WO2011008463A1 - Process for producing cemented and skinned acicular mullite honeycomb structures - Google Patents

Abstract

Cement compositions are used to form skins on ceramic honeycombs, or to cement smaller honeycombs to other honeycombs or other materials to form assemblies. The cement composition contains an inorganic filler, and either colloidal silica, colloidal alumina, or both. The inorganic filler and the colloidal materials individually or collectively supply silicon and aluminum atoms. The cement composition is fired in the presence of a fluorine source. A preferred fluorine source is residual fluorine that is contained in an acicular mullite honeycomb. Residual fluorine is released during the firing step, and facilitates the production of mullite in the cement composition as it is fired.

Description

PROCESS FOR PRODUCING CEMENTED AND SKINNED ACICULAR

MULLITE HONEYCOMB STUCTURES

This application claims benefit of United States Provisional Patent Application No. 61/221,422, filed 29 June 2009.

The present invention relates to a process for producing acicular mullite honeycomb structures having an inorganic cement layer or an inorganic skin.

Acicular mullite honeycomb structures are often used as filters in high temperature applications. These honeycombs are often used as particulate filters to remove soot particles or droplets from diesel engine exhaust. Filters of these types are frequently exposed to large, rapid changes in temperature. The temperature changes can occur during the normal operation of the vehicle, but they are especially pronounced when the filter is thermally regenerated to burn out the captured soot. These large, rapid temperature changes are sometimes referred to as "thermal shock" events.

These rapid temperature changes usually create temporary but significant temperature gradients within the honeycomb structure, which in turn lead to the creation of large localized stresses due to non-uniform thermal expansion (or thermal contraction) within the part. When these localized stresses exceed the strength of the part, the structure will relieve the stress by cracking, which can lead to part failure.

Various approaches have been tried to improve the thermal shock resistance of these honeycomb structures. In one approach, the honeycomb is made up of multiple smaller honeycombs which are cemented together. Another approach focuses on the peripheral "skin" of the honeycomb. The periphery of the part is often subjected to the highest thermally-induced stresses, especially during rapid temperature increases. As a result, cracking often initiates at the skin, from which the cracks can propagate throughout the structure and destroy the part. This skin can be removed and replaced with another ceramic material that is more compliant than the original acicular mullite skin of the honeycomb. The cement and skins are made by applying and firing a cement composition that contains a colloidal silica or alumina, filler particles and a carrier fluid. For example, USP 7,083,842 describes a ceramic honeycomb structure in which the original peripheral region of the structure is removed and replaced with an inorganic coating that is fired to form a replacement skin. The coating composition includes an inorganic binder, ceramic fibers of up to 100 microns in length, and particles having a diameter of from 0.5 to 100 microns. USP 5,914,187 describes a cement that includes an inorganic binder such as a glassy silica phase, as well as both ceramic fibers and other inorganic powders or whiskers. The powders or whiskers are used to increase the thermal conductivity of the cement. USP 7,112,233 describes a similar cement, which in this case is formulated to have a specific thermal conductivity. The cement described in USP 7,112,233 includes silica-alumina fibers which are at least 1 mm in length. According to USP 7,112,233, shorter fibers do not permit an "elastic" structure to be formed. The needed thermal conductivity is provided by including silicon carbide, silicon nitride or boron nitride particles in the cement formulation.

In one aspect, this invention is a process comprising the steps of (a) forming a ceramic honeycomb containing multiple axially-extending cells defined by intersecting walls, (b) applying to at least one surface of the ceramic honeycomb a cement composition that contains both aluminum and silicon atoms and includes (1) at least one inorganic filler, (2) a colloidal silica, colloidal alumina or mixture thereof which forms a binding phase upon firing, and (3) a carrier fluid and then (c) firing the honeycomb and cement composition at a temperature of at least 1000 ⁰C in the presence of a fluorine source.

The resulting ceramic honeycomb structure often has greater thermal shock resistance, compared to when those steps are performed sequentially. Although the invention is not limited to any theory, it is believed that mullite forms in the cement composition when it is fired in the presence of the fluorine source. Some mullite can form when the cement is fired, even in the absence of the fluorine source. However, it has been found that mullite forms faster and to a greater extent in the cement composition when a fluorine source is present. The higher mullite content of the fired cement in some cases can more closely match the coefficient of thermal expansion (CTE) of the fired cement to that of the underlying honeycomb, especially in preferred cases in which the honeycomb is acicular mullite. This closer match in CTE is believed to account for the greater shock resistance of the honeycomb structure.

The cement composition can function as a cement which adheres the honeycomb to another part of the final structure. For example, the honeycomb may be composed of two or more smaller honeycombs, which are cemented together using the cement composition to produce a larger honeycomb. The cement composition may perform such a cementing function. The cement composition of the invention may also serve to cement a honeycomb to some other structure. The cement composition may instead, or in addition, be used to produce a peripheral skin for the honeycomb structure. In a particularly preferred embodiment, this invention is a process comprising the steps of (a) forming a ceramic honeycomb containing multiple axially-extending cells defined by intersecting walls, wherein at least a portion of the ceramic honeycomb is an acicular mullite that contains at least 0.5 weight percent residual fluorine, based on the weight of the acicular mullite in the honeycomb (b) applying to at least one surface of the ceramic honeycomb a cement composition that contains both aluminum and silicon atoms and includes (1) at least one inorganic filler, (2) a colloidal silica, colloidal alumina or mixture thereof which forms a binding phase upon being fired, and (3) a carrier fluid, and then (c) exposing the honeycomb and cement composition to a temperature of at least 1200⁰C.

