Preparation And Microwave Absorbing Properties Of Coal-Based Porous Carbon Composites Loaded With Ni And Co Doped Zinc Ferrites

The ideal electromagnetic wave absorbing material should be thin,light,broadband and strong absorption.Among them,spinel ferrite is widely used because of its low coercivity,high saturation magnetization and complex permeability,low cost,simple process and excellent absorption performance.However,as a single absorption material,ferrite materials are limited to further applications due to their high density and low dielectric constant.Porous carbon materials are considered to be excellent supplementary materials for ferrites due to their excellent thermal conductivity,electrical conductivity,low density,high specific surface area and controllable pore structure.Therefore,this thesis explores the electromagnetic parameters and absorbing properties of coal-based porous carbon materials and spinel ferrite materials and their possible absorbing mechanisms.In this thesis,coal-based porous carbon（CBC）with different pore structure was prepared by using OSC as carbon precursor and KOH as activator.The effects of carbonization,activation temperature,activator ratio and activation method on the structure of coal-based porous carbon were investigated.The results show that the pore structure of CBC can be effectively adjusted by adjusting the carbonization activation conditions.The specific surface area of CBC reached 2511.6 m²·g^-1,the total pore volume was 1.57 cm³·g^-1,and the average pore size was 2.50 nm when the alkali-carbon ratio was 2.5:1 and the activation temperature was 800°C.Compared with the tubular furnace carbonization and activation,the microwave activation method is faster and simpler,and can also obtain porous carbon with various pore structures and excellent pore structure,but the reaction is more intense,and the controllability of the results is smaller than that of the conventional tubular furnace carbonization and activation reaction.Ni_xCo_0.5-xZn_0.5Fe₂O₄ samples（x=0,0.1,0.2,0.3,0.4 and 0.5）doped with Ni and Co elements were prepared by chemical coprecipitation method.The effects of different ion doping and calcination temperature on the crystal structure and microwave absorption properties of ferrite were studied.The results show that the lattice constants decrease with the increase of temperature,indicating that the appropriate increase of calcination temperature is beneficial to the densification of the lattice structure inside the grains and the more complete crystallization.The increase of Ni ion content broadens the effective bandwidth and improves the absorbing properties of the material at low frequency and low thickness.The addition of Co ions can greatly improve the reflection loss of the material.Ni_0.4Co_0.1Zn_0.5Fe₂O₄ has the best absorbing performance under the condition of low thickness.When the thickness is 2 mm,it has reached-10 d B at 18.0 GHz frequency.When the thickness is 2.5 mm,the effective frequency band is2.6 GHz,and the maximum attenuation is-11.1 d B.Although it is still difficult to meet the requirements of’thin,wide,light and strong’,the doping ratio of Ni and Co ions can effectively adjust the effective bandwidth and maximum attenuation of ferrite under different thicknesses,and improve its electromagnetic absorption performance.CBC/Ni_0.4Co_0.1Zn_0.5Fe₂O₄ composites with uniform dispersion were successfully prepared by solid phase sintering method.The effects of pore structure and ratio of CBC on the absorbing properties of composites were investigated,and the possible absorbing mechanism was explored.The results show that the composite material prepared when the pore structure of CBC is specific surface area of 913.0 m²·g^-1,total pore volume of0.40 cm³·g^-1 and micropore volume of 0.33 cm³·g^-1 reaches the maximum attenuation of-43.7 d B when the matching thickness is 1.5 mm,and the effective bandwidth is 3.9GHz.The ratio of CBC has a significant effect on the absorbing properties of the composites.When the ratio of CPC is 30%,the absorbing properties of the composites are optimal at low thickness.When the thickness is 1.3 mm,the maximum attenuation is-33.8 d B at 15.7 GHz,and the effective bandwidth is also broadened to 4.3 GHz.By adjusting the CBC ratio and the thickness of the composite coating,the absorbing requirements of each frequency band can be met.The excellent microwave absorbing properties of CBC/Ni_0.4Co_0.1Zn_0.5Fe₂O₄composites are determined by the impedance matching and attenuation constant optimized by the synergistic effect after compounding.Specifically,it is mainly caused by the dielectric loss composed of conductance loss,dipole polarization and interface polarization,eddy current loss,magnetic loss composed of natural resonance and multiple reflection and scattering of CBC porous structure.This thesis has 53 figures,14 tables,and 89 references.