Title of Invention	A METHOD OF IMPROVING RESPONSE AND RECOVERY TIME OF SEMICONDUCTING TIN OXIDE BASED GAS SENSORS IN THICK FILM FORM FOR DETECTION OF COMBUSTIBLE GASES AND SEMICONDUCTING TIN OXIDE BASED GAS SENSORS IN THICK FILM FORM MADE THEREBY
Abstract	A method of improving response and recovery time of semiconducting tin oxide based gas sensors in thick film form for detection of combustible gases and semiconducting tin oxide based gas sensors in thick film form made thereby. This invention relates to a method of improving response and recovery time of semiconducting tin oxide based gas sensors in thick film form for detection of combustible gases like methane, LPG, propane, CNG. The improved gas sensors are made by coating pre-cleaned and electroded tubular alumina substrates with a sensor coating comprising tin dioxide (SnO2), antimony oxide (Sb2O3) in the range of 0.2-0.3 wt% with respect to SnO2 and palladium in the range of 5-10 wt% with respect to SnO2 and alumina gel equivalent to 0.05wt% alumina with respect to SnO2 along with a gas forming agent such as ammonium nitrate in the range of 0.25-2 wt% with respect to SnO2 batch composition, vacuum drying and curing the sensor coating at a temperature in the range of 600°C-700°C for a time in the range of 30min to 2 h at a heating rate in the range of 50°C-75°C/h so as to get coating microstructure having controlled cracks. The response and recovery time of the thick film sensors prepared by this method are around 15 sec and Imin 15 sec., respectively, which are acceptable for real-life applications of the sensors. The invented method also imparts good adherence of the coating with the substrates. The sensors made by the method of the present invention are highly sensitive towards the combustible gases to be detected. The processing steps of the fabrication of such sensors are also simple and cost effective.

Title of Invention

A METHOD OF IMPROVING RESPONSE AND RECOVERY TIME OF SEMICONDUCTING TIN OXIDE BASED GAS SENSORS IN THICK FILM FORM FOR DETECTION OF COMBUSTIBLE GASES AND SEMICONDUCTING TIN OXIDE BASED GAS SENSORS IN THICK FILM FORM MADE THEREBY

Abstract

A method of improving response and recovery time of semiconducting tin oxide based gas sensors in thick film form for detection of combustible gases and semiconducting tin oxide based gas sensors in thick film form made thereby. This invention relates to a method of improving response and recovery time of semiconducting tin oxide based gas sensors in thick film form for detection of combustible gases like methane, LPG, propane, CNG. The improved gas sensors are made by coating pre-cleaned and electroded tubular alumina substrates with a sensor coating comprising tin dioxide (SnO2), antimony oxide (Sb2O3) in the range of 0.2-0.3 wt% with respect to SnO2 and palladium in the range of 5-10 wt% with respect to SnO2 and alumina gel equivalent to 0.05wt% alumina with respect to SnO2 along with a gas forming agent such as ammonium nitrate in the range of 0.25-2 wt% with respect to SnO2 batch composition, vacuum drying and curing the sensor coating at a temperature in the range of 600°C-700°C for a time in the range of 30min to 2 h at a heating rate in the range of 50°C-75°C/h so as to get coating microstructure having controlled cracks. The response and recovery time of the thick film sensors prepared by this method are around 15 sec and Imin 15 sec., respectively, which are acceptable for real-life applications of the sensors. The invented method also imparts good adherence of the coating with the substrates. The sensors made by the method of the present invention are highly sensitive towards the combustible gases to be detected. The processing steps of the fabrication of such sensors are also simple and cost effective.

