Ionic Conductivity of Zirconia-Scandia-Dysprosia Solid Electrolyte

Zirconia-based solid electrolytes are the preferred materials for high-temperature applications due to the combination of their electrical and mechanical properties. Zirconia-8-11 mol% scandia possess higher values of ionic conductivity at temperatures above 600 °C than other zirconia-based electrolytes [1]. At low temperatures the ionic conductivity of this system decreases due to the transition from the cubic fluorite-type phase to the ordered Sc₂Zr₇O₁₇(β) rhombohedral phase [2].

It has been reported that the cubic phase can be stabilized at room temperature by adding minor amounts of e.g. Y₂O₃, CeO₂ and Gd₂O₃[3].

In this work, small amounts of Dy₂O₃ was added to zirconia-10 mol% scandia to evaluate the effect of the additive on the phase composition and the ionic conductivity.

Zirconia-10 mol% scandia (DKKK) and Dy₂O₃(Alfa Aesar) were the starting materials. Dysprosia was added in the 1-2 mol% contents to the zirconia-scandia solid electrolyte. Cylindrical pellets were prepared by pressing followed by sintering at 1500 °C. Sintered pellets were characterized by X-ray diffraction, apparent density measurements, scanning electron microscopy observations and ionic conductivity evaluation by impedance spectroscopy.

Table 1 summarizes the relative densities, ρ_R, of sintered samples with different additive contents, x.

Table 1. Dy₂O₃ content (x, in mol%) and relative density (ρ_R) of sintered samples.

x (mol%)	ρR (%)
1	95.6 ± 0.5
1.5	95.5 ± 0.1
2	92.3 ± 0.4

The sintered density decreases with increasing Dy₂O₃ content. The relative density is higher than 92% for sintering experiments at 1500 °C for 5 h for all compositions.

The X-ray diffraction patterns of sintered pellets show that for the additive content of 1 mol% the cubic phase was partially stabilized at room temperature. A small amount of the rhombohedral phase coexists for this additive content. For dysprosia additions of at least 2 mol% the rhombohedral phase was suppressed, as shown in Fig. 1. The X-ray diffraction pattern of Fig. 1 corresponds to that of cubic zirconia-based solid electrolytes.

The ionic conductivity was determined by impedance spectroscopy in the 500-800 °C range. The Arrhenius plot of the total ionic conductivity is depicted in Fig. 2 for the sample containing 2 mol% dysprosia.

The calculated apparent activation energy is 1.2 eV, similar to that of other zirconia-scandia systems.

Stabilization of the cubic phase was successfully accomplished by small additions of dysprosia to zirconia-10 mol% scandia. The magnitude of the ionic conductivity value decreases with increasing additive content. The activation energy for oxide ion conduction is similar to that of other zirconia-scandia systems containing a second additive.

References

[1] E. C. Subbarao, Solid electrolytes and their applications, Plenum Press, New York, 1980.

[2] S. P. S. Badwal, F. T. Ciacchi, D. Milosevic, Solid State Ionics 136/137 (2000) 91.

[3] O. Yamamoto, Y. Arati, Y. Takeda, N. Imanishi, Y. Mizutani, M. Kawai, Y. Nakamura, Solid State Ionics 79 (1995) 137-142.