Preparation And Research Of High Thermal Conductivity Composite Absorbing Materials And Rare Earth Doped Melt-spun Fe-based Alloys

Electromagnetic wave technology has attracted more and more attention because of its wide application in the electronic and communication fields such as stealth technology,surveillance radar,wireless data communication and satellite communication.However,excessive electromagnetic radiation will do harm to the signal and communication quality of equipment,environment and human health.Therefore,absorbing materials to suppress harmful electromagnetic radiation have become a research hotspot in the field of electromagnetic protection.Modern electronic equipment is becoming miniaturized and integrated,and the working frequency is from MHz to GHz,resulting in higher power consumption and more attention to the heat dissipation of devices.Therefore,thermal conductivity has become one of the essential core engineering issues in the design of electronic devices.For high-power precision electronic devices,the abnormal temperature rise caused by insufficient heat dissipation will force the device to lower the frequency,which seriously affects the normal operation of the device.Therefore,the development of thermal conductive absorbing materials with excellent absorbing properties and high thermal conductivity is one of the research hotspots in the field of absorbing waves.It has the characteristics of absorbing wave and conducting heat,and has far-reaching applications in military and civil fields,which can effectively absorb electromagnetic wave while rapidly conducting heat,so that the device work in the normal temperature range.The flaky carbonyl iron powder（CIP）has high saturation magnetization,high Curie temperature and high permeability.Due to the shape anisotropy,its permeability can break Snoek’s limit and has excellent absorbing performance in low microwave frequency band.In this paper,we carry out two aspects of research:Firstly,the flaky CIP and various thermal conductive fillers were compounded to prepare a series of thermal conductive composite absorbing materials.The microstructure,electromagnetic parameters and thermal conductivity of the composite absorbing materials obtained under the doping of different types and proportions of thermal conductive fillers are studied,and the thermal conductivity and absorbing performance of the thermal conductive composite absorbing materials are evaluated.Secondly,based on the wave absorbing mechanism of"thin,light,wide and strong",the microstructure,magnetic properties and absorption properties of rare earth Sm doped melt-spun Fe Si Co alloys were studied.The main work contents and research conclusions are as follows:（1）The flaky CIP were prepared by high-energy ball milling,which was used as the substrate for ultrasonic mixing with nitride Al N,BN and Si₃N₄ceramic fillers respectively,to obtain CIP/nitride ceramic composite thermal absorbing material with uniform composition,and the effects of different proportions of nitride on the thermal conductivity and absorption properties of the composites were investigated.The XRD results show that the pure flaky CIP substrate presents a singleα-Fe phase,and the addition of nitride ceramics does not affect the formation ofα-Fe phase,while the composite sample has the characteristic peaks of bothα-Fe phase and nitride crystal,indicating that there is no chemical reaction between nitride ceramics and flaky CIP,and the composite material is a mechanical mixture.SEM shows that all samples have good sheet structure,and the addition of nitride ceramic filler increases the agglomeration phenomenon of composite samples.Compared with pure flaky CIP substrates,the complex permittivity and complex permeability of CIP/nitride ceramic composite are decreased,and the minimum reflection loss(RL_min)is also decreased,which is related to the fact that nitride is a non-magnetic and non-conductive material.Even so,each composite sample doped with nitride ceramics still retains good absorbing performance,but the reflection loss peak shifts to high frequency,and the absorption band is obviously widened.The thermal conductivity test results show that the thermal conductivity of the composite sample increases with the increase of nitride ceramic doping content.When the mass ratio of CIP to nitride is 10:2,the thermal conductivity of the CIP/Al N ceramic composite is 2.09W/（m·K）.When the coating thickness is 1mm,the RL_mincalculated by the line transmission theory is-7.3d B at 7.4GHz,and the absorption bandwidth below-5d B reaches 7GHz,indicating that the thermal conductive composite absorbing material has both good absorbing performance and good thermal conductivity.（2）Using the same preparation process,the CIP/graphite and carbide thermal conductive composite absorbing materials with uniform composition were obtained by ultrasonic mixing the flaky CIP substrate with graphite and Si C thermal conductive filler.The complex permittivity of the composite samples was significantly improved by graphite doping,and the absorption performance decreased rapidly with the increase of graphite content,and when the coating thickness is 1mm,the RL_minof all the composite samples did not reach-4d B.The complex permittivityt and complex permeability of all composite samples are decreased by Si C filler,but the composite sample basically keeps the good electromagnetic wave absorption ability of the original substrate.The thermal conductivity test shows that the higher content of graphite filler can significantly improve the thermal conductivity of the composites,and the thermal conductivity is up to 4.37W/（m·K）.Si C can also significantly improve the thermal conductivity of composite samples.When the content of Si C is 20%,the thermal conductivity of each sample is more than2W/（m·K）.The thermal conductivity of CIP/Si C composite samples of ball milling for 7h and 10h is more than 2.9W/（m·K）,and when the coating thickness is 1mm,the RL_minat 5.8GHz and 7.5GHz are-6.6d B and-6.8d B,respectively,the bandwidth is as high as 6.5GHz when less than-5d B.The results show that Si C composite CIP has good absorption and thermal conductivity,and has a practical application prospect as a thermal absorbing composite.（3）The melt-spun Sm_xFe_80-xSi₁₀Co₁₀（x=0,1,2）alloy powder absorbing material was prepared by using the melt-spuning method combined with high-energy ball milling method.The absorbing samples were characterized by XRD,SEM,VSM and VNA,and the influence of rare earth Sm doping on the absorption performance of the melt-spun Fe Si Co alloy was studied,especially the absorption performance of microwave low frequency band（around 2GHz）.XRD shows that all samples have a singleα-Fe（Co）phase,AND SEM shows that all samples have typical lamellar anisotropic structure.VSM measurements show that the saturation magnetization and coercivity of the samples increase with the increase of Sm content.When x=1,the Sm₁Fe₇₉Si₁₀Co₁₀sample has the best reflection loss.When the coating thickness is 3mm,the RL_minat 1.7GHz is-7.96d B,and the absorption bandwidth of less than-5d B is 1.2GHz.The results show that appropriate amount of rare earth Sm-doped can improve the microwave absorption performance of the melt-spun Fe Si Co alloy in L-band.