IN-SITU INFRARED AND ELECTRICAL CHARACTERIZATION OF TIN DIOXIDE GAS SENSORS IN NITROGEN OXYGEN MIXTURES AT TEMPERATURES UP TO 720-K

FT-IR spectroscopy and impedance measurements of tin dioxide sensor ma terials at working temperatures up to 450-degrees-C in atmospheres wit h varying O2/N2 ratio are used as an in situ probe to study the intera ctions at the surface of the semiconducting oxide. Every diminution in the oxygen content above the sample induces a broad IR absorption ban d (X-band) between 2300-700 cm-1 with a few small peaks in the 1400-85 0 cm-1 region of the spectrum superimposed on it. The X-band results f rom the enhanced electron concentration in the bulk of the tin dioxide domain. The fine structure is due to the absorption of several kinds of surface oxygen species associated vibration modes. The porous tin d ioxide consists of domains were the outward 'shell' is depleted of ele ctrons by the formation of adsorbed O- species on oxygen surface sites , S(O)(O-) species. In our proposed model for the impedance data this gives rise to a parallel R(p)C(p) circuit for the domain boundary char acteristics and to an R(s) parameter for the intradomain resistance. T he evolution of these IR and impedance spectroscopic effects with temp erature and oxygen content is used to set up, to confirm and refine a physicochemical operation model of tin dioxide gas sensor. This model consists of a sensitizing reaction sequence in the presence of oxygen and a gas-detection reaction sequence when a reducing gas is present. Based on this model, the principal disadvantages of this type of gas s ensor become clear. Every factor that influences the concentration of S(O)(O-) species, causes a conductance modification. If we can control and direct the nature, the number and the arrangement of the tin diox ide domains, a directed development and improvement of the sensor char acteristics is possible.