Ionic conductivity studies of ultrafine-grained yttria stabilized zirconia polymorphs

Solid State Ionics 123 (1999) 271–278

Ionic conductivity studies of ultrafine-grained yttria stabilized zirconia polymorphs a b a, R. Ramamoorthy , D. Sundararaman , S. Ramasamy * a

b

Department of Nuclear Physics, University of Madras, Guindy Campus, Madras ( Chennai) 600 025, India Physical Metallurgy Section, Metallurgy Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India Received 23 February 1999; accepted 22 March 1999

Abstract Ultrafine-grained zirconia ceramics doped with yttria in different concentrations from 2 to 12 mol% were prepared by chemical precipitation method with an average grain size of 10 nm. The ac ionic conductivity studies revealed that 3 mol% yttria stabilized zirconia (3YSZ) in tetragonal phase shows maximum conductivity both in grains and grain boundaries for a range of temperatures. In the coarser grained zirconia systems, with different yttria concentration, the conductivity maximum is usually observed in the cubic phase field with 8 mol% Y 2 O 3 . The shift of the maximum conductivity with the yttria content towards the lower concentration (3 mol%) is attributed to the grain size effect. The Arrhenius plot of the total dc conductivity is found to follow that of the grain boundaries rather than the grains, showing that the total conductivity is mainly influenced by the grain boundaries in the ultrafine-grained materials.  1999 Elsevier Science B.V. All rights reserved. Keywords: Yttria stabilized zirconia; Ionic conductivity; Grain boundaries

1. Introduction Zirconia based solid electrolyte systems doped with many rare-earth and alkaline metal oxides are well known candidates for applications such as oxygen sensors, fuel cells and catalytic membrane reactors etc. due to its high ionic conductivity at elevated temperatures ( . 1000 K). Even though a tremendous amount of research work has been carried out on these systems for many years, it is always interesting to study the ionic conductivity behavior of the zircoma based solid electrolytes. This is because, the conductivity of the phase stabilized *Corresponding author.

zirconia is very often found to depend on various factors such as composition, grain size, microstructure, crystal phase, porosity, and purity of the sample [1,2]. Among many dopant additives such as Y 2 O 3 , CaO, MgO, Gd 2 O 3 , Sc 2 O 3 etc., yttria is an effective stabilizer, which stabilizes the tetragonal and cubic phases of zirconia in the regimes of 2–4 mol% and 6–10 mol% of Y 2 O 3 respectively. It is a well known fact that the high electrical conductivity in the stabilized zirconia at elevated temperatures is due to the mobile oxygen ion vacancies created in the oxygen sub-lattice, when the lower valent dopant cation (e.g. Y 31 ) is substituted for Zr. And the ionic conductivity is found to increase with yttria con-

0167-2738 / 99 / $ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 99 )00103-4

272

R. Ramamoorthy et al. / Solid State Ionics 123 (1999) 271 – 278

centration up to 8 mol% and for higher concentrations it decreases [3,4]. However, the present study illustrates that the ionic conductivity in the grains and as well as in grain boundaries of fine grained 3 mol% Y 2 O 3 doped tetragonal zirconia is higher than that of the fully stabilized cubic zirconia. In recent years, there has been an increasing interest in ultra-fine grained ceramics studying the physical properties with respect to different microstructures [5,6] mainly due to the decisive role played by the increased number of grain boundaries in the total conductivity of the stabilized zirconia. With an ultrafine grain size, say , 1 mm, the variation of the grain and the grain boundary conductivities in a range of temperatures and the variation of activation energies with different yttria concentrations studied by the ac impedance analysis are reported in this paper.

impedance / gain phase analyzer in the frequency range 1 Hz to 1 MHz, for the temperature range 573 to 1173 K. Platinum paint was used as electrodes. The paint was applied on both the surfaces of the pellet and fired at 1000 K for 30 min. The grain and the grain boundary dc resistivities were separately determined by the complex non-linear least square (CNLS) fitting of the semi-circular arcs in the complex impedance spectra [9]. Powder XRD patterns of the sintered pellets were obtained from the high resolution Siefert X-ray diffractometer with CuK a1 radiation and quartz monochromator, for the phase confirmation analysis. Morphological observations were made by using a scanning electron microscope (SEM). For the SEM observation, the pellets were chemically etched for 5 min in concentrated HF.

