Patent Application
20160207840
The present disclosure is directed to an extrudable ceramic and method of making the extrudable ceramic.
Conventional monolithic extruded ceramic materials are brittle and can have low strength (e.g., are easily crushed). Further, due to their insulating properties they do not propagate heat well, which can result in long cooling times in thermal applications.
In some applications, such as the sequestration of carbon dioxide from gas streams and catalytic converters, monolithic extruded ceramic materials with complex structures and/or high surface area are desired. For such applications, the ability to decrease minimum dimensions, such as wall thicknesses, of ceramic structures would be beneficial. However, the low strength and poor thermal properties of conventional monolithic extruded ceramic materials can be a limiting factor for achieving the desired minimum dimensions.
Fillers are generally known for altering the physical characteristics of materials. In particular, it is known that fibers may be added to ceramics to increase strength and provide other desired properties. For example, carbide fibers are known for providing thermal conductivity and silica fibers are known for providing insulation or strength. However, fibers are not generally known for use in extruded materials because such particles may undesirably interfere with the extruder equipment.
Thus, there is a need in the art for extrudable materials that can be used to make ceramic structures with increased strength and/or increased thermal conductivity.
The present disclosure is directed to an extrudable composite. The extrudable composite comprises a pre-sintered ceramic matrix material; a plurality of particles having an aspect ratio ranging from about 1 to about 100, the plurality of particles comprising at least one particle type chosen from structurally reinforcing particles and thermal conductivity modifying particles; and a carrier liquid.
The present disclosure is also directed to an extruded ceramic composite. The composite comprises a matrix comprising a refractory ceramic and a plurality of particles embedded in the matrix. The plurality of particles have an aspect ratio ranging from about 1 to about 100, the plurality of particles comprising at least one particle type chosen from structurally reinforcing particles and thermal conductivity modifying particles. The extruded ceramic composite has an extruded shape.
The present disclosure is also directed to a method of making a ceramic composite. The method comprises mixing one or more pre-sintered ceramic matrix materials, a plurality of particles and a carrier liquid to form an extrudable composite material. The plurality of particles have an aspect ratio ranging from about 1 to about 100 and comprise at least one particle type chosen from structurally reinforcing particles and thermal conductivity modifying particles. The composite material is extruded into an extruded shape. The extruded composite material is heated to form a ceramic composite.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the present teachings, as claimed.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrates aspects of the present teachings and together with the description, serve to explain the principles of the present teachings.
It should be noted that some details of the figures have been simplified and are drawn to facilitate understanding rather than to maintain strict structural accuracy, detail, and scale.
Reference will now be made in detail to the present teachings, examples of which are illustrated in the accompanying drawings. In the drawings, like reference numerals have been used throughout to designate identical elements. In the following description, reference is made to the accompanying drawings that form a part thereof, and in which is shown by way of illustration specific examples of practicing the present teachings. The following description is, therefore, merely exemplary.
The present disclosure is directed to an extrudable ceramic composite. The composite comprises a matrix comprising a refractory ceramic. A plurality of particles are embedded in the ceramic matrix. The plurality of particles have an aspect ratio ranging from about 1 to about 100. The particles comprise at least one particle type chosen from structurally reinforcing particles and thermal conductivity modifying particles.
The ceramic matrix can comprise any suitable refractory ceramic material, including natural or synthetic ceramics. All or a portion of the ceramic matrix can be chemically active, meaning that the matrix can participate chemically in a chemical reaction, such as by acting as a catalyst, or that it can function to carry out some other desired process at the molecular level, such as by acting to separate gas molecules in a gas extraction process (e.g., as in the case of a molecular sieve). Many chemically active ceramic materials are known that can provide a suitable matrix material. For example, the refractory ceramic matrix can comprise zeolites, such as aluminosilicates, or other chemically active materials such as silica-alumina or alkali-modified amorphous silica alumina. Specific examples of zeolites include X zeolites, such as 13X zeolites, and A zeolites, such as 3A zeolites, Zeolite Socony Mobil (ZSM), chabazite, clinoptiolite and other naturally occurring or synthetic zeolites, any of which can be treated by a chemical exchange process (e.g., ion exchange) and/or can otherwise be modified, as is well known in the art.
