| Summary: | 碩士 === 國立中正大學 === 機械系 === 90 === Thin films are being widely used in various fields such as sensors, semiconductor, and tribology. Due to the development of the nanotechnology, the applications of nanoscopic thin film increase rapidly. The most fundamental mechanical properties of the thin film, which are defined on the precepts of continuum mechanics, are assumed to be independent of size. However, with a nanoscale dimension, that the properties of a material depend on the size of the system is commonly expected and observed. Obviously, the physical properties of the bulk cannot be applied to the thin film with the nanometer dimension in the thickness. Furthermore, the nature of this size dependence will further depend on the material in question. MD simulation is the main simulation method from the molecular point of view. Molecular dynamics has wide applicability such that it can simulate, in principle, at least, the phenomena at a molecular level. However, it is very time consuming due to the high time resolution smaller than at least a pico-second. In this study, instead of MD simulation, alternative approach based on the fact that in condensed systems, atoms or molecules always oscillate around the minimum-energy positions was used. The computation in this way is quasi-static, thereby greatly reducing the computing time.
This study investigated the effects of vacancies on the mechanical behaviors of nanoscopic thin film of copper. The interactions of atoms are dealt with Morse’s potential function. The results show that the pre-stress in microscopic world shouldn’t be ignored because the stress is not zero while the strain is zero. The results also show that the Young’s modulus of nanoscopic thin film decreases with increasing vacancies. Young’s modulus decreases while the thickness of thin film is decreasing. The analysis of the distribution of stress-strain shows that the stress near the free surfaces decreases while the thickness of thin film decrease. This is an indication that the mechanical properties of a nanoscopic material depend on the size.
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