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Der Chemica Sinica, 2013, 4(4):55-61
ISSN: 0976-8505
CODEN (USA) CSHIA5
A phenomenological hysteresis and enhanced ionic conductivity of solid
electrolyte Ba(NO3)2-KNO3 mixed crystals
S. Shashi Devi1 and A. Sadananda Chary2
1
Department of Physics, Vardhaman College of Engg., Shamshabad, R. R. Dist, Andhra Pradesh, India
Department of Physics, University College of Engg., Osmaniania University, Hyderabad, Andhra Pradesh, India
_____________________________________________________________________________________________
2
ABSTRACT
Ionic conductivity measurements are made in pellets of pure Ba(NO3)2, KNO3 and in different compositions of
Ba(NO3)2-KNO3 mixed crystals in the temperature range from about 100oC to 500oC. 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 KNO3 in Ba(NO3)2. Conductivity attains maximum value for a
certain intermediate composition.
Keywords: Ionic conductivity, composites, Solid electrolytes,
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INTRODUCTION
Physical properties of Ba(NO3)2 & KNO3 have been studied by various researchers previously. Studies on KNO3
include ac conductivity near the phase transition points[1], enhancement of dc Ionic conductivity in KNO3-Al2O3
composite solid electrolyte system[2], phase transition in KNO3[3] etc. Similarly investigations in Ba(NO3)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 [10],
photo elastic dispersion [11] and Ionic conductivity in single crystals of Ba(NO3)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[13-14]. 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(NO3)2 & KNO3 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 99.5% purity. Ba(NO3)2 & KNO3 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 0.46GPa by using hydraulic press. These pellets were sintered at 2/3 of
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S. Shashi Devi and A. Sadananda Chary
Der Chemica Sinica, 2013, 4(4):55-61
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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 150oC for 4hrs before the data
was recorded. A constant rate of heating of 2oC/min was maintained throught the experiment. The temperature was
recorded by Cr-Al thermocouple. A small dc voltage of 1.5V 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 rangeof120oC -490oC for Ba(NO3)2 , 90oC - 320oC for
KNO3 and for various compositions of Ba(NO3)2-KNO3 the temperature range covered is from 80oC–270oC.
Fig. 1 and 6 shows the conductivity-temperature plots for pure Ba(NO3)2 and KNO3 . Similarly, figures 2 to 5
correspond to the conductivity plots of the mixed systems of KNO3-Ba(NO3)2 with increasing mole percentages of
KNO3 in Ba(NO3)2. fig.2 shows the variation of conductivity versus temperature plot for 39 mole % of KNO3 in
Ba(NO3)2, fig.3 shows the 72 mole % of KNO3 in Ba(NO3)2, fig.4 shows the 85 mole % of KNO3 in Ba(NO3)2 and
95 mole % of KNO3 in Ba(NO3)2 are shown in fig.6. All the above figures are drawn for both heating and cooling.
0
-2 0
1
2
3
ln(σT) Ω-1cm-1 k
-4
-6
-8
Heating
-10
Cooling
-12
-14
-16
1000/T k-1
.
FIGURE 1: ln(σT) against 103/T for pure Ba(NO3)2
0
-2 0
1
2
3
4
ln(σT) Ω-1cm-1 k
-4
-6
-8
Heating
-10
Cooling
-12
-14
-16
-18
1000/T k-1
.
FIGURE 2: ln(σT) against 103/T for 39 mole% of KNO3 in Ba(No3)2
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S. Shashi Devi and A. Sadananda Chary
Der Chemica Sinica, 2013, 4(4):55-61
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Some of the important features noticed from these graphs are given below.
(i) In fig.1 the points below 120oC 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 199oC (2.11 on x-axis).
Above this the conductivity becomes once again gradual around 295oC (1.76 on x-axis). There are three distinct
regions observed in the heating curve. Starting from high temperature first region is above 295oC on (x-axis 1.76),
second region is between 295oC to 215oC (on x-axis 1.76-2.04), third region is below 215oC (on x-axis 2.04) [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 296oC (on x-axis 1.76).
(ii) In fig. 6 also points below 2.77 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 157oc (2.32), second region is between (157oC148oC) (on x-axis 2.32-2.37), third region is below
0
-2
0
1
2
3
4
ln(σT) Ω-1cm-1 k
-4
-6
Heating
-8
Cooling
-10
-12
-14
-16
1000/T k-1
.
FIGURE 3: ln(σT) against 103/T for 72 mole% of KNO3 in Ba(No3)2
148oC (on x-axis 2.37). 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 159oC (on x-axis 2.31).
