25-06-2013, 03:29 PM
SENSOR, AN APPLICATION OF ADVANCED CERAMICS
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Material Science:-
In the branch of material science we are study the relationship that exist in the structure & property of materials , as like – metals, ceramics, polymers.
Ceramics can be defined as:
Solid compounds that are formed by the application of heat, and sometimes heat and pressure, comprising at least two elements provided one of them is a non-metal or a non-metallic elemental solid. The other element(s) may be a metal(s) or another non-metallic elemental solid(s).
A somewhat simpler definition was given by Kingery who defined ceramics as- "the art and science of making and using solid articles, which have, as their essential component, and are composed in large part of inorganic non-metallic materials". In other words, what is neither a metal, a semiconductor nor a polymer is a ceramic.
General Characteristics of Ceramics:-
As a class, ceramics are hard, wear-resistant, brittle, prone to thermal shock, refractory, electrically and thermally in-sulative, intrinsically transparent. Ceramics nonmagnetic, chemically stable, and oxidation-resistant. As with all generalizations, there will be exceptions; some ceramics are electrically and thermally quite conductive, while others are even superconducting. An entire industry is based on the fact that some ceramics are magnetic. One of the main purposes of this book is to answer the question of why ceramics exhibit the properties they do. And while this goal will have to wait until later chapters, at this point it is worthwhile to list some of the applications for which ceramics have been or are being developed.
Fiber Optic Sensor:
Introduction
Advanced ceramics are valued for their hardness, high temperature strength, light weight and abrasion resistance, and hence find wide use in such applications as ballistic protective armor components, wear components of equipment in oil, gas, and mining operations, and high-performance engines. However due to the brittle nature, advanced ceramics are subjected to cracking upon impact during operation. Cracks, including invisible micro cracks, significantly degrade the strength of a ceramic component.
The traditional inspection of cracks in ceramics relies on visual inspection including the dye and the fluorescent penetrate methods. Recently nondestructive evaluation (NDE) methods such as X-ray and ultrasonic imaging have been applied to detect cracks in ceramics, but have limited sensitivities to micro-cracks. Both the visual and NDE inspection methods are laborious, time consuming, and more importantly, difficult to perform continuously during operation.
Optical fiber based sensing has received increasing attention over the last two decades for the purposes of structural health monitoring. Different sensing techniques have been developed to monitor specific parameters. Recently it has been reported that fiber Bragg grating (FBG) sensors were applied to detect cracks in carbon fiber reinforced polymer composites with a strain resolution of 1 ×10−6.
Sensing Principle of Fiber Optic Sensor
The distributed strain sensing capability of the Brillouin fiber optic sensor provides a potential tool for detecting cracks at unknown locations in materials and structures by measuring a strain profile along the entire length of a sensing fiber, rather than strains at discrete points. One class of the distributed Brillouin fiber optic sensor is based on the Brillouin loss technique whereby two counter-propagating laser beams, a pulsed Stokes beam and a continuous wave (CW) pump beam, exchange energy through an induced acoustic field. The interaction magnifies the pulsed Stokes beam at the expense of depleting the pump beam, which is then detected as a loss signal. The maximum depletion of the pump beam at a point along the fiber occurs when the frequency of the acoustic wave νB at that point matches the beat frequency of two laser beams, i.e., νp – νs = νB, where νp and νs are the frequencies of the pump and Stokes beams, respectively. The frequency of the acoustic wave, hereafter referred to as the Brillouin frequency shift, is related to the fiber properties and the laser wavelength. The sensing capability of Brillouin scattering arises from the dependence of the Brillouin frequency shift, νB, on the local acoustic velocity and refractive index in the fiber core glass, which has a linear temperature and strain dependence through
Ceramic Impedance Sensor:-
Michell Instrument’s portable dew-point meters use the Advanced Ceramic Moisture Sensor. The operation of this sensor depends on the dielectric property of water molecules absorbing onto an active porous insulating layer sandwiched between two layers of conductive material deposited on a ceramic substrate. Water has a very high dielectric compared to the dielectric of the active layer and the background of the carrier gas so it can be detected easily. The active layer is very thin – less than one micron and the porous top conductor that allows water molecules to penetrate into the active layer is less than 0.1micron thick. This allows the sensor to respond very rapidly to changes in the moisture surrounding it both when moisture decreases (drying) and increases in the sensor environment.
