The mechanism of formation of interface barriers in ZnO:Mn ceramics

The role of oxygen adsorption in the formation of varistor characteristics in ZnO:Mn ceramics was investigated. The samples were made of the mixture of ZnO powder with MnSO₄ aqueous solution, sintered in air for 3 h at 1100–1300 °C and furnace cooled. Mn content in the initial mixture was of 0.1 at.%. The density of prepared ceramics was of 5.37–5.41 g/cm³. XRD patterns indicated hexagonal wurtzite structure of obtained samples. The incorporation of manganese atoms in ZnO lattice as Mn_Zn²⁺ centres was proven by EPR spectroscopy. Current–voltage (CV) characteristics of as-prepared samples and of the same ones kept in N₂ flow, vacuum or oxygen atmosphere were compared. The increase of the sample conductivity and the decrease of CV characteristic nonlinearity after the exposure to nitrogen or vacuum were observed, whereas the action of oxygen resulted in the opposite effects. Obtained results testify that the adsorption of ambient oxygen contributes essentially to the formation of interface potential barriers in ZnO:Mn ceramics. The analysis of the available data has led to the conclusion that a thin layer of some Mn-related phase is formed at grain boundaries under sintering. This layer separates adjacent ZnO grains, which allows the diffusion of oxygen along grain boundaries and its adsorption on ZnO grain surface.