PREPARATION, MICROSTRUCTURE AND ELECTRICAL PROPERTIES OF ERBIA AND YTTRIA-DOPED TETRAGONAL ZIRCONIA

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PREPARATION, MICROSTRUCTURE AND ELECTRICAL PROPERTIES OF ERBIA AND

YTTRIA-DOPED TETRAGONAL ZIRCONIA

J. Jurado, C. Moure, P. Durán

To cite this version:

J. Jurado, C. Moure, P. Durán. PREPARATION, MICROSTRUCTURE AND ELECTRI-

CAL PROPERTIES OF ERBIA AND YTTRIA-DOPED TETRAGONAL ZIRCONIA. Journal de

Physique Colloques, 1986, 47 (C1), pp.C1-789-C1-793. �10.1051/jphyscol:19861120�. �jpa-00225517�

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PREPARATION, MICROSTRUCTURE AND ELECTRICAL PROPERTIES OF ERBIA AND YTTRIA-DOPED TETRAGONAL ZIRCONIA

J. R . JURADO, C. MOURE and P. D U R ~ N

I n s t i t u t o d e Ceramica y V i d r i o , C . S . I . C . , Dpto. d e M a t e r i a l e s Cerzmicos E s p e c i a l e s , Arganda d e l Rey, Madrid, S p a i n

RGsum6

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Des 6 c h a n t i l l o n s t r G s denses d e z i r c o n e q u a d r a t i q u e con- t e n a n t 3moles % d e Y,O, ou E r 2 0 3 o n t Gt6 p r e p a r e s p a r f r i t t a g e 2 des tempsratures mod6rGes ( 1 4 0 0 - 1 5 5 0 ' ~ ) . La m i c r o s t r u c t u r e , l e s propriGtGs mGcaniques e t l e s propriGtGs G l e c t r i q u e s de c e s gchan- t i l l o n s o n t G t G GtudiGes. Les r e l a t i o n s e n t r e l e s propriGt6s G l e e t r i q u e s , ddterminGes p a r l a mGthode du t r a c e d e s diagrammes d'im- pGdance complexe, e t l a m i c r o s t r u c t u r e o n t Et6 examinges.

A b s t r a c t - Highly d e n s i f i e d s t a b i l i z e d t e t r a g o n a l z i r c o n i a c e r a m i c s c o n t a l n i n g 3 mol% Y,03 o r E r p O B were o b t a i n e d b y n o r m a l s i n t e r i n g a t moderate temperatures ( 1 4 0 0 - 1 5 5 0 ' ~ ) . The r n i c r o s t r u c t u r e . mecha- nicalandelectricalpropertiesof s i n t e r e d samp1eswerestudied.The relationshipsbetweenelectricalproperties,asdeterminedby complex p l a n e impedance spectroscopy, a n d m i c r o s t r u c t u r e h a s b e e n e x a m i - ned.

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INTRODUCTION

Since few y e a r s t h e t e t r a g o n a l z i r c o n i a can be s t a b i l i z e d i n a d e n s e f i - ne-grained p o l y c r i s t a l l i n e body from a p a r t i a l l y s t a b i l i z e d z i r c o n i a

(PSZ) powder c o n t a i n i n g a low c o n c e n t r a t i o n of y t t r i a , provided t h a t both t h e i n i t i a l p a r t i c l e s i z e and t h e s i n t e r i n g c o n d i t i o n s have been c a r e f u l l y chosen / l / .

Mechanical p r o p e r t i e s on y t t r i a PSZ ceramics c o n t a i n i n g a m e t a s t a b l e t e - t r a g o n a l phase were r e p o r t e d by Gupta e t a 1 /2//3/. They c o n c l u d e d t h a t t h e m e t a s t a b l e phase i n t h e range of 100 t o 30% of t e t r a g o n a l c o n t e n t s , due t o i t s a b i l i t y t o undergo s t r e s s - i n d u c e d phase t r a n s f o r m a t i o n , con- t r i b u t e s both t o a high s t r e g t h -700 MPa and high f r a c t u r e toughness -6 t o 9 MPa m - 3 / 2 . They a l s o i n d i c a t e t h a t t h e r e i s a c r i t i c a l g r a i n s i - ze of 0.34 pm below which t h e m e t a s t a b l e phase i s s t a b l e .

