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Chapter 3 ' ' '
Studies on A-A -M-M-0 (A=Na or K; A =Ca or Sr; M= Ti or Zr; '
M = Nb or Ta) system
The oxide ion conductors in the system A-A'-M-M0
-0 (A=Na or K; A' =Ca or Sr; M = Ti
or Zr; M0
= Nb or Ta) were prepared by solid state reaction method. Phase identification
by X-ray powder diffraction shows they have perovskite type structure. The effects of
microstructure on the properties were studied using scanning electron microscopy (SEM).
Ionic conductivity of the oxides was measured using ac impedance spectroscopy as a
function of temperature ranging from 300-750°C.
3.1 Introduction
Inorganic oxides having high ionic conductivity are currently of great importance,
primarily because of their possible application as solid electrolytes in solid oxide fuel
cells (SOFC) [1]. They have a wide variety of other applications also like oxygen sensors
[2] and pumps [3] as well as membranes for oxygen separation [ 4]. Currently, yttria
stabilized zirconia (YSZ) is commonly used as an electrolyte in the conventional SOFCs
[5]. However, it is associated with some limitations such as high operating temperature
(1073-1273K), relatively low ionic conductivity below l000°C, problems about long term
stability of the cells, etc. Lowering the operating temperature can reduce some of these
problems.
Therefore, there is intense research going on worldwide to find new materials
h_aving high ionic conductivity at lower operating temperatures. Some useful materials
have already been developed in a few different systems such as samarium doped ceria
(SDC) [6], rare eath gallates [7] and lanthanum molybdate, La2Mo209 [8] and
NaBhV2010 [9].
Perovskite structured oxide ion conducting compositions with conductivities high
enough for consideration are developed recently. Perovskite type oxides have two sites
for aliovalent doping and thus creating vacancies in oxygen sublattice, which help the
migration of oxygen ions through the lattice [ 10-12]. Lanthanum gallate is one candidate
for application as electrolyte in SOFCs [13, 14]. Doubly doped (Sr and Mg) LaGa03
based perovskite system exhibits oxide ion conductivity higher than that of YSZ at 800°C
[15-17]. In the present study, high ionic conductivity in new perovskite type oxides: A
A0
-M-M0
-0 (A=Na or K; A0
=Ca or Sr; M= Ti or Zr; M0
= Nb or Ta) is reported.
3.2 Experimental
High temperature solid state reaction technique is used for the preparation of the
oxides in the A-A0
-M-M0
-0 (A=Na or K; A0
=Ca or Sr; M= Ti or Zr; M0
= Nb or Ta)
systems. High purity chemicals Na2C03 (99%, NICE), K2C03(99+%, Acros
Organics),CaC03 (99+%, Acros Organics), SrC03 (98.5%, OTTO KEMI), Ta205 (99.9%,
47
Acros Organics), Ti02 (99%, Merck) Zr02 (99%, Acros Organics) and Nb205(Sigma
Aldrich Chemicals) were wet mixed with acetone as the wetting medium in an agate
mortar and dried in an air oven at 100°C for I hr. The process of grinding and heating was
repeated thrice to get a homogeneous mixture.
The samples of NaA'TiM,06 (A' =Ca orSr; M' = Nb or Ta) were calcined at
1300°C in platinum crucible for l 2hrs. Calcined powders were ground well and
uniaxially pressed into compacts of 10mm diameter and 2.5mm thickness using hydraulic
press by applying a pressure of 2.5 tons. Then the pellets were sintered at 1450°C for
6hrs.
NaA'ZrM'06 (A' =Ca orSr; M' = Nb or Ta) samples were calcined at 1300°C for
12hr. The calcined powder was uniaxially pressed into pellets of 10mm diameter and
2.5mm thickness using hydraulic press by applying a pressure of 2.5 tons. Then the
pellets were sintered at 1450°C for 12hr.
KSrTi Nb06 sample was calcined at l000°C for 3hr and the calcined powder was
pelletised using hydraulic press by applying a pressure of about 2.5 tons and sintered at
1300°C for 3hr. The potassium compound shows some hygroscopic nature while
preparation.
