Microcrystalline structure evolution and pore structure formation of liquefied Chinese fir carbon fibers during different carbonization-activation processes

  Objective  To further reveal the pore structure formation mechanism of liquefied wood activated carbon fibers (ALWCFs), the microcrystalline structure evolution and pore structure formation of liquefied Chinese fir carbon fiber (LWCFs) during different carbonization-activation processes were investigated.

  Method  By controlling carbonization temperature (500−900 ℃), LWCFs with different microcrystalline structures were obtained and activated by water vapor at 800 ℃. The changes of elemental composition, microcrystalline structure and pore structure of LWCFs during different carbonization-activation processes were investigated by elemental analyzer, X-ray diffractometer and nitrogen adsorption analyzer.

  Result  With the increase of carbonization temperature, the mass fraction of carbon increased gradually, and the mass fractions of hydrogen and oxygen decreased for LWCFs, while for ALWCFs the mass fraction of carbon increased gradually, and the mass fractions of hydrogen and oxygen decreased gradually. With the increase of carbonization temperature, the axial size of turbostratic graphite-like microcrystals from LWCFs gradually increased and the structure became more densely. Furthermore, the transverse microcrystals began to grow at 900 ℃. During the activation process, the high carbonization temperature significantly promoted the erosion of axial microcrystals by water vapor, which became more seriously with increasing activation time. When activated for 20 min, the transverse size of the microcrystals increased significantly, and grew most obviously when carbonized at 900 ℃, while it did not change significantly with further activation. The pore structure data showed that the specific surface area and total pore volume of ALWCFs were enhanced with the increase of carbonization temperature, and the micropore structure significantly increased with the increase of activation time. For mesoporous structure, low carbonization temperature was beneficial to the formation of mesoporous structure in the initial activation stage. When carbonized at 900 ℃, less mesoporous structure was formed in the initial activation stage, but the mesoporous structure significantly increased with the increase of activation time, due to the gradual expansion of microporous structure.

  Conclusion  The erosion of the interior of the turbostratic graphitic-like axial microcrystals by water vapor is the main way to form the microporous structure for ALWCFs, while the mesoporous structure is mainly derived from the activation of water vapor on the crystal defect or the original crack in the initial activation stage, and from the expansion of the initial micropores in the later activation stage.