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Preparation of submicron-sized Sm2Fe17N3 fine powder with high coercivity by reduction-diffusion process
Journal of Alloys and Compounds xxx (2016) 1e7
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Preparation of submicron-sized Sm2Fe17N3 fine powder with high coercivity by reduction-diffusion process Shusuke Okada a, *, Kazuyuki Suzuki a, Eri Node a, Kenta Takagi a, Kimihiro Ozaki a, Yasushi Enokido b a
Magnetic Powder Metallurgy Research Center, National Institute of Advanced Industrial Science and Technology (AIST), 2266-98 Anagahora, Shimoshidami, Moriyama, Nagoya, 463-8560, Japan Materials Development Center, TDK Corporation, 570-2, Matsugasita, Minamihadori, Narita, 268-8588, Japan
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 July 2016 Received in revised form 14 October 2016 Accepted 30 October 2016 Available online xxx
Submicron-sized Sm2Fe17N3 powders having high coercivity were prepared by a newly-developed synthesis procedure for submicron-sized Sm2Fe17 powders and nitridation process with a proper postprocess for the powders. It was revealed that the washing step, which is performed to remove excess Ca, supplied hydrogen into the Sm2Fe17N3 crystal structure, and elongation of the crystal structure along the c-axis by the supplied hydrogen reduced coercivity. This phenomenon was observed clearly in powder with a smaller size and good dispersity. When the powders were subjected to dehydrogenation treatment, they showed high coercivity, as expected from the particle size, and coercivity of 24.7 kOe was achieved with the 0.5 mm powder. The fact that the intrinsic good thermal stability of Sm2Fe17N3 is maintained at the submicron-scale was also confirmed. In addition, the obtained powder exhibited a high maximum energy product after disintegration treatment under appropriate conditions. This study demonstrated the high potential of Sm2Fe17N3 for surpassing the performance of NdeFeeB magnets under hot environments. © 2016 Elsevier B.V. All rights reserved.
Keywords: Sm2Fe17N3 SmeFeeN Reduction-diffusion Rare-earth permanent magnet
1. Introduction With expanding use of permanent-magnet motors in electronic appliances and electric vehicles, enhancement of the magnetic properties of permanent magnets has become a very important research topic for energy saving. Currently, NdeFeeB sintered magnets are used in these motors. However, the Curie temperature of NdeFeeB magnets is about 585 K, which causes a serious decrement of magnetic properties in motors exposed to a hot environment. Dy and Tb are used to improve the temperature tolerance of NdeFeeB, but use of these heavy rare earth elements results in low remanence magnetization and can be affected by supply instability. Considering this, Sm2Fe17N3 is a promising candidate to replace NdeFeeB magnets, because Sm2Fe17N3 has a high Curie temperature (750 K) with a huge anisotropy field (~260 kOe) and comparable high saturation magnetization (1.54 T) [1e5]. However, it is difficult to manufacture Sm2Fe17N3 sintered
* Corresponding author. E-mail address: [email protected] (S. Okada).
bulk magnets, as the intermetallic alloy decomposes at around 873 K. Our research group took on the challenge of realizing sintered Sm2Fe17N3 magnets. In previous work, we reported a process for producing isotropic Sm2Fe17N3 sintered bulk magnets by a highpressure current sintering technique at below 723 K [6]. At present, we are attempting to develop a sintering process for Sm2Fe17N3 anisotropic sintered magnets. To realize anisotropic Sm2Fe17N3 sintered magnets which surpass the conventional NdeFeeB sintered magnets, it is also necessary to develop a high magnetic performance Sm2Fe17N3 powder, especially a powder having high coercivity. C. Kuhrt et al. and A. Teresiak et al. reported isotropic Sm2Fe17N3 powders having coercivity of 32 kOe and 37 kOe by using mechanical alloying, respectively [7,8]. However, the coercivity of high coercivity anisotropic Sm2Fe17N3 powders has been limited to only around 17e18 kOe [9]. Recently, Hirayama et al. reported a Sm2Fe17N3 powder with the size of 0.69 mm, which displayed coercivity of 23.2 kOe, by a polymerized-complex and reduction-diffusion process [10]. However, the values of remanent magnetization were not described, and the hysteresis curve contained a step which was thought to be attributable to a soft magnetic phase. Moreover, this submicron-sized powder appears to be
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Please cite this article in press as: S. Okada, et al., Preparation of submicron-sized Sm2Fe17N3 fine powder with high coercivity by reductiondiffusion process, Journal of Alloys and Compounds (2016), http://dx.doi.org/10.1016/j.jallcom.2016.10.306
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a strictly sintered, which would make it difficult to express a high maximum energy product (BHmax). We have also attempted to prepare a high coercivity Sm2Fe17N3 powder by reducing the particle size, and we have reported a submicron-sized Sm2Fe17 powder which was produced by reduction-diffusion of a precursor prepared by a coprecipitation technique [11]. In the present report, we describe the development of a nitridation process and post-process for submicron-sized powders. Utilizing these newly-developed processes, we succeeded in preparing submicron-sized Sm2Fe17N3 powders having high coercivity of up to 24.7 kOe without steps in the hysteresis curve. The temperature dependence of coercivity of the submicron-sized powder was also investigated. In addition, since the as-prepared submicron-sized Sm2Fe17N3 powder was partially sintered, its remanent magnetization properties were not high considering the intrinsic properties of Sm2Fe17N3. The powder was disintegrated by gentle milling, and anisotropic Sm2Fe17N3 powder with a high BHmax exceeding 40 MGOe was obtained. 2. Experimental In order to prepare the SmeFe oxide precursor, we mixed an aqueous solution of Fe(NO3)3‧9H2O and Sm(NO3)3‧6H2O, in which the molar ratio of Fe/Sm was adjusted to 5.5. To this solution, 2 mol/ L of a KOH aqueous solution was added dropwise to obtain a coprecipitate suspension. The coprecipitate suspension was continuously stirred overnight under ambient conditions. The coprecipitate was harvested by filtration, rinsed with distilled water, and dried in air at 393 K. The obtained agglomerated SmeFe oxide precursor was crushed in a mortar, pulverized by a rotation mill using 1 mm and 3 mm stainless balls in ethanol for 12 h, corrected by centrifugation, and dried in vacuo. The obtained oxide fine powder was heated at 473 K for 1 h in vacuo to remove
