WO2002081054A1 - Corps de filtre poreux et procede - Google Patents

Corps de filtre poreux et procede Download PDF

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Publication number
WO2002081054A1
WO2002081054A1 PCT/US2002/007382 US0207382W WO02081054A1 WO 2002081054 A1 WO2002081054 A1 WO 2002081054A1 US 0207382 W US0207382 W US 0207382W WO 02081054 A1 WO02081054 A1 WO 02081054A1
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WO
WIPO (PCT)
Prior art keywords
particles
range
coarse
oxide
fine
Prior art date
Application number
PCT/US2002/007382
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English (en)
Inventor
Lars T. Johannesen
Original Assignee
Corning Incorporated
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Publication date
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Publication of WO2002081054A1 publication Critical patent/WO2002081054A1/fr

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    • C04B35/56—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on carbides or oxycarbides
    • C04B35/565—Shaped ceramic products characterised by their composition; Ceramics compositions; Processing powders of inorganic compounds preparatory to the manufacturing of ceramic products based on non-oxide ceramics based on carbides or oxycarbides based on silicon carbide
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D39/00—Filtering material for liquid or gaseous fluids
    • B01D39/14—Other self-supporting filtering material ; Other filtering material
    • B01D39/20—Other self-supporting filtering material ; Other filtering material of inorganic material, e.g. asbestos paper, metallic filtering material of non-woven wires
    • B01D39/2068—Other inorganic materials, e.g. ceramics
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B28—WORKING CEMENT, CLAY, OR STONE
    • B28B—SHAPING CLAY OR OTHER CERAMIC COMPOSITIONS; SHAPING SLAG; SHAPING MIXTURES CONTAINING CEMENTITIOUS MATERIAL, e.g. PLASTER
    • B28B3/00—Producing shaped articles from the material by using presses; Presses specially adapted therefor
    • B28B3/20—Producing shaped articles from the material by using presses; Presses specially adapted therefor wherein the material is extruded
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    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
    • C04B2235/02—Composition of constituents of the starting material or of secondary phases of the final product
    • C04B2235/50—Constituents or additives of the starting mixture chosen for their shape or used because of their shape or their physical appearance
    • C04B2235/54—Particle size related information
    • C04B2235/5463—Particle size distributions
    • C04B2235/5472—Bimodal, multi-modal or multi-fraction
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
    • C04B2235/65—Aspects relating to heat treatments of ceramic bodies such as green ceramics or pre-sintered ceramics, e.g. burning, sintering or melting processes
    • C04B2235/66—Specific sintering techniques, e.g. centrifugal sintering
    • C04B2235/668—Pressureless sintering
    • C—CHEMISTRY; METALLURGY
    • C04—CEMENTS; CONCRETE; ARTIFICIAL STONE; CERAMICS; REFRACTORIES
    • C04B—LIME, MAGNESIA; SLAG; CEMENTS; COMPOSITIONS THEREOF, e.g. MORTARS, CONCRETE OR LIKE BUILDING MATERIALS; ARTIFICIAL STONE; CERAMICS; REFRACTORIES; TREATMENT OF NATURAL STONE
    • C04B2235/00—Aspects relating to ceramic starting mixtures or sintered ceramic products
    • C04B2235/70—Aspects relating to sintered or melt-casted ceramic products
    • C04B2235/96—Properties of ceramic products, e.g. mechanical properties such as strength, toughness, wear resistance

Definitions

  • the present invention relates to a method for producing a particle-based ceramic porous filter body which may be used for filtering particles from fluids.
  • Filter bodies formed of porous silicon carbide (SiC), in particular filter bodies in the form of honeycomb filter bodies, are described in European Patent No. 0 336 883.
  • the SiC filter bodies disclosed in that patent are of high thermal conductivity as well as high porosity. Thus they are particularly useful for the filtration of diesel engine exhaust since they can be regenerated without the localised heating that can damage conventional ceramic filters.
  • Patent No. 0 692 995 which specifically relates to a method for closing the ends of the channels of such filter bodies, provides a further description of processes for the production of these filters.
  • the invention provides an improvement in the method for producing a ceramic porous filter body comprising extruding a viscous plastic paste containing non-oxide ceramic particles and an organic binder to form a "green" body, solidifying the "green” body by drying, and subjecting the dried solidified body to sintering under conditions sufficient to result in a porous structure.
  • the viscous plastic paste has rheological properties of viscosity and plasticity such that the extrusion results in a extrudate substantially without flaws or cracks at an extrusion pressure of at least 30 bar and an extrusion velocity of at least 60 mm per minute.
  • the rheological properties required to realize these results include an apparent viscosity for the plastic paste within the range of 5*10 3 - 1*10 5 Pa . s, as measured by a capillary rheometer at a temperature of 35°C within a shear rate range of 0.4 - 3.0 s "1 .
