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Structure, phase transformation and magnetic properties of SmyFe100−1.5yC0.5y alloys prepared by mechanical alloying and re-milling
Journal of Alloys and Compounds 291 (1999) 276–281
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Structure, phase transformation and magnetic properties of Sm y Fe 10021.5y C 0.5y alloys prepared by mechanical alloying and re-milling a, a a a a a Dian-yu Geng *, Zhi-dong Zhang , Bao-zhi Cui , Zhi-jun Guo , Wei Liu , Xin-guo Zhao , a b Tong Zhao , Ji-wen Liu a
International Center for Materials Physics, Institute of Metal Research, Academia Sinica, Shenyang, 110015, China b Tianjin Institute of Technology, Tianjin 300191, China Received 27 April 1999; received in revised form 8 June 1999; accepted 8 June 1999
Abstract The structure, phase transformation and magnetic properties of Sm y Fe 10021.5y C 0.5y ( y510, 12, 14, 16, 18, 20) alloys prepared by mechanical alloying (MA) have been studied systematically. The Sm 2 Fe 17 Cx (Sm 2 Fe 14 C) phase tends to form when y is increased (decreased). For the same composition, the Sm 2 Fe 17 C x phase is formed more easily when the annealing temperature T a is lower. No Sm 2 Fe 14 C phase is observed in the Sm 20 Fe 70 C 10 alloy. In other alloys, annealed between 6508C and 9508C, the Sm 2 Fe 14 C and Sm 2 Fe 17 C x phases coexist under a certain condition. Re-milling and re-annealing proved to be suitable for improving the magnetic properties of the MA alloys. After annealing at 8008C for 35 min, re-milling for 1 h and then re-annealing at 6008C for 35 min, the value (BH) max 510.6 MGOe has been achieved for the MA Sm 14 Fe 79 C 7 alloy. After annealing at 8008C for 35 min, the MA Sm 20 Fe 70 C 10 alloy was re-milled for 1 h and then re-annealed at 5508C for 2 h, resulting in the value of i Hc 57.96 kOe. 1999 Elsevier Science S.A. All rights reserved. Keywords: Samarium iron carbides; Magnetic properties; Mechanical alloying
1. Introduction The discovery of high energy product magnets based on Nd 2 Fe 14 B [1,2] has stimulated the study of the corresponding series of carbides [3,4]. The structure and magnetic properties of Nd 2 Fe 14 C-type compounds were found to be close to those of their boride counterparts. Subsequently, the interstitial compounds of Sm 2 Fe 17 C x [5,6] and Sm 2 Fe 17 N x [7]-types were studied. It was found that Sm 2 Fe 17 can accommodate three carbon atoms per formula unit [5]. The carbon atoms occupy interstitial sites in Sm 2 Fe 17 C x which have a large effect on the structure of the compounds. The lattice parameters a and c, the Curie temperature and the 3d magnetic moment increase with increasing carbon atoms. Due to the outstanding values of the intrinsic magnetic properties, i.e., a Curie temperature T c ¯3978C and an anisotropy field m0 HA ¯16 T, the interstitial compound Sm 2 Fe 17 C x is a promising candidate for a new class of permanent magnetic material [5,6,8]. *Corresponding author. E-mail address: [email protected] (D.-y. Geng)
There have been reports of several preparation techniques for the interstitial compounds of the Sm 2 Fe 17 C x – type [9]: The first is to alloy directly with carbon above the melting temperature [5,8]. The second is by absorbing carbon through the gas–solid reaction of the Sm 2 Fe 17 compound with a hydrocarbon gas [10]. The third is through a solid–solid reaction with pure carbon [11]. Ternary compounds are formed upon combining Sm 2 Fe 17 with carbon atoms which occupy interstitial sites. The Curie temperature T c changes with varing C content in the Sm 2 Fe 17 Cx phase [12,13]. The interstitial Sm 2 Fe 17 C x compounds are metastable in alloys prepared by ball milling, where they can transform into a mixture of phases, including Sm 2 Fe 14 C, Sm 2 Fe 17 Cx , Sm 2 C 3 , a-Fe etc., when the annealing temperature T a varies from 6008C to 9508C [9,14]. In our previous work [15,16], structure, phase transformation and magnetic properties of Nd 2 Fe 14 C-based alloys were investigated by mechanical alloying (MA) and subsequent annealing. The effects of the phase transformation from Nd 2 Fe 17 C x to Nd 2 Fe 14 C on the magnetic properties of the alloys were discussed. In this contribution we study the phase transformation
0925-8388 / 99 / $ – see front matter 1999 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 99 )00278-9
D.-y. Geng et al. / Journal of Alloys and Compounds 291 (1999) 276 – 281
from Sm 2 Fe 14 C to Sm 2 Fe 17 C x of Sm y Fe 10021.5y C 0.5y ( y5 10, 12, 14, 16, 18, 20) alloys prepared by MA. In the present system, the hard magnetic phase is of the Sm 2 Fe 17 C x -type, which has uniaxial magnetic anisotropy at room temperature [12]. In order to improve the magnetic properties of the MA alloys, a novel procedure, including re-milling and re-annealing, is developed in this work. The organization of this paper is as follows. Section 2 contains experimental details, section 3 results and discussion. Section 3.1 deals with the structure, phase transformation and magnetic properties of MA Sm y Fe 10021.5y C 0.5y ( y510, 12, 14, 16, 18, 20) alloys upon the annealing. Section 3.2 concentrates on the effect of the re-milling and re-annealing on the structure and magnetic properties of MA Sm 14 Fe 79 C 7 and Sm 20 Fe 70 C 10 alloys. A conclusion is given in Section 4.
