Basics of radiation lenses_ absorbers

Basics of radiation lenses: absorbers

Electromagnetic radiation has become an important source of environmental pollution, directly affecting the living environment of human beings, and the United Nations Conference on the Human Environment has listed electromagnetic radiation as one of the public hazards that must be controlled. Electromagnetic radiation exists in a wide range of fields, whether in the professional workplace or in the home environment, human beings will be exposed to different degrees of electromagnetic field irradiation. Electronic products from the short-wavelength band of 30 MHz to the microwave band of 3 000 MHz are often contacted, and can cause thermal, non-thermal, and cumulative effects to the human body according to their frequency characteristics. The human eye tissue contains a large amount of water, which is easy to absorb the power of electromagnetic radiation, and the blood flow of the eye is small, so the temperature of the eyeball is easy to rise under the action of electromagnetic radiation. Elevated temperature is the main condition that causes cataracts. Electromagnetic radiation in some frequency bands also has varying degrees of damage to the cornea, aqueous humor, vitreous humor, and retina. Furthermore; Long-term exposure to low-intensity electromagnetic radiation can promote symptoms such as visual fatigue, eye discomfort and eye dryness. This article introduces the basic knowledge of radiation absorbing materials. A wave-absorbing material is one that can absorb the electromagnetic wave energy projected onto its surface and convert the electromagnetic wave energy into heat or other forms of energy through the dielectric loss of the material. Due to the different chemical composition and microstructure of various materials, the principle of wave absorption is also different. However, the absorbing properties of materials can be evaluated and analyzed by macroscopic electromagnetic theory, that is, the dielectric constant and magnetic permeability of the materials are used to evaluate the reflection and transmission characteristics of absorbing materials. The classification of absorbing materials is as follows: 1. According to the material forming process and bearing capacity, it can be divided into coating absorbing materials and structural absorbing materials. In the former, an absorbent (metal or alloy powder, ferrite, conductive fiber, etc.) is mixed with an adhesive and applied to the target surface to form an absorbing coating. The latter is a composite structure with the dual functions of bearing and absorbing waves, usually dispersing the absorbent in the layered structural material, or using polymer composite materials with high strength and good wave transmittance (such as glass fiber reinforced plastic, aramid fiber composites, etc.) as the panel, and honeycomb, corrugated or corner cone as the sandwich core. 2. According to the principle of absorption, absorbing materials can be divided into two categories: absorption type and interference type. The absorption absorbing material itself absorbs and loses radar waves, and the basic types include absorbers with basically equal compound permeability and dielectric constant, broadband absorbers with progressive impedance, and thin-layer absorbers that attenuate surface current. Interference absorption is to use the surface and bottom of the absorbing layer of the two columns of reflected waves to equalize the radiation, opposite phase of interference cancellation, the disadvantage of this kind of material is that the absorption frequency band is narrow. 3. According to the loss mechanism of the material, the absorbing material can be divided into three categories: resistive type, dielectric type and magnetic dielectric type. The electromagnetic energy of resistive absorbing materials is mainly attenuated by the resistance of materials, such as silicon carbide, graphite, etc. The main mechanism of dielectric absorbers is dielectric polarization relaxation loss, such as barium titanate. The loss mechanism of magneto-type absorbing materials is mainly ferromagnetic resonance absorption, such as ferrite, hydroxyl matrix, etc. 4. According to the research period, it can be divided into traditional absorbing materials and new absorbing materials. Ferrite, barium titanate, metal powder, graphite, silicon carbide, conductive fibers, etc. are all traditional absorbing materials. The common disadvantage of these absorbers is that they have a narrow absorption band and are denser. Among them, ferrite absorbers and metal powder absorbers have better performance. New absorbers include nanomaterials, chiral materials, conductive polymers, polycrystalline fibers, and circuit simulation absorbers.。