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Materials Chemistry and Physics 109 (2008) 482–487
Electrical transport properties of CoZn ferrite–SiO2 composites prepared by co-precipitation technique M.U. Islam a,∗ , Faiza Aen a , Shahida B. Niazi b , M. Azhar Khan a , M. Ishaque a , T. Abbas a , M.U. Rana a a b
Department of Physics, Bahauddin Zakariya University, Multan, Pakistan Department of Chemistry, Bahauddin Zakariya University, Multan, Pakistan Received 21 October 2007; accepted 17 December 2007
Abstract CoZn ferrite–SiO2 composites having general formula (1 − x)Co0.5 Zn0.5 Fe2 O4 + xSiO2 with x = 0.0–0.8 were prepared by co-precipitation technique. The X-ray diffraction analysis of the composites reveals that they are bi-phase. Room temperature resistivity increases from 105 to 109 ( cm) from x = 0.0–0.8. This drastic increase in resistivity may be attributed to the presence of pores and the segregation of Si at grain boundaries. The Arrhenius plots of these samples show that resistivity decreases as the temperature increases indicating their semi conducting behavior. Arrhenius plots show a change of slope at particular temperature (except for x = 0.8) that may be attributed to their Curie temperature. It is observed that the activation energies are small in Para-region as compared to Ferri-region and is an indication of the hopping conduction mechanism. The variation of thermopower with temperature reveals that these samples are degenerate type semiconductors. The values of activation energies calculated from log μd vs. 1000/T are slightly lower than the values of activation energies obtained from Arrhenius plots. This suggests that the conduction phenomenon is due to polaron hopping. © 2007 Elsevier B.V. All rights reserved. PACS: 75.50; 81.05; 81.16; 72.15.L; 07.85F; 68.37.R; 68.37.R; 65.80 Keywords: Ferrites; Fine particles; Co-precipitation; Electical resistivity; Thermopower
1. Introduction Advancement in the miniaturization of technology and to reduce the cost of a device is the demand of new era. Composite ferrites may fulfill this end [1–2]. Cobalt ferrite is a soft magnetic ceramic that has the potential to meet these requirements. It has been observed that CoZn ferrites exhibit important properties such as excellent chemical stability, high corrosion resistivity, magneto crystalline anisotropy, magnetostriction and magneto optical properties. These properties make this material useful in many applications like recording media with optical wave guides, magnetic static wave devices, and surface acoustic wave transducers [3]. It has been reported that complex oxides consist of transition metals such as manganese, iron, cobalt and nickel are extensively used as negative temperature coefficient thermistors for temperature measurement and control [4]. Due
to the hybridized properties of composite ferrites, these materials have attracted the attention of researchers in the recent years. NiZn ferrite–SiO2 composites have been reported. Kinetics and magnetic properties of NiZn ferrite–SiO2 composites have been studied [5]. It was observed that initial permeability increased with the increase of ferrite content and this was attributed to the increase of grain size. Electrical and magnetic properties of co-precipitation prepared ferrite composites, such as NiZn ferrite–polymer and NiZn ferrite–SiO2 (or B2 O3 ) have been reported [5]. The high electrical resistivity, low dielectric and magnetic losses are reported to be useful in absorption of microwaves applications. In the present study the samples have been prepared by co-precipitation method and electrical properties have been studied systematically. 2. Experimental procedure 2.1. Preparation of ferrite composites
∗
Corresponding author. Tel.: +92 61 9210091; fax: +92 61 9210068. E-mail address:
[email protected] (M.U. Islam).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.12.021
The composite ferrites with chemical formula (1 − x)Co0.5 Zn0.5 Fe2 O4 + xSiO2 with x = 0.0–0.8 were prepared by co-precipitation technique.
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The starting materials were metal chlorides 99.99% pure purchased from Merck. The CoCl2 ·6H2 O, ZnCl2 ·2H2 O, FeCl3 ·6H2 O and SiO2 were taken in appropriate proportions using the following equation: (1 − x) [CoCl2 ·6H2 O + ZnCl2 ·2H2 O + FeCl3 ·6H2 O] + xSiO2 → (1 − x)Co0.5 Zn0.5 Fe2 O4 + xSiO2
(1)
where x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5,0.6, 0.8 Metal chlorides with stoichiometric amounts were dissolved one by one in 150–200 ml of distilled water with constant stirring (using magnetic stirrer) until a clear solution is obtained. Proportionate amount of NaOH was used as reagent to precipitate the metals as hydroxides. Calculated amount of NaOH (9.0 gm) was dissolved in 100 ml distilled water and added slowly drop-wise into metal solution with constant stirring until precipitation is complete. The precipitates were allowed to settle down and the supernatant liquid was tested with NaOH solution for any more precipitation. Filtration of precipitates was carried out with the help of filtration flask operating on a water suction pump. The precipitates were thoroughly washed with distilled water until free from Cl− ions, which were checked with the help of AgNO3 solution. The precipitated product contained hydroxides of Co2+ , Zn2+ , and Fe3+ . It was dried in an electric oven at a temperature of 100 ◦ C for 36 h until constant weight. The dried powder was mixed homogenously in an agate pestle mortar for 2 h. Before and after this process, the pestle mortar was rinsed with acetone. Finally the ground powder was palletized by using hydraulic pressing machine (Paul-Otto Weber) under the pressure of 30 kN mm−2 on each pellet. The pellets were first pre-sintered at 800 ◦ C for 2 h, then the temperature was raised in steps of 100 ◦ C up to 1000 ◦ C for 24 h followed by air quenching.
