CHAPTER 14
MICRO-STRUCTURAL INVESTIGATION
OF GRAPHITE POWDERS FOR SENSIBLE
REACTIVITY COMPARISON
HEINRICH BADENHORST, BRIAN RAND, and WALTER FOCKE
CONTENTS
14.1 Introduction ...................................................................................................426
14.2 Materials and Methods ..................................................................................427
14.3 Discussion and Results .................................................................................427
14.3.1 Fegsem Resolution ...........................................................................427
14.5 Conclusion ....................................................................................................459
Keywords .................................................................................................................460
Acknowledgments ....................................................................................................460
References ................................................................................................................461
426 Engineering of Polymers and Chemical Complexity
14.1 INTRODUCTION
Synthetic graphite is an important industrial material and is used in many high temper-
ature applications, ranging from structural and moderator components in nuclear reac-
tors to electrodes for arc furnaces in the steel production industry. Thus, knowledge
of the high temperature oxidative behavior of graphite is essential for understanding
any structural changes which may occur due to oxidation during accidental air-ingress
situations or degradation during normal operation.
Synthetic graphite is produced via a multi-step, re-impregnation process result-
ing in very complex microstructures and porosity [8]. In general though, graphite is
considered to be a fairly simple and well understood allotrope of carbon. It is assumed
to be comprised of layered planes of hexagonally bonded carbon atoms with crystal-
lites of varying sizes and thickness. However, the closest approximation to this ideal
-
ity and large property variations found in graphitic materials, kinetic investigations
are routinely conducted on graphite samples [2-5,7,11,12] from different sources and
origins, without any examination of the material microstructure.
          



enhances the surface detail which can be resolved, making this technique ideal for
examining the morphology of graphite materials.
The aim of this Chapter is to demonstrate the necessity of visually inspecting the
microstructure of graphite materials both before and during oxidation. The microstruc-
ture not only demonstrates the complex development of the surface area during oxida-
tion but also highlights the presence of trace impurities and exposes the underlying
crystallinity. All of these factors are critical when attempting to compare the oxidative
reactivity of graphite samples from different sources and origins. Especially in cases
where the exact history of the material is not known.
Simply analyzing the purity or ash content is not enough due to the considerable
impact extremely low levels of impurities have on the oxidation rate. Furthermore,
the use of X-ray diffraction to determine the crystallinity is shown to be an inadequate
representation of the observed behavior.
Graphite samples from different origins which have been exposed to different pre-
treatments are often subjected to an oxidative reactivity comparison without any con-
sideration being given to the underlying sample microstructures. Four samples from
natural and synthetic origins were thoroughly examined before and after being par-
tially oxidized using a high resolution FEGSEM at low acceleration voltages. Despite
the fact that all four are considered to be high purity, highly crystalline samples, based
on ash content and XRD results, they exhibit widely varying reactivities. Based on the
-
tatively explain the measured differences in reaction rate. This demonstrates the clear

present in a given graphite material before comparing its reactivity to other samples.

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