The crystalline phase of the samples was examined by powder X-ray diffraction
method (XRD) with a Philips X-pert Pro X-ray diffractometer using Ni-filtered CuKa
radiation. The differential thermal analysis is carried out on the powdered samples using
Slimadzu 50H Thermal Analyzer.
The surfaces of the polished and thermally etched (near the sintering temperature
for 30 min) pellets were photographed using scanning electron microscope, JEOL JSM
5600LV to study grain morphology. For the electrical conductivity measurements the
pellets were electroded with long silver leadwires using silver paste. The measurements
were taken using Solatron 1260 Impedance Analyzer in the air and oxygen atmosphere.
48
3.3 Results and Discussion
3.3.1 AA1
TiM1
06 (A=Na or K; A'=Ca or Sr; M1
= Nb or Ta) system
3.3.1.1 XRD and Structure
Powder XRD patterns of the AA1
TiM1
06 (A=Na or K; A0
=Ca or Sr; M1
=
Nb or Ta) system is given in fig 3. l a-c. All the peaks in the patterns can be indexed on
the basis of perovskite type structure. The XRD patterns of these compounds show the
formation of a single phase pure perovskite type compound. The tolerance factor
calculated for this system is -1, the optimum tolerance factor to obtain the largest
electrical conductivity in perovskite type oxides [ 18].
NaCaTiNb06 and NaCaTiTa06 give XRD patterns which can be indexed based
on orthorhombic perovskite structure. The d spacing is in good agreement with the
JCPDS powder diffraction file (PDF) No.51-133 [19]. The prominent peaks are (101),
(200) and (220) reflections.
NaSrTiNb06, NaSrTiTa06 and KSrTiNb06 can be indexed on the basis of cubic
perovsl<ite structure. These patterns are similar to that of earlier reported XRD patterns of
perovskite type compounds such as KTa03 and are in good agreement with the PDF
No.38-1470 [20]. The strong diffraction peaks corresponds to (110), (100) and (200)
planes. The lattice parameters of these oxides are given in table 3.1.
49
0
10 20
0
0
N
30 40
0
N
N
29
�
0 N
N
<"> N
50
NaCaTiTa06
60 70 80
Fig. 3.la X-ray diffraction pattern of the NaCaTiM'o6 (M0
= Nb or Ta)
10 20 30 40
20
0
0
N
-
0
�
N
50
NaSrTiTa06
�
N
0
N
N
60 70
Fig. 3.lb X-ray diffraction pattern of the NaSrTiM0
06 (M' = Nb or Ta)
50
10 20 30 40
20
a
a
50 60
a
"'
"'
70
Fig. 3.lc X-ray diffraction pattern of KSrTi Nb06
3.3.1.2 Differential thermal analysis (DT A)
The DTA curves for the NaA'TiM'06 (A' =Ca orSr; M' = Nb or Ta) system are
shown in �g.3.2 a-d. DTA study of the new system is carried out to see whether there is
any phase change occurs during heating. There is no phase change is observed in the
DTAplots.
51
80
60
20
0
100
80
60
40
20
0
0 200 400
Temperature (°C)
600 800
Fig. 3.2a DTA curve for NaCaTiNb06
0 � � � � � � � � �
Temperature (°C)
Fig. 3.2b DTA curve for NaCaTiTa06
52
100 ,----------------�
80
60
20 /
0
0 200 400 600 800 1000
Temperrature (0
C)
Fig. 3.2c DT A curve for N aSrTiNb06
100
80
// 60
40 // 20
/0
0 200 400 600 800 1000
Temperature (0
C)
Fig. 3.2d DTA curve for NaSrTiTa06
53
3.3.1.3 SEM and microstructure
The SEM microstructure of the polished thermally etched surfaces of the sintered
pellets is shown in fig.3.3a-e. The samples show well sintered and well grown grains with
minimum porosity except in the case of KSrTiNb06• The density calculated for these
materials is in the range 4.6-4.9 glee. This can be confirmed from the microstructure.
