| With the development of low carbon or ultra low carbon steelmaking technologies andenergy saving regulations, carbon content reduction is required for the production of conventionalMgO-C refractories. However, the reduction of the carbon content usually leads to a dramaticreduction of their thermal shock resistance of MgO-C refractories. It is well known that phenolresin has been widely applied as a binder for MgO-C refractories. However, some unfavorablebehavior and performance were recognized when phenol resin was carbonized, e.g., the isotropicglassy structure for secondary carbon derived from phenol resin. This structure can lead tobrittleness and is difficult to be graphitized even at elevated temperature. Therefore, the brittlenessof secondary carbon will decrease the strength at elevated temperature and deteriorate theirthermal shock resistance of low carbon MgO-C refractories.It is realized that in order to overcome the problems mentioned above, a possible way is tooptimize the secondary carbon structure derived from the binder. In this paper, the effects of thetype and content of the catalyst, carbonization temperature, soaking time, treating atmosphere andheating rate on the structure of the pyrolytic carbon derived from transition metal doped phenolresin were investigated systematically by techniques of X-ray diffraction, scanning electronmicroscope, transmission electron microscope and energy dispersive spectrometer. The effectingmechanism of process factors on the growth of in situ carbon nanotubes (CNTs) in the secondarycarbon was analyzed. The key factors for growth kinetics of in situ CNTs in low carbon MgO-Crefractories were further explained, and the growth model of CNTs at high temperature wasestablished. The reaction of the matrix of low carbon MgO-C refractories with the in situ CNTsand the effect on the reaction of various factors were studied. The interface reaction mechanism ofthe in situ CNTs with the matrix was clarified. The effecting mechanism and relativity ofsecondary carbon structure on thermal shock resistance of low carbon MgO-C refractories werealso investigated. Some conclusions can be drawn as follows:(1) The catalytic activity of Fe, Co and Ni in the phenol resin is ranked as Ni>Co>Fe. Themassive and high aspect ratio CNTs with the size of50-100nm in diameter and of micrometerscale in length could be generated at1000℃in pyrolytic carbon derived from Ni doped phenolresin. The catalytic activity of Fe catalyst could be improved by mechanical activation orintroduction of element Ni. The massive and high degree of graphitization CNTs could begenerated at1000℃in pyrolytic carbon derived from phenol resin doped by Fe nanosheets prepared by mechanical activation. The oxidation resistance of the pyrolytic carbon is improvedsignificantly due to the formation of partly graphitized carbon. The oxidation peak temperature forthe pyrolytic carbon derived from catalyst doped phenol resin is increased by80℃, compared tothat for resin carbon without any dopants.(2) The formation process of CNTs will go through the following four stages:1) thehydrocarbon components are decomposed into the carbon atoms on the surface of the metalliccatalyst particle,2) the carbon atoms are dissolved into the metallic catalyst particle,3) the carbonatoms diffuse througn the metallic particle,4) and finally the carbon atoms will be segregated inthe case of supersaturation, thus, the CNTs were formed. During the process, catalysts doped inphenol resin serve as single metal participate in the vapor deposition reaction. The growth ofCNTs agrees with tip growth model. When Fe is served as the catalyst, the treating temperatureshave a significant influence on the morphologies of CNTs. The yield and aspect ratio of CNTsdecrease and the diameter of CNTs increases significantly as the coking temperature increases.The growth mechanism of vapor-solid (V-S) for CNTs will be transformed into avapor-liquid-solid (V-L-S) mechanism at1200℃.(3) Under a reducing atmosphere, aluminium carbide will be formed on the surface of in situCNTs through the reaction of Al vapor from metallic Al at1000℃. It is found that the reactionbetween Al vapor and CNTs is selective and Al vapor just reacts with the poor-crystallized CNTsthrough V-S model. The reaction between Al vapor and CNTs is closely related to its partialpressure. With the increases of the partial pressure of Al vapor, the interfacial reaction between Alvapor and CNTs becomes more severe, and the content of aluminium carbide increasessignificantly. After heat treating at1400℃, aluminium carbide will be transformed into corundumwhich still covers on the surface of CNTs.(4) The partial pressure of hydrocarbon components in the pore of low carbon MgO-Crefractory matrix has a significant influence on the formation of in situ CNTs. The high partialpressure of hydrocarbon components results the formation of CNTs, otherwise, carbonmicrospheres will be formed. It is found that the additives of phenol resin powders or metallicaluminium powders can increase the partial pressure of hydrocarbon components in the pore andis favorable for the formation of in situ CNTs.(5) When the catalyst of nano-sized Ni is added in MgO-C refractories, the granular MgAl2O4 spinels will be transformed into whiskers at1200℃. The existence of Ni catalyst can acceleratethe generation of Mg vapor, which will react with Al vapor and CO to form MgAl2O4spinelwhiskers. Through dissolution, diffusion and precipitation, MgAl2O4spinel crystal nucleatesdirectly and grows into whiskers from the catalytic droplets of nano-sized metallic Ni particles.The growth of MgAl2O4spinel whiskers follows a typical V-L-S growth mechanism.(6) Well-crystallized CNTs of50-100nm in diameter and of micrometer scale in length could begenerated in low carbon MgO-C refractory specimen bonded by a Fe nanosheet-modified phenolresin at1000℃. At temperature range of1000-1400℃, the mechanical properties and thermalshock resistance of the specimens with Fe nanosheets are significantly improved compared withspecimens without Fe nanosheets. These results should be attributed to the formation of in situCNTs, which creates bridging and crack deflection mechanisms in the matrix.(7) The average diameter and average aspect ratio of in situ CNTs have a close relationship tothermal shock resistance of low carbon MgO-C refractories. With the decrease of averagediameter of CNTs or the increase of average aspect ratio of CNTs, thermal shock resistance of lowcarbon MgO-C refractories are improved gradually. Based on the grey relation theory, thecorrelation study of diameter distribution interval and aspect ratio distribution interval withthermal shock resistance indicates that diameter distribution interval of <80nm or aspect ratiodistribution interval of>35have the greatest correlation index. |
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