Title of Invention	"A METHOD OF MANUFACTURING SEMICONDUCTING OXIDE BASED GAS SENSORS IN THICK FILM FORM FOR DETECTION OF COMBUSTIBLE GASES IN PRESENCE OF VOLATILE ORGANIC COMPOUNDS"
Abstract	In the present invention there is provided a method of manufacturing semiconducting oxide, such as tin dioxide, zinc oxide, based gas sensors in thick film form for detection of combustible gases, like methane, LPG, propane, CNG, in presence of highly reactive volatile organic compounds, which may emanate from sources such as domestic, industrial and automobile exhaust. The gas sensor is a double coated (bi-layer) sensor, wherein the top coating is essentially having an electrical resistance at least two orders of magnitude higher than that of the bottom coating. The bottom and top semiconducting oxide based coating compositions used are functionally graded tin dioxide based compositions. The composition of the top protective coating is different from that of the bottom gas sensing coating so that volatile organic compounds are preferentially reacted at the top layer and the detecting gases are reacted at the bottom layer. The top layer having an electrical resistance at least two orders of magnitude higher than that of the bottom layer makes the top layer remain electrically shunted from the bottom gas sensing layer. Thus the problem of volatile organic compounds (VOC) cross-sensitivity with the detecting gases is effectively overcome. The bi-layer sensors are highly sensitive towards detecting gases whereas their sensitivity is quite low towards highly reactive volatile organic compounds. Thus such sensors are capable of selectively detecting combustible gases even in the presence of highly reactive volatile organic compounds. The process steps for the manufacture of such sensors are also simple and cost-effective.

Title of Invention

"A METHOD OF MANUFACTURING SEMICONDUCTING OXIDE BASED GAS SENSORS IN THICK FILM FORM FOR DETECTION OF COMBUSTIBLE GASES IN PRESENCE OF VOLATILE ORGANIC COMPOUNDS"

Abstract

In the present invention there is provided a method of manufacturing semiconducting oxide, such as tin dioxide, zinc oxide, based gas sensors in thick film form for detection of combustible gases, like methane, LPG, propane, CNG, in presence of highly reactive volatile organic compounds, which may emanate from sources such as domestic, industrial and automobile exhaust. The gas sensor is a double coated (bi-layer) sensor, wherein the top coating is essentially having an electrical resistance at least two orders of magnitude higher than that of the bottom coating. The bottom and top semiconducting oxide based coating compositions used are functionally graded tin dioxide based compositions. The composition of the top protective coating is different from that of the bottom gas sensing coating so that volatile organic compounds are preferentially reacted at the top layer and the detecting gases are reacted at the bottom layer. The top layer having an electrical resistance at least two orders of magnitude higher than that of the bottom layer makes the top layer remain electrically shunted from the bottom gas sensing layer. Thus the problem of volatile organic compounds (VOC) cross-sensitivity with the detecting gases is effectively overcome. The bi-layer sensors are highly sensitive towards detecting gases whereas their sensitivity is quite low towards highly reactive volatile organic compounds. Thus such sensors are capable of selectively detecting combustible gases even in the presence of highly reactive volatile organic compounds. The process steps for the manufacture of such sensors are also simple and cost-effective.

