Enhancement of ionic conductivity of SrCl2 by Al2O3 dispersion

Solid State lonics 18 & 19 (1986) 1146-1149 North-Holland, Amsterdam

1146

ENHANCEMENTOF IONIC CONDUCTIVITY OF SrCI2 BY A1203 DISPERSION Satoru FUJITSU, Kunihito KOUMOTOand Hiroaki YANAGIDA Departmant of I n d u s t r i a l Chemistry, Faculty of Engineering, U n i v e r s i t y of Tokyo 7 - 3 - I , Hongo, Bunkyo-ku, Tokyo 113, JAPAN Enhancement of i o n i c c o n d u c t i v i t y was found in the system of SrCI2 and A1203 mixture. The enhancement of CI- c o n d u c t i v i t y depended on the composition and the grain size of A1203. From these r e s u l t s , i t was considered that the interface between SrClz matrix and A1203 p a r t i c l e s played a role in g i v i n g r i z e to high i o n i c c o n d u c t i v i t y . The e f f e c t i v e thickness and the c o n d u c t i v i t y of the interface layer were estimated by using a simple mixture model. The v a l i d i t y of t h i s estimation was supported by the results of measurement f o r a modeled sample. e f f e c t i v e thickness and c o n d u c t i v i t y of an

I . INTRODUCTION Many e f f o r t s to develop a high i o n i c

interface layer by using a multilayered

conductor have been devoted to the preparation

material composed of SrCI2 t h i n films and

of crystal structure which was e f f c t i v e f o r

A1203 t h i n plates.

i o n i c conduction! -3

Liang 4 found the

phenomenon that Li + c o n d u c t i v i t y in L i l was

2. EXPERIMENTALPROCEDURE

accelarated by Alz03 dispersion without any

2 - I . Sample preparation

reaction products or s o l i d s o l u t i o n .

2-I-I.

This type

of conductor is d i f f e r e n t from the conventional

Sintered compact

As s t a r t i n g materials, SrC12(99.99%,Rare

ones in that heterogeneous interfaces are

M e t a l l i c Co., Ltd.) and A1203(99.9%) of

u t i l i z e d to give rise to high i o n i c c o n d u c t i v i t y .

d i f f e r e n t reported sizes(O.3 and 3~m, Marumoto

Thus, the interface formed between a conductor

Kogyo Co.,Ltd.) were employed.

matrix and AIz03 p a r t i c l e s played a role as the

p a r t i c l e s usually aggregate i n t o larger

conduction path.

secondary p a r t i c l e s during the sample

Since t h i s phenomenon was

Alumina

preparation, so that the average p a r t i c l e size

found, s i m i l a r observations have been reported for such materials as CuCI5, Agl 6, LiBr.H207,8

was measured by a sedimentation method.

CaFz, BaFz9 and SrCI2 I0.

measurement showed that 0.3 and 3~m A1203

The present authors9'lOhave simulated the e f f e c t i v e thickness and c o n d u c t i v i t y of a high i o n i c - c o n d u c t i v i t y layer in the system of

The

p a r t i c l e s aggregated into secondary p a r t i c l e s of 2.6 and lO~m, respectively. The aqueous s o l u t i o n of SrCI2 containing

BaF2-AI203 and SrCI2-AI20~ by using a simple

A1203 powders were heated in an evaporating

m a t r i x - p a r t i c l e mixing model.

gave a s a t i s f a c t o r y explanation f o r enhancement

dish. The mixed powder were heated at 900°C f o r 4h in vacuum(m.p, of SrCI2 is 874°C).

of i o n i c c o n d u c t i v i t y of a mixed sintered

The compacts were crushed, p e l l e t i z e d under the

compact, while the v a l i d i t y of the estimation

pressure of 200-300MPa in a steel die, and

was not recognized by any other d i r e c t

sintered at 800°C f o r 4h in vacuum.

