THERMODYNAMIC STABILITY OF INTERGRANULAR AMORPHOUS FILMS IN BISMUTH-DOPED ZINC-OXIDE

It is shown that the solid-state equilibrium configuration of Bi-doped ZnO grain boundaries is a nanometer-thick amorphous film. Polycrystal line Bi-doped ZnO was investigated using high-resolution transmission electron microscopy (HRTEM) and scanning transmission electron microsc opy (STEM). The equilibrium state below the eutectic temperature and a t 1 atm total pressure was approached from three different routes: sam ples were cooled from above the eutectic temperature (T-eutectic = 740 degrees C), processed entirely below the eutectic temperature, and de segregated by high applied pressure (1 GPa) followed by annealing at a mbient pressure to restore segregation. In all instances, the final st ate is an amorphous intergranular film 1.0-1.5 nm in thickness, The re sults show that a thin intergranular film in this system has lower fre e energy than the crystal-crystal grain boundary, The implications of these results for creation of electrically active grain boundaries in zinc oxide varistors are discussed.