The use of AC potentials in electrospraying and electrospinning processes

The use of AC potentials in electrospraying and electrospinning processes was demonstrated. Carboxymethylcellulose (CMC) was electrosprayed onto semiconducting and insulating substrates using both DC and AC potentials. On the semiconducting substrate, both AC and DC methods were capable of producing significant CMC coverage. However, only the AC potential was capable of producing significant coverage on the insulating substrate, possibly due to a reduction in the amount of surface charging. In the electrospinning investigation, poly(ethylene oxide) (PEO) fibers were spun into mats using both DC and AC driving potentials. The AC potential resulted in a significant reduction in the amount of fiber ‘whipping’ and the resulting mats exhibited a higher degree of fiber alignment but were observed to contain more residual solvent. The average fiber diameter for both DC and AC-spun mats exhibited a strong dependence on solution concentration.     

Fabrication of nanofibers by a modified air-jet electrospinning method

A modified air-jet electrospinning (MAE) setup was demonstrated for contributing to the large-scale nanofibers production. With this single nozzle air-jet electrospinning device, the productivity of nanofibers can be increased more than forty times as compared with using the single-needle electrospinning (SNE) setup. When compared with other needle-less electrospinning setups, the benefits of this setup include ability to keep stable concentration of electrospun solution and to produce more uniform and thinner fibers, controlling of the jets formed speed and position, higher throughput, lower critical voltage, easier assembling, simpler operation, and so on. Four different parts of the fiber generator were, respectively, charged as electrospun electrodes to produce fibers. The distributions of the electric field with different electrodes were simulated and investigated for explaining the experimental results including the fibers productivity, the deposition area of nanofiber mats, as well as the surface morphology of the fibers. When the whole nozzle was charged, as compared with charging other electrodes, the MAE system produced thinner fibers with larger standard deviation on a much larger scale. By reduction of charged area, the received fibers presented lower productivity and thicker diameter with lower standard deviation. Especially, when a half of the nozzle was charged, the deposition area of nanofiber mats was larger than charging other electrodes. Besides, when a half of the nozzle was charged, the influences of electrospinning parameters such as applied voltage, collecting distance and the flow rate of air on nanofibers morphology were also investigated. Furthermore, based on this spinning unit, multi-nozzle air-jet electrospinning setup can be designed for larger production of nanofibers.     

The effect of electrode configuration and substrate material on the mass deposition rate of electrospinning

Poly(vinyl alcohol) (PVOH) was electrospun using different methods to charge the polymer solution. A positive high voltage relative to the collecting electrode significantly increased the fiber deposition rate. Electron microscopy showed that approximately half of the increase in fiber mass was due to thicker fibers being deposited. The current flowing from the grounded electrode was measured to determine the charge carried on the PVOH jet. This showed that for a positive voltage charging condition there is a much larger current and hence more charge carriers generated in the PVOH solution. As a result, more mass is ejected from the Taylor cone, implying that a positive voltage also produces longer fiber for a given time period. We also tested whether different substrate materials caused any variation when the charging conditions were changed. Statistically significant variation between substrates was only found when the substrate was an insulator and was expected to support a high‐deposition rate. This confirms the view that the PVOH fiber arrives at the collecting electrode carrying a charge that must be neutralized, otherwise a repulsive charge will form where the fiber is deposited and some fiber will be lost to any alternative earth. In electrospraying, charge carriers are generated using associated redox reactions. Thus, for electrospinning a lack of symmetry in these reactions may result in the generation of different quantities of charge carriers in the PVOH solution and changes in the mass deposition rate of electrospun fiber. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Surface functionalization of polymer nanofibers by ITO sputter coating

In this study, transparent conductive films of tin-doped indium oxide (ITO) were deposited onto the polyamide 6 (PA6) nanofiber substrates at room temperature. Atomic force microscopy (AFM) was employed to study the morphology of the nanofibers, respectively. The AFM results indicated a significant change in the morphology of the nanofibers before and after the ITO sputter coatings. The light transmittance and surface conductivity of the ITO-deposited nanofibers were also investigated. It was found that the surface resistivity of the PA6 nanofiber with the ITO deposition had a significant drop and the ITO deposition obviously affected the light transmittance of the PA6 nanofibers.     

