- Email: [email protected]
Synthesis of nanocrystalline hexagonal tungsten carbide via co-reduction of tungsten hexachloride and sodium carbonate with metallic magnesium
Journal of Alloys and Compounds 448 (2008) 215–218
Synthesis of nanocrystalline hexagonal tungsten carbide via co-reduction of tungsten hexachloride and sodium carbonate with metallic magnesium Jianhua Ma a,b,∗ , Yihong Du c a
College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou, Zhejiang 325027, People’s Republic of China b The Key Laboratory of Wenzhou Pen-making Industry, Wenzhou, Zhejiang 325035, People’s Republic of China c City College, Wenzhou University, Wenzhou, Zhejiang 325035, People’s Republic of China Received 21 September 2006; received in revised form 15 October 2006; accepted 16 October 2006 Available online 20 November 2006
Abstract Nanocrystalline tungsten carbide (WC) was synthesized via a simple route by the reaction of metallic magnesium with sodium carbonate and tungsten hexachloride in an autoclave at 600 ◦ C. X-ray powder diffraction pattern indicated that the product was hexagonal WC, and the cell ˚ c = 2.838 A. ˚ Transmission electron microscopy image showed that it consisted of particles with an average size of 20 nm. constant was a = 2.913 A, The product was also studied by BET and TGA. It had good thermal stability and oxidation resistance below 500 ◦ C in air. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanostructures; Chemical synthesis; X-ray diffraction; Thermal analysis
1. Introduction Carbides of transition metals have attracted considerable interest due to their desirable properties for application involving high temperatures environment. Among these carbides, hexagonal tungsten carbide is one of these carbides throughout these years. It is a typical example since it has high melting point (2800 ◦ C), high degree of hardness (Hv = 22 GPa), high modulus of elasticity (696 GPa), low coefficient of thermal expansion (5.2 m/m/K), high fracture toughness (28 MPa m1/2 ), and good wear resistance over a wide range of temperatures [1]. Besides these, tungsten carbide has very high strength for a material so hard and rigid. Its compressive strength is higher than virtually all melted and cast or forged metals and alloys. It has heat and oxidation resistance. Tungsten-base carbides perform well up to about 1000 ◦ F in oxidizing atmospheres and to 1500 ◦ F in non-oxidizing atmospheres. It also has other properties, such as thermal conductivity, electrical conductivity.
∗ Corresponding author at: College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou, Zhejiang 325027, People’s Republic of China. Tel.: +86 577 88655170; fax: +86 577 86689508. E-mail address: [email protected] (J. Ma).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.10.060
Due to these properties, WC has found a wide range of industrial applications, such as tips for cutting and drilling tools, wear-resistant parts in wire drawing, extrusion and pressing dies and wear-resistant surfaces in many types of machines [2–5]. Its extreme hardness makes it useful in the manufacture of abrasives and bearings, as a cheaper alternative to diamond. It is also used as a scratch-resistant material for jewelry including watch bands and wedding rings. Besides, WC is also finding new applications as a substitute for noble metals like Pt, Pd and Ir in catalysis industries [6,7] (as catalysts in reactions of hydrogenation, methanation and ammonium synthesis [8]). WC erosion-resistant coatings for aerospace components are another application [9,10]. The low electrical resistivity combined with chemical and thermal stability makes tungsten carbide an attractive thin film diffusion barrier in microelectronics industrials [11]. Carbides of tungsten have been prepared by a variety of methods. Conventionally, the process of manufacturing WC is performed by the direct reaction of carbon and tungsten at a high temperature (1400–1600 ◦ C) [4]. The conventional synthesis technique is not able to produce powders under 150 nm in grain size [12]. Efforts have been developed to exceed this limit [13]. Very fine WC powders are attractive for use in hard metal because fine grained alloys exhibit higher hardness than coarser
216
J. Ma, Y. Du / Journal of Alloys and Compounds 448 (2008) 215–218
