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High orientation Nd-Fe-B sintered magnets prepared by wet pressing method
Journal of Magnetism and Magnetic Materials 495 (2020) 165826
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Research articles
High orientation Nd-Fe-B sintered magnets prepared by wet pressing method ⁎
Shuai Cao, Xiaoqian Bao , Jiheng Li, Haijun Yu, Kunyuan Zhu, Xuexu Gao
T
⁎
State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, 30Xue Yuan Road, Beijing 100083, People’s Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Nd-Fe-B sintered magnets Wet pressing Orientation Magnetic properties Microstructure
Fine magnetic powder particles are usually utilized to prepare high coercivity Nd-Fe-B magnets. However, it is more difficult to obtain high orientation degree due to the larger friction between finer powder particles during magnetic-aligned compaction process, which greatly affects the performance of final magnets. In this work, a special wet pressing method is adopted to reduce the friction between the powder particles. The degree of alignment of the magnet is obviously increased and the microstructure is optimized. The remanent magnetization of the magnet shows a significant increase with negligible coercivity change, while the compressive strength is improved compared with that of the traditional method.
1. Introduction
2. Experimental
Sintered Nd-Fe-B magnets with outstanding permanent magnetic properties have found a wide range of applications [1]. Sintered permanent magnets are generally produced by a powder metallurgical approach [2], and the magnetic-aligned compaction is an important process for maximizing the magnetization of the magnetic powder to the final higher performance sintered magnets [3]. However, in recent years, magnetic-aligned compaction has become increasingly difficult due to the development of finer and high coercive permanent magnet powders [4], and the orientation degree decreases with reducing powder size [5]. It is still a hard work to reduce the friction between the powder particles and increase the degree of alignment by the PLP technology developed by M. Sagawa et al. [6]. Friction forces could be reduced by addition of liquid lubricant at powder process [7], however, a small amount of lubricant with ester or alkyl organic matter cannot overcome the large friction, while lots of lubricants will be difficult to eliminate at sintering process in traditional preparation method, which will weak remanence and magnetic energy product. It is still necessary to do further research on improving the orientation of the magnet. In this work, we propose a wet pressing method with a special double-layer mold to produce magnets with high degree of alignment. Compared with that of the traditional pressing method, both of the orientation and remanence are improved, meanwhile, the microstructures are optimized. The mechanical properties are discussed by conducting compressive strength measurement.
(Nd,Pr)30.90Mn0.12Al0.16Ga0.11Cu0.12B1.09Febal (wt.%) powders with a mean particle size of 3.0 μm were produced by strip casting, hydrogen decrepitation and jet-milling. Such powders were mixed with organic solvents to make a powder slurry, in which ethanol was the major component of the solvents and the volume ratio of powders to solvents was 1:3. A powder slurry with low viscosity was obtained due to the low solid powder content. The powder slurry was subjected to ultrasonic shock treatment for 10 min and then placed in a special doublelayer mold. The inner mold was a composite material with porous structure, of which the average pore size was less than 2 μm. And the outer mold was a high manganese steel with through-hole structure. Then, the slurry was affected under a magnetic field of 1.8T and a pressure of 25 MPa simultaneously, both of which persisted until there was no significant fluid leakage from the mold. The magnetic field stayed to the end of die pressing and its direction was perpendicular to the pressing direction. The wet pressing process and the characteristic of the mold were schematically shown in Fig. 1a. Subsequently, the green compacts obtained by the above wet pressing process were treated by cold isostatic pressing and then sintered for 2 h at 1070 °C, flowed by a two-step annealing for 2 h at 900 °C and 2 h at 500 °C, respectively. In these processes, they were all protected by a high vacuum following high purity argon. In addition, some contrast samples were prepared by the traditional vertical pressing method with the same powders and heat treatments. Magnet density was calculated using the Archimedes principle.
⁎
Corresponding authors. E-mail addresses: [email protected] (X. Bao), [email protected] (X. Gao).
https://doi.org/10.1016/j.jmmm.2019.165826 Received 5 May 2019; Received in revised form 18 August 2019; Accepted 9 September 2019 Available online 10 September 2019 0304-8853/ © 2019 Published by Elsevier B.V.
