| Most of the semiconductor gas sensors are made from MOS(Metal Oxide Semiconductors).When the gas adsorbed on the surface of MOS, the electronic property(such as conductivity, etc.) can be enhanced, which is widely used for the purpose of detecting harmful gases and what's critical is the detection technology of MOS for on-line monitoring. However, MOS also contain a lot of problems such as low sensitivity,large dispersion of data, short life, poor stability of long-term operation. Worse still, MOS are most if not all utilized in atmospheric environment in which oxygen plays a paramount role. So it is very important and significant to in-depth study on gas sensor of MOS. In this project, we have comprehensively investigated sensors based on MOS for detecting CO2 and H2 in oxygen-free atmosphere. SnO2 doped with La, Gd, Lu as well as SnO2, ZnO,WO3 loaded with noble materials like Pd, Pt were fabricated respectively for modification of the gas-sensing properties(such as sensitivity and stability) of sensing materials. The main research contents of this thesis are summarized as follows:(1) SnO2 thick films doped with atomic ratios ranging from 0 up to 4at.% La,4at.%Gd, 4at.%Lu were fabricated respectively via hydrothermal and impregnation methods. The crystal phase, morphology, and chemical composition of the SnO2-based thick films were characterized by XRD, FE-SEM, EDX, HRTEM and XPS. Results showed that our prepared samples were nanoscale spherical particles, with good dispersion and uniform particle size.(2) Sensing properties of La-SnO2, Gd-SnO2, Lu-SnO2 films, as well as the pure SnO2 film, were analyzed towards CO2 in the absence of O2. It was found that the optimal doping element was La and the best doping ratio was 4at.%. The maximum response appeared at an operating temperature of 250°C, on which condition the 4at.%La-SnO2 exhibited a remarkable improvement of response from 5.12 to 29.8 when increasing CO2 concentration from 50 to 500 ppm. Furthermore, the working mechanism underlying such enhancement in CO2-sensing functions by La additive in the absence of O2 was proposed and discussed.(3) Pd-SnO2, Pt-ZnO and Pt-WO3 sensor array probe working in the absence of O2 for on-line H2 monitoring was prepared. To intrinsically investigate their sensitization mechanisms, the changes in microstructure and surface valence of the as-prepared Pd-SnO2, Pt-ZnO and Pt-WO3 thick films were characterized by XRD, FE-SEM, HRTEM and XPS.(4) H2 sensing performances of sensor array probe working in the absence of O2 were systematically investigated. By using a brand new operating mode, high sensitivity, fast response and the long time stability without electron conductance drift towards1~10000ppm H2 were achieved. Thereafter a novel self-designed experimental platform was reported with an ability to screen materials of different gas sensing properties towards low and high concentrations of H2.In conclusion, our paper systemically introduces how to detect traces of CO2(50~500 ppm) and explores an effective strategy to solve the problem of instability towards1~10000 ppm H2 in oxygen-free atmosphere with the aid of modified MOS, which deeply enriches the sensing mechanism of MOS and establishes the foundation for developing high-quality, novel-performance sensing materials and gas sensors. |
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