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Nanocrystalline soft magnetic composite-cores with ideal orientation of the powder-flakes
ELSEVIER
Journal of Magnetism and Magnetic Materials 196-197 (1999) 327-329
Journalof magnetism and magnetic ~ l ~ materials
Nanocrystalline soft magnetic composite-cores with ideal orientation of the powder-flakes D. Nuetzel a'c'*, G. Rieger a, J. Wecker a, J. Petzold b, M.
Mueller
c
"Siemens AG, Corporate Technology, ZT MF 1, Paul-Gossen Str. 115, P.O. Box 3220, D-91050 Erlangen, Germany bVacuumschmelze GmbH, Griiner Weg 37, D-63450 Hanau, Germany Clnstitutefor Solid State and Material Research, Helmholtzstr 20, D-01171 Dresden, Germany
Abstract Amorphous Fev3,sCulNb3Sils,sB7 powders with a particle size up to 1400 ~tm were consolidated to cores by using a press-additive, which allows an ideal orientation of the flakes. The nanocrystalline cores exhibit permeabilities up to #e = 6000 and a coercive field of Hc = 8 A/m. The magnetic properties can be varied in a wide range depending on particle size and content of press-additive. ~ 1999 Elsevier Science B.V. All rights reserved. Keywords: Nanocrystalline structure; Consolidation; Powder cores; Soft magnetic properties
1. Introduction Iron-based nanocrystalline alloys exhibit excellent soft magnetic properties [1,2]. But due to process of rapid solidification the shape of these alloys is limited to ribbons and wires. However, by consolidation of amorphous powder flakes complex-shape parts can be realised. Recently, different consolidation techniques have been reported. The proposed procedures like explosive compaction [3], shock-wave compaction I-4] and static high-pressure compaction [5] with pressures up to 5 GPa are however costly and complex. The aim of this study is the preparation of compacts by a simple hot-pressing process in a low-pressure range. A special glass-based press-additive allows high densities and an ideal orientation of the flakes in the cores.
ous as-quenched state by using an ultra centrifugal mill (Retsch ZM 100) at room temperature. Afterwards the powder were sieved in fractions ranging from 20 to 1400 gm. The powder flakes were mixed with different contents of a glass-additive. The glass was a low melting, electrical isolating composite solder glass. The powders were filled into cylindrical dies (outer diameter 15 mm) with graphite inlays. The press chamber was evacuated and heated by an infrared furnace up to 500°C with an applied pressure of 500 MPa. This temperature was maintained for 300 s. The consolidated flake cores were annealed for 1 h at 520°C in vacuum to generate the nanocrystalline state and to reduce press-induced stresses. The magnetic measurements were made using a F6rster coerzimeter for the coercive force and an impedance analyser (HP 4284A) for the frequency-dependent permeability.
2. Experimental procedure
3. Results and discussion
Rapidly quenched Fev3,sCuxNb3Sils.sB7 ribbons (Vitroperm 500 ':) were ground to powder in the amorph-
In the first series of experiments the particle size was varied between 20 and 1400 gm with a fixed glass content of 5 wt%. In the second series we investigated the effect of the glass content in combination with a fixed particle size (125-150 gm).
*Corresponding author. Tel.: + 49-131-731435; fax: + 49131-732469; e-mail: [email protected].