2. Experimental details 99.9%-pure Sm, 99.8%-pure Fe powders and 99.7% graphite were mixed according to the desired composition, and sealed under argon in a hardened cylindrical steel container. Mechanical alloying (MA) was performed in a high-energy ball miller designed in our laboratory for 5 h [15–17]. The hardened steel balls used for ball milling were 12 mm in diameter. The weight ratio of balls to powders was kept at 25:1. The diameter of the cans for MA is 54 mm. The diameter of the movement of the cans is about 100 mm. The rotating rate is 15 turns / s. The MA powders were subsequently in a vacuum furnace connected to a closed glove box from 6008C to 9508C for 35 min. In order to improve the formation process of Sm 2 Fe 17 C x and consequently the magnetic properties, a re-milling and re-annealing procedure was performed, just after the first annealing process. The conditions for the re-milling were set to be the same as those for MA, with exception of the re-milling time. After re-milling for 1 h, the powders were annealed at low temperatures for a more complete reaction for forming of the hard magnetic phase Sm 2 Fe 17 C x . X-ray diffraction (XRD) analysis was performed using Cu Ka radiation with a Rigaku D/ Max-rA diffractometer equipped with a graphite crystal monochromater. Initial susceptibility xa.c. measurements were used to determine the Curie temperature and to preliminarily estimate the amount of possible magnetic phases in the specimens. For the magnetic measurement at room temperature, the powders were embedded in epoxy resin with an approximate ratio 1:1 of the magnetic powders and the resin to form magnetically isotropic magnets. The magnetic properties were measured at room temperature using a pulsed magnetometer in fields up to 8 T. The magnetization was related to the amount of magnetic powders, neglecting the dilution effect of resin and the density of the magnetic powdered samples was assumed to be 7.6 g / cm 3 .
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3. Results and discussion
3.1. Mechanical alloying and annealing As usual, mechanical alloying for 5 h in a high energy ball miller is enough for forming the amorphous materials [15–17]. Fig. 1 shows X-ray diffraction patterns of MA Smy Fe 10021.5y C 0.5y ( y510, 12, 14, 16, 18 and 20) alloys annealed at 8008C for 35 min. Fig. 2 represents the temperature dependence of the a.c. susceptibility xa.c. of the corresponding alloys. The structure of the MA Sm y Fe 10021.5y C 0.5y alloys depends sensitively on the composition and annealing temperature T a . For y510, the main phases are a mixture of Sm 2 Fe 14 C and a-Fe. It is clearly shown in Fig. 2 that Curie temperature of the Sm 2 Fe 14 C compound is 3158C. The reaction during the annealing leads from amorphous Sm 10 Fe 85 C 5 to the mixture of Sm 2 Fe 14 C and a-Fe. The amount of a-Fe phase gradually decreases with increasing y. This means that Sm and C continue to combine with Fe to form the ternary compounds. In the alloy with y518, Sm 2 Fe 17 C x and Sm 2 Fe 14 C coexist with small amounts of a-Fe. For y520, the main phases are a mixture of Sm 2 Fe 17 Cx and Sm 2 C 3 so that the reaction during the annealing can be described as: Sm 20 Fe 70 C 10 (amorphous) → Sm 2 Fe 17 C x 1Sm 2 C 3. Figs. 3 and 4 represent respectively X-ray diffraction patterns and xa.c. vs. T curves of MA Sm 14 Fe 79 C 7 alloys annealed at 650, 700, 750, 800, 850, 900 and 9508C for 35 min. At T a 56508C, the relatively low annealing temperature for the solid reaction limits complete conversion of the
Fig. 1. X-ray diffraction patterns of mechanically alloyed Sm y Fe 10021.5y C 0.5y ( y510, 12, 14, 16, 18 and 20) alloys annealed at 8008C for 35 min.
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D.-y. Geng et al. / Journal of Alloys and Compounds 291 (1999) 276 – 281
Fig. 2. Temperature dependence of a.c. susceptibility xa.c. of the samples of Fig. 1.
crystallites into Sm 2 Fe 17 C x . When T a 56508C the Curie temperature of Sm 14 Fe 79 C 7 alloys is 1718C which is higher than that of Sm 2 Fe 17 . When T a 58008C, the Sm 2 Fe 14 C phase forms perfectly. The Sm 2 Fe 17 C x and the Sm 2 Fe 14 C phases can transform into each other under certain conditions. In the present case, the following reaction can occur during annealing at various temperatures: Sm 2 Fe 14 C1a-Fe ↔Sm 2 Fe 17 C x 1 Sm 2 C 3.
Fig. 3. X-ray diffraction patterns of mechanically alloyed Sm 14 Fe 79 C 7 alloys annealed at 650, 700, 750, 800, 850, 900 and 9508C for 35 min.
Fig. 4. Temperature dependence of a.c. susceptibility xa.c. of the samples of Fig. 3.
At T a 58008C and for y520, the Sm 2 Fe 17 C x phase forms perfectly and its Curie temperature is only 1388C, indicating that the carbon content is small [6]. There is no Sm 2 Fe 14 C phase present in all samples with y520, annealed at various temperatures. The metastable Sm 2 Fe 17 carbide tends to decompose at temperatures from 650 to 8008C into other alloys with a carbon content from y510 to 18. At T a 56508C, the Sm 2 Fe 14 C and Sm 2 Fe 17 C x phases coexist in the alloys for y510, 12, 14 and 16. The Curie temperature T c of the Sm 2 Fe 17 C x phase changes with increasing annealing temperature, indicating a change of the carbon content of the phase. The Curie temperature of the Sm 2 Fe 14 C phase remains almost the same in these alloys, suggesting that the Sm 2 Fe 14 C phase in the alloys is relatively stable. From 650 to 7008C, the less perfect Sm 2 Fe 17 C x phase gradually transforms into the Sm 2 Fe 14 C phase. The reason for the imperfection of the Sm 2 Fe 17 C x phase is the fact that when annealed T a ,7008C for 35 min the crystallization process of Sm–Fe–C is not yet completely finished. When comparing with the results for the alloys with y514 and y516, one finds that when T a is relatively low, the Sm 2 Fe 17 C x phase tends to form, while the higher T a (in the range of 650,T a ,9508C) is more favourable for the formation of the Sm 2 Fe 14 C phase. On the other hand, with increasing y, the phase Sm 2 Fe 17 C x tends to be formed. This is because only small quantities of the Sm 2 Fe 17 C x phase are formed in the alloy with y516 annealed at 7008C, while it becomes the main phase for y518. It is worth noticing that although the ratio of Sm: Fe: C of the MA Sm 10 Fe 85 C 5 alloy is equal to the ratio in Sm 2 Fe 17 C, no Sm 2 Fe 17 C x phase forms in the annealed Sm 10 Fe 85 C 5 alloy.
