Journal of Alloys and Compounds 645 (2015) 436–442

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Journal of Alloys and Compounds

journal homepage: www.elsevier .com/locate / ja lcom

Temperature-dependence thermoelectric power studies of mixedNi–Cu nano ferrites

http://dx.doi.org/10.1016/j.jallcom.2015.05.0410925-8388/� 2015 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +91 9246162228.E-mail address: ravindergupta28@rediffmail.com (D. Ravinder).

Rapolu Sridhar a, D. Ravinder b,⇑, K. Vijaya Kumar c

a Vignan’s Institute of Technology & Aeronautical Engineering College, Deshmukhi, Nalgonda 508284, Telangana, Indiab Department of Physics, Osmania University, Hyderabad 50001, Telangana, Indiac Department of Physics, JNTU Hyderabad College of Engineering, Nachupally (Kondagattu), Karimnagar 505501, Telangana, India

a r t i c l e i n f o

Article history:Received 1 April 2015Received in revised form 5 May 2015Accepted 7 May 2015Available online 14 May 2015

Keywords:Ni–Cu nano ferriteX-rdTEMFTIR spectraThermoelectric powerCharge carrier concentration

a b s t r a c t

Synthesized Ni–Cu nano ferrites with the compositional formula Ni1�xCuxFe2O4 (x = 0.0, 0.2, 0.4, 0.5, 0.6,0.8 and 1.0) were prepared by the citrate-gel auto combustion technique. The prepared samples werecharacterized using X-ray difractrograms (XRD), Transmission electron microscope (TEM) and Fouriertransforms infrared (FTIR). The X-ray difractrograms (XRD) clearly exhibited the existence of single phasecubic spinel structure and the crystallite size was found in the range of 36–58 nm. TEM micrographsindicated the nanostructure nature with platelet shape. FTIR absorption spectra revealed the presenceof two significant absorption bands m1 and m2 around 580 cm�1 and 410 cm�1 characteristic of spinelferrite. This confirms the formation of single phase spinel structure with two sub-lattices tetrahedral(A) site and octahedral (B) site. The thermoelectric properties were carried out by the differential methodfrom room temperature to well beyond the transition temperature. The seebeck coefficient is negative forall the compositions showing that these ferrites behave as n-type semiconductors. The transitiontemperature determined from thermoelectric power studies, it was found that the transition temperaturedecreases with increase of Cu concentration. The values of the charge carrier concentration and chargecarrier mobility have been computed from the observed values of the seebeck coefficient. Thetemperature variation of the seebeck coefficient and carrier concentration is also discussed. On the basisof these results an explanation for the conduction mechanism in Ni–Cu nano ferrites is suggested.

� 2015 Elsevier B.V. All rights reserved.

1. Introduction

Hall effect and thermoelectric power studies are widely used inthe analysis of the conduction mechanism in semiconductors.Ferrites having low resistivity and low eddy current losses havebeen found to be the most versatile to be used for technologicalapplications and these are low mobility semiconductors. In thecase of low mobility semiconductors such as ferrites, it is some-times difficult to measure the Hall effect, hence to know aboutthe conduction mechanism in ferrites thermoelectric measure-ments is the only alternative [1]. The thermo e.m.f. and its signgives suitable information about the type of conduction in semi-conductors, i.e., they are p-type or n-type. Thermoelectric effectis the direct conversion of temperature differences to electric volt-age and vice versa, and it can be used to generate electricity, mea-sure temperature or change the temperature of objects.Thermoelectric devices can be used as temperature controllers

because the direction of heating and cooling is determined bythe polarity of the applied voltage.

During the past 10 years there has been considerable interest infinding new materials and structures for use in clear, highlyefficient cooling and energy conversion systems [2–5].Nanostructural materials have significant applications for the nextgeneration thermoelectric devices. While the thermal transportproperties of bulk materials have been intensely studied, theunderstanding of nanostructure thermoelectric properties andtheir interrelation is still incomplete.

