1、聚合物也能做成隐身材料（Polymers can also be made into stealthy materials）Stealth material is one of the key aspects of the development of stealth technology. At present, the main military powers in the world are developing several new stealth materials.AP (1) chiral materials (chiralmaterial) 1. Chiral refers
2、to an object and its mirror symmetry does not exist and cannot coincide with the object and image by any operation phenomenon. Research shows that the characteristics of chiral materials, can reduce the reflection of incident electromagnetic wave and electromagnetic wave absorption.AP (2) nano steal
3、th materials 2. In recent years, the research of nano materials to prove unceasingly thorough, the absorbing properties of nano materials have excellent, nano materials has been paid more attention by the major countries of the world, and regard it as a new generation of stealth materials for explor
4、ation and research.AP (3) conductive polymer materials 1. This material developed in recent years, it has diverse structure and low density and unique physical and chemical properties, it has attracted wide attention from the scientific community.AP (4) ceramic absorber. Ceramic absorbent than ferri
5、te of low density, good absorbing properties, high temperature resistance, but also can effectively suppress the infrared radiation signal, this kind of absorber are mainly SiC powder, SiC fiber and boron aluminum silicate. Of course, if the powder absorber size to the nanometer scale, can also be i
6、ncluded in stealth nano materials.AP (5) salt absorbent 1,2. Retinyl Schiff base polymers such as, high polarization salts in the material structure of double ion displacement with absorbing function, which has strong polarization characteristics of radar wave, by the salt absorption, energy into he
7、at dissipation, a certain type of salt can absorb a specific wavelength of radar wave.AP (6) polycrystalline iron fiber absorber 2. Polycrystalline iron fiber is a kind of low density magnetic absorbing material, can achieve a high absorption effect in a wide frequency range, and the quality loss of
8、 40% to 60%, which most magnetic absorbing materials are density problems. The fibers pass through the dual role of magnetic loss and eddy current loss to achieve the absorption of electromagnetic wave.AP (7) plasma absorbing material. The use of alpha particle emission of radioactive isotopes, the
9、surrounding air ionization and plasma formation, can absorb the energy of the electromagnetic wave. Russia has successfully developed a new plasma stealth technology, is used in the fifth generation fighter 3.In the past twenty years, the research technique and stealth stealth material has made cons
10、iderable progress, with the United States as an example: the first generation of products such as F117 stealth fighter, the second generation of products such as B2 stealth bomber has been equipped troops and used for many years, and the third generation of products such as F22 /JSF stealth aircraft
11、 will soon be equipped the troops. At present, the research technique and stealth stealth material is moving towards “thin, light, wide and strong development of nanotechnology as a cutting-edge technology of modern science, technology and research for stealth stealth materials can be prepared after
12、 absorbing material with excellent performance, promising. Study of stealth nano materials is becoming a hot research of new microwave absorbing materials.2 the nano composite stealth materialsThe properties of 4 2.1 nano materialsAP (1) surface effect. The small size of nanoparticles,High surface e
13、nergy at the surface of the atom accounted for a large proportion, with the decrease of particle size, surface atomic number ratio increased rapidly. Because the surface atomic number ratio increased, atoms are lack of coordination and high surface energy, so that these surface atoms with high activ
14、ity, extremely unstable, it is easy to combine with other atoms.AP (2) quantum size effect. The particle size decreases to a certain value, electrons near the Fermi level continuum discretization, the nano material has high nonlinear optical, catalytic properties and light catalytic specificity.AP (
15、3) small size effect. When the physical size of coherent length and other characteristics of ultrafine particles size and the wavelength or De Broglie wavelength and the superconducting state is equal to or more hours, the crystal periodic boundary condition will be destroyed, resulting in the optic
16、al, thermal, magnetic and mechanical properties of a series of.The stealth mechanism of composite stealth materials in 2.2 NMWhich was due to the nano material structure size in nanometer material, quantum size effect and surface effect and other aspects have important influence on material properti
17、es. According to the mechanism of absorbing stealth materials can be divided into electric loss and magnetic loss type. Dielectric loss type stealth materials including SiC powder, SiC fiber, short fiber, metal barium titanate ceramic body, conductive polymer and conductive graphite powder; magnetic
