Direct measurements of of grain boundary properties and mechanism of electroceramics.

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The electrical properties in very narrow region of a single grain boundary in Ladoped SrTiO_3 crystals were measured by using the micromanipulatoR.The sample having one grain boundary was prepared from semi conductive SrTiO_3 crystals having mosaic structure prepared by xenon-arc image floating zone method. The nonlinear relation of voltage and current was observed in the every grain boundaries. Although there were no electrical effects from the neighbor grains in the single grain boundary sample, the break down voltage (Vb) and nonlinear coefficient (alpha) varied 0.5 to 3.5 V and 1 to 7 respectively. The varistor characteristic was independent from the directions of crystals. After heat treatment in reduced atmosphere, the varistor characteristics was disappears. The reduced samples were re-oxidized at high temperature, then teh varistor characteristics were observed again. The voltage-current characteristics of grain boundary were changed reversibly by oxidation or reduction of the sample. The break down voltage was increase with oxidation temperature and time, but nonlinear coefficient (alpha) was almost constant.In the sample, cut the SrTiO_3 FZ-single crystal into two pic es and oxidized them and then contacted them mechanically in same direction before separation, the nonlinear relation of voltage and current was observed same as the grain boundary in the FZ-crystals.