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Plasma nitriding of Sm2Fe17
Journal of Alloys and Compounds, 193 (1993) 271-273 JALCOM 2036
271
Plasma nitriding of Sm2Fe17 Ken-ichi Machida*,
Eiji Y a m a m o t o
and Gin-ya Adachi*
Department Of Applied Chemistry, Faculty of Engineering, Osaka University, Yamadaoka 2-I, Suita, Osaka 565 (Japan)
Abstract An intermetallic compound Sm2Fel7Nx was synthesized by plasma nitriding of Sm2Fe17 in a stream of N2-H 2 mixed gas. The reaction proceeded at a lower temperature (around 423 K) than that of the conventional thermal technique, but not at room temperature. This is discussed on the basis of the difference between the reaction mechanisms of plasma and thermal nitridings.
I. Introduction Since Coey and Sun [1] reported that Sm2Fe~7N x possesses excellent magnetic properties such as high Curie temperature, strong uniaxial anisotropy and high saturation magnetization, it has received much attention as a new material for high-performance permanent magnets [2-5]. However, Sm2Fe17Nx is metastable and decomposes to SmN and c~-Fe at a high temperature above ca. 923 K [6]. It is well known that plasma techniques usually enhance chemical reactions [7]. If a plasma nitriding method is applicable to this system, one can expect to perform the nitriding of SmzFe~7 at low temperatures and avoid the accompanying thermal decomposition. In this work, the plasma nitriding of Sm2Fe~7 has been carried out using a glow-discharge, and the temperature and time dependences of products are discussed on the basis of the reaction mechanism.
through reduced copper and P205 columns, but the hydrogen gas was used without any further purification. A glass cell equipped with a stainless steel trayshaped electrode for samples and an aluminum counter electrode was used for the plasma nitriding. The tray with a diameter of ca. 18 mm was charged with the Sm2Fe~7 powder (ca. 150 mg) or disks, and then the cell was evacuated up to a background pressure below 1 x 10 -4 Torr. The plasma nitriding was performed by the glow-discharge between the electrodes under a differential pumping condition at 2 Torr of a N 2 - H 2 mixed gas with molar ratio of 1:2. The temperature was controlled by an electric furnace. The resulting nitrides were identified by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements, and the nitrogen contents were determined by using a Horiba EMGA-650 oxygen and nitrogen analyzer.
3. Results and discussion 2. Experimental details The intermetallic compound Sm2Fel7 annealed for homogenization was supplied by Santoku Kinzoku Inc. and its composition was as follows: Sm, 24.78 wt.%; Fe, 74.88 wt.% (impurities (parts per million): A1, 300; Si, 100; Cu, 100; C, 20; O, 351; N, 44). The as-obtained ingot was powdered for 30 min up to a particle size below 70 lam and was usually employed for the plasma nitriding. Disk samples (5 mm diameter x 2 mm) for XPS measurements were prepared by cutting the ingot, polishing the surface and washing with acetone. The nitrogen gas for the nitriding was purified by passing it
*To whom correspondence should be addressed.
0925-8388/93/$6.00
3.1. X R D patterns
A series of X R D patterns for the samples obtained by the plasma treatments at 423-723 K for 2 h are shown in Fig. 1, together with that of the as-obtained Sm2Fe~7 (nitrogen free). Although the nitriding of SmzFel7 did not take place up to the bulk level for X R D at room temperature, heating above 423 K accelerated the formation of Sm2FelTNx. In particular, the X R D pattern of the sample obtained at 573 K is regarded as the single phase of Sm2FelyNx, and thus this temperature is high enough to perform the plasma nitriding. Since the thermal nitriding of Sm2Fe~7 hardly proceeds at this temperature, the plasma reaction seems to lower the temperature for the formation of Sm2Fel7N x. The thermal nitriding mechanism takes place by reaction with the atomic nitrogen derived from N2 or N H 3
© 1993 ElsevierSequoia. All rights reserved
K-L Machida et al. / Plasma nitriding of Sm2Fet7
272 25
30 i
35 i
40
45
i
(a)
(303) (113)
(300)
50
i
25
30
35
40
i
I
i
45
o~
I
(220) ~ (060) (a)
(b) E
(113) ~ ,E
50
1
(300)
(303) (220) j~ Jl (060) I 1(214)
(3o3) (220) j 1~ (060)
(c) (113)
I ~(214)
(300)
(d) 2 O/degree
3'0
3;
4'0
&
Fig. 2. XRD patterns (CuK~ radiation) of the plasma-treated samples (Sm2FelTNx) at 573 K for (a) 1 h, (b) 2 h and (c) 4 h in the Nz-H2 mixed gas with molar ratio = 1:2 (2 Torr).