grained ones of the same composition, at the same toughness level [14]. More recently, other methods, such as chemical processing (carbothermal reduction of tungsten oxides or carbon-coated tungsten oxide precursors method [15]), fluid bed method, mechanical alloying, self-propagating techniques and electrochemical [16], organometallic precursor [17] and solution state methods [18], etc., have been used to prepare them. Some require high temperature and long reaction time. Kodambaka [19] performed the initial research on the use of carbon-coated precursors for enhancing productivity of tungsten carbide and succeeded in synthesizing high quality submicron WC powders at temperatures as low as 1100 ◦ C. Cao and Kear [20,21] investigated the thermo-chemical synthesis of nanophase WC powders. Nanophase WC with the particle sizes of 30 nm were produced by carburization of nanophase W powder utilizing a controlled carbon activity gas phase environment. Wang et al. [22] reported a new method that hexagonal tungsten carbide was synthesized in the thermal system of tungsten–sodium and supercritical carbon dioxide at moderate temperature (600 ◦ C) in a high-temperature stainless steel autoclave. But the pressure in the autoclave is very high due to the supercritical carbon dioxide. Nersisyan et al. [23] reported that the phase pure and sub-micrometer WC powder can be synthesized from WO3 + Mg + C + NaCl + Na2 CO3 green mixture using combustion synthesis technique. WC powder produced by this saltassisted combustion synthesis technique has a size 0.2–3 m, crystalline shape and low agglomeration degree. Kim [24] synthesized nanostructured tungsten carbide powders by the chemical vapor condensation (CVC) process using tungsten hexacarbonyl (W(CO)6 ) precursor. The loose agglomerated WC1 − x powders, which had a rounded cubic shape, were obtained by carburization with carbon from the dissociation of CO gas in the temperature range of 600–800 ◦ C. Grain size was decreased from 53 to 28 nm with increasing reaction temperature. We have consulted many references in selecting carbon source in our research. Liu et al. [25] developed a new method of medial-reduction for the synthesis of carbon nanotubes and nanobelts. In this process, Mg was used as reductant, and Na2 CO3 and CCl4 were used as carbon sources, respectively. Nascent carbon could be produced by the reaction of metallic magnesium powder and sodium carbonate in high temperature. And this nascent carbon had good reaction activity with comparison to the black carbon. In this paper, nanocrystalline hexagonal WC has been synthesized via co-reduction of tungsten hexachloride and sodium carbonate with metallic magnesium in an autoclave at 600 ◦ C. In this reaction, tungsten hexachloride is as the tungsten source and sodium carbonate is as the carbon source.
tungsten hexachloride and 0.004 mol (about 0.424 g) analytical grade sodium carbonate and 0.04 mol (about 0.972 g) metallic magnesium powders were put into a stainless steel autoclave. After sealing under argon atmosphere, the autoclave was heated at 600 ◦ C for 8 h, followed by cooling to room temperature in the furnace. The obtained product from the autoclave was washed several times with absolute ethanol, dilute HCl aqueous solution, distilled water to remove the impurities. Finally, the product was washed three times with absolute ethanol to remove water. The final product was vacuum-dried at 60 ◦ C for 12 h. Black powders were obtained. The obtained sample was analyzed by powder X-ray diffraction (XRD) on a Rigaku Dmax-␥A X-ray diffractometer using Cu K␣ radiation (wavelength ˚ The morphology of the sample was examined from transmisλ = 1.54178 A). sion electron microscopy (TEM) on a Hitachi H-800 transmission electron microscope. The specific surface area of the sample was measured by the Brunauer–Emmett–Teller (BET) method (Model ASAP 2000, Micromeritics, Norcross, GA). The average diameter of the powders (specific surface diameter) was estimated using the specific surface area. The thermogravimetric analysis was performed on a thermal analyzer (Model: TA-50) below 1000 ◦ C in air at a rate of 10 ◦ C min−1 to study its thermal stability and oxidation behavior.
3. Results and discussion Fig. 1 shows the XRD patterns of the products. The first pattern (pattern M) is about the products after synthesis (before leaching). The second pattern (pattern N) is for the as-prepared sample after purification. In the first pattern, there are many diffraction peaks which can indexed as cubic magnesium oxide (JCPDS card No. 77–2364), cubic sodium chloride (JCPDS card No. 78–0751) and hexagonal tungsten carbide (JCPDS card No. 73–0471). The impurities, such as sodium chloride and magnesium oxide, can be removed by washing with water and dilute HCl aqueous solution. In the second pattern, there are eight obvious diffraction peaks on this pattern. And all these diffraction peaks ((0 0 1), (1 0 0), (1 0 1), (1 1 0), (0 0 2), (1 1 1), (2 0 0), (1 0 2)) at different d-spacing can be indexed as hexag˚ onal WC. The refinement gives the cell constants, a = 2.913 A, ˚ which are consistent with the values reported in the c = 2.838 A, ˚ c = 2.836 A) ˚ (JCPDS card No. 73–0471). literature (a = 2.906 A, No evidences of impurities such as Mg, MgO, W and WO3 , can be found in this XRD pattern. The broadening nature of the
2. Experimental All of the manipulations were carried out in a dry glove box with flowing nitrogen gas. In a typical experiment, 0.004 mol (about 1.586 g) analytical grade
Fig. 1. XRD pattern of the as-prepared sample.
