Effect of particle size on properties of injection-molded bonded NdFeB magnet

Key words: bonded magnet; NdFeB; injection molding; particle size

Although injection molded bonded NdFeB magnets and magnetic properties as molded bonded NdFeB sintered magnets, but it has high dimensional accuracy can be made into complex shapes and thin components, heat resistance, good processing performance, high efficiency, materials high utilization, low cost, magnetic circuit design room for large, integrated with other components forming a series of advantages, to meet the magnetic and electronic components to the "small, light, thin, intelligent and high performance "the direction of trends, so in the electronic information, automobiles, audio systems, control systems, instrumentation, office automation, home appliances and other industrial areas show a good prospect, and its annual growth rate of 40% [1-s133]. Injection bonded magnet properties of the factors is an important factor, it has magnetic properties and processing of a greater impact. We study the quenched NdFeB magnetic powder particle size on bulk density and the fluidity and performance of bonded magnets, and by getting the right size to adjust the particle size distribution, in order to improve the magnetic properties of molded bonded magnet. Steyaert et al [9] studied the oxidation of different size behavior of NdFeB magnetic powder, that the finer granularity of the more easily oxidized, not conducive to the preparation of high performance NdFeB magnets. Xiao et al [10] have also been similar results, and confirmed that a smaller particle size to change the surface properties of NdFeB magnetic powder bonded magnets greatly improved magnetic properties and corrosion resistance. We study the magnetic particle size, respectively, injection molding and hot embossing of isotropic bonded NdFeB magnets and magnetic properties of density. (www.hzbst.com)The isotropic quenched NdFeB magnetic powder as raw material, anisotropic magnets prepared by hot extrusion method, and studied the magnetic orientation of their degree of granularity. Can be seen for the study were quenched NdFeB magnetic powder, while the effect of anisotropic NdFeB powder particle size and properties of the injection molding of small.
This paper on the injection molding anisotropic bonded NdFeB magnets in particle size of their preparation process, mechanical properties, magnetic and thermal properties of the system, believing that its large-scale industry to provide a theoretical reference.
 

1 Experimental
 
1.1 HDDR NdFeB magnetic properties of anisotropic Law
Anisotropic HDDR NdFeB magnetic powder method Aichi Steel Corporation of Japan to provide, powder size range 10 ~ 150 μm, irregular-shaped polygonal morphology was shown in Figure 1. The powder in the shaker on the screening into 4 levels of granularity, namely: A, <45 μm (average particle size of D50 = 36 μm); B, 45 ~ 75 μm (D50 = 60 μm); C, 75 ~ 106 μm (D50 = 85 μm) and D, 106 ~ 150 μm (D50 = 128 μm).

1.2 Sample Preparation 

Figure 1 HDDR NdFeB magnetic powder prepared by the SEM image

Fig.1 SEM image of NdFeB magnetic powder made by HDDR

First, the use of impregnation on the surface of powder coating treatment, is about to put the concentration of magnetic KH550 5% ethanol solution of silane soaked 1 h, 110 ℃ in a vacuum and dried to obtain powder have antioxidant layer. (www.hzbst.com)
Then powder coated in different loading and after drying nylon 12 powder and various additives fully mixed, and the high disk uniform mixing of melt extrusion, a system to prepare for injection or flow feeding experiment; feed injection molding machines in the magnetic field at the injection temperature 270 ℃, injection pressure 130 MPa, the injection speed and direction over 75% of the conditions of the magnetic field strength 1.25 T obtained specifications for the injection of d 20 mm × 9 mm test anisotropic NdFeB magnets sample. 
 

1.3 Performance Test
Instron 3211 capillary rheometer in the determination of feeding on the viscosity and the rheological behavior and the evaluation of the critical load of Фc; in domestic NIM200C permanent magnet bonded magnet measuring instrument to measure the remanence Br, coercivity magnetic induction bHc, iHc intrinsic coercivity and maximum energy product (BH) max and other magnetic properties; while taking advantage of the digital flux meter apparatus by pulling method under various states of the open circuit bonded magnet flux, and press calculated as the irreversible loss of flux magnet hirr:

Where B0 is bonded magnet at room temperature 25 ℃, the magnetic flux, B1 conditions for the aging treatment by cooling to room temperature, the magnet flux.
In the 500 kN universal testing machine to measure the compressive strength of the magnet σbb; determined by the drainage density of the magnet, and thus calculate the density of the magnet d; by Q = Br / [Br (p) * φ] obtained sticky Results orientation degree of the magnet Q, where Br, Br (p) respectively, the magnet and the magnetic remanence, φ is the volume fraction of magnetic powder; with a scanning electron microscope S-s20400 powder morphology.
 
