| Summary: | 博士 === 國立清華大學 === 材料科學工程學系 === 107 === The research described in this doctoral thesis involves an experimental investigation of thin film Al-Cu-Fe quasicrystal growth from a multilayer structure. The aim was to extend our knowledge about the growth mechanism and phase evolution during the heating and cooling process. First, we investigate the best annealing process to form the thin quasicrystal after depositing Al, Cu and Fe by magnetron sputtering. We show that the steps and duration of annealing have a significant impact on the formation and mechanical properties of QCs. Nanoindentation tests were conducted to measure the hardness of the thin films, and contact angle measurements were taken to measure the surface energy. Our study shows that a longer duration annealing process (15 hours) increased the amount of crystallinity of QC and yields the greatest hardness (~11 GPa) and the highest contact angle (127°). Our analysis showed that the mechanical properties and surface energy were strongly related to the phases. These results led us to investigate the phase evolution from RT to 800°C. Using in-situ XRD and in-situ TEM techniques which enable a direct observation of phase formations in the QC thin film during heating and cooling. Our analyses showed that the ternary phase is more thermodynamically stable compared to the binary phases during the heating process. We demonstrate that QC formation occurs during the cooling process, specifically at 660°C, after the sample reached a liquid state. Using high resolution XRD, we showed that the peak broadening increases monotonically with the physical scattering vector (G∥) but does not have systematic dependence on the phason momentum (G⏊). This analysis confirms that indeed we have observed the QC instead of approximant crystals and it is almost free of phason strain. To further understand the stabilizing mechanism, we study the local environment, neighboring atoms and coordination numbers (CNs), for a Al-Cu-Fe multilayer during heating and cooling. With in-situ X-ray absorption spectroscopy (XAS), we clarified the transition of the ω- Al7Cu2Fe phase to a liquid state at a high temperature which transformed into the QC phase during cooling. By analyzing the XAFS data between 700°C and 800°C, we observed a dramatic increase in atomic distance between Cu-Cu, in which Cu in the second shell relocated to the fourth shell. During the cooling process the QC phase formed by changing the atomic distances, CNs and atomic environments of Cu and Fe. The CN for Fe was changed significantly during the cooling down process and two shells of Al atoms with CN= 4 and 5 combined into a single shell (CN=9) with smaller atomic distances (Fe-Al). Overall, in this
study, we used different techniques to investigate the formation as well as the physical properties to give a clear picture of QC which could provide a better understanding of the synthesis of functional QC nanomaterials.
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