  • the paste will have a yield stress such that, when maintained at a temperature of 35°C, it will commence to flow when exposed to a pressure difference in the range of 100 - 400 kPa between the opposite ends of a rectilinear passage having an axial length of 80 mm and an inner diameter of 26 mm defined by a smooth, circularly cylindrical inner passage wall. Flow should also commence at a pressure difference of 400 - 800 kPa when the paste is maintained at a temperature of 25°C.
  • the preferred paste viscosities are within the range 8*10 3 -6*10 4 Pa s, more preferably 1» 0 4 -3 «10 4 , and still more preferably 1.5*10 4 -2.5*10 4 Pa s.
  • the yield stress is preferably such that the paste starts flowing when exposed to a pressure difference in the range of 100-300 kPa, preferably 150-250 kPa and more preferably 200-225 kPa between the opposite ends of the aforementioned passage at a paste temperature of 35°C.
  • plastic The rheological properties of these pastes can generally be described as plastic. As hereinafter more fully described, the nature of the plasticity is shown in more detail with reference to Fig. 5 of the drawings, showing the relationship between extrusion velocity and extrusion pressure. Reference will also be made to Figs. 6, 7, and 8 of the drawings showing the temperature dependency of yield point through a capillary tube with an inlet pressure drop and without an inlet pressure drop, as well as the temperature dependency of the viscosity.
  • one of the critical features of the present invention is to adjust the properties of the viscous plastic mass with respect to viscosity and plasticity in such a manner that the extrusion, especially when the extrudate is highly complex such as is the case for a honeycomb structure, will result in an extrudate substantially without flaws or cracks under production conditions.
  • These conditions are defined, as a lower limit, by an extrusion pressure of at least 30 bar and an extrusion velocity of at least 60 mm per minute, the preferred production conditions, however, being characterised by a higher extrusion pressure, such as an extrusion pressure of at least 40 bar, or an extrusion pressure of at least 50 bar or even at least 60 bar, such as up to about 70 bar.
  • the velocity with which the extrusion proceeds is preferably at least 100 mm per minute, more preferably at least 120 mm per minute and still more preferably at least 150 mm per minute.
  • the foregoing minimum conditions are established, however, while recognising that there is not necessarily a well-defined relationship between extrusion pressure and extrusion velocity. Thus it may be desired to utilise a higher extrusion pressure either to obtain a higher extrusion rate or to obtain a higher shape stability.
  • the conditions for obtaining filter bodies substantially without flaws or cracks at these production parameters are the proper rheological properties, such as viscosity and plasticity properties, of the paste extruded, that is, parameters which confer to the paste a sufficient tensile strength, ultimate strain and fracture toughness to result in a smooth high quality filter body.
  • the extruded paste will comprise both coarse and fine non- oxide ceramic particles as well as an organic binder.
  • the content of the fine particles has a considerable influence on the viscosity and extrudability of the paste, with the extrudability of the paste improving with increasing content of the fine particles.
  • the amount of fine particles in the extrusion batch can also have an important influence on the properties of the final porous filter body. It is normally preferred that the fine particles are not present in such an amount that they will tend, in the sintering process, to result in any substantial blocking of the porosities of the filter. Thus, the amount of the fine particles is preferably an amount in the range below an amount where substantial porosity reduction of the final filter body will occur in the sintering process. Porosity reduction can result from the settling of material originating from the consumed fine particles on the coarse particles outside the bridges or menisci present in the pore structure of the sintered body.
  • the amount of the fine particles is preferably an amount in the range above an amount where a considerable further strength improvement due to bridge or menisci formation between neighbouring coarse particles can take place as a result of further addition of the fine particles. It can be determined experimentally the range necessary to produce neither substantial porosity reduction, nor further strength improvement of the final filter body. Thus the preferred amount of fine particles is in the range below the amount where substantial porosity reduction of the final filter body will occur in the sintering process and above the amount where a considerable further strength improvement due to bridge or menisci formation between neighbouring coarse particles can take place as a result of further addition of the fine particles.
  • the attainment of the necessary strength during the sintering of the filter bodies requires that there be a temperature "window" in the sintering range for the bodies wherein fine particle sublimation and/or diffusional transport can occur without substantial sublimation of the larger particle fraction of the batch.
  • a window of 25°C is useful for this purpose, it is generally preferred to have a larger window, this object being achieved by selecting a fine particle fraction of a size such that the particles will be consumed wholly or partially by sublimation and/or diffusional transport at a temperature which is at least 100°C, more preferably at least 200°C, below the temperature at which the coarse particles will sublimate.