Full Text	This invention relates to a method of manufactunng semiconducting oxide based gas sensors in thick film form for detection of combustible gases in presence of volatile organic compounds The present invention particularly relates to a method of manufactunng bi-layered semiconducting oxide based gas sensors in thick film form for detection of combustible gases in presence of volatile organic compounds The present invention more particularly relates to a method of manufactunng semiconducting oxide based bi-layer sensors where the composition of the top protective coating is different from that of the bottom gas sensing coating so that volatile organic compounds are reacted pnmarily at the top layer The present invention specifically relates to a method of manufactunng bi-layered semiconducting oxide based, such as functionally graded tin dioxide composition, gas sensors in thick film form for detection of combustible gases in presence of volatile organic compounds Gas sensors based on semiconducting oxides like tin dioxide, zinc oxide can be used to detect a vanety of combustible gases like methane, propane, CNG and LPG in presence of interfacing volatile organic compounds (VOCs) Such VOCs may come out from industnal and domestic products like paints, lacquers, varnishes, cosmetics and automobile exhausts Thick film tin dioxide based sensors appeared in the market more than 30 years ago In this context, reference may be made to 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 and a book G Sberveglien (ed) "Gas Sensors - Pnnciple, Operation and Development", kluwer, Dordrecht (1992) With increasing world-wide distnbution of gas sensors for different applications, the demand for sensors fulfilling specific standards is growing in leaps and bounds Incidentally, volatile organic compounds (VOCs) are posing a menace for satisfactory sensor performance because they come out from many industnal and domestic products like paints, lacquers, varnishes, cosmetics and automobile exhausts and being highly reactive, they tend to interfere with the sensor operation In this regard, a particular concern is to avoid VOC cross-sensitivity with the detecting gases as VOCs being highly reactive, can give nse to false alarms A search of patent databases reveals a vanety of available pnor art covenng metal oxide composition based semiconducting gas sensor / detector, sensor for the detection of combustible gases, sensing combustible gases employing an oxygen-activated sensing element and semiconductor oxide gas combustibles sensor Reference may be made to European patent no 1041039, wherein a composition of metal oxides is used as a semiconducting gas detector, in particular for carbon monoxide in a hydrogen-nch atmosphere The mixture compnses two metal oxides (indium and zinc), or of three metal oxides (indium, zinc and gallium) Compo of metal oxides including oxides of indium, zinc and gallium Reference may be made to US patent no 6,046,054, titled semiconducting oxide gas sensors, wherein the selectivity of response of resistive gas sensors to specific gases or vapors is improved by the selection of specified gas-sensitive matenals which are not previously known for the applications descnbed, which include detection of hydrocarbons in the presence of CO, H2S, SO2, chlonne, NO2, CO2 (especially in low concentrations), CFC's, ammonia, free oxygen by determination of partial pressures, and numerous organic gases and vapors Reference may be made to US patent no 5,942,676, titled sensor for the detection of combustible gases, wherein the core of the sensor is compnsed of a sensitive layer based on a semiconducting metal oxide that is deposited on a ceramic substrate and for which the electrical resistance provides information on the concentration of combustible gases in a test gas The sensitive layer (3) IS compnsed of a compound (12) of sintered-together grains (15) of the semiconducting metal oxide, the surface of which is coated with gold and/or a gold alloy The semiconducting metal oxide in this case is stannic oxide (SnO2), indium oxide (In2O3), titanium oxide (Ti02) or another n-semiconducting metal oxide or metal mix oxide The gold alloy, for example, is composed of 66 mol % gold and 33 mol % palladium (Pd) Reference may be made to US patent no 5,635,136 titled apparatus for sensing combustible gases employing an oxygen-activated sensing element, wherein the combustible gas sensor having a noble metal sensor element with a surface that is heated above a critical temperature at which the surface is able to dissociate oxygen in a gas stream and adsorb the oxygen onto its surface The adsorbed oxygen present on the noble metal surface enhances the reactivity of the sensor element and permits it to react with combustible gases that otherwise would have little or no affinity for the sensor element The balance of adsorbed oxygen and combustible gas species on the sensor surface cause a change in an electncal property that is used to determine the presence or identity of a combustible gas, or to derive a concentration measurement The electncal property measured to determine the presence, identity, or concentration of combustible gas includes the work function of the noble metal surface, which may be measured by incorporating the sensor element as a component of a vanable capacitor contained in an electncal circuit adapted to measure change in current caused by the change in capacitance of the vanable capacitor Reference may be made to US patent no 5,549,871, titled sensor for combustible gases, wherein the combustible gas sensor compnses four glass insulated radially extended and symmetncal temperature sensitive elements which are coated with a porous ceramic but only two elements having a catalyst The sensor is mounted inside a temperature controlled reactor vessel in a region of no temperature gradient into which a combustible gas mixture is admitted and reacted on the catalytic surfaces The change in the resistance of the elements under the catalyst is a measure of the concentration of the combustible gas Reference may be made to US patent no 4,587,104, titled semiconductor oxide gas combustibles sensor, which is an n-type semiconductor gas detecting element The semiconductor oxide is bismuth molybdate having the composition Bi2O 3MoO3 and the gas detecting element formed therewith has high sensitivity to combustible gas The detection of the combustible gases isased upon the change of electncal conductivity of a thick film of the semiconductor oxide detecting element resulting from the combustible gas component in an oxygen-containing atmosphere Pnmarily two techniques have been tned to get around the problem of VOC cross-sensitivity with the detecting gases One of the techniques to avoid interference from unwanted vapours is to use charcoal filters Reference may be made to M Schweizer-Berbench, S Strathmann, U Weimar, R Sharma, A Senbe A Peyre-Lav & W Gopel, Sensors and actuators B 58 (1999) 318-324) Such sensors are satisfactory for detection of CO in presence of VOCs The disadvantage of this method lies in the fact that charcoal filters also adsorb most of the combustible gases and hence cannot be used satisfactonly to detect combustible gases like methane, propane, CNG & LPG in presence of VOCs The other technique is to use uncoated/coated (with Pt, Pd etc) filters of AI2O3, SiO2, WO3 on SnO2 coatings In this respect reference may be made to US patent no 4,592,967, titled gas sensor of mixed oxides A gas sensor compnsing a sintered piece composed of tin oxide, at least one of lanthanide oxides and at least one of IVa group element oxides The sintered piece may be covered or coated with a porous layer of a ceramic matenal such as silica, alumina or silica-alumina Reference may also be made to C A Papadopoulos, D S Valchos & J N Avaritsotis, Sensors and Actuators B, 32(1996)61-69) Such protecting filters can, to some extent, check alcohol cross-sensitivity The second method also has drawbacks because VOCs, in general, are either not affected by such filters or the overall sensitivity of the sensors towards the detecting gases dramatically goes down From the details of the hitherto known pnor art as given above, it is seen that there is a definite need to provide a method of manufactunng semiconducting oxide based gas sensors in thick film form for detection of combustible gases in presence of volatile organic compounds The mam object of the present invention is to provide a method of manufactunng semiconducting oxide based gas sensors in thick film form for detection of combustible gases in presence of volatile organic compounds, which obviates the drawbacks as detailed above Another object of the present invention is to provide a method of manufactunng bi-layered semiconducting oxide based gas sensors in thick film form for detection of combustible gases in presence of volatile organic compounds Yet another object of the present invention is to provide a method of manufactunng bi-layered semiconducting oxide based, such as functionally graded tin dioxide composition, gas sensors in thick film form for detection of combustible gases in presence of volatile organic compounds Still another object of the present invention is to provide a method of manufactunng semiconducting oxide based bi-layer sensors where the composition of the top protective coating is different from that of the bottom gas sensing coating Still yet another object of the present invention is to provide a method of manufactunng semiconducting oxide based bi-layer sensors so that volatile organic compounds are preferentially reacted at the top layer and the detecting gases are reacted at the bottom layer A further object of the present invention is to provide a method of manufacturing bi-layered semiconducting oxide based gas sensors, using a functionally graded tin dioxide based composition imparting good adherence to the substrates for gas sensors capable of detecting combustible gases in presence of volatile organic compounds A still further object of the present invention is to provide a method of manufactunng semiconducting oxide based bi-layer sensors so that electncal output IS collected from the terminals coming out from the bottom gas sensing layer In the present invention there is provided a method of manufactunng semiconducting oxide, such as tin dioxide, zinc oxide, based gas sensors in thick film form for detection of combustible gases, like methane, LPG, propane, CNG, in presence of highly reactive volatile organic compounds, which may emanate from sources such as domestic, industnal and automobile exhaust The gas sensor is a double coated (bi-layer) sensor, wherein the top coating is essentially having an electncal resistance at least two orders of magnitude higher than that of the bottom coating The bottom and top semiconducting oxide based coating compositions used are functionally graded tin dioxide based compositions The composition of the top protective coating is different from that of the bottom gas sensing coating so that volatile organic compounds are preferentially reacted at the top layer and the detecting gases are reacted at the bottom layer The top layer having an electncal resistance at least two orders of magnitude higher than that of the bottom layer makes the top layer remain electncally shunted from the bottom gas sensing layer Thus the problem of volatile organic compounds (VOC) cross-sensitivity with the detecting gases is effectively overcome The bi-layer sensors are highly sensitive towards detecting gases whereas their sensitivity is quite low towards highly reactive volatile organic compounds Thus such sensors are capable of selectively detecting combustible gases even in the presence of highly reactive volatile organic compounds The process steps for the manufacture of such sensors are also simple and cost-effective Accordingly, the present invention provides a method of manufactunng semiconducting oxide based gas sensors in thick film form for detection of combustible gases in presence of volatile organic compounds, which comprises providing bi-layer coating of semiconducting oxide, such as tin dioxide, zinc oxide, based compositions by known thick film technique on a cleaned and eiectroded ceramic tubular substrate, wherein the electncal resistance of the top coating is essentially atleast two orders of magnitude higher than that of the bottom coating In an embodiment of the present invention, there is provided a method of manufactunng semiconducting oxide based gas sensors in thick film form for detection of combustible gases in presence of volatile organic compounds, which consists of cleaning by known methods a ceramic tubular substrate, electroding by known methods the ends of the substrate by gold based electrode, followed by lead attaching with lead-wire, such as gold wire, gold alloy wire or platinum wire, by finng at a temperature in the range of 900-1000°C, coating the said substrate with a semiconducting oxide based composition by known thick film technique, followed by vacuum drying and cunng at a temperature in the range of 550 to 700°C for a penod in the