- Email: [email protected]
Magnetic shielding by soft magnetic materials in alternating magnetic field
,M"
Journal of Magnetism and Magnetic Materials 112 (1992) 192-194 North-Holland
Magnetic shielding by soft magnetic materials in alternating magnetic field Yasuo O k a z a k i and l~yoshi U e n o Steel Research Laboratory, N;ppon Steel Corporation, 20-1 Shintomi, Futtsu, Chiba 299-11, Japan
The magnetic shielding effect of an alternating field up to 20 kHz was examined in 3% Si steel sheets and amorphous ribbons. Not only the permeability but also the domain configuration was found to affect the ~hielding effects. The annealed Fe.based amorphous shield without field showed exceedingly high shielding effectiveness fer higher frequencies.
1. Introduction Magnetic shielding by ferromagnetic materials in a dc magnetic field has been studied theoretically and experimentally. The effectiveness of the dc magnetic shielding is based on the high permeability of the ferromagnetic materials, but magnetic shielding in an alternating field up to 20 kHz has not been studied much for ferromagnetic materials. Recently, magnetic city noise shielding has become important, where the magnetic field has various amplitudes and frequencies. Magnetic shielding for higher frequency above 10 kHz that is defined as electromagnetic wave is considered to be done effectively by metallic soft magnetic materials because of their low electric resistivities [11. The effectiveness of shielding by silicon steel sheet with Goss texture decreases with increasing frequency [2]. The magnetic shielding by soft magnetic materials is directly related to the flux distribution in the material, so the texture ~nd th,. ,,, . . . . t;,. domain ,-,~,, ,~1~,,;m,-,,~,-t~,, roles. Here, the shielding effectiveness of an alternating magnetic field in cylindrical shields of 3% Si steel sheets with different textures, and differIk I11 I1~¢
1 I 1 I L , I . ~ 1 I ~ ¢ I. It ~,~
1 1 1 ¢~,lkJ
~.J 1 IkALf f
llll~J~J&
I~ Ik.&1 I ~,
Correspondence to. Dr. Y. Okazaki, Nippon St~.ei C6rp., Electromagnetic Materials, Steel Research Lab., 20-1, Shintomi, Futtsu, Chiba 229-11, Japan. Tel.: +81-439-802316; telefax: + ~,1-439-8{12746.
ent domain configurations, was examined. The magnetic anisotrooy Fe-based amorph,ms ribbons induced by field annealing was also examined. The theoretical approach by three-dimensional analysis by the finite-eiement method was also carried out. 2. Experimental details Three different 3% Si steel sheets were selected to investigate the influence of texture and magnetic domain configuration op the alternating shielding effectiveness. 0.30 mm thick oriented (HIB) 30ZH100 [3], 0.35 mm thick doubly oriented (DO) [4] and 0.50 mm thick non-oriented (NO) 50H210 [3] were formed into cylindrical shields 100 mm in diameter and 200 mm in length, Cylinders of 1- and 6-layer Fe80.5(Si, B)19.5 amorphous ribbon (AM) of 60 mm diameter and 150 mm long were also prepared. For HIB shields with Goss texture, the easy axis of magnetization (100) was set in the radial or ,,,,:,,1 A:~,,~,:,.., In ,h,, DO ~h;,,~n /~na\ wa laid in both the radi2l and axial directions. The field-annealed AM ~hield had its easy magnetizing axis in the radial direction. 'Fhe NO shield and the AM shield annealed without field had practically no preferred orientation. All shields were str~.,-relief.annealed: for HIB and DO shields at S00°C 3 h for NO 750°C 2 h; and AM 365°C 1 h with and without dc field.
0304-8853/~)2/$05.()() ~ 1992 - Elsevier Science Pubhshers B.V. All rights rt .,red
Y. Okazaki, K. Ueno / Magnetic shielding by soft magnetic materials
The field was measured by a pick-up coil under a parallel external field, He, of 10-300 mG and 50-20 kHz. The tangential shielding factor, ST, was defined as H e / H i , where H i is the internal field. The permeability of the shield was measured only in the radial direction under dc and ac magnetization. The magnetic domains were observed by a 200 kV S T E M in the demagnetized condition. The shielding effect was simulated by dimensional finite-element method at 100 Hz and 10 kHz.
