Sintered magnets

The first generation sintered magnets were based on the binary alloy SmCo5 and such magnets still account for the bulk of those now manufactured. However, in order to meet certain cost criteria it was soon recognised that it would be to the benefit of all to provide a less expensive type of magnet based on a modofied composition and this was first achieved by partial... 

Applications Polymers are nonmagnetic materials but they can be modified by fillers. Plastic magnets, first introduced in 1955, are inferior to cast and sintered magnets but have many desirable properties such as low cost, ease of production, better uniformity and reproducibility. Plastic magnets are used in electronic instruments, communication, household utensils, and audio equipment. 

Promising results were also obtained by Ramesh et al. (1988) when A1203 was used as an additive in the fabrication of sintered magnets. 

The compositions used to manufacture sintered magnets are generally more Nd-rich and B-rich than would correspond to the stoichiometric composition R2Fe14B. The reason for using these excess concentrations was discussed in section 3.3.1. 

Most experimental observations made on the Nd-Fe-B type materials indicate that the coercivity in the corresponding sintered magnets is nucleation-controlled (fig. 47a or b) rather than pinning controlled (fig. 47c). 

Fig. 31. Dependence of MHC on the average cooling rate through the range from 800 down to 500° C for sintered magnets SmCo5 x with different Sm excess over the 1-5 ratio. (After den Broeder et al. 1974.)...

The sintered magnets are hard and brittle. They can only he machined by the techniques developed for Alnicos, ceramic magnets or semiconductors. They can he sliced with diamond impregnated blades, surface ground with carborundum or diamond grinding wheels, or cut on electric spark-erosion machines (EDM). This must he done in a demagnetized state. 

Let us now first consider the reversible losses in RE-Co magnets. By way of example, fig. 40 shows for five commercially important types of sintered magnets the reversible flux variations on heating up to 250 °C, measured at p —2.5. It is seen that the reversible loss (or the temperature coefficient of Bd, which is the average slope of such curves over a defined temperature interval), is primarily... 

Fig. 40. Reversible temperature variation of the open-circuit remanent flux at an operating permeance of p —2.5, measured on five commercially important sintered magnet types. Curves traced after thermal prestabilization by heating for 5 h at 250 ° C, at 200 ° C for Ce(Co, Cu, Fe)5. (After Li et al. 1988.)...

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