Compositions: ceramic – Ceramic compositions – Devitrified glass-ceramics
Reexamination Certificate
1999-08-11
2001-05-22
Marcantoni, Paul (Department: 1755)
Compositions: ceramic
Ceramic compositions
Devitrified glass-ceramics
C501S080000, C501S081000, C501S088000, C501S097100, C501S125000, C501S127000, C501S128000
Reexamination Certificate
active
06235665
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to the formation of ceramic articles. More particularly, it relates to the formation of porous ceramic articles.
In the production of ceramic materials for use in refractory, kiln furniture, filtration, catalytic, bone implants and other applications, it is sometimes desired to reduce the overall density of the fabricated article by introducing porosity into the article during or after fabrication. The strategy employed for reducing the mass of the article after fabrication usually involves removal of material from the article by means of grinding, drilling, routing or other mechanical methods to physically remove material from selected locations. This usually takes the form of drilling holes, routing channels, etc. Reducing the mass of the material (per unit volume of space occupied by the fabricated article) during fabrication involves using a process which introduces porosity into the ceramic. This can be accomplished by various methods described in the literature.
Some of the basic patents assigned to Selee Corporation, assignee of the present invention, disclose a method to produce a ceramic article with a high volume percent interconnected porosity by impregnating a reticulated polyurethane foam with a ceramic slurry and heating to burn of the polymer and sinter the ceramic. This method can be used to produce various pore sizes and densities. The reported strengths for various ceramic materials fabricated in this manner lie in the 500-700 psi range.
Another method to produce low density ceramic kiln furniture is taught in U.S. Pat. No. 4,812,424, whereby a porous aluminosilicate refractory aggregate is fired. The aluminum metal, alkali silicate and alkali aluminate chemical reaction producing a large volume of small gas bubbles is combined with a sodium silicate-sodium aluminate hydrogel setting reaction which traps the hydrogen gas bubbles in the ceramic. The strengths of this material are approximately in the 500-1000 psi range.
U.S. Pat. Nos. 4,814,300, 4,846,906, 4,871,495, 4,878,947, 4,923,487, 4,963,515 and 4,976,760 are extensions of this basic technology to include membranes and are used in specific markets, such as diesel particulate traps and diesel filters.
European Patent Specification Publication No. EP 0 598 783 B1 discloses a method of preparing porous refractory articles by forming a dispersion comprising particles in a liquid carrier, introducing gas into the dispersion and removing the liquid carrier to provide a solid article having pores derived from the bubbles.
U.S. Pat. No. 4,889,670 discloses a method to produce porous ceramic parts by combining a mixture of 60-90 weight percent of a particulate ceramic with 10-40 weight percent of a latex polymer, whereby the mixture is frothed by mechanical means, shaped, set and sintered to produce the porous article.
It is also well known that porosity can be introduced into a ceramic article by incorporating various types of organic particles into the ceramic body. Upon firing, these particles are oxidized and leave behind voids in the material.
OBJECTS OF THE INVENTION
It is therefore one object of this invention to provide an improved method to produce porous ceramic articles which are stronger, more thermally shock resistant, possesses uniformly dispersed and highly controlled pore sizes, and which can be made more quickly and economically than presently available techniques.
It is another object of this invention to provide an improved method to produce porous ceramic articles so that the size and size range of the pores, and the size and size range of the interconnections between the pores, can be more closely controlled than with currently available techniques.
It is still another object of this invention to provide an improved method to produce porous ceramic articles so that the volume percent of the porosity and the distribution of pores throughout the articles can be closely controlled.
It is yet another object to provide a method for producing a coated porous ceramic article which will retain its coating through a large number of thermal cycles.
It is further another object of this invention to provide an improved method to produce porous ceramic articles so that the porosity extends to and through the surface of the articles rather than forming a solid skin on the surface.
It is another object to provide a porous ceramic article in accordance with the above methods.
SUMMARY OF THE INVENTION
In accordance with one form of this invention, there is provided a method for forming a porous ceramic article. A mixture of ceramic particles and pliable organic spheres is prepared in a liquid. Preferably, a suspension of the ceramic particles and pliable organic spheres is formed. Preferably, the spheres are hollow and are made of a polymer, such as acrylic. The mixture is formed into a shaped article. The shaped article is dried. The shaped article is then fired so that the ceramic particles are bonded such as by sintering, and the pliable organic spheres are burned off, resulting in voids in the shaped article.
In accordance with another form of this invention, there is provided another method for producing porous ceramic articles. A suspension of ceramic articles and pliable organic hollow spheres are formed such that the ceramic articles and pliable hollow polymer spheres are simultaneously suspended in a liquid, preferably comprising water. A shaped article is formed after a sufficient amount of water is added. A shaped article is formed by either slip casting, pressing, extrusion, or injection molding. The shaped article is dried to remove the water. The shaped article is then fired to allow bonding of the ceramic particles such as by sintering, and to burn off the pliable organic hollow spheres, resulting in uniformly distributed voids in the shaped article.
A range of porosities of up to 95% void volume may be achieved using these methods. The size of the voids may be preselected by selecting the appropriate size polymer spheres. The amount of porosity is easily controlled by the number of polymer spheres which are added. The size range of the pores can be closely controlled by controlling the size range of the polymer spheres which are used. The distribution of the pores in the ceramic is highly uniform due to the fact that the polymer spheres in the ceramic particles are preferably simultaneously suspended by the addition of the appropriate suspending agent.
The article may be coated, for example with another ceramic composition. It has been found that the coating will stay bonded to the article through a large number of thermal cycles. Similar coatings on other substrates do not adhere as well.
In accordance with another form of this invention, there is provided a porous ceramic article having a plurality of substantially spherical shaped voids. The voids are substantially uniformly dispersed throughout the article. The voids are interconnected with one another. For an article having a theoretical density in the range from 5% to 30%, the strength of the article is in the range from 700 psi to 4500 psi.
In another form of this invention, there is provided a porous ceramic article having a plurality of substantially spherical shaped voids therein. The voids are substantially uniformly dispersed throughout the article. The voids are interconnected with one another. A substantial number of the voids intersect with at least one adjacent void. A window is formed by the intersection. The window is substantially in the shape of a circle. The average diameter of the circle is in the range from approximately 11 microns to 22 microns.
In accordance with another form of this invention, there is provided a porous ceramic article having a plurality of substantially spherical shaped voids therein. The voids are substantially uniformly dispersed throughout the article. The voids are interconnected with one another. At least a portion of the article is coated. The coating may comprise a ceramic composition, such as zirconia. The coating will remain adhered to the
Butcher Kenneth R.
Pickrell Gary R.
Carter & Schnedler, P.A.
Marcantoni Paul
Porvair Corporation
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