Mullite-aluminum titanate body and method for making same

Abstract

This invention relates to a mullite-aluminum titanate body having a low coefficient of thermal expansion of less than 15×10 −7 C −1 , a high porosity of at least 38% by volume, a median pore diameter of at least 8 microns, and a narrow pore size distribution as characterized by the relation (d 50 -d 10 )/d 50 being less than 0.50 corresponding to a high degree of interconnected porosity. The inventive ceramic body also contains at least 0.10% by weight metal oxide, the metal being either yttrium, calcium, bismuth, a lanthanide metal or combinations of thereof. The inventive ceramic body is particularly useful as a wall-flow filter for diesel exhaust. A method of fabrication is provided where the sintering temperature is between 1375°-1550° C.

1 . A ceramic body comprising phase of mullite and aluminum titanate, and at least 0.10% by weight of a metal oxide for a metal selected from the group consisting of bismuth, calcium, yttrium, lanthanides and combinations thereof, while exhibiting a set of properties including a coefficient of thermal expansion (RT-1000° C.) less than 15×10 −7  C −1 , a porosity of at least 38% by volume, a median pore diameter of at least 8 microns, and a narrow pore size distribution as characterized by the relation (d 50 -d 10 )/d 50  being less than 0.50 corresponding to a high degree of interconnected porosity.  
   
   
       2 . The ceramic body of  claim 1  wherein the metal oxide is in an amount of between 0.10% to 5.0% by weight.  
   
   
       3 . The ceramic body of  claim 2  wherein the metal is yttrium.  
   
   
       4 . The ceramic body of  claim 1  wherein the ceramic body exhibits said set of properties when sintered to a temperature of between 1375° C. to 1550° C.  
   
   
       5 . A diesel exhaust particulate filter comprising the ceramic body of  claim 1 , wherein the ceramic body is a plugged, wall-flow honeycomb body having a plurality of parallel end-plugged cell channels traversing the body from a frontal inlet end to an outlet end thereof.  
   
   
       6 . The diesel exhaust particulate filter of  claim 5  further exhibiting a coefficient of thermal expansion (RT-1000° C.) not greater than 10×10 −7  C −1 , a porosity of between 45-60% by volume, a median pore diameter of between 10-20 microns, and a narrow pore size distribution as characterized by the relation (d 50 -d 10 )/d 50  being not greater than 0.35 corresponding to a high degree of interconnected porosity.  
   
   
       7 . A method for making a mullite-aluminum titanate ceramic body comprising: 
 a. providing a mixture of inorganic raw materials comprising an alumina source, a silica source, and a titanium dioxide source, in combination with a source of a metal oxide as a sintering additive in an amount of at least 0.10% by weight super-addition, the source corresponding to an oxide of a metal selected from the group of metals consisting of bismuth, calcium, yttrium, lanthanides and combinations thereof,    b. shaping the mixture into a body; and,    c. sintering the body to a temperature of between 1375° C. to 1550° C. for a period of between 1 hour to 15 hours;    wherein the weighted average of the median particle diameters of the inorganic raw materials, D 50 , is at least 6 microns to form a pore size in the mullite-aluminum titanate ceramic body after sintering of at least 8 microns.    
   
   
       8 . The method of  claim 7  wherein the metal oxide is added to the raw material mixture in an amount between 0.10% and 5.0% by weight.  
   
   
       9 . The method of  claim 8  wherein the metal is yttrium.  
   
   
       10 . The method of  claim 7  wherein an amount of at least 0.05% by weight of molybdenum oxide or tungsten oxide is further added to the mixture.  
   
   
       11 . The method of  claim 7  wherein the alumina source is a selected from a group consisting of corundum, gamma-alumina or another transitional alumina, boehmite, alumina hydroxide (gibbsite) and mixtures thereof.  
   
   
       12 . The method of  claim 11  wherein the alumina source has a median particle diameter greater than 15 microns.  
   
   
       13 . The method of  claim 7  wherein the mixture of inorganic raw materials further includes an aluminosilicate source.  
   
   
       14 . The method of  claim 13  wherein the aluminosilicate source is selected from the group consisting of mullite, kyanite, sillimanite, kaolin, calcined kaolin, pyrophyllite, and mixtures thereof.  
   
   
       15 . The method of  claim 7  wherein the silica source is selected from the group consisting of quartz, cristobalite, zeolite, diatomaceous earth, fused silica, colloidal silica, amorphous silica, and combinations thereof.  
   
   
       16 . The method of  claim 7  wherein the titanium dioxide source is selected from the group consisting of rutile, anatase, amorphous titania, and mixtures thereof.  
   
   
       17 . The method of  claim 7  wherein the alumina source and titanium dioxide source have median particle or agglomerate diameters of at least 10 microns.  
   
   
       18 . The method of  claim 7  wherein the metal oxide source is selected from the group consisting of bismuth oxide, calcium carbonate, calcium hydroxide, calcium aluminate, calcium titanate, calcium silicate, yttrium or rare earth oxide, hydroxide, carbonate, fluoride-carbonate, aluminate, silicate, titanate, chloride, nitrate, acetate, or other soluble or insoluble salt, a mixed rare earth concentrate such as bastnasite, calcined bastnasite, or monazite, and combinations thereof.  
   
   
       19 . The method of  claim 18  wherein the metal oxide source has a median particle diameter of less than 5 microns.  
   
   
       20 . The method of  claim 7  wherein the mixture is shaped by extrusion through a die to form a honeycomb structure.