EXPERIMENTAL STUDY ON NUMBER DENSITY AND MOBILITY OF ELECTRIC CARRIER IN A CERAMIC INSULATOR,ALUMINA

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It has been attempted to measure a number density and a mobility of an electric carrier in a ceramic insulator, alumina. The specimens used are highly pure alumina polycrystals and a highly pure single crystal sapphire. Electrons are thought to be responsible for the electric conductivity in alumina and sapphire. However, the electrical conductivity in these materials are very low, namely in the order of 10^<-13>Sm^<-1>, at moderate temperatures. Thus, the measurements of the electrical conductivity as well as the number density and the mobility of the electric carrier is very difficult in general. In this experiment, we tried to use the phenomenon so-called radiation induced condutivity (RIC) to increase the electrical conductivity, thus, increasing the number density of the carrier. The RIC is the phenomenon similar to the phtoconductivity in semiconductors. It is in general very difficult to measure the electrical conductivity in radiation environments, but by refining a techniques it became possible to measure the electrical conductivity reliable in the radiation environments in this experimental study. The developed technique made it possible to measure the electrical conductivity in the range of to 10^<-11>-10^<-4>S.Using this technique, we obtained the data concerning dependence of the electrical conductivity on the crystal orientation, microstructures of the crystals, and so on. Also, we observed some non-ohmic behavior in insulators under irradiation. This non-ohmic behavior may be caused by the charge-ups of electric carriers near the boundaries between the electrodes and the insulators or at grain boundaries. Also, some charge trap effects at some point electrical impurities may be responsible. The obtained data will give us the quantitative information concerning the number density and the mobility of the electrical carrier in alumina and sapphire.