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Influence of Microstructure on the Electrical Properties of NASICON
Materials
Article in Solid State Ionics · March 2001
DOI: 10.1016/S0167-2738(01)00701-9
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Solid State Ionics 140 Ž2001. 173–179
www.elsevier.comrlocaterssi
Influence of microstructure on the electrical properties of
NASICON materials
R.O. Fuentes a,b, F.M. Figueiredo a,c,) , F.M.B. Marques a , J.I. Franco b
a
Ceramics and Glass Engineering Department, UIMC, UniÕersity of AÕeiro, 3810-193 AÕeiro, Portugal
b
PRINSO-CITEFA-UNSAM, Buenos Aires, Argentina
c
Science and Technology Department, UniÕersidade Aberta, 1269-001 Lisbon, Portugal
Received 24 May 2000; received in revised form 17 January 2001; accepted 19 January 2001
Abstract
NASICON-type compounds with the nominal formula, Na 3 Si 2 Zr1.88Y0.12 PO12 , were prepared by a typical ceramic route
with different microstructures. The samples were fired in the temperature range 1190–12358C with sintering periods between
2 and 80 h. Results showed a significant influence of the processing conditions on the microstructure, affecting both grain
and grain boundaries. Electrical conductivity was mainly controlled by the grain boundary contribution, which is strongly
dependent on the grain size and density of grain boundaries. A maximum conductivity value of about 2.7 = 10y3 S cmy1 at
room temperature was obtained with samples sintered at 12208C for 40 h. q 2001 Elsevier Science B.V. All rights reserved.
Keywords: NASICON; Electrical conductivity; Microstructure; Impedance spectroscopy
1. Introduction
NASICON stands for a well known family of
Naq super ionic conductors with the general formula
Na 1q x Zr2 Si x P3yxO 12 Ž0 - x - 3., firstly suggested
as solid electrolyte material for Naq ion-based batteries. Recently, this family of materials has attracted
the attention of researchers looking for ion-selective
electrodes or gas sensor devices w1,2x, and a NASICON-based commercial CO 2 sensor has already been
proposed w3x.
A large range of compositions was studied and
the best conductivities were obtained for x values
)
Corresponding author. Ceramics and Glass Engineering Department, UIMC, University of Aveiro, 3810-193 Aveiro, Portugal. Tel.: q351-234-370-263; fax: q351-234-425-300.
E-mail address: framos@cv.ua.pt ŽF.M. Figueiredo..
close to 2 w4–7x. Most of these studies assess the
relation between composition, structure and electrical
conductivity, and the higher values of conductivity
appear related to a monoclinic symmetry when x is
between 1.8 and 2.2. For other values, the structural
change to rhombohedral symmetry is associated to a
decrease in conductivity. Impedance spectroscopy,
used to separate grain from grain boundary
impedances, showed that the grain boundary contribution is often the major contribution to the overall
impedance w7x.
NASICON is usually synthesized by two methods: the traditional ceramic route or a sol–gel method.
The ceramic route requires higher sintering temperatures leading to segregation of a resistive monoclinic
zirconia second phase, following Na and P volatilization w8x. A liquid phase is often found along the grain
boundaries which also results in the deterioration of
0167-2738r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 7 - 2 7 3 8 Ž 0 1 . 0 0 7 0 1 - 9
174
R.O. Fuentes et al.r Solid State Ionics 140 (2001) 173–179
the electrical properties. On the contrary, the fine
and reactive sol–gel powders, requiring lower sintering temperatures, result in more conductive and homogeneous materials w9,10x.
Previous work w11x showed that good materials
can also be obtained from a traditional ceramic route
using reactive zirconia precursors. The favorable reaction kinetics at low temperatures allows lower
sintering temperatures than usually required for the
ceramic route. These materials were dense, with
small content of monoclinic zirconia, with homogenous microstructures and without any vestiges of
liquid phase. Overall, a high electrical conductivity
and a significant abatement of the grain boundary
impedance was observed w12x.
This result suggested that further improvements
could still be achieved by optimizing the microstructure. The increase of the grain size and, therefore,
the reduction of the density of grain boundaries
should result in lower grain boundary impedances.
The aim of the present work is to investigate the
effects of the sintering conditions on the microstruc-
ture and corresponding relationships with the electrical properties.
2. Experimental procedures
One NASICON-type compound with the nominal
formula Na 3 Zr1.88Y0.12 Si 2 PO12 was obtained from
solid state reaction of ŽZrO 2 . 0.97 ŽY2 O 3 . 0.03 ŽTosoh.,
Na 3 PO4 .12H 2 O ŽMerck. and SiO 2 ŽMerck.. The
zirconia-based precursor was selected for being a
highly reactive powder, with small grain size Ž- 1
mm.. Powders were ball-milled in ethanol with zirconia balls, dried in a stove at 608C, and calcined in
air at 11008C for 8 h in a closed Pt crucible to
prevent contamination. The calcined powders were
ball-milled again for 2 h and uniaxially pressed Ž98
MPa. into disk-shaped pellets. A first series of samples was sintered in air during 10 h for different
temperatures between 11908C and 12358C, while a
second series of samples was sintered at 12208C for
2, 10, 40 and 80 h. Phase composition of both
calcined powders and sintered pellets was verified by
Fig. 1. XRD patterns of three representative materials prepared under different conditions. Monoclinic zirconia is present in all cases
Žarrows..
R.O. Fuentes et al.r Solid State Ionics 140 (2001) 173–179
X-ray diffraction ŽXRD. using Cu k a radiation. Microstructural aspects were studied by scanning electron microscopy ŽSEM., based on observation of
previously polished and thermally etched Ž10 min at
11008C. samples. The density was estimated from
the samples’ weight and geometry.
Finally, the electrical conductivity was determined from impedance spectroscopy measurements
ŽHewlett Packard 4284A LCR. between 08C and
1508C, in the frequency range 20 Hz–1 MHz. Pt
electrodes were previously painted onto the pellets
surface and heat-treated at 7008C for 30 min, to
provide the necessary electrical contacts.
3. Results and discussion
3.1. Relation between microstructure and processing
conditions
Fig. 1 shows the XRD patterns of some materials.
Since no significant differences were observed, only
175
the patterns of three representative samples were
presented, corresponding to extreme processing conditions: maximum sintering temperature Ž12358C for
10 h.; maximum sintering time Ž12208C for 80 h.;
minimum temperature and time Ž12208C for 2 h..
These patterns confirm the existence of a dominant
NASICON phase, but diffraction lines typical of
monoclinic ZrO 2 are observed in all cases. This is a
result of the relatively high sintering temperature
because no zirconia was identified in powders calcined at 11008C w12x. Nevertheless, the relative intensity of the peaks is similar in all patterns suggesting preservation of structural features irrespective of
the processing conditions.
Sintering conditions, however, do have a clear
effect on microstructure. Fig. 2 shows SEM microstructures of samples sintered at different temperatures. Despite the significant grain size distribution,
results suggest a clear trend for increasing grain size
with increasing sintering temperature ŽTable 1.. The
presence of liquid phases or segregated monoclinic
Fig. 2. SEM micrographs of materials sintered at different temperatures for 10 h.
R.O. Fuentes et al.r Solid State Ionics 140 (2001) 173–179
176
Table 1
Density and average grain size Ž G . of NASICON-based materials
processed under different conditions
Sintering temperature
Ž8C.
Sintering
time Žh.
Density
Žgrcm3 .
G Žmm.
1190
1210
1220
1220
1220
1220
1230
1235
10
10
2
10
40
80
10
10
3.20
3.25
2.98
3.26
3.26
3.27
3.26
3.25
0.7"0.3
1.0"0.3
0.6"0.3
0.9"0.3
1.0"0.4
1.3"0.3
1.4"0.4
1.2"0.3
G was determined by the lineal intercept method w13,14x.
zirconia grains could not be observed, although expected from XRD. As suggested by some authors
w4,7x, zirconia could be dispersed along the grain
boundaries, possibly in a liquid phase. Further work
by transmission electron microscopy is in progress to
clarify the structural nature of the grain boundaries.
SEM micrographs of samples sintered at 12208C
for different periods of time are shown in Fig. 3. The
increase in sintering time also results in increasing
average grain size ŽTable 1., with formation of a
liquid phase along the grain boundaries. The liquid
phase, clearly identified in the sample sintered for 80
h and probably present in other samples, should have
an important role in the sintering behavior of these
materials at temperatures close to 12208C. Densification and grain growth are probably associated to a
liquid phase assisted sintering process. This assumption is consistent with the fact that all samples
sintered for more than 2 h present similar densities
ŽTable 1..
3.2. Electrical conductiÕity and microstructure
Fig. 4 shows typical impedance spectra at 08C of
NASICON sintered under different conditions. At
08C, only the grain boundary arc is clearly seen and
the bulk contribution can only be estimated from the
Fig. 3. SEM micrographs of materials at 12208C for different periods of time.
R.O. Fuentes et al.r Solid State Ionics 140 (2001) 173–179
177
temperatures. The values of r b remain almost independent at low temperatures and increase slightly at
high sintering temperatures. In the low temperature
range, the initial decrease in rgb could be ascribed to
decrease in grain boundary density, resulting from
both increasing sample density and grain size. Increasing sintering temperature results in the segregation of a resistive liquid phase, leading to more
resistive grain boundaries. If, as expected, the composition of the liquid phase is different from the
grain, the grain composition would become different
from the nominal one. The fact that zirconia is
present as a second phase suggests that the material
is becoming Zr-deficient, and these compositions
appear to be less conductive w7x.
Fig. 4. Impedance spectra of materials processed in different
sintering conditions: Ža. samples sintered at different temperatures
for 10 h, and Žb. samples sintered at 12208C for different periods
of time.
high frequency intercept of the grain boundary arc.
Simple inspection of the spectra, shown in Fig. 4,
suggests that the major differences between these
materials are mostly due to the grain boundary contribution as all high frequency intercepts of the grain
boundary arcs are close to each other. A significant
increase of the grain boundary impedance is observed in the samples sintered under extreme conditions of temperature Ž12308C. and time Ž80 h..
These general observations result obviously from
the dependence of the estimated bulk Ž r b . and grain
boundary Ž rgb . resistivities Žat 08C. on the sintering
temperature shown in Fig. 5a. This figure shows that
rgb is particularly sensitive to the sintering temperature going through a minimum at temperatures close
to 12108C, and with a sharp increase at higher
Fig. 5. Dependence of grain Ž r b . and grain boundary Ž rgb .
resistivities at 08C: Ža. samples sintered at different temperatures
for 10 h, and Žb. samples sintered at 12208C for different periods
of time.
R.O. Fuentes et al.r Solid State Ionics 140 (2001) 173–179
178
A perfectly equivalent behavior is found when
analyzing the effect of the sintering time on bulk and
grain boundary transport properties ŽFig. 5b.. Note
that SEM analysis of the sample sintered for 80 h
ŽFig. 3. indeed reveals the presence of liquid phase
along the grain boundaries. Also, changes in sample
density and grain size with sintering time ŽTable 1.
support the above mentioned explanation. Nevertheless, it is essential to improve the knowledge of the
structural and compositional characteristics of the
grain boundary in order to fully understand the sintering behavior and the electrical properties of these
materials.
The best conductivity values were found for samples sintered at 12208C for 40 h Ž2.7 = 10y3 S cmy1
at f 278C. and are better than some of the best
values reported in the literature Ž1 = 10y3 S cmy1
w2,8x.. Room for improvement is still open from
exploitation of other experimental conditions, e.g.
sintering at 12108C for longer periods of time.
The activation energy ŽTable 2. for total conductivity was estimated from Arrhenius plots shown in
Fig. 6. The effect of sintering temperature is negligible ŽFig. 6a. and values were found to be reasonably
close to each other in the range from 0.31 to 0.35
eV, in good agreement with literature data w2,8x.
However, Fig. 6b clearly shows that the activation
energy decreases with increasing sintering time. This
seems to a consequence of the formation of a liquid
phase along the grain boundaries, particularly evident in the sample sintered for 80 h ŽSEM micrograph of Fig. 3.. In this case, the activation energy
drops to about 0.26 eV.
Fig. 6. Temperature dependence of total conductivity: Ža. samples
sintered at different temperatures for 10 h, and Žb. samples
sintered at 12208C for different periods of time.
Table 2
Bulk, grain boundary and total conductivities at room temperature of the different NASICON-based materials
Conductivity ŽS cmy1 .
Sintering temperature
Ž8C.
Sintering
time Žh.
Grain
g. boundary
Total
Activation
energy ŽeV.
1190
1210
1220
1220
1220
1220
1230
1235
10
10
2
10
40
80
10
10
4.6 = 10y3
4.6 = 10y3
4.1 = 10y3
4.7 = 10y3
6.6 = 10y3
1.6 = 10y3
4.5 = 10y3
2.2 = 10y3
2.0 = 10y3
3.6 = 10y3
6.0 = 10y4
2.1 = 10y3
4.6 = 10y3
1.1 = 10y3
1.1 = 10y3
8.2 = 10y3
1.4 = 10y3
2.0 = 10y3
5.2 = 10y4
1.5 = 10y3
2.7 = 10y3
6.3 = 10y4
8.8 = 10y4
6.0 = 10y4
0.32
0.31
0.39
0.34
0.31
0.26
0.35
0.37
Activation energies were estimated from data in the range 0–1508C.
R.O. Fuentes et al.r Solid State Ionics 140 (2001) 173–179
4. Conclusions
NASICON-type materials with nominal composition, Na 3 Si 2 Zr1.88Y0.12 PO12 , can be obtained starting
from a tetragonal zirconia-based precursor, following
a traditional ceramic route. The electrical properties
are comparable to the best materials mentioned in
the literature, usually processed by a sol–gel technique.
The total electrical conductivity strongly depends
on density and nature of the grain boundaries. Proper
combination of sintering temperature and sintering
time yields large grain size and reduces the grain
boundaries density.
The sintering behavior is likely to involve a liquid
phase assisted process, as suggested by the formation
of large quantities of a resistive liquid phase along
the grain boundaries. A better knowledge of the
nature of the grain boundaries and of the sintering
mechanism is essential to optimize the processing
conditions and, therefore, the electrical properties.
Acknowledgements
Financial support from the Alfa Program ŽCEC,
Brussels. and FCT ŽPortugal. is greatly appreciated.
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