Computational¯uid dynamics calculations were carried out to ensure that a self-made tangential¯ow mode streaming potential measurement cell meets the hydrodynamic stipulations of laminar, steady and established electrolyte¯ow necessary for reproducible electrokinetic measurements. The calculations show that the cell design meets all of these conditions.Six carboxylated polysulfones with a range of different degrees of substitution (DS) from 0.26 to 1.74 carboxyl groups per polymer repeat unit were synthesized in a two-stage process of lithiation and carboxylation. Ultra®ltration membranes were made from both the unmodi®ed polysulfone and these hydrophilic materials. The zeta potentials of these membrane surfaces were determined in 0.001 M KCl solution as a function of pH. The curves show the theoretically expected pro®les for nonionic and weakly acidic materials. The growing in¯uence of the COOH dissociation on the surface charge formation is indicated by the¯attening of the curves at low pH values. The magnitude of the negative zeta potentials plateau values ranged from À52 to À20 mV. While unmodi®ed PSU has a plateau value of À52 mV this value decreases continuously with increasing DS to À20 mV for the PSU-COOH 1.74 material. It is suggested that this arises from a shift of the electrokinetic shear plane into the bulk electrolyte solution due to an extended swelling layer re¯ecting the enhanced hydrophilicity of these membrane surfaces. # 1998 Elsevier Science B.V.

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A numerical simulation is presented on the sintering of porous alumina structures prepared by a controlled sedimentation technique. By forming this functionally gradient material with a very broad powder size distribution, the samples were able to remain flat through sintering. This experimental result is reflected in the present simulation results, which incorporated particle size distribution effects. In general, sintering functionally gradient ceramics can often introduce defects. Despite these common problems, the asymmetric structures considered in this paper featured a vertical functionality of continuously overlapping broad powder size distributions in the structure. This arrangement served to homogenize sintering rates. Modelling presented in connection with this shows that such structures can be readily sintered without warpage or cracking. To demonstrate these effects, a finite element method numerical simulation was developed to model the sintering characteristics of porous asymmetric ceramic structures by incorporating the powder particle size distribution into the model as a field variable. This work presents novel advances in the sintering model such that the contributions to the desired product properties attributable to particle size distribution effects can be demonstrated. These additions to the model produce numerical results which properly match observed structural profiles of physical samples. #