WO1996016000A1

WO1996016000A1 - High temperature, thermal shock resistant ceramic structures

Info

Publication number: WO1996016000A1
Application number: PCT/US1995/015035
Authority: WO
Inventors: Jeffrey Allen Chambers; Kenneth Scott Hatton; Dennis James Landini
Original assignee: DuPont Lanxide Composites LP
Current assignee: Power Systems Composites LLC
Priority date: 1994-11-18
Filing date: 1995-11-16
Publication date: 1996-05-30
Anticipated expiration: 1997-05-18

Abstract

Improved ceramic structures with resistance to high temperatures and thermal shock are prepared by coating and/or infiltrating previously known high thermal shock resistant ceramic structures with certain ceramic compositions through a variety of processes. Such processes include, but are not limited to (1) chemical vapor infiltration, (2) pyrolysis of ceramic precursor polymer coatings, (3) vacuum infiltration of ceramic slurries, and (4) formation of a polycrystalline oxidation reaction product. Desirable properties not found in the uncoated high thermal shock resistant ceramic structures, such as corrosion resistance in certain hostile environments, microwave and inductive susceptibility, and reduced permeability in porous structures can be obtained. The improved materials are useful, for example, in industrial microwave heating and heat exchangers.

Description

HIGH TEMPERATURE, THERMAL SHOCK RESISTANT CERAMIC STRUCTURES

FIELD OF THE INVENTION

This invention relates to ceramic substrates which are coated and/or infiltrated with a second material (e.g., a ceramic and/or a preceramic polymer) such bodies having desirable high temperature performance, superior thermal shock resistance, improved corrosion resistance in hostile environments, microwave and inductive susceptibility, and reduced permeability, and methods for making the same.

More specifically, the invention relates to novel ceramic honeycomb materials which may be useful in certain industrial microwave applications, and to novel heat exchanger pans (e.g. , tubes) which may be used in a broad range of applications.

BACKGROUND OF THE INVENTION

Considerable interest has been generated by certain traditional ceramic materials because of their high temperature performance and thermal shock resistance in various industrial processes. Other advantageous properties of certain ceramics may include low thermal conductivity, low thermal expansion, and desirable resistance to creep and oxidation. Accordingly, certain ceramic materials have been found to be useful for such applications as catalyst supports and hot gas filters.

One recently developed method for making thermal shock resistant ceramic honeycomb structures was described by Forsythe (U.S. Patent No. 5,079,064) wherein glass fibers are used to create a microcracked honeycomb structure comprising alumina, mullite, and cordierite, which structure withstands high temperatures and severe thermal shocks. The subject matter of U.S. Patent No. 5,079,064 is hereby incorporated by reference. Another suitable thermal shock resistant ceramic material may be formed in a manner similar to the method disclosed in U.S. Patent No. 5,079,064; however, rather than forming a honeycomb structure, a tubular shape may be formed. The method for forming these materials is set forth in U.S. Patent Application No. 08/221,139, filed on March 31 , 1994, in the names of Connolly et. al, and entitled Ceramic Hot-Gas Filter And Process Therefor (the subject matter of which is hereby incorporated by reference). Summarizing the disclosure of this U.S. Patent Application, a glass fiber as disclosed in U.S. Patent No. 5,079,064 is obtained. Next, an alumina slurry may be prepared, for example, by charging about 7.0 liters of water and about 20.0 ml of formic acid into a mixing vessel. About 2.0 kg of fumed alumina having an average particle size of about 13-15 nm (manufactured and sold by Degussa) may be added slowly while stirring the water/formic acid. The ph of the slurry is adjusted to about 4.0 to 4.1 using formic acid. After stabilizing at this ph for about two hours, abou' 11.0 kg of A-17 alumina (average particle size 2-3 microns, manufactured and sold by Alcoa) is added to the slurry in portions and sthred overnight. Glycerol may then be added to the slurry at a level of 3.0 wt% based on the total weight of the slurry. The solids content of the dispersion is about 62-65 weight percent and the viscosity is adjusted to about 140 centpoise by adding water.

The glass fiber (for example, a 2-ply glass yarn known as S glass, designation S-2 CGI 50 1/2 636, available from Owens-Corning Fiberglass Corporation) is fed through a ball tensioner, passed through the alumina slurry, and pulled out through a 0.017 inch (0.043 cm) die to remove excess slurry. The die controls the amount of slurry applied to the yarn so that after drying, about 50-60 percent by weight is derived from the yarn. The wet yarn is then passed through a guide attached to the traverse arm of a filament winding machine and wound onto a mandrel in the shape of the hollow portion of the desired tube shape. The winding may be continued to obtain a tube having the desired outer diameter. The tube is then dried overnight at room temperature and then removed from the mandrel. Moreover, particularly preferred winding patterns are disclosed in U.S. Patent No. 5,192,597, to Forsythe, the subject matter of which is herein incorporated by reference.

Optionally, a second layer of fiber may be applied by hoop winding glass yarn onto the tube and then allowing the tube to dry overnight. In either case, the dried tube is fired in an air atmosphere at about 700 °C for about one hour. Next, the temperature of the furnace is raised to about 800°C in about 40 minutes, held for about 20 minutes, then increased to about 1300°C at a rate of about 2°C/minute. held for about 2 hours, then heated at a rate of about 1 degree C/minute to about 1380°C, held for about two hours and then cooled to about 800°C at a rate of about 5°C/minute, followed by unrestrained cooling of the furnace to about 200°C. The tube may then be removed from the furnace. The formed hot-gas filters comprise crystalline phases of corundum, mullite, cordierite and cristobalite. Moreover, the filters also comprise from about 40-70 percent substantially interconnected porosity. Such structures have been found useful as, for example, filters and more particularly as hot-gas filters. Another thermal shock resistant ceramic material is disclosed in, for example, U.S.

Patent No. 4,869,944. The subject matter of the patent is hereby incorporated by reference. This Patent discloses extruded, porous ceramic honeycomb shapes comprising cordierite, such cordierite bodies being microcracked to resist thermal shock.

