Magnetic susceptibility and kinetics of “graphitization” of glass-like carbons

Abstract

The room temperature magnetic susceptibility ( X) of glass-like carbons (GLC) from several sources has been determined as a function of heat treatment temperature (HTT) over the range 1000–3000°C. Effects of dilute ferromagnetic impurities were observed for HTT < 1500°C. The impurity (probably Fe) exists in a non-magnetic form in the carbonized GLC; transforms to a ferromagnetic state with 1000 ⩽ HTT ⩽ 1400°C; then disappears at higher HTT. The dependence of the diamagnetism of pure GLC on HTT is characterized by a slope decrease and inflection near 1500°C and increases smoothly thereafter. X ∼- 5.2 × 10 −6emu/g and is still increasing at HTT = 3000°C, and the values for different GLCs with the same HTT differ by < 10%. The evolution of X as a function of isothermal residence time (HTt) over the range 2400–3000°C for three GLCs was analyzed by the superposition method. Very high effective activation energies, 360–420 kcal/mole (~ 1500–1750 kJ/mole) were obtained and attributed to successive-activation processes of structural development required by the microstructural constraints inherent to difficult-to-graphitize carbons. Evidence was found for a small Xincrease due to the plastic volume dilation (density decrease) processes that occur in some GLCs at high HTT.