In order to make clear the mechanism of the variation in the conductivity of La
2CuO
4 with a small amount of NO
x (x=1, 2) in oxygen atmosphere, measurements were made on the conductivity of La
2CuO
4 in the stream of NO
x-0
2-N
2 gas mixtures at temperatures 773-1023K and the outlet gas was analyzed by a zirconia sensor. The conductivity and the EMF of the zirconia sensor were analyzed in relation to P(NO
x). P(O
2) and T. When NO
2 was applied to La
2CuO
4, catalytic reaction of NO
2→NO+(1/2)O
2 was found to take place. In the catalytic reaction process, excess oxygen was considered to be incorporated into La
2CuO
4. Under the steady state. the oxygen potential in La
2CuO
4 was kept much higher than that in the gas phase. resulting in the increase in the p-type conduction of La
2CuO
4. Numerical calculations were made for the oxygen potential enhanced in La
2CuO
4 as a function of P(NO
2) and T to make clear the detection limits of La
2CuO
4-type NO
2 sensors in relation to the gas phase P(O
2) and T. Under the condition studied, NO affects very little to reduce the conductivity of La
2CuO
4. The reason was discussed based on the catalytic reaction kinetics to show the direction to enhance the NO effect in La
2CuO
4.
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