WITHDRAWN: Effect of annealing temperature on structural, magnetic, elastic and dielectric properties of Ni–Cu–Zn nano ferrites

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Effect of annealing temperature on structural, magnetic, elastic and dielectric properties of Ni–Cu–Zn nano ferrites

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V.V. Awati a, S.M. Patange b,⇑, S.M. Rathod c, S.E. Shirsath d

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8 9 10 11 13 12 14 1 2 6 8 17 18 19 20 21 22 23 24 25 26 27

a

Department of Physics, C.T. Bora College, Shirur, Dist.: Pune, M.S. 412210, India Materials Research Laboratory, Shri Krishna Mahavidalaya, Gunjoti, M.S. 413613, India c Department of Physics, Abasaheb Garware College, Karve Road, Pune, M.S. 411004, India d Department of Physics, Vivekanand College, Aurangabad, M.S. 431001, India b

a r t i c l e

i n f o

Article history: Received 27 January 2016 Accepted 6 April 2016 Available online xxxx Keywords: NiCuZn ferrite TEM V.S.M. Elastic properties Dielectric properties

a b s t r a c t Nano spinel ferrite Ni0.4Cu0.1Zn0.5Fe2O4 was prepared by sol–gel auto combination method. The synthesized samples annealed with different temperatures such as as-prepared, 400 °C and 600 °C for 4 h and there are characterized by XRD, TEM, V.S.M., elastic and dielectric properties. Analysis of X-ray diffraction patterns shows as temperature increases, the crystallinity increases and XRD pattern shows the single phase cubic structure. The lattice constant ‘a’ increases with an increase in annealed temperature. TEM images revealed that crystalline size is in the range of 8–12 nm. Saturation magnetization and coercivity both increase with an increase in annealing temperature. The ultrasonic pulse transmission method is employed to find the value of longitudinal velocity (VL) and shearing velocity (VS) at room temperature. These values utilized to calculate the elastic moduli such as longitudinal (L), young’s (E), Rigidity (G), Bulk (K) and Possion’s ratio. All moduli increase with an increase in annealing temperature. These values predict bonding force between atoms. The dielectric properties such as dielectric constant (έ) and dielectric loss (e00 ) are explained on the basis of hopping mechanism. Ó 2016 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).

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44 45

Introduction

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The polycrystalline Ni–Zn ferrites (NZFOs) are soft magnetic materials, which have a wide application in electrical and electronic industries [1], multi-layer LC filters [2], magnetic temperature sensors [3] and humidity sensors [4]. Soft magnetic materials with initial particle size in the nanometer scale are now of interest because of their unique magnetic properties, which differ considerably from those of bulk materials and become technologically very important. In order to develop Multilayer Ferrite Chip Inductor (MLFCI) the NiCuZn ferrite was intensively studied in the last ten years. It is one of the widely used electronic components for the electronic products such as Cellular Phone, Notebook Computer and Video Cameras. [5,6]. Multilayer Chip Inductor [MLCI] has recently been developed as one of the key surface mounting devices [7,8]. The low temperature sintered NiCuZn ferrite is one of the most important magnetic materials for multilayer chip inductor (MLCI) applications because of their relatively low sintering temperatures, high permeability in a high frequency region, high electrical resistivity and chemical stability [9,10].

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⇑ Corresponding author. Mobile: +91 9423342372. E-mail address: [email protected] (S.M. Patange).

