Stock material or miscellaneous articles – Structurally defined web or sheet – Honeycomb-like
Reexamination Certificate
2002-10-18
2004-11-09
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Structurally defined web or sheet
Honeycomb-like
C428S188000, C428S034400, C428S323000, C428S325000, C422S180000, C422S211000, C422S222000, C055S523000, C501S088000, C501S089000, C501S153000, C501S154000
Reexamination Certificate
active
06815038
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a honeycomb structure. More particularly, the present invention relates to a honeycomb structure which has a quality higher than a required level, in all of thermal conductivity, chemical durability, low thermal expansion and mechanical strength, which can be produced at a low cost, and which is suitably used in a filter for purification of automobile exhaust gas, a catalyst carrier, etc.
BACKGROUND ART
As a filter for capturing and removing the particulate substance contained in a particle-containing fluid such as diesel engine exhaust gas, or as a catalyst carrier for loading thereon a catalyst component which purifies the harmful substance present in an exhaust gas, there is in wide use a porous honeycomb structure constituted by cell partition walls (ribs) which form combined cells being composed of a plurality of cells being adjacent to each other and a honeycomb outer wall which surrounds and holds the outermost cells located at the circumference of the combined cells. Refractory silicon carbide (SiC) is in use as the material constituting the honeycomb structure.
As such a honeycomb structure, there is disclosed, for example, a porous silicon carbide-based catalyst carrier of honeycomb structure obtained by using, as a starting material, a silicon carbide powder having a given specific surface area and containing impurities, molding the powder into a desired shape, drying the molded material and firing it in a temperature range of 1,600 to 2,200° C. (JP-A-6-182228).
There are also disclosed a process for producing a vitrifying material-containing refractory, characterized by adding a vitrifying material to an easily oxidizable material or a refractory composition containing an easily oxidizable material, mixing and kneading them together with a binder, molding the puddle, and open-firing the molded material in an oven of a non-oxidizing atmosphere, and a silicon carbide molded material obtained by adding, to a silicon carbide powder, an organic binder and an inorganic binder of clay mineral type, vitreous type or lithium silicate type and molding the mixture (JP-A-61-26550 and JP-A-8-165171). Incidentally, in JP-A-8-165171 is disclosed, as a conventional process for producing a porous silicon carbide-based sintered material, a process which comprises adding, to silicon carbide particles as an aggregate, a vitreous flux or a binder such as clayey material or the like, molding the mixture, and firing the molded material at a temperature at which the molded material is melted by the binder.
Further, there is disclosed a cordierite-based composite material wherein plate-like silicon carbide is allowed to be present in a cordierite-based matrix in an amount of 5 to 40% by weight based on the total of the two materials (JP-A-5-213665).
In case of the sintering (necking) caused by a recrystallization reaction of silicon carbide powder per se in the catalyst carrier disclosed in JP-A-6-182228, a silicon carbide component vaporizes from the surfaces of silicon carbide particles and condenses at the contact areas (neck) between the particles; as a result, the necks grow and resultantly a bonded state is obtained. However, the vaporization of silicon carbide requires a very high firing temperature, which incurs a high cost, requires high-temperature firing of a material high in thermal expansion coefficient. Thus, there is a problem that the yield in firing is reduced, thereby. Further, in the above-mentioned sintering by recrystallization of silicon carbide powder per se to produce a filter of high porosity, particularly a filter having a porosity of 50% or more, the sintering mechanism does not function sufficiently; as a result, the growth of necks is prevented and there has been a problem caused thereby, of reduced filter strength. Furthermore, the above-mentioned material is advantageous in that it has a very high thermal conductivity of 30 W/m·k or more and suppresses local heating; however, when the material is used in, for example, a catalyst-loaded filter of continuous regeneration type in which deposited particulates are oxidized and burnt, the deposition amount of particulates is small, release of heat is easy, accordingly a long time is required before the carrier is heated and a long time is required before the catalyst reaches its function-exhibiting temperature, and, therefore, there have been problems, for example, in that cinders of particulates remain and the efficiency of regeneration drops.
Also in the method of bonding a silicon carbide powder as a material with a vitreous material, disclosed in JP-A-61-26550 and JP-A-6-182228, the firing temperature may be as low as 1,000 to 1,400° C.; however, when the sintered material produced by the method is used, for example, as a material for a diesel particulate filter (DPF) which removes the particulates contained in an exhaust gas emitted form a diesel engine and when the particulates captured by and deposited on the filter are burnt for filter regeneration, the material is low in thermal conductivity and high in thermal expansion coefficient; therefore, there have been problems of easy local heating and low thermal shock resistance.
Also in the composite material and production process disclosed in JP-B-8-13706, the composite material can be obtained as a porous material; however, when it is used as a filter, it is not easy to allow the filter to have a sufficient porosity and it has been difficult to use the composite material particularly as a filter for capturing and removing the particulate substance contained in a particle-containing fluid such as diesel engine exhaust gas.
Further, the cordierite-based composite material disclosed in JP-A-5-213665 is somewhat effective for improvement of creep property and shock resistance; however, the composite material is low in silicon carbide content and has not been fully satisfactory in thermal conductivity and chemical durability.
The present invention has been made in view of the above-mentioned problems, and aims at providing a honeycomb structure which has a quality higher than a required level, in thermal conductivity, chemical durability, low thermal expansion and mechanical strength, which can be produced at a low cost, and which is suitably used in a filter for purification of automobile exhaust gas, a catalyst carrier, etc.
DISCLOSURE OF THE INVENTION
The present inventors made a study in order to achieve the above aim. As a result, the present inventors found out that by considering that silicon carbide and cordierite each have strengths and weaknesses, that is, silicon carbide is superior in high thermal conductivity and high chemical durability but is high in Young's modulus relative to strength as well as in thermal expansion coefficient, resulting in insufficient thermal shock resistance, while cordierite is superior in low thermal expansion and low cost but is insufficient in low thermal conductivity and low melting point, the material constituting the cell partition walls, etc. of honeycomb structure is allowed to contain silicon carbide and cordierite in such optimum proportions as their strong points are utilized and their weak points are made up for, whereby the above aim can be achieved. The present invention has been completed based on the finding.
According to the present invention there is provided the following honeycomb structure.
[1] A honeycomb structure constituted by
cell partition walls (ribs) which form combined cells being composed of a plurality of cells being adjacent to each other, and
a honeycomb outer wall which surrounds and holds the outermost cells located at the circumference of the combined cells,
characterized in that the cell partition walls and the honeycomb outer wall are formed by a bonded texture containing silicon carbide (SiC) as an aggregate and cordierite as a binder and that the proportion (volume %) of the silicon carbide (SiC) forming the bonded texture to the total of the cordierite and the silicon carbide (SiC) is 35 to 90%.
[2] A honeycom
Inoue Katsuhiro
Kawasaki Shinji
Morimoto Kenji
Sakai Hiroaki
Boss Wendy
Jones Deborah
NGK Insulators Ltd.
Oliff & Berridg,e PLC
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