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The electrical properties of manganese ferrite powders prepared by two different methods
Physica B 462 (2015) 80–85
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
The electrical properties of manganese ferrite powders prepared by two different methods A. Lungu a, I. Malaescu a,n, C.N. Marin a, P. Vlazan b, P. Sfirloaga b a b
West University of Timisoara, Faculty of Physics, Bd.V. Parvan No. 4, 300223 Timisoara, Romania National Institute for Research and Development in Electrochemistry and Condensed Matter, 300569 Timisoara, Romania
art ic l e i nf o
a b s t r a c t
Article history: Received 22 August 2014 Received in revised form 13 January 2015 Accepted 22 January 2015 Available online 22 January 2015
Two powder samples of manganese ferrite (MnFe2O4) with different morphology and particle size 30– 40 nm, denoted by A and B have been synthesized by different methods starting from MnCl2 4H2O and FeCl3 6H2O. Sample A was obtained by co-precipitation followed by calcination at 900 °C and sample B has been obtained by hydrothermal method. XRD analysis show that calcination leads to the occurrence of other phases than MnFe2O4, therefore the hydrothermal method gives better results. From the temperature dependence of the electrical resistivity, measured over the range 300–483 K, the activation energy, ΔE , of the investigated samples has been evaluated, resulting in 0.43 eV (for sample A) and 0.32 eV (for sample B). The conductivity mechanism in the samples was explained in terms of Mott's variable range hopping (VRH) model. The results showed that the density of states at the Fermi level is constant over the investigated temperature range, being in order of 0.788 × 1017 eV−1 cm−3 (for sample A) and 2.05 × 1017 eV−1 cm−3 (for sample B). The hopping distance, R and the hopping energy, W (parameters of VRH model) have also been computed. Room temperature values are R¼ 27.08 nm and W ¼152 meV for sample A and R ¼21.29 nm and W¼ 120 meV for sample B. & 2015 Published by Elsevier B.V.
Keywords: Manganese ferrite Co-precipitation method Hydrothermal method Electrical resistivity Activation energy
1. Introduction The manganese ferrite with spinel structure and chemical formula MnFe2O4 belongs to the family of MeFe2O4 type oxides (where Me¼ Mn, Co, Zn, Mg, etc.), which have a great applicative importance [1]. Manganese ferrite nanoparticles have drawn the attention of researchers due to the high value of the magnetic susceptibility compared to other ferrite nanoparticles and various possible applications of manganese ferrite have been proposed. In Refs. [2–4], the use of manganese ferrite nanoparticles as ultrasensitive magnetic resonance imaging (MRI) probe and magnetic drug targeting has been suggested. Also, the manganese ferrite particles have been synthesized for their use in hyperthermia applications [5]. The obtaining of materials with electromagnetic absorbent properties is another applicative area, in which the utility of MnFe2O4 nanoparticles was proven [6,7]. Ferrite compounds, in the form of a thin film of nickel manganese ferrite nanoparticles or cobalt manganese ferrite nanoparticles were found to be useful as gas sensors [8,9]. n
Corresponding author. Fax: þ40 56 592 108. E-mail address: [email protected] (I. Malaescu).
http://dx.doi.org/10.1016/j.physb.2015.01.025 0921-4526/& 2015 Published by Elsevier B.V.
In the preparation of ferrofluids, the most usual material used is magnetite (Fe3O4) [10], but to obtain ferrofluids with controllable physical properties, some mixed ferrite particles of type Mn–Zn or Mn–Fe can be used [11]. The electrical properties of ferrites are related to the distribution of cations between tetrahedral and octahedral positions of the crystal lattice and depend on their composition and microstructure, conditions and method of synthesis [12,13]. In the case of ferrites, the cations are surrounded by oxygen anions and to a first approximation they can be considered as isolated from each other. As a result, the model of localized states is more appropriate for ferrites than the band model of electrons. Conduction in ferrites is related to the direct electronic exchange between Fe2 þ and M3 þ ions (M denotes metal ions). The diffusion of d electrons from one localized state to another is possible only when their energy is larger than a minimum value, called activation energy [13,14]. In various research studies, using different calcination temperatures and different molar ratios of raw materials, the manganese ferrite was obtained starting from FeSO4 7H2O and MnSO4 H2O [1,15] or using Mn(NO3)2 4H2O and Fe(NO3)3 9H2O as reactants [12,16]. In the present paper, in the obtaining of manganese ferrite nanoparticles, MnCl2 4H2O and FeCl3 6H2O have been used. Using these raw materials, two samples have been
A. Lungu et al. / Physica B 462 (2015) 80–85
synthesized by co-precipitation and hydrothermal methods. The dependence on temperature, T, of the electrical resistivity, ρ, of these two samples has been measured in order to study the effect of the obtaining method on the electrical properties of the manganese ferrite powders. The static conductivity mechanism in samples was explained using Mott's variable range hopping (VRH) model and the activation energy (ΔE ) as well as different related parameters were computed.
