|Title of Invention||
"A DEVICE FOR SENSING AMMONIA GAS USING SINTERED ZINC OXIDE WITH RUTHENIUM COMPOUND AS SENSOR"
|Abstract||The device of the present invention works on the principle of change in electrical properties, more particularly the resistance due to the adsorption of ammonia on the surface of the sensor element. The sensor element comprises of modified zinc oxide with ruthenium (0.15%) which senses ammonia with high sensitivity and good selectivity.|
|Full Text||This invention relates to a device for sensing ammonia gas using sintered zinc oxide with ruthenium compound as sensor. This invention particularly relates to an improved gas sensing device for the detection of ammonia gas in trace amount employing surface functionalized zinc oxide. More particularly sintered zinc oxide modified with ruthenium is used as the sensor element for sensing ammonia gas. A process for the preparation of surface modified zinc oxide sensor material has been described and claimed in our copending patent application no 1092/DEL/99.
The device of the present invention works on the principle of change in electrical properties, more particularly the resistance due to the adsorption of ammonia on the surface of the sensor element. The element comprises of surface functionalized zinc oxide prepared as mentioned in our copending patent application no. 1092/DEL/99, by incorporating, the ruthenium at a range between 0.1 to 3.0% to the zinc oxide pellets. Said zinc oxide pellets obtained by mixing zinc oxide with a binder selected from polyvinyl alcohol, ethyl cellulose, polythene, teflon and calcining at a temperature ranging between 500°C to 1000°C. Subsequent immersion of such processed zinc oxide pellete in dilute solution of ruthenium halide helps incorporating ruthenium to the surface of zinc oxide, which senses ammonia with high sensitivity and good selectivity.
Ammonia gas has wide industrial uses. It is utilized in food processing, chemical technology, medical diagnosis, fertilizer industry and environmental protection. Sensing is possible by different types of techniques wherein, different sensing mechanism are involved. They 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
obtained by the decomposition of ammonia. At present, the electrolytic method using diaphragm electrodes is commonly employed for the detection of ammonia.
Presently, several types of ammonia sensors are available on the basis of above mentioned principles. Reference may be made to J. Appl. Phys., 43_, 839 (1993). However 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 such as Figaro or /Taguchi sensors illustrate this.
The commonly employed conductometric sensors for sensing ammonia use large band gap n-type semiconductors wherein the chemisorbed oxygen in different forms can reduce the number of electrons in the conduction band resulting in high resistance values for the oxide. Reference may be made to J. Appl. Phys. £& 482 (1986). Adsorption of ammonia decreases the resistance as ammonia 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 semiconductor a similar mechanism is responsible for an increase in resistance upon ammonia adsorption. The main advantage of using these types of devices are their high sensitivity, low cost, fast response time and low power consumption.
Ammonia sensors as described in the prior art have several limitations, e.g. long term drift, poisoning by humidity and temperature effects associated with such devices (JP Patent No. 1,00,19,812 dated Jan. 23, 1998 and JP Patent No. 082,71,465 dated Nov. 18, 1996). These sensors have inherent problems due to their poor selectivity since these devices respond to every oxidizable gases such as carbon monoxide, hydrogen, alcohol, liquid petroleum gas etc. Several attempts have been made in recent times to enhance the selectivity of the sensors by using filters or doping with noble metals. Other oxide materials such as Ti02, V2O5, Sn02, WO3 and Fe2O3 are also quite sensitive to ammonia 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 sensitivity values and operating temperature for various ammonia gas sensors are given in Table I below. Beside, devices based on these 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.
sensitivity is considered as a difference in conductivity with and without 1000 ppm gas concentration after normalizing with respect to the conductivity in air.
The main objective of the present invention therefore, is to provide a device for sensing ammonia gas which overcomes the drawbacks mentioned hereinbefore.
Another objective of the present invention is to provide a device for sensing ammonia gas using surface functionalized zinc oxide sensor material with better selectivity and high sensitivity. A process for the preparation of surface modified zinc oxide sensor material useful for sensing ammonia gas has been described and claimed in our copending patent application no. The inventors of the present invention have, during the course of their research observed that use of surface functionalized zinc oxide sensors prepared by the process as described and claimed in our copending patent application no. 1092/del/99 using the dilute solution route enhances the selectivity and the sensitivity of the instrument. The ruthenation of the zinc oxide creates surface states (donor-acceptor levels) in the band gap leading to improved selectivity. When the gas sensing equipment deploying the modified ruthenated zinc oxide which basically works on the principle of the change in electrical resistance due to adsorption of ammonia gases at a controlled temperature is operated in the presence of ammonia gases diluted in air by using the standard two probe technique shows higher sensitivity with good selectivity.
