Mixed Ionic Electronic Conductivity of Lal Xsrxcol Yfey03 6

8/7/2019 Mixed Ionic Electronic Conductivity of Lal Xsrxcol Yfey03 6

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M a t . R e s . B u l l . , V o l . 2 3 , p p . 5 1 - 5 8 , 1 9 88 . P r i n t e d i n t h e U S A .0 0 2 5 -5 4 0 8 / 88 $ 3 . 0 0 + . 0 0 C o p y r i g h t ( c ) 1 98 8 P e r g a m o n J o u r n a l s L t d .

MIXED IONIC-ELECTRONIC CONDUCTIVITY OF La l-x S rx C o l-y Fe y0 3- 6PEROVSKITE-TYPE OXIDES

Y. Te raok a, H. M . Zhang, K. O kam oto, and N. YamazoeDepar tme nt o f M at er ia ls Sc ience and Technology, Graduate Sc hoo l o fEng ineer i ng Sc iences , Kyushu U n i v e r s i t y , Kasuga , Fukuoka 816, Japan

( R e c e i v e d J u l y 3 1 , 1 98 7; C o m m u n i c a t e d b y M . K o i z u m i )

ABSTRACT Ion i c (a i ) and e l ec t r on i c (O e) co nd uc t i v i t i e s o f m i xed conduc-t i v e L a l - x S rx C o l - y F e y 0 3 -6 w e re s e p a ra te l y me a s u re d b y me an s o f f o u r -p ro be i o n i c d c a nd o rd i n a r y f o u r -p ro b e d c t e c h n i q u e s , r e s p e c t i v e l y .A t 1 07 3 K , f o r i n s ta n c e , a i r a n g e d i n th e o rd e r o f I - 1 0 -2 S c m-1wh i l e ae was a round 102 S cm-1 , i nd i ca t i n g tha t the p resen t ox ideswere good mixed conduc to rs w i t h i on i c t ra ns po r t number 10 -2 - 10 -4 .a i inc rease d as con tents o f Sr and Co increase d though Sr con ten t wasm o re i n f l u e n t i a l . The a p p a r e n t a c t i v a t i o n e n e r g y f o r o x i d e io ncon duc t ion as we l l as the dependences o f a i on ox ide co m pos i t ion andoxygen p a r t i a l p ressu re suggested th a t ox ide i on conduc t i on occu r redv i a a v ac an cy me c h a n i s m. T he re l a t i o n o f m ix e d c o n d u c t i v i t y t o t h eo xy ge n s e m i p e rm e a b i l i t y w as a l s o d i s c u ss e d .

MATERIALS INDEX: p e r ov sk i tes , o x ide s , l an thanum, s t r on t i um , c ob a l t ,i r o nI n t r o d u c t i o n

