Patent Application
20080242535
The present invention relates generally to silicon carbide substrates useful for filtration and/or high temperature chemical reaction processing, such as a catalytic host. The invention more particularly relates to a substantially fiber-based silicon carbide substrate and methods for producing the same.
Ceramic honeycomb substrates are commonly used in industrial and automotive applications where inherent material stability and structural integrity are needed at elevated operating temperatures. Ceramic honeycomb substrates provide high specific surface area for effective filtration and support for efficient catalytic reactions. For example, in automotive applications, ceramic substrates are used in catalytic converters to host catalytic oxidation and reduction of exhaust gases, and to filter particulate emissions.
Ceramic honeycomb substrates are typically used in a Diesel Particulate Filter (DPF) to trap diesel exhaust particles, such as soot. When used in a DPF, the ceramic honeycomb is fabricated in a wall-flow configuration by selectively plugging alternate channels to form inlet channels and outlet channels. Every other extruded channel is plugged on the inlet side, and the remaining channels are plugged on the outlet side, thereby forcing the exhaust flow into the inlet channels, through the porous ceramic material that forms the walls of the channels, and out of the filter through the outlet channels. During operation, the soot particles accumulate on the surface of the inlet channel walls, which will ultimately increase the system backpressure. The diesel engine control system monitors backpressure and other indicators, and periodically initiates a regeneration of the filter through a controlled burn-off of the accumulated soot. If the diesel engine controls fail to maintain control of the periodic filter regeneration, too much soot may accumulate and an uncontrolled regeneration may occur, which can result in extremely high temperature gradients within the honeycomb filter, leading to potential failure of substrates.
DPF substrates have been fabricated from an extruded powder-based ceramic material, such as cordierite or silicon carbide. Cordierite, 2MgO.2Al2O3.5SiO2, is a commonly used ceramic material for monolithic catalyst support applications, such as vehicular catalytic converters. Cordierite is typically formed by extruding a mixture of particles of kaolin, talc, calcined kaolin, calcined talc, alumina, aluminum hydroxide, and silica, followed by a high temperature firing process to form cordierite in-situ. The choice of raw materials and processing determines the porosity created in the side walls. The material exhibits a relatively low melting point compared to the operating temperature of a DPF during regeneration. Cordierite is a relatively inexpensive to fabricate, and has a low thermal coefficient of expansion, but the material cannot maintain structural integrity when operating temperatures exceed 1300° Celsius. That, combined with occasional cracking observed when large thermal gradients are created during regenerations, can lead to catastrophic failures.
Silicon carbide, as a material, is desirable for high temperature filtration applications since the material exhibits significantly high thermal conductivity as well as high volumetric heat capacity, that effectively reduce the magnitude of thermal gradients during regeneration in a DPF ceramic honeycomb substrate. Silicon carbide is also chemically stable and inert, and mechanically strong when bonded. Current commercial silicon carbide substrates are typically formed by extruding a mixture of silicon carbide particles and an organic binder, followed by a sintering process that burns off the binder and sinters the silicon carbide particles into a porous structure. In another example, silicon metal powder is used to bond SiC particles together. The drawback of extruding SiC powders is that the highly abrasive particles rapidly wear extrusion dies and equipment used in expensive high pressure extruders. Additionally, the sintering process requires temperatures sometimes in excess of 2000 degrees Celsius for long periods (8-12 hours or more) in an inert environment such as argon.
Porous ceramic honeycomb substrates can also be made from ceramic fibers, as disclosed in commonly owned U.S. Pat. No. 6,946,013, and commonly owned U.S. patent applications Ser. No. 10/833,298 (published as US2005/0042151) and Ser. No. 11/322,544 (published as US2006/0120937), all incorporated herein by reference. The advantage of a fibrous ceramic structure is the improved porosity, permeability, and specific surface area that results from the open network of pores created by the intertangled ceramic fibers, the mechanical integrity of the bonded fibrous structure, and the inherent low cost of extruding and curing the ceramic fiber substrates. The commercial application of this technology, however, is limited by the availability of low cost ceramic fibers. Low cost silicon carbide fibers are not readily or commercially available.
