This content has been downloaded from IOPscience. Please scroll down to see the full text. Download details: IP Address: 130.237.122.245 This content was downloaded on 20/06/2014 at 02:04 Please note that terms and conditions apply. Structural and dielectric properties of Cr-doped Ni–Zn nanoferrites View the table of contents for this issue, or go to the journal homepage for more 2011 Phys. Scr. 83 025602 (http://iopscience.iop.org/1402-4896/83/2/025602) Home Search Collections Journals About Contact us My IOPscience
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Structural and dielectric properties of Cr-doped Ni–Zn nanoferrites
View the table of contents for this issue, or go to the journal homepage for more
2011 Phys. Scr. 83 025602
(http://iopscience.iop.org/1402-4896/83/2/025602)
Home Search Collections Journals About Contact us My IOPscience
Received 11 July 2010Accepted for publication 15 December 2010Published 17 January 2011Online at stacks.iop.org/PhysScr/83/025602
AbstractCr-doped Ni–Zn ferrite nanoparticles having the general formula Ni0.5Zn0.5Crx Fe2−x O4
(x = 0.1, 0.3, 0.5) were prepared by the simplified sol–gel method. The structural anddielectric properties of the samples sintered at 750 ± 5 ◦C were studied. X-ray diffraction(XRD) patterns confirm the single-phase spinel structure of the prepared samples. Thecrystallite size calculated from the most intense peak (3 1 1) using the Debye–Scherrerformula was 29–34 nm. Scanning electron microscope images showed that the particle size ofthe samples lies in the nanometer regime. The dielectric constant (εr), dielectric loss tangent(tan δ) and ac electrical conductivity (σac) of nanocrystalline Cr–Ni–Zn ferrites wereinvestigated as a function of frequency and Cr concentration. The dependence of εr, tan δ
and σac on the frequency of alternating applied electric field is in accordance with theMaxwell–Wagner model. The effect of Cr doping on the dielectric and electric propertieswas explained on the basis of cations distribution in the crystal structure.
Nanotechnology has emerged as the most promising fieldfrom the scientific and technological applications pointof view. Nanomaterials exhibit special properties that aredifferent from their bulk counterparts [1]. The propertiesof nanostructured materials are deeply influenced by thechemical composition and microstructure of the materials,which are sensitive to the manufacturing process [2].Nanoparticles of magnetic ferrites have attracted a great dealof research interest because of their applications in permanentmagnets, drug delivery, microwave devices, ferro-fluids andhigh-density information storage [3, 4]. Spinel ferrites havethe general formula AFe2O4 (where A2+
= Fe, Co, Ni, Zn,Cr, Mn, etc) and the unit cell contains 32 oxygen atoms incubic close packing with 8 tetrahedral (Td) and 16 octahedral(Oh) occupied sites. By changing the type of divalent cation,it is possible to obtain a wide range of different physical andmagnetic properties [5].
Cr–Ni–Zn ferrite is a mixed spinel in which tetrahedral,Td (A), sites are occupied by Zn2+ and Fe3+ ions and theoctahedral, Oh (B), sites are occupied by Ni2+ and Fe3+ inthe cubic spinel lattice [6]. Ni–Zn bulk ferrites, having high
electrical resistivity, are the only materials of choice usedfor high-frequency applications. The performance of thesematerials is limited to a few MHz owing to the generationof eddy current losses at high frequencies. The frequencyof operation of these ferrites could be increased to GHz ifthe electrical resistivity of these materials is increased to agreater level keeping its saturation magnetization high [7].Higher values of electrical resistivity could be achievedby doping with proper divalent cations or by controllingtheir microstructures. Higher values of electrical resistivitycould be achieved by processing the material in the formof ultrafine particles. The ultrafine particles would providea large number of grain boundaries as barriers for electronflow and hence a reduction in the eddy current losses [7].Various methods, such as solid state reaction, sol–gel [8, 9],co-precipitation [10], sonochemical preparation [11],citrate precursor techniques [12], microemulsion and ballmilling [10], could be used to fabricate nickel-based ferritenanoparticles. Selection of an appropriate process is a keyfactor to obtain ferrites with high quality, especially for thosewith low losses at high frequency [7].
