|Title of Invention||
"A PROCESS FOR THE PREPARATION OF AN IMPROVED SENSOR MATERIAL USEFUL FOR THE DETECTION OF LIQUID PETROLEUM GAS (LPG)"
|Abstract||The present invention relates to a process for the preparation of gas sensor materials using a thin layer of zinc oxide (ZnO) with palladium (Pd) catalyst over layer. This double layer sensor structure is useful for the detection of town gas or liquid petroleum gas (LPG). It may also be used for other gases of reducing nature e.g. hydrogen, carbonmonoxide, various hydrocarbons etc. This process uses ZnO based thin film sensor material for improved surface sensitization with palladium. The improved surface sensitization process is cost effective and the desired properties of a sensor is attained at a relatively low sensor operating temperature leading to low operation cost. The sensor materials have long time stability and high electrical surface resistance useful for detection of LPG by quick reduction of surface resistance (resistive mode sensor) at a relatively low operation temperature.|
|Full Text||The present invention relates to a process for the preparation of an improved sensor useful for the detection of liquid petroleum gas (LPG).
The present invention relates particularly to a process for the preparation of gas
sensor materials using a thin layer of zinc oxide (ZnO) with palladium (P'd) catalyst ovcrlaycr. This double layer sensor structure is useful for the detection of town gas or liquid petroleum gas (LPG). It may also be used for other gases of reducing nature e.g. hydrogen, carbon-monoxide, various hydrocarbons etc.
This invention more particularly relates to a ZnO based thin film sensor material utilising an improved process of surface sensitisation with palladium. At the same time the improved surface sensitisation process is cost effective and the desired properties of a sensor is attained at a relatively low sensor operating temperature leading to low operation cost. The sensor materials have long time stability and high electrical surface resistance useful for detection of LPG by quick reduction of surface resistance (resistive mode sensor) at a relatively low operation temperature.
The leakage of potentially ha/ardous LPG in domestic kitchens and bottling plants can be big fire hazards to common people. Since the gas forms an explosive mixture with ambient air, it can lead to personal injury upon continuous exposure, even in small concentrations. Therefore to prevent the explosions caused by the gas leaks, efficient gas leak detection system and gas leak alarms are necessary. Oxide materials (e.g., Sn02, ZnO, Ti02, Fe203 etc.) in different physical forms (e.g. thick film, sintered pellet, thin film etc.) are used for detection of hazardous gases. However, these materials are rarely used as a single phase and a small amount of catalysts (palladium and platinum arc well-known active catalysts) are added to enhance the sensing action against reducing target gases. All these materials work on the principle of resistance change in presence of the target gases. The desirable properties of a sensor material involves high sensitivity (large change of sensor resistance), fast response (quick reaction to target gas), quick recovery (low time to get back to initial condition), rcproducibility in performance and stability (useful for long time operation). Along with these, the sensor fabrication
technique must be cost-el lectvee. The operation temperature lor the sensor material also must be' on the lower side which in turn reduces the power requirement during device fabrication. A low input powor requirement means lower cost ol'operation of the device.
Ilitheito. in paiticulai. thick films or sintered pellets of tin dioxide (SnO2,) have been commercially used for detection of hydrocarbons (methane, propane, butane etc.) and lPG (a mi.xtuie of propane and butane) presumably because ol lack of properly characterised alternative materials and sophisticated and costly thin film production techniques, although other oxides are also known to have substantial gas sensitivity.
In this connection, reference may be made to the following publications:
1. N. Tnguchi. Japanese Patent. 45-38200 (1962)
2. Y. Nakatini, M. Sakai. M. Matsuoka and V. lida, IEEE Trans. Components Hybrids
Manufact. 'Tcchnol. ('I IMT-5(4) (1982) 522.
.v S. Saito, M. Miyayama, K. Koumoto and II. Yanagida. "(ias sensing ebaraeteristies of Porous ZnO and PVZ.nO Ceramics". J. Am. Ccram. Soc. 68 (1) 40 (1985).
4. M. Katsura. M. Shiratori, T. Takahasi, Y. Yokomixo and N. Ichinosc, "Catalyst effect on zinc oxide semiconductor gas sensors". Chemical Sensors (cd. T. Seiyama, Tokyo, Kodansha. 1983) p. 101.
In reference 1. work is based on sintered SnO2 pellet requiring a sensor operating temperature of 300-400T to achieve the desired properties.
In reference 2. Fe-,0, was used as the sensor material at a temperature of 420"C anil no stability study was carried out.
In reference 3, ZnO thick film was used with Pt catalyst ovcrlaycr. A high
sensitivity was obtained at 350T. However, no stability study was reported.
