David Cullen and Dr. Richard Vanfleet
Introduction
The alpha, gamma, and kappa phases of aluminum oxide were analyzed using transmission electron microscopy. Micro- and nano-powders of alpha and gamma alumina were donated by Dr. Bartholomew of the BYU Chemical Engineering Department and Alpha Aesar. Kappa phase alumina nanopowders were generated by annealing from a gibbsite powder donated by Nabaltec. Electron energy-loss spectrum (EELS) with an energy resolution of 0.4 eV were recorded for the alpha and gamma phase. This energy resolution was achieved using a Tecnai F20 Analytical Scanning Transmission Electron Microscope (STEM) equipped with a monochromator, Gatan energy filter (GIF) and high resolution spectrometer in STEM mode. Electron diffraction and powder x-ray diffraction were also used to confirm the phase of micron and nanopowder alumina samples. The results from indexing peak and ring diffraction patterns proved to be ambiguous and were resolved by comparing EELS spectra with published fingerprints.
A full report of our findings can be found in the Applied Physics capstone thesis entitled “Fingerprinting the α and γ phases of alumina using electron energy-loss spectroscopy.” In this paper, the EELS fingerprints of alpha and gamma are compared with published data and discrepancies between the spectra are discussed. The significance of using a monochromator with STEM mode for nanoparticle phase identification is addressed, followed by a brief roadmap delineating the goals of future EELS phase identification research.
Results
Powders were first imaged using the Tecnai F30 TEM. We found that the powder donated by Alpha Aesar contained both alpha and gamma phase alumina. We suspect the annealed gibbsite powder has transformed to kappa alumina, but this has yet to be fully confirmed.
EELS spetrum were taken from the alpha and gamma nanopowders, with encouraging results. The 0.4 eV energy resolution greatly simplified the fingerprinting process and demonstrated the ability of the Tecnai F20 TEM to improve upon current published results.
Conclusion
We have succeeded in fingerprinting the gamma and alpha phases of alumina with an energy resolution of 0.4 eV. This was done by using a scanning transmission electron microscope with electron energy-loss spectroscopy and a monochromator. The analysis did not reveal any new features in the EELS spectra, but we were able to demonstrate the capability of using STEM mode to enhance definition and characterize alumina nanoparticles. While electron diffraction and x-ray powder diffraction can be used for phase identification, their results weren’t as robust or conclusive as the EELS fingerprints. We also began taking the first steps towards the generation and analysis of other phases of alumina. This exercise opens the door to future EELS research at BYU, including investigations of other alumina phases and the effect of crystal orientation on EELS spectra.