阅读说明：本技术 一种宽频吸波超构材料 (Broadband wave-absorbing metamaterial ) 是由 邹春荣 郭少军 沈同圣 周晓松 于 2021-09-29 设计创作，主要内容包括：本发明属于吸波材料及超材料技术领域,提出一种宽频吸波超构材料。包括周期性排布的元胞单元,元胞单元的周期分布呈现正方形,周期大小根据吸波带宽及最高吸波工作频率进行确定,以确保电磁波在材料表面不发生明显的衍射和反射；元胞单元的材料为单一介质型吸波介质,并且元胞单元沿电磁波入射传播方向分为上下两部分,上部分为轴对称梯度渐变结构,下部分为连续结构；元胞上下部分介质的厚度根据吸波带宽、介质型吸波材料的本征介电常数和损耗角正切值进行确定。本发明通过设计不同的外形轮廓的变化梯度,可以控制材料介电常数随厚度的渐变特性,从而尽可能的减小表面反射,使得电磁波有效的进入吸波材料内部,并且在传播过程中被不断吸收损耗。(The invention belongs to the technical field of wave-absorbing materials and metamaterials, and provides a broadband wave-absorbing metamaterial. The wave absorption material comprises periodically arranged cellular units, wherein the periodic distribution of the cellular units is square, and the period is determined according to the wave absorption bandwidth and the highest wave absorption working frequency so as to ensure that electromagnetic waves are not obviously diffracted and reflected on the surface of the material; the cellular unit is made of a single-medium type wave-absorbing medium and is divided into an upper part and a lower part along the incident and propagation direction of electromagnetic waves, the upper part is of an axisymmetric gradient structure, and the lower part is of a continuous structure; the thicknesses of the upper and lower parts of the medium of the cellular are determined according to the wave-absorbing bandwidth, the intrinsic dielectric constant and the loss tangent value of the medium type wave-absorbing material. According to the invention, by designing the change gradient of different outline profiles, the gradual change characteristic of the material dielectric constant along with the thickness can be controlled, so that the surface reflection is reduced as much as possible, electromagnetic waves effectively enter the wave-absorbing material, and are continuously absorbed and lost in the transmission process.)

1. A broadband wave-absorbing metamaterial comprises periodically arranged cellular units, and is characterized in that:

the periodic distribution of the cellular units is square, and the period is determined according to the wave-absorbing bandwidth and the highest wave-absorbing working frequency so as to ensure that electromagnetic waves are not obviously diffracted and reflected on the surface of the material;

the cellular units are made of a single-medium type wave-absorbing medium and are divided into an upper part and a lower part along the incident and propagation direction of electromagnetic waves, the upper part is of an axisymmetric gradient structure, and the lower part is of a continuous structure;

the thicknesses of the upper and lower medium parts of the cellular unit are determined according to the wave-absorbing bandwidth, the intrinsic dielectric constant and the loss tangent value of the medium type wave-absorbing material, the thickness of the gradient structure of the upper part of the cellular unit is 6-14 mm, and the thickness of the lower medium part of the cellular unit is 6-10 mm.

2. The broadband wave-absorbing metamaterial according to claim 1, wherein when the upper medium of the unit cell units adopts a gradient gradual change structure with a symmetrical central axis, the outline of the upper medium is conical, parabolic or horn curve, and the radius of the bottom of the gradient structure gradually increases along the incident and propagation direction of the electromagnetic wave, so that the gradually increasing dielectric constant expanding from the surface to the inside is generated, and the gradually matching effect is generated.

3. The broadband wave-absorbing metamaterial according to claim 1, wherein the dielectric type wave-absorbing dielectric material has a dielectric constant ranging from 5 to 15 and a loss tangent ranging from 0.4 to 0.8.

4. The broadband wave-absorbing metamaterial according to any one of claims 1 to 3, wherein the period of the unit cells is 4mm to 14 mm.

5. The broadband wave absorbing metamaterial according to claim 4, wherein the broadband wave absorbing material has a good broadband wave absorbing effect when the electromagnetic waves are incident at a large oblique angle.

