1. INTRODUCTION:-
Today’s modern industrialized society has brought to the world numerous goods & services, as well as a series of problems related to technological development. Ever increasing industrialization makes it absolutely necessary to constantly monitor & control air pollution in the environment, in factories, laboratories, hospitals generally technically installations. The field of chemical sensors contributes to be a topic of interest in every parts of world.
In recent years, several types of gases have been used in different areas. In fact, in many industries gases have becomes increasingly important as raw materials. For this reason among others, it has become very important to develop highly sensitive gas detectors to prevent accidents due to gas leakages, thus saving lives & equipment. Such detectors should allow continuous monitoring of the concentration of particular gases in the environment in a quantitative & selective way.
The sensors presently being developed will allow the detection of hydrogen, hydrocarbons, nitrogen oxides, carbon monoxide, oxygen, and carbon dioxide in a variety of ambient gas conditions and temperatures. The sensors are microfabricated and micromachined using Microelectromechanical Systems (MEMS) based technology to minimize size, weight, and power consumption. Nanomaterials are used to improve the sensor response and stability. A temperature detector and a heater are also included in the structure to allow stable sensor operation at a variety of temperatures. The sensor technology development also depends on the use of nanomaterials and [Silicon carbide (SiC)] as an electronic semiconductor. Mass fabrication of the sensors using silicon-processing technology is envisioned to minimize the cost per sensor.
The following list gives both constraints and requirements for an ideal chemical detector.
a) chemically selective
b) reversible
c) fast
d) highly sensitive
e) Durable
f) Non-contamination
g) Non-poisoning
h) Simple operation
i) Small size( portability)
j) Simple fabrication
k) Relative temperature insensitivity
l) Low noise
m) Low manufacturing cost
In addition, this control system should be financially accessible to potential users. Another area in which gas detectors are also very useful is the field of surface science. In fact, devices that operate on the principle of a decrease of the work function of the selective metal by adsorption of gas are the main means for the study of the surface gas interaction in metal-gas system.
2. TERMINOLOGY:-
· SENSITIVITY:-
A sensor's sensitivity indicates how much the sensor's output changes when the measured quantity changes. For instance, if the mercury in a thermometer moves 1cm when the temperature changes by 1°, the sensitivity is 1cm/1°. Sensors that measure very small changes must have very high sensitivities.
· HYSTERESIS:-
Hysteresis is an error caused by when the measured property reverses direction, but there is some finite lag in time for the sensor to respond, creating a different offset error in one direction than in the other.
· SELECTIVITY:-
Selectivity is the ability of a sensor to detect a target gas without being affected by the presence of other interfering gases.
Two of the most important issues in gas sensing devices are gas sensitivity (detection of gas concentrations at the ppm level) and gas selectivity (detection of specific gases in a mixed gas environment).
· Lower Explosive Limit (LEL) & Upper Explosive Limit (UEL):
For flammable gases that require air to support the combustion process, there are two explosive limits.
The Lower Explosive Limit (LEL) is the minimum concentration of gas in air which must be present before it is capable of being explosively ignited by an ignition source. If the gas is present in a concentration below the LEL the fuel is too lean to propagate flame through the mixture.
The Upper Explosive Limit (UEL) is the maximum concentration of gas that can be present in air if the explosion is to occur. Concentrations above the UEL are too rich in fuel and insufficient oxygen is present to permit the rapid propagation of flame front through the mixture. For example, the LEL and UEL of methane are 5% and 15% respectively.
· Short-Term Exposure Limit (STEL) & Long-Term Exposure Limit (LTEL):
In case of toxic gases, Short-Term Exposure Limit (STEL) is the concentration of gas where a ten minute TWA (Time Weighted Average) exposure is permitted. For Long-Term Exposure Limit (LTEL) the time period is eight hours. For carbon monoxide, the STEL and LTEL are 300 ppm and 50 ppm respectively.
· RESOLUTION:
The resolution of a sensor is the smallest change it can detect in the quantity that it is measuring. Often in a digital display, the least significant digit will fluctuate, indicating that changes of that magnitude are only just resolved. The resolution is related to the precision with which the measurement is made. For example, a scanning probe (a fine tip near a surface collects an electron tunneling current) can resolve atoms and molecules.
· PARTS PER MILLION (PPM):
ppm is used to express gas concentration. It can be calculated from the following relation:
C = Vg / (Va+Vg) = Vg /Vt
where Vg is the volume of the gas, Va the volume of air, Vt the total volume of air and gas and C is the concentration in volume fraction. To obtain concentration as a percentage, C is to be multiplied by 102 and to get ppm and ppb (parts per billion), C is to be multiplied by 106 and 109 respectively. The concentration in ppm(Cppm) can be converted to mg/m3 (under standard conditions of 20oC and 760 mm Hg pressure) as shown below:
Conc. in mg/m3 = (Molecular Weight / 24.04 ) X Cppm
· RESPONSE TIME:-
Response time is typically defined as the time it takes for a sensor to read a certain percentage of full-scale reading (normally 90%) after being exposed to a given gas. Similarly, recovery time is the time it takes for a sensor to come back to its original state when the target gas is removed. Drift is the slow time variation of metrological characteristics of a sensor. Material instabilities and ambient interactions as well as ageing affects are primarily responsible for the lack of stability.
3. CLASSIFICATIONS OF GAS SENSOR:-
According to detection principles, commonly used gas sensors can be classified into the following three groups.
1) Sensors based on reactivity of gas:-
Electrochemical sensors
Semiconductor sensor
Combustible gas sensor/micro calorimetric gas sensor/pellistor
Colorimetric paper tape
Chemiluminensece
2) Sensor based on physical properties of gas:-
Non dispersive infrared
Photo acoustic sensor
Thermal conductive sensor
3) Sensors based on gas sorption:-
Reactive-gate semiconductor device/
Gas-FET/Chem-FET/ISFET (ion-selective FET)
Schottky barrier /hetero contact sensor
Schottky diode sensor structure is used in very sensitive measurements. The detection of low concentrations of hydrogen and hydrocarbons can be achieved by using this basic structure
Conductive polymer sensors
Fibre optic sensors
Micro balances
4. DIFFERENT GAS SENSING PRINCIPLES :
4.1. HYDROCARBON SENSING PRINCIPLES:
The detection of hydrocarbons is important in the monitoring of aeronautic exhaust, leak detection, and fire detection. For example, emission monitoring depends on both the ability to produce a sensor that can detect the emissions as well as the supporting electronics to condition the signal produced by the sensor. The development of such monitoring equipment has been greatly enhanced by recent developments in SiC semiconductor technology. The material properties of SiC make it suitable for operation in hostile conditions which exceed the inherent limitations of Si-based electronics. In particular, the ability of SiC to operate as a semiconductor at high temperatures makes it useful in high temperature emission measuring applications. Silicon carbide device operation at temperatures higher than 600°C is made possible by its wide band gap and low intrinsic carrier concentration. Silicon devices are typically limited to operation temperatures below 350°C. Aeronautic gas emissions are often at temperatures considerably above this threshold. Furthermore, it is at these higher temperatures that catalytic effects occur that make hydrocarbon detection possible for many hydrocarbons and sensor designs even when the application temperature is at room temperature. Thus, in a Schottky diode structure, the use of SiC rather than Si is essential for high temperature emission monitoring and control.
