Negative permittivity has been widely studied in various metamaterials and percolating composites, of which the anomalous dielectric behavior was attributed to critical structural properties of building blocks.Herein,...Negative permittivity has been widely studied in various metamaterials and percolating composites, of which the anomalous dielectric behavior was attributed to critical structural properties of building blocks.Herein, mono-phase ceramics of indium tin oxides(ITO) were sintered for epsilon-negative materials in MHz-k Hz frequency regions. Electrical conductivity and complex permittivity were analyzed with DrudeLorentz oscillator model. Carriers’ characters were measured based on Hall effect and the magnitude and frequency dispersion of negative permittivity were mainly determined by carrier concentration.Temperature-dependent dielectric properties further proved the epsilon-negative behaviors were closely associated with free carriers’ collective responses. It’s found that negative permittivity of ITO ceramics was mainly caused by plasma oscillations of free carriers, while the dielectric loss was mainly attributed to conduction loss. Negative permittivity realized here was related to materials intrinsic nature and this work preliminarily determined the mechanism of negative permittivity in doped ceramics from the perspective of carriers.展开更多
Epsilon-negative materials with high thermal conductivity and low electrical conductivity are of great importance for high power microwave devices.In this work,BaTiO_(3)/Cu composites,as a class of epsilonnegative mat...Epsilon-negative materials with high thermal conductivity and low electrical conductivity are of great importance for high power microwave devices.In this work,BaTiO_(3)/Cu composites,as a class of epsilonnegative materials,are rationally designed to achieve a high thermal conductivity yet maintaining the electrical insulative character.Negative permittivity behavior induced by dielectric resonance and plasma oscillation is observed in these BaTiO_(3)/Cu composites,which can be explained by the Lorentz and Drude model respectively.An outstanding absorption ability is achieved near the zero-cross point of the permittivity.Benefiting from the positive temperature coefficient of resistance and the weak temperature dependence of thermal conductivity in BaTiO_(3)/Cu composites,sample containing 22.3 vol% of Cu content exhibits a thermal conductivity of up to 17.7 W/(m·k)and an electrical conductivity down to 0.0022(Ω cm)^(-1) at 150℃.Therefore,BaTiO_(3)/Cu composite is a promising candidate for applications in electromagnetic attenuation and thermal management.展开更多
Te mechanism of negative permittivity/permeability is still unclear in the random metamaterials,where the precise control of microstructure and electromagnetic properties is also a challenge due to its random characte...Te mechanism of negative permittivity/permeability is still unclear in the random metamaterials,where the precise control of microstructure and electromagnetic properties is also a challenge due to its random characteristic.Here silver was introduced into porous SiO_(2) microsphere matrix by a self-assemble and template method to construct the random metamaterials.Te distribution of silver was restricted among the interstices of SiO_(2) microspheres,which lead to the precise regulation of electrical percolation(from hoping to Drude-type conductivity)with increasing silver content.Negative permittivity came from the plasma-like behavior of silver network,and its value and frequency dispersion were further adjusted by Lorentz-type dielectric response.During this process,the frequency of epsilon-near-zero(ENZ)could be adjusted accordingly.Negative permeability was well explained by the magnetic response of eddy current in silver micronetwork.Te calculation results indicated that negative permeability has a linear relation with ω^(0.5),showing a relaxation-type spectrum,diferent from the“magnetic plasma”of periodic metamaterials.Electromagnetic simulations demonstrated that negative permittivity materials and ENZ materials,with the advantage of enhanced absorption(40dB)and intelligent frequency selection even in a thin thickness(0.1 mm),could have potentials for electromagnetic attenuation and shielding.Tis work provides a clear physical image for the theoretical explanation of negative permittivity and negative permeability in random metamaterials,as well as a novel strategy to precisely control the microstructure of random metamaterials.展开更多
The development of negative permittivity materials in multifunctional applications requests expansion of their operating frequency and improvement of stability of negative permittivity.Low electron density is benefici...The development of negative permittivity materials in multifunctional applications requests expansion of their operating frequency and improvement of stability of negative permittivity.Low electron density is beneficial to reduce plasma frequency so that negative permittivity is achieved in kHz region.Negative permittivity achieved by percolating composites is restricted in practicality due to its instability nature at high temperatures.To achieve temperature-stable negative permittivity in kHz region,monophase La_(1-x)Ba_(x)CoO_(3)ceramics were prepared,and the transition from dielectric to metal was elaborated in the perspective of electrical conductivity and negative permittivity.The plasma-like negative permittivity is attained in kHz region,which is interpreted by the collective oscillation of low electron density.The temperature-stable negative permittivity is based on the fact that the plasmonic state will not be undermined at high temperatures.In addition,zero-crossing behavior of real permittivity is observed in La_(0.9)Ba_(0.1)CoO_(3)sample,which provides a promising alternative to designing epsilon-near-zero materials.This work makes the La_(1-x)Ba_(x)CoO_(3)system a source material for achieving effective negative permittivity.展开更多
