The present invention relates to a method for drying ceramic articles via a microwave dryer, and in particular to methods for drying ceramic honeycomb structures via a microwave dryer that promotes uniform drying of the honeycomb structures, thereby relieving or eliminating heat-induced structural degradation of the structures.
Ceramic honeycomb structures having transverse cross-sectional cellular densities of approximately one-tenth to 100 or more cells or channels per square centimeter of honeycomb cross-section have several uses, including use as particulate filter bodies, catalyst substrates, and stationary heat exchangers. Filter applications generally require that selected cells of the structure be sealed or plugged at one or both of the respective ends thereof in a manner such that wall-flow filtration, i.e., the filtering of fluids traversing the structure by directing at least some of those fluids through porous channel walls thereof, is effected.
Ceramic honeycomb manufacture involves several known steps. In general, the honeycomb shapes are first formed, e.g., by extrusion, from water-containing plasticized mixtures of ceramic raw materials. The formed honeycombs are next dried to solidify the desired honeycomb structure, and are finally fired to sinter or reaction-sinter the ceramic raw materials into strong unitary ceramic articles.
Referring to the appended drawings, the reference numeral 8 (
To form a filter from structure 10 (
In operation, contaminated fluid is brought under pressure to an inlet face and enters the filter via those cells which have an open end at the inlet face. Because the cells are sealed at the opposite ends, i.e., the outlet face of the body, the contaminated fluid is forced through the thin porous walls 14 into adjoining cells which are sealed at the inlet face and open at the outlet face. The solid particulate contaminant in the fluid, which is too large to pass through the pore structure of the walls, is left behind and the cleansed fluid exits the filter through the outlet cells and is ready for use.
Some previous methods used for drying ceramic honeycomb structures have led to decreased structural strength due to heat-induced structural degradation. Structural strength requirements are particularly demanding for ceramic catalyst substrates and filters to be used in the mechanically harsh environment of motor vehicle exhaust emissions control systems. Nevertheless, for the mass production of such filters and substrates it is highly desirable to be able to dry the ceramic substrates rapidly and as inexpensively as possible, while maintaining structural integrity and strength.
Various drying techniques have been utilized for ceramic honeycomb manufacture in the past, including conduction heating, convection heating, and RF heating. Microwave heating has been used to achieve higher volumetric heating uniformity than conduction and/or convection heating can provide alone, while at the same time offering low operating costs and reduced processing times. However, some ceramic materials useful for constructing ceramic substrates and filters, particularly including batches for the manufacture of cordierite, mullite, aluminum titanate, and similar ceramics that include a graphite additive to increase honeycomb porosity, are more difficult to dry via microwave drying. Also problematic from a drying standpoint are honeycombs directly incorporating materials such as transition metal oxide catalysts, where the catalysts include constituents that are semiconductive or very lossy at the desired microwave drying frequency.
These drying difficulties are attributed to the inability of microwave radiation to properly penetrate into and effect uniform heating within the interior portions of such materials, due to reduced microwave permeability occasioned by the presence of graphite or other lossy materials within the ceramic batch mixtures. The consequence is that the drying of such honeycombs using microwave radiation can lead to unacceptable localized heating, which in turn leads to unstable processing, poor select rates, and lower quality ware. For example, the drying of an aluminum titanate substrate with a 30% graphite additive has produced unwanted edge heating that results in cracks and/or contour problems in the associated filter.
One possible solution to this drying problem is simply to remove damaged edge portions from the dried honeycomb parts. This solution is obviously inefficient and creates a significant amount of waste. Other solutions include changing the composition of the ceramic batch mixtures to reduce the amount of graphite or other lossy materials therein, or using multiple drying steps, or using a combination of drying methods, for example, microwave plus hot air drying, to achieve drying without structural damage. However, each of these alternatives requires accepting unwanted compromises, such as lower quality end products and/or increases in manufacturing costs.
A method for drying ceramic substrates that reduces unwanted nonuniform drying characteristics within the ceramic substrates, thereby reducing unwanted heat-induced stress cracking and structural degradation of the substrates, while simultaneously decreasing associated cycle times, and associated operating costs, is therefore desired.
