Solid State Sensors for Toxic Gases

1 Solid State Sensors for Toxic Gases .. Founder s Day Special Issue, 2005 49 Solid State Sensors for Toxic Gases Gadkari, Manmeet Kaur, Katti, Bhandarkar, Muthe and Gupta Technical Physics & Prototype Engineering Division Bhabha Atomic Research Centre Introduction rowing industrialization and ever-increasing pollutants from vehicular exhaust have resulted into increased air pollution. Further, use of cooking gas through pipe lines in modern houses, may become a severe fire hazard due to lack of proper gas-leak alarms. The problems related to air quality monitoring are important issues of the current research activity. In fact, a key component in many process controls, product development, environmental monitoring etc.

2 Is the measurement of concentration of one or the other gaseous component of the ambient. In such situations suitable Sensors can provide the necessary interface between the ambient and the back up electronic instrumentation to detect the target gas. Solid - State Sensors have dominated this field for over the past three decades [1, 2]. Suitable application of micro-fabrication technology would lead to the realization of Solid - State Sensors , which are robust, inexpensive, reliable and durable. The systems based on array of chemical Sensors and connected to an electronic microprocessor will also find applications in medical diagnostics, food industry, pharmaceuticals, and for detection of explosives. Present paper describes preparation, characterization and use of thin film Sensors for H2, H2S and NH3.

3 Hydrogen Sensors Hydrogen is a highly combustible gas as 4% V/V concentration in air forms an explosive mixture. Its detection in the ambient becomes important to prevent fire hazard in many areas where this gas is involved. In addition, measurement of H2 concentration is required in applications where this is used as a process gas. Several types of metal-oxide semiconductor thin/thick film based Sensors have been reported in the literature [3,4]. Nano-crystalline SnO2 films with grain sizes in the range 6-8 nm are shown to detect H2 at room temperature. Films of WO3:Pt prepared using sol-gel method were found to show very high sensitivity to H2, however, the films exhibited humidity dependence. Good long-term stability has been achieved in the (Sn,Ti)O2 thin film Sensors operated at 400 C.

4 High temperature operating (450-600 C) Sensors based on stabilized zirconia and ZnO films have been reported to detect H2 in the range 50-500 ppm. Schottky diode based on SiC, WO3 with Pt electrode was reported to detect H2 from room temperature to about 300 C. A new type of thermoelectric sensor has been fabricated using Pt-thin film as catalyst and NiO thick film. This device was operated at 100 C and shown to detect 500 ppm H2 with fast response/recovery times. Thermoelectric Sensors based on carbon nanotubes were also reported for H2 sensing applications. Many groups have reported catalytic combustible type Sensors (commercial name Pellistors ), useful for detection of hydrogen up to 100% LEL [1]. The exothermic reaction of hydrogen with ambient oxygen on the sensing element (containing Pt/Pd catalyst) causes a rise in its temperature as described earlier.

5 The temperature of sensing element is generally compared with that of a compensating element without the catalytic material. These Sensors are invariably made in small sizes and are operated G Solid State Sensors for Toxic Gases .. Founder s Day Special Issue, 2005 50at higher temperature of about 500 C to achieve higher sensitivity [1]. Due to higher operating temperatures, these Sensors are not suitable in applications where hydrogen concentrations near or above LEL need to be monitored. In the present paper we describe construction and working of a Pd-thin film based catalytic sensor , operating at a lower temperature of about 120 C [4]. The sensor could be used to measure H2 concentrations up to 250% LEL with a linear response. In addition, a thin film coating of Teflon (poly-tetrafluoro-ethelene) has been developed to protect the catalyst of the sensing element from environmental poisoning.

6 This resulted in increased working life of the sensor . Pellistor type Sensors are generally operated using fixed bias voltage applied to both the sensor and the compensating elements forming the two arms of a Wheatstone bridge. In this case, any change in the ambient temperature, would lead to a change in the initial temperature of the sensor , which in turn would affect the rate of hydrogen recombination. The error in measurement caused by ambient temperature changes becomes more significant for Sensors operating at lower temperatures. Therefore, in the present case, the sensor is operated keeping the temperature of the compensating element constant. The difference of sensor and compensating element temperatures is monitored and it is directly proportional to the hydrogen concentration. A simple electronic circuit is used to maintain the temperature of control element constant and to measure the temperature difference between the sensor and the compensating elements.

7 The circuit enables stable and reproducible sensor response characteristics over prolonged periods of continuous operation. The low temperature (120 C) of operation also ensures that the drift in sensitivity of the sensor due to diffusion or structural changes in the catalyst is minimal. The basic sensor consists of two heater elements fabricated using Pt-100 RTD of 2 mm x 5 mm dimensions. A thin film of about 2000 was sputter deposited on reverse side of one of the Pt-100, called active element [5]. In order to improve poison resistance of Sensors , PTFE films were deposited on the active element of some Sensors and the performance of Sensors with (type-B) and without PTFE films (type-A) was studied. PTFE films were deposited by RF sputtering technique using 40 mm diameter and 3 mm thick Teflon sheet as a sputtering target.

8 Deposition was carried out at room temperature for 1 hr at 130 Watts MHz under argon partial pressure of mbar yielding PTFE films of nearly 100 nm thickness. Fig. 1 shows a schematic diagram of the sensor and compensating elements. The sensor unit is constructed by soldering both, the active and the compensating elements on a multiple pin connector. The compensating Pt-heater element (C) forms one arm of a Whetstone bridge. The sensor element (S) is connected in series with the bridge, such that nearly same current flows through C and S. The ratio of the resistors in other side of the bridge is adjusted such that C attains an operating temperature of about 120 C. To control the temperature the voltage across C is compared with that across a standard resistor using an Operational Amplifier (OP-27).

9 If temperature of compensating element is slightly less than set value, the second Operational Amplifier (OP27) switches a Darlington pair to pass a fixed current through sensor and the bridge. In the OFF condition a PTFEPd-FILMPt-LEADSPt-HEATER SOLDER GLASSALUMINA : Schematic diagram of the active and the compensating elements of a hydrogen sensor Solid State Sensors for Toxic Gases .. Founder s Day Special Issue, 2005 51small current flows through a resistor placed across the Darlington pair. This helps to compare voltages across C and R in the OFF State . Thus the circuit works in a time proportional ON/OFF mode to maintain the temperature of the compensator throughout the operation and effectively cancels the effects of room temperature variations and any other fluctuations common to both elements.

10 When the sensor is exposed to air containing H2, the catalytic combustion of the latter occurs with the help of the active layer (Pd). The heat of combustion results in an increase in the temperature of the active element and results into a differential voltage between sensor element S and C. The differential voltage is proportional to the H2 concentration, as described earlier. For evaluation of sensor performance, hydrogen gas and air were mixed in the desired ratio and allowed to flow through the sensor housing. A mass flow controller (MFC) was used to set the H2 flow rate in 0-30 standard cc/min range, while air flow was controlled in 0-400 standard cc/min range employing a pressure regulating valve, needle valve and a rotameter. Appropriate values of the two flow rates were selected to get desired H2 concentration [6].