Background:
Usually porous ceramics are made from aluminum oxide, silicon carbide or Zirconia.? Most porous ceramics have a natural ability to fill pores by capillary action. This makes porous ceramics water accepting, thus they also are referred to as hydrophilic material. This means the pores and channels of a ceramic have a highly charged pore surface that attracts and bonds the polar molecules of water and other polar fluids. The net effect is called “wicking” — the ability to pull fluids into the material and transport that fluid by capillary forces.? The pore size directly affects the ceramic’s air entry or bubbling pressure and hydraulic conductivity. The effective pore size is determined by the minimum orifice within a channel or pore.1
Some porous ceramic have 40-50% open porosity with a tortuous pore structure and is available in pore sizes ranging from 0.25 to 90 microns. Monolithic, single grade, aluminum oxide porous ceramic is available in 6, 15, 30, 50, 60 and 90 micron pore sizes. In addition, some ceramic membranes can use a medium pore substrate with a thin coating of fine porous ceramic membrane in 0.25, 1, 3 and 6 micron pore sizes.2
How are Pores Formed?:
Extracted from US Patent 7033527 “Highly porous ceramics fabricated from preceramic polymer and expandable microspheres, and method for fabricating the same“4
Generally, porous ceramics are fabricated in accordance with two procedures as follows:
First, a ceramic is mixed with a pyrolyzable material or volatile material. Thereafter, gases are evolved by the pyrolysis of the pyrolyzable material or volatilizing the volatile material, and the evolving gases form pores in the ceramic to fabricate a porous ceramic (see, e.g., U.S. Pat. Nos. 5,358,910 and 5,750,449).
In summary, after a ceramic and a preceramic polymer are mixed with each other by a ball milling process, the mixture is molded into a desired shape. The molded body is heated to fire combustible components and volatilize volatile components contained in the preceramic polymer (pyrolysis). The ceramic components contained in the molded body are sintered by heating, and the volatile components contained in the preceramic polymer are volatilized to form pores within the molded body, thereby fabricating a final porous ceramic.
However, this method has a disadvantage that when the content of the polymeric components is not less than 50%, the shape of the molded body may collapse due to softening and pyrolysis of the polymeric components. Accordingly, it is difficult to fabricate highly porous ceramics having a porosity of 70% or more. Further, uniform distribution of pores is difficult to obtain and pore size cannot be easily controlled according to the material properties.
Second, a porous ceramic can be fabricated by lowering the sinterability of a ceramic. This method is divided into the following two procedures. The first method is carried out by sintering a ceramic below optimum sintering temperature to lower the relative density of the ceramic, thereby forming more pores within the ceramic. However, since the porous ceramic thus fabricated is not sintered at optimum sintering conditions, mechanical properties such as strength may be greatly deteriorated.
A common material used as the preceramic polymer is polyethylene microspheres.? The advantage of using PE microspheres is that Polyethylene is a simple polymer that burns out clean and leave no residue in the ceramic, unlike many other plastics that may leave some residue.? Polyethylene will auto-ignite at temperatures above 410?C.5
Another approach using polyethylene microspheres is in gelcasting of dense and porour ceramics with a mixture of natural gelatine and polyethylene microspheres. as seen in “Gelcasting of dense and porous ceramics by using a natural gelatine”, published in the Journal of Porous Materials, Volume 16, Number 4 / August, 2009.3 The abstract follows:
Abstract
An improved gel-casting procedure was successfully exploited to produce porous ceramic bodies having controlled porosity features in terms of mean pore size, total pore volume as well as pore geometry. The gel-casting process in which a natural gelatine for food industry is used as gelling agent was firstly set-up to prepare dense alumina and zirconia components. Then, commercial PE spheres, sieved to select proper dimensional ranges, were added to the starting slurries as pore-forming agent. Both alumina and zirconia porous bodies were then produced, having a porosity ranging between 40 and 50?vol%. The fired components were characterised by spherical pores surrounded by highly dense ceramic walls and struts, having a homogeneous and fine microstructure. Their mean pore size was directly dependent on the sieved fraction of the starting PE spheres selected as pore-forming phase.
Conclusion:
Polymer microspheres offer an excellent solution to creating precise pore sizes in ceramics at reasonable prices.? Polyethylene microspheres offer the added benefit of minimal residue after firing, and the availability in wide size ranges from a few micron up to 1000um (1mm).??? Highly spherical microspheres have the added benefit of creating strong pores without any stress risers that might cause fracturing of the parent ceramic.
References:
1. www.porousceramics.org
2. www.refractron.com
3. “Gelcasting of dense and porous ceramics by using a natural gelatine”, published in the Journal of Porous Materials, Volume 16, Number 4 / August, 2009, DOI 10.1007/s10934-008-9212-0
4. Highly porous ceramics fabricated from preceramic polymer and expandable microspheres, and method for fabricating the same
5. IPCS Inchem chemical resource, properties of polyethylene ICSC: 1488
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