Yttrium sesquioxide (yttria) is considered to be one of the most hopeful candidate materials for a collector electrode of the separated uranium atom in a laser uranium enrichment system. In order to confirm a required property for the electrode, the conductivity measurement of yttria was performed under a simulated condition in the presence of molten uranium. Because of the extremely low oxygen pressure in the U-UO2 system, such a condition was locally realized by placing yttria specimens between plates of titanium or vanadium at high temperature. The specimens were reduced in advance to hypostoichiometry with titanium or vanadium plates under the condition of 1600 K and 2.5 Ms in vacuo. The electrical conductivity measurement was performed by the AC four-probe method in the temperature range of about 900 similar to 1600 K and the frequency range of 60 Hz similar to 30 kHz. The electrical conductivity of hypostoichiometric yttria reduced by titanium was higher than that of intrinsic germanium at room temperature (2.1 S/m). This specimen showed the characteristic behavior in the temperature dependence of conductivity; the conductivity showed semiconductor-like behavior in the range of 1400 similar to 1600 K, while it had a minimum at about 1250 K and again increased below this temperature. That of hypostoichiometric yttria reduced by vanadium was much lower than that of the specimen reduced with titanium and followed a straight line in the Arrhenius plot above 1000 K. The conductivities of both hypostoichiometric specimens were independent of frequency, which means the conduction carrier was the electron or electron hole. From the oxygen concentration of the metals which were used for the reduction, the oxygen pressures equilibrated with the yttria specimens were estimated to discuss the relation to the measured conductivities.
