Abstract:
A method for drying a ceramic article comprises 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 while shielding the at least one end 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, and the proper drying of the entire honeycomb structure without heat-induced structural degradation is thus ensured.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    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. 
         [0002]    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. 
         [0003]    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. 
         [0004]    Referring to the appended drawings, the reference numeral  8  ( FIG. 1 ) generally designates a ceramic article of a type that is well known for applications such as catalyst substrates and diesel exhaust particulate filters. The base structure in both cases is a ceramic honeycomb  10  comprising a matrix of intersecting, thin, porous cell walls  14  surrounded by an outer wall  15 . In the illustrated example structure  10  is provided in a circular cross-sectional configuration including a first end  13 , a second end  16  and a middle portion  17 . The walls  14  extend across and between a first end face  18  and an opposing second end face  20 , and form a large number of adjoining hollow passages or channels  22  which extend between and are open at the end faces  18 ,  20  of the structure  10 . 
         [0005]    To form a filter from structure  10  ( FIGS. 2 and 3 ), one end of each of the cells  22  is sealed, a first subset  24  of the cells  22  being sealed at the first end face  18 , and a second subset  26  of the cells  22  being sealed at the second end face  20  of the substrate  10 . Either of the end faces  18 ,  20  may be used as the inlet face of the resulting filter. The structure  10  with seals is then fired to form the filter. 
         [0006]    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. 
         [0007]    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. 
         [0008]    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. 
         [0009]    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. 
         [0010]    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. 
         [0011]    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. 
       SUMMARY OF THE INVENTION 
       [0012]    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. 
         [0013]    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. 
         [0014]    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. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a perspective view of a ceramic honeycomb structure the drying of which embodies the present invention; 
           [0016]      FIG. 2  is a perspective view of the ceramic honeycomb structure with alternatively plugged channels; 
           [0017]      FIG. 3  is an end elevational view of the ceramic honeycomb structure of  FIG. 2 ; 
           [0018]      FIG. 4  is a top perspective view of a microwave dryer with a plurality of ceramic honeycomb structures located within an interior thereof; 
           [0019]      FIG. 5  is a cross-sectional top plan view of the microwave dryer of  FIG. 4 , with a plurality of ceramic structures located within the interior thereof; 
           [0020]      FIG. 6  is a cross-sectional end elevational view of the microwave dryer of  FIG. 4 , with a plurality of ceramic structures located within the interior thereof; 
           [0021]      FIG. 7  is a graph of integrated dissipation vs. length for a ceramic structure dried via conventional means; 
           [0022]      FIG. 8  is a graph of integrated dissipation vs. width for a ceramic structure dried via conventional means; 
           [0023]      FIG. 9  is a graph of integrated dissipation vs. length for a ceramic structure dried via conventional means, and a ceramic structure dried via the present inventive process; 
           [0024]      FIG. 10  is a graph of integrated dissipation vs. width for a ceramic structure dried via conventional means, and a ceramic structure dried via the present inventive process; via conventional means; 
           [0025]      FIG. 11  is a graph of integrated dissipation vs. length for three modeled sample of ceramic structures dried via the present inventive process; 
           [0026]      FIG. 12  is a graph of integrated dissipation vs. width for three modeled sample of ceramic structures dried via the present inventive process; 
           [0027]      FIG. 13  is a side perspective view of a first alternative embodiment of the present inventive method, including a pair of shield members shielding end faces of the ceramic structure; 
           [0028]      FIG. 14  is a side perspective view of a second alternative embodiment of the present inventive method, including a pair of ceramic structures positioned end-to-end; 
           [0029]      FIG. 15  is a top perspective view of a third alternative embodiment of the present inventive method, wherein the ceramic structure is spaced from the sidewalls of a microwave applicator on a support tray; and 
           [0030]      FIG. 16  is a top perspective view of a fourth alternative embodiment of the present inventive method, including multiple spaced trays. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    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  ( FIGS. 4-6 ) located within a microwave housing  32 , exposing the ceramic honeycomb structure  10  to the microwave radiation, and shielding at least one of the ends  13 ,  16  from directly receiving the microwave radiation, such that the radiation absorbed by the middle portion  17  of the ceramic structure  10  is equal to or greater than the radiation absorbed by the at least one end  13 ,  16 , as described herein. It is noted that the present inventive process may be used to process either plugged or non-plugged ceramic structures. 
         [0032]    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%. 
