Research On Preparation Of Carbon Materials With High Thermal Conductivity

Owing to their low density, low thermal expansion coefficient, excellent mechanical propertiesand high thermal conductivity, carbon materials (such as carbon fibers, C/C composites, graphitematerials, etc.) become the most promising materials in thermal management field in recentyears. However, the particularity (anisotropy) and diversity (differentia of micro-crystallite sizeand graphitic crystal orientation) of carbon materials lead to their room-temperature thermalconductivities varying widely from70~150W/m.K (general graphite materials) to1950W/m.K(vapor grown carbon fibers). Therefore, the control of the size, orientation and continuity ofgraphite crystals in carbon materials is very important for enhancing the room-temperaturethermal conductivity along a specific direction. In this thesis, ribbon-shaped pitch fibers withwidth in milimeter-scale were prepared by a melt-spinning method using a naphthalene-basedmesophase pitch as a raw material, and highly oriented ribbon-shaped graphite fibers with highthermal conductivity were otained through pre-oxidation, carbonization and graphitization.One-dimensional C/C composites were prepared by a hot-pressing method at about500oC usingthe unidirectional stacked ribbon-shaped fibers (after stabilization and low temperature treatment)as matrix and mesophase pitch as binder. The C/C composites with high thermal conductivitywere obtained by subsequent carbonization and graphitization treatment, in which carbon layersof the ribbon-shaped graphite fibers show highly preferred orientation parallel to the longitudinaldirection of the ribbon fibers. Graphite materials with natural graphite flake with highlypreferred orientation perpendicular to the hot-pressing direction were also prepared by a similarhot-pressing and heat treatment process using mesophase pitch and natural flake graphite withhigh crystallinity as raw materials. The main conclusions of the work were obatined as follows:1. Preparation, characterization and property measurements of mesophase pitch-based ribbonfibers(1) Using mesophase pitch as a raw material, ribbon-shaped pitch fibers with a flat and smoothsurface and controllable width (0.3~1.5mm) and thickness (15~30μm) were prepared by amelt-spinning method. The morphology and structure of the ribbon-shaped pitch fibers werefixed through oxidation stabilization at220~260oC for a certain time. During the followingcarbonization and graphitization treatment, the internal graphitic crystals of ribbon fibersgradually grows and develops, the crystal size gradually becomes larger, and as a result athree-dimensional ordered stacking structure of micro-crystals becomes complete. Thegraphitic layers possess a higher degree of orientation parallel to the main plane of ribbonfibers. Ribbon-shaped fibers keep their ribbon shape and do not deform (split, fold or crimp)during the whole high-temperature carbonization and graphitization process. It can overcomethe problem of easy splitting of round-shaped carbon fibers with a radial texture after hightemperature heat treatment.(2) The heat treatment temperature has an obvious influence on the anti-oxidation property andmechanical properties of ribbon-shaped carbon fibers. The crystallization degree andgraphitization degree of the ribbon-shaped carbon fibers gradually increase with the increase of heat treatment temperature, which results in a better oxidation resistance of the ribbonfibers after graphitization treatment. The anti-oxidation property of the ribbon-shaped fibersgraphitized at2800~3000oC is significantly better than that of the K-1100fibers. Themechanical properties of the ribbon-shaped pitch fibers with a width of1.5mm carbonized atlow temperature (400~700oC) are relatively low. The tensile strength and elastic modulus ofthe ribbon-shaped fibers carbonized at1000oC reach876MPa and109GPa, which furtherincrease to2.53GPa and421GPa after graphitized at3000oC.(3) The axial electric conductivity of ribbon-shaped fibers gradually improves with the increaseof heat treatment temperature. The axial room-temperature electrical resistivity of the ribbonfibers with a width of1.5mm graphitized at3000~3200oC is as low as1.05~1.08μ.m,which is lower than that of the K-1100fibers (1.17μ.m). The axial room-temperaturethermal conductivity of the ribbon fibers graphitized at3000oC calculated by the universalcorrelation of electrical resistivity and thermal conductivity of mesophase pitch-based carbonfibers is up to1100~1200W/m.K.2. Preparation, characterization and property measurements of one-dimensional ribbon fiber/Ccomposites with high thermal conductivity(1) Using the unidirectional stacked ribbon-shaped fibers as a matrix material, followed bycoating mesophase pitch binder on them, ribbon fiber/C composite materials with high bulkdensity were fabricated through one-step hot-press molding at about500oC and subsequentcarbonization and graphitization treatment. As the heat treatment temperature rises, the bulkdensity of the composites gradually increases. The bulk density of the hot-pressed sample atabout500oC is only about1.2~1.3g/cm3, which increases to1.7~1.8g/cm3after thecarbonization treatment at1000oC due to the significant volume shrinkage, and furtherincreases to1.85~1.90g/cm3after the graphitization treatment at3000oC.(2) XRD, PLM and SEM analyses show that the1.5mm wide ribbon fiber/C composite blockprepared exhibits a structural anisotropy. Ribbon-shaped fibers have been unidirectionallydistributed in the composite block and the main planes of the ribbon fibers are orderlyaccumulated along the hot-pressing direction. The internal graphitic layers possess a higherdegree of preferred orientation parallel to the longitudinal direction of the ribbon fibers. Theribbon fiber/C composite block shows obvious electrical and thermal anisotropy and hasexcellent electrical and thermal conductivities along the longitudinal direction of the ribbonfibers. The room-temperature electrical resistivities of the composite sample graphitized at3000oC along the longitudinal direction of the ribbon fibers and along the ribbon fiberaccumulation direction are1.5μ.m and22.2μ.m, respectively. The correspondingroom-temperature thermal conductivities of the sample are862W/m.K and11W/m.K. Heattreatment temperatures have an obvious influence on the thermal diffusivity and thermalconductivity of the composites. The thermal conductivity of the composites increases withthe increasing of heat treatment temperature. There is a good linear correlation between theroom-temperature thermal conductivity of the composite along the longitudinal direction ofthe ribbon fibers and its heat treatment temperature and bulk density, the correlation coefficients (0.98and0.95) of which are both high.