Improved thermal stability of TbF 3 -coated sintered Nd–Fe–B magnets by electrophoretic deposition

Using electrophoretic deposition (EPD) method, the impact of TbF 3 diffusion on the coercivity, microstructure and thermal stability of sintered Nd–Fe–B magnets with different rare earth (RE) content was investigated. In the diffused magnets with the RE content of 34 wt.%, the maximum coercivity about 28.12 kOe with less than 1.44 wt.% Tb was achieved, the coercivity temperature coefﬁcient ( β ) was improved to -0.50 %/ ◦ C from -0.58 %/ ◦ C within the temperature interval 25-160 ◦ C, and the maximum operating temperature further increased to about 160 ◦ C. It suggested that TbF 3 diffused magnets had much superior thermal stability than the annealed samples. This was attributed to the formation of the Tb-rich (Nd, Tb) 2 Fe 14 B phase in the outer region of the matrix grains and the improved Nd-rich grain boundary phase after TbF 3 diffusion. © 2018 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/


I. INTRODUCTION
Sintered Nd-Fe-B magnets have been used in many applications, particularly in traction motors of electric vehicles and wind generators, owing to their excellent magnetic properties. 1,2However, for the commercial sintered Nd-Fe-B magnets, the coercivity was only around 12 kOe, which decreased to less than 2 kOe at 200 • C, 3 and the operating temperature of the magnets with the coercivity of about 17 kOe was only around 60 • C. Therefore a high coercivity and good thermal stability of Nd-Fe-B magnets is strongly desired.4][5][6][7][8] In our previous study, 9,10 using the electrophoretic deposition (EPD) method, the optimum process condition of the TbF 3 -coated sintered Nd-Fe-B magnets has been discussed and the remarkable increment of coercivity with a small amount of Tb has been obtained.Substitution of Nd by HRE elements in the magnet through the binary alloy method can improve the coercivity and consequent thermal stability. 11However, the effect of TbF 3 diffusion on the thermal stability of sintered Nd-Fe-B magnets has not been systematically researched.In this study, using EPD technique, starting magnets with different rare earth (RE) content were designed to investigate the impact of Tb diffusion on the magnetic properties, microstructure and thermal stability in detail, and the mechanism of the improvement of the coercivity and thermal stability was studied.

II. EXPERIMENTAL
Sintered Nd-Fe-B magnets with nominal compositions of (Pr, Nd) x (Al, Cu, Co) 1.25 Fe bal B 1 (x = 29, 30, 32, and 34) (wt.%) were prepared using powder metallurgical method.Details of the TbF 3 EPD process were described in our previous paper. 9The weight ratio of the TbF 3 coatings and the magnet is 0-2.0 wt.%.The coated samples were diffused at 900 • C for 10 hrs and subsequently annealed at 490-520 • C for 2 hrs in vacuum, here we called it the (TbF 3 -EPD) diffused samples.For comparison, the as-sintered sample without any TbF 3 coating was also processed, here we called it the annealed sample.
The magnetic properties of the samples were measured by a magnetic measuring device NIM-500C and superconducting quantum interference devices (SQUID).Backscattered electron (BSE) images and elements distributions were analyzed by scanning electron microscope (SEM) (Quanta FEG 250) equipped with an energy dispersive X-ray spectrometer (EDS) (Oxford INCA system).The irreversible flux loss was estimated by measuring the flux-difference with a Helmholtz Coil.