An additional advantage of this embodiment of this invention is that two normally distinct steps in the manufacture of a ceramic honeycomb structure can be combined into one operation.

Honeycomb structures made in accordance with the invention are useful in a variety of filtration, heat exchange and catalytic applications. Because those honeycomb structures tend to have good thermal shock resistance, they are particularly useful in applications in which the structure is exposed to rapid and large changes in temperature.

Figure 1 is graph showing the coefficient of thermal expansion of an acicular mullite honeycomb, a cement (Example 1) formed in accordance with this invention, and of a comparative cement (Comparative Sample A) that is not formed in the presence of a fluorine source.

Figure 2 is a graph showing the coefficient of thermal expansion of an acicular mullite honeycomb, a cement (Example 2) formed in accordance with this invention, and of a comparative cement (Comparative Sample B) that is not formed in the presence of a fluorine source.

The ceramic honeycomb is characterized in having multiple cells that extend axially throughout the length of the honeycomb body. The cells are defined by multiple intersecting walls. The walls and the intersection points define the number of cells, as well as their cross-sectional shape and dimensions. A typical honeycomb for many filtration or catalysis applications will contain from 25 to 1000 cells/square inch (about 4 to 150 cells/square centimeter) of cross-sectional area (i.e., transverse to the longitudinal extension). Wall thicknesses are typically from 0.05 to 10 mm, preferably from 0.2 to 1 mm, although larger or smaller wall thicknesses might be used. The ceramic honeycomb may be monolithic (i.e., formed in a single piece), or may be an assembly of smaller honeycombs which are manufactured separately and then assembled together, usually using a ceramic cement. The ceramic cement in such an assembly is in some embodiments a fired cement composition as described herein.

The walls of the honeycomb preferably are porous, and a fluid can pass through the pores from one cell to one or more adjacent cells. The ceramic making up the honeycomb generally has a porosity of about 30% to 85%. Preferably, the porous ceramic has a porosity of at least about 40%, more preferably at least about 45%, even more preferably at least about 50%, and most preferably at least about 55% to preferably at most about 80%, more preferably at most about 75%, and most preferably at most about 70%. Porosities are determined by water immersion methods.

The ceramic honeycomb may be made from an inorganic material such as alumina, zirconia, silicon carbide, silicon nitride, aluminum nitride, silicon oxynitride, silicon carbonitride, mullite, cordierite, beta spodumene, aluminum titanate, strontium aluminum silicates, lithium aluminum silicates. In preferred embodiments, at least a portion of the ceramic honeycomb is an acicular mullite that contains at least 0.5 weight percent residual fluorine. If the ceramic honeycomb is monolithic, then the entire honeycomb preferably is such an acicular mullite. In cases in which the ceramic honeycomb is a cemented assembly of smaller honeycombs, at least one of the smaller honeycombs preferably is such an acicular mullite. It is preferred that all of the smaller honeycombs are acicular mullite containing at least 0.5 weight percent residual fluorine.

Acicular mullite honeycomb structures can be prepared by forming a clay from a mullite precursor, shaping the clay into the honeycomb configuration (typically by extrusion) and then mullitizing the clay. Mullitization is performed by exposing the clay to a fluorine-containing compound under conditions that the mullite precursors react with the fluorine-containing compound to form a fluorotopaz which then decomposes to form acicular mullite needles. Suitable methods for preparing acicular mullite honeycombs are described, for example, in WO 92/11219, WO 03/082773 and WO 04/096729.

As fluorotopaz decomposes to form mullite, a mass of interconnected needle-like crystals is created. The crystals are comprised mainly of a crystalline mullite, although it is possible for small quantities of other crystalline and/or glassy phases to be present. For example, the crystals may contain up to about 2 volume percent of a crystalline silica phase such as cristobalite, as described in WO 03/082773, or up to about 10 volume percent of a glassy oxide phase that may contain silicon and/or aluminum as well as one or more metals contributed by a sintering aid and/or other compounds as may be present.

The acicular mullite crystals are bonded together at points of contact to form a porous mass having essentially the same overall geometry and dimensions as the clay honeycomb. The aspect ratio of the mullite crystals is typically at least 5, preferably at least 10, more preferably at least 20. The crystals may have a mean grain diameter of from 5 to 50 microns.

Acicular mullite bodies prepared as described above tend to contain some residual fluorine. The amount of fluorine may constitute from 0.5 to about 3 weight percent of the weight of the acicular mullite. More typically, the fluorine constitutes from about 0.8 to 2 weight percent of the acicular mullite. In conventional processes, this residual fluorine is removed by heating the honeycomb to a temperature of at least 1200⁰C, preferably at least 1400⁰C, preferably in air or the presence of oxygen. In this invention, however, it is preferred that at least a portion of this residual fluorine remains in the acicular mullite honeycomb until the cement composition is applied, as described more fully below. The acicular mullite in the honeycomb should contain at least 0.5 weight percent fluorine.

