Key Engineering Materials Vols. 510-511 (2012) pp 51-57 Online available since 2012/May/14 at www.scientific.net © (2012) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/KEM.510-511.51
Structural, Electrical and Dielectric Properties of Nanocrystalline Mg-Zn Ferrites M. Anis-ur-Rehman1,a, M.A. Malik1, S. Nasir1 , M. Mubeen1, K. Khan2 and Asghari Maqsood2 1
Applied Thermal Physics Laboratory, Department of Physics, COMSATS Institute of Information Technology, Islamabad, Pakistan 2
Thermal Transport Laboratory, SCME, National University of Sciences and Technology, Islamabad, Pakistan a
[email protected]
Keywords: Dielectric constant, crystallite size, porosity, ac conductivity, X-ray diffraction
Abstract: The nanocrystalline Mg-Zn ferrites having general formula Mg1-xZnxFe2O4 (x=0, 0.1, 0.2, 0.3, 0.4, 0. 5) were prepared by WOWS sol-gel route. All prepared samples were sintered at 700˚C for 2 h. X-ray powder diffraction (XRD) technique was used to investigate structural properties of the samples. The crystal structure was found to be spinel. The crystallite size, lattice parameters and porosity of samples were calculated by XRD data analysis as function of zinc concentration. The crystallite size for each sample was calculated using the Scherrer formula considering the most intense (3 1 1) peak and the range obtained was 34-68 nm. The dielectric constant (ε), dielectric loss tangent ( ) and AC electrical conductivity of nanocrystalline Mg-Zn ferrites are investigated as a function of frequency. The dielectric constant (ε), dielectric loss tangent ( ) increased with increase of Zn concentration. All the electrical properties are explained in accordance with Maxwell–Wagner model and Koops phenomenological theory. Introduction The spinel ferrites show interesting magnetic and electrical properties in the nano crystalline form compared with those of the micrometer-size grains. Nano-phase spinel ferrites have attracted much attention due to their technological importance in varieties of fields, such as microwave devices, high speed digital tape, disk recording, ferro-fluids, catalysis, and magnetic refrigeration systems [1]. The properties of a ferrite have direct dependence on its structure. Soft ferrites have closed packed structures of O-2 ions with metal ions on the interstitial positions (tetrahedral and octahedral sites). Moreover different elements have their own site preferences. The proper knowledge of structure and site occupancies of metal ions makes properties of ferrites controllable [2]. The magnetic and electrical properties of the spinel ferrites are optimized for specific applications by the choice of the cations and their distribution between tetrahedral (A) and octahedral (B) sites of the spinel lattice as well as preparation conditions, sintering temperature, sintering time and the method of preparation. Magnesium ferrites belong to the family of soft ferrites which finds a number of applications in heterogeneous catalysis, adsorptions, sensors, antennas and in magnetic technologies [3]. The magnesium ferrites are proficient materials for the miniaturization of the size of antennas along with enhanced properties. The size of the antenna can be reduced by using a material of higher refractive index which depends upon the relative permittivity and permeability of the material [4-7]. As the ferrites have both dielectrics as well as the magnetic properties, so these are proficient materials for the design of high frequency antenna. Kong et al. reported that the relative permittivity of magnesium ferrites is slightly greater than the value of relative permeability. The impedance mismatch of these materials with environment can be overcome by making suitable substitution in magnesium ferrites [8]. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 39.41.7.90, National University of Sciences and Technology (NUST), Islamabad, Pakistan-01/07/13,10:31:40)
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In this paper, we report the synthesis and characterization of Mg–Zn ferrite nanoparticles prepared by a simplified sol–gel method. The new sol–gel method is named as the WOWS sol–gel method. WOWS stands for sol–gel without water and surfactants, which is discussed in detail in the next section. Water and surfactants are avoided in this method for better purity. In other sol–gel methods, NaOH or HNO3 is used for pH adjustment, which may appear as an impurity in the final product. pH adjustment is not needed in this method, which further improves the purity of the prepared samples. Experimental Procedure Synthesis: In the WOWS sol-gel method, Zn(NO3)2·6H2O, Mg(NO3)2·6H2O and Fe(NO3)3·9H2O were dissolved in ethylene glycol. The molar ratio of ethylene glycol to metal salts dissolved was kept 14:1 so that the salts dissolved uniformly. The solution was stirred for 30 min at room temperature to get uniform solution. Then the temperature of the solution was increased above 100 ˚C with continuous stirring until thick gel was formed. Then the temperature was raised to 300 ˚C and the thick dried gel burned slowly into fine powder. The pellets of Mg1-xZnxFe2O4 (x=0, 0.1, 0.2, 0.3, 0.4, 0.5) powder were made and sintered at temperatures of 700 ± 5C for 2 h, and were named accordingly Z-0, Z-10, Z-20, Z-30, Z-40 and Z50. Results and Discussion Structural Properties: The crystallite size of a polycrystalline material was calculated using Scherrer formula [9,10]. The X-ray diffraction patterns of the samples shown in Figure 1 indicated the formation of single spinel phase. All the peaks of cubic crystal system corresponding to space group Fd-3m were indexed with the standard pattern for MgFe2O4 reported in ICDD PDF card # 00001-1120. The crystallite size for each sample was calculated using the Scherrer formula considering the most intense (3 1 1) peak. The crystallite size of the samples was in the range 34-68 nm. The lattice parameters increased with increase of zinc concentration. It is because of the fact that zinc has greater ionic radius than magnesium.
