Pelagia Research Library

Transcription

1 Available online at wwwpelagiaresearchlibrarycom Der Chemica Sinica, 213, 4(4):551 ISSN: CODEN (USA) CSHIA5 A phenomenological hysteresis and enhanced ionic conductivity of solid electrolyte Ba(NO 3 ) 2 -KNO 3 mixed crystals S Shashi Devi 1 and A Sadananda Chary 2 1 Department of Physics, Vardhaman College of Engg, Shamshabad, R R Dist, Andhra Pradesh, India 2 Department of Physics, University College of Engg, Osmaniania University, Hyderabad, Andhra Pradesh, India ABSTRACT Ionic conductivity measurements are made in pellets of pure Ba(NO 3 ) 2, KNO 3 and in different compositions of Ba(NO 3 ) 2 -KNO 3 mixed crystals in the temperature range from about 1 o C to 5 o C The conductivity versus temperature plots in these crystals are drawn for both heating and cooling All these plots showed four regions and are reversible and have hysteresis The activation energies are calculated for all the compositions and these are found to be decreasing with increasing mole percnt of KNO 3 in Ba(NO 3 ) 2 Conductivity attains maximum value for a certain intermediate composition Keywords: Ionic conductivity, composites, Solid electrolytes, INTRODUCTION Physical properties of Ba(NO 3 ) 2 & KNO 3 have been studied by various researchers previously Studies on KNO 3 include ac conductivity near the phase transition points[1], enhancement of dc Ionic conductivity in KNO 3 -Al 2 O 3 composite solid electrolyte system[2], phase transition in KNO 3 [3] etc Similarly investigations in Ba(NO 3 ) 2 include elastic constants[4], temperature variation of lattice parameter[5], dielectric constant[6], temperature dependence of piezo optic behavior[7], temperature variation of photo elasticity[8], photo elastic constants[9], micro hardness [1], photo elastic dispersion [11] and Ionic conductivity in single crystals of Ba(NO 3 ) 2 [12] Some of these composite solid electrolytes exhibit high ionic conductivity and good mechanical properties Some of these composite solid electrolytes exhibit high ionic conductivity and good mechanical properties and are found to be promising materials for solid state batteries, fuel cells, electrodes etc[13] These composite solid electrolytes are also termed as dispersed solid electrolyte systems or heterogeneously doped materials Measurement of Ionic conductivity is a very sensitive and useful experimental tool in understanding the defect properties of crystals As well they have usefulness in a number of possible technical devices To understand these defect properties we have undertaken the study of ionic conductivity in pure and mixed pellets of alkali and alkaline earth nitrates Survey of literature indicates that there is no work on dc ionic conductivity of mixed crystals comprising of alkaline earth nitrates as against alkali nitrates in general and no such conductivity work on Ba(NO 3 ) 2 & KNO 3 mixed crystals in particular Hence in this paper we report our studies of dc ionic conductivity of pure and mixed crystals of barium nitrate and potassium nitrate MATERIALS AND METHODS The starting materials were from Qualigens fine chemicals (SQ) of 995% purity Ba(NO 3 ) 2 & KNO 3 were obtained by crushing single crystals grown by slow evaporation method The powders of the samples were mixed in the presence of acetone and were ground in an agate mortar for about an hour The pellets of 8mm diameter and 3 mm thickness were prepared at a Pressure of 46GPa by using hydraulic press These pellets were sintered at 2 / 3 of 55

