Ceramic filter and process for making it

A ceramic filter comprising:

• a base obtained by sintering starting materials consisting of i) alumina coarse particles used as aggregates and ii) alumina-zirconia mixed fine particles used as a sintering aid and containing not less than 90 wt. % of alumina, which are so blended that the alumina is in an amount of from 80 to 99 wt. % based on the total weight of the starting materials, said material being shaped before sintering, said base having an average pore size of about 10 .mu.m;
• a first intermediate layer formed on said base and having a pore size of about 1 .mu.m and thickness of about 50 .mu.m;
• a second intermediate layer formed on said first intermediate layer and having a pore size of about 0.1 .mu.m and thickness of about 20 .mu.m;
• a fine intermediate layer formed on one surface of said second intermediate layer and comprised of at least one kind of particles selected from particles of SnO.sub.2, TiO.sub.2, AnO.sub.2 or CeO.sub.2 having a particle diameter of not more than 500 .ANG., said fine intermediate layer being obtained by baking a gel at a temperature ranging from 400.degree. C. to 900.degree. C. and having a smaller pore size than said second intermediate layer and thickness of about 2 .mu.m;
• and
• a thin film formed on the surface of said fine intermediate layer and comprised of at least one particle selected from particles of SnO.sub.2, TiO.sub.2, ZnO.sub.2 or CeO.sub.2 and having a particle diameter of not more than 300 .ANG. and smaller than the particle diameter of the particles that constitute said fine intermediate layer, said thin film being obtained by baking a gel at a temperature ranging from 300.degree. C. to 800.degree. C. and having thickness of from 0.05 .mu.m to 10 .mu.m.

A process for manufacturing a ceramic filter, comprising the steps of;

• preparing a sol comprised of at least one kind of particle selected from particles of SnO.sub.2, TiO.sub.2, ZrO.sub.2 or CeO.sub.2 ;
• adding a thickening agent to said sol;
• coating the sol to which said thickening agent has been added, on one surface of a porous support made of a ceramic;
• dehydrating the coated sol into a gel;
• baking said gel at a temperature of from 400.degree. C. to 900.degree. C. to form a fine intermediate layer on the surface of said support;
• coating said sol to which said thickening agent has been added, on the surface of said fine intermediate layer;
• dehydrating the sol coated on the fine intermediate layer, into a gel;
• and
• baking the gel on the fine intermediate layer at a temperature of from 300.degree. C. to 800.degree. C. and at a temperature lower than the baking temperature for said fine intermediate layer to form a thin film on the surface of said fine intermediate layer.