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increased band gap. The color of metallic nanoparticles may change
with their size due to surface plasmon resonance.
(4) Electrical conductivity decreases with a reduced dimension due to
increased surface scattering. However, electrical conductivity of
nanomaterials could also be enhanced appreciably due to the better
ordering in microstructure.
(5) The magnetic properties of nanostructured materials are distinctly
different from those of bulk materials. The ferromagnetism of bulk
materials disappears and transfers to superparamagnetism on the
nanometre scale due to the huge surface energy.
(6) Self-purification is an intrinsic thermodynamic property of nano-
structures and nanomaterials. Any heat treatment increases the
diffusion of impurities, intrinsic structural defects and dislocations,
and one can easily push them to the surface nearby. Increased
perfection would have an appreciable impact on the chemical and
physical properties.
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Many such properties are size dependent. In other words, the properties of
nanostructured materials can be tuned considerably simply by adjusting the
size, shape or extent of agglomeration.
2.2.1 Thermodynamic Properties and Thermal Stability
Due to their large surface area, all nanomaterials possess a huge surface
energy and thus are thermodynamically unstable or metastable. Nanoma-
terials are found to have lower melting temperatures compared with their
bulk counterparts when the systems' sizes decrease below a certain critical
size. This melting point drop was found a long time ago by various
researchers. Buffat and Borel
21
presented results that are related to an
essentially thermodynamic size effect, i.e. the reduction of the melting point
of a small gold aggregate as a function of decreasing particle size as shown in
Figure 2.1. The melting point drop is generally explained by the increasing
surface energy relationship with decreasing system size. The decrease in the
phase transition temperature can be attributed to a change in the ratio of
surface energy to volume energy as a function of particle size. It is not always
clear to determine or define the melting temperature of nanomaterials. For
example, the vapor pressure of a small particle is significantly higher than
that of its bulk counterpart, and the surface properties of nanomaterials are
very different from those of the bulk materials. Evaporation from the surface
would result in an effective reduction of nanomaterial size and thus would
affect the melting temperature. For some materials, increased surface
reactivity due to a large surface to volume ratio may promote the oxidation of
the surface layer and thus change the chemical composition on the nano-
material's surface through the surface chemical reaction,
.
leading to
a change of melting temperature. Buffet and Borel
21
proposed clever
experimental
criterion to determine the size-dependent melting of
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