Som e h i g h l y n o n s t o i c h i o m e t r i c o x i d e s sh ow m ix e d c o n d u c t i v i t y b y b o t hi o n i c an d e l e c t r o n i c c ha rg e c a r r i e r s , an d m i xe d c o n d u c to r s h a v e i n c re a s i n g l ya t t r a c t e d g r e a t a t t e n t i o n i n m a n y p r a c t i c a l a p p l i c a t i o n s ( i ) . So f a r , m i xe dcondu c t i on by ox ide i ons and e lec t ron s (o r ho les ) have been repo r ted i n manyox ide systems su ch as B i203-Tb203.5 (2) , ZrO2-CeO2-Y203 (3) , among which oxy -g e n - d e f i c i e n t p e r o v s k i t e - t y p e o x id e s c o n t a in i n g t r a n s i t i o n m e t als a t B s i t e s ,e .g ., La l -xS rxC o03-6 , seem to be p rom inen t m ixed conduc to rs w i th h i gh i on i cand e l e c t r o n i c c o n d u c t i v i t i e s . A l th o u g h L nC o 03 ( Ln : r a r e e a r t h e l em e n t )i t s e l f has been k no w n t o show h ig h e l e c t r o n i c c o n d u c t i v i t y , t h e p a r t i a ls u b s t i t u t i o n o f S r2 + f o r L n 3 + b r i n g s a b o ut n o t o n l y an i n c re a s e i n e l e c t r o n i cc o n d u c t i v i t y b u t a l s o an a pp ea ra nc e o f i o n i c c o n d u c t i v i t y b y t h e c o n c o m i ta n tf o r m a t i o n o f C o4 + ( o r h o le ) and o x i d e i on v a c a n c i e s , r e s p e c t i v e l y . T h i se f f e c t a p pe a rs t o c o r r e l a t e i n t i m a t e l y w i t h t h e h ig h p e r fo r m a n c e o f t h eo x i d e s a s c a ta l y s t s i n e l e c t r o d e p ro c e s s ( 4 ) o r i n c o mp l e te o x i d a t i o n c a ta l y -s i s ( 5 , 6 ) , e l e c t r o d e m a t e r i a l o f f u e l c e l l ( 7 ) , ga s s e n s o r ( 8 ) , e tc . On t h eo th e r h an d, t h e s u b s t i t u t i o n f o r t h e B s i t e e l eme n t was a l s o r e p o r te d t o g iv e51


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5 2 Y . T E R A O K A , e t a l . V o l . 2 3 , N o . 1

rise to promotive effects. For example, the substitut ion of Fe for Co ofLal-xSrxCo03-6 reportedly enhanced the sensitivity of a combustion monitoringsensor (9) and the catalytic activity for electrochemical reduction of oxygen(4). We recently reported that La1-xSrxCo1%yFey03-6 showed excellent oxygensemipermeability at elevated temperatures (10). All these properties, i.e.,catalytic activity, electrode activi ty, gas sens itivity, oxygen semiperme-ability etc., are deeply related with the mixed conduction of the perovskite-type oxides. However, there have been few reports which evaluated theirmixed conductivities.

This paper aims at evaluating mixed conductivities of Lal-xSrxCol-y-FeyO3-a as functions of temperature and A-site as well as B-si te composi-tions. It also aims at establishing relations between mixed conductivitiesand the oxygen semipermeability.

Experimental

Lal-xSrxCol-yFey03-~ were prepared from JIS special grade reagents oflanthanum, strontium, cobalt acetates and iron nitrate. An aqueous solutiondissolving starting materials in a desired proportion was evaporated todryness, followed by the calcination at 1123 K in air for 10 h. The powderof each sample thus obtained was compressed into a disc, 20 mmdiameter and 4mm thick, under the hydrostatic pressure of 260 MPa. After the disc was cutinto a rod, approximately 4 x 4 x 12 mm, the specimen was sintered at 1473 Kor 1523 K (for Fe containing samples) in air for 5 h. The obtained specimenwas polished by an emery paper before subjecting conductivity measurements.(YOI.5)O.16(Zr02)O.84 (YSZ) was prepared from Y203 (99.99%) and Zr02 (99.9%).A mixture of Y203 and Zr02 was f i rstcalcined in air at 1573 K for 5 h.The pre-calcined oxide was ground,pressed hydrostatically (260 MPa) ina disc, 10 mm diameter and 2 mmthick, and sintered in air at 1723 Kfor 15 h.

Oxide ionic conductivity ofLal-xSrxCol-yFey03-~ was measured bymeans of a four-probe ionic dc tech-nique. The cell arrangement isschematically shown in Fig. I. Forhaving a pure ionic current suppliedto and taken out from the samplespecimen, ionic current electrodes,Pt / Lao.2SrO.8Coo.sFeo.203-6 / YSZ,were contacted to the both ends ofthe specimen mechanically as shown.Under a flow of constant current, I,voltage drop, V, across the specimenwas measured by the ionic probes, 3mmapart for each, which had essen-t ial ly the same construction as theionic current electrode with theirsharp edges coming into contact with

ionic currente~de

[ ] s a m p l e P t p l a t e[ ] m i x e d c o n d u c t o r( L a o . 2 S r o . 8 C o o . 8 F e o . 2 0 3 _ a )E l s o l i d e l e c t r o l y t e

((YOi.5)o.16(Zr02)o.84)FIG. ISchematic cell arrangement of four-probe ionic dc measurement.


