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Fabrication of multilayer chip inductors using Ni–Cu–Zn ferrites
Journal of Alloys and Compounds 414 (2006) 282–286
Fabrication of multilayer chip inductors using Ni–Cu–Zn ferrites T. Krishnaveni, B. Rajini Kanth, V. Seetha Rama Raju, S.R. Murthy ∗ Department of Physics, Osmania University, Hyderabad 500 007, India Received 23 March 2005; received in revised form 14 July 2005; accepted 18 July 2005 Available online 8 September 2005
Abstract Ni–Cu–Zn ferrite nanopowders were synthesized by co-precipitation method using microwave–hydrothermal (M-H) reaction system. The ferrite formation conditions such as pH, temperature and time were determined in detail according to the reaction conditions. The phase identification, crystallinity and morphology of the prepared samples were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Nanocrystalline ferrites with high surface area were synthesized at temperatures as low as 165 ◦ C in a short time (30 min). The nanoferrite powders were sintered at different sintering temperatures from 800 to 950 ◦ C for 6 h using conventional sintering method. The performance of the sintered Ni–Cu–Zn ferrites has been estimated from the studies of dependence of permeability spectra on the frequency and temperature. Multilayer chip inductors were fabricated from the ferrite using the screen-printing method. Inductance and quality factor of the prepared inductors were measured over a wide frequency range. © 2005 Elsevier B.V. All rights reserved. Keywords: Ferrites; Nanocrystalline materials; Microwave-hydrothermal synthesis; Dielectric properties; Multilayerchip inductors
1. Introduction Chip inductors are one of the 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., and that require small dimensions, lightweight and better functions [1]. The traditional wire-wound chip inductors can only be miniaturized to a certain limit and lack of magnetic shielding leads to the development of new materials for the multilayer chip inductors. In this process only Ni–Cu–Zn ferrites were developed as the material used for the chip components [2–4], which can be fired at 900 ◦ C or below. But, it was found that these ferrites are comparatively sensitive to stress and magnetic properties are easily changed or deteriorated by the stress caused at the internal electrode. These problems can be reduced by the preparation of Ni–Cu–Zn ferrites using nanoferrite powders and then sintering under controlled experimental conditions. Komarneni ∗
Corresponding author. E-mail address: [email protected] (S.R. Murthy).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.07.029
et al. [5–8] have used the microwave-hydrothermal (M-H) method to prepare nanoparticles of various ferrites with high surface area. However, their studies were confirmed only to simple ferrites. Therefore, in the present investigation nanopowders of Ni–Cu–Zn ferrites were synthesized using M-H method. These nanopowders were sintered at different temperatures from 800 to 950 ◦ C for 6 h using a conventional sintering method and their electrical and magnetic properties have been measured. The multilayer chip inductors (MLCIs) are usually prepared either by the thick film screen-printing method [9] or by the process of green sheet lamination [10]. The MLCIs are fabricated with printed internal conductors connected among the ferrite layers in a coil style. The monolithic structure of the multilayer chip inductors demonstrates the superb capability of miniaturization and perfect closed magnetic circuit inside the chips [11]. Hence, no cross talk occurs when they are utilized in high-density electronic circuits. In the present paper multilayer chip inductors were fabricated using the screenprinting method with the prepared Ni–Cu–Zn ferrites. The performance of prepared multilayer chip inductor was also examined.
