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Sm(Co, Fe, Cu, Zr)z sintered magnets with a maximum operating temperature of 500 °C
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 303 (2006) e396–e401 www.elsevier.com/locate/jmmm
Sm(Co, Fe, Cu, Zr)z sintered magnets with a maximum operating temperature of 500 1C Z.H. Guo, W. Pan, W. Li Central Iron & Steel Research Institute, Division of Functional Materials, 76 Xueyuan Nanlu, Haidian District Beijing 100081, China Available online 20 February 2006
Abstract The Fe content v dependence of magnetic properties of Sm(CobalFevCu0.088Zr0.025)7.5 (v ¼ 0–0.30) sintered magnets has been systematically studied. With increasing v, the remanences Br and the maximum energy products (BH)max at room temperature increase initially and reach an optimal Br of 1.055 T for v ¼ 0.27 and an optimal (BH)max of 205 kJ/m3 for v ¼ 0.21, and then decrease sharply. Fe content significantly affects the temperature coefficient of intrinsic coercivity. The lower the Fe content is, the smaller the temperature coefficient of coercivity is. A temperature coefficient of intrinsic coercivity of -0.14%/ 1C (20–500 1C) has been achieved for v ¼ 0.07, as compared with -0.23%/ 1C for v ¼ 0.21. A high temperature sintered magnet with a maximum operating temperature of 500 1C has been obtained on the composition of Sm(CobalFe0.07Cu0.088Zr0.025)7.5. The initial magnetization curve exhibits that this high temperature magnet is still controlled by a strong domain wall pinning mechanism up to 500 1C. r 2006 Elsevier B.V. All rights reserved. PACS: 75.50.ww; 75.50.vv; 75.60.ej; 75.60.ch Keywords: High temperature magnets; Temperature dependence of intrinsic coercivity; 2:17 type Sm–Co sintered magnets; Thermal stability; Coercivity mechanism
1. Introduction Since the 2:17-type Sm–Co magnet shows excellent characterizations such as high magnetic properties, outstanding thermal stability and excellent corrosion resistance [1], it has been regarded as the ideal materials in applications like servo-motors, pump couplings and sensors, particularly where the magnet is required to operate at high temperature, across a broad temperature range or in a corrosive environment. Recently, 2:17-type Sm–Co magnet has been receiving considerable attention again due to high-temperature applications [2]. For these applications, the desired maximum operating temperature is above 450 1C. Among all rare-earth permanent magnets, the 2:17-type Sm–Co magnet has the highest Curie temperature and magnetization and therefore, is the most promising candidate for high-temperature applications. Corresponding author. Tel.: +86 10 62184522; fax: +86 10 62182610.
E-mail address: [email protected] (Z.H. Guo). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.01.107
Serious efforts to improve high-temperature performance of 2:17-type Sm–Co magnets began in 1995 [3–5]. It is well established that 2:17-type Sm–Co magnet is a pinning-controlled magnet [6]. The microstructures of 2:17type Sm–Co magnets are obtained after a long and complicated heat treatment. They consist of a mixture of cellular and lamella structures [1]. The cell structures consist of a more or less well-developed network of very small cells of a rhombohedral Sm2(Co,Fe)17 matrix phase, within the much larger grains, which are separated and often completely surrounded by a hexagonal Sm(Co,Cu)5 cell boundary phase. When high coercivity magnets are heat treated to their optimum magnetic properties, the matrix phases have linear dimensions of about 100–200 nm and the boundary phases are typically 2–10 nm thick. There are also other very thin layers visible under an electron microscope that are perpendicular to the c-axis and run across many cells and cell boundaries. They belong to a third phase, the so-called ‘‘platelet phase’’ or ‘‘z-phase’’ which contains most of the Zr.
