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Electrical properties of Mn added MgCuZn ferrites prepared by microwave sintering method
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 283 (2004) 109–116 www.elsevier.com/locate/jmmm
Electrical properties of Mn added MgCuZn ferrites prepared by microwave sintering method A. Bhaskar, B. Rajini Kanth, S.R. Murthy Department of Physics, Osmania University, Hyderabad 500 007, India Received 9 April 2003; received in revised form 16 April 2004 Available online 6 July 2004
Abstract A low temperature sintered Mn added MgCuZn ferrites were prepared using microwave sintering method. The prepared samples were characterized using X-rays and metallurgical microscope. Room temperature electrical properties such as resistivity, dielectric properties, inductance, quality factor and initial permeability have been measured in the frequency range of 100 kHz–100 MHz. The initial permeability was measured in the temperature range of 300–650 K. The multilayer chip inductors were fabricated using the prepared ferrites and their electrical properties were measured. r 2004 Elsevier B.V. All rights reserved. Keywords: Microwave sintering; Electrical resistivity; Dielectric constant; Initial permeability; Q factor; Dissipation factor; Multilayer chip inductor
1. lntroduction Chip inductors are one of the groups of passive surface mount devices (SMD). They are important components for the latest electronic products, such as cellular phones, video cameras, notebook computers, hard and floppy drives, etc. that require small dimensions, lightweight and better functions [1,2]. Multilayer chip inductor (MLCI) as a key component of electronic devices confronts Corresponding author.
E-mail address: [email protected] (S.R. Murthy).
new challenges. The key materials for MLCI are soft ferrite materials with low sintering temperature (o940 C) in order to co-firing with Ag internal electrode and better electrical and magnetic properties. In this process only NiCuZn ferrites were developed as the material used in the chip components [2,3]. In the case of NiCuZn ferrites it is found that the liquid phase causes diffusion of Ag electrode material during the sintering and consequently leads to residual stress during the cooling process, which inturn results in a decrease in the permeability and magnetic properties [4]. But, in the case of MgCuZn ferrites
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it is found that the diffusion of Ag electrode material during sintering process does not lead to residual stress, thus, no deterioration of the magnetic properties are possible for these ferrites [5]. Previous investigators have reported about the substitution of some transition metal ions such as Bi2 O3 , PbO, V2 O5 , MnO2 etc., to improve the extrinsic properties of low temperature sintered NiCuZn ferrites [6–8]. It was found that these additives are generally detrimental to the electrical and magnetic properties. But small addition of Mn2þ to NiCuZn ferrites plays a crucial role in improvement of electrical properties by reducing magnetostriction effects [9]. Keeping this point in view we have also added Mn in MgCuZn ferrites. Detailed investigation of the effect of Mn on electrical properties have been under taken and obtained results are presented in this paper.
2. Experimental method Samples with the composition Mgð0:456xÞ Cu0:144 Mnx Zn0:4 Fe2 O4 with x ¼ 0; 1; 2; 4 mol% were prepared using microwave sintering method. High purity (99.999) chemical reagent powders of Fe2 O3 , MgO, MnO2 , ZnO and CuO were mixed and grounded for 6 h. the powders were calcined at 800 C for 30 min using microwave furnace. The microwave sintering process was carried out using a specially designed applicator and which consists of a domestic microwave oven having an output power level tunable upto a maximum of 800 W and operating frequency of 2.45 GHz. Then the powder was re-grounded for 10 h. To this powder, 2 wt% poly-vinyl-alcohol was added as a binder. Then the powder was pressed into pellets (12 mm diameter, 3 mm thickness) and rods (60 mm in length, 12 mm diameter) and toroids (12 mm outer diameter, 6 mm inner diameter, 4 mm thickness) at a pressure of 8 MPa for 5 min and the samples were finally sintered at 910 C/30 min in air at atmospheric pressure. The structure and microstructure of the sintered materials were examined using X-ray diffractometry and metallurgical microscope. The bulk density of the present samples were measured using the Archimedes method. The room temperature
electrical properties were measured using a impedance analyzer (HP 4194A) in frequency range from 100 kHz to 100 MHz. The ferrites sintered at 910 C/30 min were used to fabricate multilayer chip inductors by green sheet lamination and screen-printing method. The DC resistance ðrÞ, inductance ðLÞ and quality factor ðQÞ of the fabricated chip inductors were also measured.
