EP0563261A1 - Preparation et utilisation de reseaux de trichites de mullite - Google Patents

Preparation et utilisation de reseaux de trichites de mullite

Info

Publication number
EP0563261A1
EP0563261A1 EP92903140A EP92903140A EP0563261A1 EP 0563261 A1 EP0563261 A1 EP 0563261A1 EP 92903140 A EP92903140 A EP 92903140A EP 92903140 A EP92903140 A EP 92903140A EP 0563261 A1 EP0563261 A1 EP 0563261A1
Authority
EP
European Patent Office
Prior art keywords
mullite
fluorotopaz
mixture
network
whiskers
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP92903140A
Other languages
German (de)
English (en)
Inventor
Peter A. Doty
John R. Moyer
Neal N. Hughes
Burton D. Brubaker
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Dow Chemical Co
Original Assignee
Dow Chemical Co
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from US07/633,580 external-priority patent/US5098455A/en
Application filed by Dow Chemical Co filed Critical Dow Chemical Co
Publication of EP0563261A1 publication Critical patent/EP0563261A1/fr
Withdrawn legal-status Critical Current

Links

Classifications

    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
    • F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
    • F01N3/08—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous
    • F01N3/10—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by thermal or catalytic conversion of noxious components of exhaust
    • F01N3/24—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by thermal or catalytic conversion of noxious components of exhaust characterised by constructional aspects of converting apparatus
    • F01N3/28—Construction of catalytic reactors
    • F01N3/2803—Construction of catalytic reactors characterised by structure, by material or by manufacturing of catalyst support
    • F01N3/2825—Ceramics
    • F01N3/2828—Ceramic multi-channel monoliths, e.g. honeycombs
    • 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
    • B01D39/2082—Other inorganic materials, e.g. ceramics the material being filamentary or fibrous
    • B01D39/2086—Other inorganic materials, e.g. ceramics the material being filamentary or fibrous sintered or bonded by inorganic agents
    • 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
    • C04B30/00—Compositions for artificial stone, not containing binders
    • C04B30/02—Compositions for artificial stone, not containing binders containing fibrous materials
    • 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
    • C04B38/00—Porous mortars, concrete, artificial stone or ceramic ware; Preparation thereof
    • C04B38/009—Porous or hollow ceramic granular materials, e.g. microballoons
    • C—CHEMISTRY; METALLURGY
    • C30—CRYSTAL GROWTH
    • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
    • C30B29/00—Single crystals or homogeneous polycrystalline material with defined structure characterised by the material or by their shape
    • C30B29/10—Inorganic compounds or compositions
    • C30B29/34—Silicates
    • C—CHEMISTRY; METALLURGY
    • C30—CRYSTAL GROWTH
    • C30B—SINGLE-CRYSTAL GROWTH; UNIDIRECTIONAL SOLIDIFICATION OF EUTECTIC MATERIAL OR UNIDIRECTIONAL DEMIXING OF EUTECTOID MATERIAL; REFINING BY ZONE-MELTING OF MATERIAL; PRODUCTION OF A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; SINGLE CRYSTALS OR HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; AFTER-TREATMENT OF SINGLE CRYSTALS OR A HOMOGENEOUS POLYCRYSTALLINE MATERIAL WITH DEFINED STRUCTURE; APPARATUS THEREFOR
    • C30B9/00—Single-crystal growth from melt solutions using molten solvents
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
    • F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
    • F01N3/02—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for cooling, or for removing solid constituents of, exhaust
    • F01N3/021—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for cooling, or for removing solid constituents of, exhaust by means of filters
    • F01N3/022—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for cooling, or for removing solid constituents of, exhaust by means of filters characterised by specially adapted filtering structure, e.g. honeycomb, mesh or fibrous
    • F01N3/0222—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for cooling, or for removing solid constituents of, exhaust by means of filters characterised by specially adapted filtering structure, e.g. honeycomb, mesh or fibrous the structure being monolithic, e.g. honeycombs
    • 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
    • C04B2111/00—Mortars, concrete or artificial stone or mixtures to prepare them, characterised by specific function, property or use
    • C04B2111/00474—Uses not provided for elsewhere in C04B2111/00
    • C04B2111/00793—Uses not provided for elsewhere in C04B2111/00 as filters or diaphragms
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
    • F01N2330/00—Structure of catalyst support or particle filter
    • F01N2330/06—Ceramic, e.g. monoliths
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
    • F02B—INTERNAL-COMBUSTION PISTON ENGINES; COMBUSTION ENGINES IN GENERAL
    • F02B3/00—Engines characterised by air compression and subsequent fuel addition
    • F02B3/06—Engines characterised by air compression and subsequent fuel addition with compression ignition
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02T—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO TRANSPORTATION
    • Y02T10/00—Road transport of goods or passengers
    • Y02T10/10—Internal combustion engine [ICE] based vehicles
    • Y02T10/12—Improving ICE efficiencies

Definitions

  • This invention relates to: a mullite whisker- forming composition; a process for preparing mullite whiskers, particularly in the form of an interlaced, single crystal network; and the use of such a network as a regenerable filter element in applications such as a regenerable diesel exhaust gas filter.
