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Synthesis of nanocrystalline molybdenum by hydrogen reduction of mechanically activated MoO3
Int. Journal of Refractory Metals and Hard Materials 30 (2012) 128–132
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / I J R M H M
Synthesis of nanocrystalline molybdenum by hydrogen reduction of mechanically activated MoO3 M. Saghafi a, S. Heshmati-Manesh a, A. Ataie a,⁎, A.A. Khodadadi b a b
School of Metallurgy and Materials Engineering, University of Tehran, Tehran, Iran School of Chemical Engineering, University of Tehran, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 4 May 2011 Accepted 11 July 2011 Keywords: Molybdenum Nanocrystalline Mechanical activation Hydrogen reduction
a b s t r a c t Nanocrystalline molybdenum with a mean crystallite size of 50 nm was synthesized by mechanical activation of MoO3 powder and its subsequent hydrogen reduction. MoO3 powder was severely activated in a high energy planetary ball mill under a pure argon atmosphere. Temperature-programmed reduction (TPR) by hydrogen was used to investigate hydrogen reduction behavior of the powder samples activated for 5 and 20 h. It was found that by increasing the activation time, the peak temperature for the reduction was shifted slightly to lower temperatures and the peak for the reduction of MoO3 to MoO2 was completely separated from the one for the reduction of MoO2 to molybdenum. In order to evaluate the effect of mechanical activation on the reduction behavior of MoO3, the initial micron-sized powder and the sample activated for 20 h were reduced at similar conditions. It was found that the activated sample with finer particles was reduced faster than the un-milled sample. Hydrogen reduction of the non-activated MoO3 produced a very fine grained molybdenum powder but the crystallite size changes of the sample activated for 20 h was negligible during reduction. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Molybdenum and its alloys have been widely used in the chemical, metallurgical and aerospace industries [1]. High melting point, low thermal-expansion coefficient, and high thermal/electrical conductivity make molybdenum an important material for power semiconductor components, glass-melting electrodes, and high-temperature structural parts [2]. Nanosized molybdenum powder has been recognized to possess more attractive properties and therefore, its synthesis has been investigated widely in recent years. Molybdenum powder could be used as a raw material to produce nanostructured tool steel and MoSi2 [1]. Another advantage of nanosized molybdenum is its ability for consolidation. High sintering temperatures in a range of 1800– 2000 °C, with long sintering times, are required for densification of commercial molybdenum powder to above 90% of its theoretical density [3]. Recently, densification using nanosized and ultrafine molybdenum powder in high compaction pressure has been proposed to enhance the sinter-ability of molybdenum [3–5]. Generally, it has been reported that fine particle size induces enhanced sinterability because diffusion in solid state sintering is very sensitive to particle size and surface area [3]. ⁎ Corresponding author at: School of Metallurgy and Materials Engineering, University of Tehran, P.O. Box 14395-553, Tehran, Iran. Tel.: + 98 82084084; fax: + 98 88006076. E-mail address: [email protected] (A. Ataie). 0263-4368/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2011.07.014
Solid state reduction of MoO3 by various metals has been investigated earlier [6]. When metal oxides react with carbon or hydrogen one of the reaction products would be gaseous and could be readily separated from the solid reaction products. Reduction of MoO3 by carbon using high energy ball milling and subsequent heat treatment has been investigated in an earlier work and metallic molybdenum powder with oxygen and carbon impurity synthesized [7]. Reduction of MoO3 by hydrogen is one of the methods used to obtain metallic molybdenum of high purity. In this paper, the effect of mechanical activation on the reduction behavior of MoO3 by hydrogen has been systematically investigated.
2. Experimental procedure Starting material was commercially pure MoO3 (99.5%, 1–15 μm). It was activated in a high-energy planetary ball mill with a rotation speed of 300 rpm at room temperature in an argon atmosphere for 5 and 20 h. The ball to powder mass ratio was 20:1. The vial and balls were both made of hardened chromium steel. The as-received MoO3 powder and the sample activated for 20 h were reduced in a tube furnace with quartz reactor and with a heating rate of 10 °C/min at H2 in Ar flow rate. The Ar flow rate was fixed to 30 sccm in all experiments as carrier gas and to stop the reaction, the gas atmosphere was changed to only argon gas with a subsequent cooling down of the sample.
