Electrically heatable honeycomb body

Abstract: An electrically heatable honeycomb body (10) comprises a large number of passages (12) which are each defined by partition walls (11) made of an electroconductive material and which are substantially parallel to the direction of a gas flowing through the honeycomb body, and two ends which are the gas inlet and outlet sides of honeycomb body, wherein the heat generation of honeycomb body by electrification is controlled by adjusting the flow of electricity therein, e.g. by a slit (13). Those cells each constituted by a passage and partition walls, present in the vicinity of each region of current concentration are formed with a structure such that the stress caused by current concentration can be reduced. In this honeycomb body the stress caused by current concentration is reduced and thereby the rupture of its cells can be prevented.

Description: Background of the Invention (1) Field of the Invention The present invention relates to an electrically heatable honeycomb body. More particularly, the present invention relates to a honeycomb body suitable for use as a honeycomb heater for, for example, purification of the exhaust gas emitted during the cold start of automobile. (2) Description of the Related Art In recent years, metallic honeycomb structures have drawn attention as a catalyst, a catalyst carrier or the like used for purification of the nitrogen oxides (NOx), carbon monoxide (CO) and hydrocarbons (HC) present in the exhaust gases emitted from the internal engines of automobiles, etc. As the control for such exhaust gases has become stricter, development of a heater, etc. capable of reducing the emission during the cold start of engine has become more necessary. In order to respond to such necessity, the present applicant proposed heaters such as mentioned below. In JP-A-4-224 220 (1992) there was proposed a honeycomb heater comprising (a) a honeycomb structure and (b) electrodes, wherein slits as a resistance-adjusting means were provided in the honeycomb structure between the electrodes. This heater can quickly heat the low-temperature exhaust gas emitted during engine start. EP-A-502 731, corresponding to JP-A-4-277 481, there is proposed a honeycomb heater as shown in Fig. 17, wherein the partition wall "a" at the dead end of each slit 100 (where a current is concentrated most) and the partition walls "b" at the both sides of each slit (where a current is concentrated at a relatively high degree) were made thicker with an electroconductive material; and a honeycomb heater as shown in Fig. 18, wherein the passages "d", "f" and "h" (where a current is concentrated) were sealed with an electroconductive material. In these honeycomb heaters, the partition walls at the dead end of each slit (where a current is concentrated most) or at the portion surrounding each slit end (where a current is concentrated at a relatively high degree) were made thicker or sealed. It was found, however, that the portion showing a higher temperature rise during electrification was not the cells at each slit end but was the cells at the both sides of each slit end. Therefore, the above technique is insufficient for prevention of abnormally high heat generation at current-concentrating portions. In JP-A-6-66 132 (1994) there was proposed a honeycomb heater wherein the polygonal section of each cell undergoing larger thermal shock and generating a higher thermal stress was formed so as to have a side of 10 mm or less in order to relax the thermal shock and prevent the deformation, cracking, etc. of the cell. However, when this honeycomb heater was actually electrified, cell deformation occurred at the cells of the both sides of each slit end where current concentration took place, and a higher stress appeared at these cells. Therefore, it is necessary to take an appropriate countermeasure to prevent the deformation of such cells and reduce their thermal stress. In JP-A-7-163 888 (1995) there was proposed a honeycomb heater whose thickness in the direction of gas flow was made smaller at a particular position so as to allow quick local heating. In this heater, however, since the thickness changes sharply from that particular position, current concentration occurred at that position at times during electrification. As described above, when a honeycomb structure is electrified, there appears (appear) a current-concentrating portion(s). Such portion(s) is (are) the vicinity of each slit end in the case of a honeycomb structure having slit(s) and, in the case of a honeycomb structure of vortex type, the center thereof. When the honeycomb structure is subjected to repeated electrification, cell rupture occurs at the current-concentrating portion(s) of the honeycomb structure, which may finally lead to no flow of electricity. Attention is also drawn to WO 93/21430 and EP-A-465 184. Object and Summary of the Invention The present inventors made further study and, as a result, found that some of the cells at both sides of each slit end suffer high thermal stress and temperature when electrified and showed cell deformation, as shown in Fig. 19. In the techniques proposed heretofore, it had been thought that current concentration occurs at each slit end and that the modification of the slit end