The cement composition is applied to one surface of the ceramic honeycomb. As already mentioned, the cement composition may perform a cementing function, adhering the ceramic honeycomb to another honeycomb or to some other structure. The cement composition may instead, or in addition, serve as a peripheral skin for the honeycomb structure.

The cement composition contains both silicon and aluminum atoms. Its constituent components include (1) inorganic filler particles, (2) a colloidal silica, colloidal alumina or mixture thereof which forms a binding phase upon firing, and (3) a carrier fluid. The inorganic filler particles are materials which do not form a binding phase when the cement composition is fired, and thus are distinguished from the colloidal silica and/or colloidal alumina component of the composition. The inorganic filler particles instead retain their particulate nature throughout the firing process, although they may become bound by the binding phase to other particles or to the inorganic fibers. Other components may be present in the cement composition, as described more fully below. Colloidal silica and colloidal aluminum are of course sources of silicon and aluminum atoms, respectively. If colloidal silica is used by itself to form the binder phase, the cement composition must contain some additional source of aluminum atoms. This source is typically the inorganic filler particles, which may contain silicon atoms in addition to the necessary aluminum atoms. Similarly, if colloidal alumina is used by itself to form the binder phase, the cement composition must contain some additional source, of silicon atoms, which again typically will be the inorganic filler particles. In this second case, the inorganic filler may contain aluminum atoms in addition to the needed silicon atoms.

If both colloidal silica and colloidal alumina are present in the cement, another source of silicon and aluminum atoms is not needed. Nonetheless, it is preferred even in this case that the inorganic filler particles contain aluminum atoms, silicon atoms or both aluminum and silicon atoms.

The preferred inorganic filler particles are therefore aluminate, silicate or aluminosilicate materials. The filler particles may be amorphous, partially crystalline or fully crystalline. The inorganic filler particles may contain a crystalline phase that is surrounded by glass. The inorganic filler particles may also contain other elements such as rare earths, zirconium, iron, boron and alkaline earths. Examples of silicon- and/or aluminum -containing materials that can be used as the inorganic filler particles are alumina, borosilicate glass, quartz, e-glass, s-glass, silicon carbide, silicon nitride, mullite, cordierite, alumina silicates, alumina-zirconia-silicates, wollastonite, basalt and aluminum titanate. If no silicon or aluminum atoms are needed in the inorganic filler particles, other materials such as boron nitride or carbon nitride particles can be used.

It is preferred to use an aluminosilicate material that can be at least partially converted to mullite as the inorganic filler particles.

Preferably, at least a portion of the inorganic filler particles are in the form of fibers that have a diameter of from 100 nanometers to 20 microns and an aspect ratio (longest dimension divided by shortest dimension) of at least 10, preferably at least 20. A preferred fiber diameter is from 0.5 to 10 microns. A more preferred fiber diameter is from 3 to 10 microns.

The number average length of the inorganic fibers may range from 100 microns to 130 millimeters or more. The number average length is preferably at least 100 microns and more preferably at least 200 microns. The number average length is preferably no greater than 10 millimeters. The number average length may be no greater than 5 millimeters or no greater than 2 millimeters. Longer fibers, such as those having lengths of 10 mm or more, often tend to form bundles during processing. These bundles cause difficulties in applying the skin and also lead to inconsistencies in the skin composition. Therefore, longer fibers preferably are used somewhat sparingly if at all.

In some embodiments of the invention, essentially all of the fibers have a length of less than 1 mm. In other embodiments, the fibers have a bimodal or multimodal length distribution, in which one portion of the fibers are shorter fibers having a number average length of from 100 to 1000 microns, and at least one other portion of the fibers are longer fibers having a number average length of at least 1 millimeter, preferably from 1 to 100 millimeters, more preferably from 2 to 100 millimeters and even more preferably from 5 to 30 millimeters. In such embodiments, the longer fibers preferably constitute from 1 to 50, more preferably from 3 to 30 and even more preferably from 5 to 25 percent of the total weight of the inorganic fibers. Mixed length fibers provide certain advantages. The presence of a minor proportion of longer fibers tends to increase the viscosity of the cement composition, at a given fiber content in the composition. The viscosity of the cement composition should be somewhat high, so it can be applied and shaped readily without sagging or flowing off of the honeycomb before it can dry. The presence of a minor proportion of longer fibers can allow a good working viscosity to be achieved without unduly increasing the fiber content. If the fiber content becomes too high, there may not be enough colloidal silica and/or colloidal alumina in the composition to adequately bind the fibers to each other or to the underlying honeycomb. Typically, the strength of the fired cement composition tends to decrease with increasing fiber length, because the number of fibers decreases as their length increases, and fewer fibers means fewer points of intersection where they can be bound together. When a mixture of shorter and longer fibers is used, the strength of the fired cement composition is often comparable to that of a cement that contains an equivalent proportion of only short fibers. Thus, a mixture of shorter fibers and a minor proportion of longer fibers can provide significant processing benefits with little or no corresponding disadvantages.

Examples of useful organic fibers include mullite fibers, such as are available from Unifrax; alumina-zirconium-silicate fibers, such as are available from Unifrax; alumina fibers containing up to 10% by weight silica, such as are available from Saffil; γ -alumina and α-alumina + mullite fibers such as Nextel 312 or Nextel 610 fibers from 3M; γ-alumina + mullite + amorphous SiCh fibers such as Nextel 440 fibers from 3M; γ- alumina + amorphous Siθ2 fibers such as Nextel 550 fibers from 3M; quartz fibers such as are available from Saint Gobain; e-glass or s-glass fibers; borosilicate fibers such as are available from Mo-SiC Corporation; basalt fibers such as are available from Albarrie, wollastonite fibers such as are available from Fibertec, and the like.

