Preparation Of Magnetic Component/C/CG Composites As Microwave Absorbents By Recycling Of Coal Gangue

Carbon based composite microwave absorbing materials,with the many advantages of high absorption intensity,low density,excellent physical and chemical stability and wide absorption band,have broad application prospects in the fields of electromagnetic pollution protection and stealth materials.Presently,the widely reported carbon matrix has some shortages,such as long preparation period,low production,toxic raw materials and so on,which greatly limits its practical application.Coal gangue（CG）is a solid waste that is continuously discharged in the coal mining and coal washing processes.If it could be utilized as a carbon carrier to prepare composite absorbing materials,it will greatly reduce the cost and provide an effective way for the high value-added utilization of coal gangue.Herein,using coal gangue powder as matrix,cheap starch as supplementary carbon source and nitrate as magnetic particles source,the microwave absorbing composite materials Fe₃O₄/C/CG,Fe/C/CG,Co/C/CG and Ni/C/CG with different magnetic phases were prepared by simple wet chemical method combined with moderate temperature calcination.The effects of calcination temperature and initial starch content on microstructure and microwave absorbing properties were discussed.And the conclusions were following stated:（1）When the calcination temperature is not higher than 500℃,the initial starch addition has little effect on the type of magnetic phase in the composite,and the magnetic material is Fe₃O₄ in this temperature range.When the calcination temperature is 600℃,the initial starch addition has an obvious influnce on the magnetic phase of the composite:when the starch addition is less than 5 g,the magnetic particle is Fe₃O₄.While the magnetic species is Fe,when the addition amount of starch is higher than 6 g.And when the calcination temperature reaches 800℃and the initial starch addition is less than 6 g,the main magnetic phase in the sample is spinel phase of iron.And a part of Fe reacts with C to form a part of Fe₂C alloy phase.Moreover,the reaction between Fe particles and matrix is greatly weakened because of the"isolation"between Fe particles and matrix by C obtained from starch decomposition,so that the Fe particles in the high starch addition sample can be preserved at 800℃;（2）For the Fe₃O₄/C/CG composites,sample FeCG-5-500 displays the best MA performance:when the matching thickness is 3.5 mm,the minimum reflection loss at 6.8 GHz is achieved–45.0 d B,the effective bandwidth is 2.2GHz.And when the coating thickness is reduced to 2.0 mm,the effective bandwidth can reach 3.6 GHz;（3）For the Fe/C/CG composites,when the thickness of FeCG-6-600coating is 2.0 mm,the effective absorption bandwidth can reach 4.3 GHz.And when the thickness of FeCG-7-600 coating is only 1.5 mm,the minimum reflection loss is–20.2 d B at 15.6 GHz and,the effective absorption bandwidth can reach 4.6 GHz.Compared with Fe₃O₄/C/CG composite,Fe/C/CG composite has larger effective absorption bandwith;（4）At higher temperatures（700,800℃）,the Co/C/CG composites shows exceeded high value of complex permittivity,which leads to impedance mismatching and,poorer microwave absorbing properties;（5）The Ni/C/CG composites calcined at 600℃,700℃and 800℃all show excellent microwave absorbing properties,showing the"insensitivity"of temperature,that is,it is easier to prepare the materials meeting the requirements of microwave absorbing under a wider range of conditions.