Due t o t h e tetragonal-monoclinic t r a n s f o r m a t i o n t h e e l e c t r i c a l proper- t i e s of t h e t e t r a g o n a l phase cannot be s t u d i e d b e l o w t h e t r a n s f o r m a t i o n temperature, however today it i s p o s s i b l e t o perform e l e c t r i c a l measurements on t h e t e t r a g o n a l z i r c o n i a below t h e t r a n s f o r m a t i o n temperature / 4 / , / 5 / , / 6 / . The high e l e c t r i c a l c o n d u c t i v i t y and t h e h i g h mechanical s t r e n g t h of t e t r a g o n a l z i r c o n i a make t h i s m a t e r i a l s u i t a b l e f o r a p p l i - c a t i o n s a s s o l i d e l e c t r o l y t e p a r t i c u l a r l y below 6 0 0 ' ~ .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19861120

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JOURNAL DE PHYSIQUE

Most works done in this field report on yttria-doped tetragonal zirconia, and there is not publication at our knowledge about erbia-doped tetragonal zirconia ceramics. From microstructural considerations it may be assumed that the ceramic material prepared in the tetragonalzir- conia zone of the Zr02-Er203 system /7/ could, at least, exhibit thesa- me properties as yttria-doped tetragonal zirconia.

In this work the electrical and mechanical properties of two highlyden- sified tetragonal zirconias with 3% mole of Y2O3 and 3% mole Er203 are reported.

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EXPERIMENTAL PROCEDURE

The ZrOn, Y203 and Ern03 starting raw materials were obtained from ZrOC12.8H20, Y(N0,),.5H20 and Er(N03),.5H20 99.9% pure. They were dis- solved in distilled water, and this solution was poured into the neces- sary amount of aqueous ammonia to ensure that the pH during the copreci- pitation process was always 9. In this way the quantitative coprecipi- tation of oxides was achieved. After filtering the precipitated gel was then repeatly washed with distilled water until the total disappearan- ce of the Cl- ions was achieved. Substituting isopropyl alcohol for water in the gel, the coprecipitate was again washed and dried at 7 0 " ~ . After drying the coprecipitate was calcined at 500°C for 1 h.

The DTA/TGA curves were obtained from a Mettler thermoanalyser using alumina as reference material. The crystal sizeof calcined powders was calculated from X-ray line broadening with a Philips 1040 diffractome- ter. SEMI BET and sedigraph techniques were used in the powder charac- terization.

From the calcined powders, samples were isostatically pressed at -200 MPa. The samples were sintered in air at two different temperatures,

1550'~ for 6 hours (TZP-Y-1 and TZP-Er-l), and 1420°C for 1 hour (TZP- Y-2 and TZP-Er-2). The heating rate was 5"C/min in all cases. Bulk den- sities were determined by water impression. Grain-size were measuredon electron micrographs of polished and thermal etched surface. The average grain-size was determined by the interception line method /8/. The stress intensity factor KIC as described by Marshal1 et a1 /9/ was mea- sured. Electrical conductivity measurements were performed using a.c.

impedance complex plane analysis over a frequency range 5 to 107 Hz, and from 200" to 500°C. A 20 to 100 mV potential was applied and two em- bedded platinum electrodes technique was used. A Hewlett-Packard impedance analyser Model 4192A was employed. The data during both heating and cooling were collected.

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RESULTS AND DISCUSSION Powder morphology

Afer drying, the TZP-Y coprecipitated powder had a very small particle size ('10 nm) and a high specific surface area (290 m2/g). Thecalcined powder consisted of small agglomerates with an average9 size of 580 nm and a specific surface area of approximately 140 m2/g.

In the case of the TZP-Er coprecipitated powder the agglomerates size was x0.1 pm and the specific surface area 130 The calcined powder was strongly agglomerated with a large dispersion in agglomerate size.