The ceramic materials discussed above, as well as certain similar ceramic materials. are able to combine many of the desirable properties discussed above, but lack corrosion stability in strongly acidic or corrosive industrial environments (e.g., gaseous hydrogen fluoride, hydrogen chloride, or chlorine). Moreover, such ceramics may be only marginally susceptible to heating by microwave or inductive methods. Finally, because of their microcracked structures, the reramics are generally too porous for use in applications requiring low permeability. For example, such bodies may have a porosity of about 40-70 percent and sometimes higher, wherein the porosity is substantially interconnected.

SUMMARY OF THE INVENTION

This invention relates broadly to ceramic honeycomb materials useful in industrial microwave applications and to ceramic heat exchanger rubes. The bodies are formed by infiltrating some or all of the porosity of an existing ceramic body with another material (e.g., ceramic material or precursors thereof) or by coating some or all of the surfaces of an existing ceramic body with another material (e.g., ceramic material or precursors thereof). The end use for the material will determine whether some or all of the porosity of the body will be infiltrated, and/or whether some or all of the surfaces of the body will be coated.

For example, one preferred embodiment of this invention provides a novel ceramic honeycomb material useful for industrial microwave applications, and methods for making the same. In such an embodiment it may not be necessary to infiltrate any substantial amount of internal porosity in the honeycomb body. Specifically, it has been found that to increase the microwave susceptibility of a ceramic honeycomb structure it is only necessary to provide a poπion of an outer surface of the honeycomb structure with a ceramic coating. Therefore, it is possible to apply the ceramic coating material (or precursor thereof) by, for example, painting, dipping, spraying, etc., the coating onto one or more surfaces of the honeycomb structure. However, if desirable for certain applications, it is also possible to infiltrate the porosity of the honeycomb structure (which porosity may range from about 40- 70 percent and sometimes more) with the ceramic (or precursor thereof) and not affect adversely the performance of the end material.

As an example of possible uses for ceramic honeycomb structures according to this invention, a honeycomb structure could be placed downstream of a gas incinerator to remove undesirable pollutants from the gas stream. Specifically, the honeycomb may be placed into a microwave field and heated to a specified operating temperature. When the gas stream is passed through the heated honeycomb, the pollutants may be destroyed by the heat generated by the microwave heated honeycomb. Ordinary ceramic honeycombs are, typically, not microwave susceptible. Therefore, one skilled in the an will recognize the significant achievement realized by the honeycomb embodiment of the present invention. Specifically, the invention provides a ceramic honeycomb which is microwave (and inductive) susceptible. Moreover, th honeycomb body is relatively light weight and simple to manufacture. In a second embodiment of this invention it may be important to infiltrate at least a ponion of, or substantially all of, the porosity of a ceramic body. Such infiltration could occur by using another ceramic (or precursor thereof) material.

Specifically, a second embodiment of this invention relates to novel heat exchanger tubes. To operate effectively, a heat exchanger tube should be substantially free of interconnected porosity. Thus, in the present invention, merely coating a porous ceramic tube with one or more other ceramic materials (or precursors thereof) may not be sufficient (e.g., some interconnected porosity may remain accessible from the surfaces of the tube). In this regard it is more preferable to infiltrate the porosity of the tube by, for example, immersing the tube into, for example, a ceramic slurry or a liquid preceramic polymer material. Infiltration may be assisted by applying pressure or vacuum to the slurry or to the liquid preceramic polymer.

The heat exchanger tubes made according to the present invention may be used in any number of industrial heat exchanger applications (e.g., the co-generation of electricity, etc.) that those skilled in the an are familiar with. Preferred applications include such applications where the pressure that the heat exchanger rube is exposed to does not exceed about 10 atmospheres of pressure, gauge. More preferably, the pressures do not exceed about 2 atmospheres of pressure, gauge.

High temperature resistant, thermal shock resistant ceramic materials such as those discussed above have been improved by, as discussed above, either coating and/or infiltrating the materials (depending on their end use) with one or more other ceramic materials (or precursors thereof) which, for example, provide for corrosion resistance in hostile environments, increased microwave or inductive susceptibility, and reduced permeability. It is believed that thermal shock resistance in the unprocessed materials is achieved by toughening mechanisms such as microcracking. It was previously believed that if such microcracks were filled-in or sealed by one or more coatings, that the toughening mechanism would become inoperative. Thus, the expectation was that such treatments would result in poor thermal shock resistance in the processed ceramic materials. However, it has been unexpectedly discovered that when certain thermal shock resistant ceramic materials are coated and/or infiltrated by the methods described herein, the thermal shock resistance of the body remains surprisingly high.

The ceramic materials of the present invention can be prepared by several techniques. For example, when it is desirable to infiltrate at least a ponion of the porosity of a ceramic body, the ceramic body can be infiltrated by several different techniques, alone or in combination. A first technique involves the thermal decomposition of reactive gases to deposit ceramic material on at least a portion of the internal and external surfaces of the structure. This process is commonly referred to as chemical vapor infiltration (CVI). Moreover, it is possible to infiltrate at least a ponion of the porosity of ceramic bodies with materials known as preceramic polymers. Preferred preceramic polymers include, but are not limited to polysilazanes, polycarbosilanes, and polycarbosilazanes. These preceramic polymers can be pyrolyzed under controlled atmospheres to form desired ceramic material(s) (e.g., SiC, Si3N4, etc.). Such infiltration may be carried out by, for example, immersing the porous ceramic body into a liquid preceramic polymer bath for a sufficient amount of time to permit the liquid preceramic polymer to infiltrate into at least a ponion of. or substantially all of the porosity in the body. Moreover, the infiltration may be assisted by applying pressure and/or a vacuum to liquid preceramic polymer and ceramic combinations. Further, preceramic polymers can be desirably loaded or filled with a variety of ceramic materials, including but not limited to SiC, graphite, and/or S13N4.

