Conductive polymer/magnetic particle composite absorbing material

AbsorberAs one of the effective means of protecting against electromagnetic interference, it is widely used in military and civilian fields. In the military field, absorbing materials have been used for the safety protection of large weapons such as airplanes and ships since the Second World War. With the rapid development of modern sensing technology and the increasing maturity of radar detection technology, stealth has become an important indicator in today's weapon design. Absorbing materials can reduce the strength of radar detection signals to a certain extent, achieving the effect of target stealth. In civilian use, absorbing materials can not only be used to manufacture protective clothing to protect the human body from electromagnetic radiation, but also in building materials in public places, greatly reducing electromagnetic radiation in densely populated areas.

The ideal absorbing material should meet four basic requirements: wide bandwidth, thin matching thickness, light weight, and strong absorption capacity. Traditional absorbing materials such as metals and ferrites are limited by their high density, narrow effective absorption bandwidth, and poor stabilityNew type of absorbing materialFor example, carbon based materials, conductive polymers, precursor ceramics, 3D structural composites, etc. have been widely used in the field of microwave absorption in recent years due to their excellent microwave absorption characteristics, impedance matching properties, and low density. Advanced Institute Technology first briefly explained the definition, mechanism, and classification of absorbing materials, and then introduced the performance characteristics of conductive polymers and magnetic particles, as well as the research status in the field of absorbing. It also introduced the research status, existing problems, and future development trends of conductive polymer/magnetic particle composite absorbing materials.

1. Absorbing material

1.1 Absorption mechanism of absorbing materials

Absorbing materials refer to materials with the ability to attenuate electromagnetic waves, which can convert the energy of electromagnetic waves projected onto their surface into thermal energy or other forms of energy through dielectric loss, magnetic loss, and other effects.

The composition and internal microstructure of absorbing materials are closely related to the attenuation of electromagnetic wave energy. In order to obtain absorbing materials that meet the requirements, the following two requirements need to be met: firstly, electromagnetic waves should be able to easily enter the interior of the material and be consumed, reducing their reflection on the material surface and improving the impedance matching of the material; The second electromagnetic wave can be consumed in a timely manner after entering the material, reducing secondary reflection and transmission and improving its loss ability.

According to Maxwell's electromagnetic wave theory, the impedance of electromagnetic waves propagating in an infinite medium is [18-22]:

When electromagnetic waves are incident on the surface of a dielectric material from vacuum, they are divided into reflected microwaves and transmitted microwaves. The reflectivity of the dielectric material is:

In the formula: Z represents the wave impedance of the medium; Z0 represents the wave impedance of vacuum; ε. μ is the dielectric constant and magnetic permeability.

From equation (2), it can be seen that when Z=Z0, i.e. R=0, the impedance between the medium and the vacuum wave reaches the optimal match, and the incident microwave completely enters the material without any reflected microwave, i.e.:

Due to the fact that the dielectric constant ε 0 and magnetic permeability μ 0 in vacuum are both 1, the dielectric constant of the medium with μ=ε also needs to be equal to the magnetic permeability in order to achieve the result of electromagnetic waves being completely absorbed without reflection. However, in reality, such absorbing materials do not exist, so researchers generally adjust themAbsorberThe relative magnitude of the dielectric constant and magnetic permeability of the material satisfies the impedance matching condition as much as possible.

The dielectric constant ε and magnetic permeability μ of dielectric materials are in complex form:

In the formula, the real parts ε ′ and μ ′ represent the energy storage capacity of the material for electrical and magnetic energy, while the imaginary parts ε ″ and μ ″ represent the material's ability to dissipate electromagnetic waves.

When electromagnetic waves propagate in materials, they cause polarization relaxation losses and resonance absorption in the medium, which absorb and attenuate the energy of electromagnetic waves and convert it into thermal energy before dissipating it. The electric dipole moment and magnetic dipole moment inside electromagnetic wave absorbing materials undergo displacement under external electric or magnetic field conditions, manifested macroscopically as polarization and magnetization phenomena, which convert electromagnetic wave energy into other energy through molecular motion and consume it [23]. The absorption of electromagnetic wave energy by electric loss absorbing materials is due to the dielectric loss during the polarization process, which is caused by the imaginary part of the dielectric constant ε″; The absorption of electromagnetic wave energy by magnetic loss absorbing materials is due to the loss of the magnetic medium during the magnetization process, which is caused by the imaginary part of the magnetic permeability. The attenuation coefficient of electromagnetic waves in materials is expressed as [24]:

The size of ε "and μ" plays a decisive role in the electromagnetic wave absorption capacity of the material. From equation (6), it can be seen that the attenuation coefficient α is related to the dielectric and magnetic losses of the material. Increasing the ε "and μ" of the absorbing material can improve its electromagnetic wave absorption capacity. The greater the dielectric and magnetic losses of the material, the greater the attenuation coefficient, indicating that electromagnetic waves are attenuated more rapidly during transmission.

