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Core losses and magnetic properties of Mn-Zn ferrites with fine grain sizes
JH
ELSEVIER
Journal of Magnetism and Magnetic Materials 133 (1994) 500-503
~
journalof magnetism and magnetic
malerials
Core losses and magnetic properties of Mn-Zn ferrites with fine grain sizes Y. Yamamoto *, A. Makino Materials R & D Dept. Alps Electric Co. Ltd 1-3-5 Higashitakami, Nagaoka 940, Japan
Abstract
M n - Z n ferrite with fine grains was obtained by sinterlng hydrothermal-precipitated ferrite powders. The average grain sizes of the hot isostatically pressed specimens were 1.0-6.8 ixm and the relative densities were above 99.3%. The frequency at which the maximum value of /~" was obtained for the ferrites tends to increase with decreasing grain size, and the core loss at 5 mT and 1-2 MHz attained a minimum value for the ferrite with grain size of 2-3 /xm. At 80°C, which is practically the usual operating temperature, the core loss of this specimen is also considerably smaller than those of the commercial ferrites with grain sizes of 7.7-10.9 Ixm at 0.5-1 MHz. Therefore, the fine-grain ferrite is expected to be useful as a material for high-frequency transformers.
1. Introduction
Soft ferrites have been extensively used for many kinds of magnetic devices such as transformers, inductors and magnetic heads for high frequency because their electrical resistivities are higher than those of soft magnetic alloys. Recently, power supply systems operating at frequencies higher than 1 MHz are available and power ferrites are called for a review of performance requirements, particularly with respect to losses and thermal properties [1]. It is well known that most of the electromagnetic characteristics of a ferrite material depend strongly on the microstructure of the fired body. The most desirable microstructure of power ferrite driving high frequency, is a small grain size [2-5]. We have reported that M n - Z n ferrite bodies with high densities above 99.8% and fine grain sizes below 5 ixm were obtained by sintering spinel ferrite powders prepared by the hydrothermal method [6,7]. The purposes of this study are (1) to examine the dependence of magnetic properties and core losses of the grain size for M n - Z n ferrites prepared by the hydrothermal methods, and (2) to compare the frequency depen-
* Corresponding author.
dence of the core losses of this ferrite with that of commercial ferrites now in use for high-frequency transformers.
2. Experiments
The M n - Z n ferrite powders for the sintered bodies were prepared by the hydrothermal method [8]. Fig. 1 shows a TEM image of the hydrothermal ferrite powder and Table 1 shows compositions of the powder. X-ray diffraction patterns (not presented in this paper) show that the final powder has the spinel structure. The powder, consisting of 53.00 mol% Fe20 3, 30.67 mol% MnO and 16.32 mol% ZnO, was mixed with a proper quantity of an organic binder, pelletized and sintered at different temperatures, in the interval 1123-1423 K for 14.4 ks in a reduced atmosphere. The specimens used for the purpose (1) was hot isostatically pressed in argon at 108 MPa for 7.2 ks in the temperature range 1173-1323 K (Table 2). The density was measured by the Archimedes technique. The microstructure was observed with a scanning electron microscope. The complex permeability (/x', /x") was measured under a field of 0.4 A / m with a LCR meter, the coercive force ( H c) was measured
0304-8853/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0304-8853(94)00193-U
Y. Yamamoto, A. Makino /Journal of Magnetism and Magnetic Materials 133 (1994) 500-503
501
10000
....:~i
' .... '.
1000
/
....
i
.
! =:~
:~ ,
[
~
.....
!]i
],~,s:2.o~, • o G,=3.~.
-,~** x . .
10000
,,' '
=1.
100
10
Fig. 1. TEM image of the powder prepared by hydrothermal method.
1000
~ :;,
::~.
...... ~
Table 1 Composition and particle sizes of the powder prepared by the hydrothermal method Particle size Fe20 ~ MnO ZnO FeO SiO 2 CaO Na20
fixm) (mol%) (mol%) (mol%) (wt%) (wt%) (wt%) (wt%)
0.125 53.00 30.67 16.32 1.40 0.006 0.010 0.0060
i 0.05
~:,'ll
:
I
[
j°:
:'r,
1
100
:
,
, '
0.1
1 Frequency
10 10
!
