PREPARATION AND USE OF MULLITE WHISKER NETWORKS

Abstract: A process for producing mullite whiskers includes heating a non-stoichiometric mixture containing aluminum and silicon to a temperature within a range of 500 DEG C to 950 DEG C in the presence of SiF4 gas to form fluorotopaz and then to a temperature within a range of 800 DEG C to 1500 DEG C to convert the fluorotopaz into mullite. A mullite whisker-forming composition that procudes an interlaced network of single crystal mullite whiskers with a controlled permeability comprises a mixture of an aluminum-containing material with a silicon-containing material and a fluxing agent which melts at temperatures below 1000 DEG C. The ratio of aluminum atoms to silicon atoms in the resultant product is approximately non-stoichiometric. The network may, for example, be used to fabricate a regenerable exhaust gas filter element for diesel engines.

Description: PREPARATION AND USE OF MULLITE WHISKER NETWORKS 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. These fabrication techniques experienced a number of problems. First, the reaction rate was inhibited due to the slow rate at which fluorine was made available by decomposition of the solid fluorine- containing starting compound. Second, the solid fluorine-containing starting compounds were expensive. Third, the reaction formed fluorine-containing by¬ products that posed disposal problems. One common, though extremely toxic and dangerous, by-product that resulted from using AIF 3 was SiF gas. 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. Although the improved methods tend to be faster than methods using solid fluorine sources, 10 additional rate improvements are desirable. PCT publication US89/03175 discloses a process in which AIF 3 and fused Siθ 2 , or AIF 3 , Siθ 2 , and AI 2 O 3 powders are first formed into a green body of a desired *[- shape and size. 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. For certain 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. If further conversion of the mixture is desired, the process further comprises: (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. In a related embodiment 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). As an alternative, 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. In addition, 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. 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. With the inclusion of the fluxing agent, 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. When interlaced and fused together, 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. The nature and extent of the present invention will be clear from the following detailed description of the particular embodiments thereof, taken in conjunction with the appendant drawings. 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. Referring first to Figure 1, 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. Referring next to Figure 2, 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. As can be seen from Figure 2, adjacent intake and exhaust channels alternate positions along rows as well as along columns. Referring next to Figure 3 , we see where the gases enter into the upstream end 22 of the intake channel 14, flow through walls 12, and exit through the downstream ends 24 of exhaust channels 16. As in Figures 1 and 2, the plugs that seal up the ends of the channels determine whether the channel serves as an intake channel or as an or as an exhaust channel. 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, such as the one just described, 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. As noted above, there are two characteristics of the diesel exhaust filter element of the present invention that serve to reduce the pressure drop across the filter element. First, 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. Pursuant to a preferred embodiment of the present invention, 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. As can be seen from the photograph, 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. For comparison, Figure 5 shows the interlaced network mullite material of the present invention at 100X magnification. Although the 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 2Al2θ3'Siθ2 having a 2:1 ratio of alumina to silica instead of stoichiometric 3:2 mullite (3Al2θ3 # 2Siθ2) • 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. When the mullite whiskers of the present invention are grown, they 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 Siθ 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. In addition, 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. During the final stages of the present process, pyrolyzation of the fluorotopaz, or conversion of the fluorotopaz to mullite, occurs at temperatures within a range of 800-1500°C. During pyrolysis, 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. During heating in the mullite conversion range, 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. 