| The electromagnetic environment of modern equipment and facilities is becoming complex, causing more harsh requirements for shielding materials. To improve the shielding effectiveness and optimize multilayer shielding structure, iron-based gradient composite structural materials were designed and fabricated on the basis of magnetic circuit theory in this work, which can achieve both magnetic and electromagnetic shielding. The infulance of gradient microstructure evolution on shielding properties and shielding mechanisms were investigated. And preparation technology and shielding properties were also optimized.The computational formulas of magnetic shielding factor for single layer and multilayer structure shield were deduced based on the model of magnetic circuit and reluctance. According to the two cases of computational formula, Fe-Ni and Fe-Al gradient composite structural materials with different mulailayer structures were designed. The equations of shielding factor of two gradient composite structural materials were also obtained, which were used for determining the optimal gradient structures. The Fe-Ni gradient composite structural material with Fe-Ni/Fe/Fe-Ni was prepared through electroplating and diffusion heat treatment, which could be seen as “permable layer/permable layer/permable layer” structure. The Ni elemental content in Fe-Ni diffusion layer decreaed from surface to inner, showing gradient distribution. The Fe-Al gradient composite structural material with Fe/Fe-Al/Fe was prepared with vacuum hot-pressing, which also could be seen as “permable layer/non-magnetizer/permable layer” structure. The Al element diffused into the substrautes in the two sides and the Al content decreasing from the middle to the two sides.The microstructure evolution process of Fe-Ni and Fe-Al gradient composite structural materials were studied with different diffusion technologies. On the basis of one-dimensional plate model and thin film diffusion, the Ni elemental content distribution equation in Fe-Ni alloy layer was deduced. Mathematical fitthing analysis was processed by using the Ni elemental scanning data with different diffusion temperatures and time. The diffusion coefficient D at 900℃, 1000℃ and 1100℃ were obtained through mathematic calculation, which were 3.51?10-11 cm2/s, 6.48?10-11 cm2/s and 1.45?10-10 cm2/s, respectively. In addition, the Ni elemental distribution in Fe-Ni gradient structural material could be predicated accurately, which was used for optimizing diffusion technology and shielding properties. The phases of Fe-Al reaction layer in Fe-Al gradient structural material with different diffusion time were characterized. And the half of Fe-Al reaction layer was chosen as research object. After the calculation of FeAl reaction diffusion, the phase evolution process of Fe-Al reaction layer was determined, as follows: Fe2Al5/Fe?Fe2Al5/Fe Al2/Fe?Fe2Al5/Fe Al2/?-Fe(Al)/Fe ?Fe2Al5/Fe Al2/Fe Al/?-Fe(Al)/Fe? Fe Al2/Fe Al/?-Fe(Al)/Fe?Fe Al/?-Fe(Al)/Fe.The shielding properties of Fe-Ni gradient structural materials with different diffusion technologies were investigated. It was indicated that the magnetic shielding factor of Fe-Ni gradient structural material with square tube shape arrived to the maximum value, when the Ni elemental content on the surface of Fe-Ni layer was about 79%. The peak values at 1000℃-6h and 1100-3h were 27.3 and 28.9, respectively, which were increased by 7.7 and 8.2 times of pure iron substrate. The quantitative relationship among diffusion process, elemental distribution, permeability and magnetic shielding factor was established. And the computational results of magnetic shielding factor agreed well with the experimental values. As diffusion temperature increasing, t he surface Ni elemental content decreased to 79% with faster speed, the diffusion time spent for shieding factor reaching maximum get shorter. Each gradie nt Fe-Ni layer and substrate could shunt magnetic filed in parallel. The Fe-Ni alloy layer with 79% Ni element had the maximum permeability and the ability of attenuation magnetic field was the biggest, so magnetic shielding property was optimal. While, t here is no mutual coupling relation between each gradient layer and substrate. The electromagnetic shielding effectiveness of Ni/Fe/Ni laminated structure formed with electroplating was about 20 d B higher than that of pure iron substrate. After diffusion heat treatment, electromagnetic shielding effectiveness of Fe-Ni/Fe/FeNi laminated composite has a slight increasement, which arrived at 70~80d B in the frequency of 30 k Hz~1.5GHz. There was no obvious trend with different diffusion tenperatures and time. Compared with pure iron substrate, Fe-Ni gradient structural material has higher electromagnetic shielding effectiveness, which depended on absorption loss of additional Fe-Ni alloy layer and multiple reflection loss in gradient layer.The shielding properties of Fe-Al gradient structural material with different diffusion time were investigated. The magnetic shielding factor of the tabulate FeAl gradient structural material was increased by 1.6 times of pure iron plate. The magnetic shielding factor of Fe-Al laminated composite remained stable at the same level with diffusion time extension(1h~6h) and the shielding factor of composite for diffusion 10 h presented improving silightly. Fe-Al gradient structural material consisted of a couple of soft magnetic layers and Fe-Al nonmagnetic layer in middle. The magnetic filed could be shunt twice by two permable layers to achieve high shielding effectiveness and there is mutual coupling relation between permable layers. The Fe-Al soft magnetic layer formed next to substrate could also contribute to magnetic shielding effectiveness increased. The electromagnetic shielding effectiveness of gradient structural material was higher than pure iron substrate and increased with diffusion time increasing, which amounted to 80 d B in the frequency of 30 k Hz ~1.5 GHz at 10 h diffusion time. The improvement of shielding effectiveness for Fe-Al gradient structural material mainly lay on the additional multiple reflection loss in Fe-Al gradient layer.Ni coatings were deposited on the surface of Fe/Al/Fe diffusion couple and then Ni-Fe-Al gradient structural material with Fe-Ni/Fe/Fe-Al/Fe/Fe-Ni structure was formed by hot-pressing diffusion at high temperature. Comapred with Fe-Al gradient structural material, the magnetic shielding factor of Ni-Fe-Al gradient structural material with diffusion at 900 ℃ for 1h~6h were improved about 25%~42% as the Fe-Ni alloy layer on the surface can add the effect of shunting magnetic field. The electromagnetic shielding effectiveness of Ni-Fe-Al gradient structural material(900℃-1h) was about 10~20d B than that of Fe-Al gradient structural material in the frequency of 30 k Hz~1.5GHz. The additional Fe-Ni alloy could improve the absorption loss. |
PDF Full Text Request