4.The Third Generation of Rare Earth Permanent Magnets: Innovations and Advancements

This chapter discusses the third generation rare earth magnets. The 3rd generation rare earth permanent magnetic alloy has a world - record magnetic energy product, mainly due to its tetragonal crystal system of Nd₂Fe₁₄B, which broke through the hexagonal system and rhombohedral system of permanent magnetic alloys of the 1st and 2nd generations.

Nd₂Fe₁₄B compound is the base phase of RE - Fe - B family permanent magnetic alloy. This chapter stresses on the formation of Nd₂Fe₁₄B phase, analysis of micro area of Nd₂(Fe,Co)₁₄B crystal, and the in situ and dynamic observation by TEM (transmission electron microscope) on Nd₂Fe₁₄B and Nd₂(Fe,Co)₁₄B at high temperature. Research on B - rich phase in Nd - Fe - B alloy was carried out as follows: the in situ and dynamic observation on Nd_11.1Fe₈₄B_4.9 under TEM, phase analysis on Nd_11.1Fe₈₄B_4.9, and relationship between B - rich phase and coercivity. The alloy of Nd₁₅Co₆Fe_69.9Ga₂B_7.1 could raise the Curie temperature from 312°C of Nd₁₅Fe₇₇B₈ alloy to 450 - 500°C, raise the intrinsic coercivity to \(H_{c}=716 - 955\ kA/m\), and raise the maximum magnetic energy product to \((BH)_{max}=223 - 262\ kJ/m^{3}\). The in situ and dynamic observation and analysis on magnetization and high temperature phase transformation of Nd₁₅Fe₇₉B₆ and Nd_14.6Fe₆₉Co₉B_7.4 permanent magnetic alloys. Lastly, this chapter discusses the model of coercivity and nano - meter microstructure of NdFeB permanent magnetic alloy containing niobium and gallium.

Enhancing NdFeB Permanent Magnets Through Element Substitutions

NdFeB permanent magnets set a record of magnetic energy product. The product of NdFeB reaches 446.24 kJ/m³ (55.78 MGOe), \(B_{r}=1.514\ T\) (15.14 kGs) and _m\(H_{c}=694.4\ kA/m^{3}\). However, the Curie temperature is rather low and only 312°C.

Therefore, the temperature stability is poor and the working temperature is rather low (<80°C). These drawbacks limit the application range of NdFeB permanent magnets. In order to overcome these drawbacks, the effect of different elements on the properties of NdFeB permanent magnetic alloys using substitution methods. The related studies can be classified into three types:

• Substitution of other rare earth elements for Nd;
• Substitution of other metal or non - metal elements for Fe;
• Substitution of other non - metal elements for B.

The part substitution of heavy rare earth, such as Dy, Tb and Er, for Nd can increase the coercivity. However, as the cost is more expensive for Dy than for Nd, the cost of NdFeB will increase about 50% when the substitution of Dy is about 10%. To reduce the cost, the coercivity of NdFeB alloys can be increased by the substitution of rare earth oxides, Dy₂O₃ and Tb₂O₃, for Nd through an appropriate technology (Sagawa, Hirosawa, Tokuhara, et al, 1987; Pan, Ping, Liu, et al, 2003; Pan, Ma, 1994).

The substitution of most transition elements in the Period Table for Fe has been investigated. The substitution is mainly due to an increase in Curie temperatures and a decrease in temperature coefficients. From experiment results, Al, Si and Ga substitutions for Fe can achieve above goals in a certain range. In addition, once Co can be substituted for Fe in the whole composition range and, at the same time, Nd₂Fe₁₄B phase is maintained. In Nd₂Fe_{14 - x}T_xB, possible substituted elements and their numbers x are 1.5Al, 1Si, 0.2Cu, 1Ni, 3Mn, 1Cr, 1V, 0.5Nb, 0.2Zr and 0.2Ti. If iron is substituted by other elements, not only the residual inductivity will be reduced greatly but also the magnetic energy product of NdFeB will be reduced (Pan, et al, 1987; Pan, Li, Ying, et al, 1990; Pan, Li, Li, et al, 1989; Li, Ping, Ma, 1988). Cobalt substitution for Fe can significantly increase the Curie temperature, thus greatly reducing the reversible temperature coefficient and improving the thermal stability. The Curie temperature is increased in a certain scale by Ni substitution, and is not improved by Al and Si substitutions. However, an appropriate substitution of Al and Si can improve the coercivity. Mn and Cr substitutions are not of benefit to the permanent magnetic - properties due to the antiferromagnetic coupling between Fe and Mn or Cr.

