ELECTRONIC CONDUCTIVITY OF NA BETA''-ALUMINA CERAMICS AT HIGH-TEMPERATURES

The electronic conductivity of Na beta''-alumina ceramics is important for the application of this material in different energy storage and conversion devices, It is shown that at AMTEC open circuit conditions an ionic sodium transport through the Na beta''-alumina membrane and t he formation of metallic sodium in the condenser compartment takes pla ce. This phenomenon is related to a residual electronic conductivity o f the solid electrolyte under these experimental conditions. Electroni c conductivity measurements on Mg-stabilized Na beta''-alumina ceramic s at relatively high temperatures (873 K less-than-or-equal-to T less- than-or-equal-to 1073 K) were performed using the Hebb-Wagner measurem ent technique in an AMTEC device under inverse polarization conditions . Electronic conductivity of about sigma(e) = 3.4 x 10(-4) OMEGA-1 cm- 1 was found at T = 1073 K. The determined activation energy of the tra nsport of electrons is E(a) = 1.14 eV and corresponds well to the valu e obtained previously for Li-stabilized Na beta''-alumina ceramics. Ex periments in systems with two ionically blocking molybdenum electrodes yielded lower sigma(e)(T)-values. This difference is associated with an increased electronic conductivity due to a chemical interaction bet ween the Mg-stabilized Na beta''-alumina ceramics and molten sodium or sodium vapour at elevated temperatures in the AMTEC cell.