Authors: Peng Huaxin, Professor, Zhejiang University; Qin Faxiang, Researcher, Zhejiang University
Electromagnetic functional composite materials can be divided into three categories according to their application fields and functional characteristics: electromagnetic shielding, electromagnetic induction and super-composite materials. Electromagnetic shielding functional composite materials refer to composite materials that achieve shielding and electromagnetic protection effects by absorbing or reflecting most of the electromagnetic waves through the design of the structural parameters of the composite materials, such as the number of layers, the volume fraction of the functional phase, etc., such as military radar antenna covers. Similarly, the strong electromagnetic induction characteristics of composite materials under weak fields can be achieved by utilizing the sensitive response of functional phases with a small content to the external field, which has important application value in engineering structural health monitoring. The recently proposed concept of super-composite materials uses a periodically arranged functional phase to achieve the characteristics of metamaterials and thus achieve the effect of electromagnetic stealth while retaining the high structural characteristics of composite materials.
Comparative analysis at home and abroad
- Electromagnetic shielding functional composite materials
Electromagnetic interference has adverse effects on modern military and civilian equipment and human health, and the topic of shielding electromagnetic interference has great engineering application value. The latest research has developed a new type of polymer composite material containing magnetic micron wires, which can shield 98.4% of electromagnetic waves with an embedding amount of only 0.026 vol.%, and is found to be 2-4 orders of magnitude higher than the shielding protection efficiency of a single functional phase.
- Electromagnetic induction functional composite materials
The research on electromagnetic induction composite materials mainly focuses on magnetic induction and stress induction. French researchers use the method of dispersing ferromagnetic particles in epoxy resin to achieve external field control of ferromagnetic resonance. The disadvantage is that it is difficult to disperse the particles evenly. Other studies have shown that by using structural and functional integrated composite materials containing Co-based micron wires, electromagnetic induction characteristics based on structural health monitoring have been successfully achieved by regulating parameters such as micron wire size, arrangement, and embedding amount.
- Super composite materials
Some researchers have developed a super-composite material with negative dielectric parameters, but the disadvantages are the narrow operating frequency band and the complex material synthesis process. Another method is to use uniformly dispersed iron particles in Al2O3 porous ceramics to achieve the material's double negative electromagnetic properties. However, its operating frequency band is only 200MHz. Using the idea of structural and functional integrated composite materials, periodically arranged Fe-based micron wire arrays are embedded in polymer composite materials and it is found that electromagnetic double negative performance is obtained within 1-7GHz (Figure 1). Further research found that the use of doped Co-based micron wires can improve its wave transmission performance, and proposed the application value of such electromagnetic functional composite materials in radar stealth.
Figure 1 Schematic diagram of composite materials based on magnetic micron wire
Electromagnetic functional composite materials can be divided into two categories according to their composition: matrix and functional phase.
The matrix of electromagnetic functional composite materials must first meet the characteristics of high mechanical properties. Common matrix materials can be divided into three categories: metals, ceramics and polymers. It should be emphasized that due to the total reflection of metal materials to electromagnetic waves, the functional phase cannot exert its own electromagnetic properties, so it is not included in the discussion scope of the matrix.
Functional phases can be divided into three categories according to their geometric scale. Granular functional phases are often in the micrometer to nanometer scale, so complex physical or chemical methods must be used to achieve their uniform distribution, resulting in high preparation costs; one-dimensional functional phases such as ferromagnetic micron wires are generally in the mesoscopic scale of micrometers to millimeters, so they are easier to disperse and the processing cost of raw materials is also low; two-dimensional functional phases are generally lamellar, and common ones include graphene and ferromagnetic films.
Molding process of electromagnetic functional composite materials
According to different matrices, the forming process of electromagnetic composite materials can be divided into two categories: one is polymer-based composite materials, which generally adopt the steps of embedding functional phase-prepreg layer-autoclave forming. The final composite material has the advantages of few cracks, controllable functional phase arrangement, and short processing cycle; the other is ceramic-based composite materials, which generally adopt the process of infiltration of porous ceramics and subsequent sintering, and the arrangement and content of the functional phase in the final composite material are regulated by parameters such as the porosity and density of the ceramic matrix.