Electrostatic phenomena during powder handling

During powder handling operations, particles make frequent contact with surfaces often metallic, and become electrically charged due to the process of contact electrification. It is often more appropriate to describes such a contact electrification process as triboelectrification as sliding/frictional contact is invariably involved. Triboelectrification is a complex process as charge exchange involving insulating surfaces takes place. The concept of work function may be applied to some insulator materials, especially those that charge negatively as many polymeric materials do. Surface charging depends upon surface condition and some materials are sensitive to the presence of oxidising agents in the atmosphere and also to the presence of moisture. To quantify the triboelectrification process, the dynamics of particle surface contact must be fully defined and contact area accurately assessed. The dynamic behaviour of charged particles may be significantly dependent upon charge. Space charge fields which exist in charged powder clouds act upon individual particles whose motion depends upon particle electrical mobility. Even in the absence of space charge fields, charged particles experience attractive forces towards nearby neutral surfaces. Upon contact with surfaces, charged particles may adhere strongly by a combination of electrostatic and van der Waals forces. Particle/surface adhesion is important in powder coating applications and in electrostatic precipitators.     

Electrospray ionization for deposition of ultra-thin polymer layers – principle,electrophoretic effect and applications

Nebulizing of polymer solutions, in a high-voltage field under atmospheric conditions by electrospray ionization (ESI), is a comfortable way to deposit ultra-thin layers of polar or ionic polymers onto any conductive substrate materials. The substrate is grounded and the polymer solution is sprayed through a powered capillary. The formed charged droplets shrink by solvent evaporation during their way to the grounded substrate, the charges close ranks and the droplets collapse consecutively by charge repulsion, thus forming finally charged single macromolecules. After their discharging at the grounded substrate, an ultra-thin ‘quasi-monomolecular’ polymer layer is formed. It could be shown by imaging of scratches through the polymer layer by atomic force microscopy that the deposited polymer layers are dense at a thickness of about 10?nm. Carbon fibre bundles were coated with poly(allylamine) (PAAm) or poly(acrylic acid) (PAA) as potential adhesion-promoting layers in fibre–polymer composites. The polymer deposition is self-inhibiting after formation of a continuous coverage of about 200?nm for PAAm and 30?nm for PAA as result of surface charging. Continuous deposition onto such isolating layers or polymers without charging can be achieved by using current of alternating polarity. The film formation is self-healing because of the electrophoretic effect, i.e. the ion discharging occurs preferentially at non-coated areas. This electrophoretic effect of ESI was demonstrated by completely enwrapping all the carbon fibres of the roving within a distance of about 100?μm far from its outside and also at the backside of the fibre bundle with about 80% of the topside coverage, as measured by X-ray photoelectron spectroscopy and visualized using scanning electron microscopy.     

Diamond deposition on copper treated hardmetal substrates

The adhesion of diamond coatings onto hardmetal substrates is improved by a copper deposition produced by a cementation from aqueous CuSO₄ solutions. During this reaction Co is dissolved from the substrate surface and copper is deposited. To obtain homogeneous Cu deposits, the influence of CuSO₄ concentration and reaction time on cementation were investigated.During diamond deposition, Cu reduces the surface mobility of Co, which is necessary to decrease deposition of non-diamond carbon and therefore increase adhesion. Indentation tests showed the good adhesion of the diamond coatings qualitatively.Cu precipitation and diamond deposition were examined by scanning electron microscopy (SEM). The diamond quality was detected by Raman spectroscopy. Using secondary ion mass spectroscopy (SIMS) depth profiles the interface and the Cu distribution were characterized indicating that during diamond deposition Cu is dissolved and forms an intermetallic Co–Cu mixed crystal.     

