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Preparation and properties of isotropic Nd-Fe-B bonded magnets with sodium silicate binder
Accepted Manuscript Preparation and properties of isotropic Nd-Fe-B bonded magnets with sodium silicate binder W.Q. Liu, R.J. Hu, M. Yue, Y.X. Yin, D.T. Zhang PII: DOI: Reference:
S0304-8853(17)30809-0 http://dx.doi.org/10.1016/j.jmmm.2017.04.009 MAGMA 62611
To appear in:
Journal of Magnetism and Magnetic Materials
Received Date: Accepted Date:
7 March 2017 5 April 2017
Please cite this article as: W.Q. Liu, R.J. Hu, M. Yue, Y.X. Yin, D.T. Zhang, Preparation and properties of isotropic Nd-Fe-B bonded magnets with sodium silicate binder, Journal of Magnetism and Magnetic Materials (2017), doi: http://dx.doi.org/10.1016/j.jmmm.2017.04.009
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Preparation and properties of isotropic Nd-Fe-B bonded magnets with sodium silicate binder W. Q. Liu, R. J. Hu, M. Yue*, Y. X. Yin, D. T. Zhang College of Materials Science and Engineering, Beijing University of Technology, Beijing, 100124, China
Abstract: In present study, sodium silicate, a kind of heat-resistant binder, was used to prepare bonded Nd-Fe-B magnets with improved thermal stability and mechanical strength. Effect of curing temperature and curing time of the new binder to the magnetic properties, microstructure, and mechanical strength of the magnets was systematically investigated. Fracture surface morphology observation show that sodium silicate in bonded magnets could completely be cured at 175 ℃ for 40 min, and the magnets prepared under this condition exhibit optimal properties. They exhibit usable magnetic properties of Br of 4.66 kGs, Hcj of 4.84 kOe, and (BH)max of 4.06 MGOe at 200 ℃. Moreover, the magnets possess high compressive strength of 63 MPa.
Keywords: sodium silicate; bonded magnets; magnetic properties; thermal stability; mechanical strength
Corresponding author:
Tel.: +86-10-67391760 E-mail addresses: [email protected]
1. Introduction Nd-Fe-B-based permanent magnets with excellent magnetic properties are usually fabricated by sintering or bonding methods. However, sintering process is complex and the cost is expensive [1]. On the contrary, bonded magnets, thought with lower magnetic properties than those of sintered one, possess many advantages such as low
weight, complex product shapes, and high dimensional accuracy [2]. Therefore, bonded Nd-Fe-B magnets have attracted much attention [3] and been applied widely in dc motors for automobile and other industries [4]. The bonded magnet fabrication process generally involves mixing magnet powders with a polymer and produces various shapes and sizes of magnets through injection molding, calendaring, roll molding, compaction molding and extrusion molding [5-10]. Recently, a three dimensional printing process to compete with conventional injection molding techniques was used to fabricate near-net-shape isotropic Nd-Fe-B based bonded magnets, which provides a new pathway for preparing near-net-shape bonded magnets for various magnetic applications [11]. Among above technologies, compaction molding is most popular due to the fact that the magnetic loading can be as high as 80% by volume ratio, resulting in higher output than that of other techniques. Modern automobiles use many permanent magnets in various electromagnetic components (motors, sensors, actuators, etc.). One of the most essential issues of the magnets for automobiles is thermal stability of the bonded Nd-Fe-B magnets. The composition of the magnetic powders in the bonded magnets plays a critical role in determining the magnetic properties at room and high temperature. By tuning the composition modification of magnetic powders, the thermal stability of the bonded Nd-Fe-B magnets could be improved. Liu et al. investigated that the addition of Dy can substantially improve the thermal stability by decreasing both the temperature coefficients of the remanence and the coercivity [12]. Zhang et al. reported that the Nb substitution can enhance the coercivity and thermal stability [13]. Zhang et al. improved the coercivity temperature coefficient of bonded Nd-Fe-B magnets by doping MnBi powders, which has the positive coercivity temperature coefficient [14]. More importantly, polymer binder is also an important limitation to high temperature application for the bonded Nd-Fe-B magnets. Epoxy resin is a commonly used polymer binders for the bonded Nd-Fe-B magnets [15]. Due to the active epoxy groups in molecular, epoxy resin can react easily to form a kind of three dimensional networks. The three dimensional networks provide the bonded Nd-Fe-B magnets with
perfect mechanical strength and magnetic properties. However, the epoxy resin bonded magnets is restricted for higher operation temperatures due to the poor temperature resistance of epoxy resin. In order to improve working temperature of bonded Nd-Fe-B magnets, heat-resistant binder has been paid more attentions. Many investigations demonstrating that the temperature endurance of bonded magnets mixed with new heat-resistant binders such as polyamide-12 [16], polycarbonate [17], nylon 66 [18], polyamide-6 [19], and polyphenylene-sulfide (PPS) [20] were highlighted. For example, Nylon has the melting temperature characteristics of 170 ℃ [21]. Unlike Nylon, the PPS exhibits a higher melting temperature of approximately 280 ℃ [22]. The differences in polymer structure, molecular weight, and melting temperature enable PPS-bonded magnets to be used for higher temperatures. However, PPS-bonded magnets exhibit lowest magnetic properties among ethylene vinyl acetate (EVA),
