Composition, Microstructure And Mechanical Properties Of Al₂O₃-C Refractories Containing Expanded Graphite

Due to the excellent thermal shock resistance and slag resistance, the carbon containing refractories have been widely used in steelmaking industry. In recent years, with the rapid development of clean steel metallurgical technique and the requirement of low carbon and energy-saving concept, carbon containing refractories toward low carbonization is the main direction of development in refractory field. However, the simple decrease of graphite flake content would deteriorate the toughness and thermal shock resistance. The expanded graphite(EG) has promising prospect in preparing low carbon containing refractories with high properties. Firstly, the EG not only possesses similar physical and chemical properties with graphite flake, but also has excellent mechanical propertie(like graphene nanosheets) to strengthen and toughen refractories. Secondly, due to high specific surface area and large volume, the EG can promote the formation of carbon network with fewer content to prepare low carbon containing refractories. Finally, due to the loosened wormlike structure, the EG possessed high compressibility and resilience to absorb and release thermal stress, and improve the thermal shock resistance and toughness. The bad anti-oxidation, structure evolution and week interfacial bonding strenghth with the matrix dramatically limited the lage-scale application of EG in carbon containing refractories. Moreover, the strengthening and toughening mechanism of EG on material is not distinct.Based on the problems metioned above, the boron/nitrogen doped EG are firstly prepared to solve the bad anti-oxidation and structure evolution by repairing defects and removing the carbonyl and carboxyl. After that, the EG was modified using silicon evaporation to solve the bad anti-oxidation, structure evolution and weak interfacial bonding strenghth with the matrix, and form graphite-SiC heterostructure. Finally, the strengthening and toughening mechanism of EG in carbon containing refractories was studed in this thesis using the testing method in rock mechanics and discrete element method(DEM). Based on the above research, the main conclusions can be made as follows:(1) With the doping of B/N atoms at the defects on EG, the carbonyl and carboxyl was removed, and different kinds of C-B and C-N covalent bonds were formed to inprove the structural integrity and anti-oxidation of EG. Moreover, the amount of C-B and C-N covalent bonds would increased with the increase of firing temperature, which can furtherly inprove the structural integrity and anti-oxidation of EG, decrease its reactivity(2) The ordinary EG itself could strenthen and toughen the carbon containing refractories. At hiher firing temperature(≥1000 oC), the EG promoted the formation of Al N, Al4C3 and SiC ceramic phases at the cost of structural evolution, which improved the mechanical properties of material and resulted in the structural evolution of EG. Due to the better anti-oxidation and decreased structural evolution of B/N doped EG, the B/N doped EG can synertic strengthen and toughen Al2O3-C refractories with ceramic whiskers. Moreover, the addition of B/N doped EG could improve the thermal shock resistance and antioxidation of Al2O3-C refractories.(3) With the silicon as raw material, the Si Ox spheroids and β-SiC whiskers could be synsized at the sites of defected carbon atoms on EG in coke bed atmosphere and nitrogen atmosphere, repectively. The formation of the above EG modified with silicon can be assigned as follow: In coke bed atmosphere, Si(g) and SiO(g) firstly deposited at defected C atoms to form β-SiC nucleus, then form graphite-SiOx heterostructure due to the continuous deposition of Si(g) and Si O(g); In nitrogen atmosphere, Si(g) deposited defected C atoms to form β-SiC nucleus, then form graphite-β-SiC whiskers heterostructure.(4) The interfacial bonding strength between EG modified with silicon and matrix was improved in comparasion with ordinary EG. Moreover, the modified EG kept relative structural integrity at high temperature. Therefore, the modified EG and ceramic whiskers synergistically improved the mechanical properties of Al2O3-C refractories. Meanwhile, the anti-oxidation of Al2O3-C refractories was improved due to the low reactivity and less defects of siliconization EG.(5) With addition of EG in Al2O3-C refractories, the three-dimentional carbon network structure formed and the corundum aggregates were seperated to avoid stress concentration, which resulted typical intergranular fracure of material. The toughness, cold compression strength, modulus of rupture and tensile strength were drastically inproved due to the crack deflection, crack bridging, crack branching and pull-out mechanism of EG. Meanwhile, EG could absorb the thermal stress during thermal cycle to improve the thermal shock resistance of material.