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Influences of Bi2O3 on microstructure and magnetic properties of MnZn ferrite
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 320 (2008) 919–923 www.elsevier.com/locate/jmmm
Influences of Bi2O3 on microstructure and magnetic properties of MnZn ferrite Zhong Yu, Ke Sun, Lezhong Li, Yunfei Liu, Zhongwen Lan, Huaiwu Zhang State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, PR China Received 23 July 2007; received in revised form 29 August 2007 Available online 22 September 2007
Abstract MnZn ferrites were prepared by conventional oxide ceramic process. The effects of Bi2O3 on microstructure and magnetic properties of MnZn ferrites were investigated by means of characterizing the fracture surface micrograph, composition of grain boundary, magnetic properties and density by scanning electron microscope (SEM), energy dispersive X-ray spectroscope (EDS), B-H analyzer and Archimedes method, respectively. The results indicate that Bi2O3 mainly segregates and concentrates in the grain boundary regions, promotes solid-state reaction and grain growth, reduces porosity and enhances density. Optimum addition of Bi2O3 increases the permeability and saturation magnetic induction, meanwhile ensures the well frequency stability of permeability. r 2007 Elsevier B.V. All rights reserved. Keywords: MnZn ferrite; Bi2O3 additive; Microstructure; Grain boundary; Frequency stability
1. Introduction With the development of electron technology, a large number of high-permeability MnZn ferrites with excellent characteristics are employed in applications such as antielectromagnetic interference (EMI) noise filters, broad band electronic circuits transformers, integrated services digital network (ISDN), local area network (LAN), wide area network (WAN), pulse transformer, and background lighting, etc. [1]. Especially with the rapid development of information technology, the magnetic devices used in information transmission and conversion, for example, asymmetrical digital subscriber line (ADSL) and very-highbit-rate digital subscriber line (VDSL) transformers, are in dire need of miniaturization, high frequency and integration, and the network transmission speed and wide band characteristics are facing increasingly higher demand. This requires the MnZn ferrites used in such applications which have high permeability, wide range of temperature and frequency, and high stability. Recently, study on frequency characteristic of high-permeability MnZn ferrites has Corresponding author.
E-mail address: [email protected] (Z. Yu). 0304-8853/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2007.09.008
become one of the hotspots in the research field of magnetic material. During the preparation of MnZn ferrites, in addition to the formula and sintering processes, additive is an important factor influencing the material characteristics. Additives with low melting point such as MoO3, SnO2, V2O5 and Bi2O3, etc. are widely used [2–8], and the investigators are arguing whether the Bi2O3 gets into spinel phase lattice of MnZn ferrite. The object of this work is mainly to discuss the effects of Bi2O3 on the microstructure, grain boundary characteristics and magnetic properties of MnZn ferrites. This work is intent on proving that Bi2O3 cannot get into spinel phase lattice and can influence the frequency stability of permeability of MnZn ferrites. 2. Experiments MnZn ferrites with a nominal composition of Mn0.51Zn0.41Fe2.08O4 were prepared by the conventional ceramic techniques, with Fe2O3, MnCO3 and ZnO. The main composition was milled in an attritor with deionized water for 2 h. After drying, the mixture of oxide powder was homogenized and calcined at 850 1C in air for 2 h. Then 0.02% in mass fraction (same below) of V2O5, 0.02%
ARTICLE IN PRESS Z. Yu et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 919–923