high
density
polyethylene
(HDPE),
polypropylene
(PP),
polyphenylene-sulfide (PPS), polyamide-6 (PA6) and polyamide-1010 (PA1010) bonded magnets [23]. Xiao et al. showed that the thermal stability of the Nd-Fe-B alloy powders by the treatment of silane coupling agent was improved significantly [24]. Sodium silicate, as a kind of environmentally friendly binder, has good temperature resistance and a wide application in fire proof materials. For example, it can be used as wood adhesive, and shows good temperature stability between 200 to 800 ℃ [25]. However, application of sodium silicate as a binder for bonded Nd-Fe-B magnets was seldom reported. Yin et al. compared the sodium silicate and epoxy resin bonded Nd-Fe-B magnets, and results showed that the sodium silicate bonded Nd-Fe-B magnets exhibited better comprehensive properties [26]. However, effect of curing temperature and curing time on the magnetic properties, microstructure and mechanical strength of the sodium silicate bonded Nd-Fe-B magnets is not clear yet. Especially, the morphology of sodium silicate after curing in the bonded magnets need to explore, so as to know how it works on the bonded magnets. In this work, bonded Nd-Fe-B magnets were prepared with MQP-B powders and sodium silicate as the binder. Effects of curing temperature and curing time on
magnetic properties, microstructure, and mechanical strength of the bonded magnets were systematically investigated.
2. Experiments MQP-B Nd-Fe-B powders with sodium silicate (modulus of 3.1-3.4, Baume degree of 40o±1) as the binder were selected to prepare bonded Nd-Fe-B magnets. First, sodium silicate was ultrasonic oscillated for 10 minutes. The Nd-Fe-B powders were mixed with 5 wt. % sodium silicate and stirred. Subsequently, the mixed powders were moulded and densified with isostatic pressing. Finally, the magnets were obtained by curing the green compacts at a certain temperature for a certain time. Table 1 shows the experimental details of the magnets preparation conditions.
Table 1 Experimental details of preparation condition for bonded magnets Sample
Curing temperature (℃)
Curing time (min)
1#
125
40
2#
150
40
3#
175
40
4#
200
40
5#
225
40
6#
250
40
7#
175
20
8#
175
60
9#
175
80
10#
175
100
The magnetic properties of the sodium silicate bonded Nd-Fe-B magnets at 20 ℃ and 200 ℃ were tested by NIM-500C B-H hysteresis loop tracer. The compressive strength at room temperature was measured by Zwick050 electronic tensile testing
machine referring to the GB/T7314 method. The structure and chemical bond of the sodium silicate were analyzed by Fourier Transform Infrared Spectroscopy (FTIR, Nicolet 5700). The spectral range varied from 3500 to 400 cm-1. FTIR study identifies the specific functional groups. The fracture surface observation and composition analysis for crushed magnets was performed on the NOVA nano200 Scanning Electron Microscope (SEM) at 10 kV with the Energy Dispersive X-Ray (EDX).
3. Results and discussions
3.1 Magnetic properties at room temperature
Magnetic properties of the sodium silicate bonded Nd-Fe-B magnets as function of curing temperature and curing time are shown in Fig. 1. Fig.1(a) shows that the magnetic properties (Br, Hcj, (BH)max) of the bonded magnets decreases with increased curing temperature with curing time of 40 min. The Br of the bonded magnets cured at 125, 150 and 175 ℃ are similar, which are better than those of the bonded magnets cured at 200, 225 and 250℃. The Hcj of the bonded Nd-Fe-B magnets decreases continuously with increased curing temperature. At curing temperature of 175 ℃, the Br of the magnets has little change with curing time. However, the Hcj and (BH)max of the magnets decrease continuously with increased curing time, as shown in Fig. 1(b). During curing, sodium silicate on the surface of the magnetic powders forms a three-dimensional Si-O-Si structure, which has the heat resistance and strength as shown in formula (1) and (2) [27]. The curing degree of sodium silicate enhances with curing temperature. Meanwhile, the volumetric shrinkage ratio fills out, indicating the increase of the density of the bonded magnets. In addition, the Si-O-Si structure becomes more and more stable. These reasons lead to the high magnetic values. On the other hand, the bonded Nd-Fe-B magnets are cured in air; the surface of magnetic powders would be oxidized [28].The reaction between magnetic powders and oxygen
could occur easily at higher curing temperature, especially above 175 ℃. Thus, the higher the curing temperature, the more serious the oxidation would be [29]. The oxygen content of the magnets cured at 175 ℃ and 200℃ were measured by EDX as 13.53 wt. % to 15.41 wt. %, resulting in the decreased magnetic properties of the magnets with increased curing temperature. Similarly, with increased curing time, sodium silicate also occurs above reactions. The aging phenomenon and oxide etching of magnetic powders become more serious with increased curing time.