A matrix that has chemical activity throughout its bulk can be made by combining one or more chemically active matrix materials, such as any of those discussed herein, with a relatively inert ceramic matrix material. The matrix material can include a combination of chemically active ceramics and inert ceramics in any desired proportions that will provide the desired level of chemical conductivity. Examples of inert matrix materials can include inorganic oxides, such as metal oxides, semiconductor oxides, and silicates, where the inorganic oxides are not zeolites. Specific examples of inorganic oxides include titanates, aluminum oxides, silicon oxides, zirconia oxides, alumino silicates or any combination thereof. One specific example of a chemically inert material is cordierite. In some instances, the matrix material does not include cordierite.
The zeolites can be greater than 5% by weight of the ceramic matrix material, such as 10% to about 90% by weight or about 20% to about 80% by weight, based on the pre-sintered weight of the ceramic matrix material (e.g., excluding any non-ceramic matrix material ingredients such as organic binders or extrusion agents). For example, the matrix can be 30% by weight of the matrix material or more, such as 40%, or more of the matrix material by weight. In an example, substantially all of the matrix material can be a refractive ceramic zeolite. Because the bulk of the ceramic matrix is chemically active, the resulting matrix can carry out a desired chemical function, such as catalysis or gas extraction, without the need for providing an additional chemically active material to the matrix.
The ceramic matrix material can be any suitable amount of the composite. For example, the ceramic matrix material can be included in the composition in an amount that will result in about 45% by weight or more matrix material based on the total weight of the composite after sintering, such as about 55% to about 99.5%, or about 55% to about 95% by weight based on the total weight of the composite after sintering. The amount of ceramic matrix powder used in the pre-sintered clay compositions may vary. Examples can range from about 30% or more ceramic matrix material, such as about 35% to about 75%, based on the total dry weight of the composite ingredients prior to sintering.
Many extrudable products do not benefit from chemical activity. In such cases all or a portion of the ceramic matrix material can be inert. Alternatively, an active material can be added to the inert matrix after the matrix has been formed, such as by coating or plating the ceramic matrix after sintering, in order to allow the resulting structure to carry out a desired function, such as catalysis or gas extraction.
Additional optional materials, such as binders and extrusion aids, can be added to the mixture used to form the ceramic matrix. Examples of suitable inorganic binders include calcium silicate, sodium aluminates, sodium silicates, alkaline or alkaline earth metals, phosphates, alumina, clays, bentonites or borates, as well as organic binders, such as polyvinyl alcohol (“PVA”). Examples of suitable extrusion aids include at least one compound selected from methyl cellulose or hydroxyalkyl substituted methyl cellulose, such as hydroxypropyl methylcellulose. An example of a commercially available methyl cellulose based extrusion aid is METHOCEL™, available from Dow of Midland, Mich. Binders and extrusion aids are generally well known in the art and one of ordinary skill in the art would be able to determine suitable amounts of these materials for use in the compositions of the present disclosure. The sintering process that is used to fire the ceramics may completely or partially burn out any binders or extrusion aids that cannot withstand high temperatures. For example, many organic binders will be burned out at sintering temperatures of 300° C. or more, such as 400° C. or more. While residual binder can potentially remain, this can also create pores in the matrix material. The added pores can decrease the strength of the matrix, but may in some cases be desired to achieve higher porosity.
The structurally reinforcing particles employed in the composites of the present disclosure can be any suitable particles having a shape or design that is capable of providing structural reinforcement to the final ceramic structure and that can withstand the high temperatures employed during sintering. For example, the structurally reinforcing particles can comprise ceramic or other inorganic refractory materials in either fiber or non-fiber particle form. Such materials can include inorganic oxides or nonoxide inorganics, such as inorganic carbides, inorganic borides, inorganic nitrides and inorganic silicides. Where the particle is an inorganic oxide it can be made from a different inorganic oxide than is used to form the matrix. Specific materials useful as particles include semiconductor oxides, metal oxides, silica, alumina silicate, zirconia, alumina, boria, silicon carbide, semiconductor nitrides such as silicon nitrides, metal nitrides, semiconductor carbides such as silicon carbide, metal carbides, semiconductor borides such as silicon boride, metal borides, metal silicides or elemental carbon, such as in carbon fibers, carbon nanotubes or carbon powder, and mixtures thereof. An example of a commercially available material is NEXTEL™ 312, which is an alumina-boria-silica fiber available from 3M of St. Paul, Minn. Metal particles can also be employed to enhance strength. Examples include any of the refractory metal or refractory metal alloy particles described below for use in thermal conductivity modifying particles.
