| Since 1990's, the nano-materials were develop vary quickly. The nano-materials presented a series of unique electric, optic, mechanic, magnetic and catalytic capabilities, due to its special size distribution, surface effect, small size effect, macroscopic quantum tunnel effect. The rise of the nano science and technology create one new 'world' for the sensor research. The optical waveguide (OWG) sensor has extensive application in the area of environmental monitoring, industrial process control, chemical, biochemical detection, particularly, for the detection of gaseous elements in trace concentrations, due to its several merits over other types of sensors, such as:intrinsically safe detection, anti-disturbance of electromagnetism, system compactness, and high sensitivity, rapid response, easy realization for on-line continuous monitoring, good insulation, low cost etc.In this paper, combine with unique properties of nano-materials and superiority of OWG, the LiFePO4 and its doped componds were choose as sensitive materials, and studied the sensing response of these sensitive materials to Volatile Organic Compounds such as xylene and formaldehyde. This dissertation consist of six parts as followings:The first chapter is part of introduction. In this part, the characteristics and applications of nano-materials, the relationship between nano-materials and sensors, the sensor technology, the gas sensors and its classification were introduced. The Optical Waveguide gas sensor was mainly introduced. At last, sources and contents of this study work were put forward.In the second chapter, mainly describe the theory of OWG. The theories about total reflection of light, structure of planar OWG, guide mode of planar waveguide, the OWG's sensing principles (principle of evanescent wave), coupling of planar OWG and glass OWG (K+ ion exchanged OWG and Tin-diffused OWG) fabrication techniques were described in detail. The chapter three is mainly discuss about the study on the gas sensing properties of the LiFePO4 Film/Tin-diffused Glass Composite Optical Waveguide Sensor. In this study, high-pure nano-LiFePO4 powder was prepared using the hydrothermal method, and was used as a sensing material. The LiFePO4Thin films were fabricated by dip-coating methods and studied the effect of heat treating temperature on refractive index, thickness and transmittance of thin films. Meantime the best conditions to fabricate the sensing element were selected by enormous experimentations. A Glass Optical Waveguide (OWG) gas sensor was fabricated by coated a thin film of LiFePO4 on the surface of single mode Tin-diffused glass OWG This Composite OWG sensor was used to detect Volatile Organic Compounds gas such as xylene, chlorobenzene and toluene. The sensing mechanism was explained by the influence that changes of optical characteristic of the sensitive film to the height of evanescence wave. The sensor exhibit a remarkable response to 5×10-5 (50ppm) of xylene with response times less than 5s. At the low concentration (below 1×10-4, 100ppm), the sensor only has response to xylene gas.The fourth chapter is about research of gas sensing properties of the LiFe0.99Ag0.01PO4 Film/Tin-diffused Glass Optical Waveguide Sensor. The Ag doped LiFePO4 nano-powder was synthesized using the hydrothermal method, and was used as a sensing material. The LiFe0.995Ag0.005PO4Film/Tin-diffused Glass Optical Waveguide, LiFe0.99Ag0.01PO4 Film/Tin-diffused Glass Optical Waveguide, LiFeo.98Ago.o2P04 Film/Tin-diffused Glass Optical Waveguide Sensing elements was fabricated by coated a thin film of LiFeo.995Ago.oo5PO4,LiFe0.99Ag0.01PO4, LiFe0.98Ag0.02PO4 on the surface of single mode Tin-diffused glass OWG, respectively. Among them, the LiFe0.99Ag0.01PO4 Film/Tin-diffused Glass Optical Waveguide sensor exhibited good, repetitive response to formaldehyde gas, and its linear response range was10-3～10-10, the sensor had high sensitivity (it can detect10-10 of formaldehyde gas), short response time(less than 2s). When the concentration of formaldehyde gas was below 10-5(10ppm), besides formaldehyde gas, the sensor exhibited no response to other substances. In order to check the response of pure solution (PVA+VC), the Vc- PVA composite film/Tin-diffused glass OWG were fabricated by same method, and testing various VOCs using this OWG sensor. The testing results indicated that, when the concentrations of VOCs were 10-4, it exhibited no response to formaldehyde gas. From this, we can ensure that the pure solution (PVA+VC) has no influence on formaldehyde gas detection.The fifth chapter is about research of LiFe0.99Y0.01PO4 Film/Tin-Diffused Glass optical waveguide sensor. In this part, Yttrium doped LiFePO4 powder was synthesized using the hydrothermal method, and was used as a sensing material. LiFe1-XYXPO4 thin film was coated onto the surface of Tin-diffused glass OWG by spin-coater. The LiFe0.99Y0.01PO4 Film/Tin-diffused Glass Optical Waveguide, LiFe0.98Y0.02PO4 Film/Tin-diffused Glass optical waveguide sensing elements were fabricated, and were used to detect Volatile Organic Compounds gas. In result, LiFe0.99Y0.01PO4 Film/Tin-diffused Glass Optical Waveguide sensor exhibits general sensitivity and a reversible response to xylene gas, when the concentration of other VOCs were10-7(0.1ppm), the sensor has no response to xylene vapor.In chapter six summarizes the main contents of our study. |
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