Prepared PVDF Membrane And Gas Sensing Characteristics Of Electrostatic Spray Method

Electrospray has been used for decades in many fields. A new field of application for electrospray is the deposition of polymer layers and other coating materials on chemical sensors. Compared with a number of different methods existed to deposite a thin polymer layer such as spin coating, spray coating, CVD(chemical vapor deposition) and so on,the electrospray method is capable of producing coatings with much higher quality and obtaining gas sensors with high sensitivity and low detection limit. However,the morphology of film strongly depends on many factors such as applied voltage,solution concentration,collecting distance, and flow rate. Therefore, aiming at solving the problem mentioned above, the main work of this thesis is:(1) Polyvinylidene fluoride(PVDF) thin films were prepared on silicon wafer by electrospraying. It was found that the film thickness increased linearly with the increase of the solution concentration. At the concentration of 0.05wt%, the films were made up of discontinuous particles; at the concentration ranging from 0.5 to 1wt%, the films showed net-shaped microstructure; at the concentration of 5wt%, many large particles were found due to agglomeration. The net-shaped microstructure was little influenced by the applied voltage varied from 7KV to 10KV and the collecting distance.(2) PVDF thin films were deposited on the surface acoustic wave(SAW) sensors. After deposition, the SAW sensors had much higher insert loss and lower Q value, indicating that the net-shaped microstructure was not suitable for SAW sensor application.(3) PVDF thin films were electrosprayed on the quartz crystal microbalances (QCMs) to evaluate the gas sensing properties to nerve agent simulant dimethylphosphonate (DMMP). The sensors showed linear response in the DMMP concentration ranging from 10 to 50ppm. The test results were also comparied with the PVDF-QCM sensors prepared by drop coating method. When the PVDF film thickness were identical, the sensitivity of electrosprayed sensor is lower than the drop coated sensor's sensitivity, although the electrosprayed films had much higher surface area due to the net-shaped microstructure. It indicated that the adsorption of DMMP by PVDF was not governed by surface adsorption process but some body mechanism. Moreover, the electrosprayed sensors response much faster to DMMP vapors. With the increase of PVDF film thickness,this advantage became more remarkable.