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Mechanical alloying of MgNi compounds under hydrogen and inert atmosphere
Journal of
ALLOYS AND COMPOLIND$ ELSEVIER
Journal of Alloys and Compounds 231 (1995) 815-819
Mechanical alloying of Mg-Ni compounds under hydrogen and inert atmosphere J. Huot*, E. Akiba, T. Takada National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba, Ibaraki 305, Japan
Abstract
The effect of milling atmosphere on the nature and composition of milled products was investigated. Pure magnesium and the mixture 2Mg + Ni were milled in a planetary ball mill under hydrogen atmosphere. The mixture 2Mg + Ni was also milled under argon atmosphere. X-ray powder diffraction, SEM and DSC measurements were taken at certain intervals during the milling process. When milled under hydrogen, pure magnesium formed MgH2. DSC measurement indicated that the onset temperature of MgH2 decomposition was 440.7°C. Magnesium hydride was also formed when the mixture 2Mg + Ni was milled under hydrogen. The presence of nickel lowered the onset temperature of MgH2 decomposition to 225.4°C. By milling the mixture 2Mg + Ni under argon, MgzNi was produced. The presence of Mg2Ni slowed the decomposition kinetics of MgH2. Keywords: Mechanicalalloys;Mg-Ni mixture; Magnesiumhydride;Mg2Ni
1. Introduction Because of its light weight, low cost and high hydrogen capacity, magnesium is an attractive hydrogenstroragematerial. However, the kinetics of hydriding and dehydriding are slow. As reported by Reilly and Wiswall [1], Mg2Ni can act as a catalyst on the formation of MgH 2. The process of mechanical alloying (MA), by repeatedly fracturing and rewelding a mixture of powdered metals, can be seen as a new way of producing hydrogen absorbing alloys. Recently, work on Mg-Ni systems has been carried out by Ivanov's and Hagenmuller's groups [2-4]. They found that the 2Mg + Ni meclaanicaUy alloyed material is excellent for hydrogen :storage [4]. In Mg-Ni mixtures, they showed thal with increasing amount of nickel the hydriding-dehydriding kinetics are increased while the hydrogen storage capacity is diminished. They concluded that the most favorable compositions were Mg-10wt.%Ni and Mg-25wt.%Ni. In these works the milling time (5 to 30 min) was very short. This is the reason why an intermetallic compound was not formed by milling. Recently, Aoki et al. [5] have shown that grinding the intermetallic
* Corresponding
author,
0925-8388/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0 9 2 5 - 8 3 8 8 ( 95 ) 0 1 7 6 4 - X
compound Mg2Ni in argon and hydrogen atmosphere can improve the initial hydrogen absorption rate. For the last decade, this laboratory has been actively investigating the Mg-Ni hydrogen absorbing alloys [6-11]. In the present work, we have studied the production of lightweight magnesium alloys, of stoichiometry 2Mg + Ni, by mechanical alloying. Specifically, the effect of milling atmosphere on mechanical alloying of Mg-Ni mixtures was investigated. First, pure magnesium and pure nickel powders were individually milled under hydrogen. Afterward, the mixture 2Mg + Ni was milled under argon and hydrogen atmospheres. The milled materials were analyzed by X-ray powder diffraction, scanning electron microscope (SEM) and differential scanning calorimetry (DSC).
