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In situ synthesis mechanism of Al2O3–Mo nanocomposite by ball milling process
Journal of Alloys and Compounds 477 (2009) 692–695
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
In situ synthesis mechanism of Al2 O3 –Mo nanocomposite by ball milling process A. Heidarpour ∗ , F. Karimzadeh, M.H. Enayati Department of Materials Engineering, Nanotechnology and advance material Institute, Isfahan University of Technology (IUT), Isfahan 84156-83111, Iran
a r t i c l e
i n f o
Article history: Received 12 July 2008 Received in revised form 22 October 2008 Accepted 24 October 2008 Available online 9 December 2008 Keywords: Mechanosynthesis Nanocomposite Characterization Al2 O3 –Mo
a b s t r a c t Al2 O3 –Mo nanocomposites were synthesized by ball milling of aluminum and molybdenum oxide powder mixtures. The structural evaluation of powder particles after different milling times was studied by Xray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and measurement of vial temperature. The molybdenum oxide and aluminum reaction appeared to occur through a rapid combustion reaction process. As a result, an alumina matrix nanocomposite containing molybdenum particulate was formed. In final stage of milling, Mo and alumina had a crystallite size of about 28 nm and 60 nm, respectively. After annealing at 800 ◦ C for 60 min, Mo crystallite size remained constant. However, ␣-alumina crystallite size increased to 120 nm. After annealing a partial transformation of ␣-Al2 O3 into different polymorphic, ␥-Al2 O3 with a crystallite size of 50 nm was observed. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Mechanochemical synthesis (MCS); that is, chemical reactions induced by high-energy ball milling, is one of promising routes for the synthesis of different classes of materials including metals, oxides, salts, organic compounds, etc. in various combinations [1,2]. Mechanochemical reactions involving displacement reactions between a reactive metal and a metal oxide often lead to the formation of a nanocomposite structure [3–5]. Mechanochemical reactions fall into two categories [6]: namely, (a) those which occur during the mechanical activation process in which the reaction enthalpy is highly negative (e.g. adiabatic temperature Tad = 1300–1800 K) and (b) those which occur during subsequent thermal treatment and here the reaction enthalpy is only moderate (e.g. Tad < 1300 K). The first type of reaction takes place in two distinct modes, i.e., a combustion reaction or a progressive reaction. Whenever a reaction is highly exothermic, it can occur abruptly after a certain time of milling and, once started, it proceeds in a self-sustained way. In this case, the reaction requires a given time to begin. This time is called ignition time and, due to the exothermic reaction, can be determined by an increase in temperature. A number of studies have focused on the development of A12 O3 /metal nanocomposites by different routes [7–14]. Matteazzi and Caer synthesized nanometer-sized ␣-A12 O3 –M composite (M = Fe, V, Cr, Mn, Co, Ni, Cu, Zn, Nb, Mo, W, Si) by ball milling of an
∗ Corresponding author. Tel.: +98 311 3915744; fax: +98 311 3912751. E-mail address: [email protected] (A. Heidarpour). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.10.112
appropriate metal oxide and Al [15]. The present study is an effort to provide a future understanding of Al and MoO3 reaction during room-temperature ball milling. Details of phase development and structural evolution are also investigated. The presence of metallic particles in ceramics adds physical properties inherent to the metallic phase, such as electric and thermal conductivity or magnetic properties. This combination of properties makes ceramics–metals composite excellent candidates for electric, optic, and magnetic devices or chemical sensors [16]. Some properties such as wear behavior of Al2 O3 –metal composites are evaluated in order to develop structural and functional applications [17–21]. 2. Materials and methods Mixtures of commercial aluminum powder (99.7%, 30–60 m) and MoO3 powders (99.9%, ∼100 m) were milled to give Al2 O3 –26.6 vol% Mo (48.2 wt.% Mo) and Al2 O3 –15 vol% Mo (31.3 wt.% Mo) nanocomposite. Ball milling was performed in a SPEX8000 type ball mill using hardened chromium steel vial and balls under argon atmosphere. The milling was done with a ball-powder mass ratio of 7:1 without interruption. No process control agent (PCA) was used. X-ray diffraction (XRD) was used to follow the structural changes of powders during milling. A Philips diffractometer (40 kV) with Cu K␣ radiation ( = 0.15406 nm) was used for XRD measurements. The XRD patterns were recorded in the 2 range of 20–100◦ (step size 0.03◦ and time per step 1 s). The microstructure of powder particles was investigated by scanning electron microscopy (SEM) using a Philips XL30 SEM and transmission electron microscopy (TEM) by using a Philips CM200 FEG. The average particle size was obtained from SEM images by using image-analysis software. Isothermal annealing was carried out to study the thermal behavior of milled powders. Powder samples were sealed and then annealed in a conventional tube furnace. The alumina and Mo crystallite sizes were estimated by analyzing broadening of XRD peaks using Williamson–Hall formula [22]. The reaction progress was monitored by a thermocouple attached on the external surface of the vial.
