|Title of Invention||
AN IMPROVED GAS SENSING INSTRUMENT
|Abstract||An improved gas sensing instrument useful for sensing hydrocarbons which comprises a chamber (a) having inlets for gas (G) and air (H) and outlet (J), means (C&D) being provided for controlled heating of sample holder (B) and sensor (F) characterised in that using ruthenated tin oxide sensor placed inside the said chamber, the said sensor being connected to means (R&E) for measuring the resistance.|
|Full Text||This invention relates to an improved gas sensing instrument. More particularly it relates to the improved gas sensing instrument useful for sensing hydrocarbons including those present in Liquid Petroleum Gas. Still more particularly it relates to an instrument wherein a surface modified, sintered ruthenated tin oxide is used for sensing the leakages of liquefied petroleum gas (LPG) and other similar hydrocarbons.
This gas sensing instrument, working on the principle of change in resistance or other electrical properties as a result of the adsorption of hydrocarbons on the sensor element.using a surface modified ceramic material can be profitably used to sense other hydrocarbons with high sensitivity
Hydrocarbon gases are widely used as fuels for several industrial and domestic applications. For example, Liquid Petroleum Gas (LPG) is a commonly employed fuel (consisting lower hydrocarbons such as propane and butane) in households for cooking purposes and Natural gas is similarly used in several industries These gases, if not handled properly are hazardous due to explosion possibilities. For instance, the lower explosive limits (LEL) for propane and butane are 2.1 and 1.8 vol % respectively and more significantly some of the hydrocarbons are physiologically toxic (for e.g. toluene and benzene are carcinogenic ) even in parts per million(ppm) amounts in the total volume. However marry of these hydrocarbons per se have no smell and for those hydrocarbons with smell itself, it is very difficult to distinguish e.g. benzene and toluene. Therefore there is a growing demand to design efficient instruments using sensing materials which can selectively sense the gases. Efficient hydrocarbon sensors are also useful in developing automotive emission control
systems since methane, ethane etc. are also present in the exhaust emissions from automobiles along with carbon monoxide, nitric oxide and sulfur dioxide..
Different types of techniques are generally used for gas sensing. These include measurement of conductivity, e.m.f., Limiting current heat of combustion, optical absorption etc., wherein the amount of gas present in the environment is directly inferred from these measurements. For example, the conductivity of palladium doped stannic oxide shows drastic changes with the adsorption of methane and the amount of methane can be obtained with the help of calibration plots. Similarly different types of catalytic gas sensors can be used for detecting hydrocarbons such as methane and ethane based on the measurement of the heat of oxidation of the combustible gas in the presence of a catalyst. One such type, known as pellistors generally consist of a platinum coil encapsulated by a porous alumina/thona bead containing dispersed catalysts such as platinum and palladium ( Fr 2,452,108 , JP 61,262,649 ). Temperature is measured using thermistors and from the heat change the amount of hydrocarbon can be sensed. The key component in all these hydrocarbon sensors is a material which selectively responds to the hydrocarbon gas. Thus the synthesis of such materials followed by the demonstration of hydrocarbon sensitivity is an essential prerequisite in fabricating hydrocarbon sensors.
Although different types of hydrocarbon sensors are available on the basis of above mentioned principles, those involving direct electrical signals are more convenient since signal processing such as amplification can be easily conducted if necessary. More significantly, flexibility for miniaturization, compatibility with electrical circuitry for devices, minimum interference from thermal and vibrational noise etc. give other advantages and the commonly
employed semiconductor sensors using conductivity changes (Figaro or /Taguchi sensors) illustrate this.
The commonly employed conductometric sensors for sensing hydrocarbon use n-type semiconductors with a reasonably large band gap so that the chemisorbed oxygen in different forms can reduce the number of electrons in the conduction band resulting in high resistance values for the oxide. Adsorption of hydrocarbon decreases the resistance as Hydrocarbon molecules can be oxidized by the surface oxygen species leading to an increase in the density of conduction band electrons for n-type material. For p-type material a similar mechanism is responsible for an increase in resistance upon hydrocarbon adsorption. The main advantage of using these types of devices are their high sensitivity, low cost, fast response time and low power consumption.