10 The reactions for the two steps that occur in the process of the present invention are proposed as follows: * t - For the formation of fluorotopaz from one of the possible starting mixtures; 2AI 2 O3 + SiO 2 + SiF > 2Al 2 F 2 Si0 4 For the formation of mullite from fluorotopaz; 0 2Al 2 F 2 Siθ 4 > 2Al 2 θ 3 -Siθ 2 + SiF 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 θ 3 -lSiθ 2 ) , as opposed to stoichiometric mullite (3Al 2 θ3.2Siθ 2 ) . In a closed system, 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. In other words, 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. By using a closed system, it is possible to replicate the first portion of the process in order to convert unprocessed starting compounds into fluorotopaz without introducing additional SiF 4 gas. In addition, the use of a closed system minimizes, if not eliminates, release of SiF 4 to an outside atmosphere. One can replicate the first portion of the process simply by cooling the materials back down to a temperature within the range of 500-950°C and allowing some or all of the remaining starting compounds to absorb SiF 4 and form fluorotopaz. 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. Although 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, however, 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. For example, 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. In this case, 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. Since the formation of 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. When 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. In this case, 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. As an illustration, 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. The presence of 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. One can limit the number of whiskers for a given amount of fluorotopaz by reducing the number of nucleation sites from which mullite whiskers grow. 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. A study has been made of the effects of including varying amounts of this 66.9/33.1 (LiF/MgF 2 ) mole ratio fluxing agent mixture in starting materials used to prepare filters made of mullite whiskers. The results appear in the following table. The data in this table were gathered from sample disks of filter material prepared by the method given in the example described hereinbelow. In the table, the percentage weight of the flux used in the starting materials of a given sample disk is listed along with the resultant permeability of the same disk. The Effect of Flux Upon Permeability ι < 10.0 0.051 43.0 0.42 0.6 1.4 a Measured as Velocity (Pressure Drop) x (Unit Area) 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. Although 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. By way of illustration, Li 2 θ 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. The use of a fluxing agent yields large mullite whiskers in an interlaced network of single crystal mullite whiskers obtained by pyrolyzing fluorotopaz as shown in the SEM photograph of Figure 5. Figure 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 (2Al2θ3'Siθ ) 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 (3Al2θ3*2Siθ2)• 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. The following are specific examples of methods for the formation of a network of a single crystal non- stoichiometric mullite whiskers in accordance with the present invention. Example 1 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 _- placed in a furnace that was connected to a flexible plastic bag containing gaseous SiFJi. The furnace and its contents were evacuated, then back-filled with SiFL\ from the bag, and then heated to 1125°C. As the temperature approached 950°C, the bag could be seen to 10 shrink as SiF4 was absorbed by the contents of the furnace that were being converted to fluorotopaz. At about 1050°C, the bag began to expand to its original volume as fluorotopaz began to decompose and release SiFi|. Heating continued until the furnace reached 15 1125°C. This temperature was maintained for approximately one hour. The furnace was then cooled to room temperature. After cooling, the bag containing SiFij was disconnected from the furnace and an inert gas 0 was purged through the furnace until substantially all traces of SiF4 were removed. The furnace contained fused and interlaced whiskers of single crystal non- stoichiometric mullite of the composition 2Al2θ3'Siθ2. c The results of a visual inspection showed the product to be made of a material which had an enhanced porosity and good mechanical strength. 0 Example 2 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 2Al2θ3*Siθ2- 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. While our invention has been described in terms of a specific embodiment, it will be appreciated that other embodiments could readily be adapted by one skilled in the art. Accordingly, the scope of our invention is to be limited only by the following claims.