Full Text	The present invention relates to a method of improving response and recovery time of semiconducting tin oxide based gas sensors in thick film form for detection of combustible gases and semiconducting tin oxide based gas sensors in thick film form made thereby. The present invention particularly relates to a method of improving response and recovery time of semiconducting tin oxide based gas sensors in thick film form for detection of combustible gases where controlled cracks are generated at the surface of the coating to get the desired properties. This invention more particularly relates to a method of improving response and recovery time of semiconducting tin oxide based gas sensors in thick film form for detection of combustible gases where controlled cracks are generated at the surface of the coating to get the desired properties and the gas sensitivity and adherence of the coating remain above par and are not affected by the method of the present invention. With increasing world-wide distribution of gas sensors for different applications, the demand of sensors fulfilling specific standards is growing in leaps and bounds. Tin dioxide based compositions are mostly used in semiconductor gas sensors. Tin oxide is a wide band gap (3.6 eV) n-type semiconductor. Tin oxide films consist of a large number of grains connected to each other through grain boundaries. This governs the formation of schottky barriers, whose heights are dependent on the surface charge density and the barrier heights are modulated due to the adsorption of gas/odour molecules. In this context, reference may be made to a review paper: W. Gopel and K.D. Schierbaum, "SnOa sensors Current Status and Future Prospects", Sensors and actuators B, 26-27 (1995) 1-12 and a book G. Sberveglieri (ed.) "Gas Sensors -Principle, Operation and Development", kluwer, Dordrecht (1992). Such semiconductor sensors can be made in thick or thin film form. The advantages of thick film sensors are: 1) ease of fabrication and handling, 2) ruggedness and above all, 3) low price. The main disadvantages are slow response and recovery to an analyte gas. For real life applications of the sensors, the response time (the time required for a sensor to respond to a step increase in the analyte gas) should be low so that the buzzer of a gas alarm rings during gas leak without delay. Similarly, the buzzer should not go on ringing for a long time (i.e, quick recovery, the time required for a sensor to return to baseline after a response to an analyte) after the gas is withdrawn. Though information on semiconductor sensors is available in plenty in public domain, little has been said about the response time and recovery time of gas sensors. In case of thick film semiconductor sensors for detection of combustion gases, reference may be made to publications such as: 1) G. J. Li and S. Kawi, Materials Letters 34 (1998) 99-102. 2) J. C. Kim, H. K. Jun, J. S. Huh and D. D. Lee, Sensor and Actuator B 45 (1998) 271-277. 3) E. W. Williams and A. G. Keeling, J. Materials Science: materials in electronics 9 (1998) 51-54, dealt with the response and recovery time of the sensors. Reference may be made to US patent no.: 6,872,290, titled: Electrochemical gas sensor with passage for receiving gas, relating to relates to an electrochemical gas sensor having improved response time and sensitivity. The electrochemical gas sensor includes a substrate for providing a surface upon which a film of conductive material may be deposited, a pair of electrodes deposited on the substrate for permitting a measurement of current, and an electrolytic material for providing an electrical connection between the electrodes. The electrochemical gas sensor further includes a passage in the surface for holding or transporting gas received from a gas source, whereby sensing occurs at a three way interface where the gas, electrolytic material, and electrodes come in direct contact with one another. Reference may be made to US patent no.: 6,908,538, titled: Electrochemical gas sensor having a porous electrolyte, which relates to an electrochemical gas sensor having improved response time and sensitivity. The electrochemical gas sensor includes a substrate for providing a surface upon which an electrode may be placed, a first electrode placed on the surface, a thin film of electrolyte support for receiving electrolyte and having a predetermined porosity, and a second electrode deposited on the thin film for permitting a measurement of current between the first and second electrodes. Reference may be made to US patent no.: 6,929,735, wherein is disclosed a method and apparatus for providing an electrochemical gas sensor having improved response time for detecting a gas introduced into the sensor. The sensor includes a substrate having a first surface and a second surface and an electrode deposited on the first surface. The sensor also includes an ionomer membrane in contact with the first surface and the electrode. The ionomer membrane has an opening in a location proximate to the electrode for permitting gas introduced into the sensor to diffuse through the opening to simultaneously contact the electrode and the ionomer membrane within the opening. The substrate further includes at least one hole extending from the first surface to the second surface for permitting moisture to diffuse through the at least one hole to contact the ionomer membrane for enhancing sensitivity. In this disclosure there is provided an electrochemical gas sensor having an ionomer membrane in contact with the first surface and the electrode, having improved response time for detecting a gas like CO. The drawbacks of the existing methods lie in their reported high response ( 20sec) and recovery ( 2min) times. From the hitherto known prior art it is observed that there is a need to provide to provide a method of improving the response and recovery time particularly of semiconducting tin oxide based gas sensors in thick film form for detection of combustible gases. The main object of the present invention is to provide a method of improving the response and recovery time of semiconducting tin oxide based gas sensors in thick film form for detection of combustible gases. Another object of the present invention is to provide a method of improving response and recovery time of semiconducting tin oxide based gas sensors in thick film form for detection of combustible gases, where controlled cracks are generated at the surface of the coating to get the desired properties. Still another object of the present invention is to provide a method of improving response and recovery time of semiconducting tin oxide based gas sensors in thick film form for detection of combustible gases where controlled cracks are generated at the surface of the coating to get the desired properties and the gas sensitivity and adherence of the coating remain above par and are not affected in the invented method. Yet another object of the present invention is to provide a method of improving the response and recovery time of semiconducting tin oxide based gas sensors in thick film form for detection of combustible gases where the response and recovery time should be within 15sec and Imin 15sec respectively. A further object of the present invention is to provide semiconducting tin oxide based gas sensors in thick film form having improved response and recovery time for detection of combustible gases. This invention relates to a method of improving response and recovery time of semiconducting tin oxide based gas sensors in thick film form for detection of combustible gases like methane, LPG, propane, CNG. The improved gas sensors are made by coating pre-cleaned and electroded tubular alumina substrates with a sensor coating comprising tin dioxide (SnO2), antimony oxide (Sb2O3) in the range of 0.2-0.3 wt% with respect to SnO2 and palladium in the range of 5-10 wt% with respect to SnO2 and alumina gel equivalent to 0.05wt% alumina with respect to SnO2 along with a gas forming agent such as ammonium nitrate in the range of 0.25-2 wt% with respect to SnO2 batch composition, vacuum drying and curing the sensor coating at a temperature in the range of 600°C-700°C for a time in the range of 30 min to 2 h at a heating rate in the range of 50°C-75°C/h so as to get coating microstructure having controlled cracks. The response and recovery time of the thick film sensors prepared by this method are around 15 sec and Imin 15 sec., respectively, which are acceptable for real-life applications of the sensors. The invented method also imparts good adherence of the coating with the substrates. The sensors made by the method of the present invention are highly sensitive towards the combustible gases to be detected. The processing steps of the fabrication of such sensors are also simple and cost effective. The novelty of the present invention resides in enhancing the response and recovery time of the sensor, at the same time, the other important characteristics of the sensors like gas sensitivity and adherence of the coating remain unaffected in the process of generating controlled cracks. The inventive steps resides in adding a gas forming ammonium nitrate as an additive in the sensor coating composition and curing the composition at a temperature in the range of 600°C-700°C for a time in the range of 30 min to 2 hours at a heating rate in the range of 50°C-75°C/h, so that controlled cracks appear at the surface of the coating without affecting the coating adherence and gas sensitivity, whereas the response and recovery time are improved. Accordingly, the present invention provides a method of improving response and recovery time of semiconducting tin oxide based gas sensors in thick film form for detection of combustible gases, which comprises cleaning an alumina substrate, such as a tubular substrate, by hot concentrated chromic acid, hot concentrated HNOa and distilled water in a sequence followed by washing with acetone using ultrasonic vibration to obtain a clean alumina substrate devoid of oil, grease; electroding the cleaned alumina substrate ends using gold paste followed by attaching lead wires such as gold, gold alloy, platinum and curing the assembly at a temperature in the range of 900°C -1000°C for a period of 1-2 hours; characterized in that applying on the said cleaned and electroded alumina sustrate a sensor coating consisting of a sensor precursor powder composition, a gas forming agent, alumina gel and ethanol in the weight ratio ranging between 1: 0.0025 : 0.5 : 2 and 1: 0.02 : 0.5 : 2 in aqueous or nonaqueous medium; vacuum drying the said sensor coating followed by curing at a temperature in the range of 600°C-700°C for a time period in the range of 30 minutes to 2 hours, at a heating rate in the range of 50°C-75°C/hour, to obtain coating microstructure having controlled cracks. In another embodiment of the present invention, the sensor precursor powder composition consists of tin dioxide (SnO2), antimony oxide (Sb2O3) in the range of 0.2-0.3 wt% with respect to SnO2 and palladium in the range of 5-10 wt% with respect to In yet another embodiment of the present invention, the sensor precursor powder SnO2 batch composition is prepared by sonication assisted simultaneous precipitation technique using aqueous solutions of stannous chloride, antimony chloride and palladium nitrate followed by calcination at a temperature in the range of 800°C -900°C in air for a time period in the range of 1 -2 hours and maintaining the weight ratio of Sb2O3 in the range of 0.2-0.3 wt% and palladium as a metal basis in the rangeof5-10wt%. In still another embodiment of the present invention, the gas forming agent is ammonium nitrate in the range of 0.25-2 wt% with respect to the sensor precursor powder SnO2 batch composition. In still yet another embodiment of the present