Proton conductivity of Y-doped BaZrO 3: Pellets and thin films

Abstract

Y-doped BaZrO 3 (BYZ) pellets and thin films at various doping concentrations were fabricated by the cold isostatic press and 2-step sputtering techniques, respectively. By using BaCO 3, 3 mol% yttria stabilized zirconia (YSZ), nano-Y 2O 3 powders and 1 wt% ZnO as a sintering aid, 6–40 at% BYZ pellets with no second phase and relative bulk density higher than 97% can be achieved. The highest bulk and grain boundary proton conductivities of the BYZ pellets measured by the electrochemical impedance spectroscopy (EIS) technique were obtained at 10 and 20 at% BYZ, respectively. Due to the absence of the second phase and higher bulk density, our BYZ (1 wt% ZnO) pellets show higher bulk and grain boundary conductivity than several previously reported ZnO-BYZ results. The 2-step sputtering technique for BYZ film fabrication involved two sequential steps as followed: (i) DC sputtered Y onto BaZrO 3 target and (ii) RF sputtered Y-BaZrO 3 target onto SiO 2/Si substrate. EIS results of BYZ thin films also verify the dependence of conductivity on the Y doping concentration and reveal 10 2–10 3 times higher conductivities than those of BYZ pellets even with some YSZ phase present. The high conductivity may arise from several factors such as the surface conduction, low contamination and change of grain boundary structure. Two ionic conduction mechanisms were observed in the thin films (in air): (i) the proton conduction at T ≤ 450 °C and (ii) the oxide ion conduction at T > 450 °C. However, under humidified H 2 atmosphere, the proton conduction was dominant througout the temperature range of this study.