The Microstructures And Mechanical Properties Of Nano Carbon Sources Containing Low-carbon Al₂O₃-C Refractories

Traditional Al₂O₃-C refractories?with carbon content around 10-30 wt%? have been widely used as functional materials, including nozzles, slide gates and stoppers, due to their good resistance to high temperature, thermal shock and slag corrosion. Up to now, it is urgent to develop high performance low-carbon Al₂O₃-C refractories?with carbon content less than 5wt%? to satisfy the demands of high efficiency continuous casting and clean steel smelting technology. However, merely reduce the carbon content will decrease the toughness and deteriorate the thermal shock resistance. Researchers have pointed out that the building of nano structured matrix will be a feasible way for making low-carbon refractories.Based on the latest state-of-art, the nano carbon composites, including nano carbon black, carbon nanotubes?MWCNTs?, graphene oxide nano sheets and expanded graphite together with micron graphite, have shown their capability in reducing carbon content while improving the comprehensive properties. In fact, the nano carbon sources will go through the structure evolution or transformation under high temperatures, it is very important to control or suppress this process, allowing the development of intrinsic properties of nano carbon sources. The synergetic strengthening and toughening effects of nano carbon sources should be elaborated designed to further reduce the carbon content?<5 wt% or even lower than 3 wt% C?. And it is necessary to consider how to evaluate the thermal shock resistance through the acquired mechanical parameters.Aiming at the above problems, the present works were mainly focusing on the following aspects. Firstly, the structure evolutions of carbon black and their influences on the performance of Al₂O₃-C refractories were investigated. After that, the structure evolutions of multi-walled carbon nanotubes?MWCNTs? in different additives containing Al₂O₃-C refractories were studied and their influences on the mechanical properties and thermal shock resistance of Al₂O₃-C refractories were correspondingly revealed. And then, the blends of nano carbon black and commercial MWCNTs/in-situ catalytic MWCNTs were applied as nano carbon sources. The influences of these nano composites on the properties of Al₂O₃-C refractories were characterized. Additionally, Al₂O₃-C refractories containing expanded graphite together with nano carbon black with and without in-situ catalytic MWCNTs were prepared to demonstrate the synergetic effects of multiple carbon sources. At the end, the wedge splitting test together with microscopic fractographic investigations were adopted to investigate the fracture behaviors of Al₂O₃-C refractories with different carbon sources. These studies contribute to establish a relation between phase compositions, microstructures and mechanical properties or thermal shock resistance. Based on the above research work, the following concludes can be drawn:?1? Carbon black is transformed into a core-shell structure with carbon inside and a SiC shell under high temperatures or a SiC particle. The core-shell structure is formed through the vapor-solid reactions, while SiC whiskers are formed at the space between the particles, the length/diameter ratio increases with the increasing of carbon black particles size, which increase the strengths while decrease the thermals hock resistance. Therefore, the blends of nanosized N220 and submicron N990 can improve the mechanical properties and thermal shock resistance.?2? The structure evolution of MWCNTs in Al₂O₃-C matrix are mainly dominated by the SiO?g? partial pressure. The structure evolution process can be divided to several steps: MWCNTs are oxidized below 800 ?; at 1000 ?, a SiC layer is formed on the defects area of MWCNTs and partial MWCNTs transforms into SiC whiskers; with increasing temperature further, the transformation of MWCNTs is stimulated. In comparison with specimens with only silicon added, microsilica can increase the P?SiO?g??, which accelerates the growth of SiC whiskers. In contrast, B4 C additive decreases the P?SiO?g?? evidently duo to its preferential oxidation at lower temperatures, which suppresses the MWCNTs transformation and reduces the SiC whiskers amount.?3? B4 C can act as the catalyst, which accommodating the morphology of MWCNTs and catalyzing the resin carbon into MWCNTs and quasi-graphene structure. The addition of B4 C can improve the thermal shock resistance. In comparison with those containing only silicon additive, the introduction of microsilica stimulates the SiC whiskers formation and thus increases the mechanical properties and brittleness, while deteriorates the thermal shock resistance. On the contrary, B4 C additive protects MWCNTs from transformation and also catalyzes the formation of MWCNTs and quasi-graphene sheets contributes to better toughness, improving the thermal shock resistance.?4? Al₂O₃-C refractories containing nano carbon black and MWCNTs show better properties than materials composed of nano carbon black and graphite. Al₂O₃-C refractories containing 0.1wt% MWCNTs and 0.9wt% nano carbon black show comparable thermal shock resistance than the material containing 1wt% carbon black and 1wt% graphite. In consideration of the dispersion of MWCNTs, the in-situ catalytic MWCNTs could enhance the binding strength and toughness through generating a fabric structure between the residual resin and MWCNTs.?5? The nano carbon composites have the synergetic strengthening and toughening effects. The combination of worm-like expanded graphite and nano carbon black can fully take advantages of their thermal stress absorption and buffering effects, therefore, the thermal shock resistance of the refractories has been improved. On this basis, the incorporation of in-situ catalytic MWCNTs could further improve their comprehensive performances without decreasing thermal shock resistance.?6? The relationships between the fracture behavior, thermal shock resistance and microstructures of the refractories have been established. It is believed that higher strength increase the cracks initiation resistance and higher toughness can enhance the crack propagation resistance. Therefore, Al₂O₃-C refractories containing nano carbon black and expanded graphite with or without MWCNTs show good thermal shock resistance. This should be attributed to the strengthening and toughening effects of different dimensional carbon sources together with the SiC whiskers, which increase the energy dissipation mechanism and crack formation and propagation resistance.