Ultra thin zircon coating and membrane

A process to produce an ultra thin zircon coating comprising:

• A) mixing (i) a homogenous alcoholic sol resulting from hydrolysis and partial condensation of a silicon alkoxide with (ii) a homogenous solution of a zirconium compound chelated in an alcohol and (iii) in the presence of a doping agent being soluble in the resulting mixture and promoting zircon crystallization, with proportions of the sol and the solution yielding a molar ratio Si/Zr close or equal to 1, until a homogenous mixed sol is obtained;
• B) forming a matured sol containing colloidal particles and/or polymeric entities by a procedure selected from:
• (i) stirring the mixed sol for approximately 30 minutes to 24 hours to obtain the matured sol, and
• (ii) gelling the mixed sol, drying and forming a powder of the resultant gel, and mixing the powder with acidified water to obtain the matured sol;
• C) forming a coating of the matured sol on a microporous substrate;
• D) drying the sol coating sufficiently slowly to convert the sol coating to a gel coating and avoid flaws in the gel coating;
• E) firing the gel coating on the substrate at a temperature of 900.degree. to 1500.degree. C. to form an adherent ultra thin zircon layer on the microporous substrate.