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.     

Influence of processing parameters on the surface quality of electrophoretically deposited alumina coatings on foam ceramics

Previous work showed that the electrophoretic deposition of coatings with aligned pore channels is technically feasible on Al₂O₃-C foams. In the next step, the amount and size of cracks in the sintered coatings should be reduced. The study revealed the drying conditions, alumina raw materials used, the chemistry of the foam skeleton and the sintering conditions as significant influences. The best drying procedure was freeze drying after sudden freezing in liquid nitrogen. Three alumina raw materials with different particle size distributions were tested with regard to linear shrinkage, number of cracks and number of channel-like pores. The CT 9 and CL 370 showed a low number of cracks, however CT 9 possessed almost no pores. The Al₂O₃-C foam skeletons electrophoretically coated with CL 370 and sintered at 1600 °C in air showed the best results with a low number of small cracks and high number of channel-like pores.     

Physical,mechanical, and biological properties of electrophoretically deposited lithium-doped calcium phosphates

In the present work, the preparation of sintered lithium-doped tricalcium phosphates was studied, along with their physical, mechanical, and biological properties. Calcium phosphates were shaped via the use of electrophoretic deposition (EPD), using colloidally milled dispersions of hydroxyapatite (HAp) particles. The dispersions were stabilised with monochloroacetic acid. Lithium was incorporated into the structure via an addition of lithium chloride, which also served to optimise the deposition process. The dispersions were milled colloidally for periods of 0–48 h. The colloidal milling resulted in two effects: i) disintegration of the commercial HAp powder (10 µm) agglomerates, ii) unimodal distribution of the HAp particles (~ 170 nm). The fine particles of the milled HAp dispersions accelerated the deposition rate, and increased the mass of the deposit. The reduced size of the initial particles, owed to the milling, led to the superior arrangement of the particles during deposition and to reduced porosity after sintering (1050–1250 °C). The HAp decomposed into tricalcium phosphate phases during sintering. At a sintering temperature of 1250 °C, grain growth occurred, which consequently resulted in a slight degradation of the mechanical properties (reduction in hardness and Young's modulus). In contrast, the hardness and Young's modulus increased as the dispersion milling time increased (smaller grain size after sintering); however, the fracture toughness did not change. The results of the biological testing confirmed the bioactivity of the material through the growth of the apatite layer in the simulated body fluid (SBF), and the biodegradation of the prepared materials in the Tris-HCl solution. With regard to the preparation of compact lithium-doped tricalcium phosphates, the best results were obtained in the case of the sample that utilised the dispersion that was milled for 48 h, and was sintered at 1050 °C.     

Magnetic and microwave absorptive properties of electrophoretically deposited nano-CoFe2O4 as a 3D structure on carbon fibers

Cobalt ferrite nanoparticles were successfully deposited on carbon fibers as a 3D structure using electrophoretic method to study magnetic and microwave absorption properties. Three well stabilized suspensions from cobalt ferrite nanoparticles were prepared in acetone, ethanol and acetone-ethanol media: and iodine was used as a stabilizing agent. Constant voltage and time were taken into account to investigate their influence on coating morphology and thereafter microwave absorption property. Field-Emission Scanning Electron Microscopy, Differential Thermal Analysis and X-ray Diffractometer were employed to study morphology, thermal behavior and structure of powder, respectively. To investigate magnetic and reflection loss properties, Vibrating Sample Magnetometer and Vector Network Analyzer were used. Particle size distribution and zeta potential was obtained by Dynamic Light Scattering device. It was observed that by optimizing voltage amount and time to 25 V and 6 min, respectively; uniformity of coating was improved and this led to the highest attenuation of −10.25 dB in vicinity of 8–12 GHz.     

