Molecular Dynamics Study of Complex Dopant Electrolyte Nd_(2-x)Ho_xZr_2O_7: Structure, Conductivity, and Thermal Expansion

摘要

Nd_(2-x)Ho_xZr_2O_7 is an ionic conductor for solid oxide fuel cell electrolytes. Temperature and dopant composition are two main factors that affect ionic conductivity of these groups of materials. Accordingly, Nd_(2-x)Ho_xZr_2O_7 was studied in a wide range of temperature, 1,173-1,873 K, and dopant composition, x = 0.0-2.0, by molecular dynamics simulation. Arrhenius plots of ionic conductivity in region of low temperature declared that the maximum occurred at composition x = 0.6 (H_6). Surprisingly, it was observed that complexity of dopant improved ionic conductivity of the electrolyte. Moreover, radial distribution function confirmed this result. Two types of anions were observed in the electrolyte, which were dynamic oxygen ions (DOIs) and static oxygen ions (SOIs). Mean square displacement (MSD) of DOIs was higher than SOIs and enhanced oxygen ionic conductivity. Migration barrier of Nd_(2-x)Ho_xZr_2O_7 varied from 0.68 to 1.22 eV that was in the range of a good solid oxide electrolyte. Nd_(1.2)Gd_(0.8)Zr_2O_7 (G8) and Y_(0.2)Zr_(0.9)O_(2.1) (Y2) were simulated and their results of simulations were compared with H_6. Order of ionic conductivity at low temperatures was H6 > Y2 > G8. Another important factor of electrolyte is thermal expansion. Thermal expansion of Nd_(2-x)Ho_xZr_2O_7 was around a constant value in the studied temperature range.