adsorbed moisture, and then reduced at 973 K for 4 h in a hydrogen atmosphere. The hydrogen-reduced precursor and a granular metallic Ca powder were placed in an iron crucible heated at 1173 K or 1223 K for 1 h in an Ar atmosphere. After reduction-diffusion, the sample was nitrided in a NH3eH2 (1: 2 vol/vol) atmosphere at 693 K for 1 h, then annealed in a H2 atmosphere at 693 K for 1 h to adjust its nitrogen content to an appropriate level, and annealed in an Ar atmosphere for 0.5 h to remove the hydrogen adsorbed in the Sm2Fe17N3 [2]. The sample was washed with water several times and with a diluted acetic acid aqueous solution of pH 5.0 to remove the residual Ca, formed CaO and Sm-rich phase, washed again with water, and dried in vacuo to obtain submicron-sized Sm2Fe17N3 powder. The mean diameter and size distribution of the samples were determined by the arithmetic mean from their scanning electron microscopy (SEM) images; more than 200 particles were analyzed. The powder X-ray diffraction (XRD) patterns of the samples were recorded with CoKa radiation (45 kV, 40 mA) by using a silicon powder as an internal standard. The magnetic properties of the samples were determined by using a vibrating sample magnetometer (VSM) at 300 K in vacuo with a maximum magnetic field of 9 T. For the VSM measurements, the powders were oriented in resin under a static magnetic field of 2 T and measured along the easy direction. The contents of hydrogen, oxygen, and nitrogen in the samples were determined by using a combustion analyzer by an inert gas fusion method. The Sm:Fe ratio was analyzed by an X-ray fluorescence (XRF) instrument. 3. Results and discussion We previously reported a submicron-sized Sm2Fe17 fine powder which was prepared by an improved reduction-diffusion technique [11]. In that report, we showed that the reduction-diffusion
Fig. 1. SEM images, particle size distributions, and XRD patterns of Sm2Fe17N3 powder prepared by reduction-diffusion at 1223 K ((a), (b), (c)) and 1173 K ((d), (e), (f)).
Please cite this article in press as: S. Okada, et al., Preparation of submicron-sized Sm2Fe17N3 fine powder with high coercivity by reductiondiffusion process, Journal of Alloys and Compounds (2016), http://dx.doi.org/10.1016/j.jallcom.2016.10.306
S. Okada et al. / Journal of Alloys and Compounds xxx (2016) 1e7
temperature has a large influence on the particle size, and Sm2Fe17 powder with the size of 1.3 mm was obtained through reductiondiffusion at 1223 K. It was also possible to obtain submicron scale Sm2Fe17 powder by adding Ti. However, addition of Ti or other such nonmagnetic additives will inevitably reduce magnetization [12]. Therefore, in this study, we further improved the reductiondiffusion technique in order to obtain submicron-sized Sm2Fe17 powder without addition of any nonmagnetic elements. The SmeFe oxide precursor was pulverized with a rotation mill until the mean diameter of the secondary particles was <1 mm, and the Fe/Sm molar ratio of the oxide precursor was also changed. As a result of these changes, it was possible to obtain Sm2Fe17 powder with the size of approximately 0.9 mm through reduction-diffusion at 1223 K without Ti addition. Moreover, when the reductiondiffusion temperature was lowered to 1173 K, the mean diameter became even smaller, achieving 0.6 mm. No impurity phases such as SmFe3 or a-Fe phases were observed by XRD analysis. The SEM images and XRD patterns of the powders after nitridation treatment are shown in Fig. 1. The composition of samples were also confirmed to Sm2Fe17N3 by XRF and nitrogen content analyses (the actual experimental results of Fe/Sm (at.%/at.%) were about 8.4 for
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both 0.9 mm and 0.6 mm of powders). Although 0.9 mm and 0.6 mm Sm2Fe17N3 powders without impurity phases were obtained, the coercivity values of the powders were only 13.6 kOe and 15.6 kOe, respectively. These coercivities were not as high as we estimated from the well-known empirical relationship between size and coercivity [9,10]. Therefore, we analyzed the obtained Sm2Fe17N3 powders to determine why they did not exhibit the expected high coercivity. The light blue lines in Fig. 2(a), (d) show the XRD patterns of the 0.9 mm and 0.6 mm Sm2Fe17N3 powders obtained by the nitridation process with a conventional post-process. In both XRD patterns, the peaks of the (006) face of Sm2Fe17N3 appeared at a significantly lower position compared to the reference position, while the peaks of the (300) face were observed at the reference position of Sm2Fe17N3. This means the crystal structures of the obtained Sm2Fe17N3 powders were elongated along the c-axis. It is well known that c/a is affected greatly by the content of intercalated elements, which have huge effects on the anisotropy field [2,13,14]. The nitrogen contents of the Sm2Fe17N3 powders were appropriate. As another possible intercalated element, we suspected the presence of hydrogen, which expands the crystal lattice. The hydrogen contents were measured
Fig. 2. (a), (d) XRD patterns of samples prepared with/without dehydrogenation treatment, (b), (e) demagnetization curves of samples oriented in resin when measured at 303 K, (c), (f) parameters and coercivity. (Reduction-diffusion temperature: (a), (b), (c), 1223 K; (d), (e), (f), 1173 K).
