Chemistry Reference
In-Depth Information
7.4 Energy Harvesting via Fiber-based
Nanogenerators
7.4.1 PZT Nanofiber-based Nanogenerators
PZT is a good piezoelectric material; its crystalline structure is illustrated in
Figure 7.4(a). An electric polarization of PZT can shift the Zr/Ti atom up or
down and the atoms remain their positions after applying and removing an
external electric field for the piezoelectric property. In their bulk or thin film
formats, PZT can generate a higher voltage compared with other piezo-
electric materials for sensing,
70
and actuation
71
and energy harvesting ap-
plications. However, as a ceramic material, bulk PZT is more fragile
compared with organic PVDF while a recent study has shown that PZT
nanowires have very good mechanical strength.
15
On the other hand, PVDF
has superior piezoelectric properties compared with other types of polymeric
materials due to its polar crystalline structure. In nature, the PVDF polymer
consists of at least five different structural forms depending on the chain
conformation of trans (T) and gauche (G) linkages. Figure 7.4(b) shows the
crystalline structure of the a and b-phase, respectively. While the a-phase is
known as the most abundant form in nature, the b-phase is responsible for
most piezoelectric responses due to its polar structure with oriented
hydrogen and fluoride (CH
2
-CF
2
) unit cells along with the carbon backbone.
Typically, b-phase PVDF is acquired via electrical poling and mechanical
stretching processes during the manufacturing process to align the dipoles
in the crystalline PVDF structures as illustrated in Figure 7.4. However, the
poling process is typically hard to implement within an electrospinning
process due to the fibers' physical constraints.
d
n
3
r
4
n
g
|
2
.
Figure 7.4
(a) A schematic diagram showing the crystalline structures of PZT. An
electric polarization of PZT can shift the Zr/Ti atom up or down and the
atoms remain in their positions after applying and removing an external
electric field for the piezoelectric property. (b) Schematic diagrams show-
ing crystalline structures of PVDF: (top) non-polar a-phase, and (bottom)
polar b-phase. The dipoles in the non-polar, a-phase PVDF could be
stretched and oriented by an electrical field to become the polar, b-
phase structure under electrical poling and mechanical stretching.
Reprinted with permission from ref. 72. Copyright 2012, Elsevier.
Search WWH ::
Custom Search