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Influence of particle size on the properties of polymer bonded Nd-Fe-B magnets
Ad
M.a
,ournal of Magnetism and Magnetic Materials 101 (1991) 377-378
NohHo.an
7,44
'
Influence of particle size on the properties of polymer bonded N d - F e - B magnets A. H a n d s t e i n
a,
K.-H. Miiller
a,
R. Gr6ssinger
b,
H.R. Kirchmayr b and R. Krewenka b
a Institut fiir Festkiirperphysik und Festk6rperchemie im Z F W Dresden, Helmholtzstr. 20, 0-8027 Dresden, Germany b Institut fiir Physik, TU Wien, Wiedner Hauptstr. 8-10, A-1040 Vienna, Austria The particle size in polymer bonded N d - F e - B magnets has a remarkable influence on the magnetic properties. We used different fractions of particle size obtained by sieving of original and milled MQl-powders, respectively, for preparing bonded magnets. The difference in magnetic properties is caused by the oxygen content of the powder.
1. Introduction T h e N d - F e - B p o w d e r for polymer b o n d e d m a g n e t s is usually p r o d u c e d by rapid q u e n c h i n g of a melt a n d s u b s e q u e n t a n n e a l i n g a n d grinding of ribbons, respectively, to o b t a i n a high coercivity. It is well-known t h a t the m i c r o s t r u c t u r e of p e r m a n e n t m a g n e t materials det e r m i n e s t h e i r coercive force considerably. In s i n t e r e d N d - F e - B m a g n e t s the particle size of the milled powders as well as the grain size of the m a g n e t s have a strong influence o n t h e coercivity force [1]. I n this p a p e r we will investigate t h e influence of grain size of p o w d e r s m a d e from m e l t - s p u n N d - F e - B alloys on t h e m a g n e t i c p r o p e r t i e s of c o r r e s p o n d i n g polymer b o n d e d magnets.
2. Experimental T h e N d - F e - B p o w d e r was commercially p r o d u c e d by grinding of rapidly q u e n c h e d a n d a n n e a l e d ribbons ( M a g n e q u e n c h p o w d e r M Q 1 ) s t o r e d in air. In a first r o u t e the original p o w d e r was sieved to o b t a i n different fractions of particle size as shown in table 1. T h e p o w d e r fractions were b o n d e d with 5 w t % A r a l d i t AT1 ( h a r d e n e d at 160°C for 2 h). A n o t h e r c h a r g e of original p o w d e r was g r o u n d by a ball mill at air to enlarge t h e a m o u n t of finer fractions as shown in table 1. T h e s e p o w d e r fractions were b o n d e d with 5 w t % Scotch Cast Electrical R e s i n 265 ( h a r d e n e d at 177°C for 20 min). T h e gas c o n t e n t of original a n d sieved powders, respectively, was d e t e r m i n e d by a L E C O 436 device. T h e density of samples was d e t e r m i n e d by a Multivolu m e P y c n o m e t e r 1305. T h e m a g n e t i c p r o p e r t i e s of b o n d e d m a g n e t s were m e a s u r e d by a ballistic m a g n e t o m e t e r in applied fields up to / x 0 H = 7 T at r o o m t e m p e r a t u r e a n d at 4.2 K.
3. Results and discussion T h e c h a r a c t e r i s t i c values of s a m p l e s p r e p a r e d in t h e s e two regimes are given in table 1. T h e d e m a g n e t i -
Table 1 Characteristic values of polymer bonded magnets made from a) sieved MQl-powder, b) milled and sieved MQl-powder Particle size
Br (T)
k~0jHc
Content of
(T)
oxygen (wt%)
nitrogen (wt%)
0.531 0.534 0.537 0.510 0.480 0.533 0.529 0.537 0.518 0.515 0.500 0.477
1.710 1.724 1.705 1.672 1.445 1.611 1.604 1.632 1.643 1.632 1.595 1.415
0.170 0.092 0.137 0.215 0.509 0.170 0.104 0.126 0.170 0.194 0.221 0.580
0.031 0.016 0.024 0.049 0.132 0.031 0.013 0.019 0.022 0.030 0.043 0.190
(Ixm) ~) original > 163 40-163 40- 60 < 40 b) original > 100 71-100 50- 70 32- 50 20- 32 10- 20
zation curves of samples b o n d e d in Araldit AT1 are plottet in fig. 1. C o n t r a r y to the b e h a v i o u r of m a n y particle m a g n e t s [2] the coercivity j H c decreases with 3[Tl 0.6 ..-
• It
.i0/,
/'
.222
~-;
:J -IlZ
0.5 : ,uoH [TI
Fig. 1. Demagnetization curves of bonded magnets with different powder fractions prepared by rapid quenching, grinding and sieving: - non-sieved, • . . . . . > 160 ixm, - . . . . . (40-60) txm, - . . . . < 40 ixm.