Journal of Magnetism and Magnetic Materials 495 (2020) 165826
S. Cao, et al.
Fig. 1. A schematic diagram of the layout and treatment mode for wet pressing method (a), and the laser microscope mapping images of the powder slurry before poured into the mold (b).
and gravity in wet pressing process, which seems difficult to optimize when liquid is introduced. 3D maps in these figures are used to show the results better. The XRD patterns in Fig. 2(a2) and (b2) are similar but showed different relative intensity ratios of the (0 0 6) peak to the (1 0 5) peak, with the values of 1.26 and 3.02 corresponding to the magnets with traditional and wet pressing methods, and the higher value represents the higher degree of alignment [8]. Fig. 3 shows the room temperature demagnetization curves and temperature dependence of magnetic properties for the magnets produced by wet pressing method and traditional method. Both of the magnets have a high squareness factor Hk/Hcj over 93%. The magnet prepared by traditional method possesses a coercivity Hcj of 13.67 kOe and a remanence Br of 12.42 kG. When the wet pressing method is adopted, the magnet has a coercivity Hcj of 13.62 kOe and a remanence Br of 13.34 kG, at which the coercivity reduce a little and the remanence increase significantly compared to that of the traditional method, as shown in Fig. 3a. The higher remanence of magnet is mainly due to the better orientation, and it is in consistent with previous reports [9,10]. And the effect of crystal alignment on remanence follows the theoretical prediction of the Stoner–Wohlfarth model [11]. Fig. 3b depicts the temperature dependence of remanence Br and coercivity Hcj of the magnets produced by these two different methods. The temperature coefficients at 20–60 °C and 20–100 °C for the different magnets are listed in the inset table. It can be seen that the remanence of the magnet with wet pressing method is always much higher than that with the traditional method at any temperature, and the coercivities of the two different magnets are always close. At the low temperature range of 20–60 °C, the values of α and β are −0.11%/°C and −0.77%/°C for the magnet with wet pressing method, while the values of α and β are −0.12%/°C and −0.79%/°C for the magnet with traditional method. There is no significant difference between the values of α and β for the different magnets at the temperature range of 20–100 °C. It seems that the wet pressing method has little effect on the thermal stability of magnets The microstructures of the magnets produced by wet pressing method and traditional method were observed by SEM, as shown in Fig. 4. The measured surface was perpendicular to the direction of caxis. Both samples consist of a bright contrast, which indicates that the RE-rich phase and a dark contrast showing the matrix Nd2Fe14B phase. For the magnet prepared by wet pressing method (Fig. 4a), almost all the matrix phase particles are closely arranged, and some small particles are even squeezed into the triple junction corners (marked by dotted green circles). However, for the magnet prepared by traditional method (Fig. 4b), some matrix particles are not arranged tightly enough, and certain small particles are stuck among other particles and
Dispersion characteristics of powders in solvents were studied by 3D measuring laser microscope (3DLM, Olympus, OLS4100). Alignment analyses were conducted by X-ray diffractometer (XRD, Bruker, D8 Advance), and the X-ray diffraction was done on the pole surface of magnets, which was perpendicular to the direction of the alignment magnetic field (Halign). The magnetic properties were measured by a NIM-2000 magnetic measurement device. Microstructural and compositional analyses were conducted by backscattered electron scanning electron microscopy (SEM, Carl Zeiss, Supra55), electron probe microanalyzer (EPMA, JEOL, JXA-8230) and a carbon-sulphur analyzer. The mechanical properties were measured by a WDW-2000 microcomputer control electronic universal testing machine, and scratch-free final polishing was implemented on the surfaces of specimens before testing. 3. Results and discussion Fig. 1b shows the laser microscope mapping images of the powder slurry before poured into the mold. The inset depicts the image with a larger magnification. The red contrast location indicates the cores of powders in the solvents. It can be seen that the powder particles have excellent dispersion and there is no obvious agglomeration of particles. That means the introduction of liquid solvents does not lead to the deterioration of particle dispersibility. The solid-liquid ratio chosen in this study is 1:3, which is due to the fact that the settling rate of powder particles and slurry viscosity will both increase sharply with higher solid-liquid ratio values. And an ultra-low solid-liquid ratio is unnecessary because the pressing process will be completed in a rather short time. Fig. 2 shows the stereographic projections for (0 0 6) crystal planes and the XRD patterns for the magnets with the traditional method (a12) and the wet pressing methods (b1-2). The X-ray diffraction was done on the pole surfaces perpendicular to the direction of the alignment magnetic field (Halign). It can be seen that both magnets have very strong textures, as shown in Fig. 2(a1) and (b1), and the color indexes increase with the concentrations of (0 0 6) planes, meanwhile, the concentration circle lines are close to the center with the direction of orientation approaches that of Halign. There are some higher-level concentration lines (red lines) distribute at the outer layer of the map in Fig. 2(a1), and it means that some easy axes (c-axis) of Nd2Fe14B grains deviate greatly from the Halign direction. For the map of the magnet prepared with wet pressing method in Fig. 2(b1), the concentration lines are dense and concentrated except for the separation of central circle lines. The little deviation is in a specific direction, which is far different from that in Fig. 2(a1), and it is mainly due to the liquid flow 2
