EP0720738A1 - Tin oxide based gas sensors and method for their manufacture - Google Patents
Tin oxide based gas sensors and method for their manufactureInfo
- Publication number
- EP0720738A1 EP0720738A1 EP94924914A EP94924914A EP0720738A1 EP 0720738 A1 EP0720738 A1 EP 0720738A1 EP 94924914 A EP94924914 A EP 94924914A EP 94924914 A EP94924914 A EP 94924914A EP 0720738 A1 EP0720738 A1 EP 0720738A1
- Authority
- EP
- European Patent Office
- Prior art keywords
- tin
- gas sensor
- hydrolysis
- tin oxide
- polymer
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Withdrawn
Links
Classifications
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
- G01N27/02—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance
- G01N27/04—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance
- G01N27/12—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means by investigating impedance by investigating resistance of a solid body in dependence upon absorption of a fluid; of a solid body in dependence upon reaction with a fluid, for detecting components in the fluid
Definitions
- This invention relates to improvements in tin oxide based gas sensors and in particular to improvements in the method of fabricating these sensors.
- tin oxide is used to denote tin (IV) oxide ie Sn0 2 unless the context of its usage clearly shows otherwise.
- Tin oxide is an ionic solid which generally behaves as an n-type semiconductor due to slight non-stoichiometry arising from the variable oxidation state of tin. It is generally agreed that the function of tin oxide as a gas sensor depends on the initial sensitization of the sensor through adsorption of oxygen onto the surface of the tin oxide which removes conduction band electrons from the n-type material and so reduces the conductivity. When a flammable gas reaches the oxygenated tin oxide surface it removes the surface oxide species by chemical oxidation reactions, the product gases are desorbed from the surface and the electrons are returned to the conduction band. This causes an increase in conductivity of the tin oxide which, in a fabricated gas sensor is detected either by a change in the voltage across or the current through the tin oxide.
- tin oxide in order to maximise the sensitivity of a tin oxide based sensor it is desirable to produce tin oxide with a high surface area (so that the surface reactions can dominate any bulk processes) and with a low initial conductivity (so as to minimise the number of charge carriers required to produce a detectable change) .
- the use of high temperatures in conventional fabrication methods tends to reduce the surface area of the tin oxide and also tends to increase the n-type conductivity by increasing the non-stoichiometry.
- tin oxide using a Sol-Gel process a sufficiently coherent, substantially stoichiometric material can be obtained which can be formed into gas sensor heads without recourse to the use of excessive temperatures.
- the tin oxide thus produced retains a high surface area and also has a low initial conductivity and is therefore well suited to the production of highly sensitive flammable gas sensors.
- a method of fabricating tin oxide gas sensor heads comprising the following steps:
- tin alkoxide having a largely covalent character, for example tin-t-butoxide or tin-isopropoxide, in order to reduce the hydrolysis rate.
- An advantage of carrying out this hydrolysis in the presence of the polymer is that because of the coordinating effect the tin oxide nucleates around the polymer chain. The subsequent removal of this polymer leaves a 'hollow' shell of tin oxide which ensures high surface area and good porosity.
- the rate at which the solid condenses during the hydrolysis step can be further controlled by carrying out the hydrolysis in the presence of a chelating agent, such as acetylacetone, having ligands which can coordinate to the tin after hydrolysis to hinder the formation of the oxide structure, thereby increasing control of the condensation.
- a chelating agent such as acetylacetone
- the heat treatment is carried out at temperatures below about 500°C.
- the temperature employed should be as low as is practicable for control of the physical properties of the solid whilst being sufficiently high to ensure sufficient cohesion between the solid particles without causing sintering of these particles and therefore the temperature employed for heat treatment should advantageously be substantially in the range of *400°C to 500°C.
- a tin oxide based gas sensor head fabricated according to the method above described.
- nitric acid 0.5ml
- This final solution is filtered and washed with water and acetone to remove the nitric acid and any nitrates formed by the acid.
- the isolated solid is then oven-dried at ll8°C for 30 hours.
- An amount of the dried solid is then mixed with a solvent, for example THF, to form a slurry which is deposited on a standard Rosemount alumina substrate fitted with gold interdigitated electrodes and a platinum heater on the obverse side, to form the gas sensor head.
- a solvent for example THF
- the sensor head can be used at different operating temperatures which provides the opportunity for selective chemical sensing. This selectivity arises from the fact that the reactions of different flammable gases on the sensor surface have different activation energies and so their rates of reaction may be controlled by varying the supply of thermal energy to the sensor.
- the head is heated to -400°C for 48 hours in dry air. This generates a minimal amount of cohesion between the solid particles without greatly reducing the tin oxide surface area through sintering and also sensitizes the device.
- the tin oxide of the gas sensor head is then further sensitized by heating at 26 ⁇ °C for at least 10 two minute periods alternatively in 1% methane in air and then in clean air.
- Figure 1 is graphical comparison of the performance of a sensor according to the present invention against that of a commercially available sensor.
- Figure 2 shows the response of a sensor according to the present invention to concentrations of up to 100 ppb toluene in air.
- Figure 3 is a calibration curve of sensor output against toluene concentration.
