THICK-FILM GAS SENSORS

Usually a thick-film gas sensor means a metal-oxide semiconductor sensor obtained by thick-film screen-printing technology. The chemically sensitive layer consists of a paste prepared from metal-oxide powder, inorganic additives and organic binders. The paste is printed over an alumina substrate containing metal film electrodes and a back heating resistor; the paste is then sintered in a thermal or IR belt furnace. Since stannic oxide is the most frequently used material, we shall restrict our discussion to SnO2-based sensors, which are currently prepared and characterized in our laboratory. SnO2 is extremely sensitive to trace concentration of reactive gases in air, while it presents a lack of intrinsic selectivity; however, the selectivity can be improved by the addition of catalysts and promoters. The conductivity of a high-porosity n-type thick film changes in the presence of a reducing gas due to a combustion process that lowers the surface coverage of oxygen ions, causing the return of electrons to the conduction band and lowering the Schottky-barrier heights at the intergranular contacts. On both pure and Pd-doped samples measurements of conductance and capacitance have been made by varying the temperature at fixed gas concentration or by varying the concentration at fixed temperature. The conductance variations on pure and Pd-doped samples by switching periodically between dry and wet air are outlined. Experimental correlations between the energy barrier in gas and in air and the beta parameter, which relates the conductance G to the partial pressure P-R of the reducing agent (Delta G alpha P-R beta), are discussed too.