  • Mullite whiskers have been fabricated by mixing together solid forms of starting materials and pyrolyzing these materials to yield the whiskers.
  • the starting materials included various compounds having sources of aluminum and silicon.
  • the starting materials also frequently included solid fluorine-containing compounds such as AIF 3 .
  • the fluorine-containing compound was consumed during an intermediate stage of a chemical process that eventually yielded mullite whiskers.
  • Improved methods for mullite whisker production that use gaseous fluorine sources are known.
  • One such method includes the use of gaseous HF as a source of fluorine.
  • gaseous HF as a source of fluorine.
  • PCT publication US89/03175 discloses a process in which AIF 3 and fused Si ⁇ 2 , or AIF 3 , Si ⁇ 2 , and AI 2 O 3 powders are first formed into a green body of a desired
  • the green body is heated at 700°C to 950°C in a gaseous by-product atmosphere of anhydrous SiF 4 to form barlike topaz crystals.
  • the topaz is heated in the same atmosphere at 1150°C to 1700°C to convert the topaz crystals into needlelike single 0 crystal mullite whiskers that form a porous, rigid felt structure.
  • the felt has the same shape as the green body with 1.5 or less percent change in linear dimensions. This method produces very toxic gaseous reaction products. 5
  • Ceramic whiskers have traditionally been used for various filter applications due to their mechanical and thermal strengths as well as their increased permeability over other materials. Ceramic whiskers 0 made of mullite have excellent mechanical strength and thermal shock resistance. These characteristics make mullite whiskers especially useful for high temperature filtering applications.
  • high permeability is a desired characteristic of filter elements. High permeability equates to a low value for pressure drop, as measured between the input and output sides of a filter element for a gas or liquid flowing through the element. In diesel engine exhaust filter applications, for example, a low pressure drop leads to minimal loss of engine power.
  • the permeability of a filter element is proportional to the fourth power of the diameter of the whiskers contained therein. Small whisker diameters effectively and dramatically decrease the permeability of the elements due to increased whisker density. By increasing the diameter of the whiskers, the permeability can likewise be increased.
  • U.S.-A 3,992,499 discloses single crystal mullite fibrils having a diameter of 3-100 nanometers, a length of 0.05-2 micrometers and an aspect ratio within a range of 5 to 100:1.
  • the material is prepared by heating an intimate mixture of alumina and silica sources in the presence of an alkali metal salt flux to a temperature of 750°C to 1200°C.
  • Regenerable filter elements are used in an exhaust filter to collect and burn soot particles, particularly carbon particles, from diesel engine exhaust. These elements can be regenerated (returned to their original soot-free state) by conventional procedures. Regeneration causes the filter element to reach very high temperatures before the soot is burned off and the filter element cools down to normal operating temperatures. Regeneration of the filter elements is usually done on a periodic basis such as a predetermined number of hours of operation or a predetermined level of pressure drop through the filter due to soot build-up.
  • Conventional diesel exhaust filter elements are made of porous structures of cordierite or other materials that are capable of trapping the small soot particles from diesel exhausts while allowing the exhaust gases themselves to flow through the elements. Filtering conventionally begins by directing exhaust gases down long narrow channels that are plugged at their downstream ends. This forces the exhaust gases to seep through channel walls where the soot is trapped. Once on the other side of the channel walls, the exhaust gases are now in channels that are plugged at their upstream ends. The gases are thus forced out the downstream end of the filter element.
  • the aforementioned filter elements typically lack at least one of: the strength needed to endure the ride of a moving vehicle; the ability to stand up to the high temperatures reached during regeneration; or the endurance to withstand the thermal cycles resulting from numerous repetitions of the regeneration process.
  • Cordierite in particular is not very attractive because it is not as strong as mullite and it has a lower service temperature (1000°C.) than mullite (1700°C).
  • Cordierite also has a lower heat capacity per unit volume than mullite. A lower heat capacity is undesirable because it results in the filter element reaching a higher temperature during regeneration. The higher temperatures add to degradation of the element due to thermal shock experienced by the element.