In this paper, we report the synthesis and characterizationof Cr–Ni–Zn ferrite nanoparticles prepared by a new sol–gel
method. The new sol–gel method is named WOWS. WOWSstands for sol–gel without water and surfactants, which isdiscussed in detail in the next section. Water and surfactantsare avoided in this method for better purity. In other sol–gelmethods, NaOH or HNO3 is used for pH adjustment,which may appear as an impurity in the final product. pHadjustment is not needed in this method, which furtherimproves the purity of the prepared samples. To the best ofour knowledge, this sol–gel method is reported here for thefirst time.
2. Experimental procedure
2.1. Syntheses
In the WOWS sol–gel method, stoichiometric amountsof Fe(NO3).9H2O, Zn(NO3).6H2O, Cr(NO3)3.9H2O andN2NiO6.6H2O are dissolved in ethylene glycol. Better purity,better crystallinity, better morphology, ease of handling andreduced time consumption are the main advantages of thismethod. The molar ratio of ethylene glycol to the metalsalts dissolved was kept at 14 : 1 so that the salts dissolveduniformly. The solution was stirred for 30 min at roomtemperature to obtain a uniform solution. The temperature ofthe solution was then increased above 100 ◦C with continuousstirring until a thick gel was formed. The temperature of thegel was raised to 300 ◦C. The thick gel dried and burnedslowly into a fine powder.
Pellets of 13 mm diameter and 1 mm thickness wereprepared using a hydraulic press. For pellet formation, a loadof 75 kg cm−2 was applied for 5 min. Nanocrystalline powdersof Ni0.5Zn0.5Crx Fe2−x O4 (0.1, 0.3, 0.5) were pelletized andsintered at 750 ± 5 ◦C for 2 h and named accordingly as Cr-1,Cr-3 and Cr-5.
2.2. Characterization
X-ray diffraction (XRD) patterns of all the prepared sampleswere taken by a Panalytical x-ray diffractometer using Cu-Kα
radiation. The crystallite size of the samples was calculatedusing the Deby–Scherrer formula [13]
t =0.9λ
β cos θB, (1)
where t is the crystallite size, λ is the wavelength of incidentx-ray, θB is the diffraction angle and β is the full-width athalf-maximum (FWHM).
The lattice constant a for the cubic crystal system wascalculated using the equation [14]
a = d√
h2+k2+l2, (2)
where hkl are the Miller indices of the diffraction peak and dis the interplanar spacing.
The porosity (P) of the samples was determined using theformula [14]
P = 1 −dm
dx, (3)
20 30 40 50 60 70
Cr-1
Cr-3
Inte
nsi
ty (
a.u
.)
2θ degrees
(220
)
(311
)(2
22)
(400
)
(422
)
(511
)
(440
)
Cr-5
Figure 1. XRD patterns of Ni0.5Zn0.5Crx Fe2−x O4 (x = 0.1, 0.3, 0.5)sintered at 750 ◦C.
Table 1. The crystallite size (D(3 1 1)), lattice constant (a), latticevolume (V), theoretical density (dx ), measured density (dm) andporosity (P) of Ni0.5Zn0.5Crx Fe2−x O4 nanoferrites.
where dm and dx are the measured density and theoreticaldensity, respectively. The measured density was calculatedusing the relation
dm =m
πr2h, (4)
where h is the height, r is the radius and m is the mass of acylindrical pellet of the sample. The theoretical density wascalculated by using the formula
dx =8M
N V. (5)
Here, 8 is the number of formula units in a unit cell, N isAvogadro’s number, M is the molecular weight of one formulaunit and V is the volume of the unit cell.
The dielectric constant of the prepared samples wasmeasured in the frequency range 100 Hz–3 MHz using aprecession component analyzer by the capacitance method.The dielectric constant (εr) was calculated by using therelation [15]
εr =Cd
ε0 A, (6)
where d is the thickness, C is the capacitance, ε0 is thepermittivity of free space and A is the cross-sectional area ofthe pellet.