20
Mltz
Fig. 2. Change in the permeability as a function of the frequency for different grain size in M n - Z n ferrites.
3. R e s u l t a n d d i s c u s s i o n
u n d e r a field of 800 A / m with a dc B - H loop tracer, a n d the core loss was m e a s u r e d with Ryowa dc loop t r a c e r MMS-0375-2.1B. T h e samples were ring-shaped, with o u t e r a n d i n n e r d i a m e t e r s of 10 a n d 6 mm, respectively, a n d a thickness of 1.5 mm. T h e s a t u r a t i o n m a g n e t i z a t i o n (B~) was m e a s u r e d for t h e d i s k - s h a p e d sample with d i m e n s i o n s of 6 m m in d i a m e t e r a n d 1.5 m m thick u n d e r a field of 800 k A / m with a VSM. T h e resistivity ( p ) was m e a s u r e d by the four-point p r o b e m e t h o d . All m e a s u r e m e n t s were carried out at r o o m t e m p e r a t u r e except the c o m p a r i s o n of t h e core loss with c o m m e r c i a l ferrites.
T h e densities, the grain sizes a n d m a g n e t i c p r o p e r ties of the hot isostatically pressed specimens are shown in T a b l e 2. T h e average grain sizes of the specimens, which d e c r e a s e monotonically with decreasing presintering t e m p e r a t u r e , are 1 to 6.8 Ixm. T h e relative densities are above 99.3%, so it can be c o n s i d e r e d t h a t m a g n e t i c p r o p e r t i e s are little influenced by the difference of the porosity in the specimens. B s is almost the same in all specimens. H c increases m o n o t o n o u s l y as the average grain sizes b e c o m e smaller. T h e p value of the specimens increases with increasing p r e s i n t e r i n g temperature. Fig. 2 shows the frequency d e p e n d e n c e of ~' a n d / x " for different grain sizes of the ferrite r e p o r t e d in T a b l e 2. In the low-frequency r a n g e /x' is smaller for small
Table 2 Densities, grain sizes and magnetic properties of specimens prepared by sintering and hot isostatic pressing at various temperatures Presintering temperature (K)
Hot isostatic pressing temperature (K)
Relative density (%)
Grain size (Ixm)
B800 (B s) (T)
He (A m ~)
tz, a
tan 8 a
p (f~ cm)
1223 1273 1323 1373 1423
1173 1248 1298 1323 1323
99.6 99.3 99.8 99.9 99.9
1.0 2.0 3.2 5.5 6.8
0.512 0.516 0.518 0.518 0.519
132 44 24 13 11
460 1530 2690 3020 3000
0.04 0.09 0.22 0.56 0.69
1.9 3.0 4.1 8.4 8.6
a f = 1 MHz.
502
Y. Yamamoto, A. Makino /Journal of Magnetism and Magnetic Materials 133 (1994) 500-503
grain size, while the opposite is observed in the highfrequency region. Based on the frequency dependence of ~', it can be said that the frequency properties o f / z ' are improved with decreasing grain size of the ferrites. T h e / x " value decreases with decreasing grain size over the whole frequency range. The peak of a /z"-f curve, which is considered to be caused by the domain wall resonance and to give rise to the main part of the total core loss in the high-frequency range, shifts gradually to high frequency with decreasing grain size. Fig. 3 shows the frequency dependence of the core loss on grain size for the ferrites. The relation of the core loss and grain size is found clearly. A t 50 mT, the core losses in the low-frequency range decrease with increasing average grain size. The hysteresis loss is a major part of the total loss in the low-frequency range [9], the hysteresis loss is expected to be larger for ferrites with small grain size on the basis of the dependence of H c on grain size, as described above. At 5 mT, on the other hand, the core loss attains a minim u m value, and the average grain size at which the
10000
[ (measured at 80°C) ] . . . . . . . .
tx Commercial A O Commercial B ........ I [] CommereialC
,7
E
tO00
100
~'~
,
0.05
, I
I
0.1
1 Frequency
/
MHz
Fig. 4. Comparison of the core losses as a function of frequency for the ferrite with fine grain size and commercial ferrites.
