Preparation Of Porous Carbon By Microwave Irradiation From Oxidized Coal And Its Microwave Absorbing Application

In recent years,the research on the preparation of new carbon materials using coal as carbon source is increasing.The small molecule active substances of lignite extracted by oxidizing,which is an important technical means for efficient and clean utilization of lignite.And the residual solid sample is called oxidized coal.This study is based on the status of less research on oxidized coal,and in this paper,oxidized coal is prepared from Yunnan lignite.The process of oxidized coal carbonized by microwave was studied.And coal-based porous carbon was prepared using one-step carbonization and activation by microwave,the main factors affecting the structure of coal-based porous carbon were studied,and its application in lithium electricity was investigated.Furthermore,Ni-Zn ferrite loaded on porous carbon and the microwave absorbing property were studied.These studies will open up a new way for the efficient and sustainable utilization of coal energy.In study of oxidized coal carbonized by microwave,the effects of microwave parameters and minerals on the structure of carbonized product were discussed,and the catalytic graphitization of minerals in oxidized coal was analyzed.The results show that increasing microwave power and prolonging microwave time are beneficial to the improvement of ordering degree of carbonized products,carbon microcrystals developed at transverse and longitudinal direction.Minerals can catalyze and promote the orderly arrangement of carbon microcrystals.Mineral catalysis is a mixture of G-effect and A-effect.G-effect is that carbon and iron form corresponding carbides,then which are decomposed,carbon atoms are rearranged.The homogenization degree of catalytic graphitization was improved by A-component.Coal-based porous carbon was prepared by one-step carbonization and activation with microwave,using deashing oxidized coal as raw material.The influences of microwave power,microwave time and ratio of alkali/carbon on the structure of coal-based porous carbon were studied.The results show that the coal-based porous carbon has good thermal stability,few functional groups and abundant porous structure,mainly has a large number of microporous.The specific surface area,pore size distribution and pore structure of coal-based porous carbon can be controlled by adjusting microwave power,microwave time and ratio of alkali/carbon.Microwave has the advantages of high efficiency,saving energy and rapid reaction.When the ratio of alkali/carbon is 3,microwave power is 600 W and time is only 10 min,the specific surface area of coal-based porous carbon can reach 3285 m².g^-1,the total pore volume reaches 1.81 cm³.g^-1 and the micropore volume reaches 1.63 cm³.g^-1.The electrochemical performance of coal-based porous carbon as anode material for lithium ion batteries was studied.Under 100 mA·g^-1 current density,the first cycle discharge capacity reaches 955 mAh·g^-1,and after 70 cycles,the discharge capacity can still reach 698 mAh·g^-1.Even at the high current density of 2000 mA·g^-1,the discharge capacity can be maintained at about 300 mAh·g^-1.Coal-based porous carbon as anode material for lithium-ion batteries has good cycle stability,excellent rate performance and broad application prospects.The precursor powder of Ni-Zn ferrite(Ni_0.8Zn_0.2Fe₂O₄)was prepared by chemical coprecipitation.The microwave absorbing material of Ni-Zn ferrite loaded on coal-based porous carbon were prepared by microwave solid-state reaction.The effects of pore structure and proportion of coal-based porous carbon on absorbing material were investigated.The results show that the Ni-Zn ferrite has larger particles,uneven distribution on the surface of coal-based porous carbon and agglomeration.The pore structure of coal-based porous carbon has little influence on the microwave absorbing property.The pore structure has larger number of micropore volume,the microwave absorbing property is better,the influence of mesoporous volume is not obvious.The proportion of coal-based porous carbon has a significant effect on the absorbing property of loaded material.When the proportion of porous carbon is 50%,the maximum attenuation is 20.8 dB at 15.2 GHz,and effective frequency bandwidth is 4.4 GHz.Coal-based porous carbon can enlarge the microwave absorbing property of Ni-Zn ferrite.The absorbing property of the loaded material is excellent,adjusting the thickness of the coating,the loaded material can satisfy the absorbing requirement of each frequency band.