of CaO and Bi2O3 varying from 0.00% to 0.12% in steps of 0.03% were added to the calcined powder. The resultant powder with additives was wet-milled for 2 h. After drying the milled slurry, the powder was granulated with polyvinyl alcohol (PVA) and pressed into toroidal shape with dimensions of o.d.=25 mm, i.d.=12 mm and h=7 mm at 60 MPa. The toroidal cores were sintered in a computerdriven furnace at 1400 1C for 5 h and then cooled at equilibrium oxygen partial pressure. The fracture surface was investigated by scanning electron microscope (SEM) (JSM-6490LV). The elemental analysis of grain boundary and grain was carried out using energy dispersive X-ray spectroscopy (EDS) analyzer (GENESIS 2000 XMS) connected with the SEM (JSM-6490LV). The densities of MnZn ferrites were measured by Archimedes method. The magnetic properties were characterized by a B-H analyzer SY-8232 of Iwatsu make. 3. Results and discussion 3.1. Influences of Bi2O3 on the microstructure of MnZn ferrites Bi2O3 is a low-melting-point additive whose melting point is 820 1C. During sintering, it forms liquid phase and promotes solid-state reaction. Fig. 1 shows fractured surface micrographs of MnZn ferrites with different Bi2O3 contents. When adding 0.00%, 0.06%, 0.09% and 0.12% of Bi2O3, corresponding average grain sizes are 12, 15, 100 and 120 mm, respectively. It is found that grain
size of MnZn ferrites grows larger as Bi2O3 addition increases. Fig. 2 shows the density of Bi2O3-doped MnZn ferrites. It indicates that the density of MnZn ferrites rises with an increase in Bi2O3 content; however, when the content of Bi2O3 exceeds 0.06%, the density of MnZn ferrites declines. When sintered at 1400 1C, some pores exist in grain inners of MnZn ferrites without Bi2O3 addition (see Fig. 1(a)). In the sample with 0.06% Bi2O3 addition, as Bi2O3 forms liquid phase during sintering, ions diffusion accelerates owing to the process of solid-state solubilization
5.04 5.03 5.02 Density/(g.cm-3)
920
5.01 5.00 4.99 4.98 4.97 4.96 4.95 0.00
0.03
0.06 w(Bi2O3)/%
0.09
0.12
Fig. 2. Density of Bi2O3-doped MnZn ferrites.
Fig. 1. SEM micrographs of Bi2O3-doped MnZn ferrites: (a) w(Bi2O3) ¼ 0.00 wt%; (b) w(Bi2O3) ¼ 0.06 wt%; (c) w(Bi2O3) ¼ 0.09 wt%; (d) w(Bi2O3) ¼ 0.12 wt%.
ARTICLE IN PRESS Z. Yu et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 919–923
and segregation. Grains grow larger and grain boundaries become more obvious, the pores escape from grain inners along grain boundaries easily and porosity decreases. This results in most of remnant pores remains in grain boundaries and grains grow to be more dense and homogeneous (see Fig. 1(b)); therefore, the MnZn ferrite has highest density (see Fig. 2). When the content of Bi2O3 exceeds 0.06%, the liquid phase formed by Bi2O3 become overabundant and growth of grain accelerates too fast for the pores to escape from grain inners. As a result, there are many abnormal large grains which have pores in them (see Fig. 1(c) and (d)), and this induces the decline of density. From the above discussion, Bi2O3 could promote grain growth of MnZn ferrites via liquid-phase sintering and optimum addition of Bi2O3 could help to reduce porosity and enhance density.
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3.2. Analysis of grain boundary of Bi2O3-doped MnZn ferrites Backscattered electron image of a grain boundary of MnZn ferrites with 0.09% Bi2O3 addition is shown in Fig. 3. Obviously, Bi2O3 distributes along and stays at grain boundaries, which is in accordance with the research carried out by Drofenik and Znidarsic [9]. Bi3+ ion cannot get into spinel phase lattice, as its radius is 0.096 nm, which is larger than tetrahedral clearance (A site, 0.03 nm) and octahedral clearance (B site, 0.055 nm) of MnZn ferrites. EDS analyses were carried out in the grain boundary region and grain region of MnZn ferrite with 0.09% Bi2O3 addition (see Fig. 3). Fig. 4 shows the EDS analysis result of grain and grain boundary. Obviously, Bi element is not detected in grains but on the grain boundaries. In addition, the compositions of grain boundary and grain (see Table 1) also indicate that in the MnZn ferrite with 0.09% Bi2O3 addition, Bi2O3 does not exist in grains but on the grain boundaries, where the content of Bi2O3 reaches 4.42%, much higher than initial addition 0.09%. It is suggested that Bi2O3 mainly segregates and concentrates in the grain boundary regions of MnZn ferrites, and it forms liquid phase on grain boundaries during sintering which markedly promotes the grain growth.