Journal of Magnetism and Magnetic Materials 495 (2020) 165826
S. Cao, et al.
Fig. 2. The stereographic projections for (0 0 6) crystal planes and the XRD patterns for the magnets with the traditional method (a1-2) and the wet pressing methods (b1-2).
rotation when pressing and aligning in wet pressing method, and it just corresponds to the better orientation in Fig. 2. Fig. 5 depicts the EPMA mapping images of Fe, Pr, O and C elements for the magnets prepared by wet pressing method (a1-4) and traditional method (b1-4). The measured surface was perpendicular to the direction of c-axis. The characteristics of elements distribution for the two different methods are similar. The concentration distribution of Fe elements visually demonstrates the distribution of matrix phase grains as shown in (a1) and (b1). The Pr, O and C elements are mainly enriched surrounding the matrix phase grains, while some of them agglomerate in the triple junctions. The measurement results of oxygennitrogen analyzer show that the O elements contents of magnets for both methods are less than 1500 ppm. The C elements contents of magnets are 1100 ppm for the traditional method and 900 ppm for the
prevent them from getting closer (marked by dotted yellow circles). Note that there are also some highly agglomeration areas of matrix phase particles (marked by dotted blue lines) and the grain boundaries are relatively fuzzy, which mainly attribute to poor fluidity of powders in the forming process. The measurements show that the densities of the magnets are 7.60 g/cm3 for the wet pressing method and 7.52 g/cm3 for the traditional method using Archimedes principle. That means the wet pressing method can make the density of magnet closer to the limit one, and it is also one of the reasons that the remanence of magnet increases, in addition to the higher degree of orientation. It also can be seen that some microcracks occur in Fig. 4b, and this is mainly attributed to the large friction between powder particles in the traditional forming process. Thus, the better microstructure in Fig. 4a reveals that the liquid solvents lubricate the powders in the process of particles flow and
Fig. 3. The room temperature demagnetization curves (a) and temperature dependence of magnetic properties (b) for the magnets produced by wet pressing method and traditional method. The inset table shows the temperature coefficients at different temperature range for the magnets. 3
Journal of Magnetism and Magnetic Materials 495 (2020) 165826
S. Cao, et al.
Fig. 4. SEM images of the magnets produced by wet pressing method (a) and traditional method (b).
Fig. 5. EPMA mapping images for the magnets prepared by wet pressing method (a1-4) and traditional method (b1-4).
wet pressing method according to the measurement results of carbonsulphur analyzer. That means the introduction of organic solvents will not lead to a sharp increase in carbon and oxygen contents. The lower carbon content in the wet pressing method is mainly attributed to that the organic solvents dissolve and take away some quantities of antioxidants and lubricants which were certain esters and introduced at powder process. Compressive strength is used to evaluate the mechanical properties of the magnets in the study, because it can better reflect the overall crack growth behavior and defect characteristic of the brittle materials [12–14]. Compared with bending strength measured by three-point bending test for Nd-Fe-B sintered magnets, the compressive strength tends to emphasize the integrity of mechanical properties, which means that any fragile area or micro-defect of the whole magnet will reduce the compressive strength value. The schematic diagram of compressive strength test and examples of test curves for the magnets prepared by wet pressing and traditional methods are shown in Fig. 6. The tested sample had a dimensional size of Ø2.5 × 6 mm3 and the loading force was applied parallel to the c-axis of the magnet with a loading speed of 0.05 mm/min. Four samples for each magnet were selected to measure the compressive strength values and the measurement results are shown in the inset table. It can be seen that the test curve consists of three sections, and they are the part in which stress increases slowly with displacement, the part in which stress presents linear increase, and the part of stress decreasing. The first part is determined by the experimental conditions, such as sample size, sample shape, loading rate and so on. And the second part means that elastic deformation occurs in the sample, while the third part means that the sample has fractured. The
Fig. 6. The schematic diagram of compressive strength test and examples of test curves for the magnets prepared by the different methods. The inset table shows the measurement results of four samples for each magnet.