53
(440)
(422) (511)
(400)
(311)
(220)
Key Engineering Materials Vols. 510-511
Z-5
Intensity (a.u.)
Z-40 Z-30
Z-20
Z-10
Z-0 20
30
40
50
60
70
2 (degrees)
Fig. 1. XRD patterns of Mg1-xZnxFe2O4 samples sintered at 700˚C The calculated values of lattice parameter, mass density, X-ray density and porosity of Mg-Zn ferrites samples as a function of zinc concentration are given in Table 1. Dielectric Properties: The capacitance and dielectric loss tangent were recorded simultaneously using WAYNE KERR Precision Component Analyzer (WK 6440B) in the frequency range from 100 Hz to 3MHz at room temperature. The frequency dependence of the dielectric constant for all the samples is shown in Figure 2. The dielectric constant decreased initially with increase in frequency and reached a constant value at higher frequency [11]. After certain increase in frequency all the samples exhibit frequency independent behavior. The observed variation in έ can be explained on the basis of space-charge polarization. According to Maxwell–Wagner two-layer model [12, 13], the space-charge polarization is due to the inhomogeneous structure of dielectric material. It is formed by large well conducting grains separated by thin poorly conducting grain boundaries [14]. The electrons reach the grain boundary through hopping. If the resistance of the grain boundary is large, the electrons pile up at the grain boundary and produce polarization. When the frequency of the applied field is increased, the probability of the electrons to reach the grain boundary decreases. This decreases the polarization and hence the dielectric constant with the increase in frequency [15, 16].
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Dielectric Constant ()
400
Z-0 Z-10 Z-20 Z-30 Z-40 Z-50
300
200
100
0 4
6
8
10
12
14
16
ln f
Fig. 2. Dielectric constant as a function of frequency of Mg1-xZnxFe2O4 spinel ferrites The figure 2 indicated that the dielectric constant also depend upon the increase of Zn concentration in magnesium ferrite. The space charge polarization in ferrites is governed by the number of space charge carriers and the conductivity of the material and the hopping exchange of the charges between two localized states [17]. Iwauchi [18] pointed that there is strong correlation between dielectric behavior and the conduction mechanism in ferrites. Conduction mechanism in ferrites is the result of electron and hole hopping between ions of same element exiting in different valence state on equivalent lattice sites [19]. Fe3+ ions distribute in two distinct sites, but most Fe3+ ions exist in octahedral site (B-site). In addition, Fe2+ ions occupy Bsites only. The distance between two metal ions at B-site is smaller than the distance between a metal ion at B-site and another metal ion at A site (tetrahedral site) [20]. Therefore, the electron hopping between A and B-sites under normal conditions has a smaller probability compared with that of B–B hopping. Hopping between A and A sites does not exist for the simple reason that there are only Fe3+ ions at A-site and any Fe2+ ions formed during processing preferentially occupy Bsites only [21]. For the case Mg-Zn ferrites, the conduction is considered as the result of electron hopping between Fe2+ and Fe3+ in B-site [22]. Figure 2 indicated that the doping of Zn in Mg ferrite resulted in increase in dielectric constant with increase in concentration, which can be explain on the basis of cation distribution in prepared samples. The bulk magnesium ferrites have an inverse spinel with Mg2+ ion occupying all the octahedral sites [23], while the Zn ions have tetrahedral site preference. The increase of zinc concentration resulted in replacement of magnesium ions as well as migration of Fe ions from A site to B site. This resulted in increase in electron hopping between Fe2+ and Fe3+ ions, which resulted in the increase of conductivity as well as the dielectric constant of prepared Mg-Zn ferrites. Dielectric Loss Tangent: The dielectric loss tangent ( ) as a function of frequency is studied at room temperature. The graph of dielectric loss tangent with frequency is depicted in Figure 3. Again the dielectric loss tangent decreases with increase in frequency for each sample. All the samples exhibit dispersion due to Maxwell–Wagner interfacial type polarization in agreement with Koop’s phenomenological theory [12-14]. The values of tan δ depend on a number of factors such as stoichiometry, Fe2+ content and structural homogeneity, which in turn depend on the composition and sintering temperature of the samples [24]. For the present case, the decrease in conductivity of the samples with increase in zinc concentration resulted in decrease in dielectric losses.