Fig.1. Magnetic properties of the sodium silicate bonded Nd-Fe-B magnets as function of curing temperature (a) and time (b)
Na2 O nSiO2 2n 1H 2 O 2 NaOH nSiOH 4
nSiOH 4
| | Si O Si | | 2 nH2 O Si OH 4 n O O | | Si O Si | |
(1)
(2)
3.2 Structure analyses
In order to verify the relation between properties and microstructure of the sodium silicate bonded Nd-Fe-B magnets, fracture surface morphologies of the bonded Nd-Fe-B magnets cured at 125, 175 and 250 ℃ for 40 min are investigated and shown in Fig. 2. A layered structure exists in the bonded magnets cured at 125 ℃, as shown in Fig. 2(a). The pull-out phenomenon [30] of magnetic powders is responsible for the concave and convex surface and for the bare magnetic powders. These results suggest that sodium silicate is not completely cured and the binding effect is not strong. Fig. 2(b) shows enhanced merger between magnetic powders. As a result, the amount of bare magnetic powders reduces, indicating improved degree of curing. In this case, the binding effect becomes stronger. Fig. 2(c) exhibits that the unification between magnetic powders is much improved, suggesting good curing result under 250 ℃. However，the too high curing temperature may result in brittleness for sodium silicate, which contributes to the weak mechanical properties of sodium silicate bonded magnets.
Fig.2. Fracture morphology of the sodium silicate bonded Nd-Fe-B magnets cured at different temperature for 40 min: (a) 125 ℃, (b) 175 ℃, (c) 250 ℃
Fig. 3 shows that fracture surface morphology of the bonded Nd-Fe-B magnets cured at 175℃ for different time. It shows that the amount of cracks decreases with increased curing time. The pull-out phenomenon of magnetic powders results in much cracks and debris of magnetic powders and binder (Fig. 3(a)), suggesting incomplete curing. Fig. 3(b) and (c) shows that the cracks and debris of magnetic powders and
binder decrease significantly and the surface are smooth. It demonstrates that the degree of curing of sodium silicate increases.
Fig.3. Fracture morphology of the sodium silicate bonded Nd-Fe-B magnets cured at 175 ℃ for different time: (a) 20 min, (b) 40 min, (c) 80 min
To clarify the distribution of the sodium silicate in the bonded Nd-Fe-B magnets, the microstructures as well as the concentration distribution of O, Si, Nd, and Fe elements going through the two powders are examined, and the result is shown in Fig. 4. It is found that the sodium silicate transfers into filament after curing, and locates in the powders boundary and surface of the powders, indicating successful manufacturing of the silicate bonded Nd-Fe-B magnets.
Fig.4. SEM micrograph of the sodium silicate bonded Nd-Fe-B magnets (a) and the EDX results of concentration distribution of O, Si, Nd and Fe (b-e)
In order to explore the structure of sodium silicate after curing, the FTIR absorption spectral curves in 3500-400 cm-1 region of sodium silicate bonded Nd-Fe-B magnets are illustrated in Fig. 5. The peak position of about 1080 cm-1 is assigned to Si-O-Si asymmetric stretching (as-s), and peaks around 800 cm-1 and 460 cm-1corresponds to Si-O-Si symmetric stretching (s-s) and Si-O-Si bending vibration (b), respectively [31]. The FTIR results demonstrate the presence of Si-O-Si bonds, which have excellent heat resistance and strength.
Fig.5. FTIR spectra of the sodium silicate bonded Nd-Fe-B magnets
3.3 Compressive strength
Compressive strength is another important factor for the application of the bonded Nd-Fe-B magnets. Fig. 6(a) shows the compressive strength of sodium silicate bonded Nd-Fe-B magnets cured at different temperature for 40 min. The compressive strength increases at first and then decreases with increased curing temperature. Magnets cured at 175 ℃ have the highest compressive strength of 63MPa, which is bigger than that of epoxy resin bonded magnets (27 MPa) [26]. As shown in Fig.6(b), when curing temperature is 175 ℃, the compressive strength of the sodium silicate bonded Nd-Fe-B magnets increases with the extension of curing time before 40 min, but it falls off sharply with longer curing time. The surfaces of magnetic powders are expected to be covered with a thin layer of water film during the mixing process between sodium silicate and magnetic powders. Adjacent magnetic powders are connected through the film adhesive. With the increase of curing temperature and curing time, a kind of bonding bridge is formed while internal stress is eliminated [32]. In addition, the curing degree of sodium silicate increases. As a result, the Si-O-Si structure becomes more and more stable and the compressive strength of the magnets
increases. However, when the curing temperature is above 175 ℃ and the curing time is more than 40 min, the brittleness of sodium silicate becomes serious, leading to the decrease of strength.