The average grain size of this material is Sµm. The micrographs reveal single phase
morphology of the compounds without any secondary phase. The micrograph indicates
that KSrTiNb06 is porous and this affected its conductivity value.
54
a. NaCaTiNb06
c. NaSrTiNb06
b.NaCaTiTaO6
d. NaSrTiTa06
e. KSrTi NbO6
Fig. 3.3 SEM micrographs ofAA'TiM'06(A=Na or K; A' =Ca or Sr; M' =Nb or Ta) system
55
3.3.1.4 Electrical conductivity studies
Conductivity in solids can be either electronic or ionic. Electronic conductivity is
usually of the order of -10-3 Siem (semiconductors) to -10 10 Siem (metals) and such
materials will usually blackish. Ionic conductivity would be of the order of 10·5 - 10 1
Siem and such materials would be usually colorless.
The electrical conductivity of NaA'TiM,06 (A' =Ca orSr; M' = Nb or Ta) system
has been measured in the temperature range 300-750°
C in air as well as oxygen
atmosphere. The conductivity values at 700°
C are given in table 3.2. The electrical
conductivity at 700°
C is in the order of I 04 Siem for all compounds except KSrTiNb06•
The Arrhenius plots of the conductivity, cr as a function of 1/T (T=absolute
temperature) is shown in fig. 3.4a-e. The conductivity value obtained for the system in air
atmosphere is higher than the value obtained in oxygen atmosphere. In oxygen deficient
systems the conductivity takes place by the hopping of 02· ions in the vacancies. Since,
the number of vacancy decreases in oxygen atmosphere the rate of hopping of 02· ions
into vacancy sites also decreases [21]. Hence the ionic conductivity decreases in the
oxygen atmosphere.
As can be seen from the figures, the curves are approximately linear indicating
that cr nearly follows the Arrhenius type equation,
cr = cr0 exp (-Ea/kT). In this relation, cr0 represents a pre exponential factor, Ea is
the apparent activation energy of conduction process, k is the Boltzman's constant and T
is the absolute temperature.
The activation energy for conduction was calculated from the slope of the graph
in the temperature range 500-600°
C. Kilner and Brook [22] calculated that the intrinsic
activation energy for the perovskite structure should be 0.8 ± 0.2 eV. The higher
activation energy compared to fluorite structure is due to the smaller opening through
which the oxide ion must pass in the perovskite structure. It is seen that activation energy
of current carriers in strontium compound is more than the calcium substituted
compounds. The reason for this may be that the larger cation Sr2+ increases the
polarizability of ions to give the bonding character to be covalent [ 18]. Kreuer proposed
56
that the perovskite with Sr in A site absorb water in vacancies and stabilize oxide ion in
the lattice [23].
The activation energy in the oxygen atmosphere is higher than that in the air
atmosphere. This corresponds to the decrease in conductivity due to the decrease in rate
of hopping of 02• ions in the vacancy sites in the system [24]. The systems with Ta show
more activation energy. This may be due to the change in ionic radius of Nb and Ta.
The typical Cole-Cole plots of the NaA'TiM,06 (A'=ca or Sr; M' = Nb or Ta)
system at different temperatures (600-700°C) are shown in fig.3.5a-i. The intersection of
the semicircle on the low frequency side ( right of the fig.) with abscissa was taken as the
total resistance of the sample. There is only one semicircle is observed which
corresponds to the grain conduction [25]. Frequency decreases with the increasing real
component of impedance. For some of the compounds the plot doesn't pass through the
origin. This is because there are other arcs appear at higher frequencies and also Roo >O.
The presence of pores modifies the impedance plots by altering the diameter of the arcs
[26]. The distortion of the semicircle at low frequency region (right of figure) is due to
the material electrode interaction [27].
57
-3,-----------------�
-4 ·\· •"-
-5
-7
-8
'\:"-. I • NaCaTiNbO,(Alr) I "" "' -• · NaCaTiNb0,(02)
·"'·� ·""·�. ." �.,"
�:� �-"'
."'---�• ,�.