  • a very important type of a filter is a honeycomb wall flow filter
  • another aspect of the invention relates to a method for producing a ceramic porous honeycomb wall flow filter body.
  • the honeycomb wall flow filter body may be made with any desired wall thickness by the method according to the invention, including "traditional" thicknesses in the range of 0.8mm to 1.25mm
  • an important advantage of the rheological aspects of the invention is that the effective high quality production of honeycomb filters having relatively small wall thicknesses is enabled, whereby valuable new filter design possibilities are provided.
  • the cell wall thickness of a thin-walled honeycomb wall flow filter body may be in the range of 0.3-0.75mm, the particle size of the coarse particles being adapted to the wall thickness of the honeycomb so that the ratio between the average particle size (mean diameter) of the coarse particles and the wall thickness is at the most 1 :5, normally in the range of 1 :5-1 :30, such as in the range of 1 :6-1 :20, normally in the range of 1 :7-1 :20, often preferably in the range of 1 :7-1 :15, such as in the range of 1 :8- 1 :10 or 1 :8-1 :9.
  • the weight ratio between the coarse and the fine particles importantly affects the strength and porosity and the relationship between strength and porosity of the filter body. This relationship becomes particularly important at small wall thicknesses.
  • the ratio between the fine particles and the coarse particles may be anywhere between in the range between the "normal" 1 :5 or even 1 :4 on the higher side and 1 :30 on the lower side, but will, according to the present invention, preferably be in the range where the amount of the fine particles is relatively smaller, such as in the range of 1 :6- 1 :30, e.g., 1 :7-1 :30, more often, however, in the range of 1 :6-1 :15, such as in the range of 1 :8-1 :15 or often preferably in the range of 1 :9-1 :12, such
  • a wall thickness is in the range of 0.4-0.7mm, more typically in the range of 0.5- 0.7mm and most preferably in the range of 0.5-0.6mm.
  • the coarse particles are relatively small, e.g., that they have an average particle size, according to the FEPA gradation, in the range of 40-70 ⁇ m, such as in the range of 55-65 ⁇ m.
  • a suitable particle size, as defined by FEPA Mesh size, is the size of FEPA Mesh F220-240 particles.
  • FEPA stands for Federation of the European Producers of Abrasives.
  • the weight ratio between the fine and the coarse particles is in the range of 1 :9-1 :12, the strength of the filter material exceeds 25 mPa, and the permeability (air) exceeds 1.5.10 "l2 m 2 .
  • the coarse SiC material may be FEPA mesh F230, and the fine SiC material may have an average particle size in the range of 0.3-1.5 ⁇ m, preferably in the range of 0.5-1.0 ⁇ m.
  • a honeycomb filter having these preferred parameters, and a cell pitch in the range 2-2.3 mm, and a wall thickness in the range of 0,5-0,7 mm will, when used for removing soot particles from diesel exhaust, have a very high soot loading capacity. That is, the increase of the pressure loss over the filter as soot accumulation builds up will be small because the filter area is high, while, at the same time, the thermal mass of the filter is substantially the same as in more conventional filters, e.g. SiC honeycomb filters with wall thickness of 0.8 mm or higher.
  • the average size is normally in the range of 0.3-3 ⁇ m, preferably in the range of 0.3-2 ⁇ m, such as in the range of 0.5-2 ⁇ m.
  • Another way of expressing the size of the fine particles is by reference to their maximum size, where a size of 3 ⁇ m is normally the maximum suitable or permissible size, the more preferred maximum size of the fine particles being 2 ⁇ m or even 1 ⁇ m, as the smaller particles have a much higher tendency to be consumed in the sintering, such as will be explained in the following.
  • a preferred average size of the fine particles is in the range of 0.5-0.8 ⁇ m.
  • Fig. 1 is scanning electron microscope photo of a typical micro-structure of the porous filter wall according to the invention
  • Fig. 2 illustrates the flow principle of a wall flow filter
  • Fig. 3 is a graph showing the pore size of the porous structure of the filter as a function of the particle size of the coarse particles
  • Figs. 4a-4c show, schematically, the experimental capillary rheometer used in the measurements of the rheological data discussed herein
  • Fig. 5 is a graph showing pressure loss as a function of flow velocity when extruding paste for forming a filter body
  • Figs. 6 and 7 are graphs showing temperature dependency of the yield point of the paste
  • Fig. 8 is a graph showing temperature dependency of viscosity of the paste
  • Figs. 9 and 10 are graphs showing strength and permeability of the filter body produced as a function of fine contents thereof.
  • the honeycomb wall flow filter is of a very compact filter design enabling a large filter area within a restricted volume.
  • the filter defines a plurality of parallel inlet and outlet channels or passages 10 and 11 , respectively, opening alternately in opposite ends of the filter body, whereby the openings of the passages in each end defines a chessboard-like pattern.
