Fundamentals of Hierarchical Nanostructures - Hierarchical Nanostructures for Energy Devices - page 14

2.2.4 Mechanical Properties
Nanomaterials are known to have size-dependent, superior mechanical
properties over their bulk counterparts. The strength enhancement of the
nanomaterials is due to their being near single crystalline with reduced
internal and surface imperfections (impurities, structural defects and dis-
locations). The smaller the cross-section of the nanosystems, the less likely it
is to find internal imperfections such as dislocation, micro-twins, impurity
precipitates etc. 50 Thermodynamically, imperfections in crystals are highly
energetic and should be eliminated from the perfect crystal structure. Such
imperfections are easier to eliminate in smaller systems. Additionally, some
imperfections in bulk materials, such as dislocations, are often created to
accommodate stresses generated in the synthesis and processing of bulk
materials due to temperature gradient and other inhomogeneities.
Such stresses are unlikely to exist in small structures, particularly in
nanomaterials. 20 Additionally, smaller systems have fewer surface defects.
This is usually true when the materials are made through a bottom-up
approach due to less growth fluctuation. To predict the size-dependent
mechanical strength on a nanometre scale, the conventional Hall-Petch
model is not valid anymore. 51 Currently, various combined experimental-
computational approaches have been studied with in situ transmission
electron microscopy (TEM), atomic force microscopy (AFM) characterization
and molecular dynamic (MD) simulations. Figure 2.3 shows the experi-
mental stress-strain response of penta-twinned silver nanowires with
varying diameters. 52 At low strain, the nanowires are found to deform
elastically with the elastic modulus increasing with decreasing diameter.
The elastic modulus was computed from the slope of the stress-strain curves
for the initial loading regime of a strain below 2%. Nanowires with a smaller
diameter were found to exhibit moduli of up to 1.5 times the bulk value. 52
The initial yield strain, corresponding to the first plateau in the stress-strain
curve, shows a weak dependence on nanowire diameter from the nucleation
theory. 53 However, the deformation behavior of the nanowires after the
initial yielding is significantly affected by the nanowire diameter. Thinner
nanowires show a more pronounced strain hardening effect. 52
The penta-twinned nanowires' superelastic behavior with high yield
strength is attributed to the role of twin boundary confinement. 54,55 The
unique strain hardening and multiple plastic zone formation in small
diameter nanowires (D o 100 nm) is explained by dislocation nucleation
from a local stress concentrator, which leads to the formation of a linear
chain of stacking fault decahedrons (SFDs). By confining dislocation activity
to SFD chain propagation, the internal twin boundaries cause local
hardening of thin nanowires, resulting in defect insensitive structures with
significantly enhanced flow stress, ductility and strength. As nanowire
diameter becomes larger, the number of local plastic zones decreases due to
the earlier onset of necking. 52

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The superior mechanical properties of