0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8
1000/T (K1)
Fig. 3.4a logcr vs l 000/T for NaCaTiNb06 in air and oxygen atmosphere
-3
-4
� -5
t> -6 �
-7
-8 -
0.9
�--�. •�. I • NaCaTiTaO,(Alr)
\ •�
•- NaCaTiTa0,(02)
1.0 1.1
� ·,. --�.
··��=�
�,·� •-............. ,, �· •
1.2 1.3 1.4 1.it 1.6
1000IT(¥_)
1.7 1.8
Fig. 3.4b logcr vs l 000/T for NaCaTiTa06 in air and oxygen atmosphere
58
-3.0
-3.5
-4.0 /?-
�-4.5 VJ "-:; -5.0
..g -5.5
-6.0
-6.5
-7.0
•:::::: ·�. ·�--. '•� �-
I m- NaSrTINbO,(Air) \ m-- NaSrTiNb0,(02)
-�.-�-�
'• � �--�-.
'•� "-._•
•
-7.5 +-�-r--r-,--�,--,--...�--.-���-,-���...--1 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8
1000/T (K.1}
Fig. 3.4c logcr vs l 000/T for NaSrTiNb06 in air and oxygen atmosphere
-4
-5
-8
-9
I I
I •· NaSrTiTaO,(Air) \ •- NaSrTiTa0,(0
2)
0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8
1000/T (K')
Fig. 3.4d logcr vs l 000/T for NaSrTiTa06 in air and oxygen atmosphere
59
-4.0
-4.5
-5.0-
K � -5.5
t)
C)
.Q -6.0-
-6.5
-7.0
I I o I
0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8
1000/T (K1)
Fig. 3.4e log cr vs 1000/T for KSrTiNb06 in air atmosphere
-3.0e3 t--1----1--+--+---+--+---1---+--+--+----,1----1
,..... Ill
-2.5e3
8, -2.0e3
"' C:
·5
.§ -1.5e3
., " C:
� -1.0e3
o.oe,rfflm--------------_...!!!l._ _____ ..;z!II _ __J
0. 1.0e3 4.0e3 6.0e3
Impedance real (ohms)
Fig. 3.Sa Cole-Cole plot for NaCaTiNb06 in air atmosphere (a) 600°
C (b) 650°
C and (c)
700°
C
60
,.... -1.2e3
! c.'
·l -8.0e2 §.,"ffi al t -4.oe2
_ .......... _ (a) .. . ......
.,,. ....
Impedance real (ohms)
..........
2.4e3
Fig. 3.5b Cole-Cole plot for NaCaTiNb06 at 700°C in (a) oxygen and (b) air atmosphere
-6.0e4
,.;;-� -5.0e4 e. c.' l:'l -4.0e4 ·a, ..
E -; -3.0e4 " C: ..
"O
� -2.0e4 .s
6 .Oe4 8 .Oe4 1.0e5 1 .2e5
Impedance real (ohms)
Fig. 3.5c Cole-Cole plot for NaCaTiTa06 in air atmosphere (a) 600°C (b) 650
°C and (c)
700°C
61
-2.0e4t--+---+---+---+---+---+--+--+---+----
'[ -1.5e4
e.. C'
•! § -1.0e4
f -5.0e3
\
\ 4
\ O.OeH-1�------------------=--.l
O.OeO 1.0e4 2.0e4 3.0e4 4.0e4 5.0e4
Impedance real (ohms)
Fig. 3.5d Cole-Cole plot for NaCaTiTa06 at 700°C in (a) oxygen and (b) air atmosphere
-1.2e3 T"-l----+----+---+----+---+---+------1
'[ -9.0e2
e. � ·�§ -6.0e2 "' " C .. .,,,
f -3.0e2
(a)
1.0e3 2.0e3 3.0e3 4.0e3
Impedance real (ohms)
Fig. 3.5e Cole-Cole plot for NaSrTiNb06 in air atmosphere (a) 600°C (b) 650
°C and (c)
700°C
62
-8.0e2,--+---I----+---+----+--+----+---+-----i
i -6.0e2
e~
'"c:t ·4.0e2
~
~'"..§ -2.0e
(a)
O.OeO.1..---------------......--.....------l5.0e2 1.0e3 1.5e3 2.0e3 2.5e3
Impedance real (ohms)
Fig.3.5f Cole-Cole plot for NaSrTiNb06 at 700°C in (a) air and (b) oxygen atmosphere
-6.0e2 ,I--+--+--+--+--+--+--I--I--I--I--I--I--I--+--+--I--I--+--+--+--+--I
(a)
O.OeO.L-----------------~- .....-----'
-5.0e2'"'..E.t::e. ·4.0e2~~'0~ ·3.0e2
Impedance real (ohms)
Fig. 3.5 g Cole-Cole plot for NaSrTiTa06 in air atmosphere (a) 600°C (b) 650°C and (c)
700°C
63
-6.De2 r--1----1---1----1---+----1---i---1----1
j ·4.De2
e, I!' ..
