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 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.
