TETRAGONAL ZrO2-Y2O3 PART II : THE MECHANICAL AND ELECTRICAL PROPERTIES

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TETRAGONAL ZrO2-Y2O3 PART II : THE

MECHANICAL AND ELECTRICAL PROPERTIES

K. Keizer, M. van Hemert, A. Winnubst, M.A.C.G. van de Graaf, A.

Burggraaf

To cite this version:

K. Keizer, M. van Hemert, A. Winnubst, M.A.C.G. van de Graaf, A. Burggraaf. TETRAGO-

NAL ZrO2-Y2O3 PART II : THE MECHANICAL AND ELECTRICAL PROPERTIES. Journal de

Physique Colloques, 1986, 47 (C1), pp.C1-783-C1-788. �10.1051/jphyscol:19861119�. �jpa-00225515�

TETRAGONAL Zr0,-Y,O, PART I1 : THE MECHANICAL AND ELECTRICAL PROPERTIES

K. KEIZER, M. VAN HEMERT, A.J.A. WINNUBST, M.A.C.G. VAN DE GRAAF and A.J. BURGGRAAF

Twente University of Technology, Department of Chemical Engineering, Laboratory of Inorganic Chemistry, Materials Science and Catalysis, P.O. Box 217, NL-7500 AE Enschede, The Netherlands

Resume - Des c@ramiques de zircone yttriee tetragonale ave des tailles -ins comprises entre 0,l et 0,5 pm ont et6 preparees. La tenaci t@

et la resistance 2 la flexion 2 temperature ambiante sont independantes de la taille des grains et de la composition pour les phases tetrago- nales comportant plus de 5 at.% de Y01 5. L'influence du vieillissement sur les propri@t@s mecaniques est d'autant plus importante que la tail- le des grains est grande, mais pour des echantillons dont les grains font @ , l pm aucun vieillissement n'est observg. Les resistivite volu- miques et aux joints de grains sont de 1,5 8 2 fois superieures 1 cel-

les des materiaux cubiques. Une faible teneur en impuret6 diminue la resistivit6 aux joints de grains alors que la r@sistivit& en volume est profondement affectee.

Abstract - Dense tetragonal zirconia-yttria ceramics has been prepared with grain sizes between 0.1 and 0.5 urn. The fracture toughness and

flexure strength at room temperature are independent on grain size and composition in the tetragonal phase region with YO larger than 5 at.%. The ageing rate of the mechanical properties Increases with 1.5 increasing grain size, but for samples with grains of 0.1 um no ageing is observed. The bulk and grain boundary resistivities are 1.5-2 times higher than in cubic materials. A low impurity level decreases the grain boundary resistivity while the bulk resistivity is hardly affected.

I - INTRODUCTION

The ceramic zirconia-yttria system ((l-x)ZrO -xYO ₎ is well-known as a 2 . 1.2

oxygen-ion conductor/l/ for the stabilized, cublc p ase with x=16 at.%. During the last ten years this system is recognized as a tough material in the partially -stabilized and tetragonal phase region with 3cx<10 at.% YO1 5/2/. In recent papers/3,4,5/ special attention has been paid to the electriC31 resistivity of the tetragonal materials. Gupta et al. measured the resistivity at a constant frequency; Bonanos et a1./4/ and Kuwabara et a1./5/ measured the resistivity as a function of the frequency, which makes it possible to calculate the bulk

resistivity as well as the grain boundary resistivity. The value of the total

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19861119

JOURNAL DE PHYSIQUE

ionic resistivity is somewhat larger than in cubic stabilized materials, but this should be caused by a larger grain boundary resistivity/4,5/. Two factors effect this grain boundary resistivity:the grain size and the amount of impurities. The larger the grains or the smaller the content of impurities, the smaller the grain boundary resistivity. The problem is, that a material with grains larger than 0.3- 0.4 um thermally degradates/6/ with an optimum ageing rate in air at temperatures between 470K and 570K. Also high impurity levels give rise to fast ageing rates of especially the electric resistivity/7/.

If both the electrical and mechanical properties are concerne6 and the aging properties are taken into account, the required grain size should be 0.3 um or smaller and the impurity level as low as possible.

In a previous paper/8/ and in part I of this paper/9/ the preparation and characterization of tetragonal zirconia-yttria ceramics with such small grain sizes are shown. In this paper(part 11) the mechanical and electrical

conductivity properties are given with ageing effects on especiallythe mechanical properties. The results are discussed in terms of grain size, impurities and segregation effects.

I1 - EXPERIMENTAL

The zirconia powders were prepared by two different hydrous gel precipitation techniques, which are described in previous papers/lO,ll/. The characterization methods are described in part I of this paper/9/. Fracture toughness K

IC and strength a were measur d by means of a three-point bending test at a cross- head speed of 5 pm.sec-' f with samples of 1 x 3 ~ 1 5 mm. The notch width was 5 0 ~ 5 um and no precracking was applied.

