Biomedical Engineering Reference
In-Depth Information
3 vol% Al
2
O
3
, an inflection around 27
C is noted. The inflection
transformed into a peak for 7 and 12 vol% nanocomposites. The
conductivities of the 12 vol% specimen at
8
C were reduced by
approximately two and five orders of magnitude, respectively.
The dielectric phase Al
2
O
3
and impurities in LATP such as AlPO
4
are the
underlying causes for the creation of the space charge. A significant
concentration of Al
2
O
3
interacts with lithium ions in the LATP. During the
interaction, lithium ions are adsorbed onto the Al
2
O
3
surface and become a
source of space charge. The space charge then affects the transport of the
remaining lithium ions. It is evident that the adsorption of ions and resulting
space charges occurs below 27
40 and 90
8
8
C, implying a very low energy of adsorption
(
C, a desorption of lithium ion occurs,
and the transport mechanism solely depends upon the movement of ions
through the channels of the Li
1+x
Ti
2
x
Al
x
(PO)
3
material. The LATP glass-
ceramic is a single lithium ion conductor (lithium transport number
<
5 zJ). At temperatures above 27
8
1) and
its primary transport mechanism involves conduction through the channels
of an aluminum titanium phosphate network. Thus, it was anticipated that
the addition of Al
2
O
3
would result in reduced conductivities across the
temperature range due to the blocking effect. From the experimental data
presented in Fig. 15.5(a), however, it may be inferred that both blocking and
space charge effects coexist in the temperature range of
~
C.
Because of a higher thermal energy, the space charge effect is destroyed
above 27
40 to 27
8
C by the desorption of lithium ions.
The nature of the adsorption and desorption processes of lithium onto the
Al
2
O
3
surface can be investigated by measuring conductivity during heating
and cooling cycles. Figure 15.5(b) shows the Arrhenius plots of LATP 3 and
7 vol% Al
2
O
3
specimens during heating and cooling cycles. During the
heating cycle, once the lithium ions have been desorbed from the Al
2
O
3
surface above 27
8
8
C, the space charge contribution to conductivity is
eliminated. Therefore, the specimens exhibit lower conductivity across the
entire temperature range during the cooling cycle. It should also be noted
that the lithium ion diffusion coefficient at these sub-ambient temperatures
is low but significant; therefore, a re-formation of the space charge is a
distinct possibility if the specimen is kept below 27
C, but it may take a long
time (days). In the case of polymer specimens, it has been shown that
complete recovery of the conductivities at lower temperature takes from tens
to hundreds of hours (Kumar and Scanlon, 1999). It should be appreciated
that the contribution of space charge to conductivity can be quantitatively
determined by analyzing the Arrhenius plot as well as measuring
conductivity during heating and cooling cycles.
It is now evident that the presence of a dielectric phase in an ionic
conducting matrix may lead to two antagonistic influences: (a) the blocking
effect and (b) the space charge effect. The blocking effect is characterized by
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