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Differences in microstructure and magnetic properties between directly-quenched and optimally-annealed Nd–Fe–B nanocomposite materials
ARTICLE IN PRESS Physica B 405 (2010) 690–693
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Differences in microstructure and magnetic properties between directly-quenched and optimally-annealed Nd–Fe–B nanocomposite materials Hong-chao Sheng a, Xie-rong Zeng b,c,, Dong-ju Fu a, Fei Deng a a
School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an China College of Materials Science and Engineering, Shenzhen University, Shenzhen, China c Shenzhen Key Laboratory of Special Functional Materials, Shenzhen, China b
a r t i c l e in f o
a b s t r a c t
Article history: Received 1 September 2009 Received in revised form 23 September 2009 Accepted 24 September 2009
Melt-spun Nd9.5Fe81Zr3B6.5 ribbons were prepared by the single roller melt-spinning technique. The microstructure and magnetic properties of directly-quenched and optimally-annealed Nd–Fe–B nanocomposite materials were studied by X-ray diffraction, transmission electron microscopy observations, and magnetization measurements. The experimental results show that crystallizing an precursor into nanocrystallines is ineffective for obtaining better magnetic properties, while nanocomposites with excellent properties can be directly melt spinning at an optimum wheel speed. The microstructure of the ribbons at various distances from the ribbon wheel side was studied. It shows that in the inner part of the ribbons, the directly-quenched ribbons and optimally-annealed ribbons have similar characteristics of microstructure. However, in the free-surface and wheel-contacted surface the grain size of the annealed ribbons is much coarser than that of the directly quenched ribbons. & 2009 Elsevier B.V. All rights reserved.
Keywords: Nd–Fe–B Nanocomposite magnet Magnetic properties
1. Introduction Nanocomposite magnetic materials consisting of a fine mixture of magnetically hard and soft phases with nanometer size have attracted considerable interest for potential permanent magnet development since they could, by exchange interaction, lead to very high remanence values and potential energy products even with a low content of rare-earth elements [1–7]. Calculations have shown that the maximum energy product is strongly dependent on the microstructural parameters [8,9]. A uniform microstructure and a well-coupled interface between soft and hard phases are essential conditions for well-performing nanostructural materials. Making the alloys into amorphous (or partial amorphous) structure and then annealing them to crystallize into the desired microstructure is a usual way to develop a nanocomposite structure. Several investigations reported that the microstructure and hard magnetic properties of Nd–Fe–B nanocomposite materials are sensitive to the heating rate of the post-annealing imposed on the melt-spun ribbon, and rapid thermal annealing (RTA) is capable of yielding superior properties [10–12]. Another way to obtain nanocomposite magnets is to solidify the melting alloy into
Corresponding author at: College of Materials Science and Engineering, Shenzhen University, Shenzhen, 518060 China. Tel.: + 86 755 26557459; fax: + 86 755 26536239. E-mail address: [email protected] (X.-r. Zeng).
0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.09.088
the expected phases directly. Because the microstructure and hard magnetic properties of the alloys are sensitive to the Cu-wheel speed and the quenching temperature, the range of quenching parameters over which optimum direct quench magnetic properties can be obtained is usually narrow[13–15]. In this article, differences in microstructure and magnetic properties between directly quenched and annealed Nd–Fe–B nanocomposite materials were studied. 2. Experimentals Alloy ingot with compositions of Nd9.5Fe81Zr3B6.5 was prepared by vacuum induction melting and crushed into small pieces to accommodate the size of the crucible for melt spinning. A quartz nozzle with an orifice of around 0.8 mm in diameter was used and the distance between the orifice and the copper wheel surface was maintained at 3 mm during melt spinning. Directly-quenched ribbon sample and over-quenched ribbon sample were obtained by controlled melt spinning. The over-quenched ribbons were then annealed in an evacuated quartz tube for 10 min within the temperature range 670–750 1C and the optimal annealing conditions (700 1C, 10 min) were chosen so that the best hard magnetic properties were obtained in the alloy composition. The ribbon samples were characterized by X-ray diffraction (XRD) with CuKa radiation, transmission electron microscope (TEM). The magnetic properties of the ribbons were measured with a vibrating sample magnetometer using a maximum applied field of 2 T.
