Comparison of ultrafine-grain isotropic graphite prepared from microcrystalline graphite and pitch coke

Abstract

As a metamorphic product of coal, microcrystalline graphite is of ultrafine polycrystalline structure with near-isotropic and highly graphitized characteristics, which endows itself with great potential as filler for ultrafine-grain isotropic graphite (UGIG). In this work, two kinds of microcrystalline graphite based UGIG, MGG12 and MGG8, are prepared by a liquid mixing process from microcrystalline graphite filler with an average particle size of 12 and 8 μm, respectively. For comparison, pitch coke based UGIG, PCG8, is prepared as control sample from pitch coke filler with an average particle size of 8 μm using the same pitch binder and preparation method. Compared with PCG8, MGG12 and MGG8 exhibit many structure and property advantages such as higher density, higher graphitization degree, higher thermal conductivity, higher isotropy, lower coefficient of thermal expansion, smaller median pore diameter, lower porosity and better barrier property to molten fluoride salt. Notably, due to the smallest median pore diameter (0.431 μm) of MGG8 among these three kinds of UGIG, the molten salt weight gain ratio of MGG8 is only 0.13 wt% under 5 atm, which is much lower than 16.1 wt% and 15.1 wt% for PCG8 and MGG12 under 4 atm, respectively. These advantages indicate that microcrystalline graphite may have advantages over pitch coke as filler for the preparation of UGIG for molten salt applications such as molten salt reactor. In addition, the relevant mechanisms are analyzed and discussed by comparison of the structural characteristics of microcrystalline graphite and pitch coke.