Usability of electrophoretic deposition for additive manufacturing of ceramics

The potential of electrophoretic deposition of ceramic particles for additive manufacturing of 3D structures is investigated. A custom-made test setup is constructed using a hollow electrode for the supply of a ceramic suspension and a tip c electrode for focused deposition of particles on the surface of a membrane. Parameters like voltage, electrode distance, suspension flow rate, etc. are systematically varied to study their impact on the deposition yield and the morphology of the deposited structures. Experimental work is supported by computer simulations on the electrical field distribution within the electrode area. Although detrimental phenomena like electrohydrodynamic effects occur, we have shown that the technique offers opportunities for future 3D printing of ceramics.     

Continuous Free Flow Electrophoresis in an Alternating Electric Field with a Variable Buffer Flow

Abstract

A novel method of continuous free flow electrophoresis in an alternating electric field with a variable buffer flow is proposed as a basis for continuous separation of biomolecules. A model is developed to evaluate the migration of charged particles as they move with the buffer flow. Numerical calculations on the trajectory and dispersion are presented for particles with different electrophoretic mobilities.     

电极极性影响EHD强化沸腾传热的机理

The effects of electrode polarities on EHD enhancement boiling heat transfer were investigated theoretically and experimentally based on the analysis of electric field distribution affected by superheat boundary layer and charge injection. The results showed that electric field distribution was changed by the charge induced by temperature gradients in the superheat boundary layer, but the change was independent of electrode polarities. However, when electric charge injection occurred, the electrode applied positive high voltage might generate different characteristics of charge injection from that generated by applied negative high voltage. If the electric field on the surface of heat transfer increased due to charge injection, the augmentation effects would increase. The experiments demonstrated that positive high voltage gave larger enhancement factors than negative high voltage. The experimental phenomena could be interpreted well on the basis of charge injection characteristics.     

圆筒电极抑制换热表面CaCO3污垢沉积特性研究

将一圆筒柱面电极布置在换热器入口管道处,并与高压电源正极连接,研究圆筒电极形成的非匀强电场对实验段换热表面CaCO₃污垢沉积的影响。作为对比,同时分析了平行板电极（匀强电场）作用下CaCO₃污垢沉积行为。结果表明,施加平行电极与圆筒电极均可抑制换热表面CaCO₃污垢的沉积,抑垢率随施加电压（0~5000 V）的增加呈现先升高后降低的变化趋势。当施加平行电极时,最佳抑垢电压值为1000 V,最佳抑垢率为 73.27%。当施加圆筒电极时,最佳抑垢电压为500 V,最佳抑垢率为83%。因此,施加圆筒电极具有更好的抑垢效果,同时最佳抑垢电压更低,可降低电的消耗。此外,在无电场作用时,换热表面CaCO₃晶体主要为树枝状文石结构,当施加圆筒或平行电极时,CaCO₃晶体主要为块状方解石结构。     

Aerosol Charge Neutralization by a Mixing-Type Bipolar Charger using Corona Discharge at High Pressure

An aerosol neutralizer called the Mixing-type Bipolar Charger using Corona-Discharge at High Pressure (MBCCHP) was developed. In the MBCCHP, a corona discharge (High-Pressure Corona Ionizer; HPC Ionizer) induced by high frequency voltage (>100 Hz) at high pressure (>0.2 MPa) is used to generate bipolar ions at high concentration (1–3 × 10⁹ ions/cm³) that are then mixed with aerosol particles flowing in a charging chamber where no external electric field is present. The charging performance of the MBCCHP was evaluated by comparing the measured and theoretical number ratios of positively and negatively charged particles to the total number of particles, and by comparing those of negatively charged to positively charged particles for an equilibrium charge distribution. The theoretical and measured results agreed well in the particle size range of 5–80 nm. Particle loss in the MBCCHP for the size range of 5–100 nm was less than 15%, and particle generation from the electrode due to spattering or from the carrier gas containing SO_x due to chemical reaction was either negligible or not observed. The MBCCHP can effectively provide aerosol particles in the equilibrium charge state. Advantages include (1) no selective deposition of charged particles by an electric field, (2) no generation of new particles by reactive molecules, such as atmospheric pollution gases contained in a sample aerosol by chemical reactions with active species, such as OH radicals, produced by discharge, and (3) no effect of carrier gases of the sample aerosol on the ion properties.     

