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Structural and electromagnetic properties of Ni–Zn ferrites prepared by sol–gel combustion method
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Materials Chemistry and Physics 109 (2008) 109–112
Structural and electromagnetic properties of Ni–Zn ferrites prepared by sol–gel combustion method J. Azadmanjiri ∗ Islamic Azad University-Damavand branch, P.O. Box: 39715-194, Damavand, Iran Received 23 June 2007; received in revised form 30 September 2007; accepted 3 November 2007
Abstract Ni–Zn ferrite nanoparticles with composition of Ni1−x Znx Fe2 O4 (where x = 0, 0.1, 0.2, 0.3, 0.4) were prepared by the chemical sol–gel combustion method at low temperature with a final pH value of 7. The synthesized nanopowder can be densified at a temperature lower than 950 ◦ C. X-ray diffraction results showed that the dried gels synthesized from metal nitrates and citric acid transformed directly into nano-sized ferrite particles after a combustion process in air. The Zn content has a significant influence on the electromagnetic properties such as dielectric constant (ε ), dielectric loss tangent (tan δ) and complex dielectric constant (ε ). These values decrease with increasing of zinc. It is also shown that Ni–Zn powder exhibited saturation magnetization 73 emu g−1 at 950 ◦ C. © 2007 Elsevier B.V. All rights reserved. Keywords: Sol–gel processes; Magnetic properties; Combustion method
1. Introduction Spinel type ferrites including NiZn or NiCo compositions are of great interest due to their potential applications in microelectronics, magneto-optics and as a microwave device components [1–3]. Optical fiber structures composed of magnetostriction material such as nickel, metallic glass or ferrite and optical fiber have been used for the development of magnetic field sensors [4,5]. The properties of ferrite materials are known to be strongly influenced by their composition and microstructure that in turn are sensitive to the processing methods used to synthesize them. In an attempt to prepare high performance ferrites with reproducible stoichiometric compositions and desired microstructure, the present work aimed at preparing Ni–Zn ferrites using the sol–gel combustion method. The sol–gel combustion techniques for ferrite synthesis have been proved to be more convenient, since the ferrite powders with nano-sized particles can be formed directly from combustion of dried in air [6,7]. In the present study, we use the sol–gel combustion method to synthesize the reactive nano-sized powder in order to lower the sintering temperature of Ni–Zn ferrite. In this paper, we report on the ∗
Tel.: +98 2122302713; fax: +98 2122302713. E-mail address: Jalal [email protected].
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.11.001
preparation and electromagnetic properties of Ni1−x Znx Fe2 O4 ferrites (where x = 0, 0.1, 0.2, 0.3, 0.4). 2. Experimental procedure Ni–Zn ferrite powders with compositions of Ni1−x Znx Fe2 O4 (where x = 0, 0.1, 0.2, 0.3, 0.4) were synthesized by sol–gel combustion method. The detailed process can be described as follows (Fig. 1). The analytical grade Fe(NO3 )3 ·9H2 O, Zn(NO3 )2 ·6H2 O, Ni(NO3 )2 ·6H2 O and citric acid (C6 H8 O7 ·H2 O) were used as raw materials. The appropriate amount of nitrates and citric acid was first dissolved into deionized water to form a mixed solution. The molar ratio of nitrates to citric acid is 1:1. The pH value of solution was adjusted to about 7 using ammonia. Then, the mixed solution was poured into a dish and heated at 135 ◦ C under constant stirring to transform into a dried gel. Being ignited, the dried gel burnt in a self-propagating combustion way to form loose powder. The scanning electron microscopy (SEM) photograph of the as-burnt powder is also shown in Fig. 2. Then the obtained ferrite powder was mixed with an appropriate amount of 5 wt% polyvinyl alcohols as a binder for granulation. Then the granulated powders were uniaxially pressed at a pressure of 1300 kg cm−2 to form green toroidal and pellet specimens. After binder burnt out at 600 ◦ C for 1 h, the specimens were sintered at 800–950 ◦ C for 5 h in air. In order to investigate the effect of zinc on the electromagnetic properties of NiFe2 O4 ferrites, the ferrite powders with compositions of Ni1−x Znx Fe2 O4 (where x = 0, 0.1, 0.2, 0.3, 0.4) were synthesized via the above-mentioned process. The phase identification for as-burnt powders and sintered specimen was characterized by XRD and EDX with Cu K␣ radiation. The microstructure was observed by scanning electron microscopy. Magnetic and electromagnetic pro-
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J. Azadmanjiri / Materials Chemistry and Physics 109 (2008) 109–112
Fig. 1. Schematic of the preparation Ni–Zn powder. perties of sintered specimen were also measured with VSM and impedance analyzer, respectively.