ARTICLE IN PRESS Z.H. Guo et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e396–e401
In this article, we studied the magnetic properties of 2:17-type Sm–Co sintered magnets with various iron content at room and high temperature, respectively. The coercivity mechanism of 2:17-type Sm–Co magnets at high temperature were investigated by initial magnetization curves measured by a close circuit hysteresigraph. A high temperature sintered magnet with maximum operating temperature of 500 1C has been obtained on the composition of Sm(CobalFe0.07Cu0.088Zr0.025)7.5. 2. Experimental methods Magnets with nominal composition of Sm(CobalFevCu0.088Zr0.025)7.5 (v ¼ 00.30) were prepared by arc
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melting under Ar atmosphere, followed by conventional powder metallurgy processing. Sm has a great affinity for oxygen and will form Sm2Co3 whenever oxygen is available. Different manufacturing processes result in different oxygen content, so the use of effective Sm content is intended in this article. Fine powder with particle size 4 mm were aligned and pressed in a magnetic field of 1500 kA/m and then further compacted using an isostatic cool pressing. The green bodies were sintered at 1190–1250 1C for 0.5–2 h in Ar, followed by solution heat treatment at 1160–1220 1C for 2–6 h. The sintered parts were then aged at 780–850 1C for 10–40 h, followed by slow cooling to 400 1C at 0.8 1C/min. Initial magnetization and demagnetization curves were measured from room
1.2
Sm(CobalFevCu0.088Zr0.025)7.5 Sintered Magnets 1.0
0.6
0.4
v=0.16 v=0.00
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v=0.07
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J (T)
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0.2
0.0 -1200
- 1000
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H (kA/m) 1.2 Sm(CobalFevCu0.088Zr0.025)7.5 Sintered Magnets 1.0
0.6
v=0.16 v=0.00
v=0.21
v=0.04
v=0.24
v=0.07
v=0.27
v=0.13
v=0.30
J (T)
0.8
0.4
0.2
0.0 - 1200
- 1000
- 800
- 600
- 400
- 200
0
H (kA/m) Fig. 1. J-H Demagnetization curves of the Sm(CobalFevCu0.088 Zr0.025)7.5 sintered magnets at room temperature vs. Fe content v.
ARTICLE IN PRESS Z.H. Guo et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e396–e401
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temperature to 500 1C using a NIM-2000 H closed-circuit hysteresigraph. In order to obtain accurate temperature indication, a Pt100 thermal sensor was installed very close to the sample. 3. Results and discussion Fig. 1 shows the demagnetization curves of Sm(CobalFevCu0.088Zr0.025)7.5 sintered magnets at room temperature. Fig. 2 shows Br, (BH)max, bHC and Hk at room temperature vs. Fe content v. The Sm(CobalFevCuyZrx)z magnets represent a complicated system with four compositional variables (v, x, y, z) and at least five heat-treating variables [7]. The microstructures and magnetic properties of
Sm(Co,Fe,Cu,Zr)z magnet significantly depend on the processing parameters and heat treatments. In our experiment, every magnet with certain composition has been prepared by several processing parameters and heat treatments. Figs. 1 and 2 show the optimum properties of each composition. With increasing Fe content v, the remanence Br and maximum energy products (BH)max increase initially and reach optimal values of 1.055 T at v ¼ 0.27 for Br and 205 kJ/m3 at v ¼ 0.21 for (BH)max and then drop sharply with further Fe increase, as shown in Fig. 2. For Sm(Co,Fe,Cu,Zr)z magnets, Fe mainly exists in rhombohedral Th2Zn17-type Sm2(Co,Fe)17 matrix phase, which is responsible for high saturation magnetization of magnets.
1.2 1500
Sm(CobalFevCu0.088Zr0.025) 7.5 Sintered Magnets 200
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bHC,
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Sm(CobalFevCu0.088Zr0.025) 7.5 Sintered Magnets 1.0
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bHC,
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0.2
Hk 0 0.00
0.05
0.10
0.15
0.20
0.25
0.0 0.30
0
Fe content (v) Fig. 2. Effect of Fe content v on the Br, (BH)max, bHC and Hk at room temperature of Sm(CobalFevCu0.088Zr0.025)7.5 sintered magnets.