The present invention relates to a method for drying a thin-walled ceramic structure such as a honeycomb comprising providing microwave radiation from a microwave generating source, providing a ceramic honeycomb structure having a middle portion and at least one end, and exposing the ceramic honeycomb structure to the microwave radiation. The method further includes shielding at least one end of the ceramic honeycomb structure from directly receiving the microwave radiation, such that the radiation absorbed by the middle portion is equal to or greater than the radiation absorbed by the at least one end. Uniform drying of the ceramic substrate with reduced heat-induced structural degradation is thereby promoted. The radiation absorbed by the middle portion is preferably within the range of from about 0% to about 60% greater than the radiation absorbed by the at least one end, and more preferably within the range of from about 10% to about 40% greater than the radiation absorbed by the at least one end.
The present method is highly accurate and repeatable, may be completed in a relatively short cycle time, is relatively easy to perform, and results in a filter with relatively greater structural integrity with reduced deformation and degradation. The method further reduces the relative cracking and stress fractures within the desired structure produced during the drying process, reduces manufacturing costs associated with cycle times, is efficient to use, and is particularly well-adapted for the proposed use.
These and other advantages of the present invention will be further understood and appreciated by those skilled in the art by reference to the following written specification, claims, and appended drawings.
Several methods and procedures are known in the art for forming green ceramic honeycomb structures featuring a plurality of hollow passages or channels extending therethrough. The present inventive process is directed to drying such structures regardless of the specific method used to form the honeycomb shape. The present inventive method for drying ceramic honeycomb structures 10 includes providing microwave radiation from a microwave generating source 30 (
In the illustrated example, the microwave housing 32 includes a bottom wall 34, a top wall 36, and a pair of side walls 38. The microwave generating source 30 extends downwardly from the top wall 36 and is centrally located within the microwave housing 32. In the illustrated example, a plurality of ceramic structures 10 are positioned within an interior 40 of the microwave housing 32, each supported by an associated support tray 42. It is noted that the present inventive method can be accomplished either via batch style or continuous-type flow processing, and that the housing 32 may be configured to house a single structure 10, or multiple structures. Further, the structure(s) may be horizontally or vertically oriented as the drying process is completed. A pair of planar shield members 44 are positioned within the interior 40 of the microwave housing 32 and vertically above the structure 10 between the microwave generating source 30 and the ends 13, 16 of the structure 10, thereby shielding the ends 13, 16 of the ceramic structure 10 from directly receiving the microwave radiation such that the radiation absorbed by a middle portion 17 of the ceramic structure 10 is equal to or greater than the radiation absorbed at the ends 13, 16. Preferably, the amount of radiation absorbed by the middle portion is within the range of from 0% to 60% greater than the radiation absorbed by the ends 13, 16 of the structure 10, and more preferably within the range of from 10% to 40%.
As best illustrated in
As noted above, shielding the ends 13, 16 of the ceramic structure 10 results in a more even power distribution within the ceramic structure 10, and as a result, a more uniform drying thereof. As best illustrated in
Modeled examples were completed on given ceramic structures both with and without shielding.
Alternative methods for shielding the ends 13, 16 and end faces 18, 20 of the ceramic structure 10 are also contemplated. It is noted that these alternative methods may be practice simultaneously with the other methods described herein. A first alternative embodiment includes the use of shield members 60 (
A second alternative embodiment includes spacing multiple simultaneously processed ceramic structures 10 (
Other alternative embodiments include placing the trays 42 (
The present method is highly accurate and repeatable, may be completed in a relatively short cycle time, is relatively easy to perform, and results in a filter with relatively greater structural integrity with reduced deformation and degradation. The method further reduces the relative cracking and stress fractures within the desired structure produced during the drying process, reduces manufacturing costs associated with cycle times, is efficient to use, and is particularly well-adapted for the proposed use.
It will be understood from the foregoing that the specific devices and processes illustrated in the attached drawings and described in the foregoing specification are exemplary only, and that the specific dimensions and other physical characteristics relating to those embodiments are intended to be illustrative rather than limiting.
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