3. Results and discussion
of the as-burnt powder was calculated from X-ray peak broadening of the (3 1 1) peak using Scherrer’s formula. The results showed that the crystal sizes are in the range 73–80 nm for all compositions. No noticeable influence of zinc content on the particle size was observed.
As observed in experiments, after being ignited in air, the dried gel can burn in a self-propagation way to form a loose powder. XRD diffraction was performed on dried gels and asburnt ashes. The results showed that the dried gel is amorphous in nature (Fig. 3a). After combustion, the as-burnt powders are in crystalline state, as shown in Fig. 3b and e. The as-burnt powder is a single phase Ni–Zn ferrite with a spinel structure, similar to that of the as-sintered ceramic sample. This indicates that the Ni–Zn ferrite can be directly formed after the auto-combustion of the gel, without calcinations. The broad peaks in XRD patterns indicate fine particle nature of the particles. The crystal size
Fig. 2. SEM photograph of the as-burnt powder.
Fig. 3. XRD patterns for (a) dried gel and as-burnt (Ni1−x Znx Fe2 O4 ) powders (b) x = 0.1, (c) x = 0.2, (d) x = 0.3, (e) x = 0.4.
J. Azadmanjiri / Materials Chemistry and Physics 109 (2008) 109–112
111
Table 2 Composition dependence of dielectric data for mixed Ni–Zn ferrites at room temperature Sample
NiFe2 O4 Ni0.9 Zn0.1 Fe2 O4 Ni0.8 Zn0.2 Fe2 O4 Ni0.7 Zn0.3 Fe2 O4 Ni0.6 Zn0.4 Fe2 O4
Fig. 4. SEM photograph of Ni1−x Znx Fe2 O4
(x = 0.4) sintered sample at 950 ◦ C.
The SEM photograph for ferrite sintered at 950 ◦ C is shown in Fig. 4. It is to be noted that, in general, the grain size and the sizespread for samples prepared by the present method are much smaller than those for samples prepared by the conventional method [8,9]. Chemical analyses through EDX were also carried out on sol–gel combustion ferrites. The exact compositions that were verified are shown in Table 1. As can be seen, the elemental concentrations (mean of four values taken at the center and at the border) varied insignificantly, and no zinc volatilization was verified at this critical condition. The room temperature values of dielectric constant (ε ), the dielectric loss tangent (tan δ) and the complex dielectric constant (ε ) for mixed Ni–Zn ferrites at 100 kHz are given in Table 2. The values of electrical conductivity (σ) are measured at room temperature by the two-probe method. An examination of Table 2 reveals that the values of ε , tan δ and ε are found to decrease with increasing of zinc content. Before our work [10], we had studied the composition, frequency and temperature dependence of copper containing mixed ferrites of Ni1−x Cux Fe2 O4 . El Titi et al. [11] have investigated the composition, frequency and temperature dependence of CuCr ferrites. The dielectric properties Table 1 Composition of the sintered pellet ferrite for different samples (a) and for different areas (b) Sample
(a) Samples NiFe2 O4 Ni0.9 Zn0.1 Fe2 O4 Ni0.8 Zn0.2 Fe2 O4 Ni0.7 Zn0.3 Fe2 O4 Ni0.6 Zn0.4 Fe2 O4 Sample (Ni0.8 Zn0.2 Fe2 O4 )
(b) Areas A1 A2 A3 A4
σ (−1 cm−1 )
100 kHz ε
tan δ
ε
31.78 28.21 28.19 22.28 22.21
0.46 0.45 0.43 0.42 0.40
14.92 12.97 12.95 10.02 9.98
2.05 × 10−4 4.89 × 10−5 4.87 × 10−5 2.12 × 10−5 2.09 × 10−5
