Diffusion mechanism of oxygen ions in La2Zr2O7/YSZ composite ceramics

In this paper, the diffusion mechanism of oxygen ions in La2Zr2O7/YSZ (LZ/YSZ) composites was investigated experimentally and numerically. LZ/YSZ composite samples with multiple amounts of LZ were successfully synthesized via a two-step sintering process. The so-obtained samples preserved most of the tetragonal phase (82.2 mol%) in YSZ. Moreover, an adhesive grain boundary and good chemical compatibility between YSZ and LZ grains were verified in the composite samples. The measured impedance spectra revealed that pure YSZ is an excellent oxygen ionic conductor, whereas the LZ can serve as an effective oxygen ionic isolator at high temperatures as further confirmed via the molecular dynamics simulation. For LZ/YSZ composites, the measured ionic conductivity decreases nonlinearly with the increase of the volume fraction of LZ, indicating that the addition of LZ can significantly reduce the oxygen ionic conductivity of these composites. More specifically, the simulation results showed that pores and LZ crystals would reduce the diffusion coefficient of oxygen ions, especially for the oxygen ions near the LZ/YSZ interfaces or the pore surfaces. Furthermore, we demonstrated that the bulk diffusion coefficient in YSZ increases as a direct consequence of the application of tensile stress induced by the lattice distortion between LZ and YSZ. However, the ionic conductivity in the LZ/YSZ composite was not enhanced due to the dominant blocking effect' from pores and interfaces. Indeed, we experimentally proved that both LZ grains and closed pores would reduce the ionic conductivity as a result of the 'blocking effect'. (C) 2018 Elsevier B.V. All rights reserved.