2. Experimental details Elemental powders of Mg and Ni of size - 100 mesh and 99.9% pure were used. The powders were mixed inside an argon dry box. The containers (80 ml in volume) and balls (10 mm in diameter) were made of stainless steel. In all experiments, about 16 g of starting materials were used, giving a ball to powder weight ratio of 4:1. The pressure inside the containers
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J. Huot et al. / Journal of Alloys and Compounds 231 (1995) 815-819
was 1 bar for milling under argon and 5 bar when milling under hydrogen. MA was carried out in a Fritsch "pulverisette 5" at a rotation speed of 325 rpm. This corresponds to a maximum acceleration of 16 × g [12], where g is the gravity of the Earth. Liquid crystal thermal labels pasted on the outer surface of containers indicated that the maximum temperature of the containers was 45°C. At regular intervals, the containers were opened in a dry box and a small amount of powder was taken for X-ray diffraction analysis, scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). The X-ray powder diffraction measurements were carried out on a Rigaku diffractometer with a Cu Ka radiation. For some powder diffraction measurements, a compound was used to hold the samples. In these cases, the compound gives three distinct reflection lines between 20° and 30°. These lines are easily identified on the powder diffraction pattern. A Hitachi S-2350 microscope was used for SEM observations, Before microphotograph measurements, the samples were gold coated with a thickness of about 200 A. The DSC measurements were carried out in a DuPont 9 9 0 0 unit at a heating rate of 10°C min -1 under 2 bar of argon atmosphere,
3. Results and discussion
3.1. MA of pure magnesium and pure nickel under hydrogen
in grain size. After 5 h of milling under hydrogen atmosphere, hydride MgH 2 was formed. Further milling produced a slight increase in the intensity of the MgH 2 diffraction peak and a decrease in the Mg peak intensity. The SEM observations indicated that particle size distribution changed from 30-100 tzm before milling to 30-200 tzm after 25 h of milling. Fig. 2(a) shows the DSC results of the pure magnesium powder milled for 25 h under hydrogen. The decomposition of MgH 2 is indicated by the endothermic peak with an onset temperature of 440.7°C. From the peak area and the heat of formation of MgH 2 (-37 kJ per mol H) [7], the proportion of MgH 2 was calculated to be 4% by weight. MgH 2 hydride is known to be quite stable and the kinetics are slow [13]. However, it has been reported that pure magnesium particles with cracked irregular surface absorbed hydrogen rapidly [14]. Thus, the increase of surface area and the creation of defects on the surface of the particles, which are produced by the mechanical alloying, are responsible for the formation of MgH 2 under such a low temperature and hydrogen pressure. Pure nickel does not absorb hydrogen. Therefore, the effect of milling nickel powder under hydrogen is only mechanical. SEM micrographs show that the particles before milling have a round shape with a diameter from 30/~m to 100/zm. After 25 h of milling under hydrogen, the particles were more plate-like with sizes from 200/xm to 300/~m.
3.2. MA of 2Mg %Ni mixture under hydrogen Before studying the effect of MA on Mg-Ni mixtures, it is important to study the behavior of the pure metals when milled under hydrogen. Result of pure magnesium powder is shown in Fig. 1. The diffraction peaks became smaller and broader as the milling time increased. The peak broadening indicated a reduction
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The 2Mg + Ni mixture was first milled for 22 h under hydrogen atmosphere. Results of the X-ray powder diffraction measurements are shown in Fig. 3. As in the case of pure magnesium, magnesium hydride
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Fig. 1. X-ray diffraction patterns of pure magnesium powder milled under hydrogen for various times. @, Mg; &, MgH2; O, compound holder,
Fig. 2. DSC traces of various mixtures: (a) pure magnesium powder milled for 25 h under hydrogen; (b) 2Mg + Ni mixture milled for 22 h under hydrogen; (c) 2Mg + Ni mixture milled for 22 h under hydrogen, followed by 8 h under argon; (d) 2Mg + Ni mixture milled for 22 h under argon, followed by 8 h under hydrogen.
J. Huot et al. / Journal o f Alloys and Compounds 231 (1995) 815-819 . . . . . . . . .
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20 (degrees) Fig. 3. X-ray diffraction patterns of mixture 2Mg + Ni milled under hydrogen for various times. Curve (a)is sample milled 22 h under hydrogen followed by 2 h under argon. Curve (b) is sample milled 22 h under hydrogen followed by 8 h under argon. T, Ni; 0, Mg; @, Mg2Ni; A MgH2; ©, compound holder.
