Biomedical Engineering Reference
In-Depth Information
a decline (sometimes precipitous) in the ionic conductivity of nanocompo-
sites with the gradual addition of the dielectric phase and temperature.
Under appropriate conditions, the space charge effect enhances conductiv-
ity. Collectively, these two factors determine whether or not a given dopant
will have a positive space charge effect, i.e. a conductivity enhancement.
Kumar et al. (2006) have shown that blocking and space charge effects
can be delineated and their effects quantified in different temperature
regions, especially in simple systems such as a single lithium ion conductor
doped with dielectric phases, such as the LATP-Al
2
O
3
system. A delineation
of the effects in more complex systems where several conducting charge
species contribute to the conductivity may be cumbersome.
Oxygen ion conducting nanocomposites
Oxygen ion conductors are of significant commercial interest as yttria
stabilized zirconia (YSZ) is the electrolyte of choice for solid oxide fuel cells
(SOFCs). The oxygen ion transport determines, to a large extent, the
operating temperature and usable power of a SOFC. The oxygen ion
transport in YSZ has been known and investigated for decades. In fact, the
high oxygen ion conductivity of YSZ electrolytes has been a prime
motivator for the development and commercialization of SOFCs based on
the YSZ electrolyte. Yet, the SOFC community is seeking even higher
oxygen ion conductivity, especially at lower temperatures (
C) for
alleviating chemical degradation and material compatibility issues. A
number of different approaches lead to enhanced conductivity of the YSZ
electrolyte. For example, a rare earth dopant such as scandium rather than
yttrium in ZrO
2
is known to enhance the bulk conductivity of stabilized
zirconia (Dixon et al., 1963). An alternative route to enhance the bulk
conductivity of YSZ is to employ a heterogeneous dopant. These
heterogeneous dopants are insoluble in the host YSZ and remain as a
physically distinct phase in the bulk structure.
Arrhenius plots of bulk conductivities (including grain and grain
boundaries) of YSZ and YSZ-Al
2
O
3
nanocomposites are shown in Fig.
15.6. The YSZ-Al
2
O
3
nanocomposite exhibits higher conductivity by
factors ranging from 3 to 7. Again, in this case the activation energy
(2.14 eV) for oxygen ion transport in the nanocomposite is greater than the
activation energy (1.67 eV) for oxygen ion transport in YSZ. The higher
activation in the YSZ-Al
2
O
3
system is attributed to the tortuous path that
oxygen vacancies must travel to participate in the conduction process.
Generally, higher ionic conductivity of a material is associated with lower
activation energy, but in the case of space charge mediated transport, the
data generally show noncompliance with conventional wisdom.
650
8
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