Research On The Force-electrical Effect Of Synergistic Enhanced MWCNT/CB Composites

With the development of nanotechnology,carbon nanomaterials have become the research hotspot in many scientific study fields.Because of the excellent mechanical and electrical properties,carbon nanomaterials can be added into polymers to fabricate carbon nanocomposites,like multi-walled carbon nanotubes(MWCNT)/polyvinylidene fluoride(PVDF)composite.Such nanocomposites can be used to develop high performance composite strain sensors due to their piezoresistive behaviour.However,because of the limitations of material properties and fabrication processes,it is difficult to further improve the piezoresistive sensitivity of the the composites with single type of carbon nanofiller.It was found that the composites with multiple types of carbon nanofillers have better electrical conductivity and piezoresistive properties as compared with the composites with single type of carbon nanofiller.In this thesis,MWCNT with large aspect ratio and carbon black(CB)with small aspect ratio were selected as carbon nanofillers to fabricate composites.The synergistic effects of MWCNT and CB on the electrical conductivity and force-electrical piezoresistity were investigated by means of‘material preparation and test → numerical simulation → device application’.The details of research contents and results are as follows:(1)The experimental study of the force-electrical piezoresistive properties of MWCNT/CB/PVDF composite.The MWCNT/CB/PVDF composite film samples were prepared by the process of‘dissolving matrix → mixing filler → evaporating solvent → curing’.It is observed using scanning electron microscopy(SEM)that the distribution of the CB particles in the PVDF matrix is overall uniform but aggregational in small-scale local regions.The test on the mechanical,electrical,and the force-electrical piezoresistivity of the composites show that the composite has stable mechanical properties,and its electrical conductivity follows the percolation phenomenon model.A small amount of MWCNTs with large aspect ratio can improve the conductivity of the CB/PVDF composite significantly.The piezoresistive sensitivity of nanocomposite containing MWCNTs and CBs is better than that of the nanocomposite with single type of nano filler.The CB1CNT0.1 sample exhibited the best piezoresistive sensitivity in the test.Its gauge factor(GF)reached 9.71,10.72 and 12.07,corresponding to the strain of 1%,3% and5%,respectively.(2)The numerical simulation of the force-electrical piezoresistivity of the MWCNT/CB composites.Based on Kirchhoff’s law,a three-dimensional statistical resistance network simulation model is established.When the MWCNT content in the calculating cell is low,the resistivity of the cell decreases obviously with the increase of the CB loading.However,if the MWCNT content of the calculating cell is high,the resistivity of the cell does not reduce rapidly with the increase of the CB concentration.Under external strain,the MWCNTs and the CBs in the calculating cell will be repositioned,and the resistance of the cell will be recalculated,then the piezoresistive effect of the cell can be obtained.In the case that the MWCNT content is 0.073 wt.%,the best force-electrical piezoresistive sensitivity can be achieved when the CB loading is up to 0.2 wt.%.However,with the increase of the MWCNT content,the influence of the CB content on the piezoresistive sensitivity of the calculating cell increases gradually.In case the MWCNT content is 0.378 wt.%,the best force-electrical piezoresistive sensitivity can be achieved when the CB loading is only 0.025 wt.%.It is noted from the final GF results that optimal piezoresistive sensitivity can be obtained when the nanocomposite contains lower MWCNT content and proper amount of CBs.In addition,the modified three-dimensional coarse network model is attempted to improve the calculating efficiency.(3)The development of laser ablation MWCNT/CB/(polydimethylsiloxane)PDMS composite strain sensor.In this thesis,the fabrication process of the laser ablation composite strain sensor was proposed and the force-electrical piezoresistive behaviour of the sensor were tested.The results show that the sensor exhibits excellent piezoresistive sensitivity.The GF of the CNT1.0-P3.0 sample at 5% strain is as high as 513,while the GF of the CNT0.6CB1.0-P3.0 sample at 5% strain reaches 584.7.Compared with the traditional composite sensors made of uniform mixture,the piezoresistive sensitivity of the laser ablation composite strain sensor has obvious advantages.The section microscopic characterization of the ablation region shows that laser ablation can produce‘boscage-like’ conductive structure on the composite.As the laser power increases,the height of the ‘boscage-like’ ablation structure grows first and then decreases.Correspondingly,the GF of the sensor also rises to maximal value gradually and then enters a descending phase,which means,the optimum of the force-electrical piezoresistive sensitivity can be obtained at the moderate and appropriate laser power.The transmission electron microscopy(TEM)images and the X-ray diffraction spectrum(XRD)results of ablation products show that amorphous carbon was formed and adhered on the surface of the carbon nanofillers because of the laser ablation.The existance of CB can help to attach more amorphous carbon to the laser ablation conductive structure that is mainly constructed by MWCNT,so as to enhance the piezoresistive sensitivity of the ‘boscage-like’ conductive structure.In conclusion,the synergistic force-electrical piezoresistive effect of MWCNT and CB in the nanocomposites was investigated systematically in this thesis.The high piezoresistive sensitivity mechanism of the binary filler nano-carbon composites was revealed.The results are of great significance to further improve the force-electrical piezoresistivity of nano-carbon composites and to design high performance composite strain sensors.