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t h e s i d e o f t h e s p e c i m e n m e c h a n i c a l l y . O x id e i o n i c c o n d u c t i v i t y ( o i ) w asc a l c u l a t e d fr om t h e f o l l o w i n g e q u a t io n .oi : I I/V S [1]

w h e re 1 a nd S a r e t h e d i s t a n c e b e tw e e n i o n i c p ro b e s a nd t h e c r o s s - s e c t i o n a la r e a o f t h e s am p l e s p e c im e n , r e s p e c t i v e l y . T he c o n s t a n t c u r r e n t I w as r e g u -l a t e d i n a r an g e o f I - 1 0 -2 mA s o a s t o g i v e a s t e a d y - s t a t e v o l t a g e d i f f e r -e nc e o f I - I 0 mV. T he c o n d u c t i v i t y m e a s u r e m e n t c e l l w as p l a c e d i n a q u a r t z -g l a s s e q u ip p e d i n a n i n f r a r e d im a g e f u r n a c e . T he c e l l w as h e a t ed up t o 1 1 5 0K a t a r a t e o f I 0 K m i n - 1 a nd h o ld e d a t t h a t t e m p e r a t u r e f o r 30 m i n i n a na t m o s p h e re o f He , a i r o r 02 . T h e n t h e c e l l w as c o o l e d s t e p w i s e a t a r a t e o f5 K m i n - i t o t e m p e r a t u r e s a t w h i ch i o n i c c o n d u c t i v i t y was m e a su re d a f t e rh o l d i n g f o r 30 m in to e q u i l i b r a t e t h e te m p e r a t u r e o f t he s pe cim e n w i t h t h a to f s u r r o u n d i n g . To r ed u c e t h e e f f e c t s o f p o l a r i z a t i o n , m e a s u re m e n ts w e res u c c e s s i v e l y c a r r i e d o u t i n bo th fo r w a r d an d r e v e r s e d i r e c t i o n s o f c u r r e n t .The m ea s ur em e n t o f e l e c t r o n i c c o n d u c t i v i t y ( O e ) w as c a r r i e d o u t b y an o r d i -n a r y f o u r - p r o b e dc t e c h n i q u e i n a i r .

T e m p e r a t u r e p ro gra m me d d e s o r p t i o n ( T P D ) o f o x y g e n w as c a r r i e d o u t i n as t r e a m o f He f o r L a l - x S rx C o O . 4 F e o . 60 3 - 6 a t a h e a t i n g r a t e o f 1 0 K m i n - l . T hep r o c e d u r e w a s d e s c r i b e d e l s e w h e r e ( 6 ) .

