Papers by Author: Christos Argirusis

Abstract: . In the present work we used the sol-gel process to prepare Y2SiO5 precursor sols suitable for electrophoretic deposition (EPD). The sol synthesis was performed through the controlled hydrolysis of alkoxide solutions of tetraethoxysilane and yttriumoxoisopropoxide. During sol development emphasis was put on characterization of particles size and zeta potential of the formed aggregates. We succeeded in synthesizing a clear sol containing polymeric aggregates with acceptable particle charge. The electrophoretic deposition on glassy carbon or C/C-SiC slabs led to homogenous layers. At low sol concentrations micro cracks in the deposited layers were observed whereas higher concentrations led to thin and dense layers. During constant current EPD a constant voltage was recorded indicating that the deposited layer does not lead to an increase in resistivity in this kind of EPD system.

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: In this study we present the preparation of SOFC half cells by electrophoretic deposition. The cathode is deposited from a methyl ethyl ketone (MEK) suspension on dense apatite type lanthanum silicate (ATLS) pellets using a constant voltage. ATLS are a new class of electrolytes, which provides good oxygen permeability even at temperatures below 800°C. As cathode materials we used La2Ni0.8Cu0.2O4+δ (LNC) and La0.8Sr0.2Ni0.6Fe0.4O3 (LSNF). A conductive layer on the ATLS material was used to provide the necessary conductivity for electrophoretic deposition (EPD) process. For the preparation of suitable suspensions the charging behaviour and particle size of the different powders were investigated by means of zeta-potential and laser scattering measurements. When the electrophoretic mobility of the suspended particles was not sufficient iodine was added. Deposition kinetics were determined prior to preparation of the half cells. EPD parameters as well as sintering conditions were optimized in order to gain well adhering porous cathode layers.

Abstract: The present study uses electrophoretic deposition of yttrium doped barium cerate to prepare SOFC half cells on nickel/barium cerate (Ni/BaCe) and nickel / yttria stabilized zirconia (Ni/YSZ) substrates. Therefore we investigated charging and agglomeration behaviour of nanosized barium cerate powder in different solvents by means of a micro electrophoresis experiment and dynamic light scattering. Stable suspensions of barium cerate were prepared and deposition kinetics was examined prior to the preparation of thin membranes on the two cermets. Tests with different solvents and additive combinations showed that methyl-ethyl-keton (MEK) with addition of polyacrylic acid (PAA) is the best combination for the use of the suspension for electrophoretic deposition. Electrophoretic deposition in this case is a simple and low cost method to produce homogenous and dense layers in the scale of microns.

Abstract: In this work we investigated the possibility to use electrophoretic deposition from organic solvents in order to deposit catalytically active surface-stabilized nanoparticles on a substrate. This permits to functionalize a surface by depositing a catalytic layer or simply offers a way to collect the synthesized nanoparticles e.g. for subsequent characterization. As an example, Co nanoparticles were prepared using a sonochemical method, and subsequently deposited on a substrate by electrophoretic deposition. We used ionic liquids (IL) in order to increase the ζ-potential of the nano particles.

Abstract: Ionic Liquids (IL) are novel materials with a wide range of applications. The variable combination of organic cation and inorganic anion offers the possibility of tuning their properties. In the present investigation we test the application of different ionic liquids as charging agents for the preparation of stable oxide suspensions for direct use in electrophoretic deposition to produce oxide layers on different substrates. Main objective of these investigations was to prove that this substance class constitutes suitable additives for successful electrophoretic deposition (EPD). As one key parameter for the EPD process is the particle charge, a big part of our investigations involved the measurement of the zeta-potential of in alcoholic suspension under the influence of phosphonium and imidazolium based ionic liquids using a micro-electrophoresis measurement

Abstract: Yttria-stabilized zirconia (YSZ) ceramic is considered as an attractive matrix for nuclear applications, such as inert matrix for the destruction of excess plutonium or good host material for nuclear waste storage. Some actinide elements in high-level radioactive wastes can be simulated by cerium as tetravalent actinide, and gadolinium as trivalent actinide or neutron absorber. The present work is focused on the diffusion study of Ce and Gd in YSZ single crystal and high density polycrystals. A thin film of Ce or Gd was deposited either by spin-coating method or by physical vapour deposition on the surface of polished samples. The diffusion experiments were performed from 1173 to 1673 K under air. The Ce or Gd diffusion profiles were determined by secondary ion mass spectrometry. The experiments led to the determination of effective diffusion coefficient, Deff, bulk and grain boundary diffusion coefficients, DB and DGB. The dependence of these diffusion coefficients on temperature is described by means of Arrhenius equations and the diffusivity is compared with literature.