of CuCd ferrites were studied by Kolekar et al. [12]. Iwauchi [13] reported a strong correlation between the conduction mechanism and the dielectric behaviors of the ferrites starting with the supposition that the mechanism of polarization process in ferrite is similar to that the conduction process [14]. They observed that the electron exchange between Fe2+ /Fe3+ result in local displacements determining the polarization of the ferrites. A similar model is proposed for the composition dependence of the dielectric constants of mixed Ni–Zn ferrites. In this model the electron exchange between Fe2+ and Fe3+ in an n-type and the hole exchange between Ni3+ and Ni2+ in p-type ferrites results in local displacements of electrons or holes in the direction of the electric field that then cause polarization [10,14]. Table 2 reveals that the variation of dielectric constant runs parallel to the variation of available ferrous ions on octahedral sites. It is also pertinent to mention that the variation of electrical conductivity runs parallel to the variation of ferrous ion concentration. Thus, it is the number of ferrous ions on octahedral sites that plays a dominant role in the processes of conduction as well as dielectric polarization. The variation of the dielectric constant as a function of frequency for all the ferrite samples at room temperature is shown in Fig. 5. It can be seen from the figure that the dielectric constant decreases with increasing frequency. The decrease of dielectric constant with increase of frequency as observed in the case of mixed Ni–Zn ferrites is a normal dielectric behavior of spinel ferrites. The normal dielectric behavior was also observed by several investigators in the case of LiTi, NiCuZn, MgTiZn and CoZn ferrites. It can be seen from the figure that the dispersion
Elemental composition (wt%) Nickel
Zinc
Iron
25.10 22.53 19.98 17.44 14.91
0.00 2.75 5.50 8.23 10.95
47.65 47.53 47.41 47.29 47.17
Elemental composition (wt%) Nickel
Zinc
Iron
19.98 19.88 19.80 19.83
5.50 5.48 5.45 5.46
47.41 47.39 47.38 47.40
Fig. 5. Dielectric constant as a function of frequency for Ni1−x Znx Fe2 O4 ferrite sintered at 950 ◦ C.
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J. Azadmanjiri / Materials Chemistry and Physics 109 (2008) 109–112
4. Conclusions The sol–gel combustion method is convenient for synthesis of nano-sized Ni–Zn ferrites. A series of NiZn ferrite nanosized powders with composition Ni1−x Znx Fe2 O4 (where x = 0, 0.1, 0.2, 0.3, 0.4) were synthesized by this novel method. In this method gel exhibits a self-propagating behavior after ignition in air. The synthesized powders exhibited high sintering activity, and can be sintered at temperature less than 950 ◦ C. The prepared low-temperature sintered Ni–Zn ferrites possess good electromagnetic properties, as well as fine-grained microstructures, making them good materials for electronic applications with high performance and low cost. Zn content has significant influence on the electromagnetic properties, such as dielectric constant, dielectric loss tangent and magnetic properties for Ni–Zn ferrites. Fig. 6. The VSM experimental results of Ni1−x Znx Fe2 O4 (x = 0.4) sintered at (a) 800 and (b) 950 ◦ C.