Several researchers [6,7] studied the electrical and thermalpower of the spinel ferrites and have found that they have semi-conducting properties of n or p-type. Nickel and copper substitutednickel ferrites are the important class of spinel ferrites [8] andthese are the resourceful and scientifically important soft ferritematerials because of their typical ferromagnetic properties, lowconductivity and thus lower eddy current losses, high electro-chemical stability, catalytic behavior, etc. [9–12]. Ni–Cu nanoferrites are low cost materials and have important magnetic andelectrical transport properties for technological applications.

R. Sridhar et al. / Journal of Alloys and Compounds 645 (2015) 436–442 437

The substitution of Cu brings about a structural phase transitionaccompanied by the reduction in the crystal symmetry due toco-operative Jahn–Teller effect [13,14], which ultimately modifiesthe properties of nickel ferrite which are useful in many deviceapplications. As per the present authors knowledge no informationis available on thermoelectric power studies of mixed Ni–Cu nanoferrites in the literature. Moreover, there is need for through studyof thermoelectric power studies of copper substituted nickel nanoferrites as a function of composition and temperature. Therefore asystematic study of the thermo electric power of the mixed Ni–Cunano ferrite system from room temperature to well beyond Curietemperature was undertaken. The results of such a study presentedin this communication are explained on the basis of the conductionmechanism.

2. Experimental details

2.1. Materials preparation

The Ni–Cu nano ferrite powders having the compositional formulaNi1�xCuxFe2O4 (where x = 0.0,0.2,0.4,0.5,0.6,0.8and1.0) were synthesized bycitrate-gel auto combustion technique. Analytical grade raw materials (NickelNitrate (Ni(NO3)26H2O), Copper Nitrate (Cu(NO3)23H2O), Ferric Nitrate(Fe(NO3)29H2O), Citric Acid–Citrate (C6H8O7H2O) and Ammonia (NH3)) were usedto prepare the specimens of the mixed Ni–Cu nano ferrites. Prepared metal nitratesolutions were mixed with citric acid solution in 1:3 M ratio of nitrate to citric acidand dissolved in deionized water. The pH of the solution was adjusted to 7 usingammonia. The solution was heated at 80 �C to transform into gel and then ignitedin a self-propagating combustion manner to form a fluffy loose powder. Theas-burnt ferrite powders were grained by agate motor then calcined at 700 �C for5 h. the calcined ferrite powders were again grained by agate motor. As this methodis a chemical route it requires no ball milling hence little scope of contamination,and better homogeneity. The sintered powders were mixed with 2% of PVA as a bin-der and uniaxially pressed at a pressure of about 3–5 tons cm�2 to form pellets of10 mm diameter and 2–3 mm thickness. These pellets were finally sintered at1050 �C for 12 h in a programmable furnace. Silver paste was applied on both sidesof the samples to make good ohmic contact.

2.2. Measurements

The structural characterization was carried out using X-ray DiffractometerBruker (Karlsruhe, Germany) D8 advanced system with a diffracted beammonochromatic Cu Ka radiation (k = 1.5405 Å) source between the Bragg angles20–80� in steps of 0.04�/sec. Microstructure of the calcined nanocrystalline ferritesystem has been characterized by Transmission Electron Microscope (TEM,Tecnai-12, FEI, Netherlands). The FTIR absorption spectra of synthesizednano-ferrite powders were recorded at room temperature by Fourier TransformInfrared Spectroscopy (Spectrum 100, Perkin Elmer, USA) in the range of 350–4000 cm�1 with a resolution of 1 cm�1 using KBr pellet method.

Thermo electric power measurements were made as a function of compositionand temperature by the differential method [15,16] from room temperature to wellbeyond the transition temperature. The experimental arrangement with the sampleholder consists of 2 pairs of non magnetic copper electrodes among which the sam-ple is fixed to the upper electrode for additional heating to keep a temperature dif-ference about 10 �C between the 2 faces of the sample. The temperature of bothsurfaces of the sample was calculated by using the relation.

