Premium
HREM and STEM of intergranular films at zinc oxide varistor grain boundaries
Author(s) -
Chiang,
Hong Wang,
Myeong Soo Lee
Publication year - 1998
Publication title -
journal of microscopy
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.569
H-Index - 111
eISSN - 1365-2818
pISSN - 0022-2720
DOI - 10.1046/j.1365-2818.1998.00377.x
Subject(s) - grain boundary , intergranular corrosion , materials science , varistor , amorphous solid , zinc , transmission electron microscopy , eutectic system , analytical chemistry (journal) , microstructure , metallurgy , crystallography , nanotechnology , chemistry , electrical engineering , chromatography , voltage , engineering
Grain boundaries in model ZnO–Bi 2 O 3 and ZnO–Bi 2 O 3 –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 Bi 2 O 3 content, applied pressure and temperature, the grain boundary Bi excess has been systematically varied from nearly zero to Γ Bi = 1 × 10 15 cm −2 (≈ 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 Γ Bi > 8 × 10 14 cm −2 . These films range in thickness, depending on the Bi excess, from 0.6 to 1.5 nm. Similar films of ≈ 1 nm thickness are widely observed in the commercial varistor. The composition of the films is a ZnO–Bi 2 O 3 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.