M. Saghafi et al. / Int. Journal of Refractory Metals and Hard Materials 30 (2012) 128–132
129
Fig. 1. SEM images of (a) as-received and (b) 20 h activated MoO3 samples.
The morphology and phase identification of the products were examined by SEM (Cam Scan MV2300) and XRD (Philips Xpert pro) using Co kα radiation (λ = 1.78892 Å), respectively. The mean crystallite size of the samples was calculated using Scherrer formula [8]. H2-temperature programmed reduction (TPR) tests were carried out using a Quantachrome CHEMBET-3000 apparatus equipped with a TCD detector. The TPR experiments were performed on 20 mg of the samples activated for the 5 and 20 h under 10 sccm of 7.0% H2 in Ar. Prior to reduction to eliminate adsorbed gases on the surface of the powders, the samples were treated at 150 °C for 1 h in 17 sccm Ar flow and allowed to cool. Finally, temperature was increased from 50 to 850 °C with a heating rate of 10 °C/min. The consumption of H2 was quantitatively measured by time integration of the TPR profiles.
There are two reactions for the hydrogen reduction of the MoO3 [3,9]. The first reaction is assigned to the reduction process of MoO3 to MoO2 which occurs over a temperature range of 450–650 °C. The second one occurs at a temperature range of 650–800 °C, is assigned to the reduction of MoO2 to molybdenum. Both reactions take place in two distinct stages [3]. MoO3 þ H2 →MoO2 þ H2 O
ð1Þ
MoO2 þ 2H2 →Mo þ 2H2 O
ð2Þ
Fig. 1 shows SEM micrographs of the starting MoO3 powder and the one activated for 20 h. Initial MoO3 powder particles exhibited lath type morphology and particle size was homogeneously decreased after 20 h activation. Fig. 2 shows the non-isothermal H2-TPR profile for the hydrogen reduction process of the MoO3 powder samples activated for 5 and 20 h.
As it is shown in Fig. 2, by increasing the activation time to 20 h, the reduction temperature was slightly shifted to lower temperatures because of particle size refinement. Also, the peak for the reduction of MoO3 to MoO2 was completely separated from that for the reduction of MoO2 to molybdenum. On the basis of the H2-TPR results, the suitable synthesis condition to extract molybdenum powder out of MoO3 was found to be an activation time of 20 h followed by hydrogen reduction. The first and second reduction temperatures to obtain MoO2 and Mo were selected 550 °C and 850 °C, respectively. Fig. 3 shows the XRD patterns of the as-received MoO3 sample reduced at 30 sccm flow rate of hydrogen at 550 °C for 30 and 60 min. Intermediate phase of Mo4O11 forms during the first stage reduction. As it is shown, MoO3 was completely reduced to MoO2 after 60 min.
Fig. 2. H2-TPR profile of 5 and 20 h activated MoO3 samples.
Fig. 3. XRD patterns of the as-received MoO3 sample reduced at 30 sccm flow rate of hydrogen at 550 °C.
3. Results and discussion
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M. Saghafi et al. / Int. Journal of Refractory Metals and Hard Materials 30 (2012) 128–132
Fig. 4. XRD patterns of the as-received MoO3 sample after a two-stage reduction at 30 sccm flow rate of hydrogen at 550 °C for 60 min and at 60 sccm flow rate of hydrogen at 850 °C for 30, 50 and 70 min.
Fig. 4 shows the XRD patterns of the as-received MoO3 sample after a two stages reduction at 30 sccm flow rate of hydrogen at 550 °C for 60 min and at a hydrogen flow rate of 60 sccm at 850 °C for 30, 50 and 70 min. As it is shown, MoO2 was completely reduced to molybdenum after 70 min. This two-stage heat treatment was chosen since the melting point of MoO3 is 795 °C. If the reduction of MoO3 is not completed at a temperature below 795 °C, it will end up as a fused mass at elevated temperature of 850 °C [10]. As it shown in Fig. 3 some un-reacted MoO3 remains after 30 min of reduction. Therefore, a continuous increase of temperature during the reduction process is expected to be troublesome if the reduction of MoO3 to MoO2 is not completed at temperatures below 795 °C and hence, a two stage reductions would be necessary. Figs. 5 and 6 show the SEM images of the MoO2 and molybdenum powders synthesized by hydrogen reduction of the as-received MoO3 sample.
