Honeycomb structure and method of producing the same

Abstract: A honeycomb structure (1) as a substrate for a catalyst includes a ridge (3) arranged on at least one part of an outer surface of the honeycomb structure and extending in a direction of th:rough-apertures (2) thereof, and at least one notch (4) formed in the ridge. A method of producing the honeycomb structure includes steps of extruding a ceramic batch to form a honeycomb structure having a ridge (3) in the form of a shape extending at least one part of the honeycomb structure in cross-section, and removing at least one part of the ridge in directions of through-apertures of the honeycomb structure to form a notch. With the arrangement of the honeycomb structure, it is possible to prevent any shifting of the honeycomb structure in directions of through-apertures of the structure and rotation of the structure about its axis relative to a converter can without closing any ends of through-apertures.

Description: This invention relates to an assembly of an extended honeycomb catalyst support and a container therefor, and to a honeycomb structure to be used as a support for a catalyst in such an assembly, e.g. for purifying exhaust gases from internal combustion engines, as a filter for removing fine particles in exhaust gases or as a substrate of various catalysts for deodorizing and/or purifying burnt gases when fuels such as various gases or petroleum are burnt. The invention also provides a method of producing such a honeycomb structure. Fig. 1 illustrates a catalyst converter which has been generally practically used for purifying exhaust gases from an automobile loaded with the converter. In order to make the converter not susceptible to violent vibrations in use, it comprises cushion members 22-1 and 22-2 and sealing members 22-3 about a honeycomb structure having through-apertures 21-1 through which exhaust gases pass and plate members 23 on upstream and downstream sides of the structure. The cushion members and the sealing members apply forces upon the honeycomb structure in traverse or lateral directions (referred to as "radial directions" hereinafter) relative to the directions of the through-apertures 21-1 and the plate members 23 apply forces directly or through the cushion members 22-1 onto the honeycomb structure in the directions of the through-apertures 21-1. The honeycomb structure is fixed and held in this manner. With such a construction of the catalyst converter, however, the cushion members 22-1 or the plate members 23 close some apertures 21-2 of the through-apertures 21-1 so that exhaust gases do not pass through the apertures 21-2, with the result that the catalyst carried by the portions of the apertures 21-2 will be inoperative. In order to avoid this disadvantage, it has been practically proposed to hold a honeycomb structure in radial directions by seal members arranged radially outward of the honeycomb structure for the purpose of saving catalytic noble metals. Moreover, a honeycomb structure has been known which is formed on its outer circumference with barriers adjacent at least one end face of a sealing member as disclosed in Japanese Utility Model Application Laid-open No. 62-179,319. With this honeycomb structure, which has had limited practical use, being only radially supported, however, high pressure is required to radially support the structure in order to fix it against movement caused by violent vibrations generated in use. It is possible to support it radially if thicknesses of partition walls of the ceramic honeycomb structure are comparatively thick, for example, 0.30 mm to provide a high strength against external pressures. However, such a supporting of the honeycomb structure is not applicable to a honeycomb structure whose partition walls are relatively thin such as 0.15 mm to 0.20 mm and susceptible to external pressures. In the honeycomb structure disclosed in the Japanese Utility Model Application Laid-open No. 62-179,319, however, partition walls are formed separately from a main body of the honeycomb structure, so that there is a difficulty in adhesiveness there-between. In more detail, even if a thermosetting inorganic filler is used which consists mainly of a ceramic material such as alumina, silica and the like, a problem of peeling or separation exists if the material of the honeycomb structure is cordierite, which is widely used. Such peeling results from thermal shocks which are repeatedly generated in use because of larger thermal expansion of the filler than that of the honeycomb structure itself. Therefore, although the high adhesiveness can effectively prevent the members of the structure from being shifted in directions of through-apertures of the structure without increasing the holding force, if the adhesiveness becomes less, it cannot prevent the shifting of the members in the directions of through-apertures. The barriers provided on the outer circumference of the structure are effective to prevent the shifting of the members in axial directions, but they do not serve to prevent shifting in radial directions or rotation of the members. It