Preparation And Research Of Micro-preconcentrator Based On Carbon Nanotubes

With the acceleration of the industrialization process,environmental pollution problems become more serious,thus the detection and monitoring of toxic and harmful gases are of growing importance.But they are difficult to be detected efficiently by the common gases detectors available in the market due to the low concentration and great variety of toxic and harmful gases.In order to tackle this problem,the concentrator device is employed as the front end of gas analysis system to improve the sensitivity and selectivity of the monitoring device,and the limitation of the micro-analysis system.However,existing concentrators are bulky,with slow response speed,high power consumption and high cost.MEMS technology can overcome the limitation and shortcoming of traditional concentrators,significantly reduce the size,power consumption,thermal energy,and can improve the sensitivity of the gas analysis system.In this thesis,based on the existing concentrator structure,two different types of micro-preconcentrator were designed: three-dimensional grille diverter type concentrator and two-dimensional 4×4 diaphragm type concentrator.The airflow velocity and the pressure field of the three-dimensional grid structure were theoretically simulated,and the structural mechanism simulations of the two-dimensional diaphragm structure were also performed.In order to heating the substrate evenly,the present technology conventionally arranges the heating wire on the predetermined area evenly.This arrangement doesn’t fully consider the boundary condition of the heat transfer,since the heat dissipation is slow in the central part of the substrate and fast on the edges,which causes the temperature higher in the center than on the edges.In this work,the heating electrode structures were fully optimized by thermal simulations.By adjusting the line width and the adjacent spacing of the heating line,the center power of the heater is reduced and the edge power is increased,so that the uniformity of the temperature distribution on the substrate is markedly improved.Moreover,we prepared the designed micro-concentrator device by MEMS technology.The coating of the adsorbent was accomplished by air-jet process.The device was encapsulated by HS-1840 sealant.The booster pulse was heated by a trigger and a relay for 0.6s.Then,the concentrator and the heating circuit board were assembled for further device performance tests.The three-dimensional grating shunt and two-dimensional 4×4 membrane microconcentrators were tested.We add carbon nanotubes and Tenax-TA as filler to the same concentrator structure,respectively,and their adsorption performances measurements were conducted in parallel.Results indicate that the adsorption performance of carbon nanotubes is better than that of Tenax-TA.For the three-dimensional grid split type concentrator device filled with carbon nanotubes,the concentration rate of 5 mg/m3 of carbon disulfide after 80 s enrichment is 18.17;the concentration rate of 5 mg/m3 of methyl sulfide within the same enrichment time is 15.57;the concentration rate of 5 mg/m3 of dimethyl disulfide after 4 minutes is 19.01.For the plane 4×4 diaphragm type concentrator device filled with carbon nanotubes,the concentration rate of 20 mg/m3 of methyl sulfide after 5 minutes is 5.01 and the concentration rate of 20 mg/m3 of carbon disulfide after 5 minutes is 16.49.Based on the test results,we further performed thermal simulations for the substrate of poor heat conductivity to obtain a well-distributed temperature field,by which we provided some flexible methods to enhance the concentration performance of micro-preconcentrator.