6. The method for processing the broadband wave-absorbing metamaterial according to claim 1, wherein in the processing of the broadband wave-absorbing metamaterial, the upper and lower functional structures of the cell units are regarded as an integral structure, or the upper and lower structural functions are separated in consideration of reducing stress concentration formed at the connection of the upper and lower parts, and the processing is performed by adopting a 3D printing, casting and forming or machining mode to obtain good processing characteristics.

Technical Field

The invention belongs to the technical field of wave-absorbing materials and metamaterials, and particularly relates to a broadband wave-absorbing metamaterial.

Background

The electromagnetic wave-absorbing material with light weight, environmental corrosion resistance and strong wave absorption is a stealthy key material of equipment, and the key for developing a novel wave-absorbing material is how to reduce the reflection coefficient and widen the wave-absorbing bandwidth. Generally, the wave-absorbing material needs to have both impedance matching characteristic and loss characteristic to convert the electromagnetic wave energy into heat energy and dissipate the heat energy. The impedance matching characteristic means that the surface impedance value of the wave-absorbing material is matched with the impedance value of the electromagnetic wave, so that the reflectivity of the surface is the lowest, and the electromagnetic wave enters the wave-absorbing material as much as possible; the attenuation characteristic refers to the ability of the wave-absorbing material to dissipate the energy of the electromagnetic waves entering the wave-absorbing material.

Electromagnetic wave-absorbing materials generally include materials such as metal periodic structures, wave-absorbing resins, wave-absorbing ceramics, and the like, and can be classified into three types, namely, a conductive loss type, a magnetic loss type, and a dielectric loss type, according to different ways of converting incident electromagnetic wave energy into heat energy. The electric loss wave absorbing agent mainly takes carbon allotrope as main material, and comprises carbon black, graphite, carbon fiber, carbon nano tube, graphene material and the like; the magnetic loss type mainly comprises ferrite, carbonyl iron, super-metal micro powder and the like; the dielectric loss type material includes barium titanate based materials, dielectric ceramics, fiber reinforced ceramic matrix composites, and the like. Under complex service environments such as high temperature, high humidity and high salt, the corrosion resistance and the high temperature resistance of the metal periodic structure material and the magnetic wave absorbing agent are often difficult to meet the requirements; novel electrically-lossy carbon wave absorbers such as graphene, carbon nanotubes and nanofibers face the problems of uniform dispersion and accurate adjustment of electromagnetic parameters; compared with the dielectric wave-absorbing resin and the ceramic material, the dielectric wave-absorbing resin and the ceramic material have more excellent high-temperature resistance and environmental adaptability.

The dielectric wave-absorbing material mainly comprises a complex phase material and a fiber reinforced composite material. The complex phase material generally consists of a wave absorbing agent and a wave absorbing matrix, wherein a wave absorbing agent carrier is mainly a wave-transparent phase to realize impedance matching between the wave absorbing material and an electromagnetic wave transmission medium, and a dielectric wave absorbing agent is introduced into the wave-transparent phase through modes of doping, modification, heat treatment and the like to enable the wave absorbing material to become the wave absorbing material. Meanwhile, the single-layer structure wave-absorbing material often cannot achieve an ideal stealth effect, and the broadband wave-absorbing performance needs to be improved through a multi-layer design, such as a corrugated plate sandwich structure, a pyramid sandwich structure, a wave-absorbing layer structure added in a spreading layer and the like. The composite material multilayer structure generally comprises an upper surface impedance matching layer, a middle wave-absorbing layer and a lower surface reflecting layer, and has better matching property and wave-absorbing performance. At present, the wave-absorbing composite material and the composite material can realize full-frequency effective absorption of an X (8-12 GHz) wave band through precise component dielectric parameter regulation and complex multilayer structure design, but the wave-absorbing composite material and the composite material still have the limitations of weak wave-absorbing capacity, narrow absorption frequency band and the like under most conditions. Specifically, the method comprises the following steps: on one hand, a material system needs to simultaneously contain a wave-transmitting phase and a wave-absorbing phase, namely an A + B two-phase structure, and the design of matching design of components and electromagnetic parameters between two-phase materials has great difficulty. The effect is limited when a single wave-absorbing material is adopted, and particularly when the real part of the dielectric constant of the material is increased to about 10, the surface of the material generates serious impedance mismatch to cause the surface reflection to rise steeply; secondly, the requirement on the thickness of the material is harsh, although the increase of the thickness of the material is beneficial to improving the performances such as mechanics and the like, the reflectivity is increased along with the increase of the thickness of the material, so that only a thinner thickness is often adopted; and thirdly, the broadband wave absorbing efficiency is low, and the broadband wave absorbing effect of 8-18 GHz even 4-18GHz is relatively limited, which is mainly caused by the fact that the electrical property design space is small and the broadband wave absorbing is difficult to realize due to the fact that the existing broadband wave absorbing structure such as a multilayer structure is limited in structural diversity and flexibility.