A. Heidarpour et al. / Journal of Alloys and Compounds 477 (2009) 692–695
Fig. 1. XRD patterns of Al–MoO3 powder particles (stoichiometric composition) asreceived and after different milling times.
Fig. 2. XRD patterns of Al–MoO3 –Al2 O3 powder particles (non-stoichiometric composition) as-received and after different milling times.
3. Results and discussions MoO3 and Al reaction with stoichiometric composition gives an Al2 O3 based composite containing 26.6 vol% Mo according to reaction (1) [17]. 2Al(s) + MoO3 (s) = Al2 O3 (s) + Mo(s) G◦ 298 = −915, 000 J/mol;
(1)
H ◦ 298 = −932, 000 J/mol [23]
During milling in room temperature this reaction can thermodynamically occur due to its negative free energy change. H ◦ 298
Fig. 3. TEM image of the Al2 O3 –26.6 vol% Mo nanocomposite after 240 min milling.
693
Fig. 4. Temperature variation of vial during ball milling of two compositions, Al–MoO3 and Al–MoO3 –Al2 O3 .
value for Al–MoO3 reaction indicates that the Al–MoO3 reaction is highly exothermic. The XRD patterns of stoichiometric composition after different milling times are presented in Fig. 1. The diffraction patterns of initial powder mixture show several peaks corresponding to Al and MoO3 . The intensity of MoO3 and Al peaks decreased during milling so that after 130 min of milling time these peaks disappeared on XRD patterns. Meanwhile several additional peaks corresponding to Mo and alumina developed on XRD patterns. In a separate ball milling run a mixture of Al, MoO3 and Al2 O3 powders was ball milled to produce Al2 O3 –15 vol% Mo nanocomposite. Fig. 2 shows XRD patterns of this composition after different milling times. As seen, the reaction of Al and MoO3 was completed after 180 min which is longer than that obtained for stoichiometric composition (Fig. 1). This suggests that the Al–MoO3 reaction extended over a longer period in presence of additional alumina as a diluent. The broadening of the XRD peaks in Figs. 1 and 2 is due to the reduction of the crystallite size as well as the microstrain induced in powder particles. The approach of Williamson and Hall [22] was used in order to separate the two effects of crystallite size and microstrain. After 240 min of milling, Mo and alumina achieved a crystallite size of about 28 nm and 60 nm in stoichiometric composition and 15 nm and 44 nm in non-stoichiometric composition, respectively. The smaller Mo and alumina crystallite size in case of non-stoichiometric composition can be due to the lower heat released in this composition compared to that in stoichiometric composition. Matteazzi and Caer obtained ␣-A12 O3 –M nanocomposite (M = Fe, V, Cr, Mn, Co, Ni, Cu, Zn, Nb, Mo, W, Si) by ball milling
Fig. 5. XRD patterns of Al2 O3 /26.6 vol% Mo as-milled for 240 min and after subsequent annealing at 800 ◦ C for 60 min.