  • the non-oxide ceramic material to be utilized to manufacture the wall flow filters of the invention will be selected depending upon the particular filtration environment in which the filter is intended to operate.
  • the non-oxide ceramic material for the coarse particles is a ceramic material which is not an oxide and is suitably selected from the group consisting of SiC,
  • Non-oxide silicon ceramic particles that is, non-oxide materials containing silicon such as SiC, Si 3 N 4 and SiONC, among which SiC, Si 3 N are preferred, and the presently most preferred material is SiC, particularly the commercially readily available alpha SiC.
  • the fine particles are preferably particles of the same material as the coarse particles, but also useful are mixtures wherein the coarse particles are SiC or Si 3 N , and the fine particles are particles of mullite or aluminium titanate, or of mixtures thereof with SiC and/or Si 3 N . Particles of the latter can reduce the necessary sintering temperature.
  • a preferred family of ceramic pastes based on SiC particulates includes the following main elements. These compositions can be varied to some extent depending on the choice of the coarse SiC fraction, and the actual geometry of the final product required:
  • Optional sintering aids Pastes of these types are suitably produced by first mixing the coarse particles and the fine particles with the binder added in dry form, then adding the lubricant in the form of a solution or suspension in a liquid, and mixing, then adding any optional recycled material in the form of a mixture with aqueous phase, then adding water and optionally a polymeric alcohol and optionally a polyhydric alcohol, and mixing, and finally adding further aqueous phase up to the final content of aqueous phase, and mixing.
  • the extrusion is preferably performed in such a manner that the extrudate leaves the extruder at a velocity which is substantially identical all over the cross section of the extrudate, as this has been found to result in superior quality of the extrudate.
  • aqueous pastes containing a lower alcohol such as ethanol.
  • the lower alcohol has a number of functions.
  • the lower alcohol may function to secure that the gelation temperature of the paste is increased to a temperature above the extrusion temperature.
  • the lower alcohol may also have an influence on the viscosity of the aqueous phase containing the organic binder, and in this case, it is preferred that the concentration of the lower alcohol is a concentration at which a curve representing viscosity of the aqueous phase of the paste as a function of the concentration of the lower alcohol shows a substantial plateau so that minor changes in the concentration of the lower alcohol will have only a small influence on the viscosity of the aqueous phase.
  • F240-F120 grades (FEPA gradation) silicon carbide powders can be used, with the resulting pore sizes being in the range of 6-45 microns.
  • the pore size of the sintered body is determined primarily by the selection of starting powder.
  • the expected pore size can be described by the following simple linear relationship:
  • Fig. 3 of the drawings is a graph based on measurements of the average pore size of filter samples produced with different sized coarse starting powders. The volume median measured by mercury intrusion is used. This relationship has been found to be valid for particle size from approx. 45 microns up to 250 microns.
  • FCP 10-FCP 15 SiC powder materials commercially available from Norton, are normally preferred.
  • FCP (Fine Ceramic Powder) particle sizes correspond to the number of square meters of surface area per gram of the powder product sold. Table 1 below shows equivalent particle diameters for various FCP products wherein the average particle size of each sample material as well as the particle sizes corresponding to the 10% and 90% (weight) points on the particle size distribution curve (normally distributed) are reported.
  • the fine SiC powder is added primarily to sinter the coarse particles together.
  • the grain growth of the coarse particles takes place by consumption of the fines.
  • the fines are volatilised upon heating and re-deposited at the grain contacts between the coarse particles, leading to the formation of grain boundaries and a strong, integral filter product.
  • Filter pore size is also influenced by the quantity of fine powder introduced. Up to a limit of approximately 15% addition of fines, no reduction of pore size is seen as all the fines are consumed during sintering/recrystallization to build up the grain boundaries.
  • the amount of fines and the particle size of the fines are thus chosen to give a desired compromise between permeability and strength of the sintered body under the specific sintering conditions, as well as to secure the paste plasticity that is required for the extrusion production of a well-structured porous product.
  • the binder is added with the objective of obtaining the desired rheological properties of the paste, by adding plasticity to the mixture of ceramic particles, and to add strength to the 'green' products after drying.
  • the amount of the organic binder should be adjusted to the actual liquid phase content in the ceramic paste. If the binder content is not high enough, the viscosity of the paste may not be high enough to prevent a loss of liquid phase during extrusion, which is unacceptable. On the other hand, the content of the organic binder should not be so high that it results in a too high viscosity.
  • the organic binder may suitably be a cellulose or a cellulose derivative, such as a cellulose ether.
  • a cellulose ether is methyl hydroxyethyl cellulose ether, which may, for example, be present in an amount of 3-8, preferably 4.5-6 and more preferred 5.2-5.6 per cent by weight of the total amount of paste.