A Solartron 1174 frequency response analyser was used for a.c. conductivity measurements on disc-shaped samples with sputtered platinum electrodes. These measurements were carried out between frequencies of 0.1 Hz and 1 MHz and in the temperature range from 525 to 975K.

I11 - RESULTS AND DISCUSSION 111.1 Mechanical properties

In Table I some structural and mechanical properties of samples as a function of sintering parameters and aging are shown. At densities higher than 97% the grain sizes vary between 0.1 and 0.4 pm for the tetragonal materials. At yttria concentrations larger than 10 at.% and temperatures of 1660K a cubic phase appears with larger grains than in the tetragonal material even at the same sintpring conditions. The fracture toughness of ZY 6.1 varies between 6 and 8 MPam2 and the flexure strenghth between 500 and 650 MPa. Both values are independent on the sintering and aging procedures. This can be expected because no structural change occurs/9/. At lower yttria concentrations the fracture toughness (and flexure strength) tends to increase to values higher than 10 MPam2/8,12/. However the ageing behaviour becomes more critical and for ZY 4 with grain sizes of 0.4 um the ceramic material degradates to powder after thermal cycle tests or aging at 575K/12/. Yttria concentrations smaller than 5 at.% and grain sizes in the order of 0.1 pm yields thermally stable materials(

Table I). Therefore with our preparation procedure thermally stable samples with 3-4 pt.% yttria can be made, which can have a fracture toughness larger than 10 MPam2 and a flexure strength in the order of 1000 MPa. The fracture

toughness(and flexure strength) in the cubic materials (

>l3 at.%) is smaller

than in the tetragonal materials, but decreases with increasing grain size

between 0.7 and 50 pm according to the square root of the grain size/l3/. Grains

smaller than 0.5 pm are very difficult to obtain in cubic materials because of

the relatively fast grain growth in cubic materials compared with tetragonal

materials. In tetragonal materials a grain size effect on the fracture toughness

is not observed for grain sizes between 0.1 and 0.4 pm.

ZY

Zr Y 0

in at.%.

The samplkxZf j ~ b a n d ZY 6.1 are 100% tetragonal( X-ray diffraction) ; The samples ZY 13. ZY 14 and ZY17 are 100% cubic.

* This sample is twophasic with smaller,tetragonal and larger,cubic grains.

Standard deviation in the last figure for toughness and strength is given in parentheses.

sample code

ZY 5.5

ZY 6.1

ZY 11

ZY 13 ZY 14

ZY 17

111.2 Electrical properties sintering

conditions temp. (time) 1525K(3 hrs) 1660K(3 hrs) 1380K(70hrs) 1380K(70hrs) 1380K(70hrs) 1380K(70hrs) 1425K(3 hrs) 1660K(3 hrs) 1660K(3 hrs) 1525K(3 hrs) 1660K(3 hrs) 1525K(3 hrs) 1973K(16hrs) 1626K(16hrs) 1493K(16hrs)

The bulk and grain boundary resistivities for several samples were determined separately with the frequency dispersion method. In Fig.1 an example of a impedance diagram is shown for the sample ZY 5.5 at 679K in air. The grain boundary resistivity RGB is equal to the diameter of the semi-circle; The bulk resistivity RB is equal to left-hand value of the semi-circle (about 900 Ohm). The specific bulk and grain boundary resistivities at 6733 and 773K are given in Table I1 for ZY 5.5 and some samples given in literature. The bulk resistivity of the tetragonal materials is within 30% the same for our materials and materials mentioned in literature . This bulk value is 1.5-2 times higher than for cubic materials. For comparison also values of zirconia-calcia and bismuthoxide-erbiumoxide solid solutions are shown. It is quite clear that resistivities of the'last system are lower than of ZY and for the zirconia-calcia system resistivities are much larger especially at temperatures lower than 873K.

The grain boundary resistivity is dependent on the grain size. According to Kuwabara/5/ and Verkerk/l4/ a simple calculation can be made to estimate the single grain boundary resistivity by taking into account the number of grain boundaries per unit length. This means simply multiplying of the resistivity with the grain size(see Table 11). Again this specific grain boundary resistivity is lower for cubic materials. The value for our tetragonal samples is somewhat lower

flexure strength

(MPa) 540(60)

- 640(40) 590(10)

- -

510(70)

-

- -

-

- -

- grain

size (pm)

0.3 0.4 0.1 0.1 0.1 0.1 0.2 0.4 0.4 0.25 0.5*

2.3*

1.1 50

2.6 0.7

relative density

(%) 98 98 99 99 99 99 98 99.5 99.5 97 97 95 97 95 98

aging temp. (time)

- -

-

525K(1000 hrs) 925K(l000 hrs) 455K(5 hrs)

in water

-

525K(1000 hrs) -

- - -

-

fracture toughness K~~(MP~~')

9.2(3)

-

7.5(5) 7.3(1) 6.1(5) 6.8(5) 7.6(3) 6.2(5) 6.6(5) 5.2(3)

3.3(4)

1.8(1)

2.8(1)

4.1(2)

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Fig. 1 An example of an impedance diagram( ZY 5.5 measured at 406'C(679K) in air) The semicircle is represented by the inserted equivalent circuit with R and RGB are the bulk and grain boundary resistivity

respectively and CGB fs the grain boundary capacity. The mentioned B numbers in the semi-circle are equal to the log (frequency).

