Mechanism Of Radio Frequency Negative Permittivity In Barium Titanate-based Materials

Electromagnetic fields/waves are common forms of information and energy.Permittivity is one of fundamental parameters of interaction between electromagnetic fields/waves and materials.The permittivity of materials is always considered to be positive at ac and radio frequency,and it can only approach and cannot reach the permittivity of air(?-1).The permittivity lower than 1 or negative permittivity is considered as a supernormal performance,which was once thought impossible to be achieved.In fact,the permittivity of materials can be negative in the specific frequency band.In recent years,the characteristics of negative permittivity materials,such as resonance,dispersion,loss as well as novel properties such as local field,surface wave have been more and more deeply understood.Compared with the materials with negative permittivity,the interaction between the negative permittivity materials and electromagnetic field has undergone fundamental changes.The materials with negative permittivity are giving rise to many revolutionary technologies in the fields of antenna,radar,cloaking,energy storage,sensing and transistor.Usually,negative permittivity behavior can be observed in metamaterials with periodically artificial units.In fact negative permittivity behavior can also be realized based on the intrinsic properties of materials by dielectric resonance of insulator or plasma oscillation of the conductor.The realization of negative permittivity in "natural95 materials complements the physical mechanism of negative permittivity and expands the performance of materials.In this dissertation,the mechanisms based on the electrodynamics of dielectric resonance and plasma oscillation are presented.Specifically,in the radio frequency region,the dielectric resonance can be achieved by dipole resonance of ferroelectric(BaTiO3 in this work)and plasma oscillation can be obtained in the "weak" conductor(including excessively percolating composites and single-phase materials)with diluted concentration of free electrons.The characteristics of these two kinds of responses of negative permittivity in BaTiO3-based materials are systematically compared.The mechanism and regulatory principles of these two kinds of negative permittivity behaviors are clarified by combining Lorentz model,Drude model,percolation model,equivalent circuit analysis,conductivity and reactance analysis.In addition,electromagnetic attenuation performance of epsilon-negative materials is explored.The main results obtained are as follows:(1)The dielectric resonance induced by dipole resonance of barium titanate and negative permittivity behaviors are achieved in the composites consisting of barium titanate and yttrium iron garnet at radio frequency.The negative permittivity can be well fitted by Lorentz model,and a modest increase of yttrium iron garnet content,the value of negative permittivity and dielectric loss are both decreased.The Cole-Cole plots of complex permittivity around the dielectric resonance is a nearly complete circle,and with increasing of yttrium iron garnet content,the damping factor is increased,the diameter of circle is decreased.The analysis of equivalent circuit shows that dielectric resonance can be considered as a LC resonance and the amplitude and bandwidth of dielectric resonance are related to resistance(R).The conductive mechanism below resonance frequency of barium titanate/yttrium iron garnet composite is a hopping conduction.The yttrium iron garnet can realize negative permeability under the external magnetic field,therefore the barium titanate/yttrium iron garnet composites are expected to achieve double negative.(2)The negative permittivity behaviors are obtained by plasma oscillation in barium titanate/silver composites,which can be well described by Drude model.The increase in Ag content makes both the plasma frequency and collision frequency increased.Due to the collision frequency is much less than plasma frequency,the negative permittivity is decreased with Ag content increased.In the temperature range of 50?-600?,with increase of temperature,the plasma frequency of barium titanate/silver composite in increased,so the negative permittivity is decreased with the increase of temperature.(3)The negative permittivity induced by plasma oscillation is achieved in single-phase semi-metallic La0.5Sr0.5MnO3 ceramics,which can be described by Drude model.With increase of temperature,the plasma frequency is nearly constant,while the collision frequency is increased,so it shows a temperature-stable negative permittivity.The conductivity of La0.5Sr0.5MnO3 decreases with the increase of temperature,which shows a typical metal-like character.The activation energy of La0.5Sr0.5MnO3 ceramics is much smaller,indicating a typical band transport mechanism.In the frequency range of 2 MHz-5 MHz,the fluctuation in negative permittivity is less than 2.6%and the dielectric loss(tan?)is less than 0.2 in the temperature range of 50?-600?.(4)There are two kinds of negative permittivity response mechanisms in barium titanate/nickel percolating composites.When the nickel content is lower than the percolation threshold,negative permittivity induced by dielectric resonance is achieved.When the nickel content exceeds the percolation threshold,negative permittivity derived from plasma oscillation is obtained.When the nickel content is near the percolation threshold,the dielectric behavior of the composite material is affected by both of them,and the permittivity can be fitted by linear combination of Lorentz and Drude model.For the composites with plasma oscillation,with increase of free electrons' concentration,both of plasma frequency and collision frequency are increased,but when the collision frequency increases more significantly than plasma frequency,the value of negative permittivity increases with the higher concentration of free electrons.In addition,the plasma frequency affects the bandwidth of the negative permittivity,and the enhancement of the collision frequency is helpful to obtain a weakly negative permittivity.The negative permittivity caused by dielectric resonance obeys the Kramers-Kronig relations,while the negative permittivity caused by plasma oscillation violates the Kramers-Kronig relations,the main reason is that the permittivity no longer satisfies the linear relation.In addition,due to the negative permeability can be achieved by the domain wall resonance and spin resonance of nickel particles,the composites with 35.58 vol%of nickel content show negative permittivity and negative permeability simultaneously,which can be regarded as an intrinsic metamaterial.(5)Negative permittivity induced by dielectric resonance is achieved in barium titanate/copper composites when the volume fraction of copper is below the percolation threshold,and a significantly lower resonance frequency observed compared with that of barium titanate.The perfect absorption of electromagnetic waves is obtained at a certain incident angle when the permittivity is near zero,especially when the negative permittivity is slightly smaller than zero.Barium titanate/copper composites as epsilon-negative materials can achieve high thermal conductivity,low electrical conductivity and strong absorption of electromagnetic wave by combining the positive temperature coefficient resistance effect of barium titanate and dual thermal and electrical percolation behavior.