2e/degree
Fig. 1. XRD patterns (CuK~ radiation) of (a) the as-obtained sample (Sm2Fe17) and (b)-(d) the plasma-treated samples (Sm2Fe~vNx) at 423-723 K for 2 h in the N z - F I 2 mixed gas with molar ratio = 1:2 (2 Torr). and it is in agreement with the fact that NH3 is much more reactive for nitriding than Nz. This is because the tight N - N bond of the N 2 molecule and NH3 can easily provide the atomic nitrogen on the surface of the sample by the dissociation adsorption compared with N 2•
It is well known that the N 2 - H e plasma contains ion species such as N + and N H x + [8], which are so active that the nitriding should take place even at r o o m temperature. Therefore, the temperature for performing plasma nitriding is mainly needed for the migration of nitrogen to the sample bulk. The degree of crystallinity of the samples is expected to be improved by increasing the temperature for the plasma nitriding. Indeed, the sample obtained at 723 K was of better crystallinity. Figure 2 shows the X R D patterns of the samples obtained after the plasma treatment at 573 K for 1 - 4 h. Although the sample treated only for 1 h gave a broad X R D pattern, the diffraction peaks were intensified with the prolonged duration of nitriding. The X R D patterns of the samples treated for 2 and 4 h are assigned to the single phase of SmzFe]7N x. Nitrogen contents and lattice parameters of the samples shown in Fig. 2 are summarized in Table 1. The nitrogen content was increased with the reaction time and the value of x = 2.9 was observed on the sample treated for 4 h. Also, the cell lattice was expanded with the duration of plasma nitriding as follows: a = 8.54/~
TABLE 1. Nitrogen content and lattice parameters of the plasma-treated samplesa
Reaction time
Nitrogen content
Lattice parameters (A)
(h) 1
2 4
Sm2Fel7N1. 3 SmzFe17Nz. 4
SmzFelTN2.9
a
c
_b
_b
8.76 8.76
12.66 12.70
aAt 423 K in the N z - H 2 mixed gas with molar ratio= 1:2 (2 Torr). bThe XRD pattern of this sample was too broad to calculate the lattice parameters. and c = 12.43/~, SmzFe~7 (as-obtained); a = 8 . 7 6 / ~ and c = 12.70/~, SmzFe~7Nx (treated for 4 h).
3.2. X P S signals XPS signals of Sm3d, Fe2p and N l s electrons were measured on the SmzFel7N x disk samples before and after the plasma nitriding at 573 K for 20 min. After Ar + b o m b a r d m e n t s for several minutes, the as-obtained sample provided Sm3ds/2 and Fe2p3/2 signal bands with peak positions at 1082.6 and 706.4 eV, which are assigned to S m 2 0 3 and Fe, respectively [9]. This means that the Sm around the surface is oxidized. For the sample after the plasma nitriding, a series of XPS signals were observed at the same positions of 1082.3 and 706.1eV as the nitrogen-free sample, indicating that the Sm and Fe around the surface of Sm2Fe~vNx are also Sm203 and Fe. However, an additional XPS signal of nitrogen was observed at 396.0 eV, which was assigned to be of the nitrogen in nitrides such as W N or BN [9].