Moreover, the porosity of the ceramic bodies discussed above may be at least paπially, or substantially completely, infiltrated with a ceramic slurry (e.g. , an aluminum oxide slurry, a silicon carbide slurry, etc.). Similar infiltration and/or coating techniques used with the liquid preceramic polymer may be used with the ceramic slurry embodiment of the present invention. _#

Also, the porosity of the ceramic bodies discussed above may be at least paπially, or substantially completely, infiltrated with a polycrystalline oxidation reaction product formed by reacting a molten parent metal with a suitable oxidant and "growing" the polycrystalline oxidation reaction product into the porosity of the ceramic body.

Finally, any combination of the above procedures may be used to at least paπially, or substantially completely, infiltrate the porosity of the ceramic bodies.

In any case, through control of the coating and/or infiltrating methods, it is possible to add about 1 percent to about 100 percent or more, by weight, to the weight of the original ceramic structure and to reduce the permeability of the body and, moreover, to increase its microwave and inductive susceptibility. Further, with regard to the infiltration methods set forth in this invention, it is possible to reduce the porosity of a ceramic body from an initial porosity of about 40-70 percent to as low a porosity as about 15-30 percent. Additionally, after processing, the remaining porosity is no longer substantially interconnected.

DETAILED DESCRIPTION OF THE INVENTION

Examples of preferred high temperature, thermal shock resistant ceramic materials useful as a staπing material for this invention comprise, generally, gradient structures of alumina, mullite, and cordierite. These ceramic materials also comprise anywhere from about 40 to about 70 volume percent (and sometimes more), substantially interconnected porosity. Certain of these preferred bodies are available from DuPont Lanxide Composites Inc, and are known in the industry as PRD-66 honeycombs and cylindrical products. The PRD-66 honeycombs are disclosed in U.S. Patent No. 5,079,064, and have a chemical composition of about 20-40 weight percent Siθ2, about 3-6 weight percent MgO, and about 54-77 weight percent AI2O3. Moreover, the crystalline content of the honeycomb body is about 15-40 weight percent cordierite, about 15-35 weight percent corundum and about 10- 30 weight percent mullite. Furthermore, as stated above, the porosity in the body is substantially interconnected. Cylindrical bodies comprising PRD-66 are disclosed, generally, in U.S. Patent Application No. 08/221,139, discussed above. Moreover, certain other high temperature, high thermal shock resistant ceramic components having substantially interconnected porosity may also be used. A particularly attractive ceramic honeycomb material comprises an extruded cordierite honeycomb product. Such products are available from a number of companies, including, Corning, Inc.

As stated above, a number of techniques (or combinations thereof) may be used to infiltrate or coat ceramic bodies to form desirable ceramic honeycomb bodies and ceramic heat exchanger tubes in accordance with the present invention.

For example, when chemical vapor infiltration is utilized, a ceramic honeycomb or heat exchanger rube may be fabricated by placing a shaped ceramic honeycomb or tube in a chemical vapor infiltration furnace and operating under appropriate conditions required to deposit the ceramic material. The ceramic material is preferably selected from the group of materials consisting of carbides, nitrides, oxides, and carbon. More preferably, the ceramic material may be selected from the group of materials of SiC, HfC, S.3N4, BN, AI2O3, ZΓOT , Ta2θ5, T1O2, HfB2- and pyrolitic carbon, and most preferably, may comprise SiC. It should be understood by an artisan of ordinary skill that the preferred ceramic materials may be the pure compositions represented by the chemical formulas above or may be modified to include acceptable or desirable quantities of other elements or compounds.

Any number of known CVI techniques can be used to deposit one or more ceramic materials onto an acceptable substrate material. An isothermal CVI process is preferred, but a forced flow process could also be used. In one preferred embodiment of this invention, wherein the deposited/infiltrated material comprises silicon carbide, CVI can be carried out by passing appropriate precursor gases, such as methyltrichloro-silane (MTS) or dimethyl- dichloro-silane (DDS) or a combination of a silane and a hydrocarbon gas (e.g. methane, propane, propylene, etc.) over the ceramic structure. A particularly suitable CVI process and equipment for depositing silicon carbide is described in U.S. Patent No. 4,823,734, the subject maner of which is hereby incorporated by reference. The infiltration/deposition process is continued until the desired weight gain is achieved and/or until the desired amount of porosity is filled. A typical weight gain of about 80 percent may be desirable for a body comprising a PRD-66 material which is infiltrated with silicon carbide. In a second embodiment, a preceramic polymer, and more preferably, CERASET™ SN ceramer, an available material from Lanxide Corporation, is utilⁱzed to infiltrate at least a portion of the porosity of a ceramic body. The preceramic polymer may or may not be extended by the addition of ceramic powders. By proper selection of pyrolysis conditions, the same preceramic polymer can be converted to essentially pure SiC, Si3N4, silicon carbonitrides, silicon oxynitrides, etc. The preferred CERASET™ SN ceramers are disclosed in, for example, U.S. Patent Nos. 4,929,704; 5,001 ,090; 5,021,533; and 5,206,327, the subject matter of these U.S. Patents is hereby incorporated by reference. Generally, these Patents disclose methods for making certain preceramic polymers and preceramic polymers which incorporate therein at least one filler material, including, for example, ceramic paniculate filler materials.

As stated above, depending on the ultimate commercial use of the material (i.e., if it is necessary to simply coat the body or to infiltrate a substantial ponion of the porosity of the body) the preceramic polymer material may be applied to the high thermal shock resistant material in a number of different means, alone or in combination, such means including, but not limited to, dipping, brushing, painting, immersing and immersing in a vacuum or pressure.

For example, in the case where a honeycomb ceramic structure is to be coated with a preceramic polymer only (i.e., not extended by the addition of ceramic powders) an appropriate amount of CERASET™ SN ceramer can be charged into a vessel of sufficient size to contain the honeycomb ceramic structure. A required amount of dicumyl peroxide (available from Hercules Inc.) is added and stirred thoroughly until dissolved. The honeycomb ceramic structure to be coated is immersed in the CERASET™ SN ceramer/dicumyl peroxide mixture in such a manner as to wet substantially all its surfaces. The honeycomb structure can be allowed to soak for an appropriate amount of time (e.g., 5 minutes to 120 minutes) to assure complete wetting (although infiltration of the porosity of the body is not critical). The coated pan is then allowed to drain so that all channels and passageways are coated, but not filled, with the mixture. The drained structure can then be placed into an air atmosphere furnace heated to, for example, about 150-200°C and cured in the furnace for approximately 15-60 minutes, or until the coated body reaches thermal equilibrium with its surroundings (longer cure times may be necessary for larger parts). The cured, coated structure can then be pyrolyzed to the desired ceramic coating. Pyrolysis may be carried out according to the conditions described by, for example, Matsumoto, Mat. Res. Soc. Symp. Proc., (1990) Vol. 180, pp. 797-800, and in U.S. Patent No. 5,001,090. The subject matter of both of these documents is hereby incorporated by reference.

