Origin of Solid‐State Activated Sintering in Bi ₂ O ₃ ‐Doped ZnO

Activated sintering in Bi 2 O 3 ‐doped ZnO has been studied with emphasis on the mechanistic role of intergranular amorphous films. The atomic‐level microstructures and bismuth solute distributions in doped powders have been investigated using high‐resolution electron microscopy and scanning transmission electron microscopy. Densification is observed to be significant below the bulk eutectic temperature in the presence of Bi 2 O 3 concentrations as low as 0.58 mol%. Transmission electron microscopy of as‐calcined and sintered powders shows that significant neck growth and particle coarsening occur in the solid state. Intergranular amorphous films of ∼1 nm thickness, terminating in wetting menisci at sinter‐necks, are observed to form concurrently with the onset of activated sintering. In a few instances, amorphous films are also observed at surfaces of the ZnO particles. These films appear to be the free‐surface counterpart to equilibrium‐thickness intergranular films. Activated sintering in this binary system is attributed to rapid mass transport through subeutectic, equilibrium‐thickness intergranular films, with the amorphous phase also providing capillary pressure.