Study On Preparation And Characterization Of New Microwave Absorbing Materials

Carbon nanofiber (CNF) has recently the aroused a great deal of interests, it has fibrous shape, large slenderness ratio, high permittivity and conductivity, farther more, it also has many advantage, such as low density, high intensity, resistant to corrosion and so on. So carbon nanofiber composites especially the carbon-fiber structural absorbing materials have the functions of bearing the weight and minishing the radar cross section (RCS), it has been widely used in stealth planes and other invisible weapons.Carbon Nanotube (CNT) not only has the baby size effect, surface effect and the macroscopic quantum tunnel effect etc. and also has the own helical and tube structure, high electric and magnetic loss tangent and unique AB effect etc. All these characters make the carbon nanotube more suitable for absorbing materials than many other nanomaterials.The super fine alloy powder has also been the pop research microwave absorbing material. These powders often have very fine granule, the granule nearly reaches nanosize, and the crystal grain is nanosize. They have high permeability in the GHz frequency, so they are good absorbent. They are suitable to cover the shortage of the ferrite in high frequency due to the Snoek limit.In this paper, straight and helical carbon fiber, multiwall carbon nanotube were synthesized by chemical vapor deposit method. The carbon fiber and carbon nanotube were carried through the high temperature oxidized treatment and acid treatment. They were also studied by SEM, TEM, FTIR. And the effect of the catalyst on their shape was also analyzed. Their heat-resisting property was studied by DSC-TG. The permittivity and permeability of the carbon fiber and carbon nanotube were measured by vector network analyzer. Also, the effects on permittivity and permeability of the acid treatment were studied in the paper.The Fe₂₅Ni₇₅, Fe₅₀Ni₅₀ and Fe₅₀Co₅₀ were synthesized by sol-gel auto-combustion method. The morphology of these alloy powders after high temperature reduce were studied by SEM, and the structures, crystal gain sizes and crystal structure were studied by XRD. The static magnetic properties and the complex permeability spectra of these powders in the range 0.5～6GHz were measured by vibrate sample measure (VSM) and vector network analyzer.The application of the genetic algorithm (GA) on designing the wave-absorbing coats was studied in this paper. The program writed by matlab can transfer the electromagnetic parameters which measured by network analyzer, process the simulation by transmission line theory using GA. And the result is got according the need is set. The effects on optimum result of the different object function and thickness optimum method were also discussed in the paper. The new method of creating the object function is promoted (multi-constant weight method).The result shows that, the morphology, electromagnetic parameter and antioxidation of the carbon nanofibers which were synthesized different ways exist large difference. The size of catalyst has important effect on the morphology of the carbon nanofiber. The amorphous carbon in the carbon nanofiber and nanotube enhance the permittivity, but depress the antioxidized property. And the permittivity of carbon nanofiber and nanotube will depress through acid treatment.The structures of alloy powders of Fe₂₅Ni₇₅,Fe₅₀Ni₅₀ and Fe₅₀Co₅₀ synthesized by sol-gel auto-combustion method were sigle NisFe phase, FeNi phase, and FeCo phase. The crystal sizes of them were 53nm, 66nm, 36nm. These powders have high permeability on the frequency of 0.5～6GHz. Different heat treatment ways have large effect on the permeability of the alloy powders.In the wave absorbing coats design, the restricting every coat thickness method exist deficiency, and restricting general thickness method, which trace thickness in the program can eliminate this deficiency. The object function has large effect on the optimum result. It can get a more precise result to use the multi-object weight method to create object function.