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Acrylamide route for the co-synthesis of tungsten carbide–cobalt nanopowders with additives
Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Acrylamide route for the co-synthesis of tungsten carbide–cobalt nanopowders with additives Marzie Mardali a,n, Behzad Sadeghi b, Rasoul Sarraf-Mamoory a, Babak Safarbali c a
Department of Materials Engineering, Tarbiat Modares University, P.O.Box: 14115-111, Tehran, Islamic Republic of Iran Young Researchers and Elite club, Neyshabur Branch, Islamic Azad University, Khorasan Razavi, Islamic Republic of Iran c Department of Materials Engineering, Isfahan University of Technology, Isfahan, Islamic Republic of Iran b
art ic l e i nf o
a b s t r a c t
Article history: Received 23 February 2016 Accepted 23 February 2016
In this work, cemented tungsten carbide nano-particles were prepared by a chemical method called acrylamide gel. In this process, first, a xerogel containing tungsten and cobalt oxide particles was synthesized. Then, it was carburized by a hydrogen reduction heating process. Acrylamide and N, N-methylene-bis-acrylamide monomers were used as an in-situ carbon. Ammonium meta tungstate (NH4)6H2W12O40 xH2O, and cobalt nitrate Co(NO3)2 6H2O salts were used as the precursor. Both reduction and carburization reactions were carried out simultaneously and the formation of the intermediate phases of W2C, Co3W3C, and Co6W6C led to decrease in the activation barrier. Transactions of reduction and carburization processes were studied by X-ray diffraction analysis at various temperatures. Accordingly, tungsten carbide phase formation was completed at 1100 °C. The formation of W–C and V–C bonds was verified by Raman spectroscopy. SEM images showed the average nano particle size of 50 nm. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Tungsten carbide-cobalt Nanopowders Vanadium carbide Hydrogel In-situ carbon source
1. Introduction Bulk cemented tungsten carbide is widely used in demanding industrial applications such as cutting tools, mining and oil drilling, blasting, transportation, forestry tools, spray nozzles, sealing grout pumps, etc [1]. High hardness, high toughness, and wear resistance are always desirable for these applications. Furthermore, Nano-crystalline grain structure using nano-sized WC-Co powders promise dramatic improvement of these properties [2]. Currently, there are many methods which are used to carburize tungsten oxide (WO3) powder for the preparation of cemented tungsten carbide nanoparticles, such as, carbothermal hydrogen reduction [3], chemical vapor condensation by methane gas [4], self-propagating high-temperature synthesis (SHS or combustion synthesis) [5,6]. The most important disadvantage of these methods was the particle agglomeration during carbonization in high temperatures. Earlier attempts to use in-situ carbon sources took place in former [6]. It is well known that additional carbon remaining in the products causes the suppression of grain growth inhibition and high concentration of tungsten dissolved in the binder [7]. To avoid the presence of excess carbon in compounds, n
Corresponding author. E-mail address: [email protected] (M. Mardali).
in some procedures, organic compounds are used as carbon sources [8,9]. Here we try to use acrylamide gel as a source of carbon as well as network steric stabilizer for nano particles. Acrylamide gel is a new method to the synthesis of Tungsten carbide-Cobalt powders.It has the advantages such as molecularlevel combination that leads a homogeneous composition, not requiring advanced apparatus, inexpensive salts and organic materials as precursors compared with metal alcoxides, and good controlling of particle size by the polymeric net [9–11]. We could also add vanadium carbide (VC) to WC–Co powders as a grain growth inhibitor by this simple method. Therefore, homogeneity of the additive is given by this method that has more effect on grain growth suppression. In fact, in the hydrogel method, polymeric net plays a spatial precision stabilization role in nanoscale dimensions and is an in-situ carbon source for carburization. This polymeric gel is formed by co-polymerization of acrylamide (as monomer) and N,N-methylene-bis-acrylamide (as cross linker). Polymerization reaction initiates by the addition of ammonium persulphate through the production of free radicals and reaction with inactive monomers. The resulting gel is porous and size of its pores depends on polymerization conditions. The main difference between this method and sol-gel is in the use of an external agent as gully maker [9–11]. The purpose of the present work is to obtain both WC and cobalt metal nanopowders simultaneously using polymeric
http://dx.doi.org/10.1016/j.ceramint.2016.02.152 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: M. Mardali, et al., Acrylamide route for the co-synthesis of tungsten carbide–cobalt nanopowders with additives, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.152i
M. Mardali et al. / Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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monomers as a carbon source.
