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Study on high performance Sm2Fe17Nx magnets
A~£OY5
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ANDCOMFO~D5
ELSEVIER
Journal of Alloys and Compounds 222 (]995) 23-26
Study on high performance Sm2Fe17Nx magnets J.L. Wang, W.Z. Li, X.P. Zhong, Y.H. Gao, W.D. Qin, N. Tang, W.G. Lin, J.X. Zhang, R.W. Zhao, Q.W. Yan, Fu-ming Yang Mag,~etism Laboratory, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100080, People's Republic of China
Abstract
Several technological factors which affect the permanent magnetic performance of Sm2Fe~7Nx magnets, such as the phase composition (especially a-Fe), nitrogen and oxygen contents, as well as the ball milling speed and time, have been systematically examined The coercivity increases with decreasing a-Fe content. Oxidation leads to decreases in the saturation magnetization and remanence. The nitrogen content dependence of either the coercivity or remanence shows a maximum at x = 3 . The remanence and coercivity as functions of the ball milling time also show maxima. Compared with high energy ball milling, low e n e r ~ ball milling is more suitable for improving the coercivity and the remanence. SmzFelTN~ powder magnets with Br= 1.2 Y, p~o~Hc=0.8 T and (BH)m,~=25 MGOe have been obtained. Keywords: ~igh performance magnets; Phase composition; Nitrogen content; Oxygen content; Ball milling
1. Introduction
2. Experimental details
Since Coey and Hong Sun reported that the introduction of nitrogen leads to a marked improvement in the hard magnetic properties of RzFe17 compound [1], there has been a great deal of interest in the new research field of the structural and magnetic properties of the interstitial compounds, as well as rare earth iron nitride magnets [2-5]. It has been found that Sm2Fe~7Nx, with a very high saturation magnetization and strong uniaxial anisotropy, is a prominent candidate as a new permanent magnetic material. However, because the SmzFe~TNx compounds decompose into SmN and a-Fe phases at temperatures above 600 °C [1[, the present applications of the Sm2Fe~7Nx compounds are limited to be bonded magnets. Therefore, how to improve the magnetic performance of SmzFe17N~ powders is a very important research aspect. In this paper, several technological factors which affect the performance of Sm2Fe17Nx magnets, such as the phase composition (especially a-Fe), nitrogen and oxygen contents, as well as the ball milling speed and time, have been systematically examined, and the main research results are presented.
The Sm2Fe17 alloy was prepared by Ar arc melting the constituent elements with a purity of at least 99.9%, followed by annealing at 1200 °C for 4 h in vacuum, wrapped in Mo foil and sealed in quartz tubes, then quenching in air. An excess amount of Sin was added to compensate for the loss of Sm during melting. The alloy was mechanically pulverized into a fine powder with an average size of 15-30 ~m. The alloy powders were exposed in a 30% NH3+70% H2 mixed gas at a pressure of 1 atm and temperature of typically 450 °C for a period ranging from 10 min to 4 h. The nitrogen concentration was derived from the mass difference between the samples before and after nitrogenation. The Sm2Fe17Nx powders obtained were ball milled in petroleum ether for different times, by either a high or a low energy ball milling machine. Then, the powders were bonded with epoxy resin and aligned in a magnetic field of 12 kOe. X-ray diffraction with Co Kc~ radiation was used to identify the phases of the alloy. Thermomagnetic analysis (TMA) in a low field of 0.04 T was performed in the temperature range from room temperature to the Curie temperature. The magnetization curves were measured in a pulsed high magnetic field at room temperature.
0925-8388/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0925.8388(94)04906-8
24
ZL. Wang et al. / Journal of Alloys and Compounds 222 (1995) 23-26
3. Results and discussion
Co-Ka To obtain the excellent Sm2Fe17Nx magnetic powders, the composition of the parent compounds was first examined. Because Sm is a very volatile element, it is necessary to consider an excess amount of Sm to compensate for the loss of Sm during arc melting. This compensating amount of Sm depends on the amplitude of the current and on the arc melting time. To obtain the correct composition, a series of test arc melting was performed for the Sm2Fe17_x compounds with x = 4, 3.8, 3.6, 3.4, 3.2, 3.0. The alloys were melted four times and the targets were turned over after each melting. Fig. 1 shows the X-ray diffraction patterns for the different compositions. It can be seen that some SmFe3 phase appeared when the amount of Sm was richer than necessary, whereas an a-Fe peak appeared when the Sm was defficient. It has been found that the composition Sm:Fe = 2:13.6 is the optimum, for which the compensating amount of Sm is about 25%. However, the compensating amount of Sm is also related to the annealing conditions. The better conditions not only discard the Sm-rich phase but also reduce the a-Fe content. The optimum annealing conditions were found to be in 1 atm of high purity Ar and annealing at 1200 °C for 4 h, then quenching in air. X-ray diffraction and TMA show that the parent compound Sm2Fe17 prepared in this way consists of a single phase, as shown in Fig. 2(a). The oxidation leads to decreases in the saturation magnetization and remanence. To avoid oxidizing, high purity petroleum ether was chosen as the ball milling medium and fresh mechanically pulverized powders
SmzFe 17Ny
(b) V
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(a)
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Z
30
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2O Fig. 2. X-ray diffraction patterns for (a) Sm2Fe17and (b) Sm2FeITN3 with Co Ka radiation.