3. Results and discussion Fig. 1 shows the XRD patterns of the ferrite powders. As we can see from the figure, all the powder samples posses single phase with spinel structure. No second phase was detected for all samples. Fig. 2 shows the micrographs for the samples sintered at 910 C/30 min. It can be seen from the micrographs that the dense microstructures are obtained for all samples. The average grain size was obtained by line-intercept method and obtained values are presented in Table 1. The grain size is smaller than 1:5 mm for all the samples under investigation. This indicates that the present ferrites possess fine-grained microstructure and thus, exhibit good electromagnetic properties. The table also gives the values of bulk density for all the samples. It can be seen from the table that the density and grain size of samples are found to increase with an increase of Mn2þ content. DC resistivity is an important property of low temperature sintered MgCuZn ferrites utilized for MLCIs in high frequency range. The table gives the room temperature values of DC resistivity ðrÞ measured using a two-probe method. It is evident from the table that values of r increase markedly by an addition of Mn2þ . In the sample without Mn2þ content ðx ¼ 0Þ, Fe concentration is maximum at B-sites, which is responsible for electrical conduction in ferrites [10]. An addition of Mn in the system reduces the concentration of Fe2þ ions due to the presence Mn2þ and Mn3þ ions at B-sites. It is clear that the resistivity values of all the samples are higher than 109 O cm, meeting the requirement of MLCIs. The dielectric properties such as dielectric constant ð0 Þ and dissipation factor ðDÞ are
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Fig. 1. XRD patterns of sintered Mn added MgCuZn ferrites.
important for multilayer chip inductors used in high frequency range. Therefore, the dielectric constant measurements were carried out in the frequency range from 100 kHz to 100 MHz. Table 1 gives the room temperature values of dielectric constant and dissipation factor measured at 1 MHz. It can be observed from the table that the value of dielectric constant is small and remains almost constant with an addition of Mn2þ to MgCuZn ferrites. The dissipation factor is also found to be small and remains constant with an addition of Mn to MgCuZn ferrites. Fig. 3 gives the plots showing the variation of dielectric constant with frequency for all four samples at room temperature. It can be seen from the figure that the dielectric constant in the
beginning for all the samples decreases rapidly with an increase in frequency, and finally remains constant at higher frequencies. The incorporation of Mn into MgCuZn ferrites has no pronounced effect on the dielectric constant in high frequency range, but significantly increases the dielectric constant in the low frequency range. This type of behaviour was observed for a number of ferrites [11–14]. The dielectric behaviour of ferrites may be explained on the basis of that of the mechanism of the dielectric polarization process and is similar to that of the conduction process. The electronic exchange Fe2þ Ð Fe3þ gives the local displacement of electrons in the direction of applied electric field, which induces the polarization in
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ferrites [15,16]. The magnitude of exchange depends on the concentration of Fe2þ =Fe3þ ion pairs present on B site for the present ferrites. The incorporation of Mn2þ in B sites of ferrites may increase the concentration of Fe2þ =Fe3þ ion pairs due to the oxidation of Mn2þ to Mn3þ induced by Mn addition of Fe in ferrites. Therefore, the
dielectric constant for the present ferrites decreases with increasing frequency. The dielectric constant reaches a constant value depending upon the fact that beyond a fixed frequency of electric field the electron exchange does not follow the alternating field. The frequency variation of dissipation factor ðDÞ for all the samples measured at room temperature and results are presented in Fig. 4. It can be seen from the figure that value of D decreases at low frequencies and remains nearly constant at higher frequencies. When the hopping frequency becomes equal or nearly equal to that of external applied field, the value of dielectric loss increases once again at higher frequency. It is also evident, from the figure that the dissipation factor decreases with increasing of Mn content in MgCuZn ferrites. The dielectric losses in ferrites are generally reflected in the resistivity i.e., materials with low resistivity exhibit high
0.00 0.01 0.02 0.04
Dielectric Constant (ε1)
16 15 14 13 12 11 10 1
10
100
Frequency (MHz)
Fig. 2. Metallurgical micrographs of sintered ferrites with (a) 2 mol% and (b) 4 mol% of Mn.
Fig. 3. Variation of dielectric constant with frequency.
Table 1 Bulk density, average grain size and main electrical properties of Mn added MgCuZn ferrites Mn Content (mol%)
Bulk density ðg=cm3 Þ
Average grain size ðmmÞ
r 109 ðO cmÞ
mi (1 MHz)
Q (1 MHz)
0 (1 MHz)
D (1 MHz)
0 1 2 4
4.91 4.98 5.01 5.23
0.7 0.9 1.3 1.5
0.8 2.8 4.5 7.6
236 258 390 548
52 58 68 60
11 11.5 10 11.2
0.020 0.036 0.036 0.028
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Dissipation factor (D)
0.12 0.10 0.08 0.06 0.04
600 550
Initial permeability (µi)
0.00 0.01 0.02 0.04
0.14
113
500 450 400 350 300 250 200 150 100
0.02
50
0.04 0.02 0.01 0.00
0
1
10
100
1
Frequency (MHz)
100
Fig. 5. Frequency dependence of the initial permeability.