Please cite this article in press as: S. Okada, et al., Preparation of submicron-sized Sm2Fe17N3 fine powder with high coercivity by reductiondiffusion process, Journal of Alloys and Compounds (2016), http://dx.doi.org/10.1016/j.jallcom.2016.10.306
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Fig. 3. (a) SEM image of Sm2Fe17N3 powder prepared by hydrogen reduction at 1073 K and reduction-diffusion at 1173 K, (b) particle size distribution, (c) XRD patterns, (d) hysteresis curves, and (e) parameters and coercivity.
by a combustion analyzer, revealing that the powders contained high levels of hydrogen, as the hydrogen contents of the 0.9 mm and 0.6 mm Sm2Fe17N3 powders were around 760 ppm and 1500 ppm, respectively. Therefore, dehydrogenation treatment by annealing in vacuo at 473 K for 3 h was added at the end of the preparation process. As a result, the (006) peak of the XRD patterns of the Sm2Fe17N3 powders shifted to near the reference position, as shown by the orange lines in Fig. 2(a), (d). At this time, the 0.9 mm and 0.6 mm Sm2Fe17N3 powders displayed lower hydrogen contents of 200 ppm and 340 ppm and higher coercivities of 18.1 kOe and 22.8 kOe, respectively (Fig. 2(b), (e)). These coercivity values were on the expected levels for powders of these sizes. Since there was no substantial change in the nitrogen or oxygen contents with and without dehydrogenation treatment, the existence of hydrogen, which caused elongation of the Sm2Fe17N3 crystal structure along the c-axis, was identified as the reason why the powder failed to exhibit the expected high coercivity. There are two possible routes for the intercalation of hydrogen in the Sm2Fe17N3. One is the nitridation process [15], and the other is the washing step to remove excess Ca with water by the reaction Ca þ H2O / CaO þ H2. Therefore, in order to clarify the cause of hydrogen intercalation, samples of the 0.9 mm Sm2Fe17N3 powder without the washing step (samples containing Ca and CaO) and with/without dehydrogenation treatment were prepared. As a result, no substantial change in the c/a ratio was seen among the samples, and their coercivities were similar (coercivity with/ without dehydrogenation treatment was 25.4 kOe and 25.2 kOe,
respectively). This indicates that the hydrogen adsorbed in the nitridation treatment in this study was surely removed by the annealing treatment in Ar after the nitridation process [2]. Thus, the cause of intercalation of hydrogen into the Sm2Fe17N3 crystal structure was identified as the washing step to remove residual Ca. Because the reaction of Ca with H2O is exothermic, the surface of the Sm2Fe17N3 would be heated locally, and that heat would promote intercalation of hydrogen into the Sm2Fe17N3 crystal structure. As reference, the coercivity of the 0.6 mm Sm2Fe17N3 powder before the washing step was 28 kOe. Therefore, the submicronsized Sm2Fe17N3 powder should express higher coercivity exceeding 25 kOe after improvement of the washing step. The decrement of coercivity by hydrogen, which evolved during washing to remove excess Ca, into the crystal structure was also reported by A. M. Gabay et al. for submicron-sized LaCo5 [16] and NdeFeeB [17] powders. In those reports, the powders were prepared by reduction-diffusion with Ca by using high energy ballmilled rare-metal oxide and transition metal powders. Dehydrogenation treatment improved the coercivities of the LaCo5 and NdDyeFeeB powders from 5.5 kOe to 9.6 kOe and from 3.2 kOe to 6.2 kOe, respectively. As in those magnets, an increment of coercivity by removing intercalated hydrogen was also observed in the Sm2Fe17N3 in the present study, but unlike the above-mentioned reports, submicron-sized magnetic powders having very high coercivity were obtained in this study. C.N. Christodoulou et al. reported the influence of hydrogen in Sm2Fe17N3 on magnetic properties [14,15]. In the reports, the
Please cite this article in press as: S. Okada, et al., Preparation of submicron-sized Sm2Fe17N3 fine powder with high coercivity by reductiondiffusion process, Journal of Alloys and Compounds (2016), http://dx.doi.org/10.1016/j.jallcom.2016.10.306
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Fig. 4. Dependence of coercivity on mean diameter of Sm2Fe17N3 prepared by various methods [9,10,21e27].
anisotropy field of Sm2Fe17N3 was decreased by hydrogen (the anisotropy field of Sm2Fe17N3 and Sm2Fe17N3H0.8 were 140 and 100 kOe, respectively). The ratios of coercivity of with/without dehydrogenation treatment in this study were 1.33 and 1.46 for 0.9 mm and 0.6 mm Sm2Fe17N3 powder, respectively, which were well accorded with the ratio of anisotropy field of Sm2Fe17N3/ Sm2Fe17N3H0.8. Thus, the difference of anisotropy field by hydrogen is thought to the dominant factor for the difference in coercivity. A strictly sintered Sm2Fe17N3 powder with the average diameter of 0.5 mm was also prepared by changing the hydrogen reduction temperature to 1073 K, which is 100 K higher than the usual condition. A SEM image and the particle size distribution of the powder
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are shown in Fig. 3(a) and (b), respectively. The hydrogen content in the powder was smaller than that in the 0.6 mm and 0.9 mm powders, and the effect of dehydration treatment on enhancement of coercivity was also smaller (Fig. 3(c)e(e)). This suggests that adsorption of hydrogen in the crystal structure is related to the surface area of the powder. It should be noted that the obtained powder exhibited high coercivity of 24.7 kOe, in spite of the fact that it was highly sintered. This also implies the potential for further increments of the coercivity of Sm2Fe17N3 with sizes below 0.5 mm. The relationship between the mean diameter and coercivity of Sm2Fe17N3 powders is summarized in Fig. 4. This work clearly demonstrated that the coercivity of Sm2Fe17N3 powder can be increased by decreasing the mean diameter of the powder, at least in powders with an average diameter of more than 0.5 mm, which was the limit of this study. It was also shown that submicron-sized powders can be produced by the ball-milling pulverization process. However, ball-milling changed the spherical Sm2Fe17N3 powder shape to a flaky shape, which caused defects and distortions and reduced coercivity. Jet-milling with helium gas is also a potential method for preparing submicron-sized powders. There have been attempts to apply helium jet-milling to NdeFeeB powder [18,19], but as far as we know, there has only been one attempt with Sm2Fe17N3 [20]. In the unexamined Japanese patent, the minimum obtainable size was 1.54 mm even with a helium gas pressure of 0.4 MPa, and the coercivity of the powder was only 12 kOe. Considering this, the direct synthesis method, which is composed of reduction-diffusion of oxide fine powder prepared by a wet chemical technique, is the most practical method for preparing high coercivity Sm2Fe17N3 powders. The temperature coefficient of magnetic properties is also a very important parameter for motor applications, as magnets are exposed to a high temperature environment. The magnetic properties of the 0.6 mm Sm2Fe17N3 powder were measured at temperatures ranging from 303 K to 473 K as an isotropic compact mixed with a Cu powder binder. From the results shown in Fig. 5, the temperature coefficient of coercivity was determined to be 0.36%/K. A commercially-available micron-sized Sm2Fe17N3 powder having coercivity of 13 kOe at 300 K was measured by the
Fig. 5. (a) Demagnetization curves of isotropic compacts of 0.6 mm Sm2Fe17N3 powder with Cu powder binder at 303e473 K, (b) dependence of coercivity on temperature, and (c) magnetic properties.
