Study On The Rheological Properties Of Mesophase Pitch In Spinning Condition And The Carbon Fiber Monolith Materials With High Thermal Conductivity

Mesophase pitch-based carbon fiber has excellent performance such as high specific modulus,high thermal conductivity and electrical conductivity which has great properties in high-performance structural materials,high thermal conductivity carbon/carbon composite materials and thermal management materials for high-end electronic equipment.The excellent properties of mesophase pitch-based carbon fibers come from the graphite microcrystals with high orientation along the fiber axis.The structure is mainly affected by different spinning conditions?temperature,pressure,structure of spinneret,etc.?in the melt spinning processing.And it is formed by orderly arrangement of nematic disc liquid crystals of mesophase pitch.Subsequent heat treatment is the further development and improvement of the crystal structure.Therefore,melt spinning of mesophase pitch is a key processing to control and optimize the structure and properties of carbon fibers.In this thesis,the melt spinning of mesophase pitch and its rheological properties are studied.An evaluation method of spinnability is proposed,which is based on the structure and rheological characterization of mesophase pitch.The influence factors of the spinning process on the structure and performance of carbon fiber are studied.And the die swell of mesophase pitch was explored using a fiber spinning apparatus with varied capillary geometries and processing conditions.In addition,the carbon fiber monolith materials with high thermal conductivity were made by mesophase pitch-based carbon fibers and the influencing factors of its thermal conductivity were discussed.The main research work of this thesis is as follows:In this thesis,the physicochemical properties of five kinds of mesophase pitch with different raw materials and synthetic methods were studied by polarizing microscope,FTIR,elemental analysis,components,solid ¹³C-NMR,TOF,TG,capillary rheometer,rotational rheometer.According to the thermal stability and the results of capillary rheological analysis of the mesophase pitch,a method for identifying spinnability of mesophase pitch was proposed.Mesophase pitch must have excellent thermal stability.And the smoother the extrudate surface of capillary rheometer,the better the spinnability.The spinnable range of mesophase pitch was explored by combining the rheological data under oscillating mode with a large number of actual spinning tests.The region of tan?>10 and G?>40 Pa is spinnable.Furthermore,there are differences among different kinds of mesophase pitch in the good spinnability range.Taguchi's experimental design was employed in the melt spinning of molten mesophase pitch?MP-4?to produce carbon fibers.Taguchi for the four factors at three levels.The four variables in the process were the entry angle of the spinerets,the spinning temperature,the spinning speed,and the nitrogen pressure.The textures of the obtained carbon fibers?nine groups?were radial with varied crack angles,as observed by scanning electron microscopy and polarized optical imaging.The results show that thicker carbon fiber can be obtained with a smaller entry angle,a higher spinning temperature,a reduced winding speed,and an increased extrusion pressure.The winding speed was found to be the most significant factor in relation to the fiber diameter.While it was observed that thicker carbon fiber generally shows improved preferred orientation,the most important variable affecting the preferred orientation was found to be the entry angle.And the preferred orientation increases with the decrease of the entry angle.As a consequence,the crack angle was increased,and the tensile modulus was improved.After graphitization,the crack angle increased further,and the thermal conductivity increased with the increase of diameter.Die swell of a spinnable mesophase pitch?MP-4?was explored using a fiber spinning apparatus with varied capillary geometries and processing conditions.It was found that the die swell decreased with increased shear rate and increased with a rise of temperature.A nearly constant die swell value can be reached as shear rate gets high enough for different temperatures,and it decreased with increased die angles,but maintained unchanged for capillaries with different length-to-diameter ratios.For orifice die,the die swell ratios were found to be affected little by the extrusion rate and the temperatures.The texture examination of the extrudates from orifice die,dies with capillaries and the spinneret show that the extensional flow produces high molecular orientation,and the shear flow-induced tumbling structure and it was retained in extrudate because of the strong memory of meosphase pitch.Carbon bonded carbon fiber monoliths?CBCFM?were prepared from mesophase pitch-based carbon fiber and phenolic resins by a slurry molding method.The CBCFM with density from 0.09 to 0.39 g/cm³ were filled with paraffin wax to form PCM composites with high thermal conductivity,to promote heat transfer and to stabilize the shape of phase change material?PCM?.Due to the anisotropy of the CBCFM,the PCM composites had varied thermal conductivities with their directions.Results showed that the in-plane thermal conductivity of the PCM composites was markedly improved by 18.2 to 57.6 times over the pure wax,depending on the density of CBCFM composites,while the out-of-plane thermal conductivity was also increased by 3.7 to 5.5 times.The charging time of the composites with the high CBCFM density?0.39 g/cm³?was reduced to one quarter of pure paraffin,while the discharging time was about one sixth.And the in-plane thermal conductivity of the PCM composite was 13.82 W/?m·K?.After 40 cycles,the wax loadings in the PCM composites were retained at 56-70%and the loadings trends to be stable.Self-bonded carbon fiber monolith?SBCFM?with low density,high stability and high thermal conductivity was prepared by hot pressing.The moderately pre-oxidized fibers of mesophase pitch?MP-4?was raw material and without any binders.Microstructural characterizations including FTIR,TG,EDS and SEM revealed that the oxidization degree of pitch fibers is a key factor to achieve SBCFM and high performance in thermal conduction.The results of compressive strength and the microstructure of the SBCFM was revealed that a very strong interaction force between carbon fibers.The highly efficient heat transfer network was applied to phase change materials by soaking paraffin wax into the graphitized samples?self-bonded graphite fiber monolith?.An extraordinary increase of 152 folds in thermal conductivity?36.49 W/?m·K??of the composite over the pure phase change materials was obtained at a loading of 20.7 vol%carbon fibers.The results were compared with carbon fiber monolith prepared with binders,showing 2.6 times increment.Such great improvement was attributed to the elimination of interface effect caused by binders.An apparent reduction on interfacial thermal resistance by an order of magnitude was found under an interaction direct derivative analytical framework.