When the honeycomb ceramic structure is coated with CERASET™ SN ceramer which is extended by the addition of ceramic powders, an appropriate amount of CERASET™ SN ceramer may be charged into a vessel large enough to contain the honeycomb ceramic structure. Dicumyl peroxide (available from Hercules Inc.) is then added and stirred thoroughly with the ceramer until dissolved. Approximately one kilogram of a mixture of ceramic powders of various grit sizes, including but not limited to 500 grit, 320 grit, and submicron paniculate, in a nominally 20-40-40 proportion (by weight) is added and mixed thoroughly. As a dispersing agent, an appropriate amount of Mazol may be added to assist mixing. The honeycomb ceramic structure to be coated is immersed in the CERASET™ SN ceramer mixture in such a manner as to wet all its surfaces. The honeycomb can be allowed to soak for about 15-20 minutes to assure complete wetting. The coated part is then allowed to drain so that all channels and passageways are coated but not filled with the polymer. The drained structure is placed in an air atmosphere furnace at about 150-200°C and cured for approximately 30-60 minutes, or until thermal equilibrium is achieved (longer cure times may be necessary for larger parts). The cured, ceramer-coated structure can then be pyrolyzed to the desired ceramic coating. As a further preferred embodiment, a cylindrical ceramic structure may be coated with CERASET™ SN ceramer only (i.e. not extended by the addition of ceramic powders). CERASET™ SN ceramer is charged into a vessel large enough to contain the ceramic structure. Dicumyl peroxide (available from Hercules Inc.) is added and stirred thoroughly with the ceramer until completely dissolved. The ceramic structure to be coated is immersed in the CERASET™ SN ceramer mixture in such a manner as to wet all its surfaces. The ceramic structure is allowed to soak for a sufficient amount of time to permit infiltration to occur. Alternatively, a modest vacuum may be pulled to assist in causing the preceramic polymer to infiltrate the porosity of the body. The infiltrated part is then allowed to drain. If low permeability is desired, the part should be drained only to the extent required for ease of handling. The drained structure is placed in an air atmosphere furnace and heated to about 150-200°C and cured for approximately 15-60 minutes, or until thermal equilibrium with its surroundings is reached (longer cure times may be necessary' for larger parts). The cured, coated structure can then be pyrolyzed to the desired ceramic coating. As still a further embodiment a cylindrical ceramic structure may be infiltrated with a preceramic polymer which is extended by the addition of ceramic powders. For example, CERASET™ SN ceramer may be charged into a vessel large enough to contain the ceramic structure. Dicumyl peroxide (available from Hercules Inc.) is added and stirred thoroughly until dissolved. A mixture of ceramic powders of various grit sizes, including but not limited to 500 grit, 320 grit, and submicron paniculate, in a nominally 20-40-40 proportion (by weight) is added and mixed thoroughly to form a slurry. As a dispersing agent. Mazol may be added to assist mixing. The ceramic structure is immersed in the mixture in such a manner as to allow the preceramic polymer to infiltrate the porosity of the body. Alternatively, a modest vacuum may be pulled to assist in causing the preceramic polymer to infiltrate the porosity of the body. Minimal draining is recommended to ease handling if reduced permeability is desired. The structure is placed in an air atmosphere furnace and heated to about 150-200°C and cured for approximately 15-60 minutes, or until thermal equilibrium with its surroundings is reached. The cured, infiltrated structure can then be pyrolyzed to produce a ceramic material in the porosity of the ceramic body.

As discussed above, another preferred embodiment comprises infiltrating or coating a high thermal shock resistant material with a ceramic slurry comprising a ceramic powder. These structures may be fabricated by, for example, forming a paniculate laden slurry by blending ceramic powders of various grit sizes. A preferred blend comprises 500 grit SiC, 320 grit SiC, and 1200 grit SiC in a nominally 40-20-40 proportion by weight mixed into an aqueous solution. Preferred weight ratios include, 2 parts powder to 3 parts of a solution comprising water and about 1 weight percent dispersing agent and about 1 weight percent suspending agent, to 3 parts solution to 4.5 parts of powder to 1 part water. A particularly preferred embodiment comprises nominally a ratio by weight of 1 part powder to 1 part aqueous solution. If it is desired to coat the ceramic body, the ceramic material may be coated by, for example, immersing the material into the slurry to wet substantially all of its surfaces. Other suitable coating techniques include, for example, spraying, painting, brushing, etc.

Moreover, if it is desired to infiltrate the porosity of the body, then in addition to immersing the body into the ceramic slurry, the sluπy and immersed body may be subjected to a moderate vacuum. A moderate vacuum removes air from any accessible porosity in the material and assists the penetration of the slurry into the material. The ceramic material is then removed from the slurry (when a slurry is utilized) and is dried at ambient conditions or, alternatively, can be force dried an air atmosphere furnace at about 75-135 °C in air for about 15-60 minutes. The ceramic material may be reinfiltrated and/or coated in the above fashion until a desired weight gain is achieved or until the desired amount of porosity is filled. Typically, the porous bodies discussed above herein require two or three sequential dippings or infiltrations to achieve a weight gain of about 50 to 100 percent.

