Study On Absorbing Performance Of Imidazole Ionic Liquids And Ultra-Wideband Absorbers Build And Its Mechanism

In recent years,with the rapid development of electronic information technology,the problem of electromagnetic radiation has become increasingly serious which has been a new type of environmental pollution besides air pollution,water pollution and noise pollution,and it poses a great threat to electronic equipment,human health,defense civil facilities and other aspects.Electromagnetic wave absorbing material is a class of special material that consumes the electromagnetic energy by converting it into heat and other forms of energy or makes it cancel each other using electromagnetic wave interference.It can reduce electromagnetic pollution greatly and prevent the detection of military facilities and other issues.Based on the huge demand of national defense and electromagnetic protection,it has become a hot research topic and an urgent issue in various countries to develop high-performance absorbing material with strong performance,wide frequency bandwidth,thin thickness and light weight.However,the existing single material is difficult to meet the harsh requirements of the application environment.People primarily explore the new absorbing material with superior comprehensive performance by composing the traditional absorbing material which contains different loss mechanisms and looking for the ways of fabricating new materials and new structures.In this paper,the new soft functional material,ionic liquids,is introduced into the application field of absorbing materials.Ultra-wideband microwave absorber is constructed and its microwave absorbing mechanism is revealed.The primary work content is summarized as follows:Firstly,the dielectric properties and influencing factors of imidazole ionic liquids are researched.The dielectric properties of[BMIm]~+and[EMIm]~+imidazolium ionic liquids and their correlations with anions,cations,chain lengths,temperature and binary mixture system are investigated in the frequency range from 1 to 14 GHz based on the built open coaxial test system.Furthermore,the related theoretical analysis is carried out.It is found that the dielectric properties of imidazole ionic liquids have relatively small real parts and large dielectric loss angles,which are beneficial to electromagnetic impedance matching and electromagnetic dielectric loss,and possess the necessary conditions to become absorbing materials.With the increase of cation chain length and anion symmetry,both the real and imaginary parts of the ionic liquids dielectric properties decrease.In addition,increasing temperature results in an increase in dielectric properties.A binary mixture system of different ionic liquids can modulate dielectric properties.Secondly,the absorbing performance of imidazole ionic liquids and their relationship with influencing factors are studied.The relationship between the absorbing performance of ionic liquids and the types of anions and cations,temperature and coating thickness is studied based on dielectric data and single-layer homogeneous metal back plate model.The absorbing mechanism is analyzed by impedance matching theory and dielectric loss mechanism.The results show that the ionic liquids have good absorbing properties,such as[BMIm][OTf]with a minimum reflection loss of-33.1 dB at the peak frequency of 7.7 GHz.When the coating thickness is 4 mm,the absorbing band of ionic liquids is mainly concentrated in the frequency range of 6 to 12 GHz.The absorption peak frequency shifts to the low frequency region as the cation chain length decreases,the temperature rises,and the coating thickness increases.In-depth analysis of the loss mechanism shows that the conductive loss dominates in low frequency region,and the polarization loss gradually becomes the dominant factor with increasing frequency.Thirdly,a polarization-insensitive,wide incident angle and ultra-wideband microwave absorber is constructed based on ionic liquids and then studied.A method of expanding the absorbing bandwidth by introducing the top matching medium to form a multi-layer absorbing structure is proposed.Meanwhile,the theoretical analysis model is established to explore the effects of some parameters such as ionic liquid type,geometric parameters,polarization modes,and angles of incidence t on the absorbing performance of absorber.The results of free space method experiments demonstrate that the absorption frequency ranges from 9.26 to 50 GHz,the relative absorption bandwidth is 136.4%.And it has the advantages of polarization insensitivity,large incident angle and good temperature stability.The absorption mechanism of this type of absorber is explored by analyzing the electromagnetic parameter distribution,impedance matching and the unit dielectric loss at the peak absorption frequency.Fourthly,a gravity-tunable microwave absorbers based on ionic liquids is designed and studied.A method of regulating the microwave absorption performance by using gravity to change the shape of ionic liquids in absorbers is proposed.The regulation law of absorbing performance which is affected by rotation angle,polarization mode and temperature is analyzed.It is found that the absorbing performance can be greatly regulated by rotation angle.The absorber shows different electromagnetic wave absorption effects for TE and TM modes and has better regulation effect and temperature stability in both modes.The analysis of impedance matching and electromagnetic parameters at the peak frequency shows that the absorbers with different rotation angles have different impedance matching characteristics and electromagnetic resonance laws,and then realize the regulation of electromagnetic performance.Based on the above results,it can be concluded that ionic liquids can be used in the field of microwave absorption by making full use of the complex loss mechanism of conductance and dielectric polarization,and show excellent absorbing performance in X-band and C-band.At the same time,the performance of this type of absorbing materials can be flexibly designed and regulated according to the anion and cation,chain length,temperature,thickness and binary mixture system.Ultra-wideband absorbing materials and devices based on ionic liquids can be constructed by designing unit structures,which provides new ideas and methods for the design and implementation of absorbing materials.