Exfoliated-graphite-based flexible graphite sheet with a monotonic density gradient along the sheet thickness for enhanced absorption in electromagnetic interference shielding

The electromagnetic wave absorption performance of exfoliated-graphite-based flexible graphite sheets, valuable for electromagnetic interference (EMI) shielding, can be enhanced by engineering a monotonic density gradient along the sheet thickness. The gradient is introduced during fabrication by inserting a sheet of paper at the middle of the exfoliated graphite column before compaction to form flexible graphite. Upon compaction, two symmetric sheets are produced, each having a monotonic density gradient. The greater the mass, the taller the column, the lower the density at the low-density end, and the greater is the gradient. The existence of an optimum gradient for absorption enhancement is discovered, being due to the effects of absorption-enhancing impedance mismatch reduction (with the radiation incident at the low-density end of the gradient) and absorption-reducing conductive path inadequacy. For sub-optimal mass or gradient, Type 1 (with radiation incident at the low end of the gradient) gives higher absorption loss SE _A than Type 2 (with radiation incident at the high end of the gradient), while Type 3 (made without the piece of paper) gives values between those of Types 1 and 2. The Type-1 to Type-2 SE _A ratio increases with the mass or gradient up to the optimum, but decreases when the optimum is exceeded. For sub-optimal mass or gradient, SE _A increases with increasing mass or gradient for Type 1, but decreases for Type 2. The optimum occurs at mass 0.90 g, with 99 % decrease in thickness during compaction at 0.50 MPa, and thickness 0.214 mm and density 0.64 g/cm³ after compaction.