invention, the sensor coating consists of alumina gel equivalent to 0.05 wt% alumina with respect to SnO2. In a further embodiment of the present invention, the alumina gel is prepared using sol-gel technique from an aqueous solution of alumina alkoxide such as sec-butoxide (C12H27AlOs) in nitric acid and refluxing the aluminium sec-butoxide solution at a temperature in the range of 80°C - 90°C for a time period in the range of 10-20 hours and maintaining the solid content of the sol in the range of 2-5 wt%. In a still further embodiment of the present invention, the sensor coating of thickness in the range of 0.05-0.15 mm is applied on the cleaned and electroded tubular alumina sustrate by brushing. Accordingly, the present invention provides semiconducting tin oxide based gas sensors in thick film form having improved response and recovery time for detection of combustible gases made by the method as herein above described. The detailed process steps of the present invention are as follows: (a) Cleaning the tubular alumina substrates with hot concentrated chromic acid, hot concentrated HNO2, distilled water in a sequence followed by washing with acetone under ultrasonic vibration to remove oil and grease. (b) Electroding the two ends of the alumina substrates by gold-based electrodes, attaching/fixing lead wires of gold/gold-alloy/platinum and curing the assembly at a temperature in the range of 900°C -1000°C for a time period in the range of 1-2 hours. (c) Preparing the precursor powder for the sensor coating comprising tin dioxide (SnO2), 0.20-0.30 wt% antimony oxide (Sb2O3) with respect to SnO2 and 5-10 wt% palladium (as metal) with respect to SnOa by sonication assisted simultaneous precipitation technique using aqueous solutions of stannous chloride, antimony chloride and palladium nitrate followed by calcination at a temperature in the range of 800°C -900°C in air for a time period in the range of 1-2 hours. d) Preparing the alumina gel form an aqueous solution of alumina alkoxide like sec-butoxide (C12H27AlO3) in nitric acid and refluxing the solution at a temperature in the range of 80°C -90°C for a time in the range of 10-20 hours. e) Preparing the paste for sensor coating in aqueous or nonaqueous medium, which comprises the sensor precursor powder composition, ammonium nitrate, alumina gel and ethanol in the weight ratio ranging between 1: 0.0025: 0.5: 2 and 1: 0.02: 0.5:2. (f) Applying by brushing on the said cleaned and electroded tubular alumina sustrate a coating of sensor composition of thickness in the range of 0.05-0.15 mm. (g) Vacuum drying the coating followed by curing at a temperature in the range of 600°C -700°C for a time in the range of 30 min - 2 hours, at a heating rate in the range of 50°C -75°C/h, to obtain coating microstructure having controlled cracks. 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, outside diameter 1.5mm and inner diameter 1mm was cleaned subsequently with hot concentrated chromic acid, hot concentrated HNO3, distilled water followed by washing with acetone using ultrasonic vibration to remove oil, grease etc. 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 2h. A precursor powder for the sensor layer was prepared by the following steps. Firstly, 5.99 g of reagent grade stannous chloride (SnCl2, 2H2O) was dissolved in 200 mL of distilled water containing 20 drops (0.8700 g) of concentrated HC1. For complete mixing, the solution was heated to around 80°C for Ih with continuous stirring. Secondly, 0.0100 g of reagent grade Sb2O3 (0.25 wt% with respect to SnO2) was dissolved in 50 mL distilled water (at 80°C for 30 min) containing 5 drops (0.2200 g) of concentrated HC1. In the next step, 0.6665 g of reagent grade PdCl2 (10 wt% Pd metal with respect to SnO2) was taken in lOOmL distilled water containing 10 drops (0.4200 g) of concentrated HNO3 and PdCl2 was slowly dissolved by heating the mixture at 80°C under constant stirring for Ih. The three solutions were mixed at 80°C under constant stirring for Ih and added to ammonia solution under sonication and the pH of the solution was maintained at 4. The precipitate was filtered, washed with distilled water and calcined at 900°C/2h. To prepare the alumina gel, 20.5000 g of reagent grade aluminium sec-butoxide (C12H27AlO3) was taken in 150 mL water and 0.8 mL cone. HNO2 was added to it to get a pH of 5. The mixture was stirred at 80-90°C for 3 h to get complete dissolution of the secbutoxide. The solution was refluxed for 18 h to get the gel of desired consistency so that the alumina content of the gel was 3% by weight. A thick paste of the powder formulation for the coating was made by mixing the powder with alumina gel and ethanol in the weight ratio 1: 0.5: 2. The outside of the alumina tube was painted by the paste (thickness of the coating around 0.1 mm) followed by vacuum drying and curing at a temperature of 620°C for 45 min. The furnace temperature was raised at a rate of 75°C/h. After curing, the two circular ends and the extreme edges of the tube were cleaned of any coating material by rubbing with a diamond file. For measuring the gas sensitivity of the sensor coating, kanthal heating coil was put inside the tube and the sensitivity towards a gas was measured as a percentage of change in resistance (in presence of gas) by original resistance (in the ambient) at an elevated temperature. The sensors prepared in this way showed an average sensitivity of around 75% and response time of around 35 sec and recovery time of around 4min in 1000 ppm methane and 1000 ppm LPG at 350°C. Example - 2 An alumina tube of length 3mm, outside diameter 1.5mm and inner diameter 1mm was cleaned subsequently with hot concentrated chromic acid, hot concentrated HNO3, distilled water followed by washing with acetone using ultrasonic vibration to remove oil, grease etc. 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 2h. A precursor powder for the sensor layer was prepared by the following steps. Firstly, 5.99 g of reagent grade stannous chloride (SnCl2, 2H2O) was dissolved in 200 mL of distilled water containing 20 drops (0.8700 g) of concentrated HC1. For complete mixing, the solution was heated to around 80°C for Ih with continuous stirring. Secondly, 0.0100 g of reagent grade Sb2Oa (0.25 wt% with respect to SnO2) was dissolved in 50 mL distilled water (at 80°C for 30 min) containing 5 drops (0.2200 g) of concentrated HC1. In the next step, 0.6665 g of reagent grade PdCl2 (10 wt% Pd metal with respect to SnO2) was taken in lOOmL distilled water containing 10 drops (0.4200 g) of concentrated HNO3 and PdCL2 was slowly dissolved by heating the mixture at 80°C under constant stirring for Ih. The three solutions were mixed at 80°C under constant stirring for Ih and added to ammonia solution under sonication and the pH of the solution was maintained at 4. The precipitate was filtered, washed with distilled water and calcined at 900°C/2h. To prepare the alumina gel, 20.5000 g of reagent grade aluminium sec-butoxide (C12H27AlO3) was taken in 150 mL water and 0.8 mL cone. HNO3 was added to it to get a pH of 5. The mixture was stirred at 80-90°C for 3 hours to get complete dissolution of the sec-butoxide. The solution was refluxed for 18 h to get the gel of desired consistency so that the alumina content of the gel was 3% by weight. A thick paste of the powder formulation for the coating was made by mixing the powder with ammonium nitrate, alumina gel and ethanol in the weight ratio 1: 0.0025: 0.5: 2. The outside of the alumina tube was painted by the paste (thickness of the coating around 0.1 mm) followed by vacuum drying and curing at a temperature of 620°C for 45 min. The furnace temperature was raised at a rate of 75°C/h. After curing, the two circular ends and the extreme edges of the tube were cleaned of any coating material by rubbing with a diamond file. For measuring the gas sensitivity of the sensor coating, kanthal heating coil was put inside the tube and the sensitivity towards a gas was measured as a percentage of change in resistance (in presence of gas) by original resistance (in the ambient) at an elevated temperature. The sensors prepared in this way showed an average sensitivity of around 80% and response time of around 30sec and recovery time of around 3min in 1000 ppm methane and 1000 ppm LPG at 350°C. Example - 3 An alumina tube of length 3mm, outside diameter 1.5mm and inner diameter 1mm was cleaned subsequently with hot concentrated chromic acid, hot concentrated HNO3, distilled water followed by washing with acetone using ultrasonic vibration to remove oil, grease etc. 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 2h. A precursor powder for the sensor layer was prepared by the following steps. Firstly, 5.99 g of reagent grade stannous chloride (SnC12, 2H2O) was dissolved in 200 mL of distilled water containing 20 drops (0.8700 g) of concentrated HC1. For complete mixing, the solution was heated to around 80°C for Ih with continuous stirring. Secondly, 0.0100 g of reagent grade Sb2O3 (0.25 wt% with respect to SnO2) was dissolved in 50 mL distilled water (at 80°C for 30 min) containing 5 drops (0.2200 g) of concentrated HC1. In the next step, 0.6665 g of reagent grade PdCL2 (10 wt% Pd metal with respect to SnO2) was taken in lOOmL distilled water containing 10 drops (0.4200 g) of concentrated HNO3 and PdCI2 was slowly dissolved by heating the mixture at 80°C under constant stirring for Ih. The three solutions were mixed at 80°C under constant stirring for Ih and added to ammonia solution under sonication and the pH of the solution was maintained at 4. The precipitate was filtered, washed with distilled water and calcined at 900°C/2 h. To prepare the alumina gel, 20.5000 g of reagent grade aluminium sec-butoxide (C12H27AlO3) was taken in 150 mL water and 0.8 mL cone. HNO3 was added to it to get a pH of 5. The mixture was stirred at 80-90°C for 3 h to get complete dissolution of the sec-butoxide. The solution was refluxed for 18 hours to get the gel of desired consistency so that the alumina content of the gel was 3% by weight. A thick paste of the powder formulation for the coating was made by mixing the powder with ammonium nitrate, alumina gel and ethanol in the weight ratio 1: 0.005: 0.5: 2. The outside of the alumina tube was painted by the paste (thickness of the coating around 0.1 mm) followed by vacuum drying and curing at a temperature of 620°C for 45 min. The furnace temperature was raised at a rate of 75°C/h. After curing, the two circular ends and the extreme edges of the tube were cleaned of any coating material by rubbing with a diamond file. For measuring the gas sensitivity of the sensor coating, kanthal heating coil was put inside the tube and the sensitivity towards a gas was measured as a percentage of change in resistance (in presence of gas) by original resistance (in the ambient) at an elevated temperature. The sensors prepared in this way showed an average sensitivity of around 85% and response time of around 25 sec and recovery time of around 2min and 15sec in 1000 ppm methane and 1000 ppm LPG at 350°C. Example - 4 An alumina tube of length 3mm, outside diameter 1.5mm and inner diameter 1mm was cleaned subsequently with hot concentrated chromic acid, hot concentrated HNO3, distilled water followed by washing with acetone using ultrasonic vibration to remove oil, grease etc. 