D.-y. Geng et al. / Journal of Alloys and Compounds 291 (1999) 276 – 281
Fig. 5 illustrates phase transformations of mechanically alloyed Sm y Fe 10021.5y C 0.5y (10,y,20) depending on the composition and the annealing temperature. It can be concluded that the Sm 2 Fe 17 C x (Sm 2 Fe 14 C) phase tends to form when y is increased (decreased). The Sm 2 Fe 14 C and Sm 2 Fe 17 C x phases coexist for a certain range of annealing temperatures. At T a 56508C crystallization is not perfected in the MA alloys because of the low reaction kinetics that hampers a complete crystallization. The crystallization temperature of the alloys decreases with increasing y. After annealing for 35 min between 6508C and 9508C, the permanent magnetic properties of all the MA alloys are relatively low. The energy product (BH) max and the coercivity j Hc of the Sm y Fe 10021.5y C 0.5y alloys are lower than 1 MGOe and 1 kOe, respectively. The main reasons for the poor magnetic properties are as follows: The Sm 2 Fe 14 C and Sm 2 Fe 17 C x phases can transform into each other so that in fact it is difficult to form only one single phase, Sm 2 Fe 14 C or Sm 2 Fe 17 C x , for most of the annealed MA alloy. The good magnetic properties derive from the Sm 2 Fe 17 C x phase, which has a uniaxial magnetic anisotropy. However, when the T a ,7008C, the crystallization of Sm 2 Fe 17 C x is not perfect. When T a .7008C, the Sm 2 Fe 14 C phase appears in the alloys with y,16, which has easy plane anisotropy and the magnetic properties deteriorate rapidly. Although single phase Sm 2 Fe 17 C x forms when T a .7008C and y.18, the C content remains low and is not sufficient for producing a high uniaxial anisotropic field needed for obtaining hard magnetic properties. Another reason for the poor magnetic properties might the comparatively large grain sizes of the Sm 2 Fe 17 C x phase when annealed at high temperatures.
Fig. 5. Phases transformation of mechanically alloyed Sm y Fe 10021.5y C 0.5y (10,y,20) depending on the composition and the annealing temperature. All the alloys are annealed at various temperatures for 35 min.
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3.2. Remilling and annealing In order to achieve better magnetic properties, one needs not only to avoid the formation of the Sm 2 Fe 14 C phase, but also to increase the carbon content in the Sm 2 Fe 17 C x phase. As indicated above, the carbon atoms in the alloys diffuse very slowly at T a ,6508C. Increasing the annealing temperature is not helpful for increasing the carbon content in the Sm 2 Fe 17 Cx phase since the experimental results above showed that the higher the annealing temperature T a, the lower the Curie temperature T c of the Sm 2 Fe 17 C x phase. This means that the carbon content in the Sm 2 Fe 17 C x phase decreases with increasing annealing temperature. In order to overcome the difficulties above, we developed a procedure, including re-milling for a short time and re-annealing at low temperatures. Figs. 6 and 7 show the X-ray diffraction patterns and the temperature dependence of the a.c. susceptibility xa.c. of the MA Sm 14 Fe 79 C 7 alloy annealed at 8008C for 35 min. Curve 1 in the two figures represents the results for the alloys before re-milling. Curve 2 is results obtained after re-milling for 1 h while curve 3 is results obtained after re-annealing at 6008C for 35 min after the re-milling. After re-milling, the crystal grains are distorted while the size of the grains is decreased. However, re-milling for 1 h does not lead to the complete destruction of the structure of the Sm 2 Fe 17 C x phase. Due to the re-milling, more interfaces and / or the grain boundaries appear while the carbon atoms are mixed efficiently with the grains of the Sm 2 Fe 17 C x phase. During re-annealing at low temperatures, the carbon atoms can diffuse easily into the lattice of the Sm 2 Fe 17 C x phase. The extra energy provided by the re-milling is responsible for overcoming the energy barriers of the
Fig. 6. X-ray diffraction patterns of mechanically alloyed Sm 14 Fe 79 C 7 alloy (1) annealed at 8008C for 35 min, then (2) re-milled for 1 h and (3) re-annealed at 6008C for 35 min.
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Fig. 7. Temperature dependence of a.c. susceptibility xa.c. of the samples of Fig. 6.
Fig. 9. Temperature dependence of a.c. susceptibility xa.c. of the samples of Fig. 8.
diffusion. It is likely that the grain size after re-annealing is much smaller than that before the re-milling procedure. Figs. 8 and 9 show the X-ray diffraction patterns and the temperature dependence of the a.c. susceptibility xa.c. of the MA Sm 20 Fe 70 C 10 alloy annealed at 8008C for 35 min. Curve 1 in the two figures represents the results for the alloys before re-milling. Curve 2 is results obtained after ball-milling for 1 h while curve 3 is results obtained after re-annealing at 5508C for 2 h after the re-milling. The
results in Figs. 8 and 9 for the MA Sm 20 Fe 70 C 10 alloy are similar to those in Figs. 6 and 7 for the MA Sm 14 Fe 79 C 7 alloy. The re-milling and re-annealing procedure results in a more complete reaction for the formation of the hard magnetic phase Sm 2 Fe 17 C x with higher carbon content. It is clear from Fig. 9 that the Curie temperature of the Sm 2 Fe 17 C x phase is slightly higher than that before the re-milling and re-annealing procedure.