J. Ma, Y. Du / Journal of Alloys and Compounds 448 (2008) 215–218
217
Fig. 2. TEM image and SAED pattern of the as-prepared WC sample.
XRD peaks indicated that the grain sizes of the samples are on a nanometer scale. The morphology of the prepared WC sample was investigated by Transmission electron microscopy and selected area transmission electron diffraction (SAED). The TEM image and the SAED pattern are shown in Fig. 2. In Fig. 2(a), the sample shows that it consists of particles that have an average diameter of 20 nm. These particles exhibit slightly agglomerated particle morphology due to the ultrafine size. The SAED pattern of WC samples is shown in Fig. 2(b). In this pattern, three obvious diffraction rings can be found. These rings can be indexed as (0 0 1), (1 0 0) and (1 0 1). And the SAED pattern of the sample can also confirm the crystallinity of WC, in which the diffraction ring diameters again correspond well to the hexagonal WC structure. In order to get more information about the particle size of the sample, the specific surface area of the powder was measured by the Brunauer–Emmett–Teller method. In our result, the sample has a value of 14.5 m2 /g. So the average diameter of the powder (specific surface diameter) was estimated using the specific surface area on the assumption that the powder shape was spherical and the density of the powder was 15.63 g cm−3 . The estimated average diameter of the powder was about 26 nm which is a little larger than the value observed from the TEM image, suggesting the agglomeration of primary particles. We also investigated the thermal stability of these nanosized WC powders in air under different temperature. The oxidation process of nanocrystalline WC was studied at temperatures below 1000 ◦ C under the flowing air by TGA, as shown in Fig. 3. From the TGA curve, we can find that the weight gain of the sample has not changed significantly below 400 ◦ C. A slight weight loss indicates that this maybe arise from the evaporation of absorbed water on the surface of the sample. But the quantity of the adsorbed water is very small. The onset of the oxidation of the WC sample was found to begin at about 450 ◦ C, which indicates that the sample is oxidized by oxygen to form tungsten trioxide [26] and carbon dioxide. As the temperature rises, the amount of the formed tungsten trioxide becomes big-
ger, suggesting that the oxidation rate of the sample becomes faster. Finally, the formed tungsten trioxide becomes an oxide layer on the surface of the sample grain. Since the formed tungsten trioxide on the surface of the grains is protective, the oxide layer could prevent further oxidation of the underlying material. Besides, the existing of relatively large particles of WC also delays the further oxidation process. From the TGA curve, we can see that the oxidation process becomes weaker at about 650 ◦ C. With temperature increasing further from 750 ◦ C, the weight gain shows a significant increase. This may originate from that the formed protective oxide layer is destroyed and the oxidation rate increases rapidly again. Above 950 ◦ C, the weight gain remains almost constant on the TGA curve, indicating no weight change. And from the XRD pattern of the sample tested by TGA, we cannot find evidence of WC. The sample can be oxidized thoroughly at 950 ◦ C. But the sample has good thermal stability below 500 ◦ C. It was found that both reaction temperature and time have significant influence on the formation of WC. No WC could be detected in the product if the temperature was below 500 ◦ C.
Fig. 3. TGA curve of the as-prepared WC sample.
