Design And Preparation Of Fe-Ni Soft Magnetic Alloy Absorbing Materials

The aim of this dissertation is to provide theoretical evidence and practicalapplications for researching and developing new type electromagnetic wave absorbers.This investigation will show several disadvantages in the traditional types of appliedelectromagnetic wave absorbing materials whose wave absorbers are ferrite or SiCpowders: narrowing the absorbing frequency bands, enlarging their thickness, andweakening the effectiveness of their absorption. In order to make substitutions fortraditional types of electromagnetic wave absorbers, for the first time a "shell-core"structure theoretical model will be outlined and discussed. After considering materialapplications and the relationships of microstructure versus electromagneticcharacteristics, "shell-core" structure composite powders, the new type waveabsorbers which possess FeNi3, γ-(Fe, Ni) and Fe3O4 phases and were prepared byusing the method of "mechanical alloying + oxidation + re-crystallizing heattreatment" will be described. The microstructure of the "shell-core" structurecomposite powders was studied by means of modern material analyzing techniques,including XRD, MFM, FEG-SEM, EDS, and so on. It was proved that the complexmagnetic permeability benefits from the increasing average crystal grain size, thedecreasing magnetic domain width and wall thickness of the new type wave absorbers;the "shell layer" which possesses Fe3O4 phase can be effective to reduce complexelectric permittivity. Two kinds of single-layer and double-layer flat type compositescontained within the "shell-core" structure composite powders whose graingranularity was under 10μm were manufactured.These new types of electromagnetic absorbing materials, as well as the absorbingmechanism will be deeply analyzed. The double-layer absorbing physical model"match leading layer + electromagnetic dissipation layer" will be discussed andevaluated by theoretical calculations of absorbing effectiveness (AE) on flat typecomposites as they were carried out via deduced double-layer absorbing formulae andcomputer-aided design of materials. AE theoretical calculation values conformed toAE testing values of absorbing materials with laminated structures. This study - II -北京工业大学工学博士学位论文confirms the validity of our hypothesis about electromagnetic absorbing mechanismsand material simulation. Combined theoretical calculation results, the relationship ofelectromagnetic parameters, and the thickness of materials, versus AE wassummarized via material simulations. The optimization of absorbing materials wasrealized. In the frequency bands of 1MHz~1GHz, L, S, C, X and Ku, the new typeswave absorbing materials possessed of smaller minimum AE, wider wave absorbingfrequency bands and thinner material thickness were prepared. "Single gradientconversion", "bottom-layer electromagnetic dissipation" and "double-layer magneticpermeability dissipation + bottom-layer electric permittivity dissipation" are theabsorbing mechanisms of laminated composites.In addition, a new viewpoint, which combined preparation of unifying compositesamples and material simulation, was put forward and the improvement of traditionalresearch methods on wave absorbing materials was realized. This improved researchmethod not only has the ability to simultaneous test AE, electromagnetic parametersand thickness on the same sample, but supplies basic data to double-layer waveabsorbing structure calculation model. In order to solve the problem on testingelectrical conductivity, the concept of electrical conductivity with frequencyconversion is applied to the study of electromagnetic wave absorbing mechanism andthe material optimizing design. According to experimental conditions, a "one circleinductance" physical model on magnetic ring samples was established. Along with theformulae, a testing system was developed for complex relative magnetic permeability.