Porous ceramic

Details Porous ceramic Porous ceramic

2001-06-08

2004-05-18

Marcantoni, Paul (Department: 1755)

Compositions: ceramic

Ceramic compositions

Pore-forming

C501S012000, C501S103000, C501S127000, C423S608000, C423S610000, C423S624000, C423S625000

Reexamination Certificate

active

06737376

ABSTRACT:

This invention relates to a porous ceramic and to a process for producing it.
Highly porous ceramics are increasingly being used as filter systems, as implants in medical technology and as supports for catalysts. There are numerous processes for producing them, only two of which will be cited here:
The production from aerogels and the burning-out of previously admixed organic additives.
The areas of use of ceramics such as these depend on their chemical and thermal stability, on their permeability, on their specific surface and on the length of the diffusion paths to their active surface.
The materials are selected based on requirements imposed by their area of use and stability; their pore size, pore size distribution and pore shape are selected based on the requirements of permeability, diffusion path and surface area. The importance of these properties to the use of these ceramic substances is clear: good stability enables them to be used even with aggressive media and at high temperatures, whilst good permeability results in a low pressure drop during operation and thus facilitates low energy consumption; a high specific surface results in a high density of adsorption centres and/or reaction centres; short diffusion paths enable active centres to be reached by flows of material in reasonable timescales.
Unfortunately, only partial success has been achieved hitherto in optimising these four important properties simultaneously in order to obtain high stability, a low resistance to flow, a high specific surface and short diffusion paths. Good permeability cannot be achieved with randomly oriented powdered materials or random arrangements of materials which are subsequently sintered, because these are macroscopically isotropic and also exhibit a high resistance to flow (one known example of a sintered, randomly oriented powdered material is a glass frit which is fused into chemical apparatuses). Instead, macroscopically anisotropic arrangements of particles are required, and what is required in practice is therefore systems of tubes or capillaries which are open at both ends and which have aperture diameters of the same magnitude. However, it has hitherto only been possible to produce systems such as these with tube and/or capillary spacings which are large in absolute terms. The regions between the capillaries/tubes can only be reached via long diffusion paths. For this reason, a high specific internal surface of the wall material is incapable of having the desired effect, since it is only the edge regions thereof around the tubes/capillaries which can be reached by flows of substance within a reasonably short timescale.
Currently, ceramics through which capillaries or tubes pass can only be produced by extrusion methods. The smallest diameter which can thereby be achieved is 200 &mgr;m. The spacings between the capillaries/tubes are about 600 &mgr;m. commercially available. Even the patent and scientific literature contains no references to structures such as these.
It is only the document EP-A-0479553 that describes porous ceramics with a high porosity and a narrow pore size distribution which are obtained by preparing a dilute slurry of a ceramic starting material in a solution of a high molecular weight organic compound such as ammonium alginate, which can be converted into a gel by reaction with an acid or with tri- or polyvalent cations or by heating or cooling. The slurry is brought into contact with a liquid or with a gel in which the acid or the tri- or polyvalent cations are present, or is heated or cooled, in order to obtain a ceramic gel substance which is subsequently calcined. These porous ceramics exhibit improved resistance and mechanical strength and are thus suitable as high-temperature catalyst supports.
The underlying object of the present invention is therefore to provide a ceramic body with a high stability, a low resistance to flow, a high specific surface and short diffusion paths, as well as a process for the production thereof.
The present invention relates to a porous ceramic, which is produced by:
a) mixing an aqueous solution of a suitable ionotropically orientable polyanion, either with
oxides, hydroxides or hydrated oxides, which are present in the form of a sol, of the metals Al, Zr, Ti and Nb,
or with finely crystalline oxides, hydroxides or hydrated oxides, which are present in suspension, of these metals,
or with finely crystalline tricalcium phosphate or apatite which are present in suspension,
b) bringing the mixed sol obtained as in a) or the suspension obtained as in a) into contact with a solution of a salt of a di- or trivalent metal cation in order to produce an ionotropic gel body, solution of a salt of a di- or trivalent metal cation in order to produce an ionotropic gel body,
c) compacting the gel body by introducing it into electrolyte solutions which further enhance the syneresis of the polyelectrolyte which was originally formed,
d) washing the gel body with water and subsequently impregnating it with a readily volatile, water-miscible solvent,
e) freeing the anhydrous gel body obtained as in
d) from the readily volatile water-miscible solvent,
f) burning out the organic constituents from the dry gel body obtained as in e),
g) sintering the product obtained as in f).
Ionotropic gels are formed when a dilute aqueous solution of a suitable anionic polymer, for example a solution of a sodium alginate or of a sodium pectinate, or of sodium cellulose xanthogenate, sodium xanthate or sodium hyaluronate, is brought into contact with a solution of a divalent cation such as Cu
2+
or Ca
2+
or with a solution of a trivalent cation such as Al
3+
or La
3+
. This is effected, for example, by adding the solution of the polyanion drop-wise to the solution of the metal cation or by adding the solution of the metal cation drop-wise to the solution of the polyanion, or by coating one solution with the other in the absence of convection. The proportion by weight of the polyanion in the sol can range between 0.25 and 5.0 percent by weight. Proportions by weight from 0.5 to 2.0 percent by weight are particularly suitable. The concentrations of the metal salt solutions are greater than 10
−3
M and are less than the respective saturation concentration of the salt in water. Concentrations between 10
−1
and 2 M are most suitable. After the formation of a membrane-like precipitate at the phase boundary between the two liquids, which is termed the primary membrane, oriented diffusion occurs of the low molecular weight electrolyte into the solution of the polymer. However, the precipitation which continues to occur at this location does not result in amorphous precipitate, but in a gel which is structured in three dimensions.
Regularly arranged capillaries are then formed, which are of practically identical size and which are circular in cross-section, the walls of which capillaries consist of the precipitated product and the lumina of which absorb the water evolved during precipitation. The stability of the gel is therefore based on the fact that the di- or trivalent cations crosslink the molecules of the polymer with each other and thus impart a certain mechanical strength to the capillary walls. The capillaries are all parallel to each other in the direction of diffusion of the electrolyte and can reach a length of a few centimeters. The arrangement of the capillaries is almost perfectly hexagonal and their radii slowly increase in the direction of diffusion of the metal cation, with a gradient of about 5%. The diameter of the capillaries of the gel can be adjusted within wide limits via the viscosity of the polyanion and the type of polyvalent cation. The lower limit which could be achieved hitherto was about 5&mgr;m, and the upper limit was about 300 &mgr;m. If the gels are produced by coating the two solutions, their strength in most cases is sufficient to enable them to be cut—starting at the top—into slices, the smallest thickness of which is about ½ mm and maximum thickness of which can be about 2 cm. Du

Affiliated with

Also associated with

Rating

Porous ceramic does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Porous ceramic, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Porous ceramic will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3217378