I00000
minimum value of the core loss is obtained decreases with increasing frequency. It is considered that the dependence of the core loss on the grain size is almost explained by the relation between the frequency at which domain wall resonance occurs and grain size for the ferrites. Fig. 4 shows the frequency dependence of the core losses of the ferrite with fine grain size (G~ = 2.5 Ixm) and commercial ferrites (G s = 7.7-10.9 Ixm) under the condition that is generally used for the evaluation of power ferrites ( f - B m = 25 × 103 H z - T at 80°C). In the low-frequency range, up to 500 kHz, the core losses of the ferrite with fine grain size are not as good as those of commercial ferrites, but in the high-frequency range, above 500 kHz, the core losses of ferrite with fine grain size are lower than those of commercial products. Especially at 1 MHz, the core loss of the fine grain size was 153 kW m -3, about 75% lower than that of the commercial ferrites. Therefore, the ferrites with fine grain size, obtained by sintering spinel ferrite powder prepared by the hydrothermal method, are expected to be useful as material for high-frequency transformers.
10000
I000
2
lOO
10
0.1
References 1
2
5
1
2
5
10
Grain size / ~tm Fig. 3. Change in the core loss as a function of average grain size.
[1] E.C. Snelling, Proc. ICF-5 (1989) 579. [2] T. Sano, A. Morita and A. Matsukawa, Proc. ICF-5 (1989) 595.
Y. Yamamoto, A. Makino /Journal of Magnetism and Magnetic Materials 133 (1994) 500-503 [3] M. Sasaki, K. Ito, W. Ohashi and K. Watanabe, Proc. ICF-6 (1992) 811. [4] S. Nagata, Y. Takahashi, M. Yarizumi and K. Aso, Proc. IC-6 (1992) 1191. [5] R. Lebourgeois, C. Deljurie and J.P. Ganne, Proc. ICF-6 (1992) 1169. [6] A. Makino, Y. Yamamoto and N. Sakatsume, J. Jpn. Soc. Powder and Powder Metall. 39 (1992) 129.
503
[7] Y. Yamamoto, T. Kuriyama and A. Makino, Proc. ICF-6 (1992) 1214. [8] S. Komarneni, E. Fregeau, E. Breval and R. Roy, J. Am. Ceram. Soc. 71 (1988) c-26. [9] E.G. Visser, J.J. Roelofsma and G.J.M. Aaftink, Proc. ICF-5 (1989) 605.
ELSEVIER
Journal of Magnetism and Magnetic Materials 133 (1994) 500-503
~
journalof magnetism and magnetic
malerials
Core losses and magnetic properties of Mn-Zn ferrites with fine grain sizes Y. Yamamoto *, A. Makino Materials R & D Dept. Alps Electric Co. Ltd 1-3-5 Higashitakami, Nagaoka 940, Japan
Abstract
M n - Z n ferrite with fine grains was obtained by sinterlng hydrothermal-precipitated ferrite powders. The average grain sizes of the hot isostatically pressed specimens were 1.0-6.8 ixm and the relative densities were above 99.3%. The frequency at which the maximum value of /~" was obtained for the ferrites tends to increase with decreasing grain size, and the core loss at 5 mT and 1-2 MHz attained a minimum value for the ferrite with grain size of 2-3 /xm. At 80°C, which is practically the usual operating temperature, the core loss of this specimen is also considerably smaller than those of the commercial ferrites with grain sizes of 7.7-10.9 Ixm at 0.5-1 MHz. Therefore, the fine-grain ferrite is expected to be useful as a material for high-frequency transformers.
1. Introduction
Soft ferrites have been extensively used for many kinds of magnetic devices such as transformers, inductors and magnetic heads for high frequency because their electrical resistivities are higher than those of soft magnetic alloys. Recently, power supply systems operating at frequencies higher than 1 MHz are available and power ferrites are called for a review of performance requirements, particularly with respect to losses and thermal properties [1]. It is well known that most of the electromagnetic characteristics of a ferrite material depend strongly on the microstructure of the fired body. The most desirable microstructure of power ferrite driving high frequency, is a small grain size [2-5]. We have reported that M n - Z n ferrite bodies with high densities above 99.8% and fine grain sizes below 5 ixm were obtained by sintering spinel ferrite powders prepared by the hydrothermal method [6,7]. The purposes of this study are (1) to examine the dependence of magnetic properties and core losses of the grain size for M n - Z n ferrites prepared by the hydrothermal methods, and (2) to compare the frequency depen-
* Corresponding author.
dence of the core losses of this ferrite with that of commercial ferrites now in use for high-frequency transformers.
