Preparation of Al2O3, TiO2 and Hydroxyapatite Ceramics with Pores Similar to a Honeycomb Structure

Abstract

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 ...

Processing of porous ceramics: Piezoelectric materials

“…Unidirectional, aligned continuous pores were obtained 11 by using mercerized cotton thread as the fugitive fiber; a pore diameter of 165 m and 34% open porosity in the alumina body was obtained and the material still retained flexure strength as high as 155 MPa. A pore structure similar to a honeycomb results from the gelling of alginate by cross-linking with multivalent metal ions 46 ; gelling starts from the upper surface of the suspension, dispersed with Na alginate and maintained at controlled pH, and the primary gel layer works as a selective membrane for the passage of metal ions. The slurry gradually transforms into a gel with the formation of capillaries in the direction of diffusion of the metal ions.…”

Scaffolds for Hard Tissue Engineering by Ionotropic Gelation of Alginate–Influence of Selected Preparation Parameters

“…Mixing the sols with other ceramic powders such as titania, zirconia, or alumina and subsequent sintering of those gels lead to ceramics with a regularly oriented and channel‐like pore structure, but shrinkage is inherent to the sintering process and pores become too small for cell ingrowth 12,13 . Furthermore, HAP sintered at high temperatures loses the ability to be easily resorbed by osteoclasts in vivo .…”

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

“…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).…”