. 5
l -2.De2
.. () C: ..
] § D.DeD
�·-·.._, (a)
�,,,,..� ..... ....
2.De2.L.--------------------__J 4.De2 8.De2 1.2e3 1.6e3 2.De3
Impedance real (ohms)
Fig. 3.5h Cole-Cole plot for NaSrTiTa06 at 700°C in (a) air and (b) oxygen atmosphere
-4.De3 +---+---l-+---+---t-+---1---1--i--1--+---+-+---1---+---<
j -3.De3 ,:;;;
0 ,..,, I!' .. C:
·5, .§ -2.0e3
"' ()
Iii .,, �..5 -1.De3
1.0e4 1.2e4 1.4e4 1 .6e4
Impedance real (ohms)
Fig. 3.5i Cole-Cole plot for KSrTiNb06 in air atmosphere (a) 600°C (b) 650
°C and (c)
700°C
64
Table 3.1
Crystal system and lattice parameters of AA0
TiM0
06 (A=Na or K; A0=Ca or Sr; M
0
= Nb or Ta) system
Composition Crystal Lattice parameter (A)
System a b C
NaCaTiTa06 Orthorhombic 5.432 7.738 5.479
NaSrTiTa06 Cubic 3.895
NaCaTiNb06 Orthorhombic 5.483 7.728 5.445
NaSrTiNb06 Cubic 3.917
KSrTiNb06 Cubic 3.925
Table 3.2
Comparison of conductivity and activation energy ofNaA0
TiM0
06 (A0
=Ca or Sr; M0
= Nb or Ta) compounds at 700°C
Composition Conductivity Conductivity Activation Activation
(air) (S cm-1) (02) (S · cm- 1) energy (air) energy (02)
(eV) (eV)
NaCaTiNb06 1.6lxl0-4 l.43x10-4 1.068 1.171
NaCaTiTa06 1.49x10-4 1.37xl0-4 1.241 1.269
NaSrTiNb06 3.77xl0-4 3.2lxl0-4 1.083 1.180
NaSrTiTa06 2.03xl0-4 1.99xl0-4 1.262 1.310
65
3.3.2 NaA0
ZrM0
06 (A' =Ca or Sr; M0
= Nb or Ta) system
3.3.2.1 XRD studies
XRD patterns of NaA0
ZrM0
06 (A0
=Ca or Sr; M0
= Nb or Ta) system is shown in
fig.3.3.6a-c. The patterns are similar to that obtained in the earlier system; the peaks are
not sharp. The patterns of NaCaZrNb06 and NaCaZrTa06 are indexed on the basis of
orthorhombic perovskite structure. The d spacing is in good agreement with the PDF
No.51-133 [19] and prominent planes are (101), (200) and (220).
NaSrZrNb06, and NaSrZrTa06 can be indexed on the basis of cubic perovslcite
structure and are in good agreement with the PDF No.38-1470 [20]. The strong
diffraction peaks corresponds to ( 110), ( 100) and (200) planes. The lattice parameters of
these oxides are given in table 3.3.
