Development of thin dual-layer alumina-iron composite coatings for enhanced microwave absorption

Microwave absorber with wide bandwidth of absorption with less coating thickness are of primary requirement for the modern stealth application. Microwave absorbers reported till date still difficult to meet the trade-off between the thickness and the bandwidth of absorption. Thus, in this work, efforts have been made to attain a broadband microwave absorber at low coating thickness using alumina-iron composites. These composites have been synthesized by mixing the iron and alumina metal powders of particle size ~44 μm using sieve analysis, in a mortar pastel. The complex permittivity and permeability of prepared composites are determined using vector network analyser at X-band (8.2-12.4 GHz). The surface roughness of the “as prepared” pellets are characterized by atomic force microscope (AFM). The effect of alumina content in the composite on microwave absorption is analysed. The microwave absorption of alumina-iron composites is determined for single-layer absorber and found that the maximum reflection loss (RL) increases with increase in alumina content upto 35 wt% of alumina having coating thickness of 2.6 mm, further increase in alumina content decreases the maximum value of RL. The maximum RL value of -59.43 dB is achieved at 10.04 GHz for 35 wt% alumina and 65 wt% iron (35AO) composite with bandwidth of 3.36 GHz i.e., RL below -10 dB with the frequency range 8.69-12.05 GHz. Multi-layering technique using genetic algorithm (GA) is carried out in order to improve the properties like total reflection loss, coating thickness etc. The RL value has been increased to -69.03 dB with 1.81 mm total thickness when multi-layering coating of 0.99 mm thickness of 75 wt% alumina and 25 wt% iron (75AO) composite over which 0.82 mm thickness of 20 wt% alumina and 80 wt% iron (20AO) composite is applied, when all the composites have been taken for multi-layering through GA.