2. Samples 2.1. Obtaining methods Sample A was obtained by co-precipitation followed by calcination at 900 °C. The starting materials were MnCl2 and FeCl3 in aqueous solution (0.945 g MnCl2 4H2O mixed with 2.705 g FeCl3 6H2O). Given the high reactivity of manganese ions in chemical reactions, the iron content of initial chlorides is slightly larger than the stoichiometric ratio of ions in MnFe2O4 [17]. Subsequently, 2 M NaOH aqueous solutions were added with constant stirring to the above solution until pH ¼11 were reached. After adding NaOH, magnetic stirring was done for 2 h at a temperature between 80 and 90 °C. After filtering, the precipitate was washed several times with distilled water to remove chlorides until the resulted water solution reached pH ¼7. Finally, the washed precipitate was calcined at the temperature of 900 °C for four hours. Sample B was obtained by hydrothermal method. The starting materials were 0.945 g MnCl2 4H2O and 2.705 g FeCl3 6H2O in aqueous solution and the precipitation was done with 2 M NaOH aqueous solution, at pH ¼11 (as in the case of sample A). The mixture was introduced into a Morey-type autoclave and maintained at a temperature of 250 °C, for 12 h. After decantation and filtration, the resulting precipitate was washed with distilled water on a filter paper and then laid out to dry in an air oven, at 80 °C, resulting in a powder sample. 2.2. Structural and morphologic characterization The crystalline structure of the prepared samples was investigated by X-ray diffraction (XRD) using a PANalytical-X'Pert PRO MRD diffractometer. Powder morphology was observed using a PANalytical scanning electron microscope (SEM). The elemental analysis was analyzed through EDX facility of SEM. The XRD patterns of the investigated samples are presented in Fig. 1. Based on the XRD pattern of samples one can assert that
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sample A (Fig. 1(a)) consists of a mixture of MnFe2O4, FeMnO3, Mn2O7 and Fe2O3. As reported in Ref. [18], at elevated temperatures, MnFe2O4 is unstable in air and on the surface of ferrite particles, Mn2 þ ions oxidize to form Mn3 þ , leading to dissociation of MnFe2O4 and occurrence of other phases. Therefore, any preparation method which involves calcination step is not suitable for the preparation of manganese ferrite nano-particles. Fig. 1(b) shows the XRD pattern of sample B and it contains peaks that match to the standard data of the cubic spinel MnFe2O4 (JCPDS Card no. 75-0034). The XRD pattern does not contain extra peaks, indicating formation of MnFe2O4 as single phase [19]. The Scherrer formula [20] was used in order to calculate the average size of MnFe2O4 crystallites of both samples. The values obtained are approximately of 37.93 nm for sample A and 31.9 nm for sample B. Fig. 2(a) and (c) show the morphology of MnFe2O4 powder samples. From the SEM photograph (Fig. 2(a)) it is observed that sample A (obtained by co-precipitation) consists of polycrystalline grains with average size of 200–300 nm. It can also be seen in Fig. 2(c) that sample B also contains polycrystalline grains with average size of a few hundred nanometers. The results of EDX (which is a semi-quantitative analysis) are presented in a graphical form in Fig. 2(b) and (d) and confirm that both ferrite samples consist of Mn, Fe and O. The atomic ratio of elements, 1:2.04:3.81 (for sample A) and 1:1.90:3.42 (for sample B) are close to the ratio of MnFe2O4 formula.
3. Results and discussions For measurements of the electrical resistance, R, of samples at various temperatures, T, over the range 30–200 °C, a laboratory made experimental setup (whose schematic representation is shown in Fig. 3) has been used. As can be seen from Fig. 3, the ferrite powder sample (1) was inserted into an electric furnace (3), which is heated by means of an electrical resistance connected to a voltage source, u. The sample (1) is in contact with two electrodes (2), which are connected to an ohmmeter, (Ω ). The ends of the furnace are insulated with two thermo-insulating material stoppers (5) and the temperature T of the sample was measured with a thermocouple (4). The length, L and section, A of the samples had the values: L = 11 mm and A = πD2/4 = 11.34 mm2, where D = 3.8 mm represents the diameter of the glass tube in which the ferrite sample was introduced. Based on the experimental values of the electrical resistance of
Fig. 1. X-ray diffraction patterns of samples A and B.
