Experimental and numerical analysis of electrophoretic deposition on charge selective surfaces in external electric fields

Abstract

In this paper, we present chronopotentiometric results of direct current (DC) membrane electrophoretic deposition (DC-M-EPD) experiments and analyse the influence of the parameters solid concentration, particle size and electrical conductivity of colloidal fumed silica model suspensions on voltage–current curves. The expected streamline patterns are numerically modelled based on coupled mass balances, Ohmic law, Navier–Stokes and Nernst-Planck equations. The results confirm that overlimiting current condition is an obligatory condition for the formation of EPD green deposits under suitable DC-M-EPD conditions. Furthermore, micro-EPD experiments in external alternating current (AC) fields (micro-AC-EPD) using video microscopy come to the conclusion that the observed formations of microvortices are in accordance to the numerically modelled streamline patterns. Finally, the formation of AC-EPD microdeposits exhibiting a torus-like microstructure is substantially explained based on microfluidic streamline pattern analysis.     

Studying surface charge and suspension stability of hydroxyapatite powder in isopropyl alcohol to prepare stable suspension for electrophoretic deposition

Abstract

Suspension stability of hydroxyapatite powder in isopropyl alcohol was investigated by measuring zeta potential. In this study, isopropyl alcohol was used as a suspension media, and nitric acid, polyvinyl butyral and polyethylenimine were the dispersing agents. Fine grain particles with mean particle size and surface area of 2·54 μm and 5·8 m² g^?1 respectively, were prepared. Stable suspensions were made by mixing 40 g L^?1 milled powders in isopropyl alcohol. The zeta potential of the suspensions was measured by using zeta sizer equipment. The results showed that by adding nitric acid and polyvinyl butyral to the system, negative charge was introduced, while polyethylenimine donated positive charge to the surface of particles and induced electrostatic and steric stabilisation. It was concluded that the suspension which was stabilised with 4 g L^?1 polyethylenimine revealed the highest zeta potential and stability which is suitable for cathodic electrophoretic deposition.     

Polymer interface changes in electrophoretic deposition

A Monte Carlo computer simulation model for the electrophoretic deposition of polymer chains on a discrete lattice is used to study the polymer density profile, interface growth, and its dependence on field, temperature, and molecular weight. The interface width (W) decreases WE^−1/2 on increasing the field (E). Width (W) depends non-monotonically on the temperature (T): a power-law decay is followed by a power-law increase on raising the temperature. Monotonic decay of the interface width with the molecular weight is possibly a stretched exponential. Conformation and dynamics of a tracer chain is used to probe its characteristics in interface to bulk region. The root mean square (rms) displacement of the center of mass of the tracer chain shows an ultra-slow motion, Rt^ν (ν0.1–0.01 at E=0.1–1.0) as the driven chain moves deeper from interface to bulk. Longitudinal compression of the radius of gyration (R_g) of the chain increases with the field; transverse components (R_gx, R_gy) are larger than the longitudinal component (R_gz). The transverse component (R_gx(y)) becomes oscillatory due to periodic squeezing at high fields as the field competes with the polymer barriers.     

Critical particle concentration in electrophoretic deposition

The role of particle concentration in electrophoretic deposition (EPD) was investigated with two different suspension systems. The first system consisted of positively charged TiO₂ nanoparticles dispersed in isopropanol with 1 vol% water. The second system consisted of negatively charged polystyrene (PS) microbeads dispersed in isopropanol. Constant voltage EPD was performed using suspensions with variable particle concentration (0.013–0.43 vol% TiO₂ and 0.06–11.4 vol% PS). Threshold concentration values were identified for both systems after EPD at 100 V (250 V cm^?1) for 1 min. Below these values the deposited mass deviated from the trend dictated by Hamaker's equation. Higher applied voltages and longer deposition times were tested and the results suggested that the threshold concentration did not depend on those parameters. A phenomenological model of particle deposition was proposed, which accounts for the local electrochemical conditions close to the substrate in relation to particle size.     

