ZnO Grain Boundaries Research Articles

Phase Identification and Electrical Properties in ZnO–Glass Varistors

Ceramic varistors based on ZnO with lead zinc borosilicate glass instead of Bi2O3 were prepared. The effect of sintering conditions on the electrical properties was studied by sintering samples at various temperatures and cooling them at different rates. The sample sintered at 1250°C for 1 h, then furnace cooled, possessed the best electrical properties, as characterized by the highest nonlinear coefficient, lowest leakage current, and lowest degradation. The microstructure and crystal structure of the glass phase of ZnO–glass varistors were examined by means of scanning electron microscopy, transmission electron microscopy, and powder X‐ray diffractometry. The glass phase was originally amorphous, but crystallized as an intergranular layer in the sintered and furnace‐cooled samples. This crystallized phase was a zinc borate phase (5ZnO·2B2O3), which was identified by X‐ray diffractometry, transmission electron microscopy, and Auger electron spectroscopy. The zinc borate phase at the grain boundary of ZnO–glass samples enhanced the nonohmic characteristics of the ceramic varistors.

ESR study on crystallization of (Zn, Mn)O ceramics by Nd: YAG pulse laser annealing

Abstract On the sintered ZnO involving a binding dopant of MnO below 10 mol%, Electron Spin Resonance (ESR) has been investigated before and after Nd:YAG pulse laser (λ=1.064 μm) annealing. As-sintered (Zn, Mn)O showed the resolved spectrum with the hyperfine structures for Mn2+ with 0.01 to 0.05 mol%, whereas above 1.0 mol%, a broad line appeared with the half-width of 500 Oe. By laser annealing, we found that the broad line was changed into the resolved spectrum. This result suggests that the Mn2+ clusterings located at grain boundaries in ZnO are diffused into crystal grains, in which the crystallization of ceramics is induced.

Electron spin resonance study of laser-annealed (Zn,Mn)O ceramics

Investigations have been made on the effects of pulse laser annealing on the ceramics (Zn,Mn)O by means of electron spin resonance (ESR) measurements. A remarkable change in the ESR spectrum was observed after annealing by a Nd:YAG pulse laser (λ=1.064 μm, 5–7 J/cm2). It suggests that segregated Mn2+ precipitates at grain boundaries in ZnO are diffused into crystal grains by preferential heat conduction through the boundaries. The scanning electron microscope observation verified enough melting features within the ceramics.

Electron spectroscopic studies of electrically active grain boundaries in ZnO

The non-ohmic behaviour of ZnO-ceramics is based on back-to-back Schottky barriers between adjacent grains. Here grain boundaries of ZnO-varistors are studied by a variety of surface sensitive techniques to try to correlate their macroscopic electrical behaviour with the physico-chemical properties of the interfaces. Severe electrical degradation causes a large increase in leakage current at low voltages and deformes the Schottky barriers. We show for the first time that electrical ageing of interfacial potential barriers is related to chemical changes on the monolayer scale. The electromigration of oxygen ions perpendicular to the grain boundaries is responsible for the reduction of the barrier height.

Electron spectroscopic studies of electrically active grain boundaries in ZnO

Electron spectroscopic studies of electrically active grain boundaries in ZnO

Non-Ohmic Properties of ZnO-Rare Earth Metal Oxide-Co3O4 Ceramics

Zinc oxide ceramics containing rare earth metal oxide and cobalt oxide which exhibit non-ohmic current-voltage characteristics were studied. The nonlinearity was similar to that of the zinc oxide varistor containing Bi2O3 and other additives. According to microstructural observations, the intergranular layer of the ceramics was found to be a rare earth metal oxide compound without Zn and Co atoms, and its thickness was expected to be less than 100 Å. The nonlinear characteristics are attributed to the potential barrier due to the depletion layer formed in the interfacial region of the ZnO grainboundaries doped with Co atoms. Qualitative analysis of capacitance-voltage relations confirmed this model.