Sintering of zirconia–yttria ceramics studied by impedance spectroscopy
Introduction
Zirconia-based ceramics are widely used as electrochemical transducers in oxygen sensors [1]. Some of the requirements for using these ceramics in sensor devices are high density and reproducibility of the electrical properties. Hence the sintering process has to be well controlled in manufacturing the ceramic bodies. Sintering can be described briefly as the phenomenon that occurs during heating up green compacts leading to pore elimination associated to grain growth and providing well defined grain boundaries [2]. There is basically the conversion of a large number of small particles into a lower number of larger particles, that is, grain growth and solid–solid interfaces substitution for higher energy gas–solid interfaces. In most cases this is accompanied by densification. The geometric variation of the ceramic dimensions associated to sintering follows three stages: the initial stage with particles forming bonds at the particle contacts; the intermediate stage with the smoothing of the pore structure; and the final stage corresponding to closed spherical pores that shrink slowly by vacancy diffusion to grain boundaries [3], [4]. Several processes are known to occur during sintering, the solid state sintering being the most important in zirconia ceramics manufactured for mechanical, electrical and optical applications. In this case, the main sintering process involves no liquid phase formation, i.e., all constituents of the powder compacts stay in the solid state [5].
Almost 30 years ago the complex admittance technique was applied for the first time to a solid electrolyte, (ZrO2)0.9(Y2O3)0.1 [6].
The electrical conductivity of polycrystalline ceramics has two main components: bulk conductivity and internal surfaces conductivity. The former depends on the grain or intragranular mobility of charge carriers, while the latter on the blocking of charge carriers, mainly at grain boundaries. Grain boundary conductivity may be decreased by the blocking of charge carriers by other microstructure defects like pores or insulating second phase inclusions, these defects being called ‘blockers’ [7]. Mobile charge carriers can either permeate freely through grain boundaries or be blocked at certain grain boundary locations. Two blocking parameters have been proposed [8], [9]: the resistance blocking factor αR and the frequency ratio αf. The resistance blocking factor, defined as the amount of charge carriers that are blocked at grain boundaries, is determined simply by dividing the grain boundary resistivity by the total resistivity, and these values are taken from the impedance diagrams. The resistance blocking factor αR is a characteristic parameter related to the contact between grains and can be associated to grain boundary density. The grain boundary density is already known to be related to the specimen average grain size [9], [10]. Here will be taken into account that αR is proportional to the equivalent blocker area normal to the electric field. Or simply, in other words, αR is proportional to the average surface between grains, i.e., the average pore surface. The other blocking parameter, the frequency factor, is defined as the ratio of the characteristic frequency of the grain boundary resistivity to that of the grain resistivity. The characteristic frequencies are the apex frequencies in the semicircles in the impedance diagrams. Kleitz et al. have shown that αf is proportional to the average blocker thickness or to the average intergranular distance, i.e, to the average pore thickness. The product of both resistance blocking factor and frequency factor (αR.αf) is then proportional to the volume between grains, i.e. to the pore volume.
The main idea behind this investigation is the modifications that might occur in the impedance spectroscopy of ceramics during sintering. In zirconia–yttria solid electrolytes oxygen ion vacancies could be used as local probes in the microstructural evolution of the polycrystalline specimens. Starting with a green pellet full of pores and internal surfaces and ending in a fully dense specimen, electrical measurements using impedance spectroscopy should give enough details for sintering studies. Results on impedance spectroscopy of zirconia ceramics as a function of sintering parameters have been reported [2], [9], [10], [11], [12], [13], [14]. Conductivity measurements with current of variable frequency taken in zirconia–yttria solid electrolytes with different average grain sizes showed that the bulk (grain) conductivity does not depend on grain size while the grain boundary conductivity does [12], [14]. Specimens with different grain sizes were obtained by firing hot-pressed samples at different temperatures up to 2000°C. The effect of microstructure on yttria–zirconia conductivity, using the impedance spectroscopy technique has been reported, showing that the higher is the average grain size, the lower is the grain boundary resistivity, the bulk resistivity remaining constant [15], [16]. Recently, the densification process of barium titanate has been studied by impedance spectroscopy [17]. A correlation has been found between the activation energies of grain and grain boundary conductivities and the density after sintering.
Usually sintering is monitored by density or shrinkage measurements. In this paper, impedance spectroscopy is used to determine parameters for studying sintering in zirconia ceramics.
Section snippets
Experimental
ZrO2:8% mol Y2O3 powders were prepared by the coprecipitation technique using >99% pure Y2O3 and ZrO2 produced at the Zirconium Pilot Plant at this Institute. The powders were fired at 900°C for 2 hours in air. Phase analysis were evaluated by X-ray diffraction with a Bruker AXS D8 Advance and a Philips PW3710 diffractometers. Powder suspensions were prepared for observation in a JEOL JEM200C transmission electron microscope for determination of average particle size and particle agglomeration.
Results and discussion
In Fig. 1 the dilatometric curve of a ZrO2:8% mol Y2O3 green pellet is shown. The well known three stages for these solid electrolytes are identified: from room temperature to approximately 900°C, where re-adjustment of particles without shrinkage occurs, from 900°C to 1350°C with large linear retraction and high shrinkage rate, and from 1350°C upwards with grain growth and pore elimination. The maximum retraction temperature of 1350°C, with about 23% linear retraction, has been chosen for
Conclusion
Grain growth in zirconia–yttria solid electrolytes can be studied by impedance spectroscopy by following the dependence of grain and grain boundary resistivities upon sintering time in isothermal conditions. The increase in the average grain size can be accurately determined in the impedance diagrams allowing for the study of its kinetics. Impedance spectroscopy measurements during sintering on a series of well studied ceramics are under way to evaluate the contribution of the technique to the
Acknowledgements
To CNEN, FAPESP and PRONEX for financial support and to CNPq for the scholarships.
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