2. Experiments
The M n - Z n ferrite powders for the sintered bodies were prepared by the hydrothermal method [8]. Fig. 1 shows a TEM image of the hydrothermal ferrite powder and Table 1 shows compositions of the powder. X-ray diffraction patterns (not presented in this paper) show that the final powder has the spinel structure. The powder, consisting of 53.00 mol% Fe20 3, 30.67 mol% MnO and 16.32 mol% ZnO, was mixed with a proper quantity of an organic binder, pelletized and sintered at different temperatures, in the interval 1123-1423 K for 14.4 ks in a reduced atmosphere. The specimens used for the purpose (1) was hot isostatically pressed in argon at 108 MPa for 7.2 ks in the temperature range 1173-1323 K (Table 2). The density was measured by the Archimedes technique. The microstructure was observed with a scanning electron microscope. The complex permeability (/x', /x") was measured under a field of 0.4 A / m with a LCR meter, the coercive force ( H c) was measured
0304-8853/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0304-8853(94)00193-U
Y. Yamamoto, A. Makino /Journal of Magnetism and Magnetic Materials 133 (1994) 500-503
501
10000
....:~i
' .... '.
1000
/
....
i
.
! =:~
:~ ,
[
~
.....
!]i
],~,s:2.o~, • o G,=3.~.
-,~** x . .
10000
,,' '
=1.
100
10
Fig. 1. TEM image of the powder prepared by hydrothermal method.
1000
~ :;,
::~.
...... ~
Table 1 Composition and particle sizes of the powder prepared by the hydrothermal method Particle size Fe20 ~ MnO ZnO FeO SiO 2 CaO Na20
fixm) (mol%) (mol%) (mol%) (wt%) (wt%) (wt%) (wt%)
0.125 53.00 30.67 16.32 1.40 0.006 0.010 0.0060
i 0.05
~:,'ll
:
I
[
j°:
:'r,
1
100
:
,
, '
0.1
1 Frequency
10 10
!
20
Mltz
Fig. 2. Change in the permeability as a function of the frequency for different grain size in M n - Z n ferrites.
3. R e s u l t a n d d i s c u s s i o n
u n d e r a field of 800 A / m with a dc B - H loop tracer, a n d the core loss was m e a s u r e d with Ryowa dc loop t r a c e r MMS-0375-2.1B. T h e samples were ring-shaped, with o u t e r a n d i n n e r d i a m e t e r s of 10 a n d 6 mm, respectively, a n d a thickness of 1.5 mm. T h e s a t u r a t i o n m a g n e t i z a t i o n (B~) was m e a s u r e d for t h e d i s k - s h a p e d sample with d i m e n s i o n s of 6 m m in d i a m e t e r a n d 1.5 m m thick u n d e r a field of 800 k A / m with a VSM. T h e resistivity ( p ) was m e a s u r e d by the four-point p r o b e m e t h o d . All m e a s u r e m e n t s were carried out at r o o m t e m p e r a t u r e except the c o m p a r i s o n of t h e core loss with c o m m e r c i a l ferrites.
T h e densities, the grain sizes a n d m a g n e t i c p r o p e r ties of the hot isostatically pressed specimens are shown in T a b l e 2. T h e average grain sizes of the specimens, which d e c r e a s e monotonically with decreasing presintering t e m p e r a t u r e , are 1 to 6.8 Ixm. T h e relative densities are above 99.3%, so it can be c o n s i d e r e d t h a t m a g n e t i c p r o p e r t i e s are little influenced by the difference of the porosity in the specimens. B s is almost the same in all specimens. H c increases m o n o t o n o u s l y as the average grain sizes b e c o m e smaller. T h e p value of the specimens increases with increasing p r e s i n t e r i n g temperature. Fig. 2 shows the frequency d e p e n d e n c e of ~' a n d / x " for different grain sizes of the ferrite r e p o r t e d in T a b l e 2. In the low-frequency r a n g e /x' is smaller for small
Table 2 Densities, grain sizes and magnetic properties of specimens prepared by sintering and hot isostatic pressing at various temperatures Presintering temperature (K)
Hot isostatic pressing temperature (K)
Relative density (%)
Grain size (Ixm)
B800 (B s) (T)
He (A m ~)
tz, a
tan 8 a
p (f~ cm)
1223 1273 1323 1373 1423
1173 1248 1298 1323 1323
99.6 99.3 99.8 99.9 99.9
1.0 2.0 3.2 5.5 6.8
0.512 0.516 0.518 0.518 0.519
132 44 24 13 11
460 1530 2690 3020 3000
0.04 0.09 0.22 0.56 0.69
1.9 3.0 4.1 8.4 8.6
a f = 1 MHz.