2. Experimental procedure The procedure used to prepare WC-Co with VC additive nanocomposite particles can be divided into 3 subsequent stages: solution, gelation, and calcination. First acrylamide (AM) (C2H3CoNH2, Merck, Germany) and N,N′-methylene-bis-acrylamide (MBAM) (CH2CH3CONH)CH2, Merck, Germany) monomers were easily dissolved in the mixture of aqueous solutions of ammonium meta-tungstate (AMT) ((NH4)6H2W12O40 xH2O, SigmaAldrich), cobalt nitrate (Co(NO3)2 6H2O, Merck, Germany), and ammonium meta-vanadate (NH4VO3, Merck, Germany). The concentration of monomers solution was 1.2 M. Then, the resulting solution was stirred completely for 45 min and gradually heated in a water bath at 75 °C to obtain a transparent pink sol. When the temperature reached about 75 °C, 2 mL of 0.2 M ammonium persulphate aqueous solution ((NH4)2S2O8(APS), 10 wt%, Merck, Germany) was added to the mixture as the initiator. An orange semitransparent gel was rapidly produced. The gel was dried at 100 °C for 24 h to yield a xerogel. At the next stage, the obtained xerogel was grinded to form a homogenous powder. The powders were calcined at 900 and 1000 ̊ C under a hydrogen atmosphere for 1 h. To analyze the thermal decomposition and reduction process, a thermogravimetric (TG)/differential thermal analysis (DTA) experiment was carried out in hydrogen gas. The flow rate for each gas was 50 mL/min, the heating rate was 10 ̊ C/ min, and the temperature was varied from room temperature to 1100 °C. The structure of the powder was examined at room temperature by X-ray diffraction (XRD) with Cu-Kα radiation (AXS-D8, Holland). The morphology and particle size of the powder were examined using FE-SEM (F4160, Hitachi, Japan). The formations of V–C and W–C bonds were proved by Raman Spectroscopy (second harmonic of Nd:Yag laser with λ ¼532 nm).
3. Results and discussion Fig. 1(a) shows the Raman Spectroscopy surveys for V–C and W–C bonds formation in the prepared powder. As it can be seen the W–C bond position is at 337 cm 1. Raman shift of W¼C bond is also indicated at 937 cm 1. According to Fig. 1(a) as a reference, D peak of W–C bond is at 1340 cm 1 and moreover D peak of V–C
bond is at 1360 cm 1 [12]. The asymmetric Raman shift at 1353 cm 1 is assigned to the combination of D peaks of V–C and W–C bonds in Fig. 1(b). The unsymmetrical shift at 1581 cm 1 is also referred to the blend of G peaks of W–C and V–C bonds. Decomposition of these peaks is difficult due to their proximity. These results show that the reduction process is done completely because there was no oxide compounds shift. Also, X-ray diffraction pattern was used to characterzation of the phase transformation of the WC-10% Co nanocomposite powder during the reduction processing at 900 and 1100 °C (Fig. 2). It exhibits characterizing peaks of tungsten carbide, the intermediate phases and Cobalt at 900 °C. It seems that WC phase is a dominant phase. By increasing temperature to 1100 °C, the peaks of intermediate phases are eliminated and only WC and Co phase remains with a small amount of W2C phase. The average crystalline size is estimated from the Scherrer equation. This value is 32 nm at 1100 °C. Based on diffraction thermal analysis in Fig. 3, the first peak is related to the water evaporation at 100 °C. The weight loss in rang of 250–750 °C accompanied by the release of heat that is Due to exit of structural water and organic compounds from the system. The drastic change of thermal curve at 280 °C is the shift of process type from endothermic to exothermic. It seems that in addition of the crystallization of amorphous phases, ignition has taken place. This claim has been proven in the thermos gravimeteri analysis diagram (Fig. 3a). It shows the maximum weight change at 280 °C. Oxygen for combustion provided of organic compounds and mineral salts sources. The decomposition of organic phases starts in the range of 250–300 °C. Despite the removal of some of the organic compounds in the process of restoration, the carbon content is enough to carburizing of the powder. It seems that in this stage, stack of active carbon remains in the adjacency of tungsten and cobalt ions through the pores of xerogel. It makes the carburizing at higher temperatures. In the range of 450–550 °C a relatively wide exothermic peak can be seen that is due to reduction of Cobalt Oxide. At 750 °C with exit of remaining organic compounds from the system, an endothermic peak is observed in the graph. This process can also be found from the weight changes in TG diagram. In the DTA diagram, a relatively wide exothermic peak is observed in the temperature range of 900–1100 °C that shows the carburization and reduction of products. In practice, due to two simultaneous reduction and carburization process, showing a known distance between them is difficult.