218
J. Ma, Y. Du / Journal of Alloys and Compounds 448 (2008) 215–218
Heating at 550 ◦ C could produce WC, but the crystallinity was not good and metallic tungsten could be detected in the XRD pattern of the product. But if the temperature was above 700 ◦ C, the particles size increased obviously. The higher the reaction temperature the larger is the mean particle size. An optimum temperature for the formation of nanocrystalline WC is about 600 ◦ C. If the reaction time was shorter than 4 h, the reaction was incomplete and the crystallinity of WC was lower. Varying the reaction time in the range of 6–10 h at 600 ◦ C did not significantly affect the crystallinity and particle size of the asprepared WC. An optimum reaction conditions for the formation of nanocrystalline WC was 600 ◦ C for 8 h. In our present route to prepare nanocrystalline WC, we proposed the co-reduction-carbonization synthesis mechanism. Metallic magnesium powder, acting as the reductant, plays an important role in the synthesis process. At the reaction temperature, metallic magnesium powder reacts with WCl6 to produce nascent tungsten (W* ) and magnesium dichloride. According to free energy calculations, this reduction is thermodynamically ◦ spontaneously and highly exothermic (G = −1302.44 kJ/mol, ◦ H = −1362.6 kJ/mol). So this reaction generates a great deal heat and results in a high local temperature, which favors the whole reactions. At this high temperature, sodium carbonate (decomposition: at approximately 1000 ◦ C) could decompose into carbon dioxide and sodium oxide. Then the formed carbon dioxide reacts with magnesium powders to produce carbon. The formed carbon (C* ) has high reaction activity because it is nascent during the reduction process. Finally, the nascent tungsten (W* ) and nascent carbon (C* ) can react each other to produce hexagonal tungsten carbide under the high local temperature which makes for improving the diffusion rate of carbon into tungsten. According to the above possible processes, the reaction process could be illustrated as follows, WCl6 + 3Mg → W∗ + 3MgCl2
(1)
Na2 CO3 → 2Na2 O + CO2
(2)
CO2 + 2Mg → C∗ + 2MgO
(3)
∗
∗
W + C → WC
(4)
And the total reaction process can be represented as the following, WCl6 + Na2 CO3 + 5Mg = WC + 3MgCl2 + 2MgO + Na2 O (5) 4. Conclusion Nanocrystalline WC has been successfully prepared via coreduction-carbonization route by the reaction of metallic magne-
sium powder with sodium carbonate and tungsten hexachloride in an autoclave at 600 ◦ C. The product has the hexagonal WC structure. It consists of particles with an average size of 20 nm. The product has good thermal stability and oxidation resistance below 500 ◦ C. Acknowledgment This material is based upon work funded by Zhejiang Provincial Natural Science Foundation of China under Grant No. Y404292. References [1] H.J. Scussel, Friction and Wear of Cemented Carbides, ASM Handbook, vol. 18, ASM International, 1992, p. 795. [2] P. Schwartzkopf, R. Kieffer, Refractory Hard Metals-Borides, Carbides, Nitrides, and Silicides, MacMillan Company, New York, 1953. [3] T.Y. Kosolapova, Carbides: Properties, Production, and Applications, Plenum Press, New York, 1971. [4] L.E. Toth, Transition Metal Carbides and Nitrides, Academic Press, New York and London, 1971. [5] O. Hugh, Pierson Handbook of Refractory Carbides and Nitrides, William Andrew Press, New York, 1996. [6] L. Leclercq, M. Provost, H. Pastor, G. Grimblot, A.M. Hardy, L. Gengembre, J. Catal. 117 (1989) 371–383. [7] M.J. Ledoux, C.H. Pham, J. Guille, H. Dunlop, J. Catal. 134 (1992) 383–398. [8] L. Volpe, M. Boudart, J. Solid State Chem. 59 (1985) 348–356. [9] M.J. Ledoux, C.H. Pham, J. Guille, H. Dunlop, J. Catal. 117 (1989) 383. [10] D. Garg, P.N. Dyer, Mater. Res. Fall Meeting (1989). [11] S. Ghaisas, J. Appl. Phys. 70 (1991) 7626. [12] B. Zeiler, in: G. Kneringer, P. R¨odhammer, P. Wilhartitz (Eds.), Proceedings of the 14th International Plansee Seminar, vol. 4, Plansee AG, Reutte, 1997, pp. 265–275. [13] P. Seegopaul, L.E. McCandlish, F.M. Shinneman, Int. J. Refract. Met. Hard Mater. 15 (1997) 133–138. [14] W.D. Schubert, H. Neumeister, G. Kinger, Int. J. Refract. Met. Hard Mater. 16 (1998) 133–142. [15] Rasit Koc, Suneel K. Kodambaka, J. Eur. Ceram. Soc. 20 (2000) 1859–1869. [16] V.V. Malyshev, H.B. Kushkhov, V.I. Shapoval, J. Appl. Electrochem. 32 (2002) 573. [17] K.K. Lai, H.H. Lamb, Chem. Mater. 7 (1995) 2284. [18] J.A. Nelson, M.J. Wagner, Chem. Mater. 14 (2002) 1639. [19] S.K. Kodambaka, SIUC, M.S. Thesis, 1996. [20] L. Cao, B.H. Kear, Nanostruct. Mater. 5 (1995) 555. [21] L. Cao, B.H. Kear, Nanostruct. Mater. 9 (1997) 205. [22] Q. Wang, F.Y. Cao, Q.W. Chen, Mater. Chem. Phys. 95 (2006) 113–116. [23] H.H. Nersisyan, H.I. Won, C.W. Won, Mater. Lett. 59 (2005) 3950– 3954. [24] J.C. Kim, B.K. Kim, Scripta Mater. 50 (2004) 969–972. [25] J.W. Liu, M.W. Shao, Q. Tang, S.Y. Zhang, Y.T. Qian, J. Phys. Chem. B 107 (2003) 6329–6332. [26] A. Warren, A. Nylund, I. Olefjord, Int. J. Refract. Met. Hard Mater. 14 (1996) 345–353.