Please cite this article in press as: S. Okada, et al., Preparation of submicron-sized Sm2Fe17N3 fine powder with high coercivity by reductiondiffusion process, Journal of Alloys and Compounds (2016), http://dx.doi.org/10.1016/j.jallcom.2016.10.306
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Fig. 6. SEM images (a) before and (b), (c) after disintegration, and (d), (e) magnetization curves of the disintegrated powders.
same procedure, and the result was 0.39%/K. This demonstrates that the intrinsic good thermal stability of Sm2Fe17N3 can be maintained at the submicron scale. Although it may be not appropriate to compare powder magnets and sintered magnets, the coercivities of a Dy8%-NdeFeeB sintered magnet at 293 K and 453 K are 29.8 kOe and 8.6 kOe, respectively [28]. As a rough guideline, a Sm2Fe17N3 magnet exposed to a hot environment was demonstrated to require this level of coercivity. Finally, the obtained powder was disintegrated by a vortex mixer by using 1 mm and 3 mm stainless balls in hexane. Fig. 6 shows the SEM images and magnetic properties before and after disintegration of the 0.9 mm Sm2Fe17N3 powder. While the powder before disintegration was partially sintered and Mr and M90 were low considering the intrinsic high potential of Sm2Fe17N3, Mr and M90 were drastically improved by the disintegration treatment. As a result powder having a very high maximum energy product (BHmax) exceeding 40 MGOe was obtained. Thus, the reason for the low magnetization of the as-synthesized powders is attributed to the sintering of particles. The powder prepared with a longer milling time contained many non-spherical crushed fine particles, as can be seen in Fig. 6 (c), and the powder showed lower magnetic properties than the powder with a good shape. Non-spherical crushed fine particles can be assumed to have low magnetic performance because they are considered to have many surface defects, such as edges, oxides and etc. which facilitate nucleation of reverse magnetization [9,10,16]. The reason for the decrement of coercivity in the sample having the high BHmax value could also be attributed to the formation of crushed particles (Fig. 6 (b)). The disintegration treatment was also applied to the 0.6 mm Sm2Fe17N3 powder. At present, the treated powder contains many crushed particles, and a powder
having superior magnetic properties has not yet been obtained. It is expected to be possible to achieve an anisotropic Sm2Fe17N3 powder having higher coercivity by developing a method to prevent particle sintering during the reduction-diffusion reaction and a disintegrating method to prevent the formation of crushed particles. 4. Conclusions Submicron-sized Sm2Fe17N3 powders having high coercivity of more than 20 kOe and powders having a high maximum energy product of more than 40 MGOe were prepared successfully by using a newly-developed procedure for submicron-sized Sm2Fe17N3 powders and post-process. The coercivity of the Sm2Fe17N3 powder increased as the particle size decreased, at least until the average diameter of 0.5 mm. The intrinsic good temperature tolerance of Sm2Fe17N3 was also confirmed in the submicron-sized powders. The large potential of Sm2Fe17N3 magnets for surpassing the performance of NdeFeeB magnets in hot environments was demonstrated. We are currently developing a sintering process for the submicron-sized Sm2Fe17N3 powders. References [1] J.M.D. Coey, H. Sun, J. Magn. Magn. Mater. 87 (1990) L251eL254. [2] T. Iriyama, K. Kobayashi, N. Imaoka, T. Fukuda, H. Kato, Y. Nakagawa, IEEE Trans. Magn. 28 (1992) 2326e2331. [3] B.P. Hu, X.L. Rao, J.M. Xu, G.C. Liu, Y.Z. Wang, X.L. Dong, D.X. Zhang, M. Cai, J. Appl. Phys. 74 (1993) 489e494. [4] D.N. Brown, B.-M. Ma, P. Campbell, in: Paper Presented at the 17th International Workshop on Rare-Earth Permanent Magnets and Their Applications, Delaware, USA, 2002. [5] M. Komuro, S. Kawamata, F. Tajima, H. Koharagi, J. Appl. Phys. 97 (2005)
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[17] A.M. Gabay, X.C. Hu, G.C. Hadjipanayis, J. Alloys Compd. 574 (2013) 472e476. [18] Y. Une, M. Sagawa, in: Paper Presented at the 21st International Workshop on Rare-Earth Permanent Magnets and TheirApplications, Bled, Slovenia, 2010. [19] H. Sepehri-Amin, Y. Une, T. Ohkubo, K. Hono, M. Sagawa, Scr. Mater 65 (2011) 396e399. [20] K. Watanabe, T. Ishikawa, 2006, JP2006e351688. [21] S. Maekawa, M. Yamasaki, K. Fujita, T. Katamoto, K. Ikemoto, N. Sugita, 2011, JP4662061. [22] N. Imaoka, T. Iriyama, S. Itoh, A. Okamoto, T. Katsumata, J. Alloys Compd. 222 (1995) 73e77. [23] T. Iseki, Y. K., 2011, JP4862269. [24] X.B. Ma, L.Z. Li, S.Q. Liu, B.Y. Hu, J.Z. Han, C.S. Wang, H.L. Du, Y.C. Yang, J.B. Yang, J. Alloys Compd. 612 (2014) 110e113. [25] M. Yue, Y.Q. Li, R.M. Liu, W.Q. Liu, Z.H. Guo, W. Li, J. Alloys Compd. 637 (2015) 297e300. [26] J.G. Lee, S.W. Kang, P.Z. Si, C.J. Choi, J. Magn. 16 (2011) 104e107. [27] G. Hadjipanayis, D. Neil, A. Gabay, EPJ Web Conf. 40 (2013) 06006. [28] http://www.hitachi-metals.co.jp/products/auto/el/pdf/nmx.pdf.