10 20
0
0
N
30 20
NaCaZrTa06
NaCaZrNb06
40 50 60 70
Fig. 3.6a X-ray diffraction pattern ofNaCaZrM0
06 (M0
= Nb or Ta)
66
~
0,...,...~
~
00,... NaSrZrTaO~
~
~ ,...0
~
,...0 0 N ~
N ,... ~ 0~
N N~ N
• ;;.........
NaSrZrNbOe
.11. ~
10 20 30 40
29
50 60 70
Fig. 3.6b X-ray diffraction pattern ofNaSrZrM'06 (M' = Nb or Ta)
3.3.2.2. Differential thermal analysis (DTA)
The DTA curves for the NaA'ZrM'06 (A' =Ca or Sr; M' = Nb or Ta) were shown
in fig.3.7 a-d. The system is studied for determination of any phase change during
heating. There is no phase change is observed for this system.
67
80
70
60
50
:[ 40
�
20
10
0
-10 0 200 400 600 800 1000
Temperature (0C)
Fig. 3.7a DTA curve for NaCaZrNb06
0 200 400 600 800 1000
Temperature (0C)
Fig. 3.7b DTA curve for NaCaZrTa06
68
80..--- --,
70
60
50
~4OCl:b 30
20
10
o
o 200 400 600
Temperature l°C)800 1000
Fig. 3.7c DTA curve for NaSrZrNb06
70
60
50
40
~30
~0
20
10
0
·100 200 400 600 800 1000
Temperature (oC)
Fig. 3.7d DTA curve for NaSrZrTa06
69
3.3.2.3. SEM and Microstructure
The SEM micrographs of the system NaA0
ZrM0
06 (A' =Ca or Sr; M' = Nb or Ta)
were given in fig.3.3.8 a-d. The density measured for NaCaZrM0
06 (M' = Nb or Ta)
system is in the range 4.9g/cc and for the other system, NaSrZrM0
06 (M0
= Nb or Ta), it is
in the range 4.2g/cc. The micrograph confirms these results. The SEM micrograph of
NaCaZrTa06 shows pores within the grains. The micrograph indicates that NaSrZrTa06
is porous and this affected its conductivity value. The average grain size is in the range 1-
3µm.
70
I
3.3.2.4 Electrical conductivity studies
The electrical conductivity of the system NaA'ZrM'06 (A' =Ca or Sr; M' = Nb or
Ta) has been measured by impedance analyzer over a frequency range 100mHz-IOMHz
in the air as well as oxygen atmosphere. The measurement temperature range was
between 300-750°C. Fig.3.9 a-d shows Arrhenius plots of conductivity for NaA'ZrM'06
(A' ==Ca or Sr; M' == Nb or Ta) system. The graph obtained is almost straight line
indicating that the conductivity obeys the Arrhenius equation. It can be seen from the
Arrhenius plots that the electrical conductivity increases linearly with rising temperature
and no abrupt conductivity change is observed. The conductivity values at 700°C are
given in table 3.4.
The conductivity values obtained for the system in oxygen atmosphere is less than
the values obtained in air atmosphere. This is due to the decrease in the rate of diffusion
of 0 2- ions into the vacancy due to the decrease in the number of vacancy in oxygen
atmosphere [24].
The activation energy for conduction was calculated from the slope of the graph
in the temperature range 500-600°C. The activation energy of the system in oxygen
atmosphere is higher than that in air. This is expected from the decrease in ionic
conductivity.
The conductivity is a function of the elements in the structure, the size of the
elements and the degree of order or disorder in the structure. Sammells et al. [28, 29]
observed that there is a strong correlation between the conductivity and the size of the
critical radius. The critical radius refers to the radius of the smallest area through which
the oxide ion 0 2- must pass.
