation of titanium carbonitride phases via the reduction of TiO2 with carbon in the presence of nitrogen. J Mater Sci 1999;34:307–22. [16] Matyi RJ, Schwartz LH, Butt JB. Particle size, particle size distribution and related measurements of supported metal catalysts. Catal Rev Sci Eng 1987;29:41–99. [17] Bahgat M, Paek M-K, Pak J-J. Reduction investigation of WO3/NiO/Fe2O3 and synthesis of nanocrystalline ternary W-Ni-Fe alloy. J Alloys Compd 2009;472: 314–8. [18] Warren A, Nylund A, Olefjord I. Oxidation of tungsten and tungsten carbide in dry and humid atmospheres. Int J Refract Met Hard Mater 1996;14:345–53. [19] Moulder JF, Stickle WF, Sobol PE, Bomben KD. Handbook of X-ray photoelectron spectroscopy. Eden Prairie, Minnesota: Perkin Elmer; 1992.