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A. Heidarpour et al. / Journal of Alloys and Compounds 477 (2009) 692–695
Fig. 6. Morphology of as-received (a) Al and (b) MoO3 powder particles.
of a suitable metal oxide and Al [15] using Scherrer formula. They reported a smaller value of about 10 nm for crystallite sizes of alumina and metal. The discrepancies in crystallite size between the present study and those reported by Matteazzi and Caer can be due to the fact that in Scherrer method it is assumed that the whole of broadening is caused by very fine crystallite size and the effect of the lattice microstrain is ignored. The Al2 O3 –26.6 vol% Mo nanocomposite after 240 min milling was studied by TEM. Fig. 3 represents the TEM photograph and related diffraction pattern of the nanocomposite dispersed on a carbon-coated Cu grid. It can be observed that molybdenum phase
(dark region) disperses homogeneously in alumina phase and has dimensions less than 100 nm, and diffraction pattern confirmed that this composite is nanostructure. The variation of temperature of vial during ball milling can provide a future insight into the MoO3 –Al reaction mode. Fig. 4 shows the vial temperature variation with milling time for both compositions. If a reaction is highly exothermic, it can take place abruptly after a certain time of milling; and once started, it proceeds in a self-sustained way [14,24,25]. As seen after 120 min of milling time, the temperature of vials increased rapidly suggesting a combustion reaction between Al and MoO3 . This can be expected
Fig. 7. Morphology of powder particles (stoichiometric composition) after (a) 90 min, (b) 120 min and (c) 240 min of milling times.
A. Heidarpour et al. / Journal of Alloys and Compounds 477 (2009) 692–695
from the adiabatic temperature, Tad , for the reaction between MoO3 and Al, which is 1600 ◦ C [15] and higher than the critical value of 1300 ◦ C proposed by Schaffer and McCormick [25,26]. The combustion reaction between Al and MoO3 is promoted by the dynamically maintained high Al/MoO3 interface areas as well as the short-circuit diffusion path provided by the increasing number of defects such as dislocations and grain boundaries induced during ball milling [27]. It is interesting to note that the presence of extra alumina with respect to stoichiometric composition shifts the peak temperature to longer milling time and also decreases the peak temperature. In fact extra alumina acts as a diluent reducing the adiabatic temperature as well as the ignition time. Fig. 5 shows the XRD patterns of stoichiometric composition powder milled for 240 min before and after annealing at 800 ◦ C for 60 min. After annealing several peaks of ␥-Al2 O3 were appeared on XRD patterns indicating the partial transformation of ␣-Al2 O3 to ␥-Al2 O3 phase during annealing. Mo crystallite size remained unchanged after annealing due to the pinning effect of the Al2 O3 particles. In contrast, after annealing ␣-alumina crystallite size increased to 120 nm. In this case, the crystallite size of ␥-Al2 O3 which was formed during annealing was about 50 nm. The change in morphology of powder particles during milling process are shown in Figs. 6 and 7. As-received Al powder particles had random morphology with a particle size ranging from 30 m to 60 m. As-received MoO3 powder had elongated morphology with a typical length and thickness of 100 m and 20 m, respectively. The average particle size was estimated by using image-analysis tools. In initial stage of milling MoO3 was mixed with Al at the micrometer level and a homogenous and active composite is formed. Therefore, ball milling up to 90 min led to the refinement of powder particles and the average size of powder particle before combustion reaction reached about 2 m (Fig. 7a). Then, the reaction between Al and MoO3 took place abruptly after 120 min of milling time (Fig. 4) and as seen from Fig. 7b after this time, the size of powder particles increased again. In fact, exothermic reaction between Al and MoO3 increases temperature locally and causes the particle size to increase to 4 m. For longer milling time up to 240 min, the change of particle size was not significant. However, the rate of agglomeration and fragmentation was equal and particle size was constant. In this stage, the average particle size was about 2 m.