Fig. 1. (a) Reference Raman spectra for some carbides [12] (b) Raman spectrum of sample sintered at 1000 °C.
Please cite this article as: M. Mardali, et al., Acrylamide route for the co-synthesis of tungsten carbide–cobalt nanopowders with additives, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.152i
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Fig. 2. X-ray diffraction pattern of WC- Co reduced at (a) 900 °C and (b)1100 °C.
Fig. 4. Scanning electron microscope image of the sample calcined in the magnification of 1000.
Fig. 3. (a) Thermo gravimeteri analysis diagram and (b) Differential thermal analysis diagram.
As the results showed, In the temperature range of 300–750 °C organic phases removed from system to form of CO, CO2 and H2O gases. Product has a very high specific surface area due to gas emissions from raw gel resulted in a highly porous network. The increased surface area allows reduction and carburization at a lower temperature and therefore obtained much finer particles. Fig. 4 shows a SEM image of calcined sample with porous network. The Micrographs of the final powders calcined at 900 and 1100 °C are shown in Fig. 5 wherein morphology of the powders is regular and the particle size distribution was homogenous. In Fig. 5(a), the average particle size is about 50 nm. The EDX diagram of the sample reduced at 1100 °C is indicated in Fig. 6. The absence of any oxygen element and the presence of carbon in Table1 show that reduction processes are done successfully. Low weight percentage of carbon element is due to the absorption of its characterized X-ray beam by the detector.
The mechanism of polymeric cell formation in polyacrylamide gel is demonstrated in Fig. 7. Then, ammonium meta-tungstate and cobalt nitrate are dissolved into the water by the following reactions [13]: (NH4)6H2W12O40 þ H2O-H2W12O40 6 þ6NH4 þ
(1)
H2W12O40 6 þH2O-H2W6O20 6
(2)
H2W6O20 6 þ H2O-6WO4 2 þ6H þ
(3)
Co(NO3)2 þ H2O -Co(NO3) þ þNO3–
(4)
Co(NO3) þ þH2O-Co þ 2 þNO3
(5)
During the studies, electronegativity of Cobalt ions is more than hydrogen ions. So it seems that the particles in the initial cell contain a mixture of cobalt and tungsten ions. The structure of polyacrylamide gel is a net like and the viscosity of the solution containing WO4 2 and Co þ 2 ions is relatively high. Therefore, the velocity of the ions is limited, and they are confined in the polymeric cells. These mechanisms cause the
Please cite this article as: M. Mardali, et al., Acrylamide route for the co-synthesis of tungsten carbide–cobalt nanopowders with additives, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.152i
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Fig. 5. FE-SEM photographs of WC–Co nano powders reduced at (a) 900 °C, (b) 1100 °C.
Fig. 6. EDX diagram of the sample reduced at 1100 °C.
Fig. 7. Mechanism of polymeric cell formation in polyacrylamide gel [10].
Table1 Elemental characterization of calcined sample at 1100 °C.
4. Conclusions
Elt
Lin e
Int
Error
K
Kr
wt%
C Co W Au
Ka Ka La La
16.3 98.8 255.3 71.4
11.3554 0.6230 0.6230 0.6230
0.0199 0.0533 0.6383 0.2885 1.000
0.0178 0.0477 0.5712 0.2581 0.8948
7.69 4.02 60.66 27.62 100.00
decrease of agglomeration particles during drying and calcination process. The subsequent carburization sequence appeared as W-Co6W6C-Co3W3C-W2C-WC. The proposed route for the carburization of W was according to the following equations: 6W þ6 Co þ C ¼Co6W6C
(6)
Co6W6C þC ¼2Co3W3C
(7)
2Co3W3C þC ¼3 W2C þ6Co
(8)
As evident, cobalt plays the role of a catalyst for this process [14].
