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Chemical Vapor Synthesis and Characterization of Nanosized WC-Co Composite Powder and Post-treatment Taegong Ryu, Hong Yong Sohn,* Kyu Sup Hwang, and Zhigang Zak Fang Department of Metallurgical Engineering, UniVersity of Utah, Salt Lake City, Utah 84112
Nanosized WC-Co composite powders were produced by a chemical vapor synthesis (CVS) process. The reactions of vaporized chlorides (WCl6 and CoCl2) with methane-hydrogen mixtures produced nanosized WC-Co composite powder, which sometimes contained W2C or W phase. The effects of reactant concentrations, WCl6 volatilizer temperature, and H2 to CH4 molar ratio in the feed stream were investigated. The produced powder was heat-treated under hydrogen to fully carburize all tungsten carbide phases to WC and reduce carbon content down to nearly stoichiometric levels. Finally, WC-Co composite powders of particle size less than 70 nm with 0-3% excess carbon were obtained. Introduction Cemented tungsten carbide has broad industrial applications in such fields as metal working, drilling, and mining industries under severe conditions, thanks to its superior properties of high hardness and excellent wear resistance. Because of its brittleness, tungsten carbide is typically used in the form of a metal matrix composite with cobalt as the matrix. Cobalt contents in WC-Co composites are typically limited to the range of 3-30 wt % for commercial applications. The fine size of cobalt powder is also shown effective in the reduction of cobalt pooling during the consolidation process.1-7 Previous investigations8-15 have shown that the reduction of tungsten carbide grain size gives significant improvements of the mechanical properties. Nanosized powders have been produced by various methods, such as the thermochemical spray drying process,16,17 mechanical alloying (MA),18-20 and chemical vapor condensation (CVC).21 The chemical vapor synthesis (CVS) is a process for making fine solid particles by the vapor-phase chemical reactions of precursors. This process has been applied to the synthesis of metallic and intermetallic powders at the University of Utah and has proven to yield nanostructured powders having high purities and small grain sizes.22-24 The CVS has shown a unique advantage in producing nanocomposite powders of high compositional uniformity. For the preparation of nanosized tungsten carbide, tungsten compounds such as tungsten hexachloride (WCl6),25 tungsten hexafluoride (WF6),26 or tungsten hexacarbonyl (W(CO)6)27 are generally favored as the precursor because of their relatively low volatilization temperatures as well as the ease of reduction by hydrogen or thermal decomposition. Several hydrocarbons such as propane (C3H8),28 acetylene (C2H2),29 and methane (CH4)30,31 have been used as the carburizing agent. Methane is most commonly used because it is inexpensive and stable up to a high temperature.32 In this work, the CVS of WC-Co nanocomposite powder was carried out using metal chloride precursors (WCl6 and CoCl2) in a tubular reactor system. The effects of reactant concentrations, WCl6 volatilizer temperature, and molar ratio of reactant gases on WC-Co composite formation behavior were investigated. The produced composite powders, which sometimes contained W2C and/or W phase, were subjected to * To whom correspondence should be addressed. E-mail: h.y.sohn@ utah.edu. Tel.: +1-801-581-5491. Fax: +1-801-581-4937.
heat treatment under hydrogen and the resulting product was analyzed for composition and grain size. Experimental Details WCl6 (99.9%) and CoCl2 (99.7%) were used as the precursors. Hydrogen (99.9%) and methane (99.9%) were used, respectively, as the reducing and carburizing agents. The tubular reactor system consisted of two powder feeders, a vertical reactor, powder collectors, an off-gas scrubber containing a 5 wt % NaOH solution, and a Bunsen burner, as shown in Figure 1. The reactant feeding system consisting of two separate entrained-flow powder feeders, each with a vibrator, a carriergas line, a precursor container, and a precursor delivery line was specially designed to feed the precursors separately into their volatilizers in the reactor. The precursors were placed in a glass tube mounted on the powder feeder. A stainless steel tube of 3.2 mm inner diameter was used as the delivery line connected from the powder feeder to the volatilizer. The volatilizer was made of an alumina tube of 6.4 mm inner diameter with holes near the closed end to provide an outlet for the vaporized precursors into the reactor. Argon gas (99.9%) was flowed through the powder feeding system as the carrier gas as well as to flush the reactor. Methane was introduced through the delivery line into the WCl6 volatilizer to facilitate
Figure 1. Schematic diagram of the tubular reactor system: (1) entrainedflow powder feeder for WCl6, (2) entrained-flow powder feeder for CoCl2, (3) thermocouple, (4) vertical furnace, (5) alumina reactor, (6) powder collector, (7) scrubber, and (8) Bunsen burner.