ARTICLE IN PRESS Z.H. Guo et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e396–e401
iHC (kA/m)
Substituting Fe for Co significantly increases the saturation magnetization of Sm2Co17 and this is an obvious approach to improving magnetic properties of Sm(Co,Fe,Cu,Zr)z at room temperature [8]. However, an excessive substitution of Fe (vX0.24) for Co results in rapid deterioration of
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permanent magnetic properties due to formation of a considerable amount of Fe–Co soft magnetic phase, which was reported in our previous article [9]. Kumar [10] reported that the higher Fe content is, the more easily Fe–Co soft magnetic phase forms in the heat treatment
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Sm(CobalFe0.07Cu0.088Zr0.025) 7.5
2000
commercial 2:17-type Sm-Co
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Sm(CobalFe0.07Cu0.088Zr0.025) 7.5
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Br (T)
1.0 0.8
(BH)max (kJ/m3)
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100
200
300
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Temperature (°C) Fig. 3. Temperature dependence of magnetic properties for high-temperature magnet Sm(CobalFe0.07Cu0.088 Zr0.025)7.5(K) and commercial high coercivity 2:17-type Sm–Co (’).
ARTICLE IN PRESS Z.H. Guo et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e396–e401
temperature °C 350 400 450 500
Br T 0.793 0.756 0.725 0.687
iHC
H b C
(BH)max
kA/m 1101 886 764 637
kA/m 592 533 516 430
kJ/m3 117 103 95 84
1.4 1.2 1.0 0.8 0.6
J
350oC 400oC 450oC 500oC
J, B (T)
process for Sm(Co,Fe,Cu,Zr)z magnets. The magnet with v ¼ 0.24 exhibits a clear two step demagnetization curve and the iHC of magnet with v ¼ 0.30 is near to zero, as shown in Fig. 1. (BH)max decrease sharply with increasing v from 0.21 to 0.30. Fe content v also has a significant effect on demagnetization curve squareness of Sm(CobalFevCu0.088Zr0.025)7.5 sintered magnets. In Figs. 1 and 2, we can easily see the squareness is increasing with decreasing v. The magnet with v ¼ 0.07 has the best demagnetization curve squareness among these series magnets. Iron has a significant effect on the high-temperature magnetic properties of Sm(Co,Fe,Cu,Zr)z [3,4,11,12]. The lower the Fe content is, the higher the intrinsic coercivity at high temperature is. One reason why Sm(Co,Fe,Cu,Zr)z magnet with low iron content demonstrates improved hightemperature stability is that its Curie temperature is higher than that of the magnet with high iron content. For Sm2(Co1xFex)17 matrix phase, the Curie temperature TC increases with decreasing iron content; for example, TC ¼ 747 1C for x ¼ 0.5 and TC ¼ 926 1C for x ¼ 0. The temperature coefficient is the most important factor in the development of high temperature magnets. The temperature coefficient of remanence a is defined by a(T0T1) ¼ {[Br(T0)-Br(T1)]/[Br(T0) (T0T1)]} 100%, where T0 and T1 are the room temperature and operating temperature, Br(T0) and Br(T1) are the remanence at room temperature and operating temperature, respectively. The temperature coefficient of intrinsic coercivity b is defined by b(T0 T1) ¼ {[iHC(T0) iHC(T1)]/[iHC(T0) (T0 T1)]} 100%, where iHC(T0) and iHC(T1) are the intrinsic coercivity at room temperature and operating temperature, respectively. According to Kim [13,14], the maximum operating temperature is a function of the coercivity at room temperature and temperature coefficient of coercivity b and the maximum operating temperature is more effectively increased by a b decrease. Fig. 3 shows the comparison of temperature dependence of intrinsic coercivity, remanences and maximum energy products between Sm(CobalFe0.07Cu0.088Zr0.025)7.5 magnet and a commercial high coercivity 2:17-type Sm–Co magnet. For the commercial magnet, the coercivity is 2630 kA/m at 25 1C and 175 kA/m at 500 1C, which gives a temperature coefficient b of 0.20%/ 1C. For Sm(CobalFe0.07Cu0.088Zr0.025)7.5 magnet, the coercivity is 1831 kA/m at 25 1C and 637 kA/m at 500 1C, which gives a temperature coefficient b of 0.14%/ 1C. For the magnet with v ¼ 0.21, b ¼ 0.23%/ 1C. Fe content of commercial 2:17-type Sm–Co magnet is generally in the range 0.21–0.31. From these experimental results, it is easily found that b of magnet with low Fe content is much smaller than that of magnet with high Fe content. In Fig. 3, we can see that the temperature coefficients of remanence a are almost the same for Sm(CobalFe0.07Cu0.088Zr0.025)7.5 magnet and commercial 2:17-type Sm–Co magnet. The maximum energy product (BH)max of Sm(CobalFe0.07Cu0.088Zr0.025)7.5 magnet at 500 1C is higher than that of commercial Sm–Co magnet mainly due to its higher intrinsic coercivity at high
0.4 B 0.2
-1000
-800
-600
-400
-200
0
0.0
H (kA/m) Fig. 4. Demagnetization curves and magnetic properties of Sm(CobalFe0.07Cu0.088Zr0.025)7.5 sintered magnet at 350, 400, 450, and 500 1C.
