Fabrication and measurements on polymer bonded NdFeB magnets

Journal of

ELSEVIER

Materials Processing Technology Journal of Materials Processing Technology 56 (1996) 571-580

FABRICATION AND MEASUREMENTS ON

POLYMER BONDED NdFeB MAGNETS A.Z. Liu, I.Z. Rahman, M.A. Rahmana and E.R. Pettyb Magnetic Research Laboratory, "Department of Electronic & Computer Eng. Department of Materials Science Technology, University of Limerick, Ireland.

ABSTRACT This paper presents measurements on polymer bonded NdFeB permanent magnets. These permanent magnets are fabricated by mixing NdFeB ribbons and binder epoxy resin under 5 ton/cm2 pressure. The ribbons of NdFeB alloy are prepared by melt-spinning process. The performances of ribbon quenched at different rates and polymer bonded magnet are investigated by x-ray diffraction, DTA, SEM and VSM. The crystal lattice constants of the main phase Nd2Fet4B are found to be a=8.73 ~ c=12.48 ~ Effect of annealing at different temperature and time are also investigated. 1. INTRODUCTION J.J. Croat [1] in 1983 announced a permanent magnet with a.(BH)~,x of 14 MGOe with NdFeB composition which was fabricated using the melt-spinning route. This type of rapid solidification process differs markedly with the traditional process[2]. Using this method a stable, magnetically hard microstructure can be quenched directly from the molten state. The microstructure of the resulting product consists of microcrystaUites less than 0.1 micron in diameter, almost 100 times smaller than the grain size obtained in sintered materials. Owing to its extremely fine microstmcture, this type of ribbon has high thermal stability and after crushing can be formed directly into a variety of bonded magnets by compression or injection molding techniques. One inherent feature of melt-spinning NdFeB is that the magnetic properties are isotropic. As a consequence, the remanence and hence the energy products are lower than the anisotropic or sintered materials.

2. FABRICATION OF POLYMER BONDED NdFeB Ribbons are fabricated using the melt-spinning system developed during this investigation. The quench rate is varied by changing the substrate surface velocity (V,) of the copper roller while holding all other experimental parameters constant. Quench rate is a crucial factor in determining the magnetic performances of a magnet[ 1]. In Table 1, Ndt~enB s ribbons are labelled from A to G according to the quench rates. Ribbons which are made at speeds less than 13.9 rrds are found to be underquenched. The optimally quenched ribbons are formed when the quench rates are within the range 13.6 m/s< V, <95.6 m/s. The ribbons quenched at rates in excess of 25.6 m/s are found to be overquenched and exhibit amorphous properties. For the fabrication of magnet of certain size and shape, ribbons are crushed by pestle and mortar to about 0.15 mm powder which is sieved through a 0.15 mm mesh. The resulting powder is mixed with 10% or so volume of binder. The mixture is placed in a cylindrical die cavity and compacted under a pressure of 5 ton/cm2 for 5 minutes. After compacting, the sample is cured in air for a day. The samples are of disc shape of diameter 0924-0136/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved SSD10924-0136( 95 ) 01871 -B

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13 mm approximately and thickness 2 mm to 5 mm. Excess resin is squeezed out of the mold and resulted in higher volume fraction of ribbons with enhanced magnetic properties. The density of the magnets are found to be about 5 g/cm~. Table 1 Quench rates (m/s) of Nd~Fe.nB s ribbons labelled A to G.

Name A

B

C

D

E

F

G

Speed 13.9

15.7

18.2

19.9

22.5

24.1

25.6

3. RESULTS ON PHYSICAL CHARACTERIZATION

A) X-ray diffraction The crystal structure and solidification process are studied using X-ray diffraction technique. The X-ray diffraction pattern of an amorphous material consists of one or more broad diffuse peaks. The pattern is different for the diffraction pattern of crystalline materials which show a large number of fairly sharp peaks. The type of X-ray pattern obtained using the prepared samples shows that the material has disordered structure indicating the absence of unit cell that repeats itselfidenticallyat periodic intervalsin three dimensions. Different phases in the sample are identifiedby a Philips X-ray diffractomctcrand CuKot radiation.The X-ray wavelength is 1.54 A. The diffractionangle 20 is from 20 to 50 degree.