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Fig. 1 shows the hysteresis loops of Nd9.5Fe81Zr3B6.5 ribbon samples prepared by different process parameters. For the directly-quenched sample, high degree of squareness and larger magnetic properties of the ribbons are obtained and Jr = 0.98 T, Hc = 614 kA/m, and (BH)max = 146 kJ/m3 have been achieved as compared to 0.94 T, 580 kA/m, and 112 kJ/m3 in optimallyannealed sample. Thus, it is clear that the directly quenched nanocomposite material has superior magnetic properties in comparison with the annealed nanocomposite materials. Fig. 2 shows XRD patterns from the powders of the overquenched, optimally-annealed and directly-quenched ribbons. For the over-quenched sample amorphous phase and crystalline phases of both Nd2Fe14B and a-Fe can be detected. The optimally-annealed and directly-quenched ribbons have similar XRD characteristics and only hard magnetic Nd2Fe14B phase and a soft magnetic a-Fe phase can be detected, and the amorphous phase is absent.
In order to further study the differences in microstructure between directly quenched and annealed Nd–Fe–B nanocomposite materials, XRD analyses were carried out from the wheelcontacted surfaces (Fig. 3) and free surfaces (Fig. 4) of Nd9.5Fe81Zr3B6.5 alloy prepared at various conditions. The wheelcontacted part of over-quenched ribbon was obtained in fully amorphous state as indicated by the absence of Bragg peaks and presence of amorphous halo. After optimally annealed, the wheelcontacted part of the over-quenched ribbon crystallized into Nd2Fe14B and a-Fe phases, and the grains have coarser size than that of wheel-contacted part of the directly-quenched ribbon, which can be seen from the broaden peaks of the latter. Fig. 4 is XRD analyses on the free surfaces of the Nd9.5Fe81Zr3B6.5 ribbons, showing the XRD patterns of free surfaces of over-quenched, directly-quenched and optimallyannealed ribbons, respectively. From the patterns, one can clearly see that reflections such as (0 0 4), (0 0 6), and (0 0 8) become dominant in all samples, indicating that the Nd2Fe14B crystals are strongly textured in the free surface of Nd9.5Fe81Zr3B6.5 ribbons.
Fig. 1. Room temperature hysteresis loops for optimally quenched and optimally annealed Nd9.5Fe81Zr3B6.5 ribbons.
Fig. 3. XRD analyses on the wheel-contacted surfaces of the Nd9.5Fe81Zr3B6.5 ribbons prepared at various conditions.
Fig. 2. XRD analyses on the ribbon powders of the Nd9.5Fe81Zr3B6.5 alloy prepared at various conditions.
Fig. 4. XRD analyses on the free surfaces of the Nd9.5Fe81Zr3B6.5 ribbons prepared at various conditions.
3. Results
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Fig. 5. TEM micrographs taken at various distances from the ribbon wheel side on the over-quenched ribbon sample. The insets are selected area diffraction patterns. (a) wheel-contacted surface, (b) inner part and (c) free surface.
Fig. 6. TEM micrographs taken at various distances from the ribbon wheel side on the optimally-annealed sample. The insets are selected area diffraction patterns. (a) wheel-contacted surface, (b) inner part and (c) free surface.
Fig. 7. TEM micrographs taken at various distances from the ribbon wheel side on the directly-quenched sample. The insets are selected area diffraction patterns. (a) wheelcontacted surface, (b) inner part and (c) free surface.
