Mounting for metal honeycomb structures
BACKGROUND OF THE INVENTION 
     This invention relates to the art of mounting thin-walled metal honeycomb structures, such as those that would have utility as a substrate or heater in controlling emissions from a gasoline or diesel internal combustion engine. Such metal structures may be formed by extrusion from extrudable particulate metal batch materials which have the property of being able to flow or plastically deform during extrusion, while being able to become sufficiently rigid immediately thereafter so as to maintain their structural integrity, in the manner set forth in U.S. Pat. Nos. 3,790,654 and 4,758,272. Alternatively, the honeycomb structures may be fabricated from thin metal sheets such as by wrapping corrugated and uncorrugated metal sheets into a honeycomb structure as shown in U.S. Pat. Nos. 3,112,184 and 3,444,925. Further, if desired, the honeycomb structures could be made of pleated thin porous sheets of filter material whose layers are interleaved with corrugated or crimped spacers as disclosed in U.S. Pat. Nos. 2,884,091, 2,952,333 and 3,242,649. 
     The invention particularly relates to the problem of preventing degradation of the metal honeycomb substrate when utilized in and subjected to the harsh and rigorous conditions experienced in vehicular emissions control systems. When mounted in an exhaust stream of a vehicle, cellular metal honeycombs can be resistively heated for the purpose of accelerating the time in which pollutants may be converted. However, such mounted resistive heaters must withstand severe shock and vibration at elevated temperatures, which temperatures may exceed 920° C. and such forces may exceed 20 g&#39;s. Even the relatively high temperature metals utilized in such cellular substrates exhibit a rather large and unfavorable reduction in mechanical properties as the temperature rises, potentially leading to heater distortion. Such distortion could result in electrical short circuits to ground, in the closure of resistance adjusting slots which would shorten the electrical path length, and in distortion of the flow cells which would compromise the gas flow therethrough. 
     It thus has been an object of the invention to provide a mounting system for metal honeycomb structures which not only protectively supports the structure within an enclosing housing, but also electrically insulates the metal honeycomb structure from such housing. 
     SUMMARY OF THE INVENTION 
     In its simplest form, the present invention sets forth the use of non-conductive honeycomb structures for mechanically supporting and electrically insulating a metal honeycomb structure. As a result, the mechanically supported and electrically insulated metal honeycomb structure may be utilized as a resistance heater in such applications as vehicular and industrial emissions control systems. The non-conductive supporting honeycomb structures are preferably positioned on opposite sides of the metal honeycomb structure with the cells of the non-conductive honeycomb structures being opened to the cells of the metal honeycomb structure such that the longitudinal axis of the cells of the metal honeycomb structure are parallel with the longitudinal axis of the cells of the non-conductive honeycomb structure, so that exhaust gases may flow freely through such adjacent structures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic exploded view of a preferred embodiment of the invention showing a pair of recessed non-conductive support honeycomb structures on opposite sides of a metal honeycomb structure having electrode connection tabs formed thereon. 
     FIG. 2 is a schematic oblique view showing the structures of FIG. 1 in an assembled form. 
     FIG. 3 is a schematic cross-sectional view taken along line of 3--3 of FIG. 2, showing the assembly of FIG. 2 within an enclosing canister housing. 
     FIG. 4 is a schematic exploded view of a further embodiment of the invention. 
     FIG. 5 is a schematic oblique illustration of the assembled units of FIG. 4. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIGS. 1 and 2, a metallic honeycomb structure 10 of a desired peripheral configuration is shown having a plurality of webs 12 defining a plurality of cells 14 therebetween, which cells extend longitudinally through the body portion of the metallic honeycomb structure and terminate in opposed inlet and outlet faces 16, 18. A plurality of slits 20 are shown formed in the metallic honeycomb structure 10, which function as a resistance adjusting mechanism when the metallic honeycomb structure is utilized as a resistance heater. A pair of electrode connector tabs 22 are shown formed on the body portion of structure 10 at opposite sides thereof, and are continuous with an outer skin or wall 24 of the honeycomb structure. 