The Definition of the Locus of Failure on Ceramic Substrates: The Benefit of Monochromated XPS

The use of X-ray photoelectron spectroscopy (XPS) to assess the locus of failure of a photo-cured resin on an alumina substrate is reported. It is shown that the carbon 1s spectra obtained with conventional (achromatic) and monochromatic photon sources are markedly different. The spectrum obtained with the monochromatic source reveals the fine structure associated with the polymer whilst the spectrum recorded with the conventional source is distorted by differential charging. This observation has important ramifications when XPS is used for the definition of the locus of failure of organic coatings, or adhesives, applied to insulating substrates such as ceramics.     

Processing and microstructural characterization of porous biocompatible protein polymer thin films

The process of electrostatic fiber formation, or electrospinning, was used to create biocompatible thin films for use in implantable devices. The morphology of the thin films was found to depend on process parameters including solution concentration, applied electric field strength, deposition distance, and deposition time. The microstructure of the coatings was examined by Transmission Electron Microscopy (TEM) and Wide-Angle X-ray Scattering (WAXS), with electrospun filaments being weakly oriented along the fiber axis. A shish kebab model for the filament morphology was proposed. The electrospinning process was shown to be a means of creating porous thin films with structural gradients and controlled morphology that could enhance biocompatibility.     

静电纺丝法纺制聚乳酸纳米纤维无纺毡

采用静电纺丝法制备了生物降解聚乳酸(PLLA)纳米纤维无纺毡。分析了纺丝液浓度、电压、接收距离、挤出速度等因素对纤维形态的影响。结果表明:纺丝液的浓度和挤出速度对纤维直径的影响较为明显,溶液挤出速度增大,所得纤维微孔含量及尺寸也增大;适当的电压和接收距离有利于收集无液滴纤维;随着纤维直径的减小,无纺毡的孔径呈减小趋势。在PLLA质量分数为5．7％、挤出速度0．8 mL／h、接受距离 15．5 cm、电压8 kV的静电纺丝条件下,可制备纤维直径为200-400 nm的PLLA纳米纤维无纺毡。     

静电纺丝制备交联PVA纳米纤维

用静电纺丝法制备了含有交联结构的PVA纳米纤维,并通过红外光谱分析(FT-IR)、扫描电子显微镜(SEM)、热分析(TG、DTA)对制品的结构和形貌进行了表征;通过对制品进行接触角、溶胀度的测定,考察了电纺条件和加热后处理对交联度的影响以及交联度对吸水率的影响;提出了静电场力可作为动力学条件促进化学反应的新概念。     

Exhaust Aerosol of a Plasma Enhanced CVD System: II. Electrical Charging and Transport

The microelectronics industry has been concerned about the loss of product yield in its semiconductor wafer processing steps from killer defects caused by the presence and deposition of contaminant particles. Reactant gases used in plasma enhanced chemical vapor deposition (PECVD) can form nanometer sized particles from homogeneous nucleation. Once the particles grow to a few nanometers, they become negatively charged due to the collected ion and electron currents on the particle surface. A gradient of the electron and ion concentration between the sheath and bulk plasma causes an electric field to develop directed to the walls. Contaminant particles can eventually become trapped in electrostatic potential wells due to the higher charge density of positive ions near the powered electrode. The traps fill in the plasma sheath region until some particles are "leaked out" by gas drag forces. Therefore particles formed solely in the plasma volume are theorized to possess a distinct charge level from condensation particles originating within the exhaust line alone. Consequently, plasma properties determine the size and charge distribution of contaminants that exit the reactor and enter the exhaust line. The intended contribution of this research was to (1) develop the capability to monitor charged and uncharged contaminant particle fractions during a thin film deposition cycle and (2) to understand the coupled effect of particle transport and charging in an radio frequency (RF) plasma. Therefore the size distribution and absolute electrical charge were measured in the vacuum exhaust line during a SiO 2 thin film cycle using an integral mobility charge analyzer. Experiments confirm that over 50% of fine particles in the exhaust were neutral. However, a moderately charged coarse fraction was measured between 0.15 and 0.3 w m. The larger particles are hypothesized to be reactor particles gradually released from the potential wells in the plasma sheath. Process variables, including the RF power and the reactor pressure, directly affected the size and charge characteristics measured on particles in the exhaust line. The first paper in the series "Exhaust Aerosol of a Plasma Enhanced CVD System" uses an in-line exhaust sampling system to measure the size distribution of contaminant particles exiting the plasma.     