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Figure I X-ray diffractionspectrum of NdFeB alloy,sample A, D and G. Four samples are investigatedby X-ray diffraction.These are sample A, D, G and a NdFeB standard alloy. A, D, and G samples arc bonded magnets with 0.15 m m particle size. Sample A and D show only some of whole spectram lines of NdFcB alloy. Sample A displays higher intensityand more lines than sample D. This means that sample D possess higher percentage of amorphous phase than A. For the ribbons which arc quenched at rates in excess of 25.6 m/s the X-ray diffractionline looks fainter and less clear. These samples possess microcrytallitc structure and are not amorphous. Figure 1 are X-ray diffraction spectrums of NdFeB alloy, sample A, D and G. (a),(b), (c) and (d) in fig. 1 correspond to NdFcB alloy, sample A, D and G respectively. Lattice constants obtained from the d-values of (004) and (330) are as following, c = 12.48 A from the d-value

A.Z. Liu et al. / Journal (~'Materials Processing Technology 56 (1996) 571-580

of (004) and a = 8.73 A from the d-value of (330). This is consistent with the results of Croat and Sagawa (a=8.8/~, c=12.2 A) [1, 21.

B) Differential thermal analysis (DTA) Differential thermal analysis is used to determine the crystallization temperature of the samples. The heat change within the material are monitored by measuring the difference in temperature between the sample and a reference material. The instrument used is a Stanton-Redcroft 780 Series simultaneous thermogravimetric differential thermal analyzer. A heating rate of 10 °C/min is used. Minute pieces 15 mg of the samples are placed in a platinum crucible. Oxygen free nitrogen is passed through the system to prevent oxidation.

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Figure 2 DTA curve of sample C. Differential thermal analysis is carried out on three samples of Nd, sFenB s ribbons in order to determine the temperature at which erystallisation occurs. These ribbons are quenched at different rates, 13.9 m/s, 18.2 m/s and 25.6 m/s. All DTA curves show two irreversible exothermic peaks, as shown in the curve of sample C in Figure 2. This means that two crystalline phases form when the temperature is increased. The onset temperature To, of first peak occurred at 300 °C which corresponds to the bec phase of a-Fe[3, 4]. The onset temperature To2 of second peak occurred at 520-560 °C and could be due to the crystallization of the Nd~e,,B magnetic hard phase. The results of DTA can be used to guide the annealing. If the annealing temperature is in the range between T¢, and T a, magnetic hardening will not occur. After annealing at Ta, the magnetic hard phase appears. The appearance of DTA curves are markedly influenced by experimental factor, such as amount of sample and heating rate etc[5]. The position of peak is less useful than the onset temperature. The base lines which precede and follow a DTA peak are not at the same ordinate value because the heat capacity of the sample has changed.

C) Scanning electron microscopy (SEM) A JEOL JSM-840 scanning electron microscope is used on three samples of Nd,sFe,-nB8 ribbons in order to observe grain sizes. These ribbons are quenched at rates, 13.9 m/s, 18.2 m/s and 25.6 m/s. Figure 3, 4 and 5 arc SEM photographs of grain size of sample A, C and G respectively. Sample A has grain size of about 1 micrometer. Sample C has grain size of about 0.5 micrometer. Sample G has grain size of about 0.2 micrometer. Figure 6 is the case of "parallel press direction". Figure 7 is the case of "perpendicular press direction". The highly anisotropic packing is evident in the transverse view. The ribbons tend to lie face to face.

