Electrical properties of Na2SO4-based composite systems
S. M. Bobade & A. R. Kulkarni & P. Gopalan
Abstract Composite electrolytes are well-known multiphase systems and exhibit maxima in the conductivity at
certain second-phase concentration. An attempt has been
made to investigate a number of sodium sulfate (Na2SO4)based composite systems. The dispersoids that have been
used are MgO, Al2O3, and SiO2. The samples have been
characterized using impedance spectroscopy, X-ray diffraction, and differential scanning calorimetry. The maximum
conductivity has been observed for MgO dispersed system,
and the percolation threshold has been observed at 30-mol%
dispersoid, MgO concentration. Interestingly, two maxima
have been observed in case of the Na2SO4–SiO2 and Na2SO4–
Al2O3 composite systems. In the Na2SO4–SiO2 system, the
first maximum occurs at lower concentration, i.e., in the
range between 10 and 20 mol%, whereas the second occurs
at the 40-mol% dispersoid concentration. For the Na2SO4–
Al2O3 system, although slightly indistinguishable, two peaks
in the conductivity vs composition plot have been observed
around 12- and 30-mol% Al2O3 concentrations.
Keywords Composite electrolytes . Na+ conductors .
Ionic conductivities
Introduction
The enhancement in the conductivity of the host material
on dispersion of insulating phase particles has been
S. M. Bobade : A. R. Kulkarni : P. Gopalan
Department of Metallurgical Engineering and Materials Science,
Indian Institute of Technology Bombay,
Powai,
Mumbai 400076, India
reported by Liang et al., the enhancement for the LiI–
Al2O3 composite system being more a magnitude of 100
[1]. Several systems have been probed and reported
subsequently. The traditionally used dispersoids are
Al2O3, SiO2, and fly ash [1–24]. There have been few
reports where hydrated matrix–dispersoid has been used
[14]. The electrical conductivity of these systems attains a
maximum at certain concentration of dispersoid, generally
in the range between 30 and 40 mol% of dispersoid [1–23].
Sodium sulfate, a promising candidate for electrochemical devices, exhibits five polymorphs. Among these,
phases V, III, and I are well characterized. Phase V and III
are known to exhibit orthorhombic structure with space
group Fddd and Cmcm, respectively [24–26], whereas
phase I is known to exhibit a hexagonal structure, space
group P63mmc [27].
The higher conductivity of phase I has sparked a greater
interest. Sodium sulfate undergoes a sequence of phase transitions. The room temperature phase V transforms to phase I
during heating at 250 °C. However, while cooling, phase I
transforms to phase III and subsequently to phase V.
Phase III exists over a narrow temperature range, and it
has been reported that, on the absorption of water, it transforms to phase V. Under dry conditions, phase III is known to
be stable at room temperature [28–30].
Various approaches to improve the electrical properties
of Na2SO4 have recently been reviewed by Gopalan et al.
[6]. Considering the Frenkel disorder in Na2SO4, divalent
and trivalent dopants have been substituted at Na site
resulting in additional vacancies. Similar types of anion
substitution have been employed at the SO4 site as well.
The conductivity mechanism in sulfates, in particular
Li2SO4 and Na2SO4, is a subject of debate [31]. There
has been a growing belief that the percolation mechanism
operates in Na2SO4.
(220)*
(200)*
258
50 mol%
Intensity (A.U.)
40 mol%
30 mol%
20 mol%
10
20
30
(112)#
(130)#
(110)#
(020)#
(111)#
(021)#
(002)#
10 mol%
# Na2SO4-III 5 mol%
* MgO
40
50
60
70
80
30, 35, 40, and 50 mol% for each of the three dispersoids
MgO, SiO2, and Al2O3 were separately prepared at 900 °C.
The samples were heated for 6 h and were quenched in air
to room temperature. The quenched mass was then ground
thoroughly and pressed into the pellets of 10-mm diameter
using 30 kN pressure. The processed powders were also
characterized by X-ray diffraction and differential scanning
calorimetry. The pellets were fired at 800 °C and cooled
down to room temperature. The fired pellets were silverpasted and baked at 300 °C for an hour. Impedance
measurements were carried out in the temperature range
between 500 and 200 °C in the cooling cycle using
HP4192, an impedance analyzer. A frequency range
between 100 Hz and 13 MHz was employed.
