| As an indispensable material, silicon steel has been widely used in electron,electricity and military industry. The property of silicon steel which depends on thesilicon content and will be improved as the increasing of silicon content, theresistivity and permeability will increase, but the iron loss will reduce, especiallywhen the silicon content of steel increased to6.5%. In this case, silicon steel willperform some excellent soft magnetic properties such as high magnetic conductivity,low iron loss and nearly zero magnetostriction. Therefore,6.5wt%silicon steel can beused as ideal material for high-frequency core and has been widely concerned.However, the silicon steel sheets with silicon contents larger than4%are verydifficult to be produced by using traditional rolling methods because of its poorelongation.So far,6.5wt%silicon steel can be prepared through various new methods suchas the special rolling, the rapid solidification, the chemical vapor deposition (CVD),plasma chemical vaporous deposition (PCVD), Molten salt chemical reductionmethod and the powder metallurgy et al.. Only chemical vapor deposition (CVD) putforwarded by NKK Corporation of Japan has been successfully applied in asmall-scale industrial production. But the huge energy consumption andenvironmental problem limit its development of large-scale production. Based on thekey project of National Natural Science Foundation, a new idea combining theelectro-deposition of iron and iron-silicon particles and heat-treatment under magneticfields has been put forward to prepare the near-net-sharp silicon steel strips, whichoffers a kind of possibility of producing the high silicon steel strips (6.5wt%siliconsteel). However, both how to prepare composite coatings with high silicon and howmagnetic fields affect the electro-deposition process of iron and iron-silicon particlesare worthy of studying deeply.In this work, a parallel magnetic field (the direction between current and magneticfield was parallel) and a perpendicular magnetic field (the direction between current and magnetic field was perpendicular) were adopted, and the effects of magnetic fieldflux(MFD), the current densities and the arrangement of electrodes on the Fe-Sicomposition electro-deposition were investigated. The main work was done in thefollowing:(1) When without magnetic field, the effects of silicon content in Fe-Si particles(including Fe-30wt%Si particles, Fe-50wt%Si particles, Fe-70wt%Si particles and Siparticles), particle concentration, stirring speed, current densities and arrangement ofelectrodes on the Fe-Si electro-deposition process were investigated. The results showthat the silicon content of coatings for all types of particles reached a maximum whenthe current density and the stirring speed were at2A/dm2and60rpm under a particleconcentration of10g/L in vertical electrode system, the silicon content of coatings forFe-30wt%Si, Fe-50wt%Si, Fe-70wt%Si and Si particles can reach2.36wt%,1.92wt%,1.49wt%and1.23wt%. At the same time, the experimental results indicate that theconductivity of applied particles was the most important factor affecting the siliconcontent of coatings, high conductivity significantly promoted the incorporationprocess of particles. The conductive particles into the coatings were in a “wrap up”mode while that for the nonconductive particles was in an “enclosure” mode.Furthermore, when in aclinic electrode system, and the particle size and particleconcentration were2.5μm and10g/L, the silicon content of coatings for Fe-30wt%Si,Fe-50wt%Si, Fe-70wt%Si and Si particles can reach12.65wt%,21.33wt%,35.22wt%and37.94wt%respectively.(2) In this part, the Fe-Si composite electro-deposition in a horizontal magnetic field(the direction of magnetic field was in horizontal direction) was carried out. Whensupposing a parallel magnetic field with the MFD below0.5T, many microneedle-like protrusions appeared on the coating surface. With increasing the MFD, theneedle-like protrusions weakened and transformed into micro dome-shape protrusionsunder a1T magnetic field. Meanwhile, with the increasing silicon content of particles,the diameter of protrusions significantly decreased. However, the phenomena was notobvious for Si particles, the coating surface of Si particles appeared many bean-shapeprotrusions, this is mainly attributed to the increasing of local current density on cathode surface. Furthermore, the silicon content of coatings for Fe-50wt%Si,Fe-70wt%Si and Si particles significantly increased with