A gas sensor is a sensor for detecting gaseous substances and is thus a chemosensor. The chemical information in the ambient air is converted by the gas sensor into a usable electrical signal.
- 3.1 issue of chemical sensors
- 3.2 Overview of measuring principles
- 3.3 Electrical Principles 3.3.1 Resistive, chemotherapy Resistor
- 3.3.2 Capacitive
- 3.3.3 Potentiometric
- 3.3.4 Amperometric
- 3.3.5 Thermal 220.127.116.11 Thermochemically
- 18.104.22.168 Thermal- physical
- 3.4.1 Heat tint effect 22.214.171.124 Principle of operation
- 126.96.36.199 types
- 188.8.131.52 advantages
- 184.108.40.206 disadvantages
- 220.127.116.11 Principle of operation
- 18.104.22.168 types
- 22.214.171.124 advantages
- 126.96.36.199 disadvantages
- 3.5.1 Sensor layer optimization
- 3.5.2 filter
- 3.5.3 Modes 188.8.131.52 Single sensor, constant operation
- 184.108.40.206 Sensor Array
- 220.127.116.11 Virtual Multi-Sensor
Uses and economic importance
Areas of application and sample applications
- Security Technology: Explosion protection ( methane and carbon monoxide detection in mines, hydrogen detection at fuel cells), detection of gas leak ( natural gas supply, LPG ) poisoning safety ( Hardcover carbon monoxide and hydrogen sulfide monitoring), leak detection (monitoring of chemical camps with volatile organic components, solvents and coolants ), fire alarm (fire gas detection intelligent detectors), drug test chemical ( breath test for the traffic ) detection. Warfare agents ( explosives, poisonous gas). Use in gas Protection Units of ATMs.
- Emission measurement ( motor control over the oxygen sensor, vehicle diagnostics, measurement of gaseous emissions in urban transport)
- Comfort: air quality indoors (automatic ventilation flap in the car, building management systems, fume hood )
- Quality assurance: Leakage detection (solvent detection), process monitoring ( lambda probe)
There are different priorities worldwide. The carbon monoxide detection is very common in the U.S. and increasingly in Europe, while gas sensors are used primarily for the detection of natural gas ( heating systems ) in Asia. The gas sensor in the automotive industry ( air recirculation flap control ) was initially found only in Europe, now worldwide. The application in industrial safety engineering and building management ( air quality ) is primarily used in North America, Africa and Europe, and increasingly in Asia. In safety technology for public facilities, the area of application is mainly the United States. The global market for gas sensing and gas analysis systems is estimated at about 1.5 billion dollars per year, with Germany is in third place in the ranking of the world market leader in the manufacture of sensors behind the U.S. and Japan.
Requirements for gas sensors
- High selectivity: restriction of the sensor response to a target substance
- High sensitivity: detection of gas concentration desired interval, either a few percent by volume to a few ppm
- Stability: chemical, electrical, mechanical
- Long service life: 10 years and more
- For fast applications: small thermal and chemical time constants
- Low power requirement
- Simple calibration capability for the widest possible Kalibrierabständen
- Low tolerances
- Ease of use
- Low price
Depending on the application, these requirements are weighted differently.
Measurement principles and procedures
Problem of chemical sensors
The field of chemical sensors is relatively young. While sensors are waterproof and airtight for measuring physical quantities such as temperature, pressure and acceleration, a chemical sensor as the gas sensor in direct interaction with its environment is. This makes it even more vulnerable to poisoning ( environmental influences that the sensor can be insensitive ), cross sensitivity ( substances other than the target component that produce a sensor signal ), corrosion, drift and aging.
Overview of measuring principles
There are many ways of gas detection, which differ according to the type of conversion of the chemical information in an electronic. Ultimately must always be an electronic signal to be processed by the subsequent electronics can. But until that happens, one may implement complex takes place through various physical quantities and sensor principles, a selection of which is presented here. Depending on the application ( Which gas to be detected? What concentrations are expected How quickly must be done to measure what are the environmental conditions to which is exposed to the gas sensor How much does a sensor must be How is its commercial availability? What maintenance intervals are permissible? etc. ) must be selected a suitable sensor principle.
