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Journal of Physics and Chemistry of Solids 70 (2009) 340–343

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Journal of Physics and Chemistry of Solids

0022-36

doi:10.1

� Corr

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journal homepage: www.elsevier.com/locate/jpcs

Preparation, characterization and ionic conductivity studies of ZrO2 dispersedmixed halide matrix (KCl)0.9–(NaCl)0.1

Archana Gupta a, Anjan Sil b,�, N.K. Verma a

a School of Physics and Materials Science, Thapar Institute of Engineering and Technology, Patiala 147004, Punjab, Indiab Department of Metallurgical and Materials Engineering, Indian Institute of Technology Roorkee, Roorkee 247667, Uttarakhand, India

a r t i c l e i n f o

Article history:

Received 16 January 2008

Received in revised form

13 June 2008

Accepted 31 October 2008

Keywords:

C. X-ray diffraction

C. Thermogravimetric analysis(TGA)

D. Electrical conductivity

D. Microstructure

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016/j.jpcs.2008.10.029

esponding author. Tel.: +911332 285073; fax

ail addresses: asil1fmt@iitr.ernet.in, anj_sil@y

a b s t r a c t

Composite electrolytes in the system [(KCl)0.9:(NaCl)0.1]1�y:(ZrO2)y were prepared and their ionic

conductivities were studied. In our previous study on the mixed halide system (KCl)1�x:(NaCl)x,

maximum conductivity (�50 times that of the base KCl matrix) was found when x ¼ 0.1. The matrix

(KCl)0.9:(NaCl)0.1 was dispersed with different concentration of ZrO2 (powder) for the preparation

of composites and their conductivities were determined. The maximum conductivity was developed

for the composite having composition y ¼ 0.5. The matrices were prepared by melt-quench technique

and the dispersion of ZrO2 was carried out in liquid medium. The conductivity measurements

of the composites were carried out by impedance spectroscopy technique. The composite

[(KCl)0.9:(NaCl)0.1]0.5:(ZrO2)0.5 was characterized by X-ray diffraction (XRD) analysis, differential thermal

analysis (DTA), thermogravimetric analysis (TG) and scanning electron microscopy (SEM). The

conductivity of the composite [(KCl)0.9:(NaCl)0.1]0.5:(ZrO2)0.5 as a function of temperature was

also studied. The conductivity increase in the composite could be attributed to enhancement of

defect concentration in the space charge region created at the interface between the host halide and

the dispersoid.

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1. Introduction

The dispersion with fine insulating particles such as ofSnO2, Al2O3, SiO2, etc. into normal ionic conductors such asalkali-, silver- and copper-halides is a well-known method toenhance extrinsic ionic conductivity [1–3]. Such heterogeneouslydoped materials are called composite solid electrolytes ordispersed solid electrolytes. These solid electrolytes show im-mense technological promise, especially in the development ofsolid-state electrochemical devices such as solid-state powersources, sensors, fuel cells, electrochromic display devices andmemory devices, etc. [4,5].

Conductivity enhancement in two-phase composite electrolytesystems has been known for about 75 years. However, theresearch activity in this area gained impetus only after 1973,when C.C. Liang [6] reported an enhancement of �50 times in Li+

ion conduction at room temperature by dispersing ultra-fineparticles of inert Al2O3 in LiI. A large number of two-phasecomposite systems, have, so far, been investigated and theconductivity enhancement by one to three orders of magnitudewith respect to the conductivity of respective matrices has been

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found [7–9]. Several phenomenological theories were proposed tounderstand the ion transport mechanism in composite electrolytesystems [10–12]. There is no single unified model that can explainvarious experimental results on different composite electrolytesystems. However, the central feature of the majority of themodels to explain the conductivity enhancement is that there isan existence of a space charge region at the interface between thehost and dispersoid. The possible causes of the conductivityenhancement are (i) the formation of a new kinetic path via a thininterphase layer along the interface between dispersoid andhalide matrix, (ii) development of an enhanced ionic mobility dueto higher dimensional defects, which form as a result of thedispersoid–halide interaction (e.g. strain effects) and (iii) ionicconcentration enhancement due to space charges either at theinterface region, at around dislocations present or due tohomogeneous bulk doping (e.g. impurity effect, charge carrierinjection). N. Sata et al. [13] reported the preparation ofheterolayered films composed of CaF2 and BaF2 by molecularbeam epitaxy. The films exhibit increase in ionic conductivity(parallel to the interfaces) proportionally with interface densityfor interfacial spacing greater than 50 nm. Their results werefound in excellent agreement with semi-infinite space chargecalculations. S. Azad et al. [14] reported ionic conductivityimprovement in gadolinia-doped ceria- and zirconia-layeredstructures compared to individual bulk electrolytes. The increase

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A. Gupta et al. / Journal of Physics and Chemistry of Solids 70 (2009) 340–343 341

in conductivity in these systems was perhaps due to strainenhancement of either dopant solubility or oxygen vacancymobility [14].