ultimate strains refer to the total value of the strain of elastic deformation and they are similar for both of the different magnets. Note that all the compressive strength values of samples from the magnet with wet pressing method are higher than that of traditional method. Fig. 7 shows the SEM fracture morphologies of the fractured specimens after above compressive strength test. It can be seen that the samples 4
Journal of Magnetism and Magnetic Materials 495 (2020) 165826
S. Cao, et al.
Fig. 7. SEM fracture morphologies of the fractured specimens after compressive strength test. ((a) specimen from the wet pressing method; (b) specimen from the traditional method.)
present the completely brittle fracture characteristic in intergranular way. Some (Nd, Pr)-rich oxides were detected by EDS analysis in both of the samples (marked by yellow arrows), while they are obviously more in the sample from the traditional method. A crack is observed at the grain boundaries in Fig. 7(b) (marked by red arrow). That means there are more defects in the sample from the traditional method, and cracks are easier to generate and propagate compared with that of the wet pressing method. And it just corresponds to the BSE image in Fig. 4(b), in which there are more grain boundary phases in the triple junction corners, and the triple junction regions tends to be damaged firstly when suffering the stress because of the (Nd, Pr)-rich phase and some original defects such as pin-pores and inclusions of oxides formed during the sintering process preferential to aggregate in these regions [15].
[2] S. Hirosawa, Y. Matsuura, H. Yamamoto, S. Fujimura, M. Sagawa, Magnetization and magnetic anisotropy of R2Fe14B measured on single crystals, J. Appl. Phys. 59 (1986) 873–879, https://doi.org/10.1063/1.336611. [3] W.F. Li, T. Ohkubo, K. Hono, M. Sagawa, The origin of coercivity decrease in fine grained Nd–Fe–B sintered magnets, J. Magn. Magn. Mater. 321 (2009) 1100–1105, https://doi.org/10.1016/j.jmmm.2008.10.032. [4] R. Soda, K. Tanaka, K. Takagi, K. Ozaki, Simulation-aided development of magneticaligned compaction process with pulsed magnetic field, J. Powder Technol. 329 (2018) 364–370, https://doi.org/10.1016/j.powtec.2018.01.035. [5] Y. Sun, R.W. Gao, G.B. Han, G. Bai, T. Liu, B. Wang, Effects of powder flowability on the alignment degree and magnetic properties for NdFeB sintermagnets, J. Magn. Magn. Mater. 299 (2006) 82–86, https://doi.org/10.1016/j.jmmm.2005.03.086. [6] A.G. Popov, O.A. Golovnia, A.V. Protasov, Enhanced method of magnetic powder alignment for production of PLP Nd-Fe-B magnets, J. Magn. Magn. Mater. 428 (2017) 424–430, https://doi.org/10.1016/j.jmmm.2016.12.134. [7] A.G. Popov, V.S. Gaviko, N.N. Shchegoleva, O.A. Golovnia, T.I. Gorbunova, G.C. Hadjipanayis, Effect of addition of esters of fatty acids on the microstructure and properties of sintered Nd–Fe–B magnets produced by PLP, J. Magn. Magn. Mater. 386 (2015) 134–140, https://doi.org/10.1016/j.jmmm.2015.03.069. [8] X.G. Cui, C.Y. Cui, X.N. Cheng, X.J. Xu, T.Y. Ma, M. Yan, C. Wang, Effects of alignment on the magnetic and mechanical properties of sintered Nd–Fe–B magnets, J. Alloys. Compd. 563 (2013) 161–164, https://doi.org/10.1016/j.jallcom.2013.02. 093. [9] R.W. Gao, D.H. Zhang, H. Li, J.C. Zhang, Effects of the degree of grain alignment on the hard magnetic properties of sintered NdFeB magnets, Appl. Phys. A 67 (1998) 353, https://doi.org/10.1007/s003390050783. [10] J.J. Ni, W. Luo, C.C. Hu, Y.B. Yin, D. Zhang, X.L. Peng, Q. Han, Relations of the structure and thermal stability of NdFeB magnet with the magnetic alignment, J. Magn. Magn. Mater. 468 (2018) 105–108, https://doi.org/10.1016/j.jmmm.2018. 07.090. [11] T. Kawai, B.M. Ma, S.G. Sankar, W.E. Wallace, Effect of crystal alignment on the remanence of sintered NdFeB magnets, J. Appl. Phys. 67 (1990) 4610, https://doi. org/10.1063/1.344828. [12] J.D. Hogan, L. Farbaniec, T. Sano, M. Shaeffer, K.T. Ramesh, The effects of defects on the uniaxial compressive strength and failure of an advanced ceramic, Acta Mater. 102 (2016) 263–272, https://doi.org/10.1016/j.actamat.2015.09.028. [13] E. Krimsky, K.T. Ramesh, M. Bratcher, M. Foster, J.D. Hogand, Quantification of damage and its effects on the compressive strength of an advanced ceramic, Eng. Fract. Mech. 208 (2019) 107–118, https://doi.org/10.1016/j.engfracmech.2019. 01.007. [14] H. Jiang, F. Jiang, D.Y. Hu, R.Q. Wang, J. Lu, B. Li, Numerical modeling of compressive failure mechanisms in ceramic materials at high strain rates, Comput. Methods Appl. Mech. Engrg. 347 (2019) 806–826, https://doi.org/10.1016/j.cma. 2019.01.006. [15] W. Liu, J.S. Wu, Mechanical properties and fracture mechanism study of sintered Nd–Fe–B alloy, J. Alloys. Compd. 458 (2008) 292–296, https://doi.org/10.1016/j. jallcom.2007.04.277.