695
4. Conclusion Fabrication of alumina–Mo nanocomposite was investigated. It was found that MoO3 reacted with Al through a rapid combustion reaction. The reaction was completed after 120 min in stoichiometric composition. But in presence of extra alumina, as a diluent, the time of complation of MoO3 –Al reaction was longer, 170 min. After 240 min of milling time, Mo and alumina achieved a crystallite size of about 28 nm and 60 nm in stoichiometric composition and 15 nm and 44 nm in non-stoichiometric composition, respectively. After annealing Mo crystallite size remained constant, however ␣alumina crystallite size increased to 120 nm. Moreover ␣-Al2 O3 was partially transformed to ␥-Al2 O3 with a crystallite size of 50 nm. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
T.F. Grigorieva, A.P. Barinova, N.Z. Lyakhov, J. Nanoparticle Res. 5 (2003) 439. T.F. Grigorieva, M. Korchagin, N.Z. Lyakhov, KONA 20 (2002) 4. L. Takacs, Prog. Mater. Sci. 47 (2002) 355. G.B. Schaffer, P.G. McCormick, Scripta Metal. 23 (1989) 835. M. Khodaei, M.H. Enayati, F. Karimzadeh, J. Mater. Sci. 43 (2008) 132. W.J. Botta, Scripta Mater. 44 (2001) 1735. L.C. Stearns, J. Zhao, M.P. Harmer, J. Eur. Ceram. Soc. 10 (1992) 473. C.E. Borsa, N.M.R. Jones, R.J. Brook, R.I. Todd, J. Eur. Ceram. Soc. 17 (1997) 865. C.C. Anya, S.G. Roberts, J. Eur. Ceram. Soc. 17 (1997) 565. M. Sternitzke, J. Eur. Ceram. Soc. 17 (1997) 1061. T. Sekino, K. Niihara, J. Mater. Sci. 32 (1997) 3943. O. Sbaizero, G. Pezzotti, T. Nishida, Acta Mater. 46 (1998) 681. M. Nawa, T. Sekino, K. Niihara, J. Mater. Sci. 29 (1994) 3185. M. Lieberthal, D.W. Kaplan, Mater. Sci. Eng. A 302 (2001) 83. P. Matteazzi, G.L. Caer, J. Am. Ceram. Soc. 75 (1992) 2749. K.K. Chalwa, Ceramic Matrix Composites, 2nd ed., Chapman and Hall Pub, 1993. G. Portu, S. Guicciardi, C. Melandri, F. Monteverde, Wear 262 (2007) 1346. E.M.J.A. Pallone, D.R. Martin, R. Tomasi, W.J.B. Filho, Mater. Sci. Eng. A 464 (2007) 47. M. Szafran, K. Konopka, E. Bobryk, K.J. Kurzydłowski, J. Eur. Ceram. Soc. 27 (2007) 651. C. Lin, W.J. Wei, Mater. Chem. Phys. 111 (2008) 82. K. Wieczorek-Ciurowa, D. Oleszak, K. Gamrat, J. Alloys Compd. 434–435 (2007) 501. K. Williamson, W.H. Hall, Acta Metal. 1 (1953) 22. O. Kubaschewski, C.B. Alcock, Metallurgical Thermochemistry, 5th ed., Pergamon Pres, 1912. E.M.J.A. Pallone, V. Trombini, W.J. Botta, R. Tomasi, J. Mater. Process. Technol. 144 (2003) 185. G.B. Schaffer, P.G. McCormick, Mater. Lett. 9 (1990) 1014. G.B. Schaffer, P.G. McCormick, Metall. Mater. Trans. A 22 (1991) 3019. C. Suryanarayana, Prog. Mater. Sci. 46 (2001) 1.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
In situ synthesis mechanism of Al2 O3 –Mo nanocomposite by ball milling process A. Heidarpour ∗ , F. Karimzadeh, M.H. Enayati Department of Materials Engineering, Nanotechnology and advance material Institute, Isfahan University of Technology (IUT), Isfahan 84156-83111, Iran
a r t i c l e
i n f o
Article history: Received 12 July 2008 Received in revised form 22 October 2008 Accepted 24 October 2008 Available online 9 December 2008 Keywords: Mechanosynthesis Nanocomposite Characterization Al2 O3 –Mo
a b s t r a c t Al2 O3 –Mo nanocomposites were synthesized by ball milling of aluminum and molybdenum oxide powder mixtures. The structural evaluation of powder particles after different milling times was studied by Xray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and measurement of vial temperature. The molybdenum oxide and aluminum reaction appeared to occur through a rapid combustion reaction process. As a result, an alumina matrix nanocomposite containing molybdenum particulate was formed. In final stage of milling, Mo and alumina had a crystallite size of about 28 nm and 60 nm, respectively. After annealing at 800 ◦ C for 60 min, Mo crystallite size remained constant. However, ␣-alumina crystallite size increased to 120 nm. After annealing a partial transformation of ␣-Al2 O3 into different polymorphic, ␥-Al2 O3 with a crystallite size of 50 nm was observed. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Mechanochemical synthesis (MCS); that is, chemical reactions induced by high-energy ball milling, is one of promising routes for the synthesis of different classes of materials including metals, oxides, salts, organic compounds, etc. in various combinations [1,2]. Mechanochemical reactions involving displacement reactions between a reactive metal and a metal oxide often lead to the formation of a nanocomposite structure [3–5]. Mechanochemical reactions fall into two categories [6]: namely, (a) those which occur during the mechanical activation process in which the reaction enthalpy is highly negative (e.g. adiabatic temperature Tad = 1300–1800 K) and (b) those which occur during subsequent thermal treatment and here the reaction enthalpy is only moderate (e.g. Tad < 1300 K). The first type of reaction takes place in two distinct modes, i.e., a combustion reaction or a progressive reaction. Whenever a reaction is highly exothermic, it can occur abruptly after a certain time of milling and, once started, it proceeds in a self-sustained way. In this case, the reaction requires a given time to begin. This time is called ignition time and, due to the exothermic reaction, can be determined by an increase in temperature. A number of studies have focused on the development of A12 O3 /metal nanocomposites by different routes [7–14]. Matteazzi and Caer synthesized nanometer-sized ␣-A12 O3 –M composite (M = Fe, V, Cr, Mn, Co, Ni, Cu, Zn, Nb, Mo, W, Si) by ball milling of an
∗ Corresponding author. Tel.: +98 311 3915744; fax: +98 311 3912751. E-mail address: [email protected] (A. Heidarpour). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.10.112
appropriate metal oxide and Al [15]. The present study is an effort to provide a future understanding of Al and MoO3 reaction during room-temperature ball milling. Details of phase development and structural evolution are also investigated. The presence of metallic particles in ceramics adds physical properties inherent to the metallic phase, such as electric and thermal conductivity or magnetic properties. This combination of properties makes ceramics–metals composite excellent candidates for electric, optic, and magnetic devices or chemical sensors [16]. Some properties such as wear behavior of Al2 O3 –metal composites are evaluated in order to develop structural and functional applications [17–21]. 2. Materials and methods Mixtures of commercial aluminum powder (99.7%, 30–60 m) and MoO3 powders (99.9%, ∼100 m) were milled to give Al2 O3 –26.6 vol% Mo (48.2 wt.% Mo) and Al2 O3 –15 vol% Mo (31.3 wt.% Mo) nanocomposite. Ball milling was performed in a SPEX8000 type ball mill using hardened chromium steel vial and balls under argon atmosphere. The milling was done with a ball-powder mass ratio of 7:1 without interruption. No process control agent (PCA) was used. X-ray diffraction (XRD) was used to follow the structural changes of powders during milling. A Philips diffractometer (40 kV) with Cu K␣ radiation ( = 0.15406 nm) was used for XRD measurements. The XRD patterns were recorded in the 2 range of 20–100◦ (step size 0.03◦ and time per step 1 s). The microstructure of powder particles was investigated by scanning electron microscopy (SEM) using a Philips XL30 SEM and transmission electron microscopy (TEM) by using a Philips CM200 FEG. The average particle size was obtained from SEM images by using image-analysis software. Isothermal annealing was carried out to study the thermal behavior of milled powders. Powder samples were sealed and then annealed in a conventional tube furnace. The alumina and Mo crystallite sizes were estimated by analyzing broadening of XRD peaks using Williamson–Hall formula [22]. The reaction progress was monitored by a thermocouple attached on the external surface of the vial.