1.0 Sm(CobalFe0.07Cu0.088Zr0.025) 7.5 Sintered Magnets @500°C
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0.6 J (T)
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0.0
-0.2 -600 -400 -200
0
200
400
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800
1000 1200
H (kA/m)
Fig. 5. Initial magnetization curve and demagnetization curve of Sm(CobalFe0.07Cu0.088Zr0.025)7.5 sintered magnet at 500 1C.
temperature. Fig. 4 shows the demagnetization curves and magnetic properties of Sm(CobalFe0.07Cu0.088Zr0.025)7.5 sintered magnet at 350, 400, 450 and 500 1C. This high temperature magnet exhibits excellent high-temperature magnetic properties with Br ¼ 0.687 T, bHC ¼ 430 kA/m, 3 iHC ¼ 637 kA/m and (BH)max ¼ 84 kJ/m at 500 1C. Fig. 5 shows the initial magnetization and demagnetization curves of Sm(CobalFe0.07Cu0.088Zr0.025)7.5 at 500 1C. The initial magnetization curve is typical of domain-wall pinning type magnets. Te´llez-Balanco et al. [15] studied the coercivity mechanism using a micromagnetic analysis of the temperature dependence of the intrinsic coercivity and found that the coercivity of Sm(Co,Fe,Cu,Zr)z is controlled at elevated temperature (above 520 K ) by a nucleation process of reversal domain. However, our experimental results show that the coercivity of Sm(Co,Fe,Cu,Zr)7.5 magnets is still determined predominately by a domainwall pinning process up to 500 1C (773 K).
ARTICLE IN PRESS Z.H. Guo et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e396–e401
4. Conclusions Sm(CobalFevCu0.088Zr0.025)7.5 sintered magnets with higher Fe (vp0.24) content have higher remanences and maximum energy products at room temperature. An excessive substitution of Fe for Co (vX0.24) results in rapid deterioration of permanent magnetic properties due to the formation of a considerable amount of Fe–Co soft magnetic phase. Fe content strongly affects the temperature coefficient of intrinsic coercivity b. The lower the Fe content is, the smaller the b is. When the Fe content v of Sm(CobalFevCu0.088Zr0.025)7.5 decreases from 0.21 to 0.07, b (20–500 1C) is reduced from 0.23 to 0.14%/1C. A high-temperature sintered magnet with a maximum operating temperature of 500 1C has been obtained on the composition of Sm(CobalFe0.07Cu0.088Zr0.025)7.5 with Br ¼ 0.687 T, bHC ¼ 430 kA/m, iHC ¼ 637 kA/m and (BH)max ¼ 84 kJ/m3 at 500 1C. The initial magnetization curve indicates that Sm(CobalFe0.07Cu0.088Zr0.025)7.5 sintered magnets are still controlled by a strong domain wall pinning mechanism up to 500 1C. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant no. 50201004) and the
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National High Technology Research and Development Program of China (Grant no. 2003AA305930).