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A. Lungu et al. / Physica B 462 (2015) 80–85
Fig. 2. SEM photographs of samples (a) for sample A and (c) for sample B and EDX elemental analysis (b) for sample A, synthesized by co-precipitation method and (d) for sample B, obtained by hydrothermal method.
expressed by a relation of the form [22]
⎛ ΔE ⎞ ⎟ ρ = ρ0 exp⎜ ⎝ kT ⎠
(2)
where ρ0 represents the pre-exponential factor of resistivity,
Fig. 3. The experimental setup for temperature dependent.
ferrite sample, at different temperatures within the range 300– 573 K and knowing the length, L, and section, A, of the sample, we have determined the electrical resistivity, ρ , using the relation:
ρ=R
A L
(1)
The temperature dependence of the electrical resistivity of ferrite samples is shown in Fig. 4(a) and (b). From Fig. 4(a) and (b) it can be seen that electrical resistivity of the samples A and B presents an exponential dependence, which indicates that the samples have a typical semiconductor behavior [22]. Consequently, the electrical resistivity of the samples can be
k = 1.38 × 10−23 J /K , is the Boltzmann constant and ΔE is the activation energy of the ferrite sample. The experimental dependencies of ln ρ on T −1 of the investigated samples are shown in Fig. 5. The activation energy, ΔE , was determined from the linear fit of the dependence on T −1 of ln ρ . The following values were obtained: ΔE ¼0.43 eV for sample A and ΔE ¼0.32 eV for sample B. The value of activation energy obtained for sample B (the sample containing only MnFe2O4) is similar to that obtained in Ref. [21] for the ferromagnetic state of a MnFe2O4 powder. Also, ΔE of sample B compares favorably to those obtained in the papers [22,23] for similar ferrite powder samples. In the case of sample A (which contains other three phases apart from MnFe2O4 – see Fig. 1) the activation energy is larger than that obtained for sample B, but has similar values with those obtained in Ref. [21] for the paramagnetic state of a MnFe2O4 powder. By linear fitting of the experimental dependence, ln ρ(T −1) from Fig. 5, one obtains the parameter, ρ0 , corresponding to both samples, A and B. The values obtained are: ρ0(A) = 7.057 Ω m and ρ0(A) = 2.525 Ω m . The large values of pre-exponential factor, ρ0 indicates that the electric conduction in these samples can be
A. Lungu et al. / Physica B 462 (2015) 80–85
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Fig. 4. Temperature dependence of the electrical resistivity ρ of samples A (a) and B (b).
Fig. 5. Plots of ln ρ on T −1 of samples A (a) and B (b).
Fig. 6. The dependencies, ln σ (T −1/4) corresponding to the ferrite samples A (a) and B (b).
Table 1 Mott parameters calculated at three temperatures for samples A and B. Samples
A B
T0 [K]
0.244 1010 0.093 1010
N(EF) [cm 3 eV 1]
0.788 1017 2.05 1017
R [nm] 303 K
373 K
483 K
303 K
373 K
483 K
27.08 21.29
25.71 20.21
24.10 18.95
152 120
178 140
216 170
explained by the hopping process between localized states [24]. In this model, it is assumed that the hopping process would be determined by Mott's variable-range-hopping (VRH) mechanism and the static conductivity, σdc , is given by the following expression [27]:
σdc = σ0 exp⎡⎣−(T0/T )1/4 ⎤⎦
W [meV]
where T0 is characteristic temperature coefficient being a measure of the degree of disorder [26], given by the relation
T0 =
λα 3 . kN (EF )
(4) 7
(3)
In Eq. (4), λ ≅ 16.6, is a dimensionless constant [26]; α ≅ 10 cm−1 represents the degree of localization and N (EF ) is the density of the
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A. Lungu et al. / Physica B 462 (2015) 80–85
localized states at the Fermi level EF. The hopping distance, R is given by the relation,
⎛ ⎞1/4 9 R=⎜ ⎟ ⎝ 8αkTN (EF ) ⎠
(5)
and the hopping energy W is given by the relation [25]
W=
3 4πR 3N (EF )
.
(6)
Fig. 6 presents the Mott's VRH plot, ln σdc versus (1/T1/4) for the investigated samples, using the Eq. (3). By fitting the experimental dependence, ln σ (T −1/4) from Fig. 6, to a linear equation, one obtains the parameters, T0, corresponding to samples A and B. Using the values obtained for T0 and Eq. (4), we have computed the density of the localized states at the Fermi level, N (EF ) and the values obtained are shown in Table 1. These values suggest that the density of states at the Fermi level is higher in sample B than in sample A (N (EF )(sample B) > N (EF )(sample A) ). The result can be correlated with the structural composition of samples, which was obtained by X-ray diffraction (see Fig. 2): sample B consists of only one type of manganese ferrite particles (MnFe2O4), while the sample A contains a mixture of substances (MnFe2O4, FeMnO3, Mn2O7 and Fe2O3). From Eq. (5) and using the computed values of N (EF ), we have determined the average hopping distance, R, corresponding to the samples A and B, at each temperature within the range 300–483 K. Knowing the hopping distance R and using Eq. (6) we have computed the hopping energy W, for the investigated samples. The values obtained for the parameters R and W, corresponding of the samples A and B, at three temperatures, are shown in Table 1. All Mott's parameters compares favorably to those reported in Ref. [22] for nano-ferrite samples. One can observe from Table 1 that for both samples the hopping distance is smaller than the average size of crystallites (as obtained from the XRD analysis), demonstrating that the conduction is realized by electron hopping within the crystal grains. Also, both the hopping distance, R and the hopping energy, W have larger values in the case of sample A than in the case of pure sample (sample B). From Table 1, it is observed that the hopping distance, R, decreases with the increase in temperature and the hopping energy, W, increases with the increase in temperature. This may be correlated to the increase with temperature in the lattice parameters of MnFe2O4 (i.e. cell edge and cation–oxygen distance) [28]. Comparing the value of the hopping energy, W, with the computed value of the thermal energy,kT , at the same temperatures T shown in Table 1, it follows that the value of W is (3.2–3.6) times greater than the value of kT (for sample A) and (2.5–2.8) times larger than kT (for sample B). Also, the value of αR (computed at the temperatures listed in Table 1) is much larger than unity. These results are in agreement with those suggested by Mott in Refs. [25,27].