The dried ceramic material is then fired in air. Preferred firing temperatures are from about 1000°C to about 1500°C, for a period of about 1/2 hour to about 30 hours. However, any suitable firing schedule may be used. As a further embodiment, the fired ceramic material may be subsequently coated with a surface layer of another ceramic material. Such subsequent coating materials include, but are not limited to, alumina, mullite, aluminum titanate, silica, etc. This subsequent coating may be applied in any suitable manner, including, but not limited to dipping, spraying, painting, brushing, etc. After applying the subsequent coating, the material may be fired in air as discussed above. The resulting material will have thereon a relatively dense, oxide rich surface layer. Also, the porosity of the ceramic bodies discussed above may be at least paπially, or substantially completely, infiltrated with a polycrystalline oxidation reaction product formed by reacting a molten parent metal with a suitable oxidant and "growing" the polycrystalline oxidation reaction product into the porosity of the ceramic body.

Several methods for forming a suitable oxidation reaction product are described in the below listed U.S. Patents, which are owned by Lanxide Technology Co., LP. The subject matter of the below listed patents is hereby incorporated by reference.

A method for producing ceramic composite bodies having a predetermined geometry or shape is disclosed in U.S. Patent No. 5,017,526 in the names of Marc S. Newkirk et al. In accordance with the method in this U.S. Patent, the developing oxidation reaction product infiltrates a permeable preform of filler material in a direction towards a defined surface boundary. It was discovered that high fidelity is more readily achieved by providing the preform with a barrier means, as disclosed in U.S. Patent No. 4,923,832, in the names of Marc S. Newkirk et al. This method produces shaped self-supporting ceramic bodies, including shaped ceramic composites, by growing the oxidation reaction product of a parent metal to a barrier means spaced from the metal for establishing a boundary or surface. A method for tailoring the constituency of the metallic component of a ceramic matrix composite structure is disclosed in U.S. Patent No. 5,017,533, in the names of Marc S. Newkirk et al. , and entitled "Method for In Situ Tailoring the Metallic Component of Ceramic Articles and Articles Made Thereby". Moreover, U.S. Patent No. 4,818,734, in the names of Robert C. Kantner et al. , discloses methods for tailoring the constituency of the metallic component (both isolated and interconnected) of ceramic and ceramic matrix composite bodies during formation thereof to impart one or more desirable characteristics to the resulting body. Thus, desired performance characteristics for the ceramic or ceramic composite body are advantageously achieved by incorporating the desired metallic component in situ, rather than from an extrinsic source, or by post-forming techniques.

As discussed in the above U.S. Patents, novel polycrystalline ceramic materials or polycrystalline ceramic composite materials are produced by the oxidation reaction between a parent metal and an oxidant (e.g., a solid, liquid and/or a gas). In accordance with the generic process disclosed in these Patents, a parent metal (e.g., aluminum) is heated to an elevated temperature above its melting point but below the melting point of the oxidation reaction product to form a body of molten parent metal which reacts upon contact with an oxidant to form the oxidation reaction product. At this temperature, the oxidation reaction product, or at least a ponion thereof, is in contact with and extends between the body of molten parent metal and the oxidant, and molten metal is drawn or transpoπed through the formed oxidation reaction product and towards the oxidant. The transpoπed molten metal forms additional fresh oxidation reaction product in contact with the oxidant, at the surface of previously formed oxidation reaction product. As the process continues, additional metal is transpoπed through this formation of polycrystalline oxidation reaction product thereby continually "growing" a ceramic structure of interconnected crystallites. The resulting ceramic body may contain metallic constituents, such as non-oxidized constituents of the parent metal, and/or voids. Oxidation is used in its broad sense in all of above-discussed

Patents and in this application, and refers to the loss or sharing of electrons by a metal to an oxidant which may be one or more elements and/or compounds. Accordingly, elements other than oxygen may serve as an oxidant.

In certain cases, the parent metal may require the presence of one or more dopants in order to influence favorably or to facilitate growth of the oxidation reaction product. Such dopants may at least partially alloy with the parent metal at some point during or prior to growth of the oxidation reaction product. For example, in the case of aluminum as the parent metal and air as the oxidant, dopants such as magnesium and silicon, to name but two of a larger class of dopant materials, can be alloyed with aluminum, and the created growth alloy is utilized as the parent metal. The resulting oxidation reaction product of such a growth alloy, in the case of using oxygen as an oxidant, comprises alumina, typically alpha- alumina.

Novel ceramic composite structures and methods of making the same are also disclosed and claimed in certain of the above-discussed Patents which utilize the oxidation reaction to produce ceramic composite structures comprising a substantially inert filler (note: in some cases it may be desirable to use a reactive filler, e.g., a filler which is at least partially reactive with the advancing oxidation reaction product and/or parent metal) infiltrated by the polycrystalline ceramic matrix. A parent metal is positioned adjacent to a mass of permeable filler (or a preform) which can be shaped and treated to be self- supporting, and is then heated to form a body of molten parent metal which is reacted with an oxidant, as described above, to form an oxidation reaction product. As the oxidation reaction product grows and infiltrates the adjacent filler material, molten parent metal is drawn through previously formed oxidation reaction product within the mass of filler and reacts with the oxidant to form additional fresh oxidation reaction product at the surface of the previously formed oxidation reaction product, as described above. The resulting growth of oxidation reaction product infiltrates or embeds the filler and results in the formation of a ceramic composite structure of a polycrystalline ceramic matrix embedding the filler. As also discussed above, the filler (or preform) may utilize a barrier means to establish a boundary or surface for the ceramic composite structure.

The methods disclosed in the above-discussed Patents could be used to at least partially, or substantially completely, infiltrate the porosity of the ceramic bodies discussed herein. For example, a body of parent metal could be placed adjacent to a porous ceramic honeycomb or heat exchanger tube and heated to a temperature above its melting point but below the melting point of its oxidation reaction product to form a body of molten parent metal which reacts with a suitable oxidant to form an oxidation reaction product which infiltrates at least partially, or substantially completely, the porosity of the ceramic honeycomb or heat exchanger tube.

Regardless of which infiltration technique is used (when infiltration is desirable), or combinations of infiltration techniques, the present invention provides a method for reducing the porosity of a porous high temperature, thermal shock resistant ceramic material from about 40-70 percent (and sometimes higher) to anywhere to about 15-30 percent. Moreover, the interconnected porosity is substantially reduced while the material maintains its desirable thermal shock resistance.