Recently,increasing attention has been concentrated on negative permittivity with the development of the emerging metamaterials composed of periodic array structures.However,taking facile preparation into consideratio...Recently,increasing attention has been concentrated on negative permittivity with the development of the emerging metamaterials composed of periodic array structures.However,taking facile preparation into consideration,it is important to achieve negative permittivity behavior based on materials'intrinsic properties rather than their artificially periodic structures.In this paper,we proposed to fabricate the percolating composites with copper dispersed in epoxy(EP)resin by a polymerization method to realize the negative permittivity behavior.When Cu content in the composites reached to 80 wt%,the conductivity abruptly went up by three orders of magnitudes,suggesting a percolation behavior.Below the percolation threshold,the conductivity spectra conform to Jonscher's power law;when the Cu/EP composites reached to percolating state,the conductivity gradually reduced in high frequency region due to the skin effect.It is indicated that the conductive mechanism changed from hopping conduction to electron conduction.In addition,the permittivity did not increase monotonously with the increase of Cu content in the vicinity of percolation threshold,due to the presence of leakage current.Meanwhile,the negative permittivity conforming to Drude model was observed above the percolation threshold.Further investigation revealed that there was a constitutive relationship between the permittivity and the reactance.When conductive fillers are slightly above the percolation threshold,the inductive characteristic derived from conductive percolating network leads to the negative permittivity.Such epsilon-negative materials can potentially be applied in novel electrical devices,such as high-power microwave filters,stacked capacitors,negative capacitance field effect transistors and coil-free resonators.In addition,the design strategy based on percolating composites provides an approach to epsilon-negative materials.展开更多
Epsilon-near-zero(ENZ)material has been a research hotspot in recent years due to unique physical properties such as inverse Doppler effect and negative refractive index,showing great potentials in the fields of flexi...Epsilon-near-zero(ENZ)material has been a research hotspot in recent years due to unique physical properties such as inverse Doppler effect and negative refractive index,showing great potentials in the fields of flexible electronics,wearable devices,sensors,etc.The ENZ materials are mostly reported at visible,infrared and terahertz wavelengths,while the report about ENZ materials at radio frequency is rare.In this work,flexible and biocompatible poly(vinyl alcohol)/multi-walled carbon nanotubes(PVA/MWCNTs)hydrogels,which were successfully fabricated by an environmentally friendly method,were used as ENZ materials at radio frequency for the first time.Cytotoxicity experiments proved that the experimental process and products are green,environmentally friendly and biocompatible.The microstructure,crystalline structure,chemical composition and dielectric properties were investigated.Two different water states,which were free water molecules and bound water molecules,coexisted in these PVA/MWCNTs hydrogels and accounted for about 37.5 wt%and 15.9 wt%respectively by analyzing the thermogravimetric analysis curves.When the MWCNTs content reached 12 wt%and 15 wt%,the continuous conductive MWCNTs network was formed in the hydrogel,and ENZ phenomenon was observed at about 760 k Hz and 580 k Hz respectively,which was attributed to the interband transition.Considering the flexibility and non-toxicity of PVA/MWCNTs hydrogels,the ENZ properties of this structure at the radio frequency can be well used in wearable invisibility cloak,flexible electronics,skin sensors and other wearable devices.展开更多
基金supported by the National Natural Science Foundation of China(Nos.51771104,51871146,51971119)the Future Plan for Young Talent of Shandong University(No.2016WLJH40)the Innovation Program of Shanghai Municipal Education Commission(No.2019-01-07-00-10-E00053)。
文摘Negative permittivity has been widely studied in various metamaterials and percolating composites, of which the anomalous dielectric behavior was attributed to critical structural properties of building blocks.Herein, mono-phase ceramics of indium tin oxides(ITO) were sintered for epsilon-negative materials in MHz-k Hz frequency regions. Electrical conductivity and complex permittivity were analyzed with DrudeLorentz oscillator model. Carriers’ characters were measured based on Hall effect and the magnitude and frequency dispersion of negative permittivity were mainly determined by carrier concentration.Temperature-dependent dielectric properties further proved the epsilon-negative behaviors were closely associated with free carriers’ collective responses. It’s found that negative permittivity of ITO ceramics was mainly caused by plasma oscillations of free carriers, while the dielectric loss was mainly attributed to conduction loss. Negative permittivity realized here was related to materials intrinsic nature and this work preliminarily determined the mechanism of negative permittivity in doped ceramics from the perspective of carriers.