Fig. 1. XRD pattern for (1 − x)Co0.5 Zn0.5 Fe2 O4 + xSiO2 (x = 0.0) composite ferrite.
2.2. Structure determination For structure determination X-ray diffractometer equipped with copper K␣ radiation was used for diffraction. The 2θ scanning range was 20◦ –70◦ . The density was measured by Archimedes principle in toluene using the following formula [6]: D=
wt of sample in air loss in wt
× density of toluene.
(2)
where the density of toluene was 0.857 g cm−3 X-ray densities were calculated by using the relation [6]: Dx =
8M Na3
(3)
Where M is the molecular weight, N is the Avogadro’s number and ‘a’ is the lattice constant.
Fig. 2. XRD pattern for (1 − x)Co0.5 Zn0.5 Fe2 O4 + xSiO2 (x = 0.1) composite ferrite.
2.3. DC resistivity and thermopower DC resistivity was measured by two probe method [7]. A DC power supply model IP-2717 Heathkit and a very sensitive electrometer model 610C (Keithely) was used in this study. Before the measurements both sides of each sample were polished to remove oxide layer formed during sintering and scratches from their surfaces. Thermopower was measured by using differential method. The Seebeck coefficient (α) was measured using the relation [8]. LtΔt→0 =
ΔV Δt
(4)
where V(mV) is thermo e.m.f developed across the pellet due to the temperature difference t.
3. Results and discussions Figs. 1–3 show the diffraction patterns of few representative CoZn-ferrite composites sintered at 1000 ◦ C. The diffraction patterns of (1 − x)Co0.5 Zn0.5 Fe2 O4 + x(SiO2 ) composite ferrites
Fig. 3. XRD pattern for (1 − x)Co0.5 Zn0.5 Fe2 O4 + xSiO2 f (x = 0.2) composite ferrite.
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Table 1 X-ray diffraction parameters of CoZnFe2 O4 + SiO2 system for x = 0.0, 0.1 and 0.2 Si-content (x)
2θ (degrees)
´˚ d-values (A)
X = 0.0
30.1 35.5 42.9 56.7 62.6
2.97 2.52 2.10 1.62 1.48
220 311 400 333 440
fcc fcc fcc fcc fcc
0.1
30.0 32.5 35.5 43.2 48.1 56.6 62.6
2.97 2.76 2.52 2.09 1.89 1.62 1.48
220 421 311 400 420 333 440
fcc * FeSiO 3 fcc fcc fcc fcc fcc
0.2
30 35.5 43.0 47.1 53.1 53.7 56.9 61.4 62.2 63.5
2.97 2.52 2.10 1.93 1.72 1.70 1.62 1.51 1.49 1.46
220 311 400 331 422 422 333 630 440 033
fcc fcc fcc fcc fcc fcc fcc * Zn SiO 2 4 fcc * Fe SiO 2 4
hkl
Phase
* Indicates a second phase peak with h k l s = (4 2 1) in the X-ray diffraction patterns shown in Figs. 1, 2, and 3 respectively.
were indexed to confirm the phases precipitated out. The indexing of the patterns shows the presence of two phases. The 2θ-values, h k l s and phases are tabulated in Table 1. Figs. 4 and 5 show plot of mass density (D) and X-ray density (Dx ) vs. Si content, respectively. It can be observed that X-ray densities are large in magnitude than bulk densities. Xray densities depend upon the lattice constant and molecular weight of the sample. The large value of X-ray density (Dx) than mass density (D) may be due to existence of pores [9]. It was observed that mass density and X-ray density decreases linearly with Si-concentration showing that the samples has become less dense, due to increased number of pores. The grain size
Fig. 4. Plot of mass density (D) vs. Si-content (x) for (1 − x)Co0.5 Zn0.5 Fe2 O4 + (x)SiO2 composite ferrites (x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.8).
