Influence of CeO2 content on the electrical properties of Y2O3TZP ceramics

SOLID STATE IONICS

Solid State Ionics 51 (1992) 147-156 North-Holland

Influence of CeO2 content on the electrical properties of YzO3-TZP ceramics M . T . H e r n a n d e z , J.R. Jurado and P. Duran l Instituto de Cerdtmica y Vidrio (CSIC), Departamento de Materiales Cer(tmicos Especiales, 29500 Arganda del Rey, (Madrid) Spain

Received 3 July 1988; accepted for publication 7 November 1989

Yttria-tetragonal zirconia (Y-TZP) ceramics with 1.7-3.0 mol% Y203, alloyed with variable amount of CeO 2 (2 to 12 mol% CeO2) have been prepared by hydroxide coprecipitation method, ac impedance complex plane analysis and ionic conductivity domain measurements were performed. Preliminary ageing experiments in water vapour were also carried out. The total conductivity does not suffer any change when CeO2 is incorporated and the grain boundary effect is partially avoided by the incorporation of CeO2 to the tetragonal structure. The ageing effect is also improved when that oxide is incorporated at concentrations higher than 4 mol%.

1. Introduction Considering the phase diagram relationships o f the ZrOz-CeO2 system in the zirconia rich region, there exists a single tetragonal phase zone which is stable at near r o o m t e m p e r a t u r e [ 1,2]. Taking into account that fact, Sato a n d Shimada, and U r a b e et al. [ 3 - 6 ] have p r e p a r e d yttria-tetragonal zirconia ceramics containing different a m o u n t s o f CeO2. Although fracture toughness decreases as CeOz contents increases [7], its excellent ageing b e h a v i o u r [ 8,9 ], even in water v a p o u r conditions, makes these materials p r o m i s i n g candidates for structural and solid oxide fuel cell ( S O F C ) applications. The maj o r i t y o f previous work has been focused on mechanical and ageing properties o f those ceramics, but we have reviewed the literature and no electrical conductivity d a t a are available at present [ 10]. Prel i m i n a r y result indicates that CeO2 i n c o r p o r a t i o n in y t t r i a - T Z P solid electrolytes does not reduce the total electrical conductivity o f the Y203-TZP [ 10 ], and it has a beneficial influence because o f grain b o u n d ary effect was diminished. In the presence work we prepare y t t r i a - T Z P ceramics alloying with CeO2 in o r d e r to study their Author to whom correspondence should be addressed. Elsevier Science Publishers B.V.

electrical properties, by using i m p e d a n c e spectroscopy analysis and the ionic conductivity d o m a i n as a function o f oxygen partial pressure.

2. Experimental Raw material with > 99% analytical grade o f zirc o n i u m tetrabutoxide, cerium oxide and yttrium oxide were used to prepare reactive powders. A p p r o priate a m o u n t s o f z i r c o n i u m n-butoxide were dissolved in isopropyl alcohol, and y t t r i u m oxide in nitric acid. To achieve the quantitative coprecipitation o f the hydroxides the p H was a r o u n d 9. After filtering and washing the coprecipitated p o w d e r was dried at 70°C. Then the required a m o u n t o f CeO2 was a d d e d to the coprecipitate and ball-milling for several hours in isopropyl alcohol was employed. Powders, after being calcined at 800°C for two hours were ground, ball-milled again and isopressed at 2000 k g / c m 2 to prepare disc-shaped specimens for electrical measurements. Chemical spectrographic analysis have been done to detect SIO2, A120 and other m i n o r c o m p o n e n t s present in the samples. The processing route followed in this work is shown

148

M.T. Hernandez et al. / The electrical properties of YeOj-TZP ceramics

in fig. 1. Table 1 recordes all the compositions studied in this work. A Hewlett-Packard impedance analyzer model HP 4192 A and a computer program of ac impedance complex plane analysis were employed, an ac voltage ranged from 0.1 to 1 V was applied. Throughout the experiments, a temperature controllable hot sample holder for dielectric measurements was used. Each ac impedance spectrum was obtained after a mini-

mum of 15 min of temperature stabilization and then frequency sweeping of capacitance and conductance were collected. Both platinum sputtering and high conductivity silver paste contact-electrodes were applied. The specimen shape were discs of 0.5 to 1 cm in diameter and 0.2 cm in thickness. Ionic domain measurements were carried out as it is described elsewhere [ 11 ]. To measure dc conductivity a long term experiment (50 h of thermal equilibrium) was

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M.T. Hernandez et al. / The electrical properties of Y203-TZP ceramics

Table 1 Compositions of the investigated samples. Sample

2Y/TZP 8Y/FSZ 1.7 Y TZP 10 Ce 2YTZP 10Ce 2.5 Y TZP 10 Ce 3 Y TZP l0 Ce 2 Y TZP 4 Ce 2 Y TZP 6 Ce 2 Y TZP 8 Ce 2 Y TZP 10 Ce 2YTZP 12Ce

Composition (mol%) Y203

ZrOx

CeO2

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98 92 88.3 88 87.5 87 94 92 90 88 86

10 10 10 10 4 6 8 10 12

performed and a very small dc electric field (70 m V ) was applied.