in ε is analogous to Maxwell–Wagner interfacial polarization [15,16], in agreement with Koops phenomenological theory [17]. The dispersion of the dielectric constant is maximum for sample with x = 0.1. This maximum dielectric dispersion may be explained on the basis of available ferrous ions on octahedral sites. In the case of x = 0.1 the concentration of ferrous ions is higher than in other compositions of mixed Ni–Zn ferrites. As a consequence, it is possible for these ions to be polarized to the maximum possible extent. Further, as the frequency of the externally applied field increases gradually, though the number of ferrous ions is present in the ferrite material, the dielectric constant decreases from 54.59 at 100 kHz to 34.21 at 1 MHz. The reduction occurs because beyond a certain frequency of the externally applied electric field, the electronic exchange between ferrous and ferric ions, i.e., Fe2+ /Fe3+ cannot follow the alternating field. The variation of the dispersion of ε with composition for other mixed nickel–zinc ferrites explained by the fact that the electron exchange between Fe2+ and Fe3+ in an n-type semiconducting ferrite and hole exchange between Ni3+ and Ni2+ in a p-type semiconducting ferrite cannot follow the frequency of the applied alternating field beyond a critical value of the frequency. The VSM experimental results of Ni–Zn ferrite sintered at 800 and 950 ◦ C are shown in Fig. 6. It can be seen that the saturation magnetization ferrite sintered at 800 ◦ C is 62 emu g−1 and increased to 73 emu g−1 at 950 ◦ C.
References [1] R. Valenzuela, Magnetic Ceramics, Cambridge University Press, Cambridge, 1994. [2] F. Bucholtz, in: E. Udd (Ed.), Fiber Optic Sensors: An Introduction for Engineers and Scientists, John Wiley, New York, 1991. [3] A.M. Abdeen, Electric conduction in Ni–Zn ferrites, J. Mang. Magn. Mater. 185 (1998) 199–206. [4] M. Sedlar, L. Pust, Preparation of cobalt doped nickel ferrite thin films on optical fibers by dip-coating technique, Ceram. Int. 21 (1995) 21–27. [5] Y.W. Bibby, D.C. Larson, Fiber optic magnetic field sensors using metallic glass coated optical fibers, in: Proceedings of the Eighth International Conference on Optical Fiber Sensors, 1992. [6] Z. Yue, L. Li, J. Zhou, H. Zhang, Z. Gui, Mater. Sci. Eng. B64 (1999) 68. [7] Z. Yue, J. Zhou, L. Li, H. Zhang, Z. Gui, J. Magn. Magn. Mater. 208 (2000) 55. [8] K. Majima, M. Hasigawa, S. Katsuyama, H. Nagai, S. Mishima, in: T. Yamaguchi, M. Abe (Eds.), Proceedings of the Sixth International Conference on Ferrites, Tokyo and Kyoto, The Japan Society of Powder Metallurgy, Japan, 1992, p. 374. [9] S. Pyun, J.T. Baek, Am. Ceram. Soc. Bull 64 (4) (1985) 602. [10] J. Azadmanjiri, H.K. Salehani, M.R. Barati, F. Farzan, J. Mater. Lett. 61 (2007) 84–87. [11] M.A. El titi, M.A. Ahmed, M.M. Mosad, S.M. Attia, J. Magn. Magn. Mater. 150 (1995) 399. [12] C.B. Kolekar, P.N. Kamble, S.G. Kulkarni, A.S. Vaingankar, J. Mater. Sci. 30 (1995) 5784. [13] K. Iwauchi, Jpn. J. Appl. Phys. 10 (1971) 1520. [14] L.I. Rabinkin, Z.I. Novika, Ferrites Minsk (1960) 146. [15] J.C. Maxwell, Electricity and Magnetism, vol. 1, Oxford Univ. Press, Oxford, Section 328 (1929) p. 752. [16] K.W. Wagner, Ann. Phys. (Leipzig) 40 (1913) 817. [17] C.G. Koops, Phys. Rev. 83 (1951) 121.