Fig. 5. SEM images of the as-received MoO3 sample reduced at 30 sccm flow rate of hydrogen at 550 °C.
Fig. 6. SEM images of the as-received MoO3 sample after a two-stage reduction at 30 sccm flow rate of hydrogen at 550 °C for 60 min and at 60 sccm flow rate of hydrogen at 850 °C for 70 min.
M. Saghafi et al. / Int. Journal of Refractory Metals and Hard Materials 30 (2012) 128–132
Fig. 7. XRD patterns of 20 h activated MoO3 sample reduced at 30 sccm flow rate of hydrogen.
Fig. 8. XRD patterns of 20 h activated MoO3 sample after a two-stage reduction at 30 sccm flow rate of hydrogen at 550 °C for 30 min and at 60 sccm flow rate of hydrogen at 850 °C for 30 and 50 min.
By extending the hydrogen reduction of the as-received MoO3, the particle size of the reduced powders decreased. This may be explained by the fact that the transformation steps for the reaction of MoO3 to MoO2 take place via chemical vapor transport (CVT) and reduction of
131
MoO2 to molybdenum primarily occurs by oxygen transport and is partially dependent on the CVT process [3,11]. It was also reported that nucleation rate of new phase is more than its growth rate during hydrogen reduction of molybdenum oxide at the lowest possible temperatures [12]. Majumdar et al. [9] also reported that many cracks and fissures are developed from the surface to center of molybdenum during hydrogen reduction of MoO2. This behavior was predicted by the crackling core model (CCM). Formation of surface cracks is believed to be due to the tensile stress generated due to a volume decrease during oxygen removal. These cracks also decrease the particle size of products during hydrogen reduction of molybdenum oxide. Fig. 7 shows the XRD patterns of 20 h activated MoO3 powder sample reduced at 30 sccm flow rate of hydrogen at 550 °C for 10 and 30 min. It is shown that MoO3 was completely reduced to MoO2 after 30 min. The XRD results show that complete reduction of MoO3 to MoO2 in the activated sample took place in shorter periods of time as compare to the as-received sample because of the particles refinement and shortening of diffusion paths. Fig. 8 shows the XRD patterns of 20 h activated MoO3 sample after a two-stage reduction; reduced in 30 sccm flow rate of hydrogen at 550 °C for 30 min and at 60 sccm flow rate of hydrogen at 850 °C for 30 and 50 min. It is revealed that MoO2 was completely reduced to Mo after 50 min. The XRD results revealed that mechanical activation accelerates the reduction process of MoO3 to Mo. Nano size particles of MoO2 and Mo phases could be observed in the SEM images of 20 h activated MoO3 sample after reduction at 550 and 850 °C, respectively (see Figs. 9 and 10). Table 1 shows crystallite and particle size of the molybdenum powders synthesized by hydrogen reduction of the 20 h activated and as-received MoO3 sample. As it is shown synthesized molybdenum powders by hydrogen reduction of the 20 h activated MoO3 sample had smaller crystallite and particle size rather than as-received MoO3 sample. 4. Conclusion Molybdenum nano particles with a mean particle size of almost 50 nm was synthesized from 20 h activated sample after it was subjected to reduction at a hydrogen flow rate of 30 sccm at 550 °C for 30 min and at a hydrogen flow rate of 60 sccm at 850 °C for 50 min. It was found that the 20 h activated MoO3 sample with finer particles
Fig. 9. SEM images of 20 h activated MoO3 sample reduced at 30 sccm flow rate of hydrogen.
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Fig. 10. SEM images of 20 h activated MoO3 sample after a two-stage reduction at 30 sccm flow rate of hydrogen at 550 °C for 30 min and at 60 sccm flow rate of hydrogen at 850 °C for 50 min.
Table 1 Crystallite and particle sizes of the synthesized molybdenum powders. Reduced MoO3 powder
Crystallite size (from Scherrer formula)
Particle size (from SEM images)
20 h activated As-received
50 nm 87 nm
133 nm 320 nm
was reduced faster comparing with the as-received sample. Hydrogen reduction of the non-activated MoO3, produced a very fine grained molybdenum powder. Acknowledgments The financial support of this work by the Research Council of the University of Tehran and Iranian Nanotechnology Initiative is gratefully acknowledged. Also, the cooperation of Pars Molybdenum Company for supplying MoO3 is appreciated. References [1] Liu B, Gu H, Chen Q. Preparation of nanosized Mo powder by microwave plasma chemical vapor deposition method. Mater Chem Phys 1999;59:204–9.