range of 30 to 60 minutes, cleaning, if required, the coating from the two circular ends and extreme edges of the tube by means such as diamond file or emery paper, applying a top coating on the coated substrate with a semiconducting oxide based composition by known thick film technique, followed by vacuum drying and cunng at a temperature in the range of 500 to 650°C for a penod of 30 to 60 minutes In another embodiment of the present invention, the ceramic tubular substrate is such as alumina, quartz, glass In still another embodiment of the present invention, the thickness of the bottom layer is in the range of 0 05 to 0 15 mm In yet another embodiment of the present invention, the thickness of the top layer IS in the range of 0 3 to 0 6 mm In still yet another embodiment of the present invention, the bottom and top semiconducting oxide based coatings are of different functionally graded tin dioxide based compositions, wherein the electncal resistance of the top coating is two to three orders of magnitude higher than that of the bottom coating In a further embodiment of the present invention, the bottom semiconducting oxide based coating compnses 88 80 to 93 70 wt% tin dioxide (SnO2), 0 20 to 0 30 wt% antimony oxide (Sb2O3) and 6 to 11 wt% palladium (Pd as metal) along with alumina (AI2O3) gel equivalent to 1 25 tol 75 times the weight of the powder mix where the alumina content of the gel is 0 04 to 0 10 wt% In a still further embodiment of the present invention, the top semiconducting oxide based coating compnses 61 to 65% wt% tin dioxide (SnO2), 5 to 9% wt% palladium (as metal) and 26 to 34% wt% alumina oxide (AI2O3) along with alumina gel equivalent to 1 25 to1 75 times the weight of the powder mix where the alumina content in the gel is 0 04 to 0 10 wt% In a yet further embodiment of the present invention, the semiconducting oxide based coating compositions are prepared by known soft chemistry methods such as simultaneous precipitation, sonication assisted simultaneous precipitation In another embodiment of the present invention, the alumina gel is prepared by known soft chemistry method such as sol-gel technique The pnnciple behind the invention resides in the functionally graded bi-layer structure of the sensor, where the resistance of the top coating is engineered to have two to three orders of magnitude higher than that of the bottom coating Such increase in resistance is achieved by exploiting the electronic interaction between semiconducting oxides in close contact with the Lewis acid site (electron acceptor) containing oxides The volatile organic compounds (VOCs) being highly reactive, interact with adsorbed oxygen on the top layer releasing free electrons However, due to orders of magnitude higher resistance of the top coating with respect to that of bottom coating, the top coating always remains shunted to the bottom coating Less reactive gases like methane penetrate the top layer and interact with the bottom layer of adsorbed oxygen and the change in resistance is picked up by the electncal leads at the bottom The novelty of the present invention resides in the bi-layered (double-coated) structure of the sensors, where instead of trying to filter out the volatile organic compounds by any adsorbent coating, the volatile organic compounds are rather forced to react strongly at the top layer which has an electrical resistance at least two orders of magnitude higher than that of the bottom layer so that the top layer remains electncally shunted from the bottom gas sensing layer allowing the less reactive gases like methane, LPG to interact with the gas sensing bottom layer and the signal is collected from the bottom layer thereby obviating the interference of volatile organic compounds in the detection of combustible gases like methane, CNG and LPG The novelty of the present invention has been achieved by the non-obvious inventive steps, such as a) Providing a functionally graded two-layered structure of the sensors and the method of making it b) Making the electncal resistance of the top coating at least two orders of magnitude higher than that of the bottom coating, at the same time keeping the top coating reactive to volatile organic compounds and the bottom coating reactive to the detecting gases c) Collecting the signal from the gas sensing bottom coating d) Providing a formulation of tin dioxide based composition for the top coating having high reactivity towards the volatile organic compounds and at the same time depicting electncal resistance at least two orders of magnitude higher than that of the bottom coating e) Providing a formulation of tin dioxide based composition for the bottom coating having high reactivity towards the detecting combustible gases and at the same time depicting electncal resistance at least two orders of magnitude lower than that of the top coating f) The incorporation of alumina gel which imparts adherence to the coatings and improves the functionally graded nature of the coatings The detailed process steps of the present invention are as follows 1) Prepanng the precursor powder for the bottom coating compnsing 88 80 -93 70 wt% tin dioxide (SnO2), 0 20-0 30 wt% antimony oxide (Sb2O3) and 6-11 wt% palladium (as metal) by sonication assisted simultaneous precipitation technique using aqueous solutions of stannous chlonde, antimony chlonde and palladium nitrate followed by calcination at a temperature in the range of 550-700°C in air for 1-4hour 2) Prepanng the precursor powder for the top coating compnsing 61-65 wt% SnO2, 5-9 wt% Pd and 24-30 wt% AI2O3 by simultaneous precipitation technique using aqueous solutions of stannous chlonde and palladium nitrate followed by calcination at a temperature in the range of 800-950°C in air for 1-4 hour and then mixing with AI2O3 powder 3) Prepanng the alumina gel form an aqueous solution of alumina alkoxide like sec-butoxide (C12H27AIO3) in nitnc acid and refluxing the solution at 80-90°C for 10-20 hours 4) Prepanng the composition for the top layer which compnses the bottom coating precursor powder and alumina gel in the weight ratio of 1 1 25 to 1 1 75 along with a suitable amount of water or nonaqueous solvent 5) Prepanng the composition for the top layer which compnses the bottom coating precursor powder and alumina gel in the weight ratio of 1 1 25 to 1 1 75 along with a suitable amount of water or non-aqueous solvent 6) Cleaning the tubular substrates either in water and/or alcohol and/or acetone using ultrasonic vibration 7) Electroding the ends by a standard technique with Au-based electrodes, attaching lead wires of Au, Au-alloy or platinum and finng the assembly at a temperature in the range of 900-1000°C 8) Applying a bottom coating of thickness 0 05-0 15 mm on the tubular substrates by screen pnnting or brushing using nonaqueous or aqueous based paste, as obtained in step (4) above 9) Vacuum drying the bottom coating followed by cunng at a temperature in the range of 550-700°C for 30-60 minutes, when dunng heating, the furnace temperature is allowed to nse at a rate of 50-75°C/hour 10) Rubbing/cleaning the coating from the two circular ends of the tubes (if any) and extreme edges of the tubes by diamond file or emery paper before application of the top coating, so that no portion of the bottom coating is directly exposed to the ambient 11) Applying a thicker top coating (thickness 0 3-0 6 mm) of coating composition as obtained in step (5) above 12) Vacuum drying the top coating followed by cunng at a temperature in the range of 500-650°C for 30-60 minutes, when dunng heating, the furnace temperature is allowed to rise at a rate of 50-75°C/hour The following examples are given by way of illustrating the invention and the manner in which it may be carried out in actual practice However, this should not be construed to limit the scope of the present invention in any way Example 1 A precursor powder for the bottom layer was prepared by the following steps Firstly, 5 99 gram of reagent grade stannous chlonde (SnCI2, 2H2O) was dissolved in 200 mL of distilled water containing 20 drops of cone HCI For complete mixing, the assembly was heated to around 80°C with continuous stirnng Secondly, 0 0163 gram of reagent grade Sb2O3 was dissolved in 50 mL distilled water (at 80°C) containing 5 drops of Cone HCI In the next step, 0 6665 gram of reagent grade PdCl2 was taken in 100mL distilled water containing 10 drops of cone HNO3 and PdCI2 was slowly dissolved by heating the mixture at 80°C under constant stirnng for 1 hour The three solutions were mixed and added to ammonia solution under sonication and the pH of the solution has maintained at 4 The precipitate was filtered, washed with distilled water and calcined at 600°C/2 h To prepare the precursor powder for the top layer, 5 99 gram of stannous chlonde and 0 6665 gram of PdCl2 were dissolved in distilled water following the above procedure The two solutions were mixed and added to ammonia solution maintaining a pH of 9 The precipitate was filtered, washed and calcined at 950°C/2h Reagent grade alumina powder was then mixed with the calcined powder in the ratio of 3 7 (alumina calcined powder) by weight using an agate mortar and a pestle To prepare the alumina gel, 20 5 gram of reagent grade aluminium secbutoxide (C12H27AIO3) 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 secbutoxide The solution was refluxed for 18 hours to get the gel of desired consistency so that the alumina content of the gel was 0 05% by weight A thick paste of the powder formulation for the bottom coating was made by mixing the prepared precursor powder for the bottom layer with alumina gel and ethanol in the weight ratio of 1 0 1 5 5 0 A thick paste of the powder formulation for the top coating was made by mixing the prepared precursor powder for the top layer with alumina gel and ethanol in the weight ratio of 1 0 1 5 5 0 An alumina tube of length 3mm, outside diameter 1 5mm and inner diameter 1mm 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 hour The outside of the alumina tube was painted by a thick paste of the powder formulation for the bottom coating (thickness of the first coating around 0 1 mm) followed by vacuum drying and cunng at a temperature of 620°C for 45 mm The furnace temperature was raised at a rate of 75°C/hour After firing, the two circular ends and the extreme edges of the tube were cleaned of any coating matenal by rubbing with a diamond file The singly coated alumina tube was painted by a thick paste of the powder formulation for the top coating (thickness of the second coating was around 0 5 mm) followed by vacuum drying and cunng at a temperature of 600°C for 45 minutes The furnace temperature was raised at a rate of 75°C/hour For measunng the gas sensitivity of the bi-layered 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 onginal resistance (in the ambient) at an elevated temperature The double coated sensors prepared in this way showed an average sensitivity of around 90% in 1000 ppm methane or LPG at 350°C Incidentally, the sensors show a low sensitivity of around 45% when kept inside a container containing standard paint thinner ( a source of cone VOCs) whereas, under the same condition, the sensitivity in cone VOC can be as high as 95%, when the sensor is singly coated, i e , the top coating of the sensor is absent Hence, by properly designing the electronic circuit, the double coated sensors can be made selective to the detecting gases even in the presence of VOCs Example 2 A precursor powder for the bottom layer was prepared by the following steps Firstly, 5 99 gram of reagent grade stannous chlonde (SnCl2, 2H2O) was dissolved in 200 mL of distilled water containing 20 drops of cone HCI For complete mixing, the assembly was heated to around 80°C with continuous stirnng Secondly, 0 0170 gram of reagent grade Sb2O3 was dissolved in 50 mL distilled water (at 80°C) containing 5 drops of Cone HCI In the next step, 0 6685 gram of reagent grade PdCl2 was taken in lOOmL distilled water containing 10 drops of cone HNO3 and PdCl2 was slowly dissolved by heating the mixture at 80°C under constant stirnng for 1 hour The three solutions were mixed and added to ammonia solution under sonication and the pH of the solution has maintained at 4 The precipitate was filtered, washed with distilled water and calcined at 600°C/2 h To prepare the precursor powder for the top layer, 5 99 gram of stannous chlonde and 0 6665 gram of PdCl2 were dissolved in distilled water following the above procedure The two solutions were mixed and added to ammonia solution maintaining a pH of 9 The precipitate was filtered, washed and calcined at 950°C/2h Reagent grade alumina powder was then mixed with the calcined powder in the ratio of 3 7 (alumina calcined powder) by weight using an agate mortar and a pestle To prepare the alumina gel, 20 5 gram of reagent grade aluminium secbutoxide (C12H27AIO3) 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 secbutoxide The solution was refluxed for 18 hours to get the gel of desired consistency so that the alumina content of the gel was 0 05% by weight A thick paste of the powder formulation for the