In addition, investigation into the inclusion of a reactive layer, such as tin oxide (SnO2), into the Schottky diode structure has produced a sensor of increased sensitivity and stability. This SiC-based MROS (metal reactive-oxide semiconductor) approach can be expanded by adding other materials into the SiC-based Schottky diode sensor structure. Each diode, having a different reactive oxide, will have a different sensitivity to the gases to which they are exposed.
4.2. NITROGEN OXIDE SENSING PRINCIPLES:
The detection of Nitrogen oxides (NOx), such as NO and NO2, and carbon monoxide (CO) is necessary for a range of applications. There is a need for improved NOx and CO sensors for environmental monitoring, fire detection and for aeronautic emissions applications. This work uses tin oxide (SnO2) as the gas sensitive materials, processed in such a way so as to insure stable operation. A standard characteristic of SnO2 is that it undergoes drift at high temperatures due to annealing of the grains. However, nanocrystalline SnO2 provides greater stability and sensitivity at higher temperatures due to its small grain size and large surface area. By doping the SnO2 appropriately, the sensor can be primarily either NOx or CO sensitive. Another approach for CO detection is a titanium dioxide (TiO2) based system.
4.3. CARBON MONOXIDE SENSING
The detection of carbon monoxide (CO) is necessary for a range of applications. There is a need for improved CO sensors for fire detection and environmental monitoring. CO detection is also of interest in aeronautic emissions applications. This work uses tin oxide (SnO2) as the gas sensitive materials, processed in such a way so as to insure stable operation. A standard characteristic of SnO2 is that it undergoes drift at high temperatures due to annealing of the grains. By doping the SnO2 appropriately, the sensor can be primarily either CO or NOx sensitive.
4.4. HYDROGEN SENSING
In the rocket propulsion industry, hydrogen propellant leaks pose significant operational problems. In 1990, leaks on the Space Shuttle while on the launch pad temporarily grounded the fleet until the leak source could be identified. In response to this problem, efforts were made at NASA to develop microfabricated point-contact hydrogen sensors for finding the position of the leaks. One component of this program involves the fabrication of a Pd-alloy hydrogen sensors on Si substrates. The sensors include Schottky diode and a hydrogen sensitive resistor.
The complete hydrogen sensor package using PdAg or PdCr Schottky diodes and PdCr resistors (depending on the application) is now being used in commercial product. The sensor includes a temperature detector and heater for operation in a wide variety of environments. It has an extremely high sensitivity and can operate in either inert or oxygen-containing environments. The sensor’s operating range can be designed from ppm to 100%. These sensors have flown on Space Shuttle missions STS-95 and STS-96, the NASA Helios vehicle, and are part of a safety system on the [International Space Station].The basic structure can also be used for detection of toxic gases such as hydrazine.
4.5. SCHOTTKY DIODE HYDROGEN SENSOR
The above figure shows a schematic (not to scale) of NASA’s Pd13%Ag Schottky diode sensor structure. It has a MOS (metal alloy, oxide, semiconductor) type structure. The diode structure is composed of three layers: a Pd13%Ag catalyst metal film on a SiO2 oxide layer which is adherent to a n-type Si substrate. The most commonly accepted theory of sensor operation for hydrogen detection is that the hydrogen dissociates on the surface of the metal and migrates to the interface between the metal and the oxide. The resulting dipole layer changes the electronic properties of the diode. The change in the electronic properties, particularly current and capacitance measures can be correlated to the amount of hydrogen in the environment. The sensor signal can also be affected by the presence of oxygen which occupies sites on the metal surface or can interact with the hydrogen to form water. Thus, although the sensor does not need oxygen in order to operate, the sensor response may be affected by oxygen. This sensor design yields a highly sensitive sensor with a good response time.
5. TEST CIRCUITS:
Two different basic electric circuits which can be used with the gas sensors are presented on the figure below. The amplifier system (2) presents however the advantage of maintaining a constant voltage VC on the sensitive layer. A constant-current test circuit can also be used for the gas sensors, (maximum power sensor dissipation of 1 mW).
6. SENSOR ARRAYS:-
For a large number of chemical sensing applications, a single sensor is not sufficient to adequately characterize the environment. Rather, sensor arrays are necessary. The formation of an array of the sensors discussed above will detect hydrogen, hydrocarbons, nitrogen oxides, carbon monoxide, carbon dioxide, and oxygen. These gases are detected using three different platforms: Schottky diodes, resistors, and electrochemical cells. Each platform gives different information on the surrounding environment. Three different sensor arrays are presently being developed.
6.1. LEAK SENSOR ARRAY
In leak monitoring of launch vehicles, detection of low concentrations of hydrogen and other fuels is important to avoid explosive conditions that could harm personnel and damage the vehicle. Dependable vehicle operation also depends on the timely and accurate measurement of these leaks. The fuels include hydrogen and hydrocarbons while simultaneous knowledge of oxygen is important to determine if there is an explosive condition. Using the sensing technologies elsewhere on this webpage, leak detection arrays which measure hydrogen, hydrocarbons, and oxygen in a single package are being developed in the Next Generation Launch Technology program. The goal of this work is a system with the surface area of a postage stamp which simultaneously measures both fuel and oxygen with power and telemetry. Measuring hydrogen/hydrocarbons (fuel) and oxygen can give an appraisal of whether there is an explosive condition in a given region. The accompanying “smart” sensor package contains all required analog and digital electronics, including CPU, memory, and communications devices.