502
Y. Yamamoto, A. Makino /Journal of Magnetism and Magnetic Materials 133 (1994) 500-503
grain size, while the opposite is observed in the highfrequency region. Based on the frequency dependence of ~', it can be said that the frequency properties o f / z ' are improved with decreasing grain size of the ferrites. T h e / x " value decreases with decreasing grain size over the whole frequency range. The peak of a /z"-f curve, which is considered to be caused by the domain wall resonance and to give rise to the main part of the total core loss in the high-frequency range, shifts gradually to high frequency with decreasing grain size. Fig. 3 shows the frequency dependence of the core loss on grain size for the ferrites. The relation of the core loss and grain size is found clearly. A t 50 mT, the core losses in the low-frequency range decrease with increasing average grain size. The hysteresis loss is a major part of the total loss in the low-frequency range [9], the hysteresis loss is expected to be larger for ferrites with small grain size on the basis of the dependence of H c on grain size, as described above. At 5 mT, on the other hand, the core loss attains a minim u m value, and the average grain size at which the
10000
[ (measured at 80°C) ] . . . . . . . .
tx Commercial A O Commercial B ........ I [] CommereialC
,7
E
tO00
100
~'~
,
0.05
, I
I
0.1
1 Frequency
/
MHz
Fig. 4. Comparison of the core losses as a function of frequency for the ferrite with fine grain size and commercial ferrites.
I00000
minimum value of the core loss is obtained decreases with increasing frequency. It is considered that the dependence of the core loss on the grain size is almost explained by the relation between the frequency at which domain wall resonance occurs and grain size for the ferrites. Fig. 4 shows the frequency dependence of the core losses of the ferrite with fine grain size (G~ = 2.5 Ixm) and commercial ferrites (G s = 7.7-10.9 Ixm) under the condition that is generally used for the evaluation of power ferrites ( f - B m = 25 × 103 H z - T at 80°C). In the low-frequency range, up to 500 kHz, the core losses of the ferrite with fine grain size are not as good as those of commercial ferrites, but in the high-frequency range, above 500 kHz, the core losses of ferrite with fine grain size are lower than those of commercial products. Especially at 1 MHz, the core loss of the fine grain size was 153 kW m -3, about 75% lower than that of the commercial ferrites. Therefore, the ferrites with fine grain size, obtained by sintering spinel ferrite powder prepared by the hydrothermal method, are expected to be useful as material for high-frequency transformers.
10000
I000
2
lOO
10
0.1
References 1
2
5
1
2
5
10
Grain size / ~tm Fig. 3. Change in the core loss as a function of average grain size.
[1] E.C. Snelling, Proc. ICF-5 (1989) 579. [2] T. Sano, A. Morita and A. Matsukawa, Proc. ICF-5 (1989) 595.
Y. Yamamoto, A. Makino /Journal of Magnetism and Magnetic Materials 133 (1994) 500-503 [3] M. Sasaki, K. Ito, W. Ohashi and K. Watanabe, Proc. ICF-6 (1992) 811. [4] S. Nagata, Y. Takahashi, M. Yarizumi and K. Aso, Proc. IC-6 (1992) 1191. [5] R. Lebourgeois, C. Deljurie and J.P. Ganne, Proc. ICF-6 (1992) 1169. [6] A. Makino, Y. Yamamoto and N. Sakatsume, J. Jpn. Soc. Powder and Powder Metall. 39 (1992) 129.
503
[7] Y. Yamamoto, T. Kuriyama and A. Makino, Proc. ICF-6 (1992) 1214. [8] S. Komarneni, E. Fregeau, E. Breval and R. Roy, J. Am. Ceram. Soc. 71 (1988) c-26. [9] E.G. Visser, J.J. Roelofsma and G.J.M. Aaftink, Proc. ICF-5 (1989) 605.