3. Results The chemical spectrographic analysis results gave a content o f 0.08% SiO2 a n d 0.2% AIeO3. The X-ray diffraction o f the c o p r e c i p i t a t e d powder after milling and drying showed an a m o r p h o u s structure. After calcining powders exhibited a t e -

149

tragonal s y m m e t r y single phase, and a small a m o u n t o f CeO2 which disappears as firing t e m p e r a t u r e increases was also observed. The sintered temperatures were ranged between 1200-1600 ° C for various hours, and the m a x i m u m relative density reached was >~95% o f the theoretical density. W h e n sintering t e m p e r a t u r e increases above 1350 °C the presence o f small a m o u n t o f cubic phase was noted. Fig. 2 shows the representative microstructure o f the sintered samples at 1300-1400°C, and a uniform and homogeneous microstructure with an average grain size o f 0.5 ~tm (see fig. 2a) was obtained. The cubic phase a m o u n t is small and it appears like a big grains inside the tetragonal matrix (see fig. 2b). At higher sintering t e m p e r a t u r e s ( 1600 ° C ) , the concentration o f cubic phase grows up sharply and the average grain size o f tetragonal phase increases up to values o f a r o u n d 1 ~tm. Fig. 3 shows a typical i m p e d a n c e plot at 310 ° C for sample 2 Y - T Z P 10 Ce. The m a j o r feature o f this figure is the one concerning the small arc o f the grain b o u n d a r y resistivity contribution, c o m p a r i n g with much bigger o f the Y-TZP-free CeO2 [ 10 ]. The lattice conductivity is lower than that o f Y-TZP, however grain b o u n d a r y conductivity is higher and, therefore, the total conductivity is quite similar. Fig. 4a shows a representative i m p e d a n c e arc evolution as a function o f t e m p e r a t u r e for both 2 Y T Z P

Fig. 2. Microstructure of the sintered bodies: (a) tetragonal grains, (b) some large cubic grains.

M. ~ Hernandez et al. / The electrical properties o f Y203- TZP ceramics

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M. 72 Hernandez et al. / The electrical properties of Y203- TZP ceramics

specimens and those containing a CeO2 concentration lower than 8 mol%. The samples had been sintered in all cases at 1350°C. It is noted that the grain boundary resistance is very large. As temperature is raised, both semicircles (grain interior and grain boundary) overlap, grain boundary being one of the predominant factors in the conductivity behaviour. The effect of high CeO2 concentrations ( > 8 mol%) is illustrated in fig. 4B. It is observed a noticeable reduction of the grain boundary arc; its electrical resistance is almost the same as that of grain interior. At the higher experimental temperatures ( > 300 °C) both phases contribute in same way and magnitude to the total conductivity. In fig. 4C are depicted the impedance arc evolution on samples containing 8 mol% CeO2 and sintered at 1600°C. By comparison with the experimental results shown in figs. 4A ad 4B, a strong reduction of the grain boundary contribution was noticed, and it practically disappeared above 500°C. In that case, the conductivity is mainly governed by the grain interior (lattice conductivity) contribution. Another important fact to consider is the one concerning the presence in all samples, of a small electrode-electrolyte interphase arc (IF) which emerges at experimental temperatures higher than 600°C. That arc has a depression angle of about 45 °. From the arc interception on the p'-axis, grain interior, grain boundary, electrode-electrolyte interphase, and total conductivities of the 2Y-TZP-8Ce sample were evaluated. Fig. 5 shows the conductivity of the all contributions as a function of reciprocal temperature, dc conductivity curve is also shown for comparison. From that figure it can be stated that both the grain boundary and grain interior contributions are very similar. As it was expected the total conductivity is lower than grain interior and boundary conductivities, and dc conductivity is much lower than all those contributions. In order to compare the electrical behaviour of various materials with different structures, we have prepared 8Y-FSZ, monoclinic and tetragonal 2YTZP ceramics. Bulk ceramic specimens with monoclinic structure was obtained from a 2 mol% Y203TZP sample by annealing at 250°C for 8 h in water vapour. A representative ac impedance plot of that monoclinic material is shown in fig. 6 at 450°C, we have detected a unique impedance semicircle in all