2 S Shashi Devi and A Sadananda Chary Der Chemica Sinica, 213, 4(4):551 their melting point for about 24 hrs After cleaning the surfaces an electrode material (silver) was applied for good electrical contact The pellet then was mounted in a spring loaded sample holder and annealed at about 15 o C for 4hrs before the data was recorded A constant rate of heating of 2 o C/ min was maintained throught the experiment The temperature was recorded by Cr-Al thermocouple A small dc voltage of 15V was applied across the sample and the current was measured on a digital dc nano ammeter Data was recorded on at least three to four samples each of pure and mixed pellets, running a minimum of three cycles on each sample Similar experimental conditions were maintained for all the samples and the data showed a reasonably good reproducibility RESULTS AND DISCUSSION Ionic conductivity studies were made in the temperature rangeof12 o C 9 o C for Ba(NO 3 ) 2, 9 o C - 32 o C for KNO 3 and for various compositions of Ba(NO 3 ) 2 -KNO 3 the temperature range covered is from 8 o C 27 o C Fig 1 and 6 shows the conductivity-temperature plots for pure Ba(NO 3 ) 2 and KNO 3 Similarly, figures 2 to 5 correspond to the conductivity plots of the mixed systems of KNO 3 -Ba(NO 3 ) 2 with increasing mole percentages of KNO 3 in Ba(NO 3 ) 2 fig2 shows the variation of conductivity versus temperature plot for 39 mole % of KNO 3 in Ba(NO 3 ) 2, fig3 shows the 72 mole % of KNO 3 in Ba(NO 3 ) 2, fig4 shows the 85 mole % of KNO 3 in Ba(NO 3 ) 2 and 95 mole % of KNO 3 in Ba(NO 3 ) 2 are shown in fig6 All the above figures are drawn for both heating and cooling /T k -1 FIGURE 1: ln(σt) against 1 3 /T for pure Ba(NO 3) /T k -1 FIGURE 2: ln(σt) against 1 3 /T for 39 mole% of KNO 3 in Ba(No 3) 2 56

3 S Shashi Devi and A Sadananda Chary Der Chemica Sinica, 213, 4(4):551 Some of the important features noticed from these graphs are given below (i) In fig1 the points below 12 o C on the x-axis are not included where some fluctuations are present As the sample is heated there is a slow and gradual increase in conductivity which increases around 199 o C (211 on x-axis) Above this the conductivity becomes once again gradual around 295 o C (176 on x-axis) There are three distinct regions observed in the heating curve Starting from high temperature first region is above 295 o C on (x-axis 176), second region is between 295 o C to 215 o C (on x-axis 1764), third region is below 215 o C (on x-axis 24) [12] The cooling part does not retrace the heating part and shows a hysteresis in two regions Also, as the sample is cooled the data shows a steep fall at 296 o C (on x-axis 176) (ii) In fig 6 also points below 277 on the x-axis are not included where some polarization effect was present Here also we observed three distinct regions The first region is above 157 o c (232), second region is between (157 o C- 148 o C) (on x-axis 23237), third region is below /T k -1 FIGURE 3: ln(σt) against 1 3 /T for 72 mole% of KNO 3 in Ba(No 3) o C (on x-axis 237) In this case also the cooling part does not retrace the heating part and shows a hysteresis As the sample is cooled the data shows a steep fall at 159 o C (on x-axis 231) (iii) The melting point of composite system is lower than the melting points of individual components Ba(NO 3 ) 2 :KNO 3 3: cooling 1/T k -1 FIGURE 6: ln(σt) against 1 3 /T for 85 mole % of KNO3 in Ba(NO 3) 2 57

4 S Shashi Devi and A Sadananda Chary Der Chemica Sinica, 213, 4(4):551 Ba(NO3)2:KNO3 1: /T k -1 FIG 5: ln(σt) against 1 3 /T for 95 mole% of KNO 3 in Ba(NO 3) 2 TABLE-I (iv) Loop area is more for composite systems as against that of individual components and it is found to decrease with increase in mole % of KNo 3 (v) All the composite systems showed four regions same as their individual components and the conductivity is seen increase linearly with temperature and followed by a bend in conductivity Pure KNO n(σt) Ω-1cm-1 k 1/T k-1 FIGURE 6: ln(σt) against 1 3 /T for pure KNO3 TABLE I Mole Percentages Temperature range( o C) Activation Energies(eV) Ba(NO 3) % 72% % 95% KNO (vi) As the samples are cooled the data show a steep fall in conductivity (around 245 on x-axis) as against a more gradual change during the process of heating at low temperature 58

