Mechanism of a “Schottky-barrier-limited” Bi 2Sr 2CaCu 2O 8+x-based sensor for CO and NO

Abstract

A Taguchi-type sensor with a porous polycrystalline Bi 2Sr 2CaCu 2O 8+x (BSCCO) film as the active element has been developed. The electrical conductance of the film is “Schottky-barrier limited”. Its resistivity has beeen measured as a function of temperature in different CO/air and NO x/air gas mixtures. The power law that is valid for most Taguchi-type sensors is not suitable for fitting the experimental data. The linear relation between lnln S+ln S (where S is the sensitivity of the sensor) and ln P (where P is the partial pressure of the gas in air) is interpreted using a 'Schottky-barrier-limited' model. In this model, the lattice oxygen in the surface layer of this material participates in the sensing reaction. The adsorption of reducing gases decreases the concentration of elecrtron holes in the surface layer. The increase in resistivity is attributed to the rise in height of the Schottky barrier between the primary particles resulting from the lower concentration of the charge carriers. The sensing reaction for CO is proposed to be CO(g)+O xO+2h·+ad⇌(CO 2ad+Vö and that for NO NO(g)+O x o+2h·+ad⇌(NO 2) ad+Vö The model fits the experimental data for CO sensing, and satisfactory fits the data for NO sensing at high temperatures. The sensitivity to NO 2 is ascribed to the NO formed by its decomposition.