Journal of Electroceramics, 6, 209–217, 2001 C  2001 Kluwer Academic Publishers. Manufactured in The Netherlands. NO x Sensing Properties of Varistor-Type Gas Sensors Consisting of Micro p-n Junctions YASUHIRO SHIMIZU, ∗ NOBUKUNI NAKASHIMA, TAKEO HYODO & MAKOTO EGASHIRA Department of Materials Science and Engineering, Faculty of Engineering, Nagasaki University, 1-14 Bunkyo-machi, Nagasaki 852-8521, Japan Submitted May 23, 2000; Revised February 27, 2001; Accepted March 7, 2001 Abstract. NO x sensing properties of SnO 2 -x Cr 2 O 3 as a varistor-type gas sensor have been investigated in the temperature range of 200–600 ◦ C. The breakdown voltage of SnO 2 shifted to a higher electric field upon exposure to NO 2 at 300–500 ◦ C, and the largest breakdown voltage shift, i.e. the highest NO 2 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 NO 2 sensitivity of SnO 2 was superior to the NO sensitivity at every temperature, and then the SnO 2 exhibited good selectivity to NO 2 at 400 ◦ C. The breakdown voltage of Cr 2 O 3 shifted in the reverse direction upon exposure to NO and NO 2 , in comparison with those observed with SnO 2 , owing to its p-type semiconductivity. Thus, Cr 2 O 3 also exhibited certain sensitivity to both NO and NO 2 at 200 ◦ C, being more sensitive to NO 2 , though the sensitivities decreased drastically at temperatures higher than 300 ◦ C. The addition of 5.0 wt% Cr 2 O 3 to SnO 2 resulted in a significant improvement of NO and NO 2 sensitivities at 600 ◦ C, being accompanied by an increase in the breakdown voltage in air. Especially, the NO sensitivity was superior to the NO 2 sensitivity in the concentration range of 20–100 ppm, and then SnO 2 mixed with 5.0 wt% Cr 2 O 3 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 Cr 2 O 3 addition was confirmed to arise from both the decrease in the particle size of SnO 2 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 NO 2 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. Keywords: NO x gas sensor, varistor, p-n junctions, nonlinearity, breakdown voltage Introduction Nitrogen oxides, NO x (mainly NO and NO 2 ), are produced in combustion furnaces and automobile en- gines, and are typical air pollutants causing acid rain and photochemical smog. For effective control of both the combustion conditions and the NO x -eliminating systems, there have been increasing demands for devel- oping high performance, low-cost and solid-state NO x sensors. Several kinds of materials, e.g. semiconductor metal oxides and solid electrolytes, have so far been tested as a NO x sensor. NO 2 always behaves as an oxidizing gas, ∗ To whom all correspondence should be addressed. and therefore its chemisorption leads to an increase in resistance of n-type semiconductive metal oxides used as a sensor material. On the other hand, NO chemisorbs as NO - (ad) or NO + (ad) depending on both the kind of semiconductive metal oxides and the temperature [1]. Under the conditions where NO and NO 2 show the opposite electronic nature upon adsorption, the sen- sitivity to a mixture of NO and NO 2 , as in the case of the exhaust gases, becomes low or complicated, in comparison with the sensitivities to individual gases. In this case, the addition of catalysts to promote the oxidation of NO to NO 2 is a possible way to enhance the sensitivity to total NO x . Even if both NO and NO 2 behave as an oxidizing gas under certain conditions, most of semiconductive metal oxides tend to exhibit