4. Conclusions Two powder samples of manganese ferrite particles were synthesized by chemical co-precipitation method followed by calcination at 900 °C (sample A) and by hydrothermal method (sample B). The XRD analysis shows that the sample B consists of MnFe2O4 and sample A has other three phases apart of MnFe2O4. In conclusion, the hydrothermal method gives better results in the obtaining process of manganese ferrite and thermal treatment at elevated temperatures is not recommended, due to the instability of Mn2 þ ion which oxidizes to Mn3 þ . The average crystallite sizes
of MnFe2O4 are approximately of 37.93 nm for sample A and 31.90 nm for sample B. The static measurements of the electrical resistivity, over the temperature range (300–483) K, show that the resistivity ρ of samples A and B decreases with the increase in temperature, showing a semiconductor like behavior. The experimental results allowed determination of the thermal activation energy ΔE of samples, resulting in the values: ΔE ¼0.43 eV (sample A) and ΔE ¼0.32 eV (sample B), which depend on the obtaining method of particles and on their structural composition. The static conductivity mechanism in the samples over the investigated temperature range was explained using the Mott's variable range hopping (VRH) model. Based on VRH model, the density of the localized states at the Fermi level N (EF ), the hopping distance R and the hopping energy W were computed.
Acknowledgments This work was supported by the Strategic Grant POSDRU/159/ 1.5/S/137750, Project “Doctoral and Postdoctoral Programs Support for Increased Competitiveness in Exact Sciences research” cofinanced by the European Social Fund within the Sectorial Operational Program Human Resources Development 2007–2013.
References [1] S. Sam, A.S. Nesaraj, Preparation of MnFe2O4 nanoceramic particles by soft chemical routes, Int. J. Appl. Sci. Eng. 223 (2011) 223–239. [2] M. Faham, H. Shokrollahi, G. Yousefi, S. Abbasi, PEG decorated glycine capped Mn–ferrite nanoparticles synthesized by co-precipitation method for biomedical application, Adv. Mater. Res. 829 (2014) 274–278. [3] J. Lu, S. Ma, J. Sun, C. Xia, C. Liu, Z. Wang, X. Zhao, Z. Gu, , Manganese ferrite nanoparticle micellar nanocomposites as MRI contrast agent for liver imaging, Biomaterials 30 (15) (2009) 2919–2928. [4] Saqlain A. Shah, A. Majeed, M.A. Shafique, K. Rashid, Saif-Ullah Awan, Cell viability study of thermo-responsive core–shell superparamagnetic nanoparticles for multimodal cancer therapy, Appl. Nanosci. 4 (2014) 227–232. [5] A. Doaga, A.M. Cojocariu, W. Amin, F. Heib, P. Bender, R. Hempelmann, O. F. Caltun, Synthesis and characterizations of manganese ferrites for hyperthermia applications, Mater. Chem. Phys. 143 (2013) 305–310. [6] F. Tabatabaie, M.H. Fathi, A. Saatchi, A. Ghasemi, Microwave absorption properties of Mn- and Ti-doped strontium hexaferrite, J. Alloys Compd. 470 (1–2) (2009) 332–335. [7] S.H. Hosseini, S.H. Mohseni, A. Asadnia, H. Kerdari, Synthesis and microwave absorbing properties of polyaniline/MnFe2O4 nanocomposite, J. Alloys Compd. 509 (14) (2011) 4682–4687. [8] Y. Köseo|glu, I. Aldemir, F. Bayansal, S. Kahraman, H.A. Çetinkara, Synthesis, characterization and humidity sensing properties of Mn 0.2Ni0.8Fe2O4 nanoparticles, Mater. Chem. Phys. 139 (2–3) (2013) 789–793. [9] I. Sandu, L. Presmanes, P. Alphonse, P. Tailhades, Nanostructured cobalt manganese ferrite thin films for gas sensor application, Thin Solid Films 495 (1–2) (2006) 130–133. [10] R.E. Rosensweig, Ferrohydrodynamics, Cambridge University Press, Cambridge, 1985. [11] I. Malaescu, N. Ştefu, L. Gabor, Relaxation process and ferromagnetic resonance investigation of ferrofluids with Mn–Zn and Mn–Fe mixed ferrite particles, J. Magn. Magn. Mater. 