NOx Sensing Properties of Varistor-Type Gas Sensors Consisting of Micro p-n Junctions

Abstract

NO _x sensing properties of SnO₂-xCr₂O₃ as a varistor-type gas sensor have been investigated in the temperature range of 200–600°C. The breakdown voltage of SnO₂ shifted to a higher electric field upon exposure to NO₂ at 300–500°C, and the largest breakdown voltage shift, i.e. the highest NO₂ sensitivity was observed at 400°C. In contrast, the direction of the breakdown voltage shift in NO varied with temperature: the breakdown voltage shifted to a lower electric field at 300°C, but to a higher electric field at 500°C, and remained almost unchanged at 400°C. The NO₂ sensitivity of SnO₂ was superior to the NO sensitivity at every temperature, and then the SnO₂ exhibited good selectivity to NO₂ at 400°C. The breakdown voltage of Cr₂O₃ shifted in the reverse direction upon exposure to NO and NO₂, in comparison with those observed with SnO₂, owing to its p-type semiconductivity. Thus, Cr₂O₃ also exhibited certain sensitivity to both NO and NO₂ at 200°C, being more sensitive to NO₂, though the sensitivities decreased drastically at temperatures higher than 300°C. The addition of 5.0 wt% Cr₂O₃ to SnO₂ resulted in a significant improvement of NO and NO₂ sensitivities at 600°C, being accompanied by an increase in the breakdown voltage in air. Especially, the NO sensitivity was superior to the NO₂ sensitivity in the concentration range of 20–100 ppm, and then SnO₂ mixed with 5.0 wt% Cr₂O₃ was found to be the most suitable candidate for a NO sensor among the sensors tested. The increase in the breakdown voltage in air induced by the Cr₂O₃ addition was confirmed to arise from both the decrease in the particle size of SnO₂ and the formation of micro p-n junctions at grain boundaries. The decrease in the particle size was also responsible for the increased NO and NO₂ sensitivities, but the p-n junctions were suggested to play a more important role in promoting and stabilizing the chemisorption of NO at higher temperatures.