Table 1 Compositions of grain boundaries and grains of MnZn ferrite with 0.09% Bi2O3 addition
Fig. 3. Backscattered electron image of MnZn ferrite with 0.09% Bi2O3 addition.
Compositions
Bi2O3
MnO
Fe2O3
ZnO
Content (%) Grains Grain boundaries
0.00 4.42
16.39 15.50
69.91 67.24
13.70 12.84
Fig. 4. EDS analysis of MnZn ferrite with 0.09% Bi2O3 addition: (a) grain and (b) grain boundary.
ARTICLE IN PRESS Z. Yu et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 919–923
922
14000 2.5
13000 Coercive force/(A·m-1)
Initial Permeability
12000 11000 10000 9000 8000 7000
2.0
1.5
1.0
0.5
6000 5000 0.00
0.03
0.06 w(Bi2O3)/%
0.09
0.0
0.12
0.00
0.06 w(Bi2O3)/%
0.09
0.12
Fig. 7. Effect of Bi2O3 on coercivity.
420
w(Bi2O3) = 0.00%
13500 415
0.03% 0.06% 0.09% 0.12%
12000 10500
410 Permeability
Saturation magnetic inducution/mT
Fig. 5. Effect of Bi2O3 on initial permeability.
0.03
405 400
9000 7500 6000 4500
395 3000 390
1500 0.00
0.03
0.06 w(Bi2O3)/%
0.09
0.12
Fig. 6. Effect of Bi2O3 on saturation magnetic induction.
0 10
100 f/(KHz)
300
600
Fig. 8. Effect of Bi2O3 on permeability frequency characteristic.
3.3. Effects of Bi2O3 on magnetic properties of MnZn ferrites Figs. 5–8 show the magnetic properties of Bi2O3-doped MnZn ferrites. With the increase of Bi2O3 content, the initial permeability and saturation magnetic induction of MnZn ferrites increase gradually, and simultaneously reach the maximum at the Bi2O3 content of 0.06%, then decrease rapidly when the Bi2O3 content is beyond 0.06% (see Figs. 5 and 6). However, the coercivity changes contrarily. The coercivity of MnZn ferrites gradually declines with an increase in Bi2O3 content, and reaches the minimum at the Bi2O3 content of 0.06%, then increases rapidly when the Bi2O3 content exceeds 0.06% (see Fig. 7). Although Bi2O3 is helpful to improve the initial permeability and saturation magnetic induction of MnZn ferrites, it induces the deterioration of permeability frequency stability as shown in Fig. 8. From 10 to 300 kHz, the permeability of MnZn ferrite without Bi2O3
almost has no change, but those of MnZn ferrites doped with 0.03% and 0.06% Bi2O3 reduce 18.7% and 55.1%, respectively, and the Bi2O3 content beyond 0.06% implies the poorer frequency stability. As discussed previously, Bi2O3 segregates and concentrates in the grain boundary regions of MnZn ferrites, promotes the solid-phase reaction and grain growth via liquid-phase sintering. With the increase of Bi2O3 content, the grain size rises gradually, pores escape gradually along the grain boundary, the domain wall movement and domain rotation become easier, so the initial permeability and saturation magnetic induction increase gradually, and the coercivity decreases. The resonance frequency of domain wall declines, which reduces the permeability frequency stability of MnZn ferrites. In the MnZn ferrite doped with 0.06% Bi2O3, the grains are most homogeneous, the pores are the least and mainly stay at the grain boundaries, the initial permeability and saturation
ARTICLE IN PRESS Z. Yu et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 919–923
magnetic induction are the maximal, but coercivity is the minimal, meanwhile, the resonance frequency of domain wall reaches the minimum, so with the increase of frequency, the permeability declines rapidly. When the Bi2O3 content exceeds 0.06%, the excessive liquid phase quickens the ions diffusion during sintering, growth of grain accelerates too fast for the pores to escape, and a great deal of pores are engulfed in the abnormal large grains, therefore, the domain wall movement becomes difficult, which causes the initial permeability and saturation magnetic induction to decline and the coercivity to increase. Just because of the increase of the abnormal large grains and porosity, the frequency stability of permeability is deteriorated. 4. Conclusions Bi2O3 segregates and concentrates in the grain boundary regions of MnZn ferrites, where the content of Bi2O3 is much higher than initial adding content. The additive Bi2O3 enhances grain growth via liquid-phase sintering, optimum addition of Bi2O3 could reduce porosity and enhance density.
