Preparation And Regulation Of Negative Permittivity Materials With Carbonaceous Percolation Structure

The negative permittivity materials(NPMs)are developed from the metamaterials,which break through the limitation that the metamaterials need artificial periodic array to realize the special properties.By using natural materials,the negative permittivity properties are obtained.The negative permittivity is one of the intrinsic parameters,and the NPMs having broad application prospects in electromagnetic field by virtue of this special properties.The negative permittivity properties are closely related to the percolation behavior of functional phases in NPMs.Three-dimensional conductive network is formed when the content of conductive functional phase exceeds the percolation threshold.The plasma oscillation of a large number of free electrons in the conductive network leads to the negative permittivity.Therefore,the functional phase,one of the research priorities,is the key to regulate the dielectric properties of NPMs.Researchers have done many studies on NPMs with different functional phases,but some problems still exist.First of all,researchers mostly focused on the realization of negative permittivity properties in the past reports,while there was insuficient research on the influence of the dimension,morphology and composition of functional phases on the percolation and permittivity behavior of NPMs.In addition,scholars rarely studied the properties of negative permittivity materials with binary functional phase.The systematic study of different functional phases is of great scientific significance to clarify the dielectric mechanism and realize the regulation of negative permittivity properties.Moreover,the study of binary functional phases can expand the research system and promote the application of NPMs,which has important research value.Nano carbon materials contain a variety of allotropes with different morphologies,structures and three-dimensional dimensions,which provides a rich choice for the systematic study of the internal correlation between the negative permittivity properties and the microstructure.In view of the above problems,several nano carbon materials are selected in this paper.We choose nano carbon powder(CP)as the zero-dimensional functional phase,carbon nanotubes(CNTs)as the one-dimensional functional phase,graphene(GR)as the two-dimensional functional phase.Besides,carbon nanotubes and nano carbon powder are used as the binary functional phase(CNTs-CP).Four kinds of composites are prepared by hot pressing with poly(vinylidene fluoride)(PVDF)as matrix.The percolation behavior and dielectric behavior are analyzed by means of micro morphology characterization,electrical properties test and equivalent circuit fitting,Free electron theory,Drude model and plasma ocillation theory are used.The main research contents and results are as follows.(1)We characterized the microstructure of NPMs,observing the distribution of functional phases in the matrix,studying the percolation behavior of functional phases.The results show that the parameters such as dimension,morphology and composition directly affect the percolation process of functional phases,then affecting the percolation threshold of NPMs.As zero-dimensional functional phase,nano carbon powder has granular morphology and smaller three-dimensional size.Carbon nanotube has larger aspect ratio and axial size as one-dimensional functional phase.As two-dimensional functional phase,graphene has larger aspect ratio and two-dimensional planar morphology.With the changes of morphology and three-dimensional size,the percolation threshold of CP/PVDF,CNTs/PVDF and GR/PVDF composites decrease in turn.There is a synergistic effect between nano carbon powder and carbon nanotube s in the binary functional phase CNTs-CP.Carbon nanotubes are distributed between the nano carbon powder,playing the role of connector.The two materials together form a conductive path.The synergistic effect of the two makes the percolation threshold of CNTs-CP/PVDF the lowest in the four groups of experiments.(2)The AC conductivity of NMPs is analyzed.The changes of AC conductivity are explained by Power Law,free electron theory and Drude model.It shows that the percolation behavior leads to great changes in the electrical properties of the NPMs.When the content is lower than the percolation threshold,the functional phases are isolated.The conductive mechanism of NPMs is hopping conduction,which can be described by Power Law.When the content exceeds the percolation threshold,the functional phase percolates to form a three-dimensional conductive network.At this time,NPMs shows metallic conduction.The conductive mechanism can be explained by free electron theory and Drude model.(3)The changes of impedance of NPMs are studied.By using equivalent fitting circuit,the changes of electrical characteristics are described and analyzed.NPMs show capacitive characteristics and the equivalent fitting circuit is mainly composed of capacitance and resistance when the content of functional phase lower than the percolation threshold.When the content of functional phases exceeds the percolation threshold,NPMs change from capacitive characteristic to inductive characteristic.The main components of the equivalent fitting circuit are inductance and resistance.(4)The realization and regulation of the negative permittivity properties are studied.Negative permittivity appears when the content of functional phases in NPMs is higher than percolation threshold.The negative permittivity behavior can be described by free electron theory and Drude model.There is large amount of free electron in the conductive network formed by percolation of functional phases.When the external electric field reaches a specific frequency,the plasma oscillation of free electrons leads to the generation of negative permittivity.Weak negative permittivity phenomenon is observed in GR/P VDF NPM.According to the plasma oscillation theory,the absolute value of negative permittivity is positively correlated with the free electron concentration.Thus,the negative permittivity can be regulated according to this relationship.