Typical application case analysis-design and preparation of radar antenna cover
- Introduction to Radar Dome
Antenna is a precision instrument. The accuracy of its dimensions and structural integrity determine the correctness, stability and reliability of communication. As an important component of the radar communication system, the radome is a shell that protects the internal antenna and communication system from damage by harsh environments such as lightning, frost, rain erosion, static electricity, and high temperature, thereby providing a relatively safe working environment for it. The complexity of modern civil and military communication tasks has put forward new design requirements for the radar radome in addition to its basic protection function, such as its aerodynamic performance on airborne and missile-borne aircraft, improving the aiming error of precision tracking radars, and achieving electromagnetic stealth. Generally speaking, at this stage, the ideal radar radome needs to meet two basic performance requirements: first, excellent mechanical properties, including high strength, stiffness, and corrosion resistance; second, good wave transmission performance in a specific band, so as to ensure that the internal radar antenna system is not affected. In terms of material selection and technology, electromagnetic functional composite materials are usually selected at this stage. By adding functional phases to the matrix material with high mechanical properties, the requirements for electromagnetic performance are guaranteed. At the same time, through the molding process and component design, the goals of low dielectric constant, low loss, high modulus, and simple preparation process can be achieved, thereby achieving the safety, protection, conductivity, reliability, concealment and other requirements of the antenna cover, and truly realizing the integration of mechanical structure and electromagnetic function.
- Types and analysis of composite radomes
In order to develop a radar radome with ideal performance, domestic and foreign scholars have conducted long-term exploration and made great progress. Since the radome has strict requirements on the thickness of its corresponding radome wall according to the wavelength of the communication band, the radome can be divided into single-layer and multi-layer structures according to the cross-section of the radome wall. There are two main categories of composite material technology in the development of radomes:
Single layer cover
Single-layer covers are divided into thin-walled and half-wavelength wall structures. The former has a wall thickness much smaller than the wavelength of the medium, usually 1/10-1/20 of the wavelength, and is suitable for antennas with a wavelength of more than 10 cm. Its advantages are high wave transmittance and low sensitivity to the polarization direction of electromagnetic waves [22]. The latter has a wall thickness close to half of the half wavelength and is generally used for high-precision communications in narrow frequency bands, but its disadvantages are small communication bandwidth and weak adjustability. These two composite laminate structures are used in the form of skins in conjunction with rigid metals or dielectric materials on the radome. The composition of the composite material varies depending on the application. For radomes for aerospace purposes, such as missiles and manned spacecraft, there are relatively high requirements for high temperature resistance. Ceramic-based composite materials are generally used, such as boron nitride-based composite materials, silica-based composite materials, silicon aluminum oxide-nitrogen-based composite materials, etc. For radomes for aviation use, resin-based fiber-reinforced composite materials are generally used. The resin systems used are mainly epoxy resins, cyanate esters and polyimide resins, and the reinforcing fibers are mainly glass fibers, such as D-glass fibers, quartz fibers, and high-silica fibers. The most widely used is cyanate-based quartz glass fiber reinforced composite material, which has the advantages of heat resistance, corrosion resistance, low dielectric constant, and high mechanical properties. The industry generally adopts the method of laminating glass fiber-containing resin prepregs with autoclave molding to prepare them. However, due to the special geometric shape of the single-layer radome, there is often stress concentration and uneven distribution at the edge joint of the laminate, which may lead to further crack initiation and a decrease in mechanical properties. How to ensure that the requirements of electromagnetic performance are met without affecting its mechanical properties is the technical key to the application of secondary polymer composite materials in radomes.
Multi-layer cover
For multi-layer radomes, there are mainly two types: sandwich interlayer and metamaterial-based multi-layer radome. Sandwich structure radomes generally use paper honeycombs or foam polymers as the inner core and composite materials as the skin. Generally, compared with single-wall radomes, they have better dielectric properties, which can be achieved by designing and optimizing structural parameters such as their structure and interlayer thickness to obtain better electromagnetic matching with the incident wave, thereby achieving the purpose of lower loss. Foam polymers have better mechanical properties and machining properties than paper honeycombs and are more widely used. In addition, the bonding strength between the foam polymer and the skin is higher and the resistance to mechanical impact is stronger. The more representative one is PMI foam. Using a honeycomb structure as the inner core can achieve some special mechanical properties, such as zero Poisson's ratio, negative Poisson's ratio, etc., which is of engineering significance for improving the shock absorption performance of the antenna system (Figure 2).
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