| Carbon composite functional refractories such as ladle shround,submerged nozzle,monoblock stopper,and sliding plate bricks serve the continuous casting process of iron and steel production.The structures and properties of carbon composite functional refractories determine the efficiency of the continuous casting process and the quality of products.Carbon composite functional refractories are subjected to mechanical scouring of molten steel,repeated thermal shock and oxidation during the service process.Therefore,there is an urgent need for new and novel approaches that will optimize mechanical properties,thermal shock stability and oxidation resistance of carbon composite functional refractories.Because of no sintering between oxide and carbon components in carbon composite functional refractories,the bonding strength of the materials is mainly provided by the network carbon structure formed by the carbon component and the ceramic phase formed at high temperature.It is found that the glass carbon structure formed by carbonization of phenolic resin in carbon composite functional refractories leads to poor mechanical properties and oxidation resistance.Traditionally,transition metal elements have been used to catalyze the formation of carbon nanotubes/nanofibers and silicon carbide ceramic reinforcement phases to improve their properties,but the reaction temperature is needed to be above 800oC and above.However,the pyrolysis temperature of phenolic resin releases small-molecule gas and forms porous carbonization products below800oC.Therefore,it is of great significance to select appropriate catalysts to form carbon nanotubes/nanofibers and other reinforcing phases at low temperatures to regulate the formation of carbon structure and the evolution of carbon-ceramic structure for binding phase,so as to optimize the properties of materials.Compared with the traditional transition metal catalysts,copper has a lower melting point,which can affect the formation of carbon structure at low temperatures and regulate the evolution of ceramic phase at high temperatures.Based on this merit,copper is selected as a catalyst to control the evolution of carbon/carbon-ceramic structure of the combination phase.The formation of carbon structure of phenolic resin was regulated by adjusting the introduction form and amount of copper source.This paper studied the calence statvalence state change of copper particles and its effect on catalytic activity.In order to optimize the catalytic effect of Cu,the nitrogen source was introduced to regulate carbon structure and optimize oxidation resistance.The mechanical properties and oxidation resistance of phenolic resin carbonized products were improved,and the carbon structure was transformed into carbon-ceramic structure by in-situ formation of Si C ceramic reinforcement phase.The effects of atmosphere,silica powder size,C/Si ratio and copper source on the evolution of carbon-ceramic structure were investiagted,and the related strengthening mechanism was also discussed.The formation and evolution of carbon/carbon-ceramic structure of mesocarbon microbeads and lignin were regulated by copper.Finally,the effects of different binder phases on the structure and properties of carbon composite functional refractories were further studied by introducing mesocarbon microbeads and lignin with phenolic resin into carbon composite functional refractories,respectively.The main results are as follows:(1)The formation of carbon nanofibers catalyzed by copper at a low temperature regulated the formation of carbon structure of phenolic resin and improved the oxidation resistance of phenolic resin carbonized products.Carbon nanofibers were formed by the pyrolysis gas of phenolic resin catalyzed by copper under the mechanism of "dissolution-precipitation" at 500 °C covering the surface of phenolic resin carbonized products matrix and filling the holes to optimize the pore structure and improve the graphitization degree of the carbonized products.After the co-introduction of the nitrogen source,the carbon structure was not only doped with nitrogen to increase the active site of copper catalytic reaction,the graphite like carbon nitride was also created with lamellar structure which covered the surface and pores of the matrix like a "protective layer".The synergistic effect of copper catalyzed and nitrogen doping significantly improved the oxidation resistance of phenolic resin carbonized products.(2)The carbon structure of phenolic resin carbonized products changed to carbon-ceramic system,which improved its strength and oxidation resistance.The introduction of copper source could catalyze the reaction of phenolic resin carbonized products with silicon to form silicon carbide whiskers,and the formation mechanism of silicon carbide whiskers changed from a gas-liquid-solid mechanism to gas-solid mechanism.Copper catalytic reaction reduced the formation temperature of silicon carbide,increased its amount,and regulated the morphology of silicon carbide.The silicon carbide whiskers formed bridges in the matrix and pores of the phenolic resin carbonized products and filled the pores at 1200 °C,which significantly improved the strength and oxidation resistance of the phenolic resin carbonized products.(3)Copper could promote the structure evolution and strength of mesocarbon microbeads and lignin carbonized products.Granular carbonization products were formed after pyrolysis of mesocarbon microbeads,and porous network carbonization products were formed after pyrolysis of lignin.Copper could catalyze the complete reaction between silicon and carbonized products to form silicon carbide whiskers with a large aspect ratio by reducing the melting point of Cu/Si alloy.The formation temperature of silicon carbide was decreased,and the amount of silicon carbide was increased,which promoted the carbon structure of mesocarbon microbeads and lignin carbonized products to evolve into carbon-ceramic network.Finally,the strength of mesocarbon microbeads and lignin carbonized products was improved.(4)Copper catalyst was introduced into carbon composite functional refractories,and the materials’ mechanical properties,thermal shock stability and oxidation resistance were improved by regulating the formation and evolution of the structure of the bonding phase.The introduction of mesocarbon microbeads and lignin improved the cold modulus of rupture and cold crushing strength.At 1400oC,the cold modulus of rupture was increased to 24.7 MPa and 23.0 MPa,respectively.The materials containing mesocarbon microbeads had better thermal shock resistance,and the thermal shock residual strength ratio was 68%.Copper-catalyzed the formation of more Si C whiskers with a large aspect ratio and intertwined them to form a continuous carbon-ceramic network structure,which optimized the carbon-ceramic structure of the combined phase.Silicon carbide ceramics bonding improved the mechanical properties and thermal shock stability of the material through the reinforcement and toughening mechanisms such as "pull-out","crack bridging" and "crack deflection",and improved the oxidation resistance of the materiald by filling holes and reducing the internal oxygen partial pressure of the material,so as to achieve the coordinated improvement of its properties. |
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