Patent Application
20140338296
1. Field
Exemplary embodiments of the present disclosure relate to porous ceramic articles and a method of making the same. Exemplary embodiments of the present disclosure relate to porous ceramic articles having microstructure including sinter bonded or reaction bonded large pre-reacted particles and pore network structure and a method of making porous ceramic articles using pre-reacted particles.
2. Discussion of the Background
Cordierite, silicon carbide, and aluminum titanate-based honeycombs have been widely used for a variety of applications including catalytic substrates and filters for diesel and gasoline engine exhaust after treatment.
To meet increasingly stringent emission regulations for light and heavy duty vehicles, the substrate and filter materials have to be highly porous to allow gas flow through the walls without restricting the engine power, have to show high filtration efficiency for emitted particles, and, at the same time, are expected to demonstrate low back pressure. The substrates and filters also have to be able to withstand the erosive/corrosive exhaust environment and bear thermal shock during rapid heating and cooling. Regulation of CO2 emission and raising fuel cost drive miniaturization and integrated functionality in the exhaust gas after-treatment system. It may be desirable to reduce the number of components in the after-treatment system, decrease their size and implement multi-functionality of the different components. For example, integrating de-NOx catalyst and diesel oxidation catalyst (DOC) into diesel particulate filters may be desired. To reach high de-NOx efficiency, rather high loading of de-NOx catalyst is required together with a high catalyst activity at low temperature, such as can be found for Cu-zeolites. Trends and Original Equipment Manufacturers (OEMs) desires may drive zeolite catalyst loading to high levels of 200 g/l. In order to meet this loading target and preserve low pressure drop, the filter substrate may need high porosity and large pore size, for example, around 60% porosity with a median pore size of 18 μm or larger.
High porosity and large pore size that enables high de-NOx efficiency are expected to not degrade the particulate filtration efficiency. They should also not decrease the thermo-mechanical properties of the filter. Cordierite and aluminum titanate may both have low thermal expansion and are therefore suited for applications where high thermal shock resistance is required. Both materials show anisotropy in their thermal expansion with different crystallographic directions exhibiting positive and negative expansion. Due to the anisotropy in thermal expansion, mismatch strains build up between grains with different crystallographic orientation; such strains can lead to microcracking. Polycrystalline cordierite or aluminum titanate ceramics may undergo extensive microcracking during thermal cycling. Microcracks open during cooling and close, sometimes even heal during heating. This creates a hysteresis response to thermal cycling with differences between heating and cooling that can be attributed to the reversible microcrack formation and closure. As a consequence of microcracking, the overall coefficient of thermal expansion (CTE) of the ceramics may be lower than the crystallographic average CTE.
On first look, microcracking may seem beneficial; the thermal shock resistance of the material, which is proportional to the material's strength and inversely proportional to its elastic modulus and thermal expansion, is expected to be improved by microcracking. However, the material strength also decreases with increasing microcrack density. Microcrack densities in cordierite remain rather low, due to the small difference in crystallographic thermal expansion and large grain (domain) sizes required to reach the stress threshold for microcracking. As a result of a much larger anisotropy in crystallographic expansion, microcrack densities in aluminum titanate-based materials are much higher and strongly influence the ceramic article's strength.
Porous cordierite and aluminum titanate based honeycomb ceramic articles with low thermal expansion, high porosity, low Young's modulus and high strength are utilized as high-performance automotive catalytic converter substrates and diesel particulate filters. For cordierite products, raw materials such as alumina, talc, clay, magnesia, alumina and silica powders may be mixed with organic binders and pore formers. For aluminum titanate composite products, raw materials such as alumina, titania powders and raw materials for forming the “filler” phase, for example strontium oxide, alumina, silica to form feldspar (strontium aluminum silicate feldspar or “SAS”), may be mixed with organic binders, pore formers and water to form a plastic mixture. The plastic mixture may be extruded or otherwise shaped into a green body of desired shape, for example, a honeycomb, trough log or disk filter, dried, and then fired to temperatures between 1350° C. and 1450° C., depending on the raw material combination. During the drying and firing process, the raw material particles react, and form, via various intermediates, the final crystalline cordierite or alumina titanate composite. The shaped green part transforms upon firing into a solid, durable porous ceramic article. Other substrate and filter honeycomb materials or mixtures of materials that upon high temperature treatments react to form oxide or non-oxide ceramics, may include metals, intermetallics, mullite, alumina (Al2O3), zircon, alkali and alkaline-earth alumino-silicates, spinels, perovskites, zirconia, ceria, silicon nitride (Si3N4), silicon aluminum oxynitride (SiAlON), and zeolites.
Diesel particulate filters (DPF) and gasoline particulate filters (GPF) may be obtained from a honeycomb porous ceramic by plugging channels in a checkerboard pattern on one end and plugging the remaining channels at the other end to form a filter with inlet and outlet channels. The exhaust gas flows into the open inlet channels, through the wall of the honeycomb (through-wall flow) because the inlet channels are plugged at the other end and out of the outlet channels, which are plugged at the inlet end. During exhaust gas passage through the porous honeycomb wall, small particulates from the exhaust gas are deposited on the pore surface or as the soot layer on the wall surface, thus providing filtering of the exhaust gas. The soot cake of deposited particulates may be periodically burned in a regeneration cycle or continuously during passive regeneration so that the DPF or GPF has a lifetime similar to that of the vehicle. Alternative filter designs may be used, such as radial trough filters or radial disk filters, which compared to the honeycomb design with its long, narrow gas flow channels may show wider gas flow channels and a stronger radial component for the gas flow, but share the same particulate filtering of the gas when passing through the thin porous ceramic wall and offer the same opportunity for de-NOx functionality with incorporation of a suited catalyst in the wall-porosity and/or on the channel walls.