is necessary. It has been found, however, that even if overheating takes place at the slit end owing to current concentration, it causes no serious problem because the generated heat is dissipated from the slit end toward the lower-temperature-portion adjacent to the slit end and closer to the circumference of honeycomb body. Hence, the object of the present invention is to provide an electrically heatable honeycomb body having further improvement against cell rupture during repeated electrification. According to the present invention there is provided an electrically heatable honeycomb body as set out in claim 1. Brief Description of the Drawings Fig. 1 is a sectional view showing a heater containing therein a honeycomb body formed by winding foil type thin plates into a vortex shape. Figs. 2 and 3 are sectional views showing respective examples of the electrically heatable honeycomb body of the present invention wherein slits are formed in parallel to the passages for resistance adjustment. Figs. 4 and 5 are sectional views showing respective examples of the electrically heatable honeycomb body of the present invention wherein the shape of each intersection point of cell partition walls at each slit end is changed. Figs. 6 and 7 are sectional views showing respective examples of the electrically heatable honeycomb body of the present invention wherein the cells at the portion surrounding each slit end are each provided with a rib. Fig. 8 is a sectional view showing an example of the electrically heatable honeycomb body of the present invention wherein the cells at the portion sorrounding each slit end are provided with ribs. Fig. 9 is a sectional view showing an example of the electrically heatable honeycomb body of the present invention wherein the passages at the portion surrounding each slit end are sealed with an electroconductive material. Figs. 10 to 16 are sectional views each showing an example of the electrically heatable honeycomb body of the present invention wherein the cells in the vicinity of each current-concentrating portion have a structure capable of withstanding deformation and the cells at other portions have an easily deformable structure. Fig. 17 is a fragmentary view showing each slit end in a known electrically heatable honeycomb body. Fig. 18 is a fragmentary view showing each slit end in a known electrically heatable honeycomb body. Fig. 19 is a fragmentary view showing each slit end in a known electrically heatable honeycomb body. Fig. 20 is a fragmentary view showing an example of the electrically heatable honeycomb body of the present invention wherein the thicknesses of those partition walls present in the vicinity of each current-concentrating portion are made larger than the thicknesses of the partition walls at other portions. Fig. 21 is a view showing an example of the deformation of slit end during heating, in a known electrically heatable honeycomb body. Fig. 22 is a view showing an example of the electrically heatable honeycomb body of the present invention wherein the thicknesses of the partition walls in the vicinity of each current-concentrating portion are made larger to prevent deformation during heating. Fig. 23 is a view showing an example of the electrically heatable honeycomb body of the present invention wherein the cells in the vicinity of each current-concentrating portion are each provided with a rib to prevent deformation during heating. Detailed Description of the Invention In the electrically heatable honeycomb body of the present invention, improvement has been made at the portions of honeycomb body where current concentration takes place during electrification and thereby higher thermal stress, higher temperature and cell deformation are invited, by (1) lowering the temperature, (2) reducing the thermal stress and (3) minimizing the cell deformation, of said portions of honeycomb body. Also in the electrically heatable honeycomb body of the present invention having slits, improvement was made to prevent cell-to-cell contact between the cells present at the two sides of each slit end during the heat generation by electrification, by making larger the partition wall lengths of each cell present at the dead end of each slit and in the slit extension line, in the slit width direction. In the present electrically heatable honeycomb body, specific examples of the "cell structure in which the stress caused by current concentration can be reduced" are as follows. (1) The thicknesses of those partition walls present in the vicinity of each current-concentrating portion are made larger than the thicknesses of the partition walls at other portions. (2) The thicknesses of those partition walls present in the vicinity of each current-concentrating portion and in the portion extending therefrom toward the circumference of honeycomb body are gradually made larger as the partition walls are closer to the circumference. (3) At least one slit is formed in the present honeycomb body so as to control the heat generation of honeycomb body and the thicknesses of those partition walls present at the portion surrounding each slit end are made larger than the thicknesses of the partition walls at other portions. (4) At least one slit is formed in the present honeycomb body so as to