K-I. Machida et al./ Plasma nitriding of Sm2Fet7
4. Conclusions T h e p l a s m a - n i t r i d i n g o f Sm2Fe~7 lowers the r e a c t i o n t e m p e r a t u r e c o m p a r e d with the c o n v e n t i o n a l t h e r m a l technique. This is r e s p o n s i b l e for the reactive ion species such as N + a n d N H x + g e n e r a t e d in the p l a s m a process. H o w e v e r , even for the p l a s m a nitriding, heating is necessary for the m i g r a t i o n o f n i t r o g e n to the s a m p l e bulk.
References 1 J. M. D. Coey and H. Sun, J. Magn. Magn. Mater., 87(1990 L251.
273
2 H. Sun, J. M. D. Coey, Y. Otani and D. P. F. Hurley, J. Phys. Condens. Matter, 2 (1990) 6465. 3 B.-P. Hu, H.-S. Li, H. Sun and J. M. D. Coey, J. Phys. Condens. Matter, 3 (1991) 3983. 4 R. M. Ibberson, O. Moze, T. H. Jacobs and K. H. J. Buschow, J. Phys. Condens. Matter, 3 (1991) 1219. 5 Y. Otani, A. Moukarika, H. Sun, J. M. D. Coey, E. Devlin and I. R. Harris, J. Appl. Phys., 69 (1991) 6735. 6 Y. Otani, D. P. F. Hurley, H. Sun and J. M. D. Coey, J. Phys. Condens. Matter, 69 (1991) 5584. 7 F. K. McTaggert, Plasma Chemistry in Electrical Discharge, Elsevier, Amsterdam, 1967. 8 T. Shibutani, Y. Kanzaki and O. Matsumoto, J. Less-Common Met., 113 (1985) 177. 9 C. D. Wagner, W. M. Riggs, L. E. Davis, J. F. Moulder and G. E. Muilenberg, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer, Physical Electronics Division, Eden Prairie, MN, 1979, pp. 40, 41, 76, 77, 136, 137.
271
Plasma nitriding of Sm2Fe17 Ken-ichi Machida*,
Eiji Y a m a m o t o
and Gin-ya Adachi*
Department Of Applied Chemistry, Faculty of Engineering, Osaka University, Yamadaoka 2-I, Suita, Osaka 565 (Japan)
Abstract An intermetallic compound Sm2Fel7Nx was synthesized by plasma nitriding of Sm2Fe17 in a stream of N2-H 2 mixed gas. The reaction proceeded at a lower temperature (around 423 K) than that of the conventional thermal technique, but not at room temperature. This is discussed on the basis of the difference between the reaction mechanisms of plasma and thermal nitridings.
I. Introduction Since Coey and Sun [1] reported that Sm2Fe~7N x possesses excellent magnetic properties such as high Curie temperature, strong uniaxial anisotropy and high saturation magnetization, it has received much attention as a new material for high-performance permanent magnets [2-5]. However, Sm2Fe17Nx is metastable and decomposes to SmN and c~-Fe at a high temperature above ca. 923 K [6]. It is well known that plasma techniques usually enhance chemical reactions [7]. If a plasma nitriding method is applicable to this system, one can expect to perform the nitriding of SmzFe~7 at low temperatures and avoid the accompanying thermal decomposition. In this work, the plasma nitriding of Sm2Fe~7 has been carried out using a glow-discharge, and the temperature and time dependences of products are discussed on the basis of the reaction mechanism.
through reduced copper and P205 columns, but the hydrogen gas was used without any further purification. A glass cell equipped with a stainless steel trayshaped electrode for samples and an aluminum counter electrode was used for the plasma nitriding. The tray with a diameter of ca. 18 mm was charged with the Sm2Fe~7 powder (ca. 150 mg) or disks, and then the cell was evacuated up to a background pressure below 1 x 10 -4 Torr. The plasma nitriding was performed by the glow-discharge between the electrodes under a differential pumping condition at 2 Torr of a N 2 - H 2 mixed gas with molar ratio of 1:2. The temperature was controlled by an electric furnace. The resulting nitrides were identified by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) measurements, and the nitrogen contents were determined by using a Horiba EMGA-650 oxygen and nitrogen analyzer.
