Effects of thermal oxidation on electromagnetic interference shielding properties of SiC_f/SiC composites

SiC_f/SiC composites containing PyC interphase were prepared by chemical vapor infiltration and their properties were investigated for electromagnetic interference (EMI) shielding applications in the frequency of 8.2-12.4 GHz. The room temperature direct current electrical conductivity (σ_dc) and EMI shielding effectiveness (SE) were tested after thermal oxidation for various time at 900 °C. It was found that the EMI was shielded mainly by absorption for both as-received and oxidized SiC _f/SiC composites. Both σ_dc and SE of SiC _f/SiC composites decrease after thermal oxidation at 900 °C. It was proposed that the free carbon, including PyC interphase and free carbon in matrix and fibers, mainly determines the EMI SE of composites. The decrease of σ_dc and SE of composites after oxidation are attributed to the consumption of free carbon. The SE of SiC_f/SiC composites with a thickness of 2 mm remains above 15 dB after thermal oxidation for 4 h in air at 900 °C.