Title of Invention

AN IMPROVED PROCESS FOR THE PREPARATION OF CERAMIC WATERBORNE SLURRY/SLIP FOR TAPE CASTING

Abstract An improved process for the preparation of a ceramic waterborne slurry/slip for tape casting by dissolving polyvinyl alcohol powder in water at a temperature in the range of 60 - 80°C, milling the mixture thoroughly, adding ceramic material powder along with silicone emulsion and milling again, adding glycerol followed by milling to obtain a mix, warming the mix slowly to a temperature in the range of 70 - 75° C and simultaneously adding to the said mix under constant stirring a dilute aqueous boric acid solution maintained at the mix temperature, allowing the resultant slurry to cool to a temperature in the range of 45 - 50° C, prior to casting.
Full Text This invention relates to an improved process for the preparation of ceramic waterborne slurry/slip for tape casting. The slurry prepared using the process of the present invention is particularly useful in the fabrication of electrolyte retention matrix for molten carbonate fuel cell.
One of the challenges in molten carbonate fuel cell is to develop a cost-effective method of manufacturing very thin matrix for long term fuel cell operation. The electrolyte retention matrix in a molten carbonate fuel cell provides electronic isolation while maintaining ionic communication between cell electrodes as well as separation of reactants. The ceramic material used in the preparation of matrix provides structure but does not participate in the electrochemical process. To perform these functions efficiently, the matrix needs to have properties, like stable pore structure, dimensional and material stability.
The conventional process used to fabricate electrolyte matrix structure until about 1980 involved cold/hot pressing of the ceramic powder with or without alkali carbonates. The matrix structure prepared by the above said method was thick, nonuniform, poor mechanical strength and often characterised by high IR drop. Moreover these structures were difficult to produce in large sizes because large dies and presses were required and expensive. Reference may be made to Development status of Toshiba advanced paste electrolyte molten carbonate fuel cells, 1985, May 19-12, pp 162-165.
To overcome these problems associated with cold/hot pressing matrix structures an alternative tape casting process has been developed and is widely used in the electronic industry. This process involves the preparation of a

homogeneous mixture of a ceramic powder in a liquid medium (solvent) and a dispersant that provides stability. Binders and plasticizers are added to provide strength and flexibility. This homogeneous dispersion is called slurry or slip. Usually, the slurry/slip, for tape casting is prepared by milling the ceramic powder and other ingredients for about 24 h and cast over a glass plate in the form of a tape using a doctor blade assembly.
Reference may be made to Journal of Applied Electrochemistry, 20(1990) 606-10. Mould releasing agents are applied to base (glass) plate prior to casting. Reference may be made to Journal of American Ceramic Society, 77(8) (1994) 2137-44.
The major drawbacks of the method employed by various researchers are: i. Hazardous organic chemicals such as Ethyl Methyl Ketone(MEK), Toluene, Xylene, Isopropanol, Poly vinyl butyral(PVB), Dioctyl phthalate(DBP), Dibutyl. phthalate(DBP), Poly ethylene glycol(PEG), etc. are used as solvents and - plasticizers, which are toxic and not environment friendly, ii. Longer time duration of 12 to 24 hours is required for milling the different
ingredients, iii. Proper mould release agents are to be used which should be compatible
with the nature of the ceramic material and the solvent system, iv. Longer time duration greater than 24 hours are required to remove the tapes from the base plate after curing.

The main objective of the present invention is to provide an improved process for the preparation of ceramic waterborne slurry/slip for tape casting, which obviates the drawbacks as detailed above.
Another object of the present invention is to provide an improved process for the preparation of a ceramic waterborne slurry/slip for tape casting, using ceramic material powders having high surface area, so as to be useful as matrix material in molten carbonate fuel cells.
Yet another object is to provide tape casting made by the improved process of the present invention.
Accordingly, the present invention provides an improved process for the preparation of a ceramic waterborne slurry/slip for tape casting which comprises dissolving polyvinyl alcohol powder in the concentration range of 10 to 15 wt % water at a temperature in the range of 60 - 80°C, milling the mixture thoroughly, adding ceramic material powder in the range of 15 to 35 wt% on dry wt basis such as herein described along with silicone emulsion and milling again, adding glycerol in the range of 20 to 50 wt% with respect to polyvinyl alcohol followed by milling to obtain a mix, warming the mix slowly to a temperature in the range of 70 - 75° C and simultaneously adding to the said mix under constant stirring a dilute aqueous boric acid solution in the range of 0.25 to 1 wt% with respect to polyvinyl alcohol maintained at the mix temperature, allowing the resultant slurry to cool to a temperature in the range of 45 - 50° C, prior to casting.
In an embodiment of the present invention the ceramic material used is such as alumina (Al2O3), lithium aluminate (LiAIO2), zirconia (ZrO2), titania (Ti02).

In another embodiment the particle size of the ceramic material used in the range of 5 to 20 µrn.
In yet another embodiment the binder used is such as poly vinyl alcohol (PVA) powder.
In still another embodiment the water used is such as distilled water.
In yet another embodiment the binder solution concentration is in the range of 10to15wt%.
In another embodiment the binder to ceramic material ratio is in the range of 15 to 35 wt% on dry weight basis.
In still another embodiment the defoamer used is such as silicone emulsion, 7 volume % to water.
In yet another embodiment the plasticizer used is such as glycerol. In another embodiment the plasticizer to binder weight ratio is in the range of 20 to 50 wt. %.
In still another embodiment the plasticizer to water ratio is in the range of 20 to 40 wt%.
In yet another embodiment the hardener used is such as boric acid, 0.06 wt % to that of water.
In another embodiment the hardener to binder ratio is in the range of 0.25 to 1 wt%.
In yet another embodiment of the present invention, mixing of the various ingredients are effected at temperatures in the range 60-80°C, for quick dissolution of the materials and for easy removal, of-the dissolved air from the slurry.