Fig. 8. X-ray diffraction patterns of mechanically alloyed Sm 20 Fe 70 C 10 alloy (1) annealed at 8008C for 35 min, then (2) re-milled for 1 h and (3) re-annealed at 5508C for 2 h.
Fig. 10. Hysteresis loops of mechanically alloyed, annealed at 8008C for 35 min, then re-milled for 1 h (1) Sm 14 Fe 79 C 7 samples re-annealed at 6008C for 35 min and (2) Sm 20 Fe 70 C 10 samples re-annealed at 5508C for 2 h.
D.-y. Geng et al. / Journal of Alloys and Compounds 291 (1999) 276 – 281
Consequently, the magnetic properties of the MA Sm y Fe 10021.5y C 0.5y alloys (T a 58008C) can be increased by the combination of re-milling and then re-annealing at low temperature. Fig. 10 shows hysteresis loops of two alloys first annealed at 8008C for 35 min, then re-milled for 1 h and re-annealed at various temperatures. Curve(1): Sm 14 Fe 79 C 7 re-annealed at 6008C for 35 min having the following magnetic properties Ms 511.8 kGs, Mr 59.53 kGs, i Hc 52.67 kOe, (BH) max 510.7 MGOe. Curve (2): Sm 20 Fe 70 C 10 re-annealed at 5508C for 2 h, which leads to Ms 56.25 kGs, Mr 54.51 kGs, i Hc 57.96 kOe and (BH) max 52.79 MGOe. The good magnetic properties achieved in this work are mainly due to the fact that we succeeded in preparing Sm–Fe–C based magnets composed of the Sm 2 Fe 17 C x phase with high carbon content, and having a high uniaxial magnetic anisotropy at room temperature. The comparatively smaller grain size after re-milling and re-annealing also contributes to the better magnetic properties. Finally, we would like to compare our methods and results with those of Mao et al. [9,14]. First, the methods we used differ from those of Mao et al. Their first step was to prepare the Sm 2 Fe 17 alloy by melting the constituents in a cold crucible induction furnace. An excess of 15% Sm was added to compensate for Sm loss during the melting. The Sm 2 Fe 17 C x samples in their work were synthesized by solid–solid reaction of Sm 2 Fe 17 / graphite blends made by ball milling. Our first step was to synthesize the Sm 2 Fe 17 C x phase directly by mechanical alloying the appropriate amounts of the pure constituents. After the first annealing procedure at high temperatures (6508C,T a , 9508C), we applied re-milling for a short time and then re-annealing at low temperatures (below 6508C). Second, in the report of Mao et al. the metastable Sm 2 Fe 17 C x was found to decompose when it was heated to above 5008C. Our results in most alloys are in good agreement with their results. However, we obtained the Sm 2 Fe 17 C x phase without the presence of the Sm 2 Fe 14 C phase for the Sm 20 Fe 70 C 10 alloy annealed between 6508C and 9508C. Third, only the value of the coercivity was reported for the Sm–Fe–C based magnets [9]. The re-milling and re-annealing procedure developed in this work is helpful for the formation of a high carbon content in the Sm 2 Fe 17 C x phase leading to high uniaxial magnetic anistropy. Good hard magnetic properties have been achieved for the Sm 2 Fe 17 C x based magnets due to the novel procedure.
4. Conclusions The structure, phase transformation and magnetic properties of Sm y Fe 10021.5y C 0.5y ( y510, 12, 14, 16, 18, 20) alloys prepared by mechanical alloying (MA) have been studied systematically. The structure of MA Sm y Fe 10021.5y C 0.5y alloys depends sensitively on the composition and the annealing temperature (T a ). The Sm 2 Fe 17 C x (Sm 2 Fe 14 C) phase tends to form with increas-
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ing (decreasing) y. For the same composition, the Sm 2 Fe 17 C x phase is formed more easily when T a is lower. No Sm 2 Fe 14 C phase is observed in the Sm 20 Fe 70 C 10 alloy. For other alloys annealed between 6508C and 9508C, the metastable Sm 2 Fe 17 carbide tends to decompose so that Sm 2 Fe 14 C and Sm 2 Fe 17 C x phases coexist under a certain condition. A re-milling and re-annealing procedure has been developed to improve the magnetic properties of the MA alloys. The procedure is helpful for preparing Sm–Fe–C based magnets containing the Sm 2 Fe 17 C x phase of high carbon content, which has a high uniaxial magnetic anisotropy at room temperature. It also results in a comparatively smaller grain size which might contribute to the better magnetic properties. Thanks to this procedure, the high values (BH) max 510.6 MGOe and i Hc 57.96 kOe have been achieved for MA Sm 14 Fe 79 C 7 and Sm 20 Fe 70 C 10 alloys, respectively.
Acknowledgements This work has been supported by the National Natural Sciences Foundation of China under grant no.59725103 and 59831010, by the Sciences and Technology Commissions of Shenyang and Liaoning, and Tianjin Sciences Foundation under project no. 963603411.