On Differential Mobility Analyzer Resolution

The resolution of the differential mobility analyzer (DMA) is conveniently described as the ratio of the mobility at the peak of the column transfer function to the full width of the transfer function at 1/2 of its maximum value. The best resolution that can be achieved is that for nondiffusive particles, -nd=beta1, where beta is the flow rate ratio, beta = (Q a + Q s)/(Q sh + Q e) . Brownian diffusion causes particles to deviate from the ideal electrophoretic migration trajectories, thereby reducing the resolution. The relative importance of electrophoretic migration to diffusion can be expressed as a function of the migration Peclet number, which can be expressed either in terms of mobilities, dimensions, and flow rates or as Pe m ig = ( qV / k T ) f , where q is the charge on the particle, V is the applied voltage, and f is a geometry factor that accounts for nonuniformities in the electric field along the migration pathway. Expressed in this way, the performance of DMAs with different geometries, operating at different flow rates, are, in the absence of distortions in the flows and electric fields, shown to be nearly indistinguishable. Diffusion is shown to dominate at operating voltages below a critical value that is proportional to the square of the limiting resolution. Since the voltage range for DMA measurements is limited, the dynamic range decreases with increasing - nd . Because of the changing size dependence of the mobility, this limitation is more pronounced for free-molecular aerosols than for larger particles.     

Long-range attractive forces extending from alumina nanofiber surface

Aluminum oxide-hydroxide nanofibers, 2 nm in diameter and approximately 250 nm long, are electroadhesively grafted onto glass microfibers, therefore forming a macroscopic assembly of alumina nanofibers on the second solid in highly organized matter. The assembly can be viewed as a straight cylinder with rough surface and charge density of approximately 0.08 C/m². This creates a significant electric field with negligible screening (ka ? 1) in the region close to the surface of the assemblies. This field attracts nano- and micron-size particles from as far as 0.3 mm in less than a few seconds, many orders of magnitude greater than the conventional Derjaguin–Landau–Verwey–Overbeek theory that predicts only nanometer-scale effects arising from the presence of the surface. The strong electric field on the surface is then able to retain particles such as micron-size powdered activated carbon as well as much smaller particles such as fumed silica nanoparticles of 10–15 nm in diameter, viruses, atomically thick sheets of graphene oxide, latex spheres, RNA, DNA, proteins, and dyes.     

Improved field emission performance of carbon nanotube by introducing copper metallic particles

To improve the field emission performance of carbon nanotubes (CNTs), a simple and low-cost method was adopted in this article. We introduced copper particles for decorating the CNTs so as to form copper particle-CNT composites. The composites were fabricated by electrophoretic deposition technique which produced copper metallic particles localized on the outer wall of CNTs and deposited them onto indium tin oxide (ITO) electrode. The results showed that the conductivity increased from 10^-5 to 4 × 10^-5 S while the turn-on field was reduced from 3.4 to 2.2 V/μm. Moreover, the field emission current tended to be undiminished after continuous emission for 24 h. The reasons were summarized that introducing copper metallic particles to decorate CNTs could increase the surface roughness of the CNTs which was beneficial to field emission, restrain field emission current from saturating when the applied electric field was above the critical field. In addition, it could also improve the electrical contact by increasing the contact area between CNT and ITO electrode that was beneficial to the electron transport and avoided instable electron emission caused by thermal injury of CNTs.     

Centrifugal float–sink separation of fine Turkish coals in dense media

Zonguldak bituminous coal, Tunçbilek and Soma–Merkez lignites were each separated into two sub-fractions, coal rich and mineral matter rich, using a centrifugal float–sink separation technique in heavy media. An isopropyl alcohol (IPA)–carbon tetrachloride (CCl₄) mixture and a zinc chloride (ZnCl₂) solution, with a specific gravity of 1.40 g cm⁻³ at 25°C were used as dense medium liquids. The addition of surface active agents (Triton X-100 and Brij-35) to the zinc chloride solution improved the removal of minerals. The recovery and purity of the final product (float) obtained from the heavy media separation depend on such parameters as the density of the medium, rotor speed and centrifugation time. The separation efficiency of each coal differed significantly. Particle size distributions of the coals and their float and sink fractions were analysed using a Laser Particle Size Analyser. A Scanning Electron Microscope (SEM) was used to interpret the liberation of minerals from the coal particles.     

