Preparation And Properties Of Mesophase-pitch-based Carbon Foams

Mesophase-pitch was foamed, stabilized, carbonized and graphitized resulting in a novel foam material Mesophase-pitch-based carbon foam. It is an excellent engineering material due to its unique porous structure and carbon material properties. Porosity, foaming process and properties of the pitch precursor are important factors that affect the final properties of the carbon foam. In this work, mesophase-pitch was prepared by traditional pyrocondensation method and then used the as-received pitch and AR mesophase pitch as precursor to prepare carbon foams, the obtained foam was stabilized, carbonized and graphitized. On the base of researching the preparation processes of mesophase pitches and carbon foams, the text studied the effects of the properties of precursors and preparation methods on the properties of the mesophase pitch derived carbon foams. In addition, the controlling of the microcraks of the foams was discussed in detail and tried to give a principle for preparing mesophase pitch derived carbon foams.A series of mesophase pitches were prepared by the pyrocondensation under different pyrocondensation time and the as-obtained mesophase pitches were used as precursor to prepare carbon foams. The foaming process was given preliminary study, and experimental results show that the mesphase pitch prepared by long holding time exhibits fibre-like structure. Carbon foams from the mesophase pitches prepared by 4.5h and 5.5h exhibit non-uniform foam structure, and carbon foams prepared from mesophase pitches prepared above 6.5 have good foam structure.A simple method for preparing the meosphase-pitch-based carbon foams at low pressures through prolonging the soaking time in the preparation process of the mesophase pitch was disclosed. The physical properties, morphologies and the crystal structure of the as-obtained foams were investigated. Bulk density of the resultant carbon foams cover a range 0.514-0.624 g/cm3, under the preparation pressure range 0.5-2 MPa. The SEM micrographs revealed that the thermal shrinkage of the graphitized foams derived from the higher softening point mesophase pitch was less than that of the foam from the lower softening point; Optical micrographs showed that higher softening point mesophase pitch derived carbon foams exhibited better orientation and less microcracks in both junctions and ligaments; The XRD results revealed that higher softening point pitch derived graphitized foams exhibited smaller interlayer spacing and larger crystallite size. The properties of the carbon foam can be severely affected by the properties of the precursor, so it is critical to tailor the properties of the pitch precursor so as to obtain high performance and low cost products.Two kinds of mesophase pitches were prepared and used as precursor to prepare carbon foams in order to study the effects of mesophase content on the properties of the final products. The experimental results show that carbon foams with cell size uniformity can be prepared from the non-100% mesophase content pitch only under the temperature around the maximum rate of weight loss. The non-100% mesophase content pitch derived foams exhibit more microcracks and worse mesophase domain orientation in the cell walls than that of 100% mesophase content pitch derived foams. After graphitization, the foams from non-100% mesophase content pitch show lower graphitization degree and smaller crystallite sizes. The properties of the carbon foams can be severely affected by the mesophase content, and carbon foams prepared from the non-100% mesophase content pitch give a potential process to prepare foams with properties between insulating and high-thermal-conductivity carbon foams and thus maximizing the potential use of carbon foams. Graphite foams were prepared from the mesophase pitch with addition of natural graphite and the relationship between properties and structure of these foams was investigated in detail. These graphite foams possessed high specific compressive strength and experimental results show that reduced microcracks appeared on the cell walls of foam by adding of natural graphite. The specific compressive strength increased from 2.0 to 5.84 MPa/g.cm3 with the addition of 30 wt% natural graphite, and the inter layer spacing of graphite foams decreased with the increase of the natural graphite content in the pitch. In addition, the optical micrograph shows that the anisotropic domain size of the foam decreased with the addition of natural graphite and it also affects the compressive strength of the foams.The effects of different stabilization methods on the structural properties of the carbon foams were also studied; the foaming mechanism of carbon foam prepared using AR mesophase-pitch as precursor was also studied. SEM, XRD, OM, et al were used to detect the bubble shape and the morphology of the as-obtained foams. The results show that the bubble structure and structural properties of carbon foam can be controlled by the softening point of the pitch precursor. Carbon foam prepared by long-time-coking method exhibit better molecular orientation, larger mesophase domain, less microcracks and less interlayer spacing (0.3376nm and 0.3381nm) when the carbon foam were graphitized. The initial foam bubble are not uniformly dispersed in the molten pitch but start to grow at the upper section of the pitch; the cross section bubble shape at gravitational direction is spherical but at the direction perpendicular to gravitational direction is elliptical. Research show that the bulk density is the key factor to determine the bubble shape of the final mesophase-pitch-based carbon foams and the bulk density gradient exist in the as-received carbon foams.