It has been confirmed that there is an interphase epitaxy between the Nd2Fe14B phase and the Fe phase. This epitaxial relationship produces a tendency for Nd2Fe14B texturing in a direction orthogonal to its fast growth direction [16]. In addition, the diffraction peaks of the directly-quenched ribbons are obviously broadened in comparison with those in the other ribbons, which implies that thermal treatment leads to the increase of the grain size of the free surface of Nd9.5Fe81Zr3B6.5 alloy. According to the XRD results, the structure of the free surface is very different from that of wheel-contacted surface of the Nd9.5Fe81Zr3B6.5 ribbons, thus TEM specimens were prepared from three parts of the alloy ribbons, that is, the free surface, the inner part and the wheel-contacted surface. Fig. 5 is TEM images of the over-quenched ribbon, showing the wheel-contacted surface, the inner part and the free surface, respectively. From the wheel-contacted surface shown in Fig. 5(a) one can see that only a modulated contrast typical for an amorphous phase and no contrast corresponding to the precipitation of a crystalline phase is seen over the whole image, and the electron diffraction patterns also consist of only halo rings typical for an amorphous phase. On the other hand, the inner part of the ribbon is composed of very
fine Nd2Fe14B and a-Fe grains. In the case of free-surface, the crystal grains are about 80 nm, which are coarser than those of inner part and are strongly textured. At wheel-contacted part, the heat extraction is sufficient to cool the liquid below the effective glass transition temperature. However, since intermetallic compound Nd2Fe14B has poor thermal conductivity [17], the middle to upper portions of the ribbon towards the free side fail to cool below the effective glass transition temperature resulting in successively increasing growth of pre-existing nuclei through the remaining portion of the ribbon. These are believed to be the mechanism for the different structures of the Nd9.5Fe81Zr3B6.5 ribbons from free surface to wheel-contacted surface. The TEM micrographs of optimally-annealed sample are given in Fig. 6. The wheel-contacted surface shows an inhomogeneous microstructure comprised of small grains (about 40 nm) and large grains ( 480 nm). The coarse microstructure of wheel-contacted surface in the annealed sample can be attributed to lack of nucleation sites during the crystallization process. In the asquenched ribbons, the crystalline phases may act as nucleation centers during the crystallization of the amorphous phase. Lack of
ARTICLE IN PRESS H.-c. Sheng et al. / Physica B 405 (2010) 690–693
nucleation sites could lead to a coarse-grained, inhomogeneous microstructure. This coarse structure would reduce the strength of exchange coupling effect between the hard and the soft grains, leading to incomplete exchange coupling and thus to inferior magnetic properties. For the inner part of the ribbon, average grain size of samples is 20–25 nm in size and large grains could not be detected in the alloy. Significant increase in grain size of free side zone is observed when the ribbon had been annealed at optimum conditions, as can be seen by comparing Fig. 5(c) with Fig. 6(c), and the grain size of the alloy increases from about 100 nm to about 800 nm which will weaken the exchange coupling and deteriorate the magnetic properties further. Fig. 7 shows the TEM micrographs taken at various distances from the ribbon wheel side on the directly-quenched sample. Wheel side region shows refinement microstructure which is attributed to the high undercooling conditions near the Cu wheel. Further away from the wheel side surface, refinement and uniform microstructure can be observed, which is similar to that of inner part of the optimally-annealed ribbons. Large and uniform grains occur in free side region of optimally quenched ribbon which is much smaller than that of the optimallyannealed.
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wheel-contacted portion. Moreover, the grain size of the free-side portions is much smaller than that of the annealed ribbons. Thus, excellent properties were achieved for directly-quenched ribbons.
5. Conclusions Crystallizing an amorphous precursor into nanocrystallines is ineffective for obtaining better magnetic properties, while nanocomposite with excellent properties can be directly prepared by controlling the solidification process of the melt, and Jr =0.98 T, Hc =614 kA/m, and (BH)max = 146 kJ/m3 have been achieved as compared to 0.94 T, 580 kA/m, and 112 kJ/m3 in optimallyannealed sample. These are attributed to coarser structure of annealed alloy in the wheel-contacted surface and free surface.