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the temperature range tested, and that semicircle has a large depression angle (26 ° ). The monoclinic phase was identified by X-ray diffraction analysis and the entire sample transformed was also checked by X-ray diffraction. In fig. 6 is also shown the diffraction pattern obtained, it seems that a slight presence of tetragonal phase is noticed. The electrical conductivity behaviour of all mentioned ceramics is depicted in fig. 7. As it is noted, tetragonal zirconia alloying with 8 mol% of CeO2 exhibits an intermediate conductivity response between 8Y-FSZ and TZP ceramics. Fig. 8A shows the grain interior and grain boundary conductivity measurements at 335°C as a function of CeO2 concentration on samples sintered at 1350°C. Both grain interior and grain boundary conductivity of a 2 mol% Y203-TZP decreases as CeO2 increases up to 10 mol% CeO2, and then increases again for the 12 mol% CeO2 specimen. It was noted that when CeO2 concentration augments the conductivity, the differences between lattice and boundaries were reduced. Fig. 8B shows that the activation energy for conduction does not significantly

152

M. 72 Hernandez et al. / The electrical properties o f Y2Os-TZP ceramics

vary as CeO2 concentration raises, it was however noted a tendency to decrease above 12 mol% CeO2. In order to know the potential applications of these materials as solid electrolyte (particularly in SOFC systems) an important test is to determine the ionic conductivity domain. Solid electrolytes based on cerium oxides undergo a strong oxidation-reduction effects when they are subjected to a reducing environment atmosphere, due to the easy change of the oxidation state of Ce 4+ to Ce 3+ specially at high temperatures. We have chosen 10mol% CEO2-3 mol% Y203-TZP sample and fig. 9 shows log dc conductivity against log oxygen partial pressure at various constant temperatures. The isothermic experiment indicates that the ionic domain is very spacious ( 0 - 1 0 - 2 7 a t m ) at 405°C for 48 h, and it changes at 875°C ( 0 - 1 0 - ' T a t m ) for 48 h. A 3 mol% Er203-TZP sample is also illustrated for comparison. As it is well-known, TZP ceramic suffers a strong transformation tetragonal-monoclinic specially when it is subjected to an annealing in water vapour at a temperature of 150-300°C. It is an important limitation in order to use that material as structural and as solid electrolyte applications. However, it has been demonstrated [3] that in Y203-TZP alloying with CeO2 the tetragonal-monoclinic degradation can be

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M.T. Hernandez et al. I The electrical properties of Y20 ~-TZP ceramics

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stopped. To confirm that fact in our samples we have performed an annealing experiment in water vapour at 170°C for, at least, 1000 h. From fig. I0 one can

deduce that in our experimental conditions, when CeO2 additions is higher than 4 mol% the tetragonal phase is retained. On the contrary at lower CeO2

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154

M. T. Hernandez et al. / The electrical properties o f Y2Os- TZP ceramics

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The presence of cubic crystalline phase in the TZPCeO2 matrix, particularly at high sintering temperature, could be explained according to the tentative ternary phase diagram ZrO2-Y203-CeO2 in the ZrO2-rich region shown in fig. 11. The increasing of tetragonal average grain size and the presence of cubic grains leads to a decrease in densification. Urabe et al. and Sato and Shimada [5-12] have suggested to use hipping and other more sophisticated methods as the only ways to improve densification in these materials. Various interesting results were found in the electrical conductivity behaviour o f Y 2 0 3 - T Z P - C e O 2 ceramics: (i) The grain boundary effect is appreciably reduced particularly at CeO2 concentration higher than 8mo1%; (ii) the lattice conductivity decreases as CeO2 concentration increases and the total conductivity of TZP does not significantly vary as CeO2 is incorporated; (iii) the ionic conductivity domain at 875°C is quite large.

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M.T. Hernandez et al. / The electrical properties of Y2Os-TZP ceramics

