A CONCEPT OF CHEMICAL POTENTIAL PUMPING EFFECT AND THE NOx SENSING MECHANISM OF PEROVSKITE-TYPE OXIDE MIXED CONDUCTORS

摘要

A concept of chemical potential pumping effect was proposed to describe the NOx sensing mechanism of catalytic oxides with high electronic conductivity or large carrier density. When NOx decomposed on the surface of the catalytic oxides continuously, the high oxygen potential released by the decomposition of NOx covered the whole surface of the oxide under a steady state. Then, the bulk oxide was equilibrated with the steady-state high oxygen potential of the surface, which was much larger than the oxygen pressure of the environment. This effect was confirmed on the conductivity enhancement of La_2CuO_4, La_(0.5)Sr_(0.5)FeO_3 and single crystal SrTiO_3 with NO_2. The enhancement of the nonstoichiometric oxygen content by NO_2 was also confirmed for La_(0.5)Sr_(0.5)CoO_3. A direct observation of the chemical potential pumping from NO_2 was made using the solid state electrochemical cell with dense La_(0.6)Sr_(0.4)CoO_3 film as the sensing material. It was shown that the chemical potential pumping effect appears essentially on dense materials irrespective of the sample dimension. On SrTiO_3, NO_2 was found to decompose into N_2 and O_2 while on the other oxides, it decomposed into NO and O_2. NO did not decompose on the oxides studied in this work.