Highly anisotropic thermostructural properties of graphite revealed by in situ X-ray diffraction up to 2500 °C

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Abstract
High temperature crystallographic evolution of graphite determines the fundamental behavior and limitations of this critical material in extreme environments. In this study, the conical nozzle levitator was used in combination with high energy X-ray diffraction to study the crystallographic and atomic structure of graphite up to 2500 °C. The rhombohedral (3R) allotrope of carbon is related to the hexagonal (2H) allotrope of carbon through stacking faults. The annealing of these stacking faults was tracked as the 3R allotrope transformed into the 2H allotrope at elevated temperatures. Highly anisotropic lattice expansion and resulting coefficients of thermal expansion were catalogued and compared to literature values. Atomic displacement parameters were extracted using the Rietveld refinement method, revealing strong covalent bonding in the basal plane of the carbon layered structure that persisted at elevated temperatures and increasingly weak Van der Waals bonding perpendicular to the basal plane between the layers as a function of temperature. The results therein can be used to aid in modeling of directional deformation and shear behavior of graphite at elevated temperatures.