The thermal conductivity modifying particles can be any suitable particle that is capable of either increasing or decreasing thermal conductivity to within a desired range and that can withstand the high temperatures employed during sintering. For example, the thermal conductivity modifying particles can comprise at least one material chosen from inorganic nitrides such as metal nitrides and semiconductor nitrides, inorganic borides such as metal borides and semiconductor borides, silicon, silicon carbide, elemental carbon, such as carbon fibers, and metal particles. Examples of metal particles include particles made of refractory metals or alloys thereof, such as niobium, niobium alloys, molybdenum, molybdenum alloys, tantalum, tantalum alloys, tungsten, tungsten alloys, rhenium, rhenium alloys, titanium, titanium alloys, vanadium, vanadium alloys, chromium, chromium alloys, zirconium, zirconium alloys, hafnium, hafnium alloys, ruthenium, ruthenium alloys, osmium, osmium alloys, iridium and iridium alloys, and mixtures thereof. The materials can be in fiber or non-fiber form.
In some applications, such as CO2 capture, it can be desirable to have bi-directional thermal conductivity, meaning that the extruded ceramic has sufficiently good thermal conductivity so that it can be both quickly heated and quickly cooled, depending on process needs at any given time. Including at least one of silicon carbide particles, carbon particles or metal particles in the ceramic in sufficient amounts can provide desired improvements in bi-directional thermal conductivity. For instance, 5% by weight or more of either silicon carbide or carbon particles, based on the total weight of the ceramic composite, can provide desired increases in bi-directional thermal conductivity.
Using high weight percentages of particles can be problematic during the extrusion process, especially in cases where complex shapes are formed. The amount of particles included in the composite can depend on the final shape that is to be formed from the extrudable material. For relatively simple shapes or shapes that have thicker minimum dimensions, such as pipes or solid objects, the particles can be included in an amount up to about 40% by weight, such as an amount ranging from about 0.5% to about 40% by weight relative to the total weight of the sintered ceramic composition. For more complex shapes or shapes that have thinner minimum dimensions, such as thin-walled structures, the particles can be included in an amount up to about 20% by weight, such as an amount ranging from about 0.5% to about 20% by weight relative to the total weight of the sintered ceramic composition. The amount of particles added to the to pre-sintered clay compositions may vary. Example particle concentrations can range from about 0.2% by weight or more particles, such as about 1% to about 30%, or about 5% to about 20% by weight, based on the total dry weight of the composite ingredients prior to sintering.
The particles can have any suitable form that is compatible with extrusion. For example, the particles can be in the form of a powder, a uniform fiber, a chopped fiber, a milled fiber or combinations thereof. The term particle as used in the present disclosure is defined to include particles of any shape, including flakes, tubes, fibers, spherical particles, and any other symmetrically shaped or non-symmetrically shaped particle. To improve the ability of the composite materials to be extruded, the particles have aspect ratios ranging from about 1 to about 100, such as about 2 to about 50, or about 4 to about 10. The desired aspect ratios can be achieved in any suitable manner, such as by grinding, milling or chopping the fibers to have the desired size. The term aspect ratio is defined herein to be the ratio of the average length of the fibers to the average smallest fiber dimension (e.g., thickness, or in the case of cylindrical fibers, the diameter).
The particles can have at least one dimension of 100 microns or less, such as 0.001 microns to about 100 microns, or about 1 micron to about 50 microns, or about 1 micron to about 20 microns. For spherical, cylindrical or tubular shaped particles, the at least one dimension can be a diameter.
The composites of the present disclosure can be extruded into any suitable shape capable of being formed by extrusion techniques. One example of an extruded shape is a monolithic honeycomb structure 2 comprising a plurality of cells 4 having cell walls 6, as shown in
The increased strength provided by the composites of the present disclosure allows for thinner cell walls and therefore an increased cell density, while still providing acceptable crush strength for the honeycomb structure. The honeycomb shape can have one or a plurality of cell walls having a thickness, Twall, of 10 mils or less, such as about 1 mil to about 8 mils, or about 2 mils to about 5 mils. In an example, all of the walls of the honeycomb structure have a thickness of less than 10 mils, or less than 5 mils, or less than 2 mils. The monolithic honeycomb structure can have a cell density that is greater than 50 cells per square inch or more, such as 60, 100 or 400 cells per square inch or more.
