With the increase of power density of electronic devices, the electromagnetic compatibility problem of electronic devices is becoming more and more serious, and absorbing materials with shielding function have become a new trend to solve this problem.
Definition and function of absorbing materials
Absorbing materials refer to a class of functional materials that can absorb and attenuate electromagnetic wave energy incident in space, and reduce or eliminate reflected electromagnetic waves. Generally, they are composed of matrix materials and lossy media. The energy of microblogging is absorbed and consumed by small polar molecules. According to the law that electromagnetic waves propagate in the medium from low magnetic to high magnetic permeability, high-permeability ferrite is used to guide electromagnetic waves, and through resonance, a large amount of radiation energy of electromagnetic waves is absorbed, and then the energy of electromagnetic waves is converted into heat energy through coupling.
Electromagnetic waves induce a current in the material, and the current is blocked from being transmitted inside the material and converted into internal energy. The greater the conductivity, the greater the macroscopic current induced by carriers (electric current induced by electric field and eddy current induced by magnetic field), which is favorable for the conversion of electromagnetic energy into thermal energy.
The wave absorbing material is mainly a shielding film with ferrite as the base material. Its main function is to absorb coupled electromagnetic waves to prevent the superposition of electric waves and eliminate redundant electric waves in the intelligent electronic system. The wave absorbing material can be cut into shape and pasted on the touch panel Anti-electromagnetic interference EMI on the back or on the cable optimizes the performance of the touch screen.
The principle of absorbing materials
- The principle of wave absorbing material is to use magnetic microwave absorbent as the main body to convert the electromagnetic wave of electronic equipment into heat energy by means of insulation loss, magnetic loss and resistance loss to achieve the effect of reducing electromagnetic radiation. It has high magnetic permeability and wide frequency range. Features, and can be developed for specific frequency bands.
- The absorbing material has good absorption characteristics in the range of 10MHz~6GHz, which can avoid electromagnetic interference or leakage caused by secondary reflection. The products are mainly absorbing patches, and can also be processed into various shapes according to customer needs. The absorbing material can be used inside the cavity of electronic equipment such as notebook computers, mobile phones, and communication cabinets.
- The absorbing material has good electromagnetic wave absorption effect and wide absorption frequency. It can be customized according to the frequency band required by customers. It is thin and cost-effective, and has a wide range of uses and applications.
Classification of absorbing materials
At present, there are many classifications of absorbing materials, which can be roughly divided into the following categories:
One. Forming process and load-carrying capacity by material
Absorbing materials can be divided into coated absorbing materials and structural absorbing materials.
Two. According to the principle of absorbing
- Absorptive absorbing materials, which absorb and lose electromagnetic waves by themselves. The basic types include absorbers with equal complex permeability and complex permittivity, impedance gradient “broadband” absorbers and thin-layer absorbers that attenuate surface current. .
- The interference type wave absorbing material uses the same amplitude and opposite phase of the two columns of reflected waves on the surface of the wave absorbing layer and the bottom layer to perform interference cancellation.
Third. Loss Mechanism by Material
- Resistive loss. This type of absorption mechanism is related to the conductivity of the material. The greater the conductivity, the greater the macroscopic current caused by the carrier, which is conducive to the conversion of electromagnetic energy into heat energy. Such as silicon carbide, graphite, etc. are resistive absorbing materials.
- Dielectric loss, which is a kind of dielectric loss absorption mechanism related to electrodes, which converts electromagnetic energy into thermal energy and dissipates it through the “friction” effect generated by repeated polarization of the medium. Such as barium titanate is a dielectric type absorbing material.
- Magnetic loss, this type of absorption mechanism is a type of magnetic loss related to the dynamic magnetization process of ferromagnetic media, which can be refined into: hysteresis loss, damping loss, gyromagnetic eddy current, and magnetic aftereffects, etc. The main sources are Magnetic domain turning, magnetic domain wall displacement and magnetic domain natural resonance, which are similar to the hysteresis mechanism, such as ferrite and hydroxy iron, are magnetic loss absorbing materials.
Fourth. By study period
- Traditional absorbing materials, such as ferrite, barium titanate, metal powder, graphite, silicon carbide, conductive fibers, etc. are all traditional absorbing materials, among which the research and application of ferrite absorbing materials and metal powder absorbing materials More and better performance.
- New absorbing materials, including nanomaterials, chiral materials, conductive polymers, polycrystalline iron fibers and circuit simulation absorbing materials, etc., which have different absorbing mechanisms from traditional absorbing materials. Among them, nanomaterials and polycrystalline iron fibers are the two with the best performance among many new absorbing materials.
Fifth. By material
- Ferrite absorbing material, it is a composite dielectric material, the absorption of electromagnetic waves has both polarization effect and magnetic loss effect in terms of dielectric properties. It has the advantages of high absorption rate, thin coating and wide frequency band, and is widely used in various fields.