R e s u l t s a n d D i s c u s s i o nEffects of temperature on~iVTt ies

I n F i g . 2 , i o n i c ( d i )a nd e l e c t r o n i c ( ~e ) c o n du c -t i v i t i e s o f L a O . 8 S r O . 2 -C o 0 . 8 F e 0 . 2 0 3 - ~ u n d e r s e v -e r a l s u r r o u n d i n g a t m o -s p h e r e s a r e s h o w n as af u n c t i o n o f t e m p e r a t u r e .F o r m e a s u r e m e n t s o f o i , t h el o w e r l i m i t o f m e a s u r i n gt e m p e r a t u r e , 8 00 K , w asd e t e r m i n e d b y t he i o n i cc o n d u c t i v i t y o f YSZ w h i c hw a s u s e d a s a n e l e c t r o nb l o c k i n g m a t e r i a l o f t hei o n i c c u r r e n t e l e c t r o d e .o i o f L a O . 8 S r . 2 C o . 8 F e . 2 0 3 - ~i n c r e a s e d w i t h d e c r e a s i n go xy ge n p a r t i a l p r e s s u r e( P 0 2 ) o f t h e s u r r o u n d i n ga t m o s p h e r e , 02 < a i r < H e.I n e v e r y c a s e , h o w e v e r , o iw as l a r g e r b y a b o u t o n eo r d e r o f m a g n i t u d e t ha nt h a t o f Y SZ a t t h e s am et e m p e r a t u r e a s d e p i c t e dt o g e t h e r in F i g . 2. T h i s

1 31 0 210o

I0 -I( , I )

b 10 -2

O-e(ai r)

1 0 - 3

10 40,8

~ c i (He)0~" CT

, , ~ ,0,9 1.0 i,i 1,2 1,3T-1 /I0-3 K-I

FIG. 2T e m p e r a t u r e d ep en de nc e o f i o n i c a nd e l e c t r o n i cc o n d u c t i v i t i e s o f L a o .8 S r .2 C o o . 8F e o .2 0 3 - 8 .


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54 Y. TERA OKA, et al. Vol. 23, No. 1

c o n f i r m s t h a t t h e p e r o v s k i t e - t y p e o x i d e i s an e x c e l l e n t o x id e io n co n d u c t or .T he o b s e r ve d e f f e c t s o f P02 c an be u n d e r s t o o d f ro m t h e o x y g e n - s o r p t i v e p r o p -e r t i e s o f t h e o x i d e . As d e s c r i b e d i n m o re d e t a i l l a t e r , a l a r g e am o u nt o fo x yg e n c an b e d e s o rb e d f ro m o r a b s o rb e d in th e c r y s t a l l i n e l a t t i c e o f t h eo x i d e on h e a t i n g o r c o o l i n g , r e s p e c t i v e l y . T h i s d e s o r p t i o n o r a b s o r p t i o nn a t u r a l l y r e s u l t s i n f o r m a t i o n o r e x t i n c t i o n o f o x i d e i on v a c a n c i e s ( 6) .T h e r e f o r e , t h e c o n c e n t r a t i o n o f o x i d e i o n v a c a n c i e s de pe nd s o n P 02 a nd t e m -p e r a t u r e . T he o b s e r v e d d e c r e a s e i n a i w i t h i n c r e a s i n g POp c an t h u s be a t -t r i b u t e d t o a d e c r e a s e in 6 , i f o ne a ss um e s o x i d e i o n c o n d u c t i o n v i a av a c a n c y m e ch an is m . I t i s a l s o n o t e d f ro m F i g . 2 t h a t t h e a p p a r e n t a c t i v a t i o ne n e r g y f o r o x i d e i o n c o n d u c t i o n , E a, i n c r e a s e d w i t h i n c r e a s i n g P O p . T h is w asa g a in a f f e c t e d b y o xy ge n s o r p t i o n : i n o x y g e n - c o n t a i n i n g a tm o s ph e ge , th e h e a to f o x i d e i o n v a c an c y f o r m a t i o n w as a d d i t i v e l y i n c l u d e d i n E a.

It was found that ae was larger by several orders of magnitude than oiand l i t t le dependent on PO2. The magnitude of Oe as well as its monotonousdecrease with increasing temperature suggest a metallic conduction. Withsuch high ionic as well as electronic conductivities, the present perovskite-type oxide is concluded to be an excellent mixed conductor.

Effects of composition on conductivities

Effects of A si te (x) or B si te (y) subst itut ion on oi and ~e at 1073 Kare shown in Fig. 3. oi increased sharply with increasing x while i t gradu-

102

IEi0 0

b

1 0 - 2

%

ZLZf. y (air)I I I0 0,5 1.0

in La_xSrxCoo. Feo.03_6

!