Effect of suspension medium on the electrophoretic deposition of hydroxyapatite nanoparticles and properties of obtained coatings

The suspensions of hydroxyapatite (HA) nanoparticles were prepared in different alcohols. The zeta potential of HA nanoparticles was the highest in butanolic suspension (65.65 mV) due to the higher adsorption of RCH₂OH₂⁺ species via hydrogen bonding with surface P3OH group of HA. Electrophoretic deposition was performed at 20 and 60 V/cm for different times. Deposition rate was faster in low molecular weight alcohols due to the higher electrophoretic mobility of HA nanoparticles in them. The coating deposited from butanolic suspension had the highest adhesion strength and corrosion resistance in SBF solution at 37.5 °C. The surface of this coating was covered by apatite after immersion in SBF solution for 1 week.     

Influence of anionic stabilization of alumina particles in 2-propanol medium on the electrophoretic deposition and mechanical properties of deposits

The ceramic dispersions were prepared using 0.85, 1.70, 4.25, 12.75 or 21.25 wt.% of monochloroacetic, dichloroacetic or trichoroacetic acid, 15 wt.% alumina and 2-propanol. The mechanism of anionic stabilization in 2-propanolic media was described. Alumina green bodies were prepared from the stable dispersion via electrophoretic deposition (EPD). It was found that increasing dispersion conductivity significantly influenced the EPD yields. The most effective electrophoretic depositions were performed from dispersions with conductivity in range 4.0–5.3 × 10⁻⁴ S m⁻¹. Deposits with the highest green density were prepared from the dispersion stabilized by trichloroacetic acid. This behavior was explained by low voltage drop during deposition. The surface roughness was high at low dispersion conductivity and with increasing acid concentration in dispersion the surface of deposits was smoother. The mechanism of particle arrangement in deposit was discussed. Influence of stabilizer amount in the dispersion on the hardness and fracture toughness was described.     

Fabrication of titania dense layers by electrophoretic deposition in aqueous media

Fabrication of titania dense layers by electrophoresis in aqueous media has been studied according to the suspension formulation. Stable titania suspensions with negatively charged particles are obtained by adding either the strong basis (C₂H₅)₄NOH, or the Tiron molecule or a salt of polymethacrylic acid. To prevent water electrolysis at the anode which is the collecting electrode, ethanol is added as cosolvent. A concentration of 10 vol% is sufficient to avoid gaseous emission at the anode and to keep a stable suspension suitable for electrophoretic deposition (EPD). The parameters influencing the deposit kinetic of particles are studied, such as the concentration of ethanol, of solid and of dispersant, and the current intensity applied. Finally, it is possible to fabricate layers with a relative density of 60% with a very narrow size distribution of pores.     

Relationship between the process parameters and the saturation point in electrophoretic deposition

We used the electrophoretic deposition (EPD) process to fabricate a composite with glass frit and investigated the EPD parameters to find the optimum deposition time by understanding the relationship between the process parameters of zeta potential (ZP), pH, deposition yield and saturation point in a slurry. A binder and a dispersing agent were mixed properly with glass frit (0.2–25 μm, d₅₀ = 8.77 μm) in an ethyl alcohol medium for the preparation of the slurry. The pH and ZP were in an inverse relationship to each other due to the generation of H₃O⁺ ions with the addition of the dispersing agent in the slurry. The acidic nature of the slurry resulted in a decrease of the pH and an increase of the ZP. Otherwise, the pH increased with the addition of the glass frit in the slurry because H₃O⁺ ions were absorbed on the glass frit. Therefore, the OH^? ions correspondingly increased. The saturation point of EPD was strongly correlated with the variation of the pH in the slurry; this is caused by a chemical reaction between the ethyl alcohol and the ions that make up the glass frit. An adjustment of the pH variation and the saturation point in the slurry can be established with respect to the optimum deposition time in the slurry.     

Effect of HCl on electrophoretic deposition of yttria stabilized zirconia particles in organic solvents

This study investigates the electrophoretic deposition (EPD) of YSZ particles onto a metal substrate from an organic solvent, the conductivity of which was manipulated by HCl additions. The green density is dependent on electrical conductivity and deposition time. It was found that a uniform coating with up to 67% relative green density could be produced after 10 min deposition from a 20 g/L suspension with electrical conductivity in the range of 10–15 μS/cm (0.5–0.7 mM HCl concentration). Direct measurements of the green YSZ coating density were supported by micro-indentation data using a spherical indenter.     

