Preparation of AgI-Al2O3 composites with high ionic conductivity using Al2O3 aerogel and xerogel

Article abstract of DOI:10.1149/1.1394019

AgI-Al₂O₃ composites were prepared using sol-gel-derived Al₂O₃ aerogels and xerogels. The addition of Al₂O₃ aerogel or xerogel to AgI enhanced the ionic conductivity. The conductivity at room temperature showed a maximum at the composition 0.4AgI·0.6Al₂O₃ in both aerogel-containing and xerogel-containing composites and was 2.8×10^-3 S cm^-1 for the aerogel composite. The conductivity of aerogel-containing composites was larger than that of xerogel-containing composites for all compositions, suggesting that the pore size distribution of aerogels was more effective than that of xerogels for enhancing the ionic conductivity.

Full text of DOI:10.1149/1.1394019

Journal of The Electrochemical Society, 147 (11) 4061-4064 (2000)
4061
S0013-4651(00)04-072-6 CCC: $7.00 © The Electrochemical Society, Inc.
Preparation of AgI-Al O Composites with High Ionic Conductivity
2
3
Using Al O Aerogel and Xerogel
2
3
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-Al O composites were prepared using sol-gel-derived Al O aerogels and xerogels. The addition of Al O aerogel or xero-
2
3
2
3
2 3
gel to AgI enhanced the ionic conductivity. The conductivity at room temperature showed a maximum at the composition
Ϫ3
Ϫ1
0
.4AgIи0.6Al O in both aerogel-containing and xerogel-containing composites and was 2.8 ϫ 10 S cm for the aerogel
2 3
composite. The conductivity of aerogel-containing composites was larger than that of xerogel-containing composites for all com-
positions, suggesting that the pore size distribution of aerogels was more effective than that of xerogels for enhancing the ionic
conductivity.
©
2000 The Electrochemical Society. S0013-4651(00)04-072-6. All rights reserved.
Manuscript submitted April 20, 2000; revised manuscript received July 14, 2000.
In recent years, much attention has been paid to solid electrolytes
because of their potential application in many electrochemical de-
vices such as solid-state batteries, sensors, and electrochromic dis-
plays. Among the solid electrolytes, ␣-AgI crystal exhibits an ex-
wide range of compositions, and the effects of the porous structure
of xerogels and aerogels on the conductivity are discussed.
Experimental
Xerogels and aerogels were prepared via a previously reported
Ϫ1
tremely high ionic conductivity of more than 1 S cm . However, at
7
procedure. Aluminum-tri-sec-butoxide, Al(O-sec-Bu) , was used as
3
room temperature, the conductivity of ␤-AgI, the stable phase at
a starting material for Al O . Al(O-sec-Bu) and isopropyl alcohol,
Ϫ5
Ϫ1
2
3
3
temperatures lower than 147ЊC, is as low as 10 S cm . The con-
i-PrOH, were mixed and stirred at room temperature for 1 h. Ethyl
acetoacetate, EAcAc, was added to the solution as a chelating agent,
and the solution was stirred for 3 h. Water diluted with i-PrOH was
then carefully added to the solution for hydrolysis. The as-prepared
sols were kept in closed containers for gelation at 50ЊC. The molar
ratios of i-PrOH, EAcAc, and H O to Al(O-sec-Bu) were 10, 1, and
ductivity of AgI is greatly increased by the addition of certain insu-
1
-5
lators to form two-phase composites, and in the system AgI-
Al O , the highest composite conductivity was reported to be about
2
3
Ϫ3
Ϫ1
2,3
1
0
S cm at room temperature. The mechanisms of conductiv-
ity enhancement have been explained in terms of a space charge
model, the formation of highly conducting metastable phases at the
2
3
4
, respectively.
2
-4
interface between AgI and Al O , or the effect of water.
2
3
Xerogels were prepared by drying the wet gels at 50ЊC for about
Various kinds of porous alumina have been prepared using the
2
weeks. For the preparation of aerogels, the wet gels were aged in
6
sol-gel process. In the sol-gel process, wet gels are formed by
i-PrOH for at least 1 week at 50ЊC, and the alcohol was renewed sev-
eral times to wash out the remaining water in the aging process. The
aged wet gels were then supercritically dried in an autoclave at about
hydrolysis of alcoholic solutions of metal alkoxides; wet gels consist
of secondary particles, which are formed by the aggregation of pri-
mary particles with sizes of several nanometers. Xerogels are formed
by drying the wet gels at an ambient atmosphere, and aerogels are
formed by drying the wet gels under a supercritical condition of the
solvent used. Due to the supercritical drying, aerogels are porous
1
2 MPa at 270ЊC, in which the initial pressure of 4 MPa was intro-
duced by nitrogen gas. The xerogels and aerogels were heat-treated
in air at 500ЊC for 7 h to be used for the preparation of composites.
It was found from X-ray diffraction (XRD) measurements that these
heat-treated gels were amorphous. The specific surface area and pore
size distribution of these heat-treated gels were studied by the nitro-
gen-adsorption method.
6
materials with porosity higher than 90% and have large pores. We
have already reported that the microstructures of xerogels and aero-
gels made from chemically modified Al-alkoxide showed different
features in pore size distribution and in specific surface area when
different stabilizing agents such as ␤-diketones and alkanolamines
For the preparation of (1 Ϫ x)AgIиxAl O composites, AgI was
2
3
ground with the Al O xerogel or aerogel in a planetary ball mill
7
,8
2 3
were used.
The alumina used for fabricating the AgI-Al O composites has
for 15 min to 10 h, and the mixture was pelletized to a bar shape
2
3
Ϫ2
(
3 ϫ 3 ϫ 30 mm) at 400 kg cm . The pellets were sintered at
been mostly commercial powders, and only the effects of the specific
surface area or grain size of these alumina have been examined, al-
though the pore size distribution of the powders must be a very impor-
tant factor in understanding the enhancement of conductivity. Nagai
and Nishino have reported the preparation of AgI-Al O composites
4
00ЊC for 3 h.
The pellet conductivity was measured with Ag-pasted electrodes
using a Solartron 1260 impedance analyzer in the frequency range
from 10 Hz to 8 MHz. The intercept of a semicircle or arc on the real
axis of the complex-impedance plot was adopted as the bulk imped-
ance. All measurements were carried out under dry nitrogen flow.
Differential scanning calorimetry (DSC) measurements were
carried out with a Rigaku TA100 calorimeter with heating and cool-
2
3
from alkoxide-derived Al O powder and pointed out that porous
2
3
Al O having pores with a radius less than about 20 nm can be expect-
2
3
5
ed to show a remarkable enhancement in conductivity. As mentioned
above, the pore size distribution as well as the specific surface area of
porous Al O can be changed by controlling the hydrolysis-condensa-
Ϫ1
ing rates of 10ЊC min . XRD patterns were measured using a
2
3
Rigaku RINT 1100. The composite microstructures were observed
with a Hitachi S-4500 field-emission-scanning electron microscope
tion and drying processes of gel processing. Thus, superior conduct-
ing properties are expected to be achieved by using porous alumina,
with various pore size distributions prepared via the sol-gel process, as
the insulating component of AgI-Al O composites.
(
FE-SEM).
Results and Discussion
The specific surface areas of the xerogels and the aerogels pre-
2
3
In this study, the AgI-Al O composites were prepared using sol-
2
3
2
Ϫ1
gel-derived Al O aerogels and xerogels. The temperature depen-
pared in this study were about 500 m g . Figure 1 shows the pore
size distribution of the xerogels and aerogels before and after
mechanical grinding. The xerogels without mechanical grinding
have pores of size less than about 3 nm, whereas the aerogels have
2
3
dence of ionic conductivity was measured for composites with a
z
E-mail: tadanaga@ams.osakafu-u.ac.jp
Downloaded on 2015-03-08 to IP 193.61.135.80 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

