Electric Field‐Induced Microstructural Evolution in Polycrystalline Bi₂O₃‐Doped ZnO in Presence of a Secondary Liquid Phase

Herein, a spectrum of electric field–induced microstructural evolution phenomena are observed in polycrystalline Bi₂O₃‐doped ZnO, and the underlying mechanisms are revealed. An applied electric field can drive the migration of bismuth (Bi) toward the negative electrode (including the motion of the charge‐neutral Bi‐rich liquid phase via electrochemical coupling), which produces a Bi‐free zone near the anode. The field‐induced migration of Bi creates a junction between electron‐conducting ZnO and ion‐conducting Bi₂O₃‐doped ZnO (via Bi₂O₃‐enriched liquid‐like intergranular films [IGFs]), where an oxygen ion current is converted to an electron current while generating O₂ gas, generating a porosity belt. Aberration‐corrected electron microscopy further reveals the formation of three distinct types of grain boundaries (GBs). “Clean” ZnO GBs are observed near the anode, where Bi is depleted by the electric field. Bi₂O₃‐enriched liquid‐like IGFs (disordered GBs) are observed in the middle section. Near the cathode, electrochemical reduction induces an GB disorder–order transition to form fast‐moving Bi₂O₃‐enriched ordered GBs, which causes abnormal grain growth. Herein, several new field–matter interaction phenomena are discovered and an exemplar of field effects on microstructural evolution are established at multiple length scales in the presence of GB complexion (phase‐like) transitions.