Microstructural investigations of natural and synthetic graphites and semi-graphites

The present work is focused on the characterization of natural samples as end-member products of coalification process—semi-graphites and graphites, and on the characterization of manufactured graphite. To evaluate the properties of the materials a multi-methodology approach was applied using bulk characterization techniques (chemical analysis, X-ray powder diffraction, and helium density measurements), as well as techniques which permitted the identification and characterization of the different components found in the samples, such as optical microscopy, scanning electron microscopy (SEM), and micro-Raman spectroscopy. Good correlations were found between: i) chemical and Rev RIS parameters for less evolved materials (anthracite C—semi-graphite) indicating that the transformations are mainly of chemical nature in this range of coalification; and ii) optical (RMIN% and Ram) and X-ray diffraction parameters in the range of semi-graphite–graphite, showing that transformations are mainly of physical character with strong rearrangement of BSUs, increase of degree of packing and crystalline sizes. Additionally, micro-Raman spectroscopy allowed assessing the properties of all graphitic structures found in the studied samples. The data showed similarities between: i) graphite basal planes (GBP) of both natural and synthetic graphites; ii) graphite edge planes (GEP) of natural graphites, flakes and microspheres of synthetic graphites, crystalline aggregates and lamellae from semi-graphites; and dense particles from semi-graphites. The analysis performed by micro-Raman spectroscopy, optical microscopy measurements, and SEM observations showed that graphitic structures such as flakes found in synthetic graphites are indeed corresponding to the GEP of natural graphites.