Constrained sintering and thermal ageing behaviour of electrophoretically deposited Yb2Si2O7 environmental barrier coating

The oxidation of SiC and the formation of a thermally grown oxide layer (TGO) limit the lifetime of environmental barrier coatings. Thus, this paper focuses on the deposition of denser Yb₂Si₂O₇ coatings using electrophoretic deposition to reduce the TGO growth rate. The findings showed densification for Yb₂Si₂O₇ can be achieved with an optimized sintering profile (heating/cooling rate, temperature, and time). However, the addition of 1.5 wt% of Al₂O₃ to Yb₂Si₂O₇ promoted densification and lowered the required sintering temperature, 1380 °C using 2 °C/min heating/cooling rate for 10 h provided efficient coating density. Moreover, adding Al₂O₃ reduced the TGO growth rate by more than 70 % compared to the Al₂O₃-free coatings, without cracking in TGO after 150 h of thermal ageing at 1350 °C. Results within this study suggest electrophoretic deposition with Al₂O₃ addition produces promising Yb₂Si₂O₇ environmental barrier coatings on SiC substrate with low oxidation rates and increased lifetime.     

Structural changes of hydroxyapatite coating electrophoretically deposited on NiTi shape memory alloy

The surface of the NiTi shape memory alloy was functionalized through the deposition of hydroxyapatite (HAp) coatings using the electrophoretic method (EPD). The electrophoresis carried out at the voltage of 40?V during the time of 120?s did not affect the crystalline structure of the initial HAp powder and, at the same time, ensured obtaining a homogeneous layer without visible cracks or discontinuities. Next, the coatings were subjected to heat treatment at 800?°C for 2?h in vacuum, wherein the applied conditions did not affect the decomposition of the deposited hydroxyapatite. The heat treatment resulted in the formation of carbonate apatite (C-HAp) in the HAp layer and in ceramic particles’ coalescence. Changes in the morphology and roughness of the layer as well as partial decomposition of the NiTi substrate parent phase into Ti₂Ni and Ni₄Ti₃ phases were also observed.     

A study on fouling mitigation using pulsing electric fields in electrodialysis of lactate containing BSA

Fouling tendency in electrodialysis was investigated using the electrochemical and physical properties of the foulants and ion exchange membranes. It was found that bovine serum albumin (BSA), a large molecular weight protein, fouled the AMX membrane irreversibly by deposition on the membrane surface. Electrodialysis experiments of lactate with 1.0 wt% of BSA were performed using the square wave powers at different frequencies to examine the pulsing power influences as a fouling mitigation method, and the results were compared with the data obtained using the DC power. Reduced fouling potentials were observed when the square wave powers were used in the electrodialysis of lactate and confirmed the membrane fouling index for electrodialysis (EDMFI). The pulsing electric fields enhanced the mobility of the charged particles in the fouling layer and decreased the electric resistance of the electrodialysis cell. It was clearly observed that the pulsing electric fields with different frequencies reduced the fouling potentials, and consequently the power consumption was reduced significantly as a fouling mitigation method.     

外加电场对静电流化床中颗粒运动与床层粘壁的调控机制

对流化床反应器中的颗粒运动行为进行调控可达到强化反应器性能的目的。通过冷模实验研究了直流/交流电场对静电流化床中颗粒运动的影响规律与影响机制,建立了通过外加电场调控静电流化床中床层粘壁的方法。结果表明,在低场强条件下,库仑力主导外加直流电场对颗粒运动的影响,由床层壁面指向床层中心的外加直流电场使得颗粒运动强度和轴向颗粒运动分率降低,而由床层中心指向床层壁面的外加直流电场则作用相反;在高场强条件下,极化力主导外加直流电场对颗粒运动的影响,使得颗粒运动强度减弱。在外加交流电场中,无库仑力存在时,极化力仍在高电场强度下使得颗粒运动强度减弱,但当库仑力存在时,电场强度和方向的周期性改变使得颗粒发生周期性摆动,颗粒运动强度增强。在本文的实验条件下,外加交流电场是一种控制床层粘壁的良好方法。2.5 kV/cm、50 Hz的正弦交流电场使得床层粘壁下降76%。研究结果可为聚烯烃流化床反应器的安全运行和过程强化提供指导。