Andreas Mattern

Tailored Microstructures by Multi-Component Pressure Filtration

A novel apparatus for computer controlled pressure filtration has been constructed. The precise flow rate of multiple ceramic suspensions of different compositions can be controlled. Suspensions are mixed before entering the filtration chamber. By dynamically adjusting the different flow rates, phase compositions anywhere between 100% phase A to 100% phase B can be realized as a function of the filter cake height with filtration pressures of up to 10 MPa. This is suitable for producing functionally graded materials with smooth changes of composition or layered structures where the changes are steep. Gradients and layers in porous alumina and in aluminazirconia have been produced and investigated. The process is suitable for all materials that can be handled by slip casting. Introduction Slip casting and pressure slip casting have a long tradition as a forming technique for parts with complex shapes that cannot be realized by dry pressing [1]. Pressure slip casting has been limited to producing homogeneous parts with no change of suspension composition. Slip casting on plaster of paris moulds on the other hand has been successfully used to realize composite ceramics by sequentially casting layers with varying slip compositions [2,3]. Efforts to automate the suspension mixing process for realizing continuous gradients have been reported [4]. An apparatus to realize gradients driven by gas pressure filtration has been designed [5], but in practice problems with sealing at the pressures required and a high solubility of the process gas in the suspension proved problematic. Based on this process a computer controlled pressure filtration apparatus has been built that uses metering pumps to drive the filtration. It combines the precision and speed of deposition of computer controlled pressure filtration with the versatility of the manual slip stacking process on plaster of paris moulds. Pressure Filtration Apparatus A novel computer controlled pressure filtration apparatus has been constructed. The setup is shown in Fig. 1 and described in detail elsewhere [6]. High precision metering pumps (Herbert Ott GmbH, Leonberg, Germany) are used to pump water into pressure vessels with an internal, non-permeable membrane. These are used to overcome the problem that ceramic suspensions cannot be pumped due to their abrasive nature. The vessels are filled with different ceramic suspensions which are displaced by the water, thus leading to an indirect pumping. The flow rates of the pumps can be controlled by the computer and are almost independent of pressure, showing a drop of less than 10% when pumping against a pressure of 10MPa as compared to pumping against ambient. The different suspensions are mixed in a Kenics type static mixer (Chemineer inc., Dayton, Ohio, USA) before being deposited on a metal filter. By adjusting the two flow rates the composition of the mixed suspension can be controlled at any ratio between 100% suspension A to 100% suspension B. A volume element of the cake dh of composition αβ, cross section A and a solids loading cαβ is Key Engineering Materials Online: 2004-05-15 ISSN: 1662-9795, Vols. 264-268, pp 169-172 doi:10.4028/www.scientific.net/KEM.264-268.169

Corrosion of Single Crystal Cordierite by Model Diesel Particulate Filter Ashes: Mechanisms and Orientation Dependence

Cordierite Mg2Al4Si5O18 is a material for diesel particulate filters (DPF) with high potential. Its resistance to a simplified model ash has been tested on single crystals at temperatures up to 1050°C, which are realistic for use under ‘worst case’ conditions. A mixture of Ca, Mg and Zn phosphates, which are typical main constituents of many DPF ashes, was used as model ash. Single crystals were examined in order to investigate the orientation dependence of attack due to the strongly anisotropic nature of the Cordierite crystal lattice. Strong corrosion by the ash occurs at 1050 °C, connected to excessive ash melting. The molten ash attacks Cordierite by fast dissolution of the substrate with melt (super)saturation within minutes. Anisotropy of the dissolution process could not be detected. The initial kinetics are dominated by saturation effects, which slow down corrosion. The saturated melt attacks Cordierite by reaction processes leading to the formation of new crystalline phases. This process is much slower than the initial dissolution process but may significantly contribute to the destruction of Cordierite substrates if large contact areas between ash melt and Cordierite exist.