T. Krishnaveni et al. / Journal of Alloys and Compounds 414 (2006) 282–286
283
2. Experimental method Pure (99.99%) nickel nitrate [Ni(NO3 )2 ·6H2 O], copper nitrate [Cu(NO3 )2 ·3H2 O], zinc nitrate [Zn(NO3 )2 ·6H2 O] and iron nitrate [Fe(NO3 )2 ·9H2 O] were dissolved in 50 ml of deionized water. The molar ratio of powders was adjusted to obtain composition Ni0.53 Cu0.12 Zn0.35 Fe2 O4. An aqueous NaOH solution was added to this solution until the desired pH ∼9.45 was obtained. Proper control of the pH is the key factor in synthesizing the ferrites. The mixture was then treated in a Teflon-lined vessel using a microwave digestion system (Model MDS2000, CEM Corp., Mathews, NC). This system uses 2.45 GHz microwaves and can be operated at 0–100% full power (630 ± 50 W). The time, pressure and power were computer-controlled. The products obtained were filtered and washed repeatedly with deionized water, followed by drying overnight. The prepared powders were weighed and the percentage yields were calculated. All the synthesized nanocrystalline ferrite particles were characterized using powder X-ray diffractometry. Particle size and morphology were determined using transmission electron microscopy (TEM). The nanocrystalline particles of the prepared ferrite powder were mixed with an appropriate amount of 2 wt.% polyvinyl alcohol as a binder. Then the powder was uniaxially pressed at a pressure of 1500 kg/cm2 to form green pellet and toroidal specimens. After the binder burnt out at 600 ◦ C for 2 h, the compacts were conventionally sintered at 800 ◦ C/6 h (sample No. 1), 850 ◦ C/6 h (sample No. 2), 900 ◦ C/6 h (sample No. 3) and 950 ◦ C/6 h (sample No. 4) in air. The final composition of the ferrite as estimated from chemical analysis was Ni0.53 Cu0.12 Zn0.35 Fe2 O4 . The electrical properties were measured using HP4194 impedance analyzer in the frequency range of 1–100 MHz. The permeability and quality factor were calculated using the standard
Fig. 2. TEM picture of Ni–Cu–Zn ferrite.
procedure from the measured inductance and magnetic loss of the coil wound toroids. The saturation magnetization has been measured using vibrating sample magnetometer (VSM). The MLCIs were fabricated using the screen-printing method. The prepared chip inductors were fired to 890 ◦ C/4 h, 900 ◦ C/4 h, and 910 ◦ C/4 h, respectively, and the obtained results were presented in this paper.
3. Results and discussion Fig. 1 shows the powder XRD result of microwavehydrothermally synthesized sample. It is clear from the figure that the sample is nanocrystalline in nature. Fig. 2 shows the TEM analysis of the synthesized powder. The particle size of the synthesized powder is about ∼10–30 nm in size and exhibited more or less spherical morphology. Fig. 3 shows the XRD patterns for the sintered Ni–Cu–Zn ferrites. It can be seen from the figures that all the samples possess mono-phase in nature. The average value of lattice constant calculated from these X-ray data is found to be
Fig. 1. XRD patterns of M-H-synthesized nanophase Ni–Cu–Zn ferrite powders.
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Fig. 3. XRD patterns for the sintered Ni–Cu–Zn ferrites.
800 to 900 ◦ C. It is observed that the ferrites sintered at 900 ◦ C (sample No. 3) possess a high saturation magnetization value of ∼0.61 T with coercive field of 85 Oe in maximum external field of 20 kOe. Fig. 4 (a and b) shows the real (µ ) and imaginary (µ ) permeability spectra for all the samples. It can be seen from the figure that the value of the real part of permeability is found to increase from 120 to 350 with increase of sintering temperature. As sintering temperature increases, the natural resonance frequency increases, where the imaginary permeability had a maximum value shifted towards low frequency from 9 to 1 MHz. This result indicates that by increasing the sintering temperature over 900 ◦ C, large ferrite particles are obtained. The complex permeability of sintered ferrite is related to two different magnetizing mechanisms; the spin rotational magnetization and domain wall motion [15]. The permeability spectra of the sintered ferrite decomposed into the spin rotational component, χSP (ω), and the domain wall component, χDW (ω). The spin rotational component is relaxation type and its dispersion is inversely proportional to the frequency. The domain wall component is of resonance type and depends on the square of frequency. The following equations are generally used to describe the frequency variation of permeability spectra:
˚ The particle size (Dm ) of the sintered ferrites has 8.337 A. been estimated with the help of XRD patterns using Scherrer’s equation: Dm = Kλ/β cos θ
(1)
where K is a constant, β, the full width half maxima and λ is the wavelength of X-rays used, respectively. The obtained results are presented in Table 1. It can be observed from the table that the particle size of nanocrystalline powders is increased with an increase of sintering temperature. Table 1 also gives the room temperature values of bulk density (dBulk ), real part of permeability (µ ), saturation magnetization (Ms ), quality factor (Q), and dc resistivity (ρ) for the sintered ferrites. It can be seen from the table that the bulk density and particle size of samples increases with an increase of sintering temperature. The value of dc resistivity for the present ferrites is around ∼1013 cm, meeting the requirement of MLCIs [12]. The high resistivity observed in the present samples may be due to the substitution of nickel by copper that resists the oxidation of Ni+2 to Ni+3 . It decreases the hopping conduction of electron from Ni+2 to Ni+3 , thereby increasing the resistivity of the sample [13,14]. It can also be observed from the table that the value of Q is high for sample No. 3. It is evident from Table 1 that the saturation magnetization (MS ) increases with increasing sintering temperature from
µ(ω) = 1 + χSP (ω) + χDW (ω)
(2)
Table 1 Room temperature data for Ni–Cu–Zn ferrites Sample No.
