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Studies of MISiC-FET sensors for car exhaust gas monitoring
Linköping University, Department of Physics, Chemistry and Biology, Applied Physics . Linköping University, The Institute of Technology.
2005 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The increasing size of the car fleet makes it important to find ways of lowering the amounts of pollutants from each individual diesel or gasoline engine to almost zero levels. The pollutants from these engines predominantly originate from emissions at cold start, in the case when gasoline is utilized, and high NOx emissions and particulates from diesel engines.

The cold start emissions from gasoline vehicles are primarily due to a high light-off time for the catalytic converter. Another reason is the inability to quickly heat the sensor used for controlling the air-to-fuel ratio in the exhausts, also called the lambda value, which is required to be in a particular range for the catalytic converter to work properly. This problem may be solved utilizing another, more robust sensor for this purpose.

One way of treating the high NOx levels from diesel engines is to introduce ammonia in the exhausts and let it react with the NOx in a special catalytic converter to form nitrogen gas and water, which is called SCR (selective catalytic reduction). However, in order to make this system reduce NOx efficiently enough for meeting future legislations, closed loop control is required. To realize this type of system an NOx or ammonia sensor is needed.

This thesis presents the efforts made to test the SiC-based field effect sensor device both as a cold start lambda sensor for gasoline engines and as an NH3 sensor for SCR systems in diesel engines.

The MISiC (metal insulator silicon carbide) lambda sensor has proven to be both sensitive and selective to lambda, and its properties have been studied in lambda stairs both in gasoline engine exhausts and in the laboratory. There is, however, a small cross-sensitivity to CO. The influence of metal gate restructuring on the linearity of the sensor has also been investigated. The metal tends to form islands by time, which decreases the catalytic activity and thereby gives the sensor, which is binary when fresh, a linear behavior. Successful attempts to prevent the restructuring through depositing a protective layer of insulator on top of the metal were made. The influence of increasing the catalytic activity in the measurement cell was also studied. It was concluded that the location of the binary switch point of MISiC lambda sensors could be moved towards the stoichiometric value if the consumption of gases in the measurement cell was increased.

The MISiC NH3 sensor for SCR systems has been shown to be highly sensitive to ammonia both in laboratory and diesel engine measurements. The influence of other diesel exhaust gas components, such as NOx, water or N2O has been found to be low. In order to make the ammonia sensor more long-term stable experiments on samples with different types of co-sputtered Pt or Ir/SiO2 gas-sensitive layers were performed. These samples turned out to be sensitive to NH3 even though they were dense and NH3 detection normally requires porous films.

The speed of response for both sensor types has been found to be fast enough for closed loop control in each application.

Place, publisher, year, edition, pages
Institutionen för fysik, kemi och biologi , 2005.
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 931
Series
Keyword [en]
field effect sensor, gas detection, selective catalytic reduction, lambda, cold start, ammonia, silicon carbide, engine exhaust
National Category
Physical Sciences
Identifiers
URN: urn:nbn:se:liu:diva-4674ISBN: 91-85297-61-5 (print)OAI: oai:DiVA.org:liu-4674DiVA: diva2:20684
Public defence
2005-04-22, Hörsal Planck, Fysikhuset, Campus Valla, Linköpings univeristet, Linköping, 10:15 (English)
Opponent
Supervisors
Note
On the day of the ublic defence of the doctoral thesis, the status of article IV was: accepted, article V was: submitted and article VII was: manuscript.Available from: 2005-11-10 Created: 2005-11-10 Last updated: 2014-01-09
List of papers
1. Using a MISiCFET device as a cold start sensor
Open this publication in new window or tab >>Using a MISiCFET device as a cold start sensor
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2003 (English)In: Sensors and actuators. B, Chemical, ISSN 0925-4005, E-ISSN 1873-3077, Vol. 63, no 1-3, 295-303 p.Article in journal (Refereed) Published
Abstract [en]