[2] Huang HS, Hwang KS. Deoxidation of molybdenum during vacuum sintering. Metall Mater Trans A 2002;33:657–64. [3] Kim GS, Lee YJ, Kim DG, Kim YD. Consolidation behavior of Mo powder fabricated from milled Mo oxide by hydrogen-reduction. J Alloys Compd 2008;454:327–30. [4] Kim GS, Kim HG, Kimb DG, Oh ST, Suk MJ, Kim YD. Densification behavior of Mo nanopowders prepared by mechanochemical processing. J Alloys Compd 2009;469:401–5. [5] Garg P, Park SJ, German RM. Effect of die compaction pressure on densification behavior of molybdenum powders. Int J Refract Met Hard Mater 2007;25:16–24. [6] Kirshenbaum AD, Beardell AJ. Thermal analysis of the reaction of molybdenum trioxide with various metals. Thermochim Acta 1972;4:239–48. [7] Saghafi M, Ataie A, Heshmati-Manesh S. Effects of mechanical activation of MoO3/ C powder mixture in the processing of nano-crystalline molybdenum. Int J Refract Met Hard Mater 2011;29:419–23. [8] Cullity BD, Stock SR. Elements of X-ray Diffraction. 3rd ed. Upper Saddle River NJ: Prentice Hall; 2001. [9] Majumdar S, Sharma IG, Samajdar I, Bhargava P. Kinetic studies on hydrogen reduction of MoO3 and morphological analysis of reduced Mo powder. Metall Mater Trans B 2008;39:431–8. [10] Gupta CK. Chemical metallurgy principles and practice. India: Wiley-VCH Verlag GmbH & Co; 2003. [11] Schulmeyer WV, Ortner HM. Mechanisms of the hydrogen reduction of molybdenum oxides. Int J Refract Met Hard Mater 2002;20:261–9. [12] Radchenko PY, Panichkina VV, Radchenko OG. Properties of molybdenum powders obtained by reduction in moving beds. Powder Metall Met Ceram 1999;38:429–35.
Contents lists available at ScienceDirect
Int. Journal of Refractory Metals and Hard Materials j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / I J R M H M
Synthesis of nanocrystalline molybdenum by hydrogen reduction of mechanically activated MoO3 M. Saghafi a, S. Heshmati-Manesh a, A. Ataie a,⁎, A.A. Khodadadi b a b
School of Metallurgy and Materials Engineering, University of Tehran, Tehran, Iran School of Chemical Engineering, University of Tehran, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 4 May 2011 Accepted 11 July 2011 Keywords: Molybdenum Nanocrystalline Mechanical activation Hydrogen reduction
a b s t r a c t Nanocrystalline molybdenum with a mean crystallite size of 50 nm was synthesized by mechanical activation of MoO3 powder and its subsequent hydrogen reduction. MoO3 powder was severely activated in a high energy planetary ball mill under a pure argon atmosphere. Temperature-programmed reduction (TPR) by hydrogen was used to investigate hydrogen reduction behavior of the powder samples activated for 5 and 20 h. It was found that by increasing the activation time, the peak temperature for the reduction was shifted slightly to lower temperatures and the peak for the reduction of MoO3 to MoO2 was completely separated from the one for the reduction of MoO2 to molybdenum. In order to evaluate the effect of mechanical activation on the reduction behavior of MoO3, the initial micron-sized powder and the sample activated for 20 h were reduced at similar conditions. It was found that the activated sample with finer particles was reduced faster than the un-milled sample. Hydrogen reduction of the non-activated MoO3 produced a very fine grained molybdenum powder but the crystallite size changes of the sample activated for 20 h was negligible during reduction. © 2011 Elsevier Ltd. All rights reserved.