4. Conclusions In summary, the orientation degree of the magnet is greatly improved by the wet pressing method with special double-layer mold. Meanwhile, the remanence and density are enhanced compared with the traditional vertical pressing method. In addition, the compressive strength was conducted to indicate the better mechanical properties. The beneficial effects of this method are mainly attributed to that the liquid solvents lubricate the powders well in the process of particles flow and rotation when pressing and aligning in the wet pressing process. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51401021) and the State Key Laboratory Advanced Metals and Materials (No. 2016Z-14). References [1] K. Hono, H. Sepehri-Amin, Strategy for high-coercivity Nd–Fe–B magnets, Scr. Mater. 67 (2012) 530–535, https://doi.org/10.1016/j.scriptamat.2012.06.038.
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Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Research articles
High orientation Nd-Fe-B sintered magnets prepared by wet pressing method ⁎
Shuai Cao, Xiaoqian Bao , Jiheng Li, Haijun Yu, Kunyuan Zhu, Xuexu Gao
T
⁎
State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, 30Xue Yuan Road, Beijing 100083, People’s Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Nd-Fe-B sintered magnets Wet pressing Orientation Magnetic properties Microstructure
Fine magnetic powder particles are usually utilized to prepare high coercivity Nd-Fe-B magnets. However, it is more difficult to obtain high orientation degree due to the larger friction between finer powder particles during magnetic-aligned compaction process, which greatly affects the performance of final magnets. In this work, a special wet pressing method is adopted to reduce the friction between the powder particles. The degree of alignment of the magnet is obviously increased and the microstructure is optimized. The remanent magnetization of the magnet shows a significant increase with negligible coercivity change, while the compressive strength is improved compared with that of the traditional method.
1. Introduction
2. Experimental
Sintered Nd-Fe-B magnets with outstanding permanent magnetic properties have found a wide range of applications [1]. Sintered permanent magnets are generally produced by a powder metallurgical approach [2], and the magnetic-aligned compaction is an important process for maximizing the magnetization of the magnetic powder to the final higher performance sintered magnets [3]. However, in recent years, magnetic-aligned compaction has become increasingly difficult due to the development of finer and high coercive permanent magnet powders [4], and the orientation degree decreases with reducing powder size [5]. It is still a hard work to reduce the friction between the powder particles and increase the degree of alignment by the PLP technology developed by M. Sagawa et al. [6]. Friction forces could be reduced by addition of liquid lubricant at powder process [7], however, a small amount of lubricant with ester or alkyl organic matter cannot overcome the large friction, while lots of lubricants will be difficult to eliminate at sintering process in traditional preparation method, which will weak remanence and magnetic energy product. It is still necessary to do further research on improving the orientation of the magnet. In this work, we propose a wet pressing method with a special double-layer mold to produce magnets with high degree of alignment. Compared with that of the traditional pressing method, both of the orientation and remanence are improved, meanwhile, the microstructures are optimized. The mechanical properties are discussed by conducting compressive strength measurement.