  • U.S.-A 4,264,760 discloses a ceramic honeycomb filter comprising a ceramic honeycomb structural body having a multiplicity of parallel channels extending therethrough. Selected channels are sealed at one end of the body. The remaining channels are sealed at the opposite end of the body.
  • PCT/US 89/03175 discloses a process for forming a porous, rigid felt structure.
  • the structure comprises randomly oriented, single crystal mullite whiskers that are mechanically interlocked to form a rigid felt structure capable of maintaining its shape without binders.
  • the whiskers are composed of stoichiometric mullite or solid solutions of AI2O3 in stoichiometric mullite.
  • One embodiment of the present invention is a process for producing mullite whiskers or crystals using SiF 4 gas as a fluorine source.
  • SiF 4 is less expensive, safer to handle, and less demanding on production equipment than fluorine sources of previous methods for producing mullite whiskers.
  • the process preferably takes place in a closed system allowing the absorption, recovery, and re-use of SiF 4 .
  • the process also allows the use of starting materials that are less expensive and safer to handle than those of prior art methods. These materials include compounds such as aluminosilicate clay, alumina or silica.
  • the process comprises first heating a mixture containing at least aluminum and silicon atoms from ambient temperature to a temperature within a range of 500°C to 950°C in the presence of SiF 4 to form fluorotopaz from at least a portion of the mixture, whereby at least a portion of the SiF 4 is absorbed while fluorotopaz is being formed, then heating the fluorotopaz to a temperature within a range of 800°C to 1500°C to convert the fluorotopaz into non- stoichiometric mullite whiskers and SiF .
  • the SiF 4 is desirably captured for subsequent re-use.
  • the process converts various starting compounds that can be in a variety of physical forms into mullite whiskers.
  • the process further comprises:
  • step (c) cooling the mixture resulting from step (b) to a temperature within the 500°C to 950°C range, in the presence of Si 4 to form additional fluorotopaz in the same manner as in step (a) from at least a portion of the mixture resulting from step (b) (d) repeating step (b); and, optionally replicating steps (c) and (d) until a desired level of conversion to mullite whiskers is attained.
  • the mixture is heated to effect a substantially linear increase in temperature to a temperature within a range of 800°C to 1500°C.
  • Fluorotopaz formation generally occurs at intermediate temperatures within a range of 500°C to 950°C. Thereafter, when the temperature reaches the range of 800°C to 1500°C and sufficient SiF4 gas is present, the fluorotopaz is converted into mullite yielding single crystal mullite whiskers. SiF 4 gas is evolved and recovered during the production of mullite from fluorotopaz.
  • a second embodiment of the present invention is a mullite whisker-forming composition that produces an interlaced single crystal mullite whisker network having a controlled morphology and permeability.
  • the composition comprises a mixture of an aluminum- containing material in combination with a silicon- containing material and a fluxing agent that melts at temperatures below 1000°C and is capable of dissolving fluorotopaz.
  • Aluminum atoms and silicon atoms in the resultant product are present in a ratio that is approximately non-stoichiometric.
  • the fluxing agent includes at least one material that is a fluorine- containing bi-elemental molecule wherein the elemental component of the molecule other than fluorine has an ionic radius less than 0.74 Angstroms (7.4 nanometer).
  • the fluxing agent may be a compound that becomes a fluorine-containing bi-elemental molecule upon exposure to SiF 4 atom below 950°C. This allows that elemental component to substitute for at least some of the aluminum or silicon atoms within a mullite lattice resulting from the mixture.
  • the aluminum- containing material is preferably AI 2 O 3 or alpha- alumina.
  • the silicon-containing material may include silica, fused silica, an aluminosilicate mineral or kaolin. A preferred ratio of aluminum to silicon atoms in the starting materials is 4:1.
  • the fluxing agent is included to control the diameter of mullite whiskers, and desirably includes a source of magnesium, such as MgF 2 or hectorite.
  • the fluxing agent may include a lithium source, such as LiF or spodumene.
  • a preferred fluxing agent is a combination of a magnesium source and a lithium source, such as a 10-50 mole percent magnesium fluoride (MgF 2 ) and 50-90 mole percent LiF.
  • MgF 2 10-50 mole percent magnesium fluoride
  • One such preferred fluxing agent is 33% MgF 2 and 67% LiF.
  • the fluxing agent may also include the fluoride of at least one element selected from Li, Be, B, Mg, P, V, Cr, Mn, Fe,Co, Ni, Cu, Ga, Ge, Se, Mo, Sn, Sb, Te, Ta, or W. Furthermore, it is advantageous to have a fluxing agent that is a fluoride or an element that would become a fluoride upon exposure to SiF 4 gas.
