Applications Of Carbon Nanotube In Gas And Humidity Sensors

Carbon nanotube (CNT) is a new kind of one dimensional nanomaterials, which unique features in structure (small size, hollow, high surface area) make it show fantastic mechanical, electrical, and other properties. It is expected to use in field emission display device, hydrogen storage, super capacitor and many other fields. Especially, the abundant pore structures and high surface area make it one of the most promising gas and humidity sensing materials. In this thesis, applications of CNT in gas and humidity sensors are researched.Humidity sensors were fabricated by coating films of multi-walled carbon nanotube (MWNT) and hydroxyl-modified multi-walled carbon nanotube (MWNT-OH) onto ceramic substrates respectively. Sensitivity, hysteresis and response-recovery properties of the sensors were measured. The resistance of sensors increased with increasing relative humidity. MWNT-OH was more sensitive than MWNT. MWNT used in this thesis was p-type. After adsorbing H₂O, MWNT accepted electrons from H2O, carriers in MWNT decreased, and the resistance of sensor increased; the surface area of MWNT-OH was larger than that of MWNT, and it contained active groups to enhance the adsorption ability of MWNT to H₂O molecules.MWNT as doping agent was used to improve sensing properties of SnO₂ gas sensors. Indirect-heated SnO₂ gas sensors doped with different proportion of MWNT were fabricated and tested. Doped gas sensors were more sensitive than undoped ones, among which containing 5%MWNT gas sensor had the highest sensitivity and could detect 0.05ppm HCHO. Doping MWNT greatly increased effective donor density in SnO₂ film, and enhanced the adsorbing ability of SnO₂ to HCHO. Electrons in SnO₂ increased and the resistance of sensor decreased. Therefore, sensitivity of sensor increased. Doped both MWNT and Pd SnO₂ gas sensor was fabricated and its working temperature rose to 350℃. Its sensitivity was between that of doped MWNT and Pd sensor separately. Influences of ethanol, toluene, acetone, methanol, benzene and ammonia were measured, and ethanol was still the main interfering gas for formaldehyde detection.To further improve the sensitivity of gas sensor to low concentration of HCHO, enrichment by activated carbon was used. After that the sensitivity of gas sensor increased greatly and the gas sensor could detect 0.03ppm HCHO.