Fig.6. Compressive strength of the sodium silicate bonded Nd-Fe-B magnets cured at different temperature for 40 min (a) and cured at 175 ℃ for different time (b)
3.4 Magnetic properties at 200℃
Fig. 7 shows the demagnetization curves of optimal sodium silicate bonded Nd-Fe-B magnets measured under 20℃ and 200℃. It can be found that the magnets still has usable magnetic properties with Br of 4.656 kGs, Hcj of 4.844 kOe, and (BH)max of 4.057 MGOe at 200 ℃. The three-dimensional Si-O-Si structure formed in the curing process has excellent heat resistance and strength, ensuring that the bonded magnets have a certain shape and usable magnetic properties at 200℃. It is therefore, concluded that sodium silicate enables bonded Nd-Fe-B magnets to be used for higher operation temperatures. However, the epoxy resin bonded magnets could not keep their shape due to the brittleness of epoxy resin when measured at 200 ℃ by B-H hysteresis loop tracer. Therefore, the magnetic properties of epoxy resin bonded magnets could not obtain. In other words, epoxy resin bonded Nd-Fe-B magnets are not applicable at high temperature of 200 ℃.
Fig.7. Demagnetization curves of the sodium silicate bonded Nd-Fe-B magnets at 20 ℃ and 200 ℃
4. Conclusion The bonded Nd-Fe-B magnets have been fabricated with MQP-B type magnetic powders and sodium silicate as the binder by compaction molding. The effects of curing temperature and curing time on magnetic properties, microstructure, and compressive strength of the sodium silicate bonded Nd-Fe-B magnets were investigated. With increase of curing temperature and curing time, combination between magnetic powders and sodium silicate binder became tighter. Meanwhile, the Si-O-Si structure becomes stronger and more crumbly. However, as the temperature and the time further increasing, the surface of magnetic powders was seriously oxidized. The reaction between magnetic powders and oxygen etching could occur easily at higher curing temperature or time. As a result, the sodium silicate bonded Nd-Fe-B magnets cured at 175 ℃ for 40 min process the optimized comprehensive properties, and exhibit usable magnetic properties with Br of 4.656 kGs, Hcj of 4.844 kOe, and (BH)max of 4.057 MGOe at 200 ℃.
Acknowledgements This work was supported by National Natural Science Foundation of China (51371002, 51001002, and 51331003), the National High Technology Research and Development Program of China (2012AA063201), International S&T Cooperation Program of China (2015DFG52020), the National Key Research and Development Program of China (2016YFB0700902), the 2011 Cooperative Innovation Center of Beijing University of Technology.
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Figure captions Fig.1. Magnetic properties of the sodium silicate bonded Nd-Fe-B magnets as function of curing temperature (a) and time (b) Fig.2. Fracture morphology of the sodium silicate bonded Nd-Fe-B magnets cured at different temperature for 40 min: (a) 125 ℃, (b) 175 ℃, (c) 250 ℃ Fig.3. Fracture morphology of the sodium silicate bonded Nd-Fe-B magnets cured at 175 ℃ for different time: (a) 20 min, (b) 40 min, (c) 80 min Fig.4. SEM micrograph of the sodium silicate bonded Nd-Fe-B magnets (a) and the EDX results of concentration distribution of O, Si, Nd and Fe (b-e) Fig.5. FTIR spectra of the sodium silicate bonded Nd-Fe-B magnets Fig.6. Compressive strength of the sodium silicate bonded Nd-Fe-B magnets cured at different temperature for 40 min (a) and cured at 175 ℃ for different time (b) Fig.7. Demagnetization curves of the sodium silicate bonded Nd-Fe-B magnets at 20 ℃ and 200 ℃
Table captions Table 1. Experimental details of preparation condition for the bonded magnets.
To improve the working temperature of bonded Nd-Fe-B magnets, the heat-resistant binder, sodium silicate, was used to prepare new type bonded Nd-Fe-B magnets. The three-dimensional Si-O-Si structure formed in the curing process has excellent strength; it can ensure that the bonded magnets have a certain shape and usable magnetic properties when working at 200 ℃.