References [1] M. Sagawa, S. Fujimara, N. Togawa, H. Yamamoto, Y. Matsuura, J. Appl. Phys. 55 (1984) 2083. [2] J.F. Herbst, Rev. Mod. Phys. 63 (1991) 819. [3] F.R. de Boer, Y.-K. Huang, Z.-D. Zhang, D.B. de Mooij, K.H.J. Buschow, J. Magn. Magn. Mater. 72 (1988) 167. [4] F.R. de Boer, R. Verhoef, Z. Zhi-dong, D.B. de Mooij, K.H.J. Buschow, J. Magn. Magn. Mater. 73 (1988) 263. [5] X.P. Zhong, R.J. Radwanski, F.R. de Boer, T.H. Jacobs, K.H.J. Buschow, J. Magn. Magn. Mater. 86 (1990) 333. [6] D.B. De Mooij, K.H.J. Buschow, J. Less-Common Met. 142 (1988) 349. [7] J.M.D. Coey, H. Sun, J. Magn. Magn. Mater 86 (1990) L251. [8] M. Gueramian, A. Bezinge, K. Yvon, J. Muller, Solid State Commun. 64 (1987) 639. [9] O. Mao, J.O. Stom-Olsen, Z. Altounian, J. Yang, J. Appl. Phys. 79 (1996) 4619. [10] L.X. Liao, X. Chen, Z. Altounian, D.H. Ryan, Appl. Phys. Lett. 60 (1992) 129. [11] R. Skomski, C. Murray, S. Brennan, J.M.D. Coey, J. Appl. Phys. 73 (1993) 6940. ¨ [12] R. Grossinger, X.C. Kou, T.H. Jacobs, K.H.J. Buschow, J. Appl. Phys. 69 (1991) 5596. [13] J.M.D. Coey, H. Sun, Y. Otani, D.P.F. Hurley, J. Magn. Magn. Mater. 98 (1991) 76. [14] O. Mao, Z. Altounian, J. Yang, J.O. Strom-Olsen, J. Appl. Phys. 79 (1996) 5536. [15] Y.C. Sui, Z.D. Zhang, Q.F. Xiao, W. Liu, X.G. Zhao, T. Zhao, Y.C. Chuang, J. Phys. Cond. Matt. 8 (1996) 11231. [16] Y.C. Sui, Z.D. Zhang, Q.F. Xiao, W. Liu, T. Zhao, X.G. Zhao, Y.C. Chuang, J. Alloys and Compounds. 267 (1998) 215. [17] W. Liu, Q. Wang, X.K. Sun, X.G. Zhao, T. Zhao, Z.D. Zhang, Y.C. Chuang, J. Magn. Magn. Mater. 131 (1994) 413.
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Structure, phase transformation and magnetic properties of Sm y Fe 10021.5y C 0.5y alloys prepared by mechanical alloying and re-milling a, a a a a a Dian-yu Geng *, Zhi-dong Zhang , Bao-zhi Cui , Zhi-jun Guo , Wei Liu , Xin-guo Zhao , a b Tong Zhao , Ji-wen Liu a
International Center for Materials Physics, Institute of Metal Research, Academia Sinica, Shenyang, 110015, China b Tianjin Institute of Technology, Tianjin 300191, China Received 27 April 1999; received in revised form 8 June 1999; accepted 8 June 1999
Abstract The structure, phase transformation and magnetic properties of Sm y Fe 10021.5y C 0.5y ( y510, 12, 14, 16, 18, 20) alloys prepared by mechanical alloying (MA) have been studied systematically. The Sm 2 Fe 17 Cx (Sm 2 Fe 14 C) phase tends to form when y is increased (decreased). For the same composition, the Sm 2 Fe 17 C x phase is formed more easily when the annealing temperature T a is lower. No Sm 2 Fe 14 C phase is observed in the Sm 20 Fe 70 C 10 alloy. In other alloys, annealed between 6508C and 9508C, the Sm 2 Fe 14 C and Sm 2 Fe 17 C x phases coexist under a certain condition. Re-milling and re-annealing proved to be suitable for improving the magnetic properties of the MA alloys. After annealing at 8008C for 35 min, re-milling for 1 h and then re-annealing at 6008C for 35 min, the value (BH) max 510.6 MGOe has been achieved for the MA Sm 14 Fe 79 C 7 alloy. After annealing at 8008C for 35 min, the MA Sm 20 Fe 70 C 10 alloy was re-milled for 1 h and then re-annealed at 5508C for 2 h, resulting in the value of i Hc 57.96 kOe. 1999 Elsevier Science S.A. All rights reserved. Keywords: Samarium iron carbides; Magnetic properties; Mechanical alloying
1. Introduction The discovery of high energy product magnets based on Nd 2 Fe 14 B [1,2] has stimulated the study of the corresponding series of carbides [3,4]. The structure and magnetic properties of Nd 2 Fe 14 C-type compounds were found to be close to those of their boride counterparts. Subsequently, the interstitial compounds of Sm 2 Fe 17 C x [5,6] and Sm 2 Fe 17 N x [7]-types were studied. It was found that Sm 2 Fe 17 can accommodate three carbon atoms per formula unit [5]. The carbon atoms occupy interstitial sites in Sm 2 Fe 17 C x which have a large effect on the structure of the compounds. The lattice parameters a and c, the Curie temperature and the 3d magnetic moment increase with increasing carbon atoms. Due to the outstanding values of the intrinsic magnetic properties, i.e., a Curie temperature T c ¯3978C and an anisotropy field m0 HA ¯16 T, the interstitial compound Sm 2 Fe 17 C x is a promising candidate for a new class of permanent magnetic material [5,6,8]. *Corresponding author. E-mail address: [email protected] (D.-y. Geng)
There have been reports of several preparation techniques for the interstitial compounds of the Sm 2 Fe 17 C x – type [9]: The first is to alloy directly with carbon above the melting temperature [5,8]. The second is by absorbing carbon through the gas–solid reaction of the Sm 2 Fe 17 compound with a hydrocarbon gas [10]. The third is through a solid–solid reaction with pure carbon [11]. Ternary compounds are formed upon combining Sm 2 Fe 17 with carbon atoms which occupy interstitial sites. The Curie temperature T c changes with varing C content in the Sm 2 Fe 17 Cx phase [12,13]. The interstitial Sm 2 Fe 17 C x compounds are metastable in alloys prepared by ball milling, where they can transform into a mixture of phases, including Sm 2 Fe 14 C, Sm 2 Fe 17 Cx , Sm 2 C 3 , a-Fe etc., when the annealing temperature T a varies from 6008C to 9508C [9,14]. In our previous work [15,16], structure, phase transformation and magnetic properties of Nd 2 Fe 14 C-based alloys were investigated by mechanical alloying (MA) and subsequent annealing. The effects of the phase transformation from Nd 2 Fe 17 C x to Nd 2 Fe 14 C on the magnetic properties of the alloys were discussed. In this contribution we study the phase transformation
0925-8388 / 99 / $ – see front matter 1999 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 99 )00278-9
D.-y. Geng et al. / Journal of Alloys and Compounds 291 (1999) 276 – 281
from Sm 2 Fe 14 C to Sm 2 Fe 17 C x of Sm y Fe 10021.5y C 0.5y ( y5 10, 12, 14, 16, 18, 20) alloys prepared by MA. In the present system, the hard magnetic phase is of the Sm 2 Fe 17 C x -type, which has uniaxial magnetic anisotropy at room temperature [12]. In order to improve the magnetic properties of the MA alloys, a novel procedure, including re-milling and re-annealing, is developed in this work. The organization of this paper is as follows. Section 2 contains experimental details, section 3 results and discussion. Section 3.1 deals with the structure, phase transformation and magnetic properties of MA Sm y Fe 10021.5y C 0.5y ( y510, 12, 14, 16, 18, 20) alloys upon the annealing. Section 3.2 concentrates on the effect of the re-milling and re-annealing on the structure and magnetic properties of MA Sm 14 Fe 79 C 7 and Sm 20 Fe 70 C 10 alloys. A conclusion is given in Section 4.