- Physical Measurement Methods
Utilization of molecular properties for detection: molecular weight, diffusion behavior, molecular structure (magnetic properties, for example paramagnetism when oxygen sensor), molecular stability ( binding energy ) and molecular mobility.
- Chemical methods of measurement
Utilization of chemical properties such as reactivity, oxidizability and reducibility.
Resistive, chemotherapy Resistor
In resistive principles the measured gas or gas mixture directly affects the conductivity of a gas-sensitive sensor layer. This change in resistance is used as a benchmark. Examples:
- Inorganic metal oxide semiconductor ( MOX)
- Organic phthalocyanine
- Conductive polymer
Measured variable here is the capacitance of a capacitor is influenced by a gas- sensitive dielectric. Examples:
- Polymer sensors for measuring moisture
When potentiometric sensor itself a voltage generated by the sensor, which is directly measurable. Examples:
- Solid-state ion conductor ( lambda probe)
- Chemotherapeutic transistor ( polymer field-effect transistor )
An amperometric sensor provides a measurable current. example:
- Electrochemical cell ( Clark electrode, fuel cell)
- Flame ionization detector
- Photoionization detector
In thermal principles, the increase in temperature is measured directly and used, the thermal conductivity of the gas as a measurement due to a chemical reaction on the sensor surface.
On the sensor surface, chemical reactions take place, in which energy is delivered in the form of heat. This increase in temperature is measured. Examples:
- Catalytic sensor (eg Pellistor )
Direct measurement of thermal conductivity of the gas atmosphere. Examples:
- Thermal conductivity detector
In gravimetric sensors a mass change is measured. Gas molecules settle for example on the surface of a quartz crystal from and thereby change its resonance frequency. Such sensors operate on the principle of a piezoelectric sensor. Examples:
- Quartz crystal microbalance
- Surface acoustic wave
Also in optical gas sensors are physical principles, in which the optical properties of a sample chamber filled with gas to be characterized. example:
- Refractive index
- Absorption spectrum in the infrared range, IR spectroscopy
- Intensity, luminescence
Biochemical gas sensors follow the biological model of the implementation of certain substances or groups of substances. Use several of the above principles to the conversion of the signal. See main article biosensor.
Modes of common sensor principles
Heat tint effect
Principle of operation
In catalytic sensors, the sensor effect is caused by the combustion of adsorbed gases on the surface of a catalyst. In order for a chemical reaction to occur, an activation energy is required. The catalyst causes a reduction of the activation energy. Intermediate states of the reaction, which would not be possible without him form on its surface. Gas molecules are separated ( homolytic dissociation) and are the reactants on the catalyst surface. This process takes place in each vehicle catalyst. Here, for example, a raw material (nitrogen monoxide, NO) responsive to two reaction products (oxygen and nitrogen). For gas sensors, however, react mostly two starting materials for just one product, such as carbon monoxide and oxygen to carbon dioxide. The catalyst itself is not involved in the reaction, in the ideal case, is unchanged. Catalyst materials are for example, noble metals such as platinum ( Pt) or palladium ( Pd) as well as metal oxides such as manganese oxide or copper ( II) oxide.
The catalyst does not affect the energy balance of the reaction but only influences the rate at which the reaction proceeds. The rate of reaction of the starting materials to the reaction product depend on the concentration of the reactants and the temperature.
The reaction takes place only on the catalyst surface of catalytically active surface sites ( active sites ). Adsorption of Gaseous substance is exothermic here, so with the release of heat energy and the desorption endothermic ( absorbing heat energy from the environment ). If the catalyst is a substance available, which no longer desorbed from the surface, because there is not enough desorption energy is available, the catalyst is poisoned by this substance, since the active sites are blocked. Catalyst poisons include sulfur compounds (SO2, H2S) or silicone-containing substances from oil or detergents (eg, hexamethyldisiloxane (HMDS, plasticizers such as cable insulation ) ).
