Conduction Properties and Ionic Transference Behavior of CaTi1-XScxO3-δ (x=0.05, 0.1)

1. Introduction

Finding out new candidates that alternative typical YSZ electrolyte was researched for long periods to overcome the low conductivity at low temperature. dopants doped ceria-based oxide electrolytes such as Sm (SDC) or Gd (GDC) considered as attractive material due to their high ionic conductivity at low temperature but electron conduction was observed at low oxygen partial pressure because of reduction reaction of Ce⁴⁺ to Ce³⁺ [1]. In previous research, H. Iwahara el al. reported various perovskite materials can shows oxide ionic conduction behavior by partial substitution of lower valence such as La_0.8Sr_0.2AlO_3-δ, CaTi_0.8Al_0.2O_3-δ and so on[2]. T. Ishihara et al. discovered Lanthanum gallium-based oxide (La_0.8Sr_0.2Ga_0.8Mg_0.2O_3-δ) having perovskite structure was regarded as promising electrolyte due to their high stability at low temperature as well as high ionic conductivity although La and Ga are considered as expensive elements [3]. Meanwhile, S. Hashimoto el al. reported CaTi_0.9Sc_0.1O_3-δ has comparable ionic conductivity with YSZ electrolyte  at 800^oC although the fair ionic transportation number at other temperature is unclear [4].

In this research, conductivity behavior of CaTi_1-xSc_xO_3-δ (x=0.05, 0.1) was analysed as function of P(O₂) dependence at various temperature from 500^oC to 1000^oC. Based on the conductivity behaviour, the ionic transference number of CaTi_1-xSc_xO_3-δwas calculated from defect chemistry point of view for mixed-conductor.

2. Experimental Procedures

 Commercial CaCO₃, TiO₂, Sc₂O₃were used as starting materials and synthesized by a conventional solid state reaction method depending on required ratio. The mixed powders were calcined in air at 1200ºC for 10h and grounded by satellite-type ball milling process for 2h using ethanol. The calcined powder (x=0.05, 0.1) was compacted into a bar with hydrostatic pressure and sintered at 1550ºC for 10h in air.

Lattice parameter and phase characterization were investigated using X-ray diffraction (XRD) analyser with CuKα. To confirm the electrical properties of CaTi_1-xSc_xO_3-δ (x=0.05, 0.1), impedance analysis was carried out as function of oxygen partial pressure, P(O₂)(10^-30to 1bar) and temperature (1273-773K) assisted by 4-terminal method using chemical impedance meter (Hioki 8334-30, Japan).

3. Results and Discussion

 Total conductivity behavior was shown depending on P(O₂) and temperature and fitted total conductivity (solid line) based on experimental analysis were shown as Fig. 1. Fitting process was carried out to separate total conductivity by conduction parameter (σ_elec, σ_ion σ_hole) as function of P(O₂). Conductivity was increased with increasing temperature and also decreased at high P(O₂) region and increased at low P(O₂) region showing typical mixed ionic-electronic conductor (MIEC). On the contrary to this their ionic domain range was decreased with increasing temperature. These kinds of phenomenon were considered as dominant hole and electron conduction due to generation of holes and electrons at high temperature. In addition, effect of electron conduction seemed like not so strong compare with hole conduction with decreasing temperature. Moreover, conductivity was increased with increasing of Sc content. Sc seemed not to improve conductivity in case of CaTi_0.95Sc_0.05O_3-δ. But if Sc content was increased, conductivity was increased and showed high conductivity compare with YSZ, contrastively. This behavior observed more clearly low temperature than high temperature.

References

[1] B. Cales and J. Baumard, J. Phys. Chem. Sol, 45(8/9), (1984), 929-935

[2] T. Ishihara, H. Minami, H. Matsuda, H. Nishiguchi and Y. Takita, Chem. Commun, (1996), 929-930

[3] T. Takahashi and H. Iwahara, Energy Conv. 11(1971), 105-111

[4] S. Hashimoto, H. Kishimoto and H. Iwahara, S. S. Ionics, 179, (2001), 179-187