923
The addition of Bi2O3 deteriorates the permeability frequency stability. Optimum addition of Bi2O3 increases the initial permeability and saturation magnetic induction, meanwhile ensures the well frequency stability of permeability.
References [1] G. Ott, J. Wrba, R. Lucke, J. Magn. Magn. Mater. 254/255 (2003) 535. [2] A. Znidarsic, M. Limpel, M. Drofenik, IEEE Trans. Magn. 2 (1995) 950. [3] K. Ono, S. Fukunaga, Y. Matsuo, IEEE Trans. Magn. 5 (1999) 3406. [4] Y. Matsuo, K. Ono, M. Ishikura, et al., IEEE Trans. Magn. 5 (1997) 3751. [5] S. Gasiorck, J. Kulikowski, J. Magn. Magn. Mater. 2 (1982) 295. [6] P. Andrei, F.C. Ovidiu, P. Constantin, et al., J. Magn. Magn. Mater. 196/197 (1999) 362. [7] S.H. Chen, S.C. Chang, C.Y. Tsay, et al., J. Eur. Ceram. Soc. 21 (2001) 1931. [8] J. Dreikorn, L. Michalowsky, in: Proceedings of the Eight International Conference on Ferrites (ICF8), Kyoto and Tokyo, Japan, 2000, p. 554. [9] M. Drofenik, A. Znidarsic, J. Am. Ceram. Soc. 11 (1998) 2841.
Journal of Magnetism and Magnetic Materials 320 (2008) 919–923 www.elsevier.com/locate/jmmm
Influences of Bi2O3 on microstructure and magnetic properties of MnZn ferrite Zhong Yu, Ke Sun, Lezhong Li, Yunfei Liu, Zhongwen Lan, Huaiwu Zhang State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, PR China Received 23 July 2007; received in revised form 29 August 2007 Available online 22 September 2007
Abstract MnZn ferrites were prepared by conventional oxide ceramic process. The effects of Bi2O3 on microstructure and magnetic properties of MnZn ferrites were investigated by means of characterizing the fracture surface micrograph, composition of grain boundary, magnetic properties and density by scanning electron microscope (SEM), energy dispersive X-ray spectroscope (EDS), B-H analyzer and Archimedes method, respectively. The results indicate that Bi2O3 mainly segregates and concentrates in the grain boundary regions, promotes solid-state reaction and grain growth, reduces porosity and enhances density. Optimum addition of Bi2O3 increases the permeability and saturation magnetic induction, meanwhile ensures the well frequency stability of permeability. r 2007 Elsevier B.V. All rights reserved. Keywords: MnZn ferrite; Bi2O3 additive; Microstructure; Grain boundary; Frequency stability
1. Introduction With the development of electron technology, a large number of high-permeability MnZn ferrites with excellent characteristics are employed in applications such as antielectromagnetic interference (EMI) noise filters, broad band electronic circuits transformers, integrated services digital network (ISDN), local area network (LAN), wide area network (WAN), pulse transformer, and background lighting, etc. [1]. Especially with the rapid development of information technology, the magnetic devices used in information transmission and conversion, for example, asymmetrical digital subscriber line (ADSL) and very-highbit-rate digital subscriber line (VDSL) transformers, are in dire need of miniaturization, high frequency and integration, and the network transmission speed and wide band characteristics are facing increasingly higher demand. This requires the MnZn ferrites used in such applications which have high permeability, wide range of temperature and frequency, and high stability. Recently, study on frequency characteristic of high-permeability MnZn ferrites has Corresponding author.
