Piezoelectric Energy Harvesting Nanofibers - Hierarchical Nanostructures for Energy Devices - page 167

7.5.3 Enhancing Strong Piezoelectricity in Nanofibers
As the most important measure of all piezoelectric materials, piezoelectricity
in nanofibers should be investigated thoroughly as it relates strongly to their
performance, including the energy conversion eciency. However, various
processing parameters and conditions easily affect the piezoelectricity not
only in nanofibers but also in large scale thin films. For example, many of
the commercially available PZT or PVDF thin films/products have been
processed with unknown ingredients and/or process parameters. Although
there are a wide variety of studies in the literature on the piezoelectric
properties of laboratory-made piezoelectric nanofibers, many of them lack
details in processing data. Furthermore, most of the published reports only
characterize piezoelectricity of nanofibers without specific characterizations
on the specific piezoelectric mode information. Differences in material
compositions and possible experimental variations/inaccuracies could be
the fundamental reason that there are a wide range of nanogenerator results
on nanofiber nanogenerators in Table 7.1 as well as piezoelectric properties
as listed in Table 7.2. For example, molecular weight and concentration,
solvent types and mixing percentage, electrospinning methodologies, ap-
plied bias and electrode-to-collector distance could all affect the piezo-
electric properties of electrospun nanofibers and there is a wide range of
these parameters used by different groups as shown in Table 7.1. In the work
of Yee et al., 92 it is concluded that electrospinning of PVDF from DMF-
acetone solution would promote the formation of a b-phase and aligned
electrospinning using a rotation disk would result in the c-axis of the b-phase
crystallites in orientation along the fiber axis. They also suggest that the
formation of the b-phase is likely to be by the electric field instead of the
mechanical and shear force. In another study using a modified rotating disk
collector, they further suggest that effective stretching by the rotation disk
could help the formation of a highly oriented b-phase, which is quite dif-
ferent from their previous report. 93 A study by Zhong et al. on PVDF/poly-
acrylonitrile and PVDF/polysulfone nanofibers also has a different
conclusion. 94 They found that mechanical stretching is more effective than
electric poling to induce a ferroelectric phase. Clearly, these recent reports
indicate a lack of uniform and optimal processing parameters and lack of
fundamental understanding of the piezoelectric effects of nanofibers.
In several different approaches, researchers have worked on enhancing
the piezoelectric effects of nanofibers by adjusting solvents or adding other
nanomaterials. For example, Andrew and Clarke have shown that b-phase
PVDF fibers can be electrospun directly from dimethyl formamide (DMF)
solution with a maximum b-phase fraction of 0.75 51 and the addition of
well-dispersed ferrite (Ni 0.5 Zn 0.5 Fe 2 O 4 ) nanoparticles can form nanofibers
with an overall crystalline fraction made up solely of ferroelectric b- and
g-phases. 95 Huang et al. have found that adding a low concentration of
SWCNTs in the electrospinning process can induce highly oriented b-form
crystallites. 52 Studies by Costa et al. concluded that a low evaporation rate is

d n 3 r 4 n g | 2

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