EXAMPLE 1

This example describes a specific method for forming a honeycomb ceramic material useful in microwave heating applications.

Specifically, a honeycomb structure measuring about 1 inch in diameter and about 1.5 inch thick was formed according to the teachings of U.S. Patent No. 5,079,064. The honeycomb structure was then infiltrated with SiC using the chemical vapor infiltration process described in U.S. Patent No. 4,752,503, using the apparatus described in U.S. Patent No. 4,823,734. After infiltration, a weight increase of about 80 percent was noted.

EXAMPLE 2

This example demonstrates a method of forming a heat exchanger tube according to the present invention. A porous cylindrical tube having an outside diameter of about 60 mm was formed substantially as described in Example 7. Prior to infiltrating the porosity of the tube, a length of tube measuring about 2 inches was cut from the formed tube.

The 2-inch length of tube was infiltrated with essentially pure beta-SiC using the method disclosed in U.S. Patent No. 4,752,503, utilizing the apparatus disclosed. EXAMPLE 3

This example demonstrates a method which may be used for forming a honeycomb structure useful for microwave heating applications. Specifically, a honeycomb structure measuring about 1 inch in diameter and about 1 1/2 inch thick was formed according to the teaching of Example 1 of U.S. Patent No. 5,079,064. The honeycomb was dipped in a preceramic polymer slurry. Specifically, the preceramic polymer slurry comprised about 50 volume percent 500 grit SiC powder (39 CRYSTOLON™ SiC, Norton, Co.) and 50 volume percent of a preceramic polymer, available from Lanxide Corporation and known as CERASET™ SN ceramer, plus 0.5 percent to 5 percent of a free radical generator, dicumyl peroxide. The honeycomb was dipped in such a manner as to increase the original weight of the honeycomb by about 25-40 percent. The preceramic polymer slurry coated honeycomb was then heated in air to about 170°C for about 30 minutes to cure the polymer. The cured, preceramic polymer/SiC coated honeycomb was then placed in a muffle furnace which was evacuated and backfilled with argon. This evacuation and refilling process was repeated three times until an essentially pure argon atmosphere was created in the furnace. The cured, preceramic polymer/SiC coated honeycomb was then heated in argon at a rate of about 10°C/minute until a temperature of about 1000°C was reached, then the temperature was held for about one hour to pyrolyze the preceramic polymer. The structure was then allowed to cool to room temperature by convection. It should be understood that this entire preceramic polymer infiltration, curing, and pyrolysis cycle may be repeated as needed to obtain virtually any desired coating thickness.

EXAMPLE 4

This example demonstrates a method for forming a ceramic honeycomb material useful for microwave heating applications according to the present invention.

Specifically, a ceramic honeycomb structure measuring about 1 inch in diameter and about 1 1/2 inch thick was formed according to the teaching of Example 1 of U.S. Patent No. 5,079,064. The honeycomb was dipped in a liquid preceramic polymer available from Lanxide Corporation and known as CERASET™ SN ceramer, plus 0.5 percent to 5 percent of a free radical generator, dicumyl peroxide. The honeycomb was dipped in such a manner as to increase the original weight of the honeycomb by about 25-40 percent. The preceramic polymer coated honeycomb was heated in air to about 170°C for about 30 minutes to cure the preceramic polymer. The cured, preceramic polymer coated honeycomb was then placed in a muffle furnace which was evacuated and backfilled with argon. This evacuation and backfilling process was repeated three times until an essentially pure argon atmosphere was present in the furnace. The cured, preceramic polymer coated honeycomb was heated in argon at a rate of about 10°C/minute until a temperature of about 1000°C was reached, then the temperature was held for about one hour to pyrolyze the preceramic polymer. The structure was then allowed to cool to room temperature by convection. It should be understood that this entire polymer infiltration, curing, and pyrolysis cycle may be repeated as needed to obtain virtually any desired coating thickness.

EXAMPLE 5

This example demonstrates a method for forming a ceramic honeycomb material useful in microwave heating applications according to the present invention.

Specifically, a ceramic honeycomb material measuring about 1 inch in diameter and about 1 1/2 inch thick was fabricated according to the teaching of Example 1 of U.S. Patent No. 5,079,064. The honeycomb was dipped in a liquid preceramic polymer available from Lanxide Corporation and known as CERASET™ SN ceramer, plus 0.5 percent to 5 percent of a free radical generator, dicumyl peroxide. The honeycomb was dipped in such a manner as to increase the original weight of the honeycomb by about 25-40 percent. The preceramic polymer coated honeycomb was heated in air to about 170°C for about 30 minutes to cure the preceramic polymer. The cured, preceramic polymer coated honeycomb was then placed in a muffle furnace which was evacuated and backfilled with ammonia gas. This evacuation and refilling process was repeated three times until an essentially pure ammonia atmosphere was present. The cured, preceramic polymer coated honeycomb was heated in ammonia at a rate of about 10°C/minute until a temperature of about 1000°C was reached, then the temperature was held for about one hour to pyrolyze the preceramic polymer. The structure was then allowed to cool to room temperature by convection. It should be understood that this entire preceramic polymer infiltration, curing, and pyrolysis cycle may be repeated as needed to obtain virtually any desired coating thickness.

EXAMPLE 6

This example demonstrates a further method for forming a ceramic honeycomb material useful in microwave heating applications according to the present invention.

Specifically, a ceramic honeycomb structure measuring about 1 inch in diameter and about 1 1/2 inch thick was fabricated according to the teaching of Example 1 of U.S. Patent

No. 5,079,064. The ceramic honeycomb was dipped in a preceramic polymer slurry. Specifically, the preceramic polymer slurry comprised about 50 volume percent 500 grit Si3N4 powder and 50 volume percent of a preceramic polymer, available from Lanxide Corporation and known as CERASET™ SN ceramer, plus 0.5 percent to 5 percent of a free radical generator, dicumyl peroxide. The honeycomb was dipped in such a manner as to increase the original weight of the honeycomb by about 25-40 percent. The preceramic polymer slurry coated honeycomb was heated in air to about 170^βC for about 30 minutes to cure the preceramic polymer. The cured, preceramic polymer/Si3N4 coated honeycomb was then placed in a muffle furnace which was evacuated and backfilled with ammonia. This evacuation and refilling process was repeated three times until an essentially pure ammonia atmosphere was present in the furnace. The cured, preceramic polymer/Si3N4 coated honeycomb was then heated in ammonia at a rate of about 10°C/minute until a temperature of about 1000°C was reached, then the temperature was held for about one hour to. The structure was then allowed to cool to room temperature by convection. It should be understood that this entire preceramic polymer infiltration, curing, and pyrolysis cycle may be repeated as needed to obtain virtually any desired coating thickness.