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A.Z. Liu et al. / Journal of Materials Processing Technology 56 (1996) 571-580

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A.Z. Liu et al. / Journal of Materials Processing Technology 56 (1996) 571-580

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Figure 7 Ribbons stacking photographs perpendicular to press direction of bonded magnet. 4. RESULTS ON MAGNETIC CHARACTERIZATION

A) Magnetizationcurves The magnetization curves are measured by a Vibrating Sample Magnetometer(VSM). The curves of magnetic field perpendicular to the surface of ribbons and magnetic field parallel to the surface of ribbons are presented. At relatively low values of magnetic field the data points of vertical magnetization fall appreciably below those of parallel magnetization owing to the presence of a preferred direction of magnetization in the ribbons. It can be considered that the easy magnetizing axis that is c-axis is inside the ribbon. When the applied magnetic field is over 9 KOe, the curve of sample A shows that the vertical magnetization and parallel magnetization are basically of the same magnitude. This means the sample has anisotropy field of about 9 KOe. It is due to thermal motion that the vertical magnetization remains below parallel magnetization over 9 KOe field. The sample A has similar magnetizing curve behaviour as R2Fe14Balloy[6], but anisotropy field is lower. This means sample A is underquenched as examined by x-ray diffraction. The curve of sample G has the same case as sample A, but sample G only has 7.5 KOe anisotropy field and relatively high initial permeability. The reason for this is when wheel speed increases the crystalline grain size decreases. The sample approaches isotropic behaviour so that the two curve tend to be the same. Table 2 displays the difference between vertical magnetization and parallel magnetization at 12 KOe magnetic field for different samples. Table 2 The values and the difference of vertical and parallel magnetization at 12 KOe. Sample AM(emu/g) Ml(emu/g) Mj_(emu/g)

A 3 56 53

B 30 53 23

C 41 56 15

D 46 69 23

E 32 56 24

F 34 65 31

G -6 50 56

where, AM=Mi-M.L, M i is the magnetization of magnetic field parallel to the ribbon surface, M± is the magnetization of magnetic field perpendicular to the ribbon surface. From the above table it can be concluded that the sample D has maximum anisotropy field. The sample is quenched at 19.9 m/s and shows the best magnetic performances. This is consistent with the result of Croat[9]. The sample from B to F have nearly the same vertical magnetization. The sample D has the maximum saturation magnetization, of course, it is far from saturation at 12 KOe magnetic field because ribbons have

A.Z. Liu et al. / Journal of Materials Processing Technology 56 (1996) 571-580

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higher coercivity which results from finer grain size. The saturation magnetization M I of every sample are basically of the same magnitude which illustrates the magnetic performance of the material. The perpendicular magnetization M± of every sample shows a large difference in magnitude. This indicates that these samples have directional preferences towards the applied magnetic field. From the magnetization curves the extrapolated anisotropy field in these samples is found to be in the range of 2 to 6 Tesla.

B) Hysteresis loops The hysteresis loops are also measured by the VSM. The magnetic field is kept parallel to the surface of the ribbon during measurements. Table 3 shows the magnetic properties of each sample. Table 3 The remnant magnetization and coercivity of NdFeB ribbons

M~(emu/g) ~I~(KOe) •

A 36 9.64

B 51 0.91

C 32 2.55

D 43 1.60

E 56 0.65

F 36 2.00

G 30 1.00

where, M~ is the remanence magnetization and iI-I~is the intrinsic coercivity. From the above table, trend of magnetic properties as a function of quench rates are not clear. The sample A has highest coercivity. Most of loops show deviation from the origin due to the lower applied magnetic field which can not saturate the samples. Specially in third quadrant, the reverse magnetic field can not rotate more magnetic moments which have aligned along the positive field direction. The curves of sample B, E and G show little deviations. These samples display approximate saturation at 12 KOe and have lower coercivity. Some curves display a slender waist. This implies that a soft magnetic phase exist in underquenched samples[8] and a considerable amount of glassy material formed in overquenched samples leading to poor hard magnetic properties[9]. The magnetic properties of fabricated bonded magnets are listed in Table 4. These are magnetized under 1.5 T magnetic field. Table 4 Magnetic properties of bonded magnets before annealing

