DESCRIPTION OF THE DRAWINGS
FIG. 1A is a perspective view of a prior art catalytic, flow-through ceramic substrate.
FIG. 1B is a plan view of an octant section along the angle 1B-1B of FIG. 1A, illustrating the network of web walls contained therein.
FIG. 1C is a partial side, cross-sectional view taken along the line 1C-1C of the substrate of FIG. 1A.
FIG. 1D is an enlarged, cross-sectional view of the web walls circled in phantom in FIG. 1C.
FIG. 2A is a cross-sectional side view of the fast light off, slow cool-down ceramic substrate of the invention, illustrating an axial region near the inlet end of the substrate having thinner web walls.
FIG. 2B is an enlargement of the area circled in phantom in FIG. 2A, illustrating the interface between the thinner walls in the axial region near the inlet end of the substrate, and the rest of the web walls of the substrate.
FIG. 2C is a view along the line 2C-2C of a cell formed by the web walls illustrated in FIG. 2B.
FIG. 3 is a side, cross-sectional view of a second embodiment of the substrate of the invention, illustrating how the axial region of thinner web walls is spaced apart from the inlet end of the substrate.
FIG. 4 is a partial side cross-sectional view of a third embodiment of the fast light off ceramic substrate of the invention, illustrating how the web walls may be tapered along the axis of the substrate such that they are thinner near the inlet end and thicker near the outlet end.
FIG. 5 illustrates how any of the three embodiments of the invention may also incorporate a radial thickening of the web walls between the centroid of the substrate, and its outer skin.
FIG. 6 is a schematized, cross-sectional view of an embodiment of the invention that incorporates the radial web thickening illustrated in FIG. 5.
FIGS. 7A and 7B schematically illustrate how a tapered web wall embodiment of the invention such as that illustrated in FIG. 4 may be manufactured.
FIG. 8 is a graph illustrating how the substrate of the invention reduces the amount of time necessary for at least a portion of the substrate to arrive at the 250° C. necessary for effective catalyzing of automotive pollutants to occur.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
With reference now to FIGS. 1A through 1D, the invention is applicable to ceramic substrate 1 of the type used as automotive catalytic converters. Such substrates including network 3 of walls typically between about 2.0 and 6.0 mils thick which define a plurality of flow-through cell channels 7. While the cell channels 7 are illustrated as having a square cross-section in FIG. 1B, that may just as easily be octagonal, hexagonal or some other polygonal shape. The flow-through cell channels are typically between about 0.02 and 0.05 inches in width, and are arranged at a cell density of between about 300-900 cells per square inch. The substrate 1 further includes an extruded outer skin 9 which is typically cylindrical, and has a thickness of approximately three to four times that of the web walls 5. The resulting cylindrical structure of the substrate 1 has an inlet end 11, and an outlet end 13 which are opposed from one another along the axis A illustrated in FIG. 1C. In this application, the term “flow-through cells” encompasses cells designed to directly conduct a flow of exhaust gas or other fluid into the inlet end straight along the axis A illustrated in FIG. 1C and out through the outlet end, i.e., an unplugged structure.
The walls 5 are formed from a porous ceramic material, such as cordierite, mullite, silicon carbide, or aluminum titanate. As is illustrated in FIG. 1D, the web walls 5 are porous, including a number of microscopic pores 15 which tend to reduce the density of the walls 5. Typically, the web walls 5 have a porosity of between 20%40%. As is best seen with respect to FIG. 1D, the walls 5 are coated with a wash coat 17 which not only covers the outer surface of the walls 5, but penetrates the region adjacent to the outer surfaces (as is indicated in phantom). The wash coat 17 includes fine particles of a catalyst, which may include platinum, palladium or rhodium, or alloys thereof, which have been suspended in an alumina slurry. Such wash coats 17 are vacuum-deposited on the outer surfaces of the walls 5 to penetrate beyond the outer surfaces thereof, thereby increasing the surface area covered by the catalytic particles. Such vacuum-deposition of the wash coat 17, in combination with a high density of cell channels 7, maximizes area contact between automotive exhaust gases blowing straight through the cell channel 7 from the inlet to the outlet ends 13, and therefore maximizes the catalytic reaction of the wash coat in oxidizing carbon monoxide (CO) and in disassociating nitrous oxide (NOx) into nitrogen and oxygen. However, before the wash coat 17 can become effective in promoting such pollutant-neutralizing reactions, it must first reach an activation temperature of approximately 250° C. The instant invention provides a ceramic substrate that attains the 250° C. “light off” temperature faster than the prior art substrate illustrated in FIGS. 1A-1D.
