Influence of vanadium and dysprosium co-doping on phase stability, microstructure, and electrical properties of Bi2O3

Abstract

Herein, we report the synthesis of Dy–V co-doped Bi2O3 ceramics using the solid–state processing technique under atmospheric conditions. The X-ray diffraction (XRD) patterns demonstrate the stability of the cubic fluorite δ-Bi2O3 in the V-rich ceramics. However, in Dy-rich ceramics, a mixture of phases, including α and δ, gradually diminishes with increased mutual dopant concentrations, suggesting a transition to the single fcc δ-phase with Fm-3 m space group. According to the Rietveld analysis and electron density representation, it is evident that there are no impure peaks present in α-Bi2O3, which highlights the clear transition to the δ-phase polymorphs. The DTA curves for samples M4 and M7 display a distinct endothermic peak at temperatures around 724.5 and 744.5 °C, indicating a phase transition from the monoclinic α-phase to the cubic δ-phase. These peaks are also attributed to an order-disorder transition (ODT). The FESEM micrographs consistently revealed the existence of irregular and aggregated grains, with an average grain size ranging from 0.80 to 5.57 μm. The level of aggregation became more apparent with the escalation of Dy3+ doping, as opposed to the 5–20 wt% V loading. Moreover, the as-sintered pellets M2 demonstrated the absence of any pore formation compared to other samples, confirming a remarkably high degree of densification. As a result, the as-sintered pellets have a low level of void space, with an apparent porosity of no more than 2.5%. Based on the conductivity measurements and electrochemical impedance spectroscopy (EIS), Bi0.85V0.10Dy0.05O1.5 exhibits the highest electrical conductivity of 0.965 (Ω cm)−1 and an optimal activation energy of 0.537 eV at 627 °C compared to other prepared compositions. This remarkable performance is attributed to the high polarizability and mixed valence cations, especially in V5+-rich ceramics, compared to the Dy3+-rich compositions with a reduced conductivity of 0.010 to 0.097 (Ω cm)−1. The Nyquist plots indicate that impedance decreases with vanadium doping increases until it reaches Bi0.85V0.10Dy0.05O1.5. Higher Dy3+ content increases impedance, leading to lower cell performance. The typical composition can be a solid electrolyte in SOFCs operating at moderate temperatures.