Tightening of exhaust gas regulations may call for higher particulate filtration efficiency, particularly for small particle size, and for higher NOx filtration efficiency, not only in the currently established test cycles, but also in continuous real-world driving. CO2 regulations may call for use of less fuel and OEMs demand lower pressure drops, both at improved thermal shock resistance and extended lifetime of the porous ceramic honeycomb substrate. To meet these demands, substrates and filters with higher porosity, larger pore size, with thinner honeycomb walls than currently in use may be needed.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention as claimed and therefore it may contain information that does not form any part of the prior art nor what the prior art may suggest to a person of ordinary skill in the art.
Exemplary embodiments of the present disclosure provide porous ceramic articles having microstructures characterized by sinter bonded or reaction bonded engineered spheroidal particles and pore networks.
Exemplary embodiments of the present disclosure also provide a method of making porous ceramic articles using sinter bonded or reaction bonded engineered spheroidal particles.
Exemplary embodiments of the present disclosure also provide a plastic ceramic precursor batch composition for making porous ceramic articles having microstructures characterized by sinter bonded or reaction bonded engineered spheroidal particles and pore networks.
Additional features of the invention will be set forth in the description which follows, and in part will be apparent from the description, or may be learned by practice of the invention.
An exemplary embodiment discloses a method of making a porous ceramic article. The method includes forming green particles of at least 10 μm diameter, calcining the green particles to form pre-reacted particles, mixing the pre-reacted particles and a liquid vehicle to form a paste and forming the paste into a wet green body. The pre-reacted particles include at least one of dense, porous, or hollow spheroidal particles, and the pre-reacted particles include one or more phases. The method includes drying the wet green body to form a dried green body and firing the dried green body to form the porous ceramic article.
An exemplary embodiment also discloses a method of making a porous ceramic article having inverse porosity pore structure. The method includes forming green particles of at least 10 μm diameter, calcining the green particles to form pre-reacted particles, mixing the pre-reacted particles and a liquid vehicle to form a paste. The pre-reacted particles comprise at least one of dense, porous, or hollow spheroidal particles and the pre-reacted particles comprise one or more phases. The method includes forming the paste into a wet green body, drying the wet green body to form a dried green body, and firing the dried green body to form the porous ceramic article comprising inverse porosity pore structure. The porous ceramic article comprises a porosity of at least 50% and a median pore size (d50) of 10 to 30 μm.
An exemplary embodiment also discloses a porous ceramic body including a microstructure of solid matter and a network of contiguous pores with large pore necks. The porous ceramic body has a permeability of greater than or equal to 1000, a porosity greater than or equal to 50%, a median pore size (d50) greater than 10 μm, a coefficient of thermal expansion (CTE) in a range of 2×10−7 K−1 to 20×10−7K−1 from room temperature (25° C.) to 800° C., a strain tolerance greater than 0.10%, and an MOR greater than 170 psi for a honeycomb geometry (300/14) or equivalent.
An exemplary embodiment also discloses a porous ceramic body including a microstructure of sinter bonded or reaction bonded large pre-reacted particles and pore network structure exhibiting large pore necks. The sinter bonded or reaction bonded large pre-reacted particles comprise a homogeneous phase mixture or a phase distribution of reaction product layers and green phases.
An exemplary embodiment also discloses a plastic ceramic precursor batch composition for making a porous ceramic article. The plastic ceramic precursor batch includes at least one of dense, porous, and hollow pre-reacted particles, and a liquid vehicle, wherein the pre-reacted particles comprise one or more phases.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.
The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate exemplary embodiments of the disclosure, and together with the description serve to explain the principles of the claimed invention.
SEM images of AT-type batch extruded and fired ware made with spray-dried alumina/3% silica are shown in
The claimed invention is described more fully hereinafter with reference to the accompanying drawings, in which embodiments of the disclosure are shown. This claimed invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure is thorough, and will fully convey the scope of the claimed invention to those skilled in the art. In the drawings, the size and relative sizes of layers and regions may be exaggerated for clarity. Like reference numerals in the drawings denote like elements.
It will be understood that when an element or layer is referred to as being “on” or “connected to” another element or layer, it can be directly on or directly connected to the other element or layer, or intervening elements or layers may be present. In contrast, when an element or layer is referred to as being “directly on” or “directly connected to” another element or layer, there are no intervening elements or layers present. It will be understood that for the purposes of this disclosure, “at least one of X, Y, and Z” can be construed as X only, Y only, Z only, or any combination of two or more items X, Y, and Z (e.g., XYZ, XYY, YZ, ZZ).
Exemplary embodiments of the present disclosure relate to a porous ceramic article and the production thereof. The porous ceramic article may be made of aluminum titanate based materials, such as aluminum titanate solid solution (pseudobrookite) as the main phase (greater than 50 vol %) and other phases such as cordierite, feldspar, mullite, spinel, alumina, rutile, or similar oxides, cordierite, or other oxide or non-oxide ceramics, including metals, intermetallics, mullite, alumina (Al2O3), zircon, alkali and alkaline-earth alumino-silicates, spinels, perovskites, zirconia, ceria, silicon oxide (SiO2), silicon nitride (Si3N4), silicon aluminum oxynitride (SiAlON), and zeolites. Application of the porous ceramic article may include, for example, honeycomb integrated and non-integrated diesel and gasoline catalyst supports, substrates, and particulate filters. Exemplary embodiments of the present disclosure also relate to a porous ceramic article and batch compositions including pre-reacted particles, and a process for making the porous ceramic article by using at least one batch material in the form of pre-reacted particles.
The pre-reacted particles may be of selected composition, structure, size, and size distribution to impose a reaction path and microstructure development during reactive firing of the porous ceramic article. The porous ceramic article produced with the pre-reacted particles may have an engineered pore distribution and phase distribution, which may be referred to herein as the pore network structure and the microstructure. The microstructure may be characterized by phases within the solid matter and the morphology may be characterized by the shape of the matter and the shape of the pores within the ceramic article. Generally, the microstructure and morphology are characterized by matter and pore channels of two-dimensional cross sections through the three dimensional structure. The porous ceramic article made from the pre-reacted particles may have a “normal” porosity of small pore necks and large matter necks or “inverse” porosity of large pore necks and small matter necks with large pore size in the final, fired ceramic. Pore necks may be considered the channels connecting pores. In microstructures having pores created by fugitive pore formers, the pores are generally spherical or other shape with small connecting channels where the pores intersect one another, as an example of normal porosity with small pore necks. On the other hand, in microstructures having pre-reacted spheroidal particles forming the microstructure, pores may be formed between the particle material as an example of inverse porosity with large pore necks. In the latter case the pore channels are wider and show less constriction for gas flow.