  • the mullite whisker-forming composition yields a mullite whisker network wherein individual resultant mullite whisker average diameters range from 5 to 250 micrometers.
  • the mullite whisker network has a controlled permeability from 20 to 100 cm./sec./in. of water. Permeability is measured in a device which records the difference in pressure between the two sides of a disk (change in pressure in inches of water) and the rate of flow of air through the disk in cubic centimeters/minute. The ratio of the two is the permeability in centimeters per second per inch of water.
  • a third embodiment of the present invention is a regenerable exhaust gas filter element for diesel engines that comprises an interlaced network of mullite crystals grown together and automatically interlaced and fused with one another to form a rigid porous body such that soot particles carried by exhaust gas from a diesel engine flowing therethrough are trapped in the porous body. The soot particles may thereafter by burned off by conventional procedures to regenerate the element.
  • the element of the present invention is significantly stronger and more tolerant of thermal cycling than prior art elements.
  • the present filter element beneficially includes an interlaced network of single crystal mullite whiskers formed by the pyrolysis of fluorotopaz.
  • the present element preferably includes an interlaced network of single crystal mullite whiskers made from non-stoichiometric mullite crystals having about 2:1 molar ratio of alumina to silica, rather than a stoichiometric ratio as described in the prior art.
  • mullite whiskers can form a rigid structure. This structure can be formed or extruded into various shapes to produce elements capable of filtering particulate matter away from gases or liquids.
  • Illustrative filter elements include exhaust filters for removing soot from diesel exhausts, and filters for molten metals.
  • the filter element when used as a substrate for an inorganic membrane, functions as a microfilter.
  • FIGURE 1 shows a partial cut-away view of a side of a diesel exhaust filter element constructed in accordance with the present invention.
  • the channels and channel end plugs are visible as well as a cross-section of the walls separating the channels.
  • FIGURE 2 shows an end view of the diesel filter element of Figure 1 revealing the ends of the channels that are alternately plugged and unplugged.
  • FIGURE 3 depicts the channels more clearly and shows the direction of the flow of the diesel exhaust gases therethrough.
  • FIGURE 4 is a SEM photograph of a prior art cordierite diesel exhaust gas filter taken along a fractured surface of one of the element structures.
  • FIGURE 5 is a SEM photograph of a similar mullite filter element material prepared in accordance with the present invention.
  • a diesel exhaust filter is generally denoted by the numeral 10.
  • Channel walls 12 separate intake channels 14 from exhaust channels 16.
  • the intake channels are formed by plugging their downstream ends with plugs 18 while the exhaust channels are formed by plugging their upstream ends with plugs 20.
  • the view depicts the upstream end of the diesel filter as seen from its side.
  • Upstream ends 22 of the intake channels are surrounded by the channel walls 12.
  • Also surrounded by the channel walls 12 are plugs 20 for the upstream ends of the exhaust channels 16.
  • adjacent intake and exhaust channels alternate positions along rows as well as along columns.
  • the walls 12 of the filter element must be capable of trapping and retaining the soot particles commonly suspended in diesel exhaust gases.
  • the walls 12 must also allow the gases themselves to flow therethrough without excessive resistance.
  • any filter element naturally offers a certain amount of resistance to the flow of exhaust gases.
  • This resistance is due, to a small degree, to the restrictive nature of the narrow channels along which the exhaust gases must travel.
  • the resistance to the flow of exhaust gases is largely due to the finite permeability of the channel walls through which the gases are forced to flow.
  • the resistance to the flow of exhaust gases through the filter element results in a pressure drop from the upstream end of the filter to the downstream end of the filter.
  • the channel walls provide a large surface area and allow the diesel soot to be more evenly distributed throughout the available filter material.
  • the second characteristic is a greatly increased permeability that results from its especially porous nature as described in greater detail hereinbelow.
  • the walls of the filter element are made of an interlaced network of fused whiskers made of single crystal mullite.
  • the mullite is preferably non- -stoichiometric.
  • the mullite whisker network shown in the SEM photograph of Figure 5 is grown and fused together by a process similar to those of the examples given hereinbelow.
  • the individual mullite whiskers are interlaced and randomly oriented in a somewhat criss-cross fashion and are attached to one another at relatively few points along their lengths. This configuration includes relatively large open spaces between the whiskers. This is in contrast to a more orderly, compact arrangement where the whiskers are laying more or less side by side. If the whiskers are not fused together, but are formed instead into a loose mat, they are free settle into the more compact form with small open spaces between whiskers.