In an embodiment of the present invention pre-warming of the base plate such as glass is done to a temperature of around 40° C prior to the casting stage.
Accordingly the present invention provides tape casting made using the improved process of the present invention as described herein. Method of preparation of the waterborne slurry for tape casts
Alumina (AI2O3), lithium aluminate (LiAIO2), zirconia (ZrO2) powders of high surface area are used as the ceramic material. The binder, poly vinyl alcohol (PVA) powder (degree of polymerization 1700 -1800) is dissolved in distilled water at 60 -75°C and agitated with a power blender for few seconds. The composition will be 10-15 wt%. To this solution, the defoamer (few drops of silicone emulsion) and the ceramic are added and mixed again. The binder to ceramic ratio is kept in the range 15 to 35 wt% on dry weight basis. The plasticizer (glycerol) is added and once again the ingredients are mixed for some more time then ball milled for one hour. The plasticizer to binder weight ratio is kept in the range 20 to 50 wt%. The plasticizer to the liquid (water) ratio is kept in the range 20% to 40% on weight basis. The mix is warmed slowly to 70 - 75° C and during this stage, a dilute solution of the hardener material (boric acid) in the same liquid (water) maintained at the same temperature is added and mixed thoroughly. The boric acid to the PVA ratio is kept in the range 0.25 to 0.1 wt%.
This slurry formulation is used for casting tapes of thickness from 0.25 to 1.0 mm by tape casting technique using a doctor blade assembly on to a glass plate.
The following examples are given by way of illustrations and therefore should not be construed to limit the scope of the invention.



We claim :
1. An improved process for the preparation of a ceramic waterborne slurry/slip
for tape casting which comprises dissolving polyvinyl alcohol powder in the
concentration range of 10 to 15 wt % water at a temperature in the range of
60 - 80°C, milling the mixture thoroughly, adding ceramic material powder in
the range of 15 to 35 wt% on dry wt basis such as herein described along
with silicone emulsion and milling again, adding glycerol in the range of 20 to
50 wt% with respect to polyvinyl alcohol followed by milling to obtain a mix,
warming the mix slowly to a temperature in the range of 70 - 75° C and
simultaneously adding to the said mix under constant stirring a dilute
aqueous boric acid solution in the range of 0.25 to 1 wt% with respect to
polyvinyl alcohol maintained at the mix temperature, allowing the resultant
slurry to cool to a temperature in the range of 45 - 50° C, prior to casting.
2. An improved process as claimed in claim 1 wherein the ceramic material
powder used is.alumina (AI2O3), lithium aluminate (LiAIO2), zirconia (ZrO2),
titania (TiO2).
3. An improved process as claimed in claims 1-2 wherein the particle size of the
ceramic material used is in the range of 5 to 20µn.
4. An improved process as claimed in claims 1-10 wherein the glycerol to water
ratio is in the range of 20 to 40 wt%.
5. An improved process as claimed in claims 1-13 wherein the mixing of the
various ingredients are effected at temperatures in the range of 60 to 80°C.

6. An improved process for the preparation of a ceramic waterborne slurry/slip for tape casting substantially as herein described with reference to the examples.

Documents:

3153-del-1998-abstract.pdf

3153-del-1998-claims.pdf

3153-del-1998-correspondence-others.pdf

3153-del-1998-correspondence-po.pdf

3153-del-1998-description (complete).pdf

3153-del-1998-form-1.pdf

3153-del-1998-form-19.pdf

3153-del-1998-form-2.pdf

3153-del-1998-form-4.pdf

3153-del-1998-form-5.pdf


Patent Number 232592
Indian Patent Application Number 3153/DEL/1998
PG Journal Number 13/2009
Publication Date 27-Mar-2009
Grant Date 19-Mar-2009
Date of Filing 28-Oct-1998
Name of Patentee COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH
Applicant Address RAFI MARG, NEW DELHI-110001, INDIA.
Inventors:
# Inventor's Name Inventor's Address
1 SUBBARAJ DHEENADAYALAN,SCIENTIST CENTRAL ELECTRO-CHEMICAL RESEARCH INSTITUTE, KARAIKUDI-630 006, INDIA.
2 KAYAMKULATHETHU LALITHA ANITHA, RESEARCH ASSOCIATE CENTRAL ELECTRO CHEMICAL RESEARCH INSTITUTE, KARAIKUDI 630 006, INDIA.
3 RAJAM PATTABIRAMAN, SCIENTISTS CENTRAL ELECTRO CHEMICAL RESEARCH INSTITUTE, KARAIKUDI 630 006, INDIA.
4 RAMASAMY CHANDRASEKARAN, SCIENTISTS CENTRAL ELECTRO CHEMICAL RESEARCH INSTITUTE, KARAIKUDI 630 006, INDIA.
5 GANESAN PRABHU,PROJECT ASSISTANTS CENTRAL ELECTRO CHEMICAL RESEARCH INSTITUTE, KARAIKUDI 630 006, INDIA.
6 ARPUTHARAJ SAMSON NESARAJ,PROJECT ASSISTANTS CENTRAL ELECTRO CHEMICAL RESEARCH INSTITUTE, KARAIKUDI 630 006, INDIA.
7 SAVARIMUTHU JOSEPH PORES,PROJECT ASSISTANTS CENTRAL ELECTRO CHEMICAL RESEARCH INSTITUTE, KARAIKUDI 630 006, INDIA.
PCT International Classification Number B22C 9/06
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 NA