基金supported by the National Natural Science Foundation of China(Grant No.51601105,No.51803119,No.51871146)the Innovation Program of Shanghai Municipal Education Commission(Grant No.2019-01-07-00-10-E00053)+2 种基金Chenguang Program supported by Shanghai Education Development Foundation and Shanghai Municipal Education Commission(Grant No.18CG56)the support of National Research Foundation Singapore(NRF-CRP17-2017-01),for research conducted at National University of Singaporethe support from the China Scholarship Council.
文摘Epsilon-negative materials with high thermal conductivity and low electrical conductivity are of great importance for high power microwave devices.In this work,BaTiO_(3)/Cu composites,as a class of epsilonnegative materials,are rationally designed to achieve a high thermal conductivity yet maintaining the electrical insulative character.Negative permittivity behavior induced by dielectric resonance and plasma oscillation is observed in these BaTiO_(3)/Cu composites,which can be explained by the Lorentz and Drude model respectively.An outstanding absorption ability is achieved near the zero-cross point of the permittivity.Benefiting from the positive temperature coefficient of resistance and the weak temperature dependence of thermal conductivity in BaTiO_(3)/Cu composites,sample containing 22.3 vol% of Cu content exhibits a thermal conductivity of up to 17.7 W/(m·k)and an electrical conductivity down to 0.0022(Ω cm)^(-1) at 150℃.Therefore,BaTiO_(3)/Cu composite is a promising candidate for applications in electromagnetic attenuation and thermal management.
基金The authors thank Xueyan Fu for her advice on electromagnetic simulation using Computer Simulation Technology sofware.Te authors thank Yao Liu,Guohua Fan,Yunpeng Qu,and Yuliang Jiang for their advice for doing experiments and measurements.The authors acknowledge the support of the National Natural Science Foundation of China[Grants nos.51601105,51871146,and 51803119]the Young Elite Scientists Sponsorship Program by CAST(Grant no.2017QNRC001)the Innovation Program of Shanghai Municipal Education Commission.
文摘Te mechanism of negative permittivity/permeability is still unclear in the random metamaterials,where the precise control of microstructure and electromagnetic properties is also a challenge due to its random characteristic.Here silver was introduced into porous SiO_(2) microsphere matrix by a self-assemble and template method to construct the random metamaterials.Te distribution of silver was restricted among the interstices of SiO_(2) microspheres,which lead to the precise regulation of electrical percolation(from hoping to Drude-type conductivity)with increasing silver content.Negative permittivity came from the plasma-like behavior of silver network,and its value and frequency dispersion were further adjusted by Lorentz-type dielectric response.During this process,the frequency of epsilon-near-zero(ENZ)could be adjusted accordingly.Negative permeability was well explained by the magnetic response of eddy current in silver micronetwork.Te calculation results indicated that negative permeability has a linear relation with ω^(0.5),showing a relaxation-type spectrum,diferent from the“magnetic plasma”of periodic metamaterials.Electromagnetic simulations demonstrated that negative permittivity materials and ENZ materials,with the advantage of enhanced absorption(40dB)and intelligent frequency selection even in a thin thickness(0.1 mm),could have potentials for electromagnetic attenuation and shielding.Tis work provides a clear physical image for the theoretical explanation of negative permittivity and negative permeability in random metamaterials,as well as a novel strategy to precisely control the microstructure of random metamaterials.
基金supported by the National Natural Science Foundation of China(Nos.51771104,51871146,51971119)the Natural Science Foundation of Shandong Province(No.ZR2020YQ32)the Innovation Program of Shanghai Municipal Education Commission(No.2019-01-07-00-10-E00053)。
文摘The development of negative permittivity materials in multifunctional applications requests expansion of their operating frequency and improvement of stability of negative permittivity.Low electron density is beneficial to reduce plasma frequency so that negative permittivity is achieved in kHz region.Negative permittivity achieved by percolating composites is restricted in practicality due to its instability nature at high temperatures.To achieve temperature-stable negative permittivity in kHz region,monophase La_(1-x)Ba_(x)CoO_(3)ceramics were prepared,and the transition from dielectric to metal was elaborated in the perspective of electrical conductivity and negative permittivity.The plasma-like negative permittivity is attained in kHz region,which is interpreted by the collective oscillation of low electron density.The temperature-stable negative permittivity is based on the fact that the plasmonic state will not be undermined at high temperatures.In addition,zero-crossing behavior of real permittivity is observed in La_(0.9)Ba_(0.1)CoO_(3)sample,which provides a promising alternative to designing epsilon-near-zero materials.This work makes the La_(1-x)Ba_(x)CoO_(3)system a source material for achieving effective negative permittivity.
