Electric force microscopy investigations of barrier formations in ZnO-based varistors

Non‐linear coefficient of BaTiO 3 ‐doped ZnO varistor

“…2. The XRD study confirms the formation of hexagonal system of ZnO [12,13]. All diffraction peaks coincide with the earlier reported values, where lattice parameters a = 32420 Å and c = 51760 Å were comparable with the ones carried out in [12].…”

(Co, Nb, Sm)‐Doped Tin Dioxide Varistor Ceramics Sintered Using Nanopowders Prepared by Coprecipitation Method

“…Because the nonlinear current–voltage characteristics of varistor ceramics are controlled by grain‐boundary features, 1,3,4,20–22 it is necessary to point out that in the present work, we found several types of junctions in the microstructure of (Co, Nb, Sm)‐doped SnO 2 ‐based varistor: direct SnO 2 –SnO 2 junction, triple junction rich in cobalt and grain‐boundary phase rich in Sm. The direct SnO 2 –SnO 2 junction with transition metal segregated in it suggests this is the main kind of junction controlling the overall nonohmic properties 17,21,37 .…”

“…The role of Co Sn ″, Sm Sn ′ , V Sn ″″, and V Sn ″ defects (due to the reduction of Sn 4+ into Sn 2+ in the interstitial sites) would be related to the increase of the oxygen species adsorbed at the grain‐boundary interface, causing a decrease in the conductivity of the grain boundary by the donation of electrons to the O 2 adsorbed at the grain boundary. The adsorbed oxygen at the grain boundary captures electrons from the negatively charged defects at the grain boundary and thus stays at the interface 3,4,33 . Based on the models for ZnO‐based varistors proposed by Gupta and Carlson, 47 Bueno et al 48 proposed a model for SnO 2 ‐based varistors.…”

“…V aristor ceramics are technologically important for their highly nonlinear electrical characteristics. They are used as reversible solid state switches with large‐energy‐handling capabilities 1–5 . Recently, many papers have been published on new SnO 2 ‐based varistors 6–9 .…”

“…The electronic charge changes the Fermi level near the grain boundary, resulting in band bending. The electronic charges stored in an interface represent a repulsive potential for the majority carriers (the electrons in the case of an n ‐type semiconductor) across the interface 3,4,20–22 . However, the nonohmic properties are controlled by grain–grain junctions with transition metals segregated at the grain boundary 20,21 …”