SELECTIVE DETECTION OF CO AND CH4 WITH GAS SENSORS USING SNO2 DOPED WITH PALLADIUM

SnO2 is Pd doped by using two different methods. In the first method, SnO2 and PdCl2 powders are simply mixed in a mixer, then cold pressed and sintered at high temperature. In the second one, a fixation method is used: a PdCl42- complex is chemically fixed on the surface of the SnO2 powder, the fixed species subsequently being reduced to metallic Pd; powder is then cold pressed and sintered at 650 degrees C. Pd disp ersion is lower in the case of the mixing method, but electrical prope rties are about the same for the two kinds of sensors realized by Core ci Company. On the one hand, CH4 and aliphatic hydrocarbons are select ively detected at high temperature (400-450 degrees C). On the other h and, CO detection is possible at low temperature (50 degrees C, for ex ample). Nevertheless, response time is long and can be improved by two different working modes: continuous pulsed temperature plus cleaning pulse, or isothermal measurement plus cleaning pulse. By doing this, t he CO sensitivity is greatly increased, and the humidity variations ar e easy to compensate. However, the use of a carbon-based filter is nec essary in order to avoid the presence of NOx which is a great interfer ent. These two kinds of sensors can be used either as a domestic alarm in order to control CH4 or CO leaks, or as a control sensor in order to monitor car pollution.