Thermo electric power S = DE/DT

Fig. 1. X-ray diffraction pattern of mixed Ni1�xCuxFe2O4 nano ferrites.

where DE is the thermo emf produced across the sample

DT is the temperature difference.

Table 1Crystallite size and FTIR parameters of mixed Ni–Cu nano ferrites.

The charge carrier concentration was calculated using formula given by Morinand Gebella [17].

Density of charge carriers n = NA exp (�Se/K)

Sl. no. Composition Crystallite size (nm) FTIR parameters

where NA is the density of states (NA = 1022 cm�3)

m1 (cm�1) m2 (cm�2)

1 Ni Fe2 O4 58.91 569.83 414.192 Ni Cu Fe O 46.3 572.23 411.28

S is Seebeck coefficiente is charge of electron (1.6 � 10�19 Coulombs)K is Boltzmann constant (1.38 � 10�23 Joule/Kelvin).

0.8 0.2 2 4

3 Ni0.6 Cu0.4 Fe2 O4 49.2 574.42 408.554 Ni0.5 Cu0.5Fe2 O4 37.3 577.45 404.695 Ni0.4 Cu0.6Fe2 O4 36.9 577.04 407.216 Ni0.2 Cu0.8Fe2 O4 43.3 581.56 409.437 Cu Fe2 O4 36.7 584.24 404.89

From the experimental values of the electrical conductivity (r) and carrier con-centration (n), charge carrier mobility (l) was calculated by using the relation.

Charge carrier mobility l = r/ne

where e is the charge of electron.

3. Results and discussion

3.1. X-rd analysis

Fig. 1 shows the X-ray diffraction pattern of mixed Ni–Cu nanoferrite system. It can be seen from the Fig. 1 that the crystallinephases compared with PDF-4 reference data from the internationalcentre for diffraction data (ICDD). All Bragg reflections have beenindexed and all planes are the allowed planes which indicate theformation of cubic spinel structure in single phase [18]. The valuesof the crystal size vary between 36.7 nm and 58.91 nm shown inTable 1.

3.2. TEM & SEM analysis

Transmission electron microscope (TEM) micrographs ofNiFe2O4 and Ni0.2Cu0.8Fe2O4 samples are illustrated in Figs. 2aand 2b. SEM micrographs of Ni0.4Cu0.6Fe2O4 and Ni0.5Cu0.5Fe2O4

samples are illustrated in Figs. 2c and 2d. The micrographs indi-cated the nanostructure nature with platelet shape and the crystal-lite size is in nanometer.

3.3. FTIR spectra analysis

The FTIR spectra of the Ni1�xCuxFe2O4 (x = 0.0,0.2,0.4,0.5,0.6,0.8,1.0) synthesis by Citrate-gel auto combustion method areshown in Fig. 3. The spectra of all the ferrites have been used tolocate the band positions which are given in Table 1. In the presentstudy the absorption bands m1 and m2 are found to be around580 cm�1 and 410 cm�1 for all the compositions which arises dueto vibration of ions in the crystal lattices [19]. This reveals the

Fig. 2c. SEM images of Ni0.4Cu0.6Fe2O4 nano ferrite.

Fig. 2d. SEM images of Ni0.5Cu0.5Fe2O4 nano ferrite.

Fig. 3. FTIR patterns of mixed Ni1�xCuxFe2O4 nano ferrites.

Fig. 2a. TEM image of NiFe2O4 nano ferrite.

Fig. 2b. TEM images of Ni0.2Cu0.8Fe2O4 nano ferrite.

438 R. Sridhar et al. / Journal of Alloys and Compounds 645 (2015) 436–442

formation of single-phase spinel structure having two sub-latticestetrahedral (A) site and octahedral (B) site [20]. There is no absorp-tion band in spectra for all samples above frequency bands600 cm�1. This is due to complete elimination of water contentand amorphous nature. The high frequency band (m1) corresponds

to stretching vibration mode of Fe+3–O�2 in tetrahedral A-site,while the low frequency band (m2) corresponds to Me+2–O�2 vibra-tions in octahedral sites. The variation in the band positions is dueto the difference in the Fe+3–O�2distances for the octahedral andtetrahedral complexes [21].

