Alginate gelation is one of few methods that can produce porous ceramics with oriented tubular pores. Alginates are well known as inorganic polymers that can be gelled by cross-linking with multivalent metal ions. This process allows the production of structured alumina, titania, and hydroxyapatite ceramics with an approximately honeycomb structure.The primary thin layer has the function of a selective membrane through which the slurry gradually transforms into the gel.The gel-like substance was dried by various methods such as air drying, supercritical drying, and freeze drying. The main problem is very high shrinkage occurring during the transition from the wet gel to the sintered body. The samples were fired at various temperatures and had a high porosity of up to 80 %. The bulk density, tensile splitting strength, and a narrow pore and capillary size distribution were measured. The ceramics have a bimodal pore size distribution when measured by mercury porosimetry. The first peak represents the pores between the capillaries, the second is caused by the capillaries themselves. The pores between the capillaries almost disappear with increasing sintering temperature.The spatial pore structure was investigated by serial sectioning and image analysis. An increase in pore size and homogenization of pore arrangement led to the formation of capillaries.This process offers new possibilities for preparing porous ceramics with uniform parallel capillaries, which might be useful as membranes, catalyst supports, gas or chemical sensors, or for implants.The starting powder was dispersed in aqueous solution at pH 6 with Na alginate to give a slurry. It is important to control the pH to produce stable slurries. Na alginate slurry is stable at pH 5±10. The average particle sizes of the powders are shown in Table 1 (Microtrac Ultrafine Particle Analyzer; Leeds & Northrup). The isoelectric points of the materials are also given in Table 1.Then a solution of divalent metal ions (Me 2+ ) are deposited onto the surface of the slurry.The slurry can be gelled by ion exchange of Na + in the alginate with divalent metal ions such as Cu 2+ , Cd 2+ , Pb 2+ , Ca 2+ , Zn 2+ , and Sr 2+ . The gelation reaction is 2 Na alginate (aq.) + Me 2+ ® Me alginate 2 + 2 Na + (1) Immediately a primary thin gel layer is formed. The primary gel layer has the function of a selective membrane, allowing the passage of the metal ions (Me 2+ ) but not other ions (such as ions of Na alginate and Al 2 O 3 ). Owing to diffusional control of Me 2+ transport through the membrane, the slurry gradually transforms to the gel resulting in the formation of capillaries in the direction of Me 2+ diffusion. The metal ions and also the anions have an effect on the size of the capillaries. [1] The concentration of electrolyte also influenced the capillaries: the higher the concentration the smaller and fewer the capillaries.After gelation, the primary membrane and the gel bottom are cut away. To remove Me 2+ impurities, ion exchange is carried out. [2±4] Figure 1 shows ...
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