The dispersion of oxides is not the only way to enhance theconductivity of the composite solid electrolytes. The othertechniques also very effective for conductivity enhancement [15]are (i) addition of aliovalent impurities [16–18], (ii) substitution ofhost ion with larger ion to open the lattice structure [19,20] and(iii) control on preparative parameters like the applied load forpelletization, sintering temperature and soaking time period, etc.

The present article reports the conductivity studies on KCl-based composite solid electrolytes. To the best of authors’knowledge, a few investigations on conductivity behaviourof KCl-based composite solid electrolytes were reported. Theprimary objective for choosing KCl as the host matrix is that thematerial does not undergo phase transformation before melting,adequately giving a scope to study the material characteristicsover a wide range of temperatures. The modification of the matrixby way of mixed halide matrix was considered in view of the sizedifference between K+ and Na+ cations [21]. In the present study,the composite solid electrolytes were prepared by dispersing theZrO2 powder in the mixed halide matrix (KCl)0.9:(NaCl)0.1 havinghighest conductivity in the system (KCl)1�x:(NaCl)x.

Fig. 1. The SEM micrograph of composite electrolyte [(KCl)0.9:(NaCl)0.1]0.5:

(ZrO2)0.5.

2. Experimental procedure

The samples were prepared using the raw materials KCl(purity 4 99%, E. Merck), NaCl (purity 4 99%, Qualigens) andZrO2 (purity 4 99%, C.S.Zircon, particle size 1mm). The homo-geneous mixture of KCl and NaCl powders in 90:10 mol% wasmelted in the temperature range of 600–800 1C and the melt wasthen quenched to room temperature into solid mass. The solidmass was further grounded to powder. The oxide dispersion intothe mixed halide matrix powder, was carried out in a liquid(acetone) medium and the mixture was dried. The dried mixturewas compacted at a pressure level of about 10 tons in cylindricalpellets of 20 mm in diameter and 3 mm thick. The samples weresintered for 5 h at a temperature between 450 and 650 1C.

The conductivity measurements show that the compositeshaving composition [(KCl)0.9:(NaCl)0.1]0.5:(ZrO2)0.5 has the highestconductivity among all the compositions prepared in this system.The composite [(KCl)0.9:(NaCl)0.1]0.5:(ZrO2)0.5 was characterizedby XRD to detect the possibility of any new phase(s) formationduring synthesis. The X-ray diffraction (XRD) patterns at roomtemperature were recorded by an X-ray diffractometer (Model PW1140/09) using CuKa radiation. The sample was also subjected tothermal analyses (DTA and TG) by differential thermal analyser(Perkin-Elmer, Model:Pyris-Diamond) for further confirmation ofany chemical compound formation. The thermal analyses werecarried out in the temperature range of ambient to 1000 1C with ascanning rate of 5 1C/min.

In order to study the microstructure of the samplesand to examine the dispersion of ZrO2 particles into the mixedhalide matrix leading to the formation of interface betweenthe oxide and the halide, microstructural investigation of[(KCl)0.9:(NaCl)0.1]1�y:(ZrO2)y sample was done using a scanningelectron microscope (Model LEO 435 VP).

For the conductivity measurements the samples were elec-troded with conducting silver paste and the measurements wereperformed by impedance spectroscopy technique using LCR meter(HP, Model 4274A) in the frequency range of 100 Hz–100 kHz. Thebulk resistance (dc resistance) of the samples were obtained fromthe intercepts of complex impedance plot in semicircular natureon the real axis (resistance) and hence the dc conductivity valueswere calculated using the sample dimensions. The conductivity

behaviour of this composite electrolyte was studied as a functionof ZrO2 content. The activation energy (Q) for ionic transport inthe composite electrolyte having y ¼ 0.5 was determined fromthe conductivity measurements as a function of temperature(i.e. logs vs. 1/T).

3. Results and discussion

3.1. Scanning electron microscopy

The microstructural investigations of composite electrolytein the system [(KCl)0.9:(NaCl)0.1]1�y:(ZrO2)y were carried out and amicrograph of [(KCl)0.9:(NaCl)0.1]0.5:(ZrO2)0.5 composition is pre-sented in Fig. 1, which shows that the grains are interspersed withthe ZrO2 particles. The development of interfaces can also bepredicted qualitatively from the micrograph (Fig. 1). However,the existence of oxide as a separate phase was noticed as acommon feature.

