Structures and magnetic properties of Sm-Fe-N bulk magnets produced by the spark plasma sintering method
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Sm-Fe-N powders were successfully consolidated at 873 K and below by the spark plasma sintering (SPS) method. Although the decomposition temperature of the hard magnetic Sm2Fe17N3 phase has been reported to be 873 K, partial decomposition of the Sm2Fe17N3 phase was noted in the bulk materials obtained by sintering at below that temperature. The resultant bulk materials showed a coercivity of around 0.24 MAm−1, significantly lower than that of the original Sm-Fe-N powder. It was found that decomposition of the Sm2Fe17N3 phase in the SPS method was significantly lowered by the addition of a small amount of Zn powder to the Sm-Fe-N powder. The bulk material obtained by sintering a mixture of Sm-Fe-N and Zn powder (10%Zn) at 723 K exhibited high coercivity, comparable with that of the original Sm-Fe-N powder.
I. INTRODUCTION
With the advent of high-performance Nd-Fe-B permanent magnets, research and development of new permanent magnets has been largely concentrated on rareearth-based alloys.1–3 As a result of intensive research, several new rare-earth-based alloys for permanent magnets have been found.4–7 Among them, Sm-Fe-N alloy is the most promising candidate for high-performance permanent magnets.4 The superiority of Sm-Fe-N magnets arises from Sm2Fe17N3 intermetallic compound, which exhibits high saturation magnetization with a large anisotropy field and high Curie temperature.4 Sm2Fe17N3 intermetallic compound has been prepared by the production of Sm2Fe17 alloy powder and subsequent nitrogenation of the powder by a gas-solid reaction. The resultant Sm2Fe17N3 intermetallic compound has thus been produced in powder form. This magnetic powder has been used for the production of bonded magnets, which are now being increasingly used in permanent magnet applications. However, the magnetic properties of SmFe-N bonded magnets are much lower than those of the original Sm-Fe-N powder, mainly because of the existence of a large amount of nonmagnetic binder. In the production of high-performance Nd-Fe-B permanent magnets, Nd-Fe-B alloy powders have been consolidated into green compact and then sintered at a high
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0386 3130 J. Mater. Res., Vol. 22, No. 11, Nov 2007 http://journals.cambridge.org Downloaded: 04 Feb 2015
temperature.2 However, this sintering technique cannot be applied to the production of Sm-Fe-N bulk magnets because Sm2Fe17N3 intermetallic compound is not stable at high temperatures and decomposes into the ␣-Fe and SmN phases above 873 K.4 Up to now, Sm-Fe-N bulk magnets have been produced by nonconventional consolidation techniques such as shock compaction or the compression shearing method.8–10 Sm-Fe-N bulk magnets with porosities of 2%–10% have been realized by optimizing the experimental conditions.8,9 There has been no practical application of these techniques, however, because of the difficulties involved. The recently developed spark plasma sintering (SPS) method can consolidate powders at relat
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