3.4. Composition dependence of seebeck coefficient (S)

The values of the seebeck coefficient of all mixed Ni–Cu nanoferrites at room temperature were calculated from the observedvalues of the thermo emf and are given in Table 2. It can beseen from the Table 2 that the value of seebeck coefficient isnegative, indicating the majority charge carriers are electronshence the predominant conduction mechanism in these ferritesis of n-type semiconductors. The seebeck coefficient value varyfrom �150 lV/K to �40 lV/K as the copper content increasesin nickel nano ferrite as shown in Fig. 4. Similar behavior ofvariation of seebeck coefficient with composition was observedin Ni–Mg ferrites [22]. The decrease in seebeck coefficient withCu concentration explained by hopping mechanism betweenFe2+ and Fe3+ ions [23].

Fe3þ þ e� ! Fe2þ

Fig. 4. Seebeck coefficient variation with composition.

Table 2Thermoelectric power data of mixed Ni–Cu nano ferrites.

Sl. no. Composition Seebeck coefficient(S) (lV/K)

Charge carrierconcentration (n) (1022/m3)

Charge carriermobility (l) (cm2/V s)

Transitiontemperature (K)

Curie temp (K)

TS Tn Tq Tg

1 Ni Fe2 O4 �150 11.902 4.2175 � 10�09 795.40 798.25 812.08 8082 Ni0.8Cu0.2 Fe2O4 �130 11.629 2.8358 � 10�08 750.54 747.60 738.22 7363 Ni0.6Cu0.4 Fe2O4 �110 11.362 3.5024 � 10�08 730.89 737.44 730.19 7264 Ni0.5Cu0.5Fe2O4 �100 11.231 3.7437 � 10�08 689.45 693.70 688.42 6865 Ni0.4Cu0.6Fe2O4 �60 10.721 4.7037 � 10�08 680.60 678.49 676.59 6746 Ni0.2Cu0.8Fe2O4 �50 10.598 1.1914 � 10�07 664.80 668.56 665.42 6687 Cu Fe2 O4 �40 10.475 2.8095 � 10�07 645.46 639.72 646.20 648

R. Sridhar et al. / Journal of Alloys and Compounds 645 (2015) 436–442 439

3.5. Temperature dependence of the thermoelectric power (S)

Fig. 5(a)–(g) shows the variation of the thermoelectric power(S)of the Ni1�xCuxFe2O4 (x = 0.0,0.2,0.4,0.5,0.6,0.8,1.0) nano ferriteswith temperature. It can be seen that the value of thermoelectricpower for all mixed Ni–Cu nano ferrites increases with increasingtemperature up to a certain temperature, which is designed astransition temperature Ts (Kelvin). However, beyond this tempera-ture the value of S starts decreasing with increasing temperatureand the thermoelectric power found as negative for entire temper-ature range as been investigated. A similar variation of thermoelec-tric power with temperature was observed [24–26].

The seebeck coefficient variation with temperature can beexplained as, the hot surface of the material becomes positivelycharged, as it loses some of its electrons. The cold surface of thematerial becomes negatively charged due to the diffusion of freeelectrons from the hot portion. On increasing the temperature,the conduction mechanism becomes more probable that generateselectrons Fe2+, Fe3+ + e�. These electrons accumulate on the coldportion, as a result, thermo emf increases hence, increases theseebeck coefficient. After the transition temperature decrease inseebeck coefficient may be due to filling up of oxygen vacanciesand migration of ions from one site to other thereby reducing theconcentration of mobile electrons [27].

