New Anisotropic Ceramic Membranes from Chemically Fixed Dissipative Structures

Abstract

The formation of highly ordered capillaries in alginate gels is due to a dissipative convective process resulting from opposing diffusion gradients and friction. Ceramic membranes with an anisotropic pore structure have been gained from this self-organization process by incorporating inorganic particles into the gel matrix, followed by subsequent ion exchange, drying, and sintering. The aim of this study was to overcome existing preparative deficiencies and to optimize the capillary structure and surface properties with respect to specific technical applications. A new method of ion exchange was introduced, and the sintering program was improved to obtain reproducible product quality. By controlling the parameters of the self-organization reaction, the overall porosity of the ceramic membranes was adjusted to selected values between 60% and 83%. Capillary sizes were varied between 8 and 50 microm. Further modification by metal plating, particle coating, or hydrophobization led to an extended spectrum of applicability of the ceramic membranes. For the first time, anisotropic capillary ceramics have been characterized in detail as to their technical use as catalyst supports, filter membranes, or other solid-fluid and solid-gas contact processes.

Preparation and properties of 3–1 type PZT ceramics by a self-organization method

“…3(b)-(e). It is known to us that the cation-induced gelification of alginate is limited by the unidirectional diffusion of Me 2+ metal ions through alginate sol, so the generation of capillary structures is a convective process resulting from two opposing diffusion gradients and friction between the contracting alginate chains and surrounding solution [20]. Therefore, the increasing viscosity of slurries with solid loading would definitely retard the mobility of the two exchanging counter ions ([Alg − ] and [Ca 2+ ]) during the dissipative process, and hinder the even diffusion of Ca 2+ into the slurry and the uniform cross-link of Ca 2+ with NaAlg, which resulted in the increase of pore size and deformation of pore shape.…”

Preparation and Properties of Porous Alumina with Highly Ordered and Unidirectional Oriented Pores by a Self‐Organization Process

“…The following equation shows the gelation reaction: The alginate chains are cross‐linked by the calcium ions. The unidirectional diffusion of the calcium ions through the sol causes orientation and densification of the alginate chains forms capillaries which are oriented parallel to the diffusion direction . The Ca 2+ took ̃36h to completely penetrate the sols and transform it into calcium alginate gels.…”

“…This flow pattern is mapped during the cross‐linking of the polyelectrolyte chains by the cations and leads to a structure of parallel capillaries filled with aqueous solution . Ceramic membranes with an anisotropic pore structure can be formed from this self‐organization process by incorporating inorganic particles into the gel matrix, followed by subsequent ion exchange, drying, and sintering . The advantage of this method can be seen in its low cost and environmental friendliness.…”

Novel Biomaterials with Parallel Aligned Pore Channels by Directed Ionotropic Gelation of Alginate: Mimicking the Anisotropic Structure of Bone Tissue

“…Other strategies are exchange of the gelling ions or different drying procedures. Freeze, air and supercritical drying were studied when the interest on ceramic membranes aroused (Weber et al, 1997) and was further optimised Eljaouhari et al, 2006). Investigations by micro computer tomography (µCT) revealed that the pore structure was destroyed by ice crystals during freeze drying whereas the structure remained intact when water was exchanged against tert.…”

“…Only Weber et al (1997) described the synthesis of ceramic membranes based on structuring via sol-gel-process of ionotropic gelation of alginate/powder-slurries followed by calcination. Ceramic processing for membrane manufacturing was studied with Al 2 O 3 or TiO 2 including development of adapted drying regimes for the wet composites applying method inherent shrinkage, followed by heat treatment to obtain a sintered ceramic without cracks (Weber et al, 1997;Dittrich et al, 2002;Eljaouhari et al, 2006). Dittrich et al (2002) for the first time synthesised such ceramics consisting of the mineral phase of bone, hydroxyapatite, with parallel aligned pores and investigated their structure by µCT in cooperation with Goebbels et al (2002).…”