10.1021/ie800322y CCC: $40.75 2008 American Chemical Society Published on Web 10/29/2008
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mixing with volatilized WCl6, and Ar alone was flowed to carry CoCl2 powder into the CoCl2 volatilizer to avoid the deposition of reduced Co in the volatilizer. Since it had been observed during earlier experiments that an intermediate phase Co3W3C was formed when premixed powders of WCl6 and CoCl2 were fed into the reactor,33 a modification was made to feed the chlorides separately as described above. Furthermore, the WCl6 volatilizer temperature was sufficiently low to prevent the reduction and/or carburization of WCl6 by methane in the volatilizer, which would result in a deposition of solids in the volatilizer. Hojo et al.25 reported that substantial WC formation from WCl6 starts above 1000∼1200 °C, as also observed in the current work. The formation of an intermediate phase Co3W3C or Co6W6C was avoided by placing the CoCl2 volatilizer at a point where temperature was 1400 °C. Thus, tungsten was given a sufficient length of time to be carburized by the time it is carried to this position of CoCl2 volatilizer. The deposition of reduced Co in the CoCl2 volatilizer was prevented by using Ar to carry CoCl2 powder so that it can be reduced by methane away form the volatilizer. The reactor was an alumina tube of 54 mm inner diameter and 1.5 m length and placed in a vertical furnace. The produced powder was collected using a Teflon-coated polyester filter with a pore size of 1 µm. A mixture of argon, hydrogen, and methane gases was fed into the reactor to reduce the reactant precursors and carburize the reduced tungsten. The reactor was purged with an Ar flow of 2 L/min (25 °C, 86.1 kPa total pressure at Salt Lake City) for 20 min before and after each experiment. The detailed experimental conditions are discussed in the Results and Discussion section since various experimental conditions were used for specific runs. The product was analyzed by XRD (Siemens, D 5000), and the morphology was examined by TEM (JEOL, JEM-2000FXII) with energy dispersive X-ray spectrometry (EDS) that also gave distribution of the elements. The total carbon contents of the powders were measured with a Carbon Determinator (LECO, CS-444). The cobalt content in WC-Co composite powder can be adjusted by controlling the feeding rate of WCl6 or CoCl2 powder in the feed stream. In this work the Co/W ratio in the composite product was 0.8, which is equivalent to the cobalt content of 19 wt % in the product without free carbon after the post-treatment of the powder as described subsequently. The grain size of WC was calculated from the XRD pattern by applying the Scherrer equation calibrated by using standard samples.34 The Scherrer equation relates the XRD peak width and crystallite size, as follows: crystallite size )
Kλ
(B - Bex) cos θ
(1)
where K ) shape factor (0.89-1.0 taken as 0.9 in this work), λ ) X-ray wavelength (1.542 Å for Cu KR radiation), B ) full width at half-peak height (radians), θ ) peak position, and Bex ) extra broadening due to microstrain and instrumental effects (radians), obtained by the use of a standard sample. It is noted that because the powders in this study were produced in the vapor phase, no lattice strain effects were considered in this grain-size analysis. The molar ratio of the different tungsten carbide phases in the product was estimated from the XRD patterns by applying the internal standard method using standard samples, combined with the following calibration equation:34 XR IR )κ Iβ Xβ
(2)
Figure 2. X-ray diffraction patterns of the products obtained with different feeding rates of precursors in the feed stream: (a) 0.06 for WCl6 and 0.02 g/min for CoCl2, (b) 0.3 and 0.1 g/min, and (c) 0.6 and 0.2 g/min, respectively.
