The importance of the electrical contact between specimen and testing fixture in evaluating the electromagnetic interference shielding effectiveness of carbon materials

The quality of the electrical contact between the specimen and electromagnetic interference (EMI) shielding testing fixture (coaxial cable method) is found to be critical to shielding effectiveness testing. The carbon fiber (CF) mat (0.15–0.42 mm thick) and carbon nanofiber (CNF) fiber mat (4.4–4.6 mm thick) are similar in the shielding effectiveness (68–70 dB), but the former is much superior in the effectiveness per unit thickness (200 vs. 15 dB/mm), due to the clinginess of the CF mat (resulting from short fiber protrusion) promoting good electrical contact, even at zero specimen-fixture fastening torque. Flexible graphite (0.13–0.38 mm thick) is superior to CF/CNF mats in both the shielding effectiveness (110 dB) and shielding effectiveness per unit thickness (560 dB/mm), partly due to its deformability promoting the electrical contact. For the CF mat, the shielding effectiveness decreases monotonically with increasing fastening torque, due to the increase in the degree of in-plane fiber preferred orientation and the consequent degradation of the electrical contact between the specimen and testing fixture. In contrast, for both CNF mat and flexible graphite, the torque enhances the shielding. The absorption loss per unit thickness reaches 165, 14 and 500 dB/mm for CF mat, CNF mat and flexible graphite, respectively.