Porous ceramic honeycomb substrates of ceramic fibers have also been fabricated in a honeycomb form using laminations of ceramic fiber-based paper elements, as disclosed in U.S. patent application Ser. No. 10/518,373 (published as US2006/0075731), incorporated herein by reference. This method of fabrication does not have the benefit of low-cost extrusion, but the fabrication method is adaptable to the use of expensive silicon carbide fibers to provide a high temperature and robust porous substrate.
Accordingly, there is a need for fibrous ceramic honeycomb structure that possesses the thermal and mechanical properties of a silicon carbide honeycomb substrate, with the performance and fabrication cost advantage of alternative ceramic materials and fabrication processes.
The present invention provides an improved silicon carbide substrate that is formed from an in-situ formation of silicon carbide from carbon or carbonaceous fibers. A honeycomb structural body that has a plurality of through holes that form a partition wall between each channel that is formed from the lamination of members that are formed from in-situ silicon carbide. The lamination members each have through holes, that when laminated, are superimposed on one another to form the honeycomb channels. The honeycomb structure is adapted into a wall-flow configuration by sealing one of the ends of the through holes. The honeycomb structural body of the present invention can be adapted for use a filter.
Catalyst coatings can be applied to the in-situ silicon carbide fibers in order to provide catalytic reactions for oxidation and/or reduction of harmful constituents of exhaust gases, such as in a diesel particulate filter.
In an embodiment of the invention, carbon fiber is mixed into a slurry with silicon additives to form a carbon-fiber paper. The carbon-fiber paper is then subjected to a silicon carbide formation process by heating in an inert environment to a temperature that, for example, exceeds the melting point of silicon metal. In this forming step, the carbon fiber and the silicon additives react to form silicon carbide (i.e., in-situ silicon carbide).
In an alternate embodiment of the invention, carbon fiber is used to form a carbon fiber paper lamination member. The lamination member is heated in an inert environment with the addition of silicon additives, for example, in a melt-infiltration process. The carbon fiber and the silicon additives react to form in-situ silicon carbide fiber lamination members, that can then be assembled into the structural body. In further embodiments, the carbon fiber paper lamination elements can be assembled into the structural body, with the addition of silicon additives. In a forming process, the carbon fiber and the silicon additives react to form in-situ silicon carbide.
It is an object of the present invention to provide a laminated porous structural body that comprises in-situ silicon carbide fibers. In this way, the fabrication steps to form a silicon carbide porous substrate are not subjected to either the high cost of silicon carbide fibers, or the expense of processing the same. The high bonding temperatures and the difficulty of handling the extremely abrasive silicon carbide raw materials is thereby avoided.
The drawings constitute a part of this specification and include exemplary embodiments of the invention, which may be embodied in various forms.
Detailed descriptions of examples of the invention are provided herein. It is to be understood, however, that the present invention may be exemplified in various forms. Therefore, the specific details disclosed herein are not to be interpreted as limiting, but rather as a representative basis for teaching one skilled in the art how to employ the present invention in virtually any detailed system, structure or manner.
The present invention relates to a honeycomb structural body that exhibits an effective trapping efficiency, with sufficient mechanical durability and robustness for use as an exhaust filtration element. The chemical properties of the honeycomb structural body is extremely robust even at elevated temperatures that may be experienced during regeneration cycles to burn out accumulated soot and particulates. The honeycomb structural body of the present invention provides these benefits with a low inherent backpressure, even when sufficient levels of soot and particulates are accumulated in the filter. Low backpressure is an important characteristic of an exhaust filter as the performance of an internal combustion engine can be severely degraded with increased exhaust backpressure.