Please cite this article in press as: S. Okada, et al., Preparation of submicron-sized Sm2Fe17N3 fine powder with high coercivity by reductiondiffusion process, Journal of Alloys and Compounds (2016), http://dx.doi.org/10.1016/j.jallcom.2016.10.306
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Preparation of submicron-sized Sm2Fe17N3 fine powder with high coercivity by reduction-diffusion process Shusuke Okada a, *, Kazuyuki Suzuki a, Eri Node a, Kenta Takagi a, Kimihiro Ozaki a, Yasushi Enokido b a
Magnetic Powder Metallurgy Research Center, National Institute of Advanced Industrial Science and Technology (AIST), 2266-98 Anagahora, Shimoshidami, Moriyama, Nagoya, 463-8560, Japan Materials Development Center, TDK Corporation, 570-2, Matsugasita, Minamihadori, Narita, 268-8588, Japan
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 July 2016 Received in revised form 14 October 2016 Accepted 30 October 2016 Available online xxx
Submicron-sized Sm2Fe17N3 powders having high coercivity were prepared by a newly-developed synthesis procedure for submicron-sized Sm2Fe17 powders and nitridation process with a proper postprocess for the powders. It was revealed that the washing step, which is performed to remove excess Ca, supplied hydrogen into the Sm2Fe17N3 crystal structure, and elongation of the crystal structure along the c-axis by the supplied hydrogen reduced coercivity. This phenomenon was observed clearly in powder with a smaller size and good dispersity. When the powders were subjected to dehydrogenation treatment, they showed high coercivity, as expected from the particle size, and coercivity of 24.7 kOe was achieved with the 0.5 mm powder. The fact that the intrinsic good thermal stability of Sm2Fe17N3 is maintained at the submicron-scale was also confirmed. In addition, the obtained powder exhibited a high maximum energy product after disintegration treatment under appropriate conditions. This study demonstrated the high potential of Sm2Fe17N3 for surpassing the performance of NdeFeeB magnets under hot environments. © 2016 Elsevier B.V. All rights reserved.
Keywords: Sm2Fe17N3 SmeFeeN Reduction-diffusion Rare-earth permanent magnet
1. Introduction With expanding use of permanent-magnet motors in electronic appliances and electric vehicles, enhancement of the magnetic properties of permanent magnets has become a very important research topic for energy saving. Currently, NdeFeeB sintered magnets are used in these motors. However, the Curie temperature of NdeFeeB magnets is about 585 K, which causes a serious decrement of magnetic properties in motors exposed to a hot environment. Dy and Tb are used to improve the temperature tolerance of NdeFeeB, but use of these heavy rare earth elements results in low remanence magnetization and can be affected by supply instability. Considering this, Sm2Fe17N3 is a promising candidate to replace NdeFeeB magnets, because Sm2Fe17N3 has a high Curie temperature (750 K) with a huge anisotropy field (~260 kOe) and comparable high saturation magnetization (1.54 T) [1e5]. However, it is difficult to manufacture Sm2Fe17N3 sintered
* Corresponding author. E-mail address: [email protected] (S. Okada).