bottom coating was made by mixing the prepared precursor powder for the bottom layer with alumina gel and ethanol in the weight ratio of 1 0 1 5 5 0 A thick paste of the powder formulation for the top coating was made by mixing the prepared precursor powder for the top layer with alumina gel and ethanol in the weight ratio of 1 0 1 5 5 0 An alumina tube of length 3mm, outside diameter 1 5mm and inner diameter 1mm 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 hour The outside of the alumina tube was painted by a thick paste of the powder formulation for the bottom coating (thickness of the first coating around 0 8 mm) followed by vacuum drying and cunng at a temperature of 600°C for 45 mm The furnace temperature was raised at a rate of 75°C/hour After finng, the two circular ends and the extreme edges of the tube were cleaned of any coating matenal by rubbing with a diamond file The singly coated alumina tube was painted by a thick paste of the powder formulation for the top coating (thickness of the second coating is around 0 5 mm) followed by vacuum drying and cunng at a temperature of 580°C for 45 minutes The furnace temperature was raised at a rate of 75°C/hour For measunng the gas sensitivity of the bi-layered 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 onginal resistance (in the ambient) at an elevated temperature The double coated sensors prepared in this way showed an average sensitivity of around 95% in 1000 ppm methane or LPG at 350°C Incidentally, the sensors show a low sensitivity of around 48% when kept inside a container containing standard paint thinner ( a source of cone VOCs) whereas, under the same condition, the sensitivity in cone VOC can be as high as 97%, when the sensor is singly coated, i e , the top coating of the sensor is absent Hence, by properly designing the electronic circuit, the double coated sensors can be made selective to the detecting gases even in the presence of VOCs Example 3 A precursor powder for the bottom layer was prepared by the following steps Firstly, 5 99 gram of reagent grade stannous chlonde (SnCl2, 2H2O) was dissolved in 200 mL of distilled water containing 20 drops of cone HCI For complete mixing, the assembly was heated to around 80°C with continuous stirnng Secondly, 0 0153 gram of reagent grade Sb2O3 was dissolved in 50 mL distilled water (at 80°C) containing 5 drops of Cone HCI In the next step, 0 6645 gram of reagent grade PdCl2 was taken in 100mL distilled water containing 10 drops of cone HNO3 and PdCl2 was slowly dissolved by heating the mixture at 80°C under constant stirnng for 1 hour The three solutions were mixed and added to ammonia solution under sonication and the pH of the solution has maintained at 4 The precipitate was filtered, washed with distilled water and calcined at 600°C/2 h To prepare the precursor powder for the top layer, 5 99 gram of stannous chlonde and 0 6675 gram of PdCI2 were dissolved in distilled water following the above procedure The two solutions were mixed and added to ammonia solution maintaining a pH of 9 The precipitate was filtered, washed and calcined at 950°C/2h Reagent grade alumina powder was then mixed with the calcined powder in the ratio of 3 7 (alumina calcined powder) by weight using an agate mortar and a pestle To prepare the alumina gel, 20 5 gram of reagent grade aluminium secbutoxide (C12H27AIO3) 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 secbutoxide The solution was refluxed for 18 hours to get the gel of desired consistency so that the alumina content of the gel was 0 05% by weight A thick paste of the powder formulation for the bottom coating was made by mixing the prepared precursor powder for the bottom layer with alumina gel and ethanol in the weight ratio of 101550 A thick paste of the powder formulation for the top coating was made by mixing the prepared precursor powder for the top layer with alumina gel and ethanol in the weight ratio of 1 0 1 5 5 0 An alumina tube of length 3mm, outside diameter 1 5mm and inner diameter 1mm 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 hour The outside of the alumina tube was painted by a thick paste of the powder formulation for the bottom coating (thickness of the first coating around 0 1 mm) followed by vacuum drying and cunng at a temperature of 600°C for 45 mm The furnace temperature was raised at a rate of 75°C/hour After firing, the two circular ends and the extreme edges of the tube were cleaned of any coating matenal by rubbing with a diamond file The singly coated alumina tube was painted by a thick paste of the powder formulation for the top coating (thickness of the second coating is around 0 6 mm) followed by vacuum drying and cunng at a temperature of 575°C for 45 minutes The furnace temperature was raised at a rate of 50°C/hour For measunng the gas sensitivity of the bi-layered 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 onginal resistance (in the ambient) at an elevated temperature The double coated sensors prepared in this way showed an average sensitivity of around 92% in 1000 ppm methane or LPG at 350°C Incidentally, the sensors show a low sensitivity of around 42% when kept inside a container containing standard paint thinner ( a source of cone VOCs) whereas, under the same condition, the sensitivity in cone VOC can be as high as 95%, when the sensor is singly coated, i e , the top coating of the sensor is absent Hence, by properly designing the electronic circuit, the double coated sensors can be made selective to the detecting gases even in the presence of VOCs Example 4 A precursor powder for the bottom layer was prepared by the following steps Firstly, 5 99 gram of reagent grade stannous chlonde (SnCl2, 2H2O) was dissolved in 200 mL of distilled water containing 20 drops of cone HCI For complete mixing, the assembly was heated to around 80°C with continuous stirnng Secondly, 0 0168 gram of reagent grade Sb2O3 was dissolved in 50 mL distilled water (at 80°C) containing 5 drops of Cone HCI In the next step, 0 6668 gram of reagent grade PdCl2 was taken in lOOmL distilled water containing 10 drops of cone HNO3 and PdCI2 was slowly dissolved by heating the mixture at 80°C under constant stirnng for 1 hour The three solutions were mixed and added to ammonia solution under sonication and the pH of the solution has maintained at 4 The precipitate was filtered, washed with distilled water and calcined at 575°C/2 h To prepare the precursor powder for the top layer, 5 99 gram of stannous chlonde and 0 6665 gram of PdCl2 were dissolved in distilled water following the above procedure The two solutions were mixed and added to ammonia solution maintaining a pH of 9 The precipitate was filtered, washed and calcined at 960°C/2h Reagent grade alumina powder was then mixed with the calcined powder in the ratio of 3 7 (alumina calcined powder) by weight using an agate mortar and a pestle To prepare the alumina gel, 20 5 gram of reagent grade aluminium secbutoxide (C12H27AIO3) 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 secbutoxide The solution was refluxed for 18 hours to get the gel of desired consistency so that the alumina content of the gel was 0 05% by weight A thick paste of the powder formulation for the bottom coating was made by mixing the prepared precursor powder for the bottom layer with alumina gel and ethanol in the weight ratio of 1 0 1 5 5 0 A thick paste of the powder formulation for the top coating was made by mixing the prepared precursor powder for the top layer with alumina gel and ethanol in the weight ratio of 1 0 1 5 5 0 An alumina tube of length 3mm, outside diameter 1 5mm and inner diameter 1mm 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 hour The outside of the alumina tube was painted by a thick paste of the powder formulation for the bottom coating (thickness of the first coating around 0 1 mm) followed by vacuum drying and cunng at a temperature of 650°C for 30 mm The furnace temperature was raised at a rate of 75°C/hour After finng, the two circular ends and the extreme edges of the tube were cleaned of any coating matenal by rubbing with a diamond file The singly coated alumina tube was painted by a thick paste of the powder formulation for the top coating (thickness of the second coating is around 0 5 mm) followed by vacuum drying and cunng at a temperature of 620°C for 30 minutes The furnace temperature was raised at a rate of 75°C/hour For measunng the gas sensitivity of the bi-layered 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 double coated sensors prepared in this way showed an average sensitivity of around 90% in 1000 ppm methane or LPG at 350°C Incidentally, the sensors show a low sensitivity of around 47% when kept inside a container containing standard paint thinner ( a source of cone VOCs) whereas, under the same condition, the sensitivity in cone VOC can be as high as 96%, when the sensor is singly coated, i e , the top coating of the sensor is absent Hence, by properly designing the electronic circuit, the double coated sensors can be made selective to the detecting gases even in the presence of VOCs The mam advantages of the present invention are as follows 1) The method can provide sensors which can detect combustible gases like methane, LPG, propane, CNG even in the presence of highly reactive volatile organic compounds 2) As no filter is used, the method provides gas sensor of more compact size 3) As no filter is used, the method provides gas sensors where the sensitivity of the gas sensor remains high even in the presence of interfenng volatile inorganic compounds 4) The functionally graded composition can be used to make sensors which can detect combustible gases like methane, LPG, propane even in the presence of highly reactive volatile organic compounds 5) The functionally graded composition provides good adherence and hence the sensors are rugged and show reproducible behaviour 6) The process steps for the manufacture of such sensors are simple and cost-effective We claim: 1. A method of manufacturing semiconducting oxide based gas sensors in thick film form for detection of combustible gases in presence of volatile organic compounds, which comprises providing bi-layer coating of semiconducting oxide, such as tin dioxide, zinc oxide, based compositions by known thick film technique on a cleaned and electroded ceramic tubular substrate, wherein the electrical resistance of the top coating is essentially atleast two orders of magnitude higher than that of the bottom coating. 2. A method as claimed in claim 1, which consists of cleaning by known methods a ceramic tubular substrate, electroding by known methods the ends of the substrate by gold based electrode, followed by lead attaching with lead-wire, such as gold wire, gold alloy wire or platinum wire, by firing at a temperature in the range of 900-1000°C, coating the said substrate with a semiconducting oxide based composition by known thick film technique, followed by vacuum drying and curing at a temperature in the range of 550 to 700°C for a period in the range of 30 to 60 minutes, cleaning, if required, the coating from the two circular ends and extreme edges of the tube by means such as diamond file or emery paper, applying a top coating on the coated substrate with a semiconducting oxide based composition by known thick film technique, followed by vacuum drying and curing at a temperature in the range of 500 to 650°C for a period of 30 to 60 minutes. 3. A method as claimed in claim 1-2, wherein the ceramic tubular substrate is such as alumina, quartz, glass. 4. A method as claimed in claim 1-3, wherein the thickness of the bottom layer is in the range of 0.05 to 0.15 mm. 5. A method as claimed in claim 1-4, wherein the thickness of the top layer is in the range of 0.3 to 0.6 mm. 6. A method as claimed in claim 1-5, wherein the bottom and top semiconducting oxide based coatings are of different functionally graded tin dioxide based compositions, wherein the electrical resistance of the top coating is two to three orders of magnitude higher than that of the bottom coating. 7. A method as claimed in claim 1 -6, wherein the bottom semiconducting oxide based coating comprises: 88.80 to 93.70 wt% tin dioxide (Sn02), 0.20 to 0.30 wt% antimony oxide (Sb203)and 6 to 11 wt% palladium (Pd as metal) along with alumina (AI203) gel equivalent to 1.25 tol.75 times the weight of the powder mix where the alumina content of the gel is 0.04 to 0.10 wt%. 8. A method as claimed in claim 1-7, wherein the top semiconducting oxide based coating comprises: 61 to 65% wt% tin dioxide (Sn02), 5 to 9% wt% palladium (as metal) and 26 to 34% wt% alumina oxide (AI203) along with alumina gel equivalent to 1.25 tol.75 times the weight of the powder mix where the alumina content in the gel is 0.04 to 0.10 wt%. 9. A method as claimed in claim 1-8, wherein the semiconducting oxide based coating compositions are prepared by known soft chemistry methods such as simultaneous precipitation, sonication assisted simultaneous precipitation. 10. A method as claimed in claim 1-9, wherein the alumina gel is prepared by known soft chemistry method such as sol-gel technique.