The effect of electrophoretic convection for the separation of solute in the chromatography column (I)

A theoretical model has been derived in an electrophoretic packed column where an electric potential is applied to a column in the axial direction. The effect of electrophoretic convection in gel particles packed in the column significantly contributes to the separation of large polyelectrolytes because the conformation of polyelectrolyte quickly orients in the field direction. The dependence of the transport in the gel particle upon field intensity and molecular size aids in understanding the transport of polyelectrolyte in the packed column, since the convective velocity of polyelectrolyte is accelerated inside a porous gel particle. There are few convection studies of large poly-electrolyte in a column packed with porous gel particles under an electric field for the separation. Convective-diffusive transport of a large polyelectrolyte is analyzed using Peclet number described by electrophoretic mobility and diffusion coefficient measured experimentally. The separation of two different polyelectrolytes in the packed column is performed using a value ofPe _f/Pe_g of individual polyelectrolyte by molecular size and an electric field. The purpose of this paper is to study the separation of solute from a mixture in the column using the physicochemical properties in the gel particle which are measured experimentally.     

Low-frequency ferroelectric switching studies in PVDF thin films across Cu or (Ag/Cu)/PVDF/Cu capacitor structures

Ferroelectric switching dynamics of polyvinylidene fluoride (PVDF) thin films in Cu or (Ag/Cu)/PVDF/Cu capacitors are explored by varying PVDF film thickness, applied electric field amplitude (4.35–87.5 MV/m) and frequency (100 mHz–200 Hz). Comprehending spontaneous polarization and its dependence upon interfaces, an electric field is critical for organic ferroelectric memory devices. In this article, quasi-static current–voltage, and polarization–electric field measurements are used to explain the relationship between the coercive field, signal amplitude, and frequency. The observed coercivity enhancement at lower PVDF film thicknesses and with rising frequencies of the applied signal is discussed with Kolmogorov-Avrami-Ishibashi domain nucleation and growth model. The relation between domain growth and the top electrode layer is further discussed from the exponent parameters.     

Dielectrophoresis in clay syspensions

An alternating, non-uniform electric field applied to suspensions of sodium, hydrogen, and aluminium kaolinites, aluminium montmorillonite, and aluminium bentolite, caused water to flow towards the more intense part of the field. The flow rates were generally proportional to the square of the voltage, the proportionality factor being greater for subsequent applications of the voltage. This appears to be the first demonstration of dielectrophoretic dewatering of aqueous dispersions. The rather weak dielectrophoretic effect increased for more strongly bound cations, whereas the much stronger electroosmotic effect (measured in the same apparatus using a direct voltage) decreased. Dielectrophoresis arises from the surface polarisation of elastically bound cations in the electric double layer, while relatively free cations are responsible for conductivity and electroosmosis. Only in the case of aluminium bentolite, where the clay particles were probably coated with positively charged hydroxy-aluminium species, was the dielectrophoretic effect appreciable compared with electroosmosis.     

Isolation and dispersion of reduced metal particles using the surface dipole moment of F-terminated diamond electrodes