The honeycomb structural body of the present invention, is generally shown in
As shown in
Referring to
Referring now the
In a first embodiment, the in-situ silicon carbide lamination member 130 can be fabricated by the method shown in
The silicon additives 320 can be in the form of silicon metal particles or silicon oxide (silica) particles, such as colloidal silica. The fluid 330 can be water, or a solvent solution. Additives 325 can be included in the mixture, such as organic and inorganic binders, that may facilitate the subsequent paper-making process, and to provide for structural enhancement of the lamination member 130 without detracting from the overall porosity of the member. Organic binders can include, without limitation, acrylic latex, methylcellulose, carboxymethylcellulose, hydroxyethylcellulose, polyethylene glycol, phenol resin, epoxy resin, polyvinyl alcohol and styrene-butadiene rubber. At step 340, the a slurry is formed that is mixed to form an evenly distributed mixture of the fibers 310, silicon additives, 320 and the fluid 330.
In order to form silicon carbide fibers from the carbon fibers 310 and the silicon additives 320, the silicon content of the silicon particles must be provided in approximately a stoichiometric ratio to form silicon carbide, and evenly distributed throughout the lamination member 130. Silicon-based particles can be material provided in the form of silicon metal particles, fumed silicon, silicon microspheres, silica-based aerogels, polysilicon, silane or silazane polymers, or from other silicon-based compounds, such as amorphous, fumed, or colloidal silicon dioxide (silica). Colloidal silica can also be used for the silicon-based component of the additives 120. Colloidal silica is a stable dispersion of discrete particles of amorphous silica (SiO2), sometimes referred to as a silica sol. Colloidal silica is commercially available with particle sizes between 5 nm and 5 μm dispersed in an aqueous or solvent solution, typically around 30-50% solid concentration. The small particle size of colloidal silica, when mixed with the carbon fibers 310, permits a uniform distribution of the silicon-based component with the carbon fiber, so that the silica can effectively coat the surface of the individual carbon fibers. The stoichiometric ratio of silicon carbide will be attained with a ratio of three parts carbon to one part Silica (3:1), though the ratio of materials added to the mixture can include excess carbon or excess silica, for example, the mixture can be in the range of about 5:1 and 2:1 carbon:silica.
Alternatively, the silicon-based constituent of the additives 120 can be silicon metal particles with a sufficiently fine particle size to be fully and evenly dispersed during processing. Purity of the silicon is not essential for the silicon carbide formation reaction to occur, but metallic contaminants may alter the application and effectiveness of any subsequent catalyst layer. Preferably, the particle size of the silicon additives 320 is as small as commercially available. Silicon powder in the 1 to 4 μm size or silicon nanoparticles are desirable, though lower cost materials are typically associated with particles in the 30 to 60 μm size. The larger particles are sufficiently small enough to be effectively distributed for the formation of silicon carbide. The stoichiometric molar ratio of silicon carbide will be attained with a ratio of about 1:1 carbon:silicon, though the ratio can be extreme, resulting in either excess carbon or excess silicon. Excess silicon is advantageous to make up for silicon or silicon monoxide that may be lost during the process (due to volatility at high temperatures), and/or to provide available silicon for metal bonds. Additionally, excess silicon residing on the formed silicon carbide fibers can act as a protective coating, which can be advantageous when used with catalysts that include materials such as potassium that can otherwise chemically degrade the silicon carbide material.
The slurry is then subjected to a paper-making process at step 350, to form a carbon fiber paper with an even distribution of silicon particles. More specifically, a perforated mesh in which holes having a predetermined shape can be formed with mutually predetermined intervals, and the resulting matter dried in a range from 100° C. to 200° C. so that a honeycomb-shaped lamination member 130, which has through holes and a predetermined thickness is obtained.
Moreover, in the case where a lamination member is an inlet sealing member 120 or an outlet sealing member 140, that forms the end face of the structural member 100 to adapt the same in a wall-flow configuration, a mesh having holes with a predetermined shape that form a staggered pattern is formed at a predetermined thickness.
