Piezoelectric Energy Harvesting Nanofibers - Hierarchical Nanostructures for Energy Devices - page 165

pads. Under a small actuation as shown on the left side of Figure 7.12(a),
C overlap could be insignificant while under a large actuation as shown on
the right side of Figure 7.12(a), C overlap could dominate the total responses.
By switching the actuation direction upwards or downwards, the output
responses due to the electrode gap will have opposite effects as C gap between
two electrodes decreases or increases, respectively. On the other hand,
C overlap should always increase due to the increased overlapping area
under either the upward or downward actuation. Figure 7.12(b) shows
experimental results based on upward actuation and releasing to the original
flat position under small actuation (left) and large actuation (right), re-
spectively. Figure 7.12(c) shows results from the same tests based on
downward actuation and releasing to the original flat position. The speci-
men used in these tests has an electrode area of 1000 mm 1000 mm and a
cross-sectional area in a gap of 100 nm 1000 mm without depositing any
nanofiber. The output signals from small actuation (left side of figures(b)
and (c)) are smaller than 1 mV, which can be considered as noise when
compared with the typical voltage outputs of a single nanofiber at tens of
mV 22 while the clear change in the polarities of the output signals suggests
the outputs come from the changes in C gap . On the other hand, under large
actuation, the contribution of C gap is small and the changes of C overlap
dominate the output signals while either upward or downward actuation
should increase C overlap . As expected, on the right side of Figure 7.12(b) and
(c), larger outputs up to 10 mV were observed while the polarity changes are
the same in both figures under either upward or downward actuation.
Nevertheless, these effects could be easily filtered out if the 'switching
polarity criterion' is checked.

d n 3 r 4 n g | 2

.

7.5.2 Energy Conversion Eciency
The energy conversion eciency of nanofiber nanogenerators is an im-
portant measurement for energy scavenging applications. It can be calcu-
lated by comparing the output electrical energy with the input mechanical
energy from the applied strain. The electric energy W e , can be estimated by
integrating the product of output voltage and current of the nanogenerator.
The elastic strain energy can be estimated by using (in the case of a single
nanofiber):

W s ¼ 1

2 EAe 2 L 0

(7 : 2)

where E is Young's modulus of the material, A is the cross-sectional area, e is
the strain applied on the material, and L 0 is the length of the material.
Several reports have found that nanostructures could have high energy
generation eciency in nanogenerator applications, including PVDF nano-
fibers, 22 ZnO nanowires 1 and PZT nanofibers (the piezoelectric constant
for nanofibers was 0.079 mV N 1 as compared with PZT bulk material at