Disclosure of Invention

The invention aims to provide a broadband wave-absorbing superstructure, which solves the problems that the existing wave-absorbing complex-phase material and multilayer composite material are difficult to accurately regulate and control component dielectric parameters, complex in multilayer structure design and the like, the existing wave-absorbing complex-phase material needs to be composed of two phases of the latter, the wave-absorbing capacity is weak, the absorption frequency band is narrow and the like, breaks through the existing broadband wave-absorbing structure design thought based on a multilayer or sandwich structure, and provides a new design thought of periodic structuring for the design of a broadband wave-absorbing structure.

In order to achieve the above purpose and solve the above technical problems, the following technical solutions are provided:

a broadband wave-absorbing superstructure comprises periodically arranged cellular units, wherein the cellular units are in a square shape in periodic distribution, and the period size is determined according to wave-absorbing bandwidth and the highest wave-absorbing working frequency so as to ensure that electromagnetic waves are not obviously diffracted and reflected on the surface of a material;

the cellular unit is made of a single-medium type wave-absorbing medium and is divided into an upper part and a lower part along the incident and transmission direction of electromagnetic waves, the upper part of the cellular unit is of an axisymmetric gradient structure, and the lower part of the cellular unit is of a continuous structure;

the thicknesses of the upper and lower medium parts of the cell are determined according to the wave-absorbing bandwidth, the intrinsic dielectric constant and the loss tangent value of the medium type wave-absorbing material, the thickness of the gradient gradual change structure of the upper part of the cell unit is 6-14 mm, and the thickness of the lower medium part of the cell unit is 6-10 mm.

Furthermore, when the upper part of the medium of the cellular unit adopts a gradient gradual change structure with a symmetrical central axis, the outline of the upper part of the medium is conical, parabolic or horn curve, and the radius of the bottom of the gradient structure is gradually increased along the incident propagation direction of the electromagnetic wave, so that the gradually increased dielectric constant expanding from the surface to the inside is generated, and the effect of gradual matching is generated.

Furthermore, the dielectric constant range of the dielectric type wave-absorbing dielectric material is 5-15, and the loss tangent range is 0.4-0.8.

Furthermore, the period size of the unit cell is 4 mm-14 mm.

Furthermore, the broadband wave-absorbing material still has a good broadband wave-absorbing effect when the electromagnetic waves are incident at a large oblique incident angle.

Furthermore, when the broadband wave-absorbing metamaterial is processed, the upper and lower functional structures of the cellular units can be regarded as an integral structure, or the upper and lower structural functions can be separated in order to reduce stress concentration formed at the joint of the upper and lower parts, and the processing can be carried out by adopting a 3D printing, casting and forming or machining mode so as to obtain good processing characteristics.

The effective benefits of the invention are as follows:

1. the broadband wave-absorbing metamaterial provided by the invention realizes impedance matching of a low-frequency broadband through the structural units with the structuralization and gradient gradual change, reduces electromagnetic wave reflection, realizes high-efficiency wave absorption by utilizing intrinsic loss of a wave-absorbing medium, overcomes the limitation that the traditional wave-absorbing material needs a wave-transmitting bearing medium and a plurality of layers of wave-absorbing materials are complex in design by only adopting a single wave-absorbing medium material, and provides a new choice for broadband sound-absorbing materials.

