Lingke Yu

Spectroscopic evidence for a high fraction of ferroelectric phase induced in electrospun polyvinylidene fluoride fibers

Effective transformation from paraelectric to a high fraction of ferroelectric phase is crucial to produce piezoelectric materials with a high piezoelectric constant for broad applications. In polyvinylidene fluoride (PVDF) thin films, both mechanical stretching and electric poling processes have been found to be critical in the α → β phase transformation. However, in PVDF fibers fabricated by the electrospinning process, the roles of mechanical stretching and electric poling have not been well explored. Here, the properties of PVDF fibers from electrospinning and forcespinning, a mechanical spinning process without electric poling, have been characterized and analyzed by FTIR and XRD spectroscopic techniques. The results show that pure mechanical stretching in the forcespun fibers can result in a high fraction of the all-trans β-phase, at 95%. Electrospun fibers from the same material system, on the other hand, can also reach a high fraction of β-phase, at approximately 99%. These results preliminarily demonstrate that mechanical stretching is the main reason for β-phase induction in PVDF fibers. Further experiments performed in this work show that higher wt% of PVDF, lower polymer solution supply rate, and more uniformly mixed solvent systems facilitate achieving a higher level of ferroelectric β-phase in electrospun PVDF fibers.

Electrospinning-induced preferred dipole orientation in PVDF fibers

Polyvinylidene fluoride (PVDF) can be made electroactive by properly mechanical stretching and electric poling treatments of its film, which may be easily realized by single-step electrospinning. This technique is acknowledged as an effective approach to induce rich ferroelectric β-phase in electrospun PVDF fibers; however, the investigation of dipole arrangement during the electrospinning process is still lacking. Here, the piezoelectricity of β-PVDF fibers by electrospinning and forcespinning, a mechanical spinning process without static electric field bias, has been demonstrated. Results show that the electrospun fibers can generate piezoelectric voltage after deformation, while the forcespun fibers nearly show no piezoelectricity for the same condition, revealing that electric field during the electrospinning process can perform in situ poling effect and therefore induces preferred dipole orientation in electrospun PVDF fibers. Further experiments performed in this work show that piezoelectricity of the electrospun fibers increases with increasing fraction of β-phase and/or the applied electric field strength of electrospinning, which provides good guideline for preparing high-performance piezoelectric fibers.

The fast fabrication of flexible electronic devices of graphene composites.

The rapid production and accurate deposition of graphene composites are first integrated into one process, due to the strong interaction between the polymer bond with sodium dodecyl sulfonate (SDS) and graphene. It is demonstrated that tension-shear exfoliation in high viscosity fluid may get a higher graphene production rate than in N-methyl-pyrrolidone. In addition, the micro-scale patterns of graphene nanomaterials produced by this method show high electrical conductivity and superior sensitivity to pressure due to their porous structure.

Predicting Polymorphism of Electrospun Polyvinylidene Fluoride Membranes by Their Morphologies

Although electrospinning of polyvinylidene fluoride (PVDF) has been studied for more than 10 years, the crystalline phase differentiation of the electrospun mats is still normally through the combination of different characterization techniques, and the relationship between polymorphism and morphology of the fibers in electrospun PVDF membranes has never been reported. Here, we show their close relationships by conducting room-temperature electrospinning experiments on various polymer/solvent systems. The electrospun membranes full of bead-free fibers have a very high fraction of β-phase, F(β), over 90%, and high orientation, whereas the membranes comprising beads and/or a large number of beaded fibers most often result in a low fraction of β-phase (F(β) normally below 50%) and low orientation. On the other hand, electrospun membranes consisting of both bead-free fibers and a very limited number of beaded fibers showed a medium high fraction of β-phase, F(β) more than 70% but less than 90%. These findings suggest the feasibility of intuitively predicting the crystalline phase of electrospun PVDF membranes directly by their morphologies, which is obviously simple, inexpensive and convenient for future investigations.

