Porous ceramic bodies and process for their preparation

Abstract

A process for producing a porous ceramic body comprises a) mixing a coated porogen with a silicate or an oxide ceramic precursor, wherein the porogen is decomposable to gaseous decomposition products and optionally solid products upon heating, and is coated with a coating agent; b) forming a green body from the mixture obtained in step (a); and c) firing the green body obtained in step (b) to obtain the ceramic body, whereby the porogen decomposes to form pores within the ceramic body and the coating agent is deposited at the inner surface of the pores. The porogen is coated with a coating agent which, upon firing, is deposited at the inner surface of the ceramic pores, so that porous ceramics having decreased weight and improved porosity are obtained, while maintaining at the same time good mechanical strength. A green body and a porous ceramic body obtainable with the above-mentioned process are described too.

1 . A process for preparing a porous ceramic body, comprising:
 a) mixing at least a coated porogen with at least a silicate or an oxide ceramic precursor, wherein the porogen is decomposable to gaseous decomposition products and optionally solid products upon heating, and is coated with a coating agent;   b) forming a green body from the mixture obtained in step (a); and   c) firing the green body obtained in step (b) to obtain the ceramic body, whereby the porogen decomposes to form pores within the ceramic body and the coating agent is deposited at the inner surface of the pores.   
     
     
         2 . The process of  claim 1 , wherein said porogen is a carbon or organic compound. 
     
     
         3 . The process of  claim 1 , wherein said porogen is a spherical particle or a fibre. 
     
     
         4 . The process of  claim 2 , wherein said porogen is selected from the group consisting of graphite, cellulose, starch, organic polymers and mixtures thereof. 
     
     
         5 . The process of  claim 1 , wherein said coating agent is a ceramic selected from the group consisting of quartz, silica, kaolin, chamotte, mullite, alumina, zirconia, yttrium oxide, titanium oxide, magnesium oxide, calcium oxide, barium oxide and mixtures thereof. 
     
     
         6 . The process of  claim 1 , wherein said coating agent is selected from the group consisting of feldspar, glass, frits, syenites, aluminum hydroxide, aluminum oxide hydrate, aluminum nitride, silicon nitride, aluminum halides, yttrium halides, zirconium halides, silicon halides, zirconium carbonate, salts of alkaline and alkaline earth metals, and mixtures thereof. 
     
     
         7 . The process of  claim 6 , wherein said coating agent is selected from the group consisting of aluminium hydroxide, calcium carbonate, magnesium carbonate, calcium hydrogen-phosphate and mixtures thereof. 
     
     
         8 . The process of  claim 1 , wherein the coating agent is different from the ceramic precursor. 
     
     
         9 . The process of  claim 1 , wherein the amount of coating agent coating the porogen ranges from 1 to 40% wt., based on the weight of the coated porogen. 
     
     
         10 . The process of  claim 1 , wherein the amount of coated porogen in the mixture of step (a) is at least 5% wt., based on the weight of the mixture. 
     
     
         11 . The process of  claim 10 , wherein the amount of coated porogen ranges from 10 to 40% wt. 
     
     
         12 . (canceled) 
     
     
         13 . The process of  claim 1 , wherein in step (b) the green body is formed by casting, pressure casting, jiggering, extruding or press processing. 
     
     
         14 . The process of  claim 1 , wherein in step (c) the firing is carried out at a temperature ranging from 150 to 1800° C., for a time ranging from 0.5 to 70 hours. 
     
     
         15 . The process of  claim 1 , wherein the ceramic body obtained in step (c) has total porosity of at least 10%, open porosity lower than 1.5% and closed porosity of at least 8.5%. 
     
     
         16 . A green body comprising a mixture of at least a coated porogen intermixed with at least a silicate or an oxide ceramic precursor, wherein the porogen is decomposable to gaseous and optionally solid decomposition products upon heating, and is coated with a coating agent. 
     
     
         17 . The green body of  claim 16 , comprising from 10 to 40% wt. of coated porogen. 
     
     
         18 . The green body of  claim 16 , wherein the amount of coating agent coating the porogen ranges from 1 to 40% wt., based on the weight of the coated porogen. 
     
     
         19 . A porous ceramic body having total porosity of at least 10%, open porosity lower than 1.5% and closed porosity of at least 8.5%, wherein the inner surface of the pores is covered by the thermolysis product of a porogen coated with a coating agent. 
     
     
         20 . The porous ceramic body of  claim 19 , having total porosity ranging from 15 to 30%. 
     
     
         21 . The porous ceramic body of  claim 19 , having open porosity ranging from 0.01 to 1.0%. 
     
     
         22 - 24 . (canceled) 
     
     
         25 . The porous ceramic body of  claim 19 , having a bulk density reduction of at least 8%. 
     
     
         26 . The porous ceramic body of  claim 19 , having water absorption lower than 2% wt.