is also mentioned that EP-A-217493 shows a housing and a substrate therein which is formed of folded metal foil strips. The substrate has a seal extending circumferentially around it and received in grooves in the housing. It is an object of the invention to provide an assembly, a honeycomb structure and a method of producing the same which eliminate the disadvantages of the prior art to prevent shifting of the honeycomb structure relative to a converter can or vessel in directions of through-apertures of the structure as well as radial directions or rotating directions. The assembly of honeycomb catalyst support and container according to the invention is set out in claim 1. The honeycomb structure for use in such an assembly according to the invention is set out in claim 11. In another aspect of the invention, a method of producing the honeycomb structure is as set out in claim 12. In the assembly of the invention, when the honeycomb structure having a rib arranged on at least one part of the outer surface extending in the direction of the through-apertures is assembled with a sealing member to form a converter, rotation of the structure about its axis relative to the converter can is prevented by means of engagement of the sealing member and the rib. At the same time, shifting of the structure relative to the converter can is also effectively prevented by steps formed by the notch in the direction of the through-apertures. According to the method, as the rib having the notch is formed integral with the honeycomb structure, it is possible to solve the problem of adhesiveness which would occur when the rib is formed separately from the honeycomb structure. Moreover, as the notch can be formed by cutting, grinding or pattern-pressing, the honeycomb structure provided with the subject features of the invention is formed in a simple manner. The invention will be more fully understood by referring to the following detailed specification and claims taken in connection with the appended drawings. Figs. 1 and 2 are views illustrating examples of a honeycomb structure and a catalyst converter of the prior art, respectively; Figs. 3a-3d are perspective views illustrating various embodiments of honeycomb structures according to the invention, respectively; Figs. 4a and 4b are views illustrating steps of one embodiment of the method of producing the honeycomb structure according to the invention; Fig. 5 is a view showing a step of another embodiment of the method according to the invention; Figs. 6a-6c are views illustrating steps of assembling the honeycomb structure according to the invention into a catalyst converter; Figs. 7a and 7b are views illustrating shapes of honeycomb structures used in the test for comparing the invention with the prior art; and Figs. 8a-8c are views showing shapes of catalyst converters used in the test for comparing the invention with the prior art. Figs. 3a-3d illustrate in perspective views various embodiments of a honeycomb structure according to the invention, respectively. In the embodiment shown in Fig. 3a, a cylindrical honeycomb structure 1 is partially formed on its outer circumference with a rib 3 extending in parallel with through-apertures 2 and having a predetermined height. The rib 3 is partially cut to a surface in flush with the outer surface of the structure to form a notch 4. In the embodiment shown in Fig. 3b, a honeycomb structure 1 is formed fully on its outer circumference with a plurality of ribs 3 extending in parallel with through-apertures 2. Some of the ribs 3 are partially cut to a surface in flush with the outer surface of the structure to form a notch 4. In Fig. 3c, a cylindrical honeycomb structure 1 is partially formed on its outer circumference with a rib 3 extending in parallel with through-apertures 2 and having a predetermined height. A part of the rib 3 is cut to form a notch 4 consisting of a plurality of grooves adjacent each other. In the embodiment shown in Fig. 3d, edges of a rib 3 formed on a honeycomb structure 1 are smoothly connected with an outer circumferential surface of the structure 1. In contrast herewith, in the embodiment of Fig. 3a, the rib 3 forms steps or shoulders relative to the outer circumference of the structure 1. The honeycomb structures as shown in Figs. 3a, 3b and 3d are preferably manufactured according to steps shown in Figs. 4a and 4b. In other words, as shown in Figs. 4a and 4b, a ceramic batch is extruded through a dies 5 having a retaining cover 6 formed at its opening with a notch or formed in a predetermined shape to form an integral honeycomb structure having a rib 3. Thereafter, a cutting tool 7 vertically movably provided relative to the honeycomb structure 1 is lowered to a position where the cutting tool 7 is able to form a predetermined notch 4. The notch 4 is formed in this manner. The cutting tool may be rotatable. A depth of the notch 4 can be adjusted as will by changing the position where the cutting tool is lowered. In this case, by changing the shape of the cutting tool 7 the notch can be formed whose bottom is quite coincident with an outer circumferential