With reference now to FIGS. 2A through 2C, the fast light off, slow cool down ceramic substrate 20 of the invention includes a first axial region 22 having an average thermal mass (ATM1) that is between 10%-35% less than the average thermal mass of all of the walls 5 (ATMTOT), or even 10%-30% less than the ATMTOT. To this end, the walls 23 within the first axial region 22 have between about 35-55% less thermal mass than the walls 25 located in a second axial region 24 axially contiguous with the first axial region 22. As is indicated in FIG. 2B, this difference in thermal mass is achieved not only by rendering the thickness T1 of the walls 23 in region 22 less than the thickness T2 of the walls 25 in the region 24, but further by increasing the porosity P1 in the walls 23 relative to the porosity P2 of the walls 25. However, it is equally within the scope of the invention that the reduction of the thermal mass within the first axial region 22 is accomplished solely by (1) reducing the thickness of the walls 23 relative to the thickness of the walls 25, or by keeping these thicknesses equal (i.e., T1=T2), but increasing the porosity P1 of the walls 23 so that they are greater than the porosity P2 of the walls 25; which may be approximately 35%-55% greater. Thus, the invention encompasses any ceramic substrate wherein a first axial region 22 of the web walls has less thermal mass than a second, contiguous axial region 24, whether such reduction in mass is accomplished by thinner walls, higher porosity, or any combination of both. When such reduction in thermal mass is achieved completely or substantially through the provision of thinner walls 23 in the first axial region, such walls 23 may be between about 1.5 mils and 2.5 mils. Ceramic substrates 20 having web walls less than 1.5 mils in thickness are not preferred due to the mechanical weakness of such walls, and the difficulty in their manufacture. In all cases, as best shown in FIG. 2C, reinforcing fillets 26 may be provided at the corners of the cell channel 7 to improve the strength characteristics of the resulting channel 7.
FIG. 3 illustrates a second embodiment 30 of the invention wherein the first axial region 22 does not include the inlet end 11 of the ceramic substrate. Instead, the first axial region 22 is spaced away from the inlet 11 by a relatively short inlet region 32 having a length L1 which is between 5%-10% of the overall length L2 of the ceramic body. The provision of such an inlet region 32 having web walls 25 that are just as thick as the web walls 25 of the second axial region 24, but of a relatively short length advantageously affords protection against erosion of the relatively thin walls 23 present in the first axial region 22 without significantly increasing the short light off time that these thin walls 23 provide when this embodiment 30 of the invention is in operation.
FIG. 4 illustrates a third embodiment 35 of the invention having walls 37 that are tapered along their longitudinal axis such that their thickness increases from the inlet end 11 to the outlet end 13. Each of the walls 37 includes a core portion 39 formed from a first ceramic material having a thickness T3 which is preferably uniform among the longitudinal axis of the ceramic body, and tapered layers 41 of a second ceramic material overlying the core portion 39 as shown. The thickness of the tapered layers 41 linearly varies along the axis of the ceramic body at an angle such that the average thermal mass (ATM1) of a first axial region 43 is at least 20% less than the average thermal mass (ATMTOT) of all the walls 37. In this example of the third preferred embodiment 35, the thickness T3 of the tapered walls 37 at the inlet end 11 is 2.0 mils, while the thickness T4 of the tapered walls 37 at the outlet end 13 is approximately 4.5 mils. When the walls 37 are so dimensioned, the average thermal mass ATMTOT is 3.25. Hence, when the axial length of the first axial region 43 is approximately 50% the overall length of the ceramic substrate, the average thermal mass of the walls 37 is 2.625, which is approximately 80% of the ATMTOT of 3.25. Preferably, the slope of the tapered layers 41 is chosen such that the length L3 of the first axial region 43 is 50% or less than the overall length L4 of the ceramic substrate in order to ensure that the first axial region 43 will have a substantially shorter light off time than a substrate having walls of the same thermal mass, but of uniform thickness between the inlet and outlet ends 11, 13.