Full Text

This invention relates to a method of manufactunng semiconducting oxide based gas sensors in thick film form for detection of combustible gases in presence of volatile organic compounds
The present invention particularly relates to a method of manufactunng bi-layered semiconducting oxide based gas sensors in thick film form for detection of combustible gases in presence of volatile organic compounds
The present invention more particularly relates to a method of manufactunng semiconducting oxide based bi-layer sensors where the composition of the top protective coating is different from that of the bottom gas sensing coating so that volatile organic compounds are reacted pnmarily at the top layer
The present invention specifically relates to a method of manufactunng bi-layered semiconducting oxide based, such as functionally graded tin dioxide composition, gas sensors in thick film form for detection of combustible gases in presence of volatile organic compounds
Gas sensors based on semiconducting oxides like tin dioxide, zinc oxide can be used to detect a vanety of combustible gases like methane, propane, CNG and LPG in presence of interfacing volatile organic compounds (VOCs) Such VOCs may come out from industnal and domestic products like paints, lacquers, varnishes, cosmetics and automobile exhausts
Thick film tin dioxide based sensors appeared in the market more than 30 years ago In this context, reference may be made to 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 and a book G Sberveglien (ed) "Gas Sensors - Pnnciple, Operation and Development", kluwer, Dordrecht (1992)
With increasing world-wide distnbution of gas sensors for different applications, the demand for sensors fulfilling specific standards is growing in leaps and bounds Incidentally, volatile organic compounds (VOCs) are posing a menace for satisfactory sensor performance because they come out from many industnal and domestic products like paints, lacquers, varnishes, cosmetics and automobile exhausts and being highly reactive, they tend to interfere with the sensor operation In this regard, a particular concern is to avoid VOC cross-sensitivity with the detecting gases as VOCs being highly reactive, can give nse to false alarms
A search of patent databases reveals a vanety of available pnor art covenng metal oxide composition based semiconducting gas sensor / detector, sensor for the detection of combustible gases, sensing combustible gases employing an oxygen-activated sensing element and semiconductor oxide gas combustibles sensor
Reference may be made to European patent no 1041039, wherein a composition of metal oxides is used as a semiconducting gas detector, in particular for carbon monoxide in a hydrogen-nch atmosphere The mixture compnses two metal oxides (indium and zinc), or of three metal oxides (indium, zinc and gallium) Compo of metal oxides including oxides of indium, zinc and gallium
Reference may be made to US patent no 6,046,054, titled semiconducting oxide gas sensors, wherein the selectivity of response of resistive gas sensors to specific gases or vapors is improved by the selection of specified gas-sensitive matenals which are not previously known for the applications descnbed, which include detection of hydrocarbons in the presence of CO, H2S, SO2, chlonne, NO2, CO2 (especially in low concentrations), CFC's, ammonia, free oxygen by determination of partial pressures, and numerous
organic gases and vapors
Reference may be made to US patent no 5,942,676, titled sensor for the detection of combustible gases, wherein the core of the sensor is compnsed of a sensitive layer based on a semiconducting metal oxide that is deposited on a ceramic substrate and for which the electrical resistance provides information on the concentration of combustible gases in a test gas The sensitive layer (3) IS compnsed of a compound (12) of sintered-together grains (15) of the semiconducting metal oxide, the surface of which is coated with gold and/or a gold alloy The semiconducting metal oxide in this case is stannic oxide (SnO2), indium oxide (In2O3), titanium oxide (Ti02) or another n-semiconducting metal oxide or metal mix oxide The gold alloy, for example, is composed of 66 mol % gold and 33 mol % palladium (Pd)
Reference may be made to US patent no 5,635,136 titled apparatus for sensing combustible gases employing an oxygen-activated sensing element, wherein the combustible gas sensor having a noble metal sensor element with a surface that is heated above a critical temperature at which the surface is able to dissociate oxygen in a gas stream and adsorb the oxygen onto its surface The adsorbed oxygen present on the noble metal surface enhances the reactivity of the sensor element and permits it to react with combustible gases that otherwise would have little or no affinity for the sensor element The balance of adsorbed oxygen and combustible gas species on the sensor surface cause a change in an electncal property that is used to determine the presence or identity of a combustible gas, or to derive a concentration measurement The electncal property measured to determine the presence, identity, or concentration of combustible gas includes the work function of the noble metal surface, which may be measured by incorporating the sensor element as a component of a vanable capacitor contained in an electncal circuit adapted to measure change in current caused by the change in
capacitance of the vanable capacitor
Reference may be made to US patent no 5,549,871, titled sensor for combustible gases, wherein the combustible gas sensor compnses four glass insulated radially extended and symmetncal temperature sensitive elements which are coated with a porous ceramic but only two elements having a catalyst The sensor is mounted inside a temperature controlled reactor vessel in a region of no temperature gradient into which a combustible gas mixture is admitted and reacted on the catalytic surfaces The change in the resistance of the elements under the catalyst is a measure of the concentration of the combustible gas
Reference may be made to US patent no 4,587,104, titled semiconductor oxide gas combustibles sensor, which is an n-type semiconductor gas detecting element The semiconductor oxide is bismuth molybdate having the composition Bi2O 3MoO3 and the gas detecting element formed therewith has high sensitivity to combustible gas The detection of the combustible gases isased upon the change of electncal conductivity of a thick film of the semiconductor oxide detecting element resulting from the combustible gas component in an oxygen-containing atmosphere
Pnmarily two techniques have been tned to get around the problem of VOC cross-sensitivity with the detecting gases One of the techniques to avoid interference from unwanted vapours is to use charcoal filters Reference may be made to M Schweizer-Berbench, S Strathmann, U Weimar, R Sharma, A Senbe A Peyre-Lav & W Gopel, Sensors and actuators B 58 (1999) 318-324) Such sensors are satisfactory for detection of CO in presence of VOCs The disadvantage of this method lies in the fact that charcoal filters also adsorb most of the combustible gases and hence cannot be used satisfactonly to detect combustible gases like methane, propane, CNG & LPG in presence of VOCs
The other technique is to use uncoated/coated (with Pt, Pd etc) filters of AI2O3, SiO2, WO3 on SnO2 coatings In this respect reference may be made to US patent no 4,592,967, titled gas sensor of mixed oxides A gas sensor compnsing a sintered piece composed of tin oxide, at least one of lanthanide oxides and at least one of IVa group element oxides The sintered piece may be covered or coated with a porous layer of a ceramic matenal such as silica, alumina or silica-alumina
Reference may also be made to C A Papadopoulos, D S Valchos & J N Avaritsotis, Sensors and Actuators B, 32(1996)61-69) Such protecting filters can, to some extent, check alcohol cross-sensitivity The second method also has drawbacks because VOCs, in general, are either not affected by such filters or the overall sensitivity of the sensors towards the detecting gases dramatically goes down
From the details of the hitherto known pnor art as given above, it is seen that there is a definite need to provide a method of manufactunng semiconducting oxide based gas sensors in thick film form for detection of combustible gases in presence of volatile organic compounds
The mam object of the present invention is to provide a method of manufactunng semiconducting oxide based gas sensors in thick film form for detection of combustible gases in presence of volatile organic compounds, which obviates the drawbacks as detailed above
Another object of the present invention is to provide a method of manufactunng bi-layered semiconducting oxide based gas sensors in thick film form for detection of combustible gases in presence of volatile organic compounds
Yet another object of the present invention is to provide a method of manufactunng bi-layered semiconducting oxide based, such as functionally
graded tin dioxide composition, gas sensors in thick film form for detection of combustible gases in presence of volatile organic compounds
Still another object of the present invention is to provide a method of manufactunng semiconducting oxide based bi-layer sensors where the composition of the top protective coating is different from that of the bottom gas sensing coating
Still yet another object of the present invention is to provide a method of manufactunng semiconducting oxide based bi-layer sensors so that volatile organic compounds are preferentially reacted at the top layer and the detecting gases are reacted at the bottom layer
A further object of the present invention is to provide a method of manufacturing bi-layered semiconducting oxide based gas sensors, using a functionally graded tin dioxide based composition imparting good adherence to the substrates for gas sensors capable of detecting combustible gases in presence of volatile organic compounds
A still further object of the present invention is to provide a method of manufactunng semiconducting oxide based bi-layer sensors so that electncal output IS collected from the terminals coming out from the bottom gas sensing layer
In the present invention there is provided a method of manufactunng semiconducting oxide, such as tin dioxide, zinc oxide, based gas sensors in thick film form for detection of combustible gases, like methane, LPG, propane, CNG, in presence of highly reactive volatile organic compounds, which may emanate from sources such as domestic, industnal and automobile exhaust The gas sensor is a double coated (bi-layer) sensor, wherein the top coating is essentially having an electncal resistance at least two orders of magnitude higher than that
of the bottom coating The bottom and top semiconducting oxide based coating compositions used are functionally graded tin dioxide based compositions The composition of the top protective coating is different from that of the bottom gas sensing coating so that volatile organic compounds are preferentially reacted at the top layer and the detecting gases are reacted at the bottom layer The top layer having an electncal resistance at least two orders of magnitude higher than that of the bottom layer makes the top layer remain electncally shunted from the bottom gas sensing layer Thus the problem of volatile organic compounds (VOC) cross-sensitivity with the detecting gases is effectively overcome The bi-layer sensors are highly sensitive towards detecting gases whereas their sensitivity is quite low towards highly reactive volatile organic compounds Thus such sensors are capable of selectively detecting combustible gases even in the presence of highly reactive volatile organic compounds The process steps for the manufacture of such sensors are also simple and cost-effective
Accordingly, the present invention provides a method of manufactunng semiconducting oxide based gas sensors in thick film form for detection of combustible gases in presence of volatile organic compounds, which comprises providing bi-layer coating of semiconducting oxide, such as tin dioxide, zinc oxide, based compositions by known thick film technique on a cleaned and eiectroded ceramic tubular substrate, wherein the electncal resistance of the top coating is essentially atleast two orders of magnitude higher than that of the bottom coating
In an embodiment of the present invention, there is provided a method of manufactunng semiconducting oxide based gas sensors in thick film form for detection of combustible gases in presence of volatile organic compounds, which consists of cleaning by known methods a ceramic tubular substrate, electroding by known methods the ends of the substrate by gold based electrode, followed by lead attaching with lead-wire, such as gold wire, gold alloy wire or platinum wire, by finng at a temperature in the range of 900-1000°C, coating the said
substrate with a semiconducting oxide based composition by known thick film technique, followed by vacuum drying and cunng at a temperature in the range of 550 to 700°C for a penod in the range of 30 to 60 minutes, cleaning, if required, the coating from the two circular ends and extreme edges of the tube by means such as diamond file or emery paper, applying a top coating on the coated substrate with a semiconducting oxide based composition by known thick film technique, followed by vacuum drying and cunng at a temperature in the range of 500 to 650°C for a penod of 30 to 60 minutes
In another embodiment of the present invention, the ceramic tubular substrate is such as alumina, quartz, glass
In still another embodiment of the present invention, the thickness of the bottom layer is in the range of 0 05 to 0 15 mm
In yet another embodiment of the present invention, the thickness of the top layer IS in the range of 0 3 to 0 6 mm
In still yet another embodiment of the present invention, the bottom and top semiconducting oxide based coatings are of different functionally graded tin dioxide based compositions, wherein the electncal resistance of the top coating is two to three orders of magnitude higher than that of the bottom coating
In a further embodiment of the present invention, the bottom semiconducting oxide based coating compnses 88 80 to 93 70 wt% tin dioxide (SnO2), 0 20 to 0 30 wt% antimony oxide (Sb2O3) and 6 to 11 wt% palladium (Pd as metal) along with alumina (AI2O3) gel equivalent to 1 25 tol 75 times the weight of the powder mix where the alumina content of the gel is 0 04 to 0 10 wt%
In a still further embodiment of the present invention, the top semiconducting oxide based coating compnses 61 to 65% wt% tin dioxide (SnO2), 5 to 9% wt%
palladium (as metal) and 26 to 34% wt% alumina oxide (AI2O3) along with alumina gel equivalent to 1 25 to1 75 times the weight of the powder mix where the alumina content in the gel is 0 04 to 0 10 wt%
In a yet further embodiment of the present invention, the semiconducting oxide based coating compositions are prepared by known soft chemistry methods such as simultaneous precipitation, sonication assisted simultaneous precipitation
In another embodiment of the present invention, the alumina gel is prepared by known soft chemistry method such as sol-gel technique
The pnnciple behind the invention resides in the functionally graded bi-layer structure of the sensor, where the resistance of the top coating is engineered to have two to three orders of magnitude higher than that of the bottom coating Such increase in resistance is achieved by exploiting the electronic interaction between semiconducting oxides in close contact with the Lewis acid site (electron acceptor) containing oxides The volatile organic compounds (VOCs) being highly reactive, interact with adsorbed oxygen on the top layer releasing free electrons However, due to orders of magnitude higher resistance of the top coating with respect to that of bottom coating, the top coating always remains shunted to the bottom coating Less reactive gases like methane penetrate the top layer and interact with the bottom layer of adsorbed oxygen and the change in resistance is picked up by the electncal leads at the bottom
The novelty of the present invention resides in the bi-layered (double-coated) structure of the sensors, where instead of trying to filter out the volatile organic compounds by any adsorbent coating, the volatile organic compounds are rather forced to react strongly at the top layer which has an electrical resistance at least two orders of magnitude higher than that of the bottom layer so that the top layer remains electncally shunted from the bottom gas sensing layer allowing the less reactive gases like methane, LPG to interact with the gas sensing bottom layer

