Ceramic honeycomb structure having reduced stress web-skin joints

#This invention generally relates to ceramic honeycomb structures, and is specifically concerned with a honeycomb structure having reduced stress web-skin joints.

#Ceramic honeycomb structures are known in the prior art. Such structures generally comprise a plurality of webs that define a plurality of cells, and a skin surrounding the plurality of webs and having an inner surface or inner wall integrally connected to peripheral ends of the webs. Such ceramic honeycomb structures are currently favored for use as ceramic catalyst substrates for automobile exhaust systems. To maximize contact between the exhaust and the catalyst coated over the webs, high cell densities on the order of 400-1200 cells per square inch are favored. However, to minimize the pressure drop with such a structure, thin webs on the order of 0.001 to 0.005 inches are typical. During manufacture, both the plurality of webs and the skin are simultaneously extruded such that sharp-cornered or filleted joints are formed between the peripheral ends of the webs, and the inner wall of the skin. The skin is typically made considerably thicker than the webs so that the extrusion which ultimately becomes the final ceramic honeycomb structure will have sufficient mechanical strength to endure the handling and cutting procedures required to render the raw extrusion into a final product.

#Unfortunately, the sharp or slightly filleted interfaces between the thin webs forming the plurality of cells and the inner wall of the skin tend to concentrate mechanical stresses in these areas when mechanical forces are applied to the exterior of the skin during the manufacturing process. Additionally, the applicant has observed that the substantially greater thickness of the skin can create thermally-induced stresses at these interfaces, as the greater thickness of the skin inherently gives it a different coefficient of thermal expansion (CTE) than the substantially thinner cell walls, even though both walls are formed from the same ceramic material. Finally, the inventor has observed that during the extrusion process, these same differences in thickness between the webs and the skin creates differences in flow rate of the plasticized ceramic material as it is squeezed through the extrusion die that forms the honeycomb structure. It is theorized that these differences in the velocity of material flow weakens the joints between the peripheries of the outer webs and the inner wall of the skins.

#FIG. 1 is a perspective drawing of a prior art ceramic honeycomb structure that the invention applies to;

#FIG. 2 is a plan view of the area circled in phantom in the ceramic honeycomb structure of FIG. 1, illustrating the sharp edges in the interface between the peripheral ends of the webs, and the inner wall of the skin;

#FIGS. 3A, 3B and 3C are enlarged, partial plan views of prior art ceramic honeycomb structures that provide fillets in the joints between the webs or thickened webs to reduce micro-cracking in the interface between the webs and skin;

#With reference now to FIGS. 1 and 2, wherein like numerals designate like components throughout all the several figures, the ceramic honeycomb structure 1 that the invention may be applied to includes a cell array 3 formed from a plurality of individual cells 5. Each of the cells 5 is in turn defined by webs 7 integrally connected at their intersections. While the webs 7 define square-sided cells 5 in this example, other cell shapes such as hexagons are also within the scope of the invention. The cells 5 define a plurality of fluid conducting passageways extending parallel to the axis of rotation of the structure 1. Such ceramic honeycomb structures are typically used as catalytic converters for internal combustion engines. To this end, the webs 7 are coated with precious metal catalysts that promote the complete combustion of incompletely burned fossil fuel contained in the exhaust output of the engine. In order to maximize surface area contact between exhaust gases and the platinum-lined webs, high cell densities on the order of 400-1200 cells per square inch are favored. However, to minimize the pressure drop associated with such a relatively high cell density, thin webs on the order of 0.001 to 0.005 inches are generally employed.

#A skin 9 is provided around the cell array 3 in order to give the structure 1 sufficient mechanical strength to withstand the various steps of manufacture without damage to the thin webs 7. Consequently, the skin 9 is typically formed on the order of about three times the thickness of the webs 7, and the peripheral ends 13 of the webs 7 are joined to the inner wall surface 11 of the skin 9 by way of integral joints 15, best seen in FIG. 2.

#Relatively sharp corners 17 are formed at the intersections of the webs 78, and the inner wall 11 of the skin 9. Such sharp corners tend to concentrate mechanical stresses created by differences in properties and/or temperatures of use between the peripheral ends 13 of the webs 17, and the relatively thicker skin 9. These stresses can arise in the course of honeycomb use, or earlier during honeycomb manufacture when the extruded, green bodies that ultimately form the honeycombs 1 are fired in a ceramic kiln in order to form the final ceramic structures. Even though the skin 9 and webs 7 will comprise the same material (typically cordierite) after firing, differences in thermal or physical properties between the webs and skins can give rise to stresses that, in the presence of micro-cracks or otherwise, can cause portions of the skin 9 to separate from or easily flake away from the fired honeycomb structure, rendering it useless.