Sensors and Actuators B, 13- 14 (1993) 495-496 Oxygen sensing properties of T&doped N&O, 495 Michitaka Ohtaki, Jun Peng*, Koichi Eguchi and Hiromichi Arai Lkpartnvnt of MaterialsScience and Technology, Gmduate School of Engineering Sciences, Kvushu Univekty, 6-I ffisuga-keen, Kamga, Fukzokn 816 (Jopan) Introduction As an answer to the strict requirements on automobile exhaust pollution, three major pollutants in exhaust gases, carbon monoxide, nitrogen oxides, and hydro- carbons, are removed in the exhaust system by catalyst converters using noble metals. High conversion efficiency of the catalyst can be attained only when the air-to- fuel ratio (A/F) of the inlet mixture is kept in a very narrow range near the stoichiometric ratio. Concen- trationcell-type oxygen sensors fabricated with stabilized zirconia are widely used as a feedback element for controlling the engine at the stoichiometric A/F point. Nevertheless, semiconductor-type oxygen sensors uti- liig the conductivity change against oxygen partial pressure are promising owing to their simplicity in the structure, the low cost for fabrication, and the lower operation temperatures. Although there are still some problems for practical application of the semiconductor- type oxygen sensors, there have been several proposals for semiconductor-oxygen sensing materials. Among them, TiOz has been under the most intensive inves- tigations [l-3]. However, Kondo et cl. [4] have reported that Nb205 exhibits the better sensing properties than TiO, as an A/F sensing material. Nb205 has some advantages such as its high sensitivity even without a Pt catalyst on the surface, and the smaller temperature dependence on the electrical conductivity. In the present study, we investigate the oxygen sensitivity of %O, doped with TiOz attaining the maximum sensitivity at the low doping level of Ti. 1300 “C for 10 h in air. The mixture was then pulverized and uniaxially pressed into a pellet at 14 MPa. The pellet was sintered at 1300 “C for 5 h in air, and then cooled by turning the furnace off. Powder X-ray dif- fraction (XRD) was observed using Cu Ku irradiation. The electrical conductivity was measured with a standard d.c. 4-probe arrangement using Pt electrodes on the sample cut into a rectangular bar. Dependence of the conductivity on oxygen partial pressure (PoJ was mea- sured in Nz/Oz or CO/CO, gas mixtures. Sensing prop- erties to air excess ratio, A, were investigated by the conductivity measurement in the exhaust gas of com- busted GHs at various A values. Results and discussion Dependence of the electrical conductivity of oxygen deficient metal oxides on zyxwvutsrqponmlkjihgfedcbaZYXW PO, can be generally described as: go (PoYm (1) where the m value is usually in between -4 and - 6 for n-type semiconductor oxides. The absolute values of l/m, as a measure of the sensitivity to oxygen, are plotted in Fig. 1 as a function of the composition of the binary oxides, showing two maxima at 5 and 80 mol% of TiOz. Moreover, the sensitivity is higher than that of each single oxide in the region where the amount of TiO, is considerably small. A phase diagram of the Experimental 0.3 1 1 TiO,-Nb,O, binary semiconductors were prepared at nominal compositions of (TiO&(NbO,,),_, with .r= 0 to 1 from the corresponding single oxides com- mercially purchased. Properly weighed oxide powders were mixed using a ball mill for 24 h and calcined at TiOz mol% in TiOz-NbzOs *Present address: Microelectronics Institute, Xidian University, Wan 71UO71, China. Fig. 1. Variation in l/lml values at 850 ‘C as a function of the composition of TiO,mO, binary oxides. 09254005/93/$6.00 Q 1993-Elsevier Sequoia. All rights resewed