In this study, the hydrogel new method for WC-10wt% Co nanocomposite powders was reported. Controlling the size, reduction, and carburization was carried out in one step by this method. During the process, acrylamide gel was an in-situ carbon source and carburization reaction finished at 1100 °C with 32 nm mean crystallite size, and the average nanoparticles size of WC-10wt% Co is about 50 nm. The monomer/salt ratio of 3/1 provided the carbon required to carbonization process and no free carbon was remained.
Reference [1] A.S. Kurlov, A.I. Gusev, Tungsten Carbides: Structure, Properties and Application in Hardmetals, Springer International Publishing, Switzerland, Germany, 2013. [2] Z.Z. Fang, X. Wang, T. Ryu, K.S. Hwang, H.Y. Sohn, Synthesis, sintering, and mechanical properties of nanocrystalline cemented tungsten carbide – a review, Int. J. Refract. Met. Hard Mater. 27 (2) (2009) 288–299. [3] Y. Jin, X. Li, D. Liu, C. Liu, R. Yang, Phase and microstructure evolution during the synthesis of {WC} nanopowders via thermal processing of the precursor, Powder Technol. 217 (2012) 482–485.
Please cite this article as: M. Mardali, et al., Acrylamide route for the co-synthesis of tungsten carbide–cobalt nanopowders with additives, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.152i
M. Mardali et al. / Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎ [4] J.C. Kim, B.K. Kim, Synthesis of nanosized tungsten carbide powder by the chemical vapor condensation process, Scr. Mater. 50 (7) (2004) 969–972. [5] A.A. Zaytsev, I.P. Borovinskaya, V.I. Vershinnikov, I. Konyashin, E.I. Patsera, E. A. Levashov, B. Ries, Near-nano and coarse-grain WC powders obtained by the self-propagating high-temperature synthesis and cemented carbides on their basis. Part I: Structure, composition and properties of WC powders, Int. J. Refract. Met. Hard Mater. 50 (2015) 146–151. [6] W. Liu, X. Song, J. Zhang, F. Yin, G. Zhang, A novel route to prepare ultrafinegrained WC–Co cemented carbides, J. Alloys Compd. 458 (1–2) (2008) 366–371. [7] I. Konyashin, S. Hlawatschek, B. Ries, F. Lachmann, F. Dorn, A. Sologubenko, T. Weirich, On the mechanism of {WC} coarsening in WC–Co hardmetals with various carbon contents, Int. J. Refract. Met. Hard Mater. 27 (2) (2009) 234–243. [8] Y.T. Zhu, A. Manthiram, Influence of processing parameters on the formation of WC-Co nanocomposite powder using a polymer as carbon source, Compos. Part B Eng. 27 (5) (1996) 407–413.
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[9] A. Douy, Polyacrylamide gel: an efficient tool for easy synthesis of multicomponent oxide precursors of ceramics and glasses, Int. J. Inorg. Mater. 3 (7) (2001) 699–707. [10] X. Wang, R. Wang, C. Peng, T. Li, B. Liu, Polyacrylamide gel method: synthesis and property of BeO nanopowders, J. Sol-Gel Sci. Technol. 57 (2) (2011) 115–127. [11] A. Ahmad, K. Mehrdad, Tailoring Size of α-Al2O3 nanopowders via polymeric gel-net method, Iran. Polym. J. 19 (8) (2010) 615–624. [12] S. Urbonaite, L. Hälldahl, G. Svensson, Raman spectroscopy studies of carbide derived carbons, Carbon N. Y 46 (14) (2008) 1942–1947. [13] J.W. van Put, G.J. Witkamp, G.M. van Rosmalen, Formation of ammonium paratungstate tetra- and hexa-hydrate. I: stability, Hydrometallurgy vol. 34 (2) (1993) 187–201. [14] Z.G. Ban, L.L. Shaw, On the reaction sequence of WC–Co formation using an integrated mechanical and thermal activation process, Acta Mater. 49 (2001) 2933–2939.
