On the Control of Oxygen Potential in Gas Atmosphere using the Stabilized Zirconia Galvanic Cell as an Oxygen Pump

In order to control oxygen potential in gas atomosphere automatically, an apparatus was constructed utilizing an oxygen pump and oxygen sensor consist of stabilized zirconia with the porous platinum electrodes, and the performance of this apparatus was investigated.For the oxygen potential, the buffer capacity of Ar-O2 and CO-CO2 gas mixtures was evaluated and plotted as a function of oxygen partial pressure at 800°C. Because the CO-CO2 gas mixture was found to have a good buffer capacity over a wide range of oxygen potential, it was employed in the present experiment.The purified CO2, as a starting material, was filled and circulated in a closed system. Circulating gas was electrolyzed by the oxygen pump and CO2 was dissociated until the oxygen sensor showed the desired oxygen potential. A feedback control unit was used between the oxygen pump and the oxygen sensor.The oxygen potential of the circulating gas was measured as a function of temperature by an oxygen sensor at another position and it was found that the composition of circulating gas was kept constant by this control system. The control of the oxygen potential corresponding to the PO2 range 2.8×10−3∼1.5×10−20 atm was tested at 800°C using this apparatus. In the whole range, except PO2=1.15×10−6∼2.69×10−9 atm, the oxygen potential was controlled in a very stable manner. The range of the unstable oxygen partial pressure was in accord with that of minimum buffer capacity.The oxygen potential dependence of electrical resistance of a TiO2 specimen was measured employing this apparatus, and the result agreed with the other reliable data.