6.2. FIRE DETECTION ARRAY
Another type of sensor array developed by NASA and its collaborators is for the detection of fires. Fire detection on-board space and commercial aircraft is extremely important to avoid catastrophic situations and to verify the operational status of the vehicle. False alarm rates from the traditional fire detectors can be high with changes in humidity, condensation on the detector’s surface, or contamination from the contents of the vehicle often being responsible for false indications of a fire. The purpose of this work is to produce microsensors to measure a chemical signature of a fire to complement the existing fire detection technology. Two gases of particular interest in fire detection are CO and CO2 and their ratio.
6.3. HIGH TEMPERATURE ELECTRONIC NOSE /GAS SENSOR ARRAY:-
A high temperature gas sensor array, in effect – a high temperature electronic nose, is currently under development to detect a variety of gases of interest for emission and chemical process monitoring such as hydrocarbons, nitrogen oxides, carbon monoxide, carbon dioxide, and oxygen. Several of these arrays could be placed around the exit of the engine exhaust to monitor the emissions produced by the engine. The signals produced by this nose could be analyzed to determine the constituents of the emission stream, and this information then used to control those emissions. Commercial electronic noses presently exist, and there are a number of efforts to develop other electronic noses. However, some electronic noses depend significantly on the use of polymers and other lower temperature materials to detect the gases of interest. The polymers used are generally unstable above 400°C and, thus, would not be appropriate for use in harsh engine environments. Other noses use only one type of sensor and extract information about the environment using, to a significant degree, analysis of the data. This high temperature electronic nose concept uses materials which are suitable for high temperature environments and which are composed of different types of microsensor platforms (diodes, resistors, and electrochemical cells) to give wider range of inputs.
7.SEMICONDUCTOR GAS SENSORS
7.1. OVERVIEW
It was discovered decades ago that atoms and molecules interacting with semiconductor surfaces influence surface properties of semiconductors, such as conductivity and surface potential. Seiyama (1962) and Taguchi (1970) first applied the discovery to gas detection, by producing the first chemoresistive semiconductor gas sensors. Since then, semiconductor gas sensors have been widely used as domestic and industrial gas detectors for gas-leak alarm, process control, pollution control, etc.Compared to the organic (such as phenenthrene, polybenzimidole) and elemental (such as Si, Ge, GaAs, GaP) semiconductors, semiconductor oxides have been more successfully employed as sensing materials for the detection of different gases, such as CO, CO2, H2, alcohol, H2O, NH3, O2, NOx, etc. Both n-type (such as SnO2, TiO2 an ZnO) and p-type semiconductor oxides can be used as gas sensor materials. The most widely used semiconductor sensor material is SnO2. The increases of conductance of SnO2 caused by the surface reactions between pre-adsorbed surface oxygen species and reducing gases are used to detect the concentrations of reducing gases.
Many approaches have been attempted to modify the sensing properties of these semiconductor oxide gas sensors in order to achieve high sensitivity and selectivity. The enhancement of the sensing properties of the sensor materials have been mainly achieved by development of unique preparation methods to fabricate sensors (such as thin-film, thick-film, and sol-gel methods) and the addition of transition metals onto sensors. However, the progresses on this study are still not satisfactory till now. Let us discuss the phenomenon related to the operations of a semiconductor gas sensor.
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7.1.1 INTERACTION OF GAS WITH SURFACES:-
Two very important cases can be distinguished –
a) Calorimetric detection vs temperature rise produced by the heat of reaction on a catalytic surface.
b) Detection due to the change of electrical parameters, such as the change in electrical conductivity induced by adsorption or reaction of gases on the solid surfaces.
Catalytic effect plays an important role in the field of gas detection. Solid state gas sensors are directly related to the phenomenon of catalysis. Catalysis processes not only control the rate at which a chemical reaction approaches equilibrium (this considerably affects the response time in the case of gas detection) but also affect sensitivity & selectivity. The ideal catalyst is one which increases the rate of the gas surface interaction without itself becoming permanently affected by the reaction. Thus the response time will be fast and the process will be reversible & finally the sensor will possess three important properties i.e. speed, durability, reversibility.
Following factors are important in affecting catalytic reaction.
a) Transport of gases to the solid surface
b) Adsorption of gases on the solid surface
c) Reaction between the adsorbed species and/or with the solid surface
d) Desorption of the surface species & products away from the surface
e) Transport of the gaseous reactants and products away from the surface
As is well known, gas adsorption on surface occurs because the atoms or ions at the surface of the solid can not fully satisfy their valency or co-ordination requirements. This reads to a certain permanent force acting in word. Then, the adsorption of external species that happen to be in the neighborhood of the surface reduces the surface energy of the solid. One can distinguish two types of adsorption.
1) Physisorption:
Weak attraction followed by gas adsorption due to Vander walls forces .It is characterized by a low heat of adsorption.
2) chemisorption:
In the case of high surface energy, the gas may become adsorbed through an exchange of electron with the surface i.e. chemical bonds are formed.
In order to have successful gas sensors two main properties concerning the catalyst are necessary.
a) In the case where the detectivity of the sensor is due to the electrical conductivity shift, these charges must be proportional to the concentration of the detecting gas.
b) In the case of calorimetric detection, the heat of adsorption must be independent of the coverage. If these two properties are satisfied then the sensor may possess a linear response, which is one of the most important properties of a gas detection device. The linear response occurs especially in the range of low and not near the saturation level.
7.1.2 THE CHANGE IN WORK FUNCTION:-
The adsorption of atoms or molecules on a metal surface changes the distribution of charges & gives rise to changes in the work function. The formation of adsorbed negative ions or the ionization of atoms (transfer of electron into the surface) decreases/increases the work function respectively. Thus the monitoring of work function of the known metal surface under investigation can give important information concerning the nature of gas adsorption on the surface as well as information concerning the gas -surface interaction & the kinetics of the adsorption-absorption process. In fact, very often the work function change is directly related to surface coverage. Several experimental techniques have been used since 1940 for measuring work function changes upon chemisorptions, such as retarding potential vibrating capacitor & field emission.
