Title of Invention

A METHOD OF MANUFACTURING OF TIN DIOXIDE COATED CERAMIC INSULATING SUBSTRATE

Abstract

The present invention relates to a method of manufacturing of tin dioxide coated ceramic insulating substrate. The product is useful for gas sensor applications. The process steps are: cleaning by known methods a ceramic insulating substrate, electroding by known methods the ends of the substrate by gold based electrode followed by lead attaching with gold wire, gold alloy wire or platinum wire by firing at a temperature in the range of 900 - 1000°C, applying by rubbing characterized in that a fugitive conductive coating of a graphite clay mix onto the substrate, placing the substrate at the positive terminal of a conventional electrophoretic deposition set-up under a suspension containing SnO2 coating and finally firing the said coating at a temperature in the range of 500 - 1000°c for 1 4 hours to obtain coated ceramic insulating substrate.

Full Text

The present invention relates to a method of manufacturing of tin dioxide coated ceramic insulating substarte. More particularly this invention relates to method of manufacturing thick film tin dioxide useful for gas sensor applications. This invention more particularly relates to a method of fabricating thick film tin dioxide gas sensors by electrophoretic deposition (EPD) technique. Such tin dioxide films can be used to detect a variety of toxic and combustible gases like methane, carbon monoxide and LPG. Semiconducting gas sensors based on tin dioxide (ShO2) have been subjected to extensive research and development for their applications in detection of toxic and inflammable gases. In this context, reference may be made to a book: G. Sberveglieri (ed), Gas Sensors - Principles, Operation and Developments, Kluwer, Dordrecht (1992) and a review paper: W. Gopel and K. D. Schierbaum, SnO2 sensors: Current Status and Future Prospects, Sensors and Actuators B, 26-27 (1995) 1-12. Conventionally there are three processes, which can be utilized for the fabrication of gas sensors viz. sintering, thin film and thick film. In this context, reference may be made to a U. S. Patent: I. C. Chen, M. H. Tzeng, P. P. Tsai, C. F. Liaw and J. C. H. Ku, "Method of fabricating a gas sensor and the product fabricated thereby" U. S. Pat. 5,273,779, Dec. 28, 1993. The sintering technique has the following disadvantages: a) the process should be carefully monitored so as to get the desired porosity, b) not well-adapted for automation and consumes more material in fabrication and c) higher unit cost. The thin film techniques like chemical vapour deposition, spray pyrolysis, spin coating, sputtering, sol-gel processing provide good coatings; however, the capital investment is prohibitive and at the same time, unit cost is also quite high. The thick film technique is primarily based on slurry paste method and screen printing. In the slurry paste method, a paste (aqueous or organic based) is applied as a layer around the ceramic tube and impregnated with a binder like tetraethyl silicate before firing in air. This procedure leads to a porous microstructure that invests the finished product with a high sensitivity to reducing gases in air. The most widespread method of preparing thick films in electrons and sensor applications is through the well-known screen printing procedure. Rarely, flexible plastic sheet precursors are also used where an appropriately prepared powder is dispersed in an organic material to form a flexible plastic green sheet. This sheet is then cut and assemble into the required form before firing to remove the organic material to give the final ceramic component in its required form. The advantages of thick film technique are a) low unit cost and b) capital investment is also relatively low. However, the above thick film techniques have got one or more of the following drawbacks: a) uniform coating thickness may not be achieved b) fast fabrication may not be possible c) may necessitate costly and hazardous organic solvents and d) the method may not be amendable to simple automation. The main object of the present invention is to provide a method of manufacturing tin dioxide coated ceramic insulating substrate, which obviates the drawbacks as detailed above. Another object of the present invention is to provide a fugitive conducting material and the method of applying the same on the substrates for electrophoretic deposition. Still another object of the present invention is to provide a stable slurry composition for electrophoretic deposition of tin dioxide. Yet another object of the present invention is to electrophoretically deposit SnO2 powder to get the desired coatings on the substrate. Accordingly, the present invention provides a method of manufacturing of tin dioxide coated ceramic insulating substrate which comprises; cleaning by known methods a ceramic insulating substrate, electroding by known methods the ends of the substrate by gold based electrode followed by lead attaching with gold wire, gold alloy wire or platinum wire by firing at a temperature in the range