  • Figure 4 shows a SEM photograph of the fractured surface of a side portion of an individual cell wall of a cordierite diesel exhaust filter element of the prior art at 86X magnification.
  • Figure 5 shows the interlaced network mullite material of the present invention at 100X magnification.
  • magnifications are not identical, they illustrate the dramatic difference in porosity and consequent permeability.
  • the material of the present invention shown in Figure 5 has a considerable amount of open space between the whiskers, giving the network substantial porosity. This open space accounts for a large percentage of the total volume of the material.
  • the cordierite material shown in Figure 4 has very small holes that do not account for nearly as large a percentage of the total volume of the material as do the open spaces in the mullite whisker lattice shown in Figure 5.
  • the cordierite material has a substantially lower permeability than the material of the present invention.
  • a non-stiochiometric mullite structure having single crystal whiskers with individual whisker lengths of from 0.05 micrometers to 2 micrometers, individual whisker diameters of from 4 to 30 micrometers, and aspect ratios of from 10 to 50 is particularly useful for the filter element of the instant invention.
  • the preferred composition of non-stoichiometric mullite is 2Al2 ⁇ 3'Si ⁇ 2 having a 2:1 ratio of alumina to silica instead of stoichiometric 3:2 mullite (3Al2 ⁇ 3 # 2Si ⁇ 2) •
  • This composition of mullite is preferably formed by pyrolyzing fluorotopaz in the presence of catalysts and fluxes as described more fully hereinbelow.
  • the mullite whiskers of the present invention are grown and fused simultaneously. This is an advantage over a network of whiskers that are first grown in one step, and then bonded together in a later step.
  • the loose whiskers are in danger of settling into a less porous structure before they have a chance to be bonded together.
  • the mullite whiskers of the present invention grow randomly in all directions with large distances and open spacesbetween the whiskers. As the whiskers grow, they fuse with one another where they touch, thereby preserving the open nature of the whisker lattice.
  • the process of the present invention uses starting compounds or mixtures that contain at least a source of aluminum and a source of silicon.
  • Preferred starting compounds include abundantly available and inexpensive materials such as aluminosilicate clay, alumina or silica.
  • the starting compounds are heated in the presence of SiF 4 gas to form an interlaced network of mullite whiskers.
  • SiF 4 appears to behave as a catalyst because it is absorbed during a first step and subsequently evolved and, preferably, recovered during a final step of the process.
  • the present invention includes heating, in the presence of SiF 4 , the starting compounds or mixtures to a temperature within a range of from 500°C to 950°C to allow for formation of fluorotopaz (Al 2 F 2 Si ⁇ 4 ) from the starting compounds. Other gases or impurities may be present.
  • the SiF 4 provides all of the fluorine required to form fluorotopaz.
  • the present invention may be practiced by substantially linearly heating the starting compounds or mixtures, or it may be performed by heating to the fluorotopaz and mullite formation ranges, 500-950°C and 800-1500°C, respectively, as heating plateaus.
  • the present invention may be practiced by cycling between the two temperature ranges in order to convert substantially all of the aluminum and silicon in the starting compounds into mullite.
  • pyrolyzation of the fluorotopaz occurs at temperatures within a range of 800-1500°C.
  • the fluorotopaz is converted into single crystal mullite whiskers. It appears that the 500-950°C fluorotopaz formation range and the 800-1500°C mullite conversion range overlap. In practice, however, they do not overlap because the temperature used to effect conversion of fluorotopaz to mullite depends upon the vapor pressure of the SiF 4 gas. If the vapor pressure is very low, as is the case when a vacuum pump is used to remove substantially all of the SiF 4 , the necessary temperature for the reaction is low.
  • the reaction is, however, very slow at temperatures near 800°C. If large mullite whiskers are desired, the conversion of fluorotopaz preferably occurs at temperatures of 975-1500°C. If the vapor pressure is low, a higher temperature is required in order to make the reaction go forward.
  • SiF 4 is evolved and returned to the atmosphere surrounding the mullite. In other words, SiF 4 concentration is lower when conversion of fluorotopaz to mullite begins than when it nears completion. As such, temperatures must be higher when conversion nears completion than when it begins.
  • the evolved SiF 4 is preferably recaptured and used for the next reaction.
  • the mullite resulting from this reaction is substantially non-stoichiometric with a predominantly 5 2:1 ratio of the alumina component to the silica component (2Al 2 ⁇ 3 -lSi ⁇ 2 ) , as opposed to stoichiometric mullite (3Al 2 ⁇ 3.2Si ⁇ 2 ) .
  • the SiF 4 consumed in the 0 first (fluorotopaz formation) temperature range is substantially fully recovered in the second (mullite conversion) range so long as all the fluorotopaz is converted into mullite.