1. Introduction Molybdenum and its alloys have been widely used in the chemical, metallurgical and aerospace industries [1]. High melting point, low thermal-expansion coefficient, and high thermal/electrical conductivity make molybdenum an important material for power semiconductor components, glass-melting electrodes, and high-temperature structural parts [2]. Nanosized molybdenum powder has been recognized to possess more attractive properties and therefore, its synthesis has been investigated widely in recent years. Molybdenum powder could be used as a raw material to produce nanostructured tool steel and MoSi2 [1]. Another advantage of nanosized molybdenum is its ability for consolidation. High sintering temperatures in a range of 1800– 2000 °C, with long sintering times, are required for densification of commercial molybdenum powder to above 90% of its theoretical density [3]. Recently, densification using nanosized and ultrafine molybdenum powder in high compaction pressure has been proposed to enhance the sinter-ability of molybdenum [3–5]. Generally, it has been reported that fine particle size induces enhanced sinterability because diffusion in solid state sintering is very sensitive to particle size and surface area [3]. ⁎ Corresponding author at: School of Metallurgy and Materials Engineering, University of Tehran, P.O. Box 14395-553, Tehran, Iran. Tel.: + 98 82084084; fax: + 98 88006076. E-mail address: [email protected] (A. Ataie). 0263-4368/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijrmhm.2011.07.014
Solid state reduction of MoO3 by various metals has been investigated earlier [6]. When metal oxides react with carbon or hydrogen one of the reaction products would be gaseous and could be readily separated from the solid reaction products. Reduction of MoO3 by carbon using high energy ball milling and subsequent heat treatment has been investigated in an earlier work and metallic molybdenum powder with oxygen and carbon impurity synthesized [7]. Reduction of MoO3 by hydrogen is one of the methods used to obtain metallic molybdenum of high purity. In this paper, the effect of mechanical activation on the reduction behavior of MoO3 by hydrogen has been systematically investigated.
2. Experimental procedure Starting material was commercially pure MoO3 (99.5%, 1–15 μm). It was activated in a high-energy planetary ball mill with a rotation speed of 300 rpm at room temperature in an argon atmosphere for 5 and 20 h. The ball to powder mass ratio was 20:1. The vial and balls were both made of hardened chromium steel. The as-received MoO3 powder and the sample activated for 20 h were reduced in a tube furnace with quartz reactor and with a heating rate of 10 °C/min at H2 in Ar flow rate. The Ar flow rate was fixed to 30 sccm in all experiments as carrier gas and to stop the reaction, the gas atmosphere was changed to only argon gas with a subsequent cooling down of the sample.
M. Saghafi et al. / Int. Journal of Refractory Metals and Hard Materials 30 (2012) 128–132
129
Fig. 1. SEM images of (a) as-received and (b) 20 h activated MoO3 samples.
The morphology and phase identification of the products were examined by SEM (Cam Scan MV2300) and XRD (Philips Xpert pro) using Co kα radiation (λ = 1.78892 Å), respectively. The mean crystallite size of the samples was calculated using Scherrer formula [8]. H2-temperature programmed reduction (TPR) tests were carried out using a Quantachrome CHEMBET-3000 apparatus equipped with a TCD detector. The TPR experiments were performed on 20 mg of the samples activated for the 5 and 20 h under 10 sccm of 7.0% H2 in Ar. Prior to reduction to eliminate adsorbed gases on the surface of the powders, the samples were treated at 150 °C for 1 h in 17 sccm Ar flow and allowed to cool. Finally, temperature was increased from 50 to 850 °C with a heating rate of 10 °C/min. The consumption of H2 was quantitatively measured by time integration of the TPR profiles.
There are two reactions for the hydrogen reduction of the MoO3 [3,9]. The first reaction is assigned to the reduction process of MoO3 to MoO2 which occurs over a temperature range of 450–650 °C. The second one occurs at a temperature range of 650–800 °C, is assigned to the reduction of MoO2 to molybdenum. Both reactions take place in two distinct stages [3]. MoO3 þ H2 →MoO2 þ H2 O
ð1Þ
MoO2 þ 2H2 →Mo þ 2H2 O
ð2Þ
Fig. 1 shows SEM micrographs of the starting MoO3 powder and the one activated for 20 h. Initial MoO3 powder particles exhibited lath type morphology and particle size was homogeneously decreased after 20 h activation. Fig. 2 shows the non-isothermal H2-TPR profile for the hydrogen reduction process of the MoO3 powder samples activated for 5 and 20 h.