- Metal micro-powder absorbing material, usually referred to as the particle size of metal micro-powder is 0. 5～20μm. Metal micropowder absorbing materials have the advantages of high Curie temperature, good temperature stability, high magnetization in magnetic materials, large microwave permeability, and high dielectric constant, etc. Therefore, they are widely used in the field of absorbing materials. It mainly absorbs electromagnetic waves through hysteresis loss and eddy current loss. The size of the metal micropowder mainly used at present is usually 1-10 μm.
- Polycrystalline iron fiber absorbing material, its absorbing mechanism is eddy current loss and hysteresis loss. In addition, it is a good conductor and has strong dielectric loss absorption performance. Under the action of external alternating electric field, Electrons within the fibers vibrate, which partially converts electromagnetic energy into heat. Polycrystalline iron fibers have unique shape anisotropy, can achieve high absorption in a wide frequency band, and their mass is 40% to 60% lighter than traditional metal micropowder materials, and overcomes the serious defects of most magnetic materials. Polycrystalline iron fiber absorbing materials have the advantages of light weight, low areal density (can be reduced to 1.5 ~ 2kg/m2), frequency bandwidth (4 ~ 18GHz), and can be adjusted by adjusting the length, diameter, arrangement, dispersant of the fiber. The content and so on adjust the electromagnetic parameters of the material.
- Nano-wave absorbing material means that the size of the material is nano-scale ( usually 1-100nm) . The unique structure of nanomaterials makes it have the characteristics of tunneling effect, quantum effect, small size effect and interface effect. Nanomaterials are used as absorbents to make coatings, which can not only absorb electromagnetic waves well, but also have a thin coating and a wide absorption frequency band.
- The absorbing structural composite material is a rigid structural material that mixes the absorbing material and the resin foam glue fiber. The most commonly used are carbon fiber and silicon carbide fiber composite materials.
- Plasma wave absorbing materials, plasma stealth materials have a absorption frequency bandwidth of (3MHz-300GHz), do not need to change the appearance of the aircraft, are cheap, easy to maintain, and have extremely high potential application value, and have become the future development of stealth technology. trend.
Sixth. By shape
- Wedge shape, the absorber used in the microwave anechoic chamber is often made into a wedge shape (golden tower shape), mainly composed of polyurethane foam type, non-woven flame retardant type, silicate plate metal film assembly type, etc. As the frequency decreases (wavelength increases), the length of the absorber also increases greatly. The ordinary wedge absorber has an approximate relationship of L/λ≈1, so at 100MHz, the length of the wedge reaches 3000mm, which is not only difficult to achieve in terms of technology, And the effective space available in the anechoic chamber is also greatly reduced.
- Single-layer plate shape. The first absorber developed abroad is a single-layer plate shape. Later, the absorber is directly attached to the metal shielding layer, which is thin and light in weight, but has a narrow operating frequency range.
- Double-layer or multi-layer plate shape, this absorber can work in a wide range of operating frequencies and can be made into any shape. The ferrite and metal short fibers are uniformly dispersed in a suitable organic polymer resin to make a composite material, and the working frequency band can be broadened by 40% to 50%. The disadvantage is that the thickness is large, the process is complicated, and the cost is high.
- Coating type, only coating type absorbing material can be used on the surface of the aircraft. In order to broaden the frequency band, the coating of composite material is generally used. For example, when the thickness of the lithium cadmium ferrite coating is 2.5mm to 5mm, it can attenuate 8.5dB in the centimeter band; when the thickness of the spinel ferrite coating is 2.5mm, it can attenuate 24dB at 9GHz; adding chlorine to the ferrite When the thickness of the butadiene rubber coating is 1.7mm to 2.5mm, the attenuation is about 30dB at 5GHz to 10GHz.
- Structural shape, the absorbing material is mixed with engineering plastics to make it have both absorption characteristics and load capacity, which is a direction of the development of absorbing materials. Nowadays, in order to further improve the performance of absorbing materials, complex absorbers with several shapes and combinations have been developed abroad.
- It can be used in the cavity of electronic equipment such as notebook computers, mobile phones, communication cabinets, etc.
- It can reduce the radiation and noise of various electronic devices.
- It can reduce the coupling conduction radiation interference between low frequencies and the low frequency echo interference
- It can reduce the internal EMI (resonance, crosstalk) in the barrier frame.
- It is applied between the chip and the cooling module.
- Application of EMI/RFI: EMI (Electro MagneTIc Interference): The literal translation is electromagnetic interference. Three elements of electromagnetic interference: interference source, interference propagation path and sensitive equipment. The source of interference refers to the electronic equipment or system that generates electromagnetic interference. The interference propagation path includes cables, space, etc., and the sensitive equipment refers to the electronic equipment or system that is easily affected by electromagnetic interference. Transmitting frequency interference (RF Interference): Radio frequency is a high-frequency alternating current, also known as electromagnetic waves. Radio frequency interference is the interference caused by electromagnetic waves. If two electromagnetic waves with similar frequencies will be received by the receiver at the same time, it will cause interference. There will be harmonic interference near the transmitter. interfere with other receiving equipment. Electromagnetic waves at the same frequency can interfere with the radio.