The microwave absorbing properties of carbon microcoils

Carbon microcoils with coil diameter about 4 μm,pitch about 0.5-0.8 μm(A) and coil diameter about(20 μm),pitch about 1-4 μm(B) were prepared by a vapor phase catalytic cracking process.Paraffin wax/carbon microcoil composites were made by dispersing the latter in the former,which were sandwiched between epoxy glass plates and Nomex honeycomb plates.The complex permittivity(e) and the complex permeability(μ) of the composites were measured at 8.2-12.4 GHz by a reflection transmission system.Results show that the permittivity loss tangent of paraffin wax/B composites is higher than that of paraffin wax/A composites.A-coils show a better microwave absorbing property than B-coils.The microwave reflection loss of the sandwiched composites filled with A-coils is more than 10 dB at the range of 10-15 GHz,more than 5 dB at the range of 4.6-18 GHz and the maximum microwave reflection loss is 18 dB at a frequency of 12.4 GHz,which are opposite to the results expected from the loss tangent(tan δ_e=e″/e′).It is assumed that A-coils with a smaller diameter and pitch have a higher chiral parameter(ξ) value.Moreover,the value of ξ has a greater impact on the microwave absorption than the e value for the carbon microcoils,which results in better microwave absorbing properties.