Ni–Cu–Zn ferrite powder is usually synthesized by the conventional solid state reaction oxide method and calcinations at higher temperature. The oxide method has some inherent disadvantages such as chemical in homogeneity, coarser particle size etc [11,12]. Engineers have been researching high strength and ductile materials for a decade. These two properties often behave differently when composition and structure are varied [13]. Deformation and heat treatment are two basic processes used to improve the mechanical properties of the base materials. Annealing induces dislocation rearrangement, allowing stress relief. This may lead to improved ductility properties [14]. The size distribution of the grain and contamination in the grain boundaries can play an important role. It is well established that reducing the grain size is beneficial to the mechanical properties of poly-crystalline bulk materials [15]. Very small grain size improved metallic material strength and toughness at low temperature. Optimal grain size is usually found in the range of 5–10 nm [16]. A detailed study of elastic behavior of nanocrystalline NiCuZn ferrites has been undertaken and the obtained results are presented in this paper. However, according to the knowledge of the authors the reports on the specific composition of system Ni0.4Cu0.1Zn0.5Fe2O4 are not available in the literature. Ferrites are very good dielectric materials and have many technological applications ranging from

http://dx.doi.org/10.1016/j.rinp.2016.04.005 2211-3797/Ó 2016 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Please cite this article in press as: Awati VV et al. Effect of annealing temperature on structural, magnetic, elastic and dielectric properties of Ni–Cu–Zn nano ferrites. Results Phys (2016), http://dx.doi.org/10.1016/j.rinp.2016.04.005

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microwave to radio frequencies. Hence, it is important to study their dielectric behavior at different frequencies. The present investigation is also aimed to study frequency and annealed temperature dependent dielectric properties of nanocrystalline Ni2+ substituted Cu–Zn ferrite prepared by sol–gel auto-combustion technique. In this paper, we report on the synthesis process, the structural characterization, the elastic properties of as prepared and annealed temperature ferrite powder and dielectric behavior of Ni0.4Cu0.1Zn0.5Fe2O4 ferrites nano-sized powder.

96

Experimental

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Synthesis method

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Ni0.4Cu0.1Zn0.5Fe2O4 nanoferrites were prepared in a self propagating way. The precursor solution was prepared using AR grade metal nitrates of Cu(NONi(NO3)2, and Fe2(NO3)2. These nitrates were initially dissolved separately in distilled water and stirred well for 20 min at 80 °C, subsequently the precursor solution was prepared by adding all above solutions and continuously stirred for 30 min at 100 °C. An aqueous solution of citric acid mixed with metal nitrate solution then ammonia solution was slowly added to adjust the pH at 7. The mixed solution was kept on to a hot plate with continuous stirring at 100 °C. When finally all water molecules were removed from the mixture, the viscous gel began foaming. After a few minutes the gel automatically ignited and burnt with glowing flints. The auto combustion was completed within a minute, yielding brown colored ashes termed as precursor. The as prepared powders were annealed at 400 and 600 °C for 4 h to get the final product. Also the prepared ferrite mass was pressed in the form of pellets of 10 mm diameter with the help of hydraulic press by applying pressure of 60 kg/cm2 for two minutes. These pellets were sintered at 400 °C for two hours in air medium to remove binder effect.

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Characterizations

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Powder X-ray diffraction (XRD) pattern was carried out on Xray diffractometer (ModelBruker D8), with CuKa irradiation (k = 1.5405 Ǻ). The scanning step was 2°/min and scanning rate was 0.02°. 1-D detector was used for the XRD measurement. The lattice parameter, crystallite (grain) size of the prepared samples was calculated from the XRD data. Morphology, structure and crystalline size of the powder samples were studied using transmission electron microgram (TEM). V.S.M. measured at room temperature. The ultrasonic longitudinal (VL) and shear (Vs) wave velocities of all the ferrite samples were determined by the ultrasonic pulse transmission method. In this method 124.6 kHz PZT crystals and calibrated range of Interferometer were used with an accuracy of error ±5 m/s in velocity measurements. The dielectric properties such as dielectric constant (e0 ), dielectric loss (e00 ) and dielectric loss tangent (tan d) were studied as a function of frequency using LCR-Q meter (Hioki model 3532-50).

