Effect Of Thermal Carbon Texture And Fiber Orientation Of Carbon/carbon Composites On Thermoelectric Properties

C/C composites has a series of outstanding properties, with the deepening of the study of processing technology, properties and application of C/C composite, its application has been gradually extended to the field of nuclear energy, medical, military, electronics, automobile,sports activities and so on, but so far, there are few reports on the study of the thermoelectric properties.In this paper, 1D and 2D-C/C composites with pyrocarbon of ISO, LT, MT and HT textures were prepared by MCVI technology. The microstructures of C/C composites are investigated by polarizing microscope, X-ray diffraction, scanning electron microscope and Raman spectrometer. The results show that, with the strengthening of pyrolytic carbon texture(from ISO, LT, MT to HT), the orientation of the six angle carbon net is gradually strengthened, the grain size and aromatic carbon plane are increased at the same time, and the surface spacing becomes smaller. And after graphitization treatment, the La and Lc are increased, d002 surface spacing becomes smaller, the defects in the material are reduced. After graphitization, the four pyrolytic carbon represent a different graphitization tendency, the highly textured pyrocarbon is easily to be graphitized, and will form a highly ordered structure.The thermoelectric performance of the obtained C/C composites is investigated comprehensively by the thermoelectric performance tester, laser thermal conductivity instrument and electrical transport properties measurement system. The influence of different pyrocarbon texture on the Seebeck coefficient, electrical conductivity and thermal conductivity of C/C composites is studied. The results show that, regardless of 1D or 2D-C/C composites, with the strengthened of pyrolytic carbon texture, the graphite crystallite plane is enlarged gradually and the defects and electrons captured are decreased, making the number of holes lower. And thus the Seebeck coefficient, electrical conductivity and thermal conductivity is increased gradually, and the high textured material has the highest thermoelectric merit value.The influence of fiber orientation on the Seebeck coefficient, electrical conductivity andthermal conductivity of C/C composites is studied. The results show that, for 2D-C/C composites with the same pyrolytic carbon texture, due to the anisotropy of the fiber itself and the graphite crystallite of the pyrolysis carbon, the XY direction with more axial fibers is more conducive to reduce the carrier scattering and improve the carrier migration, improving the conductivity and thermal conductivity in a certain range, but also reduce the Seebeck coefficient.Finally, the influence of graphitization treatment on the Seebeck coefficient, electrical and thermal conductivity of C/C composites is also investigated, respectively. The results show that, the influence of graphitization treatment on the Seebeck coefficient, electrical conductivity and thermal conductivity of the materials is related to the graphitization temperature and the order of pyrolysis carbon texture. When the graphitization temperature is lower than 2100℃, with the increase of graphitization temperature, the grain size and crystallite size of the plane increases, defects are reduced, the carriers are given priority to with holes, so Seebeck coefficient, electrical conductivity and thermal conductivity are improved. When the graphite temperature is higher than 2100℃, the band gap becomes narrow gradually, electrons are thermally excited and transited to the conduction band, which weaken the Seebeck effect of holes.The forbidden band gap of highly ordered pyrolytic carbon narrows down, causing the stimulated electrons to jump to the conduction band and the type of carriers changed from mainly by the holes into electrons, so the C/C composites show the properties of metal the electrical and thermal conductivity become lower with the increase of test temperature, and the Seebeck coefficient is changed from positive to negative.And for other lower ordered pyrolytic carbon, the carrier type is still mainly by holes, the Seebeck coefficient, thermal conductivity and electrical conductivity are increased with the increase of test temperature.