Studies On Preparation, Structures And Characteristics Of FeSiAl Flaky Powders

In low frequency band of microwave, the ability to absorb electromagnetic wave ofmetal powders excels that of ferrite. The magnetization of magnetic powders, withanisotropic shape, such as stick, flake and fibre, is better than that of sphericity. Flakypowders, with large aspect ratios' and thicknesses less than shin depth in the frequencyrange of GHz, are beneficial to absorbing electromagnetic wave. Anti-EMI material,with alternate lays structure, prepared by mixing those powders and polymer, is worthinvestigating and using. FeSiAl powders are made by employing this technology. In thisthesis, flaky powders were prepared through designing composition, melting alloyssticks, quenching ribbons and milling powders. This thesis investigated powders'profiles, the effect of the content of Si and Al on the structures, electromagneticparameters and wave absorption ability. Simultaneously it calculated electron structuresemploying the empirical electron theory of solids and molecules, and analyzed thecapability of absorbing microwave.In the part of profiles and crystalline structures, the effect of milling time onprofiles and the formation of nanocrystalline were analyzed. The internal strainincreased with the time lasting before 70h, but it decreased when the time reached 90h,and it became lower after annealing. DO₃ superlattice ordered structure was found inas-quenched ribbons. After milling, it disappeared and a small quantity of novel Alatoms appeared. Ordering degrees increased with the increase of Si content andannealing temperature. There was no obvious exothermic or endothermic peak above300℃. DO₃ superlattice ordered structure can be obtained by melt-quenching, butdisappeared after ball milling and annealing.Firstly, based on the empirical electron theory of solids and molecules, theory bondlength D (n), pair number of bond covalent electrons in (111) and (100) planes n_α, bondlength differenceΔD/(?)(n_A)% of two planes and Bohr magnetons were particularlycalculated. The results indicate that the lattice constants changed during milling, andthen the pair number of bond covalent electrons in (111) plane changed, andmagnetization changed finally. With the increase of Si content, the pair number of bond covalent electrons in (111) and (100) planes, and hybridization level of Fe bothincreased, but Bohr magnetons decreased. Atom states of Fe atoms were changedbecause of micro solid state reaction. Results can explain electromagneticcharacteristics of powders.In the part of electromagnetic parameters, it was found that saturationmagnetizations increased with the time prolonged before 70h milling, and reachedmaximum at 70h, but decreased at 90h. In low microwave frequency range, the effect ofSi content on EM parameters was related to Al content. The values of complexpermeability were large after 70h milling. After annealing, complex permittivitydecreased greatly. The longer powders were milled, the lager complex permittivity was.In the part of microwave absorption, reflectivities and attenuation coefficients ofpowders were calculated respectively. Reflectivities were optimized, and microwaveabsorbing mechanism was discussed. After long time milling, reflectivities floatedtowards low frequency band, and the peak values decreased. Peak values ofreflectivities floated towards high frequency band with the increase of Si content. But itwas reverse to Al. The bandwidth of absorption was broadened after annealing at 300℃,but the peak value decreased. Thickness increased, peak value floated towards lowfrequency band but bandwidth decreased. Modules of complex permittivity decreasedafter annealing, but dielectric loss tangents increased. So, dielectric loss increased butmagnetic loss decreased. Alloys powders loss was magnetic. Dielectric loss becameprincipal with the increase of Si content.The following main conclusions can be drawn from this thesis: The composition ofalloy is Fe₇₄Si₁₅Al₁₁and milling time is 70h. Imaginary of complex permeability is 4.2 at3.5GHz. After optimizations, peak value of microwave absorption is below -10dB at2.6mm. When Al content reached 14at%, absorbing band floats to the range from 4 to 8GHz frequency. Bandwidth, whose peak value is below -10dB, can reach 2GHz. Theprimary loss is magnetic, and attenuation coefficient reaches 10.25ω/C at 4GHz.Bandwidth of microwave absorption can be broadened after annealing at 300℃. Latticeconstants changed, and then the pair number of bond covalent electrons in (111) planechanged, and magnetization changed finally.