BI SEGREGATION AT ZNO GRAIN-BOUNDARIES IN EQUILIBRIUM WITH BI2O3-ZNO LIQUID

Segregation of Bi at ZnO grain boundaries in the presence of a small amount of Bi-rich liquid has been studied. Below the eutectic temperature (740 degrees C), a partial monolayer of Bi segregation is observed in equilibrium with a non-wetting solid phase. Above the eutectic temperature, the behavior is more complex. Partial monolayer segregation of Bi is observed in equilibrium with a Bi-rich grain junction liquid which meets many grain boundaries with a zero contact angle. Although at larger volume fractions, the liquid fully wets and penetrates along boundaries, when the liquid fraction is small, a continuous liquid film is not detectable. The grain boundary coverage of Bi has been quantified using STEM, and the interplay between capillary pressure, temperature, and composition in determining boundary coverage and the distribution of liquid is discussed. An alternative interpretation of the segregation as the remnant of a high-temperature intergranular liquid film is considered.