Linyun Liang

Composition- and pressure-induced ferroelectric to antiferroelectric phase transitions in Sm-doped BiFeO3 system

A three-dimensional phenomenological model is proposed to describe both ferroelectricity and antiferroelectricity based on the Ginzburg-Landau-Devonshire theory. Its application to the multiferroic Sm-doped BiFeO3 system describes the temperature-, pressure-, and composition-induced ferroelectric to antiferroelectric phase transitions. The constructed temperature-composition and temperature-pressure phase diagrams show that compressive hydrostatic pressure and Sm doping have similar effects on the ferroelectric and antiferroelectric phase transitions. It is also indicated from the temperature-pressure phase diagram that the experimentally observed phase of BiFeO3 under the hydrostatic pressure from 3 GPa to 10 GPa is a PbZrO3-like antiferroelectric phase.

Phase-field modeling of three-phase electrode microstructures in solid oxide fuel cells

A phase-field model for describing three-phase electrode microstructure (i.e., electrode-phase, electrolyte-phase, and pore-phase) in solid oxide fuel cells is proposed using the diffuse-interface theory. Conserved composition and non-conserved grain orientation order parameters are simultaneously used to describe the coupled phase coarsening and grain growth in the three-phase electrode. The microstructural evolution simulated by the phase-field approach demonstrates the significant dependence of morphological microstructure and output statistic material features on the prescribed kinetic parameters and three-phase volume fractions. The triple-phase boundary fraction is found to have a major degradation in the early evolution.

Nonlinear phase field model for electrodeposition in electrochemical systems

A nonlinear phase-field model has been developed for describing the electrodeposition process in electrochemical systems that are highly out of equilibrium. Main thermodynamic driving forces for the electrode-electrolyte interface (EEI) evolution are limited to local variations of overpotential and ion concentration. Application of the model to Li-ion batteries describes the electrode interface motion and morphology change caused by charge mass transfer in the electrolyte, an electrochemical reaction at the EEI and cation deposition on the electrode surface during the charging operation. The Li electrodeposition rate follows the classical Butler-Volmer kinetics with exponentially and linearly depending on local overpotential and cation concentration at the electrode surface, respectively. Simulation results show that the Li deposit forms a fiber-like shape and grows parallel to the electric field direction. The longer and thicker deposits are observed both for higher current density and larger rate constant where the surface reaction rate is expected to be high. The proposed diffuse interface model well captures the metal electrodeposition phenomena in plenty of non-equilibrium electrochemical systems.

Phase field modeling of microstructure evolution of electrocatalyst-infiltrated solid oxide fuel cell cathodes

A phase field model is developed to examine microstructural evolution of an infiltrated solid oxide fuel cell cathode. It is employed to generate the three-phase backbone microstructures and morphology of infiltrate nano-particles [La1−xSrxMnO3 (LSM)]. Two-phase Y2O3 + ZrO2 and LSM backbones composed of 0.5–1 μm particles are first generated and then seeded with infiltrate, and evolution is compared for starting infiltrate particle diameters of 5 nm and 10 nm. The computed lifetime triple phase boundary (3PB) density of the infiltrated cathode is then compared to the cathode backbone. Results indicate that initial coarsening of infiltrate nano-particles is the primary evolution process, and infiltrate coarsening is the majority contributor to 3PB reduction. However, at all times, the infiltrated cathode possesses significantly greater 3PB length than even the uncoarsened backbone. Infiltrate particle size effects indicate that the smaller particle size produces greater 3PB length for the same infiltration...

Pressure and electric field effects on piezoelectric responses of KNbO3

The dielectric and piezoelectric properties of a KNbO3 single crystal under applied hydrostatic pressure and positive bias electric field are investigated using phenomenological Landau-Ginzburg-Devonshire thermodynamic theory. It is shown that the hydrostatic pressure effect on the dielectric and piezoelectric properties is similar to temperature, suggesting a common underlying mechanism for the piezoelectric anisotropy and its enhancement. The stable phase diagram of KNbO3 as a function of temperature and positive bias electric field is constructed. The maximum piezoelectric coefficient d33o* varying with temperature and electric field is calculated.

Interdiffusion across solid electrolyte-electrode interface

A phase-field model is developed for studying the cation interdiffusion across electrolyte-electrode interfaces in solid oxide fuel cell (SOFC) that can be contributing to long timescale performance degradation. Demonstrated on an interface between an 8%molY2O3-stabilized ZrO2 and a La0.65Sr0.3MnO3−x typically used in SOFC, time-dependent evolution of the cation interdiffusion profiles are predicted by linking the phase-field model to a diffusion equation. The simulated interdiffusion profiles agree with independent experimental data in both time and space domains at different temperatures.