Synthesis of nanocrystalline hexagonal tungsten carbide via co-reduction of tungsten hexachloride and sodium carbonate with metallic magnesium Jianhua Ma a,b,∗ , Yihong Du c a
College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou, Zhejiang 325027, People’s Republic of China b The Key Laboratory of Wenzhou Pen-making Industry, Wenzhou, Zhejiang 325035, People’s Republic of China c City College, Wenzhou University, Wenzhou, Zhejiang 325035, People’s Republic of China Received 21 September 2006; received in revised form 15 October 2006; accepted 16 October 2006 Available online 20 November 2006
Abstract Nanocrystalline tungsten carbide (WC) was synthesized via a simple route by the reaction of metallic magnesium with sodium carbonate and tungsten hexachloride in an autoclave at 600 ◦ C. X-ray powder diffraction pattern indicated that the product was hexagonal WC, and the cell ˚ c = 2.838 A. ˚ Transmission electron microscopy image showed that it consisted of particles with an average size of 20 nm. constant was a = 2.913 A, The product was also studied by BET and TGA. It had good thermal stability and oxidation resistance below 500 ◦ C in air. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanostructures; Chemical synthesis; X-ray diffraction; Thermal analysis
1. Introduction Carbides of transition metals have attracted considerable interest due to their desirable properties for application involving high temperatures environment. Among these carbides, hexagonal tungsten carbide is one of these carbides throughout these years. It is a typical example since it has high melting point (2800 ◦ C), high degree of hardness (Hv = 22 GPa), high modulus of elasticity (696 GPa), low coefficient of thermal expansion (5.2 m/m/K), high fracture toughness (28 MPa m1/2 ), and good wear resistance over a wide range of temperatures [1]. Besides these, tungsten carbide has very high strength for a material so hard and rigid. Its compressive strength is higher than virtually all melted and cast or forged metals and alloys. It has heat and oxidation resistance. Tungsten-base carbides perform well up to about 1000 ◦ F in oxidizing atmospheres and to 1500 ◦ F in non-oxidizing atmospheres. It also has other properties, such as thermal conductivity, electrical conductivity.
∗ Corresponding author at: College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou, Zhejiang 325027, People’s Republic of China. Tel.: +86 577 88655170; fax: +86 577 86689508. E-mail address: [email protected] (J. Ma).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.10.060
Due to these properties, WC has found a wide range of industrial applications, such as tips for cutting and drilling tools, wear-resistant parts in wire drawing, extrusion and pressing dies and wear-resistant surfaces in many types of machines [2–5]. Its extreme hardness makes it useful in the manufacture of abrasives and bearings, as a cheaper alternative to diamond. It is also used as a scratch-resistant material for jewelry including watch bands and wedding rings. Besides, WC is also finding new applications as a substitute for noble metals like Pt, Pd and Ir in catalysis industries [6,7] (as catalysts in reactions of hydrogenation, methanation and ammonium synthesis [8]). WC erosion-resistant coatings for aerospace components are another application [9,10]. The low electrical resistivity combined with chemical and thermal stability makes tungsten carbide an attractive thin film diffusion barrier in microelectronics industrials [11]. Carbides of tungsten have been prepared by a variety of methods. Conventionally, the process of manufacturing WC is performed by the direct reaction of carbon and tungsten at a high temperature (1400–1600 ◦ C) [4]. The conventional synthesis technique is not able to produce powders under 150 nm in grain size [12]. Efforts have been developed to exceed this limit [13]. Very fine WC powders are attractive for use in hard metal because fine grained alloys exhibit higher hardness than coarser
216
J. Ma, Y. Du / Journal of Alloys and Compounds 448 (2008) 215–218
grained ones of the same composition, at the same toughness level [14]. More recently, other methods, such as chemical processing (carbothermal reduction of tungsten oxides or carbon-coated tungsten oxide precursors method [15]), fluid bed method, mechanical alloying, self-propagating techniques and electrochemical [16], organometallic precursor [17] and solution state methods [18], etc., have been used to prepare them. Some require high temperature and long reaction time. Kodambaka [19] performed the initial research on the use of carbon-coated precursors for enhancing productivity of tungsten carbide and succeeded in synthesizing high quality submicron WC powders at temperatures as low as 1100 ◦ C. Cao and Kear [20,21] investigated the