Results and discussion
The X-ray diffraction patterns for the Na2SO4–MgO composite system are shown in Fig. 1. The X-ray diffraction patterns
confirm the existence of the matrix (Na2SO4-V) and the
dispersoid (MgO) phases. In addition to the two phases,
phase III of Na2SO4 has also been observed in the composite
90
2θ (degree)
40 mol%
30 mol%
Intensity (A. U)
The starting materials were obtained from Aldrich Chem.
The dispersoids were preheated at 1,000 °C for an hour to
remove moisture. The appropriate mole percent of Na2SO4
and dispersoid was weighed and mixed in the agate mortar
using acetone as a medium. A series containing 5, 10, 20,
10
20
30
(200)#
(130)#
(113)$
10 mol%
(220)$
(111)$
Experimental
(022)$
(131)$
(111)#
(021)#
(021)#
(112)#
20 mol%
(110)#
The composite formation is one of the approaches to
improve the electrical conductivity of the host ionic
material. In the Na2SO4–Al2O3 composite system, the
formation of the β-alumina phase, a good sodium ion
conductor, has been observed for 5.5-mol% Al2O3 dispersion [2, 3]. The composite systems Na2SO4–Y2(SO4)3–
Al2O3 [12] and Na2SO4–La2(SO4)3–Al2O3 [13] containing
40-mol% concentration of the second phase have been
investigated for sensor applications.
Based on the existing body of literature and the amount
of existing literature on Na2SO4-based composite systems,
various issues such as the choice of dispersoids in
determining the nature of interface are yet to be identified.
As a result, an attempt has been made to identify the role of
dispersoid in the conductivity enhancement.
50 mol%
(101)*
Fig. 1 Diffraction pattern of Na2SO4–MgO composite system for
various MgO concentrations
40
$ Na 2SO 4-V
5 mol%
# Na 2SO 4-III
* SiO 2
50
60
70
80
90
Fig. 2 Diffraction pattern of Na2SO4–SiO2 composite system for
various SiO2 concentrations
(012)*
259
Nominal
Na2SO4 (wt%)
Nominal
MgO (wt%)
98
97
93
89
84
78
2
3
7
11
16
22
(300)*
(214)*
(116)*
(113)*
Table 1 Semiquantitative
analysis for Na2SO4–MgO
system
(104)*
system. Thus, the system contains three phases, namely,
phases V and III of Na2SO4 and the dispersoid. It is known
that phase III is a metastable phase and transforms to the
stable room temperature phase V [29–31]. The formation of
phase III and its stability for such a long time in the
composite system may be an indication of active interaction
between the matrix and the dispersoid particles in determining the nature of an interface. It is well known that Na2SO4III, stable between 210 and 230 °C, has a higher conductivity
relative to the room temperature phase V.
The X-ray diffraction patterns for Na2SO4–SiO2 and
Na2SO4–Al2O3 systems are provided in Figs. 2 and 3,
respectively. The formation of phase III has been observed
in all the three composite systems. The appropriate peaks
have been indexed to identify the corresponding phases.
The semiquantitative analysis for Na2SO4–MgO and
Na2SO4–SiO2 composite systems is tabulated in Tables 1
and 2. The concentration that has been obtained from the
relative intensity ratio (RIR) method agrees well with
the nominal concentration of dispersoids. The variation in
the observed concentration may be attributed to the time lag
between preparation and X-ray diffraction data collection. A
similar argument is also applicable for the Na2SO4–Al2O3
system.
The conductivity as function of 1,000/T for Na2SO4–MgO
system is depicted in Fig. 4. The observed transition is
attributed to that between phase I and III. In Na2SO4, upon
cooling, the high temperature phase I, which exists above
250 °C, transforms to phase III, and thereafter phase III
transforms to phase V. In this case, it has been observed that,
after samples cooled down to room temperature, phase III
persists for a few hours. The possibility of I→III transition
while cooling the sample during impedance measurement
cannot be ruled out, and thus the transition is attributed as
being I→III.
TheconductivityfortheNa2SO4–SiO2 and Na2SO4–Al2O3
systems is provided in Figs. 5 and 6, respectively. The
maximum conductivity at 300 °C is of the order of 10−3 S/
cm. The conductivity in these systems exhibits two peaks.