Sintering temperature (◦ C/6 h)
dx (g/cm3 )
Dm (nm)
µ (at 1 MHz)
Ms (T)
ρ ( cm)
Q (at 1 MHz)
1 2 3 4
800 850 900 950
4.146 4.525 5.256 5.384
98 110 120 210
120 210 350 160
0.22 0.35 0.61 0.25
8.8 × 1013 13.4 × 1013 18.2 × 1013 8.5 × 1013
103 124 132 85
T. Krishnaveni et al. / Journal of Alloys and Compounds 414 (2006) 282–286
285
Fig. 5. Permeability versus temperature plots of Ni–Cu–Zn ferrite samples.
Fig. 4. Variation of (a) real (µ ) and (b) imaginary (µ ) part of the complex permeability with frequency for Ni–Cu–Zn ferrites.
and each component can be written as follows: χSP (ω) = KSP /[1 + j(ω/ωSP )]
(3)
2 χDW (ω) = KDW /[1 + ωDW − ω2 + jβω]
(4)
here ω is the magnetic field frequency, KSP , the static spin susceptibility, ωSP , the spin resonance frequency, KDW , the static susceptibility of the domain wall motion, and β is the damping factor of the domain wall motion. The domain wall motion contribution starts to decrease at lower frequency and the complex permeability in the higher frequency region above 100 MHz can described using the spin rotational component. The parameters such as KSP , ωSP, KDW , ωDW , and β can
be determined by numerical fitting of complex permeability spectra. Fig. 4 also shows results of calculated curves using the Eqs. (3) and (4), which are plotted as dark line for sample No. 3. A good agreement was observed between calculated and experimental values for the real (µ ) and imaginary (µ ) parts of the permeability spectra of the sample No. 3. Fig. 5 gives the plots of permeability versus temperature for all the samples. It is evident from the figure that the µi remains constant over a wide temperature range for all the samples. The value of µi remains constant with temperature and shows a peak in the vicinity of Curie temperature and then drops sharply to a small value ∼2, indicating the high homogeneity of the present samples. The temperature at which µi drops to small value is taken as Curie temperature (TC ) of the corresponding sample. The TC thus obtained for the present samples is 660 ± 1 K. For the polycrystalline samples the shape of µi –T curves depends strongly on the preparation conditions. Out of all the samples under investigation, sample No. 3 possesses good electrical and magnetic properties. Therefore, these ferrites were used to fabricate MLCIs by green sheet lamination and screen-printing processes. The chips were sintered at 890, 900, and 910 ◦ C. The size of the fabricated MLCIs is 2 mm × 1 mm × 0.5 mm. The chips were then coated with Ag terminal electrodes. Table 2 gives the room temperature values of dc resistance (R), inductance (L), and real part of the permeability (µ ) for the presently prepared
Table 2 Room temperature data of the fabricated multilayer chip inductors Sample
Sintering temperature (◦ C/h)
µ (at 1 MHz)
R ()
L (H) (at 1 MHz)
Ni–Cu–Zn (P) Ni–Cu–Zn (l) Ni–Cu–Zn (P) Ni–Cu–Zn (l) Ni–Cu–Zn (P) Ni–Cu–Zn (l)
890/4 890/12 900/4 900/12 910/4 910/12
435 450 610 590 520 650
0.115 0.104 0.208 0.115 0.156 0.215
0.41 4.98 0.43 5.78 0.40 6.61
P: present samples; l: Ref. [12].
