Effect of defect structure on gas sensitivity of LaCrO 3
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Effect of defect structure on gas sensitivity of LaCrO3 Rong-Fong Huang and Wei-Yean Howng Ceramic Technology Research Laboratory, Motorola Inc., Albuquerque, New Mexico 87113 (Received 12 December 1995; accepted 10 June 1996)
The isopropyl alcohol gas sensitivity of LaCrO3 at 250 ±C is found to depend on the amount of TiO2 content and cation stoichiometric ratio of the sample. The gas sensitivity enhancement is related to the defect structure and electrical conduction behavior of p-type, donor-doped semiconductive oxides. The high resistivity coupled with the increasing point defects by the donor dopants are responsible for the high gas sensitivity of TiO2 doped LaCrO3 . It is believed that the positively charged ionic-type defects created by dopants act as trapping sites to adsorb oxygen.
I. INTRODUCTION
Many oxide materials have been known to exhibit gas sensitivity based on their electrical conductivity change. Oxides, such as ZrO2 , have been investigated and used successfully as oxygen sensors. With proper doping, the electrical conduction of these oxides is carried out by ionic-type carriers.1 The gas sensing of this type of sensor is achieved primarily by measuring the electromotive force (EMF) generated by the sensor due to a reference oxygen partial pressure at elevated temperatures. On the other hand, semiconductive oxides have been shown to exhibit electrical conductivity dependence on hydrocarbon or reducing gas concentration.2 Unlike the ionic-type gas sensors, the semiconductive-type sensors generally operate at lower temperatures (200–500 ±C). The electronic-type defect is the main charge carrier for the conduction. The gas sensing behavior of n-type2,3 and p-type4 oxides has been studied previously. In general, the electrical resistivity decreases for n-type and increases for ptype oxides as the concentration of hydrocarbon gases is increased. The reaction kinetics are related to the operating temperature and the catalytic effect of the oxides toward specific gas species.5,6 Based on this effect, many n-type binary oxides have been successfully used in making hydrocarbon gas sensors.6–8 Previous studies have shown that certain dopants are especially effective in increasing the gas sensitivity of particular oxides. Nakatani et al.7 found that the sensitivity of Fe2 O3 can be greatly enhanced by the addition of tetravalent ions such as Ti, Zr, or Sn. Matsuzawa et al.8 reported that the sensitivity of ZnO can be increased by 3 mol % addition of Cr2 O3 . Furthermore, it has also been reported that enhanced sensitivity of TiO2 9 and ZnCr2 O4 4 can be attributed to the addition of Cr2 O3 dopant. These studies indicate that the gas sensitivity of semiconductive oxides can be influenced by the dopants. But the mechanism on how dopants enhance the gas sensitivity has not been fully understood. It would therefore be of interest to determine the mechanism J. Mater. Res., Vol. 11, No. 12, Dec 1996
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