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 2h. A precursor powder for the sensor layer was prepared by the following steps. Firstly, 5.99 g of reagent grade stannous chloride (SnCl2, 2H2O) was dissolved in 200 mL of distilled water containing 20 drops (0.8700 g) of concentrated HC1. For complete mixing, the solution was heated to around 80°C for Ih with continuous stirring. Secondly, 0.0100 g of reagent grade Sb2O3 (0.25 wt% with respect to SnO2) was dissolved in 50 mL distilled water (at 80°C for 30 min) containing 5 drops (0.2200 g) of concentrated HC1. In the next step, 0.6665 g of reagent grade PdCl2 (10 wt% Pd metal with respect to SnO2) was taken in lOOmL distilled water containing 10 drops (0.4200 g) of concentrated HNO3 and PdCl2 was slowly dissolved by heating the mixture at 80°C under constant stirring for Ih. The three solutions were mixed at 80°C under constant stirring for Ih and added to ammonia solution under sonication and the pH of the solution was maintained at 4. The precipitate was filtered, washed with distilled water and calcined at 900°C/2h. To prepare the alumina gel, 20.5000 gram of reagent grade aluminium sec-butoxide (C12H27AlO3) was taken in 150 mL water and 0.8 mL cone. HNO3 was added to it to get a pH of 5. The mixture was stirred at 80-90°C for 3 h to get complete dissolution of the secbutoxide. The solution was refLuxed for 18 h to get the gel of desired consistency so that the alumina content of the gel was 3% by weight. A thick paste of the powder formulation for the coating was made by mixing a powder with ammonium nitrate, alumina gel and ethanol in the weight ratio 1: 0.0075: 0.5: 2. The outside of the alumina tube was painted by the paste (thickness of the coating around 0.1 mm) followed by vacuum drying and curing at a temperature of 620°C for 45 min. The furnace temperature was raised at a rate of 75°C/h. After curing, the two circular ends and the extreme edges of the tube were cleaned of any coating material by rubbing with a diamond file. For measuring the gas sensitivity of the sensor coating, kanthal heating coil was put inside the tube and the sensitivity towards a gas was measured as a percentage of change in resistance (in presence of gas) by original resistance (in the ambient) at an elevated temperature. The sensors prepared in this way showed an average sensitivity of around 90% and response time of around 15 sec and recovery time of around Imin and 15sec in 1000 ppm methane and 1000 ppm LPG at 350°C. Example - 5 An alumina tube of length 3mm, outside diameter 1.5mm and inner diameter 1mm was cleaned subsequently with hot concentrated chromic acid, hot concentrated HNOs, distilled water followed by washing with acetone using ultrasonic vibration to remove oil, grease etc. 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 2h. A precursor powder for the sensor layer was prepared by the following steps. Firstly, 5.99 g of reagent grade stannous chloride (SnCl2, 2H2O) was dissolved in 200 mL of distilled water containing 20 drops (0.8700 g) of concentrated HC1. For complete mixing, the solution was heated to around 80°C for Ih with continuous stirring. Secondly, 0.0100 g of reagent grade Sb2O3 (0.25 wt% with respect to SnO2) was dissolved in 50 mL distilled water (at 80°C for l/2h) containing 5 drops (0.2200 g) of concentrated HC1. In the next step, 0.6665 g of reagent grade PdCl2 (10 wt% Pd metal with respect to SnO2) was taken in 1 OOmL distilled water containing 10 drops (0.4200 g) of concentrated HNOs and PdCl2 was slowly dissolved by heating the mixture at 80°C under constant stirring for Ih. The three solutions were mixed at 80°C under constant stirring for Ih and added to ammonia solution under sonication and the pH of the solution was maintained at 4. The precipitate was filtered, washed with distilled water and calcined at 900°C/2h. To prepare the alumina gel, 20.5000 g of reagent grade aluminium sec-butoxide (C12H27AlO3) was taken in 150 mL water and 0.8 mL cone. HNO3 was added to it to get a pH of 5. The mixture was stirred at 80-90°C for 3 h to get complete dissolution of the sec-butoxide. The solution was refluxed for 18 h to get the gel of desired consistency so that the alumina content of the gel was 3% by weight. A thick paste of the powder formulation for the coating was made by mixing a powder with ammonium nitrate, alumina gel and ethanol in the weight ratio 1: 0.01: 0.5: 2. The outside of the alumina tube was painted by the paste (thickness of the coating around 0.1 mm) followed by vacuum drying and curing at a temperature of 620°C for 45 min. The furnace temperature was raised at a rate of 75°C/h. After curing, the two circular ends and the extreme edges of the tube were cleaned of any coating material by rubbing with a diamond file. For measuring the gas sensitivity of the sensor coating, kanthal heating coil was put inside the tube and the sensitivity towards a gas was measured as a percentage of change in resistance (in presence of gas) by original resistance (in the ambient) at an elevated temperature. The sensors prepared in this way showed an average sensitivity of around 80% and response time of around 20sec and recovery time of around Imin and 25sec in 1000 ppm methane and 1000 ppm LPG at 350°C. Example - 6 An alumina tube of length 3mm, outside diameter 1.5mm and inner diameter 1mm was cleaned subsequently with hot concentrated chromic acid, hot concentrated HNO3, distilled water followed by washing with acetone using ultrasonic vibration to remove oil, grease etc. 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 2h. A precursor powder for the sensor layer was prepared by the following steps. Firstly, 5.99 g of reagent grade stannous chloride (SnCI2, 2H2O) was dissolved in 200 mL of distilled water containing 20 drops (0.8700 g) of concentrated HC1. For complete mixing, the solution was heated to around 80°C for Ih with continuous stirring. Secondly, 0.0100 g of reagent grade Sb2O3 (0.25 wt% with respect to SnO2) was dissolved in 50 mL distilled water (at 80°C for 30 min) containing 5 drops (0.2200 g) of concentrated HC1. In the next step, 0.3333 g of reagent grade PdCh (5 wt% Pd metal with respect to SnO2) was taken in lOOmL distilled water containing 10 drops (0.4200 g) of concentrated HNO3 and PdCI2 was slowly dissolved by heating the mixture at 80°C under constant stirring for Ih. The three solutions were mixed at 80°C under constant stirring for Ih and added to ammonia solution under sonication and the pH of the solution was maintained at 4. The precipitate was filtered, washed with distilled water and calcined at 900°C/2h. To prepare the alumina gel, 20.5000 g of reagent grade aluminium sec-butoxide (C12H27AlO3) was taken in 150 mL water and 0.8 mL cone. HNOs was added to it to get a pH of 5. The mixture was stirred at 80-90°C for 3 h to get complete dissolution of the sec-butoxide. The solution was refluxed for 18 h to get the gel of desired consistency so that the alumina content of the gel was 3% by weight. A thick paste of the powder formulation for the coating was made by mixing a powder with ammonium nitrate, alumina gel and ethanol in the weight ratio 1: 0.01: 0.5: 2. The outside of the alumina tube was painted by the paste (thickness of the first coating around 0.1 mm) followed by vacuum drying and curing at a temperature of 620°C for 45 min. The furnace temperature was raised at a rate of 75°C/h. After curing, the two circular ends and the extreme edges of the tube were cleaned of any coating material by rubbing with a diamond file. For measuring the gas sensitivity of the sensor coating, kanthal heating coil was put inside the tube and the sensitivity towards a gas was measured as a percentage of change in resistance (in presence of gas) by original resistance (in the ambient) at an elevated temperature. The sensors prepared in this way showed an average sensitivity of around 70% and response time of around 20sec and recovery time of around Imin and 35sec in 1000 ppm methane and 1000 ppm LPG at 350°C. The main advantages of the present invention are as follows: 1. The method can provide sensors, which can detect combustible gases like methane, LPG, propane, CNG with a fast response time (-15 sec) to a step increase in the analyte gas and the sensor quickly recovers (~1 min and 15 sec) after the gas is withdrawn. 2. This method provides gas sensor coating having better control of the final microstructure consisting of controlled cracks which help to get fast response and recovery time. 3. The method provides sensor coating of desired characteristics along with good adherence to the substrate. 4. The process is cost effective as the material requirement is low. 5. The sensitivity of such sensors towards the combustible gases like LPG, methane is high (-90% in lOOOppm gas at 350°C). We Claim: 1 . A method of improving response and recovery time of semiconducting tin oxide based gas sensors in thick film form for detection of combustible gases, which comprises cleaning an alumina substrate, such as a tubular substrate, by hot concentrated chromic acid, hot concentrated HNO3 and distilled water in a sequence followed by washing with acetone using ultrasonic vibration to obtain a clean alumina substrate devoid of oil, grease; electroding the cleaned alumina substrate ends using gold paste followed by attaching lead wires such as gold, gold alloy, platinum and curing the assembly at a temperature in the range of 900°C -1000°C for a period of 1-2 hours; characterized in that applying on the said cleaned and electroded alumina sustrate a sensor coating consisting of a sensor precursor powder composition, a gas forming agent, alumina gel and ethanol in the weight ratio ranging between 1: 0.0025 : 0.5 : 2 and 1: 0.02 : 0.5 : 2 in aqueous or nonaqueous medium; vacuum drying the said sensor coating followed by curing at a temperature in the range of 600°C-700°C for a time period in the range of 30 minutes to 2 hours, at a heating rate in the range of 50°C-75°C/hour, to obtain coating microstructure having controlled cracks. 2. A method as claimed in claim 1, wherein the sensor precursor powder composition consists of tin dioxide (SnO2), antimony oxide (Sb2O3) in the range of 0.2-0.3 wt% with respect to SnO2 and palladium in the range of 5-10 wt% with respect to SnO2. 3. A method as claimed in claim 1-2, wherein the sensor precursor powder batch composition is prepared by sonication assisted simultaneous precipitation technique using aqueous solutions of stannous chloride, antimony chloride and palladium nitrate followed by calcination at a temperature in the range of 800°C -900°C in air for a time period in the range of 1-2 hours and maintaining the weight ratio of Sb2O3 in the range of 0.2-0.3 wt% and palladium as a metal basis in the rangeof5-10wt%. 4. A method as claimed in claim 1-3, wherein the gas forming agent is ammonium nitrate in the range of 0.25-2 wt% with respect to the sensor precursor powder SnO2 batch composition. 5. A method as claimed in claim 1-4, wherein the sensor coating consists of alumina gel equivalent to 0.05 wt% alumina with respect to SnO2. 6. A method as claimed in claim 1-5, wherein the alumina gel is prepared using sol- gel technique from an aqueous solution of alumina alkoxide such as sec-butoxide (C12H27AlO3) in nitric acid and refluxing the aluminium sec-butoxide solution at a temperature in the range of 80°C - 90°C for a time period in the range of 10-20 hours and maintaining the solid content of the sol in the range of 2-5 wt%. 7. A method as claimed in claim 1-6, wherein the sensor coating of thickness in the range of 0.05-0.15 mm is applied on the cleaned and electroded tubular alumina sustrate by brushing. 8. Semiconducting tin oxide based gas sensors in thick film form having improved response and recovery time for detection of combustible gases made by the method as claimed in claims 1-7 herein above. 9. A method of improving response and recovery time of semiconducting tin oxide based gas sensors in thick film form for detection of combustible gases, substantially as herein described with reference to the examples. 10. Semiconducting tin oxide based gas sensors in thick film form having improved response and recovery time for detection of combustible gases made by the method substantially as herein described with reference to the examples.