7.2. Principles & operations of Semiconductor Sensors
The material of choice for semiconductor sensor is SnO2, though Ga2O3, WO3, ZnO, In2O3, Fe2O3 etc. are also employed. SnO2 is an oxygen deficient oxide and therefore is an n-type semiconductor. It crystallizes in the rutile structure and is by far the most studied and most successful semiconductor material for gas sensors. Such sensors often change their resistance by more than a factor of 100 upon exposure to a trace of reducing gases such as hydrogen, methane, ethanol, carbon monoxide and propane. Incidentally, the free electrons of n-type semiconductors like SnO2 are trapped by oxygen from the ambient by its electron affinity. Oxygen adsorbed on the surface of the grains extracts an electron to ionize into O- or O2- species, which increases the resistance of the sensor coating. Upon exposure to a reducing gas, the adsorbed oxygen species, being highly metastable, oxidize the reducing gas, releasing the trapped electron and subsequently lowering the resistance.
The amount of resistance change is proportional to the concentration of the reducing gas in the ambient, which is believed to be the dominant sensing mechanism of surface conductive gas sensors like SnO2. However, the concentration of charged oxygen species is limited to <1% r =" Ro" s =" (RA-RG" s =" RA" cs =" cosh[1−(x/L)]m/coshm," x="0," m=" L(k/Dk)1/2" title="Nanoparticles" world. Though humans have not yet been able to synthesize nanosensors successfully, predictions for their use mainly include various medicinal purposes and as gateways to building other nanoproducts, such as computer chips that work at the nanoscale and nanorobots. Presently, there are several ways proposed to make nanosensors, including top-down lithography, bottom-up assembly, and molecular self-assembly. A nanosensor probe carrying a laser beam penetrates a living cell to detect the presence of a product indicating that the cell has been exposed to a cancer-causing substance.
Medicinal uses of nanosensors mainly revolve around the potential of nanosensors to accurately identify particular cells or places in the body in need. By measuring changes in volume, concentration, displacement and velocity, gravitational, electrical, and magnetic forces, pressure or temperature of cells in a body, nanosensors may be able to distinguish between and recognize certain cells, most notably those of cancer, at the molecular level in order to deliver medicine or monitor development to specific places in the body. In addition, they may be able to detect macroscopic variations from outside the body and communicate these changes to other nanoproducts working within the body.
Currently, the most common mass-produced functioning nanosensors exist in the biological world as natural receptors of outside stimulation. For instance, sense of smell, especially in animals in which it is particularly strong, such as dogs, functions using receptors that sense nanosized molecules. Certain plants, too, use nanosensors to detect sunlight; various fish use nanosensors to detect minuscule vibrations in the surrounding water; and many insects detect sex pheromones using nanosensors.
There are currently several hypothesized ways to produce nanosensors. Top-down lithography is the manner in which most integrated circuits are now made. It involves starting out with a larger block of some material and carving out the desired form. These carved out devices, notably put to use in specific micro electromechanical systems used as micro sensors, generally only reach the micro size, but the most recent of these have begun to incorporate nanosized components.
Another way to produce nanosensors is through the bottom-up method, which involves assembling the sensors out of even more minuscule components, most likely individual atoms or molecules. This would involve moving atoms of a particular substance one by one into particular positions which, though it has been achieved in laboratory tests using tools such as atomic force microscopes, is still a significant difficulty, especially to do en masse, both for logistic reasons as well as economic ones. Most likely, this process would be used mainly for building starter molecules for self-assembling sensors.
Nanostructured materials present new opportunities for enhancing the properties and performance of gas sensors because of the much higher surface-to-bulk ratio in nanomaterials compared to coarse micro grained materials. In addition to processing nanostructured oxides, more fundamental work is needed to understand the role of nanostructured oxide materials on gas adsorption and conductivity.
Though nanosensor technology is a relatively new field, global projections for sales of products incorporating nanosensors range from $0.6 billion to $2.7 billion in the next three to four years. They will likely be included in most modern circuitry used in advanced computing systems, since their potential to provide the link between other forms of nanotechnology and the macroscopic world allows developers to fully exploit the potential of nanotechnology to miniaturize computer chips while vastly expanding their storage potential.
10. CONCLUSION
Gas sensor technology is an indispensable tool to create new technologies and new life-styles which are compatible with sustainable society. Further advancement of it is no doubt desired strongly worldwide. To enhance it, researchers would be requested to pay attention to a few more suggestions. Gas sensor technology is interdisciplinary indeed, so that collaborations among people working in broadly different disciplines, ranging from materials scientists to market developers, would be necessary to open new frontiers. Researchers should be well acquainted with the needs having emerged or newly emerging in industry and society. For this purpose, one should listen to opinions of users and market developers carefully. Importance of carrying out field test cannot be overstated when a new sensor device has been developed. It is important not only because it confirms the feasibility of the device in practice but also because it is a direct way to demonstrate the potentiality of the new frontier the device aims at opening. Finally, challenging spirit is a backbone of every successful researcher. I hope sincerely that gas sensor technology will be innovated to contribute more and more to the society in the future.
11.REFERENCES:-
1. A. Mandeles & C. Christofides, Physics,Chemistry & Technology of Solid States Gas Sensor Devices(john Wiley & Sons,Inc,New York,1993
2. Oxide Materials for Development of Integrated Gas Sensors—A Comprehensive Review -G. Eranna, B. C. Joshi, D. P. Runthala, and R. P. Gupta, Critical Reviews in Solid State and Materials Sciences, vol.29:111–188, 2004
3. Materials and Processing Issues in Nanostructured Semiconductor Gas Sensors by Frederic Cosandey, Ganesh Skandan, and Amit Singhal
4. Sensor Technology Handbook-John Wilson
5. Gas sensors;Principles, operation & Developments-giorgio Sberveglieri
6. Semiconducting Oxides in Gas Sensing,A Sen, Article 8, Science & Culture,May-june 2005
7. Fabrication of Sn02-Based Semiconductor Gas Sensors form Combustible and Pollutant Gases-A. Khodadadi, S.S. Mohajerzadeh,A.M. Miri and Y. Mortazavi, The 1 2th international Conference on Microelectronics Tehran, Oct. 31- Nov. 2,2000
8. Complex plane impedance plot as a figure of merit for
tin dioxide-based methane sensors -S. Chakraborty, A. Sen , H.S.
