Research On Flexible Strain Sensors Based Layer-by-layer Self-assembly Carbon Nanotube Thin Films

Recently lots of bridges and tunnels are being developed, and the security evaluating for these structures is very important. Computer in-time evaluating system has many advantages at this field, the basic of which is Sensor network system. Therefore, flexible strain sensors being able to endure the environmental changes and to be used on curved surfaces become the development trend. And sensors based on CNT thin films become a hot subject branch because of their excellent properties, which have been studied by many researchers. But related researches are mostly carried out by foreigners, and most of those researches focus on rigid substrate. And little studies is reported to analyze the applicability of CNT thin film strain sensor in different environment.This essay designed and manufactured a kind of strain sensor that is used for structure health monitoring and based on single-walled carbon nanotube network thin films deposited on flexible polyethytlene film with layer-by-layer self-assemble method. And both the films and the sensors were characterized.SWNT network thin films were developed on flexible substrate PET with layer-by-layer (LBL) self-assembly. Then the films were characterized by SEM and AFM and tested by electricity measuring devices. The mechanism of film conductivity is analyzed, and the key process factors are optimized. The experimental data show the CNT films are with good electric conductivity property, and the resistivity of CNT film of 12 layers is 1.47× 10-9M?·m. The resistance of the films will decay exponentially with the increase of layers. The affect of changing the concentration of PDDA(poly (dimethyldiallylammonium chloride) and SWNT was also measured. The light transmittance properties of these films were also tested.Strain sensors were manufactured with LBL self assembly CNT film by microelectronic process on flexible substrate. The sensors output different resistance for different strain with prompt and nearly linear response, as well as they achieve their strain sensitivity up to 4.25. The response of the sensor to tensile strain and compressive strain are all tested. Stability is evidently improved by adding protective coating on the surface of the devices, with which devices become less vulnerable to humidity and illumination. The piezoresistive effect of the CNT films and the mechanism of the affect of humidity and illumination to CNT films are discussed. These flexible sensors can be used on curved surfaces of bridges and other buildings to test stress and strain because of their flexible substrates.