Full Text

The present invention relates to a method of improving response and recovery time of semiconducting tin oxide based gas sensors in thick film form for detection of combustible gases and semiconducting tin oxide based gas sensors in thick film form made thereby.
The present invention particularly relates to a method of improving response and recovery time of semiconducting tin oxide based gas sensors in thick film form for detection of combustible gases where controlled cracks are generated at the surface of the coating to get the desired properties.
This invention more particularly relates to a method of improving response and recovery time of semiconducting tin oxide based gas sensors in thick film form for detection of combustible gases where controlled cracks are generated at the surface of the coating to get the desired properties and the gas sensitivity and adherence of the coating remain above par and are not affected by the method of the present invention.
With increasing world-wide distribution of gas sensors for different applications, the demand of sensors fulfilling specific standards is growing in leaps and bounds. Tin dioxide based compositions are mostly used in semiconductor gas sensors. Tin oxide is a wide band gap (3.6 eV) n-type semiconductor. Tin oxide films consist of a large number of grains connected to each other through grain boundaries. This governs the formation of schottky barriers, whose heights are dependent on the surface charge density and the barrier heights are modulated due to the adsorption of gas/odour molecules.
In this context, reference may be made to a review paper: W. Gopel and K.D. Schierbaum, "SnOa sensors Current Status and Future Prospects", Sensors and
actuators B, 26-27 (1995) 1-12 and a book G. Sberveglieri (ed.) "Gas Sensors -Principle, Operation and Development", kluwer, Dordrecht (1992). Such semiconductor sensors can be made in thick or thin film form. The advantages of thick film sensors are: 1) ease of fabrication and handling, 2) ruggedness and above all, 3) low price. The main disadvantages are slow response and recovery to an analyte gas.
For real life applications of the sensors, the response time (the time required for a sensor to respond to a step increase in the analyte gas) should be low so that the buzzer of a gas alarm rings during gas leak without delay. Similarly, the buzzer should not go on ringing for a long time (i.e, quick recovery, the time required for a sensor to return to baseline after a response to an analyte) after the gas is withdrawn. Though information on semiconductor sensors is available in plenty in public domain, little has been said about the response time and recovery time of gas sensors.
In case of thick film semiconductor sensors for detection of combustion gases, reference may be made to publications such as:
1) G. J. Li and S. Kawi, Materials Letters 34 (1998) 99-102.
2) J. C. Kim, H. K. Jun, J. S. Huh and D. D. Lee, Sensor and Actuator B 45 (1998)
271-277.
3) E. W. Williams and A. G. Keeling, J. Materials Science: materials in electronics 9
(1998) 51-54, dealt with the response and recovery time of the sensors.
Reference may be made to US patent no.: 6,872,290, titled: Electrochemical gas sensor with passage for receiving gas, relating to relates to an electrochemical gas sensor having improved response time and sensitivity. The electrochemical gas sensor includes a substrate for providing a surface upon which a film of conductive
material may be deposited, a pair of electrodes deposited on the substrate for permitting a measurement of current, and an electrolytic material for providing an electrical connection between the electrodes. The electrochemical gas sensor further includes a passage in the surface for holding or transporting gas received from a gas source, whereby sensing occurs at a three way interface where the gas, electrolytic material, and electrodes come in direct contact with one another.
Reference may be made to US patent no.: 6,908,538, titled: Electrochemical gas sensor having a porous electrolyte, which relates to an electrochemical gas sensor having improved response time and sensitivity. The electrochemical gas sensor includes a substrate for providing a surface upon which an electrode may be placed, a first electrode placed on the surface, a thin film of electrolyte support for receiving electrolyte and having a predetermined porosity, and a second electrode deposited on the thin film for permitting a measurement of current between the first and second electrodes.
Reference may be made to US patent no.: 6,929,735, wherein is disclosed a method and apparatus for providing an electrochemical gas sensor having improved response time for detecting a gas introduced into the sensor. The sensor includes a substrate having a first surface and a second surface and an electrode deposited on the first surface. The sensor also includes an ionomer membrane in contact with the first surface and the electrode. The ionomer membrane has an opening in a location proximate to the electrode for permitting gas introduced into the sensor to diffuse through the opening to simultaneously contact the electrode and the ionomer membrane within the opening. The substrate further includes at least one hole extending from the first surface to the second surface for permitting moisture to diffuse through the at least one hole to contact the ionomer membrane for enhancing sensitivity. In this disclosure there is provided an electrochemical gas sensor having
an ionomer membrane in contact with the first surface and the electrode, having improved response time for detecting a gas like CO.
The drawbacks of the existing methods lie in their reported high response ( 20sec) and recovery ( 2min) times.
From the hitherto known prior art it is observed that there is a need to provide to provide a method of improving the response and recovery time particularly of semiconducting tin oxide based gas sensors in thick film form for detection of combustible gases.
The main object of the present invention is to provide a method of improving the response and recovery time of semiconducting tin oxide based gas sensors in thick film form for detection of combustible gases.
Another object of the present invention is to provide a method of improving response and recovery time of semiconducting tin oxide based gas sensors in thick film form for detection of combustible gases, where controlled cracks are generated at the surface of the coating to get the desired properties.
Still another object of the present invention is to provide a method of improving response and recovery time of semiconducting tin oxide based gas sensors in thick film form for detection of combustible gases where controlled cracks are generated at the surface of the coating to get the desired properties and the gas sensitivity and adherence of the coating remain above par and are not affected in the invented method.
Yet another object of the present invention is to provide a method of improving the response and recovery time of semiconducting tin oxide based gas sensors in thick film form for detection of combustible gases where the response and recovery time should be within 15sec and Imin 15sec respectively.
A further object of the present invention is to provide semiconducting tin oxide based gas sensors in thick film form having improved response and recovery time for detection of combustible gases.
This invention relates to a method of improving response and recovery time of semiconducting tin oxide based gas sensors in thick film form for detection of combustible gases like methane, LPG, propane, CNG. The improved gas sensors are made by coating pre-cleaned and electroded tubular alumina substrates with a sensor coating comprising tin dioxide (SnO2), antimony oxide (Sb2O3) in the range of 0.2-0.3 wt% with respect to SnO2 and palladium in the range of 5-10 wt% with respect to SnO2 and alumina gel equivalent to 0.05wt% alumina with respect to SnO2 along with a gas forming agent such as ammonium nitrate in the range of 0.25-2 wt% with respect to SnO2 batch composition, vacuum drying and curing the sensor coating at a temperature in the range of 600°C-700°C for a time in the range of 30 min to 2 h at a heating rate in the range of 50°C-75°C/h so as to get coating microstructure having controlled cracks. The response and recovery time of the thick film sensors prepared by this method are around 15 sec and Imin 15 sec., respectively, which are acceptable for real-life applications of the sensors. The invented method also imparts good adherence of the coating with the substrates. The sensors made by the method of the present invention are highly sensitive towards the combustible gases to be detected. The processing steps of the fabrication of such sensors are also simple and cost effective.
The novelty of the present invention resides in enhancing the response and recovery time of the sensor, at the same time, the other important characteristics of the sensors like gas sensitivity and adherence of the coating remain unaffected in the process of generating controlled cracks.
The inventive steps resides in adding a gas forming ammonium nitrate as an additive in the sensor coating composition and curing the composition at a temperature in the range of 600°C-700°C for a time in the range of 30 min to 2 hours at a heating rate in the range of 50°C-75°C/h, so that controlled cracks appear at the surface of the coating without affecting the coating adherence and gas sensitivity, whereas the response and recovery time are improved.
Accordingly, the present invention provides a method of improving response and recovery time of semiconducting tin oxide based gas sensors in thick film form for detection of combustible gases, which comprises cleaning an alumina substrate, such as a tubular substrate, by hot concentrated chromic acid, hot concentrated HNOa and distilled water in a sequence followed by washing with acetone using ultrasonic vibration to obtain a clean alumina substrate devoid of oil, grease; electroding the cleaned alumina substrate ends using gold paste followed by attaching lead wires such as gold, gold alloy, platinum and curing the assembly at a temperature in the range of 900°C -1000°C for a period of 1-2 hours; characterized in that applying on the said cleaned and electroded alumina sustrate a sensor coating consisting of a sensor precursor powder composition, a gas forming agent, alumina gel and ethanol in the weight ratio ranging between 1: 0.0025 : 0.5 : 2 and 1: 0.02 : 0.5 : 2 in aqueous or nonaqueous medium; vacuum drying the said sensor coating followed by curing at a temperature in the range of 600°C-700°C for a time period in the range of 30 minutes to 2 hours, at a heating rate in the range of 50°C-75°C/hour, to obtain coating microstructure having controlled cracks.

