Piezoelectric Energy Harvesting Nanofibers - Hierarchical Nanostructures for Energy Devices - page 154

electrospinning to produce long PZT nanofibers for nanogenerator appli-
cations. In the device demonstrated by Chen et al., PDMS was used to cover
electrospun PZT fibers on top of comb-shaped platinum electrodes as shown
in Figure 7.5(c). 14 A post electric poling process was conducted at 140 1C for
24 hours at 4 V mm 1 between two adjacent platinum electrodes for en-
hanced piezoelectricity. When pressure was applied using a Teflon stack/
human finger, the device was able to generate an output voltage of up to
1420 mV. Zhang et al. have also demonstrated a PZT nanowire-based
nanogenerator using electrospun PZT nanofibers without using any post
poling process. 16 These PZT nanofibers were contacted on either end using
silver paste. A three-point bending test with an applied strain of 0.5% was
used to create an output voltage of 170 mV, which can be attributed to the
strain-induced charge of the PZT nanofibers.

d n 3 r 4 n g | 2

7.4.2 Randomly Distributed PVDF Fiber Nanogenerators
In addition to the aforementioned well-organized single fibers or fiber
arrays as nanogenerators, studies have shown that randomly distributed
fibers made by the far-field electrospinning process and other processes
can also be used for energy harvesting applications. The key demonstration
is that fibers made by conventional far-field electrospinning could also
form a b-phase crystalline structure without an additional poling process.
For example, Fang et al. have demonstrated a PVDF membrane type
nanogenerator with conventional electrospinning in which the PVDF
membrane was fabricated by a network of electrospun PVDF nanofibers. 21
Without an additional poling process, a 140 mm-thick PVDF membrane was
able to generate up to 7 V under compression strain at a high strain rate.
TheenergyoutputwassucienttolightupanLEDwiththehelpofa
capacitor type energy harvesting circuit. In a more fundamental study, Baji
et al. used XRD (X-ray diffraction) and FTIR (Fourier transform infrared)
measurements to show that a good portion of b-phase PVDF nanofibers did
exist in the fibers made by far-field electrospinning without a post poling
process. 65 The hysteresis loop under PFM (piezoresponse force microscopy)
further confirmed the ferroelectricity of nanofibers.

.

7.4.3 Orderly Patterned PVDF Fiber Nanogenerators
7.4.3.1 Single PVDF Fiber Nanogenerator
During the typical commercial piezoelectric PVDF thin-film production
process, a high electrical potential and mechanical stretching are applied at
a raised temperature for enhanced piezoelectricity. 79 PVDF nanofibers fab-
ricated by the conventional electrospinning process are under a high bias
voltage (410 kV) which could transform some non-polar a-phase structures
to polar b-phase structures for piezoelectricity. 46-49 The near-field electro-
spinning (NFES) process as shown in Figure 7.6(a) also possesses an