- Figure 4 shows the variation in sensitivity (defined as 100 * [conductance in gas - conductance in air] / [conductance in air] ) of two sensors to acetone with temperature.
- Figure 5 shows the response of a sensor according to the present invention to concentrations of up to approximately 36 ppm xylene.
- TGS sensor metal oxide gas sensor
- the upper trace A is representative of the output signal, in micro amps ( ⁇ A) , provided by the TGS sensor when operated under optimum conditions and the lower trace B is representative of the output signal provided by the SGS sensor, again operating under optimum conditions. / -
- the lower trace B shows that the SGS sensor provides a clearly measurable increase in the conductivity of the tin oxide, manifested as an increase in the output signal, when exposed to the toluene vapour which is reversed when exposed to clean air. This behaviour is seen to be reproducible over the five cycles depicted.
- the upper trace A shows that the TGS sensor provides little change in the output signal over the five cycles and so could not be reliably used to detect toluene vapour at this low level.
- Sensors fabricated according to the present invention are also useful in detecting other gases, for example the time response of an SGS sensor to pulses of up to approximately 36 parts per million (ppm) of xylene vapour in air is shown in Figure 5»
- pulse A corresponds to exposure to 9- PPm xylene vapour
- pulse B to 11.2 ppm
- pulse C to 14.5
- pulse D to 20.5 ppm
- pulse E to 35-2 ppm. It can be seen that the size of output signal, measured in micro amps ( ⁇ A) , is dependent on the concentration of xylene present.
Landscapes
- Chemical & Material Sciences (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Electrochemistry (AREA)
- Physics & Mathematics (AREA)
- Health & Medical Sciences (AREA)
- Life Sciences & Earth Sciences (AREA)
- Analytical Chemistry (AREA)
- Biochemistry (AREA)
- General Health & Medical Sciences (AREA)
- General Physics & Mathematics (AREA)
- Immunology (AREA)
- Pathology (AREA)
- Investigating Or Analyzing Materials By The Use Of Fluid Adsorption Or Reactions (AREA)
Abstract
Description
Claims
Applications Claiming Priority (3)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
GB939319456A GB9319456D0 (en) | 1993-09-23 | 1993-09-23 | Improvements in tin oxide based gas sensors |
GB9319456 | 1993-09-23 | ||
PCT/GB1994/001832 WO1995008764A1 (en) | 1993-09-23 | 1994-08-22 | Tin oxide based gas sensors and method for their manufacture |
Publications (1)
Publication Number | Publication Date |
---|---|
EP0720738A1 true EP0720738A1 (en) | 1996-07-10 |
Family
ID=10742281
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
EP94924914A Withdrawn EP0720738A1 (en) | 1993-09-23 | 1994-08-22 | Tin oxide based gas sensors and method for their manufacture |
Country Status (6)
Country | Link |
---|---|
EP (1) | EP0720738A1 (en) |
JP (1) | JPH09503860A (en) |
CN (1) | CN1134190A (en) |
CA (1) | CA2172515A1 (en) |
GB (2) | GB9319456D0 (en) |
WO (1) | WO1995008764A1 (en) |
Families Citing this family (1)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
JP2010032343A (en) * | 2008-07-29 | 2010-02-12 | Figaro Eng Inc | MANUFACTURING METHOD OF SnO2 GAS SENSOR, AND MANUFACTURING METHOD OF SnO2 CARRYING HAVING NOBLE METAL NANOPARTICLES |
Family Cites Families (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
GB1282993A (en) * | 1970-05-22 | 1972-07-26 | Naoyoshi Taguchi | Gas detecting devices |
JPS58180936A (en) * | 1982-04-17 | 1983-10-22 | Fuigaro Giken Kk | Element for detecting combustion state and preparation thereof |
-
1993
- 1993-09-23 GB GB939319456A patent/GB9319456D0/en active Pending
-
1994
- 1994-08-22 CA CA 2172515 patent/CA2172515A1/en not_active Abandoned
- 1994-08-22 JP JP7509619A patent/JPH09503860A/en active Pending
- 1994-08-22 WO PCT/GB1994/001832 patent/WO1995008764A1/en not_active Application Discontinuation
- 1994-08-22 GB GB9605970A patent/GB2296978B/en not_active Expired - Fee Related
- 1994-08-22 EP EP94924914A patent/EP0720738A1/en not_active Withdrawn
- 1994-08-22 CN CN 94194024 patent/CN1134190A/en active Pending
Non-Patent Citations (1)
Title |
---|
See references of WO9508764A1 * |
Also Published As
Publication number | Publication date |
---|---|
CN1134190A (en) | 1996-10-23 |
GB2296978A (en) | 1996-07-17 |
GB9605970D0 (en) | 1996-05-22 |
GB9319456D0 (en) | 1993-11-03 |
CA2172515A1 (en) | 1995-03-30 |
WO1995008764A1 (en) | 1995-03-30 |
GB2296978B (en) | 1997-12-24 |
JPH09503860A (en) | 1997-04-15 |
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Legal Events
Date | Code | Title | Description |
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PUAI | Public reference made under article 153(3) epc to a published international application that has entered the european phase |
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17P | Request for examination filed |
Effective date: 19960412 |
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