The degree of distortion seems to be the most important parameter controlling the
oxide ion conductivity and the distortion seems in turn to be closely related to the degree
of deviation from matching ionic radii of the cation dopants that are used to introduce the
oxide vacancies [30]. The charge of the B ion (in this case Ti4+, Zr4+, Nb5+ and Ta5+) is
also seems to be important in the AB03 perovskites.
72
The typical Cole-Cole plots of the NaA'TiM'06 (A'=Ca or Sr; M' = Nb or Ta)
system at different temperatures (600-700°C) are shown in fig.3.10a-h. The intersection
of the semicircle on the low frequency side (right of the fig.) with abscissa was taken as
the total resistance of the sample. There is only one semicircle which corresponds to the
grain conduction. The Cole-Cole plot of NaCaZrTa06, does not pass through the origin,
since Roo>O. Presence of pores modifies the impedance plot by altering the diameter of
the arcs [26].
-2,------------------,
-3
-4
-5
E -6~~
I:> -7~
-8
-9
-10
\-.. NaCaZrNbO,(Air) \-e- NaCaZrNbO,(O,)
0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8
10001T (K')
Fig.3.9a log cr vs 1000/T for NaCaZrNb06 in air and oxygen atmosphere
73
-2
-3
-4
-5
-6
-7 ..Q
-8
-9
.,, ., "• '•"'--
"-:',, - a- NaCaZrTaO,(Air)
, •, a- NaCaZrTa0,(02)
'• ' �-� • •
"".�. �� ·�· .�.
""�.� �·
""'·
0,9 1-0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8
1000/T (K1
)
Fig.3.9b log cr vs I 000/T for NaCaZrTa06 in air and oxygen atmosphere
..Q
-2
-3
-4
-5
-6
-7
-8
-9
-10
• NaSrZrNbO,(Air) - • - NaSrZrNb0,(0
2)
Q9 1� 1j 12 1� 1.4 1� 1� 1J 1B
1000/T (K1)
Fig.3.9c log cr vs 1000/T for NaSrZrNb06 in air and oxygen atmosphere
74
e
-2
-3
-4
-5
-6
-7
-8
-9
-10
•· NaSrZrTaO,(Air) .__. NaSrZrTaO,(O,)
...
0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8
1000/T (K"1)
Fig.3.9d log er vs l 000/T for NaSrZrTa06 in air and oxygen atmosphere
-6.0e3 +--+--+---t>----+--+---+--+--+-----+-----+-----1
,-..
..
-5.0e3
e. -4.0e3
"'
.s
i -3.0e3
� -2.0e3
-1.0e3
0.0eO 0. 4.0e3 6.0e3 8.0e3 1.0e4 1.2e4 1.4e4
Impedance real (ohms)
Fig. 3.10a Cole-Cole plot for NaCaZrNb06 in air atmosphere (a) 600°C (b) 650
°C and (c)
1oo·c
75
-6.0e2 t-----t---+---+----+---1-----11----+-----1
I 8. -4.0e2
� <:
"6, ..
..5 ... () <: {1l -2.0e2
!
I
I
5.0e2 1.0e3
Impedance real (ohms) 1.5e3
\ \
4-
4
2.0e3
Fig. 3.10b Cole-Cole plot for NaCaZrNb06 at 700°
C in (a) oxygen and (b) air
atmosphere
,-.. ..
E ..c::
-6.0e2-r--+---+----+---+----+------il---+---+---I
e. -4.0e2 (a) 2:-
-� "'..
..5 ... () <:
t ..5
(b)
8.0e2 1.2e3 2.0e3
Fig. 3.10c Cole-Cole plot for NaCaZrTa06 in air atmosphere (a) 600°
C (b) 650°
C and (c)
700°
C
76
·1.2e2,-+---+----+---+---+_--+---~f__-_...,
·1.0e2
6.0e25.0e24.0e2
Impedance real (ohms)
-2.0el
O.OeO"-----"''---1t-1-----------------.....---l
¥&.e. ·S.Oel~
.~co~ ·6.0el
g~ ·4.0et.,<>.