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Fig. 1. XRD pattern as prepared, annealed at 400 °C and 600 °C of Ni0.4Cu0.1Zn0.5Fe2O4 ferrite system.

conditions of the software program FullProf for the structure refinement. Crystal structure of the typical sample sintered at 600 °C was generated from the Rietveld refinement and the same is presented in Fig. 2. Fractional atomic coordinates and occupancy of cations obtained from the Rietveld refinement are presented in Table 1. These parameters were used to generate the crystal structure of the Ni0.4Cu0.1Zn0.5Fe2O4 (Fig. 2). The average particle size for each composition was calculated from the (3 1 1) plane using the Scherrer formula [18]. The value of crystallite size and lattice parameters deducted from the X-ray data are summarized in Table 2. Upon substitution of Ni2+ ions, the lattice parameter and crystallite size were found to increase with annealing temperature which is in agreement with TEM data. The observed lattice parameter and specific indices are characteristic of spinel structure which confirms the formation of cubic spinel structure in ferrite [19–21]. The values of crystalline size are shown in Table 2. These values are in the range of 8–15 nm. The crystalline size increases with an increase in temperature which is in agreement with TEM data. Figs. 3 and 4 show micrographs of TEM of nanocrystalline ferrite for as prepared and annealed samples at 600 °C. According to Fig. 4 the particle are seen to be in spherical form with crystallite size of 10–12 nm and electron diffraction pattern confirm the spinel structure of ferrite with several lines (2 2 0), (3 1 1),(4 0 0), (5 1 1) and (4 4 0) depicted from TEM data. The X-ray densities of the samples have been calculated using the relation given by Smith and Wijn [22].

dx ¼ 136

Results and discussion

137

XRD patterns have confirmed the spinel structure for all the samples. Fig. 1 shows XRD patterns of Ni0.4Cu0.1Zn0.5Fe2O4 ferrite. These diffraction peaks give the evidence of the formation of ferrite phase in all samples. The peak position and relative intensity of all diffraction peaks match well with the standard powder diffraction data [17]. The pattern shows the formation of single phase cubic spinel structure with a space group Fd3m and without any signature of secondary phase. The data were processed to realize the

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8M Na3

ð1Þ

where dx is the X-ray density, M is molecular weight of the sample, ‘a3’ is volume of unit cell, N is Avogadro’s number. These values are given in Table 2. The value of X-ray density decreases with an increase in annealed temperature, which is consistent with an increase in unit cell volume i.e an increase in lattice parameter. The porosity ‘P’ is calculated using the relation [23].

q  qa %Porosity ¼ x  100 qx

ð2Þ

Please cite this article in press as: Awati VV et al. Effect of annealing temperature on structural, magnetic, elastic and dielectric properties of Ni–Cu–Zn nano ferrites. Results Phys (2016), http://dx.doi.org/10.1016/j.rinp.2016.04.005

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Fig. 2. Spinel lattice crystal structure of Ni0.4Cu0.1Zn0.5Fe2O4 generated from Rietveld refined XRD pattern.

Table 1 Values of atomic coordinates (x, y, z) and occupancy (g) determined from Rietveld refinement of XRD pattern of Ni0.4Cu0.1Zn0.5Fe2O4. Atom

S1

Ni Cu Zn Fe Ni Cu Fe

S2

S3

x=y=z

Occ. (g)

x=y=z

Occ. (g)

x=y=z

Occ. (g)

0.1250 0.1250 0.1250 0.1250 0.5000 0.5000 0.5000

0.1885 (2) 0.0500 (2) 0.5000 (2) 0.2615 (2) 0.2115 (2) 0.0500 (1) 1.7385 (2)

0.1250 0.1250 0.1250 0.1250 0.5000 0.5000 0.5000

0.1645 0.0452 0.5000 0.2903 0.2355 0.0548 1.7097

0.1250 0.1250 0.1250 0.1250 0.5000 0.5000 0.5000

0.1423 (3) 0.0325 (2) 0.5000 (2) 0.3252 (2) 0.2577 (2) 0.0675 (1) 1.6748 (2)

(2) (1) (2) (1) (1) (1) (2)

Table 2 Lattice parameter ‘a’, crystalline size ‘t’, X-ray density ‘dx’, porosity ‘P’ and specific surface area ‘S’ of Ni0.4Cu0.1Zn0.5Fe2O4 system. Sr no.