0304-8853/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 7 3 6 - 7
328
D. Nuetzel et al. / Journal ~?/ Magnetism and Magnetic Materials 196- 197 (l 999) 3 , ,
The nanocrystalline cores with a glass contenl of 5 wt% exhibit densities between 90% and 93% of the theoretical density. In Fig. 1 one can see the effect of the glass-additive on the microstructure of the compacts. The glass becomes viscous at the pressing temperature of 500"C and enables the flakes to move against each other. Pores are filled, resulting in higher densities than glassfiee cores (densities: 85-88%). The glass-additive allows high densities with a low pressing pressure of 500 MPa. The particle movement leads to a regular planar orientation of the powder flakes in the cores which resuhs in good soft magnetic properties. The coercive field of the powder cores is mainly influenced by the distinct plastic deformation zone of the crack surface of the powder flakes induced by the milling process. Small particles have a higher content of such structural defects in relation to their size than larger ones leading to an enhanced domain wall pinning. This is the main reason for the increase of the coercive field of cores consisting of small particles (see Fig. 2). Obviously these structural defects cannot be completely recovered by annealing. Additionally, higher demagnetisation fields of small powder flakes also cause an increase of the coercive field. The permeability of the powder cores increases if the particle size increases (see Fig. 3). The high densities and the ideal planar orientation of the flakes in the cores ,esult in high permeabilities up to tl~ - 6000. The highest level of the permeability of the powder cores with a glass addition of 5 w t % is maintained only up to 0.3 kHz. At higher frequencies the eddy current losses reduce the permeability. But even at 100 kHz the cores show permeabilities between 200 and 500 depending on the particle size. Nanocrystalline ribbons of Vitroperm' with a thickness of 201am show an eddy current cut-off
-
3, 9
3s ~. 30 "o 26
zo .G
16
lO 5 0 56-90
125-
200-
300-
150
300
500
500710
10001400
particle size Lum]
Fig. 2. Coercive force of the cores with different particle sizes. content of glass 5 wt";,.
6000 1000
- 1400
pm
5000
'i
4000
3000
&
i
I
2000
1000
0,01
0,1
1
10
100
1000
frequency [kHz]
Fig. 3. Permeability It (H = 3 mA,,'cml of the cores with different particle sizes, content of solder glass 5 wt%.
1600 1400 1200 1000
w i t h o u t solder glass
800
600 40O 200 0 0 ,01
with s o l d e r g l a s s (5 wt. %)
~1°°"=1
Fig. 1. Microstructure of cores with and without glass additive, particle size 200-300 lam.
0,1
1
10
100
1000
frequency [kHz]
Fig. 4. Permeability t~ (H = 3 mA/cm) of cores with different contents of solder glass, particle size 125-150 I.tm.
D. Nuetzel et al. / Journal of Magnetism and Magnetic Materials 196-197 (1999) 327-329
frequency of 20-30 kHz. The drop of the permeability (at 0.05 kHz) of powder cores consisting of particles up to 300 ~tm is caused by short-circuit contacts between the flake layers. This leads to an effective increase of the particle thickness reducing the cut-off frequency. Small glass contents have only a modest isolating effect, because parts of the viscous glass are pressed out of the core during the pressing process and accumulate at the edges of the cores. The permeability of the cores in the low-frequency range decreases rapidly with an increasing glass content (see Fig. 4). Higher contents of electrically isolating (up to 5 wt%) reduce the eddy current losses caused by interparticle contact. An increase of the solder glass content by steps of 5 wt% (fixed particle size 125-150 p,m) reduces the level of the permeability by about 500. The core with 15 wt% glass shows a stable permeability of 400 up to 10 kHz and a smooth decrease at 20 kHz. Therefore,
329
an effective isolation and separation of the particles is realised at this glass content. The decrease of the permeability at 20 kHz is equal to the cut-off frequency for nanocrystalline Vitroperm' ribbons. This confirms the explanation of Fig. 3 that small particles and low glass contents reduce the eddy current cut-off frequency.
References
[1] Y. Yoshizawa, S. Oguma, K. Yamaauchi, J. Appl. Phys. 64 (1988) 6044. [2] G. Herzer, J. Magn. Magn. Mater. 112 (1992) 258. [3] M. Takagi, Y. Kawamura, Y. Kuroyama, T. Imura, Mater. Sci. Eng. 98 (1998) 457. [4] O. Heczko, P. Ruuskanen, IEEE Trans. Magn. MAG-29 (1993) 2670. [5] Y. Kawamura, M. Tagaki, M. Senoo, T. Imura, Mater. Sci. Eng. 98 (1988) 415.