10

-

IMPEDANCE D I R G R R M

L 0

-

Table 11: The bulk and grain boundary resistance( R and RGB) of our samples compared with literature values a ! a temperature of 673K and 773K.

L n

m .

W

X

a X W _I LL

r

0 6l U *

Z Y 5.5 4 0 6 C R I R

0 0 0 4

: + a m o e

CGB

..

^{" .,}

"4

P

1. Sample code as it is given in Table I.

*ZC is the zirconia-calcia system; BE is the b i s m u t h o x i d e - e r b i u m o x i d e system.

2. R (spec.)

R

d(grain size) according to /5/.

GB GB

0 . 5 1 1 . 5 2 2 . 5 3

REAL A X I S [ * l 0

sample code

ZY 5.5 ZY 17 /14/

ZY 17 /14/

ZY 6.2 /5/

ZY 5.5 ZY 17 /14/

ZY 17 /14/

ZY 6.2 /5/

ZY 6.2 /4/

ZY 6.2 /4/

ZC 15*/15/

BE 20*/16/

BE 35*/16/

grain size (pm)

0.3 0.4 2.5 0.4 0.3 0.4 2.5 0.4 0.8 0.5 10 30 30

R~

@m) 160 110 110 140 18 10 10 19 14 15 1400

0.4 6

R~~

@m) 280 110 17 590 2 2 10 1.2 50 14 - 24 - -

RGB(spec. ) 2

n m2(~10-5) 8.5 4 4 21 - -

0.7 0.4 0.3 2.3 1.1 1.2 -

- -

Temp.

(K)

673

673

673

673

773

773

773

773

773

773

773

773

773

literature/9,4,5/, but this cannot explain the difference with the cubic materials because bulk impurity levels are equal. Auger experiments show grain boundary enrichment with yttrium with factors 5 and 1.8 respectively for ZY 5.5 and ZY 17 after a heat treatment of 48 hours at 1030K/9/. This could explain the difference in grain boundary conductivity between cubic and tetragonal materials as well as the higher ageing stability in water. More experiments are necessary to verify this hypothesis.

For practical applications as a solid electrolyte the total resistance( bulk and grain boundary) is of importance. In cubic materials the grain boundary

resistance can be minimized by increasing the grain size .for instance, to 2.5 um(see Table 11) which decreases the grain boundary resistance by a factor 6. In thin- layer applications (20-30 pm) this grain size is acceptable. In tetragonal materials grains larger than 1 um cannot be allowed because of destabilization of the tetragonal phase. Even materials with grains larger than 0.5 pm are not very useful because of bad aging behaviour/9/. So grain boundary resistivity can only be lowered by a decrease of the amount of impurities in the material. Also low impurity levels give rise to a smaller increase of the resistivity with time so the ageing behaviour is better/7/.

IV - CONCLUSIONS

In the system zirconia-yttria dense, tetragonal ceramics with low impurity level were prepared with grain s&zes between 0.1 and 0.4 pm. The fracture toughness varied between 6 and 9 MPaZ and was hardly dependent on composition and grain size in this region. Very small grain sizes are however preferable under severe ageing conditions. Ageing of the samples of 0.1 pm grain size in water and at two different temperatures did not affect the structure and the mechanical properties.

The bulk and grain boundary resistivity of tetragonal zirconia-yttria samples are 1.5-2 times higher than in the stabilized cubic materials with 17 at.% YO1 5.

The grain boundary resistivity is affected by the grain size and by impurltres in the material. The grain size in tetragonal materials should not be larger than 0.3 pm because of destabilization and ageing behaviour. Therefore the impurity level should be as small as possible to obtain a small grain boundary

resistivity.

ACKNOWLEDGEMENT

We would like to thank Mr. H. van Benthem and Mrs. Y. Roman for their assistance with synthesis and mechanical experiments.

REFERENCES

/l/ Casselton,R.E., phys.stat.sol.(a) 2 (1970) 571

/2/ Gupta,T.K., Lange,F.F. and Bechtold, J.H., J.Mater.Sc. 13 (1978) 929

/3/ Gupta,T.K. ,Grekila,R.B. and Subbarao,E.C., J.Electrochem.Soc. 128 (1981) 929 /4/ Bonanos,N. ,Slotwinski ,R.K., Steele,B.C.H. and Butler ,E.P., J.Mater .Sc .Lett.

3 (1984) 245

/5/ Fu"abara,~. ,Murakami,T. ,Ashizuka,M. ,Kubota,Y. and Tsukidate ,T., J .Mater .Sc.

Lett. 4 (1985) 467

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JOURNAL DE PHYSIQUE

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