UNIDEF   23986
UNIDAD DE INVESTIGACION Y DESARROLLO ESTRATEGICO PARA LA DEFENSA
Unidad Ejecutora - UE
capítulos de libros
Título:
Resistive Thick and Thin Film Gas Sensors Built with Nanomaterials and Related Research
Autor/es:
M. P. POIASINA; N. E. WALSÖE DE RECA; CRISTIAN L. ARRIETA; M. F. BIANCHETTI
Libro:
Advances in Sensors VI
Editorial:
International Frequency Sensor Association (IFSA) Publishing, S. L.
Referencias:
Año: 2018; p. 7 - 581
Resumen:
Pure or doped SnO2 semiconductor has been used for many years to build resistive type thick film gas sensors and work performed at DEINSO-CITEDEF [1-7] is going to be described. When the size of the oxide particles decreases to the nanometric scale, important changes are observed in gas sensors properties depending on the oxide crystallite size. An increase of sensitivity (30 %-37 %) is observed in sensors built with nanocrystalline material in comparison with the sensitivity of devices built with the same material but microcrystalline. The operation temperature (To) decreases from (350-450) ºC to a range (180-200) ºC [8-10]. In sensors previously built by the authors, the microcrystalline semiconductor has been deposited on one face of an AlSiMg substrate, depositing on the opposite face, a heating circuit to reach the high operation temperature (To). Afterwards, the nanosemiconductor was deposited as a MEMS type microheater, which functioning is based on an electronic circuit that implements a switching logic. This circuit enables to measure the variation of film surface resistivity which is related to the analyte concentration and, conveniently heated, it enables to save energy. The thick film resistive sensors developed at DEINSO-CITEDEF were built with nanocrystalline pure SnO2 and with Al-doped and In-doped nanocrystalline SnO2 contributing the doping to increase the sensor selectivity to different gases. Sensor built with pure SnO2 was sensitive to H2 (g) while those sensors built with Al-doped SnO2 and In-doped SnO2 resulted sensitive to CO (g) and to VOCs (Volatile Organic Compounds, particularly, ethanol) [8-10] respectively. Pure and doped nanocrystalline SnO2 semiconductor has been also used to build high sensitivity thin film gas sensors: to detect H2 ppm in air using pure SnO2 and to detect SH2 ppm in air with CuO doped SnO2 respectively. Devices are fabricated with multilayered thin film systems. At first, evaporation and coating thin film techniques were used to build both sensors and their results were compared. Stresses generated by the films deformation were carefully studied. The thin films due to their thickness, to their deposit in multiple layers and by the thermal treatments effect produce stresses causing defects like grain boundaries, interfaces, dislocations, vacancies clusters, among others, able to accelerate the diffusional processes of gases in the sensitive material and improving the sensor performance. Stresses were characterized by XRD. The same XRD technique was used to determine the crystallite size (Scherrer equation). Thin films were also characterized by BET isotherms adsorption technique to determine the specific area, by HRTEM to study the microstructure and by SEM to measure the films thickness and the relief of surfaces. With regards to the electronic control of sensors, a double meander electronic circuit (built by MEMS) (already patented by the authors) was adapted to the thin films sensors improving it by a modified controlled, modular and portable circuit, being able to program the working or operation temperature, the sensing operation modes, the heating and the commutation time between them.Aspects and results of several previous works as performed at DEINSO-CITEDEF have been considered in this review chapter [11-13]:?The building of resistive thick films sensors enabled to detect H2 (g), CO (g) and VOCs (particularly, ethanol), analyzing the experimental procedures to synthesize and characterize the sensitive material, ?The building of resistive thin film sensors to detect H2 (g) and SH2 (g) in air, considering the experimental procedures to synthesize the sensitive thin films,?The provision to both types of sensors with a MEMS microheater and an electronic circuit with a switching logic enabling to measure the change of film resistivity (which is directly related to the analyte concentration) and to save energy when conveniently heated, ?The reviewing of the sensing mechanism in conventional resistive thick film sensitive material and the study of the stresses effect as generated in the thin films (mainly by thermal action) which increase the defects density. Stresses create defects which accelerate the gas diffusional processes in the sensitive materials causing, in consequence, a better sensor performance.