CARATTERIZZAZIONE ELETTRICA E SPETTROSCOPICA AD IMPEDENZA DI OSSIDI METALLICI NANOSTRUTTURATI PER IL RILEVAMENTO DI ALCANI VOLATILI

Chemoresistive gas sensors using semiconductor metal-oxides have proven to be excellent in important characteristics such as sensitivity, long-term stability, robustness and price. Solid solutions of mixed Sn and Ti oxides have shown promising results in combining the properties of the separate components. Basic science of these mixed oxides is far from being satisfactorily elucidated. Further progress in basic understanding is required to establish the principles to operate them as gas sensors. Knowledge of the space charge width of the nanostructured powder grains is needed for the comparison of the mean grain radius to determine sensing properties of the nanostructure. A new method has been addressed using impedance spectroscopy technique to estimate the electric permittivity of these Sn and Ti oxides, which is fundamental for the determination of the space charge width and capacitive response. A study of the sensing properties of chemoresistive metal-oxides vs. light alkanes has been undertaken under dry and wet conditions and even in presence of ethanol. Screen-printed films of pure Sn and Ti oxides and solid solutions of mixed Sn and Ti oxides have been selected for the purpose. It has been demonstrated that the films are capable of detecting 500ppm of methane or 100ppm of other light alkanes under either dry or wet condition, i.e. concentrations two levels by far lower than the alarm thresholds for such gases. Information about the working mechanism of chemical reactions on the surface has been discussed under either dry or wet condition. Ethanol is known to be a harmful interfering gas, though its concentration can be reduced to values lower than 10ppm by proper activated carbon filtering. It has been shown that, even in presence of 10ppm of ethanol, the films steadily responded to alkanes.