Structures and magnetic properties of Nd-Fe-B bulk nanocomposite magnets produced by the spark plasma sintering method
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monari Takeuchi and Hiroyuki Kageyama National Institute of Advanced Industrial Science and Technology (AIST), Ikeda, Osaka 563-8577, Japan (Received 31 March 2004; accepted 11 June 2004)
We studied the effects of the sintering temperature and applied pressure on Nd–Fe–B bulk nanocomposite magnets produced by the spark plasma sintering (SPS) method. Amorphous Nd4Fe77.5B18.5 melt-spun ribbons were successfully consolidated into bulk form by the SPS method. When sintered at 873 K under applied pressures between 30 and 70 MPa, the bulk materials consisted of nanocomposite materials with a soft magnetic Fe3B phase and hard magnetic Nd2Fe14B phase. The density and magnetic properties of the bulk materials sintered at 873 K were strongly dependent on the applied pressure during sintering. Bulk Nd4Fe77.5B18.5 nanocomposite magnets sintered at 873 K under an applied pressure of 70 MPa showed a high remanence of 9.3 kG with a high coercivity of 2.5 kOe.
I. INTRODUCTION
Since the discovery that nanocomposite magnets exhibit a higher remanence than traditional isotropic permanent magnets, nanocomposite magnets have been widely recognized as a new type of permanent magnet.1 The higher remanence in nanocomposite magnets is due to exchange coupling of the nano-sized magnetically soft and hard phases. Nanocomposite magnets are expected to show giant maximum energy products as high as 120 MGOe when the soft and hard phases are arranged in the proper way.2 These magnets are currently produced by annealing of either melt-spun ribbons or mechanically alloyed powders. 1,3 Thus, the resultant nanocomposite magnets are in powder form. From the engineering point of view, it is essential to produce nanocomposite magnets with bulk form. There are basically two processing techniques to obtain bulk nanocomposite materials, one involving dynamic compaction by the explosion or gun method,4,5 and the other involving static consolidation such as by hot pressing.6 It has been reported that bulk nanocomposite magnets of a Nd4Fe77.5B18.5 alloy can be obtained by annealing bulk amorphous materials produced by
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2004.0358 2730
http://journals.cambridge.org
J. Mater. Res., Vol. 19, No. 9, Sep 2004 Downloaded: 14 Mar 2015
shock compaction.7 In static consolidation such as hot pressing, heat exposure may increase the grain size in nanocomposite materials. However, a newly developed rapid sintering technique called the spark plasma sintering (SPS) method can consolidate powder at relatively lower temperatures in a short consolidation period.8 These characteristics make the SPS method suitable for the production of bulk nanocomposite materials. Some attempts have already been made to produce Nd–Fe–B nanocomposite magnets by the SPS method.9,10 In these studies, nanocomposite melt-spun ribbons of alloys such as Nd–Fe–Co–V–B or Nd–Dy–Fe–Co–Si–B were consolidated into bulk form by the SPS method. The reported magnetic properties of the resultant bulk material
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