Acknowledgment This work was financially supported by the Science and Technology foundation of ShenZhen China (No. 200605). References
4. Discussion Intermetallic compound Nd2Fe14B has poor thermal conductivity, which results in different microstructures of the Nd9.5Fe81Zr3B6.5 ribbons from the free surface to wheel-contacted surface. According to XRD patterns in Fig. 2 and the TEM results, for directly-quenched and optimally-annealed ribbons, the characteristics of inner parts agree well with the XRD analyses on the ribbon powders. This suggests that refinement microstructure (shown in Figs. 6 and 7b) is dominant in both of the samples. However, the microstructure of wheel-contacted and free-side portions is obviously different from that of the inner parts. For annealed-sample, the wheel-contacted and free-side parts of the ribbons have coarse microstructure. Although the amount of the coarse grains could be small, they will weaken the exchange coupling and deteriorate the magnetic properties of the Nd9.5Fe81Zr3B6.5 magnets in a way. In the case of directlyquenched ribbons, no coarse microstructure was observed in the
[1] A. Manaf, R.A. Buckley, H.A. Davies, J. Magn. Magn. Mater. 128 (1993) 302. [2] B. Yang, B.-g. Shen, T.-y. Zhao, J.-r. Sun, J. Magn. Magn. Mater. 320 (2008) 2406. [3] D. Nagahama, T. Ohkubo, T. Miyoshi, S. Hirosawa, K. Hono, Acta Materialia 54 (2006) 4871. [4] D.N. Brown, Z. Chen, P. Guschl, P. Campbell, J. Magn. Magn. Mater. 303 (2006) e371. [5] X.H. Li, Y. Guan, W. Li, J.W. Zhang, X.Y. Zhang, Mater. Lett. 61 (2007) 2728. [6] M. Yue, P.L. Niu, Y.L. Li, D.T. Zhang, W.Q. Liu, J.X. Zhang, J. Appl. Phys. 103 (2008) 07E101. [7] Y.Q. Wu, D.H. Ping, K. Hono, J. Appl. Phys. 87 (2000) 8658. ¨ [8] T. Schrefl, J. Fidler, H. Kronmuller, Phys. Rev. B 49 (1994) 6100. ¨ [9] R. Fischer, T. Schrefl, H. Kronmuller, J. Fidler, J. Magn. Magn. Mater. 153 (1996) 35. [10] J.S. Fang, T.S. Chin, Mater. Trans. JIM 37 (1996) 883. [11] Y. Gao, S. Zhang, B. Liu, J. Magn. Magn. Mater. 158 (1996) 208. [12] A. Kojima, A. Makio, A. Inoue, J. Appl. Phys. 87 (2000) 6576. [13] A. Manaf, R.A. Buckley, H.A. Davies, J. Magn. Magn. Mater. 128 (1993) 302. [14] X.-b. Du, H.-W. Zhang, C.-b. Rong, J. Magn. Magn. Mater. 271 (2004) 396. [15] H.-c. Sheng, X.-r. Zeng, X.-h. Li, J. Magn. Magn. Mater. 321 (2009) 3042. [16] X.Y. Zhang, Y. Guan, L. Yang, J.W. Zhang, Appl. Phys. Lett. 79 (2001) 2426. [17] R. Hanitsch, in: J.M.D. Coey (Ed.), Rare-Earth Iron Permanent magnets, Oxford Scientific Publications, Oxford, 1996, p. 452.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Differences in microstructure and magnetic properties between directly-quenched and optimally-annealed Nd–Fe–B nanocomposite materials Hong-chao Sheng a, Xie-rong Zeng b,c,, Dong-ju Fu a, Fei Deng a a
School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an China College of Materials Science and Engineering, Shenzhen University, Shenzhen, China c Shenzhen Key Laboratory of Special Functional Materials, Shenzhen, China b
a r t i c l e in f o
a b s t r a c t
Article history: Received 1 September 2009 Received in revised form 23 September 2009 Accepted 24 September 2009
Melt-spun Nd9.5Fe81Zr3B6.5 ribbons were prepared by the single roller melt-spinning technique. The microstructure and magnetic properties of directly-quenched and optimally-annealed Nd–Fe–B nanocomposite materials were studied by X-ray diffraction, transmission electron microscopy observations, and magnetization measurements. The experimental results show that crystallizing an precursor into nanocrystallines is ineffective for obtaining better magnetic properties, while nanocomposites with excellent properties can be directly melt spinning at an optimum wheel speed. The microstructure of the ribbons at various distances from the ribbon wheel side was studied. It shows that in the inner part of the ribbons, the directly-quenched ribbons and optimally-annealed ribbons have similar characteristics of microstructure. However, in the free-surface and wheel-contacted surface the grain size of the annealed ribbons is much coarser than that of the directly quenched ribbons. & 2009 Elsevier B.V. All rights reserved.