The ceramic composite materials of the present disclosure can provide one or more of the following advantages: increased strength or increased or decreased thermal conductivity, such as, for example, a free supporting, chemically active or inert, monolithic honeycomb structure having at least one of an increased strength under load or an increased thermal conductivity compared to the same monolithic honeycomb structure without the plurality of particles; a composite with improved bi-directional thermal conductivity; an increased surface area and/or decreased minimum dimension capable of supporting a desired weight load, such as a monolithic honeycomb structure having an increased surface area, increased cell density and/or decreased wall thickness that is capable of supporting a greater weight than the same monolithic honeycomb structure without the plurality of particles; the ability to form objects with complex ceramic shapes having increased strength by extrusion techniques; and/or reduced back pressure for ceramic matrix structures used in gas separation or extraction processes. Where structural reinforcing particles are employed, a crush strength can be improved by up to 20% or more compared to the same monolithic honeycomb structure without the plurality of particles. Where thermal conductivity modifying particles are employed, the thermal conductivity can be increased by 10% or more, such as by 40%, compared to the same monolithic honeycomb structure without the plurality of particles. The improvement that is realized can depend on the loading amounts of the particles and the ceramic matrix and particle materials used, among other things.
The ceramic composites of the present disclosure can be made by any suitable extrusion method. As shown in the flow chart of
The one or more pre-sintered ceramic matrix materials can include any materials suitable for forming the sintered ceramic matrix materials of the present disclosure. For example, the pre-sintered ceramic matrix material ingredients can include powdered forms of any of the ceramic matrix materials described herein. The powdered forms can have particles of any size suitable for forming a ceramic matrix, such as, for example, average particle sizes of 50 microns or less, such as 25 microns or 5 microns or less. Specific examples of powdered active materials for forming a chemically active matrix include powdered zeolite, including any of the zeolites described herein in powdered form, such as powdered aluminosilicates, powdered silica-alumina, powdered alkali-modified amorphous silica alumina, powdered X zeolites, such as 13X zeolites, and powdered A zeolites, such as 3A zeolites, powdered Zeolite Socony Mobil (ZSM), powdered chabazite, powdered clinoptiolite and other powdered naturally occurring or synthetic zeolites. Examples of the inert ingredients that can be used to form the ceramic matrix include powdered inorganic oxides, such as powdered metal oxides, powdered semiconductor oxides, and powdered silicate, where the inorganic oxides are not zeolites. Specific examples of powdered inorganic oxides include titanates, aluminum oxides such as beohmite, silicon oxides, zirconia oxides, alumino silicates, cordierite or any combination thereof. Suitable commercially available premixes for forming both chemically reactive matrix material and inert matrix material can be readily obtained. One of ordinary skill in the art would be able to choose appropriate ceramic matrix materials based on the specific application and desired characteristics of the extrudable material, such as the operating temperature at which the extruded product will be used, coefficient of thermal expansion matching, thermal properties desired, etc.
The carrier liquid can be water or any other suitable liquid, such as, for example, a colloidal suspension. For example, commercially available carrier liquids are known in the art. The carrier liquid and/or other solvents can be employed at a sufficient concentration to provide the desired consistency for extrusion, which may vary depending on such things as the type of extruder, piston or auger used, and the extrusion form, such as tube, monolith, and so forth. One of ordinary skill in the art would readily be able to determine the amount of carrier liquid to be employed.
Any suitable extrusion process can be used to form the composite materials of the present disclosure. For example, the wet composite material can be pushed or drawn through an extruder that includes a die of the desired cross-section. Suitable extrusion techniques are known in the art. The extrusion process can produce an object having the same or substantially the same cross-section along an entire dimension (e.g., length or width). This is illustrated in
After extrusion, the resulting greenware can be heated to form the ceramic in any desired manner. For example, heating can comprise drying the extruded composite material at a first temperature and then sintering the extruded composite material at a second temperature that is higher than the first temperature. Any suitable drying and sintering temperatures can be used. Examples of drying temperatures include temperatures ranging from about 20° C. to about 90° C. Examples of sintering temperatures include temperatures ranging from about 300° C. to about 700° C. Drying and sintering are generally carried out separately, although simultaneous drying and sintering processes can be used. Suitable drying and sintering techniques are well known in the art.