information. In t h i s study, we attempted to prove the

2 - I - 2 . M u l t i l a y e r sample

v a l i d i t y of the former estimation of the

alcohol was painted on the both surfaces of the

This estimation

0 167-2738/86/$ 03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

Strontium chloride dispersed in sec-butyl

1147

S. Fujitsu et aL / Enhancement o f ion& conductivity

A1203 thin plates(50~m thick, Mitsubishi Mining

electron probe microanalysis showed that A1203

& Cement Co.,Ltd.) and these plates were laminated upon each other. This m u l t i l a y e r

p a r t i c l e s were surrounded by the SrCI2 matrix.

sample was sandwitched by two thick A1203 plates

of ionic conductivity of SrCI2-AI203 are shown

The composition and p a r t i c l e size dependences

(l.4mm thick, Nippon Kagaku Togyo Co.,Ltd.) as

in Fig.2.

shown in F i g . l . Green m u l t i l a y e r was heated at 800°C for lh under the pressure of 20MPa in

of ionic conductivity is due to the formation

I t can be seen that the enhancement

vacuum. The obtained sample was cut into a

of high ionic conductivity interface layers between SrCI2 and AI20~ p a r t i c l e s I0. To explain

rectangular bar of ~15xlOx3.8mm f o r e l e c t r i c a l measurements and microstructure observation.

the enhancement of ionic conductivity by the interface layer, Wagner et a l . proposed the

2-2. Electrical conductivity

model based on the assumption that the interface

Sputtered Pt-Pd films and graphite paste were

layer has excess defects resulting in the

used as blocking electrodes of a sintered

formation of the space charge region.

compact and a m u l t i l a y e r sample, respectively.

model indicates that the extra conductivity

Their

The complex impedance of the sample was

is proportional to ( I / r ) ( V / I - V ) where r and

measured in the frequency range from 5Hz to

V are the p a r t i c l e radius and the volume

IOMHz by an impedance analyzer(HP-41g2A) at to 750°C in dry Nz.

200

The impedance of the sample

excluding that of the bloking electrodes was c l e a r l y known from the complex impedance plots.

fraction of A1203, respectively. Their model was v e r i f i e d f o r such systems as CuCI-AI2035, LiI-AI2035 and AgI-AI2036.

In the present case,

this relationship is also recognized in the composition range revealing the increase in conductivity with increaseing A1203 content.

//// ....

A1203 thin plate

. . . . SrCI2 layer

~

The mechanism for enhancement of conductivity in an anionic conductor might also be explained by t h e i r model.

. . . . graphite

electrode

FIGURE 1 Schematic drawing of m u l t i l a y e r sample. Fo~ measured sample, 17 thin plates were used. A whole thickness was 3.8mm.

~I0-3

u

3. RESULTS AND DISCUSSION 3.1 Sintered compact

-g § I0-"

FIGURE 3 Electrical conductivity for SrCI2-AI20~

All X-ray d i f f r a c t i o n peaks of the post heated sample were asinged to those of pure SrCI2 or A1203 with no indication of chemical reaction or solid solution formation. Both scanning electron microscope observation and

l I00

I 90

i 80

I 70

I 60

I 50

SrCl2 content(mol%)

o . . . . 2.6um • ....

10um

S. Fujitsu et al. / Enhancement o f ionic conductivity

1148

On the other hand, the authors estimated the effective thickness and conductivity of the

and the interface layer are parallel to the current flow, so that the total conductivity

interface layer by combining the Maxwell's equationlland Tiku and KrUger's equation12

(~) is expressed by the following equation:

I t was assumed that the material was composed of the dence SrCl2 matrix and spherical Al20s particles of monosize distribution possessing uniform interface layers.

Figure 3 shows the

o =VSrCl2oSrCl2+~intOint

(1)

where ~SrClzand Vint are the volume fraction of SrCl2 layers and interface layers possibly

results of this estimation compared with the

formed, and aSrCl2 and Oint the conductivity of SrCl2 and an interface layer, respactively.

conductivity of pure SrCl~. The conductivity

Conductivity of A1203 plates can be ignored as

of an interface layer is ~2 orders of magnitude

they are good insulators.

larger than that of a pure SrCl2, while the

SrCl2 layers is obtained by substrating that of the A1203 plates from the whole thickness of the sample. When the estimated values for the

thickness decreased from 0.6 to O.15~m with increase in temperature.