of 900 - 1000°C, applying by rubbing characterized in that a fugitive conductive coating of a graphite clay mix onto the substrate, placing the substrate at the positive terminal of a conventional electrophoretic deposition set-up under a suspension containing SnO2 coating an finally firing the said coating at a temperature in the range of 500 - 1000°c for 1 - 4 hours to obtain coated ceramic insulating substrate. In an embodiment of the present invention, the ceramic insulating substrates may be such as alumina and quartz. In another embodiment of the present invention, the fugitive conductive coating of graphite-clay mix consist of 10-15% of china clay and the rest graphite and the mix calcined at 800-1100°C to get a bonded mass. In yet another embodiment of the present invention, the suspension containing SnCh for electrophoretic deposition may comprise 1-5 wt% tin dioxide (SnO2) and 0.5-2 wt% of a mixture of surfactants such as Extran (E.Merck) and Labolene (Qualigen) in an aqueous medium where the pH may be adjusted in the range of 8-12 by adding triethylamine. In another embodiment of the present invention, the electrophoretic depositon may comprise maintaining a gap between the electrodes in the range of 30-50 mm, applying a voltage of 10 -100V for 10 -120 sec to form the coating, washing the coating by immersing in distilled water for 2-10 minutes followed by ambient drying at 50-60° for 1 - 2hours and repeating the deposition, washing and drying sequences, if required. The process steps of the present invention are as follows: a) Cleaning the rectangular or tubular r substrates in water or alcohol using transonic vibration. b) Electroding the ends by a standard technique with Au or Au-based electrode, attaching lead wires of Au, Au-alloy or platinum and bring the assembly at a temperature in the range of 900-1000°C. c) Applying by rubbing fugitive conductive coatings of graphite=-clay based material on the substrates. d) Placing the substrates at the positive terminal of the electrophoretic deposition set-up under an aqueous suspension containing l-5wt% SnO2, 0.5-2wt% of a mixture of surfactants such as Extran (E.Merck) and Labolene (Qualigen) and the pH adjusted in the range of 8-12 by adding triethylamine and maintaining a gap between the electrodes in the range of 30-50 mm. e) Applying a voltage in the range of 10-100V for 10-20 sec to get the coating, washing the coating by immersing in distilled water for 2-10 minutes, ambient drying at 50-60°C for 1-2 hours and repeating the deposition, washing and drying sequences, if required. f) Finally, firing the coated substrates at a temperature in the range of 500-1000°C for 1-4 hours. The novelty of the present invention resides in aqueous electrophoretic deposition of tin dioxide onto a ceramic insulating substrate by using a graphite-clay mix electrode, thereby obviating the following disadvantages of the known art i) nonuniformity of coating thickness ii) slow fabrication process iii) requirements of costly and hazardous organic solvent and iv) difficulties of automation. The inventive steps lie in: a) the material such as graphite-clay mix to be used as a fugitive conducting layer on the substrates and its method of application on the substrates b) formation of a stable slurry composition consisting of SnO2, a mixture of surfactants such as Extran (E.Merck) and Labolene (Qualigen) and triethylamine in aqueous medium. c) electrophoretically depositing SnO2 powder to get the desired coatings on the substrates. The following examples illustrate the invention and the manner in which it may be carried out in practice; however, this should not be construed to limit the scope of the present invention. Example - 1 An alumina tube of length 3mm, outer diameter 1.5mm and inner diameter 1mm was ultrasonically cleaned in acetone. The two ends of the tube were painted with gold electrode and platinum lead wires were carefully attached with the gold electrodes. The assembly was cured at 1000°C for 2 hours. After curing, a graphite-clay mass was rubbed on the outer surface of the AlOs tube to get a thin uniform coating. Then the alumina tube was placed at the positive terminal (in the center) of an electrophoretic deposition set-up where the negative terminal is of cylindrical shape (made of brass) with a gap of 35mm from the positive terminal of the center. A suspension of 2 gram of SnO2 was prepared in l00cc distilled water by adding 5 drops of triethylamine and 6 drops of Extran maintaining a pH of 10. The suspension was poured in the electrophoretic deposition set-up and the deposition was carried out at 30 V for 10 sec. The coating was washed by immersing in distilled water for 5 minutes. The coating was dried at 60°C for 1 hour and then fired at 600°C for 4 hours. The coating prepared in this way showed a sensitivity of around 65% in 500 ppm methane at 350°C which can be enhanced and made selective by the well-known technique of incorporation of catalysts and promoters in the coatings. Example - 2 An alumina tube of length 3 mm, outer diameter 1.5 mm inner diameter 1 mm