Introduction to Nanotechnology

6.1. SOLID DISORDERED NANOSTRUCTURES 147
density of states is assumed constant over an energy-range eV (electron volt), then
for small V and low 7: we obtain
which can be rewritten in the form
where
I = f=, (E,)N,(E,)eV (6.5)
I = G,,V (6.6)
and G,, is identified as the conductance. The junction, in effect, behaves in an ohmic
manner, that is, with the current proportional to the voltage.
6.1.6. Other Properties
While the emphasis of the previous discussion has been on the effect of nanosized
microstructure on mechanical and electrical properties, many other properties of
bulk nanostructured materials are also affected. For example, the magnetic behavior
of bulk ferromagnetic material made of nanosized grains is quite different from the
same material made with conventional grain sizes. Because of its technological
importance relating to the possibility of enhancing magnetic information storage
capability, this is discussed in more detail in Chapter 7.
In Chapter 4 we saw that the inherent reactivity of nanoparticles depends on the
number of atoms in the cluster. It might be expected that such behavior would also
be manifested in bulk materials made of nanostructured grains, providing a possible
way to protect against corrosion and the detrimental effects of oxidation, such as the
formation of the black silver oxide coating on silver. Indeed, there have been
some advances in this area. The nanostructured alloy Fe73B13Si9 has been found to
have enhanced resistance to oxidation at temperatures between 200 and 400°C. The
material consists of a mixture 30-nm particles of Fe(Si) and Fe2B. The enhanced
resistance is attributed to the large number of interface boundaries, and the fact that
atom diffusion occurs faster in nanostructured materials at high temperatures. In this
material the Si atoms in the FeSi phase segregate to interface boundaries where they
can then diffuse to the surface of the sample. At the surface the Si interacts with the
oxygen in the air to form a protective layer of SO2, which hinders further oxidation.
The melting temperature of nanostructured materials is also affected by grain
size. It has been shown that indium containing 4-nm nanoparticles has its melting
temperature lowered by 1 10 K.
In the superconducting phase there is a maximum current that a material can
carry called the critical current IC. When the current I exceeds that value, the
superconducting state is removed, and the material returns to its normal resistance. It