Synthesis of boron and nitrogen-doped expanded graphite as efficient reinforcement for Al2O3-C refractories

Boron and nitrogen doped expanded graphite (B-doped EG & N-doped EG) were synthesized for the first time by annealing the mixtures, obtained from the suspensions of expanded graphite (EG) with boric acid and urea, respectively. Afterwards, the doped EG were introduced into Al2O3-C refractories by partially replacing the graphite flake. Synthesized materials were characterized using SEM-EDS, XPS, XRD, TG-DSC and Raman Spectroscopy. Also, evaluation of various mechanical properties and oxidation resistance tests were conducted. Boron (B) and Nitrogen (N) were successfully introduced into the C skeleton via different C-B bonds (B4C, B-sub-C and BC2O) and C-N bonds (pyridine-N, amino-N and graphitic-N) respectively, maintaining the hexagonal graphitic lattice and lower reactivity. Compared to as-received EG, the B-doped EG and N-doped EG remained relatively intact structure in Al2O3-C refractories at high temperature due to their less defects and lower reactivity. Addition of B-doped EG and N-doped EG into Al2O3-C refractories showed improved mechanical properties such as modulus of rupture (CMOR), force, displacement and thermal shock resistance. Besides, the Al2O3-C specimens containing B-doped EG and N-doped EG exhibited significantly better oxidation resistance, which further promote the reinforcement effect of doped EG in Al2O3-C refractories during actual service condition.