Foaming Pitch-Derived Carbon Foam With Supercritical Fluid And Its Microstructure Controlled

The pitch-derived carbon foam is one of new carbon materials with low density, good corrosion resistant, good thermo-insulating property or good heat conductivity determined by the raw material and process, and low thermo expansion coefficient. Therefore it has been researched to use in various application areas, both in aerospace and domestic fields.There are tow main research directions in the research of carbon foam according to its raw material and process: thermo insulator or heat conductor. Pitch based carbon foam, especially mesophase pitch, for easy graphitization, is more suitable to be used as heat conductor. There are three methods for the pitch foaming: nitrizing under high pressure, self-cracking and adding solid foaming agents. These methods have their advantages respectively, however the morphologies and performances of the derived carbon foams show great differences. In this work, the carbon foam was prepared from a mesophase pitch and a coal-tar pitch famed with supercritical fluids which has been used in polymer foam processing.The elasticity and viscosity and chemical structure of the pitch precursors were analyzed with the thermo gravity analyzer (TG), the planar rheometer and the infrared analyzer (IR). The two kinds of pitch were foamed in a high-pressure vessel with a supercritical fluid (SCF) which has good saturant solubility with pitch. The pitch foam was oxidized and carbonized into carbon foam. The microstructures of carbon foam were analyzed with the scanning electron microscope (SEM) and X-ray diffraction (XRD). The different structured carbon foams were prepared. The results showed that different types of carbon foams with bulk density of 0.01-0.8g/cm~3, pore sized ranged in 200-1200μm homogeneously distributed, wall thickness in 5-100μm and porosity above 95% with high cell openings could be gotten with different foaming process.The influences of foaming conditions like SCF content, temperature and pressure, soak time and depressurization rate on the structures of foams were studied. Foaming mechanism was expounded from the view of system's phase equilibrium, gas nucleation and bubble growth. It could be concluded that SCF is suitable for pitch foaming with higher cell openings and better oriented cell walls. Moreover, the pore sizes, structure are easily controlled by this way.The oxidization and carbonization of the pitch foams were studied with the TG-FTIR and the element analysis. The relations between the microstructure of carbon foam and the heat-treatment conditions were analyzed. Different foaming agents were also compared in the action on the pore structure, for different satuation pressure and solubility.The foaming mechanism with supercritical fluids was built with the classic nucleus mechanism. The mini-crystal parameters of carbonized and graphitized foam were analyzed using X-ray diffraction. Compared with high-pressure nitriding, supercritical foaming process has advantage obviously for preparing carbon foam with pore structure controlling for that SCF agent could be homogeneously penetrated into the pitch and could be released in very short time controllably developing highly opened cells with less structural defects and homogenous distributed pore size. The viscosity of pitches was a main factor to influence the foam processing, assistant combined with other conditions.