dielectric loss and vice versa. Therefore, the increase of dissipation factor with increasing Mn content is also attributed to the exchange of electrons between Fe2þ and Fe3þ , which is responsible for the conduction mechanism in ferrites. The initial permeability ðmi Þ has been measured for all the ferrites in the frequency range of 100 kHz–100 MHz. Table 1 gives the room temperature values of initial permeability measured at 1 MHz. It can be observed from the table that the value of initial permeability increases with an increase of Mn content from 0 to 4 mol% in MgCuZn ferrites. The decreasing magnetostriction and disappearance of pores due to the densification causes an increase of the initial permeability for the present ferrites [17]. Fig. 5 shows the frequency dependence of initial permeability for all samples under investigation. It can be seen from the figure that the value of initial permeability for samples without and with 1 mol% of Mn decreases with an increase of frequency from 100 kHz to 80 MHz and then rapidly drops to small value. However, in the same frequency range the value of mi remains almost constant with an increase of frequency for samples with 2 and 4 mol% of Mn. The initial permeability of ferrite is expressed as, ð1Þ
where mi is the initial permeability, M s the saturation magnetization, k the crystal magnetic
70
0.04 0.02 0.01 0.00
60
Quality factor (Q)
Fig. 4. Variation of dissipation factor with frequency.
mi ¼ M 2s =ak þ bls
10
Frequency (MHz)
50 40 30 20 10 0 1
10
100
Frequency (MHz)
Fig. 6. Frequency dependence of the quality factor.
anisotropy constant, l is the magnetostriction constant and s is the internal stress and a and b are constants. Previously, it has been shown that addition of Mn2þ for Fe2þ can decrease the magnetostriction constant, resulting in the increase of permeability [18,19]. This reveals that the significant increase in permeability occurred in our ferrites may be due to decrease of magnetostriction constant induced by Mn addition [20]. The quality factor ðQÞ versus frequency plots of all the samples are shown in Fig. 6. It can be seen from the figure that the value of quality factor increases with an increase of frequency and shows a peak around 1.5 MHz. With further increase of frequency the value of Q remains constant upto 100 MHz. It is also observed from the figure that
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the value of Q increases with an increase of Mn content in MgCuZn ferrites. It has been known that the main loss mechanism in ferrite at high frequency is eddy current loss. The eddy current loss can be expressed as [18] Pe ¼ KB2 f 2 d 2 =r
ð2Þ
where ‘Pe ’ is the energy loss per unit volume, ‘K’ is a geometric constant, ‘B’ is the maximum induction, ‘f ’ is the frequency and ‘d’ is the thickness of the narrowest dimension perpendicular to magnetic flux and ‘r’ is the resistivity. From the above equation one can conclude that the eddy current loss is inversely proportional to the resistivity of the material. So, as the resistivity of the samples is increased, the eddy current loss is decreased, resulting in an increase in quality factor. Thus, the increase in quality factor is attributed to the increase in resistivity due to Mn addition. Initial permeability ðmi Þ has been measured for all samples in the temperature ðTÞ range of 300–600 K and obtained results are presented in Fig. 7. It can be seen from the figure that the value of mi increases with an increase of temperature and show a sharp peak and suddenly drops to a small value 3. It is well known that for the ferrite, mi drop to a small value at a temperature known as Curie temperature. Therefore, in the present investigation, Curie temperature ðT c Þ of the present ferrites is taken when mi drops to 3. The
Initial Permeability (µ i )
500
0.04 0.02 0.01 0.00
400
300
200
100
0 250
300
350
400
450
500
550
600
650
700
Temperature (K)
Fig. 7. Temperature dependence of initial permeability at 1 kHz.
average value of T c , thus, obtained for the present ferrites is 510 K. It can also be observed from the figure that mi –T curves are almost flat in between 300 and 390 K for the samples with 2 and 4 mol% of Mn. This shows that these ferrites have best thermal stability. The observed mi versus temperature results can be interpreted using the Globus model [21]. According to this model, if the sample is of toroidal shape, because of the existence of several easy axes in each grain of a soft cubic material, the domain walls align themselves, at best, along the direction of circles of the torus, in order to minimize the demagnetizing fields. When the sample is cooled down from Curie temperature, k1 in this region is weak, and it is easy for the domain walls to follow the torus symmetry, since the magnetization is larger than the anisotropy field. At low temperature, when k1 is stronger, the topography of the domain walls is influenced by the anisotropy directions in such a way that the parallelism between the applied field and portions of the domain wall worsens. The value of mi is thus lower than that due to an ideal toroidal symmetry configuration. From the above studies, it was concluded that the MgCuZn ferrites with 2 and 4 mol%. Mn content and sintered at 910 C/30 min possesses high density and optimum electrical properties among all the present samples. Therefore, these ferrites (2 mol% (inductor A), 4 mol% (inductor B)) were used to fabricate multilayer chip inductors by green sheet lamination and screen-printing processes [5]. The chips were sintered at 930 C/ 30 min. The fabricated multilayer chip inductors have the dimensions of 4.5 mm in length, 3.0 mm in width and 2 mm in height. The chips were then coated with Ag terminal electrodes. The room temperature DC resistance of the chip inductors was measured. The measured resistance values for the inductors A and B are 0.121 and 0.225 O, respectively. The resistance of the chip inductors increased with increasing Mn addition and inductor B exhibited higher resistance. The inductance of the inductor B ( 0:6 mHÞ is higher than that of inductor A ð 0:5 mHÞ (at 1 MHz). However, the quality factor of inductor B is smaller than that of inductor A as expected.