As a consequence of the formation of water droplets in the car engine at cold start, the fragile ZrO2 λ sensor cannot be heated until the engine is sufficiently warm. A possibility to shorten the time before closed loop λ control would decrease the exhaust emission. As a solution to this problem, the metal insulator silicon carbide field effect transistor (MISiCFET) sensor, which presumably is more thermo shock resistant than the ZrO2 sensor, could be used at cold start. The requirements for a cold start sensor are, among others, sensitivity to λ (air to fuel ratio) close to stochiometry, selectivity to λ and high speed of response. In this communication, the possibility of using the MISiCFET sensor at cold start is treated. The sensor consists of a SiC based MOSFET device with a buried channel design and a catalytic gate metal of 10 nm TaSix and 100 nm Pt. The response depends linearly on λ at 500 °C. The sensitivity of the device has been tested both in artificial atmospheres and in an engine. Two-level factorial designed experiments showed a high selectivity to λ compared to other gases such as CO, hydrocarbons, NOx and H2. The response time was found to be <10 ms at 500 °C when changing from an oxidizing to a reducing atmosphere. The MISiCFET sensor response showed interesting differences in λ stairs when the λ-value was varied by changing the oxygen, hydrogen or CO concentration. The results show that the MISiCFET sensor is a promising choice as a future cold start sensor.

Keyword
Field effect device, Gas sensor, Cold start, Exhaust gases, Silicon carbide
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-13401 (URN)10.1016/S0925-4005(03)00227-2 (DOI)
Available from: 2005-11-10 Created: 2005-11-10 Last updated: 2017-12-13
2. The speed of response of MISiCFET devices
Open this publication in new window or tab >>The speed of response of MISiCFET devices
2003 (English)In: Sensors and Actuators B: Chemical, ISSN 0925-4005, Vol. 93, no 1-3, 286-294 p.Article in journal (Refereed) Published
Abstract [en]

The metal oxide silicon carbide field effect transistor (MISiCFET) sensor has several possible car engine applications, such as an ammonia sensor in selective catalytic reduction (SCR) systems or as a lambda-sensitive device for enhancing catalytic converter efficiency. Both these applications involve closed loop control of the engine and thereby require fast sensors, that is why it is important to investigate the speed of response of the devices. The sensor consists of a SiC-based MOSFET device with a buried channel design and a catalytic gate metal, which makes it sensitive to a wide range of different gases. The selectivity and sensitivity of the sensor to a specific gas depends mainly on the choice of gate metal, its structure and the operating temperature. In this presentation, the speed of response of MISiCFET devices with many different gate metals at several operating temperatures are compared. The tests have been performed in the laboratory using the moving gas outlet (MGO) equipment. The equipment allows two gas outlets to move back and forth under the sensor, which makes it possible to change the atmosphere surrounding the sensor from synthetic air to the test gas quickly. The method is verified by changing the temperature of the device and frequency of the moving gas outlets. The test gas is either ammonia or hydrogen. The time constant of the sensors is shown to be very small; <100 ms when exposing a 25 nm porous Pt sensor to ammonia at 300 °C and <10 ms for a 10 nm TaSix 100 nm Pt device exposed to hydrogen. The temperature is found to have a large influence on the speed of response. The results show that the speed of response is well beyond the current requirements for use in both SCR and lambda control systems, respectively.

Keyword
Response time, Chemical sensor, Field effect device, Exhaust, Silicon carbide
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-13402 (URN)10.1016/S0925-4005(03)00228-4 (DOI)
Available from: 2005-11-10 Created: 2005-11-10 Last updated: 2014-01-09
3. Dependence of Pt gate restructuring on the linearity of SiC Field Effect Transistor lambda sensors
Open this publication in new window or tab >>Dependence of Pt gate restructuring on the linearity of SiC Field Effect Transistor lambda sensors
2003 (English)In: Sensor Letters, ISSN 1546-198X, Vol. 1, no 1, 37-41 p.Article in journal (Refereed) Published
Abstract [en]

To achieve an efficient conversion of pollutants from gasoline-driven cars, the lambda value, or the air-to-fuel ratio, of the exhausts that reach the catalytic converter is required to be close to 1. The composition of the exhaust gases is normally regulated with the use of a zirconia lambda sensor. However, at cold start another, more robust sensor is required, and a SiC-based field effect transistor sensor is currently being developed for this purpose. The SiC field effect transistor sensor has previously been shown to respond in either a linear or a binary fashion to changes in the lambda value, depending on parameters such as the choice of operation temperature and the area of catalytic metal. Here it is shown that the linear behavior of the sensor may appear as a result of the restructuring of the thick Pt film, which is working as its sensitive layer, when exposed to intermittent pulses of reducing and oxidizing gas mixtures. Sensors that have been used only for a short while have a continuous film and show a binary behavior, whereas samples that have been run for a longer time have seriously restructured films and show a linear behavior. A connection between the linear behavior and the decreased catalytic activity of the used films in comparison with fresh samples with a binary behavior is discussed. Some alternative methods for preventing the restructuring of the metal are suggested.