193
100 i ~ F e T He=a0mG
S T 10
3. Results and discussion
1 30.001
3.1. Frequency dependence of shielding factor
t
I
I
0.01
0.1
1
Frequency
Fig. 1 shows the shielding factors, ST, for the 3% Si steel shields. HIB shield with (100) axis parallel to the radial direction showed decreasing ST with frequency up to 10 kHz. The HIB shield with (100) in :he axis direction had low ST. The ST of the NO shield increased rapidly from 1 kHz up to 10 kHz. The ST for both HIB and NO showed fluctuations around 15 kHz. For the DO shield, the ST showed the same tendency as for the NO. Fig. 2 shows that the ST of the six-layer AM shield annealed without field increased rapidly above 1 kHz. On the contrary, the fieldannealed shield showed a iow ST in spite of its higher permeability. The HIB and field-annealed 100
"
I
SHIELD"
"
I
Cylinder
' =100raG IZ~3%Si NO
'He
t00dia*200L 103%Si HIB iE]3%Si DO
100
(KHz)
Fig. 2. Frequency curve for the shielding factor, ST, of six-layer F e - S i - B amorphous ribbon shields: o as case, [] zero-field annealed, • field annealed.
AM sheets had parallel 180° domains with large spacing of about 0.5-1 mm for HIB and 2-3 mm for AM. The NO and zero-field-annealed AM showed small and non-uniaxial 180 ° domains, as shown in fig. 3. The DO shows the typically crossed 180° domains and the domain spacing was the as same as for HIB. Considering the frequency dependence of the ST in relation to the domain configuration, the shields with large parallel 180° domains showed a decrease of ST with increase of frequency up to 10 kHz. Gn the other hand, the shields with small and random 180 ° domains showed a rapid inc:ease of ST from 1 kHz.
3.2. Calculation of the shielding factor For cylindrical shields of infinite length, the shielding factor ST can be calculated as follows. If d / D < l ar, d ~ > 1 , herc a = D / 6 t.t, t3= 2(d/8) [2]. The penetration depth, 6~ is defined as ( 1 0 7 p / 4 # f ) , where p is the specific resistance, f is the frequency, # is the permeability and d is the wall thickness,
10
1I.
...... I 10
!
I [111
0.001
0.01
:. , 3 ~ . i l U. J I
0.1
Frequency
1
I
|
1 1 lJtt~lL
I l t l n l
( KHz )
10
100
Fig. 1. Frequency curve for the shielding factor, ST, of 3% Si steel sheet: © HIB, zx NO, [] DO.
ST =
{[
+
](co h
e - co,
e
+ a ( s i n h /3 - sin/3) + ½oe(sinh/3 + sin 13) +5'
( cosh 13 + cos 13) }.
( l)
Y.. Okazaki, K, Ueno / Magnetic shielding by soft magnetic materials
194
i
Fig. 3. Magnetic domains of Fe-Si-B ainorphous ribbon: (a) zero-field annealed, (b) field annealed.
Fig. 4 shows the rapid increase of ST with frequency, calculated by eq. (l), for the HIB, NO and AM shields. These results contradicted the experimental results, which indicates an overestimation of the eddy-current effect. A three-dimensional FEM analysis was carried out to simulate the shielding effectiveness of the 3% Si steel cylinders at 100 Hz and 10 kHz, using the eddy-current analysis by the T-.Q method. fhe ST for the HIB shield with (100) in the radial direction was 50 at 100 Hz and 6 at 10 kHz, which was quite different from the calculation. One of the reasons of this discrepancy may come from the eddy-current distribution in the shield. A 3D FEM analysis revealed a current density
100 " ' ~ C ~inder(Celcu vl lated)//~ i~//~)/~
10 -
/y f r
distribution four times larger for the component parallel to the H e direction of the shield than the front, perpendicular to He component. The front part with smaller current density could make ST decrease at higher frequencies compared with the uniform current distribution in the calculation. 4. Conclusions
Alternating magnetic shielding was examined in cylindrical shields of 3% Si steel sheets and Fe-based amorphous ribbons. (1) Shields with large and straight 180° domains showed decreasing shielding factor, ST, with increasing frequency, whereas shields with small domains showed increasing ST at higher frequencies. (2) A three-dimensional finite-element method analysis showed the same shielding tendency with frequency as the experiments in the Goss-textured shields. The eddy-current density distribution varied in the shields, which might explain the lower shielding effectiveness at higher frequencies. [ eferences
I-,=,
0.001
I
0.01
I
I
I
_
.
_
_
0.1 1 10 100 Frecmuer~y ( K H z ) Fig. 4. Calculated frequency cu~'e for the shielding factor ST: ,~ }-liB, ,~ NO, [] AM zero-field annealed, 1:3, AM field annealed.
[1] J.L. V/pierre, D.R.L White and M.F. Vi:)lette, in: Electr~magnetic Compatibility Handbook (Van Nostrand Reinhold, New York, 19871 p. 320. [2] A.J, Mager, IEEE Trans. Magn. MAG-6 (1970) 67. [3] Electrical Steel Sheet Catalog, Nippon Steel Co. (1990). [4] S. Arai, M. Mizokami and M. Yabumoto, J. Appl. Phys. 67 (199{I) 5577.