III. RESULTS AND DISCUSSION
Fig. 1 shows the temperature dependence of coercivity for the annealed magnets and TbF 3 -EPD diffused samples with different x (x = 29, 30, 32, and 34).At room temperature, the coercivity of samples with different x all increased significantly after TbF 3 diffusion.The maximum increment of coercivity was 10.07 kOe in the samples with x = 30, and in the magnets with x = 34, the maximum coercivity about 28.12 kOe was achieved.It can be found that the coercivity all decreased with the increase of temperature.However, at the same temperature, coercivity of the diffused samples was much higher than that of the annealed samples with the same x.The coercivity of the annealed sample with x = 34 exhibited only 4.09 kOe at 160 • C, while the TbF 3 -EPD diffused sample still had the coercivity of 9.26 kOe, which was 126% higher than that of the annealed sample.It indicated that the thermal stability of the magnet was obviously improved through TbF 3 diffusion.From Fig. 1, we can see that with increasing the temperature, the coercivity increment of the diffused magnets gradually decreased in these samples with the same x.In the diffused samples with x = 30, the increment of coercivity was 10.07 kOe at room temperature, while the increment decreased to 5.57 kOe at 120 • C, and it was only about 2 kOe at 200 • C. In contrast, the increment of coercivity was 8.88 kOe at room temperature, then decreased to 6.34 kOe at 120 • C, and further decreased to 3.72 kOe at 200 • C in the diffused samples with x = 34.It indicated that at the elevated temperature, the coercivity increment in the diffused sample with low x reduced much more quickly than that with high x.
The cross-sectional SEM images of the annealed samples and the TbF 3 -EPD diffused samples with x = 30 and 34 are shown in Fig. 2. In the diffused magnet, at a depth of 100 µm of the magnet, the Tb-rich (Nd, Tb) 2 Fe 14 B core-shell structure, which was clearly visible in the backscattered image as a grey reaction phase surrounding the dark Nd 2 Fe 14 B grains, was well developed as shown in Fig. 2(b) and Fig. 2(e).And the core-shell distribution of Tb element was maintained to a depth of 500 µm [Fig.2(c) and Fig. 2(f)].The higher magnetocrystalline anisotropy of the core-shell phase in the outer region of the grains might prevent the nucleation of reverse magnetic domains in the Nd 2 Fe 14 B grains, which was beneficial for the coercivity enhancement. 12In the annealed sample with x = 30 [Fig.2(a)], since the rare earth content was low, the Nd-rich grain boundary phase was discontinuous and there was a direct local contact between two neighboring Nd 2 Fe 14 B grains.However, the grain boundary phase became much thicker and more continuous in the TbF 3 -EPD diffused magnet as shown in Fig. 2(c).This improved Nd-rich grain boundary phase weakened the exchange coupling between the matrix grains, which contributed to coercivity enhancement.Therefore, both the higher magnetocrystalline anisotropy of Tb-rich core-shell structure and the improved Nd-rich grain boundary phase contributed to the coercivity enhancement, which was in accordance with the result of Sepehri-Amin's. 3However, at the elevated temperature, the magnetocrystalline anisotropy field decreased, 12 hence the contribution from magnetically hardened grain surface due to Tb-rich shell and remedied grain surface due to the improved Nd-rich phase were both reduced. 13Therefore, the coercivity decreased with increasing the temperature, so did the coercivity increment.In the diffused samples with x = 30 and 34, the coercivity enhancement attributed to the Tb-rich core-shell structure was comparable, but the enhancement of the coercivity owing to the improved Nd-rich GB phase in the samples with x = 30 was much higher than that with x = 34, so the coercivity increment in the sample with x = 30 at the high temperature might decrease much more than that with x = 34.The temperature coefficient of coercivity ( β), which was defined as β = [H cj (T) H cj (T 0 )]/[H cj (T 0 ) × (T T 0 )], 14 can be deduced from Fig. 1.Here H cj (T 0 ) and H cj (T) are the coercivity at room temperature T 0 and temperature T, respectively.The β of the annealed magnets and TbF 3 -EPD diffused samples with different x in the different temperature intervals are shown in Fig. 3.In comparison with the annealed magnets, TbF 3 diffusion lowered the | β| values obviously in the samples with the same x in all temperature intervals.In the samples with x = 30, β was as worse as -0.63 %/ • C in the annealed sample in the range of 25-160 • C, after TbF 3 diffusion, β was improved to -0.57%/ • C.And the β values of the magnets with x = 34 with and without TbF 3 diffusion were -0.50 %/ • C and -0.58 %/ • C in the range of 25-160 • C, respectively.This further verified the improvement of thermal stability of the magnets with TbF 3 diffusion.Furthermore, from Fig. 3(e) we can see that | β| values decreased with increasing x in the diffused magnets in all temperature intervals.Since in the samples with high x, the fraction of the Nd-rich phase increased, and the grain boundary phase became more continuous, the microstructure parameter improved, which might contribute to the improvement of β. 15 Fig. 4 shows the variation of the irreversible flux loss (h irr ) as a function of temperature for the annealed magnets and TbF 3 -EPD diffused samples with different x.The h irr was defined by [Φ(T) Φ(T 0 )]/Φ(T 0 ), 14 where the magnetic flux Φ(T) was measured at room temperature T 0 after the magnet exposed at high temperature T for 2 hrs.According to Kato, 16 the maximum operating temperature (MOT) for a magnet was usually defined as the temperature at which the |h irr | was 3%.In the closed-circuit test, all the absolute values of h irr were less than 3% below 160 • C. In the open-circuit test, it can be seen that |h irr | increased with increasing temperature for all magnets.At the same temperature, h irr of the diffused magnet was much better than that of the annealed magnets with the same x, especially at higher temperature.In the magnets with x = 30, |h irr | of the annealed magnet appeared about 10.2% when heating to 80 • C, while it was only 1.6% after exposing to 120 • C in the diffused magnet.In contrast, in the magnets with x = 34, |h irr | of the annealed magnets was about 6.1% when heating to 120 • C.However, in the diffused magnets, |h irr | was only approximately 3.3% when the temperature further increased to 160 • C. It showed that TbF 3 diffusion significantly decreased |h irr |, especially at higher temperature.As shown in Fig. 4(e), the MOT increased from about 59.6 • C to 124.6 • C with TbF 3 diffusion in the samples with x = 30, increased by 65.0 • C. In comparison, the MOT of the diffused sample with x = 34 further increased to about 160.0 • C, which was much higher than that of the samples with x ≤ 30.It indicated that TbF 3 diffusion enlarged the MOT obviously compared with the annealed samples, suggesting that TbF 3 diffused magnets had much superior thermal stability than the annealed samples.

