Preparation And Characteristic Research Of Gas Sensing Thin Films And Gas Sensor Array

In the 21^st IT era, sensor plays a important role in our life, and more and more attention has been paid on this research topic. In order to detect more components using small and compact sensor unit, sensor array is one of the best choices to be realized. As each sensor responds to a class of gases rather than to a specific one (which is also called as over-lapping sensitivity), gas sensor arrays can make use of such over-lapping sensitivity. To process such a partial overlapping response, pattern recognition techniques are generally employed. Based on this principle, an electronic nose has been developed, which is conventionally made up of gas sensor array, data collecting card and pattern recognition, and has been widely used in many areas such as food, aerospace, environment, etc.This dissertation mainly focused on gas sensing thin films and gas sensor array. Firstly, a few gas sensing thin films such as WO₃ thin films, TCNQ and SnPc thin films and their composite films with PMMA, and PANi/PSSA thin films were studied. Then, ChemFET gas sensor array was designed and investigated. Finally, testing algorithms were designed and simulated. The main conclusions and original results are summarized as follows.1. WO₃ thin films were deposited on glass ceramics substrates with a pair of interdigitated Au-film electrodes by vacuum thermal evaporation, and then annealed at temperature between 300-600℃. It shows that crystallite phase and mean grain size varied significantly with the change of annealing temperature. As the annealing temperature increased, the general crystallite size considerably increased, and WO3 amorphous thin films began to crystallize. And finally we got WO3 thin films with hexagonal structure, which grew preferentially on the direction <100>. Annealing conditions and working temperatures play an important role in its sensitive characteristics to NO₂ The optimum conditions are as follows: annealing temperature 600℃and working temperature 200℃.2. Gold nanoparticulate thin films were built up using the electrostatic self-assembly method. And self-assembled polyaniline (PANi) composite films with poly (styrenesulfonic acid) were fabricated based on doping-induced deposition effect above aurum. The gold colloid was a monodispersive suspension and had more uniform micro-particle with more sodium citrate or NaBH₄. The colloid synthesized using NaBH₄ with ethanol has the highest performance and has a micro-particle diameter of 1.7 nm. The self-assembled PAN films above the aurum were uniform and highly adhesive to the substrates. And there was a new infrared absorbance spectrum at 1683cm^-1. It was also found out that the sensitivity of PANi/PSSA thin films increased with the number of film layer.3. TCNQ and SnPc thin films were prepared by thermal vacuum evaporation. The effects of substrate temperature on the surface morphology and the conductance of the thin film have been investigated. The results show that TCNQ is sensitive to both NO₂ and NH₃, but the response and recovery rate are not fast. SnPc is sensitive to NO₂ but insensitive to NH₃.Then, TCNQ and SnPc films as well as their composite films with poly(metheyl methacrylate) (PMMA) were prepared by spin coating method. And we found that the crystalline grain of TCNQ in TCNQ/PMMA composite film became smaller than that of the pure TCNQ thin film. Results show that TCNQ film is highly sensitive to NH₃. The SnPc/PMMA composite film is also highly sensitive to NO₂ with fast response rate, but the recovery time becomes longer with the increase of gas concentration. The results show that TCNQ particles in TCNQ/PMMA composite film become smaller and disperse more uniform, but the response to NH₃ is unstable. TCNQ film is highly sensitive to NH₃ whereas the response and recovery time are much longer.4. Based on conventional MOSFET, a novel kind of chemical field-effect transistor (ChemFET) gas sensor array have been designed and fabricated. The sensor consists of self-assembled PAN composite films with polymeric acids deposited on the gate area of MOSFET replacing metallic material. The gas-sensing property and mechanism of the ChemFET array was investigated. And the effect of temperature and humidity on the gas sensors array was also studied. The results show that the output of the array also changes with the temperature and humidity, which suggests that one-bodied sensor detecting temperature, humidity, and gas concentrations based on this kind of sensor array can be got.5. BP algorithms in ANN used in linear and nolinear systems were designed to process gas sensor array signal. The simulation of gas sensor array has been made using computer simulation method, and BP artificial neural network has been designed by using MATLAB neural network toolbox to process and verify the simulation data. By combining BP algorithms with "early stopping" method, we found that the drilling time of this new method becomes shorter than common BP algorithm. The results also show that the algorithms designed in this work have good non-linear, approximation ability and high discrimination ratio.In summary, the work in this dissertation is helpful for the development of gas sensor array, and high performance device can be anticipated based on this study.