ELECTROCHEMICAL BEHAVIOUR OF STABILIZED ZIRCONIA-PLATINUM CERMETS USING THICK FILM TECHNOLOGY

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ELECTROCHEMICAL BEHAVIOUR OF

STABILIZED ZIRCONIA-PLATINUM CERMETS USING THICK FILM TECHNOLOGY

M. Gogé, K. Heggestad, D. Especel, M. Gouet

To cite this version:

M. Gogé, K. Heggestad, D. Especel, M. Gouet. ELECTROCHEMICAL BEHAVIOUR OF STA-

BILIZED ZIRCONIA-PLATINUM CERMETS USING THICK FILM TECHNOLOGY. Journal de

Physique Colloques, 1986, 47 (C1), pp.C1-801-C1-805. �10.1051/jphyscol:19861122�. �jpa-00225519�

JOURNAL DE PHYSIQUE

Colloque CI, supplement au n°2, Tome 47, fevrier 1986 page ci-801

ELECTROCHEMICAL BEHAVIOUR OF STABILIZED ZIRCONIA-PLATINUM CERMETS USING THICK FILM TECHNOLOGY

M. GOGE, K. HEGGESTAD, D. ESPECEL and M. GOUET

Laboratoire de Thermodynamique et d'Electrochimle des Oxydes, Université Paris XII, Avenue C. de Gaulle, F-94010 Crétell, France

Résumé - Des couches de cermet zircone-platine ont été sérigraphiées sur un substrat d'alumine, avec des électrodes de platine. Leur comportement élect- rique est principalement déterminé par des phénomènes d'interfaces.

Abstract - Thick film technology was used to prepare zirconia-platinum cermets on an alumina substrate, with platinum electrodes. The electrical conductivity of these cermets is mainly determined by the interfacial process.

I - INTRODUCTION

Cermets which are mixtures of a conducting phase and an insulating phase are studied extensively for their electrical properties which are very valuable to the microelectronics industry. Systems which are considered are metal/insulator and metal/poor insulator combinations. Figure (l) shows the beheaviour of composite materials of this kind, after Guyon / l / . Note the importance of the distribution and of the shape of particles.

FA.g. 1 : Conductance o{, a mixtuJie o^

•imuZatot and conductoi matexiati [a) and oi -two dli&eAent conductor \b,c,d) veMui the. compoiiAion a^teA. /!/.

fia. 1 : Conductance, oi a mixXwie. o<

platinum and ytfUa itabitized zixconia veA&ui the compoiitlon [673 K, I 1/ VC).

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19861122

J O U R N A L DE PHYSIQUE

With t h e aim t o enlarge the contact area of platinum electrodes with stabilized zirconia electrolytes we studied compositions in the YSZ-Pt system towards the percolation point (about 80 % wt platinum). Below this point we found t h e addition of high conductive metal grains in a resistive matrix of zirconia increases dc resis- tivity of low-conductivity composite material as shown on figure (2).

Further more the dc conductivity is dependent on the oxygen partial pressure whereas neither platinum nor YSZ conductivity a r e sensitive to. This gave us evidence for a mechanism different from classical models, and which implies a control of t h e current by t h e rate of electrochemical reactions occuring a t t h e interface of grains with conductivities of different nature (electronic and ionic) as shown on figure (3).

gaseous or

r* O2

Oxide Anode Metal

Cathode Oxide

Faig.

^{Mechaninm 04} .the conthot 06 w e n t by ~ d w i o n and inten6aciae Eeactiom , and D

. e q d v a t e n t

I1 - EXPERIMENTAL

Fig. 4

G ~ o ~ & M 06 nampten

a - Aeumina 4 u b 4 M e

- P e m e e e m o d a c - ^{c m e t}

The cermets are prepared by mixing platinum paste (Comatel) with a zirconia paste (YSZ 8% Zircar + organic vehicle) so that t o obtain a screen-printable ink. The metallic electrodes a r e screen-printed from platinum paste.

We use two configurations of samples w h i c h a r e shown with their geometrical characteristics in fig. (4). The electrodes and t h e cermet a r e deposited on an alumina substrate, then sintered a t 1723 K for 5 hours. The cermet film is about 30pm thick, containing platinum particles (50% wt, average 5 p m ) in a porous (30%'vol.) YSZ matrix.

