Patent Application
20140061981
The present disclosure relates generally to ceramic filters and methods for manufacturing and using the same.
Ceramic filters, such as diesel particulate filters, may be used to collect or filter out particulate matter having a wide particle size distribution. For example, such filters have been used to collect soot that is exhausted from a diesel engine. Typically, such ceramic filters have a substantially honeycomb configuration, which includes a substantially columnar body of porous ceramic that has a plurality of holes extending in parallel with one another in a length direction of the columnar body. The columnar body typically will have a wall portion interposed between the through holes. These wall portions can serve to filter particulate matter as the exhaust is flowed through the ceramic filter.
The columnar body of the ceramic filter may be formed using, for example, a monolithic approach or a segmented approach. In the monolithic approach, a one-piece honeycomb body forms the columnar body. In the segmented approach, the columnar body may be formed by providing a plurality of smaller, elongated honeycomb segments and then applying a ceramic seal layer between adjacent segments to adhere the segments to one another.
Cordierite and silicon carbide are ceramic materials commonly used in the construction of ceramic filters. However, cordierite and silicon carbide may be disadvantageous. For example, cordierite has a low particulate matter loading capacity due to its low thermal shock resistance from low mechanical strength. Silicon carbide has higher back-pressure that increases due to a larger grain size. In addition, with silicon carbide filters, particulate matter leakage may also be high at an initial stage or regeneration stage, because the trapping performance is low before a cake-like layer is generated due to particulate matter accumulation. Silicon carbide may also not be as good of a catalyst due to its lower specific surface area. Finally, the amount of wash-coating for a silicon carbide filter for catalyzing may also be limited. For example, in some embodiments, 20 g/L is a high amount for the specific surface area.
Regarding the type of material to be used in a monolithic or segmented ceramic filter, cordierite is possible for a monolithic ceramic filter due to the material having lower thermal stress during operation. However, cordierite may not be as appropriate for segmented ceramic filters, due to its lower mechanical strength in the joining segments. On the other hand, silicon carbide is a better material to use for segmented ceramic filters due to its high mechanical strength. However, silicon carbide may not be as appropriate for monolithic ceramic filters due to its higher thermal stress during operation.
In one aspect, a process for manufacturing a ceramic filter includes mixing silicon, yttrium oxide-doped zirconia, magnesium-aluminum spinel, silicon nitride, a pore-forming material, and a binder to form a ceramic precursor; extruding the ceramic precursor into a generally honeycomb shaped monolithic filter precursor or into a single filter tube precursor; drying the filter precursor or filter tube precursor to form a dried ceramic precursor; heating the dried ceramic precursor to remove the binder; and sintering to form the silicon nitride ceramic filter. In some embodiments, the heating the dried ceramic precursor to remove the binder is conducted at a temperature of from about 200° C. to about 500° C. In any of the above embodiments, the silicon nitride ceramic filter includes β-Si3N4, ZrO2(Y2O3), MgO, and Al2O3. In any of the above embodiments, the sintering includes nitriding the silicon at a temperature of about 1300° C. to about 1500° C. in the presence of nitrogen, followed by heating at a temperature of about 1600° C. to about 1800° C. In any of the above embodiments, the ceramic precursor includes silicon from about 20 wt % to about 25 wt %, yttrium oxide-doped zirconia from about 0.1 wt % to about 3 wt %, magnesium-aluminum spinel from about 1 wt % to about 6 wt %, β-Si3N4 from about 15 wt % to about 25 wt %, pore-forming material from about 10 wt % to about 20 wt %, and organic binder from about 35 wt % to about 45 wt %. In any of the above embodiments, the silicon nitride ceramic filter includes β-Si3N4 at greater than or equal to about 93 wt %, yttrium oxide-doped zirconia at less than about 1.5 wt %, and MgO and Al2O3 at less than about 5.5 wt %.
In any of the above embodiments, the process may also include wash-coating the silicon nitride ceramic filter body with a wash-coating that includes aluminum oxide or titanium oxide. In any of the above embodiments, the wash-coating provides a coating of 20 g/L wash-coating or greater. In any of the above embodiments, the wash-coating provides a coating of 40 g/L wash-coating or greater. In any of the above embodiments, the wash-coating provides a coating of 60 g/L wash-coating or greater.