Highlights Dear reviewers, I am very pleased to summarize the highlights of this manuscript, and I really hope that the manuscript can meet the high quality of Journal of Magnetism and Magnetic Materials. The followings are the highlights:
1. Sodium silicate enables bonded Nd-Fe-B magnets to be used for higher operation temperatures. 2. The sodium silicate bonded magnets exhibit usable maximum energy product of 4.057 MGOe at 200 ℃. 3. The compressive strength of sodium silicate bonded magnets is twice bigger than that of epoxy resin bonded magnets. Thank you very much for your consideration, and I am looking forward to your comments and suggestions! Sincerely yours, Ming Yue and co-authors 07 Mar., 2017
S0304-8853(17)30809-0 http://dx.doi.org/10.1016/j.jmmm.2017.04.009 MAGMA 62611
To appear in:
Journal of Magnetism and Magnetic Materials
Received Date: Accepted Date:
7 March 2017 5 April 2017
Please cite this article as: W.Q. Liu, R.J. Hu, M. Yue, Y.X. Yin, D.T. Zhang, Preparation and properties of isotropic Nd-Fe-B bonded magnets with sodium silicate binder, Journal of Magnetism and Magnetic Materials (2017), doi: http://dx.doi.org/10.1016/j.jmmm.2017.04.009
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Preparation and properties of isotropic Nd-Fe-B bonded magnets with sodium silicate binder W. Q. Liu, R. J. Hu, M. Yue*, Y. X. Yin, D. T. Zhang College of Materials Science and Engineering, Beijing University of Technology, Beijing, 100124, China
Abstract: In present study, sodium silicate, a kind of heat-resistant binder, was used to prepare bonded Nd-Fe-B magnets with improved thermal stability and mechanical strength. Effect of curing temperature and curing time of the new binder to the magnetic properties, microstructure, and mechanical strength of the magnets was systematically investigated. Fracture surface morphology observation show that sodium silicate in bonded magnets could completely be cured at 175 ℃ for 40 min, and the magnets prepared under this condition exhibit optimal properties. They exhibit usable magnetic properties of Br of 4.66 kGs, Hcj of 4.84 kOe, and (BH)max of 4.06 MGOe at 200 ℃. Moreover, the magnets possess high compressive strength of 63 MPa.
Keywords: sodium silicate; bonded magnets; magnetic properties; thermal stability; mechanical strength
Corresponding author:
Tel.: +86-10-67391760 E-mail addresses: [email protected]
1. Introduction Nd-Fe-B-based permanent magnets with excellent magnetic properties are usually fabricated by sintering or bonding methods. However, sintering process is complex and the cost is expensive [1]. On the contrary, bonded magnets, thought with lower magnetic properties than those of sintered one, possess many advantages such as low
weight, complex product shapes, and high dimensional accuracy [2]. Therefore, bonded Nd-Fe-B magnets have attracted much attention [3] and been applied widely in dc motors for automobile and other industries [4]. The bonded magnet fabrication process generally involves mixing magnet powders with a polymer and produces various shapes and sizes of magnets through injection molding, calendaring, roll molding, compaction molding and extrusion molding [5-10]. Recently, a three dimensional printing process to compete with conventional injection molding techniques was used to fabricate near-net-shape isotropic Nd-Fe-B based bonded magnets, which provides a new pathway for preparing near-net-shape bonded magnets for various magnetic applications [11]. Among above technologies, compaction molding is most popular due to the fact that the magnetic loading can be as high as 80% by volume ratio, resulting in higher output than that of other techniques. Modern automobiles use many permanent magnets in various electromagnetic components (motors, sensors, actuators, etc.). One of the most essential issues of the magnets for automobiles is thermal stability of the bonded Nd-Fe-B magnets. The composition of the magnetic powders in the bonded magnets plays a critical role in determining the magnetic properties at room and high temperature. By tuning the composition modification of magnetic powders, the thermal stability of the bonded Nd-Fe-B magnets could be improved. Liu et al. investigated that the addition of Dy can substantially improve the thermal stability by decreasing both the temperature coefficients of the remanence and the coercivity [12]. Zhang et al. reported that the Nb substitution can enhance the coercivity and thermal stability [13]. Zhang et al. improved the coercivity temperature coefficient of bonded Nd-Fe-B magnets by doping MnBi powders, which has the positive coercivity temperature coefficient [14]. More importantly, polymer binder is also an important limitation to high temperature application for the bonded Nd-Fe-B magnets. Epoxy resin is a commonly used polymer binders for the bonded Nd-Fe-B magnets [15]. Due to the active epoxy groups in molecular, epoxy resin can react easily to form a kind of three dimensional networks. The three dimensional networks provide the bonded Nd-Fe-B magnets with