Sample

A'

C'

D'

F'

M,(emu/g) ~-I~(KOe) •

23 5.3

24 6.1

17.3 2.1

33.3 3.6

From the curves the deviation of magnetization curve from origin increased from sample to sample even when the applied magnetic field as high as 1.5 T. As one knows that the magnetic performances are dependent on applied field, which means higher applied field may give rise to higher remanence magnetization and coercivity. Comparing Table 3 and 4, it is found that remanence magnetization decreases and coercivity increases in bonded magnets except in sample A. It can be said that the fine powder arrangement randomly decreases the extent of magnetic order and the additive epoxy resin dilutes magnetic moments which accounts for the magnetic loss. At the same time, fine powder increases the difficulty for magnetic moment alignment along the applied magnetic field direction therefore increases coercivity. Figure 6 and 7 show ribbon stacking in epoxy bonded magnets which display the tendency for the ribbons to lie flat face to face, therefore, there is a high degree of radial alignment in a disc sample. In this investigation, the bonded magnets are magnetized along the direction of perpendicular the surface of the disc sample.

C) Curie temperature The Curie temperature is measured by a VSM. The experimental curve is shown in Figure 8. The sample C is used. The applied magnetic field is 10 KOe. Curie temperature is about 320 °C. The widespread points are owing to the temperature instability in the sample surroundings. For the operation of the furnace nitrogen gas flow through the heater is required that creates the instability.

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A.Z. Liu et aL / Journal of Materials Processing Technology 56 (1996) 571-580

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5. HEAT T R E A T M E N T Ribbons cooled at sufficiently high speeds are amorphous and do not exhibit hard magnetic properties. These ribbons are magnetically hardened by crystallization. The method for achieving magnetic hardening is annealing or heat treatment. The annealing process proceeds in vacuum at crystallization temperature for several minutes. Three annealing temperatures, 550, 600 and 650 °C according to DTA measurements are chosen. The annealing time is from 2 minutes to 120 minutes. Figure 9 and 10 show the results after annealing. Series 1 are the results of annealing at 550 °C, series 2 at 600 °C and series 3 at 650 °C. The results of annealing at 600 and 650 °C are basically the same. The best results are obtained when annealing time is under 10 minutes. Figure 11 is a curve of bonded magnet annealed at 600 °C for about 5 minutes. It displays the hard magnetic properties. It is magnetized in a 2.5 T pulse magnetic field.

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A.Z. Liu et al. / Journal of Materials Processing Technology 56 (1996) 571-580

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6. DISCUSSION High magnet densities between 80-85% are necessary for better magnetic properties. Investigated samples only have 65-73% of 7.5 g/cm3 full density. The pressure of 5 tons/cm2 used to compact the samples is too low. Croat considered that densities of 85% can be achieved with modest pressures of 6-7 tons/cm2[8]. Figure 6 and 7 show that the bonded magnets has low density. But these polymer bonded magnets show excellent physical tolerances with smooth and bright surfaces. One problem of rapidly solidified materials is crystallographic alignment. Because of the extremely fine grain size of these materials, crystallographic orientation is not practical. Lee[6] resolved this problem using thermomechanical means to prepare crystallographically oriented or "anisotropic" magnets. However, there are number of advantages of the isotropic materials. An orienting magnetic field is unnecessary, allowing for faster cycle time and greater flexibility in the size and intricacy of the bonded magnets which can be produced. Another major advantage of these isotropic materials is that the temperature coefficient of intrinsic coercivity is considerably lower than for oriented materials, which allows the material to be operated at significantly higher temperatures. The material can be handled for significant periods of time in air without noticeable degradation. The high thermal stability of these materials is attributed to two factors: the ftrst one is its extremely fine grain size and the second one is its low overall rare earth content. The experimental results of X-ray diffraction which are the case of discrete diffraction lines above a diffuse background is typical for a very fine microstructure. Both (006) and (004) disappears in diffraction spectrums of quenched ribbons which indicates that the tetragonal c axis does not lies normal to the ribbon surface. Magnetization curves illustrate c axis is parallel to the ribbon surface. This result is reflected in the magnetic anisotropy of the ribbons. The peak intensity indicates the amount of percentage of crystal phase. The patterns for the samples quenched at higher rates show increasingly broader reflections, indicating a progressive reduction in the average crystallite size as testified by the SEM. Differential thermal analysis illustrated the fact that there is a close correlation between the thermal behaviour of ribbons and its initial ordering or appearance of crystallinity. The SEM investigations on ribbons show that the grain size is within 0.001 mm which is smaller than 0.010.05 mm grain sizes of sintered magnets. The single domain particle size of Nd2FeI4B is estimated to be 4000 /~[10], therefore, sample D are expected to be single domain. The larger values of remanence magnetization measured can be explained by considering interactions among these single domain particles. Because of the very