In another aspect, a ceramic filter includes a monolithic or composite body comprising β-Si3N4 and about 20 g/L or more of a catalyst support coating on the surface of the β-Si3N4.
In another aspect, a porous ceramic body includes a plurality of pores, wherein at least 10% of the plurality of pores have an average diameter of 10 μm or less. In some embodiments, the porous ceramic body is constructed from silicon nitride. In some such embodiments, the silicon nitride is β-Si3N4.
The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other features, aspects, and advantages of the disclosure will become apparent from the description, the drawings, and the claims.
Various embodiments are described hereinafter. It should be noted that the specific embodiments are not intended as an exhaustive description or as a limitation to the broader aspects discussed herein. One aspect described in conjunction with a particular embodiment is not necessarily limited to that embodiment and may be practiced with any other embodiment(s).
As used herein, “about” will be understood by persons of ordinary skill in the art and will vary to some extent depending upon the context in which it is used. If there are uses of the term which are not clear to persons of ordinary skill in the art, given the context in which it is used, “about” will mean up to plus or minus 10% of the particular term.
The use of the terms “a” and “an” and “the” and similar referents in the context of describing the elements (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein may be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the embodiments and does not pose a limitation on the scope of the claims unless otherwise stated. No language in the specification should be construed as indicating any non-claimed element as essential.
A ceramic filter formed from silicon nitride is provided. Such filters exhibit high mechanical strength and shock resistance. Porous silicon nitride may be structured with controlled small grain crystals of elongated hexagonal systems. This enables high particulate matter filtration efficiency with low back pressure increases, thereby providing for higher filtering efficiencies for small particulate matter. In addition, due to the controlled micro-structure of the silicon nitride, a larger amount of wash-coating of catalysts to the surfaces of the filter may be applied at lower back pressure increases than in conventional ceramics. A higher capacity of wash-coating of a catalyst support provides the potential for superior catalyst efficiency at lower concentrations of precious metals being used as the active catalyst components. With regard to manufacturing cost, silicon nitride provides another advantage, as a monolithic filter may be readily manufactured. According to another aspect, a method of preparing the filter includes a post-reaction sintering process is that enables the use of metal Si that is lower in cost than Si3N4 powder for raw material. Other advantages may be achieved, as described herein.
In one aspect, a ceramic filter is provided having a one-body (i.e. monolithic) structure. The monolithic structure may include a plurality of through-holes running in parallel from a proximal end of the monolithic structure to a distal end of the monolithic structure. A portion of the through-holes may be obstructed, or “plugged,” at one or both of the proximal or the distal end. The monolithic structure may be of a convenient, cross-sectional shape. For example, cross-sectional shapes may include, but are not limited to, cylindrical, hexagonal, or octagonal shapes. In any of the above embodiments, the ceramic filter may be constructed of silicon nitride (SiN; Si3N4). In some embodiments, the monolithic structure of a ceramic filter appears in a near-net shape. An outer surface of the ceramic filter may be machined to control the diameter of the filter. Throughout the ceramic filters, the silicon nitride has a high surface area that may, optionally, include a coating.
As used herein, “near net shape” refers to a shape having minimum machining thickness. For example, while the monolithic embodiments are produced in the shape of the final product, while the segmented filter embodiments prior to machining are a rectangular parallelepiped. Thus, the monolithic is near-net shape, while the segmented filters are not.
In another aspect, a ceramic filter is provided having a plurality of segmented filters that are secured together by an adhesive to form a composite filter body. Each of the segmented filters may include a plurality of through-holes running in parallel from a proximal end of the segmented filter to a distal end of the segmented filter. A portion of the through-holes may be obstructed (i.e. “plugged”) at one or both of the proximal end or the distal end. For example, one end may be obstructed, while the other remains unobstructed. The segmented filters may be of a convenient shape to be accumulated in the composite filter body. For example, the shape of each individual segmented filter may be cylindrical, hexagonal, or octagonal shape. The individual segmented filters may tit together to provide the ceramic filter having an overall cylindrical, hexagonal, or octagonal shape. The adhesive joining the individual segmented filters together may be a mixture of cordierite and aluminum oxide, or other adhesive. In any of the above embodiments, the surfaces (including all interior surfaces, i.e. the surface area) of the ceramic filter may be coated. In one embodiment, the surfaces of the ceramic filter may be coated with a mixture of silicon oxide and silicon nitride. The segmented filters may be constructed of silicon nitride.