Accordingly, the present invention provides a device for sensing ammonia gas using sintered zinc oxide with ruthenium compound as sensor, which comprises a modified zinc oxide pellet containing ruthenium compound as sensor (3), in a sensor holder (4), the said sensor being provided with electrical contacts capable of being connected to means for measuring conductivity; the said sensor holder being encased in an enclosure having controllable inlets for gas (7) and air (8) and an outlet (6), the said enclosure being provided with a heater (2), a thermocouple (5) which is connected to a temperature controller to measure and control the temperature of sensor (3), and a multimeter (9) to measure the resistance.
In an embodiment of the present invention the sensor is made of sensing material which is prepared using ruthenated zinc oxide material prepared by the process described and claimed in our co-pending patent application no. NF - 25/99.
The present invention provides a device for sensing ammonia gas using surface functionalized zinc oxide as sensor. An embodiment of the device of the present invention is depicted in figure-1 of the drawings accompanying this specification. The embodiment consists of a tube T in the preferred form although any open tube withstanding upto 500°C in which the sensor '3' is kept for resistance measurement with and without known amount of detectable gas can be used, a sensor holder '4' and heater '2', a thermocouple '5' which is connected to a temperature controller to measure and control the temperature of the sensor '3', a multimeter '9' to measure the resistance, a gas inlet '7', an air inlet '8' and an air/gas outlet '6'.
The temperature of the sensor is sensed by a thermocouple connected to a temperature controller. The sensor '3' is placed in the sensor holder and the wires coming from the sensor are connected to a multimeter. Air (indicated as '8' in fig.l) is then passed in a controlled manner and the change in resistance of sensor is measured at various concentrations of the gases by controlling the quantity of the gas injected '7' in the tube either in open air or under a flow of air or nitrogen.
In particular, in the device of the present invention suitable electrical contacts are provided on the sensor by phosphor bronze/stainless steel using conducting paste of Ag, Au, Pt or RuO2. The conductivity is measured at a temperature ranging between 200°C and 400°C with or without ammonia diluted with air in the range of 10 to 10,000 ppm of volume. The device uses the appropriate electronic circuitry to get a buzzer (warning signal) or a digital readout of gaseous ammonia presence.
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.
EXAMPLE - 1
The ZnO pellet sintered at 650°C is dipped in millimolar solution of ruthenium trichloride for 1-2 lirs. and heated again at 400°C for two hours. The bluish-grey pellets, obtained were kept for gas sensitivity measurement forming two ohmic contact on the surface of the pellet using conducting silver paste. These pellets showed sensitivity 350 for 1000 ppm of ammonia at 300°C.
EXAMPLE - 2
The bluish-grey colored pellets with 0.15% ruthenium on the surface were tested for gas sensitivity after forming two ohmic contacts at the centres of both sides using conducting silver paint as described earlier. These pellets showed 160 sensitivity at 275°C for 1000 ppm of ammonia.
EXAMPLE - 3
Pellets prepared through surface modification of commercial zinc oxide with 0.55% ruthenium were tested at temperatures ranging from 150 to 300°C. These pellets showed 89 sensitivity at 250°C.
A device fabricated using these instructions is especially suitable for sensing ammonia 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 higher amount of noble metals and this reduction will lead to substantial economic gain for the manufacture of gas sensing equipment. Similarly, the sensitivity to ammonia is better than the available sensitivity values for noble metal doped sensing materials usually employed for fabricating ammonia gas sensors. For example, a typical value for sensitivity based on normalized conductivity change at 275 °C is 250 for 1000 ppm of ammonia while prior art gives only values ranging from 0.6 to 20. 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.
The present device shows reasonably good selectivity to ammonia while carbon monoxide and hydrogen shows only insignificant sensitivity. This is an important
advantages compared to the prior art since most of the materials used gives similar sensitivity for carbon monoxide. Finally, the temperature of operation is only about 200°C while most of the ammonia 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.
1. A device for sensing ammonia gas using sintered zinc oxide with
ruthenium compound as sensor, which comprises a modified zinc oxide
pellet containing ruthenium compound as sensor (3), in a sensor holder
(4), the said sensor being provided with electrical contacts capable of
being connected to means for measuring conductivity; the said sensor
holder being encased in an enclosure having controllable inlets for gas (7)
and air (8) and an outlet (6), the said enclosure being provided with a
heater (2), a thermocouple (5) which is connected to a temperature
controller to measure and control the temperature of sensor (3), and a
multimeter (9) to measure the resistance.
2. A device for sensing ammonia gas using sintered zinc oxide with
ruthenium compound as sensor substantially described hereinbefore with
reference to the examples and drawings accompanying this specification.
|Indian Patent Application Number||1084/DEL/1999|
|PG Journal Number||12/2008|
|Date of Filing||10-Aug-1999|
|Name of Patentee||COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH|
|Applicant Address||RAFI MARG, NEW DELHI-110001, INDIA.|
|PCT International Classification Number||G06K 7/00|
|PCT International Application Number||N/A|
|PCT International Filing date|