control the heat generation of honeycomb body and the thicknesses of those partition walls present at the portion surrounding each slit end are made larger at the intersection points of such partition walls. (5) In the above (4), the thicknesses of the intersection points of partition walls are made gradually larger as the intersection points are closer to the circumference of honeycomb body. (6) At least one slit is formed in the present honeycomb body so as to control the heat generation of honeycomb body and each cell present at the portion surrounding each slit end is provided with a rib for reinforcement. (7) At least one slit is formed in the present honeycomb body so as to control the heat generation of the honeycomb body and those passages present at the portion surrounding each slit end are sealed with an electroconductive material. (8) Those cells present in the vicinity of each current-concentrating portion are formed in a structure capable of withstanding deformation and the cells at other portions are formed in an easily deformable and flexible structure. (9) At least one slit is formed in the present honeycomb body so as to control the heat generation of honeycomb body and those cells present at the portion surrounding each slit end are formed in a structure capable of withstanding deformation and the cells at other portions are formed in an easily deformable and flexible structure. (10) In the above (8) or (9), the cells of a structure capable of withstanding deformation have a tetragonal (quadrilateral) section and the cells of an easily deformable and flexible structure have a hexagonal section. (11) At least one slit is formed in the present honeycomb body so as to control the heat generation of the honeycomb body and those cells present at the portion surrounding each slit end are provided in a more packed state than the cells at other portions. In the present electrically heatable honeycomb body comprising a large number of passages, the thicknesses and/or shapes of the partition walls at each honeycomb body portion which is liable to cause cell rupture during repeated electrification, have been changed to prevent cell rupture. In a known resistance-adjusting type heater as shown in Fig. 17, abnormally high heat generation is prevented by increasing the thicknesses of the partition walls "a" and "b" where current concentration takes place, or by sealing the passages of each current-concentrating portion with an electroconductive material. As mentioned previously, however, a detailed study by the present applicant indicated that the portions showing a larger temperature increase during electrification are the cells located at both sides of each slit end and that the above countermeasure for abnormally high heat generation is insufficient and may have durability problems (e.g. cracking) in long-term use. Also in a known heater as shown in Fig. 18, not only the cell at the dead end of each slit but also the few cells adjacent thereto in a direction perpendicular to the direction of slit extension are sealed with an electroconductive material. Hence, in the present electrically heatable honeycomb body, change in thickness and/or shape of partition wall was made in the portions where current concentration takes place during repeated electrification, namely, the vicinity of each slit end in the case of slit-containing honeycomb body and, for example, the center in the case of vortex type honeycomb body, whereby stress relaxation, temperature reduction, prevention of cell deformation, etc. have been made possible and the durability to cell rupture during repeated electrification has been improved. The material constituting the present honeycomb body is not restricted as long as it is a material capable of generating heat when electrified, i.e. an electroconductive material. It my be a metal or a ceramic with a metal being preferred for its mechanical strength. The honeycomb body made of a metal includes, for example, those made of stainless steel or a material having a composition represented by Fe-Cr-Al, Fe-Cr, Fe-Al, Fe-Ni, W-Co, Ni-Cr or the like. Of these honeycomb bodies, those made of Fe-Cr-Al, Fe-Cr or Fe-Al are preferred because of excellent heat resistance, oxidation resistance and corrosion resistance and low cost. The honeycomb body may be porous or nonporous. When a catalyst is loaded thereon, a porous honeycomb body is preferred because it has higher adhesivity to the catalyst layer loaded thereon and hardly gives rise to the peeling of the catalyst layer caused by the difference in thermal expansion between the honeycomb body and the catalyst layer. Next, description is made on an example of the process for production of a metallic honeycomb body used as one type of the honeycomb body of the present invention. First, a material metal powder is prepared using, for example, a Fe powder, an Al powder and a Cr powder, or a powder of an alloy thereof so that they give a desired composition. The material metal powder is then mixed with an organic binder (e.g. methyl cellulose or polyvinyl alcohol) and water. The resulting mixture is subjected to extrusion to obtain a honeycomb body of desired shape. In the above mixing, it is preferable that the material metal powder is mixed with an antioxidant (e.g. oleic acid) before being mixed with an organic binder and water, or that a material metal powder processed so as to have oxidation resistance is used. The honeycomb body is fired at 1,000-1,400°C in a non-oxidizing atmosphere. Use of a non-oxidizing atmosphere containing hydrogen is preferable because the organic binder is decomposed and removed by the help of the catalytic action of Fe, etc. and, as a result, an excellent sintered honeycomb body is obtained. When the firing temperature is lower than 1,000°C, the honeycomb body is not sintered. When the firing temperature is higher than 1,400°C, the sintered body obtained is deformed. Preferably, the surfaces of the partition walls and pores of the sintered honeycomb body is coated with a heat-resistant metal oxide such as Al 2 O 3 , Cr 2 O 3 or the like so that the honeycomb body can have improved heat resistance, oxidation resistance and corrosion resistance. In the thus obtained honeycomb body is formed, for example, at least one slit as a resistance-adjusting means. As in the prior art, the slit can be formed in various directions, various positions and various lengths. The thus obtained metallic honeycomb body is provided with an electrification means (e.g. electrodes) ordinarily at some of the partition walls at the circumference or inside the circumference, by brazing, welding or the like, to obtain a honeycomb heater capable of generating heat when electrified. The electrification means is not particularly restricted as long as it can supply an electric current from a power source to the honeycomb heater, and a preferable example is electrodes. The metallic honeycomb body, when used as a heater, is preferably formed so as to have a total resistance of 0.001-0.5 Ω. It is preferable to load a catalyst on the surfaces of the above metallic honeycomb body because when the resulting honeycomb body is used as a heater or a catalytic converter, a higher temperature can be obtained in the purification reaction (e.g. oxidation reaction) of exhaust gas. The catalyst loaded on the surfaces of the metallic honeycomb body comprises a heat-resistant carrier having a large surface area and a catalyst component loaded thereon. The heat-resistant carrier having a large surface area is preferably an inorganic oxide, and typical examples thereof are γ-Al 2 O 3 , TiO 2 , SiO 2 -Al 2 O 3 and lanthanum-cobalt of perovskite structure. The catalyst component includes, for example, noble metals such as Pt, Pd, Rh, Ir and the like; and base metals such as Cu, Cr, Co and the like. A catalyst comprising γ-Al 2 O 3 and 10-100 g/ft 3 of Pt and/or Pd loaded thereon is preferable. In the present invention, the shape of the honeycomb body is not particularly restricted. But, as an example, such a specific shape is preferable that the cell density is 6-1,500 cells/in 2 (0.9-233 cells/cm 2 ) and the partition wall thickness is 50-2,000 µm. The partition walls, etc. constituting the present honeycomb body may be porous or nonporous as mentioned above and its porosity is not restricted. However, the porosity is preferably 0-50%, more preferably less than 25% in view of the strength, oxidation resistance and corrosion resistance. When a catalyst is loaded on the honeycomb body, the porosity of the honeycomb body is preferably 5% or more in view of the adhesion to the catalyst layer. Incidentally, in the present invention, the honeycomb body refers to a one-piece structure having a large number of passages defined by partition walls. The external shape of the honeycomb body can be cylindrical, rectangular, oval or the like. The passages are preferably nearly parallel to each other from the standpoints of pressure loss, etc. The sectional shape of each passage (i.e. cell shape) may be any desired one such as circular, polygonal, corrugated or the like. In the present invention, the material used for increasing the thicknesses of particular partition walls and/or the electroconductive material used for sealing particular passages have no restriction as to their kinds as long as they are electroconductive. However, they are preferably the same material or the same type of material as the material constituting the partition walls in view of the productivity and the adhesion to the partition walls. The present invention is hereinafter described by Examples with reference to the accompanying drawings. However, the present invention is not restricted to the Examples. Fig. 1 is a sectional view showing a heater containing therein a honeycomb body formed by winding foil type thin sheets into a vortex shape. In Fig. 1, 1 is a honeycomb body having two ends, which comprises a number of metal sheets 3 provided in layers in a pattern to form a large number of gas passages 5 stretching from one end to the other end of the body. The metal sheets 3 are ordinarily constituted by planar sheets and corrugated sheets arranged alternately. In the honeycomb body 1, a number of metal sheets 3 are wound in a "S" shape, and an electric current is passed between electrodes 7 and 7' to generate heat in the body. Figs. 2 and 3 are each a sectional view showing an example of the electrically heatable honeycomb body of the present invention wherein slits are formed in parallel to the passages for resistance adjustment. 