(Nd,Pr)30.90Mn0.12Al0.16Ga0.11Cu0.12B1.09Febal (wt.%) powders with a mean particle size of 3.0 μm were produced by strip casting, hydrogen decrepitation and jet-milling. Such powders were mixed with organic solvents to make a powder slurry, in which ethanol was the major component of the solvents and the volume ratio of powders to solvents was 1:3. A powder slurry with low viscosity was obtained due to the low solid powder content. The powder slurry was subjected to ultrasonic shock treatment for 10 min and then placed in a special doublelayer mold. The inner mold was a composite material with porous structure, of which the average pore size was less than 2 μm. And the outer mold was a high manganese steel with through-hole structure. Then, the slurry was affected under a magnetic field of 1.8T and a pressure of 25 MPa simultaneously, both of which persisted until there was no significant fluid leakage from the mold. The magnetic field stayed to the end of die pressing and its direction was perpendicular to the pressing direction. The wet pressing process and the characteristic of the mold were schematically shown in Fig. 1a. Subsequently, the green compacts obtained by the above wet pressing process were treated by cold isostatic pressing and then sintered for 2 h at 1070 °C, flowed by a two-step annealing for 2 h at 900 °C and 2 h at 500 °C, respectively. In these processes, they were all protected by a high vacuum following high purity argon. In addition, some contrast samples were prepared by the traditional vertical pressing method with the same powders and heat treatments. Magnet density was calculated using the Archimedes principle.
⁎
Corresponding authors. E-mail addresses: [email protected] (X. Bao), [email protected] (X. Gao).
https://doi.org/10.1016/j.jmmm.2019.165826 Received 5 May 2019; Received in revised form 18 August 2019; Accepted 9 September 2019 Available online 10 September 2019 0304-8853/ © 2019 Published by Elsevier B.V.
Journal of Magnetism and Magnetic Materials 495 (2020) 165826
S. Cao, et al.
Fig. 1. A schematic diagram of the layout and treatment mode for wet pressing method (a), and the laser microscope mapping images of the powder slurry before poured into the mold (b).
and gravity in wet pressing process, which seems difficult to optimize when liquid is introduced. 3D maps in these figures are used to show the results better. The XRD patterns in Fig. 2(a2) and (b2) are similar but showed different relative intensity ratios of the (0 0 6) peak to the (1 0 5) peak, with the values of 1.26 and 3.02 corresponding to the magnets with traditional and wet pressing methods, and the higher value represents the higher degree of alignment [8]. Fig. 3 shows the room temperature demagnetization curves and temperature dependence of magnetic properties for the magnets produced by wet pressing method and traditional method. Both of the magnets have a high squareness factor Hk/Hcj over 93%. The magnet prepared by traditional method possesses a coercivity Hcj of 13.67 kOe and a remanence Br of 12.42 kG. When the wet pressing method is adopted, the magnet has a coercivity Hcj of 13.62 kOe and a remanence Br of 13.34 kG, at which the coercivity reduce a little and the remanence increase significantly compared to that of the traditional method, as shown in Fig. 3a. The higher remanence of magnet is mainly due to the better orientation, and it is in consistent with previous reports [9,10]. And the effect of crystal alignment on remanence follows the theoretical prediction of the Stoner–Wohlfarth model [11]. Fig. 3b depicts the temperature dependence of remanence Br and coercivity Hcj of the magnets produced by these two different methods. The temperature coefficients at 20–60 °C and 20–100 °C for the different magnets are listed in the inset table. It can be seen that the remanence of the magnet with wet pressing method is always much higher than that with the traditional method at any temperature, and the coercivities of the two different magnets are always close. At the low temperature range of 20–60 °C, the values of α and β are −0.11%/°C and −0.77%/°C for the magnet with wet pressing method, while the values of α and β are −0.12%/°C and −0.79%/°C for the magnet with traditional method. There is no significant difference between the values of α and β for the different magnets at the temperature range of 20–100 °C. It seems that the wet pressing method has little effect on the thermal stability of magnets The microstructures of the magnets produced by wet pressing method and traditional method were observed by SEM, as shown in Fig. 4. The measured surface was perpendicular to the direction of caxis. Both samples consist of a bright contrast, which indicates that the RE-rich phase and a dark contrast showing the matrix Nd2Fe14B phase. For the magnet prepared by wet pressing method (Fig. 4a), almost all the matrix phase particles are closely arranged, and some small particles are even squeezed into the triple junction corners (marked by dotted green circles). However, for the magnet prepared by traditional method (Fig. 4b), some matrix particles are not arranged tightly enough, and certain small particles are stuck among other particles and
