Study on the mechanism of increasing coercivity of (Nd, Dy) – Fe-B magnets

In this paper, two main phases (Nd, The heat treatment process, microstructure and coercivity of TbF3 sintered with Dy) – Fe-B sintered magnets were studied. The optimal primary and secondary heat treatment temperature of TbF3 sintered with Dy) – Fe-B sintered magnets were 900 ℃ and 490 ℃. After the comprehensive optimization of the diffusion process, the coercivity of TbF3 sintered magnets increased from 20.00 kOe to 29.49 The results show that f diffuses into the surface layer of the magnet, while TB diffuses into the geometric center of the magnet; TB is easier to replace nd than dy in the magnetic phase; there is no concentration gradient of TB in the grain boundary phase between the main phase grains, which indicates that the similar capillary suction between the main phase grains is one of the driving forces of TB diffusion. X-ray Diffraction analysis shows that the degree of orientation of the magnet decreases slightly after diffusion. In general, the temperature stability of the magnet is obviously improved by grain boundary diffusion. In the range of 20-150 ℃, the remanence temperature coefficient α of the magnet increases from – 0.107% / ℃ to – 0.093% / ℃, and the coercivity temperature coefficient β increases from – 0.539% / ℃ to – 0.483% / ℃.

Double main phase preparation technology has been widely used in the production of low cost sintering (Ce Nd)-Fe-B magnets and low heavy rare earth and high coercive force (Nd Dy Tb)-Fe-B magnets; dual principal phase technology makes the magnet eventually form a dual magnetic phase structure by adjusting the microstructure of the magnet, thus effectively optimizing the performance of the magnet [1,2,3,4]. At present, double principal phase cerium magnets have been widely used in high-tech fields such as permanent magnet motors and variable frequency air conditioners. The annual output of cerium magnets has reached more than 50,000 tons. In terms of the improvement technology of low-heavy rare earth double principal phase (Nd Dy Tb)-Fe-B magnetic coercive force, the grain boundary diffusion technology has become a research hotspot [5,6]. The heavy rare earth element diffused at the grain boundary is a process that can effectively improve the coercive force of magnets while maintaining high remanence and using the least heavy rare earth [7,8,9,10]. The process is to adhere the heavy rare earth elements to the magnet surface in the form of compound or elemental material through a suitable heat treatment process, so that the heavy rare earth elements diffuse along the grain boundary, the light rare earth element Nd 、Pr、 The Ce element in the outer layer of the main phase grain are replaced to form the core-shell structure of the heavy rare earth magnetic phase coating the Nd ,Pr, Ce)2Fe14B magnetic phase, increase the anisotropy field of magnetic crystals at the binding point of the metaform nucleus at the boundary of the original magnet particles, and improve the coercive force of the magnet [11,12,13].
There are many studies on grain boundary diffusion of single alloy sintered ndfeb magnets and less studies on grain boundary diffusion of double principal phase Ce magnets [14,15], while for double principal phase (Nd Dy Tb)-The article Fe-B the improvement of the magnet force technology is even more rare. For double principal phase (Nd Dy Tb)-Fe-B magnet, the diffusion channel changes, the diffusion mechanism of heavy rare earth, the influences of orientation and the order of terbium replacing the rare earth elements in magnetic phase, etc. are still unclear. In this paper, Tb F3 is selected as the diffusion source for the double principal phase (Nd Dy)-Fe-B sintered magnet, and the double principal phase (Nd Dy Tb) is further improved by optimizing the heat treatment temperature.-Fe-B the coercive force of magnets. At the same time, the kinetic process of terbium diffusion, the characteristics of permanent magnet element replacement and the changes of orientation are studied. The high temperature stability of diffusion magnets is analyzed.

Experiment

Experiment sample

The magnets selected in this experiment are prepared by using 5 wt.% Dy (Nd, Dy)-Fe-B magnetic powder and Nd-Fe-B magnetic powder without Dy mixed and utilized dual main phase method with Dy content of 2 wt.% (Nd, Dy)-Fe-B magnets. The original remanence of the magnet is 1.35 kGs, the coercive force is 20.00kOe, and the Square Degree is 98%, among them, the square degree is the ratio of knee-point coercive force Hk (corresponding external magnetic field when the flux is 0.9 times in the demagnetization curve) and internal coercive force Hcj. This magnet is cut into thin sheet with the thickness of 1.9 m, and TbF3 powder with the particle size of 0.7~1.3 μm is coated on the surface of the magnet; In order to find the best heat treatment process, the first-stage heat treatment temperature is set to 880-960 ℃, and the second-stage heat treatment temperature is set to 480-500 ℃.