Maiti Sensors and Actuators B 119 (2006) 431–434
9. Improvement of recovery time of nanostructured tin dioxide-based thick film gas sensors through surface modification S. Chakraborty , I. Mandal, I. Raya, S. Majumdara, A. Sen, H.S. Maiti, Sensors and Actuators B 127 (2007) 554–558
10. http://www.cpec.nus.edu.sg/myweb/newsletter/news4/development.html
11. www.grc.nasa.gov/WWW/SiC/discoveries.html
12. www.citytech.com
13. physicsworld.com
14. www.sciencedaily.com
--------------------------THE END---------------------------
Today’s modern industrialized society has brought to the world numerous goods & services, as well as a series of problems related to technological development. Ever increasing industrialization makes it absolutely necessary to constantly monitor & control air pollution in the environment, in factories, laboratories, hospitals generally technically installations. The field of chemical sensors contributes to be a topic of interest in every parts of world.
In recent years, several types of gases have been used in different areas. In fact, in many industries gases have becomes increasingly important as raw materials. For this reason among others, it has become very important to develop highly sensitive gas detectors to prevent accidents due to gas leakages, thus saving lives & equipment. Such detectors should allow continuous monitoring of the concentration of particular gases in the environment in a quantitative & selective way.
The sensors presently being developed will allow the detection of hydrogen, hydrocarbons, nitrogen oxides, carbon monoxide, oxygen, and carbon dioxide in a variety of ambient gas conditions and temperatures. The sensors are microfabricated and micromachined using Microelectromechanical Systems (MEMS) based technology to minimize size, weight, and power consumption. Nanomaterials are used to improve the sensor response and stability. A temperature detector and a heater are also included in the structure to allow stable sensor operation at a variety of temperatures. The sensor technology development also depends on the use of nanomaterials and [Silicon carbide (SiC)] as an electronic semiconductor. Mass fabrication of the sensors using silicon-processing technology is envisioned to minimize the cost per sensor.
The following list gives both constraints and requirements for an ideal chemical detector.
a) chemically selective
b) reversible
c) fast
d) highly sensitive
e) Durable
f) Non-contamination
g) Non-poisoning
h) Simple operation
i) Small size( portability)
j) Simple fabrication
k) Relative temperature insensitivity
l) Low noise
m) Low manufacturing cost
In addition, this control system should be financially accessible to potential users. Another area in which gas detectors are also very useful is the field of surface science. In fact, devices that operate on the principle of a decrease of the work function of the selective metal by adsorption of gas are the main means for the study of the surface gas interaction in metal-gas system.
2. TERMINOLOGY:-
· SENSITIVITY:-
A sensor's sensitivity indicates how much the sensor's output changes when the measured quantity changes. For instance, if the mercury in a thermometer moves 1cm when the temperature changes by 1°, the sensitivity is 1cm/1°. Sensors that measure very small changes must have very high sensitivities.
· HYSTERESIS:-
Hysteresis is an error caused by when the measured property reverses direction, but there is some finite lag in time for the sensor to respond, creating a different offset error in one direction than in the other.
· SELECTIVITY:-
Selectivity is the ability of a sensor to detect a target gas without being affected by the presence of other interfering gases.
Two of the most important issues in gas sensing devices are gas sensitivity (detection of gas concentrations at the ppm level) and gas selectivity (detection of specific gases in a mixed gas environment).
· Lower Explosive Limit (LEL) & Upper Explosive Limit (UEL):
For flammable gases that require air to support the combustion process, there are two explosive limits.
The Lower Explosive Limit (LEL) is the minimum concentration of gas in air which must be present before it is capable of being explosively ignited by an ignition source. If the gas is present in a concentration below the LEL the fuel is too lean to propagate flame through the mixture.
The Upper Explosive Limit (UEL) is the maximum concentration of gas that can be present in air if the explosion is to occur. Concentrations above the UEL are too rich in fuel and insufficient oxygen is present to permit the rapid propagation of flame front through the mixture. For example, the LEL and UEL of methane are 5% and 15% respectively.
· Short-Term Exposure Limit (STEL) & Long-Term Exposure Limit (LTEL):
In case of toxic gases, Short-Term Exposure Limit (STEL) is the concentration of gas where a ten minute TWA (Time Weighted Average) exposure is permitted. For Long-Term Exposure Limit (LTEL) the time period is eight hours. For carbon monoxide, the STEL and LTEL are 300 ppm and 50 ppm respectively.
· RESOLUTION:
The resolution of a sensor is the smallest change it can detect in the quantity that it is measuring. Often in a digital display, the least significant digit will fluctuate, indicating that changes of that magnitude are only just resolved. The resolution is related to the precision with which the measurement is made. For example, a scanning probe (a fine tip near a surface collects an electron tunneling current) can resolve atoms and molecules.
· PARTS PER MILLION (PPM):
ppm is used to express gas concentration. It can be calculated from the following relation:
C = Vg / (Va+Vg) = Vg /Vt
where Vg is the volume of the gas, Va the volume of air, Vt the total volume of air and gas and C is the concentration in volume fraction. To obtain concentration as a percentage, C is to be multiplied by 102 and to get ppm and ppb (parts per billion), C is to be multiplied by 106 and 109 respectively. The concentration in ppm(Cppm) can be converted to mg/m3 (under standard conditions of 20oC and 760 mm Hg pressure) as shown below:
Conc. in mg/m3 = (Molecular Weight / 24.04 ) X Cppm
· RESPONSE TIME:-
Response time is typically defined as the time it takes for a sensor to read a certain percentage of full-scale reading (normally 90%) after being exposed to a given gas. Similarly, recovery time is the time it takes for a sensor to come back to its original state when the target gas is removed. Drift is the slow time variation of metrological characteristics of a sensor. Material instabilities and ambient interactions as well as ageing affects are primarily responsible for the lack of stability.
3. CLASSIFICATIONS OF GAS SENSOR:-
According to detection principles, commonly used gas sensors can be classified into the following three groups.
