DC Ionic Conductivity Study by Two Probe Method on ( 1X ) Pb ( NO 3 ) 2 : XCeO 2 Composite Solid Electrolyte

Lead Nitrate composite solid electrolyte system in different compositions was synthesized by dispersing nano particles of CeO2 in the host Pb(NO3)2. These systems were characterized by XRD, DSC and FTIR. X-ray diffraction patterns have shown peaks corresponding to pure lead nitrate and cerium oxide in all the compositions with varying intensities and no additional or shifting of peaks were observed. DSC studies indicate uniform melting point in host and dispersed composites. FTIR studies ruled out the chemical reaction between host and dispersoid. Dc ionic conductivity studies were carried out in the temperature range 100oC to 400oC. Ionic conductivity was found to increase with mole percentage of the dispersoid with increase in temperature. Maximum enhancement was observed for 8mole% dispersed system as against the pure Pb(NO3)2 in the extrinsic region of conductivity. Enhanced conductivity in dispersed systems was thought to be due to the presence of Space Charge Layer between the host and dispersoid.


INTRODUCTION
The enhancement of ionic conductivity in inorganic ionic solids by dispersion of fine insulating and insoluble particles has been reported by several authors 1 .The presence of high ionic conductivity with negligible electronic conductivity of these systems has attracted a great deal of interest from industry and researchers due to their potential applications in solid state ionic devices in the recent past.These dispersed solid electrolyte materials have become promising candidates for devices in solid state technology.In the field of fast ionic conductors or super ionic conductors efforts are being continued for newer materials along with existing electrolyte materials to improve performance of solid state ionic devices.
Ionic conducting materials are broadly classified into two categories namely, cation based conductors and anion based conductors.In both the categories of composite solid electrolytes, the non conducting materials such as Al 2 O 3 , CeO 2 , SiO 2 and ZrO 2 are used as second phase particles for enhancement of conductivity.These second phase dispersoid particles are nearly insoluble in host under the preparation conditions.Several theories have been proposed to explain the enhancement of conductivity by forming a layer called Space Charge Layer (SCL) between the matrix of host and dispersoid.Due to enrichment of charge carriers at the host and interface interaction region in the matrix which forms as a highly conducting layer is treated as Space Charge Layer 2 .Two probe method is one of the important techniques to study ionic transport parameters through dc conductivity technique 3 .
The enhancement of ionic conductivity in nitrates of alkali and alkaline earth dispersed composites have been reported in the literature [4][5][6] .
The structures of Ba(NO 3 ) 2 , Ca(NO 3 ) 2 , Sr(NO 3 ) 2 and Pb(NO 3 ) 2 were reported to be fluorite type structure forming an isomorphous group 7 .They have a cubic structure with a space group Pa3 and belong to T h 6 class and reported that anti-Frenkel disorder is predominant in these nitrates 8 .For the present study Pb(NO 3 ) 2 was chosen as the host matrix material because of non-hygroscopic and does not exhibit any solid-solid transformation.Our survey of literature indicates that no systematic investigation has not been reported so far on Pb(NO 3 ) 2 :CeO 2 composite system.The aim of present work is to study enhancement in host Pb(NO 3 ) 2 , with the dispersion of nano size CeO 2 insulating particles, and to interpret the obtained results with suitable models/theories.

Experimental procedure
Single crystals of Pb(NO 3 ) 2 were grown by solution growth technique from the starting material (99% purity) which was obtained from Merck chemicals (USA).The crystals obtained by this method were ground in an agate mortar and then sieved to obtain uniform particle size ( » µm size).The dispersoid CeO 2 (nano size < 25nm) used was obtained from sigma Aldrich.Pb(NO 3 ) 2 and CeO 2 powders taken in particular composition were mixed in the presence of acetone for homogeneous mixing of both the constituents.Pellets of 12mm diameter and 1-2mm thickness were prepared by using steel die by applying a pressure of 0.26 GPa and they were sintered at 300 o C for about 15hours.Silver paint was employed to ensure good electrical contacts with electrodes.To study ionic conductivity by two probe method, pellets were mounted in between the electrodes of sample holder and annealed at 100 o C for 3hrs prior to the actual data recording.The temperature was measured with the help of Cr-Al thermocouple.A fixed voltage of 1.5V was applied and the current measurement was done by using Agilent digital multi meter.
Structural characterizations were performed on Expert Pro X-ray Diffractometer (XRD) with CuKa radiation and Bruker Optics, Germany Tensor 27 model spectrometer (FTIR) respectively.The thermal properties have been carried out by DSC on Q20, a computer interfaced system, with continuous heating rate of 10 o C min -1 .