EXAMPLE 7

This example demonstrates a method which may be used for forming a heat exchanger tube according to the present invention.

Specifically, a ceramic cylindrical structure was formed as follows: The filament winder used to wind the tube in this Example had a chain-driven traverse of approximately 70 inches (178 cm) (278 teeth of 0.5 inch (1.27 cm) pitch passing in a narrow loop driven and supported by 11 tooth drive sprockets at each end). The drive ratio was set such that the spindle rotated at a speed of 50 and 10/111 revolutions for each complete rotation of the chain loop for winding of the tube. The mandrel was a tube having a length of 65 inches (165 cm) and an outer diameter of 1.75 inches (4.45 cm) with end closures at each end. One of the end closures was conical with about a 30 degree taper on each side of the cone with a 0.50 inch (1.27 cm) diameter drive shaft mounted at its axis. The second end closure was hemispherical (1.75 in (4.45 cm) diameter) with a 0.25 inch (0.64 cm) drive shaft mounted at its axis. The mandrel was attached to and driven by the spindle in such a position as to be traversed along its length by the traversing yarn guide.

The mandrel was attached to and driven by the spindle via the 0.50 inch (1.27 cm) shaft and supported in a bearing at the 0.25 inch (0.64 cm) shaft. It was mounted parallel to the chain-driven traverse guide such that the guide traversed above the mandrel surface at a distance of about 0.75 inch (1.91 cm) from the surface of the mandrel and the traverse stroke extended from about 0.75 inch (1.91 cm) past the hemispherical closure onto the 0.25 inch (0.64 cm) shaft and to about 0.75 inch (1.91 cm) past the conical closure onto the 0.5 inch (1.27 cm) shaft.

An alumina slurry was prepared by charging 7.0 liters of water and 20.0 ml of formic acid in a mixing vessel. Fumed alumina having an average particle size of 13-15 nm (manufactured and sold by Degussa) (2.0 kg) was added slowly with stirring. The pH of the dispersion was adjusted to 4.0 to 4.1 using formic acid. After stabilizing at this pH for two hours, 11.0 kg of A-17 alumina (average particle size 2-3 microns, manufactured and sold by Alcoa) was added in portions and stirred overnight. Glycerol was added to the slurry' at a level of 3 wt% based on the total weight of the slurry. The solids content of the dispersion was 62-65 weight percent and the viscosity was adjusted to 140 centipoise by water addition, measured with a Brookfield viscometer (Model No. RV1) using the #1 spindle.

A 2-pIy glass yarn (150 filaments/ply) comprising 65.2% Siθ2, 23.8% AI2O3, and 10.0% MgO having a hydrophilic sizing to aid wetting by the aqueous coating composition (S glass, designation S-2 CG150 1/2 636, available from Owens-Corning Fiberglass Corporation) was fed through a ball tensioner, passed through the alumina slurry, and pulled out through a 0.017 in (0.043 cm) die to remove excess slurry. The die controlled the amount of slurry applied to the yarn so that, after drying, about 50-60% by weight of ceramic in the tube was from the slurry and about 40-50% by weight was derived from the yarn. The wet yarn was then passed through a guide attached to the traverse arm of the filament winding machine and wound onto the above-described mandrel wrapped with 2 layers of 0.002 in (0.005 cm) "Mylar" polyester film. The yarns were laid down in adjacent layers of diamond-like patterns such that the yarns forming the walls of the diamonds of each layer substantially covered the diamond shaped openings of each adjacent layer. This winding pattern is described in Forsythe, U.S. Patent No. 5,192,597, which is incorporated herein by reference. A humidity level of at least about 30% was maintained during the winding procedure. The winding was stopped after about 1000 grams of yarn were wound onto the mandrel, when the tube reached the desired outside diameter (approximately 60 mm). After drying overnight at room temperature, the filament- wound tube was removed from the mandrel. It was then fired at about 700 °C for about one hour in a muffle furnace to remove volatiles and stabilize the structure. An about 2 inch length section of the tube was immersed into a preceramic polymer slurry. Specifically, the preceramic polymer slurry comprised about 50 volume percent 500 grit SiC powder (39 CRYSTOLON™ SiC, Norton) and about 50 volume percent of a preceramic polymer, available from Lanxide Corporation and known as CERASET™ SN ceramer, plus 0.5 percent to 5 percent of a free radical generator, dicumyl peroxide. A moderate vacuum of about 76 torr was pulled over the ceramic tube which was submerged in the preceramic polymer slurry to vacuum infiltrate the slurry into the tube. The vacuum was held for about 30 minutes. The infiltrated tube was removed from the slurry and then heated in air to about 170°C for about 30 minutes to cure the preceramic polymer. The cured, preceramic polymer/SiC infiltrated tube was then placed in a muffle furnace which was then evacuated and backfilled with argon. This evacuation and refilling process was repeated three times until an essentially pure argon atmosphere was present. The cured, preceramic polymer/SiC infiltrated tube was heated at a rate of about 10°C/minute in argon until a temperature of about 1000°C was reached, then the temperature was held for about one hour to pyrolyze the preceramic polymer. The furnace was then allowed to cool to room temperature. Subsequent preceramic polymer infiltration, curing and pyrolysis may be carried out to infiltrate further the porosity of the body.

EXAMPLE 8

This example demonstrates a method which may be used of making a heat exchanger tube according to the present invention.

