Biomedical Engineering Reference
In-Depth Information
conductivity yields:
2 D
s ¼
Cz i F
ðÞ
=
RT
½
15
:
15
where C is a carrier concentration in solids, which is related to the density of
mobile ionic species. Typical ion concentration and diffusivity in a solid are
10 2 -10 3 mol m 3 and 10 8 m 2 s 1 , respectively. Inserting these values in
equation 15.15 provides ionic conductivity values in the range of 10 6 -10 0
Scm 1 . It should be noted that the ionic conductivity values are orders of
magnitude lower than the electronic conductivity of a typical metal. The
ionic charge transport is far more complex than the electronic charge
transport. For this reason, much more effort in recent years has been
devoted towards the development of ionic conductors.
15.2.5 Mixed ionic-electronic conductors
Beyond the traditional boundaries of electronic and ionic conductors, there
are solids known as 'mixed conductors', which can conduct both ions and
electrons. Both ionic and electronic species can move through mixed
conductors, leading to simultaneous transport of ions and electrons. This
attribute of mixed nanocomposites makes them very useful as electrodes -
materials critical for energy storage and power generation devices. These
mixed conductors can also catalyze electrochemical reactions.
15.3
Ionic nanocomposites
Ionic nanocomposites are of profound interest to electrochemical engineers
and scientists because of their potential application in devices such as
batteries, fuel cells, sensors, and displays. These state-of-the-art devices
employ liquid electrolytes, which are corrosive and hazardous and degrade
after brief exposure to high temperatures, thus limiting the life and
application of the devices. Ionic nanocomposites provide many advantages
over liquid electrolytes such as ease of containment, thermal and chemical
stability, and non-flammability. Nanocomposites are potentially useful for
electrochemical devices that are expected to be functional over a wide range
of temperatures.
A considerable number of papers on ionic conductivity and related
properties of nanocomposites have been published in last three decades. A
number of review papers document their developmental history and general
characteristics. Two major types of nanocomposites, polymer-ceramic and
ceramic-ceramic, will be discussed in this section.
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