102

Eoo i0b

10 -2 (02 )! I !0 0,5 1.0

Y In Lao.6Sro.4COl_yFey03_ 6FIG. 3Effects of A (lef t) and B (right) site substitution on conductivitiesat 1073 K of Lal-xSrxCol-yFey03-6.


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al ly decreased with increasing y. I t is clear that ~i is pr imar i ly con-trol led byA si te composition. Such dependences of oi on x and y coincidewell with the oxide ion conduction via a vacancy mechanism as shown below.As for Oe measured in ai r , i t depended only s l ight ly on x with a modestmaximum at x = 0.4 - 0.6, while i t monotonouly decreased as y increased. Thedependences of Oe on x and y are essentially in accordance with the resultsby Obayashi and kudo (14) and Rao et al. (15), respectively. From ~i and ae,the ionic transport number of Lal-xSrxCol-yFey03-~ at 1073 K is calculated tofal l in the range 10-2 - 10-4 at the compositions examined.

%>E

C. )oOc -OC ).0 ' )( P

' E l0( J

r ~

20

10

0300

/,..,,.;.;. ". . . .#] . . , . . / , . ,

if-


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5 6 Y . T E R A O K A , e t a l . V o l . 2 3 , N o . 1

f o l l o w s .o = (Zq)2 Cv Dv / k T [2 ]

where Cv and Dv are the numberof oxide ion vacancies per unitvolume and the diffusion coeffi-cient of oxide ion vacancies,respectively, and Zq is thecharge of car ri er , k is Bol tz-mann constant and T is absolutetemperature. Ishigaki et al.(12) reported that the tempera-ture dependence of Dv in LaCo03was expressed by the equationDv[cm2 s-l] =1.59 x 10-2 exp (Ea / R T) [3]Ea = 18 _+ 5 kc al m o l- 1 ,where R is gas constant. Whenthe mole fraction of oxide ionvacancies is assumed to be 10%(8 = 0.3), Cv is derived to beroughly equal to 5.0 x 1021cm-~, and thus ~i can be es-timated as 1 - 10-2 S cm-1 at1073 K by using Eqs. [2] and[3]. Th is estimation coincidesfairly well with the observed oishown in Fig. 3.

The apparent activationenergy (Ea) for oxide ion condu-ction in He atmosphere is shownin Fig. 5 as a function of A orB site composition. Ea rangedfrom 15.5 to 20.7 kcal mol-1,which was reasonably close tothe reported activation energyfor migration of oxide ion va-cancies in LaCo03 and LaFe03,18 5 k c a l m o l - 1 ( 1 2 , 1 3 ) .T he se r e s u l t s n o t o n l y s u p p o r tt h a t t h e o x id e i o n c o n d u c t i o no c c u r s v i a a v a c a n c y m e c h a n is m ,b u t a l s o a p p e a r t o a s s u r e t h a tt h e f o u r - p r o b e i o n i c dc t e c h -n i q u e e m p l o y e d i n t h i s s t u d yg i v e s r e l i a b l e d a ta o f i o n i cc o n d u c t i v i t y f o r a m i xe d c on d uc -t o r h a v i n g a s m a l l i o n i c t r a n s -po r t num ber .

y in Loo.6SFo.4COl.yFey03-80 0.2 0,4 0,6 0,8 1,0! ! i ! ! !

2o

1 5 i , , ,0 0 , 2 0 , 4 0 , 6 0 , 8 1 , 0x i n L O l _ x S F x C O o . 8 F e o . 2 0 3 - a

F I G . 5Effects of A and B site substitutionson the apparent activation energy foroxide ion conduction in He atmosphere.

EO0

Eg

1 0 o

i 0 - ]

10 2

colculotedfrom J*

Oin He0 in oir

0,9 i , i 1,3T-I/lo-3 K-IFIG. 6Applicability of measured ionic conduc-t ivit ies to the rate of oxygen semiperme-ation (J*) for LaO.6Sro.4Co03-6.


