Papers by Keyword: Electrophoretic Deposition (EPD)

Abstract: Recent developments demonstrated that liquid templates in the form of solid particles stabilized emulsions can be used to produce porous materials. The use of such emulsions offers the possibility to control the porous properties over a wide range of pore sizes and porosities for a variety of materials. In addition, the liquid nature of the template enables the formed products to be sintered without a low temperature debinding step. In this work, the electrophoretic deposition (EPD) of these liquid templates for the production of porous alumina is reported. The experimental parameters needed to obtain stable emulsions, their influence on the final porous properties, as well as the influence of the deposition parameters are discussed.

Abstract: We report the fabrication of p- and n-type thermoelectric oxide thick films laminated by insulating alumina using electrophoretic deposition and their thermoelectric performance. From the experimental studies performed for optimization of the thermoelectric performance in the p- and n-type mono-layers, the control of sintering temperature for densification and the usage of fine powder were effective for reducing the electrical resistivity of thermoelectric layers. These findings could be applicable also to the triple-layered thick films. When one assumes that two triple-layered films of p- and n-type thermoelectric materials are combined as unicouple of thermoelectric module, an estimated maximum output power was 20 times higher than a measured maximum output power of a previously reported multi-layered thermoelectric module. It was found that precise control of the microstructure in the thermoelectric layers is indispensable for development of the thermoelectric modules based on the electrophoretic deposition.

Abstract: The surface modification of SiC powder with an alumina precursor was achieved by a sol-gel method to apply the electrophoretic deposition (EPD) technique to the shaping of the SiC powder. The isoelectric point of the surface-modified SiC powder with sol-gel-derived alumina precursor was shifted to around pH 9, which is close to that of alumina. The alumina-coated SiC deposited on a cathodic substrate by EPD. The green compact of surface-modified SiC powder was sintered by hot pressing in an argon atmosphere at 2000oC.

Abstract: In this research aging behaviors of yttria stabilized zirconia (YSZ) in non aqueous suspensions, namely ethanol, isopropanol, n-propanol, acetylacetone and the mixture of ethanol-acetylacetone were investigated. For this purpose, electrical conductivity, electrophoretic mobility and suspension stability during aging time (6 to 7 days) were evaluated. Except for the ethanol-acetyl acetone mixture, each suspension contained 0 to 0.8 g/L iodine. It was revealed that, the alcoholic suspensions indicated lower conductivity in comparison with acetylacetone suspensions. However, acetylacetone suspension showed lower conductivity variation with time. Although iodine could improve the electrophoretic mobility of ethanol and acetylacetone suspensions, it had little effect on electrophoretic mobility of isopropanol and n-propanol suspensions. Also, it was indicated that iodine as a dispersant was not helpful for alcoholic suspensions stability. It was concluded that the mixture of acetylacetone-ethanol suspension was the best candidate in this study for electrophoretic deposition of YSZ, owing to its little behavior variation with the aging time, low conductivity and high electrophoretic mobility. High quality crack-free layers were electrophoretically deposited from this suspension on the substrate by applying 50 V/cm electrical field.

Abstract: The development of texture was studied during electrophoretic deposition in alumina suspensions containing plate shaped alumina particles. The mechanism of platelet orientation during EPD was examined with respect to the influence of the electric field, gravity and hydrodynamic forces. This was realized by using two different deposition cells, with vertically or horizontally positioned deposition electrode. The texture of the green deposit was further enhanced during sintering by templated grain growth in which the platelet shaped alumina particles were growing at the expense of the fine grained matrix. The sharp ‘fiber texture’ obtained after templated grain growth during sintering of the deposit was characterized by means of x-ray diffraction and Electron Backscatter Diffraction (EBSD).

