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Flash sintering of zinc oxide and the growth of its nanostructures
Flash sintering was first demonstrated in 2010, where a ceramic green body was rapidly densified within seconds by applying an electric field during the heating process. The ultra-fast densification can occur as current abruptly flows through the material and self-heats by Joule heating. This process has potentials for large energy savings due to the reduction in furnace temperatures and shortened sintering time compared to conventional sintering. In addition, the ultra-high heating and cooling rates, along with the impact of electric field and current leads to the formation of unique non-equilibrium features in ceramics, which could greatly enhance their properties. Despite the potential of flash sintering, there are many challenges in moving this technique towards practical applications, such as the microstructure inhomogeneity and lack of understanding of the defects characteristics.
In this dissertation, flash sintering was performed on ZnO to investigate the influence of various electrical conditions on the microstructure and defects. Detailed characterization was performed on flash sintered ZnO with and without a controlled current ramp, and contrasting types of current (DC and AC). These parameters show significant impact on the gradient microstructure and defects, and provide a way to tailor the desired characteristics for a wide range of applications. On the other hand, flash sintering of ZnO performed with a high electric field and low current density resulted in the growth of nanostructures. These nanostructures are unique compared to other growth techniques as they contain high density basal-plane stacking faults, and exhibit ultraviolet excitonic emission and red emission at room temperature. The nanostructure growth mechanism was investigated by varying the current density limit and revealed the formation of liquid phases which allowed growth by the vapor-liquid-solid mechanism. These findings present a new exciting route for flash sintering to produce highly defective nanostructures for device applications with new functionalities.