Performance test and characterization

The microstructure of magnets was observed by Phenom ProX scanning electron microscopy. The electron probe microzone analyzer EPMA-1720 series tested the element distribution of the sample. Pulsed magnetic field meter demagnetization curves of samples at different temperatures before and after diffusion. PANalyti the X’pert Pro x-ray diffraction of c al company to test the diffraction peak of the magnet perpendicular to the c-axis plane before and after diffusion. The flux change of 120 ℃ high temperature impact 2 H diffusion sample was tested by fluxmeter h T-707.

Experimental results

Effect of grain boundary diffusion temperature on magnetic properties

Choosing appropriate heat treatment temperature is the key to the diffusion of grain boundary. Low heat treatment temperature cannot make the heavy rare earth elements diffuse into the magnet, while high temperature will make it easy to diffuse into the magnetic main phase, this leads to the reduction of magnet remanence, so the appropriate heat treatment temperature is the core of grain boundary diffusion. In this experiment, the temperature range of heat treatment is set to 880-960 ℃ to observe the change of magnetic properties. According to Fig. 1(a), when the temperature is higher than a certain temperature, the remanence and coercive force decrease significantly. As is known from fig. 1 (B): under the temperature of 880 ℃-920 ℃, the coercive force basically keeps above 27.5kOe; When the temperature is higher than ℃, the coercive force and the remanence show a downtrend, it indicates that at higher temperature, heavy rare earth Tb diffuses into the magnetic phase, which is not conducive to Tb diffusing along the grain boundary; It can be seen from the Square Degree-T curve, in the temperature range of 880 ℃-910 ℃, its value fluctuates between 920 and, and its inflection point is at ℃, and then the square degree improves with the increase of heat treatment temperature, it shows that at the temperature point of 920 ℃, the microstructure has the worst uniformity. When the temperature is 900℃, the comprehensive magnetic properties of the magnet are the best, so the temperature is the optimal first-stage heat treatment temperature.

Fig.1 The demagnetization curves (a) and the coercivity, remanence, squareness (B) in relation to different heat treatment temperature
Regarding the two-stage heat treatment temperature, its temperature point is basically close to the two-stage tempering temperature of the original magnet. After two-stage heat treatment, the continuity of grain boundary phase can be improved, and the coercive force and square degree of magnet can be further improved. In the experiment, three temperature points, 480℃, 490℃ and 500℃, are chosen, and table 1 shows the corresponding coercive force and squared degree of secondary tempering temperature. As can be seen from the figure, at 480, 490 and 500 ℃, the corresponding values of coercive force are 27.81 kOe, 27.85 kOe and 27.4kOe respectively, and the square degrees are 92.8%, 94, 92,%. When the secondary tempering temperature is 490 ℃, the corresponding magnet coercive force and square degree are the largest, with the coercive force value of 27.85kOe, which is 7.85kOe higher than that of the original magnet. That is, the optimal thermal diffusion process for the magnet is the first and second heat treatment temperatures of 900 and 490 ℃ respectively. After the analysis of the microstructure, orientation and high temperature stability of the diffusion samples in this paper, all of them are about the sample with the coercive force of 27.85 kOe.
Table 1 The coercivity and squareness influence of secondary annealing temperature on demagnetization curve

Temperature (℃)	480	490	500
Coercivity (kOe)	27.81	27.85	27.4
Squareness (%)	92.8	94	92

In addition, under the condition that the temperature of the first and second heat treatment is 900 and 490 ℃ respectively, the parameters such as vacuum degree during the heat treatment are optimized. The coercive force of magnets is further improved, and Fig. 2 shows the optimal magnetic properties obtained after diffusion.

Fig.2 The optimal performance of diffusion magnet

Effect of grain boundary diffusion terbium on magnet microstructure

The diffusion magnet is cut by wire cutting along the diffusion direction, and the cross section is polished after sanding by sandpaper. After that, all the analyses on the diffusion sample are the sample. Figure 3 is the backscattering graph of polishing cross section; The larger the atomic number is, the brighter the corresponding color is; The gray-black one is magnetic phase, and the bright one is rare earth-rich phase. The Black Arrow indicates the penetration direction of the diffusion source. In the magnet part closer to the diffusion source, the rare earth-rich phase has the phenomenon of width widening and aggregation. The rare earth-rich phase of magnets far away from the diffusion source is slender. The figure shows that the rare earth-rich phase is widened and the aggregation depth is about 120 μm.