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Z Z x
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40
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50
3.0
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Fig. I. X-ray diffraction patterns for the different Sm2Fel7 ~ compositions with x= 3.8, 3.6, 3.4, 3.0.
were used for nitrogenation. However, some Sm or Cu powders were put near the sample holder to eliminate oxygen before starting the nitrogenation. The dependence of the nitrogen content and the nitrogenation time on the magnetic properties of the nitrides was examined. Fig, 3 shows the thermomagnetic curves of various samples for different nitrogenation times. It can be seen that, on increasing the nitrogenation time, the thermomagnetic curve becomes increasingly higher and the Curie temperature increases; however, when the nitrogenation time is larger than a certain value, the thermomagnetic curve diminishes and the Curie tem-
J.L. Wang et al. / Journal of Alloys and Compounds 222 (1995) 23-26 12
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Fig. 4. Demagnetization curves of the b o n d e d m a g n e t with the powders obtained by a high energy (O) or low energy (Y) ball milling machine.
Fig. 6. B, and iHc values of Sm2Fe]TN3 nitrided powder as functions of the ball milling time tbm.
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B(T) Fig. 7. Typical hysteresis loop of the Sm2FelvN3 nitrided powders.
-12
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H(kOe) Fig. 5. Demagnetization curves of bonded m a g n e t s as a function of the ball milling time for the powder obtained by low energy ball milling: 0 . 270 min; A, 180 rain; II, 160 min; Y, 150 min; O, 120 rain.
perature decreases. A nitrogenation time of 45 min at 450 °C looks to be the best choice. The nitrogen contents were determined by weighing. The nitrogen content depends on the nitrogenation time. The best therrnomagnetic curve was obtained for the nitride with a nitrogen content of about 3.0. The X-ray diffraction pattern for this nitride is shown in Fig. 2(b), where the nitride has less of the a-Fe and
Sm-rich phases, though the coercivity and remanence show maximum values. The Sm2FeaTNx powders were ball milled using a high energy ball milling machine and a low energy ballmilling machine. The high energy ball milling machine has two kinds of steel ball, whose radii are 3.98 and 4.76 mm, and the weight ratio is 1:1. The low energy ball milling machine has three kinds of steel ball, the radii being 3.00, 3.98 and 4.76 mm, and the weight ratio is 1:1:1. The ratio of the weight of the nitrided powder to the steel balls is 1:50. After ball milling, the nitrided powders were bonded with epoxy resin and aligned in a magnetic field of 1.0 T. Fig. 4 shows the demagnetization curves of the bonded magnets for the powders obtained either by the high energy or by the low energy ball milling machine. The
26
J.L. Wang et al. / Journal of Alloys and Compounds 222 (1995) 23-26
original nitride powders are the same for both curves. The ball milling time was 150 rain in both cases. It can be seen that the remanence and coercivity of the powders obtained by high energy ball milling are much lower than those for the case of low energy ball milling. This is because the high energy ball milling leads to more serious grains breaks and cracks in the grains, as well as causing greater breakdown of the crystalline structure in the surface of the particles. The surface will absorb a lot of oxygen, and form an oxidation layer and soft magnetic phase during the ball milling process and the followed procedure, so leading to decreases in the remanence and coercivity. Therefore, the low energy ball milling is more suitable for improving the magnetic performance of the nitrided powder. Fig. 5 shows the demagnetization curves of the bonded magnets as a function of the ball milling time, for the powders obtained by low energy ball-milling. The values of the magnetization in Figs. 4-6 have been calibrated to be those for the nitrided powders using an X-ray density of Sm2FeavN3 of 7.7 g .cm -3. The remanence and the coercivity change on increasing the ball milling time, such that a ball milling time of 3 h is the optimum. The values of the remanence Br and the coercivity iHc derived from the demagnetization curves as a function of the ball milling time tbm are shown in Fig. 6. It can be seen that the values of B r and iHc increase with increasing ball milling time tbm, going through their respective maxima at tbm=2.5 and 4 h, then decreasing with increasing tbm. The increases in Br and iHc may be associated with a decrease in the grain size. On increasing the ball
milling time, more grains become single-domain particles, which will increase the coercivity and make the particle orientation in the magnetic field easy, causing an increase in the remanence. The average size of the particles in the case of tbm = 3 h is about 1 /xm, and the distribution of the grain size is basically homogeneous. A longer ball milling time will lead to the destruction of the crystalline grains and will produce an amount of c~-Fe phase, which can be proved by comparing the X-ray diffraction pattern and thermomagnetic curves of samples before and after ball milling. Fig. 7 shows a typical hysteresis loop of the Sm2FelTN3 nitrided powders. By using the optimum technological process mentioned above, Sm2FelvN3 powders with Br = 1.2 T, ~ o i H c = 0.8 T and (B/-/)max = 25 MGOe have been obtained.