Materials Chemistry and Physics 109 (2008) 109–112
Structural and electromagnetic properties of Ni–Zn ferrites prepared by sol–gel combustion method J. Azadmanjiri ∗ Islamic Azad University-Damavand branch, P.O. Box: 39715-194, Damavand, Iran Received 23 June 2007; received in revised form 30 September 2007; accepted 3 November 2007
Abstract Ni–Zn ferrite nanoparticles with composition of Ni1−x Znx Fe2 O4 (where x = 0, 0.1, 0.2, 0.3, 0.4) were prepared by the chemical sol–gel combustion method at low temperature with a final pH value of 7. The synthesized nanopowder can be densified at a temperature lower than 950 ◦ C. X-ray diffraction results showed that the dried gels synthesized from metal nitrates and citric acid transformed directly into nano-sized ferrite particles after a combustion process in air. The Zn content has a significant influence on the electromagnetic properties such as dielectric constant (ε ), dielectric loss tangent (tan δ) and complex dielectric constant (ε ). These values decrease with increasing of zinc. It is also shown that Ni–Zn powder exhibited saturation magnetization 73 emu g−1 at 950 ◦ C. © 2007 Elsevier B.V. All rights reserved. Keywords: Sol–gel processes; Magnetic properties; Combustion method
1. Introduction Spinel type ferrites including NiZn or NiCo compositions are of great interest due to their potential applications in microelectronics, magneto-optics and as a microwave device components [1–3]. Optical fiber structures composed of magnetostriction material such as nickel, metallic glass or ferrite and optical fiber have been used for the development of magnetic field sensors [4,5]. The properties of ferrite materials are known to be strongly influenced by their composition and microstructure that in turn are sensitive to the processing methods used to synthesize them. In an attempt to prepare high performance ferrites with reproducible stoichiometric compositions and desired microstructure, the present work aimed at preparing Ni–Zn ferrites using the sol–gel combustion method. The sol–gel combustion techniques for ferrite synthesis have been proved to be more convenient, since the ferrite powders with nano-sized particles can be formed directly from combustion of dried in air [6,7]. In the present study, we use the sol–gel combustion method to synthesize the reactive nano-sized powder in order to lower the sintering temperature of Ni–Zn ferrite. In this paper, we report on the ∗
Tel.: +98 2122302713; fax: +98 2122302713. E-mail address: Jalal [email protected].
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.11.001
preparation and electromagnetic properties of Ni1−x Znx Fe2 O4 ferrites (where x = 0, 0.1, 0.2, 0.3, 0.4). 2. Experimental procedure Ni–Zn ferrite powders with compositions of Ni1−x Znx Fe2 O4 (where x = 0, 0.1, 0.2, 0.3, 0.4) were synthesized by sol–gel combustion method. The detailed process can be described as follows (Fig. 1). The analytical grade Fe(NO3 )3 ·9H2 O, Zn(NO3 )2 ·6H2 O, Ni(NO3 )2 ·6H2 O and citric acid (C6 H8 O7 ·H2 O) were used as raw materials. The appropriate amount of nitrates and citric acid was first dissolved into deionized water to form a mixed solution. The molar ratio of nitrates to citric acid is 1:1. The pH value of solution was adjusted to about 7 using ammonia. Then, the mixed solution was poured into a dish and heated at 135 ◦ C under constant stirring to transform into a dried gel. Being ignited, the dried gel burnt in a self-propagating combustion way to form loose powder. The scanning electron microscopy (SEM) photograph of the as-burnt powder is also shown in Fig. 2. Then the obtained ferrite powder was mixed with an appropriate amount of 5 wt% polyvinyl alcohols as a binder for granulation. Then the granulated powders were uniaxially pressed at a pressure of 1300 kg cm−2 to form green toroidal and pellet specimens. After binder burnt out at 600 ◦ C for 1 h, the specimens were sintered at 800–950 ◦ C for 5 h in air. In order to investigate the effect of zinc on the electromagnetic properties of NiFe2 O4 ferrites, the ferrite powders with compositions of Ni1−x Znx Fe2 O4 (where x = 0, 0.1, 0.2, 0.3, 0.4) were synthesized via the above-mentioned process. The phase identification for as-burnt powders and sintered specimen was characterized by XRD and EDX with Cu K␣ radiation. The microstructure was observed by scanning electron microscopy. Magnetic and electromagnetic pro-
110
J. Azadmanjiri / Materials Chemistry and Physics 109 (2008) 109–112
Fig. 1. Schematic of the preparation Ni–Zn powder. perties of sintered specimen were also measured with VSM and impedance analyzer, respectively.