  • SiF 4 need only be introduced into the system in a sufficient quantity before the starting compounds are heated to temperatures within the 500-950°C range.
  • the system can then be sealed, and the compounds subsequently heated to the temperatures necessary for the formation of fluorotopaz and then mullite.
  • the use of a closed system constitutes a preferred embodiment of the present invention.
  • the Si 4 that is absorbed is a mixture of some of the original SiF 4 and some of the SiF 4 that was released during the conversion of fluorotopaz into mullite. If enough SiF to convert substantially all of the starting compound is introduced at the start of the process of the present invention, subsequent heating to temperatures in the mullite conversion range forms additional mullite. Replication of these steps to attain a desired degree of conversion constitutes another preferred embodiment of the present invention.
  • the process of the present invention lends itself to use in a closed system, it is by no means limited to a such a system.
  • the SiF4 evolved during mullite formation could be carried away and fresh SiF 4 supplied during fluorotopaz formation.
  • the advantage to the closed system is that one batch of SiF 4 could be used over and over again saving the expense of continually replenishing the SiF 4 .
  • Also with the closed system there is no need to dispose of the SiF 4 , a highly toxic gas.
  • the starting compound or mixture used in the present invention may be heated by any advantageous method.
  • the materials can simply be heated up to a temperature within the range of 800-1500°C with a substantially linear temperature increase starting from room temperature.
  • the formation of fluorotopaz occurs rapidly when the temperature reaches the 500-900°C range.
  • Mullite is rapidly formed when the temperature reaches the range of 800-1500°C, preferably 975-1500°C. The rapidity of the process depends upon the temperature.
  • fluorotopaz and its subsequent decomposition to form mullite may occur in two separate steps, these steps may be optimized independently. For instance, it may be desirable to heat the starting compounds up to the temperature range required for the formation of fluorotopaz at a particular rate, and then remain at this temperature until a desired amount of fluorotopaz has been formed. The temperature could then be raised at a different rate until a suitable temperature within the temperature range required for the formation of mullite is attained, and then held at this temperature until substantially all the fluorotopaz has been converted into mullite. During either of these two separate steps, various parameters such as pressure could be adjusted in order to optimize the process.
  • the process of the present invention is very tolerant of the choice of starting compounds. All that is required for starting compounds is a composition containing aluminum and silicon in a molar ratio of about 4 aluminum atoms to one silicon atom. Especially useful compositions include: alumina and silica or combinations of alpha alumina and fused or other silicas; aluminosilicate or other clays; and kaolin and spodumene. Since substantially all the fluorine necessary for the formation of fluorotopaz is provided by the surrounding atmosphere of SiF 4 , no fluorine containing materials need to be included in the starting compounds.
  • the starting compounds or mixtures can be in the form of a loose powder or a compacted solid, such as a compacted greenware article.
  • a starting compound in the form of loose powder is processed by the present invention, the resultant product predominantly consists of large single crystal mullite whiskers loosely caked together. These caked whiskers are easily separated into loose whiskers which have many uses including thermal insulation and reinforcement of ceramics.
  • the starting compounds are preferably in the form of a compacted solid or greenware article.
  • the resultant product is a porous body made up of an interlaced and strongly fused network of large single crystal mullite whiskers.
  • the porous body has a shape that is substantially the same as that of the greenware article.
  • These porous bodies also have many important uses, including diesel exhaust filters, filters for molten metals and substrates for membrane submicron filters, to name a few.
  • Converting greenware made from alumina and silica in accordance with the present invention yields a porous body having a density greater than that resulting from greenware made of fluorotopaz.
  • greenware made from aluminum and silica and having a density of 50% of theoretical provides a porous body having essentially the same density.
  • the greenware made from fluorotopaz while starting with the same density, yields a porous body with a density of 36% of theoretical.
  • a fluxing agent during the formation of mullite promotes growth of a fused, interlaced network of relatively large single crystal mullite whiskers. This network is very suitable for use in filter applications with large open spaces between individual whiskers.
  • the diameter of mullite whiskers and the permeability of a filter formed from the whiskers is controlled primarily by limiting the number of whiskers that form from the fluorotopaz being pyrolyzed or converted. This makes more fluorotopaz available per whisker and allows each whisker to grow to a larger size.
  • the present invention adds a fluxing agent to a mullite whisker-forming composition before the fused interlaced network of non-stoichiometric single crystal mullite whiskers is formed.
  • the fluxing agent should be capable of dissolving fluorotopaz and is preferably an eutectic mixture of magnesium fluoride and lithium fluoride. The ratio of these two compounds may vary somewhat while still promoting large mullite whiskers.