4
062
Journal of The Electrochemical Society, 147 (11) 4061-4064 (2000)
S0013-4651(00)04-072-6 CCC: $7.00 © The Electrochemical Society, Inc.
during heating. The magnitude of the conductivity jump decreases
with an increase in x, and almost no jump is observed at x ϭ 0.7.
During cooling, no change in conductivity is observed at 147ЊC, and
the conductivity decreases sharply at about 130ЊC. The jump in con-
ductivity during heating and the drop during cooling are probably
due to the ␣-␤ transformation of AgI crystals in the matrices. The
jump and drop in conductivity were more or less observed for all
samples using xerogels. At lower x for the aerogel composites, the
temperature dependence of conductivity shows behavior similar to
that of xerogel composites. However, with increasing x, the jump
and drop in conductivity at the transition point almost disappear, and
almost no jump or drop is observed at x ϭ 0.6.
Figure 4 shows the composition dependence of conductivity for
(
1 Ϫ x) AgIиxAl O at 298 K. The conductivity increases with an
2 3
increase of Al O content for both xerogel and aerogel composites
2
3
and reaches a maximum at about x ϭ 0.6 and then decreases. These
conductivity variations are basically similar to those for the compos-
1-3
ites reported so far. The conductivity of the aerogel composites is
larger than that of the xerogel composites for all compositions and
Ϫ3
Ϫ1
Figure 1. Pore size distribution of the xerogels and aerogels before and after
mechanical grinding.
shows a maximum of 2.8 ϫ 10 S cm at x ϭ 0.6 for the aerogel
composites; this value is slightly larger than the highest conductivity
2,3
reported so far. This result suggests that the aerigel pore size dis-
tribution is effective in enhancing the composite ionic conductivity.
Figure 5 shows the DSC heating and cooling curves for
pores with a wide range of sizes up to 20-30 nm. The larger pores
observed in the aerogels are due to supercritical drying.6,7 With
mechanical grinding, pores of 20-30 nm in the aerogel mostly dis-
appear. However, the pore size distributions of the xerogels and aero-
gels, are basically maintained after mechanical grinding; the xero-
gels have pores of less than 3 nm, and the aerogels have pores with
a wide range of sizes up to 20-30 nm.
Figures 2 and 3 show the temperature dependence of conductivi-
ty for composites prepared using xerogels and aerogels, respective-
ly. The conductivity of an AgI pellet is also shown for comparison in
both figures. All the conductivity data were obtained during heating
from room temperature to 200ЊC and also cooling from 200ЊC to
room temperature. For the xerogel composites, a jump of conductiv-
ity is observed near the ␣-␤ transformation temperature of 147ЊC
0
.6AgIи0.4Al O composites using xerogel and aerogel, together
2 3
with AgI crystals for comparison. The DSC curves for the AgI crys-
tals show endothermic (at 147ЊC) and exothermic (140ЊC) peaks due
to the ␣-␤ transformation during heating and the cooling, respective-
ly. Compared with the AgI crystals, the composite exothermic peaks
are located at lower temperatures, and the endothermic peaks are
located at slightly higher temperatures. In the cooling curve for the
xerogel composite, a small exothermic peak with an onset tempera-
ture at about 140ЊC, which is the same temperature as that of pure
AgI, and a large peak with an onset temperature at about 130ЊC are
observed. For the aerogel sample, a broad exothermic peak with
onset temperature at about 130ЊC is observed only in the cooling
curve. Because these exothermic peaks are attributable to the ␣-␤
Figure 2. Temperature dependence of conductivity for composites prepared
Figure 3. Temperature dependence of conductivity for composites prepared
using xerogel.
using aerogel.
Downloaded on 2015-03-08 to IP 193.61.135.80 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