FIG. 5 illustrates a fourth embodiment 45 of the invention wherein the thermal mass of the walls 47 increases in a radial direction from a centroid C of the ceramic substrate. Such a thickening of the web walls 47 results in cell channels 49 having a smaller cross-sectional area toward the outermost region of the substrate, which in turn encourages the flow of the exhaust gases through the center-most region, where the walls 47 are the thinnest. Such a radially-oriented thickening of the walls is applicable to any one of the three previously discussed embodiments of the invention, and advantageously increases the mechanical strength of the substrate around its exterior regions with relatively little trade-off in the reduction of the light off time associated with walls 47 in the central region of the substrate. The rate of radial thickening of the walls 49 may be substantially linear along the radius, or non-linear, wherein most of the thickening occurs near the outer skin 9. Additionally, such radial thickening may occur in discrete steps along the radius, rather than increasing with each point along the radius. FIG. 5 illustrates the thinnest walls near the centroid, C, with thicker walls in an intermediate region, and the thickest walls adjacent the skin 9. In addition to increasing the mechanical strength of the outer portions of the substrate, such radial thickening also advantageously slows the cool-down associated with the substrate, thereby reducing thermal stresses when the flow of hot exhaust gases through the substrate is abruptly stopped.
FIG. 6 schematically illustrates an embodiment where the thickness of the web walls 47 would vary in the radial direction as illustrated in FIG. 5 combined with walls which tapered along the length as in embodiment of FIG. 4. In such an embodiment, the thinnest walls for any particular transverse cross section (parallel to the ends 11, 13), would be located in a region R1. Conversely, the flow of exhaust gases would be progressively discouraged along the axis of the substrate 45 in the region R2 complimentary to the region R1 due to the greater pressure drop associated with the thicker walls provided in region R2. Of course, optionally, there may be additional regions which allow for a more progressive change in the wall thickness in the radial direction. Each of the walls 47 would increase in the axial direction wherein they are thinnest at the inlet end 11 and thickest at the outlet end 13.
FIGS. 7A and 7B illustrate how a tapered wall embodiment similar to that illustrated in FIG. 4 might be manufactured. In this method of the invention, a substrate having web walls 5 of uniform thickness along the axis of the substrate is first fabricated. This substrate is then subjected to a gas flow 49 which may be formed from an etchant gas containing an acid, an alkali or both which creates tapered sections 50 in the web walls 5 via chemical dissolution. Alternatively, the gas flow 49 may include abrasive solid particles which create the tapered sections 50 via a “sand blasting” effect.
In an alternative method of the invention, a tapered wall embodiment such as that illustrated in FIG. 4 may be manufactured by way of the co-extrusion device disclosed in published Patent Application No. WO2006/002065 A2 entitled “Die Assembly and Method for Forming Honeycomb Filters” by Thomas Ketchum et al. and assigned to Corning, Incorporated, the entire specification of which is expressly incorporated herein by reference. The tapered wall embodiment of FIG. 4 could be manufactured by this device by providing a first precursor ceramic material to form the core 39 of the walls 37 at a uniform rate in the extrusion device, while simultaneously extruding a second precursor ceramic material to form the variable thickness layers 41 at a variable rate through such extrusion device. In such a method, the inlet end 11 would be extruded first with little or no co-extrusion of the second precursor ceramic material. However, as the extrusion was formed, an ever-increasing amount of second precursor ceramic material would be co-extruded over the core walls 39 as more and more of the ceramic substrate was formed, until a maximum amount of second precursor ceramic material was co-extruded as the outlet end 13 of the ceramic substrate was formed. To create the linearly-tapered walls illustrated in FIG. 4, the rate of co-extrusion of the second precursor ceramic material would increase substantially linearly over the axis of the substrate 35 during the extrusion formation. While it is possible that the core portions 39 and outer layers 41 may be formed from two different precursor ceramic materials having different ceramic compositions, it is preferable that the two different precursor materials be formed of the same ceramic compound (i.e., cordierite, mullite, silicon carbide, alumina titanate) but vary only in viscosity, with the ceramic precursor material forming the outer layers 41 having a lower viscosity than the material used to form the core portions 39 of the walls 37.