IV. CONCLUSION
The coercivity, microstructure and thermal stability of TbF 3 -coated sintered Nd-Fe-B magnets with different x using EPD was systematically investigated.In the starting magnets with much higher x, the coercivity was much higher at high temperature, therefore the β and h irr achieved the better improvement.In the starting magnets with x = 34, the maximum coercivity about 28.12 kOe was achieved after TbF 3 diffusion with less than 1.44 wt.% Tb, the β values was improved to -0.50 %/ • C from -0.58 %/ • C within the temperature interval 25-160 • C, and the MOT further increased to about 160 • C in the diffused sample.It suggested we could not only significantly increase the coercivity through TbF 3 diffusion, but also effectively improve the thermal stability by using EPD technique.Both the formation of (Nd, Tb) 2 Fe 14 B phase in the outer region of the matrix grains and the improved Nd-rich grain boundary phase contributed to the coercivity enhancement and thermal stability improvement.

FIG. 2 .
FIG. 2. Cross-sectional SEM images of the samples with x = 30 and 34 (a and d) the annealed samples, and the TbF 3 -EPD diffused magnets (b and e) 100 µm from the surface, (c and f) 500 µm from the surface of the sample.

FIG. 4 .
FIG. 4. The variation of h irr as a function of temperature for the annealed magnets and TbF 3 -EPD diffused samples with different x, (a) x = 29, (b) x = 30, (c) x = 32, and (d) x = 34, and (e) The MOT of the annealed magnets and TbF 3 -EPD diffused samples with different x.