Journal of The Electrochemical Society, 147 (11) 4061-4064 (2000)
4063
S0013-4651(00)04-072-6 CCC: $7.00 © The Electrochemical Society, Inc.
Figure 6 shows XRD patterns of the aerogel composites. With
increasing x, all diffraction peaks due to ␤-AgI become broad, while
the peak position is not changed. This behavior indicates that the
average crystallite size of AgI decreases with increasing x, or that the
AgI crystallinity decreases with x after heat-treatment. The relative
intensities of the peaks at about 24, 39, and 47Њ, which are the over-
lapping peaks due to ␤- and ␥-AgI, are increased compared with
those of the peaks observed in ␤-AgI. This indicates that a small
amount of ␥-AgI was possibly formed in the composites, although
the composites were heat-treated at 400ЊC after mechanical grinding.
Figure 7 shows FE-SEM images of aerogel composite cross sec-
tions (a) before and (b) after heat-treatment at 400ЊC. Before heat-
treatment, 1 ␮m AgI grains are observed in the Al O matrix, and the
2
3
composite has a dense structure. However, after heat-treatment,
some small holes about 1 ␮m diam are observed, and AgI grains dis-
appear. This indicates that AgI diffuses into the aerogel pores with
heat-treatment, and holes are formed at the original AgI location. On
the other hand, FE-SEM images of the xerogel composites showed
that only a small number of the holes were present and AgI grains
remained after the heat-treatment.
Based on these images and other results described above, the
enhancement of the conductivity can be explained as follows. Be-
cause the aerogels have large pores of about 10-30 nm, as shown in
Fig. 1, AgI diffuses easily into the pores with heat-treatment at
4
00ЊC. This diffusion of AgI into pores gives rise to coverage of the
Figure 4. Composition dependence of conductivity for (1 Ϫ x) AgIиxAl O
composites at 298 K.
Al O surface with AgI, which implies the formation of a large-area
interface between AgI and Al O . A highly conductive region must
2
3
2 3
2 3
be formed at the interface between AgI and Al O . Thus, a very large
2
3
conductivity enhancement is observed in composites with aerogels
which have a large interfacial area between AgI and Al O . The for-
phase transformation, the deviation from the equilibrium phase trans-
formation temperature 147ЊC means that the phase transition is sup-
pressed by the addition of Al O . The small peaks observed for the
xerogel composite at about 141–147ЊC during heating and cooling
are probably due to the ␣-␤ transition of free AgI, indicating that free
AgI crystals are present in the composites. The presence of free AgI
in the xerogel composites corresponds to the large jump in conduc-
tivity near the phase transformation temperature as shown in Fig. 2.
2
3
mation of such a large interfacial area between AgI and Al O should
2
3
2
3
lead to the decrease of average AgI crystallite size. Due to the de-
crease in the crystallite size, the XRD pattern of composites with
smaller amounts of AgI showed broad peaks, as shown in Fig. 6. The
deviation of the ␣-␤ transition temperature in DSC (Fig. 5) and con-
ductivity (Fig. 3) from the equilibrium phase transformation temper-
ature 147ЊC and the disappearance of the jump and drop of conduc-
tivity during heating and cooling are also explained by this diffusion
of AgI. An interaction between AgI and Al O should result in the
2
3
suppression of the phase transformation from ␣- to ␤-AgI phase, and
thus the bulk properties of AgI were not observed in the DSC and
conductivity measurements.
On the other hand, in the xerogel composite, the pore size of the
xerogels must be too small to diffuse AgI into the pores, and free
Figure 5. DSC heating and cooling curves for 0.6AgIи0.4Al O composites
2
3
using xerogel and aerogel, together with AgI crystals for comparison. The
Ϫ1
heating and cooling rate was 10ЊC min
.
Figure 6. XRD patterns of aerogel composites.
Downloaded on 2015-03-08 to IP 193.61.135.80 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

