Investigations Of The Preparation And Magnetic Properties Of Iron-carbon Composites

Soft magnetic materials（SMMs）have the advantages of high saturation magnetization（M_s）,low coercivity（H_c）and low remanence M_r/M_s,and are widely used in power transmission,electrical engineering,magnetic storage,electronic components,aerospace and other fields.In order to improve the properties of SMMs or endow them with new functions,it is necessary to explore SMMs in many aspects,including composition and structure design,preparation process and method optimization.At the same time,amorphous carbon（a-C）and carbon nanotubes（CNTs）have excellent physical,chemical,electrical and mechanical properties,and become popular materials for the development of new composite systems,performance improvement and structural optimization.Therefore,in this paper,different kinds of carbon（a-C and CNTs）were introduced into metal iron by vacuum evaporation,magnetron sputtering and high-energy ball milling,and different forms of iron-carbon composites（film and bulk）were prepared.By studying the interaction relationship,crystal structure,microstructure and magnetic properties,the influencing factors and the optimal process conditions for the preparation of the composites were determined,and the influence law of a-C and CNTs on the magnetic properties of metal iron were revealed.The specific research contents and main results are as follows:1.a-C films were deposited on iron substrates by vacuum evaporation.By studying the composition,hybrid structure of the interface,the evolution model of carbon hybrid structure at the amorphous carbon-iron interface was established,and the interaction between a-C and Fe was preliminarily understood.The results show that the amorphous carbon diffuse at the interface,forming a certain thickness of diffusion layer.In the diffusion layer,carbon elements exist in the form of sp²-C and sp³-C hybrid structures and Fe-C bonds.With the increase of etching depth,the relative contents of the three structures change regularly.When the etching depth is shallow,the relative content of sp²-C decreases gradually,and the relative content of Fe₃C increases gradually;with the increase of etching depth,the relative content of sp²-C increases and the relative content of sp³-C decreases.This is mainly because the high-speed bombardment of argon ions and the catalysis of iron elements in the substrate make the mutual conversion between sp²-C,sp³-C and Fe₃C,indicating that high-energy implantation and iron element catalysis can promote the formation of graphitized carbon and Fe₃C in amorphous carbon films.In order to verify this conclusion,amorphous carbon films were deposited on iron substrates by direct current（DC）magnetron sputtering.The effects of sputtering power on the carbon hybrid structure in amorphous carbon films were studied.The results show that the relative content of sp²-C in amorphous carbon films increases with the increase of sputtering power.2.The structure and magnetic properties of the iron films deposited by radio frequency（RF）and DC magnetron sputtering were studied.The effects of sputtering parameters（sputtering power,working pressure,deposition time）on the growth mode,grain size,surface morphology and magnetic properties of the films were mastered.The preparation method and process conditions to achieve the best magnetic properties were determined.The results show that with the increase of sputtering power,the deposition rate of the films increases,and the growth mode gradually changes from island growth mode to columnar growth mode.The density of the films increases,M_s increases,and H_c decreases;however,the sputtering power should not be too large,otherwise more defects and coarse grains will be generated in the film,which will produce pinning effect on the magnetic domain,resulting in the decrease of M_s and the increase of H_c.The effect of working pressure on the sputtering rate can be ignored,which mainly affects the microstructure of the thin film samples.At low working pressure（0.6 Pa）,the average particle size of iron film is uniform,the vacancy defect concentration is low,and the density is high.Therefore,its M_s is high and H_c is low.The deposition time directly determines the thickness of the film.When the deposition time is short,it is not enough to form the film;when the deposition time is too long,the stacking of columnar crystals reduces the order of the films,increases dislocation and other defects,and worsens the magnetic properties.Using DC magnetron sputtering method,under the conditions of sputtering power of about 63 W,working pressure of 0.6 Pa and deposition time of about 120 s,the magnetic properties of iron films reach the best,with M_s as high as 1573 emu/cm³,H_c as low as 102 Oe and M_r/M_s as 0.856.3.Based on the optimum process conditions of a-C and Fe films,the magnetic properties of Fe/a-C multilayers were deposited by DC magnetron sputtering.The effects of the number of amorphous carbon layers and the number of cycles on their magnetic properties were studied.The results show that the magnetic properties of Fe/a-C multilayers are significantly lower than those of pure iron films.Theoretically,with the increase of the number of amorphous carbon layers,the proportion of magnetic phase in the multilayers decreases,and M_s should always decrease;but in fact,M_s first increases and then decreases.When the number of amorphous carbon layers is 4,M_sreaches the maximum value（707 emu/cm³）.With the increase of the number of amorphous carbon layers,the grain size decreases,the coupling between grains decreases,and the H_c decreases.With the increase of the number of cycles,M_s of the multilayers gradually decreases,and H_c first increases and then decreases.4.The magnetic properties of Fe-CNTs composites prepared by mechanical ball milling combined with cold pressing technology were studied.The effects of CNTs content,applied saturation magnetic field strength B_m and working frequencyfon their magnetic permeability and magnetic loss were recognized.The suitable working conditions and the optimal CNTs content of the composites were determined.The action mechanism of CNTs on its magnetic properties is revealed.The results show that compared with pure iron,the effective permeability of Fe-CNTs composite is significantly improved,and it has good high frequency stability（5 k Hz-100 k Hz）.With the increase of B_m andf,the effective permeability of Fe-CNTs composites increases gradually,up to 186%（when B_m=100 m T,μ_a=108）.Compared with pure iron,the magnetic loss of Fe-CNTs composites is significantly reduced,especially at high frequency,the maximum increase of magnetic loss is-89%（when B_m=20 m T,f=100k Hz,P_s=178.2 k W/m³）.However,with the increase of CNTs content（1wt%→3wt%）,the effective permeability gradually decreases and the magnetic loss gradually increases,indicating that excessive CNTs will worsen the magnetic properties of the composite,and the optimal CNTs addition is 1wt%.The results of magnetic loss separation show that the decrease of eddy current loss of Fe-CNTs composites is much greater than that of hysteresis loss,indicating that CNTs mainly reduces the total magnetic loss by reducing eddy current loss.