In one aspect, a ceramic filter is provided. The filter may be a monolithic or composite body of β-Si3N4. The filter may also include a catalyst support coating on the β-Si3N4. The support coating may be present at about 20 g/L. In other words, about 20 g of the catalyst support coating is added for every about 1 L of ceramic material. In some embodiments, the catalyst support coating on the surface of the β-Si3N4 is present at about 40 g/L or more. In other embodiments, the catalyst support coating on the surface of the β-Si3N4 is present at about 60 g/L or more. The catalyst support coating may include materials such as aluminum oxide and titanium oxide.
In another aspect, a porous ceramic body is provided. The body includes a plurality of pores, where at least 10% of the pores have an average diameter of 10 μm or less. In some embodiments, the ceramic body is constructed of silicon nitride. In other embodiments, the silicon nitride is β-Si3N4. These and other embodiments and/or aspects will be further described by reference to the figures included herewith.
In one embodiment, a portion of the through-holes 120,121 may be obstructed at either the proximal end 110,111 or at the distal end 140,141. In some such embodiments both ends may be open, one end or the other end is obstructed, or both ends may be obstructed. Alternatively, the proximal end 110,111, or the distal end 140,141 may be substantially obstructed or substantially open. As used herein, obstructed is intended to mean that the through-hole is plugged to free passage, although due to micro-structure of the ceramic it may be gas or liquid permeable. As used herein, “substantially” means that the through-hole is less than 100% obstructed due to imperfections, but it was intended to be obstructed.
As will be observed from
In the case of either the monolithic or composite ceramic filter bodies, the ceramic may be silicon nitride. Silicon nitride has a mechanical strength that is superior to cordierite by approximately 200%, as measured by a compression strength method. The monolithic ceramic files of silicon nitride also has a lower cost of manufacture than those of silicon carbide (SiC). However, the composite filters have the advantage of lower mechanical stress in the diameter axis (i.e. latitudinal direction) because the bonding materials(adhesives) act as a stress absorber.
Generally, silicon nitride is a thermally resistant ceramic, exhibiting high mechanical strength. Furthermore, porous silicon nitride may be structured with controlled small grain crystals having an elongated hexagonal shape. This enables high particulate matter filtration efficiency with low back pressure, as well as increased and higher filtering efficiencies for smaller particulate matter. Also, due to the micro-structure of silicon nitride, a larger amount of wash-coating may be applied with lower back pressure when compared to conventional materials. A higher capacity of wash-coating provides superior catalyst efficiency with a lower amount of precious metals used as active components of the catalyst. In terms of thermal stress levels during operation of a ceramic filter, silicon nitride is comparable to cordierite, the typical industry standard material at this point in time due to its lower thermal expansion and lower elastic modulus.
The material that is used to construct the ceramic filters heavily influences filter performance. For example, filtering performance of the particulate matter and the durability of long term operation of the ceramic filter are greatly affected by the ceramic that is used. Basic performance requirements include, but are not limited to, (1) thermal resistance to particulate matter burning; (2) mechanical strength against cyclic thermal stress; (3) chemical stability against loaded materials such as particulate matter and ash; and (4) sufficient porosity for particulate matter filtering. It has been determined that silicon nitride may be configured to provide a highly advantageous ceramic filter.
Silicon nitride can provide a columnar micro-structure that allows for the achievement of certain advantages.
