Chemistry Reference
In-Depth Information
nanofibers and PDMS was used to encapsulate the whole device. The lift-off
process was performed to remove the photoresist (PR). The device was de-
signed to amplify the current output by using the parallel connection to
collect possible current generations from each individual nanofiber. A single
energy harvester was composed of a total of 500 sections formed by 50
parallel fibers and 10 pairs of electrodes and the monitored peak current was
about 30 nA. Key accomplishments in this work include: capability to place
aligned nanofibers in an orderly manner, possibility to achieve higher
electrical outputs in scavenging energy by using nanofibers connected in
parallel, and feasibility of electric poling after the nanofiber deposition
process using the comb-shaped electrodes.
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7.4.3.3 PVDF Fiber Nanogenerators by the Hollow Cylindrical
Near-field Electrospinning Process
A hollow cylindrical near-field electrospinning (HCNFES) process has been
proposed to address production and performance issues encountered pre-
viously in either far-field electrospinning (FFES) or near-field electrospin-
ning (NFES) processes.
80,81
By the introduction of a rotating glass tube
collector, PVDF fibers with small diameters, smooth surface morphology,
high density, and good piezoelectricity have been accomplished using the
HCNFES process as a potential building block to construct energy har-
vesters. In the HCNFES process, a rotating glass tube collector (diameter:
20 mm; thickness: 1 mm; length: 200 mm) is used to collect the electrospun
fibers. A copper foil is placed in the internal wall of the glass tube and an
electrical brush is attached as a grounding electrode. The glass tube collector
significantly reduces the occurrence of short circuit. By employing a DC
motor to turn the tube collector, and controlling the uniaxial movement, this
method is able to rapidly and continuously collect electrospun PVDF fibers.
In the prototype experiments, a high voltage of 10-16 kV is used and the tip-
to-tube distance is about 0.5 mm. The tube collector rotates at velocities of
900-1900 rpm and the corresponding tangential speed on the surface is
942.3-1989.3 mm s
1
. The X-Y control platform has a motion speed of
2mms
1
and a travel distance of 50 mm. The fiber arrays fabricated by
HCNFES can have controllable structural thickness based on layer-by-layer
assembly as show in Figure 7.8(a) and (b). After a long period of collection
time, there could be slight randomness as illustrated in Figure 7.8(b). The
main reason could be the presence of residual charges on the electrospun
fibers. One can easily remove the tube collector and extract the nonwoven
fiber fabric (NFF). Furthermore, it is observed that under higher mechanical
stretching speed, ultra-thin PVDF fibers (less than 1 mm) can be constructed
and the extra mechanical stretching seems to help alleviate the surface
defects such as holes and voids. When the flexible textile-fiber-based PVDF
harvester is subjected to different stretch-release cycling frequencies of
2-7 Hz with 0.05% strain, electricity can be generated and recorded.
.
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