2. Experimental details 99.9%-pure Sm, 99.8%-pure Fe powders and 99.7% graphite were mixed according to the desired composition, and sealed under argon in a hardened cylindrical steel container. Mechanical alloying (MA) was performed in a high-energy ball miller designed in our laboratory for 5 h [15–17]. The hardened steel balls used for ball milling were 12 mm in diameter. The weight ratio of balls to powders was kept at 25:1. The diameter of the cans for MA is 54 mm. The diameter of the movement of the cans is about 100 mm. The rotating rate is 15 turns / s. The MA powders were subsequently in a vacuum furnace connected to a closed glove box from 6008C to 9508C for 35 min. In order to improve the formation process of Sm 2 Fe 17 C x and consequently the magnetic properties, a re-milling and re-annealing procedure was performed, just after the first annealing process. The conditions for the re-milling were set to be the same as those for MA, with exception of the re-milling time. After re-milling for 1 h, the powders were annealed at low temperatures for a more complete reaction for forming of the hard magnetic phase Sm 2 Fe 17 C x . X-ray diffraction (XRD) analysis was performed using Cu Ka radiation with a Rigaku D/ Max-rA diffractometer equipped with a graphite crystal monochromater. Initial susceptibility xa.c. measurements were used to determine the Curie temperature and to preliminarily estimate the amount of possible magnetic phases in the specimens. For the magnetic measurement at room temperature, the powders were embedded in epoxy resin with an approximate ratio 1:1 of the magnetic powders and the resin to form magnetically isotropic magnets. The magnetic properties were measured at room temperature using a pulsed magnetometer in fields up to 8 T. The magnetization was related to the amount of magnetic powders, neglecting the dilution effect of resin and the density of the magnetic powdered samples was assumed to be 7.6 g / cm 3 .
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3. Results and discussion
3.1. Mechanical alloying and annealing As usual, mechanical alloying for 5 h in a high energy ball miller is enough for forming the amorphous materials [15–17]. Fig. 1 shows X-ray diffraction patterns of MA Smy Fe 10021.5y C 0.5y ( y510, 12, 14, 16, 18 and 20) alloys annealed at 8008C for 35 min. Fig. 2 represents the temperature dependence of the a.c. susceptibility xa.c. of the corresponding alloys. The structure of the MA Sm y Fe 10021.5y C 0.5y alloys depends sensitively on the composition and annealing temperature T a . For y510, the main phases are a mixture of Sm 2 Fe 14 C and a-Fe. It is clearly shown in Fig. 2 that Curie temperature of the Sm 2 Fe 14 C compound is 3158C. The reaction during the annealing leads from amorphous Sm 10 Fe 85 C 5 to the mixture of Sm 2 Fe 14 C and a-Fe. The amount of a-Fe phase gradually decreases with increasing y. This means that Sm and C continue to combine with Fe to form the ternary compounds. In the alloy with y518, Sm 2 Fe 17 C x and Sm 2 Fe 14 C coexist with small amounts of a-Fe. For y520, the main phases are a mixture of Sm 2 Fe 17 Cx and Sm 2 C 3 so that the reaction during the annealing can be described as: Sm 20 Fe 70 C 10 (amorphous) → Sm 2 Fe 17 C x 1Sm 2 C 3. Figs. 3 and 4 represent respectively X-ray diffraction patterns and xa.c. vs. T curves of MA Sm 14 Fe 79 C 7 alloys annealed at 650, 700, 750, 800, 850, 900 and 9508C for 35 min. At T a 56508C, the relatively low annealing temperature for the solid reaction limits complete conversion of the
Fig. 1. X-ray diffraction patterns of mechanically alloyed Sm y Fe 10021.5y C 0.5y ( y510, 12, 14, 16, 18 and 20) alloys annealed at 8008C for 35 min.
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Fig. 2. Temperature dependence of a.c. susceptibility xa.c. of the samples of Fig. 1.
crystallites into Sm 2 Fe 17 C x . When T a 56508C the Curie temperature of Sm 14 Fe 79 C 7 alloys is 1718C which is higher than that of Sm 2 Fe 17 . When T a 58008C, the Sm 2 Fe 14 C phase forms perfectly. The Sm 2 Fe 17 C x and the Sm 2 Fe 14 C phases can transform into each other under certain conditions. In the present case, the following reaction can occur during annealing at various temperatures: Sm 2 Fe 14 C1a-Fe ↔Sm 2 Fe 17 C x 1 Sm 2 C 3.
Fig. 3. X-ray diffraction patterns of mechanically alloyed Sm 14 Fe 79 C 7 alloys annealed at 650, 700, 750, 800, 850, 900 and 9508C for 35 min.
Fig. 4. Temperature dependence of a.c. susceptibility xa.c. of the samples of Fig. 3.
