Naomi Furuse

Characterization of Single Grain Boundaries in a Bi-Doped ZnO Varistor Using a Focused Ion Beam System

This report presents a new method for electrically characterizing single grain boundary junctions of ZnO varistors. For the first time to our knowledge, a focused ion beam system was used to deposit tungsten microelectrodes on two adjacent ZnO grains. Gallium ions irradiated onto the ZnO surface lowered the resistivity of the ZnO, and therefore highly nonlinear current-voltage (I–V) characteristics of single grain boundaries could be obtained after etching the gallium-irradiated ZnO surface with HClO4 solution. Breakdown voltages in the I–V characteristics were observed in the range from 2.6 V to 3.2 V within the same ZnO varistor. The grain boundary with the breakdown voltage of 3.2 V was structurally different from that of 2.6 V. The boundary with the breakdown voltage of 3.2 V contained precipitates and a thin, amorphous Bi-segregated layer about 1 nm thick. On the other hand, the boundary with the voltage of 2.6 V contained no precipitate but rather, a thin, amorphous Bi-segregated layer about 0.5 nm thick. It was suggested that the electrical characteristics of grain boundary junctions within the ZnO varistors are strongly dependent upon the microstructure of the grain boundary.

Development of advanced arresters for GIS from 66 kV to 500 kV

New zinc oxide elements with a varistor voltage about twice that of conventional ones have been developed by reducing the ZnO grain size with a new additive, making it possible to halve the series number of elements in metal oxide surge arresters. The advanced arresters with these elements for GIS are smaller in volume by 40 to 60% compared with conventional ones. These advanced MOAs increase the flexibility in positioning MOAs and optimizing the GIS layout. This paper describes the performance of the new elements, the specification and construction of the advanced MOAs, and the results of typical evaluation tests.