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small and decreases with an increase of frequency. From these studies it is expected that a good quality chip inductor can be obtained using the Ni–Cu–Zn ferrites with 12 mol% Cu.
Acknowledgements The authors are very much thankful to DRDO, New Delhi for its financial assistance and to S. Komarneni, Materials Research Institute, The Penn State University, USA for his great help to synthesize the samples.
Fig. 6. Frequency variation of L and Q for Ni–Cu–Zn ferrite MLCI sintered at 900 ◦ C/4 h.
inductors. For the sake of comparison the values of R, L, and µ for the Ni–Cu–Zn ferrite chip inductors prepared using conventionally sintered powders [12] were incorporated in the same table. It can be seen from the table that the initial permeability, resistance and inductance of the chip inductors increased with the increase of sintering temperature and the inductor sintered at 900 ◦ C/4 h exhibited higher values. Present MLCIs possess high value of resistance with low inductance than the MLCIs fabricated earlier. Fig. 6 gives the plot of frequency variation of inductance (L) and quality factor (Q) for the Ni–Cu–Zn ferrite chip inductor sintered at 900 ◦ C/4 h. It can be seen from the figure that the inductance remains constant in the frequency range of 1–20 MHz and increases at higher frequencies. The Q value is
References [1] S.R. Murthy, Bull. Mater. Sci. 24 (2001) 379. [2] T. Nakamura, J. Magn. Magn. Mater. 168 (1997) 285. [3] J.Y. Hsu, W.S. Ko, H.D. Shen, C.J. Chen, IEEE Trans. Magn. 30 (1994) 4875. [4] M. Fujimoto, J. Am. Ceram. Soc. 77 (1994) 2873. [5] S. Komarneni, R. Roy, Q.-H. Li, Mater. Res. Bull. 27 (1992) 1393. [6] S. Komarneni, Q.-H. Li, K. Stephenson, R. Roy, J. Mater. Res. 6 (1993) 3176. [7] S. Komarneni, R. Roy, Q.-H. Li, J. Mater. Res. 11 (1996) 1866. [8] S. Komarneni, V.C. Menon, R. Roy, Q.-H. Li, F. Ainger, J. Am. Ceram. Soc. 79 (1996) 1409. [9] A. One, T. Muruno, N. Kaihara, Nikken New Mater. 116 (1992) 10–38 (Japanese). [10] US patent No. 4,543,553. [11] M. Takaya, Jpn. Electron. Eng. 38. (1986). [12] S.R. Murthy, J. Mater. Sci. Lett. 21 (2002) 657. [13] F. Wang, Y.R. Wang, T.C.K. Yang, C.F. Chen, C.A. Lu, C.Y. Hnang, J. Magn. Magn. Mater. 220 (2000) 129. [14] Zhenxig Yue, Ji Zhou, Longtu Li, Zhilun Gui, J. Magn. Magn. Mater. 55 (2001) 208. [15] T. Nakamura, J. Appl. Phys. 88 (2000) 348.
Fabrication of multilayer chip inductors using Ni–Cu–Zn ferrites T. Krishnaveni, B. Rajini Kanth, V. Seetha Rama Raju, S.R. Murthy ∗ Department of Physics, Osmania University, Hyderabad 500 007, India Received 23 March 2005; received in revised form 14 July 2005; accepted 18 July 2005 Available online 8 September 2005
Abstract Ni–Cu–Zn ferrite nanopowders were synthesized by co-precipitation method using microwave–hydrothermal (M-H) reaction system. The ferrite formation conditions such as pH, temperature and time were determined in detail according to the reaction conditions. The phase identification, crystallinity and morphology of the prepared samples were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Nanocrystalline ferrites with high surface area were synthesized at temperatures as low as 165 ◦ C in a short time (30 min). The nanoferrite powders were sintered at different sintering temperatures from 800 to 950 ◦ C for 6 h using conventional sintering method. The performance of the sintered Ni–Cu–Zn ferrites has been estimated from the studies of dependence of permeability spectra on the frequency and temperature. Multilayer chip inductors were fabricated from the ferrite using the screen-printing method. Inductance and quality factor of the prepared inductors were measured over a wide frequency range. © 2005 Elsevier B.V. All rights reserved. Keywords: Ferrites; Nanocrystalline materials; Microwave-hydrothermal synthesis; Dielectric properties; Multilayerchip inductors