The porous ceramic article having the pre-reacted particles according to an exemplary embodiment of the present disclosure provides improved diesel particulate filter (DPF), gasoline particulate filter (GPF), catalyst support, substrate, and combined substrate particulate filter product properties compared to porous ceramic articles made from powder batches without pre-reacted particles. Porous ceramic articles made from batches with pre-reacted particles exhibited large pore size and high porosity, good strength and low coefficient of thermal expansion (CTE) that enable, low pressure drop at higher washcoat and catalyst loading. Thus, exemplary embodiments of the present disclosure enable integration of high selective catalytic reduction (SCR) catalyst loading and high de-NOx catalyst efficiency at low pressure drop, high filtration efficiency, and good thermal shock resistance.
Use of spray-dried particles for good particle packing and narrow pore size distributions was attempted for making porous bodies applied through processes, such as pressing into shape and natural sintering. However, green spray-dried powders did not successfully survive extrusion processes. In fact, the inventors have run into many roadblocks and dead ends in attempting to use green spray-dried powders successfully in extrusion processes. Despite a broad exploration of different binders it seemed impossible to make the spray-dried particle strong enough to survive the pressures of extrusion. Trials were conducted to extrude green spray-dried alumina-talc, alumina-clay and graphite (GR) with binder and use them as raw material in ceramic precursor batches. Material properties in these trials were little changed over powder raw material. In these attempts, the spray-dried particles were used as spray-dried green and broke up into powder or small fragments under the shear stresses of screw and extrusion die.
According to other exemplary embodiments, particles can be made by pre-reacting fine powders with a polymer followed by breaking up of the polymer, such as by grinding, and calcining (firing) the ground particles to form pre-reacted particles. Likewise, a slurry may be made of the fine powders, then compacted to form particles, such as by drying. The dried particles may then be calcined (fired) to form the pre-reacted particles. Other methods may include spray-drying, spin drying, and atomizing the slurry to form green particles that may then be calcined to form the pre-reacted particles.
The green particles may have spheroidal, such as (i) spherical (ii) ellipsoidal, and (iii) toroidal (torus like with or without a center hole) shapes as shown in a schematic of
The pre-reacted particles are then incorporated in the batch and mixed with the other batch constituents 130. The batch is extruded 140, dried 150, and fired 160 to form the porous ceramic article. In the case of pre-fired, partially or fully pre-reacted spray-dried materials, firing temperatures can be lower or durations shorter. For fully pre-reacted spray-dried materials, very short or low temperature firing schedules can be implemented, for example, when a low firing binder may be sintered.
According to an exemplary embodiment of the disclosure, fine powders and soluble constituents may be mixed in a slurry with water, and any of binder, dispersant, surfactant, and anti-foam agent. The slurry is then suspended in a carrier gas and atomized at the top of the spray dryer. Parameters such as nozzle size, temperature, pressure, and solid loading may be varied. Fine powders, for example, particles of less than 1 μm, or soluble constituents may be used.
Hollow and solid spray-dried particles of different sizes, size distribution and compositions may be made by using different settings of the spray-dryer and different starting materials according to exemplary embodiments of the disclosure. Green particles may be dense or contain different levels of porosity, ranging from dense over porous to hollow, and also different pore sizes.
According to exemplary embodiments alpha alumina or boehmite may be used as an alumina source, colloidal silica suspension may be used as a source for silica, fine titania as a source for titania, and fine magnesium oxide as a source for magnesia. Other inorganics, such as strontium carbonate, calcium carbonate, and lanthanum carbonate may be jet-milled to less than 1 μm particle size and added to the slurry. Lanthanum acetate, boron oxide and other sintering aids may be added in the form of an aqueous solution to the slurry.
Exemplary embodiments of combinations of inorganic powders spray-dried to form green particles include alumina (fine alpha alumina or boehmite) with 1.5 to 15% silica, alumina with different sinter additives such as B, Mg, Y, Fe, etc., alumina-silica mixtures with different sinter additives such as B, Mg, La, Y, Fe, etc., titania defining compositions, such as alumina with different levels of silica, alumina/titania mixtures, aluminum titanate composition, feldspar composition, and full aluminum titanate (AT) batch compositions (aluminum titanate and feldspar phases) with complete final AT inorganic composition or with a small deficiency in alumina or silica or (alumina+silica). Spray-dried full batch compositions may also contain sintering aids such as lanthanum oxide, ceria, yttria, zirconia, boron oxide, alkali oxides, etc.
Spray-dried powders may be pre-fired at different temperatures for different times, in a regular box or tube furnace in a crucible, sinter box, or on a setter, or in a rotary calciner according to exemplary embodiments. Conditions for static firing of alumina/silica-based dried green powders include firing top temperatures between 1200° C. and 1600° C. and 1 h to 15 h hold time. Conditions for static firing of full AT-based composition green powders include temperatures between 1200° C. and 1600° C. At lower reaction temperatures, aluminum titanate may not be formed; at temperatures greater than 1300° C., aluminum titanate was formed.
In a static setting, the green powders may sinter together at high temperature and at long hold times and thus, may be broken apart prior to further use, for example, as a batch constituent. Sieving or low energy milling may be used to break the loosely sintered agglomerates.
Rotation of green powders during pre-firing avoids sintering together and may provide better preservation of particle shape. An industrial rotary calciner may be used for firing powders. For example, rotary calcining conditions of alumina/silica-based green particles may include, for example, 1000-1650° C. As another example, rotary calcining conditions of the full batch AT spray-dried particles may include, for example, 1000-1480° C.