静电纺丝过程中的非稳定性研究

通过静电纺聚丙烯腈纳米纤维,研究了静电纺丝过程中的非稳定性。结果表明:静电纺丝电压和接受距离是影响静电纺丝过程中的非稳定性因素,电压和接受距离增大,纳米纤维摆动加大,电压增加到一定量后,纳米纤维的摆幅在一定范围内,不会无限扩大,趋于一个固定值,接收距离超过一定范围后,纳米纤维出现不规则扰动,出现彼此缠绕。     

Atmospheric plasma application to improve adhesion of electrospun nanofibers onto protective fabric

Nylon 6 electrospun nanofibers were deposited on plasma-pretreated woven fabric substrates with the objective of improving adhesion between them. The prepared samples were evaluated for adhesion strength and durability of nanofiber mats by carrying out peel strength, flex resistance, and abrasion resistance tests. The test results showed significant improvement in the adhesion of nanofiber mats on woven fabric substrates due to atmospheric plasma pretreatment. The samples also exhibited good flex and abrasion resistance characteristics. X-ray photoelectron spectroscopy and water contact angle analyses indicate that plasma pretreatment introduces radicals, increases the oxygen content on the substrate surface, and leads to formation of active chemical sites that may be responsible for enhanced cross-linking between the substrate fabric and the electrospun nanofibers, which in turn increases the adhesion properties. The work demonstrates that the plasma treatment of the substrate fabric prior to deposition of electrospun nanofiber mats is a promising method to prepare durable functional materials.     

Preparation of aligned polyetherimide fiber by electrospinning

Fifteen to 20 wt % polyetherimide (PEI) solutions with 1-methyl-2-pyrrolidinone (NMP) were prepared. The electrical conductivity and surface tension of the solutions were determined. The fiber spinning technique of electrospinning was optimized in order to prepare unidirectionally aligned, structurally oriented nanofiber tows. The morphology of the PEI fibers was investigated using field emission scanning electron microscopy (FESEM). The well-aligned fibers with diameters between 0.58 and 0.90 μm (FESEM) were collected by electrospinning 20 wt % PEI solutions with NMP in the range of 8–10 kV onto a target rotating with a surface velocity 9.8 m/s. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Supracolloidal Self-Assembly of Micro-Hosts and -Guests on Substrates

In this paper, we demonstrate a microcolloidal analogue of supramolecular host–guest system consisting of polymer microrings as micro-hosts and spherical particles as micro-guests. Polystyrene microrings with accurately controlled hollow cavities are fabricated by a two-step selective plasma etching method. The diameter and depth of the hollow ring can be controlled by different etching stage. The surface charge of microrings can be modified through sulfonation and/or layer-by-layer deposition of polyelectrolytes. Through electrostatic interactions, polystyrene microrings on substrates are used as micro-hosts to capture oppositely charged spherical micro-guests with assembly yields up to 98.7%. The micro-host/guest compounds are found to be stabled in aqueous solution even after peeled off from the substrate.     