Next, at step 360, the lamination member is subjected to an elevated thermal environment to form silicon carbide from the carbon fibers 310 and the silicon additives 320. At this step, the dried paper, i.e., the carbon fiber and the silicon-additives are heated in an environment sufficient to form silicon carbide from the carbon fibers. Organic binders are pyrolized and decomposed during this forming step 360, while leaving the fibrous structure in generally the same relative position within the paper.
The chemical reaction during this final phase of the forming step 360 is generally described to be:
C+Si→SiC
though when the silicon-based component is silica, the reaction can be described to be:
3C+SiO2→SiC+2CO2
It is to be appreciated that in this reaction, intermediate transitionary compounds may form before stable SiC is formed.
The above reaction will take place when the structure is heated to a temperature of about 1400 to 1800 degrees Celsius, for approximately 2 to 4 hours or more, in an inert environment. When silicon metal is included as the silicon-additives 320, the silicon particles will melt at above 1414 degrees Celsius, which will then wet to, and coat the carbon fibers to convert into silicon carbide. This wetting is optimized in vacuum atmosphere conditions where silicon metal will spontaneously wet elemental carbon, including the fiber itself or wetting of a residual carbon layer remaining from the burn out of a binder additive.
When silica is used as the silicon additive 320, there is a solid state (solid-solid) reaction that goes on that is diffusion dependent:
3C+SiO2→SiC+2CO2
There may be a secondary reaction is that the SiO2 first vaporizes to SiO, and this then reacts with the carbon to form silicon carbide, thus resulting in the following gas-solid reaction:
2C+2SiO→2SiC+O2
An inert environment is necessary to ensure the absence of oxygen to prevent the oxidation of the carbon into carbon dioxide. The resulting structure is generally silicon-carbide fibers in an intertangled and overlapping relationship, forming an open network of pores. It can be appreciated that the resulting microstructure formed within the substrate is largely based on the intertangled fiber architecture originally composed of the carbon or organic fibers, and the formation of silicon carbide during the forming step 360 does not substantially change the relative position of the fibers.
The forming step 360 can be carried out in a conventional batch or continuous furnace or kiln. The inert environment can be maintained by purging the furnace or kiln with nitrogen, argon, helium, neon, forming gas and mixtures thereof, or any inert gas or gaseous mixture. It is important to have a little to none partial pressure of oxygen, so as to prevent adverse reactions from occurring that can lead to oxidation and volatilization of the reactive species. Alternatively, the forming step 360 can be performed in a vacuum environment, which would typically require a vacuum of 200.0 torr or less. The forming step 360 can be performed by a sequential progression through multiple batch or continuous kilns, or the sequence of heating steps, i.e., drying, binder burnout, and reaction formation, can be performed in a single facility that can maintain the sequential temperature environments in a manual or automatic fashion.
At step 370, the in-situ silicon carbide lamination members 130, and the inlet sealing member 120 and outlet sealing member 140, are laminated into the honeycomb structural body 100 to form a filter assembly 200. In this assembly step, catalytic materials can be applied through the application of a washcoat and catalyst materials, as further described below.
The in-situ silicon carbon fibers are aligned generally in parallel with the main face of the lamination member 130. When the structural body 100 is formed at step 370, a substantial portion of the fibers are aligned along the face perpendicular to the forming direction of the through holes in comparison with those aligned along the horizontal face with respect to the forming direction of the through holes. Therefore, since the honeycomb structural body 100 permits exhaust gases to pass through the wall portion more easily, it is possible to minimize the impact of the filter assembly 200 on system backpressure, and to allow particulates to penetrate deep into the lamination members 130 (i.e., a depth filter).