  • the compounds are preferably mixed in a proportion of approximately 66.9 mole percent LiF and 33.1 mole percent MgF 2 to provide a composition that melts at about 750°C.
  • the data presented in the table show that permeability of the sample disk is dramatically affected by the amount of fluxing agent used in the starting materials.
  • the maximum permeability results from using starting materials with approximately 1.5% by weight of the fluxing agent with a 66.9/33.1 mole ratio of LiF to MgF 2 although other ratios are useful.
  • the data also show that the present invention controls the permeability of a filter made of interlaced single crystal non-stoichiometric mullite whiskers.
  • LiF and Mg 2 are preferred flux compound components, other fluxing agents can be used. Satisfactory fluxing agents provide ions that migrate to the surface of growing mullite crystals as well as a liquid path for those migrating ions. The ions should have ionic radii that allow them to substitute for Al or Si in the mullite lattice. The ionic radii of Al+3 and Si+3 are 0.50 Angstroms (5 nanometer (nm)) and 0.41 Angstroms (4.1 nm) respectively. Li+ and Mg+2 have ionic radii of 0.60 Angstroms (6 nm) and 0.65 Angstroms (6.5 mn) respectively and are known to work successfully as fluxing agents.
  • Bi+3 has an ionic radius of 0.74 Angstroms (7.4 nm), but bismuth oxide is not effective as a flux. It is apparent that the ionic radius of the active ion of a successful bi-elemental flux must be less than 0.74 Angstroms (7.4 nm) . Satisfactory fluxing agents must also melt at temperatures less than 1000°C.
  • Elements that have sufficiently small ionic radii and whose fluorides, or whose compounds that become fluorides upon exposure to SiF 4 at or below 950°C, can be used as fluxing agents.
  • Suitable elements are Li, Be, B, Mg, P, V, Cr, Mn Fe, Co, Ni, Cu, Ga, Ge, As, Se, Mo, Sn, Sb, Te, Ta, or W.
  • Li 2 ⁇ and MgO may react with Si 4 to form MgF 2 and LiF.
  • a general process for forming a mullite whisker network having a controlled permeability includes mixing an alumi.num-containi.ng material wi.th a si.li.con- containing material and a fluxing agent.
  • the materials provide a ratio of aluminum atoms to silicon atoms that is approximately non-stoichiometric.
  • the resultant mixture is heated in the presence of a fluorine- containing source to form fluorotopaz.
  • the fluorotopaz is thereafter pyrolyzed to form mullite whiskers.
  • Heating of the mixture from room temperature to 1500°C occurs at a rate within a range of from 0.25 to 15°C per minute in the presence of SiF 4 .
  • Other fluorine-containing sources such as AIF 3 , Na 2 , NaF, NaF, NH3F 2 , HF, a 2 SiF 6 , NH 4 F, SbF 3 , LiF, or MgF 2 may be used in conjunction with, or in place of, SiF 4 if appropriate equipment or procedures are employed.
  • FIG. 5 depicts the network formed in Example 2 with the aid of a fluxing agent. In the absence of a fluxing agent, as in Example 1, the mullite whiskers are comparatively smaller. Examples 1 and 2 represent different aspects of the present invention.
  • Mullite is an excellent material for diesel filter applications for many reasons. It has a high thermal shock resistance. This allows the filter to survive high temperatures reached during regeneration without deteriorating. It also allows the filter to withstand repeated cycling from use temperatures to temperatures experienced during regeneration. It also has a high thermal conductivity. This allows the filter element to conduct the large amount of heat produced during regeneration to its surroundings before the element has a chance to get too hot.
  • Non-stoichiometric 2:1 mullite (2Al2 ⁇ 3'Si ⁇ ) is particularly useful because it has a measured thermal conductivity of 12.4 W/mK versus 4.0 W/mK for stoichiometric 3:2 mullite (3Al2 ⁇ 3*2Si ⁇ 2)• Mullite, particularly non- stoichiometric mullite, has a greater heat capacity than other candidate materials such as cordierite. This enables a filter to operate at a lower temperature and tends to decrease the thermal shock experienced by the filter material.
  • the high mechanical strength of mullite is also advantageous in diesel filter applications because the filter is better able to withstand the vibrations encountered when attached to a moving vehicle.
  • a mixture containing 3.4 parts by weight of alpha alumina and 1 part by weight of fused (amorphous) silica was dry-blended by tumbling.
  • the mixture was
  • Thin disks were prepared by dry-pressing fluorotopaz powders at 500 psi.
