| The proportion of castables in total refractories is increasing owing to its easy construction,no requirement for high temperature firing and good integrity.The thermal shock resistance and slag corrosion resistance of refractories could be remarkably improved by the incorporation of carbon,thus,the research and development of carbon containing castables has attracted more and more attention.However,the application of flake graphite in refractory castables was hampered due to the poor water wettability of graphite.Aim at this problem,in this thesis,SiC-SiO2 coated graphites were firstly prepared via sol-gel combined in situ carbonthermal reduction method using Fe?NO3?3·9H2O as catalyst precursor,flake graphites and tetraethoxysilane as raw materials.The effects of amounts of catalyst,C/Si molar ratio and reaction temperature,etc.,on the synthesis of coated graphites were investigated.According to experimental oxidation phenomena,the oxidation models of graphite were built and calculated,and the mechanisms of oxidation morphology evolution of graphite were proposed based on density functional theory?DFT?.Secondly,SiC coated graphites were prepared via microwave-assisted molten salt synthesis using NaCl/NaF as molten salt,flake graphites and Si powders as raw materials;the effects of salt/reactant mass ratio,C/Si molar ratio and reaction temperature,etc.,on the synthesis of coated graphites were investigated.Then,SiC coated graphites were fabricated via low-temperature catalytic reaction method respectively using Fe?NO3?3·9H2O,Co?NO3?2·6H2O and Ni?NO3?2·6H2O as catalyst precursors,flake graphites and Si powders as raw materials.The effects of amounts of catalyst,C/Si molar ratio and reaction temperature,etc.,on the synthesis of coated graphites were investigated.The water wettability and oxidation resistance of as-prepared coated graphites were also respectively studied by sessile drop and Kissinger method.Meanwhile,the configurations of CO?g?and SiO?g?molecules absorbed on catalysts Fe,Co and Ni nanoclusters were built,and the activation mechanisms of the nanoclusters on reactants were also proposed based on the DFT calculation results.Finally,Al2O3-C based castables were manufactured using SiC coated graphites as carbon source.The influences of amounts of as-prepared coated graphites on the mechanical properties and high temperature performances of Al2O3-C based castables were investigated.The results indicated that:?1?SiC-SiO2 coated graphites were successfully prepared via sol-gel combined carbonthermal reduction method using Fe?NO3?3·9H2O as catalyst precursor,flake graphites and tetraethoxysilane as raw materials under the conditions of C/Si molar ratio of 5.6/1,2.0wt%of catalyst and 2h firing at 1673K.Comparing with the water contact angle?111°?and apparent oxidation activation energy?157.1kJ/mol?of raw graphite,the water contact angle of the as-prepared coated graphite decreased to 81°and apparent oxidation activation energy increased to 178.3kJ/mol.DFT calculation results showed that the structure of graphite was most stable when the carbon atoms prefered to be removed in term of hexagonal ring or line during the oxidation of graphite.Moreover,after introducing one Si-C bond into the surface of graphite,the adsorption ability of graphite to H2O was increased based on the DFT calculation results.?2?SiC coated graphites were successfully prepared via microwave-assisted molten salt synthesis using NaCl/NaF as molten salt,flake graphites and Si powders as raw materials under the conditions of NaCl/NaF mass ratio of 20/1,salt/reactants mass ratio of 2/1,C/Si molar ratio of 3/1 and 2h firing at 1423K.The reaction temperature was about 150K lower than that sample prepared by conventional molten salt method.The zeta potential of as-prepared coated graphites reached to-31mV compared to the raw graphites?-7mV?under same pH value?pH=6?,and the apparent oxidation activation energy of coated graphites increased to 166.7kJ/mol.?3?SiC coated graphites were successfully fabricated via low-temperature catalytic reaction respectively using Fe?NO3?3·9H2O,Co?NO3?2·6H2O and Ni?NO3?2·6H2O as catalyst precursors,flake graphites and Si powders as raw materials fired at 1623K.The reaction temperature was about 150K lower than that without catalyst.Using 0.60.8wt%Fe?NO3?3·9H2O as catalyst precursor,the water contact angle and apparent oxidation activation energy of as-prepared coated graphite were 54°and 191.2kJ/mol,respectively.The water contact angle and apparent oxidation activation energy of as-prepared coated graphite were respectively 25°and 193.6kJ/mol for the sample using 0.40.6wt%Co?NO3?2·6H2O as catalyst precursor.When 0.6wt%Ni?NO3?2·6H2O was used as catalyst precursor,the water contact angle and apparent oxidation activation energy of as-prepared coated graphite were 38°and 291.0kJ/mol,respectively.The water wettability and oxdidation resistance of SiC coated graphites were obviously higher than that of raw graphite.DFT calculation results suggested that the CO?g?and SiO?g?could absorb on the Fe,Co and Ni nanoclusters via chemical bond owing to the charge transfer between them.Thus,the bond lengths of C-O and Si-O were increased,and the activated CO?g?and SiO?g?molecules facilitated the synthesis of SiC.?4?The mechanical properties and high temperature performances of Al2O3-C castables were markedly improved by the addition of SiC coatd graphites.Compared to the castables containing raw graphites,the cold compressive strength and cold modulus of rupture of Al2O3-C castables prepared using coated graphites increased 109.3%and25.0%,respectively;the residual modulus of rupture ratio of samples after one cycle of water thermal shock at 1373K increased from 16.7%to 28.9%;and the oxidation area of samples after oxidation at 1473K for 30min decreased from 69.7%to 56.0%;the slag corrosion area of samples after slag corrosion at 1773K for 2h decreased from 16.1%to9.1%.Meanwhile,the slag corrosion resistance of Al2O3 castables was improved after modified by carbon nanofibers. |
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