Electrical and gas sensing properties of binder-free bead of SnO/sub 2/ powder

An aqueous paste of commercial powder of SnO/sub 2/, containing no binding additives, was manually molded into a spherical bead of -3 mm diameter and was allowed to dry in air at the tips of two parallel copper wires, inserted there in the bead while in the wet state. The bead was positioned in a self-fabricated thermostatically controlled glass reactor and subjected to heat treatment in the thermal cycle, performed in the temperature range 150 -350 degree C in the ambient of nitrogen, static air, and dynamic air either clean or loaded with varying amounts of the automobile exhaust gases. It was noted that thermal cycle significantly affected the shape of the electrical resistance vs temperature trace of the bead, irrespective of the nature of the reactor ambient. The observed results were ascribed to the dependence of the bulk and surface properties of the bead material to the given treatment and ambient composition in a sensitive manner. Moreover, it was found that the presence of automobile exhaust gases caused a reduction in the resistance of the bead matelial at the given temperature. The latter facts were explained on the basis of the interaction of the reducing components of the automobile gases with the oxygen molecules, present on the surface of the bead in the adsorbed state. (author)