The thickness of

thickness and conductivity of an interface layer are used, the total conductivity of a 0.6

multilayer sample can be calculated by Eq(1).

-0.5 0.4 E i

(A)

v

0.3

v~ 0

0.2

o

-l

S

r

C

,

,

,

l

2

I

~

O.l

I

50~m

-2 I

.0

,

1.2

I

,

1.4 1.6 IO00/T(K)

,

,

1.8

FIGURE 3 oT-I/T plots for pure SrCl2 and interface layer, and thickness of interface layer.

FIGURE 4 Cross section of a multilayer sample and characteristic X-ray(SrL) intensity along line (A).

3-2. Multilayer sample

Calculated conductivity is s l i g h t l y higher than

In order to evaluate the v a l i d i t y of the estimation for a sintered compact, the modeled multilayered sample was subjected to conductiv-

the measured conductivity as shown in Fig.5. This small difference might be due to the assumption of the ideal condition of the sample,

i t y meatsurement. The cross section of a multilayered sample is shown in Fig.4, where SrCl2 layers and Alz03 plates are clearly distinguished. In this sample, the SrCl2 layer

and imperfect contact between A1203 plates and SrCl2 layers or an experimental error to measure the thickness of SrClz would have caused the difference. Though conductivity values were not perfectly coincident with each other, the

S. Fujitsu et al. / Enhancement o f ionic conductivity

l 149

Ceramics andGl~ass(Ma~cel Dekker,New York, 1974).

2-

C.C.Liang, J.Electrochem,Soc., 120 (1973) 1289. 5. T.Jow and J.B.Wagner,Jr., J.Electrochem.Soc., 127(1979)1963. 6. K.Shahi and J.B.Wagner,Jr., J.Electrochem. Soc., 128(1981)6.

v

'E

O

cz~

7. O.Nakamura and J.B.Goodenough, Solid State Ionics, 7(1982) ll9.

Fo

SrCI ~ ~ . ~~ o

8. T.Asai and S.Kawai, Chem.Lett., (1984)I125. 9. S.Fujitsu, K. Koumoto, H.Yanagida and T.Kanazawa., J.Mat.Sci., 20(1985)2103.

-2

l

.0

J

1.2

i

~

I

l

1.4 1.6 l OOO/T(K)

i

i

1.8

FIGURE 5 oT-I/T plots for calculated and measured results for multilayer sample compared with the conductivity of SrCl2. Activation energy of multilayer sample at 400-600°C is 1.08eV. activation energy agreed well with that measured at 400-600°C. Consequently, the present results led us to believe that an interface layer of submicron order is formed between the SrC12 matrix and A1203 particles and its high conductivity causes the enhancement of conduction in the mixture system. ACKNOWLEDGEMENT The authors wish to thank Dr. Masayuki Nagai, Musashi Institute of Technology, for helpful discussions. This work was partially supported by a grant-in-Aid for Scientific Research from the Japanese Ministry of Education, Science and Culture(No.58470054). REFERENCES I. W.van Gool ed., Fast Ion Transport in Solid (North-Holland,New York,1973). 2. P.Hagenmuller and W.van Gool ed., Sold Electrolytes(Academic Press,New York,1978). 3. N.M.Tallan ed., Electrical Conductivity in

lO.S.Fujitsu, H.Kobayashi, K.Koumoto and H.Yanagida, J.Electrochem.Soc., to be published. ll.J.C.Maxwell, Electricity and Magnetism, Vol.l,3rd edn(Hey~ood, London, 1965) p.440. II

12.S.K.Tiku and F.A.Kroger, J.#~er.Ceram.Soc., 63(I 980)183.