Journal of Magnetism and Magnetic Materials 196-197 (1999) 327-329
Journalof magnetism and magnetic ~ l ~ materials
Nanocrystalline soft magnetic composite-cores with ideal orientation of the powder-flakes D. Nuetzel a'c'*, G. Rieger a, J. Wecker a, J. Petzold b, M.
Mueller
c
"Siemens AG, Corporate Technology, ZT MF 1, Paul-Gossen Str. 115, P.O. Box 3220, D-91050 Erlangen, Germany bVacuumschmelze GmbH, Griiner Weg 37, D-63450 Hanau, Germany Clnstitutefor Solid State and Material Research, Helmholtzstr 20, D-01171 Dresden, Germany
Abstract Amorphous Fev3,sCulNb3Sils,sB7 powders with a particle size up to 1400 ~tm were consolidated to cores by using a press-additive, which allows an ideal orientation of the flakes. The nanocrystalline cores exhibit permeabilities up to #e = 6000 and a coercive field of Hc = 8 A/m. The magnetic properties can be varied in a wide range depending on particle size and content of press-additive. ~ 1999 Elsevier Science B.V. All rights reserved. Keywords: Nanocrystalline structure; Consolidation; Powder cores; Soft magnetic properties
1. Introduction Iron-based nanocrystalline alloys exhibit excellent soft magnetic properties [1,2]. But due to process of rapid solidification the shape of these alloys is limited to ribbons and wires. However, by consolidation of amorphous powder flakes complex-shape parts can be realised. Recently, different consolidation techniques have been reported. The proposed procedures like explosive compaction [3], shock-wave compaction I-4] and static high-pressure compaction [5] with pressures up to 5 GPa are however costly and complex. The aim of this study is the preparation of compacts by a simple hot-pressing process in a low-pressure range. A special glass-based press-additive allows high densities and an ideal orientation of the flakes in the cores.
ous as-quenched state by using an ultra centrifugal mill (Retsch ZM 100) at room temperature. Afterwards the powder were sieved in fractions ranging from 20 to 1400 gm. The powder flakes were mixed with different contents of a glass-additive. The glass was a low melting, electrical isolating composite solder glass. The powders were filled into cylindrical dies (outer diameter 15 mm) with graphite inlays. The press chamber was evacuated and heated by an infrared furnace up to 500°C with an applied pressure of 500 MPa. This temperature was maintained for 300 s. The consolidated flake cores were annealed for 1 h at 520°C in vacuum to generate the nanocrystalline state and to reduce press-induced stresses. The magnetic measurements were made using a F6rster coerzimeter for the coercive force and an impedance analyser (HP 4284A) for the frequency-dependent permeability.
2. Experimental procedure
3. Results and discussion
Rapidly quenched Fev3,sCuxNb3Sils.sB7 ribbons (Vitroperm 500 ':) were ground to powder in the amorph-
In the first series of experiments the particle size was varied between 20 and 1400 gm with a fixed glass content of 5 wt%. In the second series we investigated the effect of the glass content in combination with a fixed particle size (125-150 gm).
*Corresponding author. Tel.: + 49-131-731435; fax: + 49131-732469; e-mail: [email protected].