ARTICLE IN PRESS A. Bhaskar et al. / Journal of Magnetism and Magnetic Materials 283 (2004) 109–116
It can be seen from the Figs. 8 and 9 that the value of inductance for inductors A and B remains constant with an increase of frequency upto 10 MHz and increases at higher frequencies. The quality factor ðQÞ for both the fabricated chip inductors decreases with an increase of frequency. Table 2 gives the comparison data of present chip inductors with that of NiCuZn chip inductors [22]. It can be seen from the table that the values of inductance ðLÞ for present chip inductors are found to be higher than that of chip inductor fabricated using NiCuZn ferrites. Similarly, the values of initial permeability ðmi Þ are also higher for present chip inductors. This shows that,
Inductor A
115
Table 2 Comparison data for chip inductors Sample
Sintering temperature
Inductance ðLÞ
Initial permeability ðmi Þ (at 1 MHz)
ð CÞ
ðmHÞ (at 1 MHz)
NiCuZn Ferrite Chip Inductor
910/6 h
0.45
580
MgCuZn Ferrite Chip Inductor A
910/30 min
0.50
600
MgCuZn Ferrite Chip Inductor B
910/30 min
0.60
650
80
60
0.4
40
20
Quality factor (Q)
Inductance (µH)
0.5
0.3
presently fabricated chip inductors are better than the chip inductors fabricated using NiCuZn ferrites.
4. Summary 0
0.2 0.1
1
10
Frequency (MHz)
Fig. 8. Frequency variation of inductance ðLÞ and quality factor ðQÞ for Fabricated multilayer chip inductor A.
Inductor B
60
50
Inductance (µH)
0.60
40 0.55 30 0.50
20
Quality factor (Q)
0.65
10
0.45
0 0.40 0.1
1
10
Frequency (MHz)
Fig. 9. Frequency variation of inductance ðLÞ and quality factor ðQÞ for Fabricated multilayer chip inductor B.
A series of Mn added MgCuZn ferrites were prepared using microwave sintering method. The powders were calcined at 800 C/30 min and finally samples were sintered at 910 C/30 min. The prepared samples were characterized using X-rays and metallurgical microscope. With increasing Mn content, the initial permeability is significantly increased, while the electrical resistivity and quality factor are slightly increased. The dielectric constant and dissipation factor are also affected by the incorporation of Mn. The increase of permeability due to the incorporation of Mn may be attributed to the decrease of magnetostriction constant induced by Mn addition. The change of other electrical properties such as dielectric constant; dissipation factor may be caused by the electron exchange between Fe2þ and Fe3þ . MgCuZn ferrites with 2 and 4 mol% Mn content and sintered at 910 C/30 min possess high density and electrical properties are also found to be optimum among all the present samples. Therefore, these ferrites were used to fabricate
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multilayer chip inductors by green sheet lamination and screen-printing processes. It was found that the fabricated chip inductors show better properties than that of chip inductors fabricated using NiCuZn ferrites.
Acknowledgements The financial support received from the Department of Science and Technology (DST), New Delhi, is gratefully acknowledged. References [1] A. Ono, T. Muruno, N. Kaihara, Jpn. Electron. Eng. 28 (1991) 5. [2] T. Nomura, A. Nakano, in: Proceedings of ICF 6, Japan Society of Powder and Powder Metallurgy, 1992, p. 1198. [3] T. Nakamura, J. Magn. Magn. Mater. 265 (1997) 168. [4] S.R. Murthy, Bull. Mater. Sci. 24 (2001) 379. [5] A. Bhaskar, Ph.D. Thesis Development of low temperature microwave sintered Mn2þ added MgCuZn ferrites, Osmania University, Hyderabad, India, 2002, p. 130.
[6] J.H. Nam, H.H. Jung, J.Y. Shin, J.H. Oh, IEEE Trans. Magn. 31 (1995) 3985. [7] H.J. Leim, H.B. Im, IEEE Trans. Magn. 18 (1982) 1541. [8] D.C. Khan, M. Misra, A.R. Das, J. Appl. Phys. 53 (1982) 2722. [9] Z. Yue, J. Zhou, L. Li, Z. Gui, J. Magn. Magn. Mater. 233 (2001) 224. [10] Zhenxing Yue, Ji Zhou, Longtu Li, Zhou Gui, J. Magn. Magn. Mater. 264 (2003) 258. [11] M.A. El Hiti, J. Magn. Magn. Mater. 164 (1996) 196. [12] H. Haberey, H. Wijn, Phys. Stat. Sol. (a) 26 (1968) 231. [13] P.V. Reddy, T.S. Rao, J. Less-Common Metals 105 (1985) 63. [14] C. Prakash, J. Bijal, J. Less-Common Metals 107 (1985) 51. [15] C.B. Kolekar, P.N. Kamble, S.G. Kulkurni, A.S. Vaingamkar, J. Mater. Sci. 30 (1995) 5784. [16] K. Iwanchi, Jpn. J. Appl. Phys. 10 (1971) 1520. [17] Xiwei Qi, Ji Zhou, Zhenxing Yue, Zhilum Gui, Longtu Li, J. Magn. Magn. Mater. 251 (2002) 316. [18] G.F. Dionne, R.G. West, J. Appl. Phys. 61 (1987) 3868. [19] P.S.A. Kumar, J.J. Shrotri, C.E. Deshpandes, J. Appl. Phys. 81 (1997) 47. [20] J.H. Nam, W.G. Han, J.H. Oh, J. Appl. Phys. 81 (8) (1997) 4794. [21] A. Globus, J. Phys. 38 (1997) C1 -1. [22] S.R. Murthy, J. Mater. Sci. Lett. 21 (2002) 657.