The dielectric loss tangent (tan δ) of the prepared sampleswas measured using the relation
tan δ =ε′′
ε′, (7)
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Phys. Scr. 83 (2011) 025602 S Nasir et al
Figure 2. SEM micrographs of Ni0.5Zn0.5Crx Fe2−x O4 (x = 0.1, 0.3, 0.5) sintered at 750 ◦C.
where ε′′ is the imaginary part and ε′ is the real part of thedielectric constant.
The ac conductivity in the frequency range100 Hz–3 MHz was determined using the values of dielectricconstant (εr) and dielectric loss tangent (tan δ) in therelation [16]
σac = ωε0εr tan δ, (8)
where σac is the ac conductivity and ω is the angularfrequency.
3. Results and discussion
3.1. Structural properties
The XRD patterns of the prepared samples shown in figure 1show the formation of single-phase spinel structure. Allthe peaks of the cubic crystal system corresponding tothe space group Fd-3m were indexed with the standardpattern for Ni0.5Zn0.5CrFeO4 reported in ICDD PDF cardno. 00-043–0555. The calculated values of the crystallitesize, lattice parameter, mass density, theoretical density andporosity of Ni–Zn–Cr ferrite samples are presented in table 1.The crystallite size of the samples was in the range 29–34 nm.
Scanning electron microscope (SEM) images of all theprepared samples are presented in figure 2. They indicate thatthe particle size of the samples lies in the nanometer regimehaving a spherical shape and a narrow size distribution.
3.2. Dielectric properties
The dielectric constant of all samples measured in thefrequency range 100 Hz–3 MHz is shown in figure 3.The dielectric constant εr of all the samples decreasedwith increasing frequency. The decrease of εr is rapid atlower frequencies and showed almost frequency-independentbehavior at higher frequencies. The observed variation in εr
is in accordance with the Maxwell–Wagner model [17, 18].The space-charge polarization in ferrites is due to theirinhomogeneous structure. The inhomogeneous structure offerrites consists of conducting grains with poorly conductinggrain boundaries [19]. Also, the decrease of dielectric constantwith increasing frequency can be attributed to the electronexchange between Fe2+ and Fe3+ ions [20]. The decreaseof dielectric constant with frequency is as can be expectedbecause any species contributing to polarizability is found to
4 6 8 10 12 14 16
0
100
200
300
ln f (Hz)
Die
lect
ric
Co
nst
ant
( ε r ) Cr-1
Cr-3 Cr-5
Figure 3. Dielectric constant (εr) as a function of frequency forNi0.5Zn0.5Crx Fe2−x O4 spinel ferrites.
show the behavior of lagging behind the applied field at higherand higher frequencies [4].
Figure 4 shows that the dielectric constant also dependson Cr concentration in Ni–Zn ferrites. The space-chargepolarization in ferrites is governed by the number ofspace-charge carriers in the material by hopping exchange ofthese charges between two localized states [19]. Iwauchi [20]has pointed that there is a strong correlation between thedielectric behavior and the conduction mechanism in ferrites.
The conduction mechanism in ferrites is a result of theelectrons and holes hopping between ions of the same elementexisting in different valence states on octahedral sites [7, 21].The dielectric constant of Cr-1 sample is maximum, whichthen decreases with increasing Cr concentration. Polarizationin ferrites is through a mechanism similar to the conductionprocess by electron exchange between Fe2+ and Fe3+; the localdisplacement of electrons in the direction of the applied fieldoccurs and these electrons determine the polarization [4].
In the case of Ni–Zn ferrites, the cations of Zn have A-sitepreference, whereas those of Ni and Cr ions prefer B-site. Fe2+
occupy B-site, whereas Fe3+ occupy both A- and B-site.The increase of Cr3+ concentration results in the
replacement of Fe3+ ions on the octahedral site, whichresults in a decrease in electrons hopping between Fe2+ andFe3+ ions. For high-frequency applications, low dielectricconstant materials are preferred because of dielectric loss andskin effect. At high frequency, the dielectric loss becomes
3
Phys. Scr. 83 (2011) 025602 S Nasir et al
0.1 0.3 0.520
40
60
80
100
120
Die
lect
ric
Co
nst
ant
(ε r )
Cr Concentration (x)
Figure 4. Dielectric constant (εr) of Ni0.5Zn0.5Crx Fe2−x O4 at afrequency of 1 kHz as a function of Cr concentration.