0312-8853/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved
378
A. Handstein et al. / Polymer bonded MQ1 magnets
3 IT] /" ¢.
ZSS~
S/.1 /2 .ff
r
-5
"
--Z / /" -,I
-0.Z
Fig. 2. Demagnetization curves of two bonded-powder magnets at 293 and 4.2 K (signs as in fig. 1). decreasing mean particle size of compacted powders already at values around 100 ;zm. The demagnetization curves of the other charge of bonded magnets (see table 1) behave similarly to those of fig. 1. The oxygen content increases monotonously with decreasing particle size of powder fractions as shown in table 1. A drastic rise of oxygen content was found for particle sizes smaller than about 40 I~m. Obviously the particle surfaces are chemically and mechanically disturbed. The smaller the particles made from flakes are the smaller the nucleation fields for demagnetization modes in the particles. In all cases the increasing oxygen content is accompanied with a decrease of remanence. In connection with this observation the bonded samples with smaller particles show a dip in the demagnetization curve (see fig. 1) which is assumed to be due to a reduction of nucleation fields of internal demagnetization modes [3] for some of the bonded particles. At low temperatures an additional dip in the second-quadrant demagnetization curves is caused by the well known spin reorientation transition of the Nd2Fe14B-phase below about 140 K [4], as shown in fig. 2 for T = 4.2 K. Comparing the coereivities of these two magnets at 4.2 K and room temperature, we note that the increase of coercivlty of the bonded magnet with particles smaller than 40 ~zm is strongly diminished in the low temperature range caused by spin reorientation and oxide phases [3]. The investigation of the anisotropy field H A by the single point detection method [5] showed that H A of these bonded magnets is unchanged for all powder fractions and takes the value of NdzFe14B near 8 T.
In comparison to this behaviour the coercivity of compacted powders prepared by milling of melted ingots increases with decreasing particle size whereas the coercivity of the resulting sintered magnets passes through a maximum and then decreases drastically [6,7]. Sintered magnets show a different behaviour due to oxidation processes of neodymium during sintering. It should be noted that the powders for sintered magnets are prepared to the exclusion of oxygen. Compared to sintered magnets the coercivities of the milled powders are small. By crushing sintered N d - F e - B magnets for bonded materials, Stadelmaier et al. [8] found a strong diminu: tion of j H c already at particle sizes greater than 100 Ixm. They could show that this loss is mainly caused by a mechanical damage of particles. On the other hand, a recover of j H c by annealing of crushed particles from sintered magnets should be possible for modified N d - F e - B alloys with two or more substitutions [9]. We suppose that the reduction of intrinsic coercivity of the bonded magnets with smaller particles is due to oxide phases causing an easier nucleation of internal demagnetizing processes. This work was supported by the Austrian Ministry of Research project "Magnetic Materials" and by the Austrian Science Foundation project No. 7327. References
[1] B.M. Ma and R.F. Krause, Proc. of 5th Int. Symp. on Magnetic Anisotropy and Coercivity in RE-Transition Metal Alloys, Bad Soden (1987) p. 209. [2] E. Kneller, in: Magnetism and Metallurgy, ed's. A.E. Berkowitz and E. Kneller (Academic Press, New York and London, 1969) p. 366. [3] K.-H. Miiller, A. Handstein, D. Eckert and J. Schneider, Phys. Stat. Sol. (a) 99 (1987) K61. [4] D. Givord, H.S. Li and R. Perrier de la Bfithie, Solid State Commun. 51 (1984) 857. [5] G. Asti and S. Rinaldi, J. Appl. Phys. 45 (1974) 3600. [6] C.N. Christodolou, J. Schlup and C.C. Hadjipanayis, J. Appl. Phys. 61 (1987) 3760. [7] P. Nothnagel, K.-H. Miiller, D. Eckert and A. Handstein, J. Magn. Magn. Mater. 101 (1991) 379. [8] H.H. Stadelmaier and N.C. Liu, Mater. Lett. 4 (1986) 304. [9] M. Li, K.J. Strnat and H.F. Mildrum, J. Appl. Phys. (1991) in press.
