Investigation of Electron Irradiation Induced Amorphous to Crystalline Phase Transformations in Nanostructured Ceramics

The effects of electron irradiation induced amorphous-to-crystalline (a-to-c) phase transformations were investigated for 5 different nanostructured ceramic materials. Nanoporous Al₂O₃, TiO₂ nanotubes, ZrO₂ nanotubes, nanoporous Nb₂O₅, and nanoporous Ta₂O₅ were analyzed in situ using transmission electron microscopy (TEM) techniques, where the imaging 200kV electron beam was used also as an electron irradiation source. TEM techniques for analysis include high-resolution transmission electron microscopy (HR-TEM), selected area electron diffraction (SAED), and electron energy-loss spectroscopy (EELS). Each material was examined starting from an as-grown amorphous state, with experiments using diffuse, intermediate, and concentrated beam conditions and various durations of irradiation. Flux of irradiating electrons, as controlled by the spread of the electron beam, showed to have considerable and consistent impact on a-to-c phase transformations for all materials. Diffuse beam experiments, where the electron beam was spread to an area of 30-80μm², with low electron flux consistently observed little to no a-to-c phase transformations, even over longer time scales. Intermediate beam experiments, where the electron beam was spread to an area of .08-.03μm², observed consistent a-to-c phase transformation across all materials, though at varying rates and producing crystallites of various sizes between materials. Concentrated beam experiments, where the electron beam was spread to an area of .003μm², consistently observed rapid a-to-c phase transformation, and also consistently produced larger crystallites compared to intermediate beam experiment counterparts. Comparison between materials showed a trend in susceptibility of crystallization under electron irradiation, with Nb₂O₅ as the least susceptible to electron irradiation induced crystallization, followed by Ta₂O₅, then TiO₂, then ZrO₂, and Al₂O₃ as the most susceptible to electron irradiation induced crystallization. The area of observed crystalline phases was fairly consistent with some outliers within each experiment, though varied both with material and beam concentration. Both production and dissipation of crystalline phases were observed. Dissipation of crystalline phases into amorphous phases appeared to be loosely correlated to the size of the grain within the material. Multiple material properties from literature were compared against the trend observed between materials, though no individual material property showed clear or consistent alignment with the experimental trend observed.