HREM and STEM of intergranular films at zinc oxide varistor grain boundaries

Grain boundaries in model ZnO-Bi2O3 and ZnO-Bi2O3-CoO varistors and a commercial multicomponent varistor have been characterized by high-resolution electron microscopy (HREM) and scanning transmission electron microscopy (STEM), in order to determine the relationship between Bi grain boundary segregation and formation of thin intergranular films. By controlling Bi2O3 content, applied pressure and temperature, the grain boundary Bi excess has been systematically varied from nearly zero to GammaBi = 1 x 1015 cm-2 ( approximately 1 monolayer), as measured by HB 603 STEM using an area-scan method. HREM shows that intergranular amorphous films are clearly distinguishable in samples with GammaBi > 8 x 1014 cm-2. These films range in thickness, depending on the Bi excess, from 0.6 to 1.5 nm. Similar films of approximately 1 nm thickness are widely observed in the commercial varistor. The composition of the films is a ZnO-Bi2O3 solid solution, which is in all cases more enriched in ZnO than the bulk eutectic liquid. The Bi-doped grain boundaries in ZnO varistors therefore contain an intergranular amorphous film which has not only an equilibrium thickness, but also a distinct equilibrium composition.