References [1] K.H.J. Buschow, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, vol. 10, 1997, p. 546 Chapter.4. [2] G.C. Hadjipanayis, W. Tang, Y. Zhang, S.T. Chui, J.F. Liu, C. Chen, H. Kronmu¨ller, IEEE Trans. Magn. 36 (2000) 3382. [3] B.M. Ma, Y.L. Liang, J. Patel, D. Scott, C.O. Bounds, IEEE Trans. Magn. 32 (1996) 4377. [4] S. Liu, E.P. Hoffman, IEEE Trans. Magn. 32 (1996) 5091. [5] J.F. Liu, Y. Ding, Y. Zhang, D. Dimitar, F. Zhang, G.C. Hadjipanayis, J. Appl. Phys. 85 (1999) 5660. [6] G. Hadjipanayis, in: J.M.D. Coey (Ed.), Rare-earth Iron Permanent Magnets, Clarendon press, Oxford, 1996. [7] A.E. Ray, J. Appl. Phys. 55 (1984) 2094. [8] A.E. Ray, S. Liu, J. Mater. Eng. Perform. 1 (1992) 183. [9] Guo Zhaohui, Li Wei, Acta Metall. Sin. 38 (2002) 866. [10] K. Kumar, J. Appl. Phys. 63 (1988) R13. [11] C.H. Chen, M.S. Walmer, M.H. Walmer, S. Liu, E. Kuhl, G. Simon, J. Appl. Phys. 83 (1998) 6706. [12] J.F. Liu, Y. Ding, G.C. Hadjipanayis, J. Appl. Phys. 85 (1999) 1670. [13] A.S. Kim, J. Appl. Phys. 81 (1997) 5609. [14] A.S. Kim, J. Appl. Phys. 83 (1998) 6715. [15] J.C. Te´llez-Balanco, X.C. Kou, R. Gro¨ssinger, E. Este´vez-Rams, J. Fidler, B.M. Ma, J. Appl. Phys. 82 (1997) 3928.
Journal of Magnetism and Magnetic Materials 303 (2006) e396–e401 www.elsevier.com/locate/jmmm
Sm(Co, Fe, Cu, Zr)z sintered magnets with a maximum operating temperature of 500 1C Z.H. Guo, W. Pan, W. Li Central Iron & Steel Research Institute, Division of Functional Materials, 76 Xueyuan Nanlu, Haidian District Beijing 100081, China Available online 20 February 2006
Abstract The Fe content v dependence of magnetic properties of Sm(CobalFevCu0.088Zr0.025)7.5 (v ¼ 0–0.30) sintered magnets has been systematically studied. With increasing v, the remanences Br and the maximum energy products (BH)max at room temperature increase initially and reach an optimal Br of 1.055 T for v ¼ 0.27 and an optimal (BH)max of 205 kJ/m3 for v ¼ 0.21, and then decrease sharply. Fe content significantly affects the temperature coefficient of intrinsic coercivity. The lower the Fe content is, the smaller the temperature coefficient of coercivity is. A temperature coefficient of intrinsic coercivity of -0.14%/ 1C (20–500 1C) has been achieved for v ¼ 0.07, as compared with -0.23%/ 1C for v ¼ 0.21. A high temperature sintered magnet with a maximum operating temperature of 500 1C has been obtained on the composition of Sm(CobalFe0.07Cu0.088Zr0.025)7.5. The initial magnetization curve exhibits that this high temperature magnet is still controlled by a strong domain wall pinning mechanism up to 500 1C. r 2006 Elsevier B.V. All rights reserved. PACS: 75.50.ww; 75.50.vv; 75.60.ej; 75.60.ch Keywords: High temperature magnets; Temperature dependence of intrinsic coercivity; 2:17 type Sm–Co sintered magnets; Thermal stability; Coercivity mechanism
1. Introduction Since the 2:17-type Sm–Co magnet shows excellent characterizations such as high magnetic properties, outstanding thermal stability and excellent corrosion resistance [1], it has been regarded as the ideal materials in applications like servo-motors, pump couplings and sensors, particularly where the magnet is required to operate at high temperature, across a broad temperature range or in a corrosive environment. Recently, 2:17-type Sm–Co magnet has been receiving considerable attention again due to high-temperature applications [2]. For these applications, the desired maximum operating temperature is above 450 1C. Among all rare-earth permanent magnets, the 2:17-type Sm–Co magnet has the highest Curie temperature and magnetization and therefore, is the most promising candidate for high-temperature applications. Corresponding author. Tel.: +86 10 62184522; fax: +86 10 62182610.
E-mail address: [email protected] (Z.H. Guo). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.01.107
Serious efforts to improve high-temperature performance of 2:17-type Sm–Co magnets began in 1995 [3–5]. It is well established that 2:17-type Sm–Co magnet is a pinning-controlled magnet [6]. The microstructures of 2:17type Sm–Co magnets are obtained after a long and complicated heat treatment. They consist of a mixture of cellular and lamella structures [1]. The cell structures consist of a more or less well-developed network of very small cells of a rhombohedral Sm2(Co,Fe)17 matrix phase, within the much larger grains, which are separated and often completely surrounded by a hexagonal Sm(Co,Cu)5 cell boundary phase. When high coercivity magnets are heat treated to their optimum magnetic properties, the matrix phases have linear dimensions of about 100–200 nm and the boundary phases are typically 2–10 nm thick. There are also other very thin layers visible under an electron microscope that are perpendicular to the c-axis and run across many cells and cell boundaries. They belong to a third phase, the so-called ‘‘platelet phase’’ or ‘‘z-phase’’ which contains most of the Zr.