was ultrasonically cleaned in acetone. The two ends of the tube were painted with gold electrode and platinum lead wires were carefully attached with the gold electrodes. The assembly was cured at 1000°C for 2 hours. After curing, a graphite-clay mass was rubbed on the outer surface of an Al2O3 tube to get a thin uniform coating. Then the alumina tube was placed at the positive terminal (in the center) of an electrophoretic deposition set-up where the negative terminal is of cylindrical shape (made of brass) with a gap of 35 mm from the positive terminal of the center. A suspension of 2 gram of SnO2 was prepared in l00cc distilled water by adding 6 drops of triethylamine and 6 drops of Extran maintaining a pH of 10.5. The suspension was poured in the electrophoretic deposition set-up and the deposition was carried out at 50 V for 10 sec. The coating was washed by immersing in distilled water for 5 minutes. The coating was dried at 60°C for 1 hour and again a second coating was applied on it at 50 V for 10 sec. The coating was again washed by immersing in distilled water for 5 minutes. After drying at 60°C, the final coating was fired at 600°C for 4 hours. The coatings prepared in this way showed a sensitivity of around 60% in 500 ppm methane at 350°C which can be enhanced and made selective by the well-known technique of incorporation of catalysts and promoters in the coatings. Example - 3 An alumina tube of length 3 mm, outer diameter 1.5 mm and inner diameter 1 mm was ultrasonically cleaned in acetone. The two ends of the tubes were painted with gold electrode and platinum lead wires were carefully attached with the gold electrodes. The assembly was cured at 1000°C for 2 hours. After curing, a graphite-clay mass was rubbed on the outer surface of the AlO2 tube to get a thin uniform coating. Then the alumina tube was placed at the positive terminal (in the center) of an electrophoretic deposition set-up where the negative terminal is of cylindrical shape (made of brass) with a gap of 35 mm from the positive terminal of the center. A suspension of 2 gram of SnOa was prepared in 100 cc distilled water by adding 5 drops of triethylamine and 6 drops of Extran maintaining a pH of 10. The suspension was poured in the electrophoretic deposition set-up and the deposition was carried out at 75 V for 10 sec. The coating was washed by immersing in distilled water for 5 minutes. The coating was dried at 60°C for 1 hour and then fired at 900°C for 4 hour. The coatings prepared in this way showed a sensitivity of around 30% in 500 ppm methane at 350°C which can be enhanced and made selective by the well-known technique of incorporation of catalysts and promoters in the coatings. The main advantages of the present invention are as follows: 1) Uniform coating thickness can be achieved. 2) Fast fabrication is possible 3) No necessity of costly and hazardous organic solvents and 4) The method is amenable to simple automation. We Claim: 1. A method of manufacturing of tin dioxide coated ceramic insulating substrate which comprises; cleaning by known methods a ceramic insulating substrate, electroding by known methods the ends of the substrate by gold based electrode followed by lead attaching with gold wire, gold alloy wire or platinum wire by firing at a temperature in the range of 900 - 1000°C, applying by rubbing characterized in that a fugitive conductive coating of a graphite clay mix onto the substrate, placing the substrate at the positive terminal of a conventional electrophoretic deposition set-up under a suspension containing SnO2 coating and finally firing the said coating at a temperature in the range of 500 - 1000° C for 1 - 4 hours to obtain coated ceramic insulating substrate. 2. A method as claimed in claim 1 wherein the ceramic insulating substrates is alumina or quartz. 3. A method as claimed in claims 1-2 wherein the fugitive conductive coating of graphite clay mix consists of 10-50% of clay and the rest graphite and the mix calcined at 800- 1100°C to get a bonded mass. 4. A method as claimed in claims 1 to 3 wherein the suspension containing SnO2 for electrophoretic deposition comprises 1-5 wt % tin dioxide (SnO2) and 0.5-2 wt% of a mixture of surfactants such as Extran (E.Merek) and Labolene (Qualigen) in an aqueous medium where the pH is adjusted in the range of 8-12 by adding trietylamine. 5. A method as claimed in claims 1 to 4 wherein the electrophoretic deposition is preferably done at 10-100 V. 6. A method as claimed in claims 1 to 5 wherein the preferably time period to form coating is 10- 120 sec. 7. A method as claimed in claims 1 to 6 wherein the drying of the coating is done at 50 to 60°C for a 1 to 2 hrs. 8. A method of manufacturing of tin oxide coated ceramic substrate substantially as herein described with reference to examples.

Documents:

202-del-2001-abstract.pdf

202-del-2001-claims.pdf

202-del-2001-correspondence-others.pdf

202-del-2001-correspondence-po.pdf

202-del-2001-description (complete).pdf

202-del-2001-form-1.pdf

202-del-2001-form-19.pdf

202-del-2001-form-2.pdf

202-del-2001-form-3.pdf