     A pair of non-metallic ceramic support honeycomb structures 30 are shown having a plurality of webs 32 forming a plurality of cells 34 extending longitudinally through the support honeycomb structures 30 and terminating at their opposite ends in opposed face portions. The webs 32 and cells 34 are bounded by an outer skin or wall 36. Each support structure 30 has a face 38 which is recessed a distance d to form a recessed face portion 40, having a peripheral configuration substantially identical with that of the body portion of metallic honeycomb structure 10, and a peripheral recessed wall 42 bounding the peripheral extent of face portion 40. In addition, a pair of slots 44 are formed in each face 38 to a given distance d. 
     Referring now to FIG. 2, when the depth d of the recess formed in each face 38 of support structures 30 is equal to one-half the thickness t of the metallic honeycomb structure 10, the support structures 30 will completely enclose the body of the metallic honeycomb structure 10, when the units are assembled as shown in FIG. 2. In fact, when the distance d of the recess in each of the support structures 30 is equal to one-half of the thickness t of the metallic structure 10, the faces 38 of the support structures 30 will be abutting. Further, each of the opposed faces 16, 18 of the metallic honeycomb structure 10 will be in contact with a face portion of each of the supporting structures 30, depending upon the relative recess distances d which are formed in the face 38. 
     That is, if one support structure 30 has a recessed distance d equal to the thickness t of the metal honeycomb structure 10, such that the resulting recess could receive the entire thickness of the body of the metal structure 10, then the recess distance d in the face 38 of the other supporting structure would be zero. In such case, one of the opposed faces 16, 18 of the metallic structure 10 would be in contact with the recessed surface portion 40 in one supporting structure 30, and the other opposed face would be in contact with face 38 of the other supporting structure 30. Generally, the total recessed distances d+d in both of the supporting structures should not exceed the thickness t of the metallic structure, however since it is not necessary that the faces 38 of the support structures abut one another, the total recess distance, d+d, of both supporting structures, may be less than the thickness t of the metal honeycomb structure 10. However, if the recess depth d in one supporting structure 30 is great enough to encompass and retain the thickness t of body 10, then a second supporting structure 10 may not be required. In such case d may be greater than t to provide the necessary protective supporting enclosure. Further, the recess formed in the face 38 is configured so that the peripheral recessed wall 42 snugly receives and engages the outer wall 24 of the metallic honeycomb structure 10, with the electrode tabs 22 extending through the slots 44. For purposes of clarity, the webs and cells have not been shown in FIG. 2. 
     When the assembly of FIG. 2 is utilized as a resistance heater in an emissions control system, it is preferably encased or retained within a can or canister housing 50, having opposed openings 52 formed in the sidewalls thereof, for facilitating the flow of exhaust gases, represented by arrows A, through appropriate conduits (not shown). The metallic honeycomb structure 10, which may be in the form of a resistance heater, is insulatably enclosed within and supported by the two support honeycomb structures 30. Although the outer peripheral wall 36 may be configured to conform with the inner peripheral surface 56 of peripheral wall 54 of the enclosing housing 50, if desired, additional insulating and impact absorbing material 58 may be positioned between the inner peripheral surface 56 of the housing 50 and the outer wall 36 of the support structures 30. In order to facilitate the flow of the exhaust gases through the heater assembly of FIG. 3 and to minimize back pressure, the cells 34 of the supporting structures 30 are preferably relatively large in cross-sectional open area compared to the cross-sectional open area of cells 14 of the resistance heater body 10. Further, as shown in FIG. 3 the metal honeycomb resistance heater structure 10 may be fully encapsulated by the non-conductive ceramic honeycomb support structures 30, which not only insulate the heater body 10 from the retaining housing 50, but also mechanically support the heater body within the housing to protect the same from severe shock and vibration. 