Acknowledgment The present investigation was supported by the National Science Foundation of China.
References [11 J.M.D. Coey and Hong Sun, J. Magn. Magn. Mater., 87 (1990) L251. [2] T. Capehart, R.K. Mishra and F.E. Pinkerton, Appl. Phys. Lett., 58 (1991) 1395. [3] M. Katter, J. Wecker, L. Schultz and R. Grossinger, J. Magn. Magn. Mater., 92 (1990) L14. [4] K.H.J. Buschow, R. Coehoorn, D.B. de Mooij, K. de Waard and T.H. Jacobs, J. Magn. Magn. Mater., 92 (1990) L35. [5] O. Isnard, S. Miraglia, J.L. Soubeyroux, J. Pannetier and D. Fruchart, Phys. Rev. B, 45 (1992) 2920.
'
ANDCOMFO~D5
ELSEVIER
Journal of Alloys and Compounds 222 (]995) 23-26
Study on high performance Sm2Fe17Nx magnets J.L. Wang, W.Z. Li, X.P. Zhong, Y.H. Gao, W.D. Qin, N. Tang, W.G. Lin, J.X. Zhang, R.W. Zhao, Q.W. Yan, Fu-ming Yang Mag,~etism Laboratory, Institute of Physics, Chinese Academy of Sciences, P.O. Box 603, Beijing 100080, People's Republic of China
Abstract
Several technological factors which affect the permanent magnetic performance of Sm2Fe~7Nx magnets, such as the phase composition (especially a-Fe), nitrogen and oxygen contents, as well as the ball milling speed and time, have been systematically examined The coercivity increases with decreasing a-Fe content. Oxidation leads to decreases in the saturation magnetization and remanence. The nitrogen content dependence of either the coercivity or remanence shows a maximum at x = 3 . The remanence and coercivity as functions of the ball milling time also show maxima. Compared with high energy ball milling, low e n e r ~ ball milling is more suitable for improving the coercivity and the remanence. SmzFelTN~ powder magnets with Br= 1.2 Y, p~o~Hc=0.8 T and (BH)m,~=25 MGOe have been obtained. Keywords: ~igh performance magnets; Phase composition; Nitrogen content; Oxygen content; Ball milling
1. Introduction
2. Experimental details
Since Coey and Hong Sun reported that the introduction of nitrogen leads to a marked improvement in the hard magnetic properties of RzFe17 compound [1], there has been a great deal of interest in the new research field of the structural and magnetic properties of the interstitial compounds, as well as rare earth iron nitride magnets [2-5]. It has been found that Sm2Fe~7Nx, with a very high saturation magnetization and strong uniaxial anisotropy, is a prominent candidate as a new permanent magnetic material. However, because the SmzFe~TNx compounds decompose into SmN and a-Fe phases at temperatures above 600 °C [1[, the present applications of the Sm2Fe~7Nx compounds are limited to be bonded magnets. Therefore, how to improve the magnetic performance of SmzFe17N~ powders is a very important research aspect. In this paper, several technological factors which affect the performance of Sm2Fe17Nx magnets, such as the phase composition (especially a-Fe), nitrogen and oxygen contents, as well as the ball milling speed and time, have been systematically examined, and the main research results are presented.