As it is shown in Fig. 2, by increasing the activation time to 20 h, the reduction temperature was slightly shifted to lower temperatures because of particle size refinement. Also, the peak for the reduction of MoO3 to MoO2 was completely separated from that for the reduction of MoO2 to molybdenum. On the basis of the H2-TPR results, the suitable synthesis condition to extract molybdenum powder out of MoO3 was found to be an activation time of 20 h followed by hydrogen reduction. The first and second reduction temperatures to obtain MoO2 and Mo were selected 550 °C and 850 °C, respectively. Fig. 3 shows the XRD patterns of the as-received MoO3 sample reduced at 30 sccm flow rate of hydrogen at 550 °C for 30 and 60 min. Intermediate phase of Mo4O11 forms during the first stage reduction. As it is shown, MoO3 was completely reduced to MoO2 after 60 min.
Fig. 2. H2-TPR profile of 5 and 20 h activated MoO3 samples.
Fig. 3. XRD patterns of the as-received MoO3 sample reduced at 30 sccm flow rate of hydrogen at 550 °C.
3. Results and discussion
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M. Saghafi et al. / Int. Journal of Refractory Metals and Hard Materials 30 (2012) 128–132
Fig. 4. XRD patterns of the as-received MoO3 sample after a two-stage reduction at 30 sccm flow rate of hydrogen at 550 °C for 60 min and at 60 sccm flow rate of hydrogen at 850 °C for 30, 50 and 70 min.
Fig. 4 shows the XRD patterns of the as-received MoO3 sample after a two stages reduction at 30 sccm flow rate of hydrogen at 550 °C for 60 min and at a hydrogen flow rate of 60 sccm at 850 °C for 30, 50 and 70 min. As it is shown, MoO2 was completely reduced to molybdenum after 70 min. This two-stage heat treatment was chosen since the melting point of MoO3 is 795 °C. If the reduction of MoO3 is not completed at a temperature below 795 °C, it will end up as a fused mass at elevated temperature of 850 °C [10]. As it shown in Fig. 3 some un-reacted MoO3 remains after 30 min of reduction. Therefore, a continuous increase of temperature during the reduction process is expected to be troublesome if the reduction of MoO3 to MoO2 is not completed at temperatures below 795 °C and hence, a two stage reductions would be necessary. Figs. 5 and 6 show the SEM images of the MoO2 and molybdenum powders synthesized by hydrogen reduction of the as-received MoO3 sample.
Fig. 5. SEM images of the as-received MoO3 sample reduced at 30 sccm flow rate of hydrogen at 550 °C.
Fig. 6. SEM images of the as-received MoO3 sample after a two-stage reduction at 30 sccm flow rate of hydrogen at 550 °C for 60 min and at 60 sccm flow rate of hydrogen at 850 °C for 70 min.
M. Saghafi et al. / Int. Journal of Refractory Metals and Hard Materials 30 (2012) 128–132
Fig. 7. XRD patterns of 20 h activated MoO3 sample reduced at 30 sccm flow rate of hydrogen.
Fig. 8. XRD patterns of 20 h activated MoO3 sample after a two-stage reduction at 30 sccm flow rate of hydrogen at 550 °C for 30 min and at 60 sccm flow rate of hydrogen at 850 °C for 30 and 50 min.
By extending the hydrogen reduction of the as-received MoO3, the particle size of the reduced powders decreased. This may be explained by the fact that the transformation steps for the reaction of MoO3 to MoO2 take place via chemical vapor transport (CVT) and reduction of
131
MoO2 to molybdenum primarily occurs by oxygen transport and is partially dependent on the CVT process [3,11]. It was also reported that nucleation rate of new phase is more than its growth rate during hydrogen reduction of molybdenum oxide at the lowest possible temperatures [12]. Majumdar et al. [9] also reported that many cracks and fissures are developed from the surface to center of molybdenum during hydrogen reduction of MoO2. This behavior was predicted by the crackling core model (CCM). Formation of surface cracks is believed to be due to the tensile stress generated due to a volume decrease during oxygen removal. These cracks also decrease the particle size of products during hydrogen reduction of molybdenum oxide. Fig. 7 shows the XRD patterns of 20 h activated MoO3 powder sample reduced at 30 sccm flow rate of hydrogen at 550 °C for 10 and 30 min. It is shown that MoO3 was completely reduced to MoO2 after 30 min. The XRD results show that complete reduction of MoO3 to MoO2 in the activated sample took place in shorter periods of time as compare to the as-received sample because of the particles refinement and shortening of diffusion paths. Fig. 8 shows the XRD patterns of 20 h activated MoO3 sample after a two-stage reduction; reduced in 30 sccm flow rate of hydrogen at 550 °C for 30 min and at 60 sccm flow rate of hydrogen at 850 °C for 30 and 50 min. It is revealed that MoO2 was completely reduced to Mo after 50 min. The XRD results revealed that mechanical activation accelerates the reduction process of MoO3 to Mo. Nano size particles of MoO2 and Mo phases could be observed in the SEM images of 20 h activated MoO3 sample after reduction at 550 and 850 °C, respectively (see Figs. 9 and 10). Table 1 shows crystallite and particle size of the molybdenum powders synthesized by hydrogen reduction of the 20 h activated and as-received MoO3 sample. As it is shown synthesized molybdenum powders by hydrogen reduction of the 20 h activated MoO3 sample had smaller crystallite and particle size rather than as-received MoO3 sample. 4. Conclusion Molybdenum nano particles with a mean particle size of almost 50 nm was synthesized from 20 h activated sample after it was subjected to reduction at a hydrogen flow rate of 30 sccm at 550 °C for 30 min and at a hydrogen flow rate of 60 sccm at 850 °C for 50 min. It was found that the 20 h activated MoO3 sample with finer particles
Fig. 9. SEM images of 20 h activated MoO3 sample reduced at 30 sccm flow rate of hydrogen.