perfect mechanical strength and magnetic properties. However, the epoxy resin bonded magnets is restricted for higher operation temperatures due to the poor temperature resistance of epoxy resin. In order to improve working temperature of bonded Nd-Fe-B magnets, heat-resistant binder has been paid more attentions. Many investigations demonstrating that the temperature endurance of bonded magnets mixed with new heat-resistant binders such as polyamide-12 [16], polycarbonate [17], nylon 66 [18], polyamide-6 [19], and polyphenylene-sulfide (PPS) [20] were highlighted. For example, Nylon has the melting temperature characteristics of 170 ℃ [21]. Unlike Nylon, the PPS exhibits a higher melting temperature of approximately 280 ℃ [22]. The differences in polymer structure, molecular weight, and melting temperature enable PPS-bonded magnets to be used for higher temperatures. However, PPS-bonded magnets exhibit lowest magnetic properties among ethylene vinyl acetate (EVA),
high
density
polyethylene
(HDPE),
polypropylene
(PP),
polyphenylene-sulfide (PPS), polyamide-6 (PA6) and polyamide-1010 (PA1010) bonded magnets [23]. Xiao et al. showed that the thermal stability of the Nd-Fe-B alloy powders by the treatment of silane coupling agent was improved significantly [24]. Sodium silicate, as a kind of environmentally friendly binder, has good temperature resistance and a wide application in fire proof materials. For example, it can be used as wood adhesive, and shows good temperature stability between 200 to 800 ℃ [25]. However, application of sodium silicate as a binder for bonded Nd-Fe-B magnets was seldom reported. Yin et al. compared the sodium silicate and epoxy resin bonded Nd-Fe-B magnets, and results showed that the sodium silicate bonded Nd-Fe-B magnets exhibited better comprehensive properties [26]. However, effect of curing temperature and curing time on the magnetic properties, microstructure and mechanical strength of the sodium silicate bonded Nd-Fe-B magnets is not clear yet. Especially, the morphology of sodium silicate after curing in the bonded magnets need to explore, so as to know how it works on the bonded magnets. In this work, bonded Nd-Fe-B magnets were prepared with MQP-B powders and sodium silicate as the binder. Effects of curing temperature and curing time on
magnetic properties, microstructure, and mechanical strength of the bonded magnets were systematically investigated.
2. Experiments MQP-B Nd-Fe-B powders with sodium silicate (modulus of 3.1-3.4, Baume degree of 40o±1) as the binder were selected to prepare bonded Nd-Fe-B magnets. First, sodium silicate was ultrasonic oscillated for 10 minutes. The Nd-Fe-B powders were mixed with 5 wt. % sodium silicate and stirred. Subsequently, the mixed powders were moulded and densified with isostatic pressing. Finally, the magnets were obtained by curing the green compacts at a certain temperature for a certain time. Table 1 shows the experimental details of the magnets preparation conditions.
Table 1 Experimental details of preparation condition for bonded magnets Sample
Curing temperature (℃)
Curing time (min)
1#
125
40
2#
150
40
3#
175
40
4#
200
40
5#
225
40
6#
250
40
7#
175
20
8#
175
60
9#
175
80
10#
175
100
The magnetic properties of the sodium silicate bonded Nd-Fe-B magnets at 20 ℃ and 200 ℃ were tested by NIM-500C B-H hysteresis loop tracer. The compressive strength at room temperature was measured by Zwick050 electronic tensile testing
machine referring to the GB/T7314 method. The structure and chemical bond of the sodium silicate were analyzed by Fourier Transform Infrared Spectroscopy (FTIR, Nicolet 5700). The spectral range varied from 3500 to 400 cm-1. FTIR study identifies the specific functional groups. The fracture surface observation and composition analysis for crushed magnets was performed on the NOVA nano200 Scanning Electron Microscope (SEM) at 10 kV with the Energy Dispersive X-Ray (EDX).
3. Results and discussions
3.1 Magnetic properties at room temperature
Magnetic properties of the sodium silicate bonded Nd-Fe-B magnets as function of curing temperature and curing time are shown in Fig. 1. Fig.1(a) shows that the magnetic properties (Br, Hcj, (BH)max) of the bonded magnets decreases with increased curing temperature with curing time of 40 min. The Br of the bonded magnets cured at 125, 150 and 175 ℃ are similar, which are better than those of the bonded magnets cured at 200, 225 and 250℃. The Hcj of the bonded Nd-Fe-B magnets decreases continuously with increased curing temperature. At curing temperature of 175 ℃, the Br of the magnets has little change with curing time. However, the Hcj and (BH)max of the magnets decrease continuously with increased curing time, as shown in Fig. 1(b). During curing, sodium silicate on the surface of the magnetic powders forms a three-dimensional Si-O-Si structure, which has the heat resistance and strength as shown in formula (1) and (2) [27]. The curing degree of sodium silicate enhances with curing temperature. Meanwhile, the volumetric shrinkage ratio fills out, indicating the increase of the density of the bonded magnets. In addition, the Si-O-Si structure becomes more and more stable. These reasons lead to the high magnetic values. On the other hand, the bonded Nd-Fe-B magnets are cured in air; the surface of magnetic powders would be oxidized [28].The reaction between magnetic powders and oxygen
could occur easily at higher curing temperature, especially above 175 ℃. Thus, the higher the curing temperature, the more serious the oxidation would be [29]. The oxygen content of the magnets cured at 175 ℃ and 200℃ were measured by EDX as 13.53 wt. % to 15.41 wt. %, resulting in the decreased magnetic properties of the magnets with increased curing temperature. Similarly, with increased curing time, sodium silicate also occurs above reactions. The aging phenomenon and oxide etching of magnetic powders become more serious with increased curing time.