10 is a honeycomb body which has a large number of passages 12 each defined by partition walls 11 made of an electroconductive material and each provided substantially parallel to the direction of a gas flowing through the honeycomb body and which has two ends, i.e. a gas inlet side and a gas outlet side. In this honeycomb body, slits are formed in parallel to the passages 12. 18 are cells each constituted by a passage and partition walls. In such a honeycomb body 10, current concentration and consequent cell temperature rise take place during repeated electrification at particular portions, for example, at the center 14 in the case of the vortex type honeycomb body of Fig. 1 and at the portion 15 surrounding each slit end in the case of the slits-containing honeycomb body of Fig. 2 or 3. However, the current density and temperature at the portion 15 can be reduced by increasing the thickness of each partition wall 11 of the portion. By thus increasing the thickness of the partition wall 11, cell deformation can be reduced and stress concentration can be relaxed. In Japanese Patent Application Kokai (Laid-Open) No. 277481/1992, increase in thickness of partition wall is restricted to the cells at the dead end of each slit. In contrast, in the present Examples, improvement was made based on the finding that stress concentration takes place more at the sides of each slit end rather than at the dead end. That is, in the present Examples, the portion 15 surrounding each slit end refers to 4 or more cells, preferably 5-6 cells present at each side of the end of each slit 13 although the number of these cells differs by the cell density (size of passage) of honeycomb body. In the present Examples, the increase in partition wall thickness at the portion surrounding each slit end is preferably 20-40% of the partition wall thickness at other portions. In the Example of Fig. 2, increase in partition wall thickness is employed not only at the current-concentrating portion but also at the portion extending therefrom toward the circumference of honeycomb body, and the thickness of partition wall is gradually made larger as the partition wall is closer to the circumference. In this case, the degree of gradual increase in partition wall thickness in direction toward circumference is preferably 2-10% per one cell. Further in this case, the current density at the portion closer to the circumference is lower and the temperature at the portion is lower than that at the current-concentrating portion, whereby the heat generated at the current-concentrating portion can be quickly dissipated toward the circumference. Figs. 4 and 5 are each a sectional view showing an example of the electrically heatable honeycomb body of the present invention wherein the shape of each intersection point of cell partition walls at the portion surrounding each slit end is changed. Figs. 4 and 5 are each an example wherein the cell partition walls 11 at the portion surrounding each slit end have larger thicknesses at their intersection points 16. In these honeycomb bodies 10, stress at the intersection points where stress concentration takes place, can be relaxed and cell rupture can be avoided by making larger the thicknesses of the areas at which current concentration and consequent cell temperature rise take place during repeated electrification, namely, the thicknesses of the intersection points 16 of partition walls at the portion surrounding each slit end in the case of the slits-containing honeycomb body of Fig. 4 and, in the case of the vortex type honeycomb body of Fig. 1, the thickness of, for example, the intersection points at the center 14. It is also preferable to make gradually larger the thicknesses of intersection points 16 in a direction proceeding from each slit end toward the circumference of honeycomb body, as seen in Fig. 5. The degree of increase in thickness of intersection point at stress-concentrating portion is preferably 20-50% of the thickness before increase, i.e. the thickness of other portions. The degree of gradual increase in thickness of intersection point in a direction proceeding from each slit end toward the circumference is preferably 2-10% per one cell. In the Examples shown in Figs. 4 and 5, only the intersection points 16 of cell partition walls 11 are allowed to have larger thicknesses and, resultantly, decrease in current resistance is small. Hence, the resulting honeycomb body is suitable for use in a honeycomb heater for automobile which must have a high resistance during cold start. Figs. 6 to 8 are each a sectional view showing an example of the electrically heatable honeycomb body of the present invention wherein the cells at the portion surrounding each slit end are provided with ribs. The cells 18 forming the passages of the portion surrounding each slit end (current-concentrating portion when electrified) are provided with ribs 19 or the like for reinforcement. This results in increased number of ribs and serves for heat dissipation toward surrounding area, temperature reduction at current-concentrating portion and prevention of cell deformation at the portion, whereby stress concentration can be relaxed. In the Example of Fig. 8, linear ribs 19 are provided in the end of each slit 13 to prevent the deformation