ARTICLE IN PRESS Z.H. Guo et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e396–e401
In this article, we studied the magnetic properties of 2:17-type Sm–Co sintered magnets with various iron content at room and high temperature, respectively. The coercivity mechanism of 2:17-type Sm–Co magnets at high temperature were investigated by initial magnetization curves measured by a close circuit hysteresigraph. A high temperature sintered magnet with maximum operating temperature of 500 1C has been obtained on the composition of Sm(CobalFe0.07Cu0.088Zr0.025)7.5. 2. Experimental methods Magnets with nominal composition of Sm(CobalFevCu0.088Zr0.025)7.5 (v ¼ 00.30) were prepared by arc
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melting under Ar atmosphere, followed by conventional powder metallurgy processing. Sm has a great affinity for oxygen and will form Sm2Co3 whenever oxygen is available. Different manufacturing processes result in different oxygen content, so the use of effective Sm content is intended in this article. Fine powder with particle size 4 mm were aligned and pressed in a magnetic field of 1500 kA/m and then further compacted using an isostatic cool pressing. The green bodies were sintered at 1190–1250 1C for 0.5–2 h in Ar, followed by solution heat treatment at 1160–1220 1C for 2–6 h. The sintered parts were then aged at 780–850 1C for 10–40 h, followed by slow cooling to 400 1C at 0.8 1C/min. Initial magnetization and demagnetization curves were measured from room
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Sm(CobalFevCu0.088Zr0.025)7.5 Sintered Magnets 1.0
0.6
0.4
v=0.16 v=0.00
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v=0.04
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J (T)
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H (kA/m) 1.2 Sm(CobalFevCu0.088Zr0.025)7.5 Sintered Magnets 1.0
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v=0.27
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v=0.30
J (T)
0.8
0.4
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0
H (kA/m) Fig. 1. J-H Demagnetization curves of the Sm(CobalFevCu0.088 Zr0.025)7.5 sintered magnets at room temperature vs. Fe content v.
ARTICLE IN PRESS Z.H. Guo et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e396–e401
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temperature to 500 1C using a NIM-2000 H closed-circuit hysteresigraph. In order to obtain accurate temperature indication, a Pt100 thermal sensor was installed very close to the sample. 3. Results and discussion Fig. 1 shows the demagnetization curves of Sm(CobalFevCu0.088Zr0.025)7.5 sintered magnets at room temperature. Fig. 2 shows Br, (BH)max, bHC and Hk at room temperature vs. Fe content v. The Sm(CobalFevCuyZrx)z magnets represent a complicated system with four compositional variables (v, x, y, z) and at least five heat-treating variables [7]. The microstructures and magnetic properties of
Sm(Co,Fe,Cu,Zr)z magnet significantly depend on the processing parameters and heat treatments. In our experiment, every magnet with certain composition has been prepared by several processing parameters and heat treatments. Figs. 1 and 2 show the optimum properties of each composition. With increasing Fe content v, the remanence Br and maximum energy products (BH)max increase initially and reach optimal values of 1.055 T at v ¼ 0.27 for Br and 205 kJ/m3 at v ¼ 0.21 for (BH)max and then drop sharply with further Fe increase, as shown in Fig. 2. For Sm(Co,Fe,Cu,Zr)z magnets, Fe mainly exists in rhombohedral Th2Zn17-type Sm2(Co,Fe)17 matrix phase, which is responsible for high saturation magnetization of magnets.
1.2 1500
Sm(CobalFevCu0.088Zr0.025) 7.5 Sintered Magnets 200
1.0
bHC,
150 900 0.6
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Hk (kA/m)
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0.10
0.15
0.20
0.25
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0
Fe content (v) Fig. 2. Effect of Fe content v on the Br, (BH)max, bHC and Hk at room temperature of Sm(CobalFevCu0.088Zr0.025)7.5 sintered magnets.