surface of the honeycomb structure 1. In the embodiment shown in Figs. 4a and 4b, the notch 4 is formed by means of the cutting tool 7 immediately after the extrusion of the honeycomb structure 1. In such a case, the notch 4 can be formed by a thin wire such as piano wire without the cutting tool 7 because the extruded structure is still sufficiently soft. As an alternative, after drying or firing the structure, it may be cut by the cutting tool 7 or ground by a grinding wheel such as a diamond wheel. In producing the honeycomb structure 1 as shown in Fig. 3c, after forming a honeycomb structure 1 having a predetermined rib 3 and before drying it, a pattern tool 8 having serrations is pressed against the rib 3 at a predetermined position to form grooves by serrations of the pattern tool 8 as shown in Fig. 5. After drying or firing the structure, the notch may of course be formed by cutting or grinding. In assembling the honeycomb structures as above described into converters as shown in Figs. 6a, 6b and 6c, a honeycomb structure is encircled by a sealing material or member 9 and provided with stainless wires 12 if required. Thereafter the structure is incorporated in converter cans 10 and 11. With such an arrangement, as shown in Figs. 6a and 6b any rotation of the honeycomb structure 1 about its axis is prevented by abutment of the rib 3 against edges 9a of the sealing or holding material or member 9 and a projection 10a extending inwardly of the can 10. Moreover, the shifting of the structure 1 in directions in parallel with the through-apertures 2 is prevented by abutment of the notch 4 of the rib 3 against an end 9b of the sealing member 9 and projections 10b extending inwardly of the cans 10 and 11. In this embodiment, the sealing member 9 is fitted in the notch 4 of the rib 3. In the embodiment in Figs. 6a and 6b, a space is formed by the honeycomb structure 1 and the cans 10 and 11 spaced by the rib 3 and projections 10a and 10b. Fig. 6c illustrates another embodiment in which a can 10 is similar in shape to the honeycomb structure 1 but slightly larger than the structure 1 and diametrically divided into two halves and a sealing member 9 is filled between the honeycomb structure 1 and the can 10. The sealing member 9 is formed with slits 9c correspondingly to the notch 4 of the honeycomb structure 1 so that shifting of the honeycomb structure 1 in directions parallel to the through-apertures 2 is prevented by the notch 4 and the part of the sealing member 9 between the slits 9c and fitted in the notch 4. In this manner, the honeycomb structure 1 can be held only by the sealing member 9 without using holding means such as retains which tend to close the through-apertures, thereby saving cost for manufacturing the cans 10 and 11. Moreover, if end faces of the rib 3 and through-apertures corresponding to the notch 4 are closed by a ceramic material or the like, a catalytic precious metal can also be saved. Actual examples of the honeycomb structure according to the invention will be explained hereinafter. Example 1 A cordierite was extruded and fired to obtain honeycomb structures each having a 100 mm diameter, a 100 mm overall length and a through-aperture density of 46.5/cm². Thicknesses of partition walls were 0.20 mm except those of Comparative example 2 which were 0.23 mm. Thicknesses of outer walls were about 0.3 mm. A honeycomb structure of Embodiment 1 according to the invention had 48 ribs 3 circumferentially arranged like petals, each having a 1 mm height and 2 mm radii of curvatures at a top and a bottom as shown in Fig. 3b. Moreover, a notch was at a location 10 mm spaced from one end surface of the honeycomb structure and had a 25 mm length, a 1 mm depth and a width over three ribs 3. A thickness of the outer wall is uniformly 0.3 mm which is different from the embodiment shown in Fig. 3b whose outer wall is varied to form the ridges. A honeycomb structure of Embodiment 2 according to the invention had one rib 3 having a 1 mm height and a 20 mm width (circumferential length) and a notch 4 at a location 10 mm spaced from one end surface of the structure and having a 25 mm length and a 1 mm depth as shown in Fig. 3a. A honeycomb structure of Embodiment 3 according to the invention was similar to that of the Embodiment 2 with exception of a 0.5 mm height of a rib and a 0.5 mm depth of a notch. A honeycomb structure of Embodiment 4 according to the invention had a rib formed by extending an outer wall 0.3 mm radially outwardly over a 20 mm width (circumferential length) and a notch located spaced 10 mm from one end surface and having a 25 mm length and a 0.3 mm depth. A honeycomb structure of Embodiment 5 according to the invention had an addition to the notch of the Embodiment 4 a further notch spaced 10 mm from the other end surface and having a 25 mm length and a 0.3 mm depth. A honeycomb structure of Embodiment 6 according to the invention had two ribs circumferentially 45° spaced each having a 0.3 mm height and a 20 mm width and each formed at its center with a notch having a length of 50 mm. On the other hand, honeycomb structures of Comparative