According to exemplary embodiments the pre-reacted powders may be used as batch material together with other raw materials to match the final ceramic article (e.g., filter, substrate) inorganic composition, for example AT, AT-based composite, cordierite, cordierite composite, silicon carbide, silicon nitride, or like ceramic article inorganic compositions. Pore formers, such as graphite, polymer beads, foaming agents, starch and others with particle sizes to match the pre-reacted particle size, may be added to the batch at levels of 0-50%, for example, at levels of 5%, 10%, 20%, 25%, 30%, or 40%. To provide satisfactory rheological properties and enable a good extrusion quality, methylcellulose (3-7%) may be added as a binder and a lubricating oil package, may be added to form a green ceramic article (green ware).
Batches may be engineered to achieve high porosity and large pore size in the ceramic article through a random loose packing (low density packing) of the pre-reacted (e.g., spray-dried pre-fired) particles in agglomerates, to benefit from engineered reaction path, microstructure and reaction degree of the pre-reacted particles to control firing shrinkage during drying and firing of the green ware to form the ceramic article.
According to exemplary embodiments, a method of making a porous ceramic honeycomb may include mixing batch constituents, for example, premixing powder batch ingredients including pre-reacted particles, pore-former, and binder. These dry ingredients may be combined in a mulling pan and mulled, while batch water is added during mulling until a suitable paste texture is reached. The paste may then be formed, for example, extruded or pressed, for example, in a ram extruder or a twin screw extruder through a die into a honeycomb structure. The cell geometry of the honeycomb structure may be, for example, 300 cells per square inch (cpsi) (46.5 cells per square cm) and 14 mil (0.014 inch or 0.0356 cm) wall thickness (300/14), 300/10, 400/14, 600/9, 900/12 or other cell geometry suitable as a honeycomb filter or substrate after drying and firing. The formed green ware with different pre-reacted particle batch materials may be dried and fired to obtain the porous ceramic honeycomb.
Formed green honeycomb parts may be dried in a microwave oven, air dried, hot air died, RF dried, etc., or subjected to a combination of drying methods and times until sufficiently dried to be fired. Firing may include heating in an appropriate atmosphere at a temperature sufficient to form the final phases of the porous ceramic honeycomb and subsequent cooling. For example, firing may occur in an air atmosphere in a furnace at heating rates of 120° C./h to the maximum firing temperature, which may range from 1000° C. to 1650° C., a hold for 1 to 30 hours and cool down rate of about 10 to 160° C./hour. Heating may include slow ramp rates during debinding in low oxygen partial pressure during burnout of pore formers, surfactants, lubricants, additives, and binders.
Inverse porosity with large pore necks between matter as can be obtained by sintering reacted spherical batch particles (spray-dried pre-fired particles) in a final firing process with or without a fine, low temperature binder is demonstrated in
The pore neck 242 of
According to exemplary embodiments of the disclosure, a porous ceramic article having an inverse pore structure achieves higher permeability than a similar composition made from non pre-reacted particles. For example, the porous ceramic article may have a permeability greater than 1000 and a porosity greater than 50%. For example, the porosity may be greater than 57%, or even greater than 60%. The porous ceramic article may have a median pore size (d50) greater than 10 μm, for example, a median pore size greater than 15 μm, or even greater than 18 μm. The porous ceramic article may have a coefficient of thermal expansion from room temperature (RT) to 800° C. less than 20×10−7K−1, for example, less than 15×10−7K−1, or even less than 10×10−7K−1. Furthermore, a (300/14) honey comb body of the porous ceramic article may have a modulus of rupture (MOR) flexural strength greater than 170 psi, for example, greater than 200 psi.
According to exemplary embodiments of the disclosure, a porous ceramic article having a regular (non-inverse) pore structure achieves higher permeability than a similar composition made from non pre-reacted particles. For example, the porous ceramic article may have a permeability greater than 1000 and a porosity greater than 50%. For example, the porosity may be greater than 57%, or even greater than 60%. The porous ceramic article may have a median pore size (d50) greater than 10 μm, for example, a median pore size greater than 15 μm, or even greater than 18 μm. The porous ceramic article may have a coefficient of thermal expansion from room temperature (RT) to 800° C. less than 20×10−7K−1, for example, less than 15×10−7K−1, or even less than 10×10−7K−1. Furthermore, a (300/14) honey comb body of the porous ceramic article may have a modulus of rupture (MOR) flexural strength greater than 170 psi, for example, greater than 200 psi.
To enhance understanding of the disclosure with respect to certain exemplary and specific embodiments thereof, which are illustrative only and not intended to be limiting, the following illustrative Examples are put forth and are intended to provide a complete disclosure and description of how the articles and methods claimed herein can be made and evaluated. They are intended to be purely exemplary and are not intended to limit the scope of what the inventors regard as their invention as claimed.
Table 1 lists the ingredients used for making green particles. Table 2 summarizes examples of hollow alumina-silica green particles obtained from boehmite—3% silica slurries designated as A1, A2, and A3, in Table 1. Spray drying parameters, such as solid loading, surfactant addition, viscosity, nozzle size in mm, temperature in ° C., pressure, and inlet and outlet pressure in atmospheres, are listed in Table 2. The obtained green particles are separated into a coarse and a fine particle size fraction and are characterized by parameters such as the ratio of coarse over fine particle fraction, the average diameter of the coarse particle fraction, the average diameter of the fine particle fraction and the breadth of the particle fractions. In addition, the fraction of hollow particles as determined by SEM is indicated. Table 2 also shows the resulting pre-reacted particle average diameter of the spray-dried coarse particle fraction after firing to 1300° C.
In Table 1, Ludox AS® was used as the SiO2 (colloidal silica in water), a fine alumina powder was used, and Tritan x-100® was used as the organic additive. In Table 2, the nozzle size was 1.5 mm.