1.2 Classification of absorbing materials

According to different classification criteria, microwave absorbing materials can be classified into different categories. Based on the microwave loss mechanism, microwave absorbing materials can be classified into dielectric loss materials, magnetic loss materials, and resistance loss materials.

According to the material forming process and load-bearing capacity, microwave absorbing materials can be classified into coating type absorbing materials and structural type absorbing materials. Coated absorbing materials [25] have strong adaptability to the shape of the target object and are easy to prepare. However, due to direct coating on the outer layer of the target object in contact with the external environment, good material stability is required. Structural absorbing materials have the characteristics of light weight and high strength. Common efficient absorbing structures include laminated structure absorbing bodies, sandwich structure absorbing bodies, metamaterial absorbing bodies, etc. [26]. Structural absorbing materials can serve as structural carriers and also absorb electromagnetic waves. Among them, the laminated structure absorbing materials have been studied the most, mainly consisting of a transmission layer, an absorption layer, and a reflection layer [27]. The structural analysis diagram is shown in Figure 1.

According to the principle of absorption, microwave absorbing materials can be classified into absorptive materials and interferometric materials. Absorbing materials can directly absorb and lose electromagnetic waves, and their absorption performance is related to the dielectric and magnetic properties of the material itself.

Interference type materials utilize the principle of equal amplitude and opposite phase of two columns of reflected waves reflected by the absorption layer on the surface and bottom layers to interfere and cancel, forming microwave absorption, following the 1/4 wavelength matching model [28]. At present, interferometric absorbing materials are divided into three basic types: Fess absorbing materials, Jauman absorbing materials, and general interferometric absorbing materials. The medium satisfies equation (7) [29]:

In the formula: n=1，2，3…n; C is the speed of light in vacuum; FM is the vibration frequency of the external electromagnetic field; μ r is the magnetic permeability; ε r is the dielectric constant; TM is the thickness of the medium that satisfies the interference condition.

According to different research periods, it can be divided into traditional absorbing materials and new absorbing materials [30-31]. Traditional absorbing materials mainly include ferrite, ceramic based materials, barium titanate, etc., most of which have disadvantages such as narrow absorption band and high density; Nanomaterials, polycrystalline iron fibers, plasma stealth materials, chiral materials, conductive polymer materials, etc. belong to new types of absorbing materials. Compared with traditional absorbing materials, new absorbing materials are more suitable for requirements such as thin, light, wide, and strong.

Introduction to Conductive Polymers and Magnetic Particles

2.1 Conductive Polymer

Conductive polymer materials can be divided into two categories: structural and composite. Structural conductive polymers refer to polymer materials that have conductivity themselves or have conductivity after doping treatment; Composite conductive polymer, also known as conductive polymer composite material, refers to a multiphase composite material with both conductivity and good mechanical properties generated by compounding various conductive substances with a general polymer matrix.

Conductive polymers typically have conjugated large π systems that can be chemically or electrochemically doped to alter their conductivity for the purpose of absorbing electromagnetic waves. The doped conductive polymer chains contain free radicals, and the conductivity of the polymer comes from the transition of these dipoles. The conductivity of conductive polymers is adjustable, ranging from insulators to semiconductors and even to metal conductors, and different electrical conductivities exhibit different absorption properties [33]. Currently, conductive polymers widely used in the field of absorption include polyaniline, polypyrrole, polythiophene, and their derivatives

2.2 Magnetic particles

Magnetic materials have high magnetic losses and strong microwave absorption efficiency. Traditional magnetic particles such as ferrite and metal particles have been studied earlier both domestically and internationally, and the research theory is relatively comprehensive. Magnetic metals have high saturation strength and Curie temperature, but are limited by the Snoek limit, causing a rapid decrease in the magnetic permeability of the magnetic medium in the high-frequency range [34], resulting in a decrease in the ability to attenuate electromagnetic waves.

2.2.1 Ferrite

Ferrite refers to a compound composed of oxygen and iron elements in a certain proportion, which has excellent magnetic properties.

