PERFORMANCE AND THERMODYNAMIC PROPERTIES OF NA-SN AND NA-PB MOLTEN ALLOY ELECTRODES FOR ALKALI-METAL THERMOELECTRIC CONVERTER (AMTEC)

An alkali metal thermoelectric converter (AMTEC) testing cell was set up, and run with molten sodium-tin (Na-Sn) and sodium-lead (Na-Pb) all oy cathodes. The Na activity, the partial molar enthalpy and partial m olar entropy of sodium in molten Na-Sn and Na-Pb alloys have been dete rmined, using a Na concentration cell: Na(1)\beta ''-alumina\Na-Me(1), where Me = Sn or Pb. The thermodynamic results of these investigation s agree with those of other authors. The electric performance of these Na-Me alloy electrodes of different Na concentration and temperatures is described, measuring current-voltage characteristics and a.c. impe dance in the AMTEC test cell. The power density of the AMTEC cell with molten alloy cathodes decreases with increasing Na concentration, wit h the Na concentrations in molten alloys varying from 0.5 to 15 mol %. Maximum power densities of 0.21 to 0.15 W cm(-2) at 700 degrees C for Na-Sn molten electrodes, and 0.30 to 0.15 W cm(-2) for Na-Pb molten e lectrodes have been obtained. The a.c. impedance data demonstrated tha t the molten alloy electrodes have a smaller cell resistance, 0.3-0.35 Omega cm(-2) at 700 degrees C after 10-20 h. Comparison with the sput tered thin, porous film electrodes, showed that the contact resistance between electrode and surface of beta ''-alumina plays an important r ole on enhancing cell power density. At 700 degrees C the power densit y of an AMTEC cell with the molten Na-Pb alloy electrode can be raised to values of about 0.2 W cm(-2) current densities of 0.8 A cm(-2), bu t at cell voltages not exceeding 0.2 V. A model for the theoretical ef ficiency of the AMTEC cell with molten Na metal electrodes is also pre sented.