Its specific surface area was about 74.6 m2/g.

In both cases the calcined powder presented a tetragonal structure although the TZP-Er sample showed the presence of a small amount of monoclinic phase ( < 5 % ) . The difference in the morphologyofcalcinedpowders accounts for their compaction; thus the green density for TZP-Ywas45%

of theoretical density and 40% of theoretical density for TZP-Er. The

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the sintering experiments.

S i n t e r i n g a n d m i c r o s t r u c t u r e

After sintering both TZP-Y and TZP-Er samples ret~ined the tetragonal symmetry and the lcttice parameters were a=5.080 A, c=5.165 A and a=

=5.085

A,

^c=5.174^Arespectively, and any cubic phase was not observed.

As it is shown in Fig. 1 ( A ) , the microstructure of the TZP-Y samples was mainly composed of small grains with a very uniform distribution and an average grain-size of 0.5 pm.Occasionally, some large grains

(klpm) were also observed. The EDS microanalysis on the two types of

F i g . 1 . - SEM o f t h e C e t r a g o n a l z i r c o n i a m i c r o s t r u c t u r e .

qrains did not reveal a variation of composition and it seems reasona- ble to assume that the larger grains have grown at the expense of the smaller grains. This assumption is, on the other hand, inagreementwith the results obtained by X-ray determinations and the established phase diagram for the Zr02-Y203 system /10/.

In the case of the TZP-Er sintered samples, Fig. 1 (B), the microstructure was more uniform than that observed in the TZP-Y sintered samples.

It seems that the attrition-milling strongly influence the size andthe particle size distribution before sintering. The average grain sizewas of 0.4 um after sintering at 1 4 2 0 ' ~ for 1 hour.

Mechanical behaviour

Although it was not the scope of the present work, an initial study of the mechanical properties for the two TZP sintered samples was done. It was found that the mechanical behaviour of the both kinds of sintered

samples was very different. Thus the KIC values for the TZP-Y samples sintered at 1550°c and 1420'~ were 8 and 5.80 MPa respectively.

In the case of PZT-Er sintered samples the KIC values were6.1 and6.65 MPa m-3/2 respectively at the same temperatures. Such a mechanical behaviour seems to be in agreement to the density, grain sizeandmonocli- nic phase present in the samples. A more complete study of these properties is now in progress.

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JOURNAL DE PHYSIQUE

E l e c t r i c a l p r o p e r t i e s

A r e p r e s e n t a t i v e example of experimental complex impedance p l o t s o b t a i n ed a t 3 0 0 ' ~ i s shown i n F i g . 2 . It was found a s i g n i f i c a t i v e d i f f e r e n c e between t h e impedance p l o t s of y t t r i a and e r b i a doped t e t r a g o n a l zirco- n i a samples s i n t e r e d a t 1 5 5 0 ' ~ f o r 6 h o u r s , a n t h o s e s i n t e r e d a t 1 5 5 0 ' ~ f o r 6 hours, and t h o s e s i n t e r e d a t 1 4 2 0 ' ~ f o r 1 hour. The small arcwhich corresponds t o t h e bulk conduction c o n t r i b u t i o n appears very c l e a r a t

samples TZP-Y-1 and TZP-Er-1, whereas TZP-Y-2 and TZP-Er-2 samplesshow

an overlapping of t h e g r a i n boundary a r c s . It can be due t o : a ) Thepre- sence of monoclinic z i r c o n i a i n TZP-Y-2 and TZP-Er-2 samples, and/or b) A d i f f e r e n t d e n s i f i c a t i o n .