2. According to the broadband wave-absorbing metamaterial provided by the invention, a new broadband impedance matching method is introduced without adding a traditional bottom reflecting layer, impedance matching is realized only by a gradient gradually-changed unit structure, the broadband wave-absorbing metamaterial is obviously different from the traditional impedance matching method based on a transmission line theory, the broadband impedance matching design means of the wave-absorbing material is expanded, and the intrinsic electromagnetic loss characteristic of a medium is fully exerted.

3. The broadband wave-absorbing metamaterial provided by the invention can be used for carrying out targeted design on a wide dielectric constant range and a loss tangent value range, and the limitation that the traditional dielectric wave-absorbing material with high dielectric constant and large thickness size is difficult to achieve broadband efficient absorption is changed. For the wave-absorbing medium material, the comprehensive properties such as mechanical stability, environmental resistance and the like can be enhanced by increasing the thickness, and the integration of a multifunctional structure can be realized.

4. The broadband wave-absorbing metamaterial provided by the invention is insensitive to the incident angle and polarization of electromagnetic waves, and can still maintain effective broadband wave absorption under the condition of a large incident angle.

5. The broadband wave-absorbing metamaterial provided by the invention has a simple cellular structure, is easy to realize, has strong designability of the cellular structure, can flexibly adjust the wave-absorbing performance and the mechanical property of the material, and meets the integrated requirements of light weight and low-frequency broadband sound absorption.

Drawings

FIG. 1 is a schematic diagram of a broadband wave-absorbing metamaterial;

wherein: 1-gradient gradual change structure on the upper part of the cellular unit; 2-a continuous structure below the unit of cells;

FIG. 2 is a reflectivity curve of the broadband wave-absorbing metamaterial in the broadband range of 4 to 18GHz in example 1;

FIG. 3 is an absorption rate curve of the broadband wave-absorbing metamaterial in the broadband range of 4 to 18GHz in example 1;

FIG. 4 is a graph of the reflectance of the flat plate materials of different thicknesses in the wide frequency range of 4 to 18GHz in example 1;

FIG. 5 is an absorption curve of the flat plate materials with different thicknesses in the wide frequency range of 4-18GHz in example 1;

FIG. 6 is a reflectivity curve of the broadband wave-absorbing metamaterial in the broadband range of 4 to 18GHz in example 2;

FIG. 7 is an absorption rate curve of the broadband wave-absorbing metamaterial in the broadband range of 4 to 18GHz in example 2;

FIG. 8 is a schematic diagram of a broadband absorbing superstructure with a parabolic profile of a gradient structure in example 3;

FIG. 9 is a reflectivity curve of the broadband wave-absorbing metamaterial in the broadband range of 4 to 18GHz in example 3;

FIG. 10 is an absorption rate curve of the broadband wave-absorbing metamaterial in the broadband range of 4 to 18GHz in example 3;

FIG. 11 is a schematic diagram of a broadband absorbing superstructure in which the gradient structure has a horn-shaped profile in example 4;

FIG. 12 is a reflectivity curve of the broadband wave-absorbing metamaterial in the broadband range of 4 to 18GHz in example 4;

FIG. 13 is an absorption curve of the broadband wave-absorbing metamaterial in the broadband range of 4 to 18GHz in example 4;

FIG. 14 is a reflectivity curve of the broadband wave-absorbing metamaterial in the broadband range of 4 to 18GHz in example 5;

FIG. 15 is an absorption rate curve of the broadband wave-absorbing metamaterial in the broadband range of 4 to 18GHz in example 5;

FIG. 16 is a reflectivity curve of the broadband absorbing metamaterial in example 5 when electromagnetic waves are obliquely incident in a broadband range of 4 to 18 GHz;

FIG. 17 is a graph showing the reflectance of flat plate materials of different thicknesses in the wide frequency range of 4 to 18GHz in example 5;

FIG. 18 is an absorption curve of the flat plate materials with different thicknesses in the wide frequency range of 4 to 18GHz in example 5.

Detailed Description

For a better understanding of the present invention, reference is made to the following detailed description and accompanying drawings.