Examples were prepared which did not have ridges 3 and notches 4 as shown in Fig. 7b. These honeycomb structures of Embodiments 1-6 and Comparative Examples 1 and 2 were tested on various performances. The results are shown in Table 1. In an external pressure strength test, aluminum plates having a thickness of about 20 mm were applied through urethane sheets having a thickness of about 0.5 mm to end surfaces of a honeycomb structure and a circumferential surface of the structure was hermetically surrounded by an urethane tube having a wall thickness of about 0.5 mm. The honeycomb structure was accommodated in a vessel filled with water and the pressure in the vessel is raised slowly. The pressure in the vessel when a sound was generated due to damage of the honeycomb structure was measured. In a canning test, a ceramic mat as a holding member shown in Fig. 6b was wound about a honeycomb structure and this assembly was inserted in a jig. The jig had an inlet whose inner diameter was larger than that of an outlet to form a tapered jig. The inner diameter of the outlet of the jig was substantially the same as an inner diameter of an inlet of a steel pipe or can. The outlet of the jig was attached to the inlet of the can and the honeycomb structure was forced into the can by means of a hydraulic ram. After the honeycomb structure was removed out of the can, an external appearance of the structure was observed. The ceramic mat had a thickness of 4.9 mm. The inner diameter of the steel pipe or can was 104.4 mm. In a heated vibration test, a holding member shown in Table 1 was wound about a honeycomb structure and the assembly was forced into a steel pipe or can having an inner diameter forming a clearance between the honeycomb structure and a can with a flange welded thereto. Thereafter, hollow conical members were bolted to both ends of the can and tested. A heated gas heated at 800°C by means of a propane burner was caused to flow against the can for two minutes so as to heat the honeycomb structure. Then air at room temperature was caused to flow against the can for two minutes so as to cool the honeycomb structure. One cycle formed by such a heating and a cooling was repeated to 50 cycles while the can was subjected to vibrations of 0-20 g with 200 Hz. Thereafter, the hollow conical members were removed from the can from which the honeycomb structure was removed to observe an external appearance of the structure. From the results shown in Table 1, it is evident that the honeycomb structures resisting to the heated vibration test simulated as actually used conditions are (1) those supported in directions in parallel with through-apertures as well in a hitherto used manner, (2) those including narrow clearances between the honeycomb structures and can such as 2.2 mm or supported by high holding pressure, and (3) those of the embodiments according to the invention having ribs whose heights are more than 0.5 mm. The honeycomb structures according to the invention can be held only on their circumferential surfaces even by low holding pressures. Even the structures having the ribs whose heights are 0.3 mm according to the invention can resist to the heated vibration test depending upon conditions to be subjected. The effectiveness of the ribs having the notches according to the invention has been ascertained. It has been moreover found that catalytic noble metal can be saved approximately 8% in comparison of effective volumes of catalysts between those of Embodiments according to the invention and Comparative examples of the prior art. It is to be understood that the invention is not limited to the above embodiments and various changes and modifications may be made in the invention without departing from the scope of the invention. For example, sizes of the ribs and the notches are not limited to those illustrated and explained because they are of design choice to be determined by used conditions and widths in through-aperture directions of cushion members abutting against the ribs and notches. Although the cross-sectional shape of the ceramic honeycomb structures is correctly circular in the above embodiments, it is not limited to circular and may be for example elliptical. Moreover, the sectional shape of cells of the honeycomb structures are not limited to square as in the embodiments and the material thereof is not limited to cordierite used in the above embodiments. Furthermore, both ends of through-apertures of the honeycomb structures may of course be alternately closed. As can be seen from the above description, the honeycomb structure according to the invention comprises the rib having the notch on its outer circumferential surface to prevent shifting in through-aperture directions and rotation about its axis, thereby enhancing the reliability of a catalyst converter incorporating the honeycomb structure therein. Moreover, the honeycomb structure according to the invention can be incorporated in a converter without closing any ends of through-apertures so that all the catalytic noble metal carried by the honeycomb structure is effectively used for the purpose.