Journal of Magnetism and Magnetic Materials 112 (1992) 192-194 North-Holland
Magnetic shielding by soft magnetic materials in alternating magnetic field Yasuo O k a z a k i and l~yoshi U e n o Steel Research Laboratory, N;ppon Steel Corporation, 20-1 Shintomi, Futtsu, Chiba 299-11, Japan
The magnetic shielding effect of an alternating field up to 20 kHz was examined in 3% Si steel sheets and amorphous ribbons. Not only the permeability but also the domain configuration was found to affect the ~hielding effects. The annealed Fe.based amorphous shield without field showed exceedingly high shielding effectiveness fer higher frequencies.
1. Introduction Magnetic shielding by ferromagnetic materials in a dc magnetic field has been studied theoretically and experimentally. The effectiveness of the dc magnetic shielding is based on the high permeability of the ferromagnetic materials, but magnetic shielding in an alternating field up to 20 kHz has not been studied much for ferromagnetic materials. Recently, magnetic city noise shielding has become important, where the magnetic field has various amplitudes and frequencies. Magnetic shielding for higher frequency above 10 kHz that is defined as electromagnetic wave is considered to be done effectively by metallic soft magnetic materials because of their low electric resistivities [11. The effectiveness of shielding by silicon steel sheet with Goss texture decreases with increasing frequency [2]. The magnetic shielding by soft magnetic materials is directly related to the flux distribution in the material, so the texture ~nd th,. ,,, . . . . t;,. domain ,-,~,, ,~1~,,;m,-,,~,-t~,, roles. Here, the shielding effectiveness of an alternating magnetic field in cylindrical shields of 3% Si steel sheets with different textures, and differIk I11 I1~¢
1 I 1 I L , I . ~ 1 I ~ ¢ I. It ~,~
1 1 1 ¢~,lkJ
~.J 1 IkALf f
llll~J~J&
I~ Ik.&1 I ~,
Correspondence to. Dr. Y. Okazaki, Nippon St~.ei C6rp., Electromagnetic Materials, Steel Research Lab., 20-1, Shintomi, Futtsu, Chiba 229-11, Japan. Tel.: +81-439-802316; telefax: + ~,1-439-8{12746.
ent domain configurations, was examined. The magnetic anisotrooy Fe-based amorph,ms ribbons induced by field annealing was also examined. The theoretical approach by three-dimensional analysis by the finite-eiement method was also carried out. 2. Experimental details Three different 3% Si steel sheets were selected to investigate the influence of texture and magnetic domain configuration op the alternating shielding effectiveness. 0.30 mm thick oriented (HIB) 30ZH100 [3], 0.35 mm thick doubly oriented (DO) [4] and 0.50 mm thick non-oriented (NO) 50H210 [3] were formed into cylindrical shields 100 mm in diameter and 200 mm in length, Cylinders of 1- and 6-layer Fe80.5(Si, B)19.5 amorphous ribbon (AM) of 60 mm diameter and 150 mm long were also prepared. For HIB shields with Goss texture, the easy axis of magnetization (100) was set in the radial or ,,,,:,,1 A:~,,~,:,.., In ,h,, DO ~h;,,~n /~na\ wa laid in both the radi2l and axial directions. The field-annealed AM ~hield had its easy magnetizing axis in the radial direction. 'Fhe NO shield and the AM shield annealed without field had practically no preferred orientation. All shields were str~.,-relief.annealed: for HIB and DO shields at S00°C 3 h for NO 750°C 2 h; and AM 365°C 1 h with and without dc field.
0304-8853/~)2/$05.()() ~ 1992 - Elsevier Science Pubhshers B.V. All rights rt .,red
Y. Okazaki, K. Ueno / Magnetic shielding by soft magnetic materials
The field was measured by a pick-up coil under a parallel external field, He, of 10-300 mG and 50-20 kHz. The tangential shielding factor, ST, was defined as H e / H i , where H i is the internal field. The permeability of the shield was measured only in the radial direction under dc and ac magnetization. The magnetic domains were observed by a 200 kV S T E M in the demagnetized condition. The shielding effect was simulated by dimensional finite-element method at 100 Hz and 10 kHz.