Journal of Magnetism and Magnetic Materials 283 (2004) 109–116 www.elsevier.com/locate/jmmm
Electrical properties of Mn added MgCuZn ferrites prepared by microwave sintering method A. Bhaskar, B. Rajini Kanth, S.R. Murthy Department of Physics, Osmania University, Hyderabad 500 007, India Received 9 April 2003; received in revised form 16 April 2004 Available online 6 July 2004
Abstract A low temperature sintered Mn added MgCuZn ferrites were prepared using microwave sintering method. The prepared samples were characterized using X-rays and metallurgical microscope. Room temperature electrical properties such as resistivity, dielectric properties, inductance, quality factor and initial permeability have been measured in the frequency range of 100 kHz–100 MHz. The initial permeability was measured in the temperature range of 300–650 K. The multilayer chip inductors were fabricated using the prepared ferrites and their electrical properties were measured. r 2004 Elsevier B.V. All rights reserved. Keywords: Microwave sintering; Electrical resistivity; Dielectric constant; Initial permeability; Q factor; Dissipation factor; Multilayer chip inductor
1. lntroduction Chip inductors are one of the groups of passive surface mount devices (SMD). They are important components for the latest electronic products, such as cellular phones, video cameras, notebook computers, hard and floppy drives, etc. that require small dimensions, lightweight and better functions [1,2]. Multilayer chip inductor (MLCI) as a key component of electronic devices confronts Corresponding author.
E-mail address: [email protected] (S.R. Murthy).
new challenges. The key materials for MLCI are soft ferrite materials with low sintering temperature (o940 C) in order to co-firing with Ag internal electrode and better electrical and magnetic properties. In this process only NiCuZn ferrites were developed as the material used in the chip components [2,3]. In the case of NiCuZn ferrites it is found that the liquid phase causes diffusion of Ag electrode material during the sintering and consequently leads to residual stress during the cooling process, which inturn results in a decrease in the permeability and magnetic properties [4]. But, in the case of MgCuZn ferrites
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.05.039
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it is found that the diffusion of Ag electrode material during sintering process does not lead to residual stress, thus, no deterioration of the magnetic properties are possible for these ferrites [5]. Previous investigators have reported about the substitution of some transition metal ions such as Bi2 O3 , PbO, V2 O5 , MnO2 etc., to improve the extrinsic properties of low temperature sintered NiCuZn ferrites [6–8]. It was found that these additives are generally detrimental to the electrical and magnetic properties. But small addition of Mn2þ to NiCuZn ferrites plays a crucial role in improvement of electrical properties by reducing magnetostriction effects [9]. Keeping this point in view we have also added Mn in MgCuZn ferrites. Detailed investigation of the effect of Mn on electrical properties have been under taken and obtained results are presented in this paper.
2. Experimental method Samples with the composition Mgð0:456xÞ Cu0:144 Mnx Zn0:4 Fe2 O4 with x ¼ 0; 1; 2; 4 mol% were prepared using microwave sintering method. High purity (99.999) chemical reagent powders of Fe2 O3 , MgO, MnO2 , ZnO and CuO were mixed and grounded for 6 h. the powders were calcined at 800 C for 30 min using microwave furnace. The microwave sintering process was carried out using a specially designed applicator and which consists of a domestic microwave oven having an output power level tunable upto a maximum of 800 W and operating frequency of 2.45 GHz. Then the powder was re-grounded for 10 h. To this powder, 2 wt% poly-vinyl-alcohol was added as a binder. Then the powder was pressed into pellets (12 mm diameter, 3 mm thickness) and rods (60 mm in length, 12 mm diameter) and toroids (12 mm outer diameter, 6 mm inner diameter, 4 mm thickness) at a pressure of 8 MPa for 5 min and the samples were finally sintered at 910 C/30 min in air at atmospheric pressure. The structure and microstructure of the sintered materials were examined using X-ray diffractometry and metallurgical microscope. The bulk density of the present samples were measured using the Archimedes method. The room temperature
electrical properties were measured using a impedance analyzer (HP 4194A) in frequency range from 100 kHz to 100 MHz. The ferrites sintered at 910 C/30 min were used to fabricate multilayer chip inductors by green sheet lamination and screen-printing method. The DC resistance ðrÞ, inductance ðLÞ and quality factor ðQÞ of the fabricated chip inductors were also measured.