(iii) The melting point of composite system is lower than the melting points of individual components.
Ba(NO3)2:KNO3
3:7
0
ln(σT) Ω-1cm-1 k
-2 0
1
2
3
-4
-6
-8
Heating
-10
cooling
-12
-14
-16
1000/T k-1
.
FIGURE 6: ln(σT) against 103/T for 85 mole % of KNO3 in Ba(NO3)2
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S. Shashi Devi and A. Sadananda Chary
Der Chemica Sinica, 2013, 4(4):55-61
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Ba(NO3)2:KNO3
1:9
0
-2
0
0.5
1
1.5
2
2.5
3
ln(σT) Ω-1cm-1 k
-4
-6
-8
Heating
-10
Cooling
-12
-14
-16
1000/T k-1
.
FIG 5: ln(σT) against 103/T for 95 mole% of KNO3 in Ba(NO3)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 KNo3.
(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 KNO3
0
n(σT) Ω-1cm-1 k
-2
0
1
2
3
-4
-6
Heating
-8
Cooling
-10
-12
-14
1000/T k-1
.
FIGURE 6: ln(σT) against 103/T for pure KNO3
TABLE I
Mole Percentages
Ba(NO3)2
39%
72%
85%
95%
KNO3
Temperature range(oC)
190-270
190-270
190-270
190-270
190-270
190-270
Activation Energies(eV)
1.38
1.01
0.97
0.78
0.34
0.19
(vi) As the samples are cooled the data show a steep fall in conductivity (around 2.45 on x-axis) as against a more
gradual change during the process of heating at low temperature.
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Der Chemica Sinica, 2013, 4(4):55-61
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(vii) The variation of conductivity with composition is found to be non-linear, attaining a maximum value at 72
mole% of KNO3 in Ba(NO3)2 for three different temperatures.
Table 1 shows activation energies, obtained from DC conductivity, for Ba(NO3)2-KNO3 system in the temperature
range of 190-270oC for different mole percentages. These results are in agreement with those reported in iterature13.
All the conductivity–temperature heating plots of Ba(NO3)2 ,KNO3 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(NO3)2,Ba(NO3)2 etc., [12] anti frenkel defect is thought to be predominant, where as in case of
KNO3 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 0K 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 antiFrenkel defects. It appeared reasonable to extend this mechanism in Ba(NO3)2 that the conductivity is mainly due to
the transport of anion defects[16] and in case of KNO3 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(NO3)2 and as the mole percentage of KNO3 in Ba(NO3)2 is increased it is found to be decreasing.
In case of KNO3 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.
0
-2
ln(σT) Ω-1cm-1 k
-4
Ba(NO3)2
-6
B8-K2
-8
B5-K5
-10
B3-K7
-12
B1-K9
-14
1.8
1.9
2
2.1
2.2
.
The temperature dependence of dc ionic conductivity with reciprocal temperature from nearly 190oC 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 i.e. 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(NO3)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(NO3)2
at about 190oC and 270oC are -11.77 and -6.79 Ω-1cm-1 k where as these are -7.3 and -3.71 Ω-1cm-1 k for 72 m/o of
KNO3 in Ba(NO3)2. In case of 72,85 and 95 m/o it is observed that the conductivity values for all of them started at
around -7.3 but ends at different values i.e. at -3.71 ,-4.59 and -6.08 Ω-1cm-1k. This is the reason for lowering of
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Der Chemica Sinica, 2013, 4(4):55-61
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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.
0
-1
-2
σn(σT)
-3
-4
-5
478K
-6
503K
-7
543K
-8
-9
-10
0%
20%
40%
60%
80%
100%
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(no3)2,kNO3 and for various compositions of Ba(no3)2-kNO3. According to data furnished by the manufacturer
both Ba(no3)2 and kNO3 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(No3)2,KNO3 and in Ba(NO3)2 –KNO3 mixed crystals the suppression of conductivity
at low temperatures[18].
However in pure Ba(No3)2,KNO3 and Ba(NO3)2–KNO3 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 KNO3 mole % in Ba(NO3)2is increasing. This could be
because as the mole% of KNO3 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(NO3)2 –KNO3 composites is due to the
increased concentration of anion vacancies. Conductivity of the system increases with increasing concentration of
KNO3 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 KNO3 further increase, KNO3 particles cannot be completely covered by the interface layers simply
because the avaible Ba(NO3)2 cannot envelop all the KNO3 particles, number of non-conducting bonds increase and
therefore conductivity decreases.
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