3. Results and discussion
of the as-burnt powder was calculated from X-ray peak broadening of the (3 1 1) peak using Scherrer’s formula. The results showed that the crystal sizes are in the range 73–80 nm for all compositions. No noticeable influence of zinc content on the particle size was observed.
As observed in experiments, after being ignited in air, the dried gel can burn in a self-propagation way to form a loose powder. XRD diffraction was performed on dried gels and asburnt ashes. The results showed that the dried gel is amorphous in nature (Fig. 3a). After combustion, the as-burnt powders are in crystalline state, as shown in Fig. 3b and e. The as-burnt powder is a single phase Ni–Zn ferrite with a spinel structure, similar to that of the as-sintered ceramic sample. This indicates that the Ni–Zn ferrite can be directly formed after the auto-combustion of the gel, without calcinations. The broad peaks in XRD patterns indicate fine particle nature of the particles. The crystal size
Fig. 2. SEM photograph of the as-burnt powder.
Fig. 3. XRD patterns for (a) dried gel and as-burnt (Ni1−x Znx Fe2 O4 ) powders (b) x = 0.1, (c) x = 0.2, (d) x = 0.3, (e) x = 0.4.
J. Azadmanjiri / Materials Chemistry and Physics 109 (2008) 109–112
111
Table 2 Composition dependence of dielectric data for mixed Ni–Zn ferrites at room temperature Sample
NiFe2 O4 Ni0.9 Zn0.1 Fe2 O4 Ni0.8 Zn0.2 Fe2 O4 Ni0.7 Zn0.3 Fe2 O4 Ni0.6 Zn0.4 Fe2 O4
Fig. 4. SEM photograph of Ni1−x Znx Fe2 O4
(x = 0.4) sintered sample at 950 ◦ C.
The SEM photograph for ferrite sintered at 950 ◦ C is shown in Fig. 4. It is to be noted that, in general, the grain size and the sizespread for samples prepared by the present method are much smaller than those for samples prepared by the conventional method [8,9]. Chemical analyses through EDX were also carried out on sol–gel combustion ferrites. The exact compositions that were verified are shown in Table 1. As can be seen, the elemental concentrations (mean of four values taken at the center and at the border) varied insignificantly, and no zinc volatilization was verified at this critical condition. The room temperature values of dielectric constant (ε ), the dielectric loss tangent (tan δ) and the complex dielectric constant (ε ) for mixed Ni–Zn ferrites at 100 kHz are given in Table 2. The values of electrical conductivity (σ) are measured at room temperature by the two-probe method. An examination of Table 2 reveals that the values of ε , tan δ and ε are found to decrease with increasing of zinc content. Before our work [10], we had studied the composition, frequency and temperature dependence of copper containing mixed ferrites of Ni1−x Cux Fe2 O4 . El Titi et al. [11] have investigated the composition, frequency and temperature dependence of CuCr ferrites. The dielectric properties Table 1 Composition of the sintered pellet ferrite for different samples (a) and for different areas (b) Sample
(a) Samples NiFe2 O4 Ni0.9 Zn0.1 Fe2 O4 Ni0.8 Zn0.2 Fe2 O4 Ni0.7 Zn0.3 Fe2 O4 Ni0.6 Zn0.4 Fe2 O4 Sample (Ni0.8 Zn0.2 Fe2 O4 )
(b) Areas A1 A2 A3 A4
σ (−1 cm−1 )
100 kHz ε
tan δ
ε
31.78 28.21 28.19 22.28 22.21
0.46 0.45 0.43 0.42 0.40
14.92 12.97 12.95 10.02 9.98
2.05 × 10−4 4.89 × 10−5 4.87 × 10−5 2.12 × 10−5 2.09 × 10−5