1. Introduction Chip inductors are one of the 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., and that require small dimensions, lightweight and better functions [1]. The traditional wire-wound chip inductors can only be miniaturized to a certain limit and lack of magnetic shielding leads to the development of new materials for the multilayer chip inductors. In this process only Ni–Cu–Zn ferrites were developed as the material used for the chip components [2–4], which can be fired at 900 ◦ C or below. But, it was found that these ferrites are comparatively sensitive to stress and magnetic properties are easily changed or deteriorated by the stress caused at the internal electrode. These problems can be reduced by the preparation of Ni–Cu–Zn ferrites using nanoferrite powders and then sintering under controlled experimental conditions. Komarneni ∗
Corresponding author. E-mail address: [email protected] (S.R. Murthy).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.07.029
et al. [5–8] have used the microwave-hydrothermal (M-H) method to prepare nanoparticles of various ferrites with high surface area. However, their studies were confirmed only to simple ferrites. Therefore, in the present investigation nanopowders of Ni–Cu–Zn ferrites were synthesized using M-H method. These nanopowders were sintered at different temperatures from 800 to 950 ◦ C for 6 h using a conventional sintering method and their electrical and magnetic properties have been measured. The multilayer chip inductors (MLCIs) are usually prepared either by the thick film screen-printing method [9] or by the process of green sheet lamination [10]. The MLCIs are fabricated with printed internal conductors connected among the ferrite layers in a coil style. The monolithic structure of the multilayer chip inductors demonstrates the superb capability of miniaturization and perfect closed magnetic circuit inside the chips [11]. Hence, no cross talk occurs when they are utilized in high-density electronic circuits. In the present paper multilayer chip inductors were fabricated using the screenprinting method with the prepared Ni–Cu–Zn ferrites. The performance of prepared multilayer chip inductor was also examined.
T. Krishnaveni et al. / Journal of Alloys and Compounds 414 (2006) 282–286
283
2. Experimental method Pure (99.99%) nickel nitrate [Ni(NO3 )2 ·6H2 O], copper nitrate [Cu(NO3 )2 ·3H2 O], zinc nitrate [Zn(NO3 )2 ·6H2 O] and iron nitrate [Fe(NO3 )2 ·9H2 O] were dissolved in 50 ml of deionized water. The molar ratio of powders was adjusted to obtain composition Ni0.53 Cu0.12 Zn0.35 Fe2 O4. An aqueous NaOH solution was added to this solution until the desired pH ∼9.45 was obtained. Proper control of the pH is the key factor in synthesizing the ferrites. The mixture was then treated in a Teflon-lined vessel using a microwave digestion system (Model MDS2000, CEM Corp., Mathews, NC). This system uses 2.45 GHz microwaves and can be operated at 0–100% full power (630 ± 50 W). The time, pressure and power were computer-controlled. The products obtained were filtered and washed repeatedly with deionized water, followed by drying overnight. The prepared powders were weighed and the percentage yields were calculated. All the synthesized nanocrystalline ferrite particles were characterized using powder X-ray diffractometry. Particle size and morphology were determined using transmission electron microscopy (TEM). The nanocrystalline particles of the prepared ferrite powder were mixed with an appropriate amount of 2 wt.% polyvinyl alcohol as a binder. Then the powder was uniaxially pressed at a pressure of 1500 kg/cm2 to form green pellet and toroidal specimens. After the binder burnt out at 600 ◦ C for 2 h, the compacts were conventionally sintered at 800 ◦ C/6 h (sample No. 1), 850 ◦ C/6 h (sample No. 2), 900 ◦ C/6 h (sample No. 3) and 950 ◦ C/6 h (sample No. 4) in air. The final composition of the ferrite as estimated from chemical analysis was Ni0.53 Cu0.12 Zn0.35 Fe2 O4 . The electrical properties were measured using HP4194 impedance analyzer in the frequency range of 1–100 MHz. The permeability and quality factor were calculated using the standard
Fig. 2. TEM picture of Ni–Cu–Zn ferrite.
procedure from the measured inductance and magnetic loss of the coil wound toroids. The saturation magnetization has been measured using vibrating sample magnetometer (VSM). The MLCIs were fabricated using the screen-printing method. The prepared chip inductors were fired to 890 ◦ C/4 h, 900 ◦ C/4 h, and 910 ◦ C/4 h, respectively, and the obtained results were presented in this paper.
