Gas-Sensing Performance And Mechanism Study Based On Oxide Fibers And Zeolite-Y For VOC Detection

Gas sensors have become an essential device in the modern world. Applications range from gas detection in outspace to indoor air control, and from industrial processes monitor to medical diagnosis. In many respects, these gas sensors are making the goal of self-directed, intelligent processing become reality. The targeted gases in this thesis are volatile organic compounds (VOC), including ethanol, formaldehyde and ammonia, which are major pollution in the air and by-products during the industry processes. Advancing the gas sensor technology is explore new structure and new material for gas sensing.In the study of sensor based on titania-doped chromium oxide fibers (CTO), the fibers were charactered for different annealing temperature. The results indicate that the diameter of crystals in oxide fibers increase with the annealing temperature until destroy the structure of fibers. The sensing behavior of CTO sensor upon exposure to ethanol was characterized in this work. For the selective detection, the isopropanol and humidity is the major interference for CTO sensors. The signal parameters of the sensors and the fitting data of the sensor response were calculated based on response equations.Furthermore, undoped and NiO-doped SnO2nanofibers (NSO) nanofibers synthesized via a simple electrospinning method were investigated for formaldehyde sensing in this thesis. It is noticed that the addition of NiO causes the distortion at the surface of SnO2nanfibers, which is responsible to adjust activation energy, grain sizes and chemical states of host material. The sensors fabricated from NSO nanofibers exhibited good formaldehyde sensing properties at operating temperature200℃, and the minimum-detection-limit was down to0.08ppm. The response time and recovery time of the sensors were about50s and80s to10ppm formaldehyde, respectively. The sensor shows a good long-term stability in90days. The sensing mechanisms of NSO nanfibers to formaldehyde were discussed.For ammonia detection, this thesis examines the interaction of ammonia with Ag-Y. Impedance spectroscopy indicates that the Ag+motion within the zeolite is facilitated in the presence of ammonia. Infrared spectroscopy and X-ray photoelectron spectroscopy indicate that Ag+-NH3bonds are being formed. Temperature programmed desorption coupled with impedance spectroscopy show that the NH3bonding to Ag+is disrupted around300℃. However, in the presence of excess NH3, a fraction of the Ag+-NH3bond survives up to450℃. No cross-sensitivity was observed for O2, CO, CO2and propane. Nitric oxide showed minor interference. Water, however, did show an interference with a decrease in baseline impedance with increasing water.Influence of particle size on ionic conductivity of ceramic materials is an active area of research. In this work, we present systematic results for the first time of ionic conductivity in alkali-metal ion-exchanged faujasitic zeolites with morphologies ranging from a zeolite membrane, micron-sized, sub-micron and nano particles of zeolite. Using impedance spectroscopy, we have obtained activation energy (Eact) of cation motion with these various morphologies. Overall, the Eact decreases with Si/Al ratio. Surface modification of the zeolite particles was carried out with a silylating agent, which upon high temperature calcination should lead to formation of a monolayer Si-O-Si film on the particle surface. This surface modification had minimal influence on the Eact of micron-sized zeolites. However, Eact increased rapidly as the zeolite particle approached the nanoscale. These observations led us to propose that cation hopping across grain boundaries is relevant to ion transport, especially as the size of the crystallite approaches the nanoscale.