Dispersion characteristics of powders in solvents were studied by 3D measuring laser microscope (3DLM, Olympus, OLS4100). Alignment analyses were conducted by X-ray diffractometer (XRD, Bruker, D8 Advance), and the X-ray diffraction was done on the pole surface of magnets, which was perpendicular to the direction of the alignment magnetic field (Halign). The magnetic properties were measured by a NIM-2000 magnetic measurement device. Microstructural and compositional analyses were conducted by backscattered electron scanning electron microscopy (SEM, Carl Zeiss, Supra55), electron probe microanalyzer (EPMA, JEOL, JXA-8230) and a carbon-sulphur analyzer. The mechanical properties were measured by a WDW-2000 microcomputer control electronic universal testing machine, and scratch-free final polishing was implemented on the surfaces of specimens before testing. 3. Results and discussion Fig. 1b shows the laser microscope mapping images of the powder slurry before poured into the mold. The inset depicts the image with a larger magnification. The red contrast location indicates the cores of powders in the solvents. It can be seen that the powder particles have excellent dispersion and there is no obvious agglomeration of particles. That means the introduction of liquid solvents does not lead to the deterioration of particle dispersibility. The solid-liquid ratio chosen in this study is 1:3, which is due to the fact that the settling rate of powder particles and slurry viscosity will both increase sharply with higher solid-liquid ratio values. And an ultra-low solid-liquid ratio is unnecessary because the pressing process will be completed in a rather short time. Fig. 2 shows the stereographic projections for (0 0 6) crystal planes and the XRD patterns for the magnets with the traditional method (a12) and the wet pressing methods (b1-2). The X-ray diffraction was done on the pole surfaces perpendicular to the direction of the alignment magnetic field (Halign). It can be seen that both magnets have very strong textures, as shown in Fig. 2(a1) and (b1), and the color indexes increase with the concentrations of (0 0 6) planes, meanwhile, the concentration circle lines are close to the center with the direction of orientation approaches that of Halign. There are some higher-level concentration lines (red lines) distribute at the outer layer of the map in Fig. 2(a1), and it means that some easy axes (c-axis) of Nd2Fe14B grains deviate greatly from the Halign direction. For the map of the magnet prepared with wet pressing method in Fig. 2(b1), the concentration lines are dense and concentrated except for the separation of central circle lines. The little deviation is in a specific direction, which is far different from that in Fig. 2(a1), and it is mainly due to the liquid flow 2
Journal of Magnetism and Magnetic Materials 495 (2020) 165826
S. Cao, et al.
Fig. 2. The stereographic projections for (0 0 6) crystal planes and the XRD patterns for the magnets with the traditional method (a1-2) and the wet pressing methods (b1-2).
rotation when pressing and aligning in wet pressing method, and it just corresponds to the better orientation in Fig. 2. Fig. 5 depicts the EPMA mapping images of Fe, Pr, O and C elements for the magnets prepared by wet pressing method (a1-4) and traditional method (b1-4). The measured surface was perpendicular to the direction of c-axis. The characteristics of elements distribution for the two different methods are similar. The concentration distribution of Fe elements visually demonstrates the distribution of matrix phase grains as shown in (a1) and (b1). The Pr, O and C elements are mainly enriched surrounding the matrix phase grains, while some of them agglomerate in the triple junctions. The measurement results of oxygennitrogen analyzer show that the O elements contents of magnets for both methods are less than 1500 ppm. The C elements contents of magnets are 1100 ppm for the traditional method and 900 ppm for the
prevent them from getting closer (marked by dotted yellow circles). Note that there are also some highly agglomeration areas of matrix phase particles (marked by dotted blue lines) and the grain boundaries are relatively fuzzy, which mainly attribute to poor fluidity of powders in the forming process. The measurements show that the densities of the magnets are 7.60 g/cm3 for the wet pressing method and 7.52 g/cm3 for the traditional method using Archimedes principle. That means the wet pressing method can make the density of magnet closer to the limit one, and it is also one of the reasons that the remanence of magnet increases, in addition to the higher degree of orientation. It also can be seen that some microcracks occur in Fig. 4b, and this is mainly attributed to the large friction between powder particles in the traditional forming process. Thus, the better microstructure in Fig. 4a reveals that the liquid solvents lubricate the powders in the process of particles flow and
Fig. 3. The room temperature demagnetization curves (a) and temperature dependence of magnetic properties (b) for the magnets produced by wet pressing method and traditional method. The inset table shows the temperature coefficients at different temperature range for the magnets. 3
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Fig. 4. SEM images of the magnets produced by wet pressing method (a) and traditional method (b).