0304-8853/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 0 7 3 6 - 7
328
D. Nuetzel et al. / Journal ~?/ Magnetism and Magnetic Materials 196- 197 (l 999) 3 , ,
The nanocrystalline cores with a glass contenl of 5 wt% exhibit densities between 90% and 93% of the theoretical density. In Fig. 1 one can see the effect of the glass-additive on the microstructure of the compacts. The glass becomes viscous at the pressing temperature of 500"C and enables the flakes to move against each other. Pores are filled, resulting in higher densities than glassfiee cores (densities: 85-88%). The glass-additive allows high densities with a low pressing pressure of 500 MPa. The particle movement leads to a regular planar orientation of the powder flakes in the cores which resuhs in good soft magnetic properties. The coercive field of the powder cores is mainly influenced by the distinct plastic deformation zone of the crack surface of the powder flakes induced by the milling process. Small particles have a higher content of such structural defects in relation to their size than larger ones leading to an enhanced domain wall pinning. This is the main reason for the increase of the coercive field of cores consisting of small particles (see Fig. 2). Obviously these structural defects cannot be completely recovered by annealing. Additionally, higher demagnetisation fields of small powder flakes also cause an increase of the coercive field. The permeability of the powder cores increases if the particle size increases (see Fig. 3). The high densities and the ideal planar orientation of the flakes in the cores ,esult in high permeabilities up to tl~ - 6000. The highest level of the permeability of the powder cores with a glass addition of 5 w t % is maintained only up to 0.3 kHz. At higher frequencies the eddy current losses reduce the permeability. But even at 100 kHz the cores show permeabilities between 200 and 500 depending on the particle size. Nanocrystalline ribbons of Vitroperm' with a thickness of 201am show an eddy current cut-off
-
3, 9
3s ~. 30 "o 26
zo .G
16
lO 5 0 56-90
125-
200-
300-
150
300
500
500710
10001400
particle size Lum]
Fig. 2. Coercive force of the cores with different particle sizes. content of glass 5 wt";,.
6000 1000
- 1400
pm
5000
'i
4000
3000
&
i
I
2000
1000
0,01
0,1
1
10
100
1000
frequency [kHz]
Fig. 3. Permeability It (H = 3 mA,,'cml of the cores with different particle sizes, content of solder glass 5 wt%.
1600 1400 1200 1000
w i t h o u t solder glass
800
600 40O 200 0 0 ,01
with s o l d e r g l a s s (5 wt. %)
~1°°"=1
Fig. 1. Microstructure of cores with and without glass additive, particle size 200-300 lam.
0,1
1
10
100
1000
frequency [kHz]
Fig. 4. Permeability t~ (H = 3 mA/cm) of cores with different contents of solder glass, particle size 125-150 I.tm.
D. Nuetzel et al. / Journal of Magnetism and Magnetic Materials 196-197 (1999) 327-329
frequency of 20-30 kHz. The drop of the permeability (at 0.05 kHz) of powder cores consisting of particles up to 300 ~tm is caused by short-circuit contacts between the flake layers. This leads to an effective increase of the particle thickness reducing the cut-off frequency. Small glass contents have only a modest isolating effect, because parts of the viscous glass are pressed out of the core during the pressing process and accumulate at the edges of the cores. The permeability of the cores in the low-frequency range decreases rapidly with an increasing glass content (see Fig. 4). Higher contents of electrically isolating (up to 5 wt%) reduce the eddy current losses caused by interparticle contact. An increase of the solder glass content by steps of 5 wt% (fixed particle size 125-150 p,m) reduces the level of the permeability by about 500. The core with 15 wt% glass shows a stable permeability of 400 up to 10 kHz and a smooth decrease at 20 kHz. Therefore,
329
an effective isolation and separation of the particles is realised at this glass content. The decrease of the permeability at 20 kHz is equal to the cut-off frequency for nanocrystalline Vitroperm' ribbons. This confirms the explanation of Fig. 3 that small particles and low glass contents reduce the eddy current cut-off frequency.
References
[1] Y. Yoshizawa, S. Oguma, K. Yamaauchi, J. Appl. Phys. 64 (1988) 6044. [2] G. Herzer, J. Magn. Magn. Mater. 112 (1992) 258. [3] M. Takagi, Y. Kawamura, Y. Kuroyama, T. Imura, Mater. Sci. Eng. 98 (1998) 457. [4] O. Heczko, P. Ruuskanen, IEEE Trans. Magn. MAG-29 (1993) 2670. [5] Y. Kawamura, M. Tagaki, M. Senoo, T. Imura, Mater. Sci. Eng. 98 (1988) 415.