Electrospun Piezoelectric Materials

Piezoelectric materials are materials that generate electrical current when a mechanical stress is applied to it. Electrospinning is a highly versatile fiber fabrication technology that has been used to produce a wide variety of polymer and inorganic nanofibers. Researchers have successfully constructed both polymeric and inorganic piezoelectric devices by electrospinning. By having piezoelectric materials in the form of fibers, its properties may be tailored or enhanced by controlling the organization of the fibers. Electrospinning has been shown to increase the piezoelectricity of certain materials especially polymers. Further, electrospinning is sufficiently versatile to enable doping and mixing of materials to enhance its piezoelectric property.

Electrospinning requires the feed material to be in the form of solution or melt. For electrospinning of inorganic piezoelectric material, electrospinning is usually carried out using its precursor material in solution form followed by annealing process. Chen et al (2010) constructed vanadium-doped ZnO piezoelectric nanofiber by electrospinning. Precursors in the form of zinc acetate and vanadyl acetylacetonate was used in combination with poly(vinylpyrrolidone) (PVP) as the base solution for electrospinning. After electrospinning, the resultant nanofibers were annealed to remove the PVP and to reduce precursors to polycrystalline V-ZnO piezoelectric nanofibers. The average d₃₃ value of Zn_0.975V_0.025O nanofiber was found to be 121 pm V^-1.

While inorganic piezoelectric materials are known to exhibit better piezoelectric properties, there is an interest in constructing polymeric piezoelectric materials due to their better flexibility. Polymer such as poly( l-lactic acid) (PLLA) has been shown to exhibit piezoelectric properties after electrospinning to form fibers due to electric dipole component along the main carbon chain of PLLA polymer nanofibers that can be polarized along the direction of alignment during electrospinning process. Zhu et al (2017) was able to construct aligned electrospun PLLA nanofibrous membrane with open-circuit voltage and short-circuit current reaching 0.55 V and 230 pA respectively. Mandal et al (2011) have provided evidence of preferential orientation of CF₂ dipoles in poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE) nanofiber by electrospinning. With poly(vinylidene fluoride) (PVDF), electrospinning was able to produce fibers with greater β-phase in PVDF, which is responsible for its piezoelectric property, compared to cast film. Stretching of PVDF molecules during fiber formation may have encouraged formation of β-phase crystals. However, there are conflicting results on the influence of voltage on its polymorphism. Cozza et al (2013) reported that varying electrospinning parameters have no effect on the α/β crystal ratio. However, Sengupta et al (2017) showed with a single strand electrospun PVDF fiber that increasing voltage does lead to a general increase in its piezoelectric d₃₃ coefficient up to a value of -58.77 pm/V. He attributed this to the electric field encouraging dipole alignment of its molecules. Lei et al (2015) did a comparative study on the piezoelectricity of β-PVDF fibers by electrospinning and forcespinning, a mechanical spinning process without electric field. Their result revealed that force spun fibers showed no piezoelectricity which provide evidence that electrospinning process does have a poling effect and therefore induces preferred dipole orientation in electrospun PVDF fibers. Their study also showed increased β-phase with higher electric field strength.

Piezoelectric properties may be enhanced by encouraging crystallization of specific phases in the material. Ahn et al (2013) showed that the addition of multiwalled carbon nanotube (MWCNT) into PVDF solution increases the percentage of β-phase fraction in the resultant electrospun fibers. This has been attributed to interfacial interaction between the functional groups on the MWCNTs and the CF₂ dipole of PVDF chains.

Further increase in the electrical output from piezoelectric polymers may lies in fabricating composite fibers containing particles with greater piezoelectric characteristics. Yun et al (2016) loaded polyvinylidene fluoride (PVDF) with PZT (lead zirconate titanate, Pb(Zr_xTi _(1-x) )O₃) nanoparticles. The resultant electrospun PVDF/PZT membrane was found to be flexible and the optimum loading of PZT is 20 wt% for highest value of Pmax (maximum polarization) at 2.64 µC/cm², 4 kV/mm. This is an increase of 27% of Pmax over pure PVDF nanofiber. Pan et al (2014, 2015a) had tested the power output from aligned nanofibrous membranes made from different piezoelectric materials. Through their test on single nanofibers from near field electrospinning, they found that the maximum power from poly(γ-benzyl α, l-glutamate) (PMLG) was 138 pW while polyvinylidene fluoride (PVDF) fiber was able to generate 266 pW. Interestingly, they found that having a fiber composite made from a blend of PVDF and PMLG was able to generate higher power output of 637.81 pW and maximum peak voltage of 0.08V. The improved performance has been attributed to better dipole orientation and higher dipole density [Pan et al 2015b]. Ji and Yun (2018) used a blend of BNT-ST (0.78Bi_0.5Na_0.5TiO₃-0.22SrTiO₃) ceramic particles and poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE) copolymers for electrospinning into nanofibers. Using a rotating collector, they found that the perovskite crystal peak intensity of BNT-ST was significantly increased. Calculating the maximum strain (S_max) and the longitudinal piezoelectric coefficient (d₃₃) of BNT-ST rich and BNT-ST poor region along the fiber showed that BNT-ST ceramic phase had greater piezoelectricity than the P(VDF-TrFE) polymer phase as the rotation speed of the collector and corresponding fiber alignment increased. Liu et al (2021) blended ZnO nanoparticles into poly(vinylidenefluoride-co-trifluoroethylene) (PVDF-TrFE) solution and electrospun into nanofibers. ZnO is a dielectric material and it facilitates the nucleation of β-phase crystals in PVDF-TrFE. Annealing is also carried out on the electrospun fibers to increase the crystal size. With the addition of ZnO and annealing, Liu et al (2021) was able to achieve an optimal peak-to-peak voltage response of 1.788 V which was a 75% increase compared to that of the pristine PVDF-TrFE sensor. This electrospun PVDF-TrFE/ZnO nanofiber membrane pressure sensor is sensitive enough to pick up a human pulse with a frequency of around 1 Hz.

Having nanoparticles or other additives blended into nanofibers is not the only way to incorporate the additives. There is also a limit to the amount of additives that can be loaded into the solution to be electrospun before the electrospinning process is effected. Mirjalali et al (2023) used electrospinning and electrospraying to construct a multilayered mat with enhanced piezoelectricity. The material for electrospinning was Polyvinylidene Fluoride (PVDF) while zinc oxide nanoparticles were electrosprayed. Pristine electrospun PVDF nanofiber mat showed an output voltage of 0.24 V. A 5-layer PVDF+ZnO 50% showed a voltage of 0.91 V. This was greater than 50% ZnO blended into PVDF nanofibers which had an output voltage of 0.49 V. Further increase in ZnO concentration blended into PVDF nanofibers resulted in a decrease in the voltage. With the multilayered construct, further increase in the amount of ZnO particles is possible and a maximum voltage of 3V was recorded with 150% ZnO. The increase in voltage likely came from greater contribution of ZnO to the piezoelectric property of the mat. However, further increase in ZnO led to a decrease in the voltage which may be due to the greater thickness hindering harvesting of the excited charges.

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