where IR ) integrated intensity of R phase peak, Iβ ) integrated intensity of β phase peak, XR ) mole fraction of R phase, Xβ ) mole fraction of β phase, and κ ) constant. Results and Discussion In our previous paper,33 it was reported that nanosized WC-Co composite powders of particle sizes less than 30 nm were produced in a tubular reactor system. The degree of carburization of W in the initial reaction product was affected by the experimental conditions such as temperature, methane concentration, CoCl2 concentration, and residence time. The best condition for synthesizing WC-Co composite powder in the previous work was given as follows: reaction temperature of 1400 °C, C/W ratio in the feed stream of 2.3, CoCl2/WCl6 ratio of 1, and total gas flow rate made up of CH4, Ar, and H2 of 1.1 L/min (25 °C, 86.1 kPa) in the same reactor described above. Further, it was reported therein33 that free carbon was always present in the product since excess CH4 was used. Thus, in this work, other factors that affect the degree of carburization were investigated to minimize the amount of the free carbon in the product. In addition, the results of post-treatment of the produced powder, in which the carbon content was reduced down to nearly stoichiometric level and all tungsten carbide phases were fully carburized to WC, are presented. Effect of Reactants Concentration. The effect of reactants concentration was investigated by varying the feeding rate of WCl6 from 0.06 g/min to 0.6 g/min and that of CoCl2 from 0.02 g/min to 0.2 g/min. Other experimental conditions were as follows: The WCl6 volatilizer temperature was 440 °C, the feeding rate of CH4 was controlled to keep the C/W ratio in the feed at 2.3, the CoCl2/WCl6 ratio in the feed was 1, the reaction temperature was 1400 °C, the total gas flow rate was made up by Ar to a constant value at 1.1 L/min (25 °C, 86.1 kPa), and no H2 was used in the feed stream since CH4 alone gave a high degree of carburization of W and was enough to reduce both chlorides.34 Figure 2 shows the XRD patterns of the products obtained with different feeding rates of precursors. The product under these conditions was WC and Co together with small amounts of W2C and/or W depending on the feeding rate of each reactant. No intermediate phases such as Co3W3C or Co6W6C were present in the product. Again, the feeding arrangement discussed in the previous section prevented the unreacted precursor gases from coming into contact with each other and forming intermediate phases, even when the concentration of precursors was increased. The mole fractions of tungsten carbides phases are shown in Figure 3. The molar ratio was calculated from the XRD peak intensity of WC at 2θ of 35.6, that of W2C at 2θ of 39.5, and that of W at 2θ of 40.2, according to eq 2. The W2C phase was represented as WC0.5 to better show how much of W is in each
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Figure 5. Effect of H2/CH4 ratio in the feed stream on the mole fractions of tungsten carbides. Figure 3. Effect of precursor feeding rates on the mole fractions of tungsten carbides with CoCl2/WCl6 molar ratio ) 1.
Figure 6. Effect of H2/CH4 ratio in the feed stream on % excess carbon.
Figure 4. X-ray diffraction patterns of the products obtained with different WCl6 volatilizer temperatures: (a) 440 and (b) 540 °C.
carbide phase. As the feeding rate of WCl6 was increased from 0.06 to 0.3 g/min, W2C disappeared and a small amount of W was produced. The amount of produced W increased somewhat as the feeding rate was increased to 0.6 g/min. The grain size of WC, 25 ( 1 nm at the lowest feeding rate, increased to 30 ( 1 nm at the highest feeding rate. The degree of carburization slightly decreased with the increase of reactant concentrations. It is likely that the increase in WCl6 concentration leads to increased grain size of the reduced W, which resulted in a slower carburization rate. Since W2C and/or W, even though the amount was small, was observed at the C/W ratio of 2.3, this ratio was increased to 2.7 to enhance carburization. At this C/W ratio, the product contained negligible amounts of W2C and/or W, as will be shown below [Figure 4a]. Thus, the C/W ratio of 2.7 was used in the subsequent experiments. Effect of WCl6 Volatilizer Temperature. The effect of WCl6 volatilizer temperature was investigated by varying it from 440 to 540 °C while keeping all other conditions the same [no H2, C/W ratio of 2.7, reaction temperature of 1400 °C, total gas flow rate of 1.1 L/min (25 °C, 86.1 kPa), WCl6 feeding rate of 0.6 g/min, and CoCl2 feeding rate of 0.2 g/min]. Figure 4 shows the XRD patterns of the products obtained with different volatilization temperatures of WCl6. When WCl6 volatilizer temperature was 440 °C, the product contained negligible amounts of W2C or W. However, when WCl6 volatilizer was lowered into the reactor to the position at which the temperature was 540 °C, the product contained small amounts of W2C and W. The grain size of produced WC was 30 ( 1 nm, which was not affected by the volatilization temperature of WCl6 within the range tested. Each of the runs was repeated 2-3 times, and the product grain size and the appearance of the small amounts of W2C and W at the higher WCl6 volatilization temperature were reproducible. However, since the difference was very small, no further work to definitively determine the reasons for this was carried out. Because the degree of carburization was
Figure 7. X-ray diffraction patterns of the products obtained with different hydrogen treatment times at 900 °C using the powder produced from the tubular reactor: (a) before treatment, (b) 1, (c) 2, and (d) 4 h of holding time.