Fig.1. Magnetic properties of the sodium silicate bonded Nd-Fe-B magnets as function of curing temperature (a) and time (b)
Na2 O nSiO2 2n 1H 2 O 2 NaOH nSiOH 4
nSiOH 4
| | Si O Si | | 2 nH2 O Si OH 4 n O O | | Si O Si | |
(1)
(2)
3.2 Structure analyses
In order to verify the relation between properties and microstructure of the sodium silicate bonded Nd-Fe-B magnets, fracture surface morphologies of the bonded Nd-Fe-B magnets cured at 125, 175 and 250 ℃ for 40 min are investigated and shown in Fig. 2. A layered structure exists in the bonded magnets cured at 125 ℃, as shown in Fig. 2(a). The pull-out phenomenon [30] of magnetic powders is responsible for the concave and convex surface and for the bare magnetic powders. These results suggest that sodium silicate is not completely cured and the binding effect is not strong. Fig. 2(b) shows enhanced merger between magnetic powders. As a result, the amount of bare magnetic powders reduces, indicating improved degree of curing. In this case, the binding effect becomes stronger. Fig. 2(c) exhibits that the unification between magnetic powders is much improved, suggesting good curing result under 250 ℃. However，the too high curing temperature may result in brittleness for sodium silicate, which contributes to the weak mechanical properties of sodium silicate bonded magnets.
Fig.2. Fracture morphology of the sodium silicate bonded Nd-Fe-B magnets cured at different temperature for 40 min: (a) 125 ℃, (b) 175 ℃, (c) 250 ℃
Fig. 3 shows that fracture surface morphology of the bonded Nd-Fe-B magnets cured at 175℃ for different time. It shows that the amount of cracks decreases with increased curing time. The pull-out phenomenon of magnetic powders results in much cracks and debris of magnetic powders and binder (Fig. 3(a)), suggesting incomplete curing. Fig. 3(b) and (c) shows that the cracks and debris of magnetic powders and
binder decrease significantly and the surface are smooth. It demonstrates that the degree of curing of sodium silicate increases.
Fig.3. Fracture morphology of the sodium silicate bonded Nd-Fe-B magnets cured at 175 ℃ for different time: (a) 20 min, (b) 40 min, (c) 80 min
To clarify the distribution of the sodium silicate in the bonded Nd-Fe-B magnets, the microstructures as well as the concentration distribution of O, Si, Nd, and Fe elements going through the two powders are examined, and the result is shown in Fig. 4. It is found that the sodium silicate transfers into filament after curing, and locates in the powders boundary and surface of the powders, indicating successful manufacturing of the silicate bonded Nd-Fe-B magnets.
Fig.4. SEM micrograph of the sodium silicate bonded Nd-Fe-B magnets (a) and the EDX results of concentration distribution of O, Si, Nd and Fe (b-e)
In order to explore the structure of sodium silicate after curing, the FTIR absorption spectral curves in 3500-400 cm-1 region of sodium silicate bonded Nd-Fe-B magnets are illustrated in Fig. 5. The peak position of about 1080 cm-1 is assigned to Si-O-Si asymmetric stretching (as-s), and peaks around 800 cm-1 and 460 cm-1corresponds to Si-O-Si symmetric stretching (s-s) and Si-O-Si bending vibration (b), respectively [31]. The FTIR results demonstrate the presence of Si-O-Si bonds, which have excellent heat resistance and strength.
Fig.5. FTIR spectra of the sodium silicate bonded Nd-Fe-B magnets
3.3 Compressive strength
Compressive strength is another important factor for the application of the bonded Nd-Fe-B magnets. Fig. 6(a) shows the compressive strength of sodium silicate bonded Nd-Fe-B magnets cured at different temperature for 40 min. The compressive strength increases at first and then decreases with increased curing temperature. Magnets cured at 175 ℃ have the highest compressive strength of 63MPa, which is bigger than that of epoxy resin bonded magnets (27 MPa) [26]. As shown in Fig.6(b), when curing temperature is 175 ℃, the compressive strength of the sodium silicate bonded Nd-Fe-B magnets increases with the extension of curing time before 40 min, but it falls off sharply with longer curing time. The surfaces of magnetic powders are expected to be covered with a thin layer of water film during the mixing process between sodium silicate and magnetic powders. Adjacent magnetic powders are connected through the film adhesive. With the increase of curing temperature and curing time, a kind of bonding bridge is formed while internal stress is eliminated [32]. In addition, the curing degree of sodium silicate increases. As a result, the Si-O-Si structure becomes more and more stable and the compressive strength of the magnets
increases. However, when the curing temperature is above 175 ℃ and the curing time is more than 40 min, the brittleness of sodium silicate becomes serious, leading to the decrease of strength.
