Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Composite having voids in a component
Reexamination Certificate
1999-05-24
2003-10-21
Dunn, Tom (Department: 1725)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Composite having voids in a component
C428S312600, C427S244000, C427S316000, C427S376100, C427S376200, C427S376600
Reexamination Certificate
active
06635339
ABSTRACT:
BACKGROUND OF THE INVENTION
a) Field of the Invention
The invention relates to the field of ceramics and is directed to an open-cell expanded ceramic with high strength such as can be used, e.g., for deep-bed filters, supporting bodies for filtration, heat exchangers, regenerators, electrically heatable thermostats, catalytic substrates or supports, burner elements for surface radiant burners and volume burners, high temperature reaction chambers, acoustic dampening or reinforcing material for panels for high temperature, and to a process for the production thereof.
b) Description of the Related Art
Processes are known for the production of open-cell expanded ceramics by the Schwartzwalder method, as it is called, which is utilized commercially and is most common. According to this process, the desired structural component part is cut out of an open-cell polymer foam and subsequently impregnated with a suspension of ceramic particles and water or solvent. Afterward, the impregnated polymer foam is repeatedly pressed out mechanically and then dried. The polymer foam is then burned out and the sintering of the remaining ceramic coating is subsequently carried out (U.S. Pat. No. 3,090,094).
Open-cell expanded ceramic produced according to this process is a casting of the cell-like polymer structure of the starting material. Because the polymer foam is burned out, the remaining ceramic webs or struts are hollow.
These struts have a three-edged cross section and the cavities also have a three-edged shape in cross section. The ceramic coating is frequently cracked at the edges of the cavities. The cavities and cracks result in very low mechanical strength. Since the contraction of the ceramic coating during sintering further increases susceptibility to cracking, relatively low-contraction substances are used, although the latter have a high internal porosity after sintering. This likewise leads to a low mechanical strength (J. A. Ceram. Soc. 77 (6), 1467-72 (1994)).
In order to increase strength, it has already been attempted to provide the open-cell expanded ceramic with greater strength by applying one or more subsequent coats to the ceramic struts of the foam before or after sintering the ceramic foam. These subsequent coats are applied by impregnating either the coated polymer foam or the sintered expanded ceramic with a ceramic slurry (suspension) (e.g., GB 2097777).
A problem consists in that the excess suspension cannot be pressed out of the cells of the expanded ceramic mechanically without destroying the now rigid foam structure; for this reason, very thin suspensions with a low solid content must be used which can drip without leading to a closure of the cells and accordingly to a reduction in the number of open cells. Thin suspensions with low solid content have the disadvantage that the coating of ceramic particles is only thin and this coating is interrupted during the drying of the suspension by drying cracks or, during the sintering, by contraction cracks.
Further, the efficacy of an external multiple coating of the cell struts is only slight because the unfavorable structure of the cavities in the struts is not overcome. Further, a subsequent coating reduces the volume of the free cells of the expanded ceramic, which is disadvantageous for most applications (J. Am. Ceram. Soc. 77 (6), 1467-72 (1994), page 1467, second paragraph, left).
A process is known from EP 0369 098 wherein a presintered open-cell ceramic foam is impregnated with a suspension of colloidal refractory oxide and a refractory oxide powder under a vacuum and, after the suspension drips off, is dried and sintered. Accordingly, in addition to a coating of the cell struts (as was mentioned above), the cavities of the cell struts must also be filled with the suspension during impregnation. As was already described above, a very thin suspension must be used for the impregnation, so that only a very small proportion of ceramic particles can reach the cavities of the struts during the impregnation. Therefore, the proportion by volume of the ceramic phase after sintering is only extremely small. Further, as was mentioned above, the layers of low-concentration ceramic suspension crack repeatedly during drying and sintering, so that their reinforcing efficiency is further diminished.
It is also known to produce an open-cell carbon foam by means of CVD/CVI. In this case, the carbon foam is produced by pyrolysis of an open-cell polymer foam and is subsequently cut according to the desired geometry of the structural component part. The carbon foam is then coated with ceramic components by means of CVD/CVI (Ceram. Bull. Vol. 70, No. 6,1991, 1025-1028). A production process of this kind is very expensive and requires elaborate plant technology. The expanded ceramic produced by this process is also a casting of the cell-like polymer structure of the starting material. The ceramic struts are formed of a tight layer of the applied ceramic, wherein the struts are formed internally by the original carbon skeleton. Due to the tightness and strength of the ceramic coating, the expanded ceramic is very strong on the whole but, because of the internal carbon skeleton which has substantially lower strength than the ceramic layer, the mechanical strength is still not adequate under high loading. Further, the carbon skeleton is exposed to oxidative processes at high temperatures which greatly undermines the otherwise favorable stability of the expanded ceramic at high temperatures. For these reasons, the carbon skeleton is removed for applications at higher temperatures, but this again results in hollow ceramic struts.
Another known method for producing an open-cell expanded ceramic is direct expansion using foaming agents. For this purpose, a suspension of ceramic particles and water or a solvent is first produced. A foaming agent and polymer components are added to this suspension. This suspension is subsequently cast in a mold and the reaction of the foaming agent is initiated. This reaction brings about the development of gas bubbles which cause a foaming of the suspension. The polymer components are then cross-linked, so that the foam hardens. The polymer components are then burned out and the remaining foam is sintered (Product Brochure: Foaming Agents W 53 FL, Zschimmer & Schwartz GmbH & Co., Lahnstein). The disadvantage of this process consists in that the foaming is difficult to control.
An expanded ceramic produced by this process has a net-like structure. The ceramic struts are composed of ceramic through the entire cross section after sintering. The external geometry of an expanded ceramic of this type is limited at least in one dimension by the open mold in which the foaming takes place. The sintered expanded ceramic is very microporous because the suspension does not allow a high concentration of particles. At higher concentrations, the suspension is too heavy and foaming takes place only incompletely or not at all. Low solid concentrations lead to a very loose structure of the burned out ceramic. When the ceramic is compressed by contraction during sintering, stresses and cracks also occur, so that the strength of the ceramic is limited. If a ceramic system with low contraction is selected, the porosity is maintained which likewise results in low strength. Further, it is disadvantageous that the pore spacing is difficult to control in general and especially over the height of the mold which impairs the through-flow capacity of the expanded ceramic.
Further, a process for the production of expanded ceramic through direct expansion by means of air is also known. For this purpose, a polymer component is added to a suspension of ceramic particles and water or a solvent. Subsequently, air bubbles are introduced into the suspension by a high-speed special stirrer. The foamy suspension is then poured into a mold and the foam is hardened by cross-linking the polymer component. The polymer component is then burned out and the foam is sintered.
Only very fine foams with few open cells can be produced by the process menti
Adler Joerg
Jaunich Helmut
Standke Gisela
Stoetzel Reinhard
Stoever Heike
Dunn Tom
Frauhofer-Gesellschaft zur Forderung der Angewandten Forschung E
Ildebrando Christina
Reed Smith LLP
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