thermo-chemical synthesis of nanophase WC powders. Nanophase WC with the particle sizes of 30 nm were produced by carburization of nanophase W powder utilizing a controlled carbon activity gas phase environment. Wang et al. [22] reported a new method that hexagonal tungsten carbide was synthesized in the thermal system of tungsten–sodium and supercritical carbon dioxide at moderate temperature (600 ◦ C) in a high-temperature stainless steel autoclave. But the pressure in the autoclave is very high due to the supercritical carbon dioxide. Nersisyan et al. [23] reported that the phase pure and sub-micrometer WC powder can be synthesized from WO3 + Mg + C + NaCl + Na2 CO3 green mixture using combustion synthesis technique. WC powder produced by this saltassisted combustion synthesis technique has a size 0.2–3 m, crystalline shape and low agglomeration degree. Kim [24] synthesized nanostructured tungsten carbide powders by the chemical vapor condensation (CVC) process using tungsten hexacarbonyl (W(CO)6 ) precursor. The loose agglomerated WC1 − x powders, which had a rounded cubic shape, were obtained by carburization with carbon from the dissociation of CO gas in the temperature range of 600–800 ◦ C. Grain size was decreased from 53 to 28 nm with increasing reaction temperature. We have consulted many references in selecting carbon source in our research. Liu et al. [25] developed a new method of medial-reduction for the synthesis of carbon nanotubes and nanobelts. In this process, Mg was used as reductant, and Na2 CO3 and CCl4 were used as carbon sources, respectively. Nascent carbon could be produced by the reaction of metallic magnesium powder and sodium carbonate in high temperature. And this nascent carbon had good reaction activity with comparison to the black carbon. In this paper, nanocrystalline hexagonal WC has been synthesized via co-reduction of tungsten hexachloride and sodium carbonate with metallic magnesium in an autoclave at 600 ◦ C. In this reaction, tungsten hexachloride is as the tungsten source and sodium carbonate is as the carbon source.
tungsten hexachloride and 0.004 mol (about 0.424 g) analytical grade sodium carbonate and 0.04 mol (about 0.972 g) metallic magnesium powders were put into a stainless steel autoclave. After sealing under argon atmosphere, the autoclave was heated at 600 ◦ C for 8 h, followed by cooling to room temperature in the furnace. The obtained product from the autoclave was washed several times with absolute ethanol, dilute HCl aqueous solution, distilled water to remove the impurities. Finally, the product was washed three times with absolute ethanol to remove water. The final product was vacuum-dried at 60 ◦ C for 12 h. Black powders were obtained. The obtained sample was analyzed by powder X-ray diffraction (XRD) on a Rigaku Dmax-␥A X-ray diffractometer using Cu K␣ radiation (wavelength ˚ The morphology of the sample was examined from transmisλ = 1.54178 A). sion electron microscopy (TEM) on a Hitachi H-800 transmission electron microscope. The specific surface area of the sample was measured by the Brunauer–Emmett–Teller (BET) method (Model ASAP 2000, Micromeritics, Norcross, GA). The average diameter of the powders (specific surface diameter) was estimated using the specific surface area. The thermogravimetric analysis was performed on a thermal analyzer (Model: TA-50) below 1000 ◦ C in air at a rate of 10 ◦ C min−1 to study its thermal stability and oxidation behavior.
3. Results and discussion Fig. 1 shows the XRD patterns of the products. The first pattern (pattern M) is about the products after synthesis (before leaching). The second pattern (pattern N) is for the as-prepared sample after purification. In the first pattern, there are many diffraction peaks which can indexed as cubic magnesium oxide (JCPDS card No. 77–2364), cubic sodium chloride (JCPDS card No. 78–0751) and hexagonal tungsten carbide (JCPDS card No. 73–0471). The impurities, such as sodium chloride and magnesium oxide, can be removed by washing with water and dilute HCl aqueous solution. In the second pattern, there are eight obvious diffraction peaks on this pattern. And all these diffraction peaks ((0 0 1), (1 0 0), (1 0 1), (1 1 0), (0 0 2), (1 1 1), (2 0 0), (1 0 2)) at different d-spacing can be indexed as hexag˚ onal WC. The refinement gives the cell constants, a = 2.913 A, ˚ which are consistent with the values reported in the c = 2.838 A, ˚ c = 2.836 A) ˚ (JCPDS card No. 73–0471). literature (a = 2.906 A, No evidences of impurities such as Mg, MgO, W and WO3 , can be found in this XRD pattern. The broadening nature of the
2. Experimental All of the manipulations were carried out in a dry glove box with flowing nitrogen gas. In a typical experiment, 0.004 mol (about 1.586 g) analytical grade
Fig. 1. XRD pattern of the as-prepared sample.
