Microstructural Evolution, In Situ Formation Of Carbon Nanotubes In Lumina-Crbon Refractories And Their Mechanical Properties

Carbon containing refractories have been widely used in steelmaking and continuous castingsystems like converter, electric furnace, steel ladles, due to their excellent thermal shock and slagresistance. With a view to current―low carbon economy‖environment all over the world and therequirement to develop clean steel, carbon containing refractories toward low or ultra-lowcarbonization is the direction of development in refractory field. However, decreasing the content ofgraphite flake in the traditional carbon containing refractories can decrease the tougthness anddeteriorate the thermal shock resistance of the refractories. Carbon nanotubes （CNTs） as a kind ofnew carbon sources, possess many excellent mechanical properties. Therefore, when they arepartially or toally used to replac graphite flake and incorporated into carbon containing refractories,the problems of low toughness and bad thermal shock resistance can be solved by this way. From thepresent point of view, the main challenges which limit the usage of CNTs in carbon containingrefractories are their high cost, difficulty in homogeneous dispersion in the matrix as well asstructural transformation and so on.Based on the problems above, the microstructural evolution of multi-walled carbon nanotubes（MWCNTs） in the high-temperature and complicated enviroment is firstly studied in this thesis.Secondly, in order to slove the problem of structural evolution, MWCNTs are modified withpolycarbosilane （PCS） which pyrolyzes into SiCxOyceramic coating in situ on the surface ofMWCNTs during the heating treatment. On the other hand, the structure of pyrolysis carbon andformation mechanism of CNTs from Fe, Co and Ni nitrate doped phenolic resin are studied atdifferent treated temperatures, in order to offer another way to solve the high cost and dispersionproblem of CNTs used in carbon containing refractories. On the basis of the work above, therelationship between microstructure and mechanical properties of Al₂O₃-C refractories is studied indetail and the conclusions can be drawn as follows:（1） In high-temperature and complicated enviroment, the microstructural evolution of MWCNTs isclosely associated with the partial pressures of Si（g） and SiO（g） in the systems using different siliconsources. When Si is used as the silicon source, Si（g） partial pressure in the system is the highest andSiC reaction layer forms on the surface of MWCNTs at low treated remperature due to the depositionand reaction of Si（g）. Most of MWCNTs transform into SiC nanowires with gradually increasing thethe coking temperature. Using Si+SiO₂as the silicon source, the partial pressure of SiO（g） is thehighest and amorphous SiO₂-SiC reaction layer forms on the surface of MWCNTs due to thedeposition and reaction between SiO（g） and CO（g）. Meanwhile, the thickness of SiO₂layer increaseswith increasing the treated temperature. Using Al+SiO₂as the silicon source, the partial pressures ofSi（g） and SiO（g） are both the lowest. Only a very thin reaction layer forms on the surface ofMWCNTs even after treated at1500oC. Compared with as-received MWCNTs, the oxidationtemperatures and oxidation activation energy of the treated MWCNTs improves greatly, which isdetermined by the thickness of the reaction layer. （2） The surface of MWCNTs can be functionally modified with PCS molecules, which pyrolyzesinto SiCxOyceramic coating on the surface of MWCNTs in situ during the heating process, whichoffers a new way to solve the problem of MWCNTs transformation. The oxidation resistance ofcoated MWCNTs improves greatly compared with as-received ones, which is closely related to thePCS concentration and the treated temperatures that determine the thickness of the coating.（3） Addition of MWCNTs has a big influence on the microstructure and mechanical properties ofAl₂O₃-C refractories. The mechanical properties such as cold modulus of rupture, modulus of elastics,and deformation displacement of refractories with MWCNTs improve greatly. When the treatedtemperature is lower than1000oC, MWCNTs can strengthen and toughen the materials bythemselves. In addition, when the treated temperature is higher than1000oC, the synergeticstrengthening and toughening mechanisms of MWCNTs and ceramic whiskers can endow theAl₂O₃-C refractories with better mechanical properties compared with that containing only graphiteflake. With increasing the amount of MWCNTs, the mechanical properties of Al₂O₃-C refractoriesdecrease due to the agglomeration of MWCNTs. By comparison with as-received MWCNTs, thedispersion of MWCNTs after functionally modified is greatly improved in the matrix. Meanwhile,SiCxOyceramic coating forms in situ on the surface of MWCNTs during the heating treatmentprocess, which protects the intact structure of MWCNTs and improves the interface bond betweenMWCNTs and the matrix, leading to the improvement on the mechanical properties and oxidationresistance of Al₂O₃-C refractories.（4） The CNTs can grow in situ by the catalytic pyrolysis of transition metals nitrate doped phenolicresin, which offers a new way to solve the problems of high cost and dispersion of CNTs in one stepwhen they are used in carbon containing refractories. The graphitization degree of pyrolysis carbonof doped phenolic resin increases greatly due to the catalytic formation of graphite structureincluding the crystalline graphite structure like nano carbon and CNTs with increasing the treatedtemperature. By comparison with Fe and Co nitrate, Ni nitrate has the best catalytic activity due tothe easy existence in the form of metal Ni during the heating treatment of doped phenolic resin. Aswell, except for the formation of MWCNTs, the catalyst can also promote the growth of ceramicwhiskers such as AlN, Al4C3and SiC simultaneously in situ in Al₂O₃-C matrix specimens.（5） Based on the work above, Ni nitrate doped phenolic resin replacing as-received one isintroduced into Al₂O₃-C refractories. On the one hand, MWCNTs can form in situ due to the catalyticpyrolysis of doped phenolic resin. On the other hand, the catalyst can promote the formation of AlN,Al4C3and SiC whiskers in the matrix at the temperature higher than1000oC. The in situ formedMWCNTs as well as the synergistic effect of MWCNTs and whiskers can endow the Al₂O₃-Crefractories with much better mechanical properties at different temperature stages.