1) Sensors based on reactivity of gas:-
Electrochemical sensors
Semiconductor sensor
Combustible gas sensor/micro calorimetric gas sensor/pellistor
Colorimetric paper tape
Chemiluminensece
2) Sensor based on physical properties of gas:-
Non dispersive infrared
Photo acoustic sensor
Thermal conductive sensor
3) Sensors based on gas sorption:-
Reactive-gate semiconductor device/
Gas-FET/Chem-FET/ISFET (ion-selective FET)
Schottky barrier /hetero contact sensor
Schottky diode sensor structure is used in very sensitive measurements. The detection of low concentrations of hydrogen and hydrocarbons can be achieved by using this basic structure
Conductive polymer sensors
Fibre optic sensors
Micro balances
4. DIFFERENT GAS SENSING PRINCIPLES :
4.1. HYDROCARBON SENSING PRINCIPLES:
The detection of hydrocarbons is important in the monitoring of aeronautic exhaust, leak detection, and fire detection. For example, emission monitoring depends on both the ability to produce a sensor that can detect the emissions as well as the supporting electronics to condition the signal produced by the sensor. The development of such monitoring equipment has been greatly enhanced by recent developments in SiC semiconductor technology. The material properties of SiC make it suitable for operation in hostile conditions which exceed the inherent limitations of Si-based electronics. In particular, the ability of SiC to operate as a semiconductor at high temperatures makes it useful in high temperature emission measuring applications. Silicon carbide device operation at temperatures higher than 600°C is made possible by its wide band gap and low intrinsic carrier concentration. Silicon devices are typically limited to operation temperatures below 350°C. Aeronautic gas emissions are often at temperatures considerably above this threshold. Furthermore, it is at these higher temperatures that catalytic effects occur that make hydrocarbon detection possible for many hydrocarbons and sensor designs even when the application temperature is at room temperature. Thus, in a Schottky diode structure, the use of SiC rather than Si is essential for high temperature emission monitoring and control.
In addition, investigation into the inclusion of a reactive layer, such as tin oxide (SnO2), into the Schottky diode structure has produced a sensor of increased sensitivity and stability. This SiC-based MROS (metal reactive-oxide semiconductor) approach can be expanded by adding other materials into the SiC-based Schottky diode sensor structure. Each diode, having a different reactive oxide, will have a different sensitivity to the gases to which they are exposed.
4.2. NITROGEN OXIDE SENSING PRINCIPLES:
The detection of Nitrogen oxides (NOx), such as NO and NO2, and carbon monoxide (CO) is necessary for a range of applications. There is a need for improved NOx and CO sensors for environmental monitoring, fire detection and for aeronautic emissions applications. This work uses tin oxide (SnO2) as the gas sensitive materials, processed in such a way so as to insure stable operation. A standard characteristic of SnO2 is that it undergoes drift at high temperatures due to annealing of the grains. However, nanocrystalline SnO2 provides greater stability and sensitivity at higher temperatures due to its small grain size and large surface area. By doping the SnO2 appropriately, the sensor can be primarily either NOx or CO sensitive. Another approach for CO detection is a titanium dioxide (TiO2) based system.
4.3. CARBON MONOXIDE SENSING
The detection of carbon monoxide (CO) is necessary for a range of applications. There is a need for improved CO sensors for fire detection and environmental monitoring. CO detection is also of interest in aeronautic emissions applications. This work uses tin oxide (SnO2) as the gas sensitive materials, processed in such a way so as to insure stable operation. A standard characteristic of SnO2 is that it undergoes drift at high temperatures due to annealing of the grains. By doping the SnO2 appropriately, the sensor can be primarily either CO or NOx sensitive.
4.4. HYDROGEN SENSING
In the rocket propulsion industry, hydrogen propellant leaks pose significant operational problems. In 1990, leaks on the Space Shuttle while on the launch pad temporarily grounded the fleet until the leak source could be identified. In response to this problem, efforts were made at NASA to develop microfabricated point-contact hydrogen sensors for finding the position of the leaks. One component of this program involves the fabrication of a Pd-alloy hydrogen sensors on Si substrates. The sensors include Schottky diode and a hydrogen sensitive resistor.
The complete hydrogen sensor package using PdAg or PdCr Schottky diodes and PdCr resistors (depending on the application) is now being used in commercial product. The sensor includes a temperature detector and heater for operation in a wide variety of environments. It has an extremely high sensitivity and can operate in either inert or oxygen-containing environments. The sensor’s operating range can be designed from ppm to 100%. These sensors have flown on Space Shuttle missions STS-95 and STS-96, the NASA Helios vehicle, and are part of a safety system on the [International Space Station].The basic structure can also be used for detection of toxic gases such as hydrazine.
4.5. SCHOTTKY DIODE HYDROGEN SENSOR
The above figure shows a schematic (not to scale) of NASA’s Pd13%Ag Schottky diode sensor structure. It has a MOS (metal alloy, oxide, semiconductor) type structure. The diode structure is composed of three layers: a Pd13%Ag catalyst metal film on a SiO2 oxide layer which is adherent to a n-type Si substrate. The most commonly accepted theory of sensor operation for hydrogen detection is that the hydrogen dissociates on the surface of the metal and migrates to the interface between the metal and the oxide. The resulting dipole layer changes the electronic properties of the diode. The change in the electronic properties, particularly current and capacitance measures can be correlated to the amount of hydrogen in the environment. The sensor signal can also be affected by the presence of oxygen which occupies sites on the metal surface or can interact with the hydrogen to form water. Thus, although the sensor does not need oxygen in order to operate, the sensor response may be affected by oxygen. This sensor design yields a highly sensitive sensor with a good response time.
5. TEST CIRCUITS:
Two different basic electric circuits which can be used with the gas sensors are presented on the figure below. The amplifier system (2) presents however the advantage of maintaining a constant voltage VC on the sensitive layer. A constant-current test circuit can also be used for the gas sensors, (maximum power sensor dissipation of 1 mW).
6. SENSOR ARRAYS:-
For a large number of chemical sensing applications, a single sensor is not sufficient to adequately characterize the environment. Rather, sensor arrays are necessary. The formation of an array of the sensors discussed above will detect hydrogen, hydrocarbons, nitrogen oxides, carbon monoxide, carbon dioxide, and oxygen. These gases are detected using three different platforms: Schottky diodes, resistors, and electrochemical cells. Each platform gives different information on the surrounding environment. Three different sensor arrays are presently being developed.
6.1. LEAK SENSOR ARRAY
In leak monitoring of launch vehicles, detection of low concentrations of hydrogen and other fuels is important to avoid explosive conditions that could harm personnel and damage the vehicle. Dependable vehicle operation also depends on the timely and accurate measurement of these leaks. The fuels include hydrogen and hydrocarbons while simultaneous knowledge of oxygen is important to determine if there is an explosive condition. Using the sensing technologies elsewhere on this webpage, leak detection arrays which measure hydrogen, hydrocarbons, and oxygen in a single package are being developed in the Next Generation Launch Technology program. The goal of this work is a system with the surface area of a postage stamp which simultaneously measures both fuel and oxygen with power and telemetry. Measuring hydrogen/hydrocarbons (fuel) and oxygen can give an appraisal of whether there is an explosive condition in a given region. The accompanying “smart” sensor package contains all required analog and digital electronics, including CPU, memory, and communications devices.