In the prior art, the structural form of the broadband wave-absorbing medium is relatively fixed, the micro components of the medium material are that the wave-absorbing medium is dispersed in a wave-transparent carrier medium, and the bottom of the medium material is a total reflection metal plate when wave-absorbing tests are carried out, but the wave-absorbing medium can only effectively absorb waves in a narrow frequency band range. The composite material usually adopts a multilayer structure form, comprises a top matching layer, a middle wave-absorbing layer and a lower reflecting layer, has better matching property, but the material system which can regulate and control the resistivity, namely the dielectric constant in a larger range is limited at present.

The invention aims to provide a broadband wave-absorbing metamaterial, which comprises periodically arranged cellular units, and fig. 1 is a schematic diagram of the broadband wave-absorbing metamaterial, which comprises 4 multiplied by 4 cellular units. The cellular unit is divided into an upper part and a lower part along the incident propagation direction of the electromagnetic wave, the upper part of the cellular unit is of an axisymmetric gradient structure, and the lower part of the cellular unit is of a continuous structure. The cellular units realize continuous broadband matching with incident electromagnetic waves through a gradient gradual change structure, so that the electromagnetic waves can effectively enter the wave-absorbing material and reduce reflection, and further, the electromagnetic waves of broadband are effectively absorbed by utilizing the intrinsic high-loss characteristic of the wave-absorbing medium, so that the broadband wave-absorbing effect is achieved, and the cellular units are insensitive to the polarization and the incident angle of the electromagnetic waves.

The invention provides a broadband wave-absorbing metamaterial, which breaks through the limitation of the existing dielectric wave-absorbing material. The material is designed into a unit with periodic change, and the unit presents a gradually changed outline along the incident direction of the electromagnetic wave, so that the spatial impedance with gradient change is generated, and the impedance matching effect of a broadband can be realized; and by designing the change gradient of different appearance profiles, the gradual change characteristic of the dielectric constant of the material along with the thickness can be controlled, so that the surface reflection is reduced as much as possible, the electromagnetic waves effectively enter the wave-absorbing material, and the loss is continuously absorbed in the transmission process. In addition, the invention only adopts a single wave-absorbing medium material, does not need to use a wave-absorbing agent carrier, does not need to accurately control the internal components of the wave-absorbing medium material and the dielectric parameter value thereof, and comprehensively regulates and controls the change of the dielectric constant along the propagation direction of the electromagnetic wave through the design of a gradient appearance structure, thereby being not limited by the size and the thickness of the intrinsic dielectric constant of the medium. The invention adopts a single medium material, can also simplify the preparation process, and can be molded and prepared by various mature processes such as injection-coagulation molding, 3D printing, machining and the like, thereby realizing multiple functions such as broadband wave-absorbing stealth, structural function integration and the like.

The following are 5 examples of implementations of the invention. In the examples, dielectric constants of 5, 10 and 15 are taken to represent low, medium and high dielectric constants, respectively, and dielectric losses of 0.4 and 0.8 are taken to represent medium and high dielectric losses, respectively.