bulk magnets, as the intermetallic alloy decomposes at around 873 K. Our research group took on the challenge of realizing sintered Sm2Fe17N3 magnets. In previous work, we reported a process for producing isotropic Sm2Fe17N3 sintered bulk magnets by a highpressure current sintering technique at below 723 K [6]. At present, we are attempting to develop a sintering process for Sm2Fe17N3 anisotropic sintered magnets. To realize anisotropic Sm2Fe17N3 sintered magnets which surpass the conventional NdeFeeB sintered magnets, it is also necessary to develop a high magnetic performance Sm2Fe17N3 powder, especially a powder having high coercivity. C. Kuhrt et al. and A. Teresiak et al. reported isotropic Sm2Fe17N3 powders having coercivity of 32 kOe and 37 kOe by using mechanical alloying, respectively [7,8]. However, the coercivity of high coercivity anisotropic Sm2Fe17N3 powders has been limited to only around 17e18 kOe [9]. Recently, Hirayama et al. reported a Sm2Fe17N3 powder with the size of 0.69 mm, which displayed coercivity of 23.2 kOe, by a polymerized-complex and reduction-diffusion process [10]. However, the values of remanent magnetization were not described, and the hysteresis curve contained a step which was thought to be attributable to a soft magnetic phase. Moreover, this submicron-sized powder appears to be
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Please cite this article in press as: S. Okada, et al., Preparation of submicron-sized Sm2Fe17N3 fine powder with high coercivity by reductiondiffusion process, Journal of Alloys and Compounds (2016), http://dx.doi.org/10.1016/j.jallcom.2016.10.306
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a strictly sintered, which would make it difficult to express a high maximum energy product (BHmax). We have also attempted to prepare a high coercivity Sm2Fe17N3 powder by reducing the particle size, and we have reported a submicron-sized Sm2Fe17 powder which was produced by reduction-diffusion of a precursor prepared by a coprecipitation technique [11]. In the present report, we describe the development of a nitridation process and post-process for submicron-sized powders. Utilizing these newly-developed processes, we succeeded in preparing submicron-sized Sm2Fe17N3 powders having high coercivity of up to 24.7 kOe without steps in the hysteresis curve. The temperature dependence of coercivity of the submicron-sized powder was also investigated. In addition, since the as-prepared submicron-sized Sm2Fe17N3 powder was partially sintered, its remanent magnetization properties were not high considering the intrinsic properties of Sm2Fe17N3. The powder was disintegrated by gentle milling, and anisotropic Sm2Fe17N3 powder with a high BHmax exceeding 40 MGOe was obtained. 2. Experimental In order to prepare the SmeFe oxide precursor, we mixed an aqueous solution of Fe(NO3)3‧9H2O and Sm(NO3)3‧6H2O, in which the molar ratio of Fe/Sm was adjusted to 5.5. To this solution, 2 mol/ L of a KOH aqueous solution was added dropwise to obtain a coprecipitate suspension. The coprecipitate suspension was continuously stirred overnight under ambient conditions. The coprecipitate was harvested by filtration, rinsed with distilled water, and dried in air at 393 K. The obtained agglomerated SmeFe oxide precursor was crushed in a mortar, pulverized by a rotation mill using 1 mm and 3 mm stainless balls in ethanol for 12 h, corrected by centrifugation, and dried in vacuo. The obtained oxide fine powder was heated at 473 K for 1 h in vacuo to remove
adsorbed moisture, and then reduced at 973 K for 4 h in a hydrogen atmosphere. The hydrogen-reduced precursor and a granular metallic Ca powder were placed in an iron crucible heated at 1173 K or 1223 K for 1 h in an Ar atmosphere. After reduction-diffusion, the sample was nitrided in a NH3eH2 (1: 2 vol/vol) atmosphere at 693 K for 1 h, then annealed in a H2 atmosphere at 693 K for 1 h to adjust its nitrogen content to an appropriate level, and annealed in an Ar atmosphere for 0.5 h to remove the hydrogen adsorbed in the Sm2Fe17N3 [2]. The sample was washed with water several times and with a diluted acetic acid aqueous solution of pH 5.0 to remove the residual Ca, formed CaO and Sm-rich phase, washed again with water, and dried in vacuo to obtain submicron-sized Sm2Fe17N3 powder. The mean diameter and size distribution of the samples were determined by the arithmetic mean from their scanning electron microscopy (SEM) images; more than 200 particles were analyzed. The powder X-ray diffraction (XRD) patterns of the samples were recorded with CoKa radiation (45 kV, 40 mA) by using a silicon powder as an internal standard. The magnetic properties of the samples were determined by using a vibrating sample magnetometer (VSM) at 300 K in vacuo with a maximum magnetic field of 9 T. For the VSM measurements, the powders were oriented in resin under a static magnetic field of 2 T and measured along the easy direction. The contents of hydrogen, oxygen, and nitrogen in the samples were determined by using a combustion analyzer by an inert gas fusion method. The Sm:Fe ratio was analyzed by an X-ray fluorescence (XRF) instrument. 3. Results and discussion We previously reported a submicron-sized Sm2Fe17 fine powder which was prepared by an improved reduction-diffusion technique [11]. In that report, we showed that the reduction-diffusion
Fig. 1. SEM images, particle size distributions, and XRD patterns of Sm2Fe17N3 powder prepared by reduction-diffusion at 1223 K ((a), (b), (c)) and 1173 K ((d), (e), (f)).
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temperature has a large influence on the particle size, and Sm2Fe17 powder with the size of 1.3 mm was obtained through reductiondiffusion at 1223 K. It was also possible to obtain submicron scale Sm2Fe17 powder by adding Ti. However, addition of Ti or other such nonmagnetic additives will inevitably reduce magnetization [12]. Therefore, in this study, we further improved the reductiondiffusion technique in order to obtain submicron-sized Sm2Fe17 powder without addition of any nonmagnetic elements. The SmeFe oxide precursor was pulverized with a rotation mill until the mean diameter of the secondary particles was <1 mm, and the Fe/Sm molar ratio of the oxide precursor was also changed. As a result of these changes, it was possible to obtain Sm2Fe17 powder with the size of approximately 0.9 mm through reduction-diffusion at 1223 K without Ti addition. Moreover, when the reductiondiffusion temperature was lowered to 1173 K, the mean diameter became even smaller, achieving 0.6 mm. No impurity phases such as SmFe3 or a-Fe phases were observed by XRD analysis. The SEM images and XRD patterns of the powders after nitridation treatment are shown in Fig. 1. The composition of samples were also confirmed to Sm2Fe17N3 by XRF and nitrogen content analyses (the actual experimental results of Fe/Sm (at.%/at.%) were about 8.4 for
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both 0.9 mm and 0.6 mm of powders). Although 0.9 mm and 0.6 mm Sm2Fe17N3 powders without impurity phases were obtained, the coercivity values of the powders were only 13.6 kOe and 15.6 kOe, respectively. These coercivities were not as high as we estimated from the well-known empirical relationship between size and coercivity [9,10]. Therefore, we analyzed the obtained Sm2Fe17N3 powders to determine why they did not exhibit the expected high coercivity. The light blue lines in Fig. 2(a), (d) show the XRD patterns of the 0.9 mm and 0.6 mm Sm2Fe17N3 powders obtained by the nitridation process with a conventional post-process. In both XRD patterns, the peaks of the (006) face of Sm2Fe17N3 appeared at a significantly lower position compared to the reference position, while the peaks of the (300) face were observed at the reference position of Sm2Fe17N3. This means the crystal structures of the obtained Sm2Fe17N3 powders were elongated along the c-axis. It is well known that c/a is affected greatly by the content of intercalated elements, which have huge effects on the anisotropy field [2,13,14]. The nitrogen contents of the Sm2Fe17N3 powders were appropriate. As another possible intercalated element, we suspected the presence of hydrogen, which expands the crystal lattice. The hydrogen contents were measured
Fig. 2. (a), (d) XRD patterns of samples prepared with/without dehydrogenation treatment, (b), (e) demagnetization curves of samples oriented in resin when measured at 303 K, (c), (f) parameters and coercivity. (Reduction-diffusion temperature: (a), (b), (c), 1223 K; (d), (e), (f), 1173 K).