3. Results and discussion Fig. 1 shows the XRD patterns of the ferrite powders. As we can see from the figure, all the powder samples posses single phase with spinel structure. No second phase was detected for all samples. Fig. 2 shows the micrographs for the samples sintered at 910 C/30 min. It can be seen from the micrographs that the dense microstructures are obtained for all samples. The average grain size was obtained by line-intercept method and obtained values are presented in Table 1. The grain size is smaller than 1:5 mm for all the samples under investigation. This indicates that the present ferrites possess fine-grained microstructure and thus, exhibit good electromagnetic properties. The table also gives the values of bulk density for all the samples. It can be seen from the table that the density and grain size of samples are found to increase with an increase of Mn2þ content. DC resistivity is an important property of low temperature sintered MgCuZn ferrites utilized for MLCIs in high frequency range. The table gives the room temperature values of DC resistivity ðrÞ measured using a two-probe method. It is evident from the table that values of r increase markedly by an addition of Mn2þ . In the sample without Mn2þ content ðx ¼ 0Þ, Fe concentration is maximum at B-sites, which is responsible for electrical conduction in ferrites [10]. An addition of Mn in the system reduces the concentration of Fe2þ ions due to the presence Mn2þ and Mn3þ ions at B-sites. It is clear that the resistivity values of all the samples are higher than 109 O cm, meeting the requirement of MLCIs. The dielectric properties such as dielectric constant ð0 Þ and dissipation factor ðDÞ are
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111
Fig. 1. XRD patterns of sintered Mn added MgCuZn ferrites.
important for multilayer chip inductors used in high frequency range. Therefore, the dielectric constant measurements were carried out in the frequency range from 100 kHz to 100 MHz. Table 1 gives the room temperature values of dielectric constant and dissipation factor measured at 1 MHz. It can be observed from the table that the value of dielectric constant is small and remains almost constant with an addition of Mn2þ to MgCuZn ferrites. The dissipation factor is also found to be small and remains constant with an addition of Mn to MgCuZn ferrites. Fig. 3 gives the plots showing the variation of dielectric constant with frequency for all four samples at room temperature. It can be seen from the figure that the dielectric constant in the
beginning for all the samples decreases rapidly with an increase in frequency, and finally remains constant at higher frequencies. The incorporation of Mn into MgCuZn ferrites has no pronounced effect on the dielectric constant in high frequency range, but significantly increases the dielectric constant in the low frequency range. This type of behaviour was observed for a number of ferrites [11–14]. The dielectric behaviour of ferrites may be explained on the basis of that of the mechanism of the dielectric polarization process and is similar to that of the conduction process. The electronic exchange Fe2þ Ð Fe3þ gives the local displacement of electrons in the direction of applied electric field, which induces the polarization in
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112
ferrites [15,16]. The magnitude of exchange depends on the concentration of Fe2þ =Fe3þ ion pairs present on B site for the present ferrites. The incorporation of Mn2þ in B sites of ferrites may increase the concentration of Fe2þ =Fe3þ ion pairs due to the oxidation of Mn2þ to Mn3þ induced by Mn addition of Fe in ferrites. Therefore, the
dielectric constant for the present ferrites decreases with increasing frequency. The dielectric constant reaches a constant value depending upon the fact that beyond a fixed frequency of electric field the electron exchange does not follow the alternating field. The frequency variation of dissipation factor ðDÞ for all the samples measured at room temperature and results are presented in Fig. 4. It can be seen from the figure that value of D decreases at low frequencies and remains nearly constant at higher frequencies. When the hopping frequency becomes equal or nearly equal to that of external applied field, the value of dielectric loss increases once again at higher frequency. It is also evident, from the figure that the dissipation factor decreases with increasing of Mn content in MgCuZn ferrites. The dielectric losses in ferrites are generally reflected in the resistivity i.e., materials with low resistivity exhibit high
0.00 0.01 0.02 0.04
Dielectric Constant (ε1)
16 15 14 13 12 11 10 1
10
100
Frequency (MHz)
Fig. 2. Metallurgical micrographs of sintered ferrites with (a) 2 mol% and (b) 4 mol% of Mn.
Fig. 3. Variation of dielectric constant with frequency.