The cement composition may contain low aspect ratio inorganic filler particles in addition to or instead of the inorganic fibers described above. "Low aspect ratio" refers to an aspect ratio of less than 10. These inorganic filler particles are different from and do not include the colloidal silica and/or colloidal alumina component of the cement composition. The low aspect ratio inorganic filler particles do not form a binding phase when the cement composition is fired. The low aspect ratio inorganic filler particles instead retain their particulate nature throughout the firing process, although they may become bound by the glassy binding phase to other particles or to the inorganic fibers.

A mixtue of inorganic fibers and low aspect ratio inorganic filler particles may be present. In such cases, these low aspect ratio inorganic filler particles can be classified into two types. The first type is particles that have the same CTE or very nearly the same CTE as the inorganic fiber (i.e., differing by no more than 1 ppm/°C in the temperature range of from 100 to 600⁰C), after the firing step is completed. The comparison is performed on the basis of the fired skin composition to account for changes in CTE that may occur to the fibers and/or other particles during the firing step, due to, for example, changes in crystallinity and/or composition that may occur. Particles of this type generally have the same or nearly the same chemical composition as the inorganic fiber. A common source of this type of particle is so-called "shot" material, which is a by-product of the fiber manufacturing process and is included in many commercial grades of inorganic fibers. However, this type of particle may be supplied from other sources as well. This first type of inorganic filler particle may constitute from 0 to as much as 60% of the total weight of the inorganic filler. Preferably, this type of inorganic filler particle constitutes no more than 50%, more preferably no more than 25% and still more preferably no more than 10% to the total weight of the inorganic fillers.

The second type of inorganic filler particles have a CTE which is significantly different (i.e., different by more than 1 ppm/°C, preferably by at least 2 ppm/°C in the temperature range from 100 to 600⁰C) than that of the inorganic fibers, after the firing step is completed. Inorganic filler particles of this type, if present at all, constitute no more than 5% by weight of the solids of the cement composition. For purposes of this calculation, the "solids" are constituted by the inorganic materials in the cement composition that remain in the skin after the firing step is completed, including fillers and inorganic binding phase. One advantage of this invention is that it is not necessary to add fillers or otherwise attempt to "match" the coefficient of thermal expansion of the cement to that of the underlying honeycomb. Accordingly, the cement composition may contain no inorganic filler particles of the second type at all, or may contain only very small proportions thereof, such as, for example, from 0 to 3% or from 0 to 2% or from 0 to 1% of the solids of the cementcomposition. Examples of this second type of inorganic filler particles are alumina, silicon carbide, silicon nitride, mullite, cordierite and aluminum titanate.

In one preferred embodiment, the inorganic filler contains only the inorganic fiber, "shot" material from the inorganic fiber, and optionally the second type of inorganic filler particle, which may be present in an amount from 0 to 5% by weight of the solids of the cement composition, but essentially no (less than 5 weight percent, preferably no more than 1%) other organic filler particles of the first type. In such an embodiment, it is more preferred that the inorganic fibers constitute at least 50, at least 75 or at least 90% of the total weight of the inorganic filler, and that the "shot" material constitutes no more than 50, no more than 25 or no more than 10% of the total weight of the inorganic filler. An especially inorganic filler of this type includes only inorganic fiber and "shot" material.

In another preferred embodiment, the inorganic filler contains only the inorganic fiber and from 0-5 weight percent of the second type of inorganic filler, but no "shot" material or other inorganic filler of the first type.

The inorganic filler particles in the aggregate may constitute from about 30 to 90% by weight of the solids in the cement. A preferred amount is from 50 to 85% by weight of the solids and a more preferred amount is from 60 to 80% by weight of the solids. As mentioned before, the "solids" in the composition are those inorganic materials that remain after the firing step is completed. In most cases, the solids will be made up of the inorganic filler particles and the colloidal silica and/or colloidal alumina. Carrier fluids and organic materials generally are lost from the cement during the firing step(s). The "solids" therefore do not include any amounts of those materials. The colloidal silica and/or colloidal alumina may constitute from 10 to 70%, preferably from 15 to 50% and more preferably from 20 to 40% of the weight of the solids portion of the cement composition.

The cement composition also includes a carrier liquid. The mixture of carrier fluid and colloidal silica and/or alumina particles forms a paste or viscous fluid in which the inorganic fibers are dispersed. The fluid or semi-fluid nature of the cement composition permits it to be applied easily and to adhere well to the underlying honeycomb until the firing step is completed. The carrier liquid may be, for example, water, or an organic liquid. Suitable organic liquids include alcohols, glycols, ketones, ethers, aldehydes, esters, carboxylic acids, carboxylic acid chlorides, amides, amines, nitriles, nitro compounds, sulfides, sulfoxides, sulfones and the like. Hydrocarbons, including aliphatic, unsaturated aliphatic (including alkenes and alkynes) and/or aromatic hydrocarbons, are useful carriers. Organometallic compounds are also useful carriers. Preferably, the carrier fluid is an alcohol, water or combination thereof. When an alcohol is used it is preferably methanol, propanol, ethanol or combinations thereof. Water is the most preferred carrier fluid.