A.Z Liu et al. / Journal ~'Materials Processing Technology 56 (1996) 571-580

fine size of the gains and the clean grain boundaries, a large interaction is expected between the neighbouring magnetic moments of each grains. These interactions act as if an effective domain wall exists between neighbouring gains. For the large grains these interactions are considerably reduced leading to a reduction in remanence magnetization. In SEM observation, it is found that the grain size did not distribute uniformly in the cross fracture section, but distributed gradually from one surface to another surface. This means that the surfaces are quenched at different rates. Measurement of magnetization at low field with samples prepared under various solidification is still a new research topic although a large number of research works have been reported over the last 10 years. There are some limitations in studying permanent magnets under low applied magnetic field, because the coercivity and remanence magnetization of the NdFeB samples increase with increasing magnetizing field[2]. All the prepared samples displayed a "certain saturation magnetization Ml" value although these are quenched at various cooling rates. The measured magnetization curves demonstrated the dependence of anisotropy field on cooling rate and microstructure. Information about crystal orientation can also be interpreted from these curves. The second quadrant in the hysteresis loops determines whether a material posses good permanent magnetic properties or not. Some hysteresis loops for the prepared samples show a 'dip' in the second quadrant. These 'dips' could arise in magnetic samples for two seasons: (a) presence of soft magnetic phases and (b) presence of reversal domains in the magnetic grains. These are confirmed by the fact that in NdFeB alloys has an ironrich phase which is produced during fabricated and this can be removed by proper quenched rate and annealing[11]. The presence of reversal domains in the samples are very likely, because the measurements are performed at lower field than the saturation field. Otani[9] has confirmed that the dip decreased in a static field of 100 KOe and almost disappeared in a pulsed field of 200 KOe. In connection with this observation the bonded samples with smaller particles show a dip in demagnetization curve. MuUer[12] assumed that this is due to a reduction of nucleation fields of internal demagnetization modes. It has been found that the coercivity depends on the strength of the applied magnetic field. The coercivity increases almost linearly for fields up to 17 KOe in hysteresis loops measurements[13]. These hysteresis loops are only minor loops as interpreted by Hadjipanayis [13], which do not represent the "true state" of the samples measured. This is why the results obtain for the coercivity values of the prepared samples do not show any definite trend. The magnetic properties are extremely sensitive to changes with the microstructure. Heat treatment condition is a key factor to characterize the microstructure. The most striking effect of annealing is the sharp rise in coercivity and remanence magnetization at the onset of crystallization which raise almost to the maximum value within 2 minutes. Further annealing has a minor influence on these values. Even annealing times up to 2 hours do not drastically change it. The ribbons quenched directly to crystalline phases are in a metastable state' due to quenched-in defects and a non-equilibrium distribution of phases. Therefore, when it is crystallized the amorphous phase leads to structural changes. The change in coercivity and remanence magnetization can also occur. This highlights the fact that equilibrium thermodynamics have to taken into account when alloy elements are chosen to improve magnetic properties, at least for the preparation of bulk materials from rapidly quenched ribbons which has to start from the amorphous state. Based on experimental results, Mishra concluded that the pinning of domain walls on the crystalline boundary is the origin of the high coercive field for quenched ribbons[14]. Obviously, the total barrier density is higher for smaller, interbarrier distance which leads to higher coercivity. Also small size grain with good consistency formed during crystallization by annealing contribute to the pinning of domain walls and hence produces higher coercivity.