(3) The width of the ribbon fibers and transversal shape of fibers have a big influence on thebulk density, the room-temperature thermal diffusivity and thermal conductivity of C/Ccomposites along the longitudinal direction of the ribbon fibers. Narrow (0.5mm and0.3mmwide) ribbon fibers/C composite has a high bulk density (1.88~1.91g/cm3), and theirroom-temperature thermal diffusivity and thermal conductivity along the longitudinaldirection of the ribbon fibers reach570~580mm2/s and820~830W/m.K, respectively.However, a round-shaped fiber/C composite has a lower bulk density (1.70g/cm3), and itsthermal diffusivity and thermal conductivity along the longitudinal direction of the ribbonfibers at room temperature are554mm2/s and707W/m.K. The bulk density of the wide (1.5mm) ribbon fiber/C composite is1.86g/cm3, lower than that of the narrow ribbon fibers/Ccomposite, but the former has a significantly larger thermal diffusivity (618mm2/s) andthermal conductivity (862W/m.K).3. Preparation, characterization and performance testing of graphite materials with high thermalconductivity made with natural flake graphite(1) Using natural flake graphite and mesophase pitch powder as bone-material and binder,graphite blocks with a high bulk density were prepared through a hot-pressing method at about500oC and subsequent heat treatment at high temperature. Flake graphite particle size, themesophae pitch binder content, hot-pressing pressure and heat treatment temperature have acertain impact on the bulk density of the graphite block. The bulk density of the graphiteblocks made with86wt.%natural flake graphite (+32mesh) and14wt.%mesophase pitchpowder hot-pressed at about10MPa pressure is above1.91g/m3after2800oC graphitizationtreatment.(2) XRD, PLM and SEM analyses show that the prepared carbon (graphite) blocks have anobvious structural anisotropy, and the natural flake graphite has been stacked orderly alongthe hot-pressing direction. Except the bulk density and specific heat capacity, other physicalproperties (such as mechanical properties, electrical property, and thermal property) of thegraphite block have an obvious anisotropy. There is a significant difference between thevertical and parallel to the hot-pressing direction.(3) The particle size of natural flake graphite, mesophase pitch binder content and heat treatmenttemperature have a great influence on the room-temperature electrical resistivity of graphiteblocks along the direction perpendicular to the hot-pressing direction. The graphite blocksafter treatment at2800oC show good electrical and thermal conductivity along the directionperpendicular to the hot-pressing direction. Their room-temperature electrical resistivity andthermal conductivity are1.45μ.m and622W/m.K, respectively. However, along thedirection parallel to the hot-pressing direction, the room-temperature electrical resistivity andthermal conductivity are8.35μ.m and25W/m.K, respectively. The compressive andbending strengths of the graphite blocks are relatively low, which are1.3MPa and7.7MPa,respectively.(4) Except heat treatment temperature, the content of mesophase pitch binder and graphitic crystal orientation, other factors (such as the environmental testing temperature, thegranularity of the natural graphite flakes, the properties of pitch binder, hot-pressingtemperature and doping treatment, etc.) have also influences on the room-temperaturethermal conductivity of the graphite blocks along the direction perpendicular to thehot-pressing direction.4. Discussion on the thermal conductivity of carbon materials associated with its electricalconductivity and thermal conductivity mechanism(1) For ribbon-shaped carbon (graphite) fibers and their one dimensional C/C composites, theaxial room-temperature thermal conductivity has close relationships with the axial electricalresistivity, degree of graphitization and graphitic micro-crystallite parameters (d002, Lc, La).The correlation coefficients are all as high as above0.91. There is no obvious relationshipbetween the room-temperature thermal conductivity and electrical resistivity of graphiteblocks along the direction perpendicular to the hot-pressing direction, but theroom-temperature thermal conductivity of graphite blocks in the direction has some weakrelevance with the pitch binder derived graphitic micro-crystallite parameters (Lc and La).The correlation coefficients are0.46and0.64, respectively.(2) The axial room-temperature thermal conductivity of the ribbon-shaped fibers graphitized at3000oC reach1084~1174W/m.K calculated according to the relationship between the axialthermal conductivity and electrical resistivity of ribbon-shaped fibers, which is up to1136W/mK through back-calculating by thermal conductivity mixed formula for one dimensionalC/C composites. It is feasible to estimate the axial thermal conductivity of ribbon fibers bytesting the electrical resistivity. The room-temperature thermal conductivity of C/Ccomposites along the longitudinal direction of fiber is expected to reach890~920W/m.Kaccording to both methods.(3) The thermal conduction mechanism analyses of three kinds of carbon materials(Ribbon-shaped fibers, C/C composites and graphite materials) with high thermalconductivity show that heat treatment temperature, graphitization degree, micro-crystalliteparameters and crystal orientation have significant influences on the thermal conductivity ofcarbon materials. The micro-crystallite size of carbon materials is one of the most importantinternal factors impacting the thermal conductivity of the carbon materials. Theroom-temperature thermal conductivity along the direction perpendicular to the hot-pressingdirection is proportional to the micro-crystallite coherence length La measured by X-raydiffraction.