  • Other cellulose binders may of course be used.
  • An example of a preferred organic binder is Tylose MH300-P2 which is the trade name for an organic polymer, a cellulose derivative, methyl hydroxyethyl-cellulose ether.
  • the Tylose binder is a water soluble polymer that becomes 'saturated' when all hydroxyl groups are hydrolysed.
  • the number 300 refers to the viscosity in a 2 wt% aqueous solution.
  • Other cellulose derivatives with various other substituents may be alternatively or additionally used as the organic binder, provided their properties are found to be suitable for the purpose.
  • a lower alcohol such as ethanol will result in an increased wetting of the ceramic particles.
  • the easy vaporisation of ethanol is desirable as the green bodies are getting stable after a very short initial drying phase which already takes place immediately after extrusion.
  • a water-alcohol liquid phase is advantageous for use with the preferred binders as the addition of alcohol shifts the binder gelation temperature higher.
  • the presently preferred alcohol is ethanol. Without the addition of ethanol, undesirable gelation of the binder in the ceramic paste during extrusion has been observed.
  • the viscosity of the binder system increases with increasing ethanol content in the liquid phase, until it reaches a plateau at approximately 20-50% by weight of ethanol. With a higher ethanol content, the viscosity decreases rapidly. In practice, therefore, the ethanol content is therefore chosen at approximately 20-35 % by weight of the liquid phase.
  • the paste normally contains a lubricant, such as stearic acid or a wax, the lubricant preferably being in a finely divided form such as micro-particles of stearic acid or a wax emulsion.
  • a lubricant such as stearic acid or a wax
  • Stearic acid is particularly effective to decrease friction during extrusion, although wax and other organic materials with similar properties have been found to be usable.
  • Pristerene 4900 flakes from Unichema International constitutes a useful stearic acid source.
  • Polyhydric alcohol (optional): Ethylene glycol, 1 ,2-ethanediol, or another suitable polyhydric alcohol may be added as an optional plasticiser for the batch.
  • a preferred component of the paste from the standpoint of paste uniformity is polyvinyl alcohol.
  • This material is added as a solution in water to act as a secondary organic binder. It has been found that such a secondary organic binder, of a chemical principle different from the first binder, preferably a polymeric alcohol, can contribute advantageously to level out any minor differences in behaviour of the paste from batch to batch and thus to obtaining a uniform and highly reproducible production.
  • suitable PVA products are BDH 30573 from BDH Laboratory Supplies, Poole, BD15 1TD, England, and Rdh 63018 from Riedel-de Haen AG, 30926 Seelze, Germany.
  • the Rdh 63018 product has a viscosity of 4-6 mPa*s as a 4% by weight solution in water at 20°C.
  • Sintering aids known from the literature to be suitable for improving the properties of a sintered body can be added to the material in the form of Al- or B-containing compounds.
  • the addition should be kept at a level lower than the solid solution level of approx. 1 % atom, to make sure that no secondary phase can be formed and concentrated in the grain boundaries.
  • the addition of sintering additives also has an influence on other relevant properties of SiC, the most pronounced influence being the effect on thermal conductivity.
  • Undoped SiC has a thermal conductivity of approx. 75 W/m*k, which, according to the literature (Ceramic Bulletin, 67, No. 12, 1988, pp 1961-1963), is decreased to approx. 60 W/m*k when doped with Al.
  • Be has a thermal conductivity of 170 W/m*k or 260 W/m*k, respectively.
  • sintering additives should be added will depend on the intended use of the filter body and the particular requirements dictated by that use.
  • One important purpose for utilising such additives with SiC filter bodies is to obtain a high TSP (Thermal Shock Parameter) when producing honeycomb filters having a small wall thickness.
  • the ceramic paste should be mixed according to some general and to some specific guidelines. Also the incorporation of recycled (used) material in the process should to follow certain guidelines.
  • a desirable first step is mixing coarse and fine SiC powders with the methyl hydroxylethyl cellulose binder
  • the next step may be a dissolution of the stearic acid lubricant in heated ethanol followed by addition of the alcohol-stearic acid solution to the dry components with mixing to ensure even distribution of the precipitated micro grained stearic acid in the ceramic material.
  • the stearic acid precipitates as micro grains already distributed evenly in the ceramic material.
  • a wax-containing emulsion of wax in water can be added together with the batch water.
  • Wax both from Hoechst, are suitable for this purpose.
  • the last batch addition may be water, with or without the inclusion in the water of the optional PVA and ethylene glycol binder constituents. Mixing of the ceramic paste to secure even liquid distribution and dissolution of the organic binder in the liquid phase then follows.