E-mail address: [email protected] (Z. Yu). 0304-8853/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2007.09.008
become one of the hotspots in the research field of magnetic material. During the preparation of MnZn ferrites, in addition to the formula and sintering processes, additive is an important factor influencing the material characteristics. Additives with low melting point such as MoO3, SnO2, V2O5 and Bi2O3, etc. are widely used [2–8], and the investigators are arguing whether the Bi2O3 gets into spinel phase lattice of MnZn ferrite. The object of this work is mainly to discuss the effects of Bi2O3 on the microstructure, grain boundary characteristics and magnetic properties of MnZn ferrites. This work is intent on proving that Bi2O3 cannot get into spinel phase lattice and can influence the frequency stability of permeability of MnZn ferrites. 2. Experiments MnZn ferrites with a nominal composition of Mn0.51Zn0.41Fe2.08O4 were prepared by the conventional ceramic techniques, with Fe2O3, MnCO3 and ZnO. The main composition was milled in an attritor with deionized water for 2 h. After drying, the mixture of oxide powder was homogenized and calcined at 850 1C in air for 2 h. Then 0.02% in mass fraction (same below) of V2O5, 0.02%
ARTICLE IN PRESS Z. Yu et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 919–923
of CaO and Bi2O3 varying from 0.00% to 0.12% in steps of 0.03% were added to the calcined powder. The resultant powder with additives was wet-milled for 2 h. After drying the milled slurry, the powder was granulated with polyvinyl alcohol (PVA) and pressed into toroidal shape with dimensions of o.d.=25 mm, i.d.=12 mm and h=7 mm at 60 MPa. The toroidal cores were sintered in a computerdriven furnace at 1400 1C for 5 h and then cooled at equilibrium oxygen partial pressure. The fracture surface was investigated by scanning electron microscope (SEM) (JSM-6490LV). The elemental analysis of grain boundary and grain was carried out using energy dispersive X-ray spectroscopy (EDS) analyzer (GENESIS 2000 XMS) connected with the SEM (JSM-6490LV). The densities of MnZn ferrites were measured by Archimedes method. The magnetic properties were characterized by a B-H analyzer SY-8232 of Iwatsu make. 3. Results and discussion 3.1. Influences of Bi2O3 on the microstructure of MnZn ferrites Bi2O3 is a low-melting-point additive whose melting point is 820 1C. During sintering, it forms liquid phase and promotes solid-state reaction. Fig. 1 shows fractured surface micrographs of MnZn ferrites with different Bi2O3 contents. When adding 0.00%, 0.06%, 0.09% and 0.12% of Bi2O3, corresponding average grain sizes are 12, 15, 100 and 120 mm, respectively. It is found that grain
size of MnZn ferrites grows larger as Bi2O3 addition increases. Fig. 2 shows the density of Bi2O3-doped MnZn ferrites. It indicates that the density of MnZn ferrites rises with an increase in Bi2O3 content; however, when the content of Bi2O3 exceeds 0.06%, the density of MnZn ferrites declines. When sintered at 1400 1C, some pores exist in grain inners of MnZn ferrites without Bi2O3 addition (see Fig. 1(a)). In the sample with 0.06% Bi2O3 addition, as Bi2O3 forms liquid phase during sintering, ions diffusion accelerates owing to the process of solid-state solubilization
5.04 5.03 5.02 Density/(g.cm-3)
920
5.01 5.00 4.99 4.98 4.97 4.96 4.95 0.00
0.03
0.06 w(Bi2O3)/%
0.09
0.12
Fig. 2. Density of Bi2O3-doped MnZn ferrites.
Fig. 1. SEM micrographs of Bi2O3-doped MnZn ferrites: (a) w(Bi2O3) ¼ 0.00 wt%; (b) w(Bi2O3) ¼ 0.06 wt%; (c) w(Bi2O3) ¼ 0.09 wt%; (d) w(Bi2O3) ¼ 0.12 wt%.