Based on the analyses of X - ray diffraction and SEM with EDAX, as Co is partly substituted for Fe, there are at least four phases in Nd₁₅(Fe_{1 - x}Co_x)₇₇B₈ at x≥0.2, i.e., the tetragonal phase Nd₂(Fe, Co)₁₄B, B - rich phase Nd_{1 + ε}(Fe, Co)₄B₄ (ε≈0.3), Nd - rich phase, and Laves phase. Nd(Fe, Co)₂ phase appearing at the grain boundary of the tetragonal phase. Among them, the tetragonal phase is dominant, and the others are insignificant.

The part substitution of Co for Fe improves the temperature stability, because the exchange constants of \(J_{Co - Co}\) and \(J_{Co - Fe}\) are larger than \(J_{Fe - Fe}\). Consequently, the permanent magnetic - properties can maintain to higher temperatures. The substitution also improves the corrosion - resistant of NdFeB alloys. The magnets prepared from Nd₁₂Dy_0.6Co_5.3Zr_0.1Ga_0.1Fe_74.5B₃ has the magnetic properties: \(B_{r}=0.87\ T\), _m\(H_{c}=1530\ kA/m\) and \((BH)_{max}=137\ kJ/m^{3}\). The temperature coefficients of \(B_{r}\) is \(\alpha_{B_{r}}=-0.11\ \%/^{\circ}C\) and \(\alpha_{H_{c}}=-0.43\ \%/^{\circ}C\) (Sagawa, Hirosawa, Tokuhara, et al, 1987; Zhou, 1995).

An appropriate substitution of Mo can increase the coercivity and reduce the irreversible loss of open - circuit magnetic flux of NdFeB. Based on the analysis, Mo exists the tetragonal phase and the RE - rich phase and aggregates in the B - rich phase (Liu, Zhou, 1990).

The fast - quenched permanent magnets Nd₉Fe_{86.5 - x}Mn_xB_4.5 (\(x = 0\), 0.5 and 1) are prepared using Mn substitution. A small amount of Mn can improve the permanent magnetic properties of the fast - quenched samples. The coercivity is increased from 339.6 kA/m to 398.2 kA/m (Xie, Yin, Jiang, et al, 2002).

For Nd₆Dy₃Fe_{83 - x}Co_xNb₁Ga₁B₆, with increasing Dy concentration, the coercivity first increases, reaches the maximum at \(x = 0.5 - 0.8\) and then decreases at \(x>0.8\). The increase in the intrinsic coercivity with increasing Dy concentration is because the Dy substitution increases the anisotropy field of samples, and the decrease at \(x>0.8\) has its origin in the large grains and weak exchange interaction (Cheng, Gao, Zhu, et al, 2002).

Al substitution can increase the coercivity. The combination of Al and Co substitutions can increase both the Curie temperature and the coercivity. \(Nd_{16}(Fe_{0.96}Al_{0.04})_{77}B_{7}\) reaches the coercivity of 1,114.4 kA/m, the remanence of 1.10 T and the maximum energy product of 238 kJ/m³. \(Nd_{16}Co_{y}Fe_{77 - y}Al_{4}B_{3}\) with \(y = 0\), 2, 4 and 8 is prepared by melting, ball - mill, shaping in a applied magnetic field, sintering at high temperature, annealing and magnetization at the field of larger than 4 T. The results of magnetic measurement showed that \(B_{r}=0.94 - 1.28\ T\), and at \(y = 3\) the coercivity achieves 1,130 kA/m, and the maximum energy density maintains 242.7 kJ/m³. The Curie temperature of the magnetic alloy is 710 K due to 10 % Co substitution for Fe. It is note that a soft magnetic phase appears as the Co substitution is more than 20 %. Mössbauer spectra showed that the line - widths become broader and the hyperfine field decrease. Al atoms preferentially occupy the \(j_{2}\) site, which has strong easy - planar anisotropy. The Al substitution for Fe and Co can decrease the easy - planar anisotropy of the \(j_{2}\) site, and thus increasing easy - axial anisotropy of the tetragonal phase (Pan, Pan, Ma, 1994; Zhou, 1995; Zhang, Ma, et al, 1985).

The two - phase technology has become a new method for manufacturing sintered Nd - Fe - B magnet in recent years (Zhang, Liu, Zhao, et al, 2009). The two - phase composite, \(Nd_{2}Fe_{14}B/\alpha - Fe\), has potential good magnetic properties. The doping of Cu, Nb, Mo, Cr, Co and La can effectively modify the structure and improve magnetic properties. The doping of Cu and Mo can effectively improve properties of permanent - magnet alloys by the grains. The part substitution of other rare earth elements for Nd can increase the coercivity. The part substitution of Co for Fe can increase the saturation magnetization and the Curie temperature. The part substitutions of Al for Fe can greatly improve the magnetic properties of the alloys after heat - treatments. In the same quenched technology, amorphous state significantly increases with increasing Al concentration. The addition of Al elevates the crystallization temperature of Nd₂Fe₁₄B, restrains the growth of grains and grain boundary. As a result, the magnetic properties are improved (Cheng, Gao, Zhu, et al, 2002; Wang, Zhao, Cui, et al, 2001).