Ferrites can be classified into spinel type [35-36], garnet type [37], and magnetite type [38] according to their different structures, as shown in Figure 3. Magnetic lead ferrite has anisotropy and natural resonance, and its magnetic loss performance is the most excellent compared to the other two types. However, it has shortcomings such as narrow absorption band, poor oxidation resistance, and high density. Researchers have explored different synthesis methods to prepare ferrite materials with different structures and properties. At present, common methods include chemical coprecipitation, magnetron sputtering, electrospinning, sol gel method [39], etc.

2.2.1.1 Porous Hollow Structure

Making dense ferrite materials into porous materials can reduce the density of the material and achieve the requirement of lightweight absorbing materials. The porous structure inside the ferrite will change the domain wall area, causing changes in domain wall energy and resistance to domain wall displacement, further causing significant magnetic losses [40-41]. Mn Zn ferrite porous microspheres were prepared using the self reactive spray forming method with Fe, MnO2, Fe2O3, and ZnO as reaction systems. The surface of the microspheres was rough and filled with micropores, with a hollow structure inside, as shown in Figure 4. After calculation, the density of the material has significantly decreased. The test results of its absorption performance show that the minimum reflection loss can reach -16 dB at a frequency of 13 GHz, and the reflection loss is lower than -8 dB in the range of 10-14 GHz. It has good absorption ability in the intermediate frequency band. Mn Zn ferrite porous microsphere materials have unique hollow and porous structural characteristics, which can effectively reduce the density of the material, increase its relative volume fraction with air, improve the impedance matching of the material, and significantly attenuate the energy of electromagnetic waves; At the same time, the hollow porous structure reduces the density of the material, which can meet the requirement of lightweight absorbing materials.

2.2.1.2 Core Shell Structure

The low dielectric constant and narrow absorption bandwidth of a single ferrite to some extent limit its microwave absorption ability. Using magnetic materials as the core and dielectric materials as the shell to construct core-shell materials enables the material to achieve good impedance matching with both dielectric and magnetic losses, thereby improving its microwave absorption ability [43-44]. Prepared by hydrothermal method BaFe12O9@MoS2 The process of the core-shell structure absorbing material is shown in Figure 5. Testing its electromagnetic performance shows that when the material thickness is 1.7mm, the minimum reflection loss can reach -61 dB, which can absorb the majority of incident electromagnetic waves, with an effective absorption bandwidth of 4.4 GHz. The microwave absorption mechanism of MoS2 is shown in Figure 6. The layered structure of MoS2 has a high specific surface area, which can form multiple scattering points and enhance the attenuation of incident electromagnetic waves through multiple scattering; The formation of a core-shell structure between MoS2 and BaFe12O9 to some extent adjusts the complex dielectric constant of BaFe12O9, improves the impedance matching of the material, and allows more electromagnetic waves to enter the interior of the material, attenuating and absorbing electromagnetic wave energy through multiple reflections and scattering.

A 3D (Fe3O4/ZnO) @ C dual core core-shell structure was prepared using the solution self propagating combustion method, heat treatment method, phenolic polymerization method, and carbon thermal reduction method. The synthesis process is shown in Figure 7. Tests have shown that when the material thickness is 2mm and the frequency is 15.31 GHz, the minimum reflection loss reaches -40 dB, the effective absorption bandwidth (RL ≤ -10dB) can reach 6.5 GHz, and the high-efficiency absorption bandwidth (RL ≤ -20dB) is 3.4 GHz. Firstly, the material of the absorber has good impedance matching conditions, allowing more electromagnetic waves to enter the absorber, thereby providing the possibility of absorption; Secondly, carbon shell and zinc oxide magnetic core cause dielectric loss, while Fe3O4 magnetic core mainly generates magnetic loss to improve the impedance matching of the material; Under electromagnetic wave irradiation, structural defects on the surface of carbon shells can serve as polarization centers, and interface polarization and relative relaxation occur at the Fe3O4/C interface and ZnO/C interface. The C shell is beneficial for improving the conductivity of nanocomposites, thereby promoting the accumulation and polarization process of interface charges in nanocomposites; Finally, the porous structure in the foam absorber provides a rich channel for the scattering and propagation of electromagnetic waves, and enhances the attenuation of microwave.

2.2.2 Metal Micropowder

Magnetic micro powders have also been widely studied as absorbing materials. Common magnetic micro powder absorbing materials include Fe, Co, Ni, and their alloys. Magnetic micro powders have a high Curie temperature (770K), good temperature stability, and high magnetic permeability, which is beneficial for enhancing magnetic loss. However, due to their susceptibility to oxidation and corrosion, they are often combined with other materials to improve their chemical stability and absorption ability [47-49].