Example 1

The period of the metamorphic material cellular unit is 12mm, the thickness of the gradient structure of the upper part is 8mm, the outline can be conical, the radius of the bottom of the gradient structure of the surface layer is gradually increased along the incident propagation direction of electromagnetic waves, the thickness of the continuous medium of the lower part is 8mm, and the total thickness is 16 mm. The metamaterial adopts a single dielectric wave-absorbing medium, the dielectric constant of the metamaterial is 5.56, and the loss tangent of the metamaterial is 0.4. The ratio of the top diameter of the upper gradient structure to the cell period area is 0.8, and the ratio of the bottom diameter to the cell period is 1.0. The reflectivity and the effective absorption rate of the periodic material in a wide frequency range of 4-18GHz are analyzed by a finite element method, and the results are shown in fig. 2 and fig. 3. As can be seen from FIG. 2, the reflectivity of the periodic material is below-10 dB in the broadband range of 4-18GHz, so that the reflection of the detection electromagnetic wave can be effectively reduced, and the radar scattering area can be reduced. The intrinsic loss characteristic and the electromagnetic wave propagation characteristic of the wave-absorbing medium material are combined, the periodic structure does not contain a reflecting layer, and most incident electromagnetic waves (more than 90 percent) are transmitted or lost and absorbed. As shown in FIG. 3, the effective absorption rate of electromagnetic waves in the wide frequency range of 4 to 18GHz is mainly due to the intrinsic dielectric loss of the dielectric material, and it can be seen that the effective absorption rate of electromagnetic waves in the wide frequency range of 7.2 to 18GHz is higher than 0.8, and even if the frequency is lower, the effective absorption rate is higher than 0.55 in 4 to 7.2GHz, and increases with the increase of the frequency. The comparison shows that when the ratio of the diameter of the top of the structured medium to the period value is 0.75, the overall reflectivity of the material is smaller, the effective absorption rate is higher, and the large gradient change (the former) in a certain range is beneficial to better realizing broadband impedance matching. Fig. 4 and 5 show the reflectivity and the effective absorption rate of a pure medium plate in a wide frequency range of 4-18GHz when the thickness of the pure medium plate is 8mm and 16mm respectively, where the thickness corresponds to the thickness of a bottom continuous medium of the metamaterial and the total thickness of the metamaterial, and the comparison shows that the pure plate is difficult to reach below-10 dB in the frequency range of 4-18GHz, the effective absorption rate is less than 0.8, and the effective absorption rate is lower than that of the metamaterial designed by the invention, which indicates that the broadband wave-absorbing metamaterial provided by the invention has good broadband wave-absorbing performance.

Example 2

The dielectric constant of the metamaterial is 10, the loss tangent is 0.4, the thickness of the upper gradient structure is 8mm, the outline is conical, the radius of the bottom of the surface gradient structure in the incident propagation direction of electromagnetic waves is gradually increased, the thickness of the lower continuous medium is 8mm, and the total thickness is 16 mm. The broadband reflection and wave absorption performance of the material under two periodic structures is analyzed in a simulation mode, firstly, the cellular period is 14mm, the diameter of the gradient structure of the upper part electromagnetic wave incident end is 9.8mm, and the ratio of the gradient structure to the period is 0.7; secondly, the cell period is 10mm, the diameter of the gradient structure at the incident end of the upper part of the electromagnetic wave is 6.0mm, namely the ratio of the diameter to the period is 0.6, and the results are shown in fig. 6 and 7. As can be seen from the reflectance of fig. 6, the reflectance of the periodic material is-10 dB or less in a wide frequency band of 4 to 18GHz, the reflection of electromagnetic waves can be effectively reduced, and the overall reflectance is lower when the period is 10 mm. As shown in FIG. 7, the effective absorption rate of electromagnetic waves in the broadband range of 4 to 18GHz is mainly due to the intrinsic dielectric loss of the medium, and is higher than 0.8 in the broadband range of 6.0 to 18GHz, and higher than 0.9 in the broadband range of 8 to 18 GHz. In summary, the broadband wave-absorbing metamaterial provided by the invention has good broadband wave-absorbing performance and good designability on a periodic structure and a gradient change appearance structure.

Example 3

The dielectric constant of the metamaterial is 10, the loss tangent is 0.4, the thickness of the upper gradient structure is 8mm, the outline is parabolic, the radius of the bottom of the surface gradient structure is gradually increased along the incident propagation direction of electromagnetic waves, and the outline is shown in fig. 8. The thickness of the lower part of the continuous medium is 10mm, and the total thickness is 18 mm. The cellular period is 14mm, the diameter of the gradient structure of the upper part of the electromagnetic wave incident end is 5.6mm, namely the ratio of the gradient structure to the period is 0.4, and the broadband reflection and wave absorption performance of the metamaterial are simulated and analyzed, and the results are shown in fig. 9 and 10. It can be seen that the reflectivity of the periodic medium is below-10 dB in a broadband range of 4-18GHz, the reflection of the detection electromagnetic wave can be effectively reduced, the effective absorption rate of the electromagnetic wave is higher than 0.8 in the broadband range of 6-18 GHz, and the effective absorption rate of the electromagnetic wave is higher than 0.9 in the broadband range of 8-18 GHz. In summary, the broadband wave-absorbing metamaterial provided by the invention has good broadband wave-absorbing performance and good design flexibility in the shape structure of gradient change.