基金sponsored by the National Natural Science Foundation of China (Grant No.51803119,51871146 and 51771108)the Innovation Program of Shanghai Municipal Education Commission (Grant No.2019-01-07-00-10-E00053)+1 种基金"Chenguang Program" supported by Shanghai Education Development Foundation and Shanghai Municipal Education Commission (Grant No.18CG56)the Science and Technology Commission of Shanghai Municipality (Grant No.18DZ1112902,No.18DZ1100802)
文摘Recently,increasing attention has been concentrated on negative permittivity with the development of the emerging metamaterials composed of periodic array structures.However,taking facile preparation into consideration,it is important to achieve negative permittivity behavior based on materials'intrinsic properties rather than their artificially periodic structures.In this paper,we proposed to fabricate the percolating composites with copper dispersed in epoxy(EP)resin by a polymerization method to realize the negative permittivity behavior.When Cu content in the composites reached to 80 wt%,the conductivity abruptly went up by three orders of magnitudes,suggesting a percolation behavior.Below the percolation threshold,the conductivity spectra conform to Jonscher's power law;when the Cu/EP composites reached to percolating state,the conductivity gradually reduced in high frequency region due to the skin effect.It is indicated that the conductive mechanism changed from hopping conduction to electron conduction.In addition,the permittivity did not increase monotonously with the increase of Cu content in the vicinity of percolation threshold,due to the presence of leakage current.Meanwhile,the negative permittivity conforming to Drude model was observed above the percolation threshold.Further investigation revealed that there was a constitutive relationship between the permittivity and the reactance.When conductive fillers are slightly above the percolation threshold,the inductive characteristic derived from conductive percolating network leads to the negative permittivity.Such epsilon-negative materials can potentially be applied in novel electrical devices,such as high-power microwave filters,stacked capacitors,negative capacitance field effect transistors and coil-free resonators.In addition,the design strategy based on percolating composites provides an approach to epsilon-negative materials.
基金the financial supports from the National Natural Science Foundation of China(51871146)the Natural Science Foundation of Shanghai(22ZR1426800)+3 种基金Young Elite Scientist Sponsorship Program by China Association for Science and Technology(YESS20200257)the Innovation Program of Shanghai Municipal Education Commission(2019-01-07-0010-E00053)China National Postdoctoral Program for Innovative Talents(BX2021174)China Postdoctoral Science Foundation(2021M692033)。
文摘Epsilon-near-zero(ENZ)material has been a research hotspot in recent years due to unique physical properties such as inverse Doppler effect and negative refractive index,showing great potentials in the fields of flexible electronics,wearable devices,sensors,etc.The ENZ materials are mostly reported at visible,infrared and terahertz wavelengths,while the report about ENZ materials at radio frequency is rare.In this work,flexible and biocompatible poly(vinyl alcohol)/multi-walled carbon nanotubes(PVA/MWCNTs)hydrogels,which were successfully fabricated by an environmentally friendly method,were used as ENZ materials at radio frequency for the first time.Cytotoxicity experiments proved that the experimental process and products are green,environmentally friendly and biocompatible.The microstructure,crystalline structure,chemical composition and dielectric properties were investigated.Two different water states,which were free water molecules and bound water molecules,coexisted in these PVA/MWCNTs hydrogels and accounted for about 37.5 wt%and 15.9 wt%respectively by analyzing the thermogravimetric analysis curves.When the MWCNTs content reached 12 wt%and 15 wt%,the continuous conductive MWCNTs network was formed in the hydrogel,and ENZ phenomenon was observed at about 760 k Hz and 580 k Hz respectively,which was attributed to the interband transition.Considering the flexibility and non-toxicity of PVA/MWCNTs hydrogels,the ENZ properties of this structure at the radio frequency can be well used in wearable invisibility cloak,flexible electronics,skin sensors and other wearable devices.