A. Heidarpour et al. / Journal of Alloys and Compounds 477 (2009) 692–695
Fig. 1. XRD patterns of Al–MoO3 powder particles (stoichiometric composition) asreceived and after different milling times.
Fig. 2. XRD patterns of Al–MoO3 –Al2 O3 powder particles (non-stoichiometric composition) as-received and after different milling times.
3. Results and discussions MoO3 and Al reaction with stoichiometric composition gives an Al2 O3 based composite containing 26.6 vol% Mo according to reaction (1) [17]. 2Al(s) + MoO3 (s) = Al2 O3 (s) + Mo(s) G◦ 298 = −915, 000 J/mol;
(1)
H ◦ 298 = −932, 000 J/mol [23]
During milling in room temperature this reaction can thermodynamically occur due to its negative free energy change. H ◦ 298
Fig. 3. TEM image of the Al2 O3 –26.6 vol% Mo nanocomposite after 240 min milling.
693
Fig. 4. Temperature variation of vial during ball milling of two compositions, Al–MoO3 and Al–MoO3 –Al2 O3 .
value for Al–MoO3 reaction indicates that the Al–MoO3 reaction is highly exothermic. The XRD patterns of stoichiometric composition after different milling times are presented in Fig. 1. The diffraction patterns of initial powder mixture show several peaks corresponding to Al and MoO3 . The intensity of MoO3 and Al peaks decreased during milling so that after 130 min of milling time these peaks disappeared on XRD patterns. Meanwhile several additional peaks corresponding to Mo and alumina developed on XRD patterns. In a separate ball milling run a mixture of Al, MoO3 and Al2 O3 powders was ball milled to produce Al2 O3 –15 vol% Mo nanocomposite. Fig. 2 shows XRD patterns of this composition after different milling times. As seen, the reaction of Al and MoO3 was completed after 180 min which is longer than that obtained for stoichiometric composition (Fig. 1). This suggests that the Al–MoO3 reaction extended over a longer period in presence of additional alumina as a diluent. The broadening of the XRD peaks in Figs. 1 and 2 is due to the reduction of the crystallite size as well as the microstrain induced in powder particles. The approach of Williamson and Hall [22] was used in order to separate the two effects of crystallite size and microstrain. After 240 min of milling, Mo and alumina achieved a crystallite size of about 28 nm and 60 nm in stoichiometric composition and 15 nm and 44 nm in non-stoichiometric composition, respectively. The smaller Mo and alumina crystallite size in case of non-stoichiometric composition can be due to the lower heat released in this composition compared to that in stoichiometric composition. Matteazzi and Caer obtained ␣-A12 O3 –M nanocomposite (M = Fe, V, Cr, Mn, Co, Ni, Cu, Zn, Nb, Mo, W, Si) by ball milling
Fig. 5. XRD patterns of Al2 O3 /26.6 vol% Mo as-milled for 240 min and after subsequent annealing at 800 ◦ C for 60 min.
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A. Heidarpour et al. / Journal of Alloys and Compounds 477 (2009) 692–695
Fig. 6. Morphology of as-received (a) Al and (b) MoO3 powder particles.