The ceramic structures of the present disclosure can be used in a variety of applications. One example of such an application is a gas capture system 30, such as shown in
Carbon fibers and/or NEXTEL™ 312 ceramic fibers, available from 3M, having a diameter of 1-20 microns were ground to decrease the length of the original fibers to form particles having reduced aspect ratios of about 10 or smaller. The reduced aspect ratio particles were combined with a matrix material. The matrix material included a ceramic mixture that optionally comprised one or more binders. A 13X zeolite powder was combined with the ceramic mixture to form a dry mixture. To the dry mixture were added water and METHOCEL™, available from Dow of Midland, Mich. The percentages of ingredients in the matrix material included about 35% ceramic mixture, about 25% 13X zeolite powder, about 3.25% METHOCEL™, and the remainder water, where the percentages of matrix ingredients are based on the total weight of the pre-sintered matrix material without the weight of the fibers. The fibers were mixed with the dry matrix materials, followed by addition of the METHOCEL™ and water.
The clay mixture was extruded into a greenware form having a cell density of 81 cells per square inch (“CPSI”). The greenware was dried under high humidity, such as about 70% to about 90% humidity, for over 48 hours and sintered at a temperature of 400° C. for 4 hours.
The crush strength was measured by extruding rods of similar matrix compositions to that of Example 1, where some rods included silicon carbide fibers, some rods included ceramic fibers (NEXTEL™ 312) and some rods had no fibers. After firing, the samples were crushed by compression testing with an Instron Dual test unit.
Baseline material (no fibers) had a crush strength of 132 psi. The addition of 14 wt % ground NEXTEL™ 312 fibers to the baseline material caused a strength increase of about 12% (148 psi). The addition of 3 wt % ground silicon carbide fibers to the baseline material resulted in about a 19% increase (157 psi). The weight percent of fibers is based on the total dry weight of the composite ingredients prior to sintering.
Heat transfer was measured on 2″ disks formulated to have the matrix material of Example 1 with 0, 2.1% and 5% SiC, or 0 and 15% NEXTEL™ 312 fibers. Thermal transfer was measured at 100° F., 300° F. and 500° F. While the NEXTEL™ 312 added strength, it did not increase the heat transfer, as shown in
Carbon fibers and/or NEXTEL™ 312 ceramic fibers having a diameter of 1-20 microns are ground to decrease the length of the original fibers to form reduced aspect ratio particles. 20 wt. % boehmite, 0.3 wt. % polyvinyl alcohol, 7 wt. % METHOCEL™ and 76.7 wt % of 13X zeolite powder are mixed to form a dry mixture, where the percentages are based on the total dry weight of the ingredients employed in the matrix. The carbon and/or NEXTEL™ 312 reduced aspect ratio fibers are combined and mixed with the dry mixture. To the resulting mixture is added water in an amount that produces a clay mixture suitable for extrusion.
The clay mixture is extruded into a greenware form having a cell density of 81 cells per square inch (“CPSI”). The greenware is dried under high humidity of about 80% for over 48 hours and sintered at a temperature of 400° C. for 4 hours.
Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the disclosure are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements. Moreover, all ranges disclosed herein are to be understood to encompass any and all sub-ranges subsumed therein.
While the present teachings have been illustrated with respect to one or more implementations, alterations and/or modifications can be made to the illustrated examples without departing from the spirit and scope of the appended claims. In addition, while a particular feature of the present teachings may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular function. Furthermore, to the extent that the terms “including,” “includes,” “having,” “has,” “with,” or variants thereof are used in either the detailed description and the claims, such terms are intended to be inclusive in a manner similar to the term “comprising.” Further, in the discussion and claims herein, the term “about” indicates that the value listed may be somewhat altered, as long as the alteration does not result in nonconformance of the process or structure to the intended purpose described herein. Finally, “exemplary” indicates the description is used as an example, rather than implying that it is an ideal.
It will be appreciated that variants of the above-disclosed and other features and functions, or alternatives thereof, may be combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompasses by the following claims.