The micro-structure of silicon nitride allows it to capture more particulate matter than silicon carbide. For purposes of comparison,
Silicon nitride also exhibits a more desirable pore distribution than either silicon carbide (SiC) or cordierite as illustrated in
Due to the micro-structure of silicon nitride, there is not a distinct correlation between particulate matter trapping performance and back pressure increase and not a distinct relationship based on pore volume and wall thickness. The micro-structure of conventional ceramic filter materials such as silicon carbide, cordierite and silicon nitride are quite different. Therefore, pore distribution and its effect on filter operation may be unrelated concepts. For the micro-structure of silicon nitride, the pore distribution under 10 μm is more important than a large size pore distribution over 25 μm. A pore distribution of under 10 μm may cause the effect of increasing lower back-pressure during particulate matter accumulation. It appears that micro-pores under 10 μm may not be ventilated after the main pores are clogged by particulate matter accumulation. Furthermore, the second peak under 10 μm is a result of the process of the present disclosure, and indicates that the pore volume should be kept under 10 μm. In some embodiments, one feature of silicon nitride is a large amount of pores under 10 μm. This has advantages such as, for example, being able to apply large amounts of wash-coating for catalyzing and also being able to ventilate the filter in order to reduce the increasing of back pressure.
Silicon nitride may be configured to provide relatively low particulate matter leakage.
Silicon nitride provides a slower increase in back pressure than compared to silicon carbide.
Silicon nitride, also provides a ceramic filter with improved catalyst loading properties.
The use of silicon nitride is particularly advantageous with regarding to wash-coating.
As illustrated by the graphs in
Due to the use of silicon nitride in ceramic filter fabrication, it is not necessary to be concerned with issues such as seal layer thickness and thermal conductivity to achieve a high level of combustion performance in regeneration. Silicon nitride has a lower thermal volume material than, for example, silicon carbide. Accordingly, silicon nitride has very good combustion performance in regeneration. The micro-structure of silicon nitride enables higher porosity with same level of strength. Silicon nitride has lower specific heat and higher porosity, and that combination enables lower thermal volume. Lower thermal volume can provide good burning performance, and leaves little unburned material.
Table 1, below, provides various properties of silicon nitride, silicon carbide, and Cordierite used to make ceramic filters. The table shows the values of thermal expansion, Young's Modulus, the Temperature Gradient and the Thermal Stress per Unit.
aThermal expansion was calculated honeycomb specimen (2 cell × 2 cell × 20 mm).
bYoung's modulus was measured by honeycomb specimen (3 cell × 2 cell × 40 mm)
cTemperature gradient was measured in regeneration test with honeycomb specimen (35 × 35 × 150 mm)
dThermal stress per unit was calculated as [thermal stress] × [Young's modulus] × [temperature gradient]
Table 2 lists the porosity (%), compressive strength (MPa), thermal expansion coefficient, Young's modulus, and thermal conductivity for silicon nitride (Si3N4), silicon carbide and cordierite.
As is shown, silicon nitride yield a low thermal stress, good thermal conductivity and high strength or thermal stress. As a result, silicon nitride has a high limitation on particulate matter accumulation, low particulate matter leakage, strong thermal strength, low back pressure, and may be conveniently and efficiently be implemented in monolithic structure.
Silicon nitride may be configured to provide improved strength.
Silicon nitride also may be used to achieve better characteristics related to ash accumulation. Table 3 shows the particulate matter ratio (or particulate matter collecting efficiency) and the ash accumulation. As may be seen from the table, the ash accumulation properties for silicon nitride are better than silicon carbide, even though the particulate matter collecting efficiency may be at similar levels.
a“Good” refers to no physical damage upon visual inspection.
Silicon nitride has unique properties to provide good properties related to ash accumulation. Silicon nitride has excellent corrosion resistance against ash components such as CaSO4 and CaCO3. If the silicon nitride includes magnesium (Mg) as part of the composition from the sintering additives, it should have additional effect to increase the corrosion resistance against ash. Magnesium may be provided by using MgAl2O4 as one of sintering additives. Silicon nitride has a lot of micro-pores under 10 μm, so that the contacting section(area) is lower than large grain size materials such as silicon carbide. Both the corrosion resistance of silicon nitride and large amount of micro-pores in the micro-structure of silicon nitride have the effect of reducing ash accumulation. The present configuration does not require any specific relationship between the length and width of cells and the surface roughness of the cell walls to achieve the desired ash accumulation characteristics, as may be required with other materials.