3. Results and discussion Fig. 1 shows the powder XRD result of microwavehydrothermally synthesized sample. It is clear from the figure that the sample is nanocrystalline in nature. Fig. 2 shows the TEM analysis of the synthesized powder. The particle size of the synthesized powder is about ∼10–30 nm in size and exhibited more or less spherical morphology. Fig. 3 shows the XRD patterns for the sintered Ni–Cu–Zn ferrites. It can be seen from the figures that all the samples possess mono-phase in nature. The average value of lattice constant calculated from these X-ray data is found to be
Fig. 1. XRD patterns of M-H-synthesized nanophase Ni–Cu–Zn ferrite powders.
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T. Krishnaveni et al. / Journal of Alloys and Compounds 414 (2006) 282–286
Fig. 3. XRD patterns for the sintered Ni–Cu–Zn ferrites.
800 to 900 ◦ C. It is observed that the ferrites sintered at 900 ◦ C (sample No. 3) possess a high saturation magnetization value of ∼0.61 T with coercive field of 85 Oe in maximum external field of 20 kOe. Fig. 4 (a and b) shows the real (µ ) and imaginary (µ ) permeability spectra for all the samples. It can be seen from the figure that the value of the real part of permeability is found to increase from 120 to 350 with increase of sintering temperature. As sintering temperature increases, the natural resonance frequency increases, where the imaginary permeability had a maximum value shifted towards low frequency from 9 to 1 MHz. This result indicates that by increasing the sintering temperature over 900 ◦ C, large ferrite particles are obtained. The complex permeability of sintered ferrite is related to two different magnetizing mechanisms; the spin rotational magnetization and domain wall motion [15]. The permeability spectra of the sintered ferrite decomposed into the spin rotational component, χSP (ω), and the domain wall component, χDW (ω). The spin rotational component is relaxation type and its dispersion is inversely proportional to the frequency. The domain wall component is of resonance type and depends on the square of frequency. The following equations are generally used to describe the frequency variation of permeability spectra:
˚ The particle size (Dm ) of the sintered ferrites has 8.337 A. been estimated with the help of XRD patterns using Scherrer’s equation: Dm = Kλ/β cos θ
(1)
where K is a constant, β, the full width half maxima and λ is the wavelength of X-rays used, respectively. The obtained results are presented in Table 1. It can be observed from the table that the particle size of nanocrystalline powders is increased with an increase of sintering temperature. Table 1 also gives the room temperature values of bulk density (dBulk ), real part of permeability (µ ), saturation magnetization (Ms ), quality factor (Q), and dc resistivity (ρ) for the sintered ferrites. It can be seen from the table that the bulk density and particle size of samples increases with an increase of sintering temperature. The value of dc resistivity for the present ferrites is around ∼1013 cm, meeting the requirement of MLCIs [12]. The high resistivity observed in the present samples may be due to the substitution of nickel by copper that resists the oxidation of Ni+2 to Ni+3 . It decreases the hopping conduction of electron from Ni+2 to Ni+3 , thereby increasing the resistivity of the sample [13,14]. It can also be observed from the table that the value of Q is high for sample No. 3. It is evident from Table 1 that the saturation magnetization (MS ) increases with increasing sintering temperature from
µ(ω) = 1 + χSP (ω) + χDW (ω)
(2)
Table 1 Room temperature data for Ni–Cu–Zn ferrites Sample No.
Sintering temperature (◦ C/6 h)
dx (g/cm3 )
Dm (nm)
µ (at 1 MHz)
Ms (T)
ρ ( cm)
Q (at 1 MHz)
1 2 3 4
800 850 900 950
4.146 4.525 5.256 5.384
98 110 120 210
120 210 350 160
0.22 0.35 0.61 0.25
8.8 × 1013 13.4 × 1013 18.2 × 1013 8.5 × 1013
103 124 132 85
T. Krishnaveni et al. / Journal of Alloys and Compounds 414 (2006) 282–286
285
Fig. 5. Permeability versus temperature plots of Ni–Cu–Zn ferrite samples.