Fig.6. Compressive strength of the sodium silicate bonded Nd-Fe-B magnets cured at different temperature for 40 min (a) and cured at 175 ℃ for different time (b)
3.4 Magnetic properties at 200℃
Fig. 7 shows the demagnetization curves of optimal sodium silicate bonded Nd-Fe-B magnets measured under 20℃ and 200℃. It can be found that the magnets still has usable magnetic properties with Br of 4.656 kGs, Hcj of 4.844 kOe, and (BH)max of 4.057 MGOe at 200 ℃. The three-dimensional Si-O-Si structure formed in the curing process has excellent heat resistance and strength, ensuring that the bonded magnets have a certain shape and usable magnetic properties at 200℃. It is therefore, concluded that sodium silicate enables bonded Nd-Fe-B magnets to be used for higher operation temperatures. However, the epoxy resin bonded magnets could not keep their shape due to the brittleness of epoxy resin when measured at 200 ℃ by B-H hysteresis loop tracer. Therefore, the magnetic properties of epoxy resin bonded magnets could not obtain. In other words, epoxy resin bonded Nd-Fe-B magnets are not applicable at high temperature of 200 ℃.
Fig.7. Demagnetization curves of the sodium silicate bonded Nd-Fe-B magnets at 20 ℃ and 200 ℃
4. Conclusion The bonded Nd-Fe-B magnets have been fabricated with MQP-B type magnetic powders and sodium silicate as the binder by compaction molding. The effects of curing temperature and curing time on magnetic properties, microstructure, and compressive strength of the sodium silicate bonded Nd-Fe-B magnets were investigated. With increase of curing temperature and curing time, combination between magnetic powders and sodium silicate binder became tighter. Meanwhile, the Si-O-Si structure becomes stronger and more crumbly. However, as the temperature and the time further increasing, the surface of magnetic powders was seriously oxidized. The reaction between magnetic powders and oxygen etching could occur easily at higher curing temperature or time. As a result, the sodium silicate bonded Nd-Fe-B magnets cured at 175 ℃ for 40 min process the optimized comprehensive properties, and exhibit usable magnetic properties with Br of 4.656 kGs, Hcj of 4.844 kOe, and (BH)max of 4.057 MGOe at 200 ℃.
Acknowledgements This work was supported by National Natural Science Foundation of China (51371002, 51001002, and 51331003), the National High Technology Research and Development Program of China (2012AA063201), International S&T Cooperation Program of China (2015DFG52020), the National Key Research and Development Program of China (2016YFB0700902), the 2011 Cooperative Innovation Center of Beijing University of Technology.
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Figure captions Fig.1. Magnetic properties of the sodium silicate bonded Nd-Fe-B magnets as function of curing temperature (a) and time (b) Fig.2. Fracture morphology of the sodium silicate bonded Nd-Fe-B magnets cured at different temperature for 40 min: (a) 125 ℃, (b) 175 ℃, (c) 250 ℃ Fig.3. Fracture morphology of the sodium silicate bonded Nd-Fe-B magnets cured at 175 ℃ for different time: (a) 20 min, (b) 40 min, (c) 80 min Fig.4. SEM micrograph of the sodium silicate bonded Nd-Fe-B magnets (a) and the EDX results of concentration distribution of O, Si, Nd and Fe (b-e) Fig.5. FTIR spectra of the sodium silicate bonded Nd-Fe-B magnets Fig.6. Compressive strength of the sodium silicate bonded Nd-Fe-B magnets cured at different temperature for 40 min (a) and cured at 175 ℃ for different time (b) Fig.7. Demagnetization curves of the sodium silicate bonded Nd-Fe-B magnets at 20 ℃ and 200 ℃
Table captions Table 1. Experimental details of preparation condition for the bonded magnets.
To improve the working temperature of bonded Nd-Fe-B magnets, the heat-resistant binder, sodium silicate, was used to prepare new type bonded Nd-Fe-B magnets. The three-dimensional Si-O-Si structure formed in the curing process has excellent strength; it can ensure that the bonded magnets have a certain shape and usable magnetic properties when working at 200 ℃.
Highlights Dear reviewers, I am very pleased to summarize the highlights of this manuscript, and I really hope that the manuscript can meet the high quality of Journal of Magnetism and Magnetic Materials. The followings are the highlights:
1. Sodium silicate enables bonded Nd-Fe-B magnets to be used for higher operation temperatures. 2. The sodium silicate bonded magnets exhibit usable maximum energy product of 4.057 MGOe at 200 ℃. 3. The compressive strength of sodium silicate bonded magnets is twice bigger than that of epoxy resin bonded magnets. Thank you very much for your consideration, and I am looking forward to your comments and suggestions! Sincerely yours, Ming Yue and co-authors 07 Mar., 2017