Claims: WHAT IS CLAIMED IS: 1. A process for producing mullite whiskers, comprising: (a) heating a mixture containing at least a source of aluminum and a source of silicon and having a ratio of aluminum atoms to silicon atoms of about 4 to 1, from ambient temperature to a temperature within a range of from 500°C to 950°C in the presence of Si 4 to form fluorotopaz from at least a portion of the mixture, whereby at least a portion of the SiF 4 is absorbed while 10 the fluorotopaz is being formed; and (b) heating the mixture to a temperature of from 800°C to 1500°C to convert the fluorotopaz into non- stoichiometric mullite whiskers and SiF 4 . 15 2. A process for forming an interlaced, single crystal, mullite whisker network having a controlled permeability, comprising: (a) heating a mixture comprising an aluminum- 20 containing material, a silicon-containing material, the materials providing a ratio of aluminum atoms to silicon atoms that is approximately non-stoichiometric, and a fluxing agent that melts at a temperature below 1000°C, pc - is capable of dissolving fluorotopaz, and includes at least one fluorine-containing bi-elemental molecule wherein the elemental component of the molecule other than fluorine has an ionic radius less than 0.74 Angstroms (0.074 nanometer), in the presence of a fluorine-containing source, from ambient temperature to a temperature within the range of from 500°C to 950°C to form fluorotopaz; and (b) heating the mixture to a temperature of from 800°C to 1500°C to convert the fluorotopaz into a network of non-stoichiometric mullite whiskers, the fluxing agent promoting enlargement of the diameter of individual resultant mullite whiskers thereby providing a degree of control over network permeability. 3. A process as claimed in Claim 2 wherein the fluorine-containing source is selected from AIF 3 , SiF , Na 2 F, NaF, NH 3 F 2 , HF, Na 2 SiF 6 , NH 4 F, SbF 3 , LiF or MgF 2 . 4. A process as claimed in Claim 2 wherein the fluorine-containing source is SiF . 5. A process as claimed in Claim 1 or Claim 4 further comprising: (c) cooling the mixture resulting from step (b) to a temperature within the range of from 500°C to 950°C in the presence of SiF 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. 6. A process as claimed in any of the preceding Claims wherein the heating and formation of fluorotopaz and the conversion of fluorotopaz to mullite take place in a closed system. 7. A process as claimed in any of the preceding Claims wherein the aluminum-containing material is AI 2 O 3 or alpha alumina and the silicon- containing material is silica, fused silica or an aluminosilicate mineral. 8. A process as claimed in any one of Claims 2-7 wherein the fluxing agent comprises a combination of from 10-50 mole % MgF 2 and 50-90 mole % .LiF. 9. A process as claimed in any one of Claims 2-8 wherein the fluxing agent further comprises at least one fluoride of element selected from Be, B, P, V, Cr, Mn, Fe, Co, Ni, Cu, Ga, Ge, Se, Mo, Sn, Sb, Te, Ta, or W. 10. A process as claimed in any of the preceding Claims wherein said mixture is in the form of loose powder. 11. A process as claimed in any of Claims 1-9 wherein said mixture is in the form of a ceramic greenware article. 12. A mullite whisker-forming composition that produces an interlaced single crystal mullite whisker network having a controlled permeability and comprises a mixture comprising an aluminum-containing material in combination with a silicon-containing material in which the ratio of aluminum atoms to silicon atoms is approximately non-stoichiometric, and a fluxing agent that melts at temperatures below 1000°C. and is capable of dissolving fluorotopaz, the fluxing agent including 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 about 0.74 Angstroms (7.4 nanometer). 13. A mullite whisker-forming composition as claimed in Claim 12 wherein the aluminum-containing material is AI 2 O 3 or alpha alumina and the silicon- containing material is silica, fused silica or an aluminosilicate mineral. 14. A mullite whisker-forming composition as claimed in Claim 12 or Claim 13 wherein the ratio of aluminum and silicon atoms in the mixture is 4 to 1. 15. A mullite whisker-forming composition as claimed in any one of Claims 12-14 wherein said fluxing agent comprises a combination of from 10-50 mole % MgF 2 and 50-90 mole % LiF. 16. A mullite whisker-forming composition as claimed in Claim 15 wherein the fluxing agent further comprises at least one fluoride of element selected from Be, B, P, V, Cr, Mn, Fe, Co, Ni, Cu, Ga, Ge, Se, Mo, Sn, Sb, Te, Ta, or W. 17. An interlaced network of mullite crystals as produced by any one of Claims 2-9 or Claim-11. 18. The network of Claim 17 wherein said mullite crystals have diameters of between 4 and 30 micrometers. 19. A regenerable exhaust gas filter element for diesel engines comprising an interlaced network of mullite crystals grown together to form a rigid porous body such that soot particles carried by exhaust gas from a diesel engine flowing therethrough is trapped in the porous body, said network of mullite crystals including non-stoichiometric crystals of alumina and silica, said network of mullite crystals being automatically interlaced and fused with one another to form the porous body. 20. A method of removing soot particles from exhaust gas produced by a diesel engine comprising using a network as claimed in Claim 17 as a filter element.

Inventors: BRUBAKER BURTON D, DOTY PETER A, HUGHES NEAL N, MOYER JOHN R, BRUBAKER, BURTON, D., DOTY, PETER, A., HUGHES, NEAL, N., MOYER, JOHN, R.
Applicants: DOW CHEMICAL CO, THE DOW CHEMICAL COMPANY
Application year: 1991
Main classification: C04B35
IPC codes: B01D39, C01B33, C01B39, C04B30, C04B38, C30B9, C30B29, F01N3, F02B3