Controlled Preparation Of Mesophase Pitch-based Carbon Foams And Their Potential Applications

Carbon foams, which exhibit an interconnected structure and a large porosity made up of macrospores, are important in many areas of modern science and technology, including aerospace, heat exchanger, catalyst support and electrode material due to their remarkable properties, such as low density, high open-cell ratio, low coefficient of thermal expansion, resistance to corrosion, easy machining and other superior performances. However, the tailor of pore architecture and enforcement of mechanical performance of carbon foams for some special application were still a challenging task. This therefore, calls for research into more economic and effecitive ways to prepare high qulitiy carbon foams with controlled structure and porperties. In this thesis, we focused on these issues and developed two methods (self-foaming and supercritical foaming) to foam mesophase pitch. The pore texture (pore shape, pore size, ligament structure, and opening/closing pore) and mechanical properties have been systematically investigated by adjusting the physical chemistry parameters of mesophase pitch, self-foaming and supercritical foaming conditions. The mechanisms of self-foaming and supercritical foaming were clarified and the occurrence of micro-cracks in graphitized carbon foams was explained. Meanwhile, the applications of carbon foams in gas-solid catalysis and the water treatment were also preliminarily studied.The major innovations of the thesis were from three aspects:tailoring of pore structure (especially pore range from10to200μm), the mechanism of self-foaming and supercritical foaming and the foming mechanism for the micro-crack in the matrix. The main research contents are summaried as follows.1) Carbon foams with pore size of100to600μm were prepred by self-foaming approach, using mesophase pitches as precusors. The pore control has been systematically studied. The mechanisms of self-foaming have been revealed. Under the foaming temperature, the light molecular or pyrolysis gas released from the mesophase pitch were formed a core around the QI component, and then aggregated, grow up and finally became stable pores. During the further heating process, the interior pressure of pitch was higher than the exterior one, resulting in that some volatile materials would be driven and escape from the inner of molten pitch under the pressure difference. As a consequence, some closing pores would turn into opening pores. It was worthy to note that the pressure difference was determined by the amount and the generating rate of the light component.2) A series of carbon foams with pore size of20-200μm were prepared through supercritical toluene technique. Toluene and mesophase would firstly form a homogeneous phase at the supercritical toluene condition. When the pressure was relieved quickly, toluene should separated from the mixture phase owing to the supersaturate status. Besides the Gibbs freedom energy of interface between QI and light components was lower than that of pitch matrix, the toluene would prefer to form the core at the interface, then diffusion, coalescence, growing up and finally turned into pores.3) During the graphitization process, the thermal stress and its gradient distribution along the pore wall and ligament would lead to the occurrence of micro-cracks. The texture and shape of the micro-cracks was greatly detemined by the thermal stress, the release rate and the properties of the precursor carbons. The micro-cracks could be controlled to some content by adusting the composition of the precursor mesophase pitch.4) The homogenous dispersion of CNTs in the mesophase pitch was realized through the ultrasonic wave combined with magnetic stirring method. When the mass concentration of CNTs was3.5%, the as-prepared carbon foams owned uniform pore distribution, and the strength of pore wall was greatly enhanced and the amount of micro-cracks was greatly reduced. The compression strength was as high as4.7MPa.5) A certain amount of mesopore and macropore was created when the carbon foams were subjected to HNO3oxidaiton treatment. The oxidation could improve the hydrophilic properties, which was beneficial for the dispersion of metal catalysis. After impregnated with metal nanoparticles, carbon nanofiber could be grown on the surface of carbon foams through CVD technique, which could effectively increase the external surface area without the loss of the pore structure. MnOx-CeO2hybird oxides could be homogeneously supported on the CNFs/carbon foams composites for NO removal. When the temperature was in the range of180-220℃, the removal efficiency of NO was as high as90%.6) The hydrophilic surface of carbon foams would be obtained by mild HNO3oxidaiton. The active bacteria could be immobilized on the surface of oxidized carbon foams after the bacterination and domestication. Due to the large total bacterial count and high acitive, the immobilized microorganism exhibited high degradation efficiency for the COD, BOD and NH3-N, which was81%,81%and75%, respectively. The performance of carbon foams-based biofilm was superior to those of bioceramic particles significantly, revealing the good biocompatibility of carbon foams.