193
100 i ~ F e T He=a0mG
S T 10
3. Results and discussion
1 30.001
3.1. Frequency dependence of shielding factor
t
I
I
0.01
0.1
1
Frequency
Fig. 1 shows the shielding factors, ST, for the 3% Si steel shields. HIB shield with (100) axis parallel to the radial direction showed decreasing ST with frequency up to 10 kHz. The HIB shield with (100) in :he axis direction had low ST. The ST of the NO shield increased rapidly from 1 kHz up to 10 kHz. The ST for both HIB and NO showed fluctuations around 15 kHz. For the DO shield, the ST showed the same tendency as for the NO. Fig. 2 shows that the ST of the six-layer AM shield annealed without field increased rapidly above 1 kHz. On the contrary, the fieldannealed shield showed a iow ST in spite of its higher permeability. The HIB and field-annealed 100
"
I
SHIELD"
"
I
Cylinder
' =100raG IZ~3%Si NO
'He
t00dia*200L 103%Si HIB iE]3%Si DO
100
(KHz)
Fig. 2. Frequency curve for the shielding factor, ST, of six-layer F e - S i - B amorphous ribbon shields: o as case, [] zero-field annealed, • field annealed.
AM sheets had parallel 180° domains with large spacing of about 0.5-1 mm for HIB and 2-3 mm for AM. The NO and zero-field-annealed AM showed small and non-uniaxial 180 ° domains, as shown in fig. 3. The DO shows the typically crossed 180° domains and the domain spacing was the as same as for HIB. Considering the frequency dependence of the ST in relation to the domain configuration, the shields with large parallel 180° domains showed a decrease of ST with increase of frequency up to 10 kHz. Gn the other hand, the shields with small and random 180 ° domains showed a rapid inc:ease of ST from 1 kHz.
3.2. Calculation of the shielding factor For cylindrical shields of infinite length, the shielding factor ST can be calculated as follows. If d / D < l ar, d ~ > 1 , herc a = D / 6 t.t, t3= 2(d/8) [2]. The penetration depth, 6~ is defined as ( 1 0 7 p / 4 # f ) , where p is the specific resistance, f is the frequency, # is the permeability and d is the wall thickness,
10
1I.
...... I 10
!
I [111
0.001
0.01
:. , 3 ~ . i l U. J I
0.1
Frequency
1
I
|
1 1 lJtt~lL
I l t l n l
( KHz )
10
100
Fig. 1. Frequency curve for the shielding factor, ST, of 3% Si steel sheet: © HIB, zx NO, [] DO.
ST =
{[
+
](co h
e - co,
e
+ a ( s i n h /3 - sin/3) + ½oe(sinh/3 + sin 13) +5'
( cosh 13 + cos 13) }.
( l)
Y.. Okazaki, K, Ueno / Magnetic shielding by soft magnetic materials
194
i
Fig. 3. Magnetic domains of Fe-Si-B ainorphous ribbon: (a) zero-field annealed, (b) field annealed.
Fig. 4 shows the rapid increase of ST with frequency, calculated by eq. (l), for the HIB, NO and AM shields. These results contradicted the experimental results, which indicates an overestimation of the eddy-current effect. A three-dimensional FEM analysis was carried out to simulate the shielding effectiveness of the 3% Si steel cylinders at 100 Hz and 10 kHz, using the eddy-current analysis by the T-.Q method. fhe ST for the HIB shield with (100) in the radial direction was 50 at 100 Hz and 6 at 10 kHz, which was quite different from the calculation. One of the reasons of this discrepancy may come from the eddy-current distribution in the shield. A 3D FEM analysis revealed a current density
100 " ' ~ C ~inder(Celcu vl lated)//~ i~//~)/~
10 -
/y f r
distribution four times larger for the component parallel to the H e direction of the shield than the front, perpendicular to He component. The front part with smaller current density could make ST decrease at higher frequencies compared with the uniform current distribution in the calculation. 4. Conclusions
Alternating magnetic shielding was examined in cylindrical shields of 3% Si steel sheets and Fe-based amorphous ribbons. (1) Shields with large and straight 180° domains showed decreasing shielding factor, ST, with increasing frequency, whereas shields with small domains showed increasing ST at higher frequencies. (2) A three-dimensional finite-element method analysis showed the same shielding tendency with frequency as the experiments in the Goss-textured shields. The eddy-current density distribution varied in the shields, which might explain the lower shielding effectiveness at higher frequencies. [ eferences
I-,=,
0.001
I
0.01
I
I
I
_
.
_
_
0.1 1 10 100 Frecmuer~y ( K H z ) Fig. 4. Calculated frequency cu~'e for the shielding factor ST: ,~ }-liB, ,~ NO, [] AM zero-field annealed, 1:3, AM field annealed.
[1] J.L. V/pierre, D.R.L White and M.F. Vi:)lette, in: Electr~magnetic Compatibility Handbook (Van Nostrand Reinhold, New York, 19871 p. 320. [2] A.J, Mager, IEEE Trans. Magn. MAG-6 (1970) 67. [3] Electrical Steel Sheet Catalog, Nippon Steel Co. (1990). [4] S. Arai, M. Mizokami and M. Yabumoto, J. Appl. Phys. 67 (199{I) 5577.