In-situ Catalytic Preparation Mechanism Of Carbon Nanotube/SiC And Their Application In MgO-C Refractory

In this thesis,carbon nanotubes?CNTs?were first synthesized via in-situ catalytic pyrolysis of phenolic resin using Fe,Co and Ni monometallic/bimetallic nanoparticles as catalysts.The effects of pyrolysis temperature,types and contents of catalysts and heating rates on the growth of CNTs were investigated.Charges and bond lengths of main carbon source C₂H₄ adsorpted on Fe,Co and Ni monometallic/bimetallic nanoparticles,chemical potential change and bonding energy between CNTs and Fe,Co and Ni monometallic/bimetallic nanoparticles were calculated based on density functional theory?DFT?.Meanwhile,the melting points of Fe and Ni nanoparticles were calculated based on the molecular dynamics?MD?.According to DFT and MD calculation results,the growth mechanisms of CNTs were proposed.Then,silicon carbide powders were catalytic prepared via conventional and microwave heating using phenolic resin and Si powders as raw materials,Fe,Co and Ni nanoparticles as catalysts.The effects of temperature,types and contents of catalysts,holding times and silicon sources on the synthesis of silicon carbide were investigated and the catalytic mechanism on the transformation of Si and SiO₂ to SiC were also studied according to the DFT calculation results.Finally,CNTs/silicon carbide whiskers reinforced low carbon containing MgO-C refractories were catalytic prepared using Fe,Co and Ni nanoparticles as catalysts,and the mechanical properties of as-prepared refractories were examined.The results indicated that:?1?CNTs were prepared via catalytic pyrolysis of phenolic resin using ferric nitrate,cobalt nitrate and nickel nitrate as catalyst precursors.CNTs with tens to hundrends microns in length were synthesized at 1073¹273K for 3h with adding0.75¹.00wt%Fe catalyst in static argon atomosphere.DFT and MD calculation suggested that Fe catalysts facilitated the CNTs growth by increasing the bond length and weakening the bond strength in C₂H₄ via donating electrons to the C atoms in it;The formation of C₅ rings was the most difficult step for synthesis of CNTs.After that,the growth of CNTs spontaneously occurred;Fe was the best catalyst for CNTs growth owing to its highest melting point compared with Ni and Co.?2?CNTs were prepared via catalytic pyrolysis of phenolic resin using Fe,Co and Ni containing bimetallic nano-catalysts.CNTs with tens to hundrends microns in length could be synthesized at 1073¹273K for 3h with adding 0.75¹.00wt%Fe₅₀Co₅₀ bimetallic catalysts.The amounts of CNTs prepared with bimetallic catalysts is superior to that of prepared with the corresponding monometallic nano-catalyst.DFT calculation suggested that the FeCo,FeNi and CoNi bimetallic catalysts facilitated the CNTs growth by increasing the bond length and weakening the bond strength in C₂H₄ via donating electrons to the C atoms in it.FeCo was the best catalyst for CNTs growth owing to its relative high melting point compared with FeNi and CoNi.?3?Si C nanopowders were synthesized by conventional heating method using silicon powders and phenolic resin as raw materials,and Fe,Co and Ni nanoparticles as catalysts.Pure SiC nanopowders with diameter about 100nm could be synthesized at 1523K for 3h with adding 1.5wt%Fe or Co catalysts,and the addition of the catalysts significantly decreased the synthesis temperature of SiC?about 50¹00K?.The formation of the Si C whiskers was dominated by solid-liquid-gas-solid growth mechanism.The as-prepared Si C whiskers were about tens of nanometers in diameter and several microns in length.?4?Si C ultrafine powders were synthesized at 1673K for 3h under Ar atomosphere via a catalytic carbothermal reduction reaction method using industrial diatomite powders and phenolic resin as raw materials,and Fe,Co and Ni nanoparticles as catalysts.Phase pure Si C can be synthesized at 1673K for 3h with1.0wt%Fe as catalysts.The synthetic temperature was reduced by at least 200K compared with the conventional carbonthermal reduction process.DFT calculation further showed that the Fe catalysts played important roles in breaking the Si-O chemical bond,and thus the formation of SiC was promoted.?5?SiC nanopowders were synthesized by catalytic microwave heating method using silicon powders and phenolic resin as raw materials,and Fe,Co and Ni nanoparticles as catalyst precursor.Phase pure SiC can be synthesized at 1373K for30min with 2.0wt%Fe as catalysts.This synthesis temperature and time were100¹50K and 2.5h respectively lower than that required by the conventional heating route.Moreover,the results also indicated that the catalytic performance of Fe is better than Co and Ni.DFT calculations suggested that Fe,Co and Ni nanocatalysts will form alloy with Si first,and then elongate the bond length of Si-Si bonds and weaken its bond strength,finally activate Si powder for synthesis of SiC;the formation of alloy is favorable to the adsorption of C atom,and then accelerate the reaction process between Si and C atoms.The catalytic performance of Fe is better than Co and Ni,which is in consistent with the experimental results.?6?CNTs/silicon carbide whiskers reinforced low carbon containing MgO-C refractories were prepared through in-situ catalytic pyrolysis of phenolic resin in argon atmosphere using MgO grains and fines,silica powders and flake graphite as raw materials,and Fe,Co and Ni nanoparticles as catalysts.The introduction of Fe catalysts promoted the formation of CNTs and silicon carbide whiskers,which significantly improved mechanical properties of the refractory.The rupture strength and compressive strength of the sample adding 0.50wt%Fe catalysts were respectively 10.1MPa and 70.7MPa.In contrast,the values were only 8.5MPa and60.5MPa respectively for the sample without any catalysts.The high-temperature modulus of rupture of as-prepared refractory materials reached a maximum value of about 14.4MPa at 1473K,which was 19.0%higher than that of the sample without any catalysts.