ARTICLE IN PRESS Z.H. Guo et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e396–e401
iHC (kA/m)
Substituting Fe for Co significantly increases the saturation magnetization of Sm2Co17 and this is an obvious approach to improving magnetic properties of Sm(Co,Fe,Cu,Zr)z at room temperature [8]. However, an excessive substitution of Fe (vX0.24) for Co results in rapid deterioration of
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permanent magnetic properties due to formation of a considerable amount of Fe–Co soft magnetic phase, which was reported in our previous article [9]. Kumar [10] reported that the higher Fe content is, the more easily Fe–Co soft magnetic phase forms in the heat treatment
2500
Sm(CobalFe0.07Cu0.088Zr0.025) 7.5
2000
commercial 2:17-type Sm-Co
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Temperature (°C) 2500
Sm(CobalFe0.07Cu0.088Zr0.025) 7.5
2000
commercial 2:17-type Sm-Co
1500 1000 500 0
Br (T)
1.0 0.8
(BH)max (kJ/m3)
0.6 240 200 160 120 80 40 0
100
200
300
400
500
Temperature (°C) Fig. 3. Temperature dependence of magnetic properties for high-temperature magnet Sm(CobalFe0.07Cu0.088 Zr0.025)7.5(K) and commercial high coercivity 2:17-type Sm–Co (’).
ARTICLE IN PRESS Z.H. Guo et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e396–e401
temperature °C 350 400 450 500
Br T 0.793 0.756 0.725 0.687
iHC
H b C
(BH)max
kA/m 1101 886 764 637
kA/m 592 533 516 430
kJ/m3 117 103 95 84
1.4 1.2 1.0 0.8 0.6
J
350oC 400oC 450oC 500oC
J, B (T)
process for Sm(Co,Fe,Cu,Zr)z magnets. The magnet with v ¼ 0.24 exhibits a clear two step demagnetization curve and the iHC of magnet with v ¼ 0.30 is near to zero, as shown in Fig. 1. (BH)max decrease sharply with increasing v from 0.21 to 0.30. Fe content v also has a significant effect on demagnetization curve squareness of Sm(CobalFevCu0.088Zr0.025)7.5 sintered magnets. In Figs. 1 and 2, we can easily see the squareness is increasing with decreasing v. The magnet with v ¼ 0.07 has the best demagnetization curve squareness among these series magnets. Iron has a significant effect on the high-temperature magnetic properties of Sm(Co,Fe,Cu,Zr)z [3,4,11,12]. The lower the Fe content is, the higher the intrinsic coercivity at high temperature is. One reason why Sm(Co,Fe,Cu,Zr)z magnet with low iron content demonstrates improved hightemperature stability is that its Curie temperature is higher than that of the magnet with high iron content. For Sm2(Co1xFex)17 matrix phase, the Curie temperature TC increases with decreasing iron content; for example, TC ¼ 747 1C for x ¼ 0.5 and TC ¼ 926 1C for x ¼ 0. The temperature coefficient is the most important factor in the development of high temperature magnets. The temperature coefficient of remanence a is defined by a(T0T1) ¼ {[Br(T0)-Br(T1)]/[Br(T0) (T0T1)]} 100%, where T0 and T1 are the room temperature and operating temperature, Br(T0) and Br(T1) are the remanence at room temperature and operating temperature, respectively. The temperature coefficient of intrinsic coercivity b is defined by b(T0 T1) ¼ {[iHC(T0) iHC(T1)]/[iHC(T0) (T0 T1)]} 100%, where iHC(T0) and iHC(T1) are the intrinsic coercivity at room temperature and operating temperature, respectively. According to Kim [13,14], the maximum operating temperature is a function of the coercivity at room temperature and temperature coefficient of coercivity b and the maximum operating temperature is more effectively increased by a b decrease. Fig. 3 shows the comparison of temperature dependence of intrinsic coercivity, remanences and maximum energy products between Sm(CobalFe0.07Cu0.088Zr0.025)7.5 magnet and a commercial high coercivity 2:17-type Sm–Co magnet. For the commercial magnet, the coercivity is 2630 kA/m