Generally, the temperature of military targets is higher than the background temperature, and shows significant thermal characteristics in thermal imaging cameras. Since the radiation intensity of the target is proportional to the 4th power of the temperature, the cooling material is the most effective material to reduce the thermal radiation of the target. Insulation materials and phase change materials have been reported recently. Insulation materials are generally foam plastics and ceramic materials. The thermal insulation reduction of hollow microspheres has been reported
The temperature effect is better.
The hollow microspheres studied have the best cooling effect at 60 mesh, and when the amount is less than 20%, the hollow microspheres basically have no effect on the emissivity of the coating. Phase change materials that have emerged in recent years are now receiving more and more attention. When the phase change material undergoes a phase transition, it will cause temperature changes with endothermic and exothermic effects. Using this characteristic, the thermal radiation energy of the Japanese standard can be controlled from the temperature. The phase change material system is used on military targets by burying capsules containing phase change substances in foamed substances, dispersed in fabrics or mixed with adhesives, and reduces the heat generated by the target's stealth material. Thermal infrared radiation intensity. This kind of capsule is actually a hard-walled microsphere filled with heat-absorbing material in the cavity, which is mainly composed of phase change material and capsule wall material. Many inorganic and organic substances can be used as capsule walls, especially polymers are used most. The choice of the capsule wall material depends on the physical properties of the phase change material filled in the capsule. If the phase change material is lipophilic, the hydrophilic polymer is selected as the capsule wall material. Alkane compounds are phase change materials very suitable for making this kind of capsules, with long service life, good thermal cycle reproducibility, and obvious phase change endothermic effect. When used under high temperature conditions, low-melting eutectic metals such as low-melting solder can also be used as capsule fillers. In addition, some plastic crystals can also be used as capsule filling materials, but it does not undergo phase change after heat absorption, but temporarily changes its molecular structure. In the phase change cooling material, the cascade phase change material is used to replace the single-stage phase change material, and the heat insulation effect can be greatly improved.
(2) Ceramic cooling stealth material-the function and role of microwave attenuation ceramics
As we all know, microwave attenuation materials have been widely used in passive electronic countermeasures. For example, important military facilities on the ground, aerial vehicles, and the construction of confidential microwave isolation chambers all require a large amount of microwave attenuation materials to prevent the other party from discovering, tracking and attacking. In the microwave measurement system, as an attenuator and load, it is also widely used in waveguides and coaxial lines. In addition, it is an indispensable key material in many microwave electric vacuum devices. The addition of attenuating materials will produce cut-off, decoupling, suppression of band-edge oscillation, high-order or parasitic mode energy, and elimination of other undesigned modes. Otherwise, many microwave electric vacuum devices will not work properly or even be scrapped. In high-power microwave electric vacuum devices, the problem is more prominent. Because of the need to absorb more power, more stringent requirements are put forward for the material, and a series of problems are brought about. Therefore, this aspect has caused widespread attention in recent years. The basic requirements for high-power attenuation materials are as follows.
①With a large enough specific attenuation within a certain bandwidth, it is one of the most important properties of attenuation materials.
② Within a given frequency, the reflection at both ends of the attenuated ceramic should be small enough.
③It can bear enough power. For example, for a high-power traveling wave tube with a wavelength of 5cm and an average power of 10kW, assuming 10% reflection, the attenuating material should withstand at least 1kW average power. At this power, the basic properties of the attenuation material (including attenuation, vacuum performance, etc.) should not be damaged. In order to ensure this, the materials with low melting point and volatile substances should be as few as possible, and the porosity should be as low as possible. When selecting the matrix material, it should have good thermal conductivity and be easy to metallize, so that the microwave energy absorbed can be quickly transmitted to the outside of the tube. This is particularly important for attenuating materials for high-power devices.
④The high temperature and chemical stability are good, which ensures that the material can withstand the entire pipe-making process without changing its performance.
⑤ A certain mechanical strength to ensure that it is not damaged during use. This should be taken seriously for high-reliability devices.
⑥ When preparing materials, it is hoped that the process is simple, easy to control, good repeatability, and high yield. Attenuating materials used in electric vacuum devices usually have two structural forms: one is a thin film structure, and the other is a volume structure, that is, the thin film attenuator and volume attenuator are used to say. Thin film attenuators are generally sprayed, vacuum evaporated, and spattered in a slow-wave structure (or cavity) with a layer of magnetic substance, such as Cantan alloy, carbonyl iron powder, etc., or cracked and sprayed on the medium holding rod Resistive substances, such as carbon, graphite, etc. However, this structure is generally used for medium and low power microwave devices. Currently, high-power microwave devices mainly rely on volumetric attenuators. The thin film attenuation materials can be divided into non-metal thin film attenuation such as carbon film, silicon carbon film, etc. and metal film attenuation such as Cantan, carbonyl iron, tungsten carbide, tantalum, tantalum aluminum alloy, titanium carbide, nickel film, etc. The volume attenuation materials can also be divided into metal ceramics, silicon carbide, titanium dioxide and carburized porous ceramics. In terms of attenuation characteristics, it can also be divided into electrical attenuation (such as cermet, titanium dioxide, and carburized porous ceramics, etc.) and magnetic attenuation (such as cantan, ferrite, and alesifil, etc.).