A kinetic model based on experimental study of structural evolution of natural carbonaceous material to graphite

Abstract:

We report here new experimental kinetic data on the structural evolution of carbonaceous materials (CM) to graphite in a wide range of temperature conditions (1000 and 1450 °C) and treatment time (10 min to 115 hrs) under a pressure condition of 1GPa. The morphologies and crystallinities of natural CM, extracted from sediments in the Shimanto accretionary complex and Hidaka metamorphic belt, transformed to fully ordered graphite with increasing temperature and annealing duration. The time-temperature relations of each crystal parameter obtained using XRD analysis and micro-Raman spectroscopy demonstrated sigmoidal transformations from amorphous to graphitic structure, suggesting the complexity of chemical reactions undergoing during graphitization. To assess these kinetic processes, the results were analyzed using a superposition method in which the crystal parameters were superposed to reference temperature with non-linear regression curves. The master curves fitted by sigmoidal and power functions exhibited very good correlation coefficients of 0.940 to 0.991, suggesting the Arrhenian relation between temperature and time. On the basis of master curves and shift values, we obtained the effective activation energies of 274 +/– 9 kJmol^-1 and 339 +/– 6 kJmol^-1 for two different natural CM. When compared with the previous studies, our data gave remarkably low activation energies for natural graphitization, which can be represented in a time-temperature relation. In addition, the sigmoidal functions obtained from time-temperature relations can be extrapolated for low temperature condition at 1GPa. Our kinetic model predicts that if the CM underwent metamorphism for about one million years, it begins to crystallize at ≈ 420 °C and transform to fully ordered graphite at over ≈ 510 °C. Thus, natural graphitization could be discussed by the laboratory experiments using natural precursor materials under realistic pressure condition and time span in the Earth’s crust.