| Flexible and stretchable electronics will have normal functions when subjected to large deformation of different types,such as stretching,torsion,and bending,and thus,have broad application prospects in the fields of digital information,renewable energy,health care,biosensing,and defense industry.In particular,the flexible strain sensor breaks through the limitations of the traditional strain sensors,which have narrow working range and are made of hard and brittle materials and thus can only be used on small rigid and flat surfaces.Such flexible and soft sensors can be used in structural health monitoring,human motion detection,biomedicine,human-machine interface,and electronics.They therefore have received considerable attention recently and are promising for further development.In the study of flexible conductive materials,researchers distributed conductive nanofillers into polymer elastomers to form nanocomposites with high conductivity,low elastic modulus,and low mass density.In particular,the remarkable piezoresistive effect of this material(that is,the sensitivity of conductivity to strain)makes it an idea candidate for highly sensitive flexible strain sensing materials.So far,extensive experiments and theoretical studies have been carried out on this type of nanocomposites,which provides theoretical guidance and abundant data for eventually realizing their engineering applications.The research results show that the double-layer strain sensing film(that is,the nano-conductive material layer plus polymer elastic substrate)has high piezoresistive sensitivity and deformability,and the fabrication methods are simple,easy to control and with low cost.Due to these advantages,the strain sensing films with double-layer structure turn out to be of great interest in the research of flexible sensing films in recent years.However,up to now,the quantitative relationship between the resistance of the two-dimensional conductive layer(on the surface of the polymer substrate)and the density of the surface nanoparticles,that is,the two-dimensional conductive infiltration behavior is still lack of quantitative description;The influence of external conditions such as load frequency and load application sequence on the dynamic piezoresistive sensitivity and linearity is still lacking in-depth experimental characterization and theoretical discussion.For further development and application,this article will conduct a comprehensive and systematic study of the above key issues.Silver nanoparticles have quantum size effect and volume effect.Compared with nanotubes and silver nanowires,they have obvious advantages in light,electricity,magnetism and catalysis.They are widely used in conductive sensing,optical enhancement,catalysis,Biomedical and other fields;PDMS substrate has the advantages of low elastic modulus,strong corrosion resistance,high transparency,good mechanical properties,thermal stability and so on.In view of this,this paper will take the silver nanoparticle-PDMS double-layer flexible sensing film(AgNP/PDMS)as the specific research object.This thesis has carried out a comprehensive and systematic study of the chemical manufacturing process,conductive percolation behavior,piezoresistive sensing characteristics,and viscoelastic properties of the AgNP/PDMS double-layer flexible sensing film.First,silver nanoparticles were used as conductive fillers,PDMS was used as the substrate,and the substrate material was functionalized on the surface and silver particles were deposited by dopamine to obtain AgNP/PDMS thin films.During the preparation process,the deposition time of the silver nanoparticles can be changed to control the conductive percolation stage of the film.Through electronic morphology analysis,conductive performance test and surface silver particle mass density test,we quantitatively characterized the relationship between the surface structure and conductive percolation,and the relationship between the conductivity and silver particle density on the surface of the film.The two-dimensional surface Mathematical description has been achieved for the dynamic process of chemical deposition of silver nanoparticles and the law of conductive percolation has also been obtained.Subsequently,the viscoelastic properties of the material under different conductive seepage stages were tested,especially the tensile hysteresis effect of the polymer.This paper further studies the influence of the conductive seepage stage on the characteristics of thin film piezoresistive sensing,including sensitivity and linearity.The main purpose of the research is to reveal the relationship between the"process-structure-performance"in the AgNP/PDMS double-layer flexible sensing film,and achieve the result of optimizing the preparation process to control the sensing characteristics and obtain the best sensing characteristics.Another important point of this thesis is the dynamic piezoresistive sensing characteristics.In the experiment,the effects of frequency,loading sequence and other load conditions on the piezoresistive performance were studied,and the underlying physical mechanism was also discussed.The dynamic load piezoresistive effect plays a very important role in developing the techniques of damage monitoring,motion detection,electronic skin and so on.In the present research,scanning electron microscope,digital multimeter,miniature fatigue tension meter,high-precision electronic balance equipment and ImageJ image processing software were used to quantitatively analyze the electronic morphology of AgNP/PDMS film,conductance test,nanometer Characterization of particle mass areal density,dynamic cyclic tensile test and hysteresis rate test.Combined with the nano-conductivity mechanism of the flexible sensing film and the piezoresistive sensing theory(especially the microcrack theory),the test results of the conductive percolation,viscoelastic properties,dynamic sensing performance and influencing factors of the AgNP/PDMS film are comprehensive Analysis to reveal its physical mechanism.The research established the governing equation of the two-dimensional conductive percolation of AgNP/PDMS film,that is,the power law relationship between the resistance R and the mass area density m_d of silver particles;Quantitatively characterize the significant effects of different stages of conductive seepage and the frequency of cyclic loading on the dynamic piezoresistive sensitivity and linearity of AgNP/PDMS films,and the physical mechanism of the experimental results is discussed through the nanoparticle layer microcrack theory.The results show that mass area density at 0.32mg/cm~2 and a loading frequency of 0.5 Hz,the maximum sensitivity of the AgNP/PDMS film can be obtained at 75;at a frequency of 1 Hz or higher,the material can maintain a high piezoresistive linearity Degree.In particular,in the AgNP/PDMS thin film preparation process,the deposition time of silver particles is a key factor in controlling the characteristics of the piezoresistive sensing layer,adjusting the film piezoresistive sensitivity,linearity,and its dependence on the load frequency.These results enrich the basic theory of AgNP/PDMS flexible sensing films and deepen the understanding of the"process-structure-performance"relationship of such films.The present research thus provides an important theoretical guidance and abundant property data which are essential in the optimal design of sensing films and practical engineering applications. |
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