Conductivity enhancement and thermal properties of AgI-MO2 (M = Zr, Si) Composites using sol-gel derived aerogels

Article abstract of DOI:10.1149/1.1475687

AgI-ZrO₂ and Agl-SiO₂ composites were prepared using sol-gel derived aerogels. The conductivity at room temperature showed a maximum for the composition of 0.4Agl.0.6ZrO₂ or 0.4Agl.0.6SiO₂ and was 3.3 × 10_-3 S cm_-1 for the composite using ZrO₂ aerogels and 1.2 × 10_-4 S cm _-1 for the composite using SiO₂ aerogels. The diffusion of AgI into the micropores of the aerogel was observed for the ZrO₂ system, suggesting that the diffusion caused a decrease of the crystallinity of AgI and the formation of a high conductive phase of AgI at the AgI-ZrO₂ interface. In the SiO₂-containing system, however, the diffusion was not observed and hence, the conductivity was an order of magnitude lower than that of the ZrO₂-based system.

Full text of DOI:10.1149/1.1475687

Journal of The Electrochemical Society, 149 ͑6͒ A773-A777 ͑2002͒
A773
0
013-4651/2002/149͑6͒/A773/5/$7.00 © The Electrochemical Society, Inc.
Conductivity Enhancement and Thermal Properties of
AgI-MO „M ϭ Zr, Si… Composites Using Sol-Gel Derived
2
Aerogels
z
Kiyoharu Tadanaga, Katsuya Imai, Masahiro Tatsumisago, and Tsutomu Minami
Department of Applied Materials Science, Graduate School of Engineering, Osaka Prefecture University,
Sakai, Osaka 599-8531, Japan
AgI-ZrO and AgI-SiO composites were prepared using sol-gel derived aerogels. The conductivity at room temperature showed
2
2
Ϫ3
Ϫ1
a maximum for the composition of 0.4AgI•0.6ZrO or 0.4AgI•0.6SiO and was 3.3 ϫ 10 S cm for the composite using ZrO2
2
2
Ϫ4
Ϫ1
aerogels and 1.2 ϫ 10 S cm for the composite using SiO aerogels. The diffusion of AgI into the micropores of the aerogel
2
was observed for the ZrO system, suggesting that the diffusion caused a decrease of the crystallinity of AgI and the formation of
2
a high conductive phase of AgI at the AgI-ZrO interface. In the SiO -containing system, however, the diffusion was not observed
2
2
and, hence, the conductivity was an order of magnitude lower than that of the ZrO -based system.
2
©
2002 The Electrochemical Society. ͓DOI: 10.1149/1.1475687͔ All rights reserved.
Manuscript submitted June 11, 2001; revised manuscript received January 2, 2002. Available electronically April 25, 2002.
In recent years there has been much interest in materials which
show high ionic conductivity at room temperature, because they
have potential application in various electrochemical devices such
gel system, the difference in the magnitude of conductivity enhance-
ment in the AgI-ZrO and AgI-SiO systems is discussed.
2
2
1
as all solid-state batteries, sensors, etc. Several approaches have
been tried to increase the ionic conductivity of solid electrolytes.
For example, the conductivity enhancement in the two-phase
composites, which consist of ionically conducting and inorganic
Experimental
Zirconium tetra-n-butoxide, Zr(O-n-Bu) , was used as a start-
4
ing material for ZrO2 aerogel. Zr(O-n-Bu)4 and ethyl alcohol,
EtOH, were mixed and stirred at room temperature for 30 min. Ethyl
acetoacetate, EAcAc, was added to the solution as a chelating agent
and the solution was stirred for 3 h. A water-EtOH mixture was
added subsequently drop by drop to the solution for hydrolysis. The
sols prepared were kept in closed containers for gelation at 50°C.
The molar ratios of EtOH, EAcAc, and H O to Zr(O-n-Bu) were
kept at 10, 1, and 4, respectively. For the preparation of aerogels, the
wet gels were aged in EtOH for at least 1 week at 50°C, and the
alcohol was renewed several times to wash out the remaining water
in the aging process. The aged wet gels were then supercritically
dried in an autoclave at about 10 MPa at 270°C, in which the initial
oxide ͑insulator͒ fine particles, has been studied since Liang
2
reported the effect in LiI-Al O3 composites in 1973. Among
2
these composites, AgI-based systems, such as AgI-MxOy
(
MxOy ϭ Al O ,SiO ,ZrO ,CeO ,Fe O ), have been studied
2 3 2 2 2 2 3
3
-11
extensively.