and the signal is collected from the bottom layer thereby obviating the interference of volatile organic compounds in the detection of combustible gases like methane, CNG and LPG
The novelty of the present invention has been achieved by the non-obvious inventive steps, such as
a) Providing a functionally graded two-layered structure of the sensors and the method of making it
b) Making the electncal resistance of the top coating at least two orders of magnitude higher than that of the bottom coating, at the same time keeping the top coating reactive to volatile organic compounds and the bottom coating reactive to the detecting gases
c) Collecting the signal from the gas sensing bottom coating
d) Providing a formulation of tin dioxide based composition for the top coating having high reactivity towards the volatile organic compounds and at the same time depicting electncal resistance at least two orders of magnitude higher than that of the bottom coating
e) Providing a formulation of tin dioxide based composition for the bottom coating having high reactivity towards the detecting combustible gases and at the same time depicting electncal resistance at least two orders of magnitude lower than that of the top coating
f) The incorporation of alumina gel which imparts adherence to the coatings and improves the functionally graded nature of the coatings
The detailed process steps of the present invention are as follows
1) Prepanng the precursor powder for the bottom coating compnsing 88 80 -93 70 wt% tin dioxide (SnO2), 0 20-0 30 wt% antimony oxide (Sb2O3) and 6-11 wt% palladium (as metal) by sonication assisted simultaneous precipitation
technique using aqueous solutions of stannous chlonde, antimony chlonde and palladium nitrate followed by calcination at a temperature in the range of 550-700°C in air for 1-4hour
2) Prepanng the precursor powder for the top coating compnsing 61-65 wt% SnO2, 5-9 wt% Pd and 24-30 wt% AI2O3 by simultaneous precipitation technique using aqueous solutions of stannous chlonde and palladium nitrate followed by calcination at a temperature in the range of 800-950°C in air for 1-4 hour and then mixing with AI2O3 powder
3) Prepanng the alumina gel form an aqueous solution of alumina alkoxide like sec-butoxide (C12H27AIO3) in nitnc acid and refluxing the solution at 80-90°C for 10-20 hours
4) Prepanng the composition for the top layer which compnses the bottom coating precursor powder and alumina gel in the weight ratio of 1 1 25 to 1 1 75 along with a suitable amount of water or nonaqueous solvent
5) Prepanng the composition for the top layer which compnses the bottom coating precursor powder and alumina gel in the weight ratio of 1 1 25 to 1 1 75 along with a suitable amount of water or non-aqueous solvent
6) Cleaning the tubular substrates either in water and/or alcohol and/or acetone using ultrasonic vibration
7) Electroding the ends by a standard technique with Au-based electrodes, attaching lead wires of Au, Au-alloy or platinum and finng the assembly at a temperature in the range of 900-1000°C
8) Applying a bottom coating of thickness 0 05-0 15 mm on the tubular substrates by screen pnnting or brushing using nonaqueous or aqueous based paste, as obtained in step (4) above
9) Vacuum drying the bottom coating followed by cunng at a temperature in the range of 550-700°C for 30-60 minutes, when dunng heating, the furnace temperature is allowed to nse at a rate of 50-75°C/hour

