Daoheng Sun

Precision deposition of a nanofibre by near-field electrospinning

The deposition behaviour of an individual nanofibre on planar and patterned silicon substrates is studied using near-field electrospinning (NFES). A high-speed camera was utilized to investigate the formation and motion process of a liquid jet. Thanks to the shorter distance from the spinneret to the collector, bending instability and splitting of the charged jet in electrospinning were overcome. In NFES, a straight-line jet between the spinneret and the collector can be utilized to direct-write an orderly nanofibre. Perturbation stemming from residual charges on the collector caused the oscillation of the charged jet, and the deposition of the non-woven nanofibre on the planar substrate. With increasing collector speed, the impact of residual charges was weakened by the strong drag force from the collector and a straight-line nanofibre could be obtained. In addition, the nanofibre can be direct-written in a special pattern by controlling the motion track of the collector. Therefore, it can be concluded that a micro-strip pattern was a good guidance for nanofibre deposition, and the nanofibre deposition track followed well along the top surface of the micro-strip pattern. The position-controlled deposition of a single nanofibre provides a new aspect for applications of electrospinning.

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.

A rational design of separator with substantially enhanced thermal features for lithium-ion batteries by the polydopamine–ceramic composite modification of polyolefin membranes

A separator plays a crucial role in ensuring the safety in lithium-ion batteries (LIBs). However, commercial separators are mainly based on microporous polyolefin membranes, which possess serious safety risks, such as their thermal stabilities. Although many efforts have been made to solve these problems, they cannot yet fully ensure the safety of the batteries, especially in large-scale applications. Herein, we report a rational design of separator with substantially enhanced thermal features. We report how, by a simple dip-coating process, polydopamine (PDA) formed an overall-covered self-supporting film, both on the ceramic layer and on the pristine polyolefin separator, which made the ceramic layer and polyolefin separator appear as a single aspect and furthermore, this layer amended the film-forming properties of the separator. Combining the function of the ceramic and PDA, the developed composite-modified separator displays substantially enhanced thermal and mechanical stability, with no visual thermal shrink and can maintain its mechanical strength up to 230 °C when the polyethylene separator acts as the pristine separator.

A critical analysis of the α, β and γ phases in poly(vinylidene fluoride) using FTIR

Poly(vinylidene fluoride) (PVDF) has been widely utilized in scientific research and the manufacturing industry for its unique piezoelectric properties. In the past few decades, the vibrational spectra of PVDF polymorphic polymers via FTIR (Fourier transform infrared spectroscopy) have been extensively investigated and documented. However, reports on the analysis of α, β and γ phases often have conflicting views based on measured data. In this work, we analyze the FTIR vibrational bands of PVDF materials fabricated by different processes with detailed XRD (X-ray diffraction) characterization to identify the structural α, β and γ phases. By examining the results in this work and extensively reviewing published research reports in the literature, a universal phase identification procedure using only the FTIR results is proposed and validated. Specifically, this procedure can differentiate the three phases by checking the bands around 763 and/or 614, 1275, and 1234 cm−1 for the α, β and γ phases, respectively. The rule for assignment of the 840* and 510* cm−1 bands is provided for the first time and an integrated quantification methodology for individual β and γ phase in mixed systems is also demonstrated.

Electrohydrodynamic Deposition of Polymeric Droplets under Low-Frequency Pulsation

Circularly shaped polymeric droplets with diameter of about 20 μm have been intermittently ejected and deposited in an orderly manner on a collector from a syringe needle by means of near-field, electrohydrodynamic reactions using pulsating voltages at around 2.25 kV. The needle has an inner diameter of 100 μm and was placed 1 mm above a silicon conductor substrate to have location control for droplet depositions. Under low-frequency operation of less than 100 Hz, the deposition frequency of droplets, f(dep), has been observed to be equal to the frequency of the applied driving voltage divided by an integer, N, as small as 1. Furthermore, the diameter of the deposited droplets has been found to be linearly dependent on (Q/f(dep))(1/3), where Q is the polymer solution supply rate at around 30 nL/s. These experimentally observed droplet ejection rules under low-frequency pulsation provide useful design guidelines for controllable deposition of polymer droplets in various potential applications, including electrohydrodynamic printing.