Abstract: The preparation of mullite coatings for the oxidation protection of carbon fibre reinforced composites using a combination of sol-gel synthesis and electrophoretic deposition (EPD) has been investigated. Mullite precursor sols were synthesised by controlled hydrolysis and condensation reactions of the metal alkoxides TEOS (tetraethoxysilane) and Al(OBus)3 (aluminiumtri-sec-butylate). The structure and properties of the mullite precursor were strongly influenced by the synthesis parameters, especially by the water to TEOS ratio (rw/Si) and the pH value of the water. A variety of synthesis conditions was tested for optimising the mullite precursor sols regarding their suitability for the electrophoretic deposition. The electrokinetic behaviour of the sols and the charging of the sol particles which is necessary for a successful EPD were investigated by measurements of the Electrokinetic Sonic Amplitude (ESA signal). 29Si CP/MAS NMR measurements were used to get information about the coordination of the silicon and the homogeneity of the Al/Si distribution in the precursors. Heat-treated samples were characterised by X-ray diffraction for investigating the mullite formation. Coatings prepared by EPD and sintering at 1300 °C in Ar enabled an effective oxidation protection in the temperature range 1200 °C ≤ T ≤ 1550 °C.

Abstract: Combining sol-gel synthesis of 3/2 mullite through hydrolysis and condensation of tetraethoxysilane and aluminum-tri-sec-butylate with electrophoretic deposition (EPD) yields sufficiently thick and homogeneous layers which transform into mullite at T ≥ 1000 °C. The characterisation of the mullite precursor during synthesis was performed through electroacustic measurements. The protectiveness of the deposited mullite layers was tested in air in the temperature range 1200 °C ≤ T ≤ 1550 °C by means of isothermal thermogravimetric analysis for up to 200 hours. Comparing the oxidation rate of mullite coated C/C-Si-SiC samples to that of uncoated reference samples clearly demonstrated that mullite offers a significant improvement to the oxidation resistance of the uncoated material. At temperatures above 1600 °C the protectiveness of the deposited layer is reduced due to the existence of a liquid phase and the formation of CO bubbles above the cracks in the SiC layer. In order to prolong the protectiveness of our mullite layers at higher temperatures we deposited an additional layer from a suspension of mullite precursor with 5 wt. % of Al2O3 powder. The protectiveness of so obtained mullite and mullite/ Al2O3 layers was also tested under cyclic conditions at 1500 °C and 1550 °C. These experiments clearly demonstrated that all samples withstood at least for 4-10 cycles which were performed subsequently in different time intervals (from 2-3 days to 1 h).

Abstract: Solid oxide fuel cells with an electrode supported thin film electrolyte are a promising alternative to electrolyte supported single cells because of decreased electrolyte resistance. The electrophoretic deposition (EPD) of the electrolyte was performed on A-site deficient La0.75Sr0.2MnO3-δ (ULSM) from three different suspensions: (Y2O3)0.08(ZrO2)0.92 (YSZ), (Ce0.9Gd0.1)O1.955 (GDC) and La0.9Sr0.1Ga0.8Mg0.2O2.85 (LSGM) in acetylacetone and isopropanol. The thickness of the deposits was controlled by varying the conditions of the electrophoretic deposition. Because of porosity reduction in the cathode layer during sintering of the electrolytes we deposited porous cathodes (La0.8Sr0.2MnO3-δ (LSM) and La0.6Sr0.4Fe0.8Co0.2O3-δ (LSCF) ) as well as anodes (NiO/YSZ and NiO/GDC) on dense YSZ foils by applying a conductive layer on top of the surface part to be coated.