The cement composition contains enough of the carrier fluid to wet the colloidal silica and/or alumina and produce a paste or viscous fluid, in which the inorganic filler particles are dispersed. A useful Brookfield viscosity, as measured at 25°C using a #6 spindle at 5 rpm, is typically at least about 5, 10, 25, 50, 75 or even 100 Pa-s. The cement composition may exhibit shear-thinning behavior, such that its viscosity becomes lower at higher shear. The total amount of carrier fluid in the cement composition (including any carrier fluid that may be brought in with the colloidal silica and/or colloidal alumina) is generally from about 25% by weight to at most about 90% by weight of the entire composition. A preferred amount of carrier fluid is from 40 to 70% by weight of the entire composition.

The cement may contain other useful components in addition to the inorganic filler particles, colloidal silica and/or colloidal alumina and carrier fluid. An organic binder or plasticizer can provide desirable rheological properties to the cement composition, and therefore preferably is present. Preferably, the binder dissolves in the carrier liquid. Examples of suitable binders and organic plasticizers include cellulose ethers, such as methyl cellulose, ethyl cellulose, hydroxypropylmethyl cellulose, hydroxypropyl cellulose, carboxylm ethyl cellulose and the like; polyethylene glycol, fatty acids, fatty acid esters and the like. Other optional components include dispersants, deflocculants, flocculants, defoamers, lubricants and preservatives, such as those described in Chapters 10-12 of Introduction to the Principles of Ceramic Processing, J. Reed, John Wiley and Sons, NY, 1988. The cement composition also may contain one or more porogens. Porogens are materials specifically added to create voids in the skin after being heated to form the amorphous phase. Typically these are any particulates that decompose, evaporate or in some way volatilize during a heating or firing step to leave a void. Examples include flour, wood flour, carbon particulates (amorphous or graphitic), nut shell flour or combinations thereof.

Organic materials such as binders, plasticizers and porogens typically constitute, in the aggregate, from 0 to 15%, preferably from 1 to 10% of the total weight of the cement composition.

The cement composition is applied to at least one surface of the honeycomb. The manner of applying the cement composition not critical, and any suitable method by which the composition can be applied at the desired thickness is suitable. The cement can be applied manually or through the use of various types of mechanical apparatus. The cement composition may be applied under sub-atmospheric pressures to facilitate removal of the carrier fluid during the application process. If the cement is used to assemble multiple parts (such as multiple honeycombs) into a larger assembly, the cement is applied in any convenient manner to a surface of one or more of the parts that are being assembled, and the parts are then joined with the cement interposed between the parts.

If the cement composition is to be used to form a skin on the honeycomb (or an assembly containing the honeycomb), the composition is applied to at least a portion of the periphery of the honeycomb. Ceramic honeycombs as manufactured typically have an outer peripheral "skin", which may be simply the exterior cell walls of the peripheral cells of the honeycomb structure. It is generally preferable to remove such a skin before applying a replacement skin in accordance with this invention. At least the exterior walls of the peripheral cells of the honeycomb are removed. More typically, the removal of the "skin" is only part of a more general shaping process, in which outer portions of the ceramic honeycomb are removed to bring its cross-sectional shape and size to necessary specifications. This step of removing peripheral portions of the ceramic honeycomb exposes the interior of the axially- extending cells that remain on the periphery of the honeycomb after the removal step is completed. The cement composition is then applied to at least a portion of the newly exposed periphery of the honeycomb.

The periphery of the honeycomb usually is not smooth, and in most cases a certain proportion of the axially-extending cells around the periphery of the honeycomb will be open before the cement composition is applied to form a skin. The cement composition typically will be applied in such a manner as to fill those open cells and to form a somewhat smooth exterior surface. Therefore, the thickness of the skin usually will vary. At its thinnest points, the applied skin should be at least 1 mm in thickness, and may be as much as 25 mm thick.

The cement composition is fired after it is applied to the honeycomb. The firing step removes the carrier fluid and any organic materials (including any porogen) from the cement. The colloidal silica and/or colloidal alumina form a binding phase during the firing step.

In this invention, at least a portion of the firing step is performed at a temperature of at least 1000 ⁰C in the presence of a fluorine source. The temperature may be as high as 1600⁰C, and preferably is up to 1500⁰C. The fluorine source may be, for example, SiF4, AIF3, HF, NaϊSiFβ, NaF, NH4F, a fluorinated polymer such a fluorinated polyethylene or polytetrafluoroethene or some mixture of any two or more thereof.