Abstract: Planar zirconia green bodies with unidirectionally aligned pore channels were prepared by an electrophoretic deposition method using an effect which is usually disadvantageous for the EPD from aqueous suspensions: the formation of gas bubbles by electrolysis. Aqueous ZrO2 suspensions containing acetic acid to enable a sufficient gas generation were used for the EPD experiments. The influence of selected parameters – electrolyte content, applied voltage, and kind of deposition electrode – on the pore structure has been investigated. The green bodies were sintered at 1450 °C in air. Optical microscopy and X-ray computer tomography (CT) were used for characterising the porous structures. CT-investigations have the advantage of a three-dimensional characterisation of the samples by a non-destructive method. The resulting pore structure depended to a high degree on the experimental conditions of the EPD. The kind of the deposition electrode – platinum foil or platinum gauze on foil – had a strong influence on the arrangement of the pores. The EPD on Pt-foil led to randomly arranged pores, whereas very regular pore structures were obtained by the use of Pt-gauze.

Abstract: Anode for direct methanol fuel cell (DMFC) was fabricated on Nafion 117 membrane by electrophoretic deposition (EPD) method. An ethanol suspension containing Pt-Ru/C and Nafion ionomer was utilized, in which Pt-Ru/C and Nafion ionomer formed composite particles. The prepared electrode was tightly attached to the membrane without mechanical pressing and heat treatment. The electrode composition, i.e., the ratio between Pt-Ru/C and ionomer in the electrode, was controllable by Nafion content in the suspension, and strongly influenced on the DMFC performance. Accordingly, the anode with ionomer fraction of 26% exhibited the highest performance, which was more than twice as high as the performance attained in a standard electrode fabrication (hot press method).

Abstract: The use of carbon fibre reinforced carbon composites in oxidizing atmospheres is limited to temperatures below 400 °C. To benefit from their excellent mechanical strength that is still preserved at high temperatures, suitable oxidation protection coating systems have to be developed. Composites which are capillary infiltrated with Si and coated with SiC via chemical vapour deposition show significantly enhanced oxidation resistance. For the increase of service temperature above 1300 °C, high temperature stable materials with low oxygen diffusivities such as yttrium silicates have to complement the SiC coating. The electrophoretic deposition performed under constant current conditions leads to relatively high green densities and therefore good sinterability of the applied coatings. In this work we present the preparation of suspensions, their characterization regarding particle size and electrophoretic mobility for yttrium silicate powder prepared by the solid state method. Depending on particle charge and conductivity of the investigated suspensions iodine is employed to increase particle charge. The use of current densities between 0.5-5mA/cm² leads to smooth and homogeneous layers. Layers sintered as low as 1400 °C for 2h already show promising protection of the C/C-Si-SiC substrate during thermogravimetric analysis.

Abstract: Laminated materials are used for special applications where combination of properties of two components is needed. Ceramics is inherently brittle and above all superior properties (wear resistance, temperature durability, stiffness, low density and others) the brittleness is limiting factor for massive implementation in wide range applications. The laminated structure can be capable to overcome this handicap. Electrophoretic deposition (EPD) is the technique able to prepare ceramic laminated structures having strong interface between layers [1]. It is possible to prepare dense and crack free materials with tailored residual stresses controlled by layer thickness and deposition conditions by this technique. Crack propagation through layered composites based on Al2O3 and ZrO2 was studied. Cracks, produced by an indentation technique, propagated in direction to layer interfaces deflected towards the interface in the compressed layers and away from the interface in the layers containing tension stress [2,3]. Changes in the direction of crack propagation for the whole range of angles of incidence (0° - 90°) were described. The biggest change in the crack propagation was observed for the angle of incidence 45° for A/Z systems and was ca. 15°. The change in the crack propagation was independent on the level of residual stresses in the layers. Behaviour of indentation cracks observed in laminates under investigation was compared with the results obtained on standard SEVNB specimens having inclined the fracture plane with respect to the composite lamellar structure. This model condition helps to understand crack propagation in bulk material. All experimental work was supported by fractographical techniques enabling explanation of fracture micromechanism.

Abstract: Electrophoretic deposition of Al2O3 and ZrO2 powders from isopropanol suspension in the presence of monochloroacetic acid under constant-current conditions was studied. The similar charge and electrophoretic mobility of Al2O3 and ZrO2 in the suspensions was found. Adjusting to properly controlled kinetics of deposition deposits were prepared of pre-defined thicknesses. In view of the negative charge of Al2O3 and ZrO2 particles in the isopropanol suspensions used, the prepared layers were deposited on the anode and thus they were not affected by possible solvent electrolysis, which contributed to their defect-free and low-porosity structure.