The effect of various parameters on the spray-dried particle size distribution will be described with reference to
Solid pre-reacted particles with different sizes and compositions were made by spray-drying and pre-reacting for use as batch materials. Table 3 summarizes examples of processing solid green particles of alumina-silica composition by spray-drying. In the Examples, alumina with 3% silica slurry compositions and the listed spray-drying parameters (solid loading, viscosity, temperature, pressure, and inlet and outlet pressure) are presented. A nozzle diameter of 1.5 mm was used for all Examples in Table 3 except for the compositions of Samples 26-28 that used a 1.0 mm nozzle. A 1% surfactant (Tritan x-100®) was used in all Examples of Table 3. The achieved green particle size distributions are characterized by parameters such as the ratio of coarse over fine particles, the average diameter of the coarse particle fraction, the average diameter of the fine particle fraction and the breadth of the fine particle fraction. In addition, the fraction of solid particles as determined by SEM is indicated when differing from 100%. The results indicate that more hollow particles are obtained at 40% solid loading than at lower solid loading. The spray-dried coarse particle fraction has been fired to 1300° C.; Table 3 also lists the average diameter of the pre-reacted particles.
The impact of the spray-drying parameters on green particle size distribution is illustrated graphically in
Many Examples of green particle compositions were made by spray-drying and are presented in Table 4. Exemplary embodiments of combinations of spray-dried batch constituents include fine alpha alumina or boehmite with 1.5 to 15% silica, alumina with sinter additives such as oxides of B, Mg, Y, Fe, etc., alumina/silica with different sinter additives B-, Mg-, La-, Y-, Fe-oxide, etc., titania with various levels of silica, and feldspar-based compositions. Aluminum titanate-feldspar composite compositions (full AT batch of inorganics), a small deficiency in alumina or silica or (alumina+silica) from full AT batch, and some spray-dried full batch compositions containing sintering aids such as lanthanum oxide, according to exemplary embodiments, were also made.
Different binders, dispersants, surfactants and other organic additives may be added to the inorganic spray-drying batch. Table 5 shows compositions having additions of Triton™ X100 (SIGMA-ALDRICH®), Duramax™ D-3005 (THE DOW CHEMICAL COMPANY®), an ammonium salt of a polyelectrolyte, or Darvan-C® (R.T. VANDERBILT COMPANY, INC.), an ammonium polymethylacrylate, at levels of 1-5% to the 97% alumina/3% silica inorganic batch mixtures. In some cases, antifoaming agent (Antifoam A) was added together with Triton to avoid foaming. Spray dryer settings of 1.5 atm pressure, 65% flow, inlet temperature 300° C., outlet temperature 120° C. and a 1.5 mm tip were used. Spray-drying of compositions with the different levels of Darvan or Duramax and with small additions of Triton produced a similar spray-dried median particle size under the same spraying conditions. Smaller particle size was produced for higher Triton/Antifoam-A levels, due to a promoted agglomeration and formation of small particles in presence of the antifoaming agent as shown in
Duramax levels of 3-5% to the spray-dried alumina-based batch provided preferred particle size, shape and surface texture. The surfactant Triton may produce fine porosity in the spray-dried particles; however, no fine porosity was observed. Darvan is a dispersant that may be used in many types of ceramic slurries. No advantage in phase distribution was observed in the spray-dried particles by the addition of Darvan. At high concentration, Duramax acts as a dispersant and binder; it may introduce porosity in the spray-dried particles. These examples of binders, dispersants and surfactants are not intended to be a complete list. One of ordinary skill in the art would know that other organic binders, dispersants, and surfactants, such as carboxymethylcellulose, acrylic binders, polyethylene glycol (PEG), or polyvinyl alcohol (PVA) may be used for the same purpose.
Green spray-dried particles in the batch did not survive mixing and extrusion shear forces independent of composition and type of organic addition. This was clearly demonstrated by examining microstructure and porosity of extruded and fired parts. Porosity and median pore size of materials made with green spray-dried particles or charred spray-dried powders did not reach the porosity and pore size of standard materials that were made with commercial coarse alumina, see Table 6.
Aluminum titanate type batches with spray-dried alumina/3% silica/organic binder batch materials (97% alumina (A1000), 3% silica (Ludox) as inorganics and superaddition of surfactant) together with porosity and pore size for extruded, fired material (1410° C., 15 h) are shown in Table 6. Green, charred, or pre-fired spray-dried powders were used; spray-dried powders were made without any binder, with Triton and Antifoam, Darvan, or Duramax. The spray-dried particles were added in a batch with batch composition silica—8.78%, strontium carbonate—8.1%, calcium carbonate—1.4%, titanium dioxide—30.32%, spray-dried particles (green, charred or pre-fired)—51.2%, lanthanum oxide—0.2%, superaddition of potato starch (PS)-15%, superaddition of Methocel—4.5%. The Examples were subjected to twin-screw type mixing and 1 inch (2.54 cm) ram extrusion. Firing condition was 1410° C./15 hrs.
Example spray-dried powders were pre-fired to improve their mechanical strength so that they would survive extrusion. Pre-firing conditions were varied and covered temperatures from 1000° C. to 1600° C. and different durations from fractions of an hour (in an industrial rotary calciner) to many hours, so that the particles were not reacted, partially reacted, or fully reacted and correspondingly not densified, partially densified, or fully densified. Particle size and shape were largely preserved during pre-firing.
SEM images in
The phases present in the fired ceramic parts were identified by X-ray diffraction (XRD). A Philips PW1830® diffractometer (Co Ka radiation) was used for X-ray diffraction. Spectra were typically acquired from 20 to 100°. Rietveld refinement was used for quantification of the phase contributions.
Standard scanning electron microscopy, SEM, characterization was conducted on green and fired spray-dried particles and their polished cross-sections, on honeycomb wall surfaces and polished honeycomb wall cross sections. For the observation of polished sections, the fired ware was infiltrated with epoxy, sliced and polished. The spatial distribution of porosity and phases in presence at a microscopic level was visualized on polished sample cross sections. High resolution SEM was used to assess details of the microstructure and the phase distribution. Chemical composition of the different phases and elemental distributions were obtained from (qualitative) analysis and elemental mapping by energy dispersive X-ray spectroscopy on the SEM.