(MgH2) was formed after only 2 h of milling. After 12 h of milling, magnesium peaks were much reduced but there was no evidence of the formation of an intermetallic phase. Furthei milling reduced magnesium peak intensities but there was no formation of intermetallic phase Mg2Ni or hydride Mg2NiH 4. A steadystate, confirmed by the lack of perceptible change in the powder diffraction pattem, was reached after 22 h of milling. SEM observations indicated that particle size was 10 /zm to 200/zm. Fig. 4(a) shows a SEM micrograph of a typical particle after 22 h of milling. The particles were porous and formed by accretion of small particles of 0.1 /~m to 7 / z m in diameter. The DSC measurement is shown in Fig. 2(b). The trace shows an endothermic peak with onset at 225.4°C. Because only magnesium hydride was ideatiffed on the powder diffraction pattern, this endothermic peak was assigned to decomposition of MgH 2. From the peak area, an ;abundance of MgH 2 of 8 wt.% was calculated. The peak shape also indicates that the decomposition kinetics were slower than in the case of pure magnesium. After 22 h of milling under hydrogen, the hydrogen atmosphere was replaced by argon and milling was resumed. Fig. 3(a) shows that after only 2 h of milling under argon, the intermetallic Mg2Ni was produced, Subsequent milling raised the amount of intermetallic while the MgH 2 phase was somewhat reduced, The DSC result of the sample milled 22 h under hydrogen followed by 8 h of milling under argon is shown in Fig. 2(c). Further milling under argon increased the onset temperature of decomposition of MgH 2 from 225.4°C to 263.4°C. The decomposition peak became much broader, indicating that the reac-
(b)
Fig. 4. SEM micrographs of the mixture 2Mg + Ni milled for 22 h under (a) hydrogen atmosphere and (b) argon atmosphere.
tion proceeded at a much slower speed. Moreover, the MgH 2 abundance slightly decreased to 6 wt.%. By milling under argon, the intermetallic phase Mg2Ni was produced. The presence of this phase increased the decomposition temperature of MgH 2 and slowed the kinetics. 3.3. M A of 2Mg + Ni mixture under argon In this experiment, the mixture 2Mg + Ni was first milled under argon for 22 h. After only 4 h of milling, all the material was sticking to the inner surface of the container and on the balls. The material remained attached to the inner surface for the duration of the experiment. The samples were thereafter collected by scraping a small amount of powder from the surface. Fig. 5 shows the powder diffraction patterns of the mixture 2Mg + Ni milled under argon for various milling times. After 6 h of milling, the peak intensities of magnesium were reduced while those of nickel increased. This effect is due to the difference in the
818
J. Huot et al. / Journal of Alloys and Compounds 231 (1995) 815-819
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Fig. 5. X-ray diffraction patterns of mixture 2Mg + Ni milled under argon for various times. Curve (a) is sample milled 22 h under argon followed by 2 h under hydrogen. V, Ni; O, Mg; O, Mg2Ni; ,t, MgH:; O, compound holder.
colored and the appearance was identical to the original powder. Therefore, the milling process was effective only for the surface materials and resulted in a small amount of milled materials. By milling the mixture 2Mg + Ni under argon atmosphere, MgzNi was produced. However, because the milling was operative only on the surface materials, the production of Mg2Ni by mechanical alloying was not effective. First milling in a hydrogen atmosphere had the advantage of keeping the milled product in powder form. When milling was resumed under an argon atmosphere, the milled product had less propensity to stick to the inner surface of the container than the material first milled under argon.
4. Conclusion linear absorption coefficient between magnesium and nickel [15]. Because nickel absorbs X-rays more strongly than magnesium, when the two metals are milled and finely mixed, magnesium peaks will become weaker compared to nickel ones [15]. The intermetallic phase MgzNi appeared after 12 h and the system seems to have reached a steady-state after 22 h of milling. Particle size distribution was similar to the case of milling under hydrogen. In Fig. 4(b) a SEM micrograph of a typical particle is presented. It shows that small particles of size 5 to 15/.,m agglomerate to form a bigger particle. The system having reached a steady-state under argon atmosphere, we switched the atmosphere to hydrogen to investigate the effect of hydrogen atmosphere. Every 2 h, the milling pot was pressurized with new hydrogen. As shown in Fig. 5(a), after 8 h MgH 2 hydride was formed but there was no evidence of formation of the intermetallic hydride. Also, the DSC trace did not show the Mg2Ni hydride phase transformation from a low temperature structure to a high temperature structure [16]. The presence of MgH 2 is confirmed by the DSC measurement shown in Fig. 2(d), where the endothermic peak indicates decomposition of MgH 2. Clearly, curves 2(d) and 2(c) are almost identical. They have the same onset temperature and give the same abundance of MgH 2. In the same way, as shown in Fig. 3(b) and 5(a), the powder diffraction patterns are identical. Therefore, milling in hydrogen followed by milling in argon had the same end product as the reverse order. After completion of the experiment, the material sticking to the inner surface of the container was mechanically removed. The surface material in which the milling balls were impacted was black. This black material was present down to a depth of about 2-3 mm. U n d e r this black layer, the material was silver
In this work, we found that the hydride MgH 2 was easily produced by milling pure Mg powder under a hydrogen atmosphere. This denoted an improvement in the kinetics of hydride formation. The DSC measurement indicated that MgH 2 started to decompose at 440.7°C. When the mixture 2Mg + Ni was milled under hydrogen, the hydride MgH 2 was also easily produced, but not the intermetallic compound Mg2Ni nor the hydride Mg2NiH 4. However, in this case the presence of nickel reduced the onset temperature of MgH 2 decomposition to 225.4°C. When the milling was performed under inert argon atmosphere, the intermetallic Mg2Ni was produced. The presence of MgeNi increased the decomposition temperature of MgH 2 from 225.4°C to 263.4°C and slowed the decomposition rate.