18、 loss type stealth materials including ferrite powder, iron powder, ultrafine metal powder or nano materials etc The following are in nanometer metal powder (such as Fe, Ni etc.) and nano Si/C/N powder as an example, detailed analysis of magnetic loss type and absorbing mechanism of dielectric loss
19、type of stealth nano materials.The brand of metal powder 5 (such as Fe, Ni etc.) with decreasing particle size, especially to reach nanometer level, the conductivity is very low, the saturation magnetization of the material decreases, but the magnetization and coercivity increased sharply. In the pr
20、ocess of thinning, in the number of atoms and surface, increase the activity of nano materials, so in a certain band of electromagnetic wave radiation, atomic and electronic motion increases and promote magnetization, the electromagnetic energy into heat, thus increasing the performance of radar abs
21、orbing materials. Generally, the interaction between the electron scattering caused by the absorption of electromagnetic wave energy caused by the lattice thermal vibration field electron scattering, impurities and defects and electrons of three kinds of effects to decide.The brand of Si/C/N nano po
22、wder is closely related to the absorbing mechanism of 6 and its structure. But at present, the study of the structure and no definite conclusion, the nano Si/C/N powder by M.Suzuki et al on laser induced SiH4+C2H4+NH3 gas phase synthesis of Si (C) N solid solution structure model to explain. The the
23、ory that the nano Si/C/N powder dissolved in N, Si (N) C solid solution, and the judge has also been confirmed by experiment. The solubility of N atoms in SiC lattice substituted C atoms and the formation of charged defects. In the normal SiC lattice, the silicon atoms each carbon atom and four adja
24、cent connected by covalent bond, as each silicon atom and four carbon atoms around the formation of covalent bond. When entering the SiC C atom substitution of N atoms, because N only only trivalent, with three Si atoms and one Si atom in the remaining one cannot bond valence. Due to the thermal mot
25、ion of atoms,This electron can move in the four Si atoms around N atoms, jumping from one Si atom to another atom Si. In the process of jumping to overcome certain barriers, small area, but not from the four Si atoms. Therefore, this electron can be called “quasi free electron“. In the electromagnet
26、ic field, the “quasi free electron position“ in the region changes with the direction of electromagnetic field, resulting in displacement. The displacement of the electronic relaxation is the main reason for the loss of electromagnetic wave energy. Charged defects from a balanced position into anoth
27、er equilibrium position, to process the equivalent electric moment, electric moment in this process due to collision with the surrounding particles and thus blocked movement lags behind the emergence of a strong electric field, the polarization relaxation.The brand of nano stealth composite material
28、s because of its high absorption characteristic of electromagnetic wave, has aroused great attention of researchers, and related research and exploration work has also been launched in many countries. Although the research project is still not mature, the application has not been reported, but it ha
29、s become one of the research directions of stealth materials, the future is bright. And once applied to the actual products, will have a huge impact on politics, economy, military and other aspects of the world.2.3 the preparation method of nano materialsThe key to improving the following two method
30、s to discuss the preparation method of nano material.The 2.3.1 sol gel methodImproving the sol gel method is a new process of preparation of nano materials developed in recent years. This is a metal organic or inorganic compounds made by solution sol under certain conditions (such as heating) the de
31、hydration is a mobile sol gradually becomes thick, into a solid gel slightly elastic, then the gel is dried and roasted to obtain nanometer products. And the temperature of sintering with different materials are also different, such as the use of microwave heating instead of conventional heating, th
32、e ultrafine powder with small particle size and high purity were synthesized at low temperature and short time; also such as the use of gamma ray irradiation nano CdSe/ polyacrylamide composite powder 7. This method can prepare pore interconnected porous nano materials. The use of liquid permeable,
33、physical method and chemical deposition, pyrolysis, oxidation and reduction reaction to fill the pores to prepare composites. The specific technology and using current technology to study the preparation of nano materials, but according to the mechanism of sol-gel to there are three main types of 2.
