Size dependent percolation threshold and microwave absorption properties in nano carbon black/silicon rubber composites

Silicone rubber based lightweight composites have been prepared using nano carbon black (CB) as fillers to achieve broadband microwave absorption properties and electromagnetic interference (EMI) shielding over the 8–18 GHz frequency range. The effect of the filler material characteristics on percolation threshold and resultant electromagnetic (EM) properties of composites has been investigated. The composites are prepared by loading different filler fractions (wt. %) of three nano size CB having different characteristics, viz., CB1 (∼5–10 nm), CB2 (∼15–20 nm), and CB3 (∼30–40 nm) in a silicone rubber matrix. The volume resistivity measurements suggest low value of percolation threshold, i.e., 2 wt. % for CB1-rubber composite, 3 wt. % for CB2-rubber composites, as compared to 15 wt. % for CB3-rubber composite. Filler concentration dependent EM properties, i.e., dielectric constant and dielectric loss tangent (tan δe), are evaluated for all the CB-rubber composites. Furthermore, calculated reflection loss (RL) values for these composites indicate that 3 wt. % CB1-rubber, 5 wt. % CB2-rubber, and 18 wt. % CB3-rubber composites can provide more than 90% microwave absorption (RL > −10 dB) in X (8–12 GHz) and Ku (12–18 GHz) bands with thickness ∼2.7 and ∼1.9 mm, respectively. Interestingly, the composites with higher loading (15 wt. %) of CB1 in the rubber matrix are found to give EMI shielding effectiveness values of ∼ 42 dB over the 8–18 GHz frequency range. In conclusion, CB1-silicone rubber composites have been found to give the best performance among the three studied composites. This composite provides >90% microwave absorption over the X band with 2.7 mm thickness and over the Ku band with thickness 1.9 mm with lowest concentration of fillers, i.e., 3 wt. % CB1 in the rubber matrix, and hence found potential for development of lightweight microwave absorbers for stealth applications.