In another embodiment of the present invention, the sensor precursor powder composition consists of tin dioxide (SnO2), antimony oxide (Sb2O3) in the range of 0.2-0.3 wt% with respect to SnO2 and palladium in the range of 5-10 wt% with respect to
In yet another embodiment of the present invention, the sensor precursor powder SnO2 batch composition is prepared by sonication assisted simultaneous precipitation technique using aqueous solutions of stannous chloride, antimony chloride and palladium nitrate followed by calcination at a temperature in the range of 800°C -900°C in air for a time period in the range of 1 -2 hours and maintaining the weight ratio of Sb2O3 in the range of 0.2-0.3 wt% and palladium as a metal basis in the rangeof5-10wt%.
In still another embodiment of the present invention, the gas forming agent is ammonium nitrate in the range of 0.25-2 wt% with respect to the sensor precursor powder SnO2 batch composition.
In still yet another embodiment of the present invention, the sensor coating consists of alumina gel equivalent to 0.05 wt% alumina with respect to SnO2.
In a further embodiment of the present invention, the alumina gel is prepared using sol-gel technique from an aqueous solution of alumina alkoxide such as sec-butoxide (C12H27AlOs) in nitric acid and refluxing the aluminium sec-butoxide solution at a temperature in the range of 80°C - 90°C for a time period in the range of 10-20 hours and maintaining the solid content of the sol in the range of 2-5 wt%.
In a still further embodiment of the present invention, the sensor coating of thickness in the range of 0.05-0.15 mm is applied on the cleaned and electroded tubular alumina sustrate by brushing.
Accordingly, the present invention provides semiconducting tin oxide based gas sensors in thick film form having improved response and recovery time for detection of combustible gases made by the method as herein above described.
The detailed process steps of the present invention are as follows:
(a) Cleaning the tubular alumina substrates with hot concentrated chromic acid, hot
concentrated HNO2, distilled water in a sequence followed by washing with acetone
under ultrasonic vibration to remove oil and grease.
(b) Electroding the two ends of the alumina substrates by gold-based electrodes,
attaching/fixing lead wires of gold/gold-alloy/platinum and curing the assembly at a
temperature in the range of 900°C -1000°C for a time period in the range of 1-2
hours.
(c) Preparing the precursor powder for the sensor coating comprising tin dioxide
(SnO2), 0.20-0.30 wt% antimony oxide (Sb2O3) with respect to SnO2 and 5-10 wt%
palladium (as metal) with respect to SnOa by sonication assisted simultaneous
precipitation technique using aqueous solutions of stannous chloride, antimony
chloride and palladium nitrate followed by calcination at a temperature in the range
of 800°C -900°C in air for a time period in the range of 1-2 hours.
d) Preparing the alumina gel form an aqueous solution of alumina alkoxide like sec-butoxide (C12H27AlO3) in nitric acid and refluxing the solution at a temperature in the range of 80°C -90°C for a time in the range of 10-20 hours.
e) Preparing the paste for sensor coating in aqueous or nonaqueous medium, which comprises the sensor precursor powder composition, ammonium nitrate, alumina gel and ethanol in the weight ratio ranging between 1: 0.0025: 0.5: 2 and 1: 0.02: 0.5:2.
(f) Applying by brushing on the said cleaned and electroded tubular alumina sustrate
a coating of sensor composition of thickness in the range of 0.05-0.15 mm.
(g) Vacuum drying the coating followed by curing at a temperature in the range of
600°C -700°C for a time in the range of 30 min - 2 hours, at a heating rate in the
range of 50°C -75°C/h, to obtain coating microstructure having controlled cracks.
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, outside diameter 1.5mm and inner diameter 1mm was cleaned subsequently with hot concentrated chromic acid, hot concentrated HNO3, distilled water followed by washing with acetone using ultrasonic vibration to remove oil, grease etc. 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 2h.
A precursor powder for the sensor layer was prepared by the following steps. Firstly, 5.99 g of reagent grade stannous chloride (SnCl2, 2H2O) was dissolved in 200 mL of distilled water containing 20 drops (0.8700 g) of concentrated HC1. For complete mixing, the solution was heated to around 80°C for Ih with continuous stirring. Secondly, 0.0100 g of reagent grade Sb2O3 (0.25 wt% with respect to SnO2) was dissolved in 50 mL distilled water (at 80°C for 30 min) containing 5 drops (0.2200 g) of concentrated HC1. In the next step, 0.6665 g of reagent grade PdCl2 (10 wt% Pd metal with respect to SnO2) was taken in lOOmL distilled water containing 10 drops (0.4200 g) of concentrated HNO3 and PdCl2 was slowly dissolved by heating the mixture at 80°C under constant stirring for Ih. The three solutions were mixed at 80°C under constant stirring for Ih and added to ammonia solution under sonication and the pH of the solution was maintained at 4. The precipitate was filtered, washed with distilled water and calcined at 900°C/2h.
To prepare the alumina gel, 20.5000 g of reagent grade aluminium sec-butoxide (C12H27AlO3) was taken in 150 mL water and 0.8 mL cone. HNO2 was added to it to get a pH of 5. The mixture was stirred at 80-90°C for 3 h to get complete dissolution of the secbutoxide. The solution was refluxed for 18 h to get the gel of desired consistency so that the alumina content of the gel was 3% by weight. A thick paste of the powder formulation for the coating was made by mixing the powder with alumina gel and ethanol in the weight ratio 1: 0.5: 2. The outside of the alumina tube was painted by the paste (thickness of the coating around 0.1 mm) followed by vacuum drying and curing at a temperature of 620°C for 45 min. The furnace temperature was raised at a rate of 75°C/h. After curing, the two circular ends and the extreme edges of the tube were cleaned of any coating material by rubbing with a diamond file.
For measuring the gas sensitivity of the sensor coating, kanthal heating coil was put inside the tube and the sensitivity towards a gas was measured as a percentage of change in resistance (in presence of gas) by original resistance (in the ambient) at an elevated temperature. The sensors prepared in this way showed an average sensitivity of around 75% and response time of around 35 sec and recovery time of around 4min in 1000 ppm methane and 1000 ppm LPG at 350°C.
Example - 2
An alumina tube of length 3mm, outside diameter 1.5mm and inner diameter 1mm was cleaned subsequently with hot concentrated chromic acid, hot concentrated HNO3, distilled water followed by washing with acetone using ultrasonic vibration to remove oil, grease etc. 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 2h.
A precursor powder for the sensor layer was prepared by the following steps. Firstly, 5.99 g of reagent grade stannous chloride (SnCl2, 2H2O) was dissolved in 200 mL of distilled water containing 20 drops (0.8700 g) of concentrated HC1. For complete mixing, the solution was heated to around 80°C for Ih with continuous stirring. Secondly, 0.0100 g of reagent grade Sb2Oa (0.25 wt% with respect to SnO2) was dissolved in 50 mL distilled water (at 80°C for 30 min) containing 5 drops (0.2200 g) of concentrated HC1. In the next step, 0.6665 g of reagent grade PdCl2 (10 wt% Pd metal with respect to SnO2) was taken in lOOmL distilled water containing 10 drops (0.4200 g) of concentrated HNO3 and PdCL2 was slowly dissolved by heating the mixture at 80°C under constant stirring for Ih. The three solutions were mixed at 80°C under constant stirring for Ih and added to ammonia solution under sonication and the pH of the solution was maintained at 4. The precipitate was filtered, washed with distilled water and calcined at 900°C/2h.
To prepare the alumina gel, 20.5000 g of reagent grade aluminium sec-butoxide (C12H27AlO3) was taken in 150 mL water and 0.8 mL cone. HNO3 was added to it to get a pH of 5. The mixture was stirred at 80-90°C for 3 hours to get complete dissolution of the sec-butoxide. The solution was refluxed for 18 h to get the gel of desired consistency so that the alumina content of the gel was 3% by weight. A thick paste of the powder formulation for the coating was made by mixing the powder with ammonium nitrate, alumina gel and ethanol in the weight ratio 1:
0.0025: 0.5: 2. The outside of the alumina tube was painted by the paste (thickness of the coating around 0.1 mm) followed by vacuum drying and curing at a temperature of 620°C for 45 min. The furnace temperature was raised at a rate of 75°C/h. After curing, the two circular ends and the extreme edges of the tube were cleaned of any coating material by rubbing with a diamond file.
For measuring the gas sensitivity of the sensor coating, kanthal heating coil was put inside the tube and the sensitivity towards a gas was measured as a percentage of change in resistance (in presence of gas) by original resistance (in the ambient) at an elevated temperature. The sensors prepared in this way showed an average sensitivity of around 80% and response time of around 30sec and recovery time of around 3min in 1000 ppm methane and 1000 ppm LPG at 350°C.
Example - 3
An alumina tube of length 3mm, outside diameter 1.5mm and inner diameter 1mm was cleaned subsequently with hot concentrated chromic acid, hot concentrated HNO3, distilled water followed by washing with acetone using ultrasonic vibration to remove oil, grease etc. 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 2h.
A precursor powder for the sensor layer was prepared by the following steps. Firstly, 5.99 g of reagent grade stannous chloride (SnC12, 2H2O) was dissolved in 200 mL of distilled water containing 20 drops (0.8700 g) of concentrated HC1. For complete mixing, the solution was heated to around 80°C for Ih with continuous stirring. Secondly, 0.0100 g of reagent grade Sb2O3 (0.25 wt% with respect to SnO2) was dissolved in 50 mL distilled water (at 80°C for 30 min) containing 5 drops (0.2200 g) of concentrated HC1. In the next step, 0.6665 g of reagent grade PdCL2 (10 wt% Pd metal with respect to SnO2) was taken in lOOmL distilled water containing 10 drops
(0.4200 g) of concentrated HNO3 and PdCI2 was slowly dissolved by heating the mixture at 80°C under constant stirring for Ih. The three solutions were mixed at 80°C under constant stirring for Ih and added to ammonia solution under sonication and the pH of the solution was maintained at 4. The precipitate was filtered, washed with distilled water and calcined at 900°C/2 h.
To prepare the alumina gel, 20.5000 g of reagent grade aluminium sec-butoxide (C12H27AlO3) was taken in 150 mL water and 0.8 mL cone. HNO3 was added to it to get a pH of 5. The mixture was stirred at 80-90°C for 3 h to get complete dissolution of the sec-butoxide. The solution was refluxed for 18 hours to get the gel of desired consistency so that the alumina content of the gel was 3% by weight. A thick paste of the powder formulation for the coating was made by mixing the powder with ammonium nitrate, alumina gel and ethanol in the weight ratio 1: 0.005: 0.5: 2. The outside of the alumina tube was painted by the paste (thickness of the coating around 0.1 mm) followed by vacuum drying and curing at a temperature of 620°C for 45 min. The furnace temperature was raised at a rate of 75°C/h. After curing, the two circular ends and the extreme edges of the tube were cleaned of any coating material by rubbing with a diamond file.
For measuring the gas sensitivity of the sensor coating, kanthal heating coil was put inside the tube and the sensitivity towards a gas was measured as a percentage of change in resistance (in presence of gas) by original resistance (in the ambient) at an elevated temperature. The sensors prepared in this way showed an average sensitivity of around 85% and response time of around 25 sec and recovery time of around 2min and 15sec in 1000 ppm methane and 1000 ppm LPG at 350°C.
Example - 4
An alumina tube of length 3mm, outside diameter 1.5mm and inner diameter 1mm was cleaned subsequently with hot concentrated chromic acid, hot concentrated
HNO3, distilled water followed by washing with acetone using ultrasonic vibration to remove oil, grease etc. 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 2h.
A precursor powder for the sensor layer was prepared by the following steps. Firstly, 5.99 g of reagent grade stannous chloride (SnCl2, 2H2O) was dissolved in 200 mL of distilled water containing 20 drops (0.8700 g) of concentrated HC1. For complete mixing, the solution was heated to around 80°C for Ih with continuous stirring. Secondly, 0.0100 g of reagent grade Sb2O3 (0.25 wt% with respect to SnO2) was dissolved in 50 mL distilled water (at 80°C for 30 min) containing 5 drops (0.2200 g) of concentrated HC1. In the next step, 0.6665 g of reagent grade PdCl2 (10 wt% Pd metal with respect to SnO2) was taken in lOOmL distilled water containing 10 drops (0.4200 g) of concentrated HNO3 and PdCl2 was slowly dissolved by heating the mixture at 80°C under constant stirring for Ih. The three solutions were mixed at 80°C under constant stirring for Ih and added to ammonia solution under sonication and the pH of the solution was maintained at 4. The precipitate was filtered, washed with distilled water and calcined at 900°C/2h.
To prepare the alumina gel, 20.5000 gram of reagent grade aluminium sec-butoxide (C12H27AlO3) was taken in 150 mL water and 0.8 mL cone. HNO3 was added to it to get a pH of 5. The mixture was stirred at 80-90°C for 3 h to get complete dissolution of the secbutoxide. The solution was refLuxed for 18 h to get the gel of desired consistency so that the alumina content of the gel was 3% by weight. A thick paste of the powder formulation for the coating was made by mixing a powder with ammonium nitrate, alumina gel and ethanol in the weight ratio 1: 0.0075: 0.5: 2. The outside of the alumina tube was painted by the paste (thickness of the coating around 0.1 mm) followed by vacuum drying and curing at a temperature of 620°C for 45 min. The furnace temperature was raised at a rate of 75°C/h. After
curing, the two circular ends and the extreme edges of the tube were cleaned of any coating material by rubbing with a diamond file.
For measuring the gas sensitivity of the sensor coating, kanthal heating coil was put inside the tube and the sensitivity towards a gas was measured as a percentage of change in resistance (in presence of gas) by original resistance (in the ambient) at an elevated temperature. The sensors prepared in this way showed an average sensitivity of around 90% and response time of around 15 sec and recovery time of around Imin and 15sec in 1000 ppm methane and 1000 ppm LPG at 350°C.
Example - 5
An alumina tube of length 3mm, outside diameter 1.5mm and inner diameter 1mm was cleaned subsequently with hot concentrated chromic acid, hot concentrated HNOs, distilled water followed by washing with acetone using ultrasonic vibration to remove oil, grease etc. 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 2h.
A precursor powder for the sensor layer was prepared by the following steps. Firstly, 5.99 g of reagent grade stannous chloride (SnCl2, 2H2O) was dissolved in 200 mL of distilled water containing 20 drops (0.8700 g) of concentrated HC1. For complete mixing, the solution was heated to around 80°C for Ih with continuous stirring. Secondly, 0.0100 g of reagent grade Sb2O3 (0.25 wt% with respect to SnO2) was dissolved in 50 mL distilled water (at 80°C for l/2h) containing 5 drops (0.2200 g) of concentrated HC1. In the next step, 0.6665 g of reagent grade PdCl2 (10 wt% Pd metal with respect to SnO2) was taken in 1 OOmL distilled water containing 10 drops (0.4200 g) of concentrated HNOs and PdCl2 was slowly dissolved by heating the mixture at 80°C under constant stirring for Ih. The three solutions were mixed at 80°C under constant stirring for Ih and added to ammonia solution under sonication
and the pH of the solution was maintained at 4. The precipitate was filtered, washed with distilled water and calcined at 900°C/2h.
To prepare the alumina gel, 20.5000 g of reagent grade aluminium sec-butoxide (C12H27AlO3) was taken in 150 mL water and 0.8 mL cone. HNO3 was added to it to get a pH of 5. The mixture was stirred at 80-90°C for 3 h to get complete dissolution of the sec-butoxide. The solution was refluxed for 18 h to get the gel of desired consistency so that the alumina content of the gel was 3% by weight. A thick paste of the powder formulation for the coating was made by mixing a powder with ammonium nitrate, alumina gel and ethanol in the weight ratio 1: 0.01: 0.5: 2. The outside of the alumina tube was painted by the paste (thickness of the coating around 0.1 mm) followed by vacuum drying and curing at a temperature of 620°C for 45 min. The furnace temperature was raised at a rate of 75°C/h. After curing, the two circular ends and the extreme edges of the tube were cleaned of any coating material by rubbing with a diamond file.
For measuring the gas sensitivity of the sensor coating, kanthal heating coil was put inside the tube and the sensitivity towards a gas was measured as a percentage of change in resistance (in presence of gas) by original resistance (in the ambient) at an elevated temperature. The sensors prepared in this way showed an average sensitivity of around 80% and response time of around 20sec and recovery time of around Imin and 25sec in 1000 ppm methane and 1000 ppm LPG at 350°C.
Example - 6
An alumina tube of length 3mm, outside diameter 1.5mm and inner diameter 1mm was cleaned subsequently with hot concentrated chromic acid, hot concentrated HNO3, distilled water followed by washing with acetone using ultrasonic vibration to remove oil, grease etc. 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 2h.
A precursor powder for the sensor layer was prepared by the following steps. Firstly, 5.99 g of reagent grade stannous chloride (SnCI2, 2H2O) was dissolved in 200 mL of distilled water containing 20 drops (0.8700 g) of concentrated HC1. For complete mixing, the solution was heated to around 80°C for Ih with continuous stirring. Secondly, 0.0100 g of reagent grade Sb2O3 (0.25 wt% with respect to SnO2) was dissolved in 50 mL distilled water (at 80°C for 30 min) containing 5 drops (0.2200 g) of concentrated HC1. In the next step, 0.3333 g of reagent grade PdCh (5 wt% Pd metal with respect to SnO2) was taken in lOOmL distilled water containing 10 drops (0.4200 g) of concentrated HNO3 and PdCI2 was slowly dissolved by heating the mixture at 80°C under constant stirring for Ih. The three solutions were mixed at 80°C under constant stirring for Ih and added to ammonia solution under sonication and the pH of the solution was maintained at 4. The precipitate was filtered, washed with distilled water and calcined at 900°C/2h.
To prepare the alumina gel, 20.5000 g of reagent grade aluminium sec-butoxide (C12H27AlO3) was taken in 150 mL water and 0.8 mL cone. HNOs was added to it to get a pH of 5. The mixture was stirred at 80-90°C for 3 h to get complete dissolution of the sec-butoxide. The solution was refluxed for 18 h to get the gel of desired consistency so that the alumina content of the gel was 3% by weight. A thick paste of the powder formulation for the coating was made by mixing a powder with ammonium nitrate, alumina gel and ethanol in the weight ratio 1: 0.01: 0.5: 2. The outside of the alumina tube was painted by the paste (thickness of the first coating around 0.1 mm) followed by vacuum drying and curing at a temperature of 620°C for 45 min. The furnace temperature was raised at a rate of 75°C/h. After curing, the two circular ends and the extreme edges of the tube were cleaned of any coating material by rubbing with a diamond file.
For measuring the gas sensitivity of the sensor coating, kanthal heating coil was put inside the tube and the sensitivity towards a gas was measured as a percentage of change in resistance (in presence of gas) by original resistance (in the ambient) at an elevated temperature. The sensors prepared in this way showed an average sensitivity of around 70% and response time of around 20sec and recovery time of around Imin and 35sec in 1000 ppm methane and 1000 ppm LPG at 350°C.
The main advantages of the present invention are as follows:
1. The method can provide sensors, which can detect combustible gases like
methane, LPG, propane, CNG with a fast response time (-15 sec) to a step
increase in the analyte gas and the sensor quickly recovers (~1 min and 15 sec) after
the gas is withdrawn.
2. This method provides gas sensor coating having better control of the final
microstructure consisting of controlled cracks which help to get fast response and
recovery time.
3. The method provides sensor coating of desired characteristics along with good
adherence to the substrate.
4. The process is cost effective as the material requirement is low.
5. The sensitivity of such sensors towards the combustible gases like LPG, methane
is high (-90% in lOOOppm gas at 350°C).