Fig.3 The BSE image of the section with the diffusion magnet
Fig. 4 is the electron probe microzone analysis (EPMA) diagram of the cross section of the diffusion magnet. The Arrow direction is the diffusion direction of terbium element. Figure 4 (a) is the back-scattering diagram of the section of diffuse magnet, in which there is a rare earth rich accumulation area; In the figure, the distribution of Tb element and the rare earth rich phase in the figure is generally the same, forming a network structure; its distribution area is wider than the rare earth-rich phase, indicating that Tb is not only distributed in the rare earth-rich phase, but also distributed on the surface of magnetic particles; the main phase grain surface containing Tb increases the magneto-crystal anisotropy constant of easy-forming nucleus to improve the coercive force of the magnet. The rare earth-rich phase accumulation area circled shows that Tb does not have similar aggregation phenomenon, and the concentration of F and Nd elements is very high, indicating that F diffuses into the crystal boundary phase of the magnet during heating, compounds containing neodymium and fluorine are formed. Combined with Fig. 3, it can be inferred that the diffusion depth of F during diffusion is about 120 μm.
Figure 4 (B) is another area of the diffusion cross section, using white circles to mark the core-shell structure of the main phase grain of neodymium after diffusion; Using black circles to mark the main phase grain of dysprosium, the main phase grain does not form a similar core-shell structure. This indicates that in the process of Tb diffusion, Nd and Dy have preferentially replaced Nd elements in the magnetic phase compared with Tb.

Fig.4 The element distribution of the section with diffusion magnet

Fig.5 The BSE (a0, b0, c0), Tb distribution (a1, b1, c1) and the content identification (a2, b2, c2) images of different diffusion depth with the magnet section *

Figure 5 is the distribution diagram of back scattering (a0,b0,c0) and Tb (a1,b1,c1) from the depth of diffusion source to 0-58μm, 200-258μm and 900-958μm. And draw a regional graph (a2,b2,c2) with the “stroke” tool in photoshop to draw the boundaries of Tb with different contents; In the area with a diffusion depth of 0-58 m, pink, white, the Tb content of the black marked area is 11.10, 6.66 and wt 3.33.%; It was found that the sides of the black and white labels were very close. In the area with a depth of 200-255 μm from the diffusion source, the Tb content is also marked with black and white, which is close to 5 and 3wt.% Of the area, Tb content is 5wt.% Of the areas show small area enrichment, with a concentration of 3wt. The area of%, covering almost all the areas at the boundary boundary of the thin layer; In the scanning area with a depth of 900-958μm from the diffusion source, the content of massive rare earth-rich Tb in the scanning area also reaches 6wt.%; The depth of the geometric center of this magnet is 950μm, which is Tb diffused to the geometric center of the magnet. The diffusion depth of terbium is 950 μm much larger than that of element F is 120 μm, which indicates that Tb and F diffuse into the magnet during the diffusion process, F combines with Nd and micro element Al in the phase of grain boundary to displace the more active Tb element; Its equation is:

Further heat treatment makes the Tb element diffuse to the deeper layer of the magnet, the compound of NdAlF3 is decomposed, and the F diffusion evaporates into the air in the form of gas.
Besides, when the content of massive rare earth-rich phase Tb is about 6wt.% Tb entering the grain boundary of the main phase to form a thin layer between the grains, its concentration is about 3wt.%. There is no significant concentration difference in the thin-layer crystal boundary phase; The diffusion of the elements of fick’s first law and second law will form a gradient difference. The consistency of the concentration of terbium in the thin-layer indicates the existence of other driving forces. The grain boundary diffusion temperature 900℃ is close to the ternary eutectic temperature, the grain boundary phase is liquid, and the grain boundary phase containing terbium enters between the main particles under the action of similar capillary suction, then the consistency of Tb concentration in the thin-layer grain boundary area is formed; That is, the similar capillary suction is also one of the powers of Tb diffusion. Fig. 6 is a structural diagram of the terbium-containing grain boundary phase diffusing into the main phase between the crystals under the action of driving force, where, f is capillary suction, and the arrow indicates the direction of suction.

