Biomedical Engineering Reference
In-Depth Information
15.3.1 Polymer-ceramic nanocomposites
A polymer-ceramic nanocomposite is defined as an ionically conducting
solid material derived from a polymer and a ceramic phase. This type of
material is a subset of solid electrolytes. The discussion here will be
primarily confined to lithium ion conductors, although other types of
nanocomposite conductors (silver and sodium) have also been reported.
A broader range of polymer and ceramic chemistries could be combined
to synthesize polymer-ceramic nanocomposites. Analyses of the published
literature suggest that the incorporation of a ceramic phase into an ionically
conducting matrix leads to enhanced conductivity, cationic transport
number, and electrode-electrolyte interfacial stability. The ionic conductiv-
ity of the nanocomposite is dependent upon several variables such as the
characteristics (chemistry, size, and volume fraction) of the ceramic phase,
annealing parameters, physical and chemical properties of the polymer
matrix, degree of reactivity between polymer and ceramic phases, and
temperature.
A number of processing techniques (e.g. solvent casting, melt casting, and
hot pressing) may be employed to obtain film and bulk nanocomposite
specimens. The most convenient is the blend and hot press technique in
which the polymer, salt (of lithium), and ceramic components are mixed in a
predetermined proportion, ground in a mortar and pestle or energy milled,
hot pressed into pellet form, and then rolled into films of desired thickness.
Nanocomposite materials covering a wide range of concentration of ceramic
phases in a polymer matrix can be obtained by this technique.
The temperature dependence of conductivity of the PEO:LiBF
4
complex
and PEO:LiBF
4
(8:1)-MgO(10 wt%) nanomaterials containing micro- and
nanosize MgO is shown in Fig. 15.1 (Kumar et al., 2001). The lowest
conductivity values are associated with the PEO:LiBF
4
(8:1) complex. Near
the melting point of PEO, 68
C, a precipitous drop in conductivity begins,
and at room temperature the conductivity drops to 10
9
Scm
1
. The
specimen containing microsize (
8
5 micron) MgO exhibits much improved
conductivity as compared to the PEO:LiBF
4
complex. At around ambient
temperature, the conductivity is improved by approximately three orders of
magnitude by the incorporation of microsize MgO in the polymer complex.
The highest conductivity values are associated with the specimen containing
nanosize MgO. The conductivity of this specimen is about four orders of
magnitude higher than the PEO:LiBF
4
complex around the ambient
temperature. Furthermore, the temperature dependence of conductivity
diminishes as the MgO particles are reduced from micro- to nanosize. At
100
~
C, all three specimens possess similar conductivity values, whereas the
curves diverge as the temperature is lowered to 40
8
C.
The temperature dependence of conductivity of the PEO:LiBF
4
complex
8