A novel sacrificial-layer process based on anodic bonding and its application in an accelerometer

It is found in our experiments that the depletion layer of anodic bonding is etched faster than the bulk glass (Pyrex 7740) in hydrofluoric acid (HF). Based on this interesting phenomenon, a novel process of a sacrificial layer is proposed in this paper. In order to deeply understand and investigate the rules concerning the influence of bonding parameters on this effect, firstly the width of the depletion layer under different bonding voltages and temperatures and the selection ratio of etching are revealed. To validate the feasibility of the method, a micro-machined accelerometer is designed and fabricated. The test results of resonant frequency and sensitivity of the fabricated accelerometer are 3254.5 Hz and 829.85–844.93 mV/g, respectively. This was further evidence that the depletion layer could be used as a sacrificial layer and the removable structure could be successfully released by fast etching this layer. The important feature of this method is that only one mask is needed in the whole process and therefore it could greatly simplify the fabrication process of the device.

Piezoelectric performance of aligned PVDF nanofibers fabricated by electrospinning and mechanical spinning

Considerable attention has been paid to aligned polymer nanofibers because of their appealing mechanical and piezoelectric properties. With modified electrospinning, the aligned poly (vinylidene fluoride) (PVDF) nanofibers can be produced. However, the roles of mechanical stretching and electrical poling during this process require further investigation. Here, we utilize a rotary drum to collect the fibers by both electrospinning and mechanical drawing approaches. The fiber mats were characterized by Scanning Electron Microscope (SEM), Fourier Transform Infrared Spectrometer (FTIR) and X-Ray Diffraction (XRD), and were directly made into sensing films without post electric poling treatment. Their piezoelectricity was measured in our home-made dynamic air pressure sensor. Results show that the β-phase predominates in both electrospun and mechanically-spun nanofibers. However, sensor based on electrospun β-PVDF fiber mat presented a sensitivity of 178 mV/kPa, while only a very weak output signal was observed in the mechanically spun β-PVDF fiber mat. Therefore, mechanical stretching and electrical poling may play key roles in inducing the β-phase and orienting the dipoles in a preferential direction during electrospinning process, respectively.

Study of Thermal Electrical Modified Etching for Glass and Its Application in Structure Etching

In this work, an accelerating etching method for glass named thermal electrical modified etching (TEM etching) is investigated. Based on the identification of the effect in anodic bonding, a novel method for glass structure micromachining is proposed using TEM etching. To validate the method, TEM-etched glasses are prepared and their morphology is tested, revealing the feasibility of the new method for micro/nano structure micromachining. Furthermore, two kinds of edge effect in the TEM and etching processes are analyzed. Additionally, a parameter study of TEM etching involving transferred charge, applied pressure, and etching roughness is conducted to evaluate this method. The study shows that TEM etching is a promising manufacture method for glass with low process temperature, three-dimensional self-control ability, and low equipment requirement.

Piezoelectric properties of PVDF nanofibers via non-uniform field electrospinning

Due to its strong piezoelectricity compared with other polymers, Poly (Vinylidene Fluoride) (PVDF) has been widely utilized in areas of energy, biology and sensor. Researches have shown that both orderly stretching and polarization treatment can strengthen the piezoelectric properties of PVDF. In this work, non-uniform field electrospinning is adopted to fabricate aligned PVDF fibers. The relationship between piezoelectric properties & morphology of PVDF fibers and PETs thickness & electrospinning time was investigated. To determine the sensitivity of PVDF fibers, the effect of air pressure on piezoelectric properties was also tested. Results show that with increasing the PETs thickness, the arrangement of PVDF fibers tends to be more orderly and the piezoelectric properties obviously increases. But the electrospinning time has almost no effect on the piezoelectric properties. Moreover, the average diameters of PVDF fibers are all about 440~475nm. Therefore, we can only adjust the main parameters to strengthen the piezoelectric property of PVDF fibers.