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介观力学1: 纳米固体变温力学-局域键平均近似

已有 2506 次阅读 2011-1-23 23:19 |个人分类:介观力学|系统分类:论文交流|关键词:力学| 力学

纳米变温力学专题研究报告(Prog Mater Sci 54, 179-307, 2009)
- 弹性模量和机械强度正比于键能密度(以帕为单位)
- 熔点与单原子结合能(单键能乘以单原子键的数目)成正比
- 由于原子配位数降低引起的键收缩和键能增强不仅使键能密度升高也使原子结合能降低
- 两者竞争决定材料在小尺度下的力学行为, 有如反常Hall-Patch行为和缺陷导致的材料增韧和增强
- 在小尺度常有弹性模量增强和熔点降低的反常情况出现
-上述讨论适用于单原子链,石墨片,纳米管,纳米线,纳米颗粒,纳米空洞,以及晶界,表面等的弹性与塑性力学行为

A local bond average (LBA) approach may complement existing approximations in terms of continuum medium and quantum computations. The LBA approach correlates the measurable quantities of a specimen to the identities of its representative bonds, and the energetic responses of these bonds (bond nature, order, length and strength) to external stimuli, such as changes in temperatures and coordination environments. It is shown that the shortened and strengthened bonds between under-coordinated atoms and the consequent local strain and quantum trapping dictate, intrinsically, the mechanical behavior of systems with a high proportion of such atoms. The thermally driven softening of a substance arises from bond expansion and lattice vibrations that weaken the bonds. The competition between the energy density gain and the residual atomic cohesive energy in the relaxed surface of skin depth determines, intrinsically, the mechanical performance of a mesoscopic specimen; the competition between the activation and inhibition of the motion of atomic dislocations dominates,extrinsically, the yield strength of the specimen during plastic deformation. Therefore, the mechanical behavior of a specimen depends on its shape, size, the nature of the bonds involved, surface and interface conditions, and the temperature at which the physical properties of the specimen is measured. Excellent agreement with existent measurements of temperature dependence of surface tension, size and temperature dependence of elasticity and extensibility, and the inverse Hall–Petch relationship in nanograins have been established. Furthermore, these agreements have led to quantitative information regarding the bond identities in monatomic chains and carbon nanotubes, as well as the factors dominating the sizes at which a grain is strongest. In addition, the interface electric repulsion between nanocontacts due to the skin trapping and the associated local charge densification may providefeasible mechanism for the superfluidity, superlubricity and superhydrophobicity as widely observed. The progress made insofar evidences the essentiality of the LBA approach from the perspective of bond formation, dissociation, relaxation and vibration and the associated energetics for the exposition of thermo-mechanical behavior of low-dimensional materials. Extending the application of the approach to junction interfaces, liquid surfaces, defects and impurities, chemically adsorbed systems, amorphous states, and substances under other applied stimuli such as pressure and electric field would contribute to better knowledge of such systems and could lead to the development of even more fascinating and profitable materials.



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