The Sm2Fe17 alloy was prepared by Ar arc melting the constituent elements with a purity of at least 99.9%, followed by annealing at 1200 °C for 4 h in vacuum, wrapped in Mo foil and sealed in quartz tubes, then quenching in air. An excess amount of Sin was added to compensate for the loss of Sm during melting. The alloy was mechanically pulverized into a fine powder with an average size of 15-30 ~m. The alloy powders were exposed in a 30% NH3+70% H2 mixed gas at a pressure of 1 atm and temperature of typically 450 °C for a period ranging from 10 min to 4 h. The nitrogen concentration was derived from the mass difference between the samples before and after nitrogenation. The Sm2Fe17Nx powders obtained were ball milled in petroleum ether for different times, by either a high or a low energy ball milling machine. Then, the powders were bonded with epoxy resin and aligned in a magnetic field of 12 kOe. X-ray diffraction with Co Kc~ radiation was used to identify the phases of the alloy. Thermomagnetic analysis (TMA) in a low field of 0.04 T was performed in the temperature range from room temperature to the Curie temperature. The magnetization curves were measured in a pulsed high magnetic field at room temperature.
0925-8388/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0925.8388(94)04906-8
24
ZL. Wang et al. / Journal of Alloys and Compounds 222 (1995) 23-26
3. Results and discussion
Co-Ka To obtain the excellent Sm2Fe17Nx magnetic powders, the composition of the parent compounds was first examined. Because Sm is a very volatile element, it is necessary to consider an excess amount of Sm to compensate for the loss of Sm during arc melting. This compensating amount of Sm depends on the amplitude of the current and on the arc melting time. To obtain the correct composition, a series of test arc melting was performed for the Sm2Fe17_x compounds with x = 4, 3.8, 3.6, 3.4, 3.2, 3.0. The alloys were melted four times and the targets were turned over after each melting. Fig. 1 shows the X-ray diffraction patterns for the different compositions. It can be seen that some SmFe3 phase appeared when the amount of Sm was richer than necessary, whereas an a-Fe peak appeared when the Sm was defficient. It has been found that the composition Sm:Fe = 2:13.6 is the optimum, for which the compensating amount of Sm is about 25%. However, the compensating amount of Sm is also related to the annealing conditions. The better conditions not only discard the Sm-rich phase but also reduce the a-Fe content. The optimum annealing conditions were found to be in 1 atm of high purity Ar and annealing at 1200 °C for 4 h, then quenching in air. X-ray diffraction and TMA show that the parent compound Sm2Fe17 prepared in this way consists of a single phase, as shown in Fig. 2(a). The oxidation leads to decreases in the saturation magnetization and remanence. To avoid oxidizing, high purity petroleum ether was chosen as the ball milling medium and fresh mechanically pulverized powders
SmzFe 17Ny
(b) V
4
Oq
Z
(a)
1 ~ SrnzFe17
Z
30
40
50
60
2O Fig. 2. X-ray diffraction patterns for (a) Sm2Fe17and (b) Sm2FeITN3 with Co Ka radiation.
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T(°C) Fig. 3. Thermomagnetic curves of various samples for different nitrogenation times: O, 20 min; Y, 25 min; II, 30 min; A, 45 min; , , 60 rain.
Z Z x
30
40
20
50
3.0
80
Fig. I. X-ray diffraction patterns for the different Sm2Fel7 ~ compositions with x= 3.8, 3.6, 3.4, 3.0.
were used for nitrogenation. However, some Sm or Cu powders were put near the sample holder to eliminate oxygen before starting the nitrogenation. The dependence of the nitrogen content and the nitrogenation time on the magnetic properties of the nitrides was examined. Fig, 3 shows the thermomagnetic curves of various samples for different nitrogenation times. It can be seen that, on increasing the nitrogenation time, the thermomagnetic curve becomes increasingly higher and the Curie temperature increases; however, when the nitrogenation time is larger than a certain value, the thermomagnetic curve diminishes and the Curie tem-
J.L. Wang et al. / Journal of Alloys and Compounds 222 (1995) 23-26 12
25
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•
high energy ball-milling
b
-
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9
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9
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o
ZZ 2 0
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-t~
-9
-6
-3
0
50
Fig. 4. Demagnetization curves of the b o n d e d m a g n e t with the powders obtained by a high energy (O) or low energy (Y) ball milling machine.
Fig. 6. B, and iHc values of Sm2Fe]TN3 nitrided powder as functions of the ball milling time tbm.
[5 neeoeee+
12
~1, • • • •
270 180 160 150 120
min rain min rain min
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B(T) Fig. 7. Typical hysteresis loop of the Sm2FelvN3 nitrided powders.