J. Ma, Y. Du / Journal of Alloys and Compounds 448 (2008) 215–218
217
Fig. 2. TEM image and SAED pattern of the as-prepared WC sample.
XRD peaks indicated that the grain sizes of the samples are on a nanometer scale. The morphology of the prepared WC sample was investigated by Transmission electron microscopy and selected area transmission electron diffraction (SAED). The TEM image and the SAED pattern are shown in Fig. 2. In Fig. 2(a), the sample shows that it consists of particles that have an average diameter of 20 nm. These particles exhibit slightly agglomerated particle morphology due to the ultrafine size. The SAED pattern of WC samples is shown in Fig. 2(b). In this pattern, three obvious diffraction rings can be found. These rings can be indexed as (0 0 1), (1 0 0) and (1 0 1). And the SAED pattern of the sample can also confirm the crystallinity of WC, in which the diffraction ring diameters again correspond well to the hexagonal WC structure. In order to get more information about the particle size of the sample, the specific surface area of the powder was measured by the Brunauer–Emmett–Teller method. In our result, the sample has a value of 14.5 m2 /g. So the average diameter of the powder (specific surface diameter) was estimated using the specific surface area on the assumption that the powder shape was spherical and the density of the powder was 15.63 g cm−3 . The estimated average diameter of the powder was about 26 nm which is a little larger than the value observed from the TEM image, suggesting the agglomeration of primary particles. We also investigated the thermal stability of these nanosized WC powders in air under different temperature. The oxidation process of nanocrystalline WC was studied at temperatures below 1000 ◦ C under the flowing air by TGA, as shown in Fig. 3. From the TGA curve, we can find that the weight gain of the sample has not changed significantly below 400 ◦ C. A slight weight loss indicates that this maybe arise from the evaporation of absorbed water on the surface of the sample. But the quantity of the adsorbed water is very small. The onset of the oxidation of the WC sample was found to begin at about 450 ◦ C, which indicates that the sample is oxidized by oxygen to form tungsten trioxide [26] and carbon dioxide. As the temperature rises, the amount of the formed tungsten trioxide becomes big-
ger, suggesting that the oxidation rate of the sample becomes faster. Finally, the formed tungsten trioxide becomes an oxide layer on the surface of the sample grain. Since the formed tungsten trioxide on the surface of the grains is protective, the oxide layer could prevent further oxidation of the underlying material. Besides, the existing of relatively large particles of WC also delays the further oxidation process. From the TGA curve, we can see that the oxidation process becomes weaker at about 650 ◦ C. With temperature increasing further from 750 ◦ C, the weight gain shows a significant increase. This may originate from that the formed protective oxide layer is destroyed and the oxidation rate increases rapidly again. Above 950 ◦ C, the weight gain remains almost constant on the TGA curve, indicating no weight change. And from the XRD pattern of the sample tested by TGA, we cannot find evidence of WC. The sample can be oxidized thoroughly at 950 ◦ C. But the sample has good thermal stability below 500 ◦ C. It was found that both reaction temperature and time have significant influence on the formation of WC. No WC could be detected in the product if the temperature was below 500 ◦ C.
Fig. 3. TGA curve of the as-prepared WC sample.
218
J. Ma, Y. Du / Journal of Alloys and Compounds 448 (2008) 215–218