Keywords: Nd–Fe–B Nanocomposite magnet Magnetic properties
1. Introduction Nanocomposite magnetic materials consisting of a fine mixture of magnetically hard and soft phases with nanometer size have attracted considerable interest for potential permanent magnet development since they could, by exchange interaction, lead to very high remanence values and potential energy products even with a low content of rare-earth elements [1–7]. Calculations have shown that the maximum energy product is strongly dependent on the microstructural parameters [8,9]. A uniform microstructure and a well-coupled interface between soft and hard phases are essential conditions for well-performing nanostructural materials. Making the alloys into amorphous (or partial amorphous) structure and then annealing them to crystallize into the desired microstructure is a usual way to develop a nanocomposite structure. Several investigations reported that the microstructure and hard magnetic properties of Nd–Fe–B nanocomposite materials are sensitive to the heating rate of the post-annealing imposed on the melt-spun ribbon, and rapid thermal annealing (RTA) is capable of yielding superior properties [10–12]. Another way to obtain nanocomposite magnets is to solidify the melting alloy into
Corresponding author at: College of Materials Science and Engineering, Shenzhen University, Shenzhen, 518060 China. Tel.: + 86 755 26557459; fax: + 86 755 26536239. E-mail address: [email protected] (X.-r. Zeng).
0921-4526/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2009.09.088
the expected phases directly. Because the microstructure and hard magnetic properties of the alloys are sensitive to the Cu-wheel speed and the quenching temperature, the range of quenching parameters over which optimum direct quench magnetic properties can be obtained is usually narrow[13–15]. In this article, differences in microstructure and magnetic properties between directly quenched and annealed Nd–Fe–B nanocomposite materials were studied. 2. Experimentals Alloy ingot with compositions of Nd9.5Fe81Zr3B6.5 was prepared by vacuum induction melting and crushed into small pieces to accommodate the size of the crucible for melt spinning. A quartz nozzle with an orifice of around 0.8 mm in diameter was used and the distance between the orifice and the copper wheel surface was maintained at 3 mm during melt spinning. Directly-quenched ribbon sample and over-quenched ribbon sample were obtained by controlled melt spinning. The over-quenched ribbons were then annealed in an evacuated quartz tube for 10 min within the temperature range 670–750 1C and the optimal annealing conditions (700 1C, 10 min) were chosen so that the best hard magnetic properties were obtained in the alloy composition. The ribbon samples were characterized by X-ray diffraction (XRD) with CuKa radiation, transmission electron microscope (TEM). The magnetic properties of the ribbons were measured with a vibrating sample magnetometer using a maximum applied field of 2 T.
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691
Fig. 1 shows the hysteresis loops of Nd9.5Fe81Zr3B6.5 ribbon samples prepared by different process parameters. For the directly-quenched sample, high degree of squareness and larger magnetic properties of the ribbons are obtained and Jr = 0.98 T, Hc = 614 kA/m, and (BH)max = 146 kJ/m3 have been achieved as compared to 0.94 T, 580 kA/m, and 112 kJ/m3 in optimallyannealed sample. Thus, it is clear that the directly quenched nanocomposite material has superior magnetic properties in comparison with the annealed nanocomposite materials. Fig. 2 shows XRD patterns from the powders of the overquenched, optimally-annealed and directly-quenched ribbons. For the over-quenched sample amorphous phase and crystalline phases of both Nd2Fe14B and a-Fe can be detected. The optimally-annealed and directly-quenched ribbons have similar XRD characteristics and only hard magnetic Nd2Fe14B phase and a soft magnetic a-Fe phase can be detected, and the amorphous phase is absent.
In order to further study the differences in microstructure between directly quenched and annealed Nd–Fe–B nanocomposite materials, XRD analyses were carried out from the wheelcontacted surfaces (Fig. 3) and free surfaces (Fig. 4) of Nd9.5Fe81Zr3B6.5 alloy prepared at various conditions. The wheelcontacted part of over-quenched ribbon was obtained in fully amorphous state as indicated by the absence of Bragg peaks and presence of amorphous halo. After optimally annealed, the wheelcontacted part of the over-quenched ribbon crystallized into Nd2Fe14B and a-Fe phases, and the grains have coarser size than that of wheel-contacted part of the directly-quenched ribbon, which can be seen from the broaden peaks of the latter. Fig. 4 is XRD analyses on the free surfaces of the Nd9.5Fe81Zr3B6.5 ribbons, showing the XRD patterns of free surfaces of over-quenched, directly-quenched and optimallyannealed ribbons, respectively. From the patterns, one can clearly see that reflections such as (0 0 4), (0 0 6), and (0 0 8) become dominant in all samples, indicating that the Nd2Fe14B crystals are strongly textured in the free surface of Nd9.5Fe81Zr3B6.5 ribbons.
Fig. 1. Room temperature hysteresis loops for optimally quenched and optimally annealed Nd9.5Fe81Zr3B6.5 ribbons.