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Fig. 3. (a) SEM image of Sm2Fe17N3 powder prepared by hydrogen reduction at 1073 K and reduction-diffusion at 1173 K, (b) particle size distribution, (c) XRD patterns, (d) hysteresis curves, and (e) parameters and coercivity.
by a combustion analyzer, revealing that the powders contained high levels of hydrogen, as the hydrogen contents of the 0.9 mm and 0.6 mm Sm2Fe17N3 powders were around 760 ppm and 1500 ppm, respectively. Therefore, dehydrogenation treatment by annealing in vacuo at 473 K for 3 h was added at the end of the preparation process. As a result, the (006) peak of the XRD patterns of the Sm2Fe17N3 powders shifted to near the reference position, as shown by the orange lines in Fig. 2(a), (d). At this time, the 0.9 mm and 0.6 mm Sm2Fe17N3 powders displayed lower hydrogen contents of 200 ppm and 340 ppm and higher coercivities of 18.1 kOe and 22.8 kOe, respectively (Fig. 2(b), (e)). These coercivity values were on the expected levels for powders of these sizes. Since there was no substantial change in the nitrogen or oxygen contents with and without dehydrogenation treatment, the existence of hydrogen, which caused elongation of the Sm2Fe17N3 crystal structure along the c-axis, was identified as the reason why the powder failed to exhibit the expected high coercivity. There are two possible routes for the intercalation of hydrogen in the Sm2Fe17N3. One is the nitridation process [15], and the other is the washing step to remove excess Ca with water by the reaction Ca þ H2O / CaO þ H2. Therefore, in order to clarify the cause of hydrogen intercalation, samples of the 0.9 mm Sm2Fe17N3 powder without the washing step (samples containing Ca and CaO) and with/without dehydrogenation treatment were prepared. As a result, no substantial change in the c/a ratio was seen among the samples, and their coercivities were similar (coercivity with/ without dehydrogenation treatment was 25.4 kOe and 25.2 kOe,
respectively). This indicates that the hydrogen adsorbed in the nitridation treatment in this study was surely removed by the annealing treatment in Ar after the nitridation process [2]. Thus, the cause of intercalation of hydrogen into the Sm2Fe17N3 crystal structure was identified as the washing step to remove residual Ca. Because the reaction of Ca with H2O is exothermic, the surface of the Sm2Fe17N3 would be heated locally, and that heat would promote intercalation of hydrogen into the Sm2Fe17N3 crystal structure. As reference, the coercivity of the 0.6 mm Sm2Fe17N3 powder before the washing step was 28 kOe. Therefore, the submicronsized Sm2Fe17N3 powder should express higher coercivity exceeding 25 kOe after improvement of the washing step. The decrement of coercivity by hydrogen, which evolved during washing to remove excess Ca, into the crystal structure was also reported by A. M. Gabay et al. for submicron-sized LaCo5 [16] and NdeFeeB [17] powders. In those reports, the powders were prepared by reduction-diffusion with Ca by using high energy ballmilled rare-metal oxide and transition metal powders. Dehydrogenation treatment improved the coercivities of the LaCo5 and NdDyeFeeB powders from 5.5 kOe to 9.6 kOe and from 3.2 kOe to 6.2 kOe, respectively. As in those magnets, an increment of coercivity by removing intercalated hydrogen was also observed in the Sm2Fe17N3 in the present study, but unlike the above-mentioned reports, submicron-sized magnetic powders having very high coercivity were obtained in this study. C.N. Christodoulou et al. reported the influence of hydrogen in Sm2Fe17N3 on magnetic properties [14,15]. In the reports, the
Please cite this article in press as: S. Okada, et al., Preparation of submicron-sized Sm2Fe17N3 fine powder with high coercivity by reductiondiffusion process, Journal of Alloys and Compounds (2016), http://dx.doi.org/10.1016/j.jallcom.2016.10.306
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Fig. 4. Dependence of coercivity on mean diameter of Sm2Fe17N3 prepared by various methods [9,10,21e27].