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Re lation to oxygen sem iperm eabilityAs we reported previously (10), the rate of oxygen semipermeability ofLal-xSrxCol-yFey03-6 increased as the contents of Sr and Co increased, thoughSr content was more inf luen tial . This trend in oxygen semipermeabilityagrees well with that in oxide ion conductivi ty shown in Fig. 3. Figure 6compares the ai values of LaO.6SrO.4Co03-6 measured in this study with thosecalculated from the rate of oxygen semipermeability by assuming the followingequation.

oilS cm-l] = 5.7 x 103 (Jd/T)[log (p'/p")]-I [4]where J is the rate of oxygen semipermeability [cm3(STP) min-1 cm-2], d isthe thickness of the disc [cm], T is temperature [K] , and P' and P" are theoxygen part ial pressures of the atmospheres separated by the perovskite-typeoxide membrane, 21 and 0.1 kPa, respectively. The calculated values are seento be close to the measured ones. Such a coincidence is reasonable becausethe oxygen semipermeability of Lal-xSrxCol-yFey03-6, a suff ic ient ly highelectronic conductor, is controlled by the ionlc conductivity.

References1 . H .L . T u l l e r , No ns to i ch iom et r i c Ox ides , Chap t. 6 , p .271 , Academ ic P ress ,New York(1981) .2 . T . Esaka and H. lwah ara, J . Ap p l . E lec t roch em ., 15, 447(1985) .3 . B . Ca les and J .F . Baumard , J. E lec t roc he m . Soc . , 131 , 2407(19 84) .4. N. M iura , Y. Sh im izu, N. Yamazoe and T. Seiyama, Nippon Kagaku K a ish i ,1985, 644.5. T. Nakamura, M. M isono, T. Uch i j im a and Y. Yo ned a, Nippon Kagaku K a is hi ,1980, 1679.6. Y. Te rao ka, S. Furukawa, N. Yam azoe and T. Seiyama, Nippon KagakuKa i sh i , 1985 , 1529 .7 . O . Yamam oto , Y . Taked a , R . Kanno and M. Noda, So l i d S ta t e l o n i c s , 22 ,241(1987) .8 . H. Obay ash i and T. Ku do , N ippon Kagaku K a is h i , 1980, 1568.9 . Y . Yamamura, Y . N inomiya and S. Sek ido, Proceedings o f th e In te rn at io n a lMe eting on Chemical S enso rs, p.187, K odansha and Els ev ier(1 98 3).

I0 . Y . Te rao ka , H .M. Zhang , S . Fu rukawa and N . Yamazoe , Chem. L e t t . , 198 5 ,1743.I i . Y . Te rao ka , H .M. Zhang and N. Yamazoe , Chem. L e t t . , 1985 , 1367 .1 2. I . I s h i g a k i , S . Y a m a u c h i , J . M i z u s a k i , K. F u e k i a nd H. T a ma ru , J. S o l i dS ta te C h e m. , 5 4 , 1 0 0 (1 9 8 4 ) .1 3. I . I s h i g a k i , S. Y a m a u c h i , J . M i z u s a k i , K. F u e k i , H. N a i t o a n d T . A d a c h i ,


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J . S o l i d S t a t e C h e m . , 5 5 , 5 0 ( 1 9 8 4 ) .14. H. Obayashi and T. Ku do, Denk i K aga ku, 44, 503(197 6) .15. C.N.R. Rao, Om Pa rkash and P. G an gu ly , J . S o l i d S ta te Chem., 15,186(1975) .16 . Y. Te rao ka , M. Y os h im ats u , N . Yamazoe and T . Se iyam a, Chem. L e t t . , 1984 .893.

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Mixed Ionic Electronic Conductivity of Lal Xsrxcol Yfey03 6