In preferred embodiments, the fluorine source is residual fluorine contained in an acicular mullite honeycomb or a mixture of that residual fluorine and an additional fluorine source as described in the preceding paragraph. In such a case, the firing temperature preferably is at least 1200⁰C and is more preferably at least 1400°C. At this high firing temperature, residual fluorine is released from the acicular mullite honeycomb, possibly in the form of SiF4. The released fluorine or SiF4 is believed to contribute to mullite formation in the cement under the firing conditions. As a result of the high temperature firing step, a significant quantity of mullite tends to form from the silicon- and aluminum -containing components of the cement. Although some mullite formation is common, even when the cement is fired at lower temperatures (i.e., below 1400⁰C, especially below 1200°C), when the higher firing temperatures are used, more mullite tends to form in the cement than is seen when lower temperatures are used, or when the acicular mullite honeycomb does not contain residual fluorine. In addition, mullite formation may occur more rapidly when the firing step is performed in accordance with this invention. In a preferred firing regimen, the honeycomb and applied cement composition are heated at a rate of no greater than 20°C/minute, preferably no greater than 10°C/minute and still more preferably no greater than 5°C/minute, from ambient temperature up to at least 1000^°C (or at least 1200⁰C, when the fluorine source is an acicular mullite honeycomb). The gradual heating rate is intended to help prevent thermal shocks and also to provide time for the carrier fluid and any organic materials to be removed. It desired, the assembly may be held at one or more intermediate temperatures for a period. This may be desirable, for example, to remove the carrier fluid, organic binders and/or porogens in some predetermined sequence, to allow some chemical reaction to take place, or for some other reason. Once the assembly reaches the necessary temperature, it is preferably held at or above that that temperature for a period of from 5 minutes to 10 hours. This allows time for the fluorine source to react with the cement composition to produce mullite, and, in preferred embodiments, allows time for residual fluorine to escape from the acicular mullite honeycomb. In the preferred embodiments, it is preferred to reduce the residual fluorine in the acicular honeycomb to less than 0.5 weight percent, more preferably less than 0.1 weight percent, of the acicular mullite. The assembly, with the cement now being fired, is then cooled to ambient temperature, preferably at some gradual cooling rate (such as no greater than 10 or 20°C/minute) to prevent damage from thermal shock.

The mullite content of the fired cement will of course depend somewhat on the amount of silicon atoms, aluminum atoms and fluorine that were available in the starting materials. The fired cement may contain as much as 85% by weight mullite. More typically, the fired cement contains from about 45 to 80% mullite, or from 45 to 75% mullite. The mullite formation has little effect on the morphology of the fired cement. The inorganic filler particles largely maintain their particulate or fibrous nature, and are bound together via a binding phase that is mainly formed from colloidal the silica and/or colloidal alumina component of the cement. Mullite may be present in the filler particles or fibers, in the binding phase, or both.

The fired cement is usually porous. The porosity of the fired cement may be from 10 to 90%, and is more typically from 40 to 70%.

The fired cement typically has a modulus that is significantly lower than that of the honeycomb. The modulus of the fired cement may be, for example in the range of 3 to 25% of that of the ceramic material in the honeycomb. It is believed that this lower modulus imparts high crack resistance of the fired cement. The modulus of the fired cement can be measured by forming 8 mm X 4 mm X 40 mm test bars from the cement composition, firing the blocks and measuring modulus using the Grindosonic impulse excitation apparatus following ASTM Standard C 1259-98, Standard Test Method for Dynamic Young's Modulus, Shear Modulus, and Poisson's Ratio for Advanced Ceramics by Impulse Excitation of Vibration.

Because the fired cement tends to resist cracking, a honeycomb made in accordance with the invention tends to exhibit excellent thermal shock resistance, whether the cement is used as a skin, to adhere constituent parts of the honeycomb together to form an assembly, or both. A suitable method of evaluating thermal shock resistance is described in the following examples. In this method, the structure is subjected to increasingly harsh thermal cycles, and inspected for cracking after each of the cycles.

A honeycomb produced in accordance with the invention can be used as a particulate filter, especially for removing particulate matter from power plant (mobile or stationary) exhaust gases. A specific application of this type is a soot filter for an internal combustion engine, especially a diesel engine.

Functional materials can be applied to the honeycomb, before or after applying and firing the cement composition, using various methods. The functional materials may be organic or inorganic. Inorganic functional materials, particularly metals and metal oxides, are of interest as many of these have desirable catalytic properties, function as sorbents or perform some other needed function. One method of introducing a metal or metal oxide onto the composite body is by impregnating the honeycomb with a solution of a salt or acid of the metal, and then heating or otherwise removing the solvent and, if necessary calcining or otherwise decomposing the salt or acid to form the desired metal or metal oxide.

Thus, for example, an alumina coating or a coating of another metal oxide is often applied in order to provide a higher surface area upon which a catalytic or sorbent material can be deposited. Alumina can be deposited by impregnating the honeycomb with colloidal alumina, followed by drying, typically by passing a gas through the impregnated body. This procedure can be repeated as necessary to deposit a desired amount of alumina. Other ceramic coatings such as titania can be applied in an analogous manner.

Metals such as barium, platinum, palladium, silver, gold and the like can be deposited on the composite body by impregnating the honeycomb (the internal walls of which are preferably coated with alumina or other metal oxide) with a soluble salt of the metal, such as, for example, platinum nitrate, gold chloride, rhodium nitrate, tetraamine palladium nitrate, barium formate, followed by drying and preferably calcination. Catalytic converters for power plant exhaust streams, especially for vehicles, can be prepared from the skinned honeycomb in that manner.

Suitable methods for depositing various inorganic materials onto a honeycomb structure are described, for example, in US 205/0113249 and WO2001045828. These processes are generally in relation to the skinned honeycomb of this invention.

In an especially preferred embodiment, alumina and platinum, alumina and barium or alumina, barium and platinum can be deposited onto the honeycomb in one or more steps to from a filter that is simultaneously capable of removing particulates such as soot, NOx compounds, carbon monoxide and hydrocarbons from a power plant exhaust, such as from vehicle engines.