Acknowledgment One of the authors (J.H.) is supported by a fellowship from the Science and Technology Agency of Japan.
References [1] J.J. Reilly and R.H. Wiswall, Jr., Inorg. Chem., 7 (1968) 2254. [2] E. Ivanov, I. Konstanchuk, A. Stepanov and V. Boldyrev, J. Less-Common Met., 131 (1987)25. [3] M.Y. Song, E. Ivanov, B. Darriet, M. Pezat and E Hagenmuller, J. Less-Common Met., 131 (1987) 71. [4] M.Y. Song, E.I. Ivanov, B. Darriet, M. Pezat and E Hagenmuller, Int. J. Hydrogen Energy, 10 (1985) 169. [5] K. Aoki, H. Aoyagi, A. Memezawa and T. Masumoto, J. Alloys Comp., 204
(1994)L7.
J. Huot et al. / Journal of Alloys and Compounds 231 (1995) 815-819
[6] E. Akiba, K. Nomura, S. ()no and Y. Mizuno, J. Less-Common Met., 83 (1982) L43. [7] S. Ono, Y. Ishido, K. Imanari, T. Tabata, Y.K. Cho, R. Yamamoto and M. Doyama, J. Less-Common Met., 88 (1982) 57. [8] E. Akiba, K. Nomura, S. Ono and Y. Mizuno, J. Less-Common Met., 89 (1983) 145. [9] Y. Ishido, S. Ono and E. Akiba, J. Less-Common Met., 120 (1986) 163. [10] E. Akiba, Y. Ishido, H. Hayakawa, S. Shin and K. Nomura, Z. Phys. Chem. N.F., 164 (1989) 1319.
819
[11] K. Nomura, S. Fujiwara, H. Hayakawa, E. Akiba, Y. Ishido and S. Ono, J. Less-Common Met., 169 (1991) 9. [12] U. Mizutani, T. Takeuchi, T. Fukunaga, S. Murasaki and K. Kaneko, J. Mater. Sci. Lett., 12 (1993) 629. [13] N. G6rard and S. Ono, in L. Schlapbach (ed.), Hydrogen in lntermetallic Compounds H, Springer, Berlin, 1992, Chapter 4. [14] B. Vigeholm, J. Kj¢ller, B. Larsen and A.S. Pedersen, J. LessCommon Met., 89 (1983) 135. [15] L. Li and J. Tang, J. Alloys Comp., 209 (1994) L1. [16] S. Ono, H. Hayakawa, A. Suzuki, K. Nomura, N. Nishimiya and T. Tabata, J. Less-Common Met., 88 (1982) 63.
