Influence of binders on the sensing and electrical characteristics of WO3-based gas sensors

The effects of various binders added in the solidification process, such as polyvinyl alcohol (PVA), silica sol and Al2O3, on the sensing and electrical characteristics of WO3-based n-type semiconductor gas sensors are investigated. Grain sizes show a slight variation according to the employed binders. In the case of WO3 films fabricated with silica sol or Al2O3 binder, some residue of binders at grain boundary and agglomerates of WO3 grains are observed. The electron concentration of WO3 film around room temperature and the temperature dependence of film resistance in normal air do not show any systematic dependence on the employed binders. In NOx gas, however, the optimum operation temperature and the sensitivity of WO3 sensors at that temperature are observed to depend greatly on the employed binders. The resistance of the WO3 films shows an exponential temperature dependence in NOx gas in the temperature range of 110-375 degrees C, and the increase of film resistance in NOx gas is observed to depend greatly on the binders added in WO3 films. Sensitivity to various ambient gases does not show any systematic variation. All these results mean that the binders affect the sensing characteristics of WO3-based gas sensors mainly through the modification of chemical conditions at grain boundary rather than the modification of grain size and electron concentration. (C) 1999 Elsevier Science S.A. All rights reserved.