The Arrhenius p l o t s ( l o g aT v s Temperature) of t h e b u l k a n d g r a i n boundary c o n d u c t i v i t i e s ( s e e Fig. 3 ) were l i n e a r over t h e t e m p e r a t u r e r a n g e i n v e s t i g a t e d . The v a l u e s of 0 0 (pre-exponential f a c t o r ) , t h e a3000cand

F i g . 2 . - Complex i m p e d a n c e p Z o t f o r t e - F i g . 3 . - B u l k a n d g r a i n bounda- t r a g o n a l z i r c o n i a s a t 3 0 0 ' ~ . r y c o n d u c t i v i t y p l o t o f t e t r a -

g o n a l z i r c o n i a s .

t h e a c t i v a t i o n e n t h a l p y f o r both bulk and g r a i n boundary c o n d u c t i v i t i e s of a l l samples a r e r e p o r t e d i n Table I ; corresponding v a l u e s o b t a i n e d f o r PSZ, FSZ and TZP r e p o r t e d by Bonanos e t a 1 /5/ a r e a l s o given f o r comparison. As Table I i l l u s t r a t e s t h e TZP samples with h i g h e s t conduc- t i v i t y a t 3 0 0 ' ~ corresponds t o TZP-Y-1. As shown by Bonanos e t a 1 / 5 / , t h e pre-exponential f a c t o r f o r bulk c o n d u c t i v i t y i s lower i n a l l TZPre- p o r t e d , u t t o d a t e , than i n PSZ and FSZ, and t h a t i s a t t r i b u t a b l e t o t h e lower a c t i v a t i o n e n t h a l p y . Sbch a behaviour i s being a l s o e x h i b i t e d f o r both e r b i a and y t t r i a t e t r a g o n a l z i r c o n i a , a l t h o u g h t h e y s h o w a n ac- t i v a t i o n e n t h a l p y s i g n i f i c a n t l y lower t h a n o t h e r s / 4 / , / 5 / . It c o u l d b e r e l a t e d w i t h t h e p r o c e s s i n g , s i n t e r i n g and developed m i c r o s t r u c t u r e s . From e l e c t r i c a l d a t a r e p o r t e d up t o d a t e / 4 / , / 5 / , /6/ it can be dedu- ced a tendency t o d e c r e a s e t h e AHa. I t i s now r e p o r t i n g an AHa lower than t h o s e p r e v i o u s l y published /4/, /5/ ( s e e Table I).Moreover a black f u l l y d e n s i f i e d TZP-Y body w i l l be r e p o r t e d elsewhere with an a c t i v a - t i o n e n t h a l p y a s low a s 0.68 eV and it was a l s o observed t h a t t h e a c t i - v a t i o n e n t h a l p y d e c r e a s e s a s a f u n c t i o n of t h e c o l o u r i n t e n s i t y i n t h e TZP-Y samples. This behaviour i s l i k e l y involving t h a t t h e c o n t r i b u t i o n of t h e a s s o c i a t i o n enthalpy (AHAl) t o t h e bulk c o n d u c t i v i t y t r e n d s t o zero and, a s consequence, a h i g h d e f f e c t i v e s t r u c t u r e i s b e i n g produced.

Such a s t r u c t u r e c r e a t e c o l o u r c e n t e r p a r t i c u l a r l y i n yttria-dopedTZP.

I t i s , t h e r e f o r e , suggested t h a t t h e s e high d e f f e c t i v e s t r u c t u r e s o b s e r - ved i n TZP-Y and TZP-Er prepared i n t h i s w o r k a r e t h e r e s p o n s i b l e f o r t h e low a c t i v a t i o n e n t h a l p y f o r t h e conduction.

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o3o0OC(GB) v a l u e s f o r b o t h s a m p l e s i s 1 . 9 2 , w h i c h s u g g e s t s t h a t t h e 03000C(GB) d i f f e r e n c e s c o u l d a c c o u n t f o r g r a i n s i z e a l o n e a s a s s u m e d b y B o n a n o s e t a 1 / 4 / . I n t h e TZP-Er s a m p l e s a n o p p o s i t e r e s u l t w a s d e t e c t - e d , t h u s t h e " B r i c k L a y e r M o d e l " p r o p o s e d b y V a n D i j k a n d B u r g r a a f / 1 1 / c o u l d b e a p p r o p i a t e d t o e x p l a i n a l l t h e d a t a h e r e i n r e p o r t e d ; n e v e r t h e - l e s s o t h e r f a c t o r s shouldbeconsideredforamoreaccuratelyexplanation.

