| Summary: | 博士 === 逢甲大學 === 機械與航空工程研究所博士班 === 100 === Recently, the isotropic and anisotropic thin films are materials with widely application. Especially, the multi-layered metal thin film is widely used in engineering applications. In addition to the multi-layered thin film, laser micromachining and processing, the real application of the high power ultra-short pulse lasers could well extend across all disciplines of physics, chemistry, biology, medicine, and optical technology. However, the ultrafast laser heating will lead to thermal damage on the front surface of a single-layer thin-film. Some researchers suggested that a padding layer added to a two-layered or a multi-layered thin-film could reduce both thermal damage and the mechanical damage of the front surface of a metal film. However thermal resistance always exists at the interface during the material processing. It may lead to thermal stress concentration at the interface and further result in delamination among layers.
First of all, the primary goal of this study was to explore the physical mechanisms of the nonlinear microscale heat transfer and thermoelasticity inside an isotropic multi-layered thin-film (MLTF) irradiated by an ultrafast laser heating. The two-temperature models and hot-electron blast force model were employed in this study. Secondly, three nonlinear formulations were used to compute the thermal resistance at the interface in an isotropic MLTF. Finally, a comparison between the coordinate transformation and direct methods were made to investigate the macro- to micro-scale heat conduction in an anisotropic material.
Numerical results show that the phonon heat transfer effect plays a more important role in an MLTF than that in a single layer. In a multi-layered thin-film, the mechanism of the absorbed energy in the padding layer diffused back into the surface layer is important for the ultrafast thermoelasticity. For imperfect contact situation, a highly non-equilibrium thermal resistance is induced in an MLTF for three different thermal resistance models. The magnitude of the thermal resistance for both electron and the lattice system predicted by those three models are quite different. For imperfect contact situation, the compressive stress may exceed the yielding strength of gold, 1.24 GPa for the lower value of κ (a parameter used to describe the contact situation at the interface between an MLTF). For an anisotropic thin-film, the CT method can be a good candidate to explore the microscale heat conduction mechanism in an anisotropic thin-film imposed by ultrafast pulse laser heating. The anisotropic heat conduction mechanism becomes more important excited by multiple consecutive pulses with high power femtosecond laser. The multiple consecutive pulses with large separation time will spend much time to reach the thermal equilibrium.
After understanding those mechanisms in an isotropic and anisotropic thin film, thermal damage and mechanical damage of the metal films can be minimized. In addition, the effect of thermal resistance existing between layers on temperature and thermal stress fields were be examined. The proposed methods can be extended to the practical industries, e.g., optical technology, nanocomposite, and high-temperature super-conductor.
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