These types of sensors as described in the prior art have several limitations, such as long term drift, poisoning by humidity and temperature effects associated with such devices. There are also problems associated with their lack of selectivity since these devices respond to every oxidizable gases such as CO, hydrogen, alcohol, LPG etc. Several attempts have been made in recent times to enhance the selectivity of the instruments by using filters or doping with noble metals. The present sensors used for detecting hydrocarbons are obtained by doping SnO2 - In2O3 with Pd (JP 59,353 , JP 57, 175, 948) , SnO2 In2O3 with Pt and Pd (JP 59,97,047) SnO2 -TiO2 with Nb and Pd. Other oxide materials such as V2O5, ZnO, WO3 and Fe2O3 are also quite sensitive to hydrocarbons after appropriate doping. However they show large cross-sensitivity and the lack of specificity cannot be -completely compensated by the use of doping or additives. The devices based on the materials are quite
expensive due to the use of noble metals such as Au, Pd or Pt in amount varying 1 to 8% by weight.
The main object of the present invention is to provide an improved gas sensing instrument useful for sensing hydrocarbons.
Another object of the present invention, is to provide an instrument using modified sensors which will overcome the drawbacks mentioned hereinbefore.
Yet another object of the present invention is to provide an instrument using a cheaper gas sensor with better selectivity and sensitivity.
During the course of our research it was observed that use of surface ruthenated tin oxide sensors prepared by the dilute solution route enhances the selectivity and the sensitivity of the instrument. The ruthenation of the tin oxide creates surface states (donor-acceptor levels) in the band gap leading to improved selectivity. In our copending patent application no. we have described and claimed a process for the preparation surface modified hydrocarbon sensor materials. The improved gas sensing instrument of the present invention using the modified ruthenated tin oxide which basically works on the principle of the change in electrical resistance due to adsorption of hydrocarbon gases at a controlled temperature is operated in the presence of hydrocarbon gases diluted in air by using the standard two probe technique it shows higher sensitivity and selectivity.
The present invention provides for an improved gas
sensing instrument using the ruthenated tin oxide as sensor, an embodiment of which has been shown m figure-1 of the drawing accompanying this specification which comprises of a .sample tube *A' in the preferred form although any open tube at 200°C to 300°C in which the sample is kept for resistance measurements of gas can be used, a sample holder 'B' and heater 'C', a thermocouple 'D'
which is connected to a temperature controller to measure and control the temperature of the sample, and the sensor 'F' and a multimeter E' to measure the resistance (i and H are the gas and air inlets respectively and J is the outlet for the gas and air
According, the present invention provides An improved gas sensing instrument useful for sensing hydrocarbons which comprises a chamber (a) having inlets for gas (G) and air (H) and outlet (J), means (C&D) being provided for controlled heating of sample holder (B) and sensor (F) characterised in that using ruthenated tin oxide sensor placed inside the said chamber, the said sensor being connected to means (R&E) for measuring the resistance.
In an embodiment of the present invention the sensor is made of sensing material which is prepared using ruthenated tin oxide material prepared In the process fully described in our co-pending patent application no.255'Del 97
The temperature of the sample is sensed by a thermocouple connected to a temperature controller. The sample 'D' is placed in the sample holder and the wnes coming from the sample are connected to a multimeter. Air is then passed in a Controlled manner and the change in resistance of sample is measured at various concentrations of the gases by controlling the quantity of the gas injected in the sample tube either in open air or under a flow of air or nitrogen.
In particular the instrument has suitable electrical contacts provided by phosphor bronze-stainless steel using conducting paste of Ag. Au, Pt or RuO2 The conductivity is measured at a temperature ranging from 200 - 350 °C with or without hydrocarbon diluted with air in the range of 10 to 10,000 ppm of volume
The instruments uses the appropriate electronic circuitry to get a buzzer (warning signal) or a digital readout of gaseous hydrocarbons presence.