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Fig. 10. SEM images of 20 h activated MoO3 sample after a two-stage reduction at 30 sccm flow rate of hydrogen at 550 °C for 30 min and at 60 sccm flow rate of hydrogen at 850 °C for 50 min.
Table 1 Crystallite and particle sizes of the synthesized molybdenum powders. Reduced MoO3 powder
Crystallite size (from Scherrer formula)
Particle size (from SEM images)
20 h activated As-received
50 nm 87 nm
133 nm 320 nm
was reduced faster comparing with the as-received sample. Hydrogen reduction of the non-activated MoO3, produced a very fine grained molybdenum powder. Acknowledgments The financial support of this work by the Research Council of the University of Tehran and Iranian Nanotechnology Initiative is gratefully acknowledged. Also, the cooperation of Pars Molybdenum Company for supplying MoO3 is appreciated. References [1] Liu B, Gu H, Chen Q. Preparation of nanosized Mo powder by microwave plasma chemical vapor deposition method. Mater Chem Phys 1999;59:204–9.
[2] Huang HS, Hwang KS. Deoxidation of molybdenum during vacuum sintering. Metall Mater Trans A 2002;33:657–64. [3] Kim GS, Lee YJ, Kim DG, Kim YD. Consolidation behavior of Mo powder fabricated from milled Mo oxide by hydrogen-reduction. J Alloys Compd 2008;454:327–30. [4] Kim GS, Kim HG, Kimb DG, Oh ST, Suk MJ, Kim YD. Densification behavior of Mo nanopowders prepared by mechanochemical processing. J Alloys Compd 2009;469:401–5. [5] Garg P, Park SJ, German RM. Effect of die compaction pressure on densification behavior of molybdenum powders. Int J Refract Met Hard Mater 2007;25:16–24. [6] Kirshenbaum AD, Beardell AJ. Thermal analysis of the reaction of molybdenum trioxide with various metals. Thermochim Acta 1972;4:239–48. [7] Saghafi M, Ataie A, Heshmati-Manesh S. Effects of mechanical activation of MoO3/ C powder mixture in the processing of nano-crystalline molybdenum. Int J Refract Met Hard Mater 2011;29:419–23. [8] Cullity BD, Stock SR. Elements of X-ray Diffraction. 3rd ed. Upper Saddle River NJ: Prentice Hall; 2001. [9] Majumdar S, Sharma IG, Samajdar I, Bhargava P. Kinetic studies on hydrogen reduction of MoO3 and morphological analysis of reduced Mo powder. Metall Mater Trans B 2008;39:431–8. [10] Gupta CK. Chemical metallurgy principles and practice. India: Wiley-VCH Verlag GmbH & Co; 2003. [11] Schulmeyer WV, Ortner HM. Mechanisms of the hydrogen reduction of molybdenum oxides. Int J Refract Met Hard Mater 2002;20:261–9. [12] Radchenko PY, Panichkina VV, Radchenko OG. Properties of molybdenum powders obtained by reduction in moving beds. Powder Metall Met Ceram 1999;38:429–35.