The peak that can be attributed to the percolation threshold
has been observed in the range between 30- and 40-mol%
dispersoid concentration. An additional peak is visible in
the lower concentration range, i.e., in the range between 10and 20-mol% dispersoid concentration. The percolation
threshold for Na2SO4–SiO2 composite system has been
observed at 40-mol% concentration, whereas for Na2SO4–
Al2O3 composite system the threshold occurs at 30-mol%
concentration. The conductivity isotherms for Na2SO4–
MgO, Na2SO4–SiO2, and Na2SO4–Al2O3 systems are
provided in Figs. 7, 8, and 9, respectively. Theconductivity
value at 300 °C for Na2SO4–Al2O3 system is similar to the
50 mol%
40 mol%
(002)#
(112)#
(200)#
(130)#
(110)#
(111)#
(021)#
Intensity (A. U)
30 mol%
20 mol%
10
20
(022)$
(131)$
(113)$
(220)$
(111)$
10 mol%
30
40
5 mol%
50
60
70
80
90
Fig. 3 Diffraction pattern of Na2SO4–Al2O3 composite system for
various Al2O3 concentrations
Series
number
1
2
3
4
5
6
Na2SO4
(mol%)
95
90
80
70
60
50
MgO
(mol%)
5
10
20
30
40
50
Experimental compositions
Phase V
(wt%)
Phase III
(wt%)
MgO
(wt%)
16
08
14
14
21
10
82
89
81
75
67
79
2
3
5
11
12
11
260
Table 2 Semiquantitative
analysis for Na2SO4–SiO2
system
Series
number
1
2
3
4
5
6
Na2SO4
(mol%)
95
90
80
70
60
50
SiO2
(mol%)
5
10
20
30
40
50
SiO2 dispersed system. InthecaseofNa2SO4–Al2O3 system,
although conductivity exhibits two peaks, they are merged
and appear as a single peak within the limit of error. The
characteristic threshold of the conductivity of the composite system, which generally occurs in the range
between 30- and 40-mol% concentration of the second
phase, has been observed at 30 mol% for Na2SO4–MgO
system and 40 and 30 mol% for Na2SO4–SiO2 and
Na2SO4–Al2O3 systems, respectively. The additional peak
at lower concentration, in the case of Na2SO4–SiO2 and
Na2SO 4–Al 2O3 systems, cannot be attributed to the
percolation threshold, as there is rare possibility of
forming continuous path in the system at such a lower
concentration.
The conductivity behaviors for 5- and 40-mol% dispersoid concentrations for all the systems are shown in Figs. 10
and 11, respectively. The conductivity for MgO dispersed
composite system is the highest followed by SiO2 and
Al2O3. It suggests that the fraction of second phase not only
influences the conductivity of the host but also the nature of
the dispersoid. In this case, the dielectric constant of all the
Nominal
Na2SO4 (wt%)
Nominal
SiO2 (wt%)
Experimental compositions
Phase V
(wt%)
Phase III
(wt%)
SiO2
(wt%)
98
96
90
85
78
70
2
4
10
15
22
30
18
44
51
53
27
19
80
52
41
33
49
52
2
4
8
14
24
29
three dispersoids varies in the range between 8 and 10.
Thus, there is no significant difference between the
dielectric constant of the dispersoids. However, the ionic
radii of the cations in the dispersoid are different. The Mg2+
cation has radii close to that for the Na+ cation. The second
difference between the dispersoid cations is the site
preference to be accommodated in the host. In addition,
the valency difference does exist. The ionic radii of the Na+
cation for VI and XII coordination are 1.02 and 1.39 Å,
respectively. The radii of Mg2+, Al3+, and Si4+ are 0.72,
0.535, and 0.4 Å in coordination VI, whereas Si4+ for
coordination IV is 0.26. The ionic radius of S6+ in
coordination IV is 0.12 Å. On the basis of coordination
and ionic radii, the local solubility of Mg2+ cation may be
higher than Al3+ and Si4+ at Na site. Moreover, the most
possible site that Si4+ will prefer is S6+. The degree of
defect diffusion will thus be higher for Mg2+ cation leading
to a higher induced defect at interface. At lower concentration, namely, 5-mol% dispersoid, Al3+ and Mg2+ have a
similar effect, as the conductivity is not significantly
different. However, for 40-mol% concentration of disper-
-1
-1
-2
log σ (S/cm)
-3
-4
-5
-3
-4
-5
-6
-6
-7
-7
1.6
1.8
2.0
2.2
2.4
2.6
-1
(1000/T) K
Fig. 4 Log σ as a function of 1,000/T for Na2SO4–MgO composite
system
Na2SO4-SiO2
100 : 00
95 : 05
