Graphitization of Oak-Tree-Based White Charcoals by High Temperature Heat Treatment.

Oak-tree-based white charcoals were subjected to high-temperature heat treatment at up to 2400 °C to analyze changes in their surface morphology and internal structure using scanning electron microscopy and transmission electron microscopy. When the treatment temperature was increased, micropores became smaller and disappeared, but macropores and mesopores remained, resulting in an increase in average pore size. At treatment temperatures of 2000 °C or higher, all the pores disappeared and the internal structure changed into a dense graphite-like structure. The X-ray diffraction patterns of charcoals heat-treated at 1800 °C or higher in an argon atmosphere exhibited a sharp peak near 2θ = 26.5°, and Raman spectroscopy showed clear D and 2D bands near 1360 and 2680 cm⁻¹, respectively, indicating that carbon graphite crystals were developing. At 2400 °C for 10 min., the interlayer distances (d₀₀₂ and d₁₀₀), L_c and L_a of the graphite crystallites were 0.34, 0.21, 23.00, and 6.13 nm, respectively. The presence of the D band and the I_G/(I_G + I_D) ratio confirmed that the newly developed structure was turbostratic. The Brunauer–Emmett–Teller (BET) adsorption isotherm of the as-received charcoals exhibited peculiar characteristics in which Types I and IV were mixed. This result is due to low-pressure hysteresis, in which nitrogen is embedded in the crevices of charcoal during adsorption and is hardly desorbed during desorption. This low-pressure hysteresis disappeared as increasing the temperature, the adsorption isotherm of charcoal treated at 2400 °C was Type II, and the specific surface area was 8.45 m²/g, indicating that the charcoal was completely transformed to nonporous graphite. [ABSTRACT FROM AUTHOR]