As noted above, also provided is a process for manufacturing a ceramic filter. The process also include preparation of a silicon nitride ceramic filed via a post-reaction sintering process. The process includes mixing silicon, yttrium oxide-doped zirconia, Mg—Al spinel (Mg Al2O4), silicon nitride, a pore-forming material, and a binder to form a ceramic precursor, extruding the ceramic precursor into a generally honeycomb shaped monolithic filter precursor or into a single filter tube precursor, drying the filter precursor or filter tube precursor to form a dried ceramic precursor, heating the dried ceramic precursor to remove the binder (i.e. “debindering”), sintering to form the silicon nitride ceramic filter (i.e. “post-reaction sintering”). The silicon nitride that is mixed in the process acts as a core of the crystal block after sintering.
According to various embodiments, the pore-forming material may be any kind of organic particle. According to various embodiments, the drying may be conducted in a microwave dryer. According to various embodiments, the debindering may be conducted at a temperature of from about 200 to about 500° C. The silicon nitride ceramic filter contains (β-Si3N4, ZrO2(Y2O3), MgO, and Al2O3.
According to various embodiments, the sintering includes two steps at different temperatures. A first step of nitriding of the silicon is conducted at a temperature of about 1300° C. to about 1500° C. in the presence of nitrogen. The initial nitriding produces an α-Si3N4. A second step of porosity control is then conducted at a temperature of about 1600° C. to about 1800° C., and at this temperature the α-Si3N4 is converted to β-Si3N4. The sintering provides a dense body of silicon nitride and the other materials, and provides the silicon nitride in the elongated columnar crystals described above.
In the ceramic precursor, the silicon is present from about 20 wt % to about 25 wt %, the yttrium oxide-doped zirconia is present from about 0.4 wt % to about 3 wt %, the β-Si3N4 is present from about 15 wt % to about 25 wt %, the pore-forming material is present from about 10 wt % to about 20 wt %, and the organic binder, as well as water, if present, is present from about 35 wt % to about 45 wt %. The final silicon nitride product contains the β-Si3N4 at greater than or equal to about 93 wt %, yttrium oxide-doped zirconia at less than about 1.5 wt %, and MgO and Al2O3 at less than about 5.5 wt %.
The process may also include a cell-plugging step, where any given cell is plugged or obstructed at one end. The cell-plugging thus results in a checkerboard like pattern on the proximal or distal faces of the segment or the monolithic form. The materials may then again be dried and subjected to a heat treatment. Where the ceramic is a segment for forming a composite ceramic filter, the segments are then bonded together in a bonding step by providing a layer of adhesive between the segments and then heating them together to bond them. In some embodiments, the adhesive includes cordierite and aluminum oxide. Additional heat treatment and machining may then be conducted on the ceramic body. This process may be further explained by reference to the figures, and the following description.
The mixed materials are then extruded in step 2606 into segments with a generally honeycomb shape. The extruded segment may be, for example, a rectangular or prism-like shape. The segment preferably has round, chamfered corners. In some embodiments, the segmented corner has a radius of greater than 2.6 mm. In conditions of 260 cpsi, the radius of the segmented corner R is over 2.6 mm, and the three cells at the corner may have deformation and be reduced in cell area. Silicon nitride has, however, lower back pressure during operation and such a reduction of cell area has no deleterious effects. Actually, a larger radius of greater than 2.6 mm has a good effect in increasing the mechanical strength of bonded segments.
After extrusion, the segment is subjected to a drying step 2608 (using a micro-wave dryer) and a de-bindering step 2610 (e.g., at 200-500° C.).
The segment is subsequently subjected to a sintering step 2612 (or post-reaction sintering). A unique sintering process known as “post reaction sintering” has been adopted. It can include two steps of temperature zones for: (1) nitriding metal silicon (1300-1450° C.) and (2) porosity controlling (1650-1800° C.). The post reaction sintering may be utilized to develop a dense body, and may also be modified to develop a porous body in order to generate elongated columnar crystals.