Heating at 550 ◦ C could produce WC, but the crystallinity was not good and metallic tungsten could be detected in the XRD pattern of the product. But if the temperature was above 700 ◦ C, the particles size increased obviously. The higher the reaction temperature the larger is the mean particle size. An optimum temperature for the formation of nanocrystalline WC is about 600 ◦ C. If the reaction time was shorter than 4 h, the reaction was incomplete and the crystallinity of WC was lower. Varying the reaction time in the range of 6–10 h at 600 ◦ C did not significantly affect the crystallinity and particle size of the asprepared WC. An optimum reaction conditions for the formation of nanocrystalline WC was 600 ◦ C for 8 h. In our present route to prepare nanocrystalline WC, we proposed the co-reduction-carbonization synthesis mechanism. Metallic magnesium powder, acting as the reductant, plays an important role in the synthesis process. At the reaction temperature, metallic magnesium powder reacts with WCl6 to produce nascent tungsten (W* ) and magnesium dichloride. According to free energy calculations, this reduction is thermodynamically ◦ spontaneously and highly exothermic (G = −1302.44 kJ/mol, ◦ H = −1362.6 kJ/mol). So this reaction generates a great deal heat and results in a high local temperature, which favors the whole reactions. At this high temperature, sodium carbonate (decomposition: at approximately 1000 ◦ C) could decompose into carbon dioxide and sodium oxide. Then the formed carbon dioxide reacts with magnesium powders to produce carbon. The formed carbon (C* ) has high reaction activity because it is nascent during the reduction process. Finally, the nascent tungsten (W* ) and nascent carbon (C* ) can react each other to produce hexagonal tungsten carbide under the high local temperature which makes for improving the diffusion rate of carbon into tungsten. According to the above possible processes, the reaction process could be illustrated as follows, WCl6 + 3Mg → W∗ + 3MgCl2
(1)
Na2 CO3 → 2Na2 O + CO2
(2)
CO2 + 2Mg → C∗ + 2MgO
(3)
∗
∗
W + C → WC
(4)
And the total reaction process can be represented as the following, WCl6 + Na2 CO3 + 5Mg = WC + 3MgCl2 + 2MgO + Na2 O (5) 4. Conclusion Nanocrystalline WC has been successfully prepared via coreduction-carbonization route by the reaction of metallic magne-
sium powder with sodium carbonate and tungsten hexachloride in an autoclave at 600 ◦ C. The product has the hexagonal WC structure. It consists of particles with an average size of 20 nm. The product has good thermal stability and oxidation resistance below 500 ◦ C. Acknowledgment This material is based upon work funded by Zhejiang Provincial Natural Science Foundation of China under Grant No. Y404292. References [1] H.J. Scussel, Friction and Wear of Cemented Carbides, ASM Handbook, vol. 18, ASM International, 1992, p. 795. [2] P. Schwartzkopf, R. Kieffer, Refractory Hard Metals-Borides, Carbides, Nitrides, and Silicides, MacMillan Company, New York, 1953. [3] T.Y. Kosolapova, Carbides: Properties, Production, and Applications, Plenum Press, New York, 1971. [4] L.E. Toth, Transition Metal Carbides and Nitrides, Academic Press, New York and London, 1971. [5] O. Hugh, Pierson Handbook of Refractory Carbides and Nitrides, William Andrew Press, New York, 1996. [6] L. Leclercq, M. Provost, H. Pastor, G. Grimblot, A.M. Hardy, L. Gengembre, J. Catal. 117 (1989) 371–383. [7] M.J. Ledoux, C.H. Pham, J. Guille, H. Dunlop, J. Catal. 134 (1992) 383–398. [8] L. Volpe, M. Boudart, J. Solid State Chem. 59 (1985) 348–356. [9] M.J. Ledoux, C.H. Pham, J. Guille, H. Dunlop, J. Catal. 117 (1989) 383. [10] D. Garg, P.N. Dyer, Mater. Res. Fall Meeting (1989). [11] S. Ghaisas, J. Appl. Phys. 70 (1991) 7626. [12] B. Zeiler, in: G. Kneringer, P. R¨odhammer, P. Wilhartitz (Eds.), Proceedings of the 14th International Plansee Seminar, vol. 4, Plansee AG, Reutte, 1997, pp. 265–275. [13] P. Seegopaul, L.E. McCandlish, F.M. Shinneman, Int. J. Refract. Met. Hard Mater. 15 (1997) 133–138. [14] W.D. Schubert, H. Neumeister, G. Kinger, Int. J. Refract. Met. Hard Mater. 16 (1998) 133–142. [15] Rasit Koc, Suneel K. Kodambaka, J. Eur. Ceram. Soc. 20 (2000) 1859–1869. [16] V.V. Malyshev, H.B. Kushkhov, V.I. Shapoval, J. Appl. Electrochem. 32 (2002) 573. [17] K.K. Lai, H.H. Lamb, Chem. Mater. 7 (1995) 2284. [18] J.A. Nelson, M.J. Wagner, Chem. Mater. 14 (2002) 1639. [19] S.K. Kodambaka, SIUC, M.S. Thesis, 1996. [20] L. Cao, B.H. Kear, Nanostruct. Mater. 5 (1995) 555. [21] L. Cao, B.H. Kear, Nanostruct. Mater. 9 (1997) 205. [22] Q. Wang, F.Y. Cao, Q.W. Chen, Mater. Chem. Phys. 95 (2006) 113–116. [23] H.H. Nersisyan, H.I. Won, C.W. Won, Mater. Lett. 59 (2005) 3950– 3954. [24] J.C. Kim, B.K. Kim, Scripta Mater. 50 (2004) 969–972. [25] J.W. Liu, M.W. Shao, Q. Tang, S.Y. Zhang, Y.T. Qian, J. Phys. Chem. B 107 (2003) 6329–6332. [26] A. Warren, A. Nylund, I. Olefjord, Int. J. Refract. Met. Hard Mater. 14 (1996) 345–353.