Example 4

The dielectric constant of the metamaterial is 5, the loss tangent is 0.65, the thickness of the continuous medium at the lower part is 8mm, the cell period is 4mm, the outline is horn-shaped, the radius of the bottom of the surface layer gradient structure is gradually increased along the incident propagation direction of electromagnetic waves, the outline is shown in figure 11, the ratio of the diameter of the gradient structure at the incident end of the electromagnetic waves at the upper part of the cell to the period is 0.3, the broadband reflection and wave absorption performance of the metamaterial under the condition that the gradient structures at the upper part of the cell are respectively 6mm, 10mm and 14mm is analyzed, and the result is shown in figure 12 and figure 13. It can be seen that the reflectivity of the periodic material is below-10 dB in a broadband range of 8-18 GHz, and the effective absorption rate of electromagnetic waves in the broadband range of 8-18 GHz is higher than 0.8; with the thickness of the gradient structure of the upper part increased from 6mm to 14mm, the broadband reflectivity of the metamaterial gradually decreases, and the wave absorbing rate is increased, so that a basis can be provided for broadband wave absorbing design.

Example 5

The dielectric constant of the metamaterial is 15, the loss tangent is 0.8, the cell period is 8mm, the thickness of the lower continuous medium is 6mm, the thickness of the upper gradient structure of the cell is 10mm, the outline is conical, and the radius of the bottom of the surface gradient structure is gradually increased along the incident propagation direction of electromagnetic waves. The simulation analysis shows the broadband reflection and wave absorption performance under the condition that the ratio of the diameter of the gradient structure of the incidence end of the partial electromagnetic wave on the cellular structure to the period is 0.5 and 0.65 respectively, and the results are shown in fig. 14 and 15. It can be seen that the reflectivity of the periodic material is below-10 dB in a broadband range of 4-18GHz, and the effective absorption rate of the electromagnetic waves in the broadband range of 4-18GHz is higher than 0.85. When the ratio of the diameter of the incident end of the upper part of the gradient structure to the period is 0.65, the effective absorption rate of the electromagnetic wave in the broadband range of 4-18GHz is higher than 0.85, and the excellent broadband wave-absorbing effect is realized. By adopting the existing design method, the electromagnetic reflection coefficient of a broadband of the high-dielectric-constant high-loss material is generally difficult to be less than-10 dB, but when the electromagnetic reflection coefficient of the material reaches-4 dB, about 36% of electromagnetic wave energy can be attenuated and absorbed by the material, namely, the material has a good wave absorbing effect, and the structure of the invention can break through the limitation and achieve the effective absorption of the broadband. FIG. 16 is a reflectivity curve of the metamaterial in a broadband range of 4-18GHz when the electromagnetic wave is obliquely incident (incident angles are 30 degrees and 45 degrees), and it can be seen that the metamaterial still maintains effective broadband wave absorption under the oblique incident condition, and the broadband reflectivity of 4-18GHz is still less than-10 dB when the incident angle is 30 degrees; when the incident angle of the electromagnetic waves is 45 degrees, the broadband reflectivity is still less than-10 dB except for a few frequency points close to 4GHz, which shows that the metamaterial provided by the invention has excellent broadband wave-absorbing performance under the condition of large-angle oblique incidence of the electromagnetic waves. Fig. 17 and 18 show the reflectivity and the effective absorption rate of a pure medium plate in a wide band range of 4 to 18GHz when the thickness of the pure medium plate is 6mm and 16mm, respectively, where the thickness corresponds to the total thickness of a bottom continuous medium of the metamaterial and the metamaterial, and it can be known by comparison that the pure plate is difficult to reach less than-5 dB in the frequency range of 4 to 18GHz, the effective absorption rate is less than 0.6, and both are lower than the metamaterial designed by the present invention, which indicates that the broadband wave-absorbing metamaterial provided by the present invention has good broadband wave-absorbing performance.