Preparation Of Fractal Structure Silver For Flexible Electrode Application

Electronic devices have become a necessity for people's daily life,and the emergence of flexible and wearable functions further promotes people's lives,which is one of the future development trends for electronic devices.The research of flexible and wearable electronic devices is necessary,because conventional electronic products are susceptible to be damaged during mechanical deformation such as bending,twisting,and stretching.As an important part of flexible and stretchable electronic devices,flexible electrodes are the critical component to facilitate the development of flexible and wearable electronic devices.Indium tin oxide(ITO)is a widely used conductive material,but it is unfavorable to prepare flexible electrodes due to its brittleness and high cost.Therefore,a variety of conductive materials have been studied for flexible electronic devices,such as metal nanomaterials,carbon nanomaterials,conductive polymers and so on.Among these conductive materials,silver nanomaterials are considered to be potential candidates due to their high electrical conductivity and good stability.Silver nanomaterials have a variety of structures,and the structures that commonly used for conductive materials are nanoparticles and nanowires,while fractal silver particles that presenting a special three-dimensional structure are less used as conductive fillers.Compared with other nanostructures,fractal silver particles have many micron-scale branches in three-dimensional space,and each branch has many nano-scale tips.This means that adjacent silver particles have a rich contact point,which makes it easier to connect to each other to form a conductive network.Therefore,in this paper,high-crystallinity fractal silver particles are synthesized by liquid phase reduction method,and they have been used as flexible materials to prepare flexible electrodes and flexible printed electrodes with high conductivity and mechanical stability.The main research contents are as follows:(1)The fractal structure silver particles with high purity are synthesized by liquid phase reduction.Although there are many methods for preparing fractal silver particles,most of these methods have some disadvantages,such as complex reaction conditions,low yield,and difficult to control morphology.In this work,the reaction conditions for synthesizing the silver particles of the fractal structure by the liquid phase reduction method are very simple and mild.The whole reaction process is completed at room temperature.Moreover,the yield of fractal structure silver particles could be controlled by adjusting the volume of the reaction solution,which is helpful to achieve large-scale mass production.(2)Preparation and properties of fractal structure silver particle flexible electrodes.A conductive network that composed of fractal structure silver particles is formed on a polyethylene terephthalate(PET)film by a coating method,and then embedded in the surface of a polydimethylsiloxane(PDMS)elastomer to prepare a flexible electrode.After detecting the performance of flexible electrodes,it is found that the sheet resistance reduces as the thickness of the conductive layer of the flexible electrode increases.When the thickness of the conductive layer reaches 60 ?m,the sheet resistance of the flexible electrode could be reduced to 0.92 ? sq-1.It is found that the flexible electrode shows good mechanical stability under several flexible conditions,such as stretching to 100%,bending and twisting to 180?.And under bending and twisting conditions,the electrical resistance of the flexible electrode reduces.Then,it is found that the change in the resistance of the flexible electrode is negligible through the tape peeling test cycles,indicating that it displays good mechanical stability.The experimental results show that the fractal silver particles have good potential as conductive materials in the field of flexible electronics.(3)Preparation and properties of fractal structure silver particle flexible printed electrodes.The synthesized silver particles are uniformly mixed with the prepared polymer matrix to prepare a water-soluble conductive ink,and then the ink is screen printed on the surface of the paper and the PET film to prepare the flexible printed electrode.It can be observed that the sheet resistance of the printed electrodes decreases with the increasement of the solid content of silver particles in the conductive ink.When the solid content of silver is 60%,the sheet resistance is only 0.23 ? sq-1.After comparing the influence of paper and PET film as the substrate on the resistance of the printed electrode,it is found that the electrode resistance of the printed surface of the paper is smaller,indicating that the paper is more favorable for preparing a highly conductive printed electrode.The effect of the printed line width on the conductivity of the electrode is compared.The results show that the resistance of the printed electrode decreases as the line width increases,but the conductivity is almost invariable.After that,the influence of the bending strain on the conductivity of the printed electrode is tested.It is found that the electrical properties of the printed electrode are stable,and the resistance in the bent state is always lower than the initial resistance.During the tape peeling test cycles,it is found that although the printed electrode exhibits a good adhesion ability,but it eventually fails with the cycles of peeling increases.Therefore,it is necessary to improve the printed electrode to enhance its tolerance.(4)Preparation and properties of embedded printed electrodes.The printed electrode is prepared on a PET film,and then the printed electrode is embedded in a PDMS elastomer to produce an embedded printed electrode.Although the sheet resistance of the embedded printed electrode slightly increases compared with the printed electrode,but embedded structure makes the printed electrode to be transferred to the stretchable substrate,breaking through the mechanical properties of the flexible substrate.The embedded printed electrode displays good stability through 30% stretching cycles and 180? bending cycles,and its resistance remains almost unchanged during the tape peeling test cycles.The embedded printed electrode not only extends its mechanical properties from flexible to stretchable while maintaining the original electrical properties of the printed electrode,but also enhances the mechanical resistance of the printed electrode.