234 (2) (2001) 299–305. [12] M.A. Ahmed, N. Okasha, S.I. El-Dek, Preparation and characterization ofnanometric Mn ferrite via different methods, Nanotechnology 19 (2008) 065–603. [13] Sangeeta Thakur, S.C. Katyal, M. Singh, Improvement in electric and dielectric properties of nanoferrite synthesized via reverse micelle technique, Appl. Phys. Lett. 91 (2007) 262–501. [14] A.A. Samokhvalov, A.G. Rustamov, Sov. Phys. Solid State 7 (1965) 961. [15] V.M. Bujoreanu, L. Diamandescu, M. Brezeanu, On the structure of manganese ferrite powder prepared by coprecipitation from MnO2 and FeSO4 7H2O, Mater. Lett. 46 (2000) 169–174. [16] Han Aijun, Liao Juanjuan, Ye Mingquan, Li Yan, Peng Xinhua, Preparation of nano-MnFe2O4 and its catalytic performance of thermal decomposition of ammonium perchlorate, Chin. J. Chem. Eng. 19 (6) (2011) 1047–1051. [17] N.N. Greenwood, A. Earnshaw, Chemistry of the Elements, Pergamon Press, Oxford (1984) 82–87.
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[18] Nasser Y. Mostafa, M.M. Hessiena, Abdallah A. Shaltouta, Hydrothermal synthesis and characterizations of Ti substituted Mn-ferrites, J. Alloys Compd. 529 (2012) 29–33. [19] L. Zhen, K. He, C.Y. Xu, W.Z. Shao, Synthesis and characterization of singlecrystalline MnFe2O4 nanorods via a surfactant-free hydrothermal route, J. Magn. Magn. Mater. 320 (2008) 2672–2675. [20] Y. Duan, H. Ma, X. Li, S. Liu, Z. Ji, The microwave electromagnetic characteristics of manganese dioxide with different crystallographic structures, Physica B 405 (7) (2010) 1826–1831. [21] B. Aslibeiki, Nanostructural, magnetic and electrical properties of Ag doped Mn/ferrite nanoparticles, Curr. Appl. Phys. 14 (2014) 1659–1664. [22] Humaira Anwar, Asghari Maqsood, Temperature dependent structural and electrical analysis of Mn–Zn nano-ferrites, J. Pak. Mater. Soc. 4 (2) (2010) 81–93. [23] S.A. Mazen, N.I. Abu-Elsaad, Temperature dependence of electrical
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Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
The electrical properties of manganese ferrite powders prepared by two different methods A. Lungu a, I. Malaescu a,n, C.N. Marin a, P. Vlazan b, P. Sfirloaga b a b
West University of Timisoara, Faculty of Physics, Bd.V. Parvan No. 4, 300223 Timisoara, Romania National Institute for Research and Development in Electrochemistry and Condensed Matter, 300569 Timisoara, Romania
art ic l e i nf o
a b s t r a c t
Article history: Received 22 August 2014 Received in revised form 13 January 2015 Accepted 22 January 2015 Available online 22 January 2015
Two powder samples of manganese ferrite (MnFe2O4) with different morphology and particle size 30– 40 nm, denoted by A and B have been synthesized by different methods starting from MnCl2 4H2O and FeCl3 6H2O. Sample A was obtained by co-precipitation followed by calcination at 900 °C and sample B has been obtained by hydrothermal method. XRD analysis show that calcination leads to the occurrence of other phases than MnFe2O4, therefore the hydrothermal method gives better results. From the temperature dependence of the electrical resistivity, measured over the range 300–483 K, the activation energy, ΔE , of the investigated samples has been evaluated, resulting in 0.43 eV (for sample A) and 0.32 eV (for sample B). The conductivity mechanism in the samples was explained in terms of Mott's variable range hopping (VRH) model. The results showed that the density of states at the Fermi level is constant over the investigated temperature range, being in order of 0.788 × 1017 eV−1 cm−3 (for sample A) and 2.05 × 1017 eV−1 cm−3 (for sample B). The hopping distance, R and the hopping energy, W (parameters of VRH model) have also been computed. Room temperature values are R¼ 27.08 nm and W ¼152 meV for sample A and R ¼21.29 nm and W¼ 120 meV for sample B. & 2015 Published by Elsevier B.V.