Claims: An assembly of an extruded honeycomb catalyst support (1) having axially extending passages (2) through it and a container (10) therefor, characterised in that the support (1) has at least one integrally formed axially extending protruding rib (3) at its periphery providing circumferential abutment faces, said rib (3) having at least one notch (4) providing an axial abutment face, the support (1) is held against axial and rotational movement in the container (10) by said abutment faces. An assembly according to claim 1, wherein said notch (4) has a depth such that its bottom surface is flush with the adjacent outer surface of the honeycomb structure (1). An assembly according to claim 1 or claim 2, wherein there are a plurality of said ribs (3) on the outer surface of the honeycomb structure and said notch (4) extends over at least two adjacent ribs (3). An assembly according to claim 1, wherein said notch (4) is provided by a plurality of grooves adjacent each other in the axial direction. An assembly according to any one of claims 1 to 4 wherein said rib (3) has its lateral edges smoothly joining with the adjacent outer surface of the honeycomb structure. An assembly according to any one of claims 1 to 5, wherein the container (10) in which the support (1) is incorporated is formed with at least one projection (10b) extending inwardly and abutting against the rib (3) of the support. An assembly according to any one of claims 1 to 6, wherein the support (1) is provided with a sealing member (9) encircling it, said sealing member (9) being fitted in the notch (4) of the rib (3). An assembly according to claim 1 wherein the container (10) in which the support (1) is located is similar in shape to the support (1) but slightly larger than the support (1) and is diametrally divided into two halves, and a sealing member (9) is located between the support (1) and the container (10) and is formed with slits (9c) correspondingly to the notch (4) so that part of the sealing member between the slits (9c) is fitted in the notch (4). An assembly according to claim 1 wherein said support has two said ribs (3) circumferentially spaced. A honeycomb structure (1) for use as a support for a catalyst in an assembly according to any one of claims 1 to 9, having a rib (3) arranged on at least one part of an outer surface of the honeycomb structure and extending in the direction of the through-apertures thereof, and at least one notch (4) formed in said rib (3). A method of producing a honeycomb structure according to claim 10 comprising the steps of extruding a ceramic batch to form a honeycomb structure (1) having an axially extending rib (3) in the form of a shape extending over at least one part of the periphery of the honeycomb structure as seen in cross-section, and removing at least one part of the rib (3) of the honeycomb structure to form a notch (4). A method according to claim 11, wherein said notch (4) is formed by one of cutting and grinding. A method according to claim 11, wherein said notch (4) is formed as a plurality of grooves by pressing a pattern tool having serrations against the rib of the honeycomb structure while it is still soft after extrusion of the honeycomb structure. A method according to claim 11, wherein said notch is formed after one of drying and firing of the honeycomb structure.

Inventors: HORIKAWA OSAMU, IKESHIMA KOICHI, HORIKAWA, OSAMU, IKESHIMA, KOICHI
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1989
Main classification: F01N3
IPC codes: B01D46, B28B11, B01D53, F01N3, B01J35, C04B38