Electrophoretic deposition of doped ceria: Effect of solvents on deposition microstructure

Electrophoretic deposition of doped ceria has been carried out in non-aqueous solvent to prepare coatings on different substrates and free standing films. It has been found that uneven deposition occurred in ethanol, while in butanol deposition yield is low having very little variation with deposition time. On the other hand, good deposit obtained in acetyl acetone medium, but had a porous structure. The best result however was obtained in mixed solvent. Effect of adding charge modifying additives in the ceria suspension on the deposit microstructure has been studied. Mechanism of charging in the non-aqueous medium to modify the surface properties of the suspended particles has been discussed.     

Nanoclay plating of cellulosic fiber surfaces

A basic problem in making cellulose‐reinforced composites is achieving a dispersion of cellulosic fibers in an often olephinic polymer matrix. Drying cellulosic fibers results in the formation of fiber flocs/nodules because of their strong interfiber bonding, and this makes the hydrophilic cellulosic fibers difficult to disperse in a hydrophobic matrix material. One common approach to alleviate floc formation is to adsorb cationic surfactant onto anionically charged cellulose, which reduces the interfiber bonding, decreases floc formation and gives better compatibility with the matrix. In this report, a different approach is taken, namely to adsorb nanoclays onto the cellulosic fibers, and thereby reduce the natural hydrogen‐bonding affinity between fibers. In a second report, the same technology will be shown to be advantageous to decrease floc formation in oleophinic composites reinforced with cellulosic fibers. This article summarizes experiments aimed at optimizing the chemistry of deposition of montmorillonite clay onto fiber surfaces. The aim was to optimize the chemical conditions for the heterodeposition of the anionic clay onto cationically charged fluff pulp. The experiments were designed to provide a theoretical framework for the deposition of the nanoclay onto the pulp fibers. High M_w p‐DADMAC and an exfoliated clay (achieved by passing the clay through a homogenizer) were used. As expected, a certain degree of charge overcompensation by adding an electrolyte was necessary to bring about deposition. The adsorbed amount of clay could be calculated from the charge balance between the overcompensated charge and the net clay charge, constituting the theoretical framework for nanoclay heterodeposition. As expected, montmorillonite clay greatly destroyed the joint strength between fibers (determined by evaluating the strength of paper made from treated fibers). The surface coverage (determined by ESCA) was shown to be a linear function of the attached amount of clay, and ～3% clay was required to fully cover the fiber surfaces. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Numerical analysis of frictional charging and electrostatic interaction of particles

Triboelectrification due to frictional contacts between particles and surfaces is prevalent in many powder handling processes. Aiming to explore the friction-induced electrostatic charging behavior, a discrete element method (DEM) is developed for the first time in the current article, in which a frictional charging model and electrostatic interaction models are implemented. The charge accumulation on both the particles and the surface in a rotational container is then analyzed numerically and experimentally to evaluate the developed DEM. The numerical results for the frictional electrification between insulant particles and an insulant wall agree well with the experimental measurement. It is also shown that both the net charge on the particles and the degree of the particle dispersion are a function of the charging time. Moreover, it is revealed that the friction-induced particle charge enhances particle dispersion, and increases the granular temperature due to the electrostatic interactions.     

Chemical vapour deposition of diamond on nitrided chromium using an oxyacetylene flame

In this paper we present our first preliminary results on chemical vapour deposition (CVD) of diamond onto nitrided chromium using an oxyacetylene flame. Polycrystalline diamond films were obtained after deposition at very low substrate temperatures (<400°C). At these low temperatures there was extremely weak bonding, or no bonding at all, between the deposited layer and the substrate. To obtain stronger bonding, four growth experiments were carried out at initially higher substrate temperatures (700–1000°C). Whilst growth continued, the substrate temperatures were lowered step by step to 250°C. It was observed that on lowering the substrate temperature by more than about 500°C from the initial temperature, delamination occurred, suggesting that the thermal stresses exceeded the bonding strength. Subsequently, adherent diamond coatings were grown on the freshly exposed substrate surface whilst further lowering the substrate temperature. These diamond coatings were characterized using scanning electron microscopy and the adhesion of the diamond coatings to the substrates was assessed by means of the scotch tape test.