Specifically, a ceramic tube was formed according to Example 7. An about 2 inch length section of the tube was immersed into a preceramic polymer slurry. The preceramic polymer slurry comprised about 50 volume percent 500 grit Si3N4 powder and 50 volume percent of a preceramic polymer, available from Lanxide Corporation and known as CERASET™ SN ceramer, plus 0.5 percent to 5 percent of a free radical generator, dicumyl peroxide. A moderate vacuum of about 76 torr was pulled over the tube which was submerged in the preceramic polymer slurry to vacuum infiltrate the slurry into the tube. The vacuum was held for about 30 minutes. The infiltrated tube was then heated in air to about 170°C for about 30 minutes to cure the preceramic polymer. The cured, preceramic polymer Si3N4 infiltrated tube was then placed in a muffle furnace which was evacuated and backfilled with ammonia gas. This evacuation and refilling process was repeated three times until an essentially pure ammonia atmosphere was present. The cured, preceramic polymer/Si3N4 infiltrated tube was then heated at a rate of about 10°C/minute in ammonia until a temperature of about 1000°C was reached, then the temperature was held for about one hour to pyrolyze the preceramic polymer. The structure was then allowed to cool to room temperature by convection. Subsequent preceramic polymer infiltration, curing and pyrolysis may be carried out to infiltrate further the porosity of the body.

EXAMPLE 9

This example demonstrates a method of forming a ceramic heat exchanger tube according to the present invention. Specifically, a ceramic tube in the shape of a heat exchanger was fabricated essentially in accordance with Example 7. The tube, measuring about 4 feet in length and having an outside diameter of about 2 inches, was then immersed into a paniculate laden slurry comprising about 50 weight percent SiC powder (comprising about 40 percent by weight 500 grit, 20 percent by weight 320 grit, and about 40 percent 1200 grit SiC (39 CRYSTOLON™ SiC, Norton, Co.)) and about 50 weight percent of an aqueous solution comprising water, about 1 percent by weight DARVAN™ 821 -A dispersing agent (R. T. Vanderbilt Co., Inc., Norwalk, CT) and about 1 percent by weight SUPERLOID suspending agent (KELCO, a division of Merck & Co., Inc., Clark, NJ). A vacuum of about 76 torr was pulled over the slurry for about 10 minutes. After about 10 minutes, the vacuum was released and the now infiltrated tube was removed from the slurry and placed in an air atmosphere oven at about 100°C for about 30 minutes. The tube was then removed from the oven, cooled for about 15 minutes and was then subjected to the above infiltration and heating procedure two more times. A weight gain of about 85 percent was noted.

The tube was then placed in an air atmosphere furnace. The furnace temperature was increased from about room temperature to about 1000°C at a rate of about 75 °C per hour. After about 4 hours the furnace was turned off and allowed to cool to room temperature.

EXAMPLE 10

This example demonstrates a further method of forming a ceramic heat exchanger tube according to the present invention.

A ceramic heat exchanger tube was fabricated essentially as described in Example 9. After removing the tube from the furnace, however, the following additional steps were carried out. The tube was lightly coated with an alumina containing slurry which comprised by weight about 30 percent 1000 grit alumina and about 70 percent isopropanol alcohol. The coating was applied using a commercially available spray gun. The coated tube was then placed into an air atmosphere furnace. The furnace temperature was increased from about room temperature to about 1250°C and held at about 1250°C for about 4 hours. After about 4 hours, the furnace was turned off and cooled to room temperature and the tube was removed from the furnace. It was noted that a dense alumina surface layer had formed on the tube.

Claims

WHAT IS CLAIMED IS:

1. A method for making a ceramic heat exchanger tube comprising: providing a ceramic tube having at least some porosity therein; infiltrating said porosity with at least one of the following, at least one ceramic contaimng slurry, at least one ceramic precursor material, and at least one oxidation reaction product.

2. The method of claim 1 , wherein said porosity is infiltrated with at least one of a ceramic containing slurry and a ceramic precursor material, and, after infiltration, said ceramic heat exchanger tube is heated.

3. The method of claim 1 , wherein said ceramic containing slurry comprises silicon carbide.

4. The method of claim 1 , wherein said ceramic precursor material comprises at least one preceramic polymer.

5. The method of claim 4, wherein said preceramic polymer comprises at least one of polysilazane, polycarbosilane, and polycarbosilazanes.

6. The method of claim 4, wherein said preceramic polymer further comprises at least one ceramic filler material.

7. The method of claim 5, wherein after said infiltrating, said preceramic polymer is pyrolyzed to form at least one ceramic material.

8. The method of claim 1 , wherein said oxidation reaction product comprises alumina.

9. A method of forming a microwave susceptible ceramic honeycomb structure comprising: providing a ceramic honeycomb structure; coating at least one surface of said ceramic honeycomb structure with at least one at least one ceramic containing slurry, at least one ceramic precursor material.

10. The method of claim 9, wherein said at least one ceramic containing slurry comprises silicon carbide.

11. The method of claim 9, wherein said at least one ceramic precursor material comprises at least one preceramic polymer.

12. The method of claim 11, wherein said at least one preceramic polymer comprises at least one polysilazane, polycarbosilane, and polycarbosilazane.

13. The method of claim 11, wherein said at least one preceramic polymer further comprises at least one ceramic filler material.

14. A method of increasing the microwave susceptibility of a ceramic honeycomb structure comprising: providing a ceramic honeycomb structure; and coating at least a ponion of at least one surface of said ceramic honeycomb structure with at least one ceramic material.

15. The method of claim 14, wherein said at least one ceramic material is provided by a chemical vapor infiltration process.

16. The method of claim 14, wherein said at least one ceramic material is provided in the form of a ceramic containing slurry.

17. The method of claim 14, wherein said at least one ceramic material is initially provided as a liquid preceramic polymer material which is subsequently pyrolyzed to form said at least one ceramic material.

18. The method of claim 17, wherein said liquid preceramic polymer comprises at least one of polysilazanes, polycarbosilanes, and polycarbosilazanes.

19. The method of claim 17, wherein said liquid preceramic polymer further comprises at least one filler material.

20. The method of claim 16, wherein said ceramic containing slurry comprises at least one material selected from silicon carbide and alumina.