Fig. 3. XRD analyses on the wheel-contacted surfaces of the Nd9.5Fe81Zr3B6.5 ribbons prepared at various conditions.
Fig. 2. XRD analyses on the ribbon powders of the Nd9.5Fe81Zr3B6.5 alloy prepared at various conditions.
Fig. 4. XRD analyses on the free surfaces of the Nd9.5Fe81Zr3B6.5 ribbons prepared at various conditions.
3. Results
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Fig. 5. TEM micrographs taken at various distances from the ribbon wheel side on the over-quenched ribbon sample. The insets are selected area diffraction patterns. (a) wheel-contacted surface, (b) inner part and (c) free surface.
Fig. 6. TEM micrographs taken at various distances from the ribbon wheel side on the optimally-annealed sample. The insets are selected area diffraction patterns. (a) wheel-contacted surface, (b) inner part and (c) free surface.
Fig. 7. TEM micrographs taken at various distances from the ribbon wheel side on the directly-quenched sample. The insets are selected area diffraction patterns. (a) wheelcontacted surface, (b) inner part and (c) free surface.
It has been confirmed that there is an interphase epitaxy between the Nd2Fe14B phase and the Fe phase. This epitaxial relationship produces a tendency for Nd2Fe14B texturing in a direction orthogonal to its fast growth direction [16]. In addition, the diffraction peaks of the directly-quenched ribbons are obviously broadened in comparison with those in the other ribbons, which implies that thermal treatment leads to the increase of the grain size of the free surface of Nd9.5Fe81Zr3B6.5 alloy. According to the XRD results, the structure of the free surface is very different from that of wheel-contacted surface of the Nd9.5Fe81Zr3B6.5 ribbons, thus TEM specimens were prepared from three parts of the alloy ribbons, that is, the free surface, the inner part and the wheel-contacted surface. Fig. 5 is TEM images of the over-quenched ribbon, showing the wheel-contacted surface, the inner part and the free surface, respectively. From the wheel-contacted surface shown in Fig. 5(a) one can see that only a modulated contrast typical for an amorphous phase and no contrast corresponding to the precipitation of a crystalline phase is seen over the whole image, and the electron diffraction patterns also consist of only halo rings typical for an amorphous phase. On the other hand, the inner part of the ribbon is composed of very
fine Nd2Fe14B and a-Fe grains. In the case of free-surface, the crystal grains are about 80 nm, which are coarser than those of inner part and are strongly textured. At wheel-contacted part, the heat extraction is sufficient to cool the liquid below the effective glass transition temperature. However, since intermetallic compound Nd2Fe14B has poor thermal conductivity [17], the middle to upper portions of the ribbon towards the free side fail to cool below the effective glass transition temperature resulting in successively increasing growth of pre-existing nuclei through the remaining portion of the ribbon. These are believed to be the mechanism for the different structures of the Nd9.5Fe81Zr3B6.5 ribbons from free surface to wheel-contacted surface. The TEM micrographs of optimally-annealed sample are given in Fig. 6. The wheel-contacted surface shows an inhomogeneous microstructure comprised of small grains (about 40 nm) and large grains ( 480 nm). The coarse microstructure of wheel-contacted surface in the annealed sample can be attributed to lack of nucleation sites during the crystallization process. In the asquenched ribbons, the crystalline phases may act as nucleation centers during the crystallization of the amorphous phase. Lack of
ARTICLE IN PRESS H.-c. Sheng et al. / Physica B 405 (2010) 690–693
nucleation sites could lead to a coarse-grained, inhomogeneous microstructure. This coarse structure would reduce the strength of exchange coupling effect between the hard and the soft grains, leading to incomplete exchange coupling and thus to inferior magnetic properties. For the inner part of the ribbon, average grain size of samples is 20–25 nm in size and large grains could not be detected in the alloy. Significant increase in grain size of free side zone is observed when the ribbon had been annealed at optimum conditions, as can be seen by comparing Fig. 5(c) with Fig. 6(c), and the grain size of the alloy increases from about 100 nm to about 800 nm which will weaken the exchange coupling and deteriorate the magnetic properties further. Fig. 7 shows the TEM micrographs taken at various distances from the ribbon wheel side on the directly-quenched sample. Wheel side region shows refinement microstructure which is attributed to the high undercooling conditions near the Cu wheel. Further away from the wheel side surface, refinement and uniform microstructure can be observed, which is similar to that of inner part of the optimally-annealed ribbons. Large and uniform grains occur in free side region of optimally quenched ribbon which is much smaller than that of the optimallyannealed.