Please cite this article as: M. Mardali, et al., Acrylamide route for the co-synthesis of tungsten carbide–cobalt nanopowders with additives, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.152i
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Acrylamide route for the co-synthesis of tungsten carbide–cobalt nanopowders with additives Marzie Mardali a,n, Behzad Sadeghi b, Rasoul Sarraf-Mamoory a, Babak Safarbali c a
Department of Materials Engineering, Tarbiat Modares University, P.O.Box: 14115-111, Tehran, Islamic Republic of Iran Young Researchers and Elite club, Neyshabur Branch, Islamic Azad University, Khorasan Razavi, Islamic Republic of Iran c Department of Materials Engineering, Isfahan University of Technology, Isfahan, Islamic Republic of Iran b
art ic l e i nf o
a b s t r a c t
Article history: Received 23 February 2016 Accepted 23 February 2016
In this work, cemented tungsten carbide nano-particles were prepared by a chemical method called acrylamide gel. In this process, first, a xerogel containing tungsten and cobalt oxide particles was synthesized. Then, it was carburized by a hydrogen reduction heating process. Acrylamide and N, N-methylene-bis-acrylamide monomers were used as an in-situ carbon. Ammonium meta tungstate (NH4)6H2W12O40 xH2O, and cobalt nitrate Co(NO3)2 6H2O salts were used as the precursor. Both reduction and carburization reactions were carried out simultaneously and the formation of the intermediate phases of W2C, Co3W3C, and Co6W6C led to decrease in the activation barrier. Transactions of reduction and carburization processes were studied by X-ray diffraction analysis at various temperatures. Accordingly, tungsten carbide phase formation was completed at 1100 °C. The formation of W–C and V–C bonds was verified by Raman spectroscopy. SEM images showed the average nano particle size of 50 nm. & 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: Tungsten carbide-cobalt Nanopowders Vanadium carbide Hydrogel In-situ carbon source
1. Introduction Bulk cemented tungsten carbide is widely used in demanding industrial applications such as cutting tools, mining and oil drilling, blasting, transportation, forestry tools, spray nozzles, sealing grout pumps, etc [1]. High hardness, high toughness, and wear resistance are always desirable for these applications. Furthermore, Nano-crystalline grain structure using nano-sized WC-Co powders promise dramatic improvement of these properties [2]. Currently, there are many methods which are used to carburize tungsten oxide (WO3) powder for the preparation of cemented tungsten carbide nanoparticles, such as, carbothermal hydrogen reduction [3], chemical vapor condensation by methane gas [4], self-propagating high-temperature synthesis (SHS or combustion synthesis) [5,6]. The most important disadvantage of these methods was the particle agglomeration during carbonization in high temperatures. Earlier attempts to use in-situ carbon sources took place in former [6]. It is well known that additional carbon remaining in the products causes the suppression of grain growth inhibition and high concentration of tungsten dissolved in the binder [7]. To avoid the presence of excess carbon in compounds, n
Corresponding author. E-mail address: [email protected] (M. Mardali).
in some procedures, organic compounds are used as carbon sources [8,9]. Here we try to use acrylamide gel as a source of carbon as well as network steric stabilizer for nano particles. Acrylamide gel is a new method to the synthesis of Tungsten carbide-Cobalt powders.It has the advantages such as molecularlevel combination that leads a homogeneous composition, not requiring advanced apparatus, inexpensive salts and organic materials as precursors compared with metal alcoxides, and good controlling of particle size by the polymeric net [9–11]. We could also add vanadium carbide (VC) to WC–Co powders as a grain growth inhibitor by this simple method. Therefore, homogeneity of the additive is given by this method that has more effect on grain growth suppression. In fact, in the hydrogel method, polymeric net plays a spatial precision stabilization role in nanoscale dimensions and is an in-situ carbon source for carburization. This polymeric gel is formed by co-polymerization of acrylamide (as monomer) and N,N-methylene-bis-acrylamide (as cross linker). Polymerization reaction initiates by the addition of ammonium persulphate through the production of free radicals and reaction with inactive monomers. The resulting gel is porous and size of its pores depends on polymerization conditions. The main difference between this method and sol-gel is in the use of an external agent as gully maker [9–11]. The purpose of the present work is to obtain both WC and cobalt metal nanopowders simultaneously using polymeric
http://dx.doi.org/10.1016/j.ceramint.2016.02.152 0272-8842/& 2016 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: M. Mardali, et al., Acrylamide route for the co-synthesis of tungsten carbide–cobalt nanopowders with additives, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.152i
M. Mardali et al. / Ceramics International ∎ (∎∎∎∎) ∎∎∎–∎∎∎
2
monomers as a carbon source.