of CuCd ferrites were studied by Kolekar et al. [12]. Iwauchi [13] reported a strong correlation between the conduction mechanism and the dielectric behaviors of the ferrites starting with the supposition that the mechanism of polarization process in ferrite is similar to that the conduction process [14]. They observed that the electron exchange between Fe2+ /Fe3+ result in local displacements determining the polarization of the ferrites. A similar model is proposed for the composition dependence of the dielectric constants of mixed Ni–Zn ferrites. In this model the electron exchange between Fe2+ and Fe3+ in an n-type and the hole exchange between Ni3+ and Ni2+ in p-type ferrites results in local displacements of electrons or holes in the direction of the electric field that then cause polarization [10,14]. Table 2 reveals that the variation of dielectric constant runs parallel to the variation of available ferrous ions on octahedral sites. It is also pertinent to mention that the variation of electrical conductivity runs parallel to the variation of ferrous ion concentration. Thus, it is the number of ferrous ions on octahedral sites that plays a dominant role in the processes of conduction as well as dielectric polarization. The variation of the dielectric constant as a function of frequency for all the ferrite samples at room temperature is shown in Fig. 5. It can be seen from the figure that the dielectric constant decreases with increasing frequency. The decrease of dielectric constant with increase of frequency as observed in the case of mixed Ni–Zn ferrites is a normal dielectric behavior of spinel ferrites. The normal dielectric behavior was also observed by several investigators in the case of LiTi, NiCuZn, MgTiZn and CoZn ferrites. It can be seen from the figure that the dispersion
Elemental composition (wt%) Nickel
Zinc
Iron
25.10 22.53 19.98 17.44 14.91
0.00 2.75 5.50 8.23 10.95
47.65 47.53 47.41 47.29 47.17
Elemental composition (wt%) Nickel
Zinc
Iron
19.98 19.88 19.80 19.83
5.50 5.48 5.45 5.46
47.41 47.39 47.38 47.40
Fig. 5. Dielectric constant as a function of frequency for Ni1−x Znx Fe2 O4 ferrite sintered at 950 ◦ C.
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J. Azadmanjiri / Materials Chemistry and Physics 109 (2008) 109–112
4. Conclusions The sol–gel combustion method is convenient for synthesis of nano-sized Ni–Zn ferrites. A series of NiZn ferrite nanosized powders with composition Ni1−x Znx Fe2 O4 (where x = 0, 0.1, 0.2, 0.3, 0.4) were synthesized by this novel method. In this method gel exhibits a self-propagating behavior after ignition in air. The synthesized powders exhibited high sintering activity, and can be sintered at temperature less than 950 ◦ C. The prepared low-temperature sintered Ni–Zn ferrites possess good electromagnetic properties, as well as fine-grained microstructures, making them good materials for electronic applications with high performance and low cost. Zn content has significant influence on the electromagnetic properties, such as dielectric constant, dielectric loss tangent and magnetic properties for Ni–Zn ferrites. Fig. 6. The VSM experimental results of Ni1−x Znx Fe2 O4 (x = 0.4) sintered at (a) 800 and (b) 950 ◦ C.