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wheel-contacted portion. Moreover, the grain size of the free-side portions is much smaller than that of the annealed ribbons. Thus, excellent properties were achieved for directly-quenched ribbons.
5. Conclusions Crystallizing an amorphous precursor into nanocrystallines is ineffective for obtaining better magnetic properties, while nanocomposite with excellent properties can be directly prepared by controlling the solidification process of the melt, and Jr =0.98 T, Hc =614 kA/m, and (BH)max = 146 kJ/m3 have been achieved as compared to 0.94 T, 580 kA/m, and 112 kJ/m3 in optimallyannealed sample. These are attributed to coarser structure of annealed alloy in the wheel-contacted surface and free surface.
Acknowledgment This work was financially supported by the Science and Technology foundation of ShenZhen China (No. 200605). References
4. Discussion Intermetallic compound Nd2Fe14B has poor thermal conductivity, which results in different microstructures of the Nd9.5Fe81Zr3B6.5 ribbons from the free surface to wheel-contacted surface. According to XRD patterns in Fig. 2 and the TEM results, for directly-quenched and optimally-annealed ribbons, the characteristics of inner parts agree well with the XRD analyses on the ribbon powders. This suggests that refinement microstructure (shown in Figs. 6 and 7b) is dominant in both of the samples. However, the microstructure of wheel-contacted and free-side portions is obviously different from that of the inner parts. For annealed-sample, the wheel-contacted and free-side parts of the ribbons have coarse microstructure. Although the amount of the coarse grains could be small, they will weaken the exchange coupling and deteriorate the magnetic properties of the Nd9.5Fe81Zr3B6.5 magnets in a way. In the case of directlyquenched ribbons, no coarse microstructure was observed in the
[1] A. Manaf, R.A. Buckley, H.A. Davies, J. Magn. Magn. Mater. 128 (1993) 302. [2] B. Yang, B.-g. Shen, T.-y. Zhao, J.-r. Sun, J. Magn. Magn. Mater. 320 (2008) 2406. [3] D. Nagahama, T. Ohkubo, T. Miyoshi, S. Hirosawa, K. Hono, Acta Materialia 54 (2006) 4871. [4] D.N. Brown, Z. Chen, P. Guschl, P. Campbell, J. Magn. Magn. Mater. 303 (2006) e371. [5] X.H. Li, Y. Guan, W. Li, J.W. Zhang, X.Y. Zhang, Mater. Lett. 61 (2007) 2728. [6] M. Yue, P.L. Niu, Y.L. Li, D.T. Zhang, W.Q. Liu, J.X. Zhang, J. Appl. Phys. 103 (2008) 07E101. [7] Y.Q. Wu, D.H. Ping, K. Hono, J. Appl. Phys. 87 (2000) 8658. ¨ [8] T. Schrefl, J. Fidler, H. Kronmuller, Phys. Rev. B 49 (1994) 6100. ¨ [9] R. Fischer, T. Schrefl, H. Kronmuller, J. Fidler, J. Magn. Magn. Mater. 153 (1996) 35. [10] J.S. Fang, T.S. Chin, Mater. Trans. JIM 37 (1996) 883. [11] Y. Gao, S. Zhang, B. Liu, J. Magn. Magn. Mater. 158 (1996) 208. [12] A. Kojima, A. Makio, A. Inoue, J. Appl. Phys. 87 (2000) 6576. [13] A. Manaf, R.A. Buckley, H.A. Davies, J. Magn. Magn. Mater. 128 (1993) 302. [14] X.-b. Du, H.-W. Zhang, C.-b. Rong, J. Magn. Magn. Mater. 271 (2004) 396. [15] H.-c. Sheng, X.-r. Zeng, X.-h. Li, J. Magn. Magn. Mater. 321 (2009) 3042. [16] X.Y. Zhang, Y. Guan, L. Yang, J.W. Zhang, Appl. Phys. Lett. 79 (2001) 2426. [17] R. Hanitsch, in: J.M.D. Coey (Ed.), Rare-Earth Iron Permanent magnets, Oxford Scientific Publications, Oxford, 1996, p. 452.