34、Improving the traditional colloid type (a). The precipitation process control of metal ions in solution, the formation of the particles are agglomerated into larger particles and sedimentation, stable homogeneous sol, followed by evaporation of the solvent (dehydration) gel. Adriana S.Albuquerque8 e
35、t al Ni0.5Zn0.5Fe2O4 nanoparticles in SiO2 glass phase forward using the traditional colloid method, to get the required amount through the magnetic properties and annealing temperature change of ferrite.AP (b) inorganic polymer. The soluble polymer in water or organic phase of the sol-gel process,
36、the metal ion is uniformly dispersed in the gel. The commonly used polymer of polyvinyl alcohol, stearic acid, polyacrylamide etcWang Li et al PVA sol-gel method had Ni1-xZnxFe2O4 (x = 0 1) nanoparticles, which have high purity, fine grain, low temperature heat treatment. Gang Xiong9 et al Ba4Co2Fe3
37、6O60 was prepared by stearic acid gel powder 10 25nm size, and with the increase of heat treatment temperature, the powder from the spherical shape into a cube.AP (c) complex. The use of complexing agent metal ions to form complexes, then through sol-gel process to form complex gel. The commonly use
38、d complexing agent citric acid etc Liu Changkun used the sol-gel method to decompose citric acid complex have an average diameter of 30nm and CoFe2O4 ultrafine particles uniformly dispersed.Which was compared with other traditional inorganic material preparation method, sol-gel method has low sinter
39、ing temperature, uniform particle size distribution and other advantages, but it also has a long reaction time, easy cracking and other defects of 10 gel. These are worth us paying enough attention in the application of this method.The 2.3.2 11 phase reaction method of laser induced chemical vaporIm
40、proving the laser induced chemical vapor reaction method is the use of a laser to induce the activation and reaction system, a method to synthesize the high grade nano materials. The basic principle is: the use of laser beam irradiation of high power laser in the reaction gas, the reaction gas throu
41、gh the laser photon absorption, heating, gas molecules or atoms in an instant activation, reaction gas molecules or atoms to obtain chemical reaction temperature in a very short period of time, the rapid completion of reaction, nucleation and condensation, the growth process, so as to prepare nano p
42、articles corresponding material. Therefore, a simple method is to use the energy that the laser heating reaction system of laser photons, a method to prepare nano particles. Usually, the incident laser beam perpendicular to the reaction gas flow, reaction gas molecular or atomic absorption of laser
43、photons after be heated rapidly, according to estimates from the J S Haggerty, the rate of laser heating for 106108C/s, heating to the reaction time of the highest temperature is less than 104s. By the reaction flow heating will form a stable distribution of the flame in the reaction zone, the flame
44、 center temperature is much higher than that of the corresponding chemical reaction temperature, the reaction will be completed within 103s. Generation of nuclear particles in the carrier gas blowing quickly out of the reaction zone, the transient growth process to the collection chamber.Gas molecul
45、ar frequency improving the incident laser can lead to chemical reaction depends on the incident light absorption coefficient of light is associated with incident light frequency. In order to ensure the rapid condensation of nuclear particles prepared by the reaction generated in the process of obtai
46、ning ultrafine particles, need cold wall reactor. The commonly used water cooled reactor wall and transparent radiation reactor wall. This is conducive to a large temperature gradient distribution in the reaction chamber, accelerate the formation of nuclear particle condensation, inhibit the excessi
47、ve growth of. In addition, in order to prevent particle collision, adhesion or agglomeration, sintering, need of inert gases in the reactor, so that the particle size of nano particles is guaranteed. In addition, the control is added to the reaction gas composition, can be prepared nano composite ma
48、terial.Improving the laser method and conventional heating method preparing nanoparticles have great difference, which is mainly for the:AP (a) reactor wall, no pollution potential.AP (b) raw material gas molecules directly or indirectly absorbed photon energy after rapid reaction.AP (c) with select
49、ive reaction.AP (d) reaction zone conditions can be accurately controlled.AP (E) laser energy is highly concentrated, the reaction zone and the surrounding environment between the large temperature gradient, is conducive to the formation of nuclear particle rapid condensation.Which was due to the technical advantages of the laser method, therefore, this method can be made uniform, high purity and ultrafine particles of various size, narrow distribution. Despite the high cost of the problem, but this method has also started towards industrialization, after all, t