10) Rubbing/cleaning the coating from the two circular ends of the tubes (if any) and extreme edges of the tubes by diamond file or emery paper before application of the top coating, so that no portion of the bottom coating is directly exposed to the ambient
11) Applying a thicker top coating (thickness 0 3-0 6 mm) of coating composition as obtained in step (5) above
12) Vacuum drying the top coating followed by cunng at a temperature in the range of 500-650°C for 30-60 minutes, when dunng heating, the furnace temperature is allowed to rise at a rate of 50-75°C/hour
The following examples are given by way of illustrating the invention and the manner in which it may be carried out in actual practice However, this should not be construed to limit the scope of the present invention in any way
Example 1
A precursor powder for the bottom layer was prepared by the following steps Firstly, 5 99 gram of reagent grade stannous chlonde (SnCI2, 2H2O) was dissolved in 200 mL of distilled water containing 20 drops of cone HCI For complete mixing, the assembly was heated to around 80°C with continuous stirnng Secondly, 0 0163 gram of reagent grade Sb2O3 was dissolved in 50 mL distilled water (at 80°C) containing 5 drops of Cone HCI In the next step, 0 6665
gram of reagent grade PdCl2 was taken in 100mL distilled water containing 10 drops of cone HNO3 and PdCI2 was slowly dissolved by heating the mixture at 80°C under constant stirnng for 1 hour The three solutions were mixed and added to ammonia solution under sonication and the pH of the solution has maintained at 4 The precipitate was filtered, washed with distilled water and calcined at 600°C/2 h
To prepare the precursor powder for the top layer, 5 99 gram of stannous chlonde and 0 6665 gram of PdCl2 were dissolved in distilled water following the above procedure The two solutions were mixed and added to ammonia solution maintaining a pH of 9 The precipitate was filtered, washed and calcined at 950°C/2h Reagent grade alumina powder was then mixed with the calcined powder in the ratio of 3 7 (alumina calcined powder) by weight using an agate mortar and a pestle
To prepare the alumina gel, 20 5 gram of reagent grade aluminium secbutoxide (C12H27AIO3) 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 secbutoxide The solution was refluxed for 18 hours to get the gel of desired consistency so that the alumina content of the gel was 0 05% by weight
A thick paste of the powder formulation for the bottom coating was made by mixing the prepared precursor powder for the bottom layer with alumina gel and ethanol in the weight ratio of 1 0 1 5 5 0 A thick paste of the powder formulation for the top coating was made by mixing the prepared precursor powder for the top layer with alumina gel and ethanol in the weight ratio of 1 0 1 5 5 0
An alumina tube of length 3mm, outside diameter 1 5mm and inner diameter 1mm 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 hour
The outside of the alumina tube was painted by a thick paste of the powder formulation for the bottom coating (thickness of the first coating around 0 1 mm) followed by vacuum drying and cunng at a temperature of 620°C for 45 mm The furnace temperature was raised at a rate of 75°C/hour After firing, the two circular ends and the extreme edges of the tube were cleaned of any coating matenal by rubbing with a diamond file
The singly coated alumina tube was painted by a thick paste of the powder formulation for the top coating (thickness of the second coating was around 0 5 mm) followed by vacuum drying and cunng at a temperature of 600°C for 45 minutes The furnace temperature was raised at a rate of 75°C/hour
For measunng the gas sensitivity of the bi-layered 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 onginal resistance (in the ambient) at an elevated temperature The double coated sensors prepared in this way showed an average sensitivity of around 90% in 1000 ppm methane or LPG at 350°C Incidentally, the sensors show a low sensitivity of around 45% when kept inside a container containing standard paint thinner ( a source of cone VOCs) whereas, under the same condition, the sensitivity in cone VOC can be as high as 95%, when the sensor is singly coated, i e , the top coating of the sensor is absent Hence, by properly designing the electronic circuit, the double coated sensors can be made selective to the detecting gases even in the presence of VOCs
Example 2
A precursor powder for the bottom layer was prepared by the following steps Firstly, 5 99 gram of reagent grade stannous chlonde (SnCl2, 2H2O) was dissolved in 200 mL of distilled water containing 20 drops of cone HCI For complete mixing, the assembly was heated to around 80°C with continuous stirnng Secondly, 0 0170 gram of reagent grade Sb2O3 was dissolved in 50 mL distilled water (at 80°C) containing 5 drops of Cone HCI In the next step, 0 6685 gram of reagent grade PdCl2 was taken in lOOmL distilled water containing 10 drops of cone HNO3 and PdCl2 was slowly dissolved by heating the mixture at 80°C under constant stirnng for 1 hour The three solutions were mixed and added to ammonia solution under sonication and the pH of the solution has maintained at 4 The precipitate was filtered, washed with distilled water and calcined at 600°C/2 h
To prepare the precursor powder for the top layer, 5 99 gram of stannous chlonde and 0 6665 gram of PdCl2 were dissolved in distilled water following the above procedure The two solutions were mixed and added to ammonia solution maintaining a pH of 9 The precipitate was filtered, washed and calcined at 950°C/2h Reagent grade alumina powder was then mixed with the calcined powder in the ratio of 3 7 (alumina calcined powder) by weight using an agate mortar and a pestle
To prepare the alumina gel, 20 5 gram of reagent grade aluminium secbutoxide (C12H27AIO3) 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 secbutoxide The solution was refluxed for 18 hours to get the gel of desired consistency so that the alumina content of the gel was 0 05% by weight
A thick paste of the powder formulation for the bottom coating was made by mixing the prepared precursor powder for the bottom layer with alumina gel and ethanol in the weight ratio of 1 0 1 5 5 0 A thick paste of the powder formulation for the top coating was made by mixing the prepared precursor powder for the top layer with alumina gel and ethanol in the weight ratio of 1 0 1 5 5 0
An alumina tube of length 3mm, outside diameter 1 5mm and inner diameter 1mm 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 hour
The outside of the alumina tube was painted by a thick paste of the powder formulation for the bottom coating (thickness of the first coating around 0 8 mm) followed by vacuum drying and cunng at a temperature of 600°C for 45 mm The furnace temperature was raised at a rate of 75°C/hour After finng, the two circular ends and the extreme edges of the tube were cleaned of any coating matenal by rubbing with a diamond file
The singly coated alumina tube was painted by a thick paste of the powder formulation for the top coating (thickness of the second coating is around 0 5 mm) followed by vacuum drying and cunng at a temperature of 580°C for 45 minutes The furnace temperature was raised at a rate of 75°C/hour
For measunng the gas sensitivity of the bi-layered 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 onginal resistance (in the ambient) at an elevated temperature The double coated sensors prepared in this way showed an average sensitivity of around 95% in 1000 ppm methane or LPG at 350°C Incidentally, the sensors show a low sensitivity of around 48% when kept inside a container containing standard paint thinner ( a source of cone VOCs) whereas, under the same condition, the sensitivity in
cone VOC can be as high as 97%, when the sensor is singly coated, i e , the top coating of the sensor is absent Hence, by properly designing the electronic circuit, the double coated sensors can be made selective to the detecting gases even in the presence of VOCs
Example 3
A precursor powder for the bottom layer was prepared by the following steps Firstly, 5 99 gram of reagent grade stannous chlonde (SnCl2, 2H2O) was dissolved in 200 mL of distilled water containing 20 drops of cone HCI For complete mixing, the assembly was heated to around 80°C with continuous stirnng Secondly, 0 0153 gram of reagent grade Sb2O3 was dissolved in 50 mL distilled water (at 80°C) containing 5 drops of Cone HCI In the next step, 0 6645 gram of reagent grade PdCl2 was taken in 100mL distilled water containing 10 drops of cone HNO3 and PdCl2 was slowly dissolved by heating the mixture at 80°C under constant stirnng for 1 hour The three solutions were mixed and added to ammonia solution under sonication and the pH of the solution has maintained at 4 The precipitate was filtered, washed with distilled water and calcined at 600°C/2 h
To prepare the precursor powder for the top layer, 5 99 gram of stannous chlonde and 0 6675 gram of PdCI2 were dissolved in distilled water following the above procedure The two solutions were mixed and added to ammonia solution maintaining a pH of 9 The precipitate was filtered, washed and calcined at 950°C/2h Reagent grade alumina powder was then mixed with the calcined powder in the ratio of 3 7 (alumina calcined powder) by weight using an agate mortar and a pestle
To prepare the alumina gel, 20 5 gram of reagent grade aluminium secbutoxide (C12H27AIO3) 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 secbutoxide The solution was refluxed for 18 hours to get the gel of desired consistency so that the alumina content of the gel was 0 05% by weight
A thick paste of the powder formulation for the bottom coating was made by mixing the prepared precursor powder for the bottom layer with alumina gel and ethanol in the weight ratio of 101550 A thick paste of the powder formulation for the top coating was made by mixing the prepared precursor powder for the top layer with alumina gel and ethanol in the weight ratio of 1 0 1 5 5 0
An alumina tube of length 3mm, outside diameter 1 5mm and inner diameter 1mm 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 hour
The outside of the alumina tube was painted by a thick paste of the powder formulation for the bottom coating (thickness of the first coating around 0 1 mm) followed by vacuum drying and cunng at a temperature of 600°C for 45 mm The furnace temperature was raised at a rate of 75°C/hour After firing, the two circular ends and the extreme edges of the tube were cleaned of any coating matenal by rubbing with a diamond file
The singly coated alumina tube was painted by a thick paste of the powder formulation for the top coating (thickness of the second coating is around 0 6 mm) followed by vacuum drying and cunng at a temperature of 575°C for 45 minutes The furnace temperature was raised at a rate of 50°C/hour
For measunng the gas sensitivity of the bi-layered 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 onginal resistance (in the ambient) at an elevated temperature The double coated sensors
prepared in this way showed an average sensitivity of around 92% in 1000 ppm methane or LPG at 350°C Incidentally, the sensors show a low sensitivity of around 42% when kept inside a container containing standard paint thinner ( a source of cone VOCs) whereas, under the same condition, the sensitivity in cone VOC can be as high as 95%, when the sensor is singly coated, i e , the top coating of the sensor is absent Hence, by properly designing the electronic circuit, the double coated sensors can be made selective to the detecting gases even in the presence of VOCs
Example 4
A precursor powder for the bottom layer was prepared by the following steps Firstly, 5 99 gram of reagent grade stannous chlonde (SnCl2, 2H2O) was dissolved in 200 mL of distilled water containing 20 drops of cone HCI For complete mixing, the assembly was heated to around 80°C with continuous stirnng Secondly, 0 0168 gram of reagent grade Sb2O3 was dissolved in 50 mL distilled water (at 80°C) containing 5 drops of Cone HCI In the next step, 0 6668 gram of reagent grade PdCl2 was taken in lOOmL distilled water containing 10 drops of cone HNO3 and PdCI2 was slowly dissolved by heating the mixture at 80°C under constant stirnng for 1 hour The three solutions were mixed and added to ammonia solution under sonication and the pH of the solution has maintained at 4 The precipitate was filtered, washed with distilled water and calcined at 575°C/2 h
To prepare the precursor powder for the top layer, 5 99 gram of stannous chlonde and 0 6665 gram of PdCl2 were dissolved in distilled water following the above procedure The two solutions were mixed and added to ammonia solution maintaining a pH of 9 The precipitate was filtered, washed and calcined at 960°C/2h Reagent grade alumina powder was then mixed with the calcined powder in the ratio of 3 7 (alumina calcined powder) by weight using an agate mortar and a pestle
To prepare the alumina gel, 20 5 gram of reagent grade aluminium secbutoxide (C12H27AIO3) 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 secbutoxide The solution was refluxed for 18 hours to get the gel of desired consistency so that the alumina content of the gel was 0 05% by weight
A thick paste of the powder formulation for the bottom coating was made by mixing the prepared precursor powder for the bottom layer with alumina gel and ethanol in the weight ratio of 1 0 1 5 5 0 A thick paste of the powder formulation for the top coating was made by mixing the prepared precursor powder for the top layer with alumina gel and ethanol in the weight ratio of 1 0 1 5 5 0
An alumina tube of length 3mm, outside diameter 1 5mm and inner diameter 1mm 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 hour
The outside of the alumina tube was painted by a thick paste of the powder formulation for the bottom coating (thickness of the first coating around 0 1 mm) followed by vacuum drying and cunng at a temperature of 650°C for 30 mm The furnace temperature was raised at a rate of 75°C/hour After finng, the two circular ends and the extreme edges of the tube were cleaned of any coating matenal by rubbing with a diamond file
The singly coated alumina tube was painted by a thick paste of the powder formulation for the top coating (thickness of the second coating is around 0 5 mm) followed by vacuum drying and cunng at a temperature of 620°C for 30 minutes The furnace temperature was raised at a rate of 75°C/hour
For measunng the gas sensitivity of the bi-layered 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 double coated sensors prepared in this way showed an average sensitivity of around 90% in 1000 ppm methane or LPG at 350°C Incidentally, the sensors show a low sensitivity of around 47% when kept inside a container containing standard paint thinner ( a source of cone VOCs) whereas, under the same condition, the sensitivity in cone VOC can be as high as 96%, when the sensor is singly coated, i e , the top coating of the sensor is absent Hence, by properly designing the electronic circuit, the double coated sensors can be made selective to the detecting gases even in the presence of VOCs
The mam advantages of the present invention are as follows
1) The method can provide sensors which can detect combustible gases like methane, LPG, propane, CNG even in the presence of highly reactive volatile organic compounds
2) As no filter is used, the method provides gas sensor of more compact size
3) As no filter is used, the method provides gas sensors where the sensitivity of the gas sensor remains high even in the presence of interfenng volatile inorganic compounds
4) The functionally graded composition can be used to make sensors which can detect combustible gases like methane, LPG, propane even in the presence of highly reactive volatile organic compounds
5) The functionally graded composition provides good adherence and hence the sensors are rugged and show reproducible behaviour
6) The process steps for the manufacture of such sensors are simple and cost-effective