         [0033]    As best illustrated in  FIG. 6 , the shield members  44  are adjustable in several directions with respect to the ceramic structure  10  being processed, including a vertical direction  48  and a horizontal direction  50 . Adjustment in the vertical direction  48  allows an operator to adjust the vertical distance of separation X between the uppermost portion of the ceramic structure  10  and the shield member  44 . Preferably, the distance X is less than or equal to 1.5 times the wavelength of the microwave radiation, more preferably within the range of 1.5 to 1.0 times the wavelength of the microwave radiation, and most preferably is about 0.5 times the wavelength of the microwave radiation. Adjustment in the horizontal direction  50  allows the operator to adjust the amount of overlap Y each shield member  44  has with the associated ceramic structure  10 . Preferably, the amount of overlap Y is within the range of from 0% to 30% of the overall length of the structure  10 , and more preferably is within the range of from 0% to 10% of the overall length of the structure  10 . Further, the relative angle θ between each shield member  44  and a longitudinal axis  53  of the ceramic structure  10  is also adjustable in a direction  51 . Preferably, the angle θ is within the range of from 0° to 5°, and more preferably is about 0°. The adjustability of the shield members  44  allow fine tuning of the positions of the shield members  44  with respect to the ceramic structure  10  to optimize the drying thereof. 
         [0034]    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  FIG. 7 , the integrated dissipation of the power absorbed by a structure subjected to microwave radiation within a conventional microwave drying, i.e., a drying that does not provide shielding, results in a power absorption that is significantly greater at the ends of the structure than an the middle portion thereof. Similarly,  FIG. 8  illustrates that the power absorbed near the side wall  15  of the structure is also significantly greater than that absorbed near the center thereof. 
         [0035]    Modeled examples were completed on given ceramic structures both with and without shielding.  FIGS. 9 and 10  illustrate integrated dissipation vs. length of the structure, and integrated dissipation vs. width of the structure, respectively, for an unshielded sample  52  and a shielded sample  54 . Further, modeled examples were completed on three variations of system configurations utilized for processing a given ceramic structure.  FIGS. 11 and 12  illustrate integrated dissipation vs. length of the structure, and integrated dissipation vs. width of the structure, respectively, of the three examples A-C. Example A included the modeling of a 36 inch in length structure with the distance X of the shield members  44  above the structure  10  being 10 inches, the overlap Y of the shield members  44  with the structure  10  being 10 inches, the angle θ between the shield members  44  and the structure  10  being 0°, and the number of structures  10  within the interior  40  of the housing  32  being 5. Example B included the modeling of a 20 inch in length structure with a distance X of 10 inches, an overlap distance Y of 18 inches, an angle θ of 0°, and 5 structures  10  simultaneously located within the interior  40  of the housing  32 . Example C included the modeling of a 36 inch in length structure  10  with a distance X of 20 inches, an overlap distance Y of 10 inches, an angle θ of 0°, and  5  structures  10  simultaneously located within the interior  40  of the housing  32 . It is clear from the integrated power dissipation along the length and width of the structures that the shielded process reduces the edge heating effect. Moreover, the integrated dissipation along the major axis ( FIG. 10 ) shows a more uniform heating as compared to the end heating occurring without shielding. 
         [0036]    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  ( FIG. 13 ) spaced from the end faces  18 ,  20  of the structure  10 . In the illustrated example, the shield members  60  are placed within the tray  42  that supports and carries the structure  10  through the housing  32 . Preferably, the shield members  60  are spaced a distance A from the associated end face  18 ,  20  of less than or equal to one quarter of the wavelength of the microwave radiation. 
         [0037]    A second alternative embodiment includes spacing multiple simultaneously processed ceramic structures  10  ( FIG. 14 ) a distance B from one another. In the illustrated example, two structures  10  are placed within the same tray  42  such that the distance A between the corresponding end faces  18 ,  20  reduces or eliminates access thereto by the drying microwave radiation. Preferably, the distance B is less than or equal to about one quarter of a wavelength of the microwave radiation. 
         [0038]    Other alternative embodiments include placing the trays  42  ( FIG. 15 ) relative to the sidewalls of a microwave applicator housing  32  ( FIG. 5 ) such that the distance between the ends  18 ,  20  of honeycomb structures  10  and the associated sidewalls  38  ( FIG. 5 ) is preferably less than about one half the wavelength of the microwave radiation. It is also useful to space multiple trays  42  ( FIG. 16 ) within the interior  40  of a microwave applicator housing  32  such that the distance D between the trays  42  will provide a spacing of about one half of the wavelength of the microwave radiation between the honeycomb structures  10 . 
         [0039]    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. 
         [0040]    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.