At T a 58008C and for y520, the Sm 2 Fe 17 C x phase forms perfectly and its Curie temperature is only 1388C, indicating that the carbon content is small [6]. There is no Sm 2 Fe 14 C phase present in all samples with y520, annealed at various temperatures. The metastable Sm 2 Fe 17 carbide tends to decompose at temperatures from 650 to 8008C into other alloys with a carbon content from y510 to 18. At T a 56508C, the Sm 2 Fe 14 C and Sm 2 Fe 17 C x phases coexist in the alloys for y510, 12, 14 and 16. The Curie temperature T c of the Sm 2 Fe 17 C x phase changes with increasing annealing temperature, indicating a change of the carbon content of the phase. The Curie temperature of the Sm 2 Fe 14 C phase remains almost the same in these alloys, suggesting that the Sm 2 Fe 14 C phase in the alloys is relatively stable. From 650 to 7008C, the less perfect Sm 2 Fe 17 C x phase gradually transforms into the Sm 2 Fe 14 C phase. The reason for the imperfection of the Sm 2 Fe 17 C x phase is the fact that when annealed T a ,7008C for 35 min the crystallization process of Sm–Fe–C is not yet completely finished. When comparing with the results for the alloys with y514 and y516, one finds that when T a is relatively low, the Sm 2 Fe 17 C x phase tends to form, while the higher T a (in the range of 650,T a ,9508C) is more favourable for the formation of the Sm 2 Fe 14 C phase. On the other hand, with increasing y, the phase Sm 2 Fe 17 C x tends to be formed. This is because only small quantities of the Sm 2 Fe 17 C x phase are formed in the alloy with y516 annealed at 7008C, while it becomes the main phase for y518. It is worth noticing that although the ratio of Sm: Fe: C of the MA Sm 10 Fe 85 C 5 alloy is equal to the ratio in Sm 2 Fe 17 C, no Sm 2 Fe 17 C x phase forms in the annealed Sm 10 Fe 85 C 5 alloy.
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Fig. 5 illustrates phase transformations of mechanically alloyed Sm y Fe 10021.5y C 0.5y (10,y,20) depending on the composition and the annealing temperature. It can be concluded that the Sm 2 Fe 17 C x (Sm 2 Fe 14 C) phase tends to form when y is increased (decreased). The Sm 2 Fe 14 C and Sm 2 Fe 17 C x phases coexist for a certain range of annealing temperatures. At T a 56508C crystallization is not perfected in the MA alloys because of the low reaction kinetics that hampers a complete crystallization. The crystallization temperature of the alloys decreases with increasing y. After annealing for 35 min between 6508C and 9508C, the permanent magnetic properties of all the MA alloys are relatively low. The energy product (BH) max and the coercivity j Hc of the Sm y Fe 10021.5y C 0.5y alloys are lower than 1 MGOe and 1 kOe, respectively. The main reasons for the poor magnetic properties are as follows: The Sm 2 Fe 14 C and Sm 2 Fe 17 C x phases can transform into each other so that in fact it is difficult to form only one single phase, Sm 2 Fe 14 C or Sm 2 Fe 17 C x , for most of the annealed MA alloy. The good magnetic properties derive from the Sm 2 Fe 17 C x phase, which has a uniaxial magnetic anisotropy. However, when the T a ,7008C, the crystallization of Sm 2 Fe 17 C x is not perfect. When T a .7008C, the Sm 2 Fe 14 C phase appears in the alloys with y,16, which has easy plane anisotropy and the magnetic properties deteriorate rapidly. Although single phase Sm 2 Fe 17 C x forms when T a .7008C and y.18, the C content remains low and is not sufficient for producing a high uniaxial anisotropic field needed for obtaining hard magnetic properties. Another reason for the poor magnetic properties might the comparatively large grain sizes of the Sm 2 Fe 17 C x phase when annealed at high temperatures.
Fig. 5. Phases transformation of mechanically alloyed Sm y Fe 10021.5y C 0.5y (10,y,20) depending on the composition and the annealing temperature. All the alloys are annealed at various temperatures for 35 min.
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3.2. Remilling and annealing In order to achieve better magnetic properties, one needs not only to avoid the formation of the Sm 2 Fe 14 C phase, but also to increase the carbon content in the Sm 2 Fe 17 C x phase. As indicated above, the carbon atoms in the alloys diffuse very slowly at T a ,6508C. Increasing the annealing temperature is not helpful for increasing the carbon content in the Sm 2 Fe 17 Cx phase since the experimental results above showed that the higher the annealing temperature T a, the lower the Curie temperature T c of the Sm 2 Fe 17 C x phase. This means that the carbon content in the Sm 2 Fe 17 C x phase decreases with increasing annealing temperature. In order to overcome the difficulties above, we developed a procedure, including re-milling for a short time and re-annealing at low temperatures. Figs. 6 and 7 show the X-ray diffraction patterns and the temperature dependence of the a.c. susceptibility xa.c. of the MA Sm 14 Fe 79 C 7 alloy annealed at 8008C for 35 min. Curve 1 in the two figures represents the results for the alloys before re-milling. Curve 2 is results obtained after re-milling for 1 h while curve 3 is results obtained after re-annealing at 6008C for 35 min after the re-milling. After re-milling, the crystal grains are distorted while the size of the grains is decreased. However, re-milling for 1 h does not lead to the complete destruction of the structure of the Sm 2 Fe 17 C x phase. Due to the re-milling, more interfaces and / or the grain boundaries appear while the carbon atoms are mixed efficiently with the grains of the Sm 2 Fe 17 C x phase. During re-annealing at low temperatures, the carbon atoms can diffuse easily into the lattice of the Sm 2 Fe 17 C x phase. The extra energy provided by the re-milling is responsible for overcoming the energy barriers of the
Fig. 6. X-ray diffraction patterns of mechanically alloyed Sm 14 Fe 79 C 7 alloy (1) annealed at 8008C for 35 min, then (2) re-milled for 1 h and (3) re-annealed at 6008C for 35 min.
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Fig. 7. Temperature dependence of a.c. susceptibility xa.c. of the samples of Fig. 6.