-12
-9
-~ -6
-3
0
H(kOe) Fig. 5. Demagnetization curves of bonded m a g n e t s as a function of the ball milling time for the powder obtained by low energy ball milling: 0 . 270 min; A, 180 rain; II, 160 min; Y, 150 min; O, 120 rain.
perature decreases. A nitrogenation time of 45 min at 450 °C looks to be the best choice. The nitrogen contents were determined by weighing. The nitrogen content depends on the nitrogenation time. The best therrnomagnetic curve was obtained for the nitride with a nitrogen content of about 3.0. The X-ray diffraction pattern for this nitride is shown in Fig. 2(b), where the nitride has less of the a-Fe and
Sm-rich phases, though the coercivity and remanence show maximum values. The Sm2FeaTNx powders were ball milled using a high energy ball milling machine and a low energy ballmilling machine. The high energy ball milling machine has two kinds of steel ball, whose radii are 3.98 and 4.76 mm, and the weight ratio is 1:1. The low energy ball milling machine has three kinds of steel ball, the radii being 3.00, 3.98 and 4.76 mm, and the weight ratio is 1:1:1. The ratio of the weight of the nitrided powder to the steel balls is 1:50. After ball milling, the nitrided powders were bonded with epoxy resin and aligned in a magnetic field of 1.0 T. Fig. 4 shows the demagnetization curves of the bonded magnets for the powders obtained either by the high energy or by the low energy ball milling machine. The
26
J.L. Wang et al. / Journal of Alloys and Compounds 222 (1995) 23-26
original nitride powders are the same for both curves. The ball milling time was 150 rain in both cases. It can be seen that the remanence and coercivity of the powders obtained by high energy ball milling are much lower than those for the case of low energy ball milling. This is because the high energy ball milling leads to more serious grains breaks and cracks in the grains, as well as causing greater breakdown of the crystalline structure in the surface of the particles. The surface will absorb a lot of oxygen, and form an oxidation layer and soft magnetic phase during the ball milling process and the followed procedure, so leading to decreases in the remanence and coercivity. Therefore, the low energy ball milling is more suitable for improving the magnetic performance of the nitrided powder. Fig. 5 shows the demagnetization curves of the bonded magnets as a function of the ball milling time, for the powders obtained by low energy ball-milling. The values of the magnetization in Figs. 4-6 have been calibrated to be those for the nitrided powders using an X-ray density of Sm2FeavN3 of 7.7 g .cm -3. The remanence and the coercivity change on increasing the ball milling time, such that a ball milling time of 3 h is the optimum. The values of the remanence Br and the coercivity iHc derived from the demagnetization curves as a function of the ball milling time tbm are shown in Fig. 6. It can be seen that the values of B r and iHc increase with increasing ball milling time tbm, going through their respective maxima at tbm=2.5 and 4 h, then decreasing with increasing tbm. The increases in Br and iHc may be associated with a decrease in the grain size. On increasing the ball
milling time, more grains become single-domain particles, which will increase the coercivity and make the particle orientation in the magnetic field easy, causing an increase in the remanence. The average size of the particles in the case of tbm = 3 h is about 1 /xm, and the distribution of the grain size is basically homogeneous. A longer ball milling time will lead to the destruction of the crystalline grains and will produce an amount of c~-Fe phase, which can be proved by comparing the X-ray diffraction pattern and thermomagnetic curves of samples before and after ball milling. Fig. 7 shows a typical hysteresis loop of the Sm2FelTN3 nitrided powders. By using the optimum technological process mentioned above, Sm2FelvN3 powders with Br = 1.2 T, ~ o i H c = 0.8 T and (B/-/)max = 25 MGOe have been obtained.
Acknowledgment The present investigation was supported by the National Science Foundation of China.
References [11 J.M.D. Coey and Hong Sun, J. Magn. Magn. Mater., 87 (1990) L251. [2] T. Capehart, R.K. Mishra and F.E. Pinkerton, Appl. Phys. Lett., 58 (1991) 1395. [3] M. Katter, J. Wecker, L. Schultz and R. Grossinger, J. Magn. Magn. Mater., 92 (1990) L14. [4] K.H.J. Buschow, R. Coehoorn, D.B. de Mooij, K. de Waard and T.H. Jacobs, J. Magn. Magn. Mater., 92 (1990) L35. [5] O. Isnard, S. Miraglia, J.L. Soubeyroux, J. Pannetier and D. Fruchart, Phys. Rev. B, 45 (1992) 2920.