The Formation,Microstructure And Defects Of Mesophase Pitch Based Carbon Materials

Mesophase pitch is considered as an excellent precursor for the preparation of a wide range of advanced carbon materials and used widely to a number of advanced fields.Their quality has largely determined the performance of final carb on material products.Therefore,studies on the formation and structure(including microstructure and defects)of mesophase carbon materials are of great significance.In this study,a petroleum pitch was used as the raw material for the preparation of mes ophase pitch samples with different microstructural types using a few customized equipments,such as high temperature centrifugation and high temperature high pressure reactor.Based on successful preparation of pitch samples,it is possible to study the microstructure of mesophase pitches in various states of coalescence by means of HR-SEM,PLM,digital imaging and analyzing techniques.Furthermore,more systematical investigations were made possible on the microstructure and defects of graphitizi ble carbon materials based on mesophase pitches.At the same time explorations were made on the effects of high-temperature centrifugation conditions on coalescence of mesophase spheres.Here are the main contents and conclusions of this study.1)The separation of mesophase spheres and preparation of mesophase pitches with different microstructures: A petroleum pitch containing some mesophase spheres was used as the raw material and separated by high temperature centrifugation in different experimental conditions.The objective was to explore the influence of acceleration and temperature on the separation,contact and coalescence of mesophase spheres.Results indicated that high temperature centrifugation technique was capable of separating mesophase pitches from isotropic pitches completely.Pitches samples with mesophase spheres in varied connecting situations were made at relatively lower temperatures ranging from 220 to 320 ℃.Pitch samples with even optical texture and over 95% of mesophase content were obtained easily under centrifugal condition of high temperature and acceleration(T ≥320℃,F≥50G).What’s more,mesophase samples with different types of optical textures,including mosaic,domain,coarse-flow or narrow-flow ones,were obtained from mosaic mesophase sa mples separated at 320℃ after heating treatment at different time and temperatures.Those samples were valuable for the followed studies on defects and microstructures of mesophase pitches.As to the impact of different heating treatment on the final texture of mesophase pitches,it suggested that domain and line-flow mesophase pitches with characteristics of low softening point,100% mesophase content and even optical texture were respectively available on the condition of 300 ℃and 480 min,and 400℃and 120 min.While narrow-flow mesophase pitches with relatively higher soft ening point were obtained on the condition of long heating time at high temperature(400℃,480min).An analysis was made on the physical characteristics of above mesophase pitches with different structures,such as softening point,thermal stability and resolution.Take the thermal stability as an example: narrow-flow pitches > coarse-flow one > domain one > mosaic one.2)The micro structures of mesophase spheres formed during the connect ion of spheres: Pitch samples with spheres connect ed in different stages were obtained through high temperature centrifugation at 220 ℃and 50 G and examined by SEM after solvent extraction,pre-oxidization and carbonization treatments.Th is technique made it possible to observe mesophase pitch samples at single lamella level.It was shownthat Brooks-Taylor spheres tend to dock with each other almost at any locations around a sphere at any angles including acute,normal and parallel situations.While the polars of spheres and contacting points were normally at the same plane.Furthermore,with the sample kept in a fixed position,dynamic variations of extinction contours(or the color)from Brooks-Taylors mesophase spheres were observed in four different modes(see chapter 4),in order to analyze the disclinations of mesophase spheres in-depth.Results indicated that the center of an O-type disclination can be reconstructed by O-O pair(see chapter 4)from two polar positions of Brooks-Taylor mesophase spheres.Thus a new method was found to comfirm the lamella orientation of Brooks-Taylor mesophase spheres.3)The microstructures of mesophase carbon materials: HRSEM was employed to observe and analyze the lamella structural features of mesophase-based carbon materials in different coalescence stages.It revealed different structural defects,including various disclinations,dislocations,voids and cracks,which were formed at different connecting angles,spacial position s and numbers of neighboring spheres during the in-contact stage.A schematic diagram was developed to illustrate the whole process of the bulk mesophase formation from Brooks-Taylor mesophase spheres.4)The defects and microstructures of mesophase pitch by PLM: Based on the results from direct observation of mesophase carbon materials by SEM at nearly single lamella,mesophase samples with four different optical textures were systematically studied by PLM in combination with advanced digital imaging techniques.U/Y pair structures as part of optical textures were extensively observed in the mesophase pitches,varying in shapes,sizes,directions and positions with mesophase pitches.The isochromatic area of mesophase pitches observe d under PLM was mostly formed in closed areas where Y and U disclinations were connected alternatedly.Thus,it is possible to confirm not only the lamella oreintations and disclination defects,but also the textures around for mesophase pitches by PLM alone.