Electromagnetic shielding response of magnetite elastomeric composites: Source and filler content dependence

The structural and electromagnetic shielding properties of NBR rubber filled with magnetite (Fe3O4) were studied. Rubber composite materials with ore magnetite and synthetic magnetite have been produced separately in order to observe its differences. Samples have been fabricated using a rubber mill and different magnetite ratios. Also, carbon black was added in the same ratio in all samples, not only to improve the mechanical properties of the composites, but also to enhance its dielectric permittivity and reduce its electric resistivity. This kind of composites are used for the manufacture of electronic elements and especially for the production of electromagnetic shielding materials in both passive defense and civil construction applications; buildings, vehicles or airplanes. FESEM-FIB micrographs of the fillers showed the differences in size and shape of the fillers, which influenced the distribution of the particles inside the matrix and therefore the final properties of the composite. Elemental distribution imaging of the composites exhibited the formation of both synthetic magnetite and carbon black aggregates, these structures affect the samples resistivity and the electromagnetic losses due to the conductivity at high frequencies. Electromagnetic shielding properties of the composites were measured in the X-band (8−12 GHz) and K-band of frequency (18–26.5 GHz), all samples showed attenuations greater than −26 dB in both frequency bands. Peaks of maximum attenuation and wider bandwidths were found in the K-band due to the electronic polarization that increased the permittivity of the samples at this high frequency band. The magnetite source (mineral or artificial), its ratio in the rubber matrix and the composite thickness determined the electromagnetic properties of the composite materials.