..5
Fig. 3.10d Cole-Cole plot for NaCaZrTa06 at 700°C in (a) oxygen and (b) air atmosphere
-1.6e5t---I--------+---+--~---+---+---j
·1.4e5
¥ ·t.2e5
e~ -1.0e5..c'6>~ -8.0e4
.,g -6.0e4...".,r -4.0e4
(c)
t .Oe5 2.0e5
(a)
4.0e5
Impedance real (ohms)
FIg. 3.10e Cole-Cole plot for NaSrZrNb06in air atmosphere (a) 600°C (b) 650°C and (c)
700°C
77
·8.0e4t-----t---I---+----+---+----t----+-----t
-' ' _A.- . - (a).... ' ..,....~,
~ -6.0e4
ec- .Illl! I'0~ -4.0e4 ~
..~ Ii ,..§ -2.0e4 ;.. •__,~..
(b)
1.0e5
Impedance real (ohms) .
\,\~..\
1.5e5 2.0e5
Fig. 3.10f Cole-Cole plot for NaSrZrNb06at 700°C in (a) oxygen and (b) air atmosphere
·6.0e5 +---I---+--I--_+_-+----+--I--+---I--+--_+_-1
-5.0e5
~.£:e -4.0e5c-..<:'0~ -3.0e5
OJ<><:-l!l -2.0e5OJ
.§
O.OeO.OeO 2.0e5 4.0e5 6.0e5 8.0e5
Impedance real (ohms)1.0e6 1.2e6
Fig. 3.10g Cole-Cole plot for NaSrZrTa06 in air atmosphere (a) 600°C (b) 650°C and (c)
700°C
78
-4.0e5 t---+---+--l-+---+--+--+-+---+--+--+---+-t---+--+-i
~ -3.0e5..ce.i'5~ -2.0e5
.."c:.....,..t -1.0e5
_ ' .....' _ , _ , ...... (a)
....-' --,.,.- .... ,.....~
'.,.~
\...O.Oe_----------_~-------l .......
o.OeO 1.0e5 2.0e5 3.0e5 4.0e5 5.0e5 6.0e5 7.0e5 S.Oe5- .. -. .
Impedance real (ohms)
Fig. 3.10h Cole-Cole plot for NaSrZrTa06 at 700°C in (a) air and (b) oxygen atmosphere
Table 3. 3
Comparison of conductivity and activation energy of NaA'ZrM'06 (A'=Ca or Sr;
M' = Nb or Ta) compounds at 700°C
Composition Crystal System Lattice parameter (A)
a b c
NaCaZrTa06 Orthorhombic 5.532 7.746 5.553
NaSrZrTa06 Cubic 3.932
NaCaZrNb06 Orthorhombic 5.584 7.734 5.543
NaSrZrNb06 Cubic 3.946
79
Table 3. 4
Comparison of conductivity and activation energy of NaA'zrM'o6 (A'=ca or Sr;
M' = Nb or Ta) compounds at 700°C
Composition Conductivity Conductivity Activation Activation
(air) ( S cm-1) (02) (S cm-1) energy (air) energy (02)
(eV) (eV)
NaCaZrNb06 3.62 xl0-J 2.73 xl0-j 1.858 2.124
NaCaZrTa06 1.78 X 10-J l.17xlo-j 2.157 2.201
NaSrZrNb06 2.56x 10-J 2.31xl0-j 1.893 2.198
NaSrZrTa06 1.22 X 104 l .09 x 10-4 2.321 2.524
3.4 Conclusion
The A-A'-M-M'-o (A=Na or K; A' =Ca or Sr; M= Ti or Zr; M'= Nb or Ta)
systems were prepared and electrical properties were studied. The X-ray diffraction study
shows the formation of perovskite structured compounds with single phase. The high
value of ionic conductivity is obtained for the NaA'ZrM'06 (A'=ca or Sr; M' = Nb or Ta)
system, except NaSrZrTa06 compound. The conducting species in this system is the 02-
ions. From the ionic conductivity studies, it can be concluded that this systems, especially
Zr substituted compounds, are potential candidates as solid electrolytes.
80
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