Samples

Lattice Parameter ‘a’ ±0.002 Å

1 2 3

S1 (As prepared) S2 (400 °C) S3 (600 °C)

8.3225 8.3624 8.3962

Crystalline Size ‘t’ (nm) XRD

TEM

8 10 12

7 11 13

186

where qx is X-ray density and qa is the bulk density of nano ferrites and specific surface area ‘S’ was calculated in g/cm3 using the relation [24].

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6000 S¼ Dqm

184 185

187

ð3Þ

X-ray density ‘dx’ (gm/cm)

Porosity ‘P’ (%)

Specific surface area ‘S’ (m2/gm)

5.4926 5.4443 5.3487

23 20 18

53.93 32.27 21.12

where D is the diameter of the particle and qm is the measured density. The porosity ‘P’ is important parameter in deciding material for gas sensing applications. The values of porosity are shown in Table 2. It is observed that porosity decreases with an increase in annealing temperature due to a decrease in X-ray density. Values of specific surface area are also tabulated in Table 2, which shows

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Fig. 3. (a) TEM micrographs of as prepared samples (b) SAED images of as prepared samples.

Fig. 4. (a) TEM micrographs of sample annealed at 600 °C temperature (b) SAED images of sample annealed at 600 °C temperature.

Fig. 5. Hysteresis loop for Ni0.4Cu0.1Zn0.5Fe2O4 ferrite annealed at different temperature.

Fig. 6. Variation of saturation magnetization and coercivity with samples.

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Fig. 7. Variation of average (Vm), shear (VS) and longitudinal (VL) wave velocity Samples.

Fig. 9. Variation of Debye temperature (QE) with samples.

Fig. 8. Variation of elastic moduli longitudinal modulus (L), rigidity modulus (G), Bulk modulus (K) and Young’s modulus (E) with samples.

Fig. 10. Variation of dielectric constant with frequency for Ni0.4Cu0.1Zn0.5Fe2O4 nano ferrites.

that specific surface area (S) lies in the range from 53.93 gm/cm3 to 21.12 gm/cm3. The decrease in S is due to an increase in particle size. Magnetic properties like saturation magnetization (Ms) and coercivity (Hc) influence particle size, lattice constant, micro structure and bulk density of the ferrites [25]. V.S.M. measure at room temperature hysteresis loops for the samples annealed at different temperatures are shown in Fig. 5. From V.S.M. data to calculate the saturation magnetization (Ms) and coercivity (Hc) annealed at different temperature these values are represented in Fig. 6. It is observed that both magnetic properties is increasing with annealing temperature was attributed to improved grain size, crystallinity of samples similar results observed in NiCuZn ferrites [26,27]. Longitudinal (VL) and shear (VS) wave velocities of samples were measured at room temperature using ultrasonic pulse transmission technique [28]. The values of these velocities are shown in Fig. 7. It is observed that VL and VS increases with an increase in annealing temperature. Using these values Young’s modulus (E), Rigidity modulus (G), Bulk modulus (K), Longitudinal modulus (L)

and Poisson’s ratio were calculated [29]. The values of elastic moduli and annealing temperature are represented in Fig. 8. It can be observed from Fig. 8 that the values of Young’s modulus (E), Rigidity modulus (G), Bulk modulus (K) and longitudinal modulus (L) increase with an increase in temperature. This behavior of elastic moduli is attributed to inter atomic binding between Ni, Zn, Cu and Fe ions in material. As the annealing temperature increases the bonding force increases and results into inter molecular forces which cause to increase the elastic moduli. In the present system, the increase in the densities and elastic constants with temperature may be attributed to the increase in the grain size. The similar results are reported for other ferrites system [30]. The values of Poisson’s ratio (r) were found to be constant (r = 0.249) for all samples, a similar result is reported for other ferrites [31]. The values of Poisson’s ratio lie in the range from 1 to 0.5 which is consistent with the theory of isotropic elasticity [27]. The mean velocity was used to calculate the Debye temperature (hE):

 1 h 3qqN 3 hE ¼  Vm k 4pM

ð4Þ

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where h is Planck’s constant, k is Boltzmann’s constant, N is Avogadro’s Number, M is Molecular weight of the specimen, q is number of atoms in molecule, q is density of the sample and Vm is the mean sound velocity. The variation of Debye temperature with different composition of samples is shown in Fig. 9, which depicts that Debye temperature increases with an increase in annealing temperature. The dielectric constant (έ) was calculated using the formula.

