Investigating carbonization and graphitization using electron energy loss spectroscopy (EELS) in the transmission electron microscope (TEM)

Abstract

Electron energy loss spectroscopy (EELS) in the transmission electron microscope (TEM) is explored as a useful characterization technique in the study of carbonization and graphitization of organic precursors. A model series of carbon materials was prepared from highly graphitizable petroleum pitch heat treated in the range 200-2730 C. Initial characterization was performed using the established techniques of X-ray diffraction (XRD), He pycnometry, TEM, electron diffraction and high-resolution lattice imaging (HREM). EELS in the TEM was then examined. Two routes are presented to quantify the change in the proportion of sp 2 type hybridization accompanying the heat treatment as the material transforms to the graphitic state. Both routes suggest an initial relative sp 2 content of $70%, rapidly increasing to $90% during mesophase development and carbonization, and then slowly increasing to 100% during graphitization. The peak position of the bulk valence plasmon ( þ ) is shown to be an excellent measure of the degree of graphitic character, and its fundamental dependence upon sample density () is confirmed. The appearance and definition of features within the core loss region representing the density of unoccupied * states are demonstrated to be an excellent measure of the extent of order. Finally, a method is established by which to extract the C-C bond length from core loss EELS spectra with an accuracy of AE0.1 pm. This method suggests an average bond length of 1.44 Å in samples with low heat treatment temperatures, decreasing to the theoretical length of 1.42 Å as both the heteroatom content and proportion of non-sp 2 -type hybridized carbon atoms decrease.

Micro to nanostructural observations in neutron irradiated nuclear graphites PCEA and PCIB

“…1 eV. This implied a slight reduction in density following neutron irradiation, possibly as a result of an increase in low density material at crystallite grain boundaries following crystallite fragmentation (as observed from XRD and Raman) and/or an increase in nanoporosity [37]. However, the data was also highly variable over different areas.…”

Effect of Lubricant Additives on the WDLC Coating Structure When Tested in Boundary Lubrication Regime

“…There are publications suggesting ways to process EELS data, claiming advantages and drawback of the methods [4,5,24]. Zhang et al [4] suggested a three Gaussian's peak fitting for the core loss; Daniels et al [24] suggested the data processing for low loss. Both methods are used in this study for the core loss and low loss spectra collected, respectively.…”

“…Both EELS and Raman are historically used to characterize the carbon based materials [3,4,24,25]. The bulk material properties can be easily investigated by Raman in a qualitative way for a micrometre resolution [3].…”

“…Both methods are used in this study for the core loss and low loss spectra collected, respectively. Both these papers considered the possible contribution of C-H * in their method [4,24]. As a significant degree of hydrogenation is maintained in the original WDLC coating structure, the method suggested by Zhang et al [4] for three Gaussian's fitting method is chosen.…”

“…In these studies, this feature is used as an indicator/a measure of hydrogen content in the coating material (1s C-H * ) [4,24].…”