Magnet Stability:

According to the Table in Section 3A, ceramic ferrites have a worse reversible temperature coefficient of B_r than any of the rare earth magnets, and yet they are not susceptible to the corrosion which affects both the samarium-cobalt and neodymium-iron-boron types. The reason for this difference lies in the materials' grain structures, which were described in Section 2A.

Ceramic ferrites are made from powder which has been milled to fine particles of approximately single domain size, which effectively contain no boundaries. Rare-earth magnets, however, are made from powder which is deliberately milled only to a grain size which is an order of magnitude larger than their single domain size. Since domain walls now exist within grains, the grain boundaries must provide "pinning" of the walls to create the magnet's intrinsic coercivity. To do this, the grain boundaries are composed of deviations from the magnet's primary composition.

Rare-earth magnets:

Corrosion in a rare-earth magnet originates at its surface, and if no coating is used for protection, oxygen will diffuse into the magnet causing a metallurgical change. This diffusion is obviously a function of time, but the rate will also increase with temperature. The relationship with temperature is highly non-linear, such that there is a critical temperature above which uncoated magnets experience sufficient corrosion to noticeable degrade their magnetic performance. This temperature is around 150^oC and 250^oC for fully dense SmCo₅ and Sm₂Co₁₇ respectively. This difference illustrates that additional cobalt in the alloy reduces the oxygen diffusion. Conversely, the lack of cobalt in neodymium-iron-boron makes surface oxidation a much more severe problem in these magnets. At elevated temperatures, the product of oxidation in samarium-cobalt magnets is mostly Sm₂O₃, while in neodymium-iron-boron it is mostly Nd₂O₃. At high humidity, neodymium-iron-boron also absorbs hydrogen from the atmosphere into its surface causing it to disintegrate.

Neodymium-iron-boron magnets:

In Neodymium-iron-boron magnets, the primary composition in the highly magnetic grain interiors is Nd₂Fe₁₄B, while the necessary "pinning" in the grain boundaries is produced by Nd-rich phases. As this micrograph (courtesy of Shin-Etsu Chemical Co., Ltd.) shows, corrosion progresses from a magnet's surface (at the left) selectively through the Nd-rich grain boundaries. As was noted above with samarium-cobalt, additional Co in the alloy reduces oxygen diffusion, and it is also true that the partial substitution of Co in Nd₂Fe₁₄B reduces surface oxidation. In this case, it is because the added Co is mainly segregated into the grain boundaries (where the corrosion occurs), where it forms an intermetallic compound with the Nd-rich phase that directly hinders the formation of Nd₂O₃.

Actually, it is now understood (B.M. Ma et al, "A New Aspect on the Corrosion Resistance of Sintered NdFeB Magnets - is High Oxygen Content Necessary?", Proc. 13^th Int'l Workshop on RE Magnets and Their Applications, Birmingham, Sept. 1994, pp. 309-318) that the mechanism controlling corrosion in neodymium-iron-boron is more general than this. For example, the data below left shows that corrosion declines with a reduction in the Total Rare Earth (TRE) content of the alloy, which basically means that the Nd-rich grain boundaries are becoming smaller. The problem is that these regions provide "pinning" for the material's intrinsic coercivity H_ci, which is indeed seen to collapse as a critical minimum TRE is reached in the diagram below right.

In Section 2A, two distinctly different process routes were described by which neodymium-iron-boron magnets are manufactured. The first method involves sintering, which produces a nucleation-type magnet whose grain boundaries must provide "pinning", while also being the sites for corrosion (diagram a at right, from Section 2A). The second method involves rapid quenching, which produces powder particles having an extremely fine microstructure that develop magnetocrystalline anisotropy rather than nucleation (diagram c). Again the Nd-rich grain boundaries deviate from the primary Nd₂Fe₁₄B composition, but less so than in the sintered case, making rapidly quenched material less susceptible to corrosion. Furthermore, the fine microstructure in the rapidly quenched powder allows the Nd-rich phases to be uniformly and finely distributed throughout the particles, while the coarse grains in the sintered material tend to concentrate the Nd-rich phase regions; these clustered Nd-rich phases lower the energy barrier for corrosion in sintered neodymium-iron-boron magnets, compared to those made from rapidly quenched powder.

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