We Claim:
1 . A method of improving response and recovery time of semiconducting tin oxide based gas sensors in thick film form for detection of combustible gases, which comprises cleaning an alumina substrate, such as a tubular substrate, by hot concentrated chromic acid, hot concentrated HNO3 and distilled water in a sequence followed by washing with acetone using ultrasonic vibration to obtain a clean alumina substrate devoid of oil, grease; electroding the cleaned alumina substrate ends using gold paste followed by attaching lead wires such as gold, gold alloy, platinum and curing the assembly at a temperature in the range of 900°C -1000°C for a period of 1-2 hours; characterized in that applying on the said cleaned and electroded alumina sustrate a sensor coating consisting of a sensor precursor powder composition, a gas forming agent, alumina gel and ethanol in the weight ratio ranging between 1: 0.0025 : 0.5 : 2 and 1: 0.02 : 0.5 : 2 in aqueous or nonaqueous medium; vacuum drying the said sensor coating followed by curing at a temperature in the range of 600°C-700°C for a time period in the range of 30 minutes to 2 hours, at a heating rate in the range of 50°C-75°C/hour, to obtain coating microstructure having controlled cracks.
2. A method as claimed in claim 1, wherein the sensor precursor powder composition consists of tin dioxide (SnO2), antimony oxide (Sb2O3) in the range of 0.2-0.3 wt% with respect to SnO2 and palladium in the range of 5-10 wt% with respect to SnO2.
3. A method as claimed in claim 1-2, wherein the sensor precursor powder batch composition is prepared by sonication assisted simultaneous precipitation technique using aqueous solutions of stannous chloride, antimony chloride and palladium nitrate followed by calcination at a temperature in the range of 800°C -900°C in air for a time period in the range of 1-2 hours and maintaining the weight
ratio of Sb2O3 in the range of 0.2-0.3 wt% and palladium as a metal basis in the rangeof5-10wt%.
4. A method as claimed in claim 1-3, wherein the gas forming agent is ammonium
nitrate in the range of 0.25-2 wt% with respect to the sensor precursor powder SnO2
batch composition.
5. A method as claimed in claim 1-4, wherein the sensor coating consists of alumina
gel equivalent to 0.05 wt% alumina with respect to SnO2.
6. A method as claimed in claim 1-5, wherein the alumina gel is prepared using sol-
gel technique from an aqueous solution of alumina alkoxide such as sec-butoxide
(C12H27AlO3) in nitric acid and refluxing the aluminium sec-butoxide solution at a
temperature in the range of 80°C - 90°C for a time period in the range of 10-20 hours
and maintaining the solid content of the sol in the range of 2-5 wt%.
7. A method as claimed in claim 1-6, wherein the sensor coating of thickness in the
range of 0.05-0.15 mm is applied on the cleaned and electroded tubular alumina
sustrate by brushing.
8. Semiconducting tin oxide based gas sensors in thick film form having improved
response and recovery time for detection of combustible gases made by the method
as claimed in claims 1-7 herein above.
9. A method of improving response and recovery time of semiconducting tin oxide
based gas sensors in thick film form for detection of combustible gases, substantially
as herein described with reference to the examples.
10. Semiconducting tin oxide based gas sensors in thick film form having improved response and recovery time for detection of combustible gases made by the method substantially as herein described with reference to the examples.