3.2. XRD analyses

X-ray diffraction analyses of (KCl)0.9:(NaCl)0.1 matrix and[(KCl)0.9:(NaCl)0.1]0.5:(ZrO2)0.5 composite samples were carriedout and the patterns are given in Fig. 2(a) and (b). The intensepeaks in the XRD patterns of the chemical constituents KCl, NaCland ZrO2 lie in the angular (2y) range of 271 and 751, the patternsin the Fig. 2, are presented for this specified range. The patternswere analysed and the peaks were identified using JCPDS data file.It can be seen from the Fig. 2(a), that besides a very few minorpeaks of NaCl, majority of the peaks belong to KCl, indicating thatthe lattice structure of the mixed halide matrix is primarily of KCltype. The XRD pattern of the composite solid electrolyte inFig. 2(b), indicates that ZrO2 is present in the material as aseparate phase and no additional peaks showing a new phaseformation were observed. Therefore, there is no substantialevidence for new compound formation either in the matrix or incomposites prepared. It can be seen from the patterns in Fig. 2(a)and (b), that for composite, the peaks of KCl have lower intensitiesthan that in mixed matrix. The cause of lower intensity may beattributed to the lower proportion (mole fraction) of the matrixphase in the composite as compared to pure matrix.

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3.3. Thermal analysis

The thermal analysis of the sample having composition of[(KCl)0.9:(NaCl)0.1]0.5:(ZrO2)0.5 was carried out using TG and DTAtechniques. Traces of TG, DTG and DTA as a function of temperatureare shown in Fig. 3, which shows that there is no significant loss ofmass upto the temperature of about 742 1C and this is supported by

Fig. 2. The X-ray diffraction patterns for (a) mixed halide matrix (KCl)0.9:(NaCl)0.1

and (b) composite electrolyte [(KCl)0.9:(NaCl)0.1]0.5:[ZrO2]0.5 sintered samples.

Fig. 3. (a) TG, (b) DTG and (c) DTA traces for the com

the endothermic event (though small) in the DTA trace. The thermalevent around 742 1C is an indication of beginning of the matrixmelting. It may be seen from the phase diagram of KCl–NaCl system[22] given in Fig. 4, that the melting of the material (KCl)0.9:(NaCl)0.1

occurs at around 742 1C. Thereafter, the loss of mass occurs steadilytill the end of the temperature range. It can be further noticed thatthe weight of the residue material at the end of the temperature ope-ration, is quite significant i.e. 6.18 mg for the composite sample signi-fying the presence of only oxide component that remains as residueafter melting of the halide matrix in composite. This observation alsosupports that the oxide exists as a separate phase in the composite.

3.4. Electrical conductivity of [(KCl)0.9:(NaCl)0.1]1�y:[ZrO2]y

composite electrolyte

The dc conductivities (s) of the samples in the system[(KCl)0.9:(NaCl)0.1]1�y:[ZrO2]y were determined from respectivecomplex impedance data plots. These s values were plotted as afunction of composition (y) in Fig. 5, which shows that themaximum conductivity can be obtained for y ¼ 0.5 and theenhancement at this composition becomes one order of magni-tude in relation to the matrix. This enhancement could beattributed to the formation of space charge layer at the interfacesbetween the host matrix and the dispersoid.

3.5. Conductivity as a function of temperature

The impedance measurements as a function of temperaturewithin the range of 250–450 1C at different frequencies were

posite electrolyte [(KCl)0.9:(NaCl)0.1]0.5:[ZrO2]0.5.

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Fig. 4. Phase diagram of NaCl:KCl system.

Fig. 5. The composition dependence of the conductivity for [(KCl)0.9:(NaCl)0.1]1�y:

(ZrO2)y system at room temperature as a function of y (0.0pyp0.6).

Fig. 6. The variation of conductivity as a function of inverse temperature for

[(KCl)0.9:(NaCl)0.1]0.5: [ZrO2]0.5 sample.

A. Gupta et al. / Journal of Physics and Chemistry of Solids 70 (2009) 340–343 343

carried out for the composite sample [(KCl)0.9:(NaCl)0.1]0.5:[ZrO2]0.5. The dc conductivities were derived from the compleximpedance spectra registered at different temperatures and wereplotted in Fig. 6 as a function of temperature (T) in the terms oflogs vs. 1/T. It can be seen from the Fig. 6, that by increasingtemperature from 250 to 450 1C, the s increases from0.00461�10�6O�1 cm�1 (at 250 1C) to about 7.45�10�6O�1 cm�1

(at 450 1C) showing the requirement of activation energy for iontransport Q ¼ 0.57 eV. This Q value is found to be comparable with

other reported electrolyte systems such as PbX2:Al2O3 (whereX ¼ Cl, Br, I), KCl:ZrO2 and AgX-based systems [23–26].

4. Conclusion

Fifty mol% of ZrO2 dispersion in the mixed halide matrix(KCl)0.9:(NaCl)0.1 resulted in maximum conductivity enhance-ment. The conductivity increase is �1 order of magnitude relativeto the (KCl)0.9:(NaCl)0.1. The possibility of new compound forma-tion due to sintering of composite electrolyte samples was ruledout by DTA, TG and XRD analyses of the samples. Therefore, thephenomenon of interfacial space charge formation is the probableexplanation of the conductivity enhancement. Three orders ofconductivity enhancement were observed with the compositeelectrolyte at 450 1C.

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