2. Experimental procedure The procedure used to prepare WC-Co with VC additive nanocomposite particles can be divided into 3 subsequent stages: solution, gelation, and calcination. First acrylamide (AM) (C2H3CoNH2, Merck, Germany) and N,N′-methylene-bis-acrylamide (MBAM) (CH2CH3CONH)CH2, Merck, Germany) monomers were easily dissolved in the mixture of aqueous solutions of ammonium meta-tungstate (AMT) ((NH4)6H2W12O40 xH2O, SigmaAldrich), cobalt nitrate (Co(NO3)2 6H2O, Merck, Germany), and ammonium meta-vanadate (NH4VO3, Merck, Germany). The concentration of monomers solution was 1.2 M. Then, the resulting solution was stirred completely for 45 min and gradually heated in a water bath at 75 °C to obtain a transparent pink sol. When the temperature reached about 75 °C, 2 mL of 0.2 M ammonium persulphate aqueous solution ((NH4)2S2O8(APS), 10 wt%, Merck, Germany) was added to the mixture as the initiator. An orange semitransparent gel was rapidly produced. The gel was dried at 100 °C for 24 h to yield a xerogel. At the next stage, the obtained xerogel was grinded to form a homogenous powder. The powders were calcined at 900 and 1000 ̊ C under a hydrogen atmosphere for 1 h. To analyze the thermal decomposition and reduction process, a thermogravimetric (TG)/differential thermal analysis (DTA) experiment was carried out in hydrogen gas. The flow rate for each gas was 50 mL/min, the heating rate was 10 ̊ C/ min, and the temperature was varied from room temperature to 1100 °C. The structure of the powder was examined at room temperature by X-ray diffraction (XRD) with Cu-Kα radiation (AXS-D8, Holland). The morphology and particle size of the powder were examined using FE-SEM (F4160, Hitachi, Japan). The formations of V–C and W–C bonds were proved by Raman Spectroscopy (second harmonic of Nd:Yag laser with λ ¼532 nm).
3. Results and discussion Fig. 1(a) shows the Raman Spectroscopy surveys for V–C and W–C bonds formation in the prepared powder. As it can be seen the W–C bond position is at 337 cm 1. Raman shift of W¼C bond is also indicated at 937 cm 1. According to Fig. 1(a) as a reference, D peak of W–C bond is at 1340 cm 1 and moreover D peak of V–C
bond is at 1360 cm 1 [12]. The asymmetric Raman shift at 1353 cm 1 is assigned to the combination of D peaks of V–C and W–C bonds in Fig. 1(b). The unsymmetrical shift at 1581 cm 1 is also referred to the blend of G peaks of W–C and V–C bonds. Decomposition of these peaks is difficult due to their proximity. These results show that the reduction process is done completely because there was no oxide compounds shift. Also, X-ray diffraction pattern was used to characterzation of the phase transformation of the WC-10% Co nanocomposite powder during the reduction processing at 900 and 1100 °C (Fig. 2). It exhibits characterizing peaks of tungsten carbide, the intermediate phases and Cobalt at 900 °C. It seems that WC phase is a dominant phase. By increasing temperature to 1100 °C, the peaks of intermediate phases are eliminated and only WC and Co phase remains with a small amount of W2C phase. The average crystalline size is estimated from the Scherrer equation. This value is 32 nm at 1100 °C. Based on diffraction thermal analysis in Fig. 3, the first peak is related to the water evaporation at 100 °C. The weight loss in rang of 250–750 °C accompanied by the release of heat that is Due to exit of structural water and organic compounds from the system. The drastic change of thermal curve at 280 °C is the shift of process type from endothermic to exothermic. It seems that in addition of the crystallization of amorphous phases, ignition has taken place. This claim has been proven in the thermos gravimeteri analysis diagram (Fig. 3a). It shows the maximum weight change at 280 °C. Oxygen for combustion provided of organic compounds and mineral salts sources. The decomposition of organic phases starts in the range of 250–300 °C. Despite the removal of some of the organic compounds in the process of restoration, the carbon content is enough to carburizing of the powder. It seems that in this stage, stack of active carbon remains in the adjacency of tungsten and cobalt ions through the pores of xerogel. It makes the carburizing at higher temperatures. In the range of 450–550 °C a relatively wide exothermic peak can be seen that is due to reduction of Cobalt Oxide. At 750 °C with exit of remaining organic compounds from the system, an endothermic peak is observed in the graph. This process can also be found from the weight changes in TG diagram. In the DTA diagram, a relatively wide exothermic peak is observed in the temperature range of 900–1100 °C that shows the carburization and reduction of products. In practice, due to two simultaneous reduction and carburization process, showing a known distance between them is difficult.
Fig. 1. (a) Reference Raman spectra for some carbides [12] (b) Raman spectrum of sample sintered at 1000 °C.
