| With improvement of living standards, people have paid more and more attention to the harmful effects that result from living environment, and they will have to spend most of their time working or studying indoor. Naturally, problem of the house air condition also arouses plenty of people's concerns. Formaldehyde is one of the main air pollutants indoor. The effective methods to monitor formaldehyde have been demanded for house atmospheric environmental real-time measurement and control, and the gas sensor is believed to be an ideal means to detect formaldehyde. At present, the research about formaldehyde gas sensor has already got some achievement. However, the problems of performance, structure, practicality and cost of sensor have not been solved. As a result, how to detect formaldehyde accurately is the urgent problem, and the study in paper is committed to develop an novel indoor formaldehyde gas sensor which have the properties of higher sensitivity, lower operating temperature, better selectivity, simple structure, easy measurement, small bulk, low energy consumption and low cost.In the thesis, sintering heater-type gas sensors of Ag-LaFeO3,CNTs modified Ag-LaFeO3,Zr modified Ag-LaFeO3and Zn modified Ag-LaFeO3were fabricated by sol-gel couple with microwave chemical synthesis process. The structure, composition, micromorphology and gas sensing properties of specimens were characterized by TG-DSC, XRD, FT-1R, TEM, XPS, nanoparticle size analyzer and gas sensor tester. And the impact of different preparation conditions on sensors performance was studied. The main results were summarized as following:(1) The structure of Ag-LaFeO3powders, which include Ag and LaFeO3phases, were orthorhombic perovskite. Ag-LaFeO3sensors prepared as the sintered temperature of800℃. AgNO3:La(NO3)3·6H2O(molar ratio) of1:99exhibited the best performance. And it showed the highest sensitivity(20.0) to lppm formaldehyde at the operating temperature of90℃. It also performed better linearity and selectivity.(2) CNTs modified Ag-LaFeO3sensors prepared as sintered temperature of800℃ AgNO3:La(NO3)3·6H2O(molar ratio) of1:99and CNTs:Ag-LaFeO3(mass ratio) of1%exhibited the best performance. It showed a higher sensitivity(12.72) to1ppm formaldehyde at the operating temperature of70℃. It also performed better linearity and selectivity.(3) The1%CNTs modified Ag-LaFeO3(1:99) sensor fabricated by using cetyl trimethyl ammonium bromide performed worse than that we expected before, especially in sensitivity and selectivity.(4) All of Zr,Zn modified Ag-LaFeO3(1:99) sensor performed worse than that we expected before. The sensitivity, optimum operating temperature, selectivity and linearity of sensors exhibited worse than Ag-LaFeO3(]:99) sensors.All of experiments in the thesis were based on the mechanism as following:(1) As Ag could reduce contact barrier, enhance surface effect and increase the adsorbed oxygen of surface. Therefore, the sensitivity and optimum operating temperature of matrix material would be enhanced by the means of Ag modification.(2) The matrix material would have the multi-gap structure by the means of CNTs modification. This structure could enhance gas adsorption. Thereby, the optimum operating temperature of sensor could be reduced.(3) As the previous conclusions of our group showed that Zr, Zn doped LaFeO3sensors exhibited higher sensitivity and better selectivity since the Zr, Zn properties of grain refinement and strong affinity of formaldehyde.The novelties of the work are mainly reflected in composition of sensing material, preparation method and sensor performance:Ag, Zr, Zn and CNTs modified LaFeO3formaldehyde gas sensors were fabricated by sol-gel couple with microwave chemical synthesis process. The optimal sensor which exhibited the best performance showed a higher sensitivity to1ppm formaldehyde than hierarchically assembled porous ZnO scnsor to300ppm formaldehyde about55%, a lower optimum operating temperature than SnO2-TiO2-Ag sensor about80%, and a better selectivity. |
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