Stock material or miscellaneous articles – Structurally defined web or sheet – Honeycomb-like
Reexamination Certificate
2000-07-21
2003-06-24
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Structurally defined web or sheet
Honeycomb-like
C428S174000, C428S188000, C428S325000, C428S332000, C264S630000, C264S631000
Reexamination Certificate
active
06582796
ABSTRACT:
The invention relates to new monolithic structures with parallel passages that are plugged alternately on one end face or the other of said monolithic structures that can be used in particular as particle filters for the exhaust gases of diesel engines. The invention also relates to the production of such structures.
PRIOR ART
The exhaust gases of internal combustion engines and in particular those of diesel engines contain soots or particles that pollute the atmosphere and can be very injurious to health. Various methods have been considered for attempting to solve this problem. It is proposed in particular to collect these particles in filters that consist of porous materials that are placed in the exhaust line of the engine.
Filters that consist of honeycomb-type monoliths made of refractory materials, such as cordierite or silicon carbide, were already described in the prior art.
These monoliths comprise a number of passages that are separated by porous walls, whereby said passages are alternately plugged at one end or the other to force the gas flow to diffuse through said walls.
The monoliths of this type that are produced from cordierite have the drawback of exhibiting premature aging caused by poor heat dissipation during the combustion of the soots that are retained, combined with a weak mechanical resistance.
With regard to silicon carbide monoliths already described previously, their production is difficult.
Actually, a standard technique consists in using as a starting material two silicon carbide powders of different grain sizes that are mixed, extruded, dried and brought to a very high temperature, on the order of 2100-2500° C., in general under an inert atmosphere; under these conditions, an abnormal growth of the largest crystals is observed, and the finest crystals undergo evaporation-condensation, which finally causes undesirable modifications of the properties of the monolith that is thus produced.
More recently, a production process was proposed that is carried out starting from silicon powder and carbon or an organic material that, by thermal decomposition, will provide the carbon that is necessary for combining with silicon. The production according to such a method also requires a final calcination stage at a high temperature, generally greater than 1650° C. and most often ranging up to 2100-2400° C., under an inert atmosphere.
DESCRIPTION OF THE INVENTION
The Monoliths
The invention proposes a new monolith that can be used as a particle filter for the exhaust gases of the diesel engines, whereby this monolith has the advantage of being able to be produced by a process in which the calcination stage calls for using neither a temperature of more than 1650° C. nor an inert atmosphere.
The monolith according to the invention can be defined in that it consists of a solid that consists of a porous refractory material whose outside volume is delimited by a cylindrical surface (in the horizontal direction) whose generatrix rests on the two end faces of any shape, for example square, rectangular, polygonal, circular or elliptical. A number of passages that are parallel to one another and to the generatrix and that empty into the end faces of said monolith pass through the solid, whereby these passages can have a square, rectangular or triangular or else polygonal section.
The monolith of the invention basically consists of a material that comprises
70 to 97% by mass, preferably 82 to 90% by mass, of &agr; and/or &bgr; crystallographic-type silicon carbide that has at least one particle size and preferably at least two particle sizes;
and 3 to 30% by mass, preferably 18 to 15% by mass, of at least one bonding ceramic phase in the form of a micronic powder and/or particles that are obtained by atomization, comprising at least one simple oxide that is selected from among, for example, B
2
O
3
, Al
2
O
3
, SiO
2
, MgO, K
2
O, Li
2
O, Na
2
O, CaO, BaO, TiO
2
, ZrO
2
and Fe
2
O
3
and/or at least one mixed oxide that is selected from among, for example, the alkaline aluminosilicates (of Li, Na or K) or alkaline-earth aluminosilicates (of Mg, Ca, Sr or Ba), clays, bentonite, feldspars or other natural silico-aluminous materials.
The material that constitutes the monolith—i.e., the walls that separate the passages—in general has a porosity of 35 to 65%, preferably 40 to 60%.
The distribution of pores is essentially monopopulated and can be centered between, for example, 5 and 60 micrometers, preferably 10 to 40 micrometers, and even more preferably 15 to 35 micrometers.
Actually, the size of the pores depends essentially on the size of the particles of silicon carbide and that of the grains of the ceramic binder that is used, as will be described latter, in particular in connection with the procedure for production of the monoliths of the invention.
The silicon carbide that has the primary portion of the material that constitutes the monoliths of the invention preferably has several particle sizes, for example from two to five particle sizes. Thus, it is possible to use designated silicon carbides, according to the FEPA 42 F 1984 standard by the symbols of F 100 to F 1200, which corresponds to particles of a mean size of 3 to 125 micrometers. In the case of a silicon carbide with two particle sizes, it is possible to consider, for example, the presence of a major proportion (for example on the order of 90% by mass) of silicon carbide in the form of particles of 15 to 125 micrometers and a minor proportion (for example on the order of 10% by mass) of silicon carbide in the form of particles that have a mean size that is less than 15 micrometers.
The bonding ceramic phase that is present in the material that constitutes monolithic structures of the invention preferably has an overall composition that is adjusted to exhibit an expansion coefficient that is close to the one of silicon carbide, nearly 50%, preferably nearly 30% and even more preferably nearly 25%.
In general, the passages that traverse the monolith are, at each end, alternately open or closed, such that for each open passage at one end, the adjacent passages are closed, whereby the gas flow that penetrates the monolith is then forced to diffuse through the porous walls that separate the passages. For example, for the square-section passages, the end faces of the monolith have the appearance of a checkerboard.
The monolith can have a number of cells (or passages) of about 50 to 400, more particularly about 100 to 200 per square inch (or about 7.75 to 62 per cm
2
, more particularly about 15.5 to 31 per cm
2
). The passages have approximately a cross section of about 0.5 to 9 mm
2
, more particularly 1.5 to 4 mm
2
, taking into account a thickness of the walls that separate the passages of about 0.3 to 1.5 mm, more particularly 0.5 to 0.8 mm.
The monoliths of the invention have exceptional properties of mechanical resistance and thermal shock resistance.
Production of the Monolith
The monolith of the invention can be produced by any suitable operating procedure that comprises in particular a stage of mixing the components resulting in a homogeneous product in the form of a thickened paste, an extrusion stage of said product through a suitable die to form the honeycomb-type monolith, a stage for drying the monolith that is obtained, then a calcination stage, whose advantage, according to the invention, is that it is carried out at a temperature that does not exceed 1650° C., preferably less than 1550° C., and under an atmosphere that contains oxygen. The operation that aims at plugging every other passage at each end of the monolith can be carried out at any stage of the production, either on the crude monolith, just after the extrusion, or on the dried monolith, or on the monolith that underwent a calcination stage.
A particular operating procedure is described in more detail below.
In the first stage, the silicon carbide with a suitable particle size, the bonding phase that consists of at least one simple oxide and/or at least one mixed oxide as defined above, and in general a pore-forming agent, are mixed
Courty Philippe
Dementhon Jean-Baptiste
Joulin Jean Pierre
Pourchet Fabienne
Institut Francais du Pe'trole
Millen White Zelano & Branigan P.C.
Xu Ling
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