In reference 4. a thick layer of Pel was stacked on a thick layer of ZnO by using ZnO semiconducting paste and catalytic paste (noble metal chloride was dissolved in dcioni/.cd water). The sensor operating temperature of the sensor clement was 350"C.
The main objective of the present invention is to provide a process for the preparation of an improved sensor useful for the detection of liquid petroleum gas which obviates some of the drawbacks of the sensors developed earlier.
Another objective of the present invention is ^q provide a simple and cost
effective technique of depositing a sensitiser (catalytic) layer on ZnO thin film necessary for effective detection.
Yet another objective is to provide a sensor material which can effectively detect the presence of low levels of LPG concentration at relatively low operating temperatures.
Accordingly, the present invention provides a process for the preparation of an improved sensor useful for the detection of liquid petroleum gas which comprises depositing on a nonconducting surface a thin film of thickness in the range of 1.5-6.0 um of ZnO by known methods, depositing on the said ZnO coated substrate a thin film of palladium by 5-20 dippings in a solution of 0.05-0.20 wt% palladium chloride in alcohol-water mixture of concentration in the range of 50-100 vol% alcohol, in such a manner so as to allow each dipping of 2-3 seconds followed by drying of the dipped substrate after each dip, heating the resultant coated substrate at a temperature above 250°C for a period of at least 10 hours in the presence of air.
The zinc oxide thin film is prepared from sodium or ammonium zincate bath using a chemical dipping technique which involves multiple dipping of the substrate in zincate bath kept at room temperature and hot water bath maintained near boiling point (96-WC). The film thickness was built up by increasing the y.iuuatc bath concentrations (0.075-0.15M) and/or number of dippings (50-200). Both the range of concentration and number of dippings were optimised during the course of the work in order to get
adherent, good quality ZnO films. One cycle of dipping consists of alternate dipping of two seconds duration in each bath. Post-deposition heat treatment at 200°C in air for 30 minutes resulted in single phase ZnO thin film. The fabrication technique is simple to perform and economic.
The sensor of the present investigation was prepared by forming a palladium catalyst layer on the ZnO film. The thin Pd layer is'deposited following a simple technique which involves dipping of the ZnO film in a solution of 0.05 - 0.20 wt% palladium chloride (PdCU) in alcohol or water/alcohol mixture. The surface loading of Pd can be easily varied by increasing the number of dippings. After each dipping of 2-3 seconds in PdCl2 solution, the ZnO film is taken out, volatile alcohol is allowed to evaporate and the film is dried in air prior to performing the next dipping. 5-20 dipings are required for long term stability of the sensor surface. For higher number of dippings, a continuous layer of palladium is formed resulting in low resistive and unstable surface and hence unsuitable for gas sensing. ZnO film with PdCl2 overlaycr is then heated in air at 250"C for 24 hours.
The present invention provides an improved fabrication technique for preparing sensor materials having high surface resistance, long time stability, ability to detect low levels of LPG concentration at relatively low operating temperatures of 225-275°C. The fabrication process comprises:
(i) formation of a layer of ZnO film of around 1.5-6.0 (Am thick on tubular glass substrates (1 mm internal diameter, 2 mm external diameter and 5 mm long) by 100-200
dipping cycles alternatively in 0.10-0.15M ammonium or sodium zincatc bath and hot
j,. water bath kept at 96-98°C. Post deposition heat treatmenfof the deposited film at 200°C
for 30 minutes resulting in single phase ZnO film.
(ii) successive dipping (5-20 dips) of the ZnO film in a palladium chloride (PdCl2, 0.05-0.20 wt%) solution in alcohol or water/alcohol mixture followed by heat treatment of the Pd-sensitised ZnO to get a thin oyerlayer of Pd and PdO/PdO2.
The double layer sensor structure thus formed works on the principle of resistance change in presence of gas ambicnts. The sensor material of the present investigation shows a sharp decrease of surface resistance at 225-275°C operation temperature range. The percentage reduction of sensor resistance (which is the sensitivity of a sensor) is 80-95% for 2 seconds exposure to 100% LEL (lower explosive limit) of LPG. The sensor surface resistance in air is 2-10 MΩ at a temperature of 225-275"C. The response time of the present sensor is 2 seconds which enables fast detection of the target gas.
The power required to operate the sensor material at the requisite temperature is maintained by nichrome wire (40-50 SWG) used in the form of a heating coil. The nichrome heating coil is inserted within the cylindrical glass tube substrate and it requires an electrical power of 0.9-1.1 watt for maintaining the sensor operation temperature in the range of 225-275°C.
