Effects of Cerium on Magnetic Properties of Sintered R-Fe-B Permanent Magnets

As an element in Lanthanide, Cerium has the highest reserve in global rare earth resources (up to $40 \sim 50$ % of total RE). To make the best use of Ce will improve the balance of rare-earth applications. Originating from the mixed valence characteristics of Ce ions in rare-earth transition compounds, its physical and chemical properties usually deviate from the trend of whole RE family. Ce exists in Ce
2
Fe
14
B almost as Ce
⁴⁺
ion. The lack of 4f electron leads Ce
⁴⁺
to make no contribution to magnetism, especially the magneto-crystalline anisotropy. Small ion radius shrinks the Fe-Fe distances and consequently decreases the exchange interaction of Fe sub-lattice, which makes Ce
2
Fe
14
B have lower Curie temperature $\mathrm {T_{C}}$ than other R
2
Fe
14
B compounds. The $\mathrm {T_{C}}$ of Ce
2
Fe
14
B is 422 K which is 164 K less than Nd
2
Fe
14
B. J
s
is 1.17 T, 0.43 T less than Nd
2
Fe
14
B. The anisotropic field $\mu _{0} \mathrm {H_{a}}$ is 3.6T, only 47% of Nd
2
Fe
14
B. In addition to decreasing the intrinsic magnetic properties in Ce
2
Fe
14
B, Ce also makes heavy damage on the coercivity of sintered Ce-Fe-B magnet. Because the conventional powder metallurgic procedure is in favor of forming CeFe
2
Laves phase instead of Ce-rich phase that magnetically decouple Ce
2
Fe
14
B grains. Even so, Ce
2
Fe
14
B still has comparable J
s
to that of SmCo
5
and reasonable $\mu _{0} \mathrm {H_{a}}$ for permanent magnet development. To add La, the RE element with the largest ion radius, is possible to improve the intrinsic magnetic properties of (La
x
Ce
1-x
)
2
Fe
14
B [1]. The lattice parameters increase linearly with Ce content x which gives $\mathrm {a}= 0.8763 +0.0033\mathrm {x}($ nm) and $\mathrm {c}= 1.2135 +0.0114\mathrm {x}($ nm). As a result, $\mathrm {T_{C}}$ do increase linearly with $\mathrm {T_{C}}= 427.4 +59.0\mathrm {x}(\mathrm {K})$. Unfortunately the J
s
at the temperature of 5K do not indicate the same increase with lattice expansion and keeps approximately a constant of 1.51 T. The practical way of preparing Ce-contained permanent magnet is to let Ce replace Nd to its upper limit. Rapidly quenching routine is straight forward to get rid of the necessity of Ce-rich phase. As shown in Fig.1 [2], even though all the permanent magnetic parameters drop linearly with Ce content x, isotropic [(Nd
0.75
Pr
0.25
)
1-x
Ce
x
]
11.65
Fe
82.75
B
5.6
powder can still realize reasonable $\mathrm {H_{cJ}}$ and $(\mathrm{BH)_{max}}$ for certain applications. For example, $\mathrm {B_{r}} =8.54$ kG, $\mathrm {H_{cJ}} =9.4$ kOe and $(\mathrm{BH)_{max}}=14.8$ MGOe when Ce replaces Nd-Pr by 20 at% $(\mathrm {x} = 0.2)$. At $\mathrm {x}=0.5$, $\mathrm {H_{cJ}}$ keeps about 6.8 kOe with $(\mathrm{BH)_{max}}$ of 11.6 MGOe. And the B
r
is 7.82 kG. $\mathrm {T_{C}}$ at this composition is reasonably high as 509 K. In preparation of sintered magnets containing Ce, dual-alloy or dual main-phase techniques are usually applied. With Ce-rich/Nd-lean alloy and Nd-rich/Ce-lean alloy as starting materials, sintered (Ce,Nd)-Fe-B magnet is commercialized with high ratio of performance to cost. The key point is to let plenty of Nd-rich phase magnetically decouple (Ce,Nd)
2
Fe
14
B grains. As shown in Table 1 [3], by mixing strip casted Nd-Fe-B and (Ce-Nd)-Fe-B alloy powder with different ratio, the sintered magnet with 45wt% Ce and 55wt% Nd in total rare earth composition gives $\mathrm {B_{r}} =12.4$ kG, $\mathrm {H_{cJ}}=6.2$ kOe and $(\mathrm{BH)_{max}} =33.4$ MGOe. At Ce of 20wt%, the correspondent parameters are 13.7 kG, 12.0 kOe and 45.0 MGOe, which is good enough to many applications. The melting points of R-rich phase decrease with Ce composition. This implies that sintering temperature for high Ce-containing magnet can be lower than low Ce-containing magnet. Another example is to directly use misch-metal (MM) as raw material [4]. The typical composition of MM is La 27.06 wt%, Ce 51.46 wt%, Pr 5.22 wt%, and Nd 16.16 wt%. When MM to total RE ratio is 21.5%, $\mathrm {B_{r}}= 12.09$ kG, $\mathrm {H_{cJ}}= 10.70$ kOe and $(\mathrm{BH)_{max}}= 34.04$ MGOe. For magnet only with MM, $\mathrm {H_{cJ}}$ is as low as 0.46 kOe. $(\mathrm{BH)_{max}}$ is just 2.40 MGOe. Microstructure observation shows that RFe
2
grains, in which Ce is the main component of R, distribute in triangle junction area formed by neighbor R
2
Fe
14
Bgrains. The lack of inter-granular R-rich phase is responsible for low $\mathrm {H_{cJ}}$ in sintered MM-Fe-B magnet.