3. Results and discussion 2. Experimental procedure

3.1. Crystal phases and microstructure The nanocrystalline powders of yttria doped zirconia were prepared by the chemical precipitation method. A mixture of aqueous solutions of zirconium oxy chloride (ZrOCl 2 ? 8H 2 O) and yttrium nitrate (Y(NO 3 ) 3 ? 6H 2 O) were taken in a flat bottom flask fitted with a reflux condenser. The solution was hydrolyzed for a period of 140 h [7]. The resultant acidic sol was neutralized and coagulated with ammonia solution. A white precipitate was formed and the particles were washed, dried and then ground for 5 min in an agate mortar. The powders were obtained with different mole percentages of yttria (viz. 2, 3, 4.5, 6, 9, and 12 mol%). The obtained powders were annealed at 1073 K for 1 h. The initial average grain size of the annealed powders is found to be around 10 nm as determined from the line width of the X-ray diffraction (XRD) peaks using the Scherrer formula [8]. The crystalline YSZ powders were compacted into disc shaped pellets with 8 mm diameter and 1.5–2 mm thickness by applying the uniaxial pressure of 780 MPa. The pellets were sintered at 1673 K for 15 min, at the heating rate of 400 K / h. The densities of all the pellets are found to be nearly 93% of the theoretical density. The ac ionic conductivity measurements were done on the sintered pellets by the Solartron SI 1260

The X-ray diffraction analysis on the sintered YSZ samples with different Y 2 O 3 concentrations was made for the crystal phase identification. Fig. 1 shows the powder XRD spectra of 2, 3, 4.5, and 6 mol% Y 2 O 3 doped ZrO 2 sintered at 1673 K in air for 15 min. The 2YSZ exhibits predominant tetragonal phase with a small percentage of monoclinic. The percentage of monoclinic phase was calculated ¯ from the X-ray intensities of 111(t), 111(m) and 111(m) peaks [10] and is found to be about 12%. The monoclinic phase of ZrO 2 is usually considered unfavorable for the ionic conduction. 3YSZ shows a purely tetragonal phase and for further higher concentrations of yttria, viz. 4.5, 6 mol% etc., only cubic phase is observed. However, it is believed that 4.5 mol% Y 2 O 3 –ZrO 2 may be in a phase mixture of cubic and tetragonal. The incorporation of increasing yttria content into the lattice is evidently observed from the increase in the unit cell volume with yttria concentration. Fig. 2 shows the variation of the unit cell volume (both tetragonal and cubic) with yttria concentration. The cell volume increases monotonically up to 9 mol% Y 2 O 3 and then decreases in the case of 12 mol%. The reason for the decrease of cell volume in the case of 12 mol% Y 2 0 3 is not clear.

R. Ramamoorthy et al. / Solid State Ionics 123 (1999) 271 – 278

Fig. 1. Powder XRD spectra of YSZ with different yttria concentration sintered at 1673 K for 15 min.

Fig. 2. Variation of unit cell volume with yttria concentration.

273

274

R. Ramamoorthy et al. / Solid State Ionics 123 (1999) 271 – 278

The largest volume of the cubic unit cell with 9 mol% yttria may be one of the reasons to exhibit highest conductivity near 8 mol% of Y 2 O 3 in the coarser grained yttria–zirconia system as reported in the literature [11,12]. The morphological features of the sintered pellets were observed in SEM and found to be similar for all concentrations of yttria. Fig. 3 shows a typical SEM micrograph of 4.5YSZ sintered pellet. A uniform grain size distribution with the size ranging from 0.5 to 1 mm is observed.

3.2. Conductivity Fig. 4 shows the complex impedance (CI) spectra obtained at 573 K for YSZ samples with different yttria concentrations 2, 3, 4.5, 6, and 12 mol%. Two semi-circular arcs corresponding to different relaxation processes in the grains and grain boundaries were observed in all the samples. The diameter of the semi-circle (I), a measure of the grain contribution to the total resistivity, is comparatively small in 3YSZ than in the other set of the samples. Fig. 5 shows the CI spectra of the YSZ pellets at 773 K. As the temperature is increased, the grain boundary and the electrode–electrolyte polarization effects dominate than the bulk electrolytic polarization. In principle, the behavior of the grains of a high purity electrolyte should appear to be a simple resistance not showing any polarization phenomenon (the capacitance value to be zero) [13]. The intragFig. 4. Complex impedance spectra of YSZ at 573 K (the semicircle I corresponds to the intragrain polarization and II corresponds to the grain boundary polarization).

Fig. 3. Typical microstructure of 4.5YSZ sintered at 1673 K for 15 min.

rain polarizations observed at temperatures 573 K in the investigated samples may be due to the spatial inhomogeneity within the grains at low temperatures. The intragrain and the intergrain polarization effects are observed via the semi-circular arcs in the complex impedance plane. The overall impedance behavior is mainly constituted of the contributions from the grain boundaries and the impurity phases, may be, present at the grain boundaries. The intergranular porosity largely alters the diameter of the grain boundary arc in the impedance spectrum and nevertheless, it does not introduce any new features. Since

R. Ramamoorthy et al. / Solid State Ionics 123 (1999) 271 – 278

Fig. 5. Complex impedance spectra of YSZ at 773 K (the semicircle III corresponds to the electrolyte–electrode interface polarization).