Fabrication of c-axis oriented zinc oxide by electrophoretic deposition in a rotating magnetic field

The fabrication of c-axis oriented zinc oxide was attempted by electrophoretic deposition (EPD) in a rotating magnetic field. The EPD was conducted in a small container which was placed on a turntable arranged in a superconducting magnet. The suspension was rotated at 0–90 rpm in a 12 T magnetic field during the deposition. The deposits were dried and then sintered at 1400 °C for 2 h. The degree of the c-axis orientation was evaluated by the Lotgering factor calculated from the X-ray diffraction data.     

Thermal residual stress gradients in an alumina–zirconia composite obtained by electrophoretic deposition

In order to understand the mechanical behavior of layered composites with compositional gradient, it is necessary to determine their state of residual stresses. Compositionally graded materials can offer the advantage of eliminating abrupt changes in composition between layers having different thermal expansion coefficient. The existence of a compositional gradient can reduce discontinuities in thermal residual stresses, something beneficial from the point of view of the mechanical properties.We present here a study of the microstructure and state of residual stressses in a layered material made of homogeneous layers of alumina and alumina–zirconia separated by thin (less than 300 μm) intermediate compositionally graded layers. The composite was obtained by controlled deposition of powders from solution using an electrophoretic deposition (EPD) method. The phase distribution and compositional gradient in the sintered composite were investigated using scanning electron microscopy (SEM). Thermal residual stresses generated during cooling after sintering were measured by using fluorescence ruby luminiscence piezo-spectroscopy and the profile of hydrostatic stress on alumina was determined at steps of about 300 μm along the direction of the compositional gradient, and at steps of about 30 μm in the compositionally graded layers. The obtained profile of hydrostatic stresses on alumina grains follows closely the profile of compositional changes along the layered composite. The presence of thin intermediate graded layers reduce significantly changes in stress in the layered composite.     

A note on the deposition of aerosol particles in a channel due to diffusion and electric charge

Flow suspensions of charged particles near the entrance to a parallel plate channel has been solved using a boundary layer analysis. Results for plug flow and Poiseuille flow are given.     

Beta-sialon phosphor deposits fabricated by electrophoretic deposition (EPD) process in a magnetic field

Phosphor deposits of β-sialon:Eu²⁺ were prepared by electrophoretic deposition (EPD) process within a magnetic field. Under the action of the magnetic force, which was parallel to the direction of the electric field of the EPD, the β-sialon:Eu²⁺ crystals were aligned along the c-axis of the hexagonal cell unit to form an oriented deposit via the EPD fabrication. Higher orientation degree was obtained at longer depositing time (300 s) and stronger applied magnetic field (12 T). The oriented deposit aligned along the c-axis obtained higher relative deposit density than the randomly fabricated deposit. Due to the improved relative density, the oriented deposit prepared within the magnetic field possessed an enhanced external quantum efficiency (η_ex). Also, because of different relative densities of the deposits prepared within and without the magnetic field, they presented different chromaticity coordinates.     

Effect of acids and bases on electrophoretic deposition of alumina and zirconia particles in 2-propanol

The effects of acids (monochloroacetic, dichloracetic, trichloroacetic and sulfuric acids) and bases (diethanolamine, triethanolamine, piperidine) on electrophoretic mobility and electrophoretic deposition (EPD) of particles of amphoteric Al₂O₃ and ZrO₂ in anhydrous 2-propanol were studied. It was found that the ζ-potential of Al₂O₃ and ZrO₂ particles had in acidic and alkaline 2-propanol media opposite charge to that in aqueous medium. This phenomenon was explained by the low dissociation constants of acids and bases in 2-propanol. This enables electrosteric stabilization of Al₂O₃ and ZrO₂ particles by acid anions and base cations. Similar electrophoretic behaviour of Al₂O₃ and ZrO₂ particles in 2-propanolic dispersions stabilized by MCAA allowed the preparation of compact, regular layered laminates with high cohesion at the interface of layers.     

Mechanical properties of textured,multilayered alumina produced using electrophoretic deposition in a strong magnetic field

The mechanical properties of ceramics materials can be tailored by designing their microstructures. It had generally been difficult to utilize a magnetic field for controlling the texture of diamagnetic ceramics because of their extremely small susceptibility; however, the possibility of controlling the texture by a magnetic field occurred with the development of superconducting magnets. We demonstrate in this study that alumina/alumina laminar composites with alternate crystalline-oriented layers are produced by electrophoretic deposition in a strong magnetic field and that the bending strength of the laminar composite depended on the direction of the multilayered microstructure.     

