Tunable Negative Permittivity In Conductive Filler/Polypyrrole Meta-composites

Meta-composites are an emerging area of metamaterials research.They can achieve negative electromagnetic parameters by using their random microstructures,which break the limitation of periodic unit structure of metamaterials.The unique physical properties and compatible microstructures endow meta-composites extraordinary research value and the potential for applications in some fields including military,communication,and energy.Negative permittivity is an important intrinsic parameter of meta-composites,and the precise regulation of it is the predominant part of meta-composites research.Plasma oscillation theory and dielectric resonance theory are commonly used to explain the generation of negative permittivity.Also,negative permittivity is closely related to electrical percolation;therefore,the construction of percolation networks provides a fast and efficient technical method for the regulation of negative permittivity.Among various types of meta-composites,polymer-based meta-composites have attracted widespread attention because of their advantages of low cost,simple fabrication process,and highly tunable performance.So far,there are still some problems that have not been solved in the field of meta-composites.For example,strong negative permittivity is detrimental to practical applications of meta-composites,and weakly negative permittivity has been sparsely explored.Therefore,it is necessary to pay closer attention to works toward the regulation of weakly negative permittivity.Additionally,high filler content will harm the performances of meta-composites.How to achieve negative permittivity with low filler content need to be explored.This thesis discusses three strategies to the solvation of the above two problems,with three meta-composites fabricated by different types of conductive fillers.The negative permittivity of these meta-composites can be modulated by efforts of control and optimization of microstructures.The main research content and results are as follows:(1)To achieve the weakly negative permittivity,titanium nitride is selected as the conductive filler,and then titanium nitride/polyethylene terephthalate composites are prepared via a hot-pressing method.Conductive networks are gradually formed in the composites when increasing the content of titanium nitride.This causes the electrical percolation,further leading to the change of the conductive mechanism from hopping conduction to electron conduction.Meanwhile,when the content of titanium nitride increases,the permittivity rises at the beginning,then decreases and turns into negative finally.When the volume fraction of titanium nitride reaches 85 vol%,the weakly negative permittivity with the value between-50 and 0 is obtained.Further Increasing the volume fraction causes the appearance of strong negative permittivity.(2)To obtain meta-composites with low filler content,carbon nanotubes and polypyrrole are used as conductive fillers and substrates respectively,and carbon nanotube/polypyrrole composites are prepared by in-situ polymerization method.As the mass fraction of carbon nanotubes climbs,the AC conductivity of composites increases rapidly.Reactance and equivalent circuit analysis indicate that conductive networks have been constructed when the content of carbon nanotube increases to 3 wt%.For samples of polypyrrole and the composites with 1 wt%carbon nanotubes,the permittivity changes from positive to negative under the influence of frequency due to the Lorentz dielectric resonance.When the content of carbon nanotubes increases to 3 wt%or higher,plasmon resonance related negative permittivity is achieved in the whole test frequency range(10 MHz-1 GHz).(3)To find an equilibrium between the weakly negative permittivity and low filler content,tungsten carbide is selected as the conductive filler,and the core-shell structured tungsten carbide/polypyrrole composites are prepared by the in-situ polymerization method.The core-shell structure promotes the uniform distribution of tungsten carbide particles in the composites,and conductive networks are formed when the content reaches 20 wt%.The AC conductivity of composites shows the hopping conduction mechanism and obeys the power law before the construction of conductive networks.In contrast,after the networks have been built,it shows the electron conduction mechanism and can be influenced by the skin effect.When the tungsten carbide content is 20 wt%and 40 wt%,the permittivity value is between-500 and-50,which means the weakly negative permittivity is achieved.This can be explained by both multiple electric field theory and plasmon resonance theory.Furthermore,control samples without core-shell structure are fabricated for comparative analysis,and the effect of core-shell structure on negative permittivity is discussed.