Comparative analysis of graphite oxidation behaviour based on microstructure

Abstract

Two unidentified powdered graphite samples, from a natural and a synthetic origin respectively, were examined. These materials are intended for use in nuclear applications, but have an unknown treatment history since they are considered proprietary. In order to establish a baseline for comparison, the samples were compared to two commercial flake natural graphite samples with varying impurity levels. The samples were characterized by conventional techniques such as powder X-ray diffraction, Raman spectroscopy and X-ray fluorescence. The results indicated that all four samples were very similar, with low impurity levels and good crystallinity, yet they exhibit remarkably different oxidation behaviours. The oxidized microstructures of the materials were examined using high-resolution scanning electron microscopy at low acceleration voltages.The relative influence of each factor affecting the oxidation was established, enabling a structured comparison of the different oxidative behaviours. Based on this analysis, it was possible to account for the measured differences in oxidative reactivity. The material with the lowest reactivity was a flake natural graphite which was characterized as having highly visible crystalline perfection, large particles with a high aspect ratio and no traces of catalytic activity. The second sample, which had an identical inherent microstructure, was found to have an increased reactivity due to the presence of small catalytic impurities. This material also exhibited a more gradual reduction in the 2 oxidation rate at higher conversion, caused by the accumulation of particles which impede the oxidation.The sample with the highest reactivity was found to be a milled, natural graphite material, despite its evident crystallinity. The increased reactivity was attributable to a smaller particle size, the presence of catalytic impurities and extensive damage to the particle structure caused by jet milling. Despite displaying the lowest levels of crystalline perfection, the synthetic graphite had an intermediate reactivity, comparable to that of the highly crystalline but contaminated sample. The absence of catalytic impurities and the needle coke-derived particle structure were found to account for this behaviour.This work illustrates that the single most important factor when comparing unknown graphite materials from different origins is an assessment of the oxidized microstructure.This approach has the added benefit of identifying further potential processing steps and limitations for material customization.

The influence of three different intercalation techniques on the microstructure of exfoliated graphite

“…Since graphite is very sensitive to especially metallic contaminants during oxidation it is not unexpected to find large amounts of damage when the exfoliated sample (A) is oxidized, as demonstrated in Figure 7. This image is typical of contaminated natural graphite [34], the edges are chaotic and ragged due the catalytic effect of the impurity. Consequently this material has a higher oxidative reactivity compared to the purified material (B).…”

Wissenswertes zur Charakterisierung und Aufbereitung von Rohgrafiten

“…Gleichsam als "Nebenprodukt" dieser Forschungsaktivitäten konnte der aus dem Untertagebau der Grafitbergbau Kaisersberg GmbH stammende mikrokristalline Grafit dem Typus der Semi-Grafite zugordnet werden [9]. Diese Unterscheidung ist nicht nur aus mineralogischer Sicht von Interesse, denn die Stufen der kristallinen Ordnung beeinflussen die Eigenschaften der Grafite wie auch der daraus erzeugten industriellen Produkte in wesentlichem Ausmaß [12][13][14].…”

Graphite oxidation and SEM as a tool for microstructural investigation

“…3(a) are not flat and exhibit a characteristic curved or bent shape. These formations are caused by the beneficiation process which involves milling of the raw material [8] to liberate the graphite particles from their constituent ores, mostly silicate based minerals. This damage is unavoidable due to the soft nature of the material.…”