The DC currents were measured using a programmable digital multimeter ( ~ e i t h l e y

mod. 195) on samples (A), and the complex impedance measurements with a frequency

response analyzer ( ~ o l a r t r o n mod. 1174) on samples (B). The oxygen partial pressure

was controlled using oxygen-nitrogen mixtures of known composition and monitored a t

the inlet and outlet of the furnace with a ZrO gauge.

111 - RESULTS AND DISCUSSION DC measurements.

On freshly prepared cells Pt / YSZ-Pt / Pt the results are not reproducible, and t h e conductance is very low. After some cycles the current-voltage characteristics a r e stable, no more hysteresis is observed and t h e conductance of the cell is much higher. This technique was proposed by Giir, Raistrick and Huggins /2/.

Figure (5) shows the oxygen pressure dependance of t h e current through cermet cells, when a constant voltage of 1 volt is applied. Though the oxygen pressure has influence neither on t h e electronic conductivity of platinum, nor on the ionic conductivity of yttria stabilized zirconia, the resistance increases when t h e oxygen pressure decreases. The current follows the relation:

where k is a constant term and n a parameter which increases from 0.2 t o 0.5 when t h e temperature changes from 523 t o 773 K. The value of n suggests that t h e charge transfer is the limiting step a t 773 K (n=0.5), while a t lower temperatures several steps can take part t o the control of the current. Figure (6) shows that the electrical DC resistance is thermally activated. For comparison t h e plot for a pure bulk YSZ is also shown, after data from Badwal, Bannister and Garret /3/.

Gas diffusion is not the determining step: With t h e Knudsen diffusion hypothesis t h e

gas diffusion coefficient is independent t h e pressure and proportional t o T ~ / ~ (Dletz 141). This leads t o resistances increasing with T ~ / In t h e bulk diffusion ~

hypothesis, the diffu ion coefficient is proportionnal t o ~~7~ , so the resistance should vary with T -lY2, which disagree with experimental plors.

,

pure

zlrconia

Fa. ⁵

F i g .

DC

06

U O ~ A I I A nvlloon m o ~ ~ t r n o I I UnPtl v e t ( s u ~ * e & h o d

tem~ehatuhe

JOURNAL DE PHYSIQUE

A.C. measurements

The impedances a r e measured with a 100 mV sine wave between 10 mHz and I M H z . Figure (7) shows the plots of Bode's diagrams for different temperatures. At each temperature, three ranges can be distinguished:

Fd.

06

fleAAune - ^{l a & .}

In t h e high frequency range, the impedance falls t o a minimum value which is assumed t o b e the bulk resistance of t h e electrolyte. It varies with the temperature and not with the oxygen pressure. The further increase a t around 0.5 or 1 MHz is caused by geometrical capacity.

In t h e intermediate frequency range, for all studied temperatures the admittance increases linearly with the frequency. This is interpreted as an effect of t h e oxide grain boundary capacity. Its value is 50 nF a t every temperature. The value generally calculated for pure zirconia is about 10 nF /5/,/6/.

In the low frequency range we examined t h e imaginary part of the admittance as a function of the square root of the frequency and noted that t h e relation is not linear in any case. This means that there is no Warburg impedance; thus t h e limiting step is not a diffusion mechanism.

IV - CONCLUSION

The conductivity of non-percolated YSZ-Pt cermets shows a strong dependence on t h e

oxygen pressure. This is due to the variation in impedance a t the metaI/oxide

interfaces. Thus these cermets shows a "pseudoionic

conductivity and provides a

lens effect on the interfacial phenomena, Neither gaseous diffusion nor electroactive

species diffusion appears to be t h e limiting step for t h e overall transfer.

V - REFERENCES

/ I / Guyon E, Pour l a Science 63 (1982) 14

/2/ Giir, TM., Raistrick, I.D., and Huggins, R.A. J.Electrochem.Soc. 127 (1980) 2620 /3/ dadwal, S.P.S., Bannister, M.J., and Garret, W.G. Adv. in Ceram. 12 (1984) 393 141 Dietz, H. Solid S t a t e Ionics 6 (1981) 175

/5/ Badwal, S.P.S. J. Materials science 19 (1984) 1767

161 Verkerk,M.J., Middelhuis,B.J., and Surgraaf,A.J. Solid S t a t e Ionics 6 (1982) 159

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