Oxygen Sensor:-
An oxygen sensor detects the amount of oxygen in a vehicle’s exhaust gases and sends a signal to the engine computer (ECM), which adjusts the air/fuel mixture to the optimal level. Too much oxygen in the exhaust gases indicates a lean mixture, which can cause performance problems, including misfire. Too little oxygen indicates a rich mixture, which wastes fuel and results in excess emissions. Either condition can shorten the life of the catalytic converter. Almost all gasoline powered vehicles newer than1986 have at least one oxygen sensor, and 1996and newer vehicles have at least two oxygen sensors. Not only are properly functioning oxygen sensors good for the environment, but they can save money in fuel costs, too- Replacing a worn-out oxygen sensor will do more than improve your vehicle’s performance and reduce harmful exhaust emissions. It can save over $100 a year in gasoline costs.
Thin Film Sensor:-
The fabrication of the thin film sensors is completed in a clean room to minimize possible contamination. The class 1000 clean room at the LeRC (NASA Lewis Research Center) contains state-of-the-art facility including several thin film sputter deposition and evaporation systems, wire bonding systems, etching systems, equipment for photolithography processes, and a surface profiler. The fabrication process of thin film sensor systems on a particular substrate material needs to be tailored to ensure good adhesion and no chemical interaction between the sensor and the substrate material. For an electrically conductive metal substrate, such as super alloy materials, a MCrAlY coating is first deposited onto the substrate by electron beam evaporation or by sputter deposition. M can represent Fe, Co, Ni, or a combination of Co and Ni. With heat treatment, this coating forms a stable, adherent, electrically insulating alumina layer. An additional layer of alumina is sputter deposited or electron beam evaporated onto the surface to fill any pinholes or cracks that may be present in the grown oxide. Electrically conductive ceramic materials such as silicon carbide are thermally oxidized to form a stable, adherent silicon dioxide layer which is followed by another layer of alumina of the thickness needed to obtain the required insulation resistance. The thickness of the thermally grown oxide and sputter deposited alumina layers are approximately 2-3μm and 5-8 mm, respectively. The sensors are fabricated onto the alumina layer; in the case of electrically insulating materials such as silicon nitride, aluminum oxide and mullite, the sensor is fabricated directly onto its surface. For those applications that require a protective overcoat, a coating of alumina is deposited either by sputtering or evaporation onto the sensor to a thickness of approximately 2-3 mm.
Conclusion:-
This paper (review article) has presented the use of the ceramics sensing technology and the first effort in applying this technology for various types of field of industry & also domestic purpose by advanced ceramics. In this review article -fiber optic sensors, temperature sensor, ion conducting sensor, thin-film sensor, oxygen sensor, micro sensor, ceramics-impendence sensor etc are discussed.
[attachment=56002]
Material Science:-
In the branch of material science we are study the relationship that exist in the structure & property of materials , as like – metals, ceramics, polymers.
Ceramics can be defined as:
Solid compounds that are formed by the application of heat, and sometimes heat and pressure, comprising at least two elements provided one of them is a non-metal or a non-metallic elemental solid. The other element(s) may be a metal(s) or another non-metallic elemental solid(s).
A somewhat simpler definition was given by Kingery who defined ceramics as- "the art and science of making and using solid articles, which have, as their essential component, and are composed in large part of inorganic non-metallic materials". In other words, what is neither a metal, a semiconductor nor a polymer is a ceramic.
General Characteristics of Ceramics:-
As a class, ceramics are hard, wear-resistant, brittle, prone to thermal shock, refractory, electrically and thermally in-sulative, intrinsically transparent. Ceramics nonmagnetic, chemically stable, and oxidation-resistant. As with all generalizations, there will be exceptions; some ceramics are electrically and thermally quite conductive, while others are even superconducting. An entire industry is based on the fact that some ceramics are magnetic. One of the main purposes of this book is to answer the question of why ceramics exhibit the properties they do. And while this goal will have to wait until later chapters, at this point it is worthwhile to list some of the applications for which ceramics have been or are being developed.
Fiber Optic Sensor:
Introduction
Advanced ceramics are valued for their hardness, high temperature strength, light weight and abrasion resistance, and hence find wide use in such applications as ballistic protective armor components, wear components of equipment in oil, gas, and mining operations, and high-performance engines. However due to the brittle nature, advanced ceramics are subjected to cracking upon impact during operation. Cracks, including invisible micro cracks, significantly degrade the strength of a ceramic component.
The traditional inspection of cracks in ceramics relies on visual inspection including the dye and the fluorescent penetrate methods. Recently nondestructive evaluation (NDE) methods such as X-ray and ultrasonic imaging have been applied to detect cracks in ceramics, but have limited sensitivities to micro-cracks. Both the visual and NDE inspection methods are laborious, time consuming, and more importantly, difficult to perform continuously during operation.
Optical fiber based sensing has received increasing attention over the last two decades for the purposes of structural health monitoring. Different sensing techniques have been developed to monitor specific parameters. Recently it has been reported that fiber Bragg grating (FBG) sensors were applied to detect cracks in carbon fiber reinforced polymer composites with a strain resolution of 1 ×10−6.