Zn is added to Nd₂Fe₁₄B by mechanical alloying. During ball - mill, Zn is added to ball - milled machine and is homogenously mixed with Nd₂Fe₁₄B. Then the mixture is annealed in high vacuum ((2.0 - 4.0)×10^-5 Torr or (2.67 - 5.33)×10^-3 Pa). The addition of Zn of 5% (wt) can significantly increase the coercivity. As the addition of 20%, \(H_{c}\) reaches the maximum; it is about 2.3 times (from 75.3 kA/m to 179 kA/m), as compared to original sample. Zn is mainly introduced to the Nd - rich phase and increases the pinning to the reverse - phase domains.

The addition of Sn or FeSn₂ can increase \(_{m}H_{c}\). For the addition of 25%, \(_{m}H_{c}\) is 43.78 kA/m and increases 60%, as compared to the original powders.

Using Zn, Sn, etc. of low melting point as bond, processing heat treatment at near the melting point, and using ball miller to add Zn and Sn in pre - melting of Nd₂Fe₁₄B and Sm₂Co₁₇ can obtain above results (Xiong, Xiong, 1993).

After a small amount of Nb is substituted for Fe, the coercivity of rare earth iron permanent - magnet alloys is increased. Most of Nb exists in phase as impurity. Based on the analysis of TEM, besides the Nd - rich phase, B - rich phase (Nd_{1 + ε}Fe₄B₄, \(ε = 0 - 1.3\)) and main phase Nd₂Fe₁₄B, one observed a large amount of fine phase with homogeneous distribution exists stacking fault exists in the interior of the grains. In addition, Laves phase Fe₂Nb is also observed in Nd₂Fe₁₄B. The addition of small amount of Nb can fine the grains, and thus increase \(_{m}H_{c}\) and \(H_{c}\). The sintering technology of sample containing Nb is different from that of Nd₂Fe₁₄B₇. The aging temperature is 550 - 900°C. It is better to be divided into three steps for aging treatment, 900°C for 2 h and 600°C for 1 - 2 h. Another effect of Nb addition is to restrain the of \(\alpha - Fe\) phase. Moreover, the addition of Nb can reduce the irreversible loss. Fe₂Nb has MgMoO₄ structure with lattice parameters \(a = 0.482\ nm\) and \(c = 0.787\ nm\). The Laval phase exists in the matrix of permanent - magnet alloy as particles, which contain Nb of 22% - 44% and have diameter of 2μm. The Laval phase is non - magnetic, is structure with main phase Nd₂Fe₁₄B and is more Nb than main phase. As Nb> 4%, the main phase Nd₂Fe₁₄B is destroy. Based on the experience of the author, it is better for the addition of Nb to be 1% - 2% (at.) (Pan, Ping, Liu, et al, 2003; Liu, Pan, Luo, et al, 1991).

The substitution of a small amount of Ga for Fe can greatly increase coercivity and reduce the irreversible loss. The irreversible loss is 1.8 % for the aging at 200°C and is 1.2 % for the aging 400°C. The sample shows good stability. The combining substitution of Ga and Nb can significantly improve the stability of permanent - magnet alloys. Nd₁₆Co₁Fe₆₀Ga₆ has relatively large coercivity when the annealing at 640°C is used and the addition of Ga is 2% (at.) (Pan, Li, Li, et al, 1989; Liu, Pan, Luo, et al, 1991).

Substitution of Gd for Nd in Nd_{1 - x}Gd_xFe_6.5B decreases \(\alpha - Fe\) precipitation. Nd - Fe - B with Gd substitution has less degradation of 4πM and \(H_{c}\). No more than 5% Gd substitution can get proper magnetic performance (Liu, Zhao, Zhao, et al, 2010).

Zhou Guojun, et al have applied CALPHAD (Calculation of Phase Diagram) to assess thermodynamical Nd - B binary system which gives guide in composition design of Nd - Fe - B alloy (Zhou, Zeng, 2010).

