Nanocrystalline and Nanostructured High-Performance Permanent Magnets
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Nanocrystalline and Nanostructured High-Performance Permanent Magnets D. Goll, W. Sigle, G.C. Hadjipanayis1 and H. Kronmuller ¨ Max-Planck-Institut f¨ur Metallforschung, Heisenbergstr. 1, D-70569 Stuttgart, Germany 1 University of Delaware, Newark, U.S.A. ABSTRACT The rather complex correlation between the microstructure and the magnetic properties is demonstrated for two types of high-quality RE-TM permanent magnets (pms), namely nanocrystalline RE2 Fe14 B (RE = Nd,Pr) and nanostructured Sm2(Co,Cu,Fe,Zr)17 pms. The detailed analysis of this correlation for both pm materials leads to a quantitative comprehension of the hardening mechanism enabling the optimization of their magnetic properties and temperature dependences. In the case of RE2 Fe14B, isotropic bonded pms are fabricated showing maximum energy products in the order of 90 kJ/m3 . In the case of Sm2(Co,Cu,Fe,Zr)17 , magnets with excellent hightemperature magnetic properties are tailored. Hereby, the investigations in addition provide important clues to the evolution of the characteristic microstructural and magnetic properties and to the role of the involved elements. INTRODUCTION Modern high-quality permanent magnets (pms) are based on RE-TM intermetallic compounds of rare earth (RE = Nd,Pr,Sm) and transition metals (TM = Fe,Co) [1–4]. With these compounds outstanding intrinsic magnetic properties, large magnetocrystalline anisotropy constants K1 > 6 J/m3 , large spontaneous polarizations J > : T and Curie temperatures T > C as S C well as optimized microstructures can be realized. A large spontaneous polarization is a prerequisite for high remanences JR , whereas a large magnetocrystalline anisotropy constant may result in large coercivities both ensuring high maximum energy products BH max JR2 = 0 (0 ;7 Vs/Am) as long as the condition 0HC > : JR holds. The ternary systems RE2 Fe14B (RE = Nd,Pr) [5] are currently considered to be the highest performance pm materials at all because of their very large spontaneous polarization of 1.6 T resulting in maximum energy products BH max up to 15 times larger than for ferrites. The only disadvantage of such mag C. Nevertheless, nanocrystalline nets are their comparatively low Curie temperatures of TC magnetic materials of this system have opened a new class of pm materials which are optimally suitable for high-performance polymer bonded magnets. Pms which supply the highest BH max values at elevated temperatures (beyond which RE2 Fe14B is no longer viable) are based on the quintary system Sm2(Co,Cu,Fe,Zr)17 [5]. With such magnets coercivities as large as 1 T can be realized even at 500 C which makes them suitable for high temperatures [6,7]. For a further improvement of the performance of polymer bonded magnets and the hightemperature magnetic properties, especially the temperature stability of the coercivity, the most important prerequisite for both hard magnetic materials is the detailed analysis of the hardening mechanisms and in the case of Sm2(Co,Cu,Fe,Zr)17 additionally of its formation. It is n
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