in ε is analogous to Maxwell–Wagner interfacial polarization [15,16], in agreement with Koops phenomenological theory [17]. The dispersion of the dielectric constant is maximum for sample with x = 0.1. This maximum dielectric dispersion may be explained on the basis of available ferrous ions on octahedral sites. In the case of x = 0.1 the concentration of ferrous ions is higher than in other compositions of mixed Ni–Zn ferrites. As a consequence, it is possible for these ions to be polarized to the maximum possible extent. Further, as the frequency of the externally applied field increases gradually, though the number of ferrous ions is present in the ferrite material, the dielectric constant decreases from 54.59 at 100 kHz to 34.21 at 1 MHz. The reduction occurs because beyond a certain frequency of the externally applied electric field, the electronic exchange between ferrous and ferric ions, i.e., Fe2+ /Fe3+ cannot follow the alternating field. The variation of the dispersion of ε with composition for other mixed nickel–zinc ferrites explained by the fact that the electron exchange between Fe2+ and Fe3+ in an n-type semiconducting ferrite and hole exchange between Ni3+ and Ni2+ in a p-type semiconducting ferrite cannot follow the frequency of the applied alternating field beyond a critical value of the frequency. The VSM experimental results of Ni–Zn ferrite sintered at 800 and 950 ◦ C are shown in Fig. 6. It can be seen that the saturation magnetization ferrite sintered at 800 ◦ C is 62 emu g−1 and increased to 73 emu g−1 at 950 ◦ C.
References [1] R. Valenzuela, Magnetic Ceramics, Cambridge University Press, Cambridge, 1994. [2] F. Bucholtz, in: E. Udd (Ed.), Fiber Optic Sensors: An Introduction for Engineers and Scientists, John Wiley, New York, 1991. [3] A.M. Abdeen, Electric conduction in Ni–Zn ferrites, J. Mang. Magn. Mater. 185 (1998) 199–206. [4] M. Sedlar, L. Pust, Preparation of cobalt doped nickel ferrite thin films on optical fibers by dip-coating technique, Ceram. Int. 21 (1995) 21–27. [5] Y.W. Bibby, D.C. Larson, Fiber optic magnetic field sensors using metallic glass coated optical fibers, in: Proceedings of the Eighth International Conference on Optical Fiber Sensors, 1992. [6] Z. Yue, L. Li, J. Zhou, H. Zhang, Z. Gui, Mater. Sci. Eng. B64 (1999) 68. [7] Z. Yue, J. Zhou, L. Li, H. Zhang, Z. Gui, J. Magn. Magn. Mater. 208 (2000) 55. [8] K. Majima, M. Hasigawa, S. Katsuyama, H. Nagai, S. Mishima, in: T. Yamaguchi, M. Abe (Eds.), Proceedings of the Sixth International Conference on Ferrites, Tokyo and Kyoto, The Japan Society of Powder Metallurgy, Japan, 1992, p. 374. [9] S. Pyun, J.T. Baek, Am. Ceram. Soc. Bull 64 (4) (1985) 602. [10] J. Azadmanjiri, H.K. Salehani, M.R. Barati, F. Farzan, J. Mater. Lett. 61 (2007) 84–87. [11] M.A. El titi, M.A. Ahmed, M.M. Mosad, S.M. Attia, J. Magn. Magn. Mater. 150 (1995) 399. [12] C.B. Kolekar, P.N. Kamble, S.G. Kulkarni, A.S. Vaingankar, J. Mater. Sci. 30 (1995) 5784. [13] K. Iwauchi, Jpn. J. Appl. Phys. 10 (1971) 1520. [14] L.I. Rabinkin, Z.I. Novika, Ferrites Minsk (1960) 146. [15] J.C. Maxwell, Electricity and Magnetism, vol. 1, Oxford Univ. Press, Oxford, Section 328 (1929) p. 752. [16] K.W. Wagner, Ann. Phys. (Leipzig) 40 (1913) 817. [17] C.G. Koops, Phys. Rev. 83 (1951) 121.