Keywords: Manganese ferrite Co-precipitation method Hydrothermal method Electrical resistivity Activation energy
1. Introduction The manganese ferrite with spinel structure and chemical formula MnFe2O4 belongs to the family of MeFe2O4 type oxides (where Me¼ Mn, Co, Zn, Mg, etc.), which have a great applicative importance [1]. Manganese ferrite nanoparticles have drawn the attention of researchers due to the high value of the magnetic susceptibility compared to other ferrite nanoparticles and various possible applications of manganese ferrite have been proposed. In Refs. [2–4], the use of manganese ferrite nanoparticles as ultrasensitive magnetic resonance imaging (MRI) probe and magnetic drug targeting has been suggested. Also, the manganese ferrite particles have been synthesized for their use in hyperthermia applications [5]. The obtaining of materials with electromagnetic absorbent properties is another applicative area, in which the utility of MnFe2O4 nanoparticles was proven [6,7]. Ferrite compounds, in the form of a thin film of nickel manganese ferrite nanoparticles or cobalt manganese ferrite nanoparticles were found to be useful as gas sensors [8,9]. n
Corresponding author. Fax: þ40 56 592 108. E-mail address: [email protected] (I. Malaescu).
http://dx.doi.org/10.1016/j.physb.2015.01.025 0921-4526/& 2015 Published by Elsevier B.V.
In the preparation of ferrofluids, the most usual material used is magnetite (Fe3O4) [10], but to obtain ferrofluids with controllable physical properties, some mixed ferrite particles of type Mn–Zn or Mn–Fe can be used [11]. The electrical properties of ferrites are related to the distribution of cations between tetrahedral and octahedral positions of the crystal lattice and depend on their composition and microstructure, conditions and method of synthesis [12,13]. In the case of ferrites, the cations are surrounded by oxygen anions and to a first approximation they can be considered as isolated from each other. As a result, the model of localized states is more appropriate for ferrites than the band model of electrons. Conduction in ferrites is related to the direct electronic exchange between Fe2 þ and M3 þ ions (M denotes metal ions). The diffusion of d electrons from one localized state to another is possible only when their energy is larger than a minimum value, called activation energy [13,14]. In various research studies, using different calcination temperatures and different molar ratios of raw materials, the manganese ferrite was obtained starting from FeSO4 7H2O and MnSO4 H2O [1,15] or using Mn(NO3)2 4H2O and Fe(NO3)3 9H2O as reactants [12,16]. In the present paper, in the obtaining of manganese ferrite nanoparticles, MnCl2 4H2O and FeCl3 6H2O have been used. Using these raw materials, two samples have been
A. Lungu et al. / Physica B 462 (2015) 80–85
synthesized by co-precipitation and hydrothermal methods. The dependence on temperature, T, of the electrical resistivity, ρ, of these two samples has been measured in order to study the effect of the obtaining method on the electrical properties of the manganese ferrite powders. The static conductivity mechanism in samples was explained using Mott's variable range hopping (VRH) model and the activation energy (ΔE ) as well as different related parameters were computed.
2. Samples 2.1. Obtaining methods Sample A was obtained by co-precipitation followed by calcination at 900 °C. The starting materials were MnCl2 and FeCl3 in aqueous solution (0.945 g MnCl2 4H2O mixed with 2.705 g FeCl3 6H2O). Given the high reactivity of manganese ions in chemical reactions, the iron content of initial chlorides is slightly larger than the stoichiometric ratio of ions in MnFe2O4 [17]. Subsequently, 2 M NaOH aqueous solutions were added with constant stirring to the above solution until pH ¼11 were reached. After adding NaOH, magnetic stirring was done for 2 h at a temperature between 80 and 90 °C. After filtering, the precipitate was washed several times with distilled water to remove chlorides until the resulted water solution reached pH ¼7. Finally, the washed precipitate was calcined at the temperature of 900 °C for four hours. Sample B was obtained by hydrothermal method. The starting materials were 0.945 g MnCl2 4H2O and 2.705 g FeCl3 6H2O in aqueous solution and the precipitation was done with 2 M NaOH aqueous solution, at pH ¼11 (as in the case of sample A). The mixture was introduced into a Morey-type autoclave and maintained at a temperature of 250 °C, for 12 h. After decantation and filtration, the resulting precipitate was washed with distilled water on a filter paper and then laid out to dry in an air oven, at 80 °C, resulting in a powder sample. 2.2. Structural and morphologic characterization The crystalline structure of the prepared samples was investigated by X-ray diffraction (XRD) using a PANalytical-X'Pert PRO MRD diffractometer. Powder morphology was observed using a PANalytical scanning electron microscope (SEM). The elemental analysis was analyzed through EDX facility of SEM. The XRD patterns of the investigated samples are presented in Fig. 1. Based on the XRD pattern of samples one can assert that
81
sample A (Fig. 1(a)) consists of a mixture of MnFe2O4, FeMnO3, Mn2O7 and Fe2O3. As reported in Ref. [18], at elevated temperatures, MnFe2O4 is unstable in air and on the surface of ferrite particles, Mn2 þ ions oxidize to form Mn3 þ , leading to dissociation of MnFe2O4 and occurrence of other phases. Therefore, any preparation method which involves calcination step is not suitable for the preparation of manganese ferrite nano-particles. Fig. 1(b) shows the XRD pattern of sample B and it contains peaks that match to the standard data of the cubic spinel MnFe2O4 (JCPDS Card no. 75-0034). The XRD pattern does not contain extra peaks, indicating formation of MnFe2O4 as single phase [19]. The Scherrer formula [20] was used in order to calculate the average size of MnFe2O4 crystallites of both samples. The values obtained are approximately of 37.93 nm for sample A and 31.9 nm for sample B. Fig. 2(a) and (c) show the morphology of MnFe2O4 powder samples. From the SEM photograph (Fig. 2(a)) it is observed that sample A (obtained by co-precipitation) consists of polycrystalline grains with average size of 200–300 nm. It can also be seen in Fig. 2(c) that sample B also contains polycrystalline grains with average size of a few hundred nanometers. The results of EDX (which is a semi-quantitative analysis) are presented in a graphical form in Fig. 2(b) and (d) and confirm that both ferrite samples consist of Mn, Fe and O. The atomic ratio of elements, 1:2.04:3.81 (for sample A) and 1:1.90:3.42 (for sample B) are close to the ratio of MnFe2O4 formula.