Fig. 5. EPMA mapping images for the magnets prepared by wet pressing method (a1-4) and traditional method (b1-4).
wet pressing method according to the measurement results of carbonsulphur analyzer. That means the introduction of organic solvents will not lead to a sharp increase in carbon and oxygen contents. The lower carbon content in the wet pressing method is mainly attributed to that the organic solvents dissolve and take away some quantities of antioxidants and lubricants which were certain esters and introduced at powder process. Compressive strength is used to evaluate the mechanical properties of the magnets in the study, because it can better reflect the overall crack growth behavior and defect characteristic of the brittle materials [12–14]. Compared with bending strength measured by three-point bending test for Nd-Fe-B sintered magnets, the compressive strength tends to emphasize the integrity of mechanical properties, which means that any fragile area or micro-defect of the whole magnet will reduce the compressive strength value. The schematic diagram of compressive strength test and examples of test curves for the magnets prepared by wet pressing and traditional methods are shown in Fig. 6. The tested sample had a dimensional size of Ø2.5 × 6 mm3 and the loading force was applied parallel to the c-axis of the magnet with a loading speed of 0.05 mm/min. Four samples for each magnet were selected to measure the compressive strength values and the measurement results are shown in the inset table. It can be seen that the test curve consists of three sections, and they are the part in which stress increases slowly with displacement, the part in which stress presents linear increase, and the part of stress decreasing. The first part is determined by the experimental conditions, such as sample size, sample shape, loading rate and so on. And the second part means that elastic deformation occurs in the sample, while the third part means that the sample has fractured. The
Fig. 6. The schematic diagram of compressive strength test and examples of test curves for the magnets prepared by the different methods. The inset table shows the measurement results of four samples for each magnet.
ultimate strains refer to the total value of the strain of elastic deformation and they are similar for both of the different magnets. Note that all the compressive strength values of samples from the magnet with wet pressing method are higher than that of traditional method. Fig. 7 shows the SEM fracture morphologies of the fractured specimens after above compressive strength test. It can be seen that the samples 4
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Fig. 7. SEM fracture morphologies of the fractured specimens after compressive strength test. ((a) specimen from the wet pressing method; (b) specimen from the traditional method.)
present the completely brittle fracture characteristic in intergranular way. Some (Nd, Pr)-rich oxides were detected by EDS analysis in both of the samples (marked by yellow arrows), while they are obviously more in the sample from the traditional method. A crack is observed at the grain boundaries in Fig. 7(b) (marked by red arrow). That means there are more defects in the sample from the traditional method, and cracks are easier to generate and propagate compared with that of the wet pressing method. And it just corresponds to the BSE image in Fig. 4(b), in which there are more grain boundary phases in the triple junction corners, and the triple junction regions tends to be damaged firstly when suffering the stress because of the (Nd, Pr)-rich phase and some original defects such as pin-pores and inclusions of oxides formed during the sintering process preferential to aggregate in these regions [15].
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4. Conclusions In summary, the orientation degree of the magnet is greatly improved by the wet pressing method with special double-layer mold. Meanwhile, the remanence and density are enhanced compared with the traditional vertical pressing method. In addition, the compressive strength was conducted to indicate the better mechanical properties. The beneficial effects of this method are mainly attributed to that the liquid solvents lubricate the powders well in the process of particles flow and rotation when pressing and aligning in the wet pressing process. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51401021) and the State Key Laboratory Advanced Metals and Materials (No. 2016Z-14). References [1] K. Hono, H. Sepehri-Amin, Strategy for high-coercivity Nd–Fe–B magnets, Scr. Mater. 67 (2012) 530–535, https://doi.org/10.1016/j.scriptamat.2012.06.038.
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