使用EXCEL进行PET装置热媒蒸发器的设计计算

通过对热媒蒸发器的特点进行分析,选取了合适的计算模型和公式,使用EXCEL电子表格的单变量求解和宏工具,方便地实现了热媒蒸发器的设计型计算。为实现程序自动迭代运算,文中还列出了通过EXCEL电子表格的数据处理功能对气相热媒和液相热媒参数在所使用的温度区间内进行连续化归纳的计算公式。     

Application of constant current pulse to suppress bubble incorporation and control deposit morphology during aqueous electrophoretic deposition (EPD)

Electrophoretic deposition (EPD) from aqueous suspension generally forms deposits containing enormous pores because of evolution of hydrogen gas at cathode and oxygen at anode due to electrolysis of water on application of DC electric field. We have demonstrated through this investigation on aqueous EPD of alumina suspension as an example that application of pulsed DC instead of the conventionally used continuous DC is a convenient and effective way to control and suppress the amount of bubble incorporation in the deposit. Bubble-free deposits of reasonable yield were obtained at suitable pulse widths and/or duty cycle. The deposit yield and bubble incorporation decreased progressively with decrease in the pulse width and duty cycle of the applied pulse current. A characteristic deposition window was found in the current vs. pulse width plot within which smooth and bubble-free deposits are obtained. The window is wider at low-applied currents compared to that at higher currents implying that it is easier to control the pulsed EPD at lower applied currents. No deposition occurred below the window whereas deposits with incorporated bubbles formed above the window. Possible mechanisms involved in pulsed EPD have been advocated on the basis of amount of hydrogen evolved/pulse due to the electrolysis of water. The discrete amount of H₂ evolved/pulse was higher for larger pulse widths leading to incorporation of more bubbles and vice versa. This was verified by monitoring the gain in weight of palladium (Pd) electrode used as cathode for electrolysis experiment since it is known to absorb hydrogen.     

Fabrication of particle and composition gradients by systematic interaction of sedimentation and electrical field in electrophoretic deposition

The systematic interaction of sedimentation and electrical field in electrophoretic deposition allows the tailoring of specific properties of deposited green bodies. This technique permits a selective deposition of the nanosized fraction of conventional powders with broad or non-monomodal particle size distribution, thus making preceding classification obsolete. Potential applications are coatings with a very smooth surface or the replication of microstructures or moulds which are filled with nanosized particles and subsequently with coarser particles as support in one process step. Also, graded structures can be fabricated with regard to particle size distribution, porosity and composition (e.g. zirconia toughened alumina). In this paper, the interaction of sedimentation and electrical field in electrophoretic deposition is described. In addition the effectiveness of the combined process will be shown.     

On the role of the indifferent electrolyte LiCl in electrophoretic deposition of hydroxyapatite from 2-propanol dispersions

In the present work, the effect of an indifferent electrolyte (LiCl) on the electrical conductivity of dispersions for the electrophoretic deposition (EPD) of thick layers (>1 mm) of hydroxyapatite (HA) with low surface roughness was studied. The indifferent electrolyte was added at concentrations of 0.10 and 0.25 g/L into dispersions, which contained HA particles, 0–21.25 wt% monochloroacetic acid (MCAA) and 2-propanol. The stability of the dispersions, which was determined by measuring the zeta potential, increased with the MCAA content in the dispersion (up to −31.4 mV). An addition of LiCl slightly reduced the stability of the dispersions but significantly increased their electrical conductivity. The increase in electrical conductivity caused the decrease in deposition rate during the EPD, which affected the deposition yields and deposit thickness. The relative density of the HA deposits increased with the MCAA content in the dispersion and further increased when 0.10 g/L of LiCl was added. Moreover, using a highly conductive dispersion, thick HA layers were deposited with low surface roughness and no surface cracks. An optimum content of indifferent electrolyte in the dispersion enables the preparation of relatively dense and thick (>2 mm) HA deposits with a low surface roughness (~3 µm) via EPD, which can be used in medical applications.