ALLOYS AND COMPOLIND$ ELSEVIER
Journal of Alloys and Compounds 231 (1995) 815-819
Mechanical alloying of Mg-Ni compounds under hydrogen and inert atmosphere J. Huot*, E. Akiba, T. Takada National Institute of Materials and Chemical Research, 1-1 Higashi, Tsukuba, Ibaraki 305, Japan
Abstract
The effect of milling atmosphere on the nature and composition of milled products was investigated. Pure magnesium and the mixture 2Mg + Ni were milled in a planetary ball mill under hydrogen atmosphere. The mixture 2Mg + Ni was also milled under argon atmosphere. X-ray powder diffraction, SEM and DSC measurements were taken at certain intervals during the milling process. When milled under hydrogen, pure magnesium formed MgH2. DSC measurement indicated that the onset temperature of MgH2 decomposition was 440.7°C. Magnesium hydride was also formed when the mixture 2Mg + Ni was milled under hydrogen. The presence of nickel lowered the onset temperature of MgH2 decomposition to 225.4°C. By milling the mixture 2Mg + Ni under argon, MgzNi was produced. The presence of Mg2Ni slowed the decomposition kinetics of MgH2. Keywords: Mechanicalalloys;Mg-Ni mixture; Magnesiumhydride;Mg2Ni
1. Introduction Because of its light weight, low cost and high hydrogen capacity, magnesium is an attractive hydrogenstroragematerial. However, the kinetics of hydriding and dehydriding are slow. As reported by Reilly and Wiswall [1], Mg2Ni can act as a catalyst on the formation of MgH 2. The process of mechanical alloying (MA), by repeatedly fracturing and rewelding a mixture of powdered metals, can be seen as a new way of producing hydrogen absorbing alloys. Recently, work on Mg-Ni systems has been carried out by Ivanov's and Hagenmuller's groups [2-4]. They found that the 2Mg + Ni meclaanicaUy alloyed material is excellent for hydrogen :storage [4]. In Mg-Ni mixtures, they showed thal with increasing amount of nickel the hydriding-dehydriding kinetics are increased while the hydrogen storage capacity is diminished. They concluded that the most favorable compositions were Mg-10wt.%Ni and Mg-25wt.%Ni. In these works the milling time (5 to 30 min) was very short. This is the reason why an intermetallic compound was not formed by milling. Recently, Aoki et al. [5] have shown that grinding the intermetallic
* Corresponding
author,
0925-8388/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0 9 2 5 - 8 3 8 8 ( 95 ) 0 1 7 6 4 - X
compound Mg2Ni in argon and hydrogen atmosphere can improve the initial hydrogen absorption rate. For the last decade, this laboratory has been actively investigating the Mg-Ni hydrogen absorbing alloys [6-11]. In the present work, we have studied the production of lightweight magnesium alloys, of stoichiometry 2Mg + Ni, by mechanical alloying. Specifically, the effect of milling atmosphere on mechanical alloying of Mg-Ni mixtures was investigated. First, pure magnesium and pure nickel powders were individually milled under hydrogen. Afterward, the mixture 2Mg + Ni was milled under argon and hydrogen atmospheres. The milled materials were analyzed by X-ray powder diffraction, scanning electron microscope (SEM) and differential scanning calorimetry (DSC).
2. Experimental details Elemental powders of Mg and Ni of size - 100 mesh and 99.9% pure were used. The powders were mixed inside an argon dry box. The containers (80 ml in volume) and balls (10 mm in diameter) were made of stainless steel. In all experiments, about 16 g of starting materials were used, giving a ball to powder weight ratio of 4:1. The pressure inside the containers
816
J. Huot et al. / Journal of Alloys and Compounds 231 (1995) 815-819
was 1 bar for milling under argon and 5 bar when milling under hydrogen. MA was carried out in a Fritsch "pulverisette 5" at a rotation speed of 325 rpm. This corresponds to a maximum acceleration of 16 × g [12], where g is the gravity of the Earth. Liquid crystal thermal labels pasted on the outer surface of containers indicated that the maximum temperature of the containers was 45°C. At regular intervals, the containers were opened in a dry box and a small amount of powder was taken for X-ray diffraction analysis, scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). The X-ray powder diffraction measurements were carried out on a Rigaku diffractometer with a Cu Ka radiation. For some powder diffraction measurements, a compound was used to hold the samples. In these cases, the compound gives three distinct reflection lines between 20° and 30°. These lines are easily identified on the powder diffraction pattern. A Hitachi S-2350 microscope was used for SEM observations, Before microphotograph measurements, the samples were gold coated with a thickness of about 200 A. The DSC measurements were carried out in a DuPont 9 9 0 0 unit at a heating rate of 10°C min -1 under 2 bar of argon atmosphere,
