Fox Thorpe

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.

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.

The environmental conical nozzle levitator system enables cooling trace experiments above 4000 K. These cooling trace experiments were used to determine the melting points through observation of thermal arrest of group IV and V transition metal diborides (TiB ₂ , T _m = 3313 ± 33 K; ZrB ₂ , T _m = 3404 ± 38 K; HfB ₂ , T _m = 3529 ± 33 K; and TaB ₂ , T _m = 3284 ± 10 K; note: mean ± 2 standard error). Temperature measurements were conducted utilizing dual single‐color pyrometers at wavelengths of 0.9 and 0.65 µm. This system utilizes aerodynamic levitation and dual laser heating to achieve temperatures approaching 4000 K. The samples were synthesized from commercial powders using ceramic gel‐casting to produce high‐density spherical components. The measurements obtained were compared to previously reported melting temperature values and were generally found to be in agreement with most of the published values.