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QQ2.02 - Computational Modeling of Fabrication of Nanoneedle Involving Phase Transformation in AgGa Alloy 
April 22, 2014   1:45pm - 2:00pm

A phase field model for representing fabrication process of AgGa alloy nanoneedle has been proposed. Meticulous analysis of AgGa needle growth is explained by using a model of phase transformation. Experimental observations expound a detailed process of silver-coated tip dipped into a drop of liquid gallium at room temperature, and then AgGa needle is formed spontaneously. The phase field model is employed to describe the fabrication and then supply reliable information on material science. A convenience of the phase field model is expressing a microstructure evolution in terms of both temporal and spatial dependence of continuum functions. In addition to these improvements, the computational method can provide a general framework for simulating many complicated micro- or nano-structure formations in real alloys. The proposed model has the phase field foundation and incorporates the phenomenological phase transformation. Changes of the free energy of the system is driving force for the phase transformation and mechanism of diffusion is considered to be important factor on influencing micro- or nano-structural evolution of the needle. The evolution of needle-shaped precipitates is modeled based on the free energy increase or the Gibbs-Thomson effect at the end of the needle. The excess free energy sets up a diffusive flow of solute along the axis of the needle that sustains the lengthening. Therefore, a growth rate and morphology of the needle is conceived base on the mathematical models to quantify and optimize the needle growth process. Quantitative observations develop a clear understanding of the growth mechanism as well as an improved control of the growth in a desired direction, the length and diameter of the needle. The computational method aims to investigate mechanical behaviors and test self-assembly process of special materials into useful micro- or nano- structures. An objective of detailed analysis of AgGa needle growth in a selected orientation based on the phase field model and driven by the phase transformation has profound significance for the process of alloy solidification and growth. The theoretical model provides an efficient verification on the experimental work and encourages the current approach to define the geometries required in actual practice.

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Keynote Address
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