Stock material or miscellaneous articles – Structurally defined web or sheet – Honeycomb-like
Reexamination Certificate
1999-05-25
2002-03-05
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Structurally defined web or sheet
Honeycomb-like
C428S117000, C428S118000
Reexamination Certificate
active
06352756
ABSTRACT:
BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT
The present invention relates to a honeycomb structure for use as a catalyst carrier for purifying automobile exhaust gas. More particularly, the present invention relates to a thin-walled honeycomb structure that has thin partition walls, a light weight, a small loss of pressure, and an improved mechanical strength. All improvements by virtue of reinforcing a circumferential portion of the honeycomb structure. Thereby preventing damage to the honeycomb structure during its manufacturing or its handling. The invention further relates to a method for reinforcing such a thin-walled honeycomb structure.
There has conventionally used, as a carrier for catalysts of automobile exhaust gas(hereinafter referred to as a catalyst for purifying exhaust gas), a ceramic honeycomb structure having (a) numerous cell passages defined by a plurality of partition walls and (b) a circumferential wall surrounding the cell passages(hereinafter referred to as a honeycomb structure). That is, the conventionally used exhaust gas catalysts have been produced by coating a honeycomb structure with a &ggr;-alumina to form a layer thereof, and loading a catalytic component such as a noble metal or the like into inner surfaces of pores of thus formed &ggr;-alumina layer.
More concretely, a honeycomb structure has usually been produced by the steps of extrusion-molding a material, which mainly becomes cordierite when it is fired, through a nozzle having lattice-like slits so as to form integratedly a honeycomb portion and a circumferential wall, and subsequently drying and firing thus molded article. Thus obtained honeycomb structure is transferred to a step of forming the catalytic layer by first coating the inner surfaces of numerous cell passages with &ggr;-alumina so as to form a &ggr;-alumina layer thereon, and then loading a noble metal component, as a catalytic component, such as platinum, rhodium, or palladium into inner surfaces of pores of thus formed &ggr;-alumina layer. Then, the honeycomb structure is subjected to baking treatment at a temperature of about 600° C. so as to back the catalytic component thereon to give a catalyst for purifying exhaust gas. Thus produced catalyst for purifying exhaust gas is housed in a metallic container with aid of a cushioning material. The metallic container, i.e., a converter is connected to an exhaust pipe by means of welding, bolting, or the like, to set up an engine for an automobile, etc.
Regulations on exhaust gases have become stricter year by year, especially in developed countries, due to environmental problems. To cope with these stricter regulations, an ever-lasting improvement in purification ability is required for a catalyst for purifying exhaust gas. On the other hand, a desire to lower fuel charge and increase output of power has been evident in the fields of engine development. Because of such a situation, the reduction in pressure loss during operation has been required in the case of the catalyst for purifying exhaust gas. In the case of the catalyst for purifying exhaust gas, so as to solve problems mentioned above, there have become evident such a strong movement that the improvement in the performance of the catalyst for purifying exhaust gas at the time of warming up of the engine has been tried by increasing a passage area of the cell passages so as to reduce pressure loss. Furthermore, lightening the weight of the catalyst for purifying exhaust gas itself so as to reduce its heat capacity by making the partition walls thin without decreasing the number of cells as well. Hitherto, a honeycomb structure having partition walls of 0.15 mm or more in the thickness was most popular. However, a honeycomb structure having partition walls having of 0.13 or less, particularly, 0.11 mm or less in the thickness has recently come to be popular.
However, to make partition walls of a honeycomb structure thinner causes a problem that the extremities(hereinafter sometimes referred to as a corner) of circumferential portions of the honeycomb structure are often broken during manufacturing, handling, or conveying the honeycomb structure, or housing the honeycomb structure into the container for converter so as to set it in an engine because the structural strength of the honeycomb structure is consequently decreased, particularly in the circumferential portions of the honey-comb structure. This phenomenon becomes evident when the partition walls of the honeycomb structure becomes thinner. This is because damage occurs in a honeycomb structure more frequently when partition walls in a honeycomb structure are thinned. Note that the honeycomb structure is liable to break when an external force such as a mechanical shock is applied thereto during transportation or the like, even in the case of the honeycomb structure having thicker partition walls. This is because a ceramic material is inherently brittle. Thus, the breaking of the honeycomb structure was also occasionally reported. Because of its very low frequency, however, it has not particularly been regarded as a problem.
Further, the frequency of the deformation in partition walls during extrusion-molding has remarkably increased as partition walls become thinner; while in the case of the conventional honeycomb structures having such thicker partition walls as the thickness of 0.15 mm or more, such a problem is no so serious. This is because the circumferential wall has a thickness of at least 0.3 mm, and therefore, the strength in the circumferential portions can be ensured to a certain degree. The deformation in partition walls tends occurs mainly at the vicinity of the circumferential walls in the circumferential portions. This is due to the failure to attain uniform extrusion-molding because of the unbalance in the fluidity of raw material between the honeycomb portions and the vicinity of the circumferential partition walls, when the circumferential walls are thicker than partition walls. Such thickening ensures strength in the circumferential portion.
A similar phenomenon when using cordierite to deform the partition walls is also observed when a ceramic material such as alumina, mullite, silicon nitride, silicon carbide, or zirconia is subjected to an extrusion-molding. This is because, as a starting material, a material prepared by mixing and kneading said material with water and a binder is used as well. Since the deformation in partition walls is mainly attributed to buckling derived from compressive load, a similar problem is also observed in not only a honeycomb structure having square cells, but also a honeycomb structure having rectangular, triangular, or hexagonal shape.
Some proposals have been made to solve the various problems derived by making partition walls thinner in a honeycomb structure. First, it has been proposed to thin the circumferential walls from 0.3 mm to 0.1 mm. Thus the thickness of the circumferential walls approximates the thickness of the partition walls, thereby improving the balance in the flow amount of raw material during molding. In this case, however, the strength of the circumferential wall is not sufficient. In other words, when circumferential walls are too thin, breaking starts at the circumferential walls due to insufficient rigidity. A circumferential wall thickness of at least 0.1 mm, desirably at least 0.15 mm, is sufficient just to house the honeycomb structure in a container kept under a uniform and static external pressure. However, such a circumferential wall thickness is not sufficient to resist external pressure, such as mechanical shock during transport or the like.
On the other hand, there has been made such a proposal that the strength against mounting pressure of the circumferential walls would increase if the thickness of the circumferential walls is thickened. Thus, a cordierite honeycomb structure having square cells, a partition wall thickness of 0.11 mm, and a circumferential wall thickness of at least 0.4 mm was prepared so as to increase strength. Contrary to expectation, however, i
Ichikawa Yukihito
Ikeshima Koichi
Boss Wendy
NGK Insulators Ltd.
Parkhurst & Wendell L.L.P.
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