of the cells 18 present at the portion surrounding each slit end. Fig. 9 is a sectional view showing an example of the electrically heatable honeycomb body of the present invention wherein the passages at the portion surrounding each slit end are sealed with an electroconductive material. The passages 12 of several cells at the portion surrounding each slit end (current-concentrating portion when electrified) are sealed with an electroconductive material 17. In Fig. 9, the passages 12 (shadowed) of cells at the portions surrounding each slit end, i.e. the portion where temperature rise occurs owing to current concentration, are sealed with an electroconductive material 17. Also in this case, as in the Examples of Figs. 6 to 8, there takes place heat dissipation toward the surrounding area, temperature reduction at the current-concentrating portion and prevention of cell deformation at the portion, whereby stress concentration can be relaxed. The number of passages sealed with an electroconductive material 17 is preferably about 4-6 at one side of each slit end. Incidentally, sealing of the passages at each current-concentrating portion with an electroconductive material is also disclosed in Japanese Patent Application Kokai (laid-Open) No. 277481/1992. As mentioned previously, the technique in the literature is insufficient as a countermeasure for prevention of abnormal heat generation and may have durability problems (e.g. cracking) in long-term use. The literature also gives a case of sealing, with an electroconductive material, the several passages at each slit end, arranged in a line perpendicular to the slit direction. Figs. 10 to 16 are each a sectional view showing an example of the electrically heatable honeycomb body of the present invention wherein the cells in the vicinity of each current-concentrating portion have a structure (e.g. a tetragon) capable of withstanding deformation and the cells at other portions have an easily deformable structure. Herein, the "cell structure capable of withstanding deformation" refers to a cell structure of a polygonal shape having a small number of sides, for example, a triangle or a tetragon; or a cell structure of a high cell density, for example, 270-1,650 cells/in 2 (40-250 cells/cm 2 ). In contrast, the "easily deformable cell structure" refers to a cell structure of a polygonal shape having a large number of sides, for example, a hexagonal or higher polygon; or a cell structure of a cell density lower than that of the cell structure capable of withstanding deformation, for example, 200-550 cells/in 2 (31-85 cells/cm 2 ). Fig. 10 is an example wherein, in the cells 18 present at the both sides of the end of each slit 13, those partition walls 20 facing the slit 13 are made so as to form a straight line. Fig. 11 is an example wherein, in the cells 18 present at the both sides of the end of each slit 13, those partition walls 20 facing the slit 13 are made so as to form a straight line and, moreover, the few cells 21 present in the extension line of the slit 13 are made so as to have a tetragonal shape. Fig. 12 is an example wherein the cells 18 at the portion surrounding the end of each slit 13 are made so as to have a rhombic shape. Figs. 13 and 14 are each an example wherein, in the cells 18 at the portion surrounding the end of each slit 13, the cell density is increased. Fig. 15 is an example wherein, in the cells 18 at the portion surrounding the end of each slit 13, those partition walls 20 facing the slit 13 are made so as to form a straight line and, moreover, the few cells at each end of the straight portion of slit are made so that the angles of their apexes change gradually and each cell at the furthest positions from the ends of the slit straight portion has a nearly regular hexagonal shape. Fig. 16 is an example wherein, in the cells 18 at the portion surrounding the end of each slit 13, those partition walls 20 facing the slit are made so as to have an arc shape. Thus, cell deformation and higher stress caused by current concentration and temperature rise, which appear in the vicinity of each current-concentrating portion of honeycomb body or at the portion surrounding each slit end, of slits-containing honeycomb body, can be prevented by allowing the cells 18 present in the above vicinity or at the above portion to have a structure capable of withstanding deformation, for example, a tetragonal shape and the cells present at other portions to have an easily deformable structure. Tetragonal cells are disclosed also in the above-mentioned Japanese Patent Application Kokai (Laid-Open) No. 277481/1992, but one same tetragonal cell structure is used in all portions, i.e. at current-concentrating portions and other portions. In such a structure (one same cell structure in all portions), stress appears and concentrates in the vicinity of each current-concentrating portion. Some of the present Examples employs a higher cell density in the vicinity of each current-concentrating portion, wherein the heat generated at the current-concentrating portion is dissipated toward the surrounding area and decrease in temperature is resulted. Examples of the effects of the honeycomb bodies described in the above Examples are shown below. Simulation tests were made using model