Fig. 9. Temperature dependence of a.c. susceptibility xa.c. of the samples of Fig. 8.
diffusion. It is likely that the grain size after re-annealing is much smaller than that before the re-milling procedure. Figs. 8 and 9 show the X-ray diffraction patterns and the temperature dependence of the a.c. susceptibility xa.c. of the MA Sm 20 Fe 70 C 10 alloy annealed at 8008C for 35 min. Curve 1 in the two figures represents the results for the alloys before re-milling. Curve 2 is results obtained after ball-milling for 1 h while curve 3 is results obtained after re-annealing at 5508C for 2 h after the re-milling. The
results in Figs. 8 and 9 for the MA Sm 20 Fe 70 C 10 alloy are similar to those in Figs. 6 and 7 for the MA Sm 14 Fe 79 C 7 alloy. The re-milling and re-annealing procedure results in a more complete reaction for the formation of the hard magnetic phase Sm 2 Fe 17 C x with higher carbon content. It is clear from Fig. 9 that the Curie temperature of the Sm 2 Fe 17 C x phase is slightly higher than that before the re-milling and re-annealing procedure.
Fig. 8. X-ray diffraction patterns of mechanically alloyed Sm 20 Fe 70 C 10 alloy (1) annealed at 8008C for 35 min, then (2) re-milled for 1 h and (3) re-annealed at 5508C for 2 h.
Fig. 10. Hysteresis loops of mechanically alloyed, annealed at 8008C for 35 min, then re-milled for 1 h (1) Sm 14 Fe 79 C 7 samples re-annealed at 6008C for 35 min and (2) Sm 20 Fe 70 C 10 samples re-annealed at 5508C for 2 h.
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Consequently, the magnetic properties of the MA Sm y Fe 10021.5y C 0.5y alloys (T a 58008C) can be increased by the combination of re-milling and then re-annealing at low temperature. Fig. 10 shows hysteresis loops of two alloys first annealed at 8008C for 35 min, then re-milled for 1 h and re-annealed at various temperatures. Curve(1): Sm 14 Fe 79 C 7 re-annealed at 6008C for 35 min having the following magnetic properties Ms 511.8 kGs, Mr 59.53 kGs, i Hc 52.67 kOe, (BH) max 510.7 MGOe. Curve (2): Sm 20 Fe 70 C 10 re-annealed at 5508C for 2 h, which leads to Ms 56.25 kGs, Mr 54.51 kGs, i Hc 57.96 kOe and (BH) max 52.79 MGOe. The good magnetic properties achieved in this work are mainly due to the fact that we succeeded in preparing Sm–Fe–C based magnets composed of the Sm 2 Fe 17 C x phase with high carbon content, and having a high uniaxial magnetic anisotropy at room temperature. The comparatively smaller grain size after re-milling and re-annealing also contributes to the better magnetic properties. Finally, we would like to compare our methods and results with those of Mao et al. [9,14]. First, the methods we used differ from those of Mao et al. Their first step was to prepare the Sm 2 Fe 17 alloy by melting the constituents in a cold crucible induction furnace. An excess of 15% Sm was added to compensate for Sm loss during the melting. The Sm 2 Fe 17 C x samples in their work were synthesized by solid–solid reaction of Sm 2 Fe 17 / graphite blends made by ball milling. Our first step was to synthesize the Sm 2 Fe 17 C x phase directly by mechanical alloying the appropriate amounts of the pure constituents. After the first annealing procedure at high temperatures (6508C,T a , 9508C), we applied re-milling for a short time and then re-annealing at low temperatures (below 6508C). Second, in the report of Mao et al. the metastable Sm 2 Fe 17 C x was found to decompose when it was heated to above 5008C. Our results in most alloys are in good agreement with their results. However, we obtained the Sm 2 Fe 17 C x phase without the presence of the Sm 2 Fe 14 C phase for the Sm 20 Fe 70 C 10 alloy annealed between 6508C and 9508C. Third, only the value of the coercivity was reported for the Sm–Fe–C based magnets [9]. The re-milling and re-annealing procedure developed in this work is helpful for the formation of a high carbon content in the Sm 2 Fe 17 C x phase leading to high uniaxial magnetic anistropy. Good hard magnetic properties have been achieved for the Sm 2 Fe 17 C x based magnets due to the novel procedure.
4. Conclusions The structure, phase transformation and magnetic properties of Sm y Fe 10021.5y C 0.5y ( y510, 12, 14, 16, 18, 20) alloys prepared by mechanical alloying (MA) have been studied systematically. The structure of MA Sm y Fe 10021.5y C 0.5y alloys depends sensitively on the composition and the annealing temperature (T a ). The Sm 2 Fe 17 C x (Sm 2 Fe 14 C) phase tends to form with increas-
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ing (decreasing) y. For the same composition, the Sm 2 Fe 17 C x phase is formed more easily when T a is lower. No Sm 2 Fe 14 C phase is observed in the Sm 20 Fe 70 C 10 alloy. For other alloys annealed between 6508C and 9508C, the metastable Sm 2 Fe 17 carbide tends to decompose so that Sm 2 Fe 14 C and Sm 2 Fe 17 C x phases coexist under a certain condition. A re-milling and re-annealing procedure has been developed to improve the magnetic properties of the MA alloys. The procedure is helpful for preparing Sm–Fe–C based magnets containing the Sm 2 Fe 17 C x phase of high carbon content, which has a high uniaxial magnetic anisotropy at room temperature. It also results in a comparatively smaller grain size which might contribute to the better magnetic properties. Thanks to this procedure, the high values (BH) max 510.6 MGOe and i Hc 57.96 kOe have been achieved for MA Sm 14 Fe 79 C 7 and Sm 20 Fe 70 C 10 alloys, respectively.
Acknowledgements This work has been supported by the National Natural Sciences Foundation of China under grant no.59725103 and 59831010, by the Sciences and Technology Commissions of Shenyang and Liaoning, and Tianjin Sciences Foundation under project no. 963603411.
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