3. Results and discussion
3.1. MA of pure magnesium and pure nickel under hydrogen
in grain size. After 5 h of milling under hydrogen atmosphere, hydride MgH 2 was formed. Further milling produced a slight increase in the intensity of the MgH 2 diffraction peak and a decrease in the Mg peak intensity. The SEM observations indicated that particle size distribution changed from 30-100 tzm before milling to 30-200 tzm after 25 h of milling. Fig. 2(a) shows the DSC results of the pure magnesium powder milled for 25 h under hydrogen. The decomposition of MgH 2 is indicated by the endothermic peak with an onset temperature of 440.7°C. From the peak area and the heat of formation of MgH 2 (-37 kJ per mol H) [7], the proportion of MgH 2 was calculated to be 4% by weight. MgH 2 hydride is known to be quite stable and the kinetics are slow [13]. However, it has been reported that pure magnesium particles with cracked irregular surface absorbed hydrogen rapidly [14]. Thus, the increase of surface area and the creation of defects on the surface of the particles, which are produced by the mechanical alloying, are responsible for the formation of MgH 2 under such a low temperature and hydrogen pressure. Pure nickel does not absorb hydrogen. Therefore, the effect of milling nickel powder under hydrogen is only mechanical. SEM micrographs show that the particles before milling have a round shape with a diameter from 30/~m to 100/zm. After 25 h of milling under hydrogen, the particles were more plate-like with sizes from 200/xm to 300/~m.
3.2. MA of 2Mg %Ni mixture under hydrogen Before studying the effect of MA on Mg-Ni mixtures, it is important to study the behavior of the pure metals when milled under hydrogen. Result of pure magnesium powder is shown in Fig. 1. The diffraction peaks became smaller and broader as the milling time increased. The peak broadening indicated a reduction
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The 2Mg + Ni mixture was first milled for 22 h under hydrogen atmosphere. Results of the X-ray powder diffraction measurements are shown in Fig. 3. As in the case of pure magnesium, magnesium hydride
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Fig. 1. X-ray diffraction patterns of pure magnesium powder milled under hydrogen for various times. @, Mg; &, MgH2; O, compound holder,
Fig. 2. DSC traces of various mixtures: (a) pure magnesium powder milled for 25 h under hydrogen; (b) 2Mg + Ni mixture milled for 22 h under hydrogen; (c) 2Mg + Ni mixture milled for 22 h under hydrogen, followed by 8 h under argon; (d) 2Mg + Ni mixture milled for 22 h under argon, followed by 8 h under hydrogen.
J. Huot et al. / Journal o f Alloys and Compounds 231 (1995) 815-819 . . . . . . . . .
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(MgH2) was formed after only 2 h of milling. After 12 h of milling, magnesium peaks were much reduced but there was no evidence of the formation of an intermetallic phase. Furthei milling reduced magnesium peak intensities but there was no formation of intermetallic phase Mg2Ni or hydride Mg2NiH 4. A steadystate, confirmed by the lack of perceptible change in the powder diffraction pattem, was reached after 22 h of milling. SEM observations indicated that particle size was 10 /zm to 200/zm. Fig. 4(a) shows a SEM micrograph of a typical particle after 22 h of milling. The particles were porous and formed by accretion of small particles of 0.1 /~m to 7 / z m in diameter. The DSC measurement is shown in Fig. 2(b). The trace shows an endothermic peak with onset at 225.4°C. Because only magnesium hydride was ideatiffed on the powder diffraction pattern, this endothermic peak was assigned to decomposition of MgH 2. From the peak area, an ;abundance of MgH 2 of 8 wt.% was calculated. The peak shape also indicates that the decomposition kinetics were slower than in the case of pure magnesium. After 22 h of milling under hydrogen, the hydrogen atmosphere was replaced by argon and milling was resumed. Fig. 3(a) shows that after only 2 h of milling under argon, the intermetallic Mg2Ni was produced, Subsequent milling raised the amount of intermetallic while the MgH 2 phase was somewhat reduced, The DSC result of the sample milled 22 h under hydrogen followed by 8 h of milling under argon is shown in Fig. 2(c). Further milling under argon increased the onset temperature of decomposition of MgH 2 from 225.4°C to 263.4°C. The decomposition peak became much broader, indicating that the reac-
(b)
Fig. 4. SEM micrographs of the mixture 2Mg + Ni milled for 22 h under (a) hydrogen atmosphere and (b) argon atmosphere.
