Fabrication of ultra low thermal expansion cordierite structures

Abstract

Disclosed is sintered ceramic article that exhibits a primary crystalline phase of cordierite and analytical oxide composition, in weight percent, of 49-53% SiO 2 , 33-38% Al 2 O 3 , 12-16% MgO and exhibits a coefficient of thermal expansion no greater than about 4.0×10 −7 /° C. over the temperature range of about 25° C. to about 800° C. and a transverse-I ratio of not less than about 0.92. Also disclosed is a method for producing a sintered cordierite ceramic article involving preparing a plasticizable raw material, comprising a magnesium source, a SiO 2 -forming source and an additional component of either: (a) a clay-free, Al 2 O 3 -forming source having a surface area of greater than about 5 m 2 /g; or, (b) a clay and Al 2 O 3 -forming source combination wherein the clay comprises no greater than about 30%, by weight, of the total inorganic mixture, and the Al 2 O 3 -forming source exhibits a surface area of greater than about 40 m 2 /g. The mixture is formed into a green body substrate of the desired configuration and subsequently dried and fired for a time and at temperature sufficient to form a structure having the aforementioned CTE and I-ratio properties.

We claim:  
     
         1 . A ceramic article having sintered phase composition, by weight, comprising at least about 93% cordierite, consisting essentially of about, by weight, 49-53% SiO 2 , 33-38% Al 2 O 3 , 12-16% MgO and which exhibits a coefficient of thermal expansion in at least one direction no greater than about 4.0× 10   −7 /° C. over the temperature range of about 25° C. to about 800° C. and a transverse I-ratio of not less than about 0.92.  
     
     
         2 . A ceramic article in accordance with  claim 1  which exhibits a coefficient of thermal expansion no greater than about 3.0×10 −7 /° C. over the temperature range of about 25° C. to about 800° C.  
     
     
         3 . A ceramic article in accordance with  claim 1  that exhibits a transverse I-ratio of at least about 0.93.  
     
     
         4 . A ceramic article in accordance with  claim 1  which further exhibits an axial I-ratio of less than about 0.41.  
     
     
         5 . A ceramic article in accordance with  claim 1  wherein the ceramic article comprises a honeycomb configuration.  
     
     
         6 . A ceramic article in accordance with  claim 1  wherein the article exhibits a porosity greater than about 15%.  
     
     
         7 . A method of producing a ceramic body having cordierite as its primary phase, comprising the following steps: 
 selecting raw materials to form a plasticizable inorganic raw material mixture having a chemical composition consisting essentially of about, by weight, 49-53% SiO 2 , 33-38% Al 2 O 3 , 12-16% MgO, the mixture comprising a magnesium-source comprising a platy talc having a morphology index of greater than 0.75 and an SiO 2  forming source comprising either crystalline or amorphous silica and one of the two following additional components, 
 (a) a substantially clay-free Al 2 O 3  forming source having a surface area of greater than about 5 m 2 /g;  
 (b) a combination of clay and an Al 2 O 3 -forming source wherein the clay comprises no greater than about 30% clay, by weight, of total inorganic mixture, and the Al 2 O 3  forming source exhibits a surface area of greater than about 40 m 2 /g  
   adding an organic binder system, including water to the inorganic mixture and kneading the mixture and thereafter forming a green body.    drying the green body and thereafter firing the green body at a time and at a temperature between 1380° C. and 1450° C. to result in sintered ceramic body having at least 93% cordierite which exhibits a transverse I-ratio of not less than about 0.92 and a coefficient of thermal expansion no greater than about 4.0×10 −7 /° C. over the temperature range of about 25° C. to about 800° C.    
     
     
         8 . The method of  claim 7  wherein the Al 2 O 3 -forming source in component (a) comprises a highly reactive cc-alumina or aluminum trihydrate having a median particle diameter of no greater than about 1 μm.  
     
     
         9 . The method of  claim 7  wherein the Al 2 O 3 -forming source in component (b) is selected from the group consisting of a transition alumina and aluminum oxyhydroxide having a median particle diameter no greater than about 1 μm.  
     
     
         10 . The method of  claim 7  wherein the coefficient of thermal expansion of the ceramic body is no greater than about 3.0×10 −7 /° C.  
     
     
         11 . The method of  claim 7  in which the green body is a honeycomb monolith.  
     
     
         12 . The method of  claim 7  wherein the organic binder system comprises a cellulose ether binder component selected from the group consisting of methylcellulose, methylcellulose derivatives, and combinations thereof, a surfactant component selected from the group consisting of, stearic acid, sodium stearate, ammonium stearate, ammonium lauryl sulfate, lauric acid, oleic acid, palmitic acid and combinations thereof and a solvent comprising water.  
     
     
         13 . The method of  claim 7  wherein the inorganic raw material mixture comprises 100 parts by weight and about 0.2 to 2 parts by weight of the surfactant component, about 2.5 to 5 parts by weight of the cellulose ether binder component, and about 20 to 50 parts by weight of the water.