THESIS
2012
xix, 133 p. : ill. ; 30 cm
Abstract
The stiffness of material has additional size-dependence when the solids are in nanoscale. It is
reported that the elastic modulus of solids appeared to be larger than its conventional value
when the size of solids is small. Although the strain gradient theory from continuum
mechanics was developed to explain this phenomenon, the stiffening mechanism of the
material is still unclear. In this thesis, we chose the molecular dynamics method to analyze the
nanoscale behavior of solids and determined the origin of the elastic stiffening of polymers
and nanowires from molecular simulation. A hypothesis was proposed to connect the current
continuum mechanics theory with the molecular behavior of polymers. The stiffening
mechanism of the solids with macromolecular molecular structure an...[
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The stiffness of material has additional size-dependence when the solids are in nanoscale. It is
reported that the elastic modulus of solids appeared to be larger than its conventional value
when the size of solids is small. Although the strain gradient theory from continuum
mechanics was developed to explain this phenomenon, the stiffening mechanism of the
material is still unclear. In this thesis, we chose the molecular dynamics method to analyze the
nanoscale behavior of solids and determined the origin of the elastic stiffening of polymers
and nanowires from molecular simulation. A hypothesis was proposed to connect the current
continuum mechanics theory with the molecular behavior of polymers. The stiffening
mechanism of the solids with macromolecular molecular structure and crystal molecular
structure was discussed.
To validate this stiffening mechanism, simulation of elastic behavior of polymers and
nanowires were carried out. Our hypothesis was supported by the simulation results. The
additional rotation of molecules generates additional deformation energy, which leads to
size-dependent stiffening. The conventional elastic modulus E
0 and the high order length scale parameter l
2 from our simulation results have been benchmarked with reported data. During
the investigation of polymers, we further discussed the influences of molecular structure on
their conventional and high order mechanical properties. The E
0 and l
2 were found to vary
linearly with entanglement density or the crosslink density of chains. The results also showed
that l
2 is linearly correlated with E
0. Once a reference E
0 and the correlation with l
2 are known,
l
2 and the size-dependence of a solid with different molecular structure can be predicted.
In the nanowire part, the significance of the directionality of the molecular bonding was
discussed during the stiffening mechanism investigation. The surface lattice distortion is
claimed to be account for the size-stiffening in tension. The bond rotation from strain gradient
should also be considered to explain the size-dependence in bending. It is also found that the
performance of piezoelectric ZnO nano-generator is able to be improved by the size-stiffening
of the ZnO nanowire. In addition, we further analyzed the effect of the dislocation in the
crystalline nanowire. The results show that rotations induced by the screw dislocations in
nanowires can cause the size-dependent deformation. The piezoelectric performance of ZnO
nano-generator can be benefit from screw dislocation, too. Conclusively, there are three
factors that can cause the size-stiffening of the crystalline nanowire: surface lattice distortion,
strain gradient and screw dislocation. When properly engineered, the mechanical and electric
properties of nanowire can be controlled based on the requirement of industry.
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