  • the mixing of the ceramic paste is suitably performed in a planetary mixer, such as a commercial R series Eirich mixer adapted for mixing of ceramic pastes. During the mixing, the mixture undergoes a phase transition from a non-uniform blend to a uniform, coherent mass.
  • a planetary mixer such as a commercial R series Eirich mixer adapted for mixing of ceramic pastes.
  • Figs. 4a-4c of the drawings show a longitudinal cross-section of a capillary design for an experimental capillary rheometer useful for generating the necessary data.
  • Fig. 4a shows an extruder outlet comprising a base part 13, which may be connected to the extruder (not shown) and an outlet tube 14a.
  • the outlet tube 14a defines an inner part 15a defining a cylindrical passage with a large diameter and an outer part 16a defining a cylindrical passage with a smaller diameter.
  • the outer part 16a may be replaced by outer parts 16b and 16c having a shorter length, but the same diameter as illustrated in Figs. 4b and 4c.
  • a pressure transducer 17 for measuring the pressure within the base part 13 is arranged in a transverse bore or pocket.
  • the aspect ratio is in the same range as that of the actual extrusion dies, thereby simulating the actual process during the determination of the rheological properties of the paste.
  • the end-effects are known not to be negligible with the extrusion dies being used for the production of porous filter bodies, it would also be quite inappropriate to base the evaluation of the compound on steady shearing viscosity only. Since these compounds have non-Newtonian fluid characteristics, it was decided to perform as many measurements as possible at different shear rates to evaluate the viscosity change with shear rate. The compounds were expected to exhibit plastic behavior including a well defined yield stress and, after a short nonlinear region, a linear relationship between shear stress and shear rate. Capillary measurements were therefore conducted at different shear rates in the entire temperature range from approximately 20 C to approximately 40 C.
  • ⁇ a ⁇ w / ⁇ w wherein ⁇ w is the wall shear stress and ⁇ w is the wall shear rate.
  • n' d(ln ⁇ w )/d(ln(8V/d c )), i.e., n' is the slope of the graph of ln ⁇ w versus ln(8V/do).
  • Fig. 5 of the drawings shows the rheological properties of the paste of Example II assessed by measuring the pressure loss through a capillary tube having a length of 80 mm and a diameter of 26 mm as a function of flow velocity.
  • the graph is obtained by measuring how the extrusion pressure through the capillary tube depends on extrusion velocity at four different temperatures in a range covering the actual operating temperature, when extruding the plastic compound. As the graphs are based on interpolated data, some actual measurement points are plotted, which confirms that the interpolations are accurate.
  • the graphs shown in Figs. 6, 7, and 8 are also based on data taken from the Example II paste.
  • Fig. 6 illustrates the temperature dependency of the yield point of this paste when extruding through a capillary having a diameter of 26 mm and a length of 240 mm including the entrance pressure drop.
  • the yield point of the paste decreases continuously when the temperature increases, the decrease being most pronounced in the range from approximately 26 °C to approximately 30°C. Over that range the pressure is reduced by approximately 33% corresponding to a decrease of 1.25 Bar/°C.
  • the yield point does not have an inflexion point nor reach a 'plateau like' temperature range.
  • Fig. 6 and Fig. 7 of the drawing characterising this paste a qualitative impression of the elastic and plastic properties of the Example II compound can be made.
  • the dominant pressure drop is the pressure drop arising from viscous flow in the capillary
  • the dominant pressure drop in the higher temperature range is the inlet pressure drop, which is related to elastic properties.
  • Fig. 7 illustrates the temperature dependency of the yield point of this paste by extrusion through a capillary having an inner diameter of 26 mm and a length of 80 mm.
  • the graph shows that the yield point decreases continuously when the temperature increases, most pronounced in the range from approximately 26 °C to approximately 30°C, where the pressure is reduced with approximately 46% corresponding to a decrease of 0.7 Bar/°C. Again the yield point does not have an inflexion point nor reach a 'plateau like' temperature range.
  • the viscosity of the paste has been calculated in the temperature range from approximately 25-40°C.
  • the temperature dependency of the viscosity is shown in Figure 8, with the viscosity of the paste is based on the experimental results described in Table 2.
  • Fig. 8 illustrates the temperature dependency of viscosity, measured with a capillary rheometer principle.
  • the graph shows that the viscosity decreases continuously when the temperature increases. This effect is most pronounced in the range from approximately 26 °C to approximately 30°C, where the viscosity is reduced with approximately 33% corresponding to a decrease of 3.5510 3 Pa s/°C. Like the yield point, the viscosity of this paste does not have an inflexion point nor reach a 'plateau like' temperature range.
  • Example III A further example of a plastic paste that. is particularly well suited for the production of a thin-walled monolith for a honeycomb wall flow filter body is set forth as Example III below:
  • a paste having the composition of Example III can be extruded to provide thin-walled honeycomb bodies useful for forming wall flow filters by conventional means.
