Biomedical Engineering Reference
In-Depth Information
conduction in heterogeneous solids. Many developments have occurred
since these pioneering works. What is perhaps a major development in the
last 40 years is the recognition and accumulation of experimental evidence
of the existence of electrically active interfaces. These interfaces also have a
major influence on the bulk electrical properties of solids.
The application of nanocomposites in energy storage and power
generation devices is in its infancy and is expected to grow as we gain a
more thorough understanding of their properties and usefulness. The
development of many alloys followed the need to manufacture high power
density internal combustion engines. Similarly, growth, interest, and
development in nanocomposites will follow the need for energy production,
transmission, and use in devices for energy harvesting and sensors.
15.2 Electrical properties
15.2.1 Fundamental relationships
The electrical properties of nanocomposites may be approached from two
different points of view. Engineers may consider them primarily as
components in energy storage batteries, power generating fuel cells, and
electrical circuits. Engineers are interested in specific properties and physical
characteristics so that the batteries, fuel cells, and circuit will perform at
high efficiency, last longer, and be cost efficient. Scientists view electrical
properties in terms of quantitative understanding of their electronic and
ionic character. A practical researcher must take a position somewhere in
the middle to reconcile the two points of view. A researcher should thus be
able to correlate and explain the effects of composition, structure, and
external environment on the bulk properties.
The application of an electrical field to a nanocomposite leads to a
generation of current that reaches an equilibrium direct current value either
rapidly or slowly. One can express the equilibrium in terms of number of
charged particles present, n, and their drift velocity, v. The current density, j,
is defined as the charge transported through a unit area in a unit time. The
equilibrium current density is expressed as:
￿ ￿ ￿ ￿ ￿ ￿
j
¼
nzev
½
15
:
1
where z is the valency and e is the electronic charge. The conductivity is
defined by:
s ¼
j
=
E
½
15
:
2
where E is the electric field strength. Combining equations 15.1 and 15.2
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