RESULTS AND DISCUSSION
X-ray diffractograms of various compositions are shown in Fig. 1 and peaks indicate that the system is in crystalline form and no new peaks were noticed except those belong to host and dispersoid which reveals that CeO 2 remains as a separate phase in (1-X)Pb(NO 3 ) 2 :XCeO 2 composite.This XRD study also confirms that the present system is a two phase system.The lattice constant of pure lead nitrate is estimated as 7.9233A o which is matched with literature value 9 .For confirmation of phase formation, XRD peaks are indexed and compared with reported JCPDS data which is shown in Table .1.The average crystallite sizes are estimated from XRD data, for Pure Lead Nitrate as 353.53nm and for dispersoid CeO 2 is 85.05nm, by means of Debye-Scherer formula.The average density percentage for all composite samples after annealing is estimated to be around 90%. Fig. 2 shows DSC curves of pure lead nitrate and composite systems of lead nitrate.The DSC spectrum recorded in the temperature range 50 -500 o C. The strong endothermic peak which corresponds to melting point of lead nitrate is observed nearly at 454 o C. The endothermic peak remains unaffected, except slight shifting with increase of dispersoid concentration.This suggest that there is no solubility of CeO 2 in Pb(NO 3 ) 2 and thus Pb(NO 3 ) 2 :CeO 2 system form a two phase system , which is in conformity with XRD results.DSC curves of pure Lead Nitrate and its various compositions shows no phase transitions occur before its melting.The Endothermic peaks in pure Lead Nitrate after melting point indicates decomposition nature of it.
FTIR spectra of pure and composite samples have been recorded in the wave number range 600-4000 cm -1 (shown 600-2500cm -1 ) shown in Fig. 3. FTIR is a powerful tool to identify functional groups present in composites by monitoring the vibrational energy levels of molecules and to identifying the nature of bonding between molecules 10 .The spectrum of Pb(NO 3 ) 2 shows a strong absorption band at 721 (v strong), 803(v strong), 1013,1072,1287,1770 cm -1 .These values are in good agreement with earlier reported values and these bands are ascribed to presence due to isotopitically different NO 3 -ions 11 .And beyond 2600 cm -1 a few very weak absorptions are observed (not shown in graph) are due to weak interaction of hydrogen bond with pure Lead Nitrate.The other compositions also show similar absorption peaks at corresponding wave numbers, except with slight fluctuation at 1287 cm -1 .data were represented by Arrhenius relation given by s = s o exp(-E a /K B T), where s o is pre-exponential factor, E a is activation energy, K B is Boltzmann's constant and T is absolute temperature.From conductivity graph it is clear that, while increasing the concentration of CeO 2 the enhancement of conductivity was found increase and reaching to a saturation value at 8mole% of dispersoid and there onwards decrement of conductivity was recorded.

Variation dc ionic conductivity
The maximum enhancement of conductivity recorded was about one order with respect to pure Pb(NO 3 ) 2 system.Fig. 5 shows variation of conductivity with different mole% of CeO 2 dispersed systems for few typical temperatures indicates a systematic variation of conductivity as a function of CeO 2 .Similar observations made on some anion-Frenkel defect systems such as CaF 2 , PbF 2 , Ba(NO 3 ) 2 , Pb(NO 3 ) 2 [12][13][14][15][16] where the conductivity varied systematically as function of dispersoid for different insulating particles.
From the above observation, it is clear that dispersion of CeO 2 in host Pb(NO 3 ) 2 responsible for considerable enhancement in conductivity.To interpret the enhancement in conductivity with CeO 2 dispersion in host Pb(NO 3 ) 2 , Random Resistor Network [RRN] model taken into account, out of available models. 17According to this model, from the macroscopic point of view at interface region a highly conducting layer exist between host and dispersoid and predicts the existence of two critical concentrations, namely, P c ' and P c ''.At P c ' the high conducting layers begin to form connected pathways and at P c ‹ §‹ § the connected pathways get disrupted as they begin to form closed loops due to increased concentrations of insulating bonds.Fig. 5 shows that conductivity raises initially, passes through maximum at 8mole% of CeO 2 composite and subsequently drops to higher mole% of CeO 2 composite.This appears to be consistent with RRN model for (1-X) Pb(NO 3 ) 2 :XCeO 2 composite system.
The transport parameters are given in Table .2for all Pb(NO 3 ) 2 -CeO 2 composites studied in this work.The variation of activation energy (E a ) is consistent with the generally observed relation for ionic conductors i.e. the increase in conductivity is almost invariably associated with a decrease in activation energy.The activation energy initially decreases from pure Pb(NO 3 ) 2 to 8mole% of CeO 2 composite, where conductivity is maximum in extrinsic region and there after enhancement of activation energy is observed, where the conductivity recorded decrement.The variation of activation energy (E a ) vs. mole percentage is shown in Fig. 6.
In composite solid electrolytes, it is widely believed that the presence of excess defects at the host matrix and dispersoid particle interface contributes to high conductivity in composite solid electrolytes.Formation of space charge layer between host and dispersed particles is thought to be responsible for conductivity enhancement process.In Fluorite structured ionic solids, ionic mobility requires anti-Frenkel defects 7,18 .The enhancement in conductivity with heterogeneous dispersion can be explained within the frame work of space charge model proposed by Maier's semi quantitative model 19 .Macroscopically generation of excess anion vacancies at matrix-particle interface due to formation of space charge layer is more appropriate for this composite solid electrolyte system.

CONCLUSION
XRD and DSC studies confirm that the composite (1-X)Pb(NO 3 ) 2 :XCeO 2 is biphasic system.FTIR studies show the presence of NO 3 -ion.The maximum enhancement of conductivity is about one order of magnitude for 8mole% dispersed electrolyte with respect to host Pb(NO 3 ) 2 .The conductivity enhancement in extrinsic region due to formation of space charge layer between host Pb(NO 3 ) 2 and CeO 2 is thought to be responsible.With the formation of space charge an internal field developed at interface which causes acceleration in transport of conducting ions.

Fig. 3 : 2 Fig. 3 :
Fig.3: FTIR spectra of pure and different compositions of CeO 2 Fig.3: FTIR spectra of pure and different compositions of CeO 2