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 the formation of metallic sodium in the condenser compartment takes place. This phenomenon is related to a residual electronic conductivity of the solid electrolyte under these experimental conditions. Electronic conductivity measurements on Mg-stabilized Na beta''-alumina ceramics 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 measurement 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 transport of electrons is E(a) = 1.14 eV and corresponds well to the value obtained previously for Li-stabilized Na beta''-alumina ceramics. Experiments 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 between the Mg-stabilized Na beta''-alumina ceramics and molten sodium or sodium vapour at elevated temperatures in the AMTEC cell.