Stock material or miscellaneous articles – Structurally defined web or sheet – Honeycomb-like
Reexamination Certificate
1999-06-02
2001-06-05
Lorin, Francis J. (Department: 1775)
Stock material or miscellaneous articles
Structurally defined web or sheet
Honeycomb-like
C264S631000
Reexamination Certificate
active
06242072
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a honeycomb structural body made of cordierite, which is used as a catalyst carrier in an exhaust gas purification apparatus for an internal combustion engine.
2. Description of the Related Art
Catalyst carriers for exhaust gas purification apparatuses have conventionally employed a honeycomb structural body
9
such as shown in
FIGS. 15A and 15B
, wherein partition walls
90
made of cordierite or the like are arranged in a honeycomb fashion to form multiple cells
99
. An exhaust gas purification function is exhibited by carrying an exhaust gas purifying catalyst on the surface of the partition walls
90
of the honeycomb structural body
9
.
However, with the enforcement of stricter exhaust gas standards for automobiles in recent years, it has become necessary to achieve more rapid activation of the catalysts in exhaust gas purification apparatuses. Specifically, research has been carried out on lowering heat capacity and pressure loss by reducing the thickness of the partition walls in honeycomb structural bodies. Other studies have focused on moving the position where the catalyst carrier is mounted to be closer to the engine than usual to provide a higher-than-conventional temperature of the exhaust gas to be purified.
These conventional honeycomb structural bodies, however, have been associated with the following problems.
Specifically, when the partition walls of a conventional honeycomb structural body are narrowed to a thickness of about 100 &mgr;m, an inconvenience results as the edges of the honeycomb structural body, i.e. the ends of the partition walls are abraded and damaged by the flow of exhaust gas. It is this narrowing of the partition walls beyond the conventional thickness which is believed to cause the lower partition wall strength and lower durability with respect to exhaust gas flow than is common.
As a strategy for fortifying partition walls when they are narrowed, the overall porosity of the partition walls has been lowered for greater density. However, while this increased density produces a strength-enhancing effect, it has led to such inconveniences as a higher heat capacity due to increased weight of the partition walls or poorer catalyst carrier properties.
In light of these problems of the prior art it is an object of the present invention to provide a honeycomb structural body with a lower-than-conventional heat capacity and excellent durability.
SUMMARY OF THE INVENTION
The first aspect of the invention resides in a honeycomb structural body having partition walls composed mainly of cordierite arranged in a honeycomb fashion,
the honeycomb structural body being characterized in that a fortified section with a higher strength than the general section is provided at one or both ends in the direction of length of the partition walls.
Most worthy of note according to the invention is that the aforementioned fortified section is provided at one or both ends in the direction of length of the partition walls.
The fortified section can be provided by various methods including thickening or densification of the walls, and coating or substitution with a fortifying material, as described later.
As mentioned above, the fortified section is provided at one or both ends. When a fortified section is provided at only one end, it is provided at the end on the side with which the exhaust gas impacts when the honeycomb structural body is used as a catalyst carrier.
When provided at both ends, there is no need for directional management of the honeycomb structural body during assembly of the exhaust gas purification apparatus, and therefore production can be accomplished more practically.
The region for formation of the fortified section can be appropriately set based on the size, etc. of the honeycomb structural body. That is, since formation of the fortified section contributes to improved durability of the honeycomb structural body but also brings the disadvantage of greater heat capacity, the region is appropriately set for the intended durability. For example, the fortified section may be formed at the edge in a region of about 1-10 mm from the end of the partition walls.
The major component composing the partition walls is preferred to be cordierite. Cordierite has the general chemical composition SiO
2
:45-55 wt %, Al
2
O
3
:33-42 wt %, MgO:12-18 wt %.
The function of the present invention will now be explained.
As mentioned above, the honeycomb structural body of the invention has a fortified section provided at an end of the partition walls. Consequently, when the honeycomb structural body is used as a catalyst carrier it is possible to vastly improve the durability of the partition walls against the flow of exhaust gas, as compared to the prior art.
That is, when the honeycomb structural body is used as a catalyst carrier, the exhaust gas first impacts with the partition walls of the honeycomb structural body. Because fortified section is provided at the end of the partition walls as explained above, they are stronger than the general section. Consequently, they have greatly improved durability against an exhaust gas flow as compared to honeycomb structural bodies of the prior art which possess no fortified section. This means that the overall thickness of the partition walls can be made narrower than according to the prior art, while maintaining durability against the flow of exhaust gas.
The fortified section is provided only at an end of the partition walls. Consequently, even with an increase in heat capacity owing to formation of the fortified section, the region of increase is limited only to the end where the fortified section has been formed. As a result, when the heat capacity is lowered by narrowing of the partition walls, etc., an adequate effect of reduced heat capacity can be achieved at the general section, so that the overall heat capacity of the honeycomb structural body can be vastly reduced with respect to the prior art. In other words, it is possible to achieve both reduced heat capacity by narrowing, and improved durability.
Consequently, according to the present invention there is provided a honeycomb structural body with a lower-than-conventional heat capacity and excellent durability.
The fortified section is preferably provided with improved strength by an increased density with respect to the general sections. That is, partition walls composed mainly of cordierite are usually formed in a porous state having numerous pores in the interior. It is thus possible to reduce the porosity at one or both of the ends of the partition walls with respect to the general section for increase in the density of the fortified section.
Since in such a case the fortified section can be provided without greatly increasing the partition wall thickness, it is possible to prevent an increase in resistance to passage of exhaust gas.
The increase in density described above is preferably accomplished so that the porosity is in the range of 5-35%. If the porosity is less than 5% the strength is improved but the thermal shock resistance may be impaired. On the other hand, if it exceeds 35% a problem arises in that the effect of improved strength at the fortified sections is reduced.
The process for forming the fortified section by increase in density may be a process whereby a portion of the starting material for the partition walls is coated and fired onto the partition walls after extrusion molding of the honeycomb structural body, as explained in the embodiments which follow.
The fortified section can be given enhanced strength by making them thicker than the general section. In this case as well, it is possible to fortify only the end in the manner described above, and to simultaneously obtain the two effects of lower heat capacity and improved durability.
Thickening of the end in such cases is carried out in a balanced fashion so as not to overly increase the resistance to the passage of an exhaust gas.
The thickening is preferably carried out to achieve a 20-400%
Andou Yoshiyasu
Ito Keiji
Murata Masakazu
Obata Takashi
Tanaka Masakazu
Denso Corporation
Lorin Francis J.
Pillsbury & Winthrop LLP
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