  • the powders contained various amounts of a flux and a binder.
  • the flux was a mixture containing 33-1 mole % MgF2 and 66.9 mole % LiF.
  • the diameter of the pellets was 2.86 cm (1.125 inches).
  • the pellets were fired at 600°C for 3 hours to burn out the binder. They were then heated in an atmosphere of SiF4 to 700°C at a rate of 10°C/minute.
  • the disks were then heated to 950°C at a rate of 3°C/minute, then to 1050°C at the rate of 1°C/minute, and finally to 1102°C at 0.5°C/minute. They were held at 1120°C for 4 hours.
  • the resultant disks were tested and found to be made of fused and interlaced whiskers of single crystal non- stoichiometric mullite of the composition 2Al2 ⁇ 3*
  • the materials made in the above examples exhibit porosity and excellent permeability.
  • the resultant mullite network can easily be formed into diesel filter element in any desired shape or configuration.

Abstract

Un procédé de préparation de trichites de mullite consiste à chauffer un mélange non-stoechiométrique contenant de l'aluminium et du silicium jusqu'à une température comprise entre 500 °C et 950 °C en présence du gaz composé de SiF4 pour former du fluorotopaze, puis jusqu'à une température comprise entre 800 °C et 1500 °C pour convertir le fluorotopaze en mullite. Une composition formant des trichites de mullite, et qui produit un réseau entrelacé de trichites de mullites à monocristal présentant une perméabilité contrôlée, comprend un mélange d'un matériau contenant de l'aluminium et d'un matériau contenant du silicium, ainsi qu'un agent de fluxage qui fond à des températures inférieures à 1000 °C. Le rapport d'atomes d'aluminium et d'atomes de silicium dans le produit qui en résulte est approximativement non-stoechiométrique. Le réseau peut être utilisé notamment pour fabriquer un élément filtrant de gaz d'échappement pouvant être régénéré, pour des moteurs diesel.
EP92903140A 1990-12-21 1991-12-13 Preparation et utilisation de reseaux de trichites de mullite Withdrawn EP0563261A1 (fr)

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Application Number Priority Date Filing Date Title
US63357690A 1990-12-21 1990-12-21
US63357990A 1990-12-21 1990-12-21
US07/633,580 US5098455A (en) 1990-12-21 1990-12-21 Regenerable exhaust gas filter element for diesel engines
US633580 1990-12-21
US633579 1990-12-21
US633576 1996-04-17

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ES2158750B1 (es) * 1998-05-29 2002-04-16 Consejo Superior Investigacion Procedimiento para la obtencion de materiales ceramicos porosos compuestos de mullita en forma de fibras cortas monocristalinas.
WO2004096729A2 (fr) * 2003-04-24 2004-11-11 Dow Global Technologies Inc. Corps ameliores de mullite poreuse et procedes de formation de ces derniers
CN1845780B (zh) * 2003-08-29 2010-06-16 陶氏环球技术公司 改进的柴油机排气滤清器
US7341970B2 (en) 2004-03-31 2008-03-11 Corning Incorporated Low thermal expansion articles
JP2011525889A (ja) * 2008-06-27 2011-09-29 ダウ グローバル テクノロジーズ エルエルシー 多孔質の針状ムライト体の作製方法
RU2011115066A (ru) 2008-09-18 2012-10-27 ДАУ ГЛОБАЛ ТЕКНОЛОДЖИЗ ЭлЭлСи (US) Способ изготовления пористых муллит-содержащих композитов
CN102471173B (zh) 2009-06-29 2014-02-19 陶氏环球技术有限责任公司 用于制备胶接的和表皮化的针状莫来石蜂窝结构体的方法
CN103562154A (zh) 2011-03-29 2014-02-05 陶氏环球技术有限责任公司 制造多孔莫来石-铝假板钛矿复合材料的方法
CN103649012B (zh) * 2011-07-06 2017-05-24 陶氏环球技术有限责任公司 制造多孔针状莫来石体的方法
US9527775B2 (en) * 2011-12-09 2016-12-27 Vecor Ip Holdings Limited Percolated mullite and a method of forming same
DE112013005817T5 (de) 2012-12-05 2015-09-24 Dow Global Technologies Llc Poröse Mullitkörper mit verbesserter thermischer Beständigkeit
WO2014088618A1 (fr) 2012-12-05 2014-06-12 Dow Global Technologies Llc Corps de mullite poreux ayant une stabilité thermique améliorée
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CN110550965B (zh) * 2019-09-20 2021-12-10 中国民航大学 一种莫来石晶须低温生长增韧型磷酸铝基高温胶制备方法
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