2.2.2.1 Carbonyl iron powder

Carbonyl iron powder has advantages such as a high Curie temperature point (about 770 ℃), good thermal stability, strong magnetic loss ability, low cost, and simple preparation method. Compared to other magnetic materials, carbonyl iron has a larger saturation magnetization value and the Snoke limit is located at higher frequencies, making it more suitable for application in a wider frequency range [50-51].

Li et al. [52] prepared a modified adsorbent by depositing copper particles on carbonyl iron powder (CIP) using ultrasonic chemical copper plating method. Subsequently, a non-woven coating absorbing material containing 85% modified CIP was prepared. Compared with the initial CIP, the composite magnetic permeability and dielectric constant of the modified CIP treated with ultrasonic chemical plating process increased. The material thickness was 2mm, and the frequency was in the range of 8-12 GHz, with a minimum reflection loss of -8.43 dB; When the material thickness is 2.08mm and the frequency is 9.35 GHz, the minimum reflection loss is -26 dB. The interface between copper particles deposited on CIP and CIP contributes greatly to improving microwave performance. When copper particles are tightly arranged on CIP, the electromagnetic properties of the CIP surface change, resulting in local small conductive currents in the absorbing material and causing electromagnetic energy loss.

2.2.2.2 Nano nickel powder

Nano metallic nickel powder, with its smaller size and larger specific surface area, exhibits superior performance in many aspects compared to bulk materials [53]. In addition, nickel metal powder has excellent conductivity and magnetic properties, and is widely used in magnetic fluids [54-56], high-efficiency catalysts [57], high-performance electrode materials [58], and absorbing materials. Two methods were used to prepare Al/Ni SiC composite materials with aluminum as the matrix and nickel and silicon carbide particles as reinforcements: the first method used chemical plating to deposit nano Ni particles on SiC particles, and then mixed with Al powder; The second method is to mix SiC with Ni, and then mix the synthesized composite powder with Al. By changing the mass fraction of SiC Ni composite material, it was found that the aluminum sample with a mass fraction of 10% Ni SiC had the best microwave absorption value in the galvanized sample, while the aluminum sample with a mass fraction of 5% Ni SiC had the best microwave absorption value in the mixed sample. Tests have shown that at a frequency of approximately 10.45 GHz, the absorption loss of some samples has increased by approximately 12 dB; At a frequency of approximately 12.7 GHz, the absorption loss of some samples increased by about 17 dB. This may be due to the good distribution of Ni SiC in the Al matrix without agglomeration. The good mixing between Al and Ni SiC powders enhances the distribution and has a positive effect on microwave absorption; By reinforcing aluminum with hard ceramic SiC particles, the particle size is minimized. These particles act as internal spheres to reduce particle size and increase surface area, thereby promoting microwave absorption. This phenomenon is mainly caused by defects, vacancies, and interface induced polarization losses.

2.2.2.3 Nano copper powder

The small size effect, surface effect, and quantum tunneling effect of nano copper powder endow it with special properties in electrical, magnetic, mechanical, and other fields [60-62]. Its conductivity is similar to silver but its price is low, so it is widely used. Advanced Institute Technology has prepared Fe/Cu composite materials by using chemical plating method to deposit copper particles on carbonyl iron plates. Copper elements are uniformly distributed on the grain boundaries of sheet-like carbonyl iron while maintaining the internal structure of the iron powder. Exploring the effect of chemical plating time on microwave absorption performance, the results showed that with the increase of chemical plating time, the reflection loss showed a decreasing trend. As the electroplating time increases, the peak reflectivity loss decreases from -32.2 dB to -11.5 dB. The effective absorption bandwidth has been reduced from 7 GHz to 1.3 GHz. Due to the growth and deposition of copper on the carbonyl iron crystal structure, internal defects are greatly improved. Copper itself has high dielectric properties, resulting in an increase in the dielectric constant of the sample. As time goes on, copper particles are difficult to adhere to, and when the copper particle content is too high, it will cause a decrease in the impedance matching performance of the material, leading to a decrease in its absorption ability.

3 Conductive polymer/magnetic particle composite absorbing material

The performance of good microwave absorbing materials mainly depends on the effective complementarity of dielectric loss and magnetic loss, as well as reasonable structural parameter design. Traditional magnetic metal particles such as ferrite and carbonyl iron have relatively high dielectric and magnetic losses in the discovered absorbing materials [64]. At present, there are mainly shortcomings in the research of magnetic particles, such as narrow frequency band, unsatisfactory absorption performance, high specific gravity, high density, poor stability, and high filling rate [65]; Conductive polymer based absorbing materials have attracted widespread attention due to their simple synthesis, light weight, and low cost. However, the microwave loss mechanism of conductive polymers is mainly due to poor dielectric loss impedance matching performance. The composite of conductive polymers and magnetic materials reduces the impedance matching of the material through the coordination effect between electromagnetic properties, thereby exhibiting good microwave absorption ability

3.1 Ferrite/Conductive Polymer Absorbing Materials

Various composite materials based on magnetic ferrite and conductive polymers provide a good choice for improving absorption efficiency and broadening absorption frequency range while effectively combining magnetic loss, dielectric loss, and interface loss.

