Correlation Between Microstructure And Thermal Conductivity Of Pitch-based Carbon Fibers

The mesophase pitch-based carbon fiber,which is made from petroleum pitch or coal pitch with abundant and cheap raw materials,has been widely used in the military,industrial,and new energy fields due to their high modulus and high thermal conductivity.In order to optimize and improve the performance of carbon fibers,it is necessary to study the microstructure of the mesophase pitch-based carbon fibers and the relationship between performances.It can provide a basis for the preparation of high-performance carbon fibers and is of great significance for the development and application of high-performance materials.This paper focuses on the structure of pitch-based carbon fibers and studies the relationship between the microstructure,structural parameters and thermal conductivity of three kinds of domestic carbon fibers plus the K-1100 carbon fiber.A new program is developed,and the number of carbon fiber tows can be obtained with the program based on image procession.This paper mainly includes the following aspects:(1)The morphology,crystallite size and graphitization degree of three kinds of domestic pitch-based carbon fibers and K-1100 were characterized,using scanning electron microscopy(SEM),X-ray diffractometer(XRD),Raman spectrometer(Raman),electron probe microanalyzer(EPMA)and X-ray microscope.The thermal conductivity of three kinds of domestic pitch-based carbon fibers was tested by indirect method,and the relationship between microstructure and thermal conductivity was studied.The results show that the pre-oxidative fibers with uneven oxygen distribution and different content of oxygen exhibit different morphology,crystalline size,graphitization degree and thermal conductivity after heat treatment.In the three domestic fibers,the differences of the content of oxygen between the surface and core of domestic sample 1 pre-oxidative fiber was much more obvious.After heat treatment,most carbon fiber have holes in the core or exhibit inter-lamellar adhesion,and the microcrystalline size was the smallest.The graphitization degree of the surface and core was the largest and the overall graphitization degree was the lowest,and the thermal conductivity of the single carbon fiber was the smallest.The research on the hole connectivity by micron CT in sample 1 shows that small holes around 1 micron have connectivity in the test range,while large holes around 5 do not.The difference of oxygen content between the surface layer and the core of the pre-oxidative fiber of domestic sample 2 was less obvious.After heat treatment,new micromorphology appeared,the surface layer is in the form of onion while the core is flat layer radial,the microcrystalline size was the largest,the graphitization degree was the highest and the thermal conductivity of the single carbon fiber was the largest.There was no obvious difference in the oxygen content in the surface and core of the domestic sample 3 pre-oxidative fiber,but the overall oxygen content was relatively high.After heat treatment,it cracked along the axial direction and formed circular sector shape.The microcrystalline size,graphitization degree and thermal conductivity were between sample 1 and sample 2.The microcrystalline size,graphitization degree and other microstructure and thermal conductivity of k-1100 carbon fiber are the best compared to the three domestic carbon fibers.Therefore,the thermal conductivity of carbon fiber has less connection with whether the carbon fiber is cracked or not,but the thermal conductivity is related to the radial oxygen distribution and the content of oxygen of the pre-oxidative fiber.When the oxygen is not homogeneous distributed in the radial direction of the pre-oxidative fiber,holes appear in the pitch-based carbon fiber after heat treatment,the microcrystalline size is small,the graphitization degree is low and the thermal conductivity is poor.(2)The relationship between the structural parameters and the thermal conductivity of pitch-based carbon fibers were studied.The radius distributions of the carbon fibers with and without crack were illustrated,and the relationship between the cracking angle and the radius was studied.It is found that the radius of the cracked sample is larger,but there is an overlap between the two radii of the two kinds of fibers.There are no observable differences between the distributions of areas of the carbon fibers with and without radial cracking.It can be drawn that the radius of the carbon fiber increase after cracking.By studying the relationship between the angle and the area of cross-section of carbon fiber,it can be seen the area of carbon fibers decreases with the increase of the cracking angle,thus the smaller the area(the smaller the radius)of carbon fibers,the larger the cracking angle after heat treatment.The relationship between the structural parameters of the carbon fiber monofilament and its thermal conductivity was analyzed.There was no obvious relationship between the radius,the cracking angle and thermal conductivity of the carbon fibers.(3)A new computer program is developed based on digital processing of optical microscope.The developed program was applied to calculate the number of carbon fiber tows automatically.The algorithm approach mainly consists of four parts:image acquisition,image binarization,image morphological processing and fiber marking.Nine digital photos of carbon fiber tows were used for test.The result shows that the error of the calculation is about 1%for the image with number of carbon fiber bundles less than 1000,As the number of fibers increase,the error increase.When the number of fibers is 1468,the error reaches 7.1%.According to the analysis,the main reason is that there are holes in the epoxy resin during embedding,and the fiber at the edge of the hole cracked into small pieces during the grinding process,which leads to the recognizing error.On the other hand,when the number of fibers in a tow is large,it is necessary to enclose all the fibers into the one image at a low magnification with optical microscope,resulting in less pixels of each fiber which cause the error.Compared with manual counting,this approach can significantly improve the accuracy of the number counting of fibers in a tow and enhance the efficiency of production and research.