Electrical contacts on the sensor were made at the two ends of the cylindrical substrate, by using silver (Ag) paste curing at around 200°C. The sensor element was mounted on a polycarbonate base with copper (Cu) pins (12 mm long) as electrodes. The
mounted sensor was covered with a cocylindrical polycarbonate cover (14 mm internal
! the LPG leaking in a domestic environment to come in contact with the sensor element.
The uniqueness of the present invention lies in the fact that the improved fabrication process gives rise to sensors which can detect 0.4 vol.% LPG in air (which is equivalent to 25% LEL of LPG) at a low operation temperature of 250"C.
The following examples illustrates the invention in the manner in which it may be carried out in practice. However, this should not be construed to limit the scope of the present invention.
A thin layer of ZnO was formed on glass substrate by dipping it in 0.125M ammonium y.incate bath kept at room temperature (~30"C) and hot water bath maintained at 98"C. The thickness of the film was approximately 4.3 f.tm after 200 dippings. The ZnO film was then dipped in 0.1 wt% PdCl2 solution in alcohol. Fifteen (15) dippings were performed and the film is then subjected, to air anneal at 250°C in air for 24 hours. The surface resistance at 250°C was around 2 MΩ(the variation of surface resistance within ±10%) and it was found to be stable for continuous 30 days operation. Nichrome wire (48 SWG) was used to maintain the operating temperature of the sensor element. The
sensor element exhibited a high sensitivity of 86% to 0.8 vol% LPG at 250°C. The
, response time was less than 2 seconds.
A thin layer of ZnO of thickness ~4.6 jam was formed from 0.125M sodium zincale bath at room temperature (~30°C) and hot water bath maintained at 98°C.. Ten (10) dippings were performed in 0.1 wt% PdCl2 solution in alcohol and water followed by air anneal at 250"C in air for 24 hours. The surface resistance at 260"C was 4 MQ (the variation of surface resistance within ±10%) and it was found to be stable for continuous 30 days operation. Nichrome wire (48 SWG) was used to maintain the requisite operating temperature. The sensor element exhibited a high sensitivity of 82% to 1.0 vol% LPG at 260°C. The response time was about 2 seconds.
A thin layer of ZnO was formed on glass substrate by dipping it in 0.125M sodium zincate bath at room temperature (~30°C) and hot water bath maintained at 98°C. The thickness of the film was -4.7 jam after 200 dippings. The ZnO film was then dipped in 0.1 wt% PdCl2 solution in alcohol. Fifty (50) such dippings were performed
and the film was subjected to post depostion air anneal at 250"C in air. The surface rcsislance of Ihc film at 250°C was 285 KΩ and it continuously degraded to 20 KQ after seven (7) days. The formation of a continuous layer of 1MO on the surfaee of ZnO makes the surface unstable and hence ineffective for long time operation.
A thin layer of ZnO of thickness ~4.4 um is fabricated from ammonium zincate batli at room temperature (~30"C) and hot water bath maintained at 98°CThc ZnO film was then dipped in 0.1 wt% PdCl, solution in alcohol. The ZnO film was then dipped in 0.1 wt% PdCl2 solution in alcohol. 30 dippings were performed. The surface resistance of the film at 250°C was 580 KQ and it was reduced was to 370 KQ after 7 days of continuous operation. The surface is thus unstable and hence not suitable for sensor operation.
The main advantages of the sensor of the present invention arc as follows:
1. Temperature stable material for long time operation.
2. Low cost and easy to perform.
3.. A stable and relatively low temperature operating LPG sensing material.
4. A process to make a LPG sensor material wherein the material shows high sensitivity to LPG when held at a relatively low temperature of ,at least 225°C.
1. A process for the preparation of an improved sensor material useful for the
detection of LPG which comprises;
a) depositing a thin film layer of ZnO of thickness 1-5-6.0 urn on a non conducting
surface such as glass substrate by known methods,
b) depositing on the said ZnO coated substrate characterized in that a thin film of
palladium by 5-20 dippings in a solution of 0.05-0.20 wt% palladium chloride in
alcohol-water mixture of concentration in the range of 50-100% alcohol in such a
manner that allowing each dipping for 2-3 seconds followed by drying,
c) heating the coated subtracted obtained in step b) at a temperature above 250°C for
a period of at least 10 hours in presence of air to obtain sensor material.
2. A process for the preparation of an improved sensor material useful for the
detection of liquid petroleum gas (LPG) substantially as herein described with
reference to the examples.
|Indian Patent Application Number||535/DEL/1999|
|PG Journal Number||12/2008|
|Date of Filing||08-Apr-1999|
|Name of Patentee||COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH|
|Applicant Address||RAFI MARG, NEW DELHI- 110001, INDIA.|
|PCT International Classification Number||G01N 027/26|
|PCT International Application Number||N/A|
|PCT International Filing date|