increasing the MFD. For theSi particles, the silicon content can reach39.8wt%in1T from1.23wt%withoutmagnetic field. This is mainly attributed to the Micro-MHD effect. When for verticalelectrode system in perpendicular magnetic field, the coatings with silicon contentsmore than10wt%can be obtained for50wt%Si and70wt%Si particles, whereas thatfor Si particles was a smooth increase. At the same time, the silicon content ofcoatings for Fe-30wt%Si reached a maximum when the MFD was in0.1T, but whichseemed a descend trend with increasing the MFD. Therefore, in this case, themagnetic field gradient force was the most important factor affecting the particlecontent of coatings. When adopting aclinic electrode system, the silicon content ofcoatings significantly descended as the increasing of MFD, and the fluctuationbecame larger for the particle with high silicon content. The Fe-Si particles with lowsilicon content have relative high susceptibility and led to a larger attracted force forthe iron electrode to Fe-Si particles which can retard the particles’ migratingbehaviors under the MHD effect and the hydrogen evolution. The silicon content ofcoatings for Fe-30wt%Si particles from12.65wt%in0T to11wt%in1T, whereas thatfor Si particles sharply decreased from37.94wt%in0T to3.42wt%in0T.(3) In this part, the magnetic field with the MFD larger than1T was used. Whenusing the vertical system in perpendicular magnetic field, many striped protrusionconsisted with Fe-Si particles appeared on the coating surface. Meanwhile, the siliconcontent of coatings for Fe-50wt%Si and Fe-70wt%Si particles increased withincreasing the MFD. When in aclinic electrode system, dome-shape protrusions alsoappeared on the coating surface for Fe-30wt%Si and Fe-50wt%Si particles in parallelmagnetic field. At the same time, owing to the effects of turbulence of electrolytecaused by MHD effect and the hydrogen evolution, the silicon content of coatingsdescended for all types of particles decreased obviously.(4) The effects of magnetic field on cathode efficiency were investigated. Whenwithout magnetic field, the current efficiency significantly increased with increasingcurrent density. However, after imposing an external magnetic field, the current efficiency was considerably descended with increasing the MFD. At the same time,when using the vertical electrode system, the currency efficiency in the perpendicularmagnetic field was lower than that in the parallel magnetic field. Furthermore, thevertical electrode system had a lower current efficiency than the aclinic electrodesystem in the same perpendicular magnetic field.(5) The effects of MFD on the structure of composite coating were investigated.The XRD results show that the orientation degree of <100> in samples parallel tomagnetic direction was enhanced for Fe-Si particles when supposing a parallelmagnetic field with the MFD was below2T. This is because the magnetic anisotropyenergy of the easy-magnetization-axis magnetized along <100> was being loweststate, so, during the process of electrodepostion, the grains parallel to magneticdirection at <100> could promote the increasing of total area of this oriented grainsand further made the structure of coatings form a certain orientation degree for it’slower extra diving force of grain boundary migration. However, with increasing MFD,the orientation degree of <100> decreased, this is mainly attributed to the strongconvection induced by MHD effect.(6) Based on the above experimental results, it can be confirmed that theapplication of magnetic field has a grate influence on the Fe-Si compositeelectrodeposition. Therefore, it is necessary to study the transport process ofelectrolyte by electrochemical analysis method. The results show that the applicationof magnetic field increased the limit current of cathode, and dischargepotential of Fe2+shifted towards positive direction. Through the analysis of AC impedance, it can befound that the electron transfer resistance was decreased after imposing an externalmagnetic field. This is mainly attributed to the MHD effects that decreased thethickness of diffuse layer and promoted the Fe2+to migrate to cathode surface.To conclude, the application of magnetic field in Fe-Si compositeelectro-deposition could enhance the mass transport process and electron transferprocess and influence the nucleation and growth of composite film. This is mainlyattributed to the interaction of MHD effect and field gradient force. |
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