The method of the invention further includes methods for forming the first and second embodiments of the invention illustrated in FIGS. 2A through 2C and in FIG. 3. In such a method, after drying and cutting of the extruded “green body” having web walls 5 of uniform thickness throughout the axis A, but before the firing of the green body, the substrate is placed into contact with a reagent that chemical removes or dissolves a portion of the web walls such that the walls are thinned or increased in porosity, or both. Such contact may be implemented by dipping a portion of the substrate into a liquid solution, or may be done via a flow of reactive gas through the substrate, the difference from the first described method being that the walls 25 in the second axial portion 24 may be masked off with a coating to insulate them from contact with such a liquid or gaseous chemical reagent.
Alternatively, after drying and cutting of the “green body”, but before the firing of the same, ozone gas may be conducted through the cell channels 7 in lieu of a liquid or gaseous chemical agent. Where the ozone contacts the green, unmasked substrate in the first axial region 22, it oxidizes and removes the binder that is within the precursor ceramic material. After a desired portion of the binder has been removed, the web walls are lightly abraded (by, for example, abrasive particles entrained in air, but needing much less contact than in the first-described method wherein “sand blasting” was exclusively relied upon) so as to remove the portion of the ceramic material from which the binder has been removed and thus make the web walls 23 thinner. Of course, any of the aforementioned methods may also be done after the firing of the green body if desired. One example of liquid chemical reagents that can be used to implement the aforementioned method are hydrochloric acid (HCl) and sodium hydroxide (NaOH). The first axial region 22 is first immersed or otherwise exposed to hydrochloric acid. When the green body is formed of cordierite, the hydrochloric acid removes mostly only MgO and Al2O3—very little of the silicon dioxide (SiO2) is removed. To remove the silicon dioxide, the walls 23 of the first axial region 22 of ceramic substrate are next immersed in or exposed to a solution of sodium hydroxide (NaOH). When the length of the substrate 20 is four inches, the first one inch of the substrate is exposed to the aforementioned two-step leeching process. Such a method results in a 10% weight reduction, which corresponds to a 35% weight reduction in the one inch length in the first axial portion 22 relative to the balance of the web walls. Either of the embodiments of FIGS. 2 and 3 may optionally be formed by a co-extrusion in the similar manner as the FIG. 4 embodiment by alternative and intermittently starting and stopping the flow of the second precursor.
FIG. 8 illustrates how the light off time of a thin-wall cordierite-containing flow through catalyzed substrate having 900 cells/inch and 2.5 mil wall thickness is substantially reduced when the average thermal mass ATM1 of only the first one inch section of a four inch long ceramic honeycomb is reduced to a value that is substantially lower than the average thermal mass ATMTOT of all the web walls. In this particular example, the ATM1 was 45% less than the ATM2 of the balance of the web walls of the substrate. As can be seen from the graph, such a reduction in the average thermal mass in the one inch long first axial section 22 reduces the light off time by an amount 52 from about 24 seconds to about 18.5 seconds when measured at a length, x, where x=L/7, and L is the total length of the substrate. Thus, the light off time to 275 degrees C. is reduced by an amount of greater than 10%, or by even greater than 20% or more as compared to a like substrate without the reduced thermal mass. Since the great majority of automotive pollutants generated by an engine having a catalytic converter occurs during the light off time, the 20+% time reduction that the graph of FIG. 8 indicates approximately a 20+% reduction in pollutant emissions. Moreover, such a substantial reduction in pollutant emissions is accomplished by the invention with no substantial reduction in the mechanical strength of the resulting honeycomb.
While this invention has been described with respect to several preferred embodiments, various additions, modifications and variations of the invention will become evident to persons of ordinary skill in the art. All such variations and additions are encompassed within the scope of the invention, which is limited only by the claims appended hereto and equivalents thereof.
Patent Application
20070238256
