PROPERTIES AND APPLICATIONS OF HIGH ... - Vacuumschmelze

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cogging torque in N cm 6 4 2 0 -2 -4 -6 butterfly magnet skewed arc straight segment 180° 190° 200° 210° 220° angle 230° 240° 250° 260° 270° Figure 9: Cogging torques of 8-pole brushless dc-motors with rotors assembled from straight segments, skewed arc segments or novel butterfly magnets, respectively.
4 Corrosion behaviour of sintered RE magnets In comparison to sintered Sm-Co magnets Nd-Fe-B magnets are more sensitive to corrosion. As a consequence all finished VACODYM magnets get a final surface treatment. Such magnets have been applied at ambient conditions (room temperature, humidity up to 50 %, no condensation of moisture) without any problems for more than 10 years in telephone transducers, for instance. In humid atmosphere, however, the Nd-rich constituents may be converted to Nd-hydroxides and some Hydrogen is released. The Hydrogen reacts immediately with the Nd-rich constituents and forms Ndhydrides. In such a humid environment Nd-hydrides are not stable and are transformed into Ndhydroxides. Since additional Hydrogen comes into existence the reaction cycle continues [9]. The formation of Nd-hydrides results in a strong volume increase of the Nd-rich constituents, so that cracks occur along the grain boundaries. The oxidation is accelerated at high temperatures. In the worst case the surface of the sintered Nd-Fe-B magnets disintegrates. As has been demonstrated by A. S. Kim [10] and B. Grieb [11] some time ago, the corrosion behaviour of Nd-Fe-B magnets can be improved by proper additions of Co, Nb, V, Mo, Cu, Ga etc. By these additions the Nd-rich constituents are transformed into more noble intermetallic compounds, see Fig. 10 inset, for instance Nd3(Co,Cu), (Nd,Dy)5(Co,Cu,Ga)3, Nd6(Fe,Co)13Ga [9 - 13]. Since the free electrochemical potential of these compounds were determined to be about 800 mV higher than that of pure Nd-metal, such Nd-Fe-B magnets are much more stable in hot and humid climates [9]. The corrosion behaviour is similar to pure iron or steels. 4.1 Corrosion tests There are many different corrosion tests. A very common test is the Highly Accelerated Stress Test (HAST) which is similar to the standard IEC 68-2-66. The test samples are stored at 130 °C, 95 % humidity, 2.6 bar water vapour pressure. Under these conditions no condensation of vapour occurs. The mass losses per surface area of the magnets are monitored in dependence of the exposition time. Standard Nd- Fe-B magnets suffer mass-losses of about 10 to 100 mg/cm² after 10 days, whereas the mass-losses of VACODYM 6xx magnets with an improved corrosion resistance amount to
Page 1 and 2: PROPERTIES AND APPLICATIONS OF HIGH
Page 3 and 4: max. energy density in kJ/m³ 450 4
Page 5 and 6: 2.3 Irreversible polarization losse
Page 7: 3.2 Special shapes for the reductio
Page 11 and 12: 5 Sintered Nd-Fe-B magnets with hig
Page 13: tures between 50 and 150 °C. For a

PROPERTIES AND APPLICATIONS OF HIGH ... - Vacuumschmelze

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