ARTICLE IN PRESS Z. Yu et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 919–923
and segregation. Grains grow larger and grain boundaries become more obvious, the pores escape from grain inners along grain boundaries easily and porosity decreases. This results in most of remnant pores remains in grain boundaries and grains grow to be more dense and homogeneous (see Fig. 1(b)); therefore, the MnZn ferrite has highest density (see Fig. 2). When the content of Bi2O3 exceeds 0.06%, the liquid phase formed by Bi2O3 become overabundant and growth of grain accelerates too fast for the pores to escape from grain inners. As a result, there are many abnormal large grains which have pores in them (see Fig. 1(c) and (d)), and this induces the decline of density. From the above discussion, Bi2O3 could promote grain growth of MnZn ferrites via liquid-phase sintering and optimum addition of Bi2O3 could help to reduce porosity and enhance density.
921
3.2. Analysis of grain boundary of Bi2O3-doped MnZn ferrites Backscattered electron image of a grain boundary of MnZn ferrites with 0.09% Bi2O3 addition is shown in Fig. 3. Obviously, Bi2O3 distributes along and stays at grain boundaries, which is in accordance with the research carried out by Drofenik and Znidarsic [9]. Bi3+ ion cannot get into spinel phase lattice, as its radius is 0.096 nm, which is larger than tetrahedral clearance (A site, 0.03 nm) and octahedral clearance (B site, 0.055 nm) of MnZn ferrites. EDS analyses were carried out in the grain boundary region and grain region of MnZn ferrite with 0.09% Bi2O3 addition (see Fig. 3). Fig. 4 shows the EDS analysis result of grain and grain boundary. Obviously, Bi element is not detected in grains but on the grain boundaries. In addition, the compositions of grain boundary and grain (see Table 1) also indicate that in the MnZn ferrite with 0.09% Bi2O3 addition, Bi2O3 does not exist in grains but on the grain boundaries, where the content of Bi2O3 reaches 4.42%, much higher than initial addition 0.09%. It is suggested that Bi2O3 mainly segregates and concentrates in the grain boundary regions of MnZn ferrites, and it forms liquid phase on grain boundaries during sintering which markedly promotes the grain growth.
Table 1 Compositions of grain boundaries and grains of MnZn ferrite with 0.09% Bi2O3 addition
Fig. 3. Backscattered electron image of MnZn ferrite with 0.09% Bi2O3 addition.
Compositions
Bi2O3
MnO
Fe2O3
ZnO
Content (%) Grains Grain boundaries
0.00 4.42
16.39 15.50
69.91 67.24
13.70 12.84
Fig. 4. EDS analysis of MnZn ferrite with 0.09% Bi2O3 addition: (a) grain and (b) grain boundary.
ARTICLE IN PRESS Z. Yu et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 919–923
922
14000 2.5
13000 Coercive force/(A·m-1)
Initial Permeability
12000 11000 10000 9000 8000 7000
2.0
1.5
1.0
0.5
6000 5000 0.00
0.03
0.06 w(Bi2O3)/%
0.09
0.0
0.12
0.00
0.06 w(Bi2O3)/%
0.09
0.12
Fig. 7. Effect of Bi2O3 on coercivity.
420
w(Bi2O3) = 0.00%
13500 415
0.03% 0.06% 0.09% 0.12%
12000 10500
410 Permeability
Saturation magnetic inducution/mT
Fig. 5. Effect of Bi2O3 on initial permeability.
0.03
405 400
9000 7500 6000 4500
395 3000 390
1500 0.00
0.03
0.06 w(Bi2O3)/%
0.09
0.12
Fig. 6. Effect of Bi2O3 on saturation magnetic induction.
0 10
100 f/(KHz)
300
600
Fig. 8. Effect of Bi2O3 on permeability frequency characteristic.