in the ultrafine grained materials, the surface atoms will have higher diffusion coefficient than that of the bulk, the grain boundaries are expected to exhibit higher conductivity and lower activation energy than that of the grains. But in the present study the observed lesser conductivity in the grain boundaries must be due to the impurity phases and porosity. The variation of the grain and the grain boundary conductivities with yttria concentration is shown in Fig. 6. At 573 K, the 3YSZ shows maximum conductivity in grains and when the measurement temperature is increased, the maximum conductivity

275

regime is shifted slightly to the higher concentration of Y 2 O 3 dopant. There is no monotonic variation in the conductivity with the dopant concentration even though all the samples were processed at similar experimental conditions. Since the 2YSZ sample is of a mixture of both monoclinic and tetragonal phases, it shows lesser conductivity. The pure tetragonal phase is found to exhibit high conductivity in the grains at 573 K as evident from the case of 3YSZ. It is a general opinion that the fully stabilized cubic zirconia is more favorable for easy ionic conduction. And also it is reported in literature that about 8 mol% yttria doped cubic zirconia exhibits highest conductivity rather with other concentrations of yttria [11,12]. But in the present study, the lattice conductivity is found maximum in 3YSZ and 4.5YSZ at 573 and 973 K respectively and relatively lesser conductivity in the other samples with higher yttria concentration. And also the magnitudes of the lattice, grain boundary and total ionic conductivities for all the concentrations of yttria are found to be slightly lower (about a fraction of an order) compared to that of the coarser grained samples. The confined passage of the conducting species within the grains due to its smallness results in lesser conductivity in the fine-grained system. The ionic conductivity is directly proportional to the jump attempt frequency (g ) and the square of the jump distance (Jd ). The jump attempt frequency will be smaller in lower concentrations and the jump distance will be lesser in higher concentrations of yttria resulting a maximum in conductivity at particular concentration of yttria. It is observed to be 8 mol% Y 2 O 3 in coarser grained zirconia and here is 3 mol%. Thus the ion jump distance and the jump frequency must be influenced by the size of the grains. In the case of the ionic conductivity at the grain boundaries also, 3YSZ shows the maximum value at all the temperatures as shown in Fig. 6. The yttria doped tetragonal zirconia polycrystalline (Y-TZP) materials were generally considered unsuitable as solid electrolytes because of their lesser grain boundary conductivity than that of the fully stabilized cubic zirconia [4]. In the present investigation, ultra fine grained 3Y-TZP is found to exhibit higher grain boundary conductivity than in cubic YSZ. However the magnitude of the total dc conductivity is one

R. Ramamoorthy et al. / Solid State Ionics 123 (1999) 271 – 278

276

Fig. 6. Variation of conductivity in grains and grain boundaries with yttria concentration.

order less than that in the coarser grained material. Elimination or significant reduction of the impurity phases at the grain boundaries, pores etc. by improved fabrication procedures without considerably altering the grain size, the ionic conductivity is expected to show enhanced values suitable for low temperature applications.

3.3. Activation energy The Arrhenius plots of grain, grain boundary and total dc conductivities of YSZ pellets are shown in Fig. 7a, b and c respectively. In Fig. 7a, a constant slope up to about 973 K is observed and beyond that a continuous change in the slope towards the lower activation energy ( ¯ 0.5 eV) is observed. Whereas in the case of conductivity at the grain boundaries, no slope change is observed (Fig. 7b). Thus the decreasing activation energy with increasing temperature is not associated with the ionic conduction in the boundaries but in the grains only. There are different reports attributing different causes to the change of activation energy, such as trapping of oxygen vac-

ancies, defect interactions, formation of vacancy clusters, and space charge layers etc. at lower temperatures [14–16]. For all the compositions of YSZ, the decrease of activation energy at high temperatures is observed indicating that it is the intrinsic property of the YSZ. The formation of vacancy clusters and defects association have been realized by the negative association energies of various clusters in stabilized zirconia relatively at lower temperatures [17]. The dissociation of the vacancy clusters at high temperatures may be a plausible explanation for this decrease in EA . But it seems that the vacancy interactions and the formation of vacancy clusters do not take place at the grain boundaries evidently showing no variation in EA with increasing temperature. Similarly no slope change is observed in the Arrhenius plots of the total dc conductivity in all the samples. Thus it is evident that the total dc conductivity of ultrafine grained YSZ is determined not by the lattice conductivity rather by the grain boundary conductivity. From Fig. 7, it is observed that there is a general trend that the grain conductivity and the total dc

R. Ramamoorthy et al. / Solid State Ionics 123 (1999) 271 – 278

277

Table 1 Activation energies for ionic conduction in grains, grain boundaries and for total ionic conductivity in ultrafine grained YSZ systems with different yttria concentration Y 2 O 3 Concentration (mol%)