at 25 1C and 175 kA/m at 500 1C, which gives a temperature coefficient b of 0.20%/ 1C. For Sm(CobalFe0.07Cu0.088Zr0.025)7.5 magnet, the coercivity is 1831 kA/m at 25 1C and 637 kA/m at 500 1C, which gives a temperature coefficient b of 0.14%/ 1C. For the magnet with v ¼ 0.21, b ¼ 0.23%/ 1C. Fe content of commercial 2:17-type Sm–Co magnet is generally in the range 0.21–0.31. From these experimental results, it is easily found that b of magnet with low Fe content is much smaller than that of magnet with high Fe content. In Fig. 3, we can see that the temperature coefficients of remanence a are almost the same for Sm(CobalFe0.07Cu0.088Zr0.025)7.5 magnet and commercial 2:17-type Sm–Co magnet. The maximum energy product (BH)max of Sm(CobalFe0.07Cu0.088Zr0.025)7.5 magnet at 500 1C is higher than that of commercial Sm–Co magnet mainly due to its higher intrinsic coercivity at high
0.4 B 0.2
-1000
-800
-600
-400
-200
0
0.0
H (kA/m) Fig. 4. Demagnetization curves and magnetic properties of Sm(CobalFe0.07Cu0.088Zr0.025)7.5 sintered magnet at 350, 400, 450, and 500 1C.
1.0 Sm(CobalFe0.07Cu0.088Zr0.025) 7.5 Sintered Magnets @500°C
0.8
0.6 J (T)
e400
0.4
0.2
0.0
-0.2 -600 -400 -200
0
200
400
600
800
1000 1200
H (kA/m)
Fig. 5. Initial magnetization curve and demagnetization curve of Sm(CobalFe0.07Cu0.088Zr0.025)7.5 sintered magnet at 500 1C.
temperature. Fig. 4 shows the demagnetization curves and magnetic properties of Sm(CobalFe0.07Cu0.088Zr0.025)7.5 sintered magnet at 350, 400, 450 and 500 1C. This high temperature magnet exhibits excellent high-temperature magnetic properties with Br ¼ 0.687 T, bHC ¼ 430 kA/m, 3 iHC ¼ 637 kA/m and (BH)max ¼ 84 kJ/m at 500 1C. Fig. 5 shows the initial magnetization and demagnetization curves of Sm(CobalFe0.07Cu0.088Zr0.025)7.5 at 500 1C. The initial magnetization curve is typical of domain-wall pinning type magnets. Te´llez-Balanco et al. [15] studied the coercivity mechanism using a micromagnetic analysis of the temperature dependence of the intrinsic coercivity and found that the coercivity of Sm(Co,Fe,Cu,Zr)z is controlled at elevated temperature (above 520 K ) by a nucleation process of reversal domain. However, our experimental results show that the coercivity of Sm(Co,Fe,Cu,Zr)7.5 magnets is still determined predominately by a domainwall pinning process up to 500 1C (773 K).
ARTICLE IN PRESS Z.H. Guo et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e396–e401
4. Conclusions Sm(CobalFevCu0.088Zr0.025)7.5 sintered magnets with higher Fe (vp0.24) content have higher remanences and maximum energy products at room temperature. An excessive substitution of Fe for Co (vX0.24) results in rapid deterioration of permanent magnetic properties due to the formation of a considerable amount of Fe–Co soft magnetic phase. Fe content strongly affects the temperature coefficient of intrinsic coercivity b. The lower the Fe content is, the smaller the b is. When the Fe content v of Sm(CobalFevCu0.088Zr0.025)7.5 decreases from 0.21 to 0.07, b (20–500 1C) is reduced from 0.23 to 0.14%/1C. A high-temperature sintered magnet with a maximum operating temperature of 500 1C has been obtained on the composition of Sm(CobalFe0.07Cu0.088Zr0.025)7.5 with Br ¼ 0.687 T, bHC ¼ 430 kA/m, iHC ¼ 637 kA/m and (BH)max ¼ 84 kJ/m3 at 500 1C. The initial magnetization curve indicates that Sm(CobalFe0.07Cu0.088Zr0.025)7.5 sintered magnets are still controlled by a strong domain wall pinning mechanism up to 500 1C. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant no. 50201004) and the
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National High Technology Research and Development Program of China (Grant no. 2003AA305930).
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