The mechanism of formation of interface barriers in ZnO:Mn ceramics
- PDF / 821,724 Bytes
- 5 Pages / 595.276 x 790.866 pts Page_size
- 84 Downloads / 198 Views
The mechanism of formation of interface barriers in ZnO:Mn ceramics Valentyna Nosenko1 · Nadija Korsunska1 · Igor Vorona1 · Tetyana Stara1 · Volodymyr Bondarenko1 · Olexandr Melnichuk2 · Lyudmyla Melnichuk2 · Andriy Kryvko3 · Iryna Markevich1 Received: 29 January 2020 / Accepted: 15 April 2020 © Springer Nature Switzerland AG 2020
Abstract 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 M nSO4 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 M nZn2+ 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. Keywords ZnO:Mn ceramics · Interface barriers · Oxygen adsorption
1 Introduction ZnO-based ceramics are widely used as surge absorbers in electronic circuits due to excellent nonlinearity of their current–voltage (CV) characteristics and high energy absorption capability. These properties were proved to result from the formation of double back-toback Schottky barriers at grain boundaries (GBs) under sintering ZnO powder with small amounts of other oxides (such as those of Bi, Co, Sb, Cr etc.) known as “varistor formers” [1]. However, the mechanism of barrier formation and the role of additives have not been
reliably elucidated. It was supposed that the additives promoted the appearance of high-resistance layer at GBs [1–7] and it was experimentally shown that oxygen played the key role in this process. It was found that a thin layer of excess oxygen was present at the interfaces in ceramics sintered in air [1]. The annealing in vacuum resulted in the disappearance of this layer, which was accompanied by the reduction of barrier height and the increase of leakage current Lc, whereas the following annealing in air restored both the oxygen layer and the initial characteristics [1]. The main role of oxygen in the appearance of Schottky barriers at GBs was also stated
* Valentyna Nosenko, [email protected]; Nadija Korsunska, [email protected];
Data Loading...