Following the sintering step 2612, a cell-plugging step 2614 is conducted in a conventional fashion. Typically, each cell will be plugged on only one end. The cell-plugging results in a checkerboard like pattern on the end of the segment. Next, a drying step 2616 (e.g., at 60° C.), and then a heat treatment step 2618 (e.g., at 200° C. at 4 hours) are conducted.
The segments are then bonded together in a bonding step 2620. Typically a sealing layer of adhesive is disposed between the segments to bond them together.
A further heat treatment step 2622 (e.g., 200° C. at 4 hours) is then conducted, followed by a pre-plugging step 2624 on the machined sections. Certain cells may be pre-plugged with, for example, a sealing layer material. The cells that will form the outer surface of the assembly after outer machining, are filled with the sealing layer material. As a result, when the outer machining is complete, the outer surface will be essentially uniform and leakage will be decreased.
A further heat treatment step 2626 (e.g., at 200° C. at 4 hours) is then conducted, followed by an outer machining step 2628. The outer machining may be performed, for example, by a turning lathe. Thereafter, the outer surface is painted in a painting step 2630 and the assembly is subjected to a final heat treatment step 2632 (e.g., at 650° C. at 4 hours). An optional inspection step 2634 may be conducted.
While certain embodiments have been illustrated and described, it should be understood that changes and modifications may be made therein in accordance with ordinary skill in the art without departing from the technology in its broader aspects as defined in the following claims.
The embodiments, illustratively described herein may suitably be practiced in the absence of any element or elements, limitation or limitations, not specifically disclosed herein. Thus, for example, the terms “comprising,” “including,” “containing,” etc. shall be read expansively and without limitation. Additionally, the terms and expressions employed herein have been used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the claimed technology. Additionally, the phrase “consisting essentially of” will be understood to include those elements specifically recited and those additional elements that do not materially affect the basic and novel characteristics of the claimed technology. The phrase “consisting of” excludes any element not specified.
The present disclosure is not to be limited in terms of the particular embodiments described in this application. Many modifications and variations may be made without departing from its spirit and scope, as will be apparent to those skilled in the art. Functionally equivalent methods and compositions within the scope of the disclosure, in addition to those enumerated herein, will be apparent to those skilled in the art from the foregoing descriptions. Such modifications and variations are intended to fall within the scope of the appended claims. The present disclosure is to be limited only by the terms of the appended claims, along with the full scope of equivalents to which such claims are entitled. It is to be understood that this disclosure is not limited to particular methods, reagents, compounds compositions or biological systems, which can of course vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting.
In addition, where features or aspects of the disclosure are described in terms of Markush groups, those skilled in the art will recognize that the disclosure is also thereby described in terms of any individual member or subgroup of members of the Markush group.
As will be understood by one skilled in the art, for any and all purposes, particularly in terms of providing a written description, all ranges disclosed herein also encompass any and all possible subranges and combinations of subranges thereof. Any listed range may be easily recognized as sufficiently describing and enabling the same range being broken down into at least equal halves, thirds, quarters, fifths, tenths, etc. As a non-limiting example, each range discussed herein may be readily broken down into a lower third, middle third and upper third, etc. As will also be understood by one skilled in the art all language such as “up to,” “at least,” “greater than,” “less than,” and the like, include the number recited and refer to ranges which may be subsequently broken down into subranges as discussed above. Finally, as will be understood by one skilled in the art, a range includes each individual member.
All publications, patent applications, issued patents, and other documents referred to in this specification are herein incorporated by reference as if each individual publication, patent application, issued patent, or other document was specifically and individually indicated to be incorporated by reference in its entirety. Definitions that are contained in text incorporated by reference are excluded to the extent that they contradict definitions in this disclosure.
Other embodiments are set forth in the following claims.
This application claims the benefit of priority to U.S. Provisional Patent Application No. 61/696,561, filed on Sep. 4, 2012, which is incorporated herein by reference in its entirety for any and all purposes.
| Number | Date | Country | |
|---|---|---|---|
| 61696561 | Sep 2012 | US |
