Foaming Mesophase Pitch By Supercritical Solvent And Mathematical Simulation Of Homogeneous Nucleation

Due to low density, corrosion resistance, low thermal expansion coefficient, high tensile strength, high thermal conductivity, pitch-based carbon foams have a broad application prospect in the field of aviation and aerospace as well as civilian. Mesophase pitch-based carbon foams prepared by supercritical solvent, become an excellent light thermal conductive carbon material as a developed three dimensional net-like and highly graphitized structure. This study focused on choosing the proper solvent to improve the size uniformity and distributing uniformity of bubbles, and had done a systemic research of various factors influencing foaming process. This paper mainly studied the effects of initial pressure, foaming temperature, depressurization rate, and solvent proportion on bubble density and average bubble size. The experiments were carried out in a high pressure reactor, and various curves were made based on the experiments.In order to analyze the nucleation process by supercritical solvent theoretically, we have made the mathematical simulation of homogeneous nucleation process, referring to Colton and Suh's classical nucleation theory and Kim's cluster model of gas molecule. And we got the theory model of homogeneous nucleation by supercritical solvent through regression of parameters. A self- made micro foaming equipment that could be cooled easily was applied for carrying out accurate experiments, and a mathematical model was established based on received data. This study gave the validation to the simulated model under instructions. Comparison between theory and experimental results showed that the variability of the bubble density following saturation pressure and depressurization time agreed well with the mathematical model when the saturation pressure was less than 5.0MPa and depressurization time was less than 5.0s, but real bubble densities were smaller than that simulated nucleus densities due to unavoidable coalescence. When saturation pressure was much high or depressurization time was very long, the bubble densities of foams were much smaller than the simulated nucleus densities due to strong coalescence for bubbles' excessive growth.Through the systematic study of all effective factors, we obtained the range of best foaming conditions. The results showed that supercritical fluid ( SCF ) was suitable for preparing mesophase pitch foams with cell size in the range of 200～800μm, either communicating or closed, and different cell density. Carbon foams with hexagon cell shape, cell diameter of 400～800μm, cell density of 10～16×10³cm^-3 and a ligament-mesh structure were prepared at a temperature of 588～593K, initial pressure of 2.0～3.0MPa, depressurization rate of 1.0～1.5MPa/s and solvent proportion of 30%.