90 : 10
80 : 20
70 : 30
60 : 40
50 : 50
-2
log σ (S/cm)
Na2SO4-MgO
100 : 00
95 : 05
90 : 10
80 : 20
70 : 30
60 : 40
50 : 50
40 : 60
1.6
1.8
2.0
2.2
2.4
2.6
-1
(1000/T) K
Fig. 5 Log σ as a function of 1,000/T for Na2SO4–SiO2 composite
system
261
-2
-2
log σ (S/cm)
-3
-4
-5
-6
-3
log σ (S/cm)
Na2SO4-Al2O3
100 : 00
95 : 05
90 : 10
80 : 20
70 : 30
60 : 40
50 : 50
-4
-5
300 C
-7
400 C
1.6
1.8
2.0
2.2
2.4
2.6
10
20
-1
30
40
50
60
mol% SiO2
(1000/T)K
Fig. 6 Log σ as a function of 1,000/T for Na2SO4–Al2O3 composite
system
Fig. 8 Conductivity isotherms for Na2SO4–SiO2 composite system
soid, a significant difference is observed. It appears that the
induced defect through defect reaction across the interface
is higher for Al2O3 relative to SiO2. Thus, a percolation
threshold exists at 30 mol% for the Al2O3 dispersed system,
whereas the same is observed for 40-mol% SiO2. However,
the order of enhancement in the conductivity is identical. It
suggests that the interaction between matrix and dispersoid
for the case of Al2O3 may be relatively more. As the
conductivity of composite system depends on distribution
of second-phase particles, a slight agglomeration may lead
to shifting of percolation threshold. However, the defect
reactions at interface, in case of MgO and Al2O3, are
predominantly cation vacancy and cation diffusion on the
sodium site in the matrix, whereas in case of SiO2
predominant interaction between matrix and dispersoid is
at anion (SO4)2− site. The enhancement in conductivity by a
factor of 35, which has been observed for SiO2 and Al2O3
dispersed system, suggests that the effect of both dispersoids is similar. The defect reaction for three dispersoids
can be summarized below.
0
Na2 SO4 !
MgNa þ VNa
2þ
Mg
0
Na2 SO4 !
Al
3þ
Na þ 2VNa
00
Na2 SO4 !
ðSiO4 ÞSO4 þ V
4
SO4
SiO4
Thus, various possible defect reactions that can occur at
the interface result into modifying the width of the space
charge region. The excess defect concentration due to
diffusion of cation or anion from dispersoid into matrix and
-2.4
-1.8
300 C
400 C
-3.0
log σ (S/cm)
-2.4
log σ (S/cm)
ð1Þ
Al
-3.0
-3.6
400 C
300 C
20
30
40
mol% MgO
50
-4.2
-4.8
-4.2
10
-3.6
60
Fig. 7 Conductivity isotherms for Na2SO4–MgO composite system
10
20
30
40
50
60
mol% Al2O3
Fig. 9 Conductivity isotherms for Na2SO4–Al2O3 composite system
262
Conclusions
-1
Na2SO4
MgO 5 mol%
SiO2 5 mol%
Al2O3 5 mol%
-2
log σ (S/cm)
-3
-4
-5
-6
-7
1.6
1.8
2.0
2.2
The enhancement in the conductivity of Na2SO4 is observed
for all the three dispersoids, namely, MgO, SiO2, and Al2O3.
The maximum conductivity has been analyzed for MgO
dispersed system, which indicates the role of local solubility
of cations from dispersoid into the matrix. Existence of
phase III in the composite system may be an indication of
formation of an interfacial phase in the composite system. It
appears that MgO is a good choice among the dispersoids
employed in this work. Two peaks in the conductivity vs
composition plot have been observed for SiO2 and Al2O3
composite systems. The peak at lower concentration may be
the result of highly conducting interfacial phase.
2.4
-1
(1000/T) K
Fig. 10 Log σ as function of 1,000/T for pure Na2SO4 and various
composite system containing 5-mol% dispersoid concentration
vice versa cannot be ruled out. Furthermore, an increase in
the defect concentration at interface is compensated by
creating defect in the bulk of dispersoid and matrix as well.
Thus, the conductivity enhancement of the composite
system is likely to be a result of defect diffusion at the
interface. Thus, the percolation threshold and the enhancement in the conductivity may be determined by defect types
in the matrix and the dispersoid.
-1
Na2SO4
MgO 40 mol%
SiO2 40 mol%
Al2O3 40 mol%
-2
log σ (S/cm)
-3
-4
-5
-6
-7
1.6
1.8
2.0
2.2
2.4
-1
(1000/T) K
Fig. 11 Log σ as function of 1,000/T for pure Na2SO4 and various
composite system containing 40-mol% dispersoid concentration
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