Carbon Foams by Solid State Foaming of Short Vermicelli Bonded by Phenol-Formaldehyde for Thermal Insulation and Electromagnetic Interference Shielding

Abstract

A simple method of production of carbon foams by randomly assembling short vermicelli rods and bonding them using phenol-formaldehyde (PF) followed by foaming in the solid state during carbonization has been reported. The brittle vermicelli becomes ductile in the range of 200–250 °C during carbonization, which facilitates the nucleation and growth of bubbles in the solid state due to the water vapor generated by the -OH condensation of starch. The dense PF coating on the vermicelli surface facilitates the enlargement of cells by restricting the escape of water vapor during the solid state foaming. The dense carbon produced from the PF binds the carbonized vermicelli rods, affording structural integrity and mechanical strength. The foam body consists of open intervermicelli void space and closed cellular pores in the carbonized vermicelli rods. A broad cell size range of 4–142 μm with an average value of 29.2 μm is observed. The density, compressive strength, and thermal conductivity of the carbon foam are modulated in the ranges of 0.30–0.34 g cm–3, 0.63–1.6 MPa, and 0.162–0.222 W m–1 K–1, respectively, by changing the PF solution concentration in the range of 50–90 vol %. The carbon foams exhibit absorption-dominated electromagnetic interference shielding with total shielding effectiveness and specific shielding effectiveness in the ranges of 47.5–65.6 dB and 155.2–192.9 dB cm3 g–1, respectively, for the X band (8.2–12.4 GHz).