6.2. FIRE DETECTION ARRAY
Another type of sensor array developed by NASA and its collaborators is for the detection of fires. Fire detection on-board space and commercial aircraft is extremely important to avoid catastrophic situations and to verify the operational status of the vehicle. False alarm rates from the traditional fire detectors can be high with changes in humidity, condensation on the detector’s surface, or contamination from the contents of the vehicle often being responsible for false indications of a fire. The purpose of this work is to produce microsensors to measure a chemical signature of a fire to complement the existing fire detection technology. Two gases of particular interest in fire detection are CO and CO2 and their ratio.
6.3. HIGH TEMPERATURE ELECTRONIC NOSE /GAS SENSOR ARRAY:-
A high temperature gas sensor array, in effect – a high temperature electronic nose, is currently under development to detect a variety of gases of interest for emission and chemical process monitoring such as hydrocarbons, nitrogen oxides, carbon monoxide, carbon dioxide, and oxygen. Several of these arrays could be placed around the exit of the engine exhaust to monitor the emissions produced by the engine. The signals produced by this nose could be analyzed to determine the constituents of the emission stream, and this information then used to control those emissions. Commercial electronic noses presently exist, and there are a number of efforts to develop other electronic noses. However, some electronic noses depend significantly on the use of polymers and other lower temperature materials to detect the gases of interest. The polymers used are generally unstable above 400°C and, thus, would not be appropriate for use in harsh engine environments. Other noses use only one type of sensor and extract information about the environment using, to a significant degree, analysis of the data. This high temperature electronic nose concept uses materials which are suitable for high temperature environments and which are composed of different types of microsensor platforms (diodes, resistors, and electrochemical cells) to give wider range of inputs.
7.SEMICONDUCTOR GAS SENSORS
7.1. OVERVIEW
It was discovered decades ago that atoms and molecules interacting with semiconductor surfaces influence surface properties of semiconductors, such as conductivity and surface potential. Seiyama (1962) and Taguchi (1970) first applied the discovery to gas detection, by producing the first chemoresistive semiconductor gas sensors. Since then, semiconductor gas sensors have been widely used as domestic and industrial gas detectors for gas-leak alarm, process control, pollution control, etc.Compared to the organic (such as phenenthrene, polybenzimidole) and elemental (such as Si, Ge, GaAs, GaP) semiconductors, semiconductor oxides have been more successfully employed as sensing materials for the detection of different gases, such as CO, CO2, H2, alcohol, H2O, NH3, O2, NOx, etc. Both n-type (such as SnO2, TiO2 an ZnO) and p-type semiconductor oxides can be used as gas sensor materials. The most widely used semiconductor sensor material is SnO2. The increases of conductance of SnO2 caused by the surface reactions between pre-adsorbed surface oxygen species and reducing gases are used to detect the concentrations of reducing gases.
Many approaches have been attempted to modify the sensing properties of these semiconductor oxide gas sensors in order to achieve high sensitivity and selectivity. The enhancement of the sensing properties of the sensor materials have been mainly achieved by development of unique preparation methods to fabricate sensors (such as thin-film, thick-film, and sol-gel methods) and the addition of transition metals onto sensors. However, the progresses on this study are still not satisfactory till now. Let us discuss the phenomenon related to the operations of a semiconductor gas sensor.
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7.1.1 INTERACTION OF GAS WITH SURFACES:-
Two very important cases can be distinguished –
a) Calorimetric detection vs temperature rise produced by the heat of reaction on a catalytic surface.
b) Detection due to the change of electrical parameters, such as the change in electrical conductivity induced by adsorption or reaction of gases on the solid surfaces.
Catalytic effect plays an important role in the field of gas detection. Solid state gas sensors are directly related to the phenomenon of catalysis. Catalysis processes not only control the rate at which a chemical reaction approaches equilibrium (this considerably affects the response time in the case of gas detection) but also affect sensitivity & selectivity. The ideal catalyst is one which increases the rate of the gas surface interaction without itself becoming permanently affected by the reaction. Thus the response time will be fast and the process will be reversible & finally the sensor will possess three important properties i.e. speed, durability, reversibility.
Following factors are important in affecting catalytic reaction.
a) Transport of gases to the solid surface
b) Adsorption of gases on the solid surface
c) Reaction between the adsorbed species and/or with the solid surface
d) Desorption of the surface species & products away from the surface
e) Transport of the gaseous reactants and products away from the surface
As is well known, gas adsorption on surface occurs because the atoms or ions at the surface of the solid can not fully satisfy their valency or co-ordination requirements. This reads to a certain permanent force acting in word. Then, the adsorption of external species that happen to be in the neighborhood of the surface reduces the surface energy of the solid. One can distinguish two types of adsorption.
1) Physisorption:
Weak attraction followed by gas adsorption due to Vander walls forces .It is characterized by a low heat of adsorption.
2) chemisorption:
In the case of high surface energy, the gas may become adsorbed through an exchange of electron with the surface i.e. chemical bonds are formed.
In order to have successful gas sensors two main properties concerning the catalyst are necessary.
a) In the case where the detectivity of the sensor is due to the electrical conductivity shift, these charges must be proportional to the concentration of the detecting gas.
b) In the case of calorimetric detection, the heat of adsorption must be independent of the coverage. If these two properties are satisfied then the sensor may possess a linear response, which is one of the most important properties of a gas detection device. The linear response occurs especially in the range of low and not near the saturation level.
7.1.2 THE CHANGE IN WORK FUNCTION:-
The adsorption of atoms or molecules on a metal surface changes the distribution of charges & gives rise to changes in the work function. The formation of adsorbed negative ions or the ionization of atoms (transfer of electron into the surface) decreases/increases the work function respectively. Thus the monitoring of work function of the known metal surface under investigation can give important information concerning the nature of gas adsorption on the surface as well as information concerning the gas -surface interaction & the kinetics of the adsorption-absorption process. In fact, very often the work function change is directly related to surface coverage. Several experimental techniques have been used since 1940 for measuring work function changes upon chemisorptions, such as retarding potential vibrating capacitor & field emission.