REFERENCES

/ l / . G u p t a T.K., B e c h t o l d J . H . , K u z n i c k i R . C . , C a d o f K.H. a n d R o s s i n g B.R. J . Mater. S c i . 1 2 , ( 1 9 7 7 ) 2 4 2 1 .

/ 2 / . G u p t a T.K., L a n g e F . E , B e c h t o l d J.H. J . Mater. S c i . 2 ( 1 9 7 8 ) 1 4 6 4 . / 3 / . L a n g e F . F . , J . 1 4 a t e r . S c i . 1 7 . ( 1 9 8 2 ) 2 2 5 .

/ 4 / . G u p t a T . K . , G r e k i l a R.B., ~ u b b a r a o E.C. J. E l e c t r o c h e m . S o c .

128,

( 1 9 8 1 ) 9 2 9 .

TABLE 1

( B ) B u l k , (GB) G r a i n B o u n d a r y .

/ 5 / . B o n a n o s N . , S l o t w i n s k i R.K., S t e e l e B.C.H., B u t l e r E.P. J. Mater.

S c i e n c e L e t t e r s

3,

( 1 9 8 4 ) 2 4 5 .

/ 6 / . M o u r e C . , J u r a d o J . R . , D u r 6 n P . P r o c e e d i n g M e e t i n g B r i t i s h C e r a m i c S o c i e t y . D e c e m b e r 1 9 5 4 , L o n d o n .

/ 7 / . P a s c u a l C . , D u r 5 n P . J . Mater. S c i . 1 6 , ( 1 9 8 1 ) 3 0 6 7 . /8/. F u l l m a n R.L.. T r a n s . AIME,

197,

( 1 9 5 3 4 4 7 .

/9/. M a r s h a l 1 D.B., E v a n s A.G. J . Amer. C e r a m . S o c . 6 4 ( 1 9 8 1 ) C - 1 8 2 . / I O / . P a s c u a Z C., D u r 6 n P. J. Amer. C e r a m . S o c .

66,

( % 8 3 ) 2 3 .

/ 1 1 / . V a n D i j k T . , B u r g r a a f A.J. P h y s . S t a t u s S o l i d i ( A ) ,

63,

( 1 9 8 1 ) 2 2 9 . AH (GB)

e V

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k J / m o l

0 . 9 8 9 4 . 4

1 . 0 2 9 8 . 2

1 . 0 1 9 7 . 3

1 . 0 9 1 0 5 . 0

1 . l 5 1 1 0 . 8

1 . 1 2 1 0 7 . 9 SAMPLE

TZP-Y-I

TZP-Y-2

TZP-Er-1

TZP-Er-2

TZP Ref ( 4 1

PSZ 4 . 7 % Y 2 0 3

R e f ( 4 ) FSZ 6 . 0 % Y 2 0 3

R e f ( 4 )

0 0 ( B ) ? T Q-'cm- T

i . i 3 x i o 5

3 . 2 0 ~ 1 0 ~

6 . 4 0 x 1 0 4

2 . 6 2 x 1 0 4

5 . 5 0 x 1 0 5

6 . 1 0 ~ 1 0 ~

6 . 8 0 x 1 0 6

0 3 0 0 ° c ( B ) R - l c m - l

1 , 1 1 ~ 1 0 - ~

6 . 4 6 x 1 0 - ~

2 . 5 9 x 1 0 - ~

2 . 1 1 ~ 1 0 - ~

7 . 7 0 x 1 0 - ~

3 . 8 5 ~ 1 0 - ~

5 . 5 6 ~ 1 0 ~ ~ AH (B) eV

-

k J / m o l

o

- 8 2 7 9 . 0

0 . 9 0 8 6 . 7

0 . 8 8 8 4 - 8

0 . 8 4 8 0 - 9

0 . 9 2 8 8 . 6

1 . 0 7 1 0 3 . 1