EA (in grain) (eV)

EA (in grain boundary) (eV)

EA (total) (eV)

2 3 4.5 6 9 12

0.98 0.97 1.12 1.02 1.17 1.22

1.20 1.21 1.27 1.20 1.31 1.35

1.17 1.12 1.27 1.18 1.29 1.32

conductivity values seem to converge at higher temperatures and the variation in the conductivity of YSZ with the dopant concentration is almost insignificant. That means the effect of the concentration of vacancies is overcome by the thermal vacancies produced at high temperatures. The values of the activation energy for ionic conduction in the grain, grain boundaries and for the total dc conductivity for different yttria concentration is given in Table 1. The EA is found to increase with increasing yttria concentration from 3 mol%. The 4.5YSZ shows higher activation energy than that of the 6YSZ, this may be because 4.5YSZ seems to be in the partially stabilized state, i.e. in a phase mixture of both tetragonal and cubic.

4. Conclusions

Fig. 7. Arrhenius plots of (a) conductivity in grains, (b) conductivity in grain boundaries and (c) total dc conductivity in YSZ.

In summary, ultrafine-grained 3Y-TZP ceramic is found to have enhanced electrical properties compared to fully stabilized cubic zirconia with increased concentrations of the dopant. This uniqueness of the 3YSZ must be attributed, considering the experimental and processing conditions of the samples, only to the grain size effect. In the ultrafine grained material system, with increased number of oxygen vacancies, the long range ion jump distance is restricted by the large number of grain boundaries, consequently the local short range jump process of the oxygen ions lead to less conductivity with increased concentration of yttria more than 3 mol%. The total ionic conductivity and the conduction mechanism is mainly influenced by the grain boundaries rather the grains.

278

R. Ramamoorthy et al. / Solid State Ionics 123 (1999) 271 – 278

Large number of grain boundaries and porosity lead to greater resistivity in the fine grained samples. Since there is no significant change in the total dc conductivity at high temperatures among different dopant concentrations, if the magnitude of the grain boundary conductivity is improved by modifying the grain boundary structures, the ultrafine-grained tetragonal 3YSZ will be the novel candidate for the electrical applications at all temperatures.

Acknowledgements One of the authors (RR) thank the CSIR, Govt. of India for the award of the SRF (award No. 9 / 115(393)-96 EMR I dated 12.8.96). The facilities made available under UGC-SAP and COSIST schemes are gratefully acknowledged.

References [1] M.J. Verkerk, B.J. Middelhuis, A.J. Burggraaf, Solid State Ionics 6 (1982) 159.

[2] N.M. Beekmans, L. Heyne, Electrochem. Acta 2 (1976) 303. [3] S.P.S. Badwal, in: R.W. Cahn, P. Haasen, E.J. Kramer (Eds.), Materials Science and Technology, A Comprehensive Treatment, Vol. 11, VCH, Weinheim, 1994, p. 567, (Volume editer: M.V. Swain). [4] S.P.S. Badwal, M.V. Swain, J. Mater. Sci. Lett. 4 (1985) 487. [5] C.S. Chen, M.M.R. Boutz, B.A. Boukamp, A.J.A. Winnubst, K.J. deVries, A.J. Burggraaf, Mater. Sci. Eng. A168 (1993) 231. [6] M.M.R. Boutz, C.S. Chen, L. Winnubst, A.J. Burggraaf, J. Am. Ceram. Soc. 77 (1994) 2632. [7] Y. Murase, E. Kato, J. Cryst. Growth 50 (1980) 509. [8] B.D. Cullity, in: Elements of X-ray Diffraction, 2nd ed, Addison-Wesley, Reading, (1977), p. 102. [9] J. Macdonald, W.B. Johnson, in: J.R. Macdonald (Ed.), Impedance Spectroscopy, John Wiley, New York, (1987), p. 7. [10] H.K. Schmid, J. Am. Ceram. Soc. 70 (1987) 367. [11] T.H. Etsell, S.N. Flengas, Chem. Rev. 70 (1970) 339. [12] T. Takahashi, in: J. Hladik (Ed.), Physics of Solid Electrolytes, Vol. 2, Academic Press, London, 1972, p. 989. [13] J.E. Bauerle, Phys. Chem. Solids 30 (1969) 2657. [14] H. Nafe, Solid State Ionics 13 (1984) 255. [15] E.C. Subbarao, H.S. Maiti, Solid State Ionics 11 (1984) 317. [16] S.P.S. Badwal, F.T. Ciacchi, S. Rajendran, J. Drennan, Solid State Ionics 109 (1998) 167. [17] A. Dwivedi, A.N. Cormack, Phil. Mag. A61 (1990) 1.