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Z-0 Z-10 Z-20 Z-30 Z-40 Z-50
6
tan
4
2
0 4
6
8
10
12
14
16
ln f
Fig. 3. Dielectric loss tangent (tan δ) as a function of frequency of Mg1-xZnxFe2O4 spinel ferrites AC Electrical Conductivity: The AC conductivity in the range of 100 Hz to 3 MHz of these samples was calculated from the values of dielectric constant and dielectric loss factor using the relation [12]. (1) where the AC conductivity, ω is is the angular frequency, is the permittivity of free space, is the dielectric constant and is the dielectric loss factor of the samples. Frequency dependent σac is shown in Figure 4. It is noted that σac increases with the increase in frequency. The frequency dependent σac can be explained on the basis of Maxwell–Wagner two lattice model [12,13]. At lower frequency, the grain boundaries are more active, hence the hopping frequency of electrons between Fe3+ and Fe2+ ions is less. At higher frequencies, the conductive grains become more active by promoting the hopping of electrons between Fe3+ and Fe2+ ions therefore increasing the hopping frequency. So we observe the increase in conductivity with the increase in frequency. -9
Z-0 Z-10 Z-20 Z-30 Z-40 Z-50
-10
ln
ac
-11
-12
-13
-14
-15 4
6
8
10
12
14
16
lnf
Fig. 4. Graph of AC conductivity ( ) as a function of frequency in Mg1-xZnxFe2O4 spinel ferrites
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The variation of AC electrical conductivity, dielectric constant and dielectric loss factor as at 10 kHz and 1 MHz for all the samples is given in the Table 1. The decrease AC conductivity of the Mg-Zn ferrite samples with increase in zinc concentration can be explained on the basis of cation distribution as elaborated above. Table 1. Particle size (D(311)), lattice constant (a), Porosity (P), Dielectric constant (ε′), Dielectric loss tangent (tan δ) and AC electrical conductivity (σac) of Mg1-xZnxFe2O4 ferrites as a function of zinc concentration Parameter
Z-0
Z-10
Z-20
Z-30
Z-40
Z-50
D(311) (nm)
56
68
68
56
42
34
a (Å )
8.36
8.36
8.38
8.39
8.40
8.40
P (%)
73
67
69
68
71
72
ε at 10 kHz
19.85
29.79
32.65
34.01
51.18
43.52
ε at 1 MHz
9.78
12.02
12.28
12.65
15.75
13.59
tan δ at 10 kHz
0.60
0.65
0.53
1.13
0.64
0.70
tan δ at 1 MHz
0.19
0.25
σac (S/m) at 10 kHz
1.054×10
σac (S/m) at 1 MHz
1.64×10-5
-6
1.71×10
0.27 -6
2.65×10-5
1.53×10
0.27 -6
2.93×10-5
3.4×10
0.23 -6
3.02×10-5
2.89×10
0.19 -6
2.69×10-6
3.20×10-5
2.28×10-6
Conclusion The doping of zinc in co-precipitated MgFe2O4 caused appreciable changes in structural and electrical properties. The XRD results showed the formation of cubic spinel structure. The dependence dielectric properties of Mg-Zn ferrites on the frequency of alternating applied field is in accordance with Maxwell–Wagner interfacial type polarization in agreement with Koop’s phenomenological theory. The increase in the values dielectric constant, dielectric loss tangent and AC electrical conductivity of polycrystalline Mg-Zn ferrites with increase in zinc concentration is explained on the basis of cation distribution. The dielectric constant of the prepared samples lies in the range of 10-14 at 3 MHz, which makes these materials suitable for the size miniaturization of high frequency antenna. Acknowledgement The authors would like to acknowledge Higher Education Commission (HEC), Islamabad, Pakistan for providing financial support for this work through NRPU # 893. Mr. A. Abdullah is acknowledged for his help in preparation of the manuscript. References [1] [2] [3] [4] [5] [6] [7] [8]
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Advanced Materials XII 10.4028/www.scientific.net/KEM.510-511
Structural, Electrical and Dielectric Properties of Nanocrystalline Mg-Zn Ferrites 10.4028/www.scientific.net/KEM.510-511.51
