Stock material or miscellaneous articles – Structurally defined web or sheet – Honeycomb-like
Reexamination Certificate
2002-04-23
2003-12-02
Zimmerman, John J. (Department: 1775)
Stock material or miscellaneous articles
Structurally defined web or sheet
Honeycomb-like
C428S018000, C055S523000, C502S034000, C502S527190, C422S180000, C422S222000
Reexamination Certificate
active
06656564
ABSTRACT:
The present invention relates to a ceramic honeycomb structure. More particularly, the present invention relates to a ceramic honeycomb structure capable of balancing the disadvantages incurred by the increased pressure loss and the decreased thermal shock resistance against the advantages brought about by the increased isostatic strength and the cell wall shape and honeycomb external shape of higher accuracy, and which is suitably used, for example, as a carrier for catalyst for automobile exhaust gas purification. The ceramic honeycomb structure of the present invention is also used suitably as a filter for diesel particulates or the like, as a chemical reactor (e.g. a catalyst carrier for fuel cell reformer), or as a heat exchanger.
BACKGROUND ART
As catalysts for purification of automobile exhaust gas, there are used so-called honeycomb catalysts wherein a catalyst component is loaded on the surfaces of the cell walls of a ceramic honeycomb carrier (honeycomb structure). In these catalysts, since the axial direction strength of the honeycomb carrier is higher than its strength in the sectional (diameter) direction, the honeycomb carrier was held in the axial direction. In this holding manner, in order to prevent the breakage of the honeycomb carrier occurring at around the periphery of its ends in axial direction holding, the thickness of the cell walls (ribs) near the circumference of the honeycomb carrier was made larger than the thickness of the cell walls in the inner portion of the honeycomb carrier to increase the anti-pressure strength of the honeycomb carrier in the axial direction.
Recently, however, the higher output adopted in engines has required a lower pressure loss of honeycomb catalyst and the stricter regulation employed for exhaust gas has needed the effective utilization of whole catalyst carrier; therefore, it has been started to hold the honeycomb carrier mainly at its outer surface, in place of holding in the axial direction. Another reason for this is that the stricter regulation for exhaust gas has invited a larger catalyst volume and an increased catalyst mass and, as a result, the holding in the axial direction has become unable to give a sufficient holding area and promise sufficient holding relative to engine vibration.
Meanwhile, in order to enhance the purification ability of catalyst, it has been started to make thinner the thickness of the cell walls of a honeycomb carrier and decrease the weight of the honeycomb carrier and thereby reduce the heat capacity of a catalyst and enhance its purification ability (warm-up property).
The above use of cell walls of thinner thickness tends to result in a lower fracture strength against the external pressure which the honeycomb carrier receives at the outer surface.
In order to meet the recent even stricter regulation for exhaust gas, it has been aimed to improve the conditions of engine combustion and the purification ability of catalyst. As a result, the temperature of exhaust gas has become higher year by year and the thermal shock resistance required for a honeycomb carrier has become stricter.
Thus, due to the thinning of the cell walls, the employment of holding honeycomb carrier at the outer surface, the increase in temperature of exhaust gas and the like, the setting of cell wall thickness and honeycomb outer wall thickness, the increase in the isostatic strength of honeycomb structure, and the higher accuracies of honeycomb external shape and cell wall shape have become important tasks to be achieved.
In connection with the above, there was proposed, in JP-A-54-110189, a honeycomb carrier structure whose cell walls are made thinner at a given ratio from the outermost peripheral cell wall towards the center of the cross-section. In this structure, since use of a thin wall in the entire honeycomb carrier is impossible, the total mass of the honeycomb carrier is inevitably large, posing a problem in the warm-up property of the honeycomb carrier. This structure is undesirable also in pressure loss.
There was also proposed, in JP-A-54-150406 and JP-A-55-147154, a structure wherein the walls of the cells near the circumference of the structure are made thicker than those of the inner cells. However, no mention is made on the thickness of the outer wall or on the specific relation between different cell wall thickness therein.
In these honeycomb structures of the prior art, since the thickness of inner cell walls is as large as 0.15 mm or more and the holding is made in the axial direction, the thickness of the honeycomb outer wall was not a problem. One may merely point out that too large an outer wall thickness gives a low thermal shock resistance, if forced to do so.
Further in WO 98/05602 was proposed a ceramic honeycomb structure wherein the average cell wall thickness T is 0.05 to 0.13 mm, the average outer wall thickness is larger than T, W>T (W is an average width of contact between cell wall and outer wall), and 0.7≧−(T/4)+0.18.
This ceramic honeycomb structure exhibits some effect in prevention of peripheral chipping during handling; however, it was not fully satisfactory in increased pressure loss, reduced thermal shock resistance, increase in isostatic strength, and the improvements in the accuracies of cell wall shape and honeycomb structure external shape.
No in-depth investigation has hitherto been made particularly on the improvements in the accuracies of cell wall shape and honeycomb structure external shape. That is, a ceramic honeycomb structure is generally molded by extruding, for example, a cordierite raw material for ceramic through a die having lattice-shaped slits; then dried; and fired to become a product. When a smaller cell wall thickness is employed, the cell walls tend to deform during molding, owing to the cause mentioned later and resultantly the fired material obtained showed no satisfactory isostatic strength while this did not happen when the cell wall thickness was as large as 0.15 mm or more. Nevertheless, no sufficient investigation has been made. The deformed cell walls are destroyed at the deformed sites by a small force. That is, when cell walls do not deform and are molded at a high accuracy, they theoretically become sites of compression stress when a pressure is applied to the outer surface of honeycomb structure, and the destruction of honeycomb structure takes place owing to the buckling of cell wall or outer wall. Meanwhile, when cell walls have deformed, a bending stress (a stress in tensile direction) is generated at the deformed sites, resulting in easy destruction. In general, materials are less resistant to tensile strength than to compression stress and, in ceramic materials, in particular, the ratio (about 1/10) of tensile strength to compression strength is very small as compared with that (about 1/3) of metal materials. Therefore, when there is deformation of cell walls, destruction takes place at a strength considerably lower than a strength at which destruction takes place ordinarily.
The present invention has bee made in view of the above problems, and aims at providing a ceramic honeycomb structure capable of balancing the disadvantages incurred by the increased pressure loss and the decreased thermal shock resistance against the advantages brought about by the increased isostatic strength and the cell wall shape and honeycomb external shape of higher accuracy, and which is suitable particularly as, for example, a carrier for catalyst for automobile exhaust gas purification.
DISCLOSURE OF THE INVENTION
In order to achieve the above object, the present inventor made a study including various tests mentioned later, with considering the thinner cell walls recently employed in honeycomb carriers. As a result, the following was found out. That is, the adoption of a thick wall only in the cells near the circumference of honeycomb structure as seen in the prior art is insufficient and attention must be paid also to the extrudability of honeycomb structure; therefore, the designing of a honeycomb structure need be made while payin
Ichikawa Yukihito
Kondo Takahiro
Miyazaki Makoto
Shirai Masahiro
Boss Wendy
NGK Insulators Ltd.
Parkhurst & Wendel L.L.P.
Zimmerman John J.
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