Table 1 Bulk density, average grain size and main electrical properties of Mn added MgCuZn ferrites Mn Content (mol%)
Bulk density ðg=cm3 Þ
Average grain size ðmmÞ
r 109 ðO cmÞ
mi (1 MHz)
Q (1 MHz)
0 (1 MHz)
D (1 MHz)
0 1 2 4
4.91 4.98 5.01 5.23
0.7 0.9 1.3 1.5
0.8 2.8 4.5 7.6
236 258 390 548
52 58 68 60
11 11.5 10 11.2
0.020 0.036 0.036 0.028
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Dissipation factor (D)
0.12 0.10 0.08 0.06 0.04
600 550
Initial permeability (µi)
0.00 0.01 0.02 0.04
0.14
113
500 450 400 350 300 250 200 150 100
0.02
50
0.04 0.02 0.01 0.00
0
1
10
100
1
Frequency (MHz)
100
Fig. 5. Frequency dependence of the initial permeability.
dielectric loss and vice versa. Therefore, the increase of dissipation factor with increasing Mn content is also attributed to the exchange of electrons between Fe2þ and Fe3þ , which is responsible for the conduction mechanism in ferrites. The initial permeability ðmi Þ has been measured for all the ferrites in the frequency range of 100 kHz–100 MHz. Table 1 gives the room temperature values of initial permeability measured at 1 MHz. It can be observed from the table that the value of initial permeability increases with an increase of Mn content from 0 to 4 mol% in MgCuZn ferrites. The decreasing magnetostriction and disappearance of pores due to the densification causes an increase of the initial permeability for the present ferrites [17]. Fig. 5 shows the frequency dependence of initial permeability for all samples under investigation. It can be seen from the figure that the value of initial permeability for samples without and with 1 mol% of Mn decreases with an increase of frequency from 100 kHz to 80 MHz and then rapidly drops to small value. However, in the same frequency range the value of mi remains almost constant with an increase of frequency for samples with 2 and 4 mol% of Mn. The initial permeability of ferrite is expressed as, ð1Þ
where mi is the initial permeability, M s the saturation magnetization, k the crystal magnetic
70
0.04 0.02 0.01 0.00
60
Quality factor (Q)
Fig. 4. Variation of dissipation factor with frequency.
mi ¼ M 2s =ak þ bls
10
Frequency (MHz)
50 40 30 20 10 0 1
10
100
Frequency (MHz)
Fig. 6. Frequency dependence of the quality factor.
anisotropy constant, l is the magnetostriction constant and s is the internal stress and a and b are constants. Previously, it has been shown that addition of Mn2þ for Fe2þ can decrease the magnetostriction constant, resulting in the increase of permeability [18,19]. This reveals that the significant increase in permeability occurred in our ferrites may be due to decrease of magnetostriction constant induced by Mn addition [20]. The quality factor ðQÞ versus frequency plots of all the samples are shown in Fig. 6. It can be seen from the figure that the value of quality factor increases with an increase of frequency and shows a peak around 1.5 MHz. With further increase of frequency the value of Q remains constant upto 100 MHz. It is also observed from the figure that
ARTICLE IN PRESS A. Bhaskar et al. / Journal of Magnetism and Magnetic Materials 283 (2004) 109–116
114
the value of Q increases with an increase of Mn content in MgCuZn ferrites. It has been known that the main loss mechanism in ferrite at high frequency is eddy current loss. The eddy current loss can be expressed as [18] Pe ¼ KB2 f 2 d 2 =r
ð2Þ
where ‘Pe ’ is the energy loss per unit volume, ‘K’ is a geometric constant, ‘B’ is the maximum induction, ‘f ’ is the frequency and ‘d’ is the thickness of the narrowest dimension perpendicular to magnetic flux and ‘r’ is the resistivity. From the above equation one can conclude that the eddy current loss is inversely proportional to the resistivity of the material. So, as the resistivity of the samples is increased, the eddy current loss is decreased, resulting in an increase in quality factor. Thus, the increase in quality factor is attributed to the increase in resistivity due to Mn addition. Initial permeability ðmi Þ has been measured for all samples in the temperature ðTÞ range of 300–600 K and obtained results are presented in Fig. 7. It can be seen from the figure that the value of mi increases with an increase of temperature and show a sharp peak and suddenly drops to a small value 3. It is well known that for the ferrite, mi drop to a small value at a temperature known as Curie temperature. Therefore, in the present investigation, Curie temperature ðT c Þ of the present ferrites is taken when mi drops to 3. The
Initial Permeability (µ i )
500
0.04 0.02 0.01 0.00
400
300
200
100
0 250
300
350
400
450
500
550
600
650
700
Temperature (K)
Fig. 7. Temperature dependence of initial permeability at 1 kHz.