Porosity, median pore diameter and pore size distribution were determined by mercury intrusion porosimetry from an Autopore™ IV 9500 porosimeter with software from Micromeritics®. The mercury intrusion porosimetry method uses the capillary law with non-wetting liquid and cylindrical pores as may be expressed with the Washburn equation D=−(1/P)4y Cos Φ, where D is the pore diameter, P is the applied pressure, y is the surface tension and Φ is the contact angle. The volume of mercury is directly proportional to the pressure. Data reduction used the differential and log differential to calculate the first derivative of the cumulative specific intrusion volume as a function of calculated log diameter. Mercury porosimetry was used to calculate the permeability. Permeability is the gas flow rate through a material under applied pressure. In the Autopore device, pressure is increased and the mercury fills smaller and smaller pores until a critical pressure is reached where the mercury spans the sample, as may be expressed as k [in millidarcys]=1/226 (Lc)2 σ/σo, where σ is the conductivity at length Lc and σo is the conductance in the pore. The mercury porosity data can further be used to deduce a tortuosity. The tortuosity factor characterizes the efficiency of gas interaction with the surface during its transport through a porous medium. Tortuosity is strongly dependent on the pore interconnectity. The gas interaction with the material internal surface is greater the larger the tortuosity factor is. J. Hager derived an expression for material permeability based on a capillary bundle model in which pores are homogeneously distributed in random directions. Using the Hagen-Poiseuille correlation for fluid flow in cylindrical geometries, and making substitutions with measurable parameters, and combining with Darcy's law, an expression can be derived for material permeability in terms of total pore volume, material density, pore volume distribution by pore size, and material tortuosity. The total pore volume, material density, and pore volume distribution by pore size are obtainable from mercury porosimetry tests. Katz and Thompson also derived an expression for material permeability based on measurements obtainable from mercury porosimetry and which does not depend on knowledge of material tortuosity. Combining the Hager and Katz-Thompson expressions provides a means for determining tortuosity from data collected by mercury porosimetry.
Thermal expansion was measured for bar-shaped samples with dimension 0.25″×0.25″×2″ (0.623×0.623×5.08 cm) during heating from room temperature to 1200° C. at a rate of 4° C./min and subsequent cooling to room temperature. Unless otherwise noted herein, the longitudinal axis of the test bars was oriented in the direction of the honeycomb channels, thus providing the thermal expansion in the axial direction of the honeycomb parts. Unless otherwise noted herein, room temperature as stated herein refers to 25° C. Average thermal expansion coefficients for various temperature ranges are listed in the tables and in the text are CTE20-800 in K−1, the average thermal expansion coefficient from room temperature to 800° C., defined as L(800° C.)−L(20° C.)/780° C. as average thermal expansion coefficient in the temperature range from room temperature to 800° C., CTE20-1000 in K−1, the average thermal expansion coefficient from room temperature to 1000° C., defined as L(1000° C.)−L(20° C.)/980° C. as average thermal expansion coefficient in the temperature range from room temperature to 1000° C., CTE500-900 in K−1, the average thermal expansion coefficient from 500 to 900° C., defined as L(900° C.)−L(500° C.)/400° C. as average thermal expansion coefficient in the temperature range from 500° C. to 800° C.
The strength of the ceramics was tested using a transverse bending technique where test specimens were loaded to failure in using either three or four bending points. The maximum stress prior to failure is often referred to as the modulus of rupture or MOR. MOR, measured using four point flexure with a lower span (L) of two inches (fifty and four fifths millimeter) and an upper span (U) of three quarters of an inch (nineteen millimeters). The specimen geometry for the 4-point flexure tests was two and one half inches (sixty three and one half millimeters) in length, one half inch (twelve and seven tenths millimeters) in width (b) and one quarter inch (six and two fifths millimeters) thick (d). The force-measuring system used was equipped with a read-out of the maximum force (P) and a calibrated load cell. The MOR value was calculated using the ASTM flexure strength equation for a rectangular specimen. All specimens tested had a square cellular (honeycomb) structure with the channels in the direction of the length of the honeycomb. The actual material strength independent of the structure of the body, often referred to as the wall strength, was determined by modifying the traditional MOR equation to account for the cellular structure of the honeycomb test bar, using ASTM standard C1674-08.
Bar-shaped samples with dimension 5″×1″×0.5″ (12.7×2.54×1.27 cm) and the longitudinal axis being oriented in the direction of the honeycomb channels were used to measure the elastic modulus by flexural resonance frequency. Samples were heated to 1200° C. and cooled back to room temperature. For each temperature the elastic modulus was directly derived from the resonance frequency and normalized for sample geometry and weight per ASTM C 1198-01.
Referring back to Table 6, materials made from spray-dried batch powders that were obtained with various levels of different dispersants, surfactants and binders are shown. The AT batch included 8.78% silica, 8.1% strontium carbonate, 1.4% calcium carbonate, 30.32% titanium dioxide, 51.2% of the spray-dried green compositions of Table 6 (alumina/silica), 0.2% lanthanum oxide, 15% superaddition potato starch, 4.5% superaddition methocel. The spray-dried powders were incorporated in the batch as green (as-spray-dried), charred, or pre-reacted powders. Table 6 compares porosities of fired extruded (300/13) materials made from batches with commercial raw materials, green spray-dried particles, charred spray-dried particles and pre-fired spray-dried particles. A batch with AT-type inorganic composition and pore former package was used, in which alumina and a small fraction of silica were replaced by spray-dried batch powder of same composition. Batch additions are listed in Table 6 together with the resulting porosity of the extruded fired material. The batch was subjected to twin-screw type mixing and 1″ (2.54 cm) ram extruded. The dried extruded articles were fired at 1410° C./15 hr.