The seebeck coefficient transition temperature Ts (Kelvin)values for each composition are listed in Table 2. Fig. 6 showsthe variation of seebeck coefficient transition temperature withcomposition. The measured transition temperature Ts (Kelvin) areclose and good agreement with curie temperature Tc (Kelvin) val-ues from dc resistivity and loria technique which are reported

[28], thereby indicating that the change in the behavior of thethermoelectric power with temperature may be due to magnetictransition, where the material becomes the paramagnetic. Thus,it is clear that incase of Ni–Cu nano ferrites the thermoelectricpower is exhibiting a clear-cut and well-defined transition at theCurie temperature. The value of thermoelectric power is maximumat Ts (Kelvin). This clearly indicates that the magnetic ordering hasa marked influence on the thermoelectric property of these ferritesamples.

3.6. Composition dependence of the charge carrier concentration (n)and the charge carrier mobility (l)

The computed values of charge carrier concentration and thecharge carrier mobility values for the different composition ofmixed Ni–Cu nano ferrites are included in Table 2. It can be seenthat the value of the carrier concentration vary from11.902 � 1022 to 10.475 � 1022 m�3 and the charge carrier mobilityvary from 4.2175 � 10�09 to 2.8095 � 10�07 cm2/V s. Such low val-ues of mobility have been reported by several researches [29,30].Among all the mixed Ni–Cu nano ferrites, the specimen with theNiFe2O4 exhibits height value of seebeck coefficient and charge car-rier concentration. It can also be seen from the Table 2 that thevalue of charge carrier mobility increases with the increases Cuconcentration. Fig. 7 shows the variation of charge carrier concen-tration and charge carrier mobility with composition.

3.7. Temperature variation of the charge carrier concentration (n)

The temperature dependent charge carrier concentration isshown in Fig. 8. It can be seen from the figure that the charge car-rier concentration increases with increase in temperature up to acertain temperature, which is designed as Tn (Kelvin). However,beyond this temperature the value of charge carrier concentrationstarts decreasing with increasing temperature for all the ferrites. Itcan be seen from Table 2 the values of transition temperature areby different technique are in good agreement obtained. Similarvariations of Tn and Tc were also observed by other researchers[31,32].

4. Conclusions

Homogeneous and reactive nanostructured Ni–Cu nano ferriteswere prepared by citrate-gel auto combustion method. X-raydiffraction patterns shows the formation of the single phased cubicspinel structure without any impurity peak and crystallite size varyin the range of 36–58 nm are obtained. TEM micrographs indicatedthe nanostructure nature with platelet shape. FTIR absorptionspectra revealed the presence of two significant absorption bandsm1 and m2 around 580 cm�1 and 410 cm�1 characteristic of spinelferrite, this confirms the formation of single phase spinel structure

Fig. 5. (a–g) Temperature dependence thermoelectric power of mixed Ni–Cu nano ferrites.

440 R. Sridhar et al. / Journal of Alloys and Compounds 645 (2015) 436–442

Fig. 6. Seebeck and carrier concentration transition temperature variation withcomposition.

Fig. 7. Charge carrier concentration and charge carrier mobility variation withcomposition.

Fig. 8. Temperature dependent charge carrier concentration of Ni1�xCuxFe2O4 nanoferrites.

R. Sridhar et al. / Journal of Alloys and Compounds 645 (2015) 436–442 441

with two sub-lattices tetrahedral (A) site and octahedral (B) site. Inthe present investigation Ni–Cu nano ferrites are classified asn-type semiconductors. The decrease in seebeck coefficient wasdue to increase in resistivity for that particular composition.Among all the nano ferrites the NiFe2O4 exhibits height value ofseebeck coefficient and charge carrier concentration. The value of

charge carrier mobility increases with the increases Cu concentra-tion. The values of transition temperature are by different tech-nique are in good agreement obtained.

Acknowledgements

One of the authors (D.R.) is grateful to Prof. Sayanna, Head ofthe department for his encouragement to carry out this researchwork. One of the authors (R.S.) is grateful to Prof. N.Venkateswarlu, principal, Vignan’s Institute of Technology &Aeronautical Engineering College, Hyderabad. The author (K.V.K.)is grateful to Dr. M. Thirumala Chary, Principal, Professor & HODof Chemistry, JNTUH College of Engineering, Nachupally,Karimnagar (Dist).

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