Fig.6 Structure diagram of RE-rich phase containing terbium diffusing into the gap between the main phase grains under the action of driving force
Fig.7 is the X-ray diffraction pattern perpendicular to the c axis plane before and after magnet diffusion. Diffraction peaks of sintered (Nd)-Fe-B magnets (105) and (006) twin planes at 2θ of 36.4 and 43.2, respectively, I(006)/I(105) represents the degree of orientation of the magnet, the ratio of the two peaks of the original magnet is 1.33, and the ratio is reduced to 1.3 after diffusion. From the element analysis, in the process of grain boundary diffusion, Tb replaces Nd to enter the main phase, the lattice parameter of Nd 2Fe14B is a = 0.880μm c = 1.22μm Tb 2Fe14B. The lattice parameter is a = μm c = 1.205 μm; the reduction of the lattice parameter c may make the c axis of the main phase grain offset the orientation direction, resulting in a slightly lower degree of orientation of the magnet.

Fig.7 The XRD curves of non-diffusion and diffusion samples

Effect of grain boundary diffusion Tb on high temperature stability of magnets

Two parameters describing the magnet’s temperature stabilization are residual magnetic temperature coefficient α and coercive force temperature coefficient, and the formula is:

In the formula, αbr and ∆hcj represent the temperature coefficients of residual magnetism and coercive force in the T0-T temperature range. Br(T0) and B(T) they are respectively T0 and T, which are the corresponding residual magnetic of the two temperature points; Hcj(T0) and Hcj(T) are the coercive force of the corresponding points. According to the calculation of this formula, we find that αbr and ∆hcj are negative; The larger they are, the smaller the absolute value is and the better the temperature stability of the corresponding magnet is. Figure 8(a) and 8 (B) are the demagnetization curves of the original magnet and the magnet after diffusion at different temperatures. According to the formula, the original magnet is between 20-150 ℃, Α is-0.107%/℃,β is-0.539%/℃. After diffusion magnet, between 20-150 ℃, α is-0.093%/℃,β is-0.483%/℃. This indicates that grain boundary diffusion obviously improves the stability of the magnet.

Fig. 8 The B- H curves under various temperature for the magnets with and without diffusion
Another way to evaluate the thermal stability of the sample is to test the magnetic flux loss. That is, after testing the magnetic flux at room temperature and placing the sample in a high-temperature oven for high-temperature impact test, the smaller the difference between the magnetic flux of the test sample, the better the stability of the magnet. 9 is the result of the magnetic flux of the diffused sample. Black lines and red lines respectively represent the magnetic flux of the diffused magnet after 120℃, 2h fully open-circuit high-temperature impact. Green Line is the percentage of Magnetic flux loss. It can be seen from the figure that the Magnetic flux loss (Magnetic flux loss-ML) of a magnet is within 1% after high temperature impact. It shows that the diffuse sample has good high temperature stability.

Fig. 9 The magnetic flux change of diffusion magnet before and after high temperature impact

Conclusion

In this paper, the grain boundary diffusion heat treatment process is optimized, and the influence of grain boundary diffusion on the microstructure, orientation and high temperature stability of magnets is analyzed. The main conclusions are:

• (1) the best condition of grain boundary diffusion is found for the sintered magnet with double main phase (Nd, Dy)-Fe-B. The optimal added value of coercive force of magnets is 9.49 kOe. The diffusion depth of F is about 120 μm Tb and diffused to the position of 950 μm in the geometric center of the magnet.
• (2) heavy rare earth Tb is basically 6wt in the block grain boundary phase at diffusion depth of 900μm and 200μm.%; However, the Tb content at the grain boundary is 3 wt.%, indicating that the similar capillary suction between the two magnetic grains is one of the powers of Tb diffusion.
• (3) heavy rare earth Tb is easier to enter the magnetic phase of Nd2Fe14B instead of Dy2Fe14B; Tb diffuses into the inside of the magnetic phase, which promotes the change of the lattice constant of the magnet, may be the reason why the orientation of the magnet is slightly reduced.
• (4) within the temperature range of 20-150 ℃, the grain boundary diffusion increases the temperature coefficient of magnet remanence from-0.107%/℃ to-0.093%/℃, the temperature coefficient of coercive force is increased from-0.539%/℃ to-0.483%/℃. After diffusion, the magnetic flux loss of the sample is less than after 120℃, 2-hour full-open-circuit high temperature impact.

Author: Xiaoning Shi,Xiangdong Wang,Minggang Zhu,Xiaoping Chen,Dongliang Zhao,Zhonghua Wei

Source: China Permanent Magnet Manufacturer – www.rizinia.com
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