  • the pastes are first pre-extruded to provide slugs of material, these desirably being aged for several days for conditioning and equilibration purposes.
  • the aging period should be sufficiently long to achieve full reaction of the organic binder, thereby obtaining the required viscosity and plasticity necessary for the re-extrusion of the ceramic paste/intermediate cylindrical bodies into the desired cylindrical honeycomb bodies.
  • a screw extruder adapted for the extrusion of porous SiC is then used to form the slugs into thin-walled honeycomb structures.
  • the extruder is suitably an auger extruder designed for the extrusion of coarse grained materials in complex shapes. Rotational movement of the ceramic paste within the extruder should be suppressed, a result achieved by using a double auger in combination with noodle dies and mass knives.
  • the auger and the linings in the extruder are suitably provided with heating or cooling means to control batch temperature and reduce thermal gradients therein. High temperatures can induce undesirable gelling , while excessive temperature gradients lead to non- uniform extrusions.
  • Conventional honeycomb extrusion dies desirably including conventional wear coatings to extend die life, can be used.
  • the extruder and batch slug may be preheated to an optimal extrusion temperature, typically in the 25-35°C. range, so that the extrusion can be initiated at pressures in the range of 5-10 bar. Thereafter continuous extrusion can take place effectively at extrusion rates producing 50-150mm of extruded product length per minute at extrusion pressures of 30-50 bar and extrusion temperatures of 25-35°C, with pastes having the rheological properties hereinabove described.
  • Extruded honeycomb ceramic bodies produced as described may be dried and fired using known techniques. To control shrinkage and the resultant possibility of cracking, drying should be carried out in such a way as to maximize the uniformity of the drying rate across all sections of the extruded pieces. Firing the bodies to achieve non-oxide particle sintering can be carried out in graphite-insulated furnaces, most preferably in the firing range 2200°C to
  • Fig. 9 of the drawings illustrates the relationship between the strength and permeability of a sintered silicon carbide product and the amount and particle size of a fine silicon carbide fraction included in a batch containing both coarse and fine silicon carbide powders.
  • the effects illustrated are for the case of a batch comprising varying proportions of Norton FCP-15 fine SiC powder having an average particle size of 0.5 ⁇ m.
  • a fines content of 9.09 wt% can maximize the level of permeability provided while still retaining a sintered product strength as high as that obtained through the inclusion of 16.67 wt% of the same fine powder in the batch.
  • FIG. 1 of the drawings is a scanning electron micrograph showing the pore micro-structure of a sintered wall portion of a typical silicon carbide honeycomb filter body provided in accordance with the invention.
  • the porous filter wall comprises coarse particles bonded together in contact points, thereby defining the porosity of the filter.
  • the coarse particles are fragments of larger crystallites, which upon crushing form irregular particles.
  • the low aspect ratio of these particles typically in the range of 1 :2, are only changed slightly during the sintering process.

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  • Filtering Materials (AREA)

Abstract

La présente invention concerne des corps de filtres en céramique frittée non oxyde à résistance et perméabilité élevées, dont les pores présentent une fourchette de dimensions particulièrement adaptée au filtrage des gaz (par exemple, des gaz d'échappement d'un moteur diesel), que l'on produit par la cuisson de préformes des filtres comprenant des mélanges choisis de particules non oxydes grossières et fines afin d'obtenir le frittage des particules en des corps de filtres en céramique poreuse comportant une pluralité de canaux parallèles d'entrée (10) et de sortie (11). L'extrusion des pâtes plastiques visqueuses de viscosité contrôlée à des pressions d'extrusion d'au moins 30 bars et à des vitesses d'extrusion de 60 mm par minute permet de conférer une intégrité structurelle élevée aux préformes précitées et d'obtenir des mélanges extrudés dépourvus de craquelures et autres défauts.
PCT/US2002/007382 2001-04-09 2002-03-11 Corps de filtre poreux et procede WO2002081054A1 (fr)

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US60/282,516 2001-04-09

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Cited By (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1640351A1 (fr) * 2003-05-29 2006-03-29 Ngk Insulators, Ltd. Procede de production d' une structure en nid d 'abeilles et particules en carbure de silicium utilisees pour produire une telle structure
EP1925353A1 (fr) * 2005-08-17 2008-05-28 Chen, Qinghua Support de filtre de capture de particules ceramique alveolaire, ensemble filtrant de capture de particules et dispositif filtrant de capture de particules composes des supports, de meme que procedes de fabrication idoines
WO2009017642A1 (fr) * 2007-07-31 2009-02-05 Corning Incorporated Compositions pour l'application à des corps en nid d'abeilles en céramique
EP2098493A1 (fr) * 2006-12-27 2009-09-09 Hitachi Metals, Ltd. Processus de fabrication de structure alvéolaire céramique à base de titanate d'aluminium
WO2010001062A2 (fr) 2008-07-04 2010-01-07 Saint-Gobain Centre De Recherches Et D'etudes Europeen Melange de grains pour la synthese d'une structure poreuse du type titanate d'alumine
FR2936512A1 (fr) * 2008-09-30 2010-04-02 Saint Gobain Ct Recherches Procede de fabrication d'un materiau poreux en sic.