3. Results and discussion 2. Experimental details The intermetallic compound Sm2Fel7 annealed for homogenization was supplied by Santoku Kinzoku Inc. and its composition was as follows: Sm, 24.78 wt.%; Fe, 74.88 wt.% (impurities (parts per million): A1, 300; Si, 100; Cu, 100; C, 20; O, 351; N, 44). The as-obtained ingot was powdered for 30 min up to a particle size below 70 lam and was usually employed for the plasma nitriding. Disk samples (5 mm diameter x 2 mm) for XPS measurements were prepared by cutting the ingot, polishing the surface and washing with acetone. The nitrogen gas for the nitriding was purified by passing it
*To whom correspondence should be addressed.
0925-8388/93/$6.00
3.1. X R D patterns
A series of X R D patterns for the samples obtained by the plasma treatments at 423-723 K for 2 h are shown in Fig. 1, together with that of the as-obtained Sm2Fe~7 (nitrogen free). Although the nitriding of SmzFel7 did not take place up to the bulk level for X R D at room temperature, heating above 423 K accelerated the formation of Sm2FelTNx. In particular, the X R D pattern of the sample obtained at 573 K is regarded as the single phase of Sm2FelyNx, and thus this temperature is high enough to perform the plasma nitriding. Since the thermal nitriding of Sm2Fe~7 hardly proceeds at this temperature, the plasma reaction seems to lower the temperature for the formation of Sm2Fel7N x. The thermal nitriding mechanism takes place by reaction with the atomic nitrogen derived from N2 or N H 3
© 1993 ElsevierSequoia. All rights reserved
K-L Machida et al. / Plasma nitriding of Sm2Fet7
272 25
30 i
35 i
40
45
i
(a)
(303) (113)
(300)
50
i
25
30
35
40
i
I
i
45
o~
I
(220) ~ (060) (a)
(b) E
(113) ~ ,E
50
1
(300)
(303) (220) j~ Jl (060) I 1(214)
(3o3) (220) j 1~ (060)
(c) (113)
I ~(214)
(300)
(d) 2 O/degree
3'0
3;
4'0
&
Fig. 2. XRD patterns (CuK~ radiation) of the plasma-treated samples (Sm2FelTNx) at 573 K for (a) 1 h, (b) 2 h and (c) 4 h in the Nz-H2 mixed gas with molar ratio = 1:2 (2 Torr).
2e/degree
Fig. 1. XRD patterns (CuK~ radiation) of (a) the as-obtained sample (Sm2Fe17) and (b)-(d) the plasma-treated samples (Sm2Fe~vNx) at 423-723 K for 2 h in the N z - F I 2 mixed gas with molar ratio = 1:2 (2 Torr). and it is in agreement with the fact that NH3 is much more reactive for nitriding than Nz. This is because the tight N - N bond of the N 2 molecule and NH3 can easily provide the atomic nitrogen on the surface of the sample by the dissociation adsorption compared with N 2•
It is well known that the N 2 - H e plasma contains ion species such as N + and N H x + [8], which are so active that the nitriding should take place even at r o o m temperature. Therefore, the temperature for performing plasma nitriding is mainly needed for the migration of nitrogen to the sample bulk. The degree of crystallinity of the samples is expected to be improved by increasing the temperature for the plasma nitriding. Indeed, the sample obtained at 723 K was of better crystallinity. Figure 2 shows the X R D patterns of the samples obtained after the plasma treatment at 573 K for 1 - 4 h. Although the sample treated only for 1 h gave a broad X R D pattern, the diffraction peaks were intensified with the prolonged duration of nitriding. The X R D patterns of the samples treated for 2 and 4 h are assigned to the single phase of SmzFe]7N x. Nitrogen contents and lattice parameters of the samples shown in Fig. 2 are summarized in Table 1. The nitrogen content was increased with the reaction time and the value of x = 2.9 was observed on the sample treated for 4 h. Also, the cell lattice was expanded with the duration of plasma nitriding as follows: a = 8.54/~
TABLE 1. Nitrogen content and lattice parameters of the plasma-treated samplesa
Reaction time
Nitrogen content
Lattice parameters (A)
(h) 1
2 4
Sm2Fel7N1. 3 SmzFe17Nz. 4
SmzFelTN2.9
a
c
_b
_b
8.76 8.76
12.66 12.70
aAt 423 K in the N z - H 2 mixed gas with molar ratio= 1:2 (2 Torr). bThe XRD pattern of this sample was too broad to calculate the lattice parameters. and c = 12.43/~, SmzFe~7 (as-obtained); a = 8 . 7 6 / ~ and c = 12.70/~, SmzFe~7Nx (treated for 4 h).