The spray-dried powders were either not pretreated (green), charred in nitrogen to 500° C., or pre-reacted by firing in air for 5 h at 1300° C. A standard AT batch Comparative Example with commercial particulate alumina A10 with 10 μm median particle size extruded and fired under the same conditions produced about 50% porosity and 15 μm pore size.
Mercury intrusion porosimetry showed that green, unfired spray-dried powders (without any surfactant addition) produced slightly lower porosity (48%) with significant smaller median pore size, d50=9 μm than the Comparative Example. Charring the spray-dried particle in nitrogen to 500° C. promoted further loss in porosity with a resulting 43% porosity, 9 μm pore size. In contrast, for the pre-fired spray-dried particles, the porosity was increased to 54.4% with median pore size of 22 μm. The gain compared to use of commercial coarse alumina raw material was a 4-5% increase in porosity and 6-7 μm increase in pore size.
Materials obtained from green or charred spray-dried batch materials with various organic additions (Darvan, Duramax, Triton) showed little change in porosity, indicating that green and charred spray-dried particles do not survive the shear during twin screw-like mixing and extrusion. Use of spray-dried, pre-fired particles containing Darvan or Duramax yielded a significant increase in porosity and pore size. Both, porosity and pore size increased with the amount of Darvan or Duramax in the slurry. For 5% addition, 53% and 58% porosity, 21 μm and 23 μm median pore size were obtained with 5% Darvan and Duramax, respectively. This is a significant gain in porosity (3% and 8%, respectively) and a significant gain in median pore size (5 and 7 μm, respectively). It may be very difficult to obtain such high porosity and large pore size with commercial raw materials at such low pore former level.
Regular and inverse pore structures (see
Materials with high porosity and regular porosity morphology were also obtained by exemplary embodiments of spray-dried composition and firing schedule.
Some Examples were made by using two pre-reacted batch materials, such as Example No. H4 from pre-fired spray-dried AlSi Example No. 78 particles and pre-fired spray-dried TiSi Example No. 111 particles.
High porosity Example H5 with 57% porosity after firing to 1427° C./15 h was made from the same spray-dried powder compositions as Example H4, but the spray-dried alumina-based powder had been fired to higher pre-firing temperature.
Properties of Examples having spray-dried, pre-fired alumina/silica batch material are summarized in Tables 11-13. The Examples were obtained by 1″ and 2″ ram extrusions as indicated in Tables 11-13, and fired to the indicated temperature. It can be seen that the Examples cover a wide porosity and pore size range.
For constant inorganic batch composition and use of spray-dried alumina/3% silica, it was shown that the use of organic additions in the spray-drying slurry had an impact on the spray-dried particle shape and the porosity of the pre-fired spray-dried particles. Table 14 shows that use of larger amount of organic binder in the spray-drying slurry produces fine porosity in the pre-fired spray-dried particles and contributes in the final batch to an increase in porosity. Type and quantity of the organic used in the spray-drying slurry do not only have an impact on the porosity in the final material, but also affect the microstructure. Porosity in the pre-fired spray-dried particles allows faster transport and matter exchange during the reaction so that different microstructures are obtained. Phase distribution and grain size in the reacted microstructures control the level of microcracking and the microcrack distribution and thus impact the thermal expansion of the final material.
Material properties of Examples that were obtained by spray-drying and pre-firing close to full inorganic batch mixture are summarized in Tables 15 and 16. Table 15 shows 1″ ram extrusions and Table 16 shows 2″ ram extrusions. Comparative Example PDG of AT made from commercial, non-spray-dried batch materials is also presented.
Table 17 presents exemplary Examples showing the evolution of honeycomb physical properties as function of honeycomb firing temperature for Batch including Particle Example No. 42 (Table 4, fired at 1300° C.), 15% potato starch (PS), and 8% graphite pore formers.
A comparison of Example 2″ filters made from pre-fired, spray-dried materials W11, W15, and W10 (Table 11) with Comparative Example AT (CW7) made with commercial raw materials was conducted. The fired honeycombs were plugged with cement to provide a bare filter. All filters had close geometries; for better comparison, data were normalized to common filter length 6″ (15.24 cm), diameter 2″ (5.1 cm), and wall thickness 13 mil (0.33 mm) in geometry (300/13). The three spray-dried Examples include normal and inverse porosity materials. The bare pressure drop of the Example filters made from spray-dried materials was found to be lower than the Comparative Example filter. Coated pressure drop was observed to follow this trend.
According to exemplary embodiments of the disclosure, 8% gain in porosity were demonstrated over Comparative Samples having the same composition. Inverse porosity with 63% porosity and above and median pore size of 15 μm and more were demonstrated. Sinter-bonded, reaction-bonded materials show less microcracking than Comparative Samples, with little or no CTE hysteresis and CTE in the range of 20-30×10−7K−1. For certain embodiments of spray-dried compositions, pre-firing and firing conditions, enhanced microcracking and lower CTE were achieved. CTE <10×10−7K−1 was achieved for several materials. Example No. H7 made from alumina/6% silica spray-dried batch material produced CTE=7×10−7K−1 at 58% porosity and median pore size 15 μm; Example Nos. H20 and H21 with spray-dried alumina/silica showed CTE=13×10−7K−1 at 60% porosity and median pore size 19 μm. Example No. H82 with 60% porosity had a CTE of 9.5×10−7K−1.
Alumina with different levels of silica from 3%-18% or silica and lanthanum was spray-dried and pre-fired at various temperatures in the Examples. Materials with high porosity (55%-61%) and with pore size of 16-20 μm were obtained with CTE of 13-15×10−7 K−1. While the porosity was unaffected by the silica level, the median pore size decreased with increasing silica content. The median pore size of the AT ware was affected by the spray-dried particle size, exhibiting a decrease for particle sizes below 15 μm.
Example articles were ram extruded as 2″ parts and fired under low oxygen pressure for polymer burn out, followed by firing in air. Bare filter performance was tested. Pressure drop measurements of bare filters showed a 27% decrease in pressure drop compared to Comparative Examples of AT-type compositions with the same filter geometry. Filtration efficiency of parts with more than 60% porosity and large pore size was similar to that of Comparative Examples of AT-type compositions with 50% porosity and 15 μm median pore size.
