Micronstructural Transformation Of Carbon Nanofibers At High Temperature And Its Homojunction's Characterization

With the development in nanoscience and nanotechnology, one-dimensional carbon nanomaterials (1-DCNMs), including carbon nanotubes (CNTs) and carbon nanofibers (CNFs), have attracted much attentions over the last decade due to their unique properties and potential use in broad fields. Up to now, there are many processes have been used for synthesizing various forms of 1-DCNMs. It is well known that different synthesis methods and conditions will produce variant 1-DCNMs'morphology and microstructures. Furthermore, the microstructural transformation can also occur when 1-DCNMs is post heat-treated at high temperature. Generally, the 1-DCNMs property depends upon its morphologies and microstructures, which further exhibits the distinguished potential applications.Recently, as a promising process, flame has been widely applied in mass-production of 1-DCNM in varied morphologies. In the present work, CNTs and CNFs were successfully synthesized from liquid flames. And the systematical studies were carried out for the novel amorphous solid-cored CNFs including the microstructural transformation at high temperature, mechanism and properties.This dissertation includes ten chapters.Chapter one is an introduction, including original of the subject, significance, major contents and author's major contribution, and an overall review for the progress and situation of 1-DCNMs and their microstructural transformation at high temperature, one-dimensional nanoscale carbon heterojunctions/homojunctions and 1-DCNMs synthesized in flames, etc.As a kind of newly developed rapid sintering process, spark plasma sintering (SPS) technique is used in this dissertation. In order to reflect interdisciplinary researches of SPS technique and nanoscience and nanotechnology, as well as make analysis and discussion in following chapters, the technical features, equipments structures, sintering mechanism and applications of SPS technology are introduced briefly in chapter two.Chapter three illustrates the experimental materials and methods used in this dissertation, on the emphasis of the flame synthesis methods of 1-DCNMs and the heat-treated process of the amorphous "solid-cored" CNFs. Meanwhile, the synthesis method of one-dimensional nanoscale "crystalline-amorphous" carbon homojunctions (1-DNCHJs) and fabrication route of CNTs compacted bulk materials are introduced also in detail. In addition, the characterization techniques for morphologies and microstructures, measurements of properties are also demonstrated.In chapter four, the synthesizing method, microstructures and physical properties of the amorphous "solid-cored" CNFs are investigated. The amorphous "solid-cored" CNFs are synthesized from ethanol flame with different substrate preparing methods. It is found that the CNFs are less-ordered and their solid core is composed of small-sized graphene sheets. Moreover, single CNF's semiconductor behavior exhibits in itsⅠ-Ⅴand R-T curves. Meantime, the liquefied petroleum gas (LPG) is used for the first time to synthesize the CNFs which provides a mass-produced route to synthesize CNFs. In addition, by using Ni/Fe metallic salts as the catalyst precursors, the CNTs/CNFs can be easily formed and the "hollow-center" and "solid-center" growth model of CNTs and CNFs brought out by our group are re-confirmed.Generally, the graphitizing temperature for regular micron-sized carbon fibers is above 2000℃, therefore, special and expensive heat treatment equipments must be applied. In chapter five, the influence of high temperature treatment effect on the microstructural transformation of the amorphous "solid-cored" CNFs is studied. It is found that the CNFs microstructural transformation from the disordered amorphous into crystalline strip-shaped structure can occur at low temperature around 1000℃. It is believed that the result of CNFs' nano-effects such as high surface activity and nanoscale effect. Therefore, the graphitization process of the CNFs can be taken part in common commercial tube furnace.In chapter six, novel "crystalline-amorphous" structural 1-DNCHJs is symmetrically investigated including the preparation, fresh microstructure, formation mechanism and electrical transport property, when using a SPS technology which is prior in heating and cooling rate, high-energy and low-voltage sparking pulse-current, compared with conventional heat-treatment. The studies of the electrical transport property show that the nanoscale homojunction exhibits a typical rectification behavior. The formation mechanism of the homojunction is proposed:1) spark plasma and high localized temperature could be momentarily generated by the high-energy, low-voltage sparking pulse-current at the tips of the CNFs; 2) with the highly localized temperature at the tip of the amorphous CNF, the amorphous microstructure is transformed into crystalline structure; 3) the transformation gradually occurs along the CNF starting from the tip, and resulting in a final "crystalline-amorphous" homojunction. In addition, it is worthy of a note that the microstructure of the homojunction can be precisely controlled by optimizing the parameters of the pulse, the treatment time and temperature.Chapter seven focuses on theoretically study of the microstructure and physical properties of the amorphous "solid" CNFs. In this chapter, the microscopic atom configurations of CNFs have been constructed by the reverse Monte Carlo method. Moreover, the simulation error of the configuration is carefully studied to ensure the similarity between simulant and real situations.In chapter eight, on the fundament of the former model, simulation and calculation are fixed on the Fermi Level of amorphous carbon and conductive mechanism of 1-DNCHJs. Based on the results of the calculation, it is pointed out that the rectification behavior of 1-DNCJs is resulted from the Schottky contact between the crystalline and amorphous carbon. Besides, theⅠ-Ⅴrectifying curve is fitted with the formulas in the conventional Metal-Semiconductor contact theory and the results are discussed in the end of this chapter.In chapter nine, the fabrication route, microstructure and physical properties of CNTs bulk materials are explained. Under uniaxial pressure during heat-treating process in SPS system, the CNTs compacted bulk materials and aligned in order are created. Their morphologies and microstructures turn out to be undamaged during the sintering process. Based on the conventional measurement, studies of CNTs bulk materials' property show that the bulk materials exhibit a metal-to-insulator Peierls phase transition at 202 K. It is also found that the electrical conductivity of the bulk material displays anisotropy behavior. The compact bulk materials would be wildly used to explore the intrinsic properties of CNTs.Chapter ten is the conclusions of all the research works mentioned in this dissertation. The last part of the dissertation lists author's published papers finished during the doctorate study period.