Rare Earth Magnets: Materials

Rare Earth Magnets: Materials

Rare Earth Magnets: Materials Rare-earth permanent magnets (RPMS) are based on intermetallic compounds of rare-earth metals (R) and the transition met...

52KB Sizes 1 Downloads 39 Views

Rare Earth Magnets: Materials Rare-earth permanent magnets (RPMS) are based on intermetallic compounds of rare-earth metals (R) and the transition metals iron or cobalt. The auspicious combination of properties of the rare-earth sublattice and the 3d sublattice lead to the spectacular development in hard magnetic materials: the former providing the high magnetic anisotropy, the latter the high Curie temperature and magnetization. The two most relevant classes of RPMs in application are based on (i) samarium and cobalt which exhibit very high coercivities and low temperature coefficients, and (ii) neodymium, iron, and boron which are unrivalled in terms of its maximum energy product. In the present article a brief review will be given of these two compounds, but also of the other recently discovered hard magnetic iron-rich compounds such as ThMn -type compounds and the interstitial solid solutions of"#nitrogen and carbon in R Fe -type compounds. The most # "( relevant intrinsic magnetic properties and micromagnetic parameters (for definition see Hard Magnetic Materials, Basic Principles of) of these compounds are listed in Table 1. Summaries of novel developments in manufacturing routes, with special emphasis on nanostructured magnets, will be given in the following sections. The development of NdFeB-type alloys as starting materials for permanent magnets was triggered by both Sumitomo Special Metals (using the powder metallurgical route, Sagawa et al. 1984) and General

Motors (employing the rapid quenching method, Croat et al. 1984). The usefulness of the tetragonal Nd Fe B-type compounds (see Fig. 1) led to num# detailed "% erous investigations of the Nd–Fe–B system. Three stable ternary phases can be found: Nd Fe B # "% (Φ), Nd +εFe B (η), and Nd Fe B ( ρ). An instructive " % % & # ' presentation of the ternary Nd–Fe–B system with low boron concentrations is given in the liquidus projection shown in Fig. 2, which marks the binary phases, ternary phases, and the important reactions (elucidated in the caption). The Φ phase is located in a region where γ-Fe is the primary solidification product. The peritectic formation of the Φ phase at 1180 mC will be incomplete in a standard casting process as residues of primary Fe dendrites are left in the Φ grains, the latter in turn being surrounded by neodymiumrich liquid which will solidify at its eutectic temperature of 630 mC. The presence of the iron crystals is detrimental to the magnet’s performance. This has important consequences for the manufacturing of this class of magnets. The analogous PrFeB-based compounds are of particular relevance for hot-deformed magnets and low-temperature applications as they show no spin reorientation down to 4.2 K (which occurs at 135 K for Nd Fe B), but have also been used # "% and HDDR (see Magnets: more recently for melt-spun HDDR Processed) materials. The intrinsic magnetic properties of the related Nd Fe C are very similar to those of the corre# "% borides: the magnetocrystalline anisotropy sponding is slightly higher and the Curie temperature and

Table 1 Structure type, intrinsic magnetic properties (Curie temperature TC, anisotropy field HA l (2K j 4K )\µ MS, anisotropy " #wall! width δ , and constant K , saturation magnetization MS, upper limit of energy density (BH )max l µ M#s \4), domain w " critical single-domain particle size dc of important permanent magnet compounds and! for comparison α-Fe and F B #$ ' (note: cubic structure) as well as Fe B (in-plane anisotropy). $ TC µ HA K µ MS µ MS#\4 δw dc ! " −$) ! ! m−$) (kJ Compound Structure type (K) (T) (MJ m (T) (nm) (µm) Nd Fe B # "% Pr Fe B Nd# Fe"% C # "% SmCo & Sm Co # "( Sm Fe N Sm#Fe"(C $ $ N Sm#Fe"(. V $ #' ( #.$ % (Sm . Zr . ) (Fe !. (& Co .! )#& Nx ! ( NdFe V! $N"! "! # y Sm(Fe Ti) α-Fe "" Fe B Fe#$B ' $

Tetragonal Nd Fe B Nd#Fe"%B Nd#Fe"%B # "% Hexagonal CaCu & Rhombohedral 1 Th Zn # "( Th Zn Th#Zn"( # "( Monoclinic Nd (Fe,V) $ #* Hexagonal TbCu ( Tetragonal ThMn ThMn"# b.c.c. "# Cubic Tetragonal

585

6.7

5

1.60

516

4.2

0.3

565 532 993

8.7 9.5 40

5

1.56 1.41 1.05

484 396 219

"4 "4 3.7

" 0.3 " 0.3 1.7

1100

6.5

3.3

1.30

336

8.6

0.5

8.8 7.4

1.54 1.45 1.33

472 418 352

3.6

0.4 " 0.3 " 0.3

1.7

575

" 0.3

1.11

245

" 0.3

1.14 2.16 1.7 1.62

259 928

749 668 683 877 743 584 1043 698 786

21 16 12

17

7.7 10 10.5 " 0.4

4.8 0.046 0.01 –0.32

4.0 30

0.4 0.007

20

1