Study On Graphite Nanoplatelets Exfoliated By Three-rollers-milling And Its Application In Low Carbon MgO-C Refractories

The high carbon content（10–20 wt.%）in current commercialized carbon-containing refractories leads to serious complications,such as inferior oxidation resistance,high thermal conductivity,waste of resources,high carbon pick-up in molten steel,and corresponding degradation of the mechanical properties of the products.In order to meet the high requirements for high-quality steel in the modern defense military and aerospace fields,it is time to solve the problem of carbon content in refractories for ultra-low carbon clean steel smelting,and a new generation of low-carbon,ultra-low-carbon（<5 wt.%,Ideally<3 wt.%）carbon-containing refractories are extremely urgent.In this thesis,based on the theoretical analysis,we innovatively propose to employ a Three-Roller Milling（TRM）to exfoliating and thinning the flakes graphite（FG）from micro-sized to nano-sized in phenol-formaldehyde resin（PF）binder,and the graphite nanoplatelets（GNPs）and GNPs/PF mixtures were prepared,which were used as carbon source to developed the low carbon and ultra-low-carbon MgO-C refractories.According to the effective surface area covering oxide aggregates,at the same carbon contents,the GNPs are equivalent to the effect of dozens,hundreds or thousands of FG,therefore,the carbon content of carbon-containing refractories can be significantly reduced,and meanwhile not only did it retain the excellent performance of high carbon-containing refractories but also it can eliminate the disadvantages of the product.Van Der Waals’force between the carbon atoms layers of FG could be overcome attributed to the shear-forced and extruding of TRM as well as the high viscosity of PF.So,the GNPs were prepared from exfoliation FG.Moreover,employed XRD,SEM,TEM and Laser particle size analysis,the effects of exfoliating times and FG content on exfoliated products were studied.The results show that with the increasing of exfoliation times the FG was constantly thinned and crushed,similarly,the FG contents retained a lower level the better the exfoliation effect is.Finally,after exfoliation for 16times,the GNPs exhibited average sizes of 10–30μm and thicknesses of 30–50 nm.The GNPs/PF mixture exfoliated for 16 times as the raw materials was cured at180°C and carbonized at 1000°C in a tube furnace in Ar.By means of XRD and SEM,the phase composition of GNPs/PF composite with different carbon content,the existing form of GNPs at high temperature,the dispersion of GNPs in GNPs/PF composite were studied by using pure PF as control specimen.The results show that GNPs remained the original morphology and dispersed uniformly in GNPs/PF mixture after treatment at 1000°C.In addition,after pyrolysis,PF was transformed into amorphous glass carbon（GC）and many micropores with different sizes appeared on substrate.Moreover,carbon nanotubes（CNTs）with diameter of 20-40 nm were grown by adding 1 wt.%Ni（NO₃）₂·6H₂O as catalyst in GNPs/PF mixture and pyrolysis at1000°C in Ar.The hydrocarbon and carbon oxide gas produced by PF pyrolysis was converted into CNTs with the Ni catalyst,and CNTs was obtained by in-situ growth with V-S and tip-growth mechanism on PF substrate;the in-situ growth of CNTs is beneficial to improve the thermal weight residual carbon ratio.Low-carbon-containing MgO-C refractories were prepared at 260°C with various size of fused magnesia（MgO）,metal Al powder and Si powder,GNPs and GNPs/PF,and phenolic resin.Base on a control specimen only with adding FG,the above prepared GNPs or GNPs/PF mixture were used to partially or completely replace FG.The Archimedes drainage method,three-point bending test method,and static crucible method tested with national standards and combined with SEM,Mapping element analysis,and EDS line scanning element analysis,were used to investigate effects of nano-carbon on the apparent porosity（AP）,bulk density（BD）cold compression strength（CCS）,hot modulus of rupture（HMOR）,the oxidation resistance and slag erosion resistance in MgO-C refractories.The results show that the use of GNPs could not only effectively improve the thermo-mechanical properties and slag erosion resistance of the refractories but also could drastically reduce the carbon content.Especially,the addition of both GNPs and GNPs/PF mixtures improved the best comprehensive performance gains of MgO-C refractories,and the values of CCS and HMOR were increased 96.4%（48.4 MPa）and 15.4%（2.1 MPa）,respectively.In addition,the oxidation resistance and slag erosion resistance of the specimen was significantly improved.