3.3. Effects of Bi2O3 on magnetic properties of MnZn ferrites Figs. 5–8 show the magnetic properties of Bi2O3-doped MnZn ferrites. With the increase of Bi2O3 content, the initial permeability and saturation magnetic induction of MnZn ferrites increase gradually, and simultaneously reach the maximum at the Bi2O3 content of 0.06%, then decrease rapidly when the Bi2O3 content is beyond 0.06% (see Figs. 5 and 6). However, the coercivity changes contrarily. The coercivity of MnZn ferrites gradually declines with an increase in Bi2O3 content, and reaches the minimum at the Bi2O3 content of 0.06%, then increases rapidly when the Bi2O3 content exceeds 0.06% (see Fig. 7). Although Bi2O3 is helpful to improve the initial permeability and saturation magnetic induction of MnZn ferrites, it induces the deterioration of permeability frequency stability as shown in Fig. 8. From 10 to 300 kHz, the permeability of MnZn ferrite without Bi2O3
almost has no change, but those of MnZn ferrites doped with 0.03% and 0.06% Bi2O3 reduce 18.7% and 55.1%, respectively, and the Bi2O3 content beyond 0.06% implies the poorer frequency stability. As discussed previously, Bi2O3 segregates and concentrates in the grain boundary regions of MnZn ferrites, promotes the solid-phase reaction and grain growth via liquid-phase sintering. With the increase of Bi2O3 content, the grain size rises gradually, pores escape gradually along the grain boundary, the domain wall movement and domain rotation become easier, so the initial permeability and saturation magnetic induction increase gradually, and the coercivity decreases. The resonance frequency of domain wall declines, which reduces the permeability frequency stability of MnZn ferrites. In the MnZn ferrite doped with 0.06% Bi2O3, the grains are most homogeneous, the pores are the least and mainly stay at the grain boundaries, the initial permeability and saturation
ARTICLE IN PRESS Z. Yu et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 919–923
magnetic induction are the maximal, but coercivity is the minimal, meanwhile, the resonance frequency of domain wall reaches the minimum, so with the increase of frequency, the permeability declines rapidly. When the Bi2O3 content exceeds 0.06%, the excessive liquid phase quickens the ions diffusion during sintering, growth of grain accelerates too fast for the pores to escape, and a great deal of pores are engulfed in the abnormal large grains, therefore, the domain wall movement becomes difficult, which causes the initial permeability and saturation magnetic induction to decline and the coercivity to increase. Just because of the increase of the abnormal large grains and porosity, the frequency stability of permeability is deteriorated. 4. Conclusions Bi2O3 segregates and concentrates in the grain boundary regions of MnZn ferrites, where the content of Bi2O3 is much higher than initial adding content. The additive Bi2O3 enhances grain growth via liquid-phase sintering, optimum addition of Bi2O3 could reduce porosity and enhance density.
923
The addition of Bi2O3 deteriorates the permeability frequency stability. Optimum addition of Bi2O3 increases the initial permeability and saturation magnetic induction, meanwhile ensures the well frequency stability of permeability.
References [1] G. Ott, J. Wrba, R. Lucke, J. Magn. Magn. Mater. 254/255 (2003) 535. [2] A. Znidarsic, M. Limpel, M. Drofenik, IEEE Trans. Magn. 2 (1995) 950. [3] K. Ono, S. Fukunaga, Y. Matsuo, IEEE Trans. Magn. 5 (1999) 3406. [4] Y. Matsuo, K. Ono, M. Ishikura, et al., IEEE Trans. Magn. 5 (1997) 3751. [5] S. Gasiorck, J. Kulikowski, J. Magn. Magn. Mater. 2 (1982) 295. [6] P. Andrei, F.C. Ovidiu, P. Constantin, et al., J. Magn. Magn. Mater. 196/197 (1999) 362. [7] S.H. Chen, S.C. Chang, C.Y. Tsay, et al., J. Eur. Ceram. Soc. 21 (2001) 1931. [8] J. Dreikorn, L. Michalowsky, in: Proceedings of the Eight International Conference on Ferrites (ICF8), Kyoto and Tokyo, Japan, 2000, p. 554. [9] M. Drofenik, A. Znidarsic, J. Am. Ceram. Soc. 11 (1998) 2841.