Thanks to a chemical reaction, a particle of a state having a higher energy state to a lower energy over, the energy difference can be dispensed in the form of heat. This results in the static case, an increase in temperature of the system. That is, the measurement of the temperature can thus be used to determine the reaction rate, which in turn is dependent on the concentration of the reactants present. This effect is utilized in so-called pellistors by the heat of reaction is detected by measuring the change in resistance of a platinum wire, due to the elevated temperature. The heating wire simultaneously serves as a temperature sensor ( RTD ). By pellistors flammable gases leave (eg methane, propane, butane, hydrogen, ) prove. Non-combustible gases are not detected by a Pellistor A., because the effect is too small. Therefore, the Pellistor is primarily for high gas concentrations in the low percent range and is often used to enforce limits ( explosion limit ), for example, used in explosion protection.
The classic design of a pellistor is a resistance heater embedded in the form of a coiled platinum wire in a ceramic bead. This bead is coated with a catalytically active substance. The reaction heat penetrates through the surface of the pearl to the platinum wire. By heating this changes its resistance, which serves as a benchmark. See explosimeters. To the power consumption required for heating will reduce underway to also micromechanically produce pellistors in silicon technology. Here, a silicon chip is processed so as carrier for the sensor layer is a very thin (a few microns) membrane remains. The main advantages of this membrane structure are: low power demand, since because of the thin membrane (i.e. high thermal resistance ), less energy is required to reach the operating temperature. Mass production in batch process.
- Linear sensor response as a function of the gas concentration
- Relatively easy to calibrate ( one-point calibration)
- Low sensitivity in the percent range, since relatively large gas concentrations are necessary for a sufficiently large amounts of heat.
- Low selectivity: Each gas burns on the catalyst surface of the pellistor and a measurable heat of reaction causes, is registered by the increase in resistance as a pellistor, so that selective determination of the type of gas is difficult.
Metal-oxide semiconductor gas sensors ( MOX)
Principle of operation
Some metallic oxides, such as tin (IV ) oxide ( SnO2 ) change under gas influence its conductivity. Other materials are zinc oxide, titanium dioxide, or organic semiconductor materials such as MePTCDI. In tin dioxide sensors oxygen vacancies in the crystal lattice act as an n -type doping of the material. This effect is metal oxide semiconductor gas sensors ( MOX) as a basis. Oxygen molecules from the ambient air adsorb on the sensor surface. This causes oxygen ions, which in turn can react irreversibly with a combustible gas. For example, the molecules make a reducing gas to the surface of the semiconductor, to react with the adsorbed oxygen. An example is the reaction of carbon monoxide to carbon dioxide. Due to the energetic position of the adsorption sites on the Zinndioxidoberfläche ( in the band model below the Fermi level ), the oxygen takes electrons from the semiconductor interior and is thus negatively charged. Characterized ( SnO2 ) is reduced in case of an n-type semiconductor, the carrier density, it is a depletion zone of electrons and the conductivity in the boundary zone is lowered. In the band model is called a band bending. The energetic position of the reaction product is above the Fermi level and not occupied, thereby again, an electron is emitted to the semiconductor, the reaction product is desorbed and the conductivity increases again. Is formed as a balance between adsorption and desorption of oxygen, carbon monoxide and carbon dioxide. This leads to a change in the oxygen surface coverage of the sensor, whereby the amount of band bending ( energy barrier ) and thus changes the conductivity of the sensor, resulting in a change in resistance is macroscopically measured.
Reducing gases such as carbon monoxide and hydrogen in the n- HL cause an increase in the conductivity, while oxidizing gases reduce the conductivity. The amount of change in the conductivity is dependent on the gas concentration.
