Nd-Fe-B magnets

The experimental value of the saturation magnetisation at room temperature for practically the whole family of compounds R2Fci4B (R = rare earth) can be explained on the basis of a ferromagnetic coupling between both Fe sublattices and the rare-earth sublattice. Some corn- 

The potential economic and technological impact of these materials prompted the organisation of a collective effort by the Commission of the European Communities in 1985, known as Concerted European Action in Magnets , CEAM. Many discoveries have resulted from this effort. 

Nd2Fei4B permanent magnets can be prepared by induction melting, followed by liquid-phase sintering of powder, in a similar way to SmCoj materials. On cooling the melt, the first solid formed is y-Fe. To prevent 

The calciothermic reduction method can also be used to prepare the Nd2Fej4B sintering powder. The reaction in this case is  

As in the synthesis of SmCoj, excess Ca is eliminated after rinsing, crushing, milling and drying, the powder is ready for compaction and sintering. 

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The Nd-Fe-B magnets are based on the intermetallic compound of the formula Nd2Fei4B. The interest in these magnets is due to the fact they are stronger than other materials and of low cost. The magnetic energies and costs of the most common magnetic materials as of 1986 are presented in Table 12.12. 

POSTER TITLE Magnetic and microstructural characteristics of sintered Nd-Fe-B magnets produced by formation of a Mo intermetallic compound phase... 

N. Leventis and X. Gao, Magnetohydrodynamic electrochemistry in the field of Nd-Fe-B magnets. Theory, experiment, and application in self-powered flow delivery systems, AtioL Chem., 73 (2001) 3981-3992. 

Ohashi et al. (1987) investigated the effect of Co substitution in Nd-Fe-B magnet alloys and found that corrosion resistance of the corresponding magnets was markedly improved. This was attributed by these authors to the segregation of the Co atoms into the Nd-rich phase where it led to the formation of an Nd-Co(Fe) intermetallic compound. The improved corrosion resistance was ascribed to the disappearance of free Nd metal, associated with the eutectic composition. 

Nd-Fe-B magnets. Both quantities are of approximately the same size in SmCo5 magnets, whereas Dc exceeds the grain diameter in melt-spun Nd-Fe-B. For this reason the coercivity mechanism in sintered and melt-spun magnets will be discussed separately, together with the corresponding microstructures. 

Most authors agree that Nd-Fe-B magnets will replace Sm-Co and Alnico to a far larger extent than they will replace the ferrites (Baran 1984, Cartocetti 1984,... 

A well-known method to increase the coercivity in Nd-Fe-B magnets is to substitute the heavy rare-earth (HREE) elements like Dysprosium (Dy) and Terbium (Tb) for Nd (Dhakal et al. 2014 Hirosawa et al. 1986 Hirota et al. 2006). Substitution with dysprosium also raises the Curie-point for these magnets, so that they can be used at higher temperatures. The amount of dysprosium in neodymium magnets may vary from 0.8 to 1.2 % (NdFeB-info.com 2015). 

In this review, we will point out some basic aspects which are of great relevance to the understanding of the effects of the third elements in fiiese materials, such as B in Nd-Fe-B magnets and N and/or C in Sm2Fei7N3 and Sm2Fei7Cjc magnets. 

Fig. 9. Schematic of the microstmctuie of a real sintered Nd-Fe-B magnet large grains create large stey fields incomplete magnetic sepaiatimi 1 grain boundaries.

The effects of additives on the properties of sintered Nd-Fe-B magnets, and ideas for optimizing the magnetic properties, are discussed next. 

Fig. 10. Idealized microstructure of sintered Nd-Fe-B magnets the magnet is divided into uniform grains grain boundaries consist of a nonmagnetic Nd-rich liquid-forming phase of monolayer thickness.

Fig. 11. Microstructure of a sintered Nd-Fe B magnet T, main Nd2FeMB phase T, Nd, Fe4B4 phase Nd, Nd-rich phase.

Concerning the industrial application of the HDDR method, Ikegami et al. (1998) studied the thermal properties of Nd-Fe-B magnets prepared by this method. Figure 20 shows the effect of Dy additions on the temperature dependence of coercivity with optimum concentrations of Co, Ga, and Zr additives. The temperature coefficient of coercivity of anisotropic Nd2Fei4B-based magnets is typically about -0.5%/K. They also prepared samples of various coercivities, and found that the flux loss after treatment... 

Fig. 24. Three types of nanostructure Nd Fe-B magnets and their typical compositions (Hirosawa and Kanekiyo...

The microstructure of sintered Nd-Fe—B magnets is mainly composed of the Nd2Fei4B phase, the Ndi.iFe4B4 phase, and the Nd-rich phase at grain boundaries. The corrosion mechanism of the sintered body has been discussed in several papers. Nakamura et al. (1989) revealed that the Nd-rich phase has a more negative electric potential than the matrix Nd2Fei4B phase, the Ndi.iFe4B4 phase and, if it exists, the Fe-B phase. They also pointed out that the Nd-rich phase is easily dissolved in water. 

Kim and Jacobson (1987), Jacobson and Kim (1987) and Kim (1989) studied and discussed the corrosion of Nd—Fe-B magnets in a number of solutions such as H2Cr04, HF, H3PO4, and in an autoclave at high temperature. They indicated that the oxidation of the magnets shows parabolic weight gain in dry and humid air at temperatures between room temperature and 423 K, and that the oxidation rate at 423 K is smaller than that at room temperature. This reduction can be explained by the fact that the formation of... 

It is concluded that the corrosion of Nd-Fe-B-based magnets proceeds mainly in the grain-boundary phase, especially the Nd-rich phase. Increasing the corrosion resistance of the grain-boundary Nd-rich phase is probably the most effective approach for the preparation of corrosion-resistant Nd-Fe-B magnets. 

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