The effect of surfactants on the diameter and morphology of electrospun ultrafine nanofiber

Different surfactants are introduced to study the diameter and morphology transformation characteristics of electrospun nanofiber. Surfactants increase the net charge density and instability motion of charged jet. The instability motion provides a good way to stretch the charged jets into finer ones, by which the beaded structures are also prevented. Ultrafine nanofiber with average diameter less than 65nm can be fabricated. The nanofiber diameter decreases with the increase of surfactant concentration in polymer solution. The nanofibers with anionic surfactant sodium dodecyl sulfate (SDS) have the smallest diameter. The cationic surfactant hexadecyl trimethyl ammonium bromide (HTAB) plays the best role to prevent the formation of beaded structures in nanofibers, and helps to increase the uniformity of electrospun nanofiber. The effects of surfactants on the nanofiber diameter and morphology have been studied, which would promote the industrial application of ultrafine polymeric nanofibers.

Design and Experiment of a Jetting Dispenser Driven by Piezostack Actuator

To make up for the insufficiency and instability of contact dispensers that are used for fluid dispensing in microelectronic packaging, a noncontact jetting dispenser driven by a piezostack actuator is introduced in this paper. After describing the structural components and operating principle of the dispensing mechanism, a fluid model is presented to discuss the dynamic properties of the fluid and analyze the key parameters of the proposed dispenser. The ANSYS simulation software is used to design the displacement amplifier, which is an important component of the dispenser. The maximum displacement output of 323 μm is obtained by optimizing. Subsequently, the dynamic behavior of the displacement amplifier is measured by an optical displacement sensor. The displacement change with the driving voltage amplitude and frequency is also investigated; the maximum displacement is 320 μm, and the error between the simulation and the measurement result is just 0.75%. In order to verify the practicality of dispenser, experiments are conducted to examine the effects of the driving voltage, backpressure, working temperature, and distance between the nozzle and the collector on the jetting performance and droplet diameter. The dispenser can dispense droplets uniformly and steadily. Its maximum jetting frequency is 65 Hz, and droplets of 1.07-mm diameter are produced by a stainless steel nozzle of 0.25-mm diameter in the experimental study, with the variation of the droplet diameter being within ±2%.

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.

Electrospun nanofibrous membrane for air filtration

Nanofibers have a large potential in air filtration applications, so this paper explores the performance of electrospun nanofiber membrane compared to traditional filtration fabrics. Poly (ethylene oxide) (PEO) and Polyvinyl Alcohol (PVA) were electrospun into nanofibrous membranes and analyzed their filtration attributes. Experimentation revealed that nanofibrous membranes have higher filtration efficiency than traditional filtration fabrics, such as meltblown and needle filtration material. In addition, nanofibrous membranes under the same electrospinning process but with different materials had similar high filtration efficiency, while their permeability had obvious difference. We suggest that different structure in the nanofiber membrane should cause this difference. Our work proves that there is a large potential for nanofiber membranes to utilize in air filtration area.

Polyethylene terephthalate/poly (vinylidene fluoride) composite separator for Li-ion battery

Electrospun nanofiber membranes have been proved to enhance performances of Li-ion batteries, but their poor mechanical strength hinders their industrial application. This paper combines meltblown polyethylene terephthalate (PET) nonwoven and electrospun poly (vinylidene fluoride) (PVDF) membrane together to improve the mechanical property via hot-pressing, wherein a dried 3 wt% PVDF solution coating on PET nonwoven is used as a binder. The hot-pressing temperature is optimized to be 145 °C and the composite PET/PVDF separator exhibits an excellent mechanical property, whose transverse and longitudinal tensile strength could reach 13.70 and 34.85 MPa respectively. Compared with a commercial PP separator, the hot-pressed PET/PVDF membrane separator shows better wettability, higher thermal shrinkage and improved electrochemical performance as well.