lower when the WCl6 volatilizer was at 540 °C, the subsequent experiment was performed at the volatilizer temperature of 440 °C. Effect of H2/CH4 Ratio in the Feed Stream. Since excess CH4 was used to synthesize WC-Co composite powder, free carbon was always present in the product. Thus, hydrogen was added as the reducing gas as well as to reduce the amount of free carbon in the reaction. The effect of hydrogen concentration in the feed stream was investigated by controlling the H2/CH4 ratio from 0 to 4.3 under otherwise identical conditions [C/W ratio of 2.7, reaction temperature of 1400 °C, total gas flow rate of 1.1 L/min (25 °C, 86.1 kPa), WCl6 feeding rate of 0.6 g/min, and CoCl2 feeding rate of 0.2 g/min]. Figure 5 shows the mole fractions of tungsten carbides obtained with different H2/CH4 ratios, indicating that the degree of carburization decreased as the H2/CH4 ratio was increased. It is likely that hydrogen addition inhibited the decomposition of CH4 into C and H2. Thus, as the feeding rate of H2 increased, the C potential decreased, which resulted in a lower degree of carburization. The weight percentage of carbon in the product was converted to percent excess carbon, which is defined as the percentage of the excess carbon over the corresponding stoichiometric amount if all W was present as WC. The % excess carbon in the product decreased from 67.8% to -17.5% as the H2/CH4 ratio in the feed stream was increased from 0 to 4.3, as shown in Figure 6. The grain size of produced WC decreased from 30 ( 1 nm with no H2 to 26 ( 1 nm with H2/CH4 ) 4.3.
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Figure 8. Effect of hydrogen heat treatment time at 900 °C on percent excess carbon.
Tungsten carbide powder is used mainly to make bulk components by consolidation process. Therefore, the presence of incompletely carburized W2C and W phases can be tolerated because they can be fully carburized during the subsequent sintering process. These phases can also be fully carburized during the separate post-treatment of the produced composite powder as discussed in the following section. Post-treatment of Produced Composite Powders. The produced composite powder was placed in a ceramic boat, which in turn was placed in a tube furnace under hydrogen. The effect of hydrogen heat treatment on the product composition, grain growth, and carbon content was investigated. The experimental conditions were as follows: The amount of powder treated was 2 g, the treatment temperature was 900 °C, which was low enough to prevent rapid grain growth of particles, the ramping rate of tube furnace to increase the temperature up to 900 °C was 10 °C/min, and the cooling rate to the room temperature was also 10 °C/min. Hydrogen was flowed at a rate of 0.5 L/min (25 °C, 86.1 kPa) after the temperature reached 900 °C. The result showed that the unreacted W2C and W phases were carburized to the fully carburized WC phase after the powder was treated by hydrogen, as shown in Figure 7. It is likely that the carburization of W2C and W phases to the fully carburized WC phase is caused by highly mobile hydrocarbons, mainly CH4 formed during the hydrogen heat treatment, which react with W2C and W.
The % excess carbon in the product, 112% before the treatment, was down to 7% after 1 h of post-treatment, 3% after 2 h, and 0% after 4 h, as shown in Figure 8. Up to 4% excess free carbon is not only tolerated but often needed in the subsequent consolidation process to remove the small amount of oxygen present on the particle surface. Therefore, a posttreatment of 2 h under the conditions used in this work is sufficient. The grain size of WC, 30 ( 1 nm before the treatment, increased to 36 ( 1 nm after 4 h of treatment. The particle size of composite powder before and after the hydrogen heat treatment was examined by TEM micrographs, as shown in Figure 9a,b. EDS maps of the composite powder obtained after the hydrogen heat treatment are shown in Figure 9c,d. The actual particle size, as opposed to the grain size, of the composite powder was less than about 40 nm before the treatment and less than about 70 nm after the treatment for 4 h in hydrogen. It is noted that these values are from measurements on a rather limited number of particles. EDS maps also indicate that WC and Co particles were uniformly mixed in the product, which verifies the significant feature of the CVS process to produce very uniformly mixed powders, as mentioned earlier. Conclusions Uniformly mixed nanosized WC-Co composite powders were produced by the chemical vapor synthesis process in the WCl6-CoCl2-H2-CH4 system. The degree of carburization increased with decreasing reactant concentrations and H2/CH4 ratio in the feed stream within the ranges tested. The amount of excess carbon in the composite powder was reduced as the hydrogen concentration was increased in the feed stream. By hydrogen heat treatment of the composite powder, the excess carbon in the product was completely removed and small amounts of unreacted W2C and W phases were fully carburized to the WC phase. The cobalt content in WC-Co composite powder also can be adjusted by controlling the feeding rate of WCl6 or CoCl2 powder in the feed stream. In this work, the Co/W ratio in the composite product was 0.8, which is
Figure 9. TEM micrographs with EDS maps of WC-Co composite powders: (a) WC-Co composite powder obtained from the tubular reactor, (b) WC-Co composite powder after hydrogen heat treatment for 4 h at 900 °C, (c) EDS map of post-treated sample (tungsten, black area), and (d) EDS map of posttreated sample (cobalt, black area).