4
064
Journal of The Electrochemical Society, 147 (11) 4061-4064 (2000)
S0013-4651(00)04-072-6 CCC: $7.00 © The Electrochemical Society, Inc.
AgI, which does not form an effective interface with Al O , remains
2
3
in the composites. This induces the appearance of the bulk proper-
ties of AgI in DSC and conductivity measurements, and the enhance-
ment of conductivity is smaller than for the aerogel samples.
The small amounts of ␥-AgI, which were formed during the
grinding process, may contribute to the conductivity enhancement in
these composites. However, the conductivity of ␥-AgI is in the order
Ϫ5
Ϫ1 10
of 10 S cm
, and thus the contribution of ␥-AgI to the conduc-
tivity of the composites must be small.
Conclusion
AgI-Al O composites were prepared using sol-gel-derived
2
3
Al O aerogels and xerogels. The ionic conductivity was enhanced
2
3
by the addition of Al O aerogel and xerogel to AgI. The conductiv-
2
3
ity reached a maximum at the composition of 0.4AgIи0.6Al O for
2
3
both composites with xerogel and aerogel, and reached 2.8 ϫ
Ϫ3
Ϫ1
1
0
S cm at room temperature for the aerogel composites. The
conductivity of the AgI-Al O aerogel composites was larger than
2
3
that of AgI-Al O xerogel composites for all compositions. It was
2
3
shown that diffusion of AgI into aerogel pores of size of 10–30 nm
played an important role in the conductivity enhancement.
Acknowledgments
This study was supported by the “Research for the Future” Pro-
gram from the Japan Society for the Promotion of Science and a
Grant-in-Aid for Scientific Research on Priority Areas (B) from
Ministry of Education, Science, Sports, and Culture of Japan.
Osaka Prefecture University assisted in meeting the publication costs of
this article.
References
1
2
.
.
K. Shahi and J. B. Wagner, Jr., J. Electrochem. Soc., 128, 6 (1981).
N. F. Uvarov, E. F. Hairetdinov, and N. B. Bratel, Russ. J. Electrochem., 29, 1231
(1993).
3
4
.
.
N. F. Uvarov, E. F. Hairetdinov, B. B. Bokhonov, and N. B. Bratel, Solid State Ion-
ics, 86-88, 573 (1996) .
N. F. Uvarov, V. P. Isupov, V. Sharma, and A. K. Shukla, Solid State Ionics, 51, 41
(1992).
5
6
.
.
M. Nagai and T. Nishino, J. Electrochem. Soc., 138, L49 (1991).
C. J. Brinker and G. W. Scherer, Sol-Gel Science: The Physics and Chemistry of
Sol-Gel Processing, Academic Press, New York (1990).
7
8
9
.
.
.
K. Tadanaga, T. Iwami, T. Minami, and N. Tohge, J. Ceram. Soc. Jpn., 103, 582
(
1995).
K. Tadanaga, S. Ito, T. Minami, and N. Tohge, J. Non-Cryst. Solids, 201, 231
1996).
J. A. Schmidt, J. C. Bazan, and L. Vico, Solid State Ionics, 27, 1 (1988).
(
Figure 7. FE-SEM images of a cross section of aerogel composites (a) before
and (b) after heat-treatment at 400ЊC.
10. H. Hosino and M. Shimoji, J. Phys. Chem. Solids, 35, 321 (1974).
Downloaded on 2015-03-08 to IP 193.61.135.80 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).