PROPERTIES AND APPLICATIONS OF HIGH ... - Vacuumschmelze

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permanent magnets increases proportional to the Curie temperature of the magnet materials, see Fig. 1. max. operating temperature in °C 600 500 400 300 200 100 0 Hard Ferrites (Nd,Dy) 2Fe 14B Sm 2(Co, Cu,Fe,Zr) 17 SmCo 5 AlNiCo 0 200 400 600 800 1000 Curie temperature in °C Figure 1: Maximum operating temperatures of permanent magnets in dependence on their Curie temperatures. The shaded lines give the lowest or the highest operating temperatures for different permanent magnet materials, respectively. Most of the RE magnets are produced by powder metallurgy in many different grades and shapes. The alloys are melted from RE metals, Fe and Fe-B master-alloys in vacuum-induction furnaces. The polycrystalline microstructure of the ingots implies, that the magnetic moments are distributed randomly. In order to achieve a well defined texture, powder metallurgy is applied. The alloys are crushed and milled to a fine alloy powder, that must consist of single crystals. In general alloy powders with an average particle size in the range 3 to 5 µm meet this requirement very well. Such alloy powders can be aligned by a magnetic field, compacted by cold isostatic pressing (CIP) to blocks, by axial (AP) or transverse (TP) field die-pressing to net-shape parts and sintered to compact anisotropic magnet materials [1 - 3]. For many applications the maximum energy density is decisive. At room temperature Nd-Fe-B as well as Nd-Dy-Fe-TM-B magnets with strong coercivities HcJ >24 kA/cm, TM: transition metals, have got higher maximum energy densities than SmCo5 or Sm2(Co,Cu,Fe,Zr)17 magnets, see Fig. 2. But at temperatures of approximately 120 °C sintered Sm2(Co,Cu,Fe,Zr)17 magnets are superior.
max. energy density in kJ/m³ 450 400 350 300 250 200 150 100 50 0 0 50 100 150 200 250 300 350 temperature in °C SmCo5 SmCo5 Sm2(Co,Cu,Fe.Zr)17 Sm2(Co,Cu,Fe,Zr) 17 Nd2Fe14B (Nd,Dy)2(Fe,Co)14B (Nd,Dy) 2(Fe,Co) 14B Figure 2: Maximum energy density (BH)m of RE magnets in dependence on the temperature. 2 Temperature stability of sintered RE Magnets The temperature stability of permanent magnets is determined by reversible and irreversible polarization losses of the magnet material and of the load line B/µ0H of the magnet or of the magnet assembly, respectively. 2.1 Reversible polarization changes The reversible polarization changes are represented by the temperature coefficient of the remanent polarization TC(Br), see Fig. 3. For temperatures between 20 and 100 °C typical temperature coefficients TC(Br) range from -0.08 to -0.11 %/K for sintered Nd-Dy-Fe-Co-TM-B magnets and amount to -0.04 or -0.03 %/K for sintered SmCo5 or Sm2(Co,Cu,Fe,Zr)17 magnets, respectively. 2.2 Irreversible polarization losses due to the temperature dependence of the coercivity HcJ . Irreversible polarization losses are mainly caused by the temperature dependence of the coercivity HcJ and the thermal aftereffect. With increasing temperature the coercivity HcJ of RE magnets decreases and as a consequence the magnetization of grains, which have smaller coercivities, may be reversed. Such irreversible polarization losses do not depend on time, see Fig. 4. Hence irreversible polarization losses can be anticipated by an annealing treatment of the magnetized magnets. As a rule of thumb, such ageing treatments are performed at temperatures 10 to 20 K above the maximum operating temperature in order to stabilize the magnet or the magnet assembly, respectively. Typical temperature coefficients of the coercivity HcJ range from -0.50 to -0.75 %/K for sintered Nd- Dy-Fe-Co-TM-B magnets and amount to -0.14 to -0.25 %/K for sintered SmCo5 or -0.15 to -0.18 %/K for Sm2(Co,Cu,Fe,Zr)17 magnets for temperatures between 20 and 100 °C. Minor polarization losses are due to the thermal aftereffect or magnetic viscosity, respectively, and depend on time logarithmically [4]. Fig. 4 gives the relative irreversible polarization losses ∆Jirr/J in dependence on the exposition time at a temperature of 130 °C for sintered Nd14.3DyFe76.7B8 magnets
Page 1: PROPERTIES AND APPLICATIONS OF HIGH
Page 5 and 6: 2.3 Irreversible polarization losse
Page 7 and 8: 3.2 Special shapes for the reductio
Page 9 and 10: 4 Corrosion behaviour of sintered R
Page 11 and 12: 5 Sintered Nd-Fe-B magnets with hig
Page 13: tures between 50 and 150 °C. For a

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