7. CONCLUSIONS Fabricated polymer bonded magnets show excellent physical tolerance, smooth and bright surface. Magnets can be handled for a significant period of time in air without noticeable degradation. This is due to a lower rare earth content. Measured magnetic parameters are lower than the reported values, because (a) measurements have been performed at a lower field and (b) the density of the prepared bulk magnets have reached only 65-73% of the 100% bulk density of NdFeB solidified alloys (7.5 g/cm3). This investigation also demonstrated the fact that it is possible to fabricate NdFeB polymer bonded magnet through short cut routes. Magnetic measurements complimented by X-ray diffraction, DTA, SEM etc. have illustrated and highlighted various factors affecting the hard magnetic properties of the fabricated magnets.

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ACKNOWLEDGEMENTS This work was partly supported by SPS LABORATORIES, NAAS, IRELAND and EOLAS ( THE IRISH SCIENCE AND TECHNOLOGY AGENCY ) under contact number He/87/189. The authors also thank the technicians in the Department of Materials Science & Technology, University of Limerick for DTA, SEM measurements and developments of the photographs.

REFERENCES

[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

J.J Croat, J.F. Herbst, R.W.Lee and F.E. Pinkerton, J.Appl.Phys.55(6), 15 March 1984, pp2078-2082. M. Sagawa, S. Fujimura, N. Togawa, H. Yamamoto and Y. Matsuura, New material for permanent magnets on a base of Nd and Fe, J. Appl. Phys. 55(6), 15 Mar. 1984, pp2083-2087. M. Sagawa, S. Hirosawa, H. Yamamoto, Y. Matsuura, S. Fujimura, H. Tokuhara and K. Hiraga, IEEE Trans. Magn., 22 (1986) pp910-912. Z.X. Wang, B.L. Yu, Y.Z. Wang, J.H. Huang, L. Yin and M.Y. Feng, Materials science and engineering, 99 (1988) pp123-126. James W. Dodd and Kenneth H. Tonge, "Thermal methods", John Wiley + sons,1987, ppll0-160. R.W. Lee, Appl. Phys. Lett., 46(1985) 790. S. Sinnema, R.J. Radwanski, J.J.M. Franse, D.B. de Mooij And K.H.J. Buschow, Magnetic properties of ternary rare-earth compounds of the type R2Fe~4B,J. of MMM, 44(1984) pp333-341. J.J. Croat, Manufacture of NdFeB permanent magnets by rapid solidification, J. of the less-common metals, 148(1989) pp7-15. Y. Otani, H. Miyajima, S. Chikazumi, S. Hirosawa And M. Sagawa, Magnetization processes in NdFeB permanent magnets, J. of MMM, 60(1986) pp168-170. G.C. Hadjipanayis and W. Gong, Magnetic hysteresis in melt-spun NdFeA1BSi alloys with high remanence, J. Appl. Phys. 64(10), 15 Nov. 1988. K.H. Muller, A. Handsten, J. Schneider & D. Eckert, The dip in demagnetization curves of sintered NdFeB permanent magnets, Acta Phys. Polonica, Vol. A72 (1987), pp89-92. K.H. Muller, A. Handstein, D. Eckert et.al, Phys. Stat. Sol. (a) 99 (1987) K61. G.C. Hadjipanayis, R.C. Hazelton & K.R. Lawless, Cobalt-free permanent magnet materials based on iron-rare-earth alloys, J. Appl. Phys. 55(6), 15 March 1984, pp2073-2077. R.K. Mishra, J. of MMM, 54-57(1986) pp450.