General Overview Of Solid State Gas Sensor
A sensor is a device that produces a measurable response to a change in a physical condition, such as temperature or thermal conductivity, or to a change in chemical concentration. Sensors are particularly useful for making in-situ measurements such as in industrial process control. It plays an important role in any measurement and automation application. Factors such as accuracy, calibration, cost, environmental, range, repeatability and resolution are to be considered while choosing a sensor. Based upon the applications, input signal, conversion mechanism, material used for construction, production technologies or sensor characteristics such as cost, accuracy or range, the sensors are classified. Depending on the input signal provided to sensor, the sensors are classified into six different types: Thermal, Electrical, Magnetic, Optical, Mechanical and Chemical Sensors. Due to increasing vehicle and industrial emissions, there is a necessity of continuous monitoring of various pollutant, toxic, refining, combustible and process gases such as carbon monoxide (CO), carbon dioxide (CO2), hydrocarbons (HC), oxides of nitrogen (NOx) etc. in the air. Inhalation of these gases beyond certain concentration is harmful to human health. Hence gas sensors have been designed and developed which detects the presence or measures the concentration of a particular atom, molecule or ion in an atmosphere. The electrical conductivity of the sensor changes corresponding to the composition of surrounding gas atmosphere. Gas sensors are divided into three groups based on the sensing technology such as: optic, spectroscopic and solid state sensors.
Optical sensor measures absorption spectra after the target gas has been stimulated by light. To analyse the absorbed spectra, complex measurement system with a monochromatic excitation source and an optical sensor is required and also employs expensive analytical techniques (such as infrared spectroscopy, ultraviolet fluorescence, chromatography, etc. ).
In spectroscopic systems direct analysis of the molecular mass or vibrational spectrum of the target gas is made. The composition of the different gases is measured and employs Gas chromatography and mass spectrometry. Spectroscopic and optic systems are too expensive for domestic use and sometimes difficult to implement in reduced spaces such as car engines. However a solid state sensor has advantages due to their fast sensing response, simple implementation and low prices. They are portable, consume less power, inexpensive because of the considerable production of semiconductor materials.
Solid state gas sensor
Based on variety of principles and materials, solid state gas sensors are widely used in several domestic, commercial and industrial gas sensing systems during past few decades. On the basis of several physical effects, the solid state sensors are classified as chemiresisitive, chemical field effect transistors (ChemFET), calorimetric, potentiometric and amperometric type of gas sensor.
Semiconductor gas sensors also known as chemiresistive gas sensors, are typically made up of metal oxides. Atoms and molecules of the gas to be measured interact with surface of metal oxide and influence surface properties such as conductivity and surface potential. Metal oxide semiconductors are deposited in the form of thick or thin films, are used as sensing layer to sense gases. The metal oxide semiconductor devices are commonly used due to their several advantages such as low cost, small size, measurement simplicity, durability, ease of fabrication, low detection limits (< ppm levels), long-life and resistant to poisoning. New generation also employs flexible gas sensor due to the advantages of low cost, lightweight, flexibility and portability.
Sensing mechanism of solid state gas sensor
A gas sensor senses a particular gas and converts into measurable output. The gas sensing is carried out through the surface chemical processes due to gas-solid interactions. The sensing mechanism is due to the features of electro-physical and chemical properties, catalytic activity, thermodynamic stability and adsorption/desorption properties of the surface. The optimization involved in the mentioned features of gas sensor has resulted in major advances in sensor technology where material’s responses are improved. The sensor electrical output is measured in the form of current, resistance or capacitance.
Basic characteristics: In chemiresistive sensor, change (increase or decrease) in resistance depends on the nature of sensor material (p-type or n-type) and the gas (reducing or oxidizing) when exposed to the molecules of analysing gas. The response curve of a sensor is characterised by following five parameters which are explained below:
- Sensitivity is defined as the resistance of the sensor in air to the resistance of the sensor upon exposure to the target gas.
- Selectivity is defined as the ratio of the sensitivity of one analyte gas relative to another analyte gas under same conditions.
- Stability is a characteristic that specifies repeatability of device measurements after a long use.
- Response Time is defined as the time interval over which resistance attains a fixed percentage (usually 90%) of final value when the sensor is exposed to full scale concentration of the gas.
- Recovery Time is defined as time interval over which the sensor resistance reduces to 10% of the saturation value when the sensor is exposed to full scale concentration of the gas and then placed in clean air. A good sensor should have a small recovery time so that sensor can be used again.
There are several factors leading to gas sensor’s instability, such as design errors, structural changes, phase shifts, poisoning during chemical reactions and influence of the surrounding environment. In order to make the sensor more stable and reduce the response time, the following methods should be considered such as identifying metals with chemical and thermal stability, optimizing elemental composition and grain size of sensing materials and utilizing specific technology during surface pre-treatment of sensors.
