Liquid purification or separation – Filter – Material
Reexamination Certificate
2000-05-31
2003-04-15
Kim, John (Department: 1723)
Liquid purification or separation
Filter
Material
C055S523000, C210S500260, C210S510100, C264S044000, C504S300000, C504S145000
Reexamination Certificate
active
06547967
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the field of ceramics and concerns a ceramic network, such as can be used, for example, as a deep-bed filter, more particularly as a molten metal filter, as a support for filtration, heat exchanger, regenerator, electrically heatable thermostat, catalyst support, burner element for radiant heaters and space heaters, reaction chamber fill element, sound absorber, stiffening element for panels, or as a ceramic reinforcement material for metal matrix composites (MMC's), and a method for the production and utilization thereof.
2. Discussion of Background Information
Ceramic networks in the form of open-cell ceramic foams are known.
Methods are known for the manufacture of such open-cell ceramic foams using the so-called “Schwartzwalder method,” which is used industrially and is the most common. This method is described in U.S. Pat. No. 3,090,094. In accordance with this method, the desired component is cut from an open-cell polymer foam and subsequently impregnated with a suspension of ceramic particles and water or solvent. Then the impregnated polymer foam is mechanically squeezed one or more times, and subsequently dried. Next the polymer foam is burned out, followed by sintering of the remaining ceramic coating.
Open-cell ceramic foam manufactured by this method is a replication of the cell-like polymer structure of the starting material. As a result of burning out the polymer foam, the remaining ceramic struts are hollow. The cross-section of these struts is three-cornered, and the shape of the cavities is also three-cornered in cross-section. The ceramic coating is often cracked at the edges of the cavities. The cavities and the cracks result in a very low mechanical strength. Because the susceptibility to cracking is increased even further by shrinkage of the ceramic coating during sintering, relatively low-shrink materials are used, but they exhibit high internal porosity after sintering. This likewise results in low mechanical strength. See J. Am. Ceram. Soc. 77(6), 1467-72 (1994).
Thus, the ceramic foams manufactured from polymer foams with the aforementioned method have cavities with a concave, three-cornered cross-section inside the ceramic struts. See (Cahn, R. W., Haasen, P., Kramer, E. J. (ed.): Material Science and Technology, Vol. 11, VCH 1994, p. 474. The shape of this cavity is very unfavorable for the mechanical strength of the struts in the ceramic foam, since the load-bearing area of the points of the triangles is only very small. Due to the susceptibility of the brittle ceramic to the formation of cracks, the very sharply pointed shape of the three-cornered cavities is also problematic, since cracks nearly always form starting from there, further decreasing the strength of the ceramic strus. See (J. Am. Ceram. Soc. 77(6), 1467-72 (1994). Consequently, the foams produced with the Schwartzwalder method have a low mechanical strength, which is disadvantageous for the aforementioned applications as well as for the handling and transport of such ceramic foams.
The foam materials used for molding are produced by foaming a mixture of various chemical components. During the reaction of the fluid components with one another, a gas is produced, which causes gas bubbles to form and grow in the fluid. Moreover, the starting components polymerize, increasing the viscosity of the fluid. At the end of the reaction, a solid polymer forms that contains a large number of gas bubbles (polymer foam). The size of the bubbles in the polymer foam can be controlled within certain limits by the choice of the starting components and by regulating the reaction.
By a subsequent treatment known as reticulation, the membranes separating the gas bubbles are completely removed by chemical or thermal means, creating the open-celled polymer foam required for manufacture of the ceramic. This foam now consists only of polymer struts that have formed between three adjacent gas bubbles. See Klemper D. and Frisch K. C. (Ed.): Handbook of Polymeric Foams and Foam Technology, Hanser 1991, p. 24.
As a result of the nature of gas bubble foaming, the surfaces of the polymer foam are always concave in shape. Thus, the cross-sections of the polymer struts forming the foam have the shape of triangles with concave sides having very sharply angled points. See Klemper D. and Frisch K. C. (Ed.): Handbook of Polymeric Foams and Foam Technology, Hanser 1991, p. 28/29. This is considered a law of nature for all foamed materials.
Also, the gas bubbles that occur during foaming of the polymers cannot be created in unlimited size. When the gas bubbles are too large, the foam collapses before polymerization has brought about solidification of the foam. See Klemper D. and Frisch K. C. (Ed.): Handbook of Polymeric Foams and Foam Technology, Hanser 1991, p. 9. The upper limit for the most commonly used polymer foam of polyurethane flexible foam is approximately 5 pores per inch (approximately 5 mm maximum cell size). Hence this also presents a limitation on the possibilities for using polymer foam for the manufacture of ceramic foam.
It is further known that the foam used is generally polyurethane foam. See (Am. Ceram. Soc. Bull. 71 (11) (1992). However, a disadvantage of the use of polyurethane as the starting structure for ceramic foam manufacture is that gases which are toxic or hazardous to health, e.g., isocyanates or hydrogen cyanide, can be released during the necessary thermal decomposition of the polyurethane. See J. Polymn. Sci. C, 23, pp. 117-125 (1968)
To somewhat mitigate the problems of mechanical strength, DE 35 40 449 and DE 35 39 522 propose applying multiple coatings to the polyurethane foam used. This increases the thickness of the ceramic struts and thus the mechanical strength of the sintered ceramic foam as well.
The increased process cost for the multiple coating is problematic. Furthermore, the ceramic coating has only low strength prior to sintering, and consequently the mechanical loading of the coated polymer foam necessary for separating the excess suspension during multiple coating frequently leads to new defects in the coating. In principle, however, multiple coating also does not eliminate the disadvantage mentioned of unfavorably shaped concave three-cornered cavities of the struts.
It is likewise known to use ceramic fibers as monofilaments or multifilaments for the manufacture of porous ceramics: which fibers can be laid, knitted, sewn or glued. See IChemE Symposium Series No. 99, pp 421-443 (1986); MTZ Motortechnische Zeitschrift 56(2), pp 88-94 (1995).
A disadvantage here is that such ceramic fibers are difficult and expensive to produce, and thus are very expensive, and are difficult to process since they are very brittle. For example, knitting techniques can be used only to a limited degree here. Hence, only a limited selection of ceramic materials may be used for such fibers, which makes it difficult or next to impossible to modify the properties of the porous ceramic produced therefrom. Moreover, such porous structures are flexible since the fibers are not joined to one another at the contact points. This is disadvantageous in the case of filtration or mechanical loads, since these ceramics are not very stiff overall and, in addition, fiber abrasion is produced, especially with multifilaments.
Joining of such fibers can also be undertaken, see U.S. Pat. No. 5,075,160; although this is only of interest for the typical applications if ceramic joining is created. This, too, is difficult and expensive to achieve, generally using CVD or CVI techniques, but the choice of materials is again very limited.
In addition, it is known to manufacture open-pored materials from polymer fibers, natural fibers, or carbon fibers, and then to convert them directly to a ceramic material, e.g., by pyrolysis or with the addition of other chemical elements through the fluid or gas phase and reaction of the fibers with these elements. However, the conversion of these starting fibers to open-pored ceramics is complicated
Adler Jörg
Heymer Heike
Standke Gisela
Franhofer-Gesellschaft zur Forderung der Angewandten Forschung E
Greenblum & Bernstein P.L.C.
Kim John
LandOfFree
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