Abstract
The effects of drag imposed by extrinsic ionic species and point defects on the grain boundary motion of ionic polycrystalline ceramics were quantified for the generality of electrical, chemical, or structural driving forces. In the absence of, or for small driving forces, the extended electrochemical grain boundary remains pinned and symmetrically distributed about the structural interface. As the grain boundary begins to move, charged defects accumulate unsymmetrically about the structural grain boundary core. Above the critical driving force for motion, grain boundaries progressively shed individual ionic species, from heavier to lighter, until they display no interfacial electrostatic charge and zero Schottky potential. Ionic p–n junction moving grain boundaries that induce a finite electrostatic potential difference across entire grains are identified for high velocity grains. The developed theory is demonstrated for Fe-doped SrTiO3. The increase in average Fe concentration and grain boundary crystallographic misorientation enhances grain boundary core segregation and results in thick space charge layers, which leads to a stronger drag force that reduces the velocity of the interface. The developed theory sets the stage to assess the effects of externally applied fields such as temperature, electromagnetic fields, and chemical stimuli to control the grain growth for developing textured, oriented microstructures desirable for a wide range of applications.
Highlights
The properties of polycrystalline ceramics for applications such as solid oxide fuel cells, actuators, sensors, capacitors, and rechargeable batteries are dictated by the underlying point defects and their interactions with the grain boundaries that develop during processing
Chemical segregation and space charge was demonstrated to contribute to the drag forces opposing grain boundary motion
The study on grain boundary motion of Fe-doped SrTiO3 performed demonstrates that: (1) at low velocities or small driving forces, the defect profiles of 1⁄2 VÁOÁ and 1⁄2 Fe0Ti defects are symmetrically distributed about the structural grain boundary core; (2) at intermediate velocities, a larger number of oxygen vacancies accumulate on one side of the grain boundary and the iron defects are unable to keep up, enabling the interface to completely break away from the localized space charge; and (3) above the critical driving force or high velocities, the grain boundaries have no effective charge and exhibit zero Schottky potential
Summary
The properties of polycrystalline ceramics for applications such as solid oxide fuel cells, actuators, sensors, capacitors, and rechargeable batteries are dictated by the underlying point defects and their interactions with the grain boundaries that develop during processing. All of the solute drag theories and phase field descriptions focus on the thermochemistry of metallic systems and miss to incorporate the structural and electrochemical contributions to describe grain boundary coarsening kinetics, including the effect of drag in ionic solids that would allow an accurate rationalization of sintering and grain growth of ionic ceramic materials[37]. The developed model provides a rational basis to understand the drag effects of solute and point defects on the grain boundary motion in ionic solids.
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