NOVEL ROOM-TEMPERATURE CARBON-MONOXIDE SENSOR UTILIZING RATE OF CHANGE OF RESISTANCE IN THICK-FILMS OF TIN OXIDE

A room temperature thick-film semiconductor gas sensor has recently be en developed in this laboratory for the measurement of carbon monoxide (CO) concentrations down to 10 ppm. Novel features of the fabrication process are the addition of a catalytic additive to enhance the sensi tivity to CO, and a binding agent which results in an open porous stru cture to increase the effective surface area. The resistance R0 of sen sors before exposure to CO varies in the range 100 kOMEGA-30 MOMEGA, a nd the rate of change of sensor resistance dR/dt after exposure to CO is large, attaining a maximum value after several minutes. It is shown that the average rate of change of sensor resistance dR/dtBAR is dire ctly proportional to R0 with proportionality constant 10(-3) s-1. Circ uitry is described, based on detection of the rate of change of sensor resistance, for a CO gas sensor capable of detecting small concentrat ions at the onset of a fire, before smoke particles are emitted.