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Patent Number

268577

Indian Patent Application Number

674/DEL/2006

PG Journal Number

36/2015

Publication Date

04-Sep-2015

Grant Date

04-Sep-2015

Date of Filing

10-Mar-2006

Name of Patentee

COUNCIL OF SCIENTIFIC & INDUSTRIAL RESEARCH

Applicant Address

ANUSANDHAN BHAWAN, RAFI MARG, NEW DELHI - 110001, INDIA.

Inventors:

#	Inventor's Name	Inventor's Address
1	HALDER ASHIM KUMAR	CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, P.O. - JADAVPUR UNIVERSITY, KOLAKATA 700032.
2	DAS NANDINI	CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, P.O. - JADAVPUR UNIVERSITY, KOLAKATA 700032.
3	CHAKRABORTY SHIRSHENDU	CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, P.O. - JADAVPUR UNIVERSITY, KOLAKATA 700032.
4	GUHA ANIMESH KUMAR	CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, P.O. - JADAVPUR UNIVERSITY, KOLAKATA 700032.
5	DASGUPTA SUBRATA	CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, P.O. - JADAVPUR UNIVERSITY, KOLAKATA 700032.
6	SEN AMARNATH	CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, P.O. - JADAVPUR UNIVERSITY, KOLAKATA 700032.
7	MAITI HIMADRI SHEKHAR	CENTRAL GLASS & CERAMIC RESEARCH INSTITUTE, P.O. - JADAVPUR UNIVERSITY, KOLAKATA 700032.

PCT International Classification Number

G11C 7/10

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA