AFM study of the incorporation of particles during electrodeposition

The incorporation of particles in electrodeposited coatings strongly depends on the surface properties of these particles. It is shown that hydrophilic particles do not incorporate due the presence of a hydration layer on their surface. This hydration layer prevents hydrophilic particles from making “real” contact with the electrode and hence particles and electrode remain separated by a small gap. During metal deposition, metal ions diffuse into this gap and are reduced on the electrode underneath the particle. This causes the particles to be pushed up by the moving metal/electrolyte interface, instead of incorporating in the growing metal deposit. This phenomenon is called “riding” and the extent to which particles ride depends on the hydrophilicity of the particles and on parameters which govern the diffusion of metal ions in the gap. It is shown that riding of hydrophilic particles can be suppressed by applying an external pressure on the hydration layer which destabilizes this layer. However, even in absence of an external pressure, destabilization of the hydration layer occurs if the metal grows more or less conformally around the particle. This is due to the increase of attractive surface forces with metal deposit thickness. Atomic force microscopy pull-off measurements of partially incorporated particles show that a critical deposit thickness is needed to incorporate a particle irreversibly. The critical deposit thickness decreases with increasing hydrophobicity of the particle.status: publishe

Synthesis and characterization of composite coatings for thermal actuation

The synthesis and thermal expansion of metal coatings containing phase change material (PCM) prepared by electrolytic deposition were investigated. Such a composite combines the thermomechanical properties of the PCM with the high thermal conductivity of the metal, and can be used as a thermal actuator material. This study used paraffin as the PCM and copper as the metal matrix. The paraffin was first microencapsulated by emulsion polymerization to obtain microcapsules with a diameter of 1 - 5 µm containing 90 vol% paraffin to facilitate the incorporation of paraffin in copper by electrocodeposition. The microcapsules were added to a copper sulfate bath up to a concentration of 500 g/L. The electrocodeposition was performed at room temperature with a current density between 2 and 5 A dm(-2). These composites were examined by scanning electron microscopy, differential scanning calorimetry, and vertical dilatometry. Coatings with 40 vol % of microcapsules and a heat capacity of 12 kJ kg(-1) during phase transformation were obtained. The thermal expansion of the composite showed a sharp increase in a small temperature range above the melting point. Although this behavior is ideal for thermal actuators, the effect decreased by thermal cycling. This remarkable thermomechanical behavior is explained by a thermoelastoplastic model for two-phase composites.status: publishe

Study of the deposit resistance during electrophoretic deposition

Deposition experiments in a Hull cell showed that high conductivity suspensions yield uniform deposits while low conductivity suspensions result in non-uniform deposits. This difference in deposition behavior is related to the resistance increase of the deposit during EPD. Impedance measurements during EPD showed that the ratio of the deposit resistance to the suspension resistance increases much more for high than for the low conductivity suspensions. They also showed that the total resistance of the EPD cell dropped almost to the suspension resistance after the electric field was turned off. This means that the deposit has no inherent resistance, but that its resistance during polarization is caused by the interaction of ions with the deposit and by the depletion of ions at the deposition electrode. The change in ion concentrations near the deposition electrode changes the acid/base properties of the particles in the deposit, as proven by adsorbed pH indicators on the particles. The change in acid/base behavior is quasi irreversible and results in a memory effect of the deposit resistance when the voltage is reapplied.status: publishe

Design of functional properties in electrogalvanised coatings

It is well-known that the texture of electrogalvanised coatings has an important effect on the final functional properties such as corrosion performance and deformation behaviour. The use of codeposition to incorporate exogenous particles might enhance some properties even further and even have an influence on properties which have been considered as out of reach before. The codeposition of zinc and polymeric particles has been described in the open literature, showing the possibility to improve paint adhesion and corrosion properties. In this paper the properties of electrogalvanised layers with anorganic particles like graphite and SiC (Silicon Carbide) will be described. The effect of the particles on functional properties will be assessed.status: publishe

The effect of electrolyte conductivity on electrophoretic deposition

In this work, the influence of electrolyte conductivity on the electrophoretic deposition of alumina particles from ethanol suspensions was studied. Deposition experiments in a Hull cell showed that high-conductivity ethanol-based suspensions yield uniform deposits, while low-conductivity suspensions result in nonuniform deposits. The difference in the deposition, behavior is due to the resistance increase over the deposit during polarization. Impedance measurements during electrophoretic deposition showed that during EPD the relative deposit resistance increases much faster for the high- than for the low-conductivity suspension. The impedance measurements also showed that the resistance increase dropped almost to the suspension resistance after the electric field was turned off. This means that the resistance over the deposit is caused by the interaction of ions with the deposit and by the depletion of ions at the deposition electrode. Negatively charged ions are depleted in the deposit by migration toward the positively charged counterelectrode, while positively charged ions undergo electrochemical reactions at the deposition electrode. This change in ion concentrations near the deposition electrode changes the acid/base properties of the particles in the deposit, as proven by adsorbed pH indicators on the particles. The change in acid/base behavior is quasi-irreversible and results in a memory effect of the deposit resistance when the voltage is reapplied.status: publishe

Growth of sputter-deposited gold nanoparticles in ionic liquids

The growth of gold nanoparticles (NPs) synthesized by sputter deposition on an ionic liquid surface is studied in situ in the bulk phase of the ionic liquids (ILs) 1-butyl-3-methylimidazolium dicyanamide [C1C4Im][N(CN)2], 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide [C1C4Im][Tf2N], 1-butyl-3-methylimidazolium tetrafluoroborate [C1C4Im][BF4], 1-butyl-3- methylimidazolium hexafluorophosphate [C1C4Im][PF6] and 1-butyl-3-methylimidazolium triflate [C1C4Im][TfO]. It is found that primary nanoparticles with a diameter smaller than 2.5 nm are present in the sample immediately after sputtering. Growth of these primary particles proceeds after the end of the sputtering process and stops when the nanoparticles reach a certain size. Depending on the viscosity of the ionic liquid this growth process can proceed several hours to several days. The growth speed is fastest for the least viscous ionic liquid and follows the trend [C1C4Im][N(CN)2] > [C1C4Im][Tf2N] > [C1C4Im][TfO] > [C1C4Im][BF4] > [C1C4Im][PF6]. It is also found that a higher concentration of sputtered gold results in faster growth of the gold nanoparticles. A discussion on the growth mechanism of sputtered gold NPs is included.status: publishe

Electrodeposition of silver-carbon coatings with low contact resistance and wear rate

Composite coatings of silver with self lubricating carbon nano particles were made by electrocodeposition. Ultrasonic disintegration during deposition was used to prevent coagulation of particles in the electrolyte in order to obtain a high and uniform particle incorporation. The effect of current density, particle concentration and rotation speed on particle incorporation was studied. Depending on the deposition parameters, a particle incorporation of 14 vol% could be obtained. The contact resistance of the coatings was measured during reciprocating fretting in order to study their behavior for electrical connector applications. It was found that there was a threshold value of 9 vol% carbon black particles in the coating above which the friction coefficient, the wear rate and the electrical properties drastically change. When there is less than 9 vol% of carbon black, the friction coefficient equals the friction coefficient of pure silver, the wear rate is large and the contact resistance shows spikes above 100 Ω during fretting. With more than 9 vol% of carbon black, a friction coefficient of 0.2 is obtained which is 4 times smaller than below the threshold value, the wear rate is lowered by a factor 14 and the contact resistance during fretting is below 1 mΩ without spikes.status: publishe

Direct numerical simulation of near-wall turbulent flow and ionic mass transport in electrochemical reactors using a hybrid finite element/spectral method

An experimental study and direct numerical simulation of near-wall turbulent flow and ionic mass transport was performed in a cylindrical electrochemical reactor for Reynolds numbers up to 18,000 (friction Reynolds number of 1245). The experimental part involved the determination of velocity gradients close to the wall of a rotating cylinder. These velocity gradients are determined by electrochemical mass transport measurements to a rotating cylinder electrode and to micro-electrodes embedded in a rotating cylinder. Simulation of the fluid flow with passive scalar is accomplished using a hybrid finite element method (FEM) in meridian planes coupled to a Fourier expansion in the azimuthal direction. It was shown that the method reproduces the turbulent flow statistics with high accuracy. Due to its high parallel efficiency and the possibility of stretched meshes in finite element planes near the wall, this hybrid method is suitable to study laboratory electrochemical systems. Coupling with mass and energy transport equations allows prediction of the concentration and temperature fields. This feature makes the model suitable for large-scale design and optimization of different electrochemical processes where accurate prediction of near-wall turbulence and ionic mass transport is required.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Metallic and bi-metallic Janus nanofibers: electrical and self-propulsion properties

Aligned polymeric nanofibers obtained via electrospinning were sputter-coated with metals on both their front and back sides. When both sides were coated with the same metal (gold, silver or platinum/palladium alloy), the nanofibers behaved as pure conductors. However, coating front and back of the fibers with different metals causes a potential difference when they are immersed in an aqueous solution. This potential difference propelled the Janus fibers in the direction perpendicular to their long axis when hydrogen peroxide was present in the solution.status: publishe