Gas Sensing Materials
Because of the unique electronic structure, the metal oxide semiconductor materials are widely used in sensing applications. The metal oxides are classified as n-type and p-type based on majority charge carriers. The p-type metal oxide materials are NiO, CuO and n-type metal oxide materials are ZnO, SnO2, In2O3, WO3. These materials sense oxidizing gases such as oxygen (O2), carbon dioxide (CO2), nitrogen dioxide (NO2) and reducing gases such as sulphur dioxide (SO2), carbon monoxide (CO), hydrogen (H2), hydrogen sulphide (H2S), ammonia (NH3), ethanol (C2H5OH). Metal oxide semiconductor sensor reacts with target gas and undergoes reduction and oxidation process. This process causes exchange of electrons from sensor with target gas at certain characteristic rate, thereby sensor’s resistance changes. The response of metal oxide gas sensor can be improved by using metal additives/doping, by reducing grain size, or by altering operating temperature and humidity. Several materials are synthesized and developed to enhance the sensing characteristics of the sensor. Metal oxide semiconductor materials based gas sensor is designed with variations in the composition of the semiconductor oxide materials and film deposition methods. Along with metal oxide semiconductor material, various additives are used to improve the material sensitivity and selectivity and decrease response time and operating temperature of the sensitive layer.
Tin dioxide (SnO2) is identified as a suitable metal oxide gas sensor with suitable dopant to increase sensor’s performance for sensing CO2 gas. SnO2 is an n-type semiconductor that has a wide band gap of 3. 6 eV at 300K. The working principle of SnO2 as gas sensor comes from changes in the electrical conductivity of SnO2 grains, which results from the reaction between oxygen and gas reduction. SnO2 is used for gas sensing applications due to advantages such as high sensitivity, low cost, fast response and recovery speed. Dopant such as iron, manganese, cobalt etc. , and additives such as Pt, Ag, Pd, Au, CuO can be added with SnO2 sensing material to improve response.
Thin film based gas sensors
Thin films are thin layers of materials ranging from fractions of a nanometer to several micrometers in thickness. Depending upon the applications, materials are deposited either as thin film or thick film on substrates. The properties of thin films differ significantly as compared to thick film materials with substrate temperature, rate of deposition and pressure. The applications and properties of a given material determine the most suitable technique for the deposition of thin film of the material [9]. Synthesis of nanostructured metal oxide thin films with and without the doping elements has been studied to obtain smaller grain size. Thin films based sensor are advantageous due to their increased surface to volume ratio, carrier concentration and enhanced catalytic activity facilitate its interaction with larger number of gas molecules. The thin film formation process involves several steps such as thermal accommodation, adsorption of metal atoms on the substrate and nucleation followed by crystallization or the formation of microstructure. Thus, the thin films synthesized from the techniques like paste/slurry, chemical and physical vapour deposition techniques, etc. result in different electrical, optical and magnetic properties. Besides the nature and properties of the thin film, the environmental conditions such as operating temperature and humidity determine the sensing performance and characteristics of the thin film gas sensors. However it is observed that, researchers are working on sensing behaviour of the metal oxide materials especially of thin film form, the relation between sensing characteristics and the most influencing parameters namely the operating temperature, type and concentration of gas molecules.
Sensing of carbon dioxide (CO2) gas: The linear bonded atoms of carbon dioxide (CO2) molecule have stable structure and there is no lone pair of electrons to bond. At low temperatures the dissociated hydroxyl and hydrogen ions from water molecules at the surface of metal oxide react with gaseous CO2 to form carbonate ions. With the intermediate products, formate ions and bicarbonate ions. At high temperatures, CO2 molecules interact with the layered oxygen ions to directly form carbonate ions. Thus, the consumption of electrons by each CO2 molecule during its interaction with the metal oxide surface leads to reduction in conduction, which can be used for sensing CO2. In this research work, using thin film SnO2, carbon dioxide gas which is one of the air pollutant is sensed. CO2 is safe for human’s up to 5000 ppm and is dangerous when it reaches concentrations of 40,000 ppm. A short term exposure of 30,000 ppm is bearable. The level of carbon dioxide in the atmosphere is changing every year. The present level of carbon dioxide in the atmosphere is over 400 ppm. Hence there is a need for developing sensor to sense carbon dioxide in the Earth’s atmosphere as a means of prevention of increasing air pollutant.
Design of gas sensing setup
Gas Sensor chamber has to be designed with side glass plates, provided with top & bottom plates made of fiberglass. The chamber is made airtight by rubber beading. The chamber house has sample holder and dc fan in order to distribute the gas molecules uniformly throughout the chamber. Gas from the cylinder is made to flow through a flow meter to the chamber through the gas valves provided at the top of the chamber. The electrometers such as digital meters of high accuracy are to be used. The provision is made in sensor setup for the measurement of electrical resistance of external circuit.