Please cite this article as: M. Mardali, et al., Acrylamide route for the co-synthesis of tungsten carbide–cobalt nanopowders with additives, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.152i
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Fig. 2. X-ray diffraction pattern of WC- Co reduced at (a) 900 °C and (b)1100 °C.
Fig. 4. Scanning electron microscope image of the sample calcined in the magnification of 1000.
Fig. 3. (a) Thermo gravimeteri analysis diagram and (b) Differential thermal analysis diagram.
As the results showed, In the temperature range of 300–750 °C organic phases removed from system to form of CO, CO2 and H2O gases. Product has a very high specific surface area due to gas emissions from raw gel resulted in a highly porous network. The increased surface area allows reduction and carburization at a lower temperature and therefore obtained much finer particles. Fig. 4 shows a SEM image of calcined sample with porous network. The Micrographs of the final powders calcined at 900 and 1100 °C are shown in Fig. 5 wherein morphology of the powders is regular and the particle size distribution was homogenous. In Fig. 5(a), the average particle size is about 50 nm. The EDX diagram of the sample reduced at 1100 °C is indicated in Fig. 6. The absence of any oxygen element and the presence of carbon in Table1 show that reduction processes are done successfully. Low weight percentage of carbon element is due to the absorption of its characterized X-ray beam by the detector.
The mechanism of polymeric cell formation in polyacrylamide gel is demonstrated in Fig. 7. Then, ammonium meta-tungstate and cobalt nitrate are dissolved into the water by the following reactions [13]: (NH4)6H2W12O40 þ H2O-H2W12O40 6 þ6NH4 þ
(1)
H2W12O40 6 þH2O-H2W6O20 6
(2)
H2W6O20 6 þ H2O-6WO4 2 þ6H þ
(3)
Co(NO3)2 þ H2O -Co(NO3) þ þNO3–
(4)
Co(NO3) þ þH2O-Co þ 2 þNO3
(5)
During the studies, electronegativity of Cobalt ions is more than hydrogen ions. So it seems that the particles in the initial cell contain a mixture of cobalt and tungsten ions. The structure of polyacrylamide gel is a net like and the viscosity of the solution containing WO4 2 and Co þ 2 ions is relatively high. Therefore, the velocity of the ions is limited, and they are confined in the polymeric cells. These mechanisms cause the
Please cite this article as: M. Mardali, et al., Acrylamide route for the co-synthesis of tungsten carbide–cobalt nanopowders with additives, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.152i
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4
Fig. 5. FE-SEM photographs of WC–Co nano powders reduced at (a) 900 °C, (b) 1100 °C.
Fig. 6. EDX diagram of the sample reduced at 1100 °C.
Fig. 7. Mechanism of polymeric cell formation in polyacrylamide gel [10].
Table1 Elemental characterization of calcined sample at 1100 °C.
4. Conclusions
Elt
Lin e
Int
Error
K
Kr
wt%
C Co W Au
Ka Ka La La
16.3 98.8 255.3 71.4
11.3554 0.6230 0.6230 0.6230
0.0199 0.0533 0.6383 0.2885 1.000
0.0178 0.0477 0.5712 0.2581 0.8948
7.69 4.02 60.66 27.62 100.00
decrease of agglomeration particles during drying and calcination process. The subsequent carburization sequence appeared as W-Co6W6C-Co3W3C-W2C-WC. The proposed route for the carburization of W was according to the following equations: 6W þ6 Co þ C ¼Co6W6C
(6)
Co6W6C þC ¼2Co3W3C
(7)
2Co3W3C þC ¼3 W2C þ6Co
(8)
As evident, cobalt plays the role of a catalyst for this process [14].
In this study, the hydrogel new method for WC-10wt% Co nanocomposite powders was reported. Controlling the size, reduction, and carburization was carried out in one step by this method. During the process, acrylamide gel was an in-situ carbon source and carburization reaction finished at 1100 °C with 32 nm mean crystallite size, and the average nanoparticles size of WC-10wt% Co is about 50 nm. The monomer/salt ratio of 3/1 provided the carbon required to carbonization process and no free carbon was remained.
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Please cite this article as: M. Mardali, et al., Acrylamide route for the co-synthesis of tungsten carbide–cobalt nanopowders with additives, Ceramics International (2016), http://dx.doi.org/10.1016/j.ceramint.2016.02.152i