The Study Of The Gas Sensor Based On Nano-zeolite Film

The sensitive characteristics of nano-sized zeolite have been systematically studied in this thesis.The quartz crystal microbalance(QCM) gas sensor and impedance sensor based on zeolite film have been researched to detect the nerve agent sarin stimulant dimethylmethylphosphonate(DMMP).One side,the influence of physical and chemical characters of zeolite to its sensitivity have been studied,the absorption and desorption characteristics have been optimized;the other side,this research provides a new approach to the quick detection of DMMP gas.In the first part of this thesis,three different kinds of nano-sized zeolites have been synthesized:Silicalite-1,ZSM-5 and Cu-ZSM-5.The Silicalite-1 and ZSM-5 zeolites are prepared by hydrothermal method.Cu-ZSM-5 zeolite is prepared by ion-exchange method.The surface morphology of zeolite films are studied using SEM.The copper content of Cu-ZSM-5 is studied using Energy Dispersive X-ray(EDX).Five Cu-ZSM-5 zeolites with different copper content have been synthesized.The growth of zeolite film on the Au substrate is using sell-assembled method.In the second part,the QCM gas sensor based on zeolite film has been developed. The sensitivity,selectivity and repeatability of the sensor have been tested.The comparison among the sensitive characters of Silicalite-1,ZSM-5 and Cu-ZSM-5 zeolites have been present.According to the affinity between Cu²⁺ and organophosphonate,a new method that modifying zeolite with Cu has been present in this thesis.The results show that this method increases the sensitivity to DMMP effectively:For Silicalite-1,the minimum detectable concentration is 3ppm and the sensitivity is 1Hz/ppm;For ZSM-5,the minimum detectable concentration is 1ppm and the sensitivity is 2.1Hz/ppm;For Cu-ZSM-5,the minimum detectable concentration is 0.3ppm and the sensitivity is 8.8Hz/ppm.When the copper content in Cu-ZSM-5 zeolite is more than 2%,the absorption reaches a saturation status.As a result of the polar absorption and chemical absorption,Cu-ZSM-5 zeolite has much better selectivity than Silicalite-1 zeolite.In order to get good repeatability of the sensor,the desorption characteristics of the zeolites have been studied.A novel desorption method for strong polar molecule DMMP using high frequency alternate electric fields is proposed.The traditional desorption methods have some disadvantages:the desorption ratio of nitrogen purging method is too low,and the desorption time of high temperature treatment is too long. The result shows that the desorption time decreases from 20mins to 60s and the ratio of desorption up to 85%by using the method with the high frequency alternate electric field against traditional desorption methods.In the end of this thesis,an impedance gas sensor based on zeolite film has been developed.Silicalite-1 and Cu-ZSM-5 zeolites are assembled on the micro-electrode. In order to solve the problem that the resistivity of Silicalite-1 is too high,Cu-ZSM-5 is used as the sensitive material to test the impedance curve.The result shows that the impedance of Cu-ZSM-5 film is only 1/10 smaller than Silicalite-1 film,because the existence of Cu²⁺ provides high ion activity and the Cu-ZSM-5 has larger capacity. The changes of the electric parameters of zeolites are studied using equivalent circuit, and the simulation result is closely with the experiment result.