Fig. 5. Plot of X-ray density (Dx ) vs. Si-content (x) for (1 − x)Co0.5 Zn0.5 Fe2 O4 + (x)SiO2 composite ferrites (x = 0.0,0.1,0.2,0.3,0.4,0.5,0.6,0.8).
was measured using Scherrer formula. Grain size ranges from 13 to 27 nm. Fig. 6 shows plot of room temperature resistivity vs. Si-concentration for all the samples. It can be observed that as SiO2 concentration increases, resistivity increases linearly and drastically from 105 to109 cm for x = 0.0–0.8. This increase in resistivity may be due to the segregation of SiO2 on the grain boundaries and hinders the hopping process between Co2+ + Fe3+ ⇔ Co3+ + Fe2+ [10]. The high values of resistivity of these samples are useful in microwave absorption. Fig. 7 shows the plot of log ρ vs. 1000/T for all the samples of Co0.5 Zn0.5 Fe2 O4 + SiO2 composites in the temperature range (305–473 K). The resistivity of these ferrite composites is observed to decrease with rising temperature according to the
Fig. 6. Plot of room temperature resistivity vs. Si-content (x) for (1 − x)Co0.5 Zn0.5 Fe2 O4 + (x)SiO2 ferrites (x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.8).
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Fig. 7. Plots of log ( cm) vs. 1000/T (K−1 ) for (1 − x)Co0.5 Zn0.5 Fe2 O4 + (x)SiO2 composites ferrites (x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.8).
following relation [11]: Ea ρ = ρ0 exp kB T
(5)
where Ea is the activation energy of the charge carriers, ρ0 is constant, T is the absolute temperature and kB is the Boltzmann’s constant. It can be observed that for x = 0.3, 0.4, 0.5 Arrhenius plots show no change in slope but for all other samples, the Arrhenius plots show two regions of conduction with different activation energies called (Para- and Ferri-regions) [12]. The activation energies are listed in Table 2. The temperature where the change of slope takes place may be their Curie temperature. The conduction phenomenon in Para-region (at high temperature) is due to polaron hopping and the conduction phenomenon in Ferri-region (at lower temperature) may be due to impurities. It may be noted that the activation energy of Para-region (at high temperature) is smaller than the activation energy of Ferri-region (at lower temperature), which shows the hopping conduction at high temperature [4]. The plot of thermoelectric power coefficient (α) vs. temperature (K) is shown in Fig. 8. For the compositions x = 0.0, 0.4, 0.8
the value of (α) remains positive which indicates that majority carriers are P-type i.e. holes. For the composition x = 0.5, 0.6, the value of (α) remains negative which indicates that majority of carriers are electrons. For the compositions x = 0.1, 0.2, 0.3, the thermoelectric power coefficient has both positive and negative values. Hence this shows that both types of charge carriers are taking part in the conduction. All these samples are degenerate type semiconductors [8,13–15]. The mobility of the charge carriers was calculated from the Seebeck coefficient and resistivity data using the following relation, provided Seebeck coefficient is measured in units of (2.3k/e): μd = exp
α/2.3(kB /e) 2N0 eρ
(6)
where N0 is the concentration of Fe3+ ions on the octahedral sites, ρ, the resistivity at temperature T (K), kB , the Boltzmann’s constant and, e, the charge on the electron. The plot of log μd vs. temperature is shown in Fig. 9. The activation energy calculated from log μd vs. 1000/T (K−1 ) was slightly lower than that activation energy measured from temperature dependent resistivity. Comparison of these values is shown in the Table 2. This con-
Table 2 Activation energy (eV) obtained from Arrhenius plots for Ferri- and Para-regions and from mobility for (1 − x)CoZnFe2 O4 + (x)SiO2 samples (x = 0.0,0.1,0.2,0.3,0.4,0.5,0.6,0.8) Si-content (x)
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.8
Activation energy (eV) from electrical resistivity Para-region
Ferri-region
0.02 0.09 0.09 0.11 0.14 0.01 0.16 0.23
0.05 0.15 0.49 – – – 0.23 0.32
Activation energy (eV) from mobility (μd)
0.03 0.11 0.09 0.12 0.13 0.01 0.22 0.26
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Fig. 8. Plot of α (mV k−1 ) vs. T (K) for (1 − x)Co0.5 Zn0.5 Fe2 O4 + (x)SiO2 composites ferrites (x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.8).