5 S Shashi Devi and A Sadananda Chary Der Chemica Sinica, 213, 4(4):551 (vii) The variation of conductivity with composition is found to be non-linear, attaining a maximum value at 72 mole% of KNO 3 in Ba(NO 3 ) 2 for three different temperatures Table 1 shows activation energies, obtained from DC conductivity, for Ba(NO 3 ) 2 -KNO 3 system in the temperature range of 197 o C for different mole percentages These results are in agreement with those reported in iterature 13 All the conductivity temperature heating plots of Ba(NO 3 ) 2,KNO 3 and their mixed systems showed three regions[15] Here the high temperature region (intrinsic region) is not recorded to avoid any risk of damaging the sample as we took the readings both while heating and cooling Based on the observations cited above, we are lead to believe that the first two regions correspond to extrinsic and extrinsic associated regions respectively In Fluorite type materials like Sr(NO 3 ) 2,Ba(NO 3 ) 2 etc, [12] anti frenkel defect is thought to be predominant, where as in case of KNO 3 Frenkel is predominant defect These defects are point defects and in crystal presence of a point defect introduces distortions If the imperfection is a vacancy, the bonds that the missing atom would have formed with its neighbours will be absent, this gives rise to elastic strains and this factor tends to increase the energy of the crystal Vacancies are always present at any temperature between K and melting point of the crystal As the temperature is increased, the number of vacancies also increases The anti-frenkel disorder consisting of anion interstitials and their vacancies and the Frenkel disorder involving equal number of cation interstitials and their vacancies If the cation and anion are of different sizes we expect the energies to put either of them into interstitial positions to differ considerably Consequently electrical conductivity in mixed crystal depends predominantly on Frenkel or anti- Frenkel defects It appeared reasonable to extend this mechanism in Ba(NO 3 ) 2 that the conductivity is mainly due to the transport of anion defects[16] and in case of KNO 3 it is due to the cation defects In case of composite systems, the monovalent potassium replaces the divalent barium substitutionally giving rise to the creation of an anion vacancy[12] This process can be considered responsible for the observed increase in the conductivity The activation energies calculated for pure and mixed systems also reveal the same The activation energy is found to be maximum for Ba(NO 3 ) 2 and as the mole percentage of KNO 3 in Ba(NO 3 ) 2 is increased it is found to be decreasing In case of KNO 3 it is found to be minimum These results suggest that the lowering of activation energy could be due to the increased concentration of defects in the interfacial layer The conductivity studies on mixed crystals[17] indicated that the concentration of vacancies in mixed crystals exceeds that in pure components ln(σt) Ω-1cm-1 k Ba(NO3)2 B8-K2 B5-K5 B3-K7 B1-K The temperature dependence of dc ionic conductivity with reciprocal temperature from nearly 19 o C to melting point of composite systems for pure and 39,72,85 and 95 mole percent of KNO3 is shown in figure7 In composite systems the enhancement in conductivity is observed to increase with m/o[18] with a threshold at 72 mole percent where from enhancement starts falling with further increase in mole percent ie for 85 and 95 m/o The maximum enhancement at 72 mole percent is observed to be about 3 orders of magnitude with respect to pure Ba(NO 3 ) 2 in the extrinsic conduction region This type of enhancement in conductivity for an intermediate composition is also observed in KBr-KI mixed crystals[19] It may be noticed from the plot that conductivity values for pure Ba(NO 3 ) 2 at about 19 o C and 27 o C are and 79 Ω -1 cm -1 k where as these are -73 and -371 Ω -1 cm -1 k for 72 m/o of KNO 3 in Ba(NO 3 ) 2 In case of 72,85 and 95 m/o it is observed that the conductivity values for all of them started at around -73 but ends at different values ie at -371,59 and 8 Ω -1 cm -1 k This is the reason for lowering of 59