Cu particles that have been reductively generated at the oxidized surface of a boron-doped diamond electrode (O-BDD) can be removed from the electrode's surface by the repulsive electrostatic force of the surface dipole moment during a potential cycle of a solution of Cu²⁺ ions. The objective of this study was to isolate various metal particles other than Cu by use of a fluorine-terminated BDD surface (F-BDD) with a stronger surface dipole moment than O-BDD, and to clarify the mechanism of the metal particles’ separation from the electrode.During the potential cycle treatment of Cu²⁺ ions using F-BDD, the reionization of the reduced Cu could be suppressed in the presence of dissolved oxygen, and the Cu particles were separated from the electrode surface as CuO. A similar result was seen with O-BDD. The degree of separation of the Cu particles could be drastically enhanced by raising the upper potential limit in the potential cycle from +0.2 to +0.8 V. By setting the upper potential to a potential greater than the metal-metal oxide equilibrium line in the potential-pH equilibrium diagram of the Cu-water system (Pourbaix Diagram), oxidation of the reduced metal surface by reaction with dissolved oxygen could be accelerated and the surface of metal particles could be insulated. The Cu particles were forced from the BDD surface by the electrostatic repulsion from the surface dipole moment of F-BDD. Also, it turned out that the physical adsorption of chloride ions (Cl⁻) on the electrode surface intensified the electrostatic repulsive force between the F- or O-BDD surface and the metal particles, and thus increased the degree of the metal particles’ separation.For Zn with a metal-metal oxide equilibrium potential of approximately −0.8 V at pH 7, complete separation of the Zn particles was achieved with F-BDD by setting the upper potential limit to +0.8 V (vs. Ag/AgCl), decreasing the Zn²⁺ concentration (1/10 that of Cu²⁺) and optimizing the sweep rate to minimize the Zn particle diameter. This result was hard to achieve with O-BDD. The separation of metal particles other than Cu was achieved with F-BDD because of its stronger surface dipole moment. However, with noble type metals (such as Pt and Ni) that have metal-metal oxide equilibrium potentials higher than the upper potential limit for potential cycling with F-BDD (+0.8 V), the surfaces of the reduced metal particles were not insulated and metal particles were deposited on the BDD surface. In order to isolate metal particles from the electrode, it was necessary to apply a maximum potential over that of the metal-metal oxide equilibrium line in the Pourbaix Diagram during the potential cycle.     

Induced-charge electrophoretic motion of ideally polarizable particles

The induced-charge electrophoretic (ICEP) motion of ideally polarizable particles is numerically studied in this paper. A complete three-dimensional multi-physics model is set up to simulate the transient ICEP motion of ideally polarizable, spherical particles in an unbounded liquid. The study shows the nonlinear induced zeta potential on the particle's surface causing a varying slipping (electroosmotic flow) velocity along the particle's surface, and hence producing microvortexes in the liquid. ICEP particle-particle interactions are also studied. The simulations show that a low pressure zone between the two polarizable conducting particles will be induced if the external electric field is applied parallel along the imaginary line connecting the two particles, resulting in an attracting effect between the two particles. Oppositely, a high pressure zone is induced between the two particles if the applied field is perpendicular to the imaginary line connecting the two particles, giving a repelling effect. The ICEP attracting or repelling effects depend on the particles’ separation distance, the electric field strength and the particle size.     

Electrochemical generation of hydrogen peroxide using surface area-enhanced Ti-mesh electrodes

Ti-mesh electrodes coated with Ti were obtained by using an electrophoretic deposition (EPD) method. The Ti coating was porous and showed a good adherence to the Ti-mesh surface, due to sintering of Ti particles during thermal treatment at 900 °C. The Ti-coated mesh electrode has a BET surface area of 3.5 m²/g, about four times larger than that of the bare electrode. The surface area-enhanced Ti-mesh electrode was applied in electrical generation of hydrogen peroxide. It was shown that the rate of hydrogen peroxide generation increased drastically compared to the fresh electrode, since the larger electrode surface area enhanced not only current density, but also the oxygen mass transfer rate.     

COMPARISON OF LIQUID-LIQUID DISPERSIONS FORMED BY A STIRRED TANK AND ELECTROSTATIC SPRAYING*

Two methods of producing liquid-liquid dispersions are compared in terms of the dispersed phase drop-size, energy requirements, and other properties. In the first method, a stirred-tank contactor, used for laboratory bioprocessing studies, was employed. Experiments were conducted using a l0cm-diameter cylindrical tank, stirred by one or two 5cm-diameter 6-blade Rushton-turbine impellers. The transient drop-size distribution of kerosene in water was measured by a video technique. It was found that (irpar; the drop-size had not reached steady state even after 10 hrs of agitation, and (iirpar; the drop-size produced by one impeller was smaller than that produced by two impellers. In the second method, aqueous droplets were electrohydrodynamically generated at the tip of a metal capillary under the influence of a pulsed, direct-current (dcrpar; voltage. The capillary tube was located co-axially at the center of another tube made of a dielectric (teflonrpar; wall. Kerosene was pumped between the capillary and the outer tube. An electric field was formed between the electrically-grounded capillary tube and an electrified electrode mounted on the external surface of the outer dielectric tube. Positive, sinusoidal-type voltage pulses in the range of 10-25 kV at frequency between 3.4 and 3.7 kHz were applied and the electric current was measured. The size of the drops ejected from the capillary was measured by a laser light scattering facility and found to be in the range 1 to 100 μm. Single and multiple spraying cones were observed depending on the aqueous-phase flow-rate. Smaller drop-size was obtained when multiple-cone spraying occurred. Energy calculations showed that dilute dispersions can be produced more efficiently by electrostatic spraying than by mechanical agitation.     