Fig. 9. Plot of log μd vs. 1000/T for (1 − x)Co0.5 Zn0.5 Fe2 O4 + x(SiO2 ) composites (x = 0.0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.8).
firms that the conduction phenomenon in these ferrites may be due to polaron hopping [16]. Ferrites are low-mobility semiconductors that is the influence of temperature on the concentration of the charge carriers is very small and the activation energy is associated with the mobility of charge carriers, rather than with their numbers [17–19]. 4. Conclusions In the present study the effect of SiO2 on the electrical properties of Co0.5 Zn0.5 Fe2 O4 ferrites was investigated. The following conclusions may be drawn. • The indexing of samples from XRD patterns reveals the presence of two phases in samples.
• The room temperature resistivity drastically increases from 105 to 109 ( cm), which may be attributed to the grain boundary resistance due to segregation of Si at grain boundaries. • Temperature dependent DC resistivity decreases with the increase of temperature exhibiting semi-conducting behavior. • Activation energy obtained from mobility is close to the activation energy obtained from Arrhenius equations which shows the polaron hopping process. • Thermopower coefficient (α) of samples shows that both type of charges take part in conduction process. The value of (α) varies with temperature for all the samples showing degenerate type semi-conducting behavior of these samples. • The mobility increases with the increase of temperature. It may be due to the fact that as the temperature increases, charge carriers start hopping from one site to another and hence conduction increases.
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Acknowledgements The authors are thankful to Prof. Dr. Farhat Saleemi, Dean Faculty of science Lahore College for Women University, Lahore for her kind help in taking XRD-patterns. Authors are also thankful to Mr. Farooq Bashir for his cooperation in taking XRD-patterns. References [1] A.A. Sattar, A.H. Wafik, H.M. EL-Sayed, J. Mater. Sci. 36 (2001) 4703–4706. [2] M. Dikeakos, L.D. Tung, T. Veres, A. Stancu, L. Spinu, F. Normandil, Mat. Res. Soc. Symp. Proc. 734 (2003) 1–6. [3] A. Medina-Boudri, D. Bueno-Baques, L. Fuents-Cobas, M. Miki-Yoshida, J. Matutes-Aquino, J. Appl. Phys. 87 (9) (2000) 6235–6237. [4] Takashi Yokoyama, Yoshiaki Abe, Takeshi Meguro, Katsutoshi Komeya, Kazuyuki Kondo, Shinobu Kaneko, Tadashi Samamoto, Jpn. J. Appl. Phys. 35 (1996) 5775–5780. [5] Xinhua He, Qingqiu Zhang, Zhiyuan Ling, Mater. Lett. 57 (2003) 3031–3036. [6] Tahir Abbas, M.U. Islam, M. Ashraf Ch., Mod. Phys. Lett. B 9 (22) (1995) 1419–1426.
487
[7] Misbah-ul-Islam, M. Ashraf Chaudhry, T. Abbas, M. Umar, Mater. Chem. Phys. 48 (1997) 227–229. [8] B.V. Bhise, V.C. Mahajan, M.G. Patil, S.D. Lotke, S.A. Patil, Indian J. Pure Appl. Phys. 33 (1995) 459–462. [9] M.U. Islam, M. Ashraf Ch, T. Abbas, M. Umar, Mater. Chem. Phys. 48 (1997) 227–229. [10] T.G.W. Stijntjes, A. Broese Van Groenou, R.F. Pearson, J.E. Knowles, P. Rankin, Ferrites, in: Proceeding of International Conference, Japan, 1970, pp. 194–198. [11] M.U. Islam, T. Abbas, M. Ashraf Ch, Abdul Wahid, J. Nat. Sci. Math. 38 (1 & 2) (1998) 195–198. [12] M.U. Islam, T. Abbas, M. Ashraf Ch., Mater. Lett. 53 (2002) 30–34. [13] M.U. Islam, A.Y. Abbasi, T. Abbas, M.A. Chaudhry, A. Zubair Chaudhry, J. Res. (Sci.), Bahauddin Zakariya University, Multan, Pakistan 14 (1) (2003) 103–107. [14] M.U. Islam, T. Abbas, Shahida B. Niazi, Zubair Ahmad, M. Sadia Sabeen, Ch. Ashraf, Solid State Commun. 130 (2004) 353–356. [15] M.A. Henaish, S.A. Olofa, M.M. Barkat, T. Tawfik, J. Thermal Anal. 37 (1991) 605–611. [16] B. Viswanathan, V.R.K. Murthy, Ferrite Materials, Narosa Publishing House, New Delhi, 1990. [17] Z. Simsa, N. Andrejev, Czech J. Phys. B19 (1969) 1389–1399. [18] B.V. Bhise, S.D. Lotke, S.A. Patil, Phys. Stat. Sol. 157 (1996) 411–419. [19] A.A. Ghani, Nuoyocimento, Mod. Phys. Lett. B 9 (22) (1995) 1419–1426.