anisotropy field of Sm2Fe17N3 was decreased by hydrogen (the anisotropy field of Sm2Fe17N3 and Sm2Fe17N3H0.8 were 140 and 100 kOe, respectively). The ratios of coercivity of with/without dehydrogenation treatment in this study were 1.33 and 1.46 for 0.9 mm and 0.6 mm Sm2Fe17N3 powder, respectively, which were well accorded with the ratio of anisotropy field of Sm2Fe17N3/ Sm2Fe17N3H0.8. Thus, the difference of anisotropy field by hydrogen is thought to the dominant factor for the difference in coercivity. A strictly sintered Sm2Fe17N3 powder with the average diameter of 0.5 mm was also prepared by changing the hydrogen reduction temperature to 1073 K, which is 100 K higher than the usual condition. A SEM image and the particle size distribution of the powder
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are shown in Fig. 3(a) and (b), respectively. The hydrogen content in the powder was smaller than that in the 0.6 mm and 0.9 mm powders, and the effect of dehydration treatment on enhancement of coercivity was also smaller (Fig. 3(c)e(e)). This suggests that adsorption of hydrogen in the crystal structure is related to the surface area of the powder. It should be noted that the obtained powder exhibited high coercivity of 24.7 kOe, in spite of the fact that it was highly sintered. This also implies the potential for further increments of the coercivity of Sm2Fe17N3 with sizes below 0.5 mm. The relationship between the mean diameter and coercivity of Sm2Fe17N3 powders is summarized in Fig. 4. This work clearly demonstrated that the coercivity of Sm2Fe17N3 powder can be increased by decreasing the mean diameter of the powder, at least in powders with an average diameter of more than 0.5 mm, which was the limit of this study. It was also shown that submicron-sized powders can be produced by the ball-milling pulverization process. However, ball-milling changed the spherical Sm2Fe17N3 powder shape to a flaky shape, which caused defects and distortions and reduced coercivity. Jet-milling with helium gas is also a potential method for preparing submicron-sized powders. There have been attempts to apply helium jet-milling to NdeFeeB powder [18,19], but as far as we know, there has only been one attempt with Sm2Fe17N3 [20]. In the unexamined Japanese patent, the minimum obtainable size was 1.54 mm even with a helium gas pressure of 0.4 MPa, and the coercivity of the powder was only 12 kOe. Considering this, the direct synthesis method, which is composed of reduction-diffusion of oxide fine powder prepared by a wet chemical technique, is the most practical method for preparing high coercivity Sm2Fe17N3 powders. The temperature coefficient of magnetic properties is also a very important parameter for motor applications, as magnets are exposed to a high temperature environment. The magnetic properties of the 0.6 mm Sm2Fe17N3 powder were measured at temperatures ranging from 303 K to 473 K as an isotropic compact mixed with a Cu powder binder. From the results shown in Fig. 5, the temperature coefficient of coercivity was determined to be 0.36%/K. A commercially-available micron-sized Sm2Fe17N3 powder having coercivity of 13 kOe at 300 K was measured by the
Fig. 5. (a) Demagnetization curves of isotropic compacts of 0.6 mm Sm2Fe17N3 powder with Cu powder binder at 303e473 K, (b) dependence of coercivity on temperature, and (c) magnetic properties.
Please cite this article in press as: S. Okada, et al., Preparation of submicron-sized Sm2Fe17N3 fine powder with high coercivity by reductiondiffusion process, Journal of Alloys and Compounds (2016), http://dx.doi.org/10.1016/j.jallcom.2016.10.306
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Fig. 6. SEM images (a) before and (b), (c) after disintegration, and (d), (e) magnetization curves of the disintegrated powders.
same procedure, and the result was 0.39%/K. This demonstrates that the intrinsic good thermal stability of Sm2Fe17N3 can be maintained at the submicron scale. Although it may be not appropriate to compare powder magnets and sintered magnets, the coercivities of a Dy8%-NdeFeeB sintered magnet at 293 K and 453 K are 29.8 kOe and 8.6 kOe, respectively [28]. As a rough guideline, a Sm2Fe17N3 magnet exposed to a hot environment was demonstrated to require this level of coercivity. Finally, the obtained powder was disintegrated by a vortex mixer by using 1 mm and 3 mm stainless balls in hexane. Fig. 6 shows the SEM images and magnetic properties before and after disintegration of the 0.9 mm Sm2Fe17N3 powder. While the powder before disintegration was partially sintered and Mr and M90 were low considering the intrinsic high potential of Sm2Fe17N3, Mr and M90 were drastically improved by the disintegration treatment. As a result powder having a very high maximum energy product (BHmax) exceeding 40 MGOe was obtained. Thus, the reason for the low magnetization of the as-synthesized powders is attributed to the sintering of particles. The powder prepared with a longer milling time contained many non-spherical crushed fine particles, as can be seen in Fig. 6 (c), and the powder showed lower magnetic properties than the powder with a good shape. Non-spherical crushed fine particles can be assumed to have low magnetic performance because they are considered to have many surface defects, such as edges, oxides and etc. which facilitate nucleation of reverse magnetization [9,10,16]. The reason for the decrement of coercivity in the sample having the high BHmax value could also be attributed to the formation of crushed particles (Fig. 6 (b)). The disintegration treatment was also applied to the 0.6 mm Sm2Fe17N3 powder. At present, the treated powder contains many crushed particles, and a powder
having superior magnetic properties has not yet been obtained. It is expected to be possible to achieve an anisotropic Sm2Fe17N3 powder having higher coercivity by developing a method to prevent particle sintering during the reduction-diffusion reaction and a disintegrating method to prevent the formation of crushed particles. 4. Conclusions Submicron-sized Sm2Fe17N3 powders having high coercivity of more than 20 kOe and powders having a high maximum energy product of more than 40 MGOe were prepared successfully by using a newly-developed procedure for submicron-sized Sm2Fe17N3 powders and post-process. The coercivity of the Sm2Fe17N3 powder increased as the particle size decreased, at least until the average diameter of 0.5 mm. The intrinsic good temperature tolerance of Sm2Fe17N3 was also confirmed in the submicron-sized powders. The large potential of Sm2Fe17N3 magnets for surpassing the performance of NdeFeeB magnets in hot environments was demonstrated. We are currently developing a sintering process for the submicron-sized Sm2Fe17N3 powders. References [1] J.M.D. Coey, H. Sun, J. Magn. Magn. Mater. 87 (1990) L251eL254. [2] T. Iriyama, K. Kobayashi, N. Imaoka, T. Fukuda, H. Kato, Y. Nakagawa, IEEE Trans. Magn. 28 (1992) 2326e2331. [3] B.P. Hu, X.L. Rao, J.M. Xu, G.C. Liu, Y.Z. Wang, X.L. Dong, D.X. Zhang, M. Cai, J. Appl. Phys. 74 (1993) 489e494. [4] D.N. Brown, B.-M. Ma, P. Campbell, in: Paper Presented at the 17th International Workshop on Rare-Earth Permanent Magnets and Their Applications, Delaware, USA, 2002. [5] M. Komuro, S. Kawamata, F. Tajima, H. Koharagi, J. Appl. Phys. 97 (2005)
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