honeycomb bodies wherein the thicknesses of partition walls present in the vicinity of each current-concentrating portion were made larger. In the honeycomb body of Fig. 20 where the thickness of partition walls in the vicinity of each current-concentrating portion was made 30% larger than those at other portions, the stress in the vicinity (the shadowed areas of Fig. 20) of each current-concentrating portion was reduced to about 70% of the stress in the shadowed areas of Fig. 19 (conventional honeycomb body). In the honeycomb bodies of Figs. 2 and 3, as compared with the conventional honeycomb body, there was also temperature decrease in each current-concentrating portion. With respect to the cell deformation during electrification, there was cell deformation at the portion surrounding each slit end in the conventional honeycomb body of Fig. 21; in contrast, in the honeycomb bodies of the present invention, there was substantially no deformation as seen in Fig. 22. Also when the cells at the portion surrounding each slit end were provided with ribs for reinforcement, there was no deformation as seen in Fig. 23. As described above, in the honeycomb bodies according to the present invention, change was made in the thickness and shape of partition walls present at each portion where cell rupture tends to occur during repeated electrification. As a result, the stress generated by current concentration can be reduced and the cell rupture caused by generated stress can be prevented. In one form of the honeycomb body according to the present invention, the partition wall lengths of cells at the dead end of each slit and in the slit extension line were made larger in the slit width direction. As a result, the cell-to-cell contact, which occurs between the cells present at the both sides of the end of each slit during electrification and heat generation, can be prevented and stable heat generation without short-circuiting is made possible.

Claims: An electrically heatable honeycomb body comprising: a large number of parallel gas flow passages (12) extending through the honeycomb body between opposite ends thereof which passages are each defined by partition walls (11) made of an electroconductive material, a peripheral surface extending between said ends, and at least one slit (13) extending into the body to a dead end to control the flow path of electrical current within the honeycomb body, said slit causing a current-concentration portion (15) in the honeycomb body when electrical current is passed for heat generation, characterized in that a plurality of cells each constituted by a said passage and partition walls, present along at least one side of said slit towards said dead end, are formed as a structure reducing the stress caused by current concentration at said current-concentration portion. A honeycomb body according to claim 1, wherein the thickness of the partition walls of said cells are larger than the thicknesses of the partition walls at other portions of the body. A honeycomb body according to claim 1 or 2, wherein the thickness of the partition walls of said cells along the side of said slit and in the portion extending therefrom toward said peripheral surface of the honeycomb body are gradually larger as the partition walls are closer to the peripheral surface. A honeycomb body according to claim 1 wherein the thicknesses of the partition walls of said cells present at the portion surrounding the slit dead end are larger at intersection points of such partition walls. A honeycomb body according to claim 4 wherein the thicknesses of said intersection points of partition walls are gradually larger as the intersection points are closer to the periphery of the honeycomb body. A honeycomb body according to claim 1, wherein each said cell present at a portion surrounding the slit dead end is provided with a rib for reinforcement. A honeycomb body according to claim 1, wherein those passages present at the portion surrounding the slit dead end are sealed with an electroconductive material. A honeycomb body according to claim 1 wherein said cells present along the side of said slit are less deformable and less flexible than cells at other portions. A honeycomb body according to claim 8 wherein said cells along the side of said slit have a tetragonal section and said cells at other portions have a hexagonal section. A honeycomb body according to claim 1, wherein said cells along the side of said slit end have a higher density (number per unit cross-sectional area) than the cells at other portions. A honeycomb body according to claim 1, wherein at least one of said ends of the body is hollow at the center. An electrically heatable honeycomb body according to claim 1 wherein partition wall lengths of each said cell adjacent the dead end of the slit and in an extension line of the slit are larger in a slit width direction to prevent cell-to-cell contact between the cells present at the both sides of the slit end during the heat generation by electrical current.

Inventors: HASHIMOTO SHIGEHARU, KATO YASUSHI, WATANABE YOSHINOBU, HASHIMOTO, SHIGEHARU, KATO, YASUSHI, WATANABE, YOSHINOBU
Applicants: NGK INSULATORS LTD, NGK INSULATORS, LTD.
Application year: 1996
Main classification: H05B3
IPC codes: F01N3, B01J35, H05B3