The above consideration applies for single crystal HL- material. As described is the effect of the conductivity increase or reduction in the marginal zone, the grain boundary of the crystal. To increase the sensitivity of many of these grain boundaries are needed, which is why one attempted in the manufacture of sensor to provide the HL- material, a polycrystalline structure as possible. In order that as many gas molecules reach the grain boundaries, the surface also should be as porous. The electron flow through the sensor is then dominated by the energy barrier at the grain boundaries. The height of the energy barrier is in turn dependent on the gas concentration, so that experimental results in the following context:
G: conductance, G0: Grundleitwert, c: gas concentration, r: empirically determined exponent
This relationship, however, is extremely simple and is valid only under controlled laboratory conditions for a gas. The complex chemical reaction mechanisms that take place on the sensor still the subject of current research.
For the manufacture of the sensitive layers of two technologies most commonly used: thin-film and thick-film technology. In thin-film technology layer thicknesses are usually in the range of 10 nm to 5 microns, which can be applied by physical or chemical methods such as thermal evaporation, sputtering or chemical vapor deposition. Thick film technology called coating processes by which layers between 10 microns and 80 microns are produced. An example is the screen printing method in which a paste-like mass with a blade through a stencil on the sensor carrier material ( substrate) is applied. This preparation method is used to produce porous layers as possible. The porosity increases the surface area of the sensor material, which passes over the gas to the grain boundaries, and thereby the sensitivity is increased. Just as there are different methods for producing the sensing layer, there are various technologies for the sensor substrate. Traditional construction is to use a ceramic substrate, on which the gas-sensitive metal oxide layer is applied. In comparison, there are also MOX sensors with microstructured structure in Si technology with membrane structure. The ceramic substrate is several hundred microns thick, and requires about 5000 times as compared with the thinner silicon membrane a much higher heating power ( W ≈ 2 with ceramic substrates, ≈ 80 mW in membrane structures). The membrane is usually made of silicon dioxide or silicon nitride and has a low heat capacity and a high heat resistance. Characterized the heating power introduced by the heater heats only the quasi membrane while the chip frame almost remains at ambient temperature. The structure in silicon technology thus reduces the required heating capacity and enables mass production in batch process.
- Example: The Taguchi sensor
The most famous sensor is developed by N. Taguchi " Figaro sensor " on tin oxide. The TGS 813 is especially used as proof of natural gas, methane gas, the TGS 822 is designed to detect alcohol, ammonia, etc. Already in the year 1988, for example, 400,000 shares of the sensor TGS 813 sold ( good evidence for natural gas and methane). The sensitivity of the Taguchi sensor is critically dependent on the average grain size of the tin oxide and this should not exceed a grain size of 10 nm. In very dry air, the sensitivity of the TGS822 is very limited on ethanol. The change in voltage drops sharply. In the area of 30-60 % humidity of the TGS822 ethanol even to changes in humidity, however, reacts only very marginal. If only one gas is present, the provisions of the relative concentration can be determined by calibration.
- Inexpensive mass production by
- High sensitivity in the ppm range
- Long life (depending on type (classic, micromechanically ) )
- Non-linear sensor response as a function of the gas concentration
- High drift and aging (depending also by design: classic, micro- mechanical)
- Calibration due to non- linearity difficult
- Cross-sensitivities eg humidity
- Low selectivity
- High energy consumption due to required operating temperature (1-4 W) ( reduced by miniaturization and thin-film technology )
Modes and increase selectivity
Most Funktionsprinzien of gas sensors are usually very broad band, that is, the sensor responds to a lot of various substances from the environment alike. The aim of the manufacturer but it is to produce highly selective sensors.
Sensor layer optimization
Selectivity is a possibility to produce the targeted sensor layer optimization for a target component or group of substances, for example, by the addition of catalyst materials.
Another method for increasing the selectivity is to use filters. These are mounted in front of the sensitive layer in the sensor housing and filter unwanted components ( cross- sensitivity).