Sensing Principle of Fiber Optic Sensor
The distributed strain sensing capability of the Brillouin fiber optic sensor provides a potential tool for detecting cracks at unknown locations in materials and structures by measuring a strain profile along the entire length of a sensing fiber, rather than strains at discrete points. One class of the distributed Brillouin fiber optic sensor is based on the Brillouin loss technique whereby two counter-propagating laser beams, a pulsed Stokes beam and a continuous wave (CW) pump beam, exchange energy through an induced acoustic field. The interaction magnifies the pulsed Stokes beam at the expense of depleting the pump beam, which is then detected as a loss signal. The maximum depletion of the pump beam at a point along the fiber occurs when the frequency of the acoustic wave νB at that point matches the beat frequency of two laser beams, i.e., νp – νs = νB, where νp and νs are the frequencies of the pump and Stokes beams, respectively. The frequency of the acoustic wave, hereafter referred to as the Brillouin frequency shift, is related to the fiber properties and the laser wavelength. The sensing capability of Brillouin scattering arises from the dependence of the Brillouin frequency shift, νB, on the local acoustic velocity and refractive index in the fiber core glass, which has a linear temperature and strain dependence through
Ceramic Impedance Sensor:-
Michell Instrument’s portable dew-point meters use the Advanced Ceramic Moisture Sensor. The operation of this sensor depends on the dielectric property of water molecules absorbing onto an active porous insulating layer sandwiched between two layers of conductive material deposited on a ceramic substrate. Water has a very high dielectric compared to the dielectric of the active layer and the background of the carrier gas so it can be detected easily. The active layer is very thin – less than one micron and the porous top conductor that allows water molecules to penetrate into the active layer is less than 0.1micron thick. This allows the sensor to respond very rapidly to changes in the moisture surrounding it both when moisture decreases (drying) and increases in the sensor environment.
Oxygen Sensor:-
An oxygen sensor detects the amount of oxygen in a vehicle’s exhaust gases and sends a signal to the engine computer (ECM), which adjusts the air/fuel mixture to the optimal level. Too much oxygen in the exhaust gases indicates a lean mixture, which can cause performance problems, including misfire. Too little oxygen indicates a rich mixture, which wastes fuel and results in excess emissions. Either condition can shorten the life of the catalytic converter. Almost all gasoline powered vehicles newer than1986 have at least one oxygen sensor, and 1996and newer vehicles have at least two oxygen sensors. Not only are properly functioning oxygen sensors good for the environment, but they can save money in fuel costs, too- Replacing a worn-out oxygen sensor will do more than improve your vehicle’s performance and reduce harmful exhaust emissions. It can save over $100 a year in gasoline costs.
Thin Film Sensor:-
The fabrication of the thin film sensors is completed in a clean room to minimize possible contamination. The class 1000 clean room at the LeRC (NASA Lewis Research Center) contains state-of-the-art facility including several thin film sputter deposition and evaporation systems, wire bonding systems, etching systems, equipment for photolithography processes, and a surface profiler. The fabrication process of thin film sensor systems on a particular substrate material needs to be tailored to ensure good adhesion and no chemical interaction between the sensor and the substrate material. For an electrically conductive metal substrate, such as super alloy materials, a MCrAlY coating is first deposited onto the substrate by electron beam evaporation or by sputter deposition. M can represent Fe, Co, Ni, or a combination of Co and Ni. With heat treatment, this coating forms a stable, adherent, electrically insulating alumina layer. An additional layer of alumina is sputter deposited or electron beam evaporated onto the surface to fill any pinholes or cracks that may be present in the grown oxide. Electrically conductive ceramic materials such as silicon carbide are thermally oxidized to form a stable, adherent silicon dioxide layer which is followed by another layer of alumina of the thickness needed to obtain the required insulation resistance. The thickness of the thermally grown oxide and sputter deposited alumina layers are approximately 2-3μm and 5-8 mm, respectively. The sensors are fabricated onto the alumina layer; in the case of electrically insulating materials such as silicon nitride, aluminum oxide and mullite, the sensor is fabricated directly onto its surface. For those applications that require a protective overcoat, a coating of alumina is deposited either by sputtering or evaporation onto the sensor to a thickness of approximately 2-3 mm.
Conclusion:-
This paper (review article) has presented the use of the ceramics sensing technology and the first effort in applying this technology for various types of field of industry & also domestic purpose by advanced ceramics. In this review article -fiber optic sensors, temperature sensor, ion conducting sensor, thin-film sensor, oxygen sensor, micro sensor, ceramics-impendence sensor etc are discussed.