Additional Examples of exemplary embodiments of the disclosure comprising cordierite spray-dried, pre-fired raw materials were made.
Table 18 shows material properties of Examples Cor1 and Cor2. The Examples showed very little microcracking. Example Cor1 had a CTE of 14×10−7K−1 (RT to 800° C.) and Example Cor2 had a CTE of 16×10−7K−1 (RT to 800° C.). The MOR of Example Cor1 was 740 psi and the MOR of Example Cor2 was 1130 psi.
Thus, exemplary embodiments of the disclosure provide higher porosity and larger pore size porous ceramic articles by use of pre-reacted particles compared to standard powder raw materials. Porosity above 55% or even above 65% can be achieved with median pore sizes in a range between 10 and 30 micrometers. The exemplary process provides filters with porosity of 60% and more, with median pore size of 20 μm or more, at relatively low cost and with control of raw material and pore former particle size and size distribution. Exemplary embodiments of the disclosure enable use of large size particles, narrow particle size distributions, mechanically robust, and combinations of one or more batch materials pre-reacted to obtain an advantageous batch material packing with large pores and high porosity that can be preserved during firing to result in higher porosity, larger pore size materials. The larger the particles and the more homogeneous in size, the larger are porosity and pore size in the porous ceramic article. Broad particle size distributions that have a negative impact on porous ceramic article properties by producing broad pore size distributions and thus low material strength can be avoided.
The exemplary embodiments of the disclosure enable high porosity and large pore size in porous ceramic articles at reasonable cost. For example, spray-dried porous alumina-based, pre-fired batch materials can be made at considerable cost savings with tailored property advantages over other sources of large particle size alumina with a narrow particle size distribution. Spray-dried particles of narrow particle size distribution produce a natural low density packing. Both spray-drying and rotary-calcining are high throughput, low cost industrial processes that can be used to engineer the required batch materials.
The exemplary embodiments of the disclosure enable the reduction in the levels of pore former for comparable porosity of a porous ceramic article and the probability of concomitant firing cracks. Generally, in articles made with powder batches, high porosity can be created only by use of high pore former levels, which require long firing cycles to accommodate the pore former burnout exothermic and endothermic events and also increase the risk of forming firing cracks. Thus, exemplary embodiments of the disclosure enable faster firing times.
The exemplary embodiments of the disclosure enable low levels of microcracking, low CTE, and high strength in porous ceramic articles. The exemplary embodiments of the disclosure enable inverse porosity having large pore necks and small material necks compared to general powdered batch reaction-sintered material that forms an interconnected pore structure with small necks. Small necks may limit permeability and gas flow and control the pressure drop. Materials with very large necks in the pore structure produce improved permeability and thus provide low pressure drop filters.
According to exemplary embodiments of the disclosure, pre-reacted particles can be made in a wide range of sizes and compositions that can contain single or several batch components. According to exemplary embodiments of the disclosure, when several components are combined as a tight mixture in spray-dried particles and pre-fired, the spray-dried powder mixture can be reacted to an intermediate product or product mixture during pre-firing that, during firing of the extruded batch, promotes another reaction path and different final phase distribution, grain size or, in short, different microstructure than a mixed powder batch of the same composition.
The exemplary embodiments of the disclosure enable the structuring of the extruded batch with mixed spray-dried particles not fully reacted to intermediates during pre-firing to act as small batch reactors and induce different reaction paths and yield different engineered microstructures than a mixed powder batch of the same composition.
The exemplary embodiments of the disclosure enable the use of pre-reacted batch constituents to control the final microstructure, its coarseness and phase distribution. Exemplary embodiments of the disclosure enable the use of pre-reacted materials to better control firing and fired properties. Exemplary embodiments of the disclosure also enable the contribution of reaction-related thermal events to be suppressed or decreased, and reaction-related shrinkage events to also be suppressed or decreased, for example, when using fully reacted spray-dried, pre-fired batch materials.
Reference throughout this specification to exemplary embodiments and similar language throughout this specification may, but do not necessarily, refer to the same embodiment. Furthermore, the described features, structures, or characteristics of the subject matter described herein with reference to an exemplary embodiment may be combined in any suitable manner in one or more exemplary embodiments. In the description, numerous specific details are provided, such as examples of controls, structures, processes, compositions, articles, etc., to provide a thorough understanding of embodiments of the subject matter. One skilled in the relevant art will recognize, however, that the subject matter may be practiced without one or more of the specific details, or with other methods, components, materials, and so forth. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the disclosed subject matter.
The schematic flow chart diagrams and method schematic diagrams described above are generally set forth as logical flow chart diagrams. As such, the depicted order and labeled steps are indicative of representative embodiments. Other steps and methods may be conceived that are equivalent in function, logic, or effect to one or more steps, or portions thereof, of the methods illustrated in the schematic diagrams. Additionally, the format and symbols employed are provided to explain the logical steps of the schematic diagrams and are understood not to limit the scope of the methods illustrated by the diagrams. Although various arrow types and line types may be employed in the schematic diagrams, they are understood not to limit the scope of the corresponding methods. Indeed, some arrows or other connectors may be used to indicate only the logical flow of a method. For instance, an arrow may indicate a waiting or monitoring period of unspecified duration between enumerated steps of a depicted method. Additionally, the order in which a particular method occurs may or may not strictly adhere to the order of the corresponding steps shown.
It will be apparent to those skilled in the art that various modifications and variations can be made in the present disclosure without departing from the spirit or scope of the claimed invention. Thus, it is intended that the present claimed invention cover the modifications and variations of this disclosure provided they come within the scope of the appended claims and their equivalents.
This application claims the benefit of priority under 35 U.S.C. §119 of U.S. Provisional Application Ser. No. 61/825,251 filed on May 20, 2013, the content of which is relied upon and incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61825251 | May 2013 | US |