US7815994B2 (en) * 2004-09-30 2010-10-19 Ibiden Co., Ltd. Method for producing porous body, porous body, and honeycomb structure

Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5700373A (en) * 1992-09-17 1997-12-23 Coors Ceramics Company Method for sealing a filter

Patent Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5700373A (en) * 1992-09-17 1997-12-23 Coors Ceramics Company Method for sealing a filter

Cited By (19)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP1640351A1 (fr) * 2003-05-29 2006-03-29 Ngk Insulators, Ltd. Procede de production d' une structure en nid d 'abeilles et particules en carbure de silicium utilisees pour produire une telle structure
EP1640351A4 (fr) * 2003-05-29 2009-11-11 Ngk Insulators Ltd Procede de production d' une structure en nid d 'abeilles et particules en carbure de silicium utilisees pour produire une telle structure
US7815994B2 (en) * 2004-09-30 2010-10-19 Ibiden Co., Ltd. Method for producing porous body, porous body, and honeycomb structure
EP1925353A1 (fr) * 2005-08-17 2008-05-28 Chen, Qinghua Support de filtre de capture de particules ceramique alveolaire, ensemble filtrant de capture de particules et dispositif filtrant de capture de particules composes des supports, de meme que procedes de fabrication idoines
EP1925353A4 (fr) * 2005-08-17 2013-09-04 Yunnan Filter Environment Prot S & T Co Ltd Support de filtre de capture de particules ceramique alveolaire, ensemble filtrant de capture de particules et dispositif filtrant de capture de particules composes des supports, de meme que procedes de fabrication idoines
US8894916B2 (en) 2006-12-27 2014-11-25 Hitachi Metals, Ltd. Method for producing aluminum-titanate-based ceramic honeycomb structure
EP2098493A1 (fr) * 2006-12-27 2009-09-09 Hitachi Metals, Ltd. Processus de fabrication de structure alvéolaire céramique à base de titanate d'aluminium
EP2098493A4 (fr) * 2006-12-27 2011-05-25 Hitachi Metals Ltd Processus de fabrication de structure alvéolaire céramique à base de titanate d'aluminium
JP5267131B2 (ja) * 2006-12-27 2013-08-21 日立金属株式会社 チタン酸アルミニウム質セラミックハニカム構造体の製造方法
US8435441B2 (en) 2007-07-31 2013-05-07 Corning Incorporated Compositions for applying to ceramic honeycomb bodies
WO2009017642A1 (fr) * 2007-07-31 2009-02-05 Corning Incorporated Compositions pour l'application à des corps en nid d'abeilles en céramique
FR2933399A1 (fr) * 2008-07-04 2010-01-08 Saint Gobain Ct Recherches Melange de grains pour la synthese d'une structure poreuse du type titanate d'alumine
WO2010001062A3 (fr) * 2008-07-04 2010-03-18 Saint-Gobain Centre De Recherches Et D'etudes Europeen Melange de grains pour la synthese d'une structure poreuse du type titanate d'alumine
WO2010001062A2 (fr) 2008-07-04 2010-01-07 Saint-Gobain Centre De Recherches Et D'etudes Europeen Melange de grains pour la synthese d'une structure poreuse du type titanate d'alumine
US8399376B2 (en) 2008-07-04 2013-03-19 Saint-Gobain Centre De Recherches Et D'etudes Europeen Particle blend for synthesizing a porous structure of the aluminum titanate type
FR2936512A1 (fr) * 2008-09-30 2010-04-02 Saint Gobain Ct Recherches Procede de fabrication d'un materiau poreux en sic.
JP2012504092A (ja) * 2008-09-30 2012-02-16 サン−ゴバン サントル ドゥ ルシェルシェ エ デトゥードゥ ユーロペン 多孔質ＳｉＣ材料の製造方法
CN102171163A (zh) * 2008-09-30 2011-08-31 欧洲技术研究圣戈班中心 制造SiC多孔材料的方法
WO2010037963A1 (fr) * 2008-09-30 2010-04-08 Saint-Gobain Centre De Recherches Et D'etudes Europeen Procede de fabrication d'un materiau poreux en sic

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