The following examples are provided to illustrate the invention, but are not intended to limit the scope thereof. All parts and percentages are by weight unless otherwise indicated.

Example 1

A cement composition is prepared by mixing 42.0 wt% of ball milled aluminum silicate fiber (HP-95-SAB-T60, available from Thermal Ceramics Inc., Augusta, GA), 13.5 wt% of colloidal alumina (AL20SD, available from Nyacol Nano Technologies, Inc., Ashland, MA), 40.5 wt% of water, 2 wt% methyl cellulose (METHOCEL A15LV, available from The Dow Chemical Co. Midland, MI), and 2 wt% polyethylene glycol 400 (available from Alfa Aesar, Ward Hill, MA) to obtain a uniform mixture.

A portion of the cement composition is applied onto as-mullitized acicular mullite honeycomb segments, which are joined together using the mixture as a cement to form a larger honeycomb assembly. The as-mullitized acicular mullite honeycombs contain 1- 1.4% by weight residual fluorine.

Another portion of the cement composition is applied onto the periphery an as- mullitized acicular mullite honeycomb to form a skin coating.

A third portion of the cement composition is formed into blocks for material property measurements.

The honeycomb assembly, coated honeycombs and cement blocks are fired together by heating them tol400°C at a rate of 2°C/minute, holding at 1400⁰C for 6 hours and then cooling slowly to room temperature. Residual fluorine is removed from the acicular mullite honeycombs during this firing step, and a binding phase is simultaneously formed. The resulting materials are referred to collectively as Example 1.

A fourth portion of the cement composition is applied to heat-treated acicular mullite honeycombs that contain less than 0.5% by weight residual fluorine. The acicular mullite honeycombs are then joined together to form a larger honeycomb assembly. A fifth portion of the cement composition is applied as a skin over a heat- treated acicular mullite honeycomb. A sixth portion of the cement composition is formed into blocks. The honeycomb assembly, skinned honeycomb and blocks are fired together in the same manner as Example 1. The fired materials are referred to collectively as Comparative Sample A.

X-ray diffraction (XRD) on the cement and skin of Example 1 shows that they contain 69.7% mullite, 16.4% cristobalite and 13.9% aluminum oxide. The cement and skin of Comparative Sample A contain only 47.4% mullite, wheres the cristobalite and aluminum oxide phases are larger (being 26.0% and 26.6%, respectively). Therefore, firing in the presence of an acicular mullite having residual fluorine increases mullite formation by about 47%.

The higher mullite content of the fired cement and skin results in a closer chemical composition match of with the acicular mullite honeycomb, and in a closer match of CTE, as shown in Figure 1. In Figure 1, the CTE of Example 1 over the temperature range from about 25°C to 800⁰C is indicated by line 1, whereas that of Comparative Sample A is indicated by line A. The thermal expansion of cement and skin fired in presence of an acicular mullite honeycomb containing residual fluorine is closer to that of mullite substrate, compared to when the acicular mullite honeycomb has little residual fluorine, as in Comparative Sample A. The closer thermal expansion match with mullite substrate is due to the increase of mullite phase in the cement and skin fired in presence of fluorine and can result in improved thermal shock performance.

The porosity of the cement and the skin of Example 1 are measured by water intrusion methods and found to be 64%, which is nearly identical to that of Comparative Sample A. Therefore, the co-firing of the cement and skin with the removal of residual fluorine from the acicular mullite honeycomb does not result in changes in the microstructure or porosity of the cement and the skin. The elastic modulus of the fired cement composition is measured by Grindosonic method. Cement bars with 8 mm x 4 mm x 40 mm dimensions are cut from fired cement blocks. Cement bars fired in presence of fluorine (Example 1) has an elastic modulus of 4.9 GPa, whereas the elastic modulus of the Comparative Sample A cement bars is nearly the same (4.7 GPa). The acicular mullite in the honeycombs has an elastic modulus of 23.6 GPa. The fired cement composition is more compliant than the underlying honeycomb and therefore helps relieve the thermomechanical stresses generated in thermal shock situation.

Example 2 and Comparative Sample B

Example 1 and Comparative Sample A are repeated, this time using a cement composition prepared by mixing 48.8 wt% of ball milled aluminum silicate fiber (PS3400 fiber, available from Unifrax LLC, Niagara Falls, NY), 11.9 wt% of colloidal alumina (AL20SD, available from Nyacol Nano Technologies, Inc, Ashland, MA), 35.9 wt% of water, 1.7 wt% methyl cellulose (METHOCEL A15LV, available from The Dow Chemical Co., Midland, MI), and 1.7 wt% polyethylene glycol 400 (available from Alfa Aesar, Ward Hill, MA). For Example 2, this cement composition is used to join and skin acicular mullite honeycombs that contain 1-1.4% residual fluorine, and to form cement blocks. For Comparative Sample B, the honeycombs are previously heat-treated to reduce residual fluorine to below 0.1%. The materials are then fired as described with respect to Example 1 and Comparative Sample A, to form Example 2 and Comparative Sample B, respectively.

