Kirsten Moritz

Electrophoretically Deposited Porous Ceramics and their Characterisation by X-Ray Computer Tomography

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.

Electrophoretic Deposition of Ceramic Powders-Influence of Suspension and Processing Parameters

The electrophoretic deposition (EPD) of AlN, Al2O3 and SiC, respectively, from ethanolic suspensions stabilised by suitable organic additives has been investigated. A main topic of this paper is the EPD in the direction of the gravitational force. Using an AlN suspension, the deposit weight depending on the experimental parameters was determined. The deposition was carried out under constant voltage or constant current conditions. Several factors influencing the effective electric field strength will be discussed. It could be shown that the conductivity of the suspension results mainly from the free ionic species. By EPD from the different suspensions, green bodies with monomodal pore size distributions were produced. The electrophoretic deposition of alumina from an ethanolic and from an aqueous suspension of the same composition led to similar results regarding the homogeneity of the particle packing in the deposits. The SiC suspension was also used for infiltrating woven carbon fibre mats by EPD in order to fabricate ceramic matrix composites.

Electrophoretic Method for Fabricating Porous Ceramics – Application to Different Oxide Materials

Electrophoretic deposition (EPD) with controlled gas evolution by electrolysis was used for producing ceramic green bodies with unidirectional pore channels. The method was applied to alumina, alumina-toughened zirconia, and mullite. It requires stable aqueous suspensions with sufficiently high particle charge for the electrophoresis and appropriate amounts of electrolyte for the gas bubble formation. An anionic polyelectrolyte and ammonia were shown to be a suitable additive combination for all three oxide powders. The amount of polyelectrolyte was chosen for each of the powders on the basis of electroacoustic measurements of the zeta potential. The addition of ammonia solution was necessary in order to obtain the desired porous structures.

Electrophoretic Deposition of Mullite Based Oxygen Diffusion Systems on C/C-Si-SiC Composites

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).

Synthesis and Electrophoretic Deposition of Mullite Precursor Sols for the Oxidation Protection of Carbon Fibre Reinforced Composites

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.