Core shell structural materials are composite materials with ordered assembly structures formed by chemical bonds or other interactions. Barium ferrite (BaFe12O19)/polyaniline (PANI) core-shell nanocomposites were prepared by in-situ polymerization method. The thickness of the PANI layer was adjusted by increasing the content of the starting monomer. Tests showed that the optimized core-shell nanocomposite with a shell thickness of 30-40 nm had a minimum reflection loss of -28 dB and an effective absorption bandwidth of 3.8 GHz (11.8-15.6 GHz) at a frequency of 12.8 GHz with a material thickness of 2mm. The surface of ferrite nanoparticles is coated with a polyaniline shell, which achieves good free space impedance matching characteristics, effectively utilizing the combined effects of magnetic resonance loss, eddy current loss, conductivity loss, and interface resistance loss. These mechanisms work together to enable the material to absorb more electromagnetic wave energy.

Hollow structure not only allows materials to have a larger specific surface area and lower density, but also its internal space can accommodate a large number of guest molecules of different sizes [69]. Ji et al. [70] utilized hydrofluoric acid and gamma rays -Fe2O3@SiO2 @The PEDOT core-shell nanocomposite material reacted to construct hollow γ - rays -Fe2O3@PEDOT The preparation process of core-shell nanocomposites is shown. Measure the electromagnetic parameters and microwave absorption performance of materials in the frequency range of 2-18 GHz, compared to γ -Fe2O3@SiO2 @PEDOT core-shell nanomaterials, hollow gamma -Fe2O3@PEDOT The microwave absorption capability of core-shell nanocomposites has been significantly improved, with a minimum reflection loss of -44.7 dB at a frequency of 12.9 GHz and an effective absorption bandwidth of 4.3 GHz. When microwaves pass through the PEDOT layer from the air, the PEDOT shell will experience dielectric loss; Secondly, the incident microwave undergoes multiple reflections and diffuse scattering in the inner cavity, leading to electromagnetic energy attenuation; On the other hand, the incident wave penetrates the PEDOT layer into the internal hollow space and then penetrates the Fe2O3 pores into the hollow space. The attenuation of electromagnetic energy is caused by the synergistic effect of the dielectric loss of the PEDOT shell, the magnetic loss of Fe2O3, and the hollow core-shell structure; In addition, the size effect of hollow and nanocomposite materials can improve their absorption performance, thus effectively attenuating electromagnetic waves through multiple reflections and absorption in hollow core-shell structures.

Fe3O4 and Fe2O3 nanoparticles have excellent magnetic properties, but their corrosion resistance and thermal stability are poor, and they are prone to side reactions during the reaction process, resulting in a decrease in the absorption performance of the composite material [71]. The hollow glass microspheres (HMG) composites coated with Ni0.7Zn0.3Fe2O4 particles were prepared by solution gel method using nickel zinc ferrite with good magnetism, corrosion resistance and thermal stability, and then the ternary composites (HMG/Ni0.7Zn0.3Fe2O4/PTh) were synthesized by in-situ polymerization. The process is shown in Figure 10. The test results show that the conductivity and saturation magnetization of HMG/Ni0.7Zn0.3Fe2O4/PTh reach 6.87 x 10-5 S/cm and 11.627 emu/g. The ternary composite material has good microwave absorption characteristics, with a minimum reflection loss of -13.79 dB at a frequency of 10.51 GHz; In the X-band (8.2-12.4 GHz), the absorption bandwidth with RL ≤ -10 dB can reach 2.6 GHz (9.4-12.0 GHz). PTh increases the dielectric loss of composite materials, improving their impedance matching; On the other hand, electromagnetic waves can be reflected multiple times in hollow glass microspheres after passing through the coating, and absorbed multiple times by Ni Zn ferrite and PTh. It enhances the absorption of electromagnetic waves entering the medium and avoids secondary reflection of electromagnetic waves; In addition, hysteresis loss, cavity effect, etc. can also cause electromagnetic wave attenuation, further improving the microwave absorption capacity of composite materials.