3.2. X P S signals XPS signals of Sm3d, Fe2p and N l s electrons were measured on the SmzFel7N x disk samples before and after the plasma nitriding at 573 K for 20 min. After Ar + b o m b a r d m e n t s for several minutes, the as-obtained sample provided Sm3ds/2 and Fe2p3/2 signal bands with peak positions at 1082.6 and 706.4 eV, which are assigned to S m 2 0 3 and Fe, respectively [9]. This means that the Sm around the surface is oxidized. For the sample after the plasma nitriding, a series of XPS signals were observed at the same positions of 1082.3 and 706.1eV as the nitrogen-free sample, indicating that the Sm and Fe around the surface of Sm2Fe~vNx are also Sm203 and Fe. However, an additional XPS signal of nitrogen was observed at 396.0 eV, which was assigned to be of the nitrogen in nitrides such as W N or BN [9].
K-I. Machida et al./ Plasma nitriding of Sm2Fet7
4. Conclusions T h e p l a s m a - n i t r i d i n g o f Sm2Fe~7 lowers the r e a c t i o n t e m p e r a t u r e c o m p a r e d with the c o n v e n t i o n a l t h e r m a l technique. This is r e s p o n s i b l e for the reactive ion species such as N + a n d N H x + g e n e r a t e d in the p l a s m a process. H o w e v e r , even for the p l a s m a nitriding, heating is necessary for the m i g r a t i o n o f n i t r o g e n to the s a m p l e bulk.
References 1 J. M. D. Coey and H. Sun, J. Magn. Magn. Mater., 87(1990 L251.
273
2 H. Sun, J. M. D. Coey, Y. Otani and D. P. F. Hurley, J. Phys. Condens. Matter, 2 (1990) 6465. 3 B.-P. Hu, H.-S. Li, H. Sun and J. M. D. Coey, J. Phys. Condens. Matter, 3 (1991) 3983. 4 R. M. Ibberson, O. Moze, T. H. Jacobs and K. H. J. Buschow, J. Phys. Condens. Matter, 3 (1991) 1219. 5 Y. Otani, A. Moukarika, H. Sun, J. M. D. Coey, E. Devlin and I. R. Harris, J. Appl. Phys., 69 (1991) 6735. 6 Y. Otani, D. P. F. Hurley, H. Sun and J. M. D. Coey, J. Phys. Condens. Matter, 69 (1991) 5584. 7 F. K. McTaggert, Plasma Chemistry in Electrical Discharge, Elsevier, Amsterdam, 1967. 8 T. Shibutani, Y. Kanzaki and O. Matsumoto, J. Less-Common Met., 113 (1985) 177. 9 C. D. Wagner, W. M. Riggs, L. E. Davis, J. F. Moulder and G. E. Muilenberg, Handbook of X-ray Photoelectron Spectroscopy, Perkin-Elmer, Physical Electronics Division, Eden Prairie, MN, 1979, pp. 40, 41, 76, 77, 136, 137.