M.a
,ournal of Magnetism and Magnetic Materials 101 (1991) 377-378
NohHo.an
7,44
'
Influence of particle size on the properties of polymer bonded N d - F e - B magnets A. H a n d s t e i n
a,
K.-H. Miiller
a,
R. Gr6ssinger
b,
H.R. Kirchmayr b and R. Krewenka b
a Institut fiir Festkiirperphysik und Festk6rperchemie im Z F W Dresden, Helmholtzstr. 20, 0-8027 Dresden, Germany b Institut fiir Physik, TU Wien, Wiedner Hauptstr. 8-10, A-1040 Vienna, Austria The particle size in polymer bonded N d - F e - B magnets has a remarkable influence on the magnetic properties. We used different fractions of particle size obtained by sieving of original and milled MQl-powders, respectively, for preparing bonded magnets. The difference in magnetic properties is caused by the oxygen content of the powder.
1. Introduction T h e N d - F e - B p o w d e r for polymer b o n d e d m a g n e t s is usually p r o d u c e d by rapid q u e n c h i n g of a melt a n d s u b s e q u e n t a n n e a l i n g a n d grinding of ribbons, respectively, to o b t a i n a high coercivity. It is well-known t h a t the m i c r o s t r u c t u r e of p e r m a n e n t m a g n e t materials det e r m i n e s t h e i r coercive force considerably. In s i n t e r e d N d - F e - B m a g n e t s the particle size of the milled powders as well as the grain size of the m a g n e t s have a strong influence o n t h e coercivity force [1]. I n this p a p e r we will investigate t h e influence of grain size of p o w d e r s m a d e from m e l t - s p u n N d - F e - B alloys on t h e m a g n e t i c p r o p e r t i e s of c o r r e s p o n d i n g polymer b o n d e d magnets.
2. Experimental T h e N d - F e - B p o w d e r was commercially p r o d u c e d by grinding of rapidly q u e n c h e d a n d a n n e a l e d ribbons ( M a g n e q u e n c h p o w d e r M Q 1 ) s t o r e d in air. In a first r o u t e the original p o w d e r was sieved to o b t a i n different fractions of particle size as shown in table 1. T h e p o w d e r fractions were b o n d e d with 5 w t % A r a l d i t AT1 ( h a r d e n e d at 160°C for 2 h). A n o t h e r c h a r g e of original p o w d e r was g r o u n d by a ball mill at air to enlarge t h e a m o u n t of finer fractions as shown in table 1. T h e s e p o w d e r fractions were b o n d e d with 5 w t % Scotch Cast Electrical R e s i n 265 ( h a r d e n e d at 177°C for 20 min). T h e gas c o n t e n t of original a n d sieved powders, respectively, was d e t e r m i n e d by a L E C O 436 device. T h e density of samples was d e t e r m i n e d by a Multivolu m e P y c n o m e t e r 1305. T h e m a g n e t i c p r o p e r t i e s of b o n d e d m a g n e t s were m e a s u r e d by a ballistic m a g n e t o m e t e r in applied fields up to / x 0 H = 7 T at r o o m t e m p e r a t u r e a n d at 4.2 K.
3. Results and discussion T h e c h a r a c t e r i s t i c values of s a m p l e s p r e p a r e d in t h e s e two regimes are given in table 1. T h e d e m a g n e t i -
Table 1 Characteristic values of polymer bonded magnets made from a) sieved MQl-powder, b) milled and sieved MQl-powder Particle size
Br (T)
k~0jHc
Content of
(T)
oxygen (wt%)
nitrogen (wt%)
0.531 0.534 0.537 0.510 0.480 0.533 0.529 0.537 0.518 0.515 0.500 0.477
1.710 1.724 1.705 1.672 1.445 1.611 1.604 1.632 1.643 1.632 1.595 1.415
0.170 0.092 0.137 0.215 0.509 0.170 0.104 0.126 0.170 0.194 0.221 0.580
0.031 0.016 0.024 0.049 0.132 0.031 0.013 0.019 0.022 0.030 0.043 0.190
(Ixm) ~) original > 163 40-163 40- 60 < 40 b) original > 100 71-100 50- 70 32- 50 20- 32 10- 20
zation curves of samples b o n d e d in Araldit AT1 are plottet in fig. 1. C o n t r a r y to the b e h a v i o u r of m a n y particle m a g n e t s [2] the coercivity j H c decreases with 3[Tl 0.6 ..-
• It
.i0/,
/'
.222
~-;
:J -IlZ
0.5 : ,uoH [TI
Fig. 1. Demagnetization curves of bonded magnets with different powder fractions prepared by rapid quenching, grinding and sieving: - non-sieved, • . . . . . > 160 ixm, - . . . . . (40-60) txm, - . . . . < 40 ixm.
