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 degrees 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 degrees 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 degrees C or higher in an argon atmosphere exhibited a sharp peak near 2 theta = 26.5 degrees, 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 degrees C for 10 min., the interlayer distances (d002 and d100), Lc and La of the graphite crystallites were 0.34, 0.21, 23.00, and 6.13 nm, respectively. The presence of the D band and the IG/(IG + ID) 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 degrees C was Type II, and the specific surface area was 8.45 m2/g, indicating that the charcoal was completely transformed to nonporous graphite.