It was reported elsewhere [10] that the grain boundary effect in 2Y-TZP seems to be governed by an impurity mechanism. In a similar way Badwal and Drennan [ 13 ] also reported a decrease in the grain boundary resistance for a 10 mol% Y z O 3 - F S Z cer a m i c s . They have suggested that if grain boundary density decreases boundary impurity concentration would increase and therefore grain boundary resistance would also increase. That occurs at sintering temperature up to 1500°C, and above that temperature grain boundaries dewet and clean of impurities, and the grain boundary resistance decreases. In our case for YzO3-TZP-CeO2 materials at sintering temperatures lower than 1500 °C grain boundary resistance decreases when the amount of CeO2 is superior to 6 mol%, and it could be due to the growth of the average grain size, and when sintering temperature was raised up to 1600 °C the grain boundary resistance tends to vanish. These experimental results could be interpreted as follows: As the sintering temperatures and CeO2 concentration are raised, both the cubic phase amount and the tetragonal grain size increased and as a consequence a constrained matrix was obtained. In such a situation the impurities, which in the beginning were located at the grain boundaries, are rejected towards the microstructural triple points and, in that way, the grain boundaries will be cleaned and, therefore, the grain boundary effect will be diminished. At this time, it is very difficult, to consider some other roles of the C e O 2 incorporation. One of them might be that C e O 2 could segregate into the boundaries to form a second phase or glassy phase with higher conductivity. Another could be that CeO2 acts like a dragging phase and, if this is so, then it could have the effect of removing silica impurity into the boundaries. High resolution transmission electron microscopy observations are now in progress to elucidate the CeO2 role into the boundaries, as well as to observe by EDAX the influence of other minor components like SiO2, A l 2 0 , etc. The lattice conductivity decreases as CeO2 is introduced into the tetragonal phase. A similar effect was also observed by Cales and Baumard [14] in Y z O 3 - F S Z - C e O 2 ceramics. Since the introduction of ceria does not involve the formation of extra oxygen vacancies in the material, that ionic conductiv-

15 5

ity decreasing could be related to a mismatching of the average cationic radii of Zr 4+, y3+, Ce4+ involved. On the other hand, almost the same activation energy for conduction was found in all the samples investigated, and it could be concluded that the incorporation of CeO2 to the tetragonal structure does not negatively change either the high electrical behaviour of the TZP or the conduction mechanism expected. For application purposes, the ionic conductivity domain detected for samples like 2 Y203-TZP-10 CeO2 is very large (below 10-~8 oxygen partial pressure) and it could be considered as a hopeful result for SOFC devices. Sato et al. [8] have reported that the phase transformation in water vapour ageing experiments is controlled by the chemical reaction between Z r - O Zr bonds on the surface and water solvent. The water adsorbed on the surface reacted with Z r - O - Z r bonds to form ZrOH groups. In our case, as CeO2 is incorporated, it is possible that Z r - O - C e bonds could be formed and that should avoid its breaking out. On the other hand in the case of the ternary Y203-YZPCeO2 the bonds energy is probably higher than that of the binary Y203-TZP and, in such a way, the active points in the sample surface are strongly diminished. Consequently the tetragonal-monoclinic phase transformation will be stopped.

Acknowledgement The authors are indebted to J. Jim6nez, F. A1mendros and M. Solana for their useful assistance throughout this work. We also are very grateful to EEC JOULE PROGRAM (Project JOUE-0044-C for financing this work.

References [ 1 ] V. Longo and S. Roitti, Ceramurgia Int. 1 ( 1971 ) 4. [2] F.F. Lange, J. Mater. Sci. 17 (1982) 255. [ 3 ] T. Sato and M. Shimada, J. Mater. Sci. 20 ( 1985 ) 3988. [4] T. Sato, S. Ontaki, T. Endo and M. Shimada, J. Mater Sci. 5 (1986) 1140. [ 5 ] K. Urabe, A. Nakajima, H. Ikawa and S. Udagawa, Advances in Cermics, Vol. 24A (Am. Ceram. Soc., Columbus, OH, 1991 ) pp. 345-355.

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M.T. Hernandez et al. / The electrical properties of Y203-TZP ceramics

[6] K. Urabe, T. Noma, A. Saeki, M. Yoshimura and S. Somiya, Zirconia Ceramics 8, eds. S. Somiya and M. Yoshimura (Uchida Rokakuho, Tokyo, 1986) pp. 45-62. [ 7 ] K. Tsukuma and M. Shimada, J. Mater Sci. 20 ( 1985 ) 1178. [ 8 ] T. Sato, S. Ontaki, T. Endo and M. Shimada, J. Am. Ceram. Soc. 68 (1985) C-320. [9] T.W. Coyle, W.S. Coblenz and B.A. Bender, Am. Ceram. Soc. Bull. 62 (1983) 966. [ 10] P. Dur~in, P. Recio, J.R. Jurado, C. Pascual, M.T. Hern~indez and C. Moure, J. Mater Sci. 24 (1989) 717.

[11] J.R. Jurado, C. Moure, P. Dur~in and N. Valverde, Solid State Ionics 28-30 (1988) 518. [ 12 ] T. Sato, T. Endo and M. Shimada, in: Zirconia '88, Advances in Zirconia Science and Technology, eds. S. Meriani and C. Palmonari (1999) pp. 293-300. [ 13 ] S.P.S. Badwal and J. Drennan, J. Mater Sci. 22 ( 1987 ) 3231. [ 14 ] B. Cales and J.F. Baumard, Rev. Int. Hautes Temp. Refract. Ft. 17 (1980) 137.