7.2. Principles & operations of Semiconductor Sensors
The material of choice for semiconductor sensor is SnO2, though Ga2O3, WO3, ZnO, In2O3, Fe2O3 etc. are also employed. SnO2 is an oxygen deficient oxide and therefore is an n-type semiconductor. It crystallizes in the rutile structure and is by far the most studied and most successful semiconductor material for gas sensors. Such sensors often change their resistance by more than a factor of 100 upon exposure to a trace of reducing gases such as hydrogen, methane, ethanol, carbon monoxide and propane. Incidentally, the free electrons of n-type semiconductors like SnO2 are trapped by oxygen from the ambient by its electron affinity. Oxygen adsorbed on the surface of the grains extracts an electron to ionize into O- or O2- species, which increases the resistance of the sensor coating. Upon exposure to a reducing gas, the adsorbed oxygen species, being highly metastable, oxidize the reducing gas, releasing the trapped electron and subsequently lowering the resistance.
The amount of resistance change is proportional to the concentration of the reducing gas in the ambient, which is believed to be the dominant sensing mechanism of surface conductive gas sensors like SnO2. However, the concentration of charged oxygen species is limited to <1% r =" Ro" s =" (RA-RG" s =" RA" cs =" cosh[1−(x/L)]m/coshm," x="0," m=" L(k/Dk)1/2" title="Nanoparticles" world. Though humans have not yet been able to synthesize nanosensors successfully, predictions for their use mainly include various medicinal purposes and as gateways to building other nanoproducts, such as computer chips that work at the nanoscale and nanorobots. Presently, there are several ways proposed to make nanosensors, including top-down lithography, bottom-up assembly, and molecular self-assembly. A nanosensor probe carrying a laser beam penetrates a living cell to detect the presence of a product indicating that the cell has been exposed to a cancer-causing substance.
Medicinal uses of nanosensors mainly revolve around the potential of nanosensors to accurately identify particular cells or places in the body in need. By measuring changes in volume, concentration, displacement and velocity, gravitational, electrical, and magnetic forces, pressure or temperature of cells in a body, nanosensors may be able to distinguish between and recognize certain cells, most notably those of cancer, at the molecular level in order to deliver medicine or monitor development to specific places in the body. In addition, they may be able to detect macroscopic variations from outside the body and communicate these changes to other nanoproducts working within the body.
Currently, the most common mass-produced functioning nanosensors exist in the biological world as natural receptors of outside stimulation. For instance, sense of smell, especially in animals in which it is particularly strong, such as dogs, functions using receptors that sense nanosized molecules. Certain plants, too, use nanosensors to detect sunlight; various fish use nanosensors to detect minuscule vibrations in the surrounding water; and many insects detect sex pheromones using nanosensors.
There are currently several hypothesized ways to produce nanosensors. Top-down lithography is the manner in which most integrated circuits are now made. It involves starting out with a larger block of some material and carving out the desired form. These carved out devices, notably put to use in specific micro electromechanical systems used as micro sensors, generally only reach the micro size, but the most recent of these have begun to incorporate nanosized components.
Another way to produce nanosensors is through the bottom-up method, which involves assembling the sensors out of even more minuscule components, most likely individual atoms or molecules. This would involve moving atoms of a particular substance one by one into particular positions which, though it has been achieved in laboratory tests using tools such as atomic force microscopes, is still a significant difficulty, especially to do en masse, both for logistic reasons as well as economic ones. Most likely, this process would be used mainly for building starter molecules for self-assembling sensors.
Nanostructured materials present new opportunities for enhancing the properties and performance of gas sensors because of the much higher surface-to-bulk ratio in nanomaterials compared to coarse micro grained materials. In addition to processing nanostructured oxides, more fundamental work is needed to understand the role of nanostructured oxide materials on gas adsorption and conductivity.
Though nanosensor technology is a relatively new field, global projections for sales of products incorporating nanosensors range from $0.6 billion to $2.7 billion in the next three to four years. They will likely be included in most modern circuitry used in advanced computing systems, since their potential to provide the link between other forms of nanotechnology and the macroscopic world allows developers to fully exploit the potential of nanotechnology to miniaturize computer chips while vastly expanding their storage potential.
10. CONCLUSION
Gas sensor technology is an indispensable tool to create new technologies and new life-styles which are compatible with sustainable society. Further advancement of it is no doubt desired strongly worldwide. To enhance it, researchers would be requested to pay attention to a few more suggestions. Gas sensor technology is interdisciplinary indeed, so that collaborations among people working in broadly different disciplines, ranging from materials scientists to market developers, would be necessary to open new frontiers. Researchers should be well acquainted with the needs having emerged or newly emerging in industry and society. For this purpose, one should listen to opinions of users and market developers carefully. Importance of carrying out field test cannot be overstated when a new sensor device has been developed. It is important not only because it confirms the feasibility of the device in practice but also because it is a direct way to demonstrate the potentiality of the new frontier the device aims at opening. Finally, challenging spirit is a backbone of every successful researcher. I hope sincerely that gas sensor technology will be innovated to contribute more and more to the society in the future.
11.REFERENCES:-
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2. Oxide Materials for Development of Integrated Gas Sensors—A Comprehensive Review -G. Eranna, B. C. Joshi, D. P. Runthala, and R. P. Gupta, Critical Reviews in Solid State and Materials Sciences, vol.29:111–188, 2004
3. Materials and Processing Issues in Nanostructured Semiconductor Gas Sensors by Frederic Cosandey, Ganesh Skandan, and Amit Singhal
4. Sensor Technology Handbook-John Wilson
5. Gas sensors;Principles, operation & Developments-giorgio Sberveglieri
6. Semiconducting Oxides in Gas Sensing,A Sen, Article 8, Science & Culture,May-june 2005
7. Fabrication of Sn02-Based Semiconductor Gas Sensors form Combustible and Pollutant Gases-A. Khodadadi, S.S. Mohajerzadeh,A.M. Miri and Y. Mortazavi, The 1 2th international Conference on Microelectronics Tehran, Oct. 31- Nov. 2,2000
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tin dioxide-based methane sensors -S. Chakraborty, A. Sen , H.S.
Maiti Sensors and Actuators B 119 (2006) 431–434
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10. http://www.cpec.nus.edu.sg/myweb/newsletter/news4/development.html
11. www.grc.nasa.gov/WWW/SiC/discoveries.html
12. www.citytech.com
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14. www.sciencedaily.com
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