Magnetic Properties and Atomic Occupancy of Co and Ga in NdFe(Co, Al, Ga)B Permanent-Magnetic Alloyss

NdFeB permanent - magnet alloys have excellent magnetic properties. However, as compared to Sm - Co alloys, the relatively low Curie temperature (only 312°C) and therefore poor thermal stability limit the applications of NdFeB. In order to improve the thermal stability, it is necessary to increase the Curie temperature of NdFeB. Soon after NdFeB was discovered, it has been known that Co substitution for Fe can greatly increase the Curie temperature (Sagawa, Fujimura, Tokawa, et al, 1984; Pan, et al, 1987; Xu, Ping, Li, et al, 1986; Croat, Herbst, Lee, et al, 1984; Ma, et al, 1987; Tokanaga, Tobise, Meguro, et al, 1986; Szafrańska - Miller, Ptusza, Wystocki, et al, 1987; Mizoguchi, Sakai, Niu, et al, 1986). However, the substitution reduces the coercivity of NdFeB. The coercivity can be increased by the small addition of Al and Ga in NdFeCoB. Based on Mössbauer spectra, the distributions of Co, Al and Ga atoms at the sites and the relationship between the distribution and magnetic properties, such as Curie temperature, have been studied.

Preparation Methods and Experimental Techniques for NdFe(Co, Al, Ga)B Magnets

Samples of Nd₁₆Fe_{77 - x}Co_xB₇ with \(x = 0\), 4, 8, 16 and 24, Nd₁₆Co₁Fe_{61 - x}Ga_xB₇, Nd₁₆Co₁Fe_{67 - x}Al_xB₇, and Nd₁₆Co₁Fe_{61 - x}Al_xB₇ with \(x = 0\), 1, 2, 4 and 7, were prepared by arc melting under an Ar atmosphere. The starting materials were 99.8% Nd, 99.5% B, 99.9% Fe in purity. The ingots were remelted three times in order achieve homogeneity. Then, the ingots were crashed and ball - milled to powders with about 3.5 μm. The powders were shaped in a magnetic field with 1.5 T. Finally, the samples were sintered at 1100°C and annealed at 500 - 630°C, followed by quenched at room temperature. The prepared samples were magnetized in a pulse strong magnetic field. The permanent magnetic parameters were measured using DC Parametric Measurement Instrument and the Curie temperatures were obtained from magnetic balance.

X - ray diffraction was performed using APD - 10X diffract - meter with Cu K radiation. Mössbauer spectra were collected in a conventional constant - acceleration spectrometer. The X - ray source was ⁵⁷Co in Rh matrix. The calibration was made using the spectrum of α - Fe spectrum at room temperature (Xu, Ping, Li, et al, 1986). All Mössbauer absorbers were the powdered samples and contained about 5 - 10 mg/cm² of natural iron. The Mössbauer spectra were analyzed using a singlet, a doublet and six sextets, which are associated with Nd - rich phase, B - rich phase and six inequivalent Fe sites in the tetragonal phase.

The occupancy of Fe atoms at six sites in the tetragonal phase Nd₂Fe₁₄B is \(N_{1}=2/4\), \(N_{2}=4/14\), \(N_{3}=2/14\), \(N_{4}=4/14\), \(N_{5}=1/14\) and \(N_{6}=1/14\), where \(N_{i}\) (\(i = 1 - 6\)) represents the \(j_{2}\), \(k_{2}\), \(j_{1}\), \(k_{1}\), \(a\) and \(c\) site, respectively. If it is assumed that the recoil - free fractions on the six sites are the same, the occupation fractions \(N_{i}(Fe)\), \(N_{i}(Co)\) and \(N_{i}(Al)\) of Fe, Co and Al atoms can be estimated by the following formulae:

For NdFeCoB permanent - magnet alloys,

\[ \begin{cases} N_{i}(Fe)=\frac{C_{1}}{C_{1}+C_{2}}\frac{S_{i}}{\sum_{i = 1}^{6}S_{i}}\frac{1}{N_{i}}\times100\%\\ N_{i}(Co)=100\% - N_{i}(Fe) \end{cases} \quad(4.1) \]

For NdFeCoAlB permanent - magnet alloys,

\[ \begin{cases} N_{i}(Fe)=\frac{C_{1}}{C_{1}+C_{2}+C_{3}}\frac{S_{i}}{\sum_{i = 1}^{6}S_{i}}\frac{1}{N_{i}}\times100\%\\ N_{i}(Co)=N_{i,0}(Co)=100\% - N_{i,0}(Fe)\\ N_{i}(Al)=100\% - N_{i}(Fe)-N_{i}(Co) \end{cases} \quad(4.2) \] where \(C_{1}\), \(C_{2}\) and \(C_{3}\) denote the concentrations of Fe, Co and Al in the alloys (Croat, Herbst, Lee, et al, 1984), \(S_{i}\) denotes the area of Mössbauer sub - spectrum at the \(i\) site, \(N_{i,0}(Fe)\) and \(N_{i,0}(Co)\) denote the occupancy in the alloys without Al element.

Composition and Magnetic Behavior of Nd₁₆Fe₇₇₋ₓCoₓB₇ Alloys

The magnetic properties of \(Nd_{16}Fe_{77 - x}Co_{x}B_{7}\) are listed in Table 4.1.