Fig. 4. Variation of (a) real (µ ) and (b) imaginary (µ ) part of the complex permeability with frequency for Ni–Cu–Zn ferrites.
and each component can be written as follows: χSP (ω) = KSP /[1 + j(ω/ωSP )]
(3)
2 χDW (ω) = KDW /[1 + ωDW − ω2 + jβω]
(4)
here ω is the magnetic field frequency, KSP , the static spin susceptibility, ωSP , the spin resonance frequency, KDW , the static susceptibility of the domain wall motion, and β is the damping factor of the domain wall motion. The domain wall motion contribution starts to decrease at lower frequency and the complex permeability in the higher frequency region above 100 MHz can described using the spin rotational component. The parameters such as KSP , ωSP, KDW , ωDW , and β can
be determined by numerical fitting of complex permeability spectra. Fig. 4 also shows results of calculated curves using the Eqs. (3) and (4), which are plotted as dark line for sample No. 3. A good agreement was observed between calculated and experimental values for the real (µ ) and imaginary (µ ) parts of the permeability spectra of the sample No. 3. Fig. 5 gives the plots of permeability versus temperature for all the samples. It is evident from the figure that the µi remains constant over a wide temperature range for all the samples. The value of µi remains constant with temperature and shows a peak in the vicinity of Curie temperature and then drops sharply to a small value ∼2, indicating the high homogeneity of the present samples. The temperature at which µi drops to small value is taken as Curie temperature (TC ) of the corresponding sample. The TC thus obtained for the present samples is 660 ± 1 K. For the polycrystalline samples the shape of µi –T curves depends strongly on the preparation conditions. Out of all the samples under investigation, sample No. 3 possesses good electrical and magnetic properties. Therefore, these ferrites were used to fabricate MLCIs by green sheet lamination and screen-printing processes. The chips were sintered at 890, 900, and 910 ◦ C. The size of the fabricated MLCIs is 2 mm × 1 mm × 0.5 mm. The chips were then coated with Ag terminal electrodes. Table 2 gives the room temperature values of dc resistance (R), inductance (L), and real part of the permeability (µ ) for the presently prepared
Table 2 Room temperature data of the fabricated multilayer chip inductors Sample
Sintering temperature (◦ C/h)
µ (at 1 MHz)
R ()
L (H) (at 1 MHz)
Ni–Cu–Zn (P) Ni–Cu–Zn (l) Ni–Cu–Zn (P) Ni–Cu–Zn (l) Ni–Cu–Zn (P) Ni–Cu–Zn (l)
890/4 890/12 900/4 900/12 910/4 910/12
435 450 610 590 520 650
0.115 0.104 0.208 0.115 0.156 0.215
0.41 4.98 0.43 5.78 0.40 6.61
P: present samples; l: Ref. [12].
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T. Krishnaveni et al. / Journal of Alloys and Compounds 414 (2006) 282–286
small and decreases with an increase of frequency. From these studies it is expected that a good quality chip inductor can be obtained using the Ni–Cu–Zn ferrites with 12 mol% Cu.
Acknowledgements The authors are very much thankful to DRDO, New Delhi for its financial assistance and to S. Komarneni, Materials Research Institute, The Penn State University, USA for his great help to synthesize the samples.
Fig. 6. Frequency variation of L and Q for Ni–Cu–Zn ferrite MLCI sintered at 900 ◦ C/4 h.
inductors. For the sake of comparison the values of R, L, and µ for the Ni–Cu–Zn ferrite chip inductors prepared using conventionally sintered powders [12] were incorporated in the same table. It can be seen from the table that the initial permeability, resistance and inductance of the chip inductors increased with the increase of sintering temperature and the inductor sintered at 900 ◦ C/4 h exhibited higher values. Present MLCIs possess high value of resistance with low inductance than the MLCIs fabricated earlier. Fig. 6 gives the plot of frequency variation of inductance (L) and quality factor (Q) for the Ni–Cu–Zn ferrite chip inductor sintered at 900 ◦ C/4 h. It can be seen from the figure that the inductance remains constant in the frequency range of 1–20 MHz and increases at higher frequencies. The Q value is
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