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e0 ¼

Ct eo A

ð5Þ

where C is the capacitance of pellet, t is the thickness of pellet, A is the area of sample and eo is the permittivity in free space. Fig. 10 shows the variation of dielectric constant έ with frequency for all samples. The dielectric constant έ decreases with an increase in frequency showing the dispersion in a certain range of frequency and it remains constant for higher frequencies. As annealing temperature increases dielectric constant also increases, a similar result is obtained for other ferrite [32]. The large value of έ at lower frequency has been attributed to the effect of heterogeneity of the samples [33]. The dielectric dispersion in ferrites has been explained on the basis of Maxwell–Wagner model [34] and Koop’s [35] phenomenological theory of dielectrics. At low frequencies, έ falls rapidly for the composition having a higher value of dielectric constant which indicates the large dispersion in the composition with a large value of έ. The exchange of 3d electrons between Fe3+ and Fe2+ which are localized at the metal ions, results in local displacement of electrons in the direction of field which determines the strength of polarization. The low value of dielectric constant could be due to the synthesis method which suppresses the formation of Fe2+ ions. At a low frequency, the tan d value was observed to be large and it decreases with increasing frequency and temperature. The physical significance of tan d is the energy dissipation in the dielectric system. For higher frequency range, the loss vanishes and the dipole orientation contributes to the polarization. The conduction in ferrites is reported due to the hopping of electrons upon application of electric field. The increase of Fe2+ concentration in the sample increases the hopping probability resulting in a decrease in resistivity at higher frequencies [36]. Similar results are reported for Cu–Zn ferrites by Sattar et al. [37] and El-Syed [38] suggested the conduction mechanism could be predominantly due to hopping

Fig. 11. Variation of dielectric loss with frequency for Ni0.4Cu0.1Zn0.5Fe2O4 nano ferrites.

of electrons or ions. The variation of dielectric loss (e00 ) against logarithm of frequency at room temperature is depicted in Fig. 11 for all ferrite samples. It can be seen that, all the samples show a normal dielectric behavior with frequency where, the dielectric loss decreases exponentially with the increase in frequency.

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Conclusions

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Nano particles of polycrystalline Ni0.4Cu0.1Zn0.5Fe2O4 ferrites, with an average crystalline size between 7 and 13 nm, were synthesized through an auto combustion method. The XRD peak indicates formation of nano ferrites. The observed lattice parameter and specific indices are characteristics of spinel structure with no secondary peak. The results of XRD pattern show that the lowest annealing temperature is around 400 °C. The TEM images together with SAED pattern show that after annealing and metal substitution, the nano particles are combined. The equivalent lattice outer boundary is found elongated, pore and grain edges also deviate for samples heated at 600 °C which indicates that by distributing symmetrical planes, the nano particles are arranged in a similar oriented structure. The elastic constant increases with increasing Ni2+ content. The increase in magnitude in elastic moduli with an increase in annealing temperature suggest the strengthening of inter atomic bonding. The low value of dielectric constant is attributed to the method of synthesis which suppresses the formation of Fe2+ ion. The Ni0.4Cu0.1Zn0.5Fe2O4 nanoparticles studied in this work showed improved magnetic and electric properties as compared to that of literature reports.

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Acknowledgement

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One of the authors V. V. Awati wish to thank BCUD, University of Pune, Pune for the fund provided under Research Project (Proposal No. 11SCI00265).

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