We claim:
1. A method of manufacturing semiconducting oxide based gas sensors in thick film form for detection of combustible gases in presence of volatile organic compounds, which comprises providing bi-layer coating of semiconducting oxide, such as tin dioxide, zinc oxide, based compositions by known thick film technique on a cleaned and electroded ceramic tubular substrate, wherein the electrical resistance of the top coating is essentially atleast two orders of magnitude higher than that of the bottom coating.
2. A method as claimed in claim 1, which consists of cleaning by known methods a ceramic tubular substrate, electroding by known methods the ends of the substrate by gold based electrode, followed by lead attaching with lead-wire, such as gold wire, gold alloy wire or platinum wire, by firing at a temperature in the range of 900-1000°C, coating the said substrate with a semiconducting oxide based composition by known thick film technique, followed by vacuum drying and curing at a temperature in the range of 550 to 700°C for a period in the range of 30 to 60 minutes, cleaning, if required, the coating from the two circular ends and extreme edges of the tube by means such as diamond file or emery paper, applying a top coating on the coated substrate with a semiconducting oxide based composition by known thick film technique, followed by vacuum drying and curing at a temperature in the range of 500 to 650°C for a period of 30 to 60 minutes.
3. A method as claimed in claim 1-2, wherein the ceramic tubular substrate is such as alumina, quartz, glass.
4. A method as claimed in claim 1-3, wherein the thickness of the bottom layer is in the range of 0.05 to 0.15 mm.
5. A method as claimed in claim 1-4, wherein the thickness of the top layer is in the range of 0.3 to 0.6 mm.
6. A method as claimed in claim 1-5, wherein the bottom and top semiconducting oxide based coatings are of different functionally graded tin dioxide based compositions, wherein the electrical resistance of the top coating is two to three orders of magnitude higher than that of the bottom coating.
7. A method as claimed in claim 1 -6, wherein the bottom semiconducting oxide based coating comprises: 88.80 to 93.70 wt% tin dioxide (Sn02), 0.20 to 0.30 wt% antimony

oxide (Sb203)and 6 to 11 wt% palladium (Pd as metal) along with alumina (AI203) gel equivalent to 1.25 tol.75 times the weight of the powder mix where the alumina content of the gel is 0.04 to 0.10 wt%.
8. A method as claimed in claim 1-7, wherein the top semiconducting oxide based coating comprises: 61 to 65% wt% tin dioxide (Sn02), 5 to 9% wt% palladium (as metal) and 26 to 34% wt% alumina oxide (AI203) along with alumina gel equivalent to 1.25 tol.75 times the weight of the powder mix where the alumina content in the gel is 0.04 to 0.10 wt%.
9. A method as claimed in claim 1-8, wherein the semiconducting oxide based coating compositions are prepared by known soft chemistry methods such as simultaneous precipitation, sonication assisted simultaneous precipitation.
10. A method as claimed in claim 1-9, wherein the alumina gel is prepared by known soft chemistry method such as sol-gel technique.

Documents:

642-del-2004-abstract.pdf

642-DEL-2004-Claims-(25-06-2012).pdf

642-del-2004-claims.pdf

642-DEL-2004-Correspondence Others-(25-06-2012).pdf

642-DEL-2004-Correspondence Others-(26-06-2012).pdf

642-del-2004-correspondence-others.pdf

642-del-2004-correspondence-po.pdf

642-del-2004-description (complete).pdf

642-del-2004-form-1.pdf

642-del-2004-form-18.pdf

642-del-2004-form-2.pdf

642-DEL-2004-Form-3-(25-06-2012).pdf

642-del-2004-form-3.pdf

642-del-2004-form-5.pdf

« Previous Patent

Next Patent »

Patent Number

255349

Indian Patent Application Number

642/DEL/2004

PG Journal Number

07/2013

Publication Date

15-Feb-2013

Grant Date

13-Feb-2013

Date of Filing

31-Mar-2004

Name of Patentee

COUNCIL OF SCIENTIFIC & INDUSTRIAL RESEARCH

Applicant Address

RAFI MARG, NEW DELHI-110 001, INDIA.

Inventors:

#	Inventor's Name	Inventor's Address
1	SHIRSHENDU CHAKRABORTY	CENTRAL GLASS & CERAMIC RESEARCH INSTUTUE, KOLKATA 700032, INDIA.
2	JALALUDDIN MONDAL	CENTRAL GLASS & CERAMIC RESEARCH INSTUTUE, KOLKATA 700032, INDIA.
3	AMARNATH SEN	CENTRAL GLASS & CERAMIC RESEARCH INSTUTUE, KOLKATA 700032, INDIA.
4	HIMADRI SEKHAR MAITI	CENTRAL GLASS & CERAMIC RESEARCH INSTUTUE, KOLKATA 700032, INDIA.
5	ASIM KUMAR HALDER	CENTRAL GLASS & CERAMIC RESEARCH INSTUTUE, KOLKATA 700032, INDIA.

PCT International Classification Number

G01N 27/12

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1			NA