The mechanism of conductivity enhancement is unclear even
now, but some models have been proposed. Most of them are based
on the fact that the interface of ionic conductors and inorganic
2
4
9
-11
oxides plays an important role in conductivity enhancement.
From this point of view, further conductivity enhancement is ex-
pected by controlling ionic conductor-oxide interfaces.
The sol-gel process is known to be a practical method for pre-
paring oxides with a variety of characteristics.12 In the sol-gel pro-
pressure of 4 MPa was established with nitrogen gas. In SiO aero-
2
gels, tetraethoxysilane, Si(OC H ) , was used as a starting material.
2
5 4
cess, wet gels are prepared by hydrolysis of an alcoholic solution of
metal alkoxides, and dried gels are porous materials with high po-
rosity and large specific surface area. Thus, sol-gel derived oxides
are expected to bring higher conductivity than the usual oxides in
two-phase composites, because the large specific surface area of the
gels can increase the area of ionic conductor-oxide interfaces, and
micropores of the gels must be used as ion conduction pathway.
Aerogels, which are formed by drying wet gels under a super-
critical condition of the solvent used, have larger pore size and po-
rosity than xerogels which are formed by drying wet gels under an
Si(OC H ) , EtOH, and 0.01 wt % HCl were mixed and stirred at
room temperature for 3 h. The selected molar ratios of EtOH and
HCl to Si(OC H ) were 5 and 6, respectively. The sols were kept
in closed containers at 50°C for gelation. After aging and washing
out using the same process for ZrO2 aerogels, the wet gels were
supercritically dried in an autoclave. ZrO and SiO aerogels were
heat-treated in air at 400°C for 3 h, at 500°C for 7 h, respectively,
and used for the preparation of the composites.
AgI was mixed in an agate mortar with ZrO or SiO aerogels
and ground in a planetary ball mill for 15 min. Then the mixture was
2
5 4
2
5 4
2
2
2
2
1
2
ambient atmosphere. We have already reported that the pore size
of gels influences the conductivity and thermal properties of com-
posites in the AgI-Al O system prepared using Al O aerogel and
Ϫ2
pelletized to bar shape at a pressure of 400 kg cm . The pellets of
the AgI-ZrO2 composite were heat-treated at 400°C for 3 h and
those of the AgI-SiO composite at 500°C for 3 h.
2
3
2
3
xerogel.1 These results indicated that the microstructure of aerogels
was more effective than that of xerogels for enhancing the ionic
conductivity of the composites. In other AgI-based systems, such as
AgI-MxOy (MxOy ϭ SiO ,ZrO ,CeO ,Fe O ), oxides with such
1
2
The conductivity was measured on the pellets with Ag pasted
electrodes by use of an impedance analyzer ͑Solartron 1260͒ in the
frequency range from 10 Hz to 8 MHz. All the measurements were
carried out under a flow of dry nitrogen atmosphere. The intercept of
a semicircle or arc at the real axis of the complex impedance plot
was adopted as bulk impedance, including interfacial impedance
between particles.
Differential scanning calorimetry ͑DSC͒ measurements were car-
ried out by heating and cooling cycles from 30 to 200°C with a
Rigaku TA100 DSC. X-ray diffraction ͑XRD͒ patterns were ob-
tained using a Rigaku RINT 1100. The Brunauer-Emmett-Teller
2
2
2
2
3
large specific surface area or large pore volume have not been used
for the preparation of the ion conducting composites.
In the present study, AgI-ZrO and AgI-SiO composites were
2
2
prepared using ZrO and SiO aerogels prepared through the sol-gel
2
2
method. This paper reports their microstructure, electrical, and ther-
mal properties. By comparing the results with the AgI-Al O aero-
2
3
͑
BET͒ specific surface area and pore-size distribution of the heat-
z
E-mail: tadanaga@ams.osakafu-u.ac.jp
treated aerogels were studied by the nitrogen adsorption method.
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Journal of The Electrochemical Society, 149 ͑6͒ A773-A777 ͑2002͒
Figure 1. Pore distribution of ZrO and SiO aerogels.
2
2
Results and Discussion
Figure 1 shows the pore size distribution of the ZrO and SiO
2
2
aerogels prepared in this study. The aerogels have pores with a wide
range of size from 1 to 30 nm. The 10-30 nm pores observed in the
aerogels are due to the supercritical drying.12 The specific surface
2
Ϫ1
areas of ZrO and SiO aerogels were 150 and 950 m g , respec-
2
2
tively. Since ZrO2 aerogels were partially crystallized during the
supercritical drying, the specific surface area of ZrO aerogels was
2
Figure 3. Temperature dependence of the ionic conductivity for the
smaller than that of SiO aerogels. Pores of 20-30 nm in size in the
2
(
1-x)AgI•xSiO composites.
aerogel were confirmed to disappear partly with the mechanical
2
Figure 4. Composition dependence of the conductivity at 25°C for the
(
1-x)AgI•xMxOy (MxOy ϭ ZrO , SiO ) composites using oxide aerogels.
Conductivity of the AgI-Al O aerogel composites is also shown for com-
2 3
2
2
Figure 2. Temperature dependence of the ionic conductivity for the
(
1-x)AgI•xZrO composites.
parison.
2
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Journal of The Electrochemical Society, 149 ͑6͒ A773-A777 ͑2002͒
A775
Figure 6. DSC curves of the (1-x)AgI•xSiO composites and of AgI.
2
Figure 5. DSC curves of the (1-x)AgI•xZrO composites and of AgI.
2
grinding. However, the feature of pore size distribution of the aero-
gel was basically maintained after the mechanical grinding; the
aerogels have pores with a wide range of size, up to 20-30 nm.
Temperature dependences of conductivity for some compositions
in the systems (1-x)AgI•xZrO and (1-x)AgI•xSiO are presented
2
2
in Fig. 2 and 3, respectively. The conductivity of an AgI pellet is
also shown for comparison in both figures. In 0.6AgI•0.4ZrO com-
2
posite (x ϭ 0.4) a jump of conductivity is observed near the ␣-␤
transformation temperature of 147°C during heating. The magnitude
of the conductivity jump decreased with an increase in x, and almost
no jump is observed at x ϭ 0.7, as shown in Fig. 2. During cooling,
however, no change in conductivity is observed at 147°C, and the
conductivity decreases suddenly at around 130°C in the case of x
ϭ 0.4. No drop is observed in the case of x ϭ 0.7. The jump in
conductivity during heating and the drop during cooling are due to
the ␣-␤ transformation of AgI crystals in the matrices. The disap-
pearance of the jump and drop of conductivity during heating and
cooling at x ϭ 0.7 is discussed later. At temperatures above 147°C,
the conductivity continuously decreased with x. This behavior of
conductivity in these composites is basically similar to that of con-
ductivities in the AgI-Al O system using an Al O3 aerogel and
2
3
2
11
commercial ␥-Al O .
2
3
In the AgI-SiO system, the jump and drop in conductivity were
2
clearly observed in all the samples. The observation of the jump and
drop in conductivity at high AgI contents indicates that the character
of the composite in the AgI-SiO system is different from that of the
2
11
AgI-Al O aerogel system we reported so far or the AgI-ZrO2
Figure 7. XRD patterns of the 0.4AgI•0.6MxOy (MxOy ϭ ZrO , SiO ,
2
3
2
2
system prepared in this study.
Al O ) composites and of AgI.
2 3
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A776
Journal of The Electrochemical Society, 149 ͑6͒ A773-A777 ͑2002͒
Figure 8. FE-SEM photographs of 0.4AgI•0.6ZrO composites ͑a͒ before and ͑b͒ after heat-treatment and 0.4(1-x)AgI•0.6SiO composites ͑c͒ before and ͑d͒
2
2
after heat-treatment.
The composition dependences of the conductivity at 25°C of
crystal show the endothermic ͑at 147°C͒ and the exothermic peaks
͑140°C͒ due to the ␣-␤ transformation during heating and cooling,
respectively. At x ϭ 0.4, a shoulder at the endothermic peak at
143°C in the heating run and a small exothermic peak at 140°C in
the cooling run are observed in addition to the main peaks. These
AgI-MO (M ϭ Zr, Si͒ composites are shown in Fig. 4. Results of
2
11
the AgI-Al O system with Al O aerogel are also shown for com-
2
3
2
3
parison. The maximum conductivity is observed at around x ϭ 0.6
in all the systems. The maximum conductivities in the AgI-ZrO and
2
Ϫ3
Ϫ4
Ϫ1
small shoulder and peaks were also observed in AgI-Al O compos-
AgI-SiO system are 3.3 ϫ 10 and 1.2 ϫ 10 S cm , respec-
2
3
2
11
ites using an Al O xerogel. These peaks probably indicate the
tively. The conductivity of the AgI-ZrO system is about two orders
2
3
2
8
of magnitude higher than that already reported for the same system.
presence of free AgI, because the ZrO2 aerogel has a relatively small
pore volume compared with Al2O3 aerogel in the AgI-Al2O3 com-
posite as shown in Fig. 1. Compared with pure AgI crystals, the
endothermic peaks for the composites are located at slightly higher
temperatures and the exothermic peaks are located at lower
temperatures.
The conductivity of the AgI-SiO system is also about one order of
2
6
magnitude higher than that of the system reported so far. The large
specific surface area and large pore volume of aerogels must con-
tribute to higher defect densities in space charge regions1 and thus
these higher values of conductivity were observed.
3
Figure 5 shows the DSC heating and cooling curves for
Figure 6 shows the heating and cooling curves of DSC for
(1-x)AgI•xSiO composites using a SiO aerogel together with data
for AgI crystals for comparison. Compared with AgI crystal, the
(
1-x)AgI•xZrO composites using a ZrO2 aerogel together with
2
2
2
data for AgI crystal for comparison. The DSC curves for the AgI
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Journal of The Electrochemical Society, 149 ͑6͒ A773-A777 ͑2002͒
A777
endothermic peaks during heating are located at slightly higher tem-
peratures and the exothermic peaks during cooling at lower tempera-
should result in the suppression of the phase transformation from the
␣- to ␤-AgI phase, and thus, the bulk properties of AgI were not
observed in the DSC and conductivity measurements.
tures. In the cooling curve of the SiO -based composite, only a large
2
peak with the onset temperature at around 135°C is observed and the
onset temperature is not changed with an increase in AgI contents.
In the composites with SiO aerogels, AgI did not diffuse into the
pores, and free AgI, which does not form effective interface with
2
The peaks observed in the SiO -containing composites at around
SiO aerogels, remains in the composites despite the fact that the
2
2
135-147°C during heating and cooling are due to the ␣-␤ transition
specific surface area of SiO aerogels is larger than that of ZrO or
2
2
of free AgI, indicating that most of the AgI crystals are present as
free AgI in the composites. The presence of free AgI in the SiO2
composites corresponds to the large jump in conductivity at around
the phase transformation temperature as shown in Fig. 2.
Al O , respectively. Since the Si-O bond has more covalent char-
2
3
acter than the Al-O or Zr-O bonds, defect concentration in the space-
charge region is rather small and also AgI, which is an ionic crystal,
may have small interactions with the SiO2 surface. Thus, highly
conductive regions do not form at the interface between AgI and
Figure 7 shows XRD patterns of the 0.4 AgI•0.6 MxOy compos-
ites (MxOy ϭ Al O , ZrO , and SiO ), together with data for
2
3
2
2
SiO . This induces the appearance of the bulk properties of AgI in
2
AgI crystals for comparison. All the diffraction peaks due to ␤-AgI
DSC and conductivity measurements, and the enhancement of the
are broad in the samples with Al O and ZrO , while the peaks are
2
3
2
conductivity is smaller than Al O - or ZrO -containing composites.
2
3
2
sharp for SiO -containing composites. This indicates that the aver-
2
age crystallite size or crystallinity of AgI decreases by spreading of
Conclusions
AgI-MxOy (MxOy ϭ ZrO , SiO ) composites were prepared
using the sol-gel derived aerogels. The conductivity of the compos-
ites was about one or two orders of magnitude higher than that of
reported composites. The diffusion of AgI into the micropores of the
1
0
AgI along a large surface of Al O or ZrO during sintering. In the
2
3
2
2
2
composites, AgI may present as the mixture of very small crystals,
crystals with strong mechanical stress, and amorphous phase of AgI.
Figure 8 shows field emission-scanning electron microscopy
͑
FE-SEM͒ photographs of the cross-section of the composites based
aerogel was observed in the ZrO -containing composites and the
2
on ZrO aerogels, ͑a͒ before and ͑b͒ after heat-treatment, and the
2
diffusion brought decrease of crystallinity of AgI and the formation
of a highly conductive phase of AgI, probably at around the inter-
face between AgI and ZrO . In the SiO -containing system, how-
composites using SiO2 aerogels, ͑c͒ before and ͑d͒ after heat-
treatment. Before the heat-treatment, AgI grains of 1 ␮m in size are
observed in the ZrO or SiO matrix and the composite has a dense
2
2
2
2
ever, the diffusion was not observed and thus the conductivity was
lower by an order of magnitude compared with that of the ZrO2
system.
structure. After the heat-treatment of the ZrO -containing compos-
2
ite, some small holes of about 1 ␮m in diameter are observed and
the AgI grains disappear. This indicates that AgI diffuses into pores
of the ZrO aerogels with the heat-treatment, and holes are formed
at the location where AgI crystals were originally present. Similar
Acknowledgments
2
The present study was supported by the ‘‘Research for the Fu-
ture’’ Program of the Japan Society for the Promotion of Science
and a Grant-in-Aid for Scientific Research on Priority Areas ͑B͒
from the Ministry of Education, Culture, Sports, Science and Tech-
nology of Japan.
behavior was already observed for the composites in the AgI-Al O
2
3
system using Al O3 aerogel.11 Only a small number of holes are
2
present and AgI grains remain after the heat treatment in the
AgI-SiO system.
2
From these photographs and other results described, the differ-
ence in enhancement of the conductivity between Al O , ZrO , and
References
2
3
2
SiO can be explained as follows. In the Al O and ZrO aerogels,
AgI easily diffuses into the pores upon heat-treatment at 400°C. This
1. J. Schoonman, Solid State Ionics, 135, 5 ͑2000͒.
2. C. C. Liang, J. Electrochem. Soc., 120, 12890 ͑1973͒.
2
2
3
2
3
. F. W. Poulsen, N. H. Andersen, B. Kindl, and J. Schoonman, Solid State Ionics,
Õ10, 119 ͑1983͒.
diffusion of AgI into pores gives rise to the coverage of Al O or
2
3
9
ZrO surfaces with AgI, which means the formation of large areas of
2
4. T. Jow and J. B. Wagner, Jr., J. Electrochem. Soc., 126, 1963 ͑1979͒.
5. J. B. Phipps and D. H. Whitmore, Solid State Ionics, 9Õ10, 123 ͑1983͒.
6. K. Shahi and J. B. Wagner, Jr., J. Electrochem. Soc., 128, 6 ͑1981͒.
interface between AgI and the aerogels. This interaction can be ex-
ϩ
plained by the adsorption of Ag cations on the Al O or ZrO2
2
3
7
. K. Shahi and J. B. Wagner, Jr., J. Solid State Chem., 42, 107 ͑1982͒.
surface.10 Since Al O or ZrO are ionic crystals, defect concentra-
tion in the space-charge region is rather high, and the highly con-
ductive region is thus formed at the interface between AgI and
Al O or ZrO , respectively. The formation of such large interface
2
3
2
8. M. C. R. Shastry and K. J. Rao, Solid State Ionics, 51, 311 ͑1992͒.
9. N. F. Uvarov, E. F. Hairetdinov, B. B. Bokhonov, and N. B. Bratel, Solid State
Ionics, 86-88, 573 ͑1996͒.
0. N. F. Uvarov, P. Vanek, M. Savionov, V. Zelezny, V. Studnicka, and J. Petzelt, Solid
State Ionics, 127, 253 ͑2000͒.
11. K. Tadanaga, K. Imai, M. Tatsumisago, and T. Minami, J. Electrochem. Soc., 147,
4061 ͑2000͒.
2. C. J. Brinker and G. W. Scherer, Sol-Gel Science: The Physics and Chemistry of
Sol-Gel Processing, Academic Press, New York ͑1990͒.
3. N. F. Uvarov, V. P. Isupov, V. Sharma, and A. K. Shukla, Solid State Ionics, 51, 41
͑1992͒.
1
3
1
2
3
2
areas between AgI and Al O or ZrO should lead to a decrease of
2
3
2
the average crystallite size or crystallinity of AgI. Due to the de-
crease in the crystallite size or crystallinity, the XRD pattern of the
composites with smaller amounts of AgI showed broad peaks, as
shown in Fig. 7. An interaction between AgI and Al O or ZrO
1
1
2
3
2
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