     Referring now to FIG. 4 and 5, a further embodiment of the invention is shown including the metal honeycomb structure 10 and a pair of non-metal honeycomb support structures 30. The metallic honeycomb structure 10 and the non-metallic or ceramic honeycomb support structures 30 are similar to those shown in FIGS. 1 and 2, with the exception that no recesses are formed in the faces 38 of the support structures 30. Accordingly, when the units of FIG. 4 are assembled as shown in FIG. 5, the faces 38 of the support structures 30 engage the opposed inlet and oulet faces 16 and 18 of the metallic honeycomb structure 10, and preferably the outer walls 36 of the support structures 30 form a complementary or continuous outer wall with the outer wall 24 of the metallic honeycomb structure 10. Again, it is preferred that the cross-sectional open area of cells 34 of the support structures 30 are formed relatively large with respect to the open cross-sectional area of cells 14 of the metallic structure 10, in order to minimize back pressure and facilitate the flow of exhaust gases through the assembly. 
     In order to more accurately align the faces 38 of the support structures with the faces 16 and 18 of the metallic honeycomb structure 10, one such face may be provided with one or more projections such as 60, and the cooperative face may be provided with one or more alignment recesses such as 62, so as to more easily orient and interlock the faces 38 with the faces 16 and 18. 
     The embodiment of FIGS. 4 and 5 functions the same as the embodiment of FIGS. 1 and 2, with the exception that the supporting honeycomb structures 30 of the embodiment shown in FIGS. 4 and 5 have an outer peripheral wall 36 which is configured so as to be coincident with the configuration of outer wall 24 of the metal honeycomb structure 10. Accordingly, in the embodiment of FIGS. 4 and 5, the supporting structures 32 do not overlie the outer wall 24 of the metallic honeycomb structure 10 as is the case in the embodiment of FIGS. 1 and 2. In both embodiments, however, the faces 38 of the supporting structures 30 are in close contact with the inlet and outlet faces 16, 18 of the metallic honeycomb resistor body 10, and thus the relatively high rigidity of the ceramic honeycomb support structures 30 provide improved mechanical support for the metallic heater body. 
     Whereas fibrous materials which have been customarily utilized for providing sealing between the heater and the housing are subject to breakdown and blowout due to the short length of the usual heater, the ceramic honeycomb structures 30 present a longer monolith surface for providing a durable sealing between the heater and the can. A further advantage of utilizing cellular ceramic support structures resides in the option of catalyzing the cells to assist in the conversion of the exhaust gases passing therethrough. That is, the supporting structures, themselves, may function as supplemental catalytic converters. 
     As far as this invention is concerned, it is really immaterial as to how the cellular of honeycomb structures 10 and 30 are formed. As previously pointed out, the honeycomb structures 10 and 30 may be formed by extrusion as set forth in U.S. Pat. Nos. 3,790,654 and 3,919,384, or alternatively they may be formed by wrapping a corrugated material into a honeycomb structure such as disclosed in U.S. Pat. Nos. 3,112,184 and 3,444,925. With respect to the desired composition of the metallic honeycomb structure 10, various metal powders and/or metal sheets may be utilized which produce a desired resistive heater effect when an EMF is applied, and representative compositions are set forth in U.S. Pat. No. 4,758,272 and Brundage et al. U.S application Ser. No. 07/767,889, filed Sep. 30, 1991, assigned to the assignee of the present application and incorporated herein by reference. Further, with respect to the composition of the non-metallic ceramic support honeycomb structures 30, a variety of non-conductive ceramic materials such as cordierite and those set forth in the U.S. Pat. Nos. 3,790,645, 3,919,384, 3,112,184, and 3,444,925 patents previously mentioned. 
     By sandwiching a metal honeycomb resistance heater between ceramic cellular material, or surrounding such heater with cellular ceramic material, it is possible to mechanically support the metallic honeycomb heater and electrically isolate the same from an enclosure such as a typical housing can utilized in vehicle emissions control systems. The ceramic cellular structures 30 may be retained about the metal honeycomb structure 10 by being compressed together within housing 50, or by any suitable means such as bonding, clamping and sintering. Thus, the use of non-metallic ceramic honeycomb support structures such as 30 about metallic honeycomb resistance heaters, which have an unfavorable reduction in mechanical properties at elevated temperatures, serves to protect such heaters from severe shock and vibration, particularly when utilized in a vehicular environment. 
     Although the now preferred embodiments of the invention have been disclosed, it will be apparent to those skilled in the art that various changes and modifications may be made thereto without department from the spirit and scope thereof as defined in the appended claims.