4 6 8 10 12 14 16
0.0
0.5
1.0
1.5
2.0
2.5
tan
δ
ln f (Hz)
Cr-1 Cr-3 Cr-5
Figure 5. Dielectric loss tangent (tan δ) as a function of frequencyfor Ni0.5Zn0.5Crx Fe2−x O4 spinel ferrites.
dominant; hence, the lower the dielectric constant, the lowerthe dielectric loss.
The dielectric loss tangent (tan δ) as a function offrequency was studied at room temperature. A plot ofthe dielectric loss tangent against frequency is depictedin figure 5. Again the dielectric loss tangent decreaseswith increasing frequency for each sample. All the samplesexhibit dispersion due to Maxwell–Wagner interfacial-typepolarization [17–19].
The values of tan δ depend on a number of factors suchas stoichiometry, Fe2+ content and structural homogeneity,which in turn depend on the composition and sinteringtemperature of the samples [22]. Figure 5 shows that thedielectric loss tangent of the prepared samples also depends oncomposition. In the present case, the decrease of the dielectricconstant of the samples with increasing Cr concentrationresulted in a decrease of the dielectric loss.
3.3. The ac electrical conductivity
The ac electrical conductivity (σac) of all the prepared samplesis shown in figure 6. The variation of σac as a function offrequency is in accordance with the Maxwell–Wagner modelas elaborated above [17, 18].
4 6 8 10 12 14 16
-16
-15
-14
-13
-12
-11
-10
-9
ln f (Hz)
ln σ
ac (σ a
c in
S /
m)
Cr-1 Cr-3 Cr-5
Figure 6. The ac conductivity (σac) of Ni0.5Zn0.5Crx Fe2−x O4 spinelferrites as a function of frequency.
Table 2. The dielectric constant (εr), dielectric loss tangent (tan δ)and ac electrical conductivity (σac) of Ni0.5Zn0.5Crx Fe2−x O4 ferritesas a function of cobalt concentration.
Parameter Cr-1 Cr-3 Cr-5
εr at 10 kHz 37.13 20.16 17.17εr at 1 MHz 18.83 6.68 13.79tan δ at 10 kHz 0.85 0.74 0.30tan δ at 1 MHz 0.15 0.25 0.19σac (µS m−1) at 10 kHz 2.79 1.32 0.456σac (µS m−1) at 1 MHz 25.0 14.8 23.2
The variation of ac electrical conductivity, dielectricconstant and dielectric loss factor at 10 kHz and 1 MHz forall the prepared samples is presented in table 2. The decreaseof the ac conductivity of the Ni–Zn samples with increasingCr concentration can be explained on the basis of the cationdistribution as mentioned above.
4. Conclusions
Ni–Zn–Cr ferrite nanoparticles having the general formulaNi0.5Zn0.5Crx Fe2−x O4 (x = 0.1, 0.3, 0.5) were preparedsuccessfully by the WOWS sol–gel method and sinteredat 750 ◦C for 2 h. XRD results show the formation ofsingle-phase cubic spinel structure. SEM images show theformation of nanoparticles with a spherical shape and anarrow size distribution. The particles were in the nmsize range even after sintering at 750 ◦C. Better purity,better crystallinity, better morphology, ease of handling andreduced time consumption were the main advantages ofthis method. The dependence of the dielectric propertiesand ac electrical conductivity of the prepared Ni–Zn–Crnanoferrites on the frequency of alternating applied fieldis in accordance with the Maxwell–Wagner model and theKoops phenomenological theory. The decrease of the valuesof the dielectric constant, dielectric loss tangent and acelectrical conductivity of nanocrystalline Ni–Zn–Cr ferriteswith increasing Cr concentration can be explained on thebasis of the cation distribution in the crystal structure. Thelower dielectric constant (εr) and dielectric loss tangent (tan δ)
4
Phys. Scr. 83 (2011) 025602 S Nasir et al
of these samples make them suitable materials for use inhigh-frequency applications.
Acknowledgment
We acknowledge the Higher Education Commission (HEC),Islamabad, Pakistan for providing financial support for thiswork through NRPU no. 893 and the Indigenous 5000Scholarships Program.
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