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equivalent to the cobalt content of 19 wt % in the product without free carbon after the post-treatment. Finally, WC-Co composite powder of particle size less than 70 nm was obtained. The CVS process carried out in our tubular reactor system confirmed that it is feasible to produce nanosized composite powders by this method. Acknowledgment This material is based upon work supported by the U.S. Department of Energy under Award No. DE-FC36-04GO14041 with cost sharing by Kennametal and Smith International and technical collaboration with Idaho National Laboratory. The authors extend special thanks to Messrs. Rick Riley and Tim Webb of Kennametal for the chemical analysis of powders and Dr. Anthony Griffo of Smith International for his helpful suggestions. Literature Cited (1) Upadhaya, G S. Cemented Tungsten Carbide; Noyes Publications: New York, 2002; pp 1-6. (2) Wahlberg, S.; Grenthe, I.; Muhammed, M. Nanostructured hard material composites by molecular engineering 1: Synthesis from soluble tungstate salts. Nanostruct. Mater. 1997, 9, 105. (3) Zhu, Y. T.; Manthiram, A. Influence of processing parameters on the formation of WC-Co nanocomposite powder using a polymer as carbon source. Composites, Part B. 1996, 27, 407. (4) Kear, B. H.; McCandlish, L. E. Chemical processing and properties of nanostructured WC-Co materials. Nanostruct. Mater. 1993, 3, 19. (5) Carroll, D. F. Sintering and microstructural development in WC/ Co-based alloys made with superfine WC powder. Int. J. Refract. Met. Hard Mater. 1999, 17, 123. (6) Kim, J. C.; Kim, B. K. Synthesis of nanosized tungsten carbide powder by the chemical vapor condensation process. Scr. Mater. 2004, 50, 969. (7) Krakhmalev, P. V.; Sukumaran, J.; Gåård, A. Effect of microstructure on edge wear mechanisms in WC-Co. Int. J. Refract. Met. Hard Mater. 2007, 25, 171. (8) Petersson, A.; Ågren, J. Constitutive behaviour of WC-Co materials with different grain size sintered under load. Acta Mater. 2004, 52, 1847. (9) Fang, Z.; Eason, J. W. Study of Nanostructured WC-Co Composites. Int. J. Refract. Met. Hard Mater. 1995, 13, 297. (10) Lee, G.-H.; Kang, S. Sintering of nano-sized WC-Co powders produced by a gas reduction-carburization process. J. Alloys Compd. 2006, 419, 281. (11) Fu, L.; Cao, L. H.; Fan, Y. S. Two-step synthesis of nanostructured tungsten carbide-cobalt powders. Scr. Mater. 2001, 44, 1061. (12) Nersisyan, H. H.; Won, H. I.; Won, C. W.; Lee, J. H. Study of the combustion synthesis process of nanostructured WC and WC-Co. Mater. Chem. Phys. 2005, 94, 153. (13) Wu, X. Y.; Zhang, W.; Wang, W.; Yang, F.; Min, J. Y.; Wang, B. Q.; Guo, J. D. Ultrafine WC-10Co cemented carbides fabricated by electric-discharge compaction. J. Mater. Res. 2004, 19, 2240. (14) Zawrah, M. F. Synthesis and characterization of WC-Co nanocomposites by novel chemical method. Ceram. Int. 2007, 33, 155. (15) Shi, X. L.; Shao, G. Q.; Duan, X. L.; Xiong, Z.; Yang, H. Characterizations of WC-10Co nanocomposite powders and subsequently sinterhip sintered cemented carbide. Mater. Charact. 2006, 57, 358.
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ReceiVed for reView February 26, 2008 ReVised manuscript receiVed September 30, 2008 Accepted October 2, 2008 IE800322Y