2 Results and Discussion
 
2.1 The magnetic particle size on rheological properties
        Injection molding bonded magnets were successfully depends on whether the feed flow of the molten filling of the viscosity is too high is not easy filling, while the viscosity is too low and may result in two-phase separation, while the anisotropic magnets words, the viscosity will also directly affect the final degree of orientation. Figure 1 shows 4 different size powder feed 60% of the loading and temperature 250 ℃, the melt viscosity η with the shear rate γ changes. Can be seen, the viscosity decreases with shear rate decreases, in line with pseudoplastic rheological behavior. For pseudoplastic fluid, τ = kγn, η = τ / γ, where τ is shear stress, γ the shear rate, k is a constant, η the apparent viscosity of the fluid, n for the viscous flow index, reflecting the size of binder and its feeding on the sensitivity of the shear rate. For injection molding, the general requirements in the n> 0.2 in the case of the smaller the better, n the smaller the value, then the viscosity will increase rapidly with the shear rate decreased, which for complex shapes or thin and so is not easy filling injection molding product is very beneficial. Lg γ lg η by the relationship between the fitting and obtained by the A, B, C and D are made of feed at a temperature 250 ℃, the n values were 0.34,0.40,0.48 and 0.51 (Figure 2). This shows that although at the same shear rate, the finer the powder the better compatibility with the binder, but it increases the resistance of the binder, the viscosity value increased, but the process better performance. Magnetic particle size is too coarse, and lower adhesion of the binder in the extrusion easily lead to phase separation, which can be proved from the rheological experiments, the first out of the material is very thin, and then out of the material is very thick and out of difficulties.

Figure 4 The size of powder on the thermal properties of bonded magnets
Fig.5 Effects of particle size on irreversible magnetic flux loss of magnet 

Table 2 Relations of distribution of magnetic powder and viscosity and critical powder volume fraction

 
Can be seen by a reasonable mix of different size powder can reduce the viscosity of the same loading value, which is conducive to a higher degree of orientation and density, or get a higher critical load, thereby enhancing the viscous Results The volume fraction of magnet powder, which is to obtain high-performance bonded magnets are a necessary condition. On this basis, to m (C): m (B): m (A): m (D) = 63: s2024: s2010: s203 J-powder preparation can be obtained the critical load of 73%, 67% of its obtained when the injection load of remanence Br 878 mT, the intrinsic coercivity iHc 1 212.3 kA / m, the maximum energy product (BH) max 128 kJ/m3 σbb 73 MPa compressive strength and high performance anisotropic bonded NdFeB magnet demagnetization curve shown in Figure 5. 

Figure 5, injection molding anisotropic bonded magnet demagnetization curve
Fig.5 Demagnetization curves of injection molding anisotropic bonded magnet

Japan MFP15 performance of similar grades (the Br, 750 ~ 830 mT, the intrinsic coercivity iHc to 875 ~ 1 034 kA / m, the maximum energy product (BH) max 119 kJ/m3).
 
3 Conclusion
 
1) With the magnetic particle size decreases, feed viscosity increases, but the viscous flow index n values decrease, injection process performance better.
2) NdFeB magnetic powder particle size is too thick or too thin are not conducive to injection molding anisotropic bonded NdFeB magnetic properties improved, the optimal size range is 80 ~ 100 μm, through data fitting shows that 90 μm particles in when the maximum diameter of the largest energy product.
3) the finer powder particle size, the bonded magnet flux due to the greater irreversible.
4) a reasonable level through the particle size distribution can reduce the viscosity of the feed system or increase the critical loading, prepared on this basis, high-performance anisotropic bonded NdFeB magnet, its performance for the Br 878 mT, iHc 1 212.3 kA / m, (BH) max 128 kJ/m3 and σbb 73 MPa, higher than the performance of similar products in Japan.

Report List

 1) Ma B M. Recent development in bonded NdFeB magnets[J]. Journal of Magnetism and Magnetic Materials(www.hzbst.com)
 2) developing situation of rare-earth bonded magnet in Japan
 3) Development of injection formed NdFeB magnets(www.hzbst.com)
 4)Microstructural improvement of NdFeB magnetic powder by the Zn vapor sorption treatment.-Materials Transactions(www.hzbst.com)
 5)Effect of particle size and distribution of magnetic powder and antioxidant on magnetic properties of NdFeB bonded Magnets by injection moulding.- Metallic Functional Materia.
 6)Influence of particle size adjustment on the properties of bonded NdFeB magnet
 7)Microstructural investigation of oxidized Nd-Fe-B powders: influence of particle size on the oxidation behavior- Journal of Magnetism and Magnetic Materials
 8)Polymer-bonded magnets- Effect of surface modification and particle size on the improved oxidation and corrosion resistance of magnetic rare earth fillers-Journal of Alloys and Compounds(www.hzbst.com)
 9)Effect of particle size and distribution of rapidly quenched Nd-Fe-B powder on magnetic properties of polymer bonded Nd-Fe-B magnet(www.hzbst.com)
 10)The influence of the powder particle size on the anisotropic properties of NdFeB magnets produced by single-stage hot deformation. Journal of Magnetism and Magnetic Materials.
 11)Dependence of the magnet properties on the alignment magnetic field for NdFeB bonded magnets made from anisotropic HDDR powders-Journal of Magnetism and Magnetic Materials.

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