Research On Structure And Properties Of Isotropic Graphite Materials Prepared From Untraditional Raw Materials

With the rapid development of science and technology,the industry exhibits increasing higher requirements for the homogeneity and isotropy of high-performance fine-structure graphite.Petroleum coke,pitch coke and other traditional raw materials can be used to produce fine-structure graphite products with low anisotropy through isostatic pressing technology,but they still cannot satisfy the higher isotropic requirements of fine-structure graphite in the high-end fields such as the nuclear industry,aerospace,and semiconductors.Therefore,untraditional spherical or isotropic raw materials are used to produce fine-structure graphite worldwide.In this paper,self-sintering mesocarbon microbead（MCMB）and isotropic microcrystalline graphite（MG）were used as raw materials to prepare fine-structure isotropic graphite.The effects of types and amounts of reinforcing fillers on the structure and properties of MCMB-based graphite were systematically reaserched,In addition,the structure and properties were analized with pitch content and carbonization temperature of MG-based isotropic graphite.These work will support the preparation of fine-structure isotropic graphite from untraditional raw materials.The research contents and main results are as follows:（1）The carbon fibers/MCMB isotropic graphite was prepared by modified carbon fibers and MCMB powder via mechanical mixing,isostatic pressing,carbonization and graphitization.The results show that the bulk density,flexural strength,and thermal expansion coefficient of the graphite material decrease,the resistivity and anisotropy increase as the carbon fibers content increase.The descend in the density and mechanical properties of the graphite were ascribe to large internal defects in the sample caused by carbon fibers agglomeration and low interface strength between the modified carbon fibers and MCMB.The rising anisotropy may be related to the uneven dispersion of carbon fibers.（2）The CNTs/MCMB isotropic graphite was prepared by modified carbon nanotubes（CNTs）and MCMB via desolvation,crushing,isostatic pressing,carbonization and graphitization.The results show that the bulk density and flexural strength of the graphite material increases initially and then decreases,and the thermal expansion coefficient,resistivity and anisotropy descends initially and then ascends as the amount of CNTs increase.When the content of CNTs reahed 0.25 wt.%,the bulk density of graphite material was 1.88g·cm^-3,the open porosity was 10.57%,the resistivity was 26μΩ·m,the flexural strength was18.5 MPa,compressive strength was 48.1 MPa.Thanks to the hollow structure of CNTs and their uniform distribution in graphite,the thermal expansion coefficient and anisotropy of graphite were effectively reduced.（3）TiC modified carbon nanotubes（T-CNTs）was prepared by growing TiC on the surface of CNTs via pressureless-spark plasma sintering technology.Then the T-CNTs/MCMB isotropic graphite was prepared by the same process as described in part（2）.The research found that the synergistic effect of CNTs and TiC enhances the interface bonding strength between T-CNTs and MCMB particles,which leads to the more compact graphite structure,significantly reduced defects and anisotropy.Compared with the graphite prepared by pure MCMB,when the T-CNTs content reached 0.40 wt.%,the flexural strength of the T-CNTs/MCMB isotropic graphite increased from 14.3 MPa to 24.6 MPa（it was improved by72%）,the anisotropy decreased from 1.07 to 1.02.（4）The MG-based isotropic graphite was prepared by microcrystalline graphite（MG）and modified pitch via solution blending,desolvation,crushing,isostatic pressing,carbonization and graphitization.The results show that the volume density,flexural strength and thermal expansion coefficient of MG-based isotropic graphite gradually increased as the augment in the content of pitch in the range of 26 wt.%～34 wt.%,while the resistivity and anisotropy gradually decreased.The volume density,flexural strength,and thermal expansion coefficient of the MG-based isotropic graphite material increases initially and then decreases,the resistivity and anisotropy decreases initially and then inreases as the carbonization temperature hightens.When the content of pitch was 30 wt.%,and the carbonization temperature reachs 1000℃,the MG-based graphite shows excellent performances.The reaserch also found that the anisotropy of graphite mainly depends on the uniformity of its pore distribution insteaed of its bulk density and open porosity.When the pores are unevenly distributed,more expansion can be absorbed in the direction with much pores while less expansion can be absorbed in the direction with fewer pores,which will increase the anisotropy of the graphite material.Thus,improving the homogeneity of the material is the key to reducing its anisotropy.