A second embodiment of the present invention is depicted in reference to
The fluid 330 can be water, or a solvent solution. Additives 325 can be included in the mixture, such as organic and inorganic binders, that may facilitate the subsequent paper-making process, and to provide for structural enhancement of the lamination member 130 without detracting from the overall porosity of the member. Organic binders can include, without limitation, acrylic latex, methylcellulose, carboxymethylcellulose, hydroxyethylcellulose, polyethylene glycol, phenol resin, epoxy resin, polyvinyl alcohol and styrene-butadiene rubber. At step 340, the a slurry is formed that is mixed to form an evenly distributed mixture of the fibers 310, additives, 325 and the fluid 330.
The slurry is then subjected to a paper-making process at step 350, to form a carbon fiber paper. More specifically, a perforated mesh in which holes having a predetermined shape can be formed with mutually predetermined intervals, and the resulting matter dried in a range from 100° C. to 200° C. so that a honeycomb-shaped lamination member 130, which has through holes and a predetermined thickness is obtained.
Moreover, in the case where a lamination member is an inlet sealing member 120 or an outlet sealing member 140, that forms the end face of the structural member 100 to adapt the same in a wall-flow configuration, a mesh having holes with a predetermined shape that form a staggered pattern is formed at a predetermined thickness.
Next, at step 370, the lamination members 130 and the inlet sealing member 120 and the outlet sealing member 140, all in a carbon fiber paper form, are assembled into the honeycomb structural body 100, with the addition of a silicon additive 320. The silicon additive 320 can be in the form of a colloidal suspension of silicon or silica particles applied by immersion, or the lamination members 130 can be laminated with a thin wafer of silicon interleaved between each lamination member.
Next, at step 360, the lamination member is subjected to an elevated thermal environment to form silicon carbide from the carbon fibers 310 and the silicon additives 320. At this step, the dried paper, i.e., the carbon fiber and the silicon-additives are heated in an environment sufficient to form silicon carbide from the carbon fibers. Organic binders are pyrolized and decomposed during this forming step 360, while leaving the fibrous structure in generally the same relative position within the paper.
The chemical reaction during this final phase of the forming step 360 is generally described to be:
C+Si→SiC
though when the silicon-based component is silica, the reaction can be described to be:
3C+SiO2→SiC+2CO2
It is to be appreciated that in this reaction, intermediate transitionary compounds may form before stable SiC is formed.
The above reaction will take place when the structure is heated to a temperature of about 1400 to 1800 degrees Celsius, for approximately 2 to 4 hours or more, in an inert environment. When silicon metal is included as the silicon-additives 320, the silicon particles will melt at above 1414 degrees Celsius, which will then wet to, and coat the carbon fibers to convert into silicon carbide. This wetting is optimized in vacuum atmosphere conditions where silicon metal will spontaneously wet elemental carbon, including the fiber itself or wetting of a residual carbon layer remaining from the burn out of a binder additive.
When silica is used as the silicon additive 320, there is a solid state (solid-solid) reaction that goes on that is diffusion dependent:
3C+SiO2→SiC+2CO2
There may be a secondary reaction is that the SiO2 first vaporizes to SiO, and this then reacts with the carbon to form silicon carbide, thus resulting in the following gas-solid reaction:
2C+2SiO→2SiC+O2
An inert environment is necessary to ensure the absence of oxygen to prevent the oxidation of the carbon into carbon dioxide. The resulting structure is generally silicon-carbide fibers in an intertangled and overlapping relationship, forming an open network of pores. It can be appreciated that the resulting microstructure formed within the substrate is largely based on the intertangled fiber architecture originally composed of the carbon or organic fibers, and the formation of silicon carbide during the forming step 360 does not substantially change the relative position of the fibers.
The forming step 360 can be carried out in a conventional batch or continuous furnace or kiln. The inert environment can be maintained by purging the furnace or kiln with nitrogen, argon, helium, neon, forming gas and mixtures thereof, or any inert gas or gaseous mixture. It is important to have a little to none partial pressure of oxygen, so as to prevent adverse reactions from occurring that can lead to oxidation and volatilization of the reactive species. Alternatively, the forming step 360 can be performed in a vacuum environment, which would typically require a vacuum of 200.0 torr or less. The forming step 360 can be performed by a sequential progression through multiple batch or continuous kilns, or the sequence of heating steps, i.e., drying, binder burnout, and reaction formation, can be performed in a single facility that can maintain the sequential temperature environments in a manual or automatic fashion.