The cement and skin of Example 2 contain 76.7% mullite, 8.1% cristobalite and 15.3% aluminum oxide by XRD, compared to only 69.2% mullite, 4.9% cristobalite and 25.9% aluminum oxide for Comparative Sample B. This indicates that mullite content in the cement and skin increase by 10.8% when the acicular mullite honeycomb contains residual fluorine when the cement composition is fired. The thermal expansion of cement and skin for both Example 2 and Comparative Sample B are shown in Figure 2. In Figure 1, the CTE of Example 2 over the temperature range from about 25°C to 800⁰C is indicated by line 2, whereas that of Comparative Sample B is indicated by line B. The thermal expansion of the Example 2 cement and skin is closer to that of the acicular mullite honeycomb substrate than are the cement and skin of Comparative Sample B. The closer thermal expansion match with mullite substrate is due to the increased mullite content of Example 2 and can result in improved thermal shock performance. Example 3 and Comparative Sample C

Example 1 and Comparative Sample A are again repeated, this time using a cement composition prepared by mixing 27.5 wt% of ball milled aluminum zirconium silicate fiber (Z-95-SAB-T30, available from Thermal Ceramics Inc., Augusta, GA), 16.9 wt% of colloidal alumina (AL20SD, available from Nyacol Nano Technologies, Inc., Ashland, MA), 50.6 wt% of water, 2.5 wt% methyl cellulose (METHOCEL A15LV, available from The Dow Chemical Co. Midland, MI), and 2.5 wt% polyethylene glycol 400 (available from Alfa Aesar, Ward Hill, MA).

For Example 3, this cement composition is used to join and skin acicular mullite honeycombs that contain 1-1.4% residual fluorine, and to form cement blocks. For Comparative Sample C, the honeycombs are previously heat-treated to reduce residual fluorine to below 0.5%. The materials are then fired as described with respect to Example 1 and Comparative Sample A to form Example 3 and Comparative Sample C, respectively.

The cement and skin of Example 3 contain 53.0% mullite, 13.7% crystobalite, 24.6% aluminum oxide and 8.6% zirconium oxide by XRD, compared to only 42.0% mullite, 18.0% cristobalite, 32.1% aluminum oxide and 7.9% zirconium oxide for Comparative Sample C. This indicates that mullite content in the cement and skin increase by 26.2% when the acicular mullite honeycomb contains residual fluorine when the cement composition is fired, even when the fibers in the cement composition are doped with zirconium. Example 3 cement and skin has an elastic modulus of 3.3 GPa,which is nearly unchanged from that of Comparative Sample C (3.6 GPa). These values are much lower that that of the underlying acicular mullite honeycomb, indicating greater compliance and ability to relieve thermomechanical stresses.

Claims

WHAT IS CLAIMED IS:

1. A process comprising the steps of (a) forming a ceramic honeycomb containing multiple cells defined by intersecting walls, which cells extend axially through the ceramic honeycomb, (b) applying to at least one surface of the ceramic honeycomb a cement composition, that contains both aluminum and silicon atoms and includes (1) at least one inorganic filler, (2) a colloidal silica, colloidal alumina or mixture thereof which forms a binding phase upon being fired, and (3) a carrier fluid and then (c) firing the honeycomb and cement composition at a temperature of at least 1000⁰C in the presence of a fluorine source.

2. The process of claim 1, wherein the honeycomb and cement composition are fired at a temperature of at least 1200⁰C.

3. The process of claim 2, wherein the honeycomb and cement composition are fired at a temperature of at least 1400⁰C.

4. The process of any of claims 1-3, wherein the fluorine source is SiF4.

5. The process of any of claims 1-4, wherein the filler includes at least one aluminate, silicate or aluminosilicate material.

6. The process of any of claims 1-5, wherein filler includes at least one fiber.

7. The process of claim 6, wherein the filler includes at least one fiber and at least one low aspect ratio particulate.

8. The process of any preceding claim, wherein the cement composition forms a skin on the periphery of the ceramic honeycomb.

9. The process of any of claims 1-7, wherein the cement composition cements a segment of the ceramic honeycomb to another segment of the honeycomb or to another structure.

10. A process comprising the steps of (a) forming a ceramic honeycomb containing multiple cells defined by intersecting walls, which cells extend axially through the ceramic honeycomb, wherein at least a portion of the ceramic honeycomb is an acicular mullite that contains at least 0.5 weight percent residual fluorine, based on the weight of the acicular mullite in the honeycomb (b) applying to at least one surface of the ceramic honeycomb a cement composition, that contains both aluminum and silicon atoms and includes (1) at least one inorganic filler, (2) a colloidal silica, colloidal alumina or mixture thereof which forms a binding phase upon being fired, and (3) a carrier fluid and then (c) exposing the honeycomb and cement composition to a temperature of at least 1200⁰C.

11. The process of claim 10, wherein the honeycomb and cement composition are fired at a temperature of at least 1400⁰C.

12. The process of any of claims 10-11, wherein the filler includes at least one aluminate, silicate or aluminosilicate material.

13. The process of any of claims 10-13, wherein filler includes at least one fiber.

14. The process of claim 13, wherein the filler includes at least one fiber and at least one low aspect ratio particulate.

15. The process of any of claims 10-14, wherein the cement composition forms a skin on the periphery of the ceramic honeycomb.

16. The process of any of claims 10-15, wherein the cement composition cements a segment of the ceramic honeycomb to another segment of the honeycomb or to another structure.