Multiscale Lattice–Boltzmann Approach for Electrophoretic Particle Deposition

In this work, a gas-particle flow over a structured sensor surface is numerically investigated. A system of parallel electrodes with an applied voltage is distributed on top of a nonconducting flat surface. The considered submicron particles (size range 25–200 nm) are electrically charged. The simulation takes into account the interaction between particle motion, fluid flow and electrical field causing the particles to deposit on the surface. As a result, dendrite microstructures of particles start growing on the electrode surface. To model these effects in detail the numerical simulations are carried out on a mesh with very high resolution of up to Δx = 0.5 μm. The fluid-flow is calculated with the Lattice–Boltzmann method incorporating automatic local grid refinement. The Laplacian equation describing the electrical field is solved by a finite-difference-scheme. The particle movement is calculated by the Lagrangian point-particle approach, accounting for drag force, Brownian motion, and Coulomb forces. Results of particle transport and dynamics of particle deposition are presented for different applied voltage, electrode configurations, flow velocities, and particle sizes.     

Horizontally aligned carbon nanotube field emitters fabricated on ITO glass substrates

Horizontally aligned carbon nanotube (CNT) field emitters, in which electrons are emitted from the side of CNTs, are fabricated on indium tin oxide (ITO) glass substrates by electrophoretic deposition and fissure formation techniques. A thin film of CNTs is deposited onto an ITO glass plate using an aqueous mixture of CNTs and the cationic detergent cetyltrimethylammonium bromide by applying a negative voltage to the ITO glass plate. Then, an additional layer of sodium dodecyl sulfate (SDS), an anionic detergent, is deposited on the CNT film. This is done using an aqueous solution of SDS by applying a positive voltage. Through the process of firing, CNTs with a clean surface are exposed in the fissures produced. No further treatment is needed to initiate or augment field emission. The CNT field emitters show relatively good field-emission properties such as high current density (11 mA/cm² at an applied electric field of 4.3 V/μm), low turn-on field (2.2 V/μm), and good stability (98 h for 10% degradation of current density from 400 μA/cm²).     

Separation of Colloidal Particles from Nonaqueous Media by Cross-Flow Electrofiltration

Abstract

Charge characteristics of particles in aqueous or nonaqueous slurries are known to play an important role in solids-liquid separation processes. We have been conducting a fundamental study on filtration of colloidal particles suspended in nonaqueous media, such as coal and tar sand slurries based on their charge characteristics. This paper presents results of such a study involving cross-flow electrofiltration of nonaqueous slurries. Data are reported for α-Al₂O₃ particles suspended in tetralin. The effects of feed rate, driving pressure and electrical field strength on the filtration rate, total deposition rate on the central electrode, and the efficiency of the filter are presented.

The outlet slurry concentrations were measured with a specially built X-ray densitometer. These data are analyzed by a mathematical model using a Graetz-type analysis. The rate of deposition was found to be determined mainly by the electric field. The sludge flow near the central electrode significantly affected the efficiency of separation.     

静电强化乳液中油滴(纳米尺度)聚并过程(英文)

Oil droplets in nanometer scale which are dispersed in water cannot be separated easily.An attractive technique is carried out by electrical phenomena to demulsify oil in water emulsion.In this research,non-uniform electric field or dielectrophoresis(DEP) is applied to remove sunflower oil(which is dispersed in the water).Effects of temperature,time and voltage(using AC-electric field) were considered to get the highest DEP-force(Fdi) and the best results.The oil particles sizes with average of approximately 76 nm have been shown using a ZetaSizer Nano ZS,Model ZEN 1600(Malvern Instrument Ltd.).The maximum separation efficiency of 85% is obtained at the optimum temperature of 38 ℃ and voltage of 3000 V.