3. Results and discussions For measurements of the electrical resistance, R, of samples at various temperatures, T, over the range 30–200 °C, a laboratory made experimental setup (whose schematic representation is shown in Fig. 3) has been used. As can be seen from Fig. 3, the ferrite powder sample (1) was inserted into an electric furnace (3), which is heated by means of an electrical resistance connected to a voltage source, u. The sample (1) is in contact with two electrodes (2), which are connected to an ohmmeter, (Ω ). The ends of the furnace are insulated with two thermo-insulating material stoppers (5) and the temperature T of the sample was measured with a thermocouple (4). The length, L and section, A of the samples had the values: L = 11 mm and A = πD2/4 = 11.34 mm2, where D = 3.8 mm represents the diameter of the glass tube in which the ferrite sample was introduced. Based on the experimental values of the electrical resistance of
Fig. 1. X-ray diffraction patterns of samples A and B.
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Fig. 2. SEM photographs of samples (a) for sample A and (c) for sample B and EDX elemental analysis (b) for sample A, synthesized by co-precipitation method and (d) for sample B, obtained by hydrothermal method.
expressed by a relation of the form [22]
⎛ ΔE ⎞ ⎟ ρ = ρ0 exp⎜ ⎝ kT ⎠
(2)
where ρ0 represents the pre-exponential factor of resistivity,
Fig. 3. The experimental setup for temperature dependent.
ferrite sample, at different temperatures within the range 300– 573 K and knowing the length, L, and section, A, of the sample, we have determined the electrical resistivity, ρ , using the relation:
ρ=R
A L
(1)
The temperature dependence of the electrical resistivity of ferrite samples is shown in Fig. 4(a) and (b). From Fig. 4(a) and (b) it can be seen that electrical resistivity of the samples A and B presents an exponential dependence, which indicates that the samples have a typical semiconductor behavior [22]. Consequently, the electrical resistivity of the samples can be
k = 1.38 × 10−23 J /K , is the Boltzmann constant and ΔE is the activation energy of the ferrite sample. The experimental dependencies of ln ρ on T −1 of the investigated samples are shown in Fig. 5. The activation energy, ΔE , was determined from the linear fit of the dependence on T −1 of ln ρ . The following values were obtained: ΔE ¼0.43 eV for sample A and ΔE ¼0.32 eV for sample B. The value of activation energy obtained for sample B (the sample containing only MnFe2O4) is similar to that obtained in Ref. [21] for the ferromagnetic state of a MnFe2O4 powder. Also, ΔE of sample B compares favorably to those obtained in the papers [22,23] for similar ferrite powder samples. In the case of sample A (which contains other three phases apart from MnFe2O4 – see Fig. 1) the activation energy is larger than that obtained for sample B, but has similar values with those obtained in Ref. [21] for the paramagnetic state of a MnFe2O4 powder. By linear fitting of the experimental dependence, ln ρ(T −1) from Fig. 5, one obtains the parameter, ρ0 , corresponding to both samples, A and B. The values obtained are: ρ0(A) = 7.057 Ω m and ρ0(A) = 2.525 Ω m . The large values of pre-exponential factor, ρ0 indicates that the electric conduction in these samples can be
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Fig. 4. Temperature dependence of the electrical resistivity ρ of samples A (a) and B (b).
Fig. 5. Plots of ln ρ on T −1 of samples A (a) and B (b).
Fig. 6. The dependencies, ln σ (T −1/4) corresponding to the ferrite samples A (a) and B (b).