0312-8853/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved
378
A. Handstein et al. / Polymer bonded MQ1 magnets
3 IT] /" ¢.
ZSS~
S/.1 /2 .ff
r
-5
"
--Z / /" -,I
-0.Z
Fig. 2. Demagnetization curves of two bonded-powder magnets at 293 and 4.2 K (signs as in fig. 1). decreasing mean particle size of compacted powders already at values around 100 ;zm. The demagnetization curves of the other charge of bonded magnets (see table 1) behave similarly to those of fig. 1. The oxygen content increases monotonously with decreasing particle size of powder fractions as shown in table 1. A drastic rise of oxygen content was found for particle sizes smaller than about 40 I~m. Obviously the particle surfaces are chemically and mechanically disturbed. The smaller the particles made from flakes are the smaller the nucleation fields for demagnetization modes in the particles. In all cases the increasing oxygen content is accompanied with a decrease of remanence. In connection with this observation the bonded samples with smaller particles show a dip in the demagnetization curve (see fig. 1) which is assumed to be due to a reduction of nucleation fields of internal demagnetization modes [3] for some of the bonded particles. At low temperatures an additional dip in the second-quadrant demagnetization curves is caused by the well known spin reorientation transition of the Nd2Fe14B-phase below about 140 K [4], as shown in fig. 2 for T = 4.2 K. Comparing the coereivities of these two magnets at 4.2 K and room temperature, we note that the increase of coercivlty of the bonded magnet with particles smaller than 40 ~zm is strongly diminished in the low temperature range caused by spin reorientation and oxide phases [3]. The investigation of the anisotropy field H A by the single point detection method [5] showed that H A of these bonded magnets is unchanged for all powder fractions and takes the value of NdzFe14B near 8 T.
In comparison to this behaviour the coercivity of compacted powders prepared by milling of melted ingots increases with decreasing particle size whereas the coercivity of the resulting sintered magnets passes through a maximum and then decreases drastically [6,7]. Sintered magnets show a different behaviour due to oxidation processes of neodymium during sintering. It should be noted that the powders for sintered magnets are prepared to the exclusion of oxygen. Compared to sintered magnets the coercivities of the milled powders are small. By crushing sintered N d - F e - B magnets for bonded materials, Stadelmaier et al. [8] found a strong diminu: tion of j H c already at particle sizes greater than 100 Ixm. They could show that this loss is mainly caused by a mechanical damage of particles. On the other hand, a recover of j H c by annealing of crushed particles from sintered magnets should be possible for modified N d - F e - B alloys with two or more substitutions [9]. We suppose that the reduction of intrinsic coercivity of the bonded magnets with smaller particles is due to oxide phases causing an easier nucleation of internal demagnetizing processes. This work was supported by the Austrian Ministry of Research project "Magnetic Materials" and by the Austrian Science Foundation project No. 7327. References
[1] B.M. Ma and R.F. Krause, Proc. of 5th Int. Symp. on Magnetic Anisotropy and Coercivity in RE-Transition Metal Alloys, Bad Soden (1987) p. 209. [2] E. Kneller, in: Magnetism and Metallurgy, ed's. A.E. Berkowitz and E. Kneller (Academic Press, New York and London, 1969) p. 366. [3] K.-H. Miiller, A. Handstein, D. Eckert and J. Schneider, Phys. Stat. Sol. (a) 99 (1987) K61. [4] D. Givord, H.S. Li and R. Perrier de la Bfithie, Solid State Commun. 51 (1984) 857. [5] G. Asti and S. Rinaldi, J. Appl. Phys. 45 (1974) 3600. [6] C.N. Christodolou, J. Schlup and C.C. Hadjipanayis, J. Appl. Phys. 61 (1987) 3760. [7] P. Nothnagel, K.-H. Miiller, D. Eckert and A. Handstein, J. Magn. Magn. Mater. 101 (1991) 379. [8] H.H. Stadelmaier and N.C. Liu, Mater. Lett. 4 (1986) 304. [9] M. Li, K.J. Strnat and H.F. Mildrum, J. Appl. Phys. (1991) in press.