Biomedical Engineering Reference
In-Depth Information
properties of nanocomposites for energy storage and power generation
applications.
Matter in its solid state form can be further classified as metal,
semiconductor, or dielectric. This classification is based on electrical
properties. A metal can have very high electrical conductivity - as high as
10
8
Scm
1
, whereas the conductivities of semiconductors and dielectrics are
orders of magnitude lower. Typically, a semiconductor has a conductivity
around 10 S cm
1
, whereas dielectrics have conductivity values around
10
12
Scm
1
. Man-made nanocomposites are basically a mixture of metals,
semiconductors, and dielectrics. When these types of solids (phases) are
mixed in a nanocomposite and subsequently processed (annealed, sintered,
drawn, extruded, etc.), they tend to exhibit interesting electrical properties at
interfaces and also in the bulk. Two examples of different
types of
nanocomposites are now considered.
When a metallic powder is introduced in a dielectric or ionic conducting
matrix, for example, Ni in yttria stabilized zirconia (YSZ), there is a little
change in the electrical conductivity of the nanocomposite at lower
concentrations of Ni. However, when the concentration of Ni approaches
a threshold (of electron percolation), there is a rapid increase (orders of
magnitude) in the conductivity. Physically, the ionic conducting phase
(YSZ) blocks electron transport at lower Ni concentrations. At higher
concentrations, the interparticle spacing between the Ni particles decreases
to the extent that electrons can percolate from one Ni particle to another,
leading to a conductivity enhancement by orders of magnitude. Further
increase in the concentration of the nickel phase increases conductivity, but
at a much lower rate. This type of nanocomposite is used as an anode in
solid oxide fuel cells (SOFCs).
The addition of a dielectric phase Al
2
O
3
in an ionically conducting matrix
(LiI) leads to conductivity enhancements by orders of magnitude. In this
case, the percolation of ions is stimulated by the introduction of the
dielectric phase, Al
2
O
3
. These types of nanocomposites are potentially
important for lithium batteries and electrochemical sensors.
The aforementioned examples of nanocomposites make use of the
transport of electrons and ions, which are the basic constituents of solids.
These electrons and ions of nanocomposites can also interact with molecular
species to catalyze certain chemical reactions, just as some metals (generally
precious metals) do. The electrical properties of nanocomposites are an
intriguing and important field of research. Regrettably, our basic under-
standing of these materials remains tentative. It is hoped that enough
interest will develop in the future to advance the science and technology of
electrically active nanocomposites.
The electrical properties of composites were analyzed by Maxwell (1881)
and Rayleigh (1892) - the first attempts to quantitatively explain electrical