tion proceeded at a much slower speed. Moreover, the MgH 2 abundance slightly decreased to 6 wt.%. By milling under argon, the intermetallic phase Mg2Ni was produced. The presence of this phase increased the decomposition temperature of MgH 2 and slowed the kinetics. 3.3. M A of 2Mg + Ni mixture under argon In this experiment, the mixture 2Mg + Ni was first milled under argon for 22 h. After only 4 h of milling, all the material was sticking to the inner surface of the container and on the balls. The material remained attached to the inner surface for the duration of the experiment. The samples were thereafter collected by scraping a small amount of powder from the surface. Fig. 5 shows the powder diffraction patterns of the mixture 2Mg + Ni milled under argon for various milling times. After 6 h of milling, the peak intensities of magnesium were reduced while those of nickel increased. This effect is due to the difference in the
818
J. Huot et al. / Journal of Alloys and Compounds 231 (1995) 815-819
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.
Fig. 5. X-ray diffraction patterns of mixture 2Mg + Ni milled under argon for various times. Curve (a) is sample milled 22 h under argon followed by 2 h under hydrogen. V, Ni; O, Mg; O, Mg2Ni; ,t, MgH:; O, compound holder.
colored and the appearance was identical to the original powder. Therefore, the milling process was effective only for the surface materials and resulted in a small amount of milled materials. By milling the mixture 2Mg + Ni under argon atmosphere, MgzNi was produced. However, because the milling was operative only on the surface materials, the production of Mg2Ni by mechanical alloying was not effective. First milling in a hydrogen atmosphere had the advantage of keeping the milled product in powder form. When milling was resumed under an argon atmosphere, the milled product had less propensity to stick to the inner surface of the container than the material first milled under argon.
4. Conclusion linear absorption coefficient between magnesium and nickel [15]. Because nickel absorbs X-rays more strongly than magnesium, when the two metals are milled and finely mixed, magnesium peaks will become weaker compared to nickel ones [15]. The intermetallic phase MgzNi appeared after 12 h and the system seems to have reached a steady-state after 22 h of milling. Particle size distribution was similar to the case of milling under hydrogen. In Fig. 4(b) a SEM micrograph of a typical particle is presented. It shows that small particles of size 5 to 15/.,m agglomerate to form a bigger particle. The system having reached a steady-state under argon atmosphere, we switched the atmosphere to hydrogen to investigate the effect of hydrogen atmosphere. Every 2 h, the milling pot was pressurized with new hydrogen. As shown in Fig. 5(a), after 8 h MgH 2 hydride was formed but there was no evidence of formation of the intermetallic hydride. Also, the DSC trace did not show the Mg2Ni hydride phase transformation from a low temperature structure to a high temperature structure [16]. The presence of MgH 2 is confirmed by the DSC measurement shown in Fig. 2(d), where the endothermic peak indicates decomposition of MgH 2. Clearly, curves 2(d) and 2(c) are almost identical. They have the same onset temperature and give the same abundance of MgH 2. In the same way, as shown in Fig. 3(b) and 5(a), the powder diffraction patterns are identical. Therefore, milling in hydrogen followed by milling in argon had the same end product as the reverse order. After completion of the experiment, the material sticking to the inner surface of the container was mechanically removed. The surface material in which the milling balls were impacted was black. This black material was present down to a depth of about 2-3 mm. U n d e r this black layer, the material was silver
In this work, we found that the hydride MgH 2 was easily produced by milling pure Mg powder under a hydrogen atmosphere. This denoted an improvement in the kinetics of hydride formation. The DSC measurement indicated that MgH 2 started to decompose at 440.7°C. When the mixture 2Mg + Ni was milled under hydrogen, the hydride MgH 2 was also easily produced, but not the intermetallic compound Mg2Ni nor the hydride Mg2NiH 4. However, in this case the presence of nickel reduced the onset temperature of MgH 2 decomposition to 225.4°C. When the milling was performed under inert argon atmosphere, the intermetallic Mg2Ni was produced. The presence of MgeNi increased the decomposition temperature of MgH 2 from 225.4°C to 263.4°C and slowed the decomposition rate.
Acknowledgment One of the authors (J.H.) is supported by a fellowship from the Science and Technology Agency of Japan.
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