of a suitable metal oxide and Al [15] using Scherrer formula. They reported a smaller value of about 10 nm for crystallite sizes of alumina and metal. The discrepancies in crystallite size between the present study and those reported by Matteazzi and Caer can be due to the fact that in Scherrer method it is assumed that the whole of broadening is caused by very fine crystallite size and the effect of the lattice microstrain is ignored. The Al2 O3 –26.6 vol% Mo nanocomposite after 240 min milling was studied by TEM. Fig. 3 represents the TEM photograph and related diffraction pattern of the nanocomposite dispersed on a carbon-coated Cu grid. It can be observed that molybdenum phase
(dark region) disperses homogeneously in alumina phase and has dimensions less than 100 nm, and diffraction pattern confirmed that this composite is nanostructure. The variation of temperature of vial during ball milling can provide a future insight into the MoO3 –Al reaction mode. Fig. 4 shows the vial temperature variation with milling time for both compositions. If a reaction is highly exothermic, it can take place abruptly after a certain time of milling; and once started, it proceeds in a self-sustained way [14,24,25]. As seen after 120 min of milling time, the temperature of vials increased rapidly suggesting a combustion reaction between Al and MoO3 . This can be expected
Fig. 7. Morphology of powder particles (stoichiometric composition) after (a) 90 min, (b) 120 min and (c) 240 min of milling times.
A. Heidarpour et al. / Journal of Alloys and Compounds 477 (2009) 692–695
from the adiabatic temperature, Tad , for the reaction between MoO3 and Al, which is 1600 ◦ C [15] and higher than the critical value of 1300 ◦ C proposed by Schaffer and McCormick [25,26]. The combustion reaction between Al and MoO3 is promoted by the dynamically maintained high Al/MoO3 interface areas as well as the short-circuit diffusion path provided by the increasing number of defects such as dislocations and grain boundaries induced during ball milling [27]. It is interesting to note that the presence of extra alumina with respect to stoichiometric composition shifts the peak temperature to longer milling time and also decreases the peak temperature. In fact extra alumina acts as a diluent reducing the adiabatic temperature as well as the ignition time. Fig. 5 shows the XRD patterns of stoichiometric composition powder milled for 240 min before and after annealing at 800 ◦ C for 60 min. After annealing several peaks of ␥-Al2 O3 were appeared on XRD patterns indicating the partial transformation of ␣-Al2 O3 to ␥-Al2 O3 phase during annealing. Mo crystallite size remained unchanged after annealing due to the pinning effect of the Al2 O3 particles. In contrast, after annealing ␣-alumina crystallite size increased to 120 nm. In this case, the crystallite size of ␥-Al2 O3 which was formed during annealing was about 50 nm. The change in morphology of powder particles during milling process are shown in Figs. 6 and 7. As-received Al powder particles had random morphology with a particle size ranging from 30 m to 60 m. As-received MoO3 powder had elongated morphology with a typical length and thickness of 100 m and 20 m, respectively. The average particle size was estimated by using image-analysis tools. In initial stage of milling MoO3 was mixed with Al at the micrometer level and a homogenous and active composite is formed. Therefore, ball milling up to 90 min led to the refinement of powder particles and the average size of powder particle before combustion reaction reached about 2 m (Fig. 7a). Then, the reaction between Al and MoO3 took place abruptly after 120 min of milling time (Fig. 4) and as seen from Fig. 7b after this time, the size of powder particles increased again. In fact, exothermic reaction between Al and MoO3 increases temperature locally and causes the particle size to increase to 4 m. For longer milling time up to 240 min, the change of particle size was not significant. However, the rate of agglomeration and fragmentation was equal and particle size was constant. In this stage, the average particle size was about 2 m.
695
4. Conclusion Fabrication of alumina–Mo nanocomposite was investigated. It was found that MoO3 reacted with Al through a rapid combustion reaction. The reaction was completed after 120 min in stoichiometric composition. But in presence of extra alumina, as a diluent, the time of complation of MoO3 –Al reaction was longer, 170 min. After 240 min of milling time, Mo and alumina achieved a crystallite size of about 28 nm and 60 nm in stoichiometric composition and 15 nm and 44 nm in non-stoichiometric composition, respectively. After annealing Mo crystallite size remained constant, however ␣alumina crystallite size increased to 120 nm. Moreover ␣-Al2 O3 was partially transformed to ␥-Al2 O3 with a crystallite size of 50 nm. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]
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