Preparation Of Thin Films Based On Layer-by-layer Self-assembly CNT And Its Piezoresistive Sensing Performances

Structural health monitoring(SHM) system based on sensor networks has become a hot and prospering field of civil engineering today. The foundation of constructing this system is the sensor networks consisting of large number of sensors. It is an emerging trend that the sensor that is not strictly required for attached on the surface, and can adapt to the complex environment. The carbon nanotube(CNT) based sensor which has excellent properties has drawn widespread attentions. Researches on CNT sensors either incorporating CNT into macroscopic cement, mortar, ceramic matrix as an electrically conductive reinforcement, and developing into an embedded sensor; or depositing or assembling on a silicon wafer, glass and other rigid substrates, developing into an adhesive film sensors.Here, we first modified CNT with negatively charging, and then employed layer-by-layer self-assembly(LBL SAS) method to produce CNT film on a tough PET substrate; and then we discussed the piezoresistive sensing properties of the films; Finally, we carried out a systematic study on the performances of the electrical resistivity-displacement response of CNT films and the sensing stability of the sensor under marine environment.To prepare well-dispersed multi-walled CNT solution with carboxyl groups, we treated the pristine CNT with mixed oxidant acid, after that, we observed surface morphology of CNT with SEM, and then we utilized infrared spectrometer and thermal gravimetric analysis to analyze carboxyl groups on CNT surface in a qualitative and quantitative way, respectively. Firstly, we negatively charged PET substrate by ozone process, and then we assembled the CNT solution charged with negative carboxyl group and PDDA solution charged with a positive group using LBL SAS method. Hereafter we characterized its microstructure, measured the light transmittance and the resistance properties of the film,. Experiments show that, the CNT films have good light transmission properties, For four different double-layer CNT films, the film transmittance in 500~900nm band range wavelengths are all greater than 70%. They also have good electrical conductivities, surface resistivity of 12 double-layers CNT film is up to 7.98kΩ.cm, The resistivity of CNT film exponentially decreases with double-layer increasing.The film sensor was studied on the tensile and compressive displacement with the different response through the output resistance change of the film. The results show that, 9 double-layer film performance good linearity(3.22%), sensitivity(0.12684/mm), reproducibility(3.06%) and low hysteresis(2.16%). Research the effect of loading rate on the piezoresistive properties of thin films, experiments show that, the effect of the loading rate is unimportant in the resistivity response of the film. We further studied the effects of humidity and temperature on the CNT film. Experiments show that, the importance of encapsulating a protective layer on improving the stability of the film, and the selection of better protection layer coating must be carefully. In summary, the flexible CNT film has good and stable piezoresistive effect, is expected to be used for bridges and other facilities in the uneven surface structural damage detection.