Single sensor, constant operation
The operating temperature of heat tinting or MOX sensors is between 200 ° C and 500 ° C. A single sensor is operated when the measurement problem is simple and, for example, only one group of gases to be detected (eg alarm threshold of combustible gases using a catalytic bead ). The signal processing and electronic circuitry here is often a bridge. To compensate for the influence of the ambient temperature and humidity twin sensors are usually housed in a casing, one of which was passivated.
In more complex problems, such as the analysis of gas mixtures, or detecting a target gas in front of a dominant noise background requires a substantially higher cost intelligent mode and signal processing using multivariate analysis ( principal component analysis, discriminant analysis, artificial neural network, an electronic nose ).
If different types of sensors connected together to form a sensor array, the individual sensors have different sensitivities for a particular gas. This characteristic signal patterns are generated. These arrays have the disadvantage that they are expensive, aging of the sensors to different degrees and thus subject to different drifts.
The chemical reactions which occur on the surface of a gas sensor, are dependent on the temperature so that the properties of a gas sensor of the operating temperature are dependent. So it makes sense to operate a single sensor at different operating temperatures. By a suitable temperature modulation (temperature cycle, T cycle ) is obtained from a single sensor, a virtual multi-sensor, depending on the operating temperature shows different behavior for the same gas supply and thereby enables a signal evaluation with multivariate analysis. A distinction (eg, sinusoidal) and discrete continuous temperature between cycles. Discrete temperature cycles allow rapid evaluation of seconds and allow pulsed operation, that is, intervals between the cycles, which is power efficient and is an important factor for battery-operated systems. Such short T- cycles are made possible by micro-mechanically textured substrates with membrane structure. The time constant for the thermal transient to the target temperature is such a membrane - sensors in the range of about 20 msec, while it is several seconds for sensors with a thick ceramic substrate. Due to the micro structure of the sensors and the resulting given small thermal mass (membrane) cycle times of 50 ° C to 400 ° C. in ten temperature levels are also possible in a few seconds. Thus, the gas detection is very fast, which is, for example, early warning systems is crucial. In the design of temperature cycles is to make sure that the sensor response of HL- gas sensors consists of two overlapping effects at a temperature jump in the presence of reducing or oxidizing gases. One hand, the reaction of the sensitive layer of the temperature change on the other hand, the reaction of the film to the actual gas supply and the adjustment of a new state of equilibrium on the sensor surface. The chemical reaction at a temperature level A., running much more slowly than from the rapid thermal transient response, whereby the minimal step length is determined by the chemical reaction.
A new approach for sensors based on so-called Mikrocantilevern. It involves tiny spikes, such as those used in atomic force microscopes. They are coated with a material that specifically binds the analyte. Cantilever to vibrate like a spring. If additional analyte molecules bound to the mass of the micro-cantilever and thus the frequency at which it vibrates and is taken as a measure of change. A research group coated cantilever made of silicon with a three-dimensionally ordered layer of titanium dioxide nanotubes. Titanium dioxide can bind substances well containing nitro groups, which for example is characteristic of TNT and other explosives. On a cantilever approximately 500,000 of the nanotubes can be accommodated. The sensor was able to be proven concentrations of TNT in air of less than ppt within 3 minutes. A practical use of these sensors for selective detection system for explosives or other gases is still pending.
Another method is based on so-called gold Mesoflowers, about 4 microns to gold particles, coated with silica, and serve as a carrier for tiny silver clusters. They are embedded in a protein ( albumin). When irradiated with light of a suitable wavelength, the silver clusters luminesce red. The gold increases the fluorescence. If a TNT - containing solution given up, they react with the amino groups of albumin to a Meisenheimer complex. Thereby, the red lamps, the silver cluster is extinguished. Already a concentration of 1 ppb TNT deletes the lights. In combination with surface-enhanced Raman scattering ( SERS, surface -enhanced Raman scattering ) detection limits can be achieved up to the zeptomoles range ( 10-21 mol).