In a modification to the second embodiment, the silicon additive 320 can be introduced to the carbon fibers of the laminated members 130 through melt infiltration during the forming step 360. In this alternate embodiment, the carbon fibers of the lamination members are exposed to molten silicon metal, that immediately wets to, and flows throughout the carbon fiber structure, so that the reaction to form silicon carbide can occur. The molten silicon metal is typically introduced to the carbon fiber in the forming step 360 in a vacuum kiln or inert environment, though at least a single strand of carbon fiber that is immersed in a crucible of silicon metal within the kiln.
In a variation of this modification to the second embodiment, carbon-fiber lamination members can be assembled into the structural body, and the entire structural body can have the silicon additive 320 introduced in a melt infiltration forming step 360. In this way, the silicon metal that wets over the carbon fiber to react with the carbon fiber to form in-situ silicon carbide, also provides silicon bonds and silicon carbide crystallized bonds between the fibers and between the lamination members 130.
In yet further embodiments, organic fiber-based paper can be carbonized to form a carbon-fiber paper having through holes in predetermined locations to form lamination members 130, inlet sealing members 120, and outlet sealing members. The paper materials can be made from rayon, cotton, wood, polymeric resins. The paper lamination elements are carbonized to convert the organic fiber into carbon fiber by heating the organic-fiber paper lamination element. In this embodiment, the organic fibers are converted into elemental carbon through pyrolyzation of the organic material, while maintaining the fibrous structure of the paper. The carbonization step is performed, for example, by heating the paper to approximately 1,000° C. for about four to five hours in an inert environment. The inert environment is necessary for this step so that the carbon is not oxidized after it is formed, and so that the remaining additives are not oxidized. In this alternate embodiment, the carbon fiber resulting from the carbonization step may shrink as much as 70% in diameter, and thus, the thickness of the organic fiber must be initially larger than the thickness of a carbon fiber in the first two embodiments to attain a similar structure. Using the methods described above in reference to the first or second embodiments, silicon additives 320 can be added, so that in-situ silicon carbide is formed.
Once the honeycomb structural body 100 is assembled, and the in-situ silicon carbide fibers has been completed, any number of catalysts and washcoats can be disposed within the honeycomb structural body 100 to chemically alter combustion byproducts in the exhaust stream by catalysis. Such a catalyst includes but is not limited to platinum, palladium (such as palladium oxide), rhodium, derivatives thereof including oxides, and mixtures thereof. In addition, the catalysts are not restricted to noble metals, combination of noble metals, or only to oxidation catalysts. Other suitable catalysts and washcoats include chromium, nickel, rhenium, ruthenium, silver, osmium, iridium, platinum, tungsten, barium, yttrium, neodymium, lanthanum, gadolinium, praseodymium, and gold, derivatives thereof, and mixtures thereof. Other suitable catalysts include binary oxides of palladium, aluminum, tungsten, cerium, zirconium, and rare earth metals. Other suitable catalysts include vanadium and derivatives thereof, e.g., V2O5, or silver or copper vanadates, particularly when sulfur is present in the fuel or lubricant. Further still, the substrate 510 can be configured with a combination of catalysts applied to different sections or zones to provide a multi-functional catalyst. For example, the substrate 510 can be used as a particulate filter with soot-oxidizing catalysts applied to the inlet channel walls, with a NOx adsorber, or selective catalyst reduction catalyst applied to the internal fibrous structure in the channel walls. Similar configurations can be applied to provide NOx traps or 4-way catalytic converters.
The present invention has been herein described in detail with respect to certain illustrative and specific embodiments thereof, and it should not be considered limited to such, as numerous modifications are possible without departing from the spirit and scope of the appended claims.