A device fabricated using these instructions is especially suitable for sensing LPG in trace amounts and the main advantages include lower cost due to minimum use of Ru and simplification of fabrication. For example this uses typically 0.05% of the ruthenium while the prior art employs Pt and Pd catalysts of about 4-8% and this reduction will lead to substantial economic gain for the manufacture of gas sensing equipment. Similarly the sensitivity to LPG is better than the available sensitivity values for noble metal doped sensing materials usually employed for fabricating hydrocarbon gas sensors. For example a typical value for sensitivity based on normalized conductivity change at 280 °C is 317 for 1000 ppm of LPG while prior art gives only values ranging from 0.6 to 20. (US Patent No. 4,535,315 dated Aug. 13,1995 and US Patent No. 5,470,756 dated Nov. 28, 1995). Response can be obtained for minimum value of 20 ppm as a lower threshold and the operating temperature can be as low as 200 depending on the calcination temperature and amount of Ru on the surface. Similar values for the sensitivity for various gases at this temperature are given
m Table I below
* sensitivity is considered as a difference in conductivity with and without 1000 ppm gas concentration after normalizing with respect to the conductivity in air
Selectivity to LPG is also reasonably good. Although other hydrocarbons and hydrogen give similar signals CO shows only insignificant sensitivity. This is an important advantage compared to the prior art since most of the materials used gives similar sensitivity for CO. Finally the temperature of operation is only about 200°C while most of the LPG sensors use 300°C or more and this advantage of lower operational temperature is extremely important during the fabrication of miniaturized portable devices with inbuilt heaters.
The present invention is described with reference to following examples which are illustrative only and should not be construed to limit the scope of the invention in any manner
The bluish-gray colored pellets -with 0.15% ruthenium on the surface were tested for gas sensitivity after funning two omic contacts at the centers of both sides using conducting silver paint as described earlier. These pellets showed 146 sensitivity.at 250 °C for 1000 ppm of LPG.
Pellets prepared through surface modification of commercial tin oxide with 0.6% ruthenium were tested at temperatures ranging from 180 to 300 OC. These pellets showed 130 sensitivity at 200 °C.
Fig.2 of the drawings describes the sensitivity variation wherein, sensitivity indicates the ratio of change in conductance after introduction of sensing gas to the original conductance of the sample at the same temperature, with concentration of LPG gas at an operating temperature of 200 °C for pellets calcined at three different temperatures of 600, 700 and 800 °C. The minimum response can be as low as 20 ppm as gleaned from this figure and from the change in conductance values distinction con be made even for 800 and 1000 ppm of concentration. Fig. 3 of the drawings similarly indicates the superior response time of these surface modified sensors as the resistance change with time after the exposure to 1000 ppm gas is shown for different quantities of Ru. The response time is very fast (few seconds) compared to prior art methods (few tens of second) and the recovery time for the material containing 0.06% Ru is of the order of one minute. These parameters demonstrate the superiority of our material for efficient sensing of LPG at these concentration and temperature ranges.
We claim :
1. An improved gas sensing instrument useful for sensing hydrocarbons which comprises a chamber (a) having inlets for gas (G) and air (H) and outlet (J), means (C&D) being provided for controlled heating of sample holder (B) and sensor (F) characterised in that using ruthenated tin oxide sensor placed inside the said chamber, the said sensor being connected to means (R&E) for measuring the resistance.
2 An improved gas sensing instrument useful for sensing hydrocarbons substantially as herein described with reference to the examples and drawing accompanying this specification.
|Indian Patent Application Number||256/DEL/1997|
|PG Journal Number||38/2008|
|Date of Filing||31-Jan-1997|
|Name of Patentee||COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH|
|Applicant Address||RAFI MARG, NEW DELHI-110 001, INDIA.|
|PCT International Classification Number||G01N 33/00|
|PCT International Application Number||N/A|
|PCT International Filing date|