6 S Shashi Devi and A Sadananda Chary Der Chemica Sinica, 213, 4(4):551 activation energies in case of 85 and 95 m/o Variation of ln(σt) with mole percent of KNO3, at different temperatures, is shown in figure 8 It can be seen from this figure that the maximum enhancement in conductivity occurs at 72 mole percent This is in agreement with the results obtained from figure 7 σn(σt) % 2% 4% 6% 8% 1% 478K 53K 543K Mole percentages The last region at the lower temperatures gave a small slope but was also sensitive to the thermal history As mentioned earlier, Qualigens fine chemical salts were used as starting materials for growing single crystals of Ba(no 3 ) 2, kno 3 and for various compositions of Ba(no 3 ) 2 -kno 3 According to data furnished by the manufacturer both Ba(no 3 ) 2 and kno 3 contains monovalent cation and monovalent anion impurities The contribution of these vacancies in the impurity region is quite appreciable for slow and gradual increase in conductivity Perhaps this could be the reason in pure Ba(No 3 ) 2, KNO 3 and in Ba(NO 3 ) 2 KNO 3 mixed crystals the suppression of conductivity at low temperatures[18] However in pure Ba(No 3 ) 2,KNO 3 and Ba(NO 3 ) 2 KNO 3 mixed crystals it is also observed the Hysteresis loop This Hysteresis mainly occur because of a dynamic lag between input and output Often, this effect is also referred to as hysteresis, or rate-dependent hysteresis This effect disappears as the input changes more slowly In this case it is observed that the loop area perhaps decreasing as the KNO 3 mole % in Ba(NO 3 ) 2 is increasing This could be because as the mole% of KNO 3 increases anion vacancies also increases This mechanism is probably responsible for the decreasing loop area We are trying to verify this with a suitable experiment CONCLUSION These results lead to theconclusion that the enhanced conductivity in Ba(NO 3 ) 2 KNO 3 composites is due to the increased concentration of anion vacancies Conductivity of the system increases with increasing concentration of KNO 3 because it amounts to the increase in the highly conducting bonds between these two composites Subsequently, when the total volume of the interface layers is most effectively linked together, the total number of highly conducting bonds become maximum, consequently sample would show maximum conductivity As the concentration of KNO 3 further increase, KNO 3 particles cannot be completely covered by the interface layers simply because the avaible Ba(NO 3 ) 2 cannot envelop all the KNO 3 particles, number of non-conducting bonds increase and therefore conductivity decreases REFERENCES [1] Riki Kawashima and Mitsuru Satoh MIT, Murroran,1999, [2] MV Madavarao, SNarender Reddy, ASadananda chary, Materials science,29,5 27 [3] VV Deshpanday, MD Karkhanavala and URK Rao Baba, journal of Thermal Analysis,1972,46, 613 [4] Bhagavantam S and Sundara Rao RVG, CurrSci,1948, [5] Bichile G K and Kulkarni R G, JPhys C: Solid St Phys 1975, 8,23 [6] Sirdeshmukh D B, J Phys Chem Solids,1966,27,1557 6

7 S Shashi Devi and A Sadananda Chary Der Chemica Sinica, 213, 4(4):551 [7] Krishna Rao K V and Krishna Murthy V G, Curr Sci, 1973, 42, 87 [8] Krishna Murthy V G, PhD Thesis Osmania University, India, 1964 [9] Bhimsen Sachar Seshagiri Rao T,Proc Nat Inst Sci India,195,16,235 [1] Sirdeshmukh L, Curr Sci, 1975, 44, 69 [11] Bansigir K G PhD Thesis Osmania University, India,1956 [12] K SaiBabu and T Chiranjivi, JPhys C:Solid St Phys,198, [13] JMaier, Programme Solid State Chem,1995 [14] JB Wagner Jr TTakahashi(Ed), High conductivity Solid IonicConductors-Recent Trends and Applications,World scientific,singapore,1989,146 [15] S Shashi Devi, B L Aruna, S Narender Reddy, A Sadananda Chary, Pelagia research library,212, 3(6): [16] Catlow C R A,Norgett M J and Ross T A, Jphys C:Solid StPhys,1977,1,1627 [17] HScultze, Thesis University of Gottingen,1972 [18] T Vijay Kumar, R Swarnalatha, A Sadananda Chary and S Narender Reddy, Pelagia research library, 212, 3 (5): [19] SC Jain and DC Parashar,J, Phys C(Solid State Series) 1969,2, 2,