Table 1 Mott parameters calculated at three temperatures for samples A and B. Samples
A B
T0 [K]
0.244 1010 0.093 1010
N(EF) [cm 3 eV 1]
0.788 1017 2.05 1017
R [nm] 303 K
373 K
483 K
303 K
373 K
483 K
27.08 21.29
25.71 20.21
24.10 18.95
152 120
178 140
216 170
explained by the hopping process between localized states [24]. In this model, it is assumed that the hopping process would be determined by Mott's variable-range-hopping (VRH) mechanism and the static conductivity, σdc , is given by the following expression [27]:
σdc = σ0 exp⎡⎣−(T0/T )1/4 ⎤⎦
W [meV]
where T0 is characteristic temperature coefficient being a measure of the degree of disorder [26], given by the relation
T0 =
λα 3 . kN (EF )
(4) 7
(3)
In Eq. (4), λ ≅ 16.6, is a dimensionless constant [26]; α ≅ 10 cm−1 represents the degree of localization and N (EF ) is the density of the
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localized states at the Fermi level EF. The hopping distance, R is given by the relation,
⎛ ⎞1/4 9 R=⎜ ⎟ ⎝ 8αkTN (EF ) ⎠
(5)
and the hopping energy W is given by the relation [25]
W=
3 4πR 3N (EF )
.
(6)
Fig. 6 presents the Mott's VRH plot, ln σdc versus (1/T1/4) for the investigated samples, using the Eq. (3). By fitting the experimental dependence, ln σ (T −1/4) from Fig. 6, to a linear equation, one obtains the parameters, T0, corresponding to samples A and B. Using the values obtained for T0 and Eq. (4), we have computed the density of the localized states at the Fermi level, N (EF ) and the values obtained are shown in Table 1. These values suggest that the density of states at the Fermi level is higher in sample B than in sample A (N (EF )(sample B) > N (EF )(sample A) ). The result can be correlated with the structural composition of samples, which was obtained by X-ray diffraction (see Fig. 2): sample B consists of only one type of manganese ferrite particles (MnFe2O4), while the sample A contains a mixture of substances (MnFe2O4, FeMnO3, Mn2O7 and Fe2O3). From Eq. (5) and using the computed values of N (EF ), we have determined the average hopping distance, R, corresponding to the samples A and B, at each temperature within the range 300–483 K. Knowing the hopping distance R and using Eq. (6) we have computed the hopping energy W, for the investigated samples. The values obtained for the parameters R and W, corresponding of the samples A and B, at three temperatures, are shown in Table 1. All Mott's parameters compares favorably to those reported in Ref. [22] for nano-ferrite samples. One can observe from Table 1 that for both samples the hopping distance is smaller than the average size of crystallites (as obtained from the XRD analysis), demonstrating that the conduction is realized by electron hopping within the crystal grains. Also, both the hopping distance, R and the hopping energy, W have larger values in the case of sample A than in the case of pure sample (sample B). From Table 1, it is observed that the hopping distance, R, decreases with the increase in temperature and the hopping energy, W, increases with the increase in temperature. This may be correlated to the increase with temperature in the lattice parameters of MnFe2O4 (i.e. cell edge and cation–oxygen distance) [28]. Comparing the value of the hopping energy, W, with the computed value of the thermal energy,kT , at the same temperatures T shown in Table 1, it follows that the value of W is (3.2–3.6) times greater than the value of kT (for sample A) and (2.5–2.8) times larger than kT (for sample B). Also, the value of αR (computed at the temperatures listed in Table 1) is much larger than unity. These results are in agreement with those suggested by Mott in Refs. [25,27].
4. Conclusions Two powder samples of manganese ferrite particles were synthesized by chemical co-precipitation method followed by calcination at 900 °C (sample A) and by hydrothermal method (sample B). The XRD analysis shows that the sample B consists of MnFe2O4 and sample A has other three phases apart of MnFe2O4. In conclusion, the hydrothermal method gives better results in the obtaining process of manganese ferrite and thermal treatment at elevated temperatures is not recommended, due to the instability of Mn2 þ ion which oxidizes to Mn3 þ . The average crystallite sizes
of MnFe2O4 are approximately of 37.93 nm for sample A and 31.90 nm for sample B. The static measurements of the electrical resistivity, over the temperature range (300–483) K, show that the resistivity ρ of samples A and B decreases with the increase in temperature, showing a semiconductor like behavior. The experimental results allowed determination of the thermal activation energy ΔE of samples, resulting in the values: ΔE ¼0.43 eV (sample A) and ΔE ¼0.32 eV (sample B), which depend on the obtaining method of particles and on their structural composition. The static conductivity mechanism in the samples over the investigated temperature range was explained using the Mott's variable range hopping (VRH) model. Based on VRH model, the density of the localized states at the Fermi level N (EF ), the hopping distance R and the hopping energy W were computed.
Acknowledgments This work was supported by the Strategic Grant POSDRU/159/ 1.5/S/137750, Project “Doctoral and Postdoctoral Programs Support for Increased Competitiveness in Exact Sciences research” cofinanced by the European Social Fund within the Sectorial Operational Program Human Resources Development 2007–2013.
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