Piezoelectric Energy Harvesting Nanofibers - Hierarchical Nanostructures for Energy Devices - page 145

tens of kilovolts and the needle-to-collector distance is tens of centimetres.
The formation of thin fibers is based on the uniaxial stretching of the vis-
coelastic solution by the electrostatic force for materials with suitable
properties, such as conductivity, which can be influenced by salt solutions,
and surface tension, which can be changed by adding surfactants. One
key characteristic of far-field electrospinning is the random and chaotic
distribution of nanofibers on the collector electrode.

d n 3 r 4 n g | 2

7.2.2 Modified Far-field Electrospinning
For applications such as nanofiber nanogenerators, good fiber alignment
could be necessary to improve the energy generation eciencies. Research
groups have demonstrated methods for aligned depositions of nanofibers
with modified conventional electrospinning processes. For example, Boland
et al. used a rotating collector to control the alignment of deposited nano-
fibers. 29 In their experiments, poly(glycolic acid) and collagen were electro-
spun on a cylinder collector rotating at a speed of 1000 rpm and 4500 rpm,
respectively. The use of fast-spinning collectors in the form of drums, 37
wheel like disks, 38 and wire drums 39 have all shown various levels of success
in controlling the deposition positions of nanofibers. These methods could
be potential manufacturing approaches to make continuous aligned nano-
fibers for nanogenerator applications.
Instead of using mechanical means to improve the conventional electro-
spinning process for better controlled deposition positions, researchers have
also exploited electric fields to guide the deposition locations of nanofibers,
including the usage of two parallel electrodes on the collector 40 and the
designs of one or several charged rings as auxiliary electrodes. 41 In the
process, nanofibers were attracted by the two collector electrodes and were
deposited to the left and right-side electrodes back and forth repeatedly to
give good alignment.

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7.2.3 Near-field Electrospinning
Another method to control deposition of nanofibers is the so-called near-
field electrospinning (NFES), 26 where the needle-to-collector distance is
reduced to enhance the controllability of the fiber deposition positions as
illustrated in Figure 7.1(a). The needle-to-collector distance is reduced to
mm range and the applied voltage is reduced to the order of 1 kV. The
reduction in distance and the increased electric field (as a result of a much
shorter distance) make it possible to control nanofiber deposition on the
collector by utilizing the stable liquid jet region. In the earlier stage of the
near-field electrospinning process, a dip-pen type approach was adopted as
shown in Figure 7.1(a); the experimental image photo is shown in
Figure 7.1(b). In this case, repeated dipping into the polymer solution was
necessary to obtain more polymer sources, which interrupted the deposition
process. Continuous near-field electrospinning was later developed 42 by