average value of T c , thus, obtained for the present ferrites is 510 K. It can also be observed from the figure that mi –T curves are almost flat in between 300 and 390 K for the samples with 2 and 4 mol% of Mn. This shows that these ferrites have best thermal stability. The observed mi versus temperature results can be interpreted using the Globus model [21]. According to this model, if the sample is of toroidal shape, because of the existence of several easy axes in each grain of a soft cubic material, the domain walls align themselves, at best, along the direction of circles of the torus, in order to minimize the demagnetizing fields. When the sample is cooled down from Curie temperature, k1 in this region is weak, and it is easy for the domain walls to follow the torus symmetry, since the magnetization is larger than the anisotropy field. At low temperature, when k1 is stronger, the topography of the domain walls is influenced by the anisotropy directions in such a way that the parallelism between the applied field and portions of the domain wall worsens. The value of mi is thus lower than that due to an ideal toroidal symmetry configuration. From the above studies, it was concluded that the MgCuZn ferrites with 2 and 4 mol%. Mn content and sintered at 910 C/30 min possesses high density and optimum electrical properties among all the present samples. Therefore, these ferrites (2 mol% (inductor A), 4 mol% (inductor B)) were used to fabricate multilayer chip inductors by green sheet lamination and screen-printing processes [5]. The chips were sintered at 930 C/ 30 min. The fabricated multilayer chip inductors have the dimensions of 4.5 mm in length, 3.0 mm in width and 2 mm in height. The chips were then coated with Ag terminal electrodes. The room temperature DC resistance of the chip inductors was measured. The measured resistance values for the inductors A and B are 0.121 and 0.225 O, respectively. The resistance of the chip inductors increased with increasing Mn addition and inductor B exhibited higher resistance. The inductance of the inductor B ( 0:6 mHÞ is higher than that of inductor A ð 0:5 mHÞ (at 1 MHz). However, the quality factor of inductor B is smaller than that of inductor A as expected.
ARTICLE IN PRESS A. Bhaskar et al. / Journal of Magnetism and Magnetic Materials 283 (2004) 109–116
It can be seen from the Figs. 8 and 9 that the value of inductance for inductors A and B remains constant with an increase of frequency upto 10 MHz and increases at higher frequencies. The quality factor ðQÞ for both the fabricated chip inductors decreases with an increase of frequency. Table 2 gives the comparison data of present chip inductors with that of NiCuZn chip inductors [22]. It can be seen from the table that the values of inductance ðLÞ for present chip inductors are found to be higher than that of chip inductor fabricated using NiCuZn ferrites. Similarly, the values of initial permeability ðmi Þ are also higher for present chip inductors. This shows that,
Inductor A
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Table 2 Comparison data for chip inductors Sample
Sintering temperature
Inductance ðLÞ
Initial permeability ðmi Þ (at 1 MHz)
ð CÞ
ðmHÞ (at 1 MHz)
NiCuZn Ferrite Chip Inductor
910/6 h
0.45
580
MgCuZn Ferrite Chip Inductor A
910/30 min
0.50
600
MgCuZn Ferrite Chip Inductor B
910/30 min
0.60
650
80
60
0.4
40
20
Quality factor (Q)
Inductance (µH)
0.5
0.3
presently fabricated chip inductors are better than the chip inductors fabricated using NiCuZn ferrites.
4. Summary 0
0.2 0.1
1
10
Frequency (MHz)
Fig. 8. Frequency variation of inductance ðLÞ and quality factor ðQÞ for Fabricated multilayer chip inductor A.
Inductor B
60
50
Inductance (µH)
0.60
40 0.55 30 0.50
20
Quality factor (Q)
0.65
10
0.45
0 0.40 0.1
1
10
Frequency (MHz)
Fig. 9. Frequency variation of inductance ðLÞ and quality factor ðQÞ for Fabricated multilayer chip inductor B.
A series of Mn added MgCuZn ferrites were prepared using microwave sintering method. The powders were calcined at 800 C/30 min and finally samples were sintered at 910 C/30 min. The prepared samples were characterized using X-rays and metallurgical microscope. With increasing Mn content, the initial permeability is significantly increased, while the electrical resistivity and quality factor are slightly increased. The dielectric constant and dissipation factor are also affected by the incorporation of Mn. The increase of permeability due to the incorporation of Mn may be attributed to the decrease of magnetostriction constant induced by Mn addition. The change of other electrical properties such as dielectric constant; dissipation factor may be caused by the electron exchange between Fe2þ and Fe3þ . MgCuZn ferrites with 2 and 4 mol% Mn content and sintered at 910 C/30 min possess high density and electrical properties are also found to be optimum among all the present samples. Therefore, these ferrites were used to fabricate
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multilayer chip inductors by green sheet lamination and screen-printing processes. It was found that the fabricated chip inductors show better properties than that of chip inductors fabricated using NiCuZn ferrites.
Acknowledgements The financial support received from the Department of Science and Technology (DST), New Delhi, is gratefully acknowledged. References [1] A. Ono, T. Muruno, N. Kaihara, Jpn. Electron. Eng. 28 (1991) 5. [2] T. Nomura, A. Nakano, in: Proceedings of ICF 6, Japan Society of Powder and Powder Metallurgy, 1992, p. 1198. [3] T. Nakamura, J. Magn. Magn. Mater. 265 (1997) 168. [4] S.R. Murthy, Bull. Mater. Sci. 24 (2001) 379. [5] A. Bhaskar, Ph.D. Thesis Development of low temperature microwave sintered Mn2þ added MgCuZn ferrites, Osmania University, Hyderabad, India, 2002, p. 130.
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