This is an overview of the present status of permanent magnet technology with special emphasis on the rare-earth intermetallic compounds, which represent the most advanced development in this area. The first part deals with the general principles of magnetism that form the basis of permanent magnet techniques and applications, focusing attention on the stored energy and the energy exchanged with the external world. Important intrinsic properties of hard magnetic materials for the manufacture of permanent magnets are an intense saturation magnetization, a high Curie temperature and a strong magnetic anisotropy. The latter is a key property in the development of coercivity, a necessary condition for a ferromagnetic material to be useful as a permanent magnet. Modern high energy density materials (rare-earth intermetallics) are especially characterized by a very elevated coercive field, and show an improvement of an order of magnitude in the energy product (BH)max compared to traditional high moment materials (Alnico alloys). Besides the SmCo5 and NdFeB type compounds, the most recent achievements in this class of materials include interstitial 2:17 compounds, which at present suffer from the difficulty of achieving sufficient densification and stability. However, application of these compounds to bonded type materials seems to be very promising. Other possibilities are presented by a series of different preparation routes, among which rapid solidification methods offer a wide range of opportunities. In this category, increasing attention is being paid to special nanostructured materials because of the possibility of obtaining enhanced remanence by profiting from a favorable exchange interaction between adjacent grains.
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- Permanent Magnets
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