National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-13403 (URN)10.1166/sl.2003.011 (DOI)
Available from: 2005-11-10 Created: 2005-11-10 Last updated: 2014-01-09
4. Using a MISiC-FET sensor for detecting NH3 in SCR systems
Open this publication in new window or tab >>Using a MISiC-FET sensor for detecting NH3 in SCR systems
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2005 (English)In: IEEE Sensors Journal, ISSN 1530-437X, E-ISSN 1558-1748, Vol. 5, no 5, 1099-1105 p.Article in journal (Refereed) Published
Abstract [en]

One way to decrease the emitted levels of NOx from diesel engines is to add NH3 in the form of urea to the exhausts after combustion. NH3 will react with NOx in the catalytic converter to form N2 and water, which is called selective catalytic reduction (SCR). The amount of NH3 added may be regulated through closed-loop control by using an NH3 sensor. The metal-insulator silicon-carbide field-effect transistor (MISiC-FET) sensor has previously been tested for this application and has been shown to be sensitive to NH3. Here, the sensors have been further studied in engine SCR systems. Tests on the cross sensitivity to N2O and NO2, and studies concerning the influence of water vapor have been performed in the laboratory. The difference between Ir and Pt films, with regard to catalytic activity, has also been investigated. The sensors were found to be sensitive to NH3 in diesel engine exhausts. The addition of urea was computer controlled, which made it possible to add NH3 in a stair-like fashion to the system and detect it with the MISiC-FET sensors. The presence of water vapor was shown to have the largest effect on the sensors at low levels and the NH3 response was slightly decreased by a background level of NO2.

Keyword
Ammonia, field-effect transistor (FET) sensor, iridium, platinum, selective catalytic reduction (SCR)
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-13408 (URN)10.1109/JSEN.2005.854489 (DOI)
Available from: 2005-11-10 Created: 2005-11-10 Last updated: 2017-12-13
5. The influence of catalytic activity on the phase transition governed binary switch point of MISiC-FET lambda sensors
Open this publication in new window or tab >>The influence of catalytic activity on the phase transition governed binary switch point of MISiC-FET lambda sensors
2006 (English)In: Applied Surface Science, ISSN 1530-437X, Vol. 252, no 20, 7473-7486 p.Article in journal (Refereed) Published
Abstract [en]

A metal insulator silicon carbide field effect transistor (MISiC-FET) sensor with a catalytic metal gate is currently under development for detecting the lambda value, or air-to-fuel ratio, of gasoline exhausts. It has been noticed that a change from a low to a high signal level of the sensor occurs at a lambda value above 1.00, which is an oxidizing atmosphere. The exact location of the switch point depends both on the kind of gas and gas concentrations chosen to obtain a specific lambda value. The switch point would rather have been expected at 1.00, which is at stoichiometry, irrespective of the composition of the gas mixture. The origin of this phenomenon is studied here by exposing the sensor to lambda stairs while changing different operating parameters. An increase in catalytic activity has been observed to move the switch point of the device towards a lambda value of 1.00. A similar effect is achieved when decreasing the flow or increasing the temperature of operation of the device. The behavior is explained through the introduction of mass transport limitations in the measurement cell, and the difference in diffusion constants and sticking coefficients among the gases when reaction limitation prevails.

Keyword
MISiC-FET; Catalytic activity; Binary switch point; Phase transition; Lambda
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-13409 (URN)10.1016/j.apsusc.2005.09.003 (DOI)
Available from: 2005-11-10 Created: 2005-11-10 Last updated: 2014-01-09
6. Co-sputtered metal and SiO2 layers for use in thick-film MISiC NH3 sensors
Open this publication in new window or tab >>Co-sputtered metal and SiO2 layers for use in thick-film MISiC NH3 sensors
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2006 (English)In: IEEE Sensors Journal, ISSN 1530-437X, Vol. 6, no 4, 887-897 p.Article in journal (Refereed) Published
Abstract [en]

High-temperature metal-insulator-silicon-carbide (MISiC) sensors are currently under development for use as NH3 sensors in selective-catalytic-reduction (SCR) systems in diesel engines or non-SCR (NSCR) systems in boilers. The detection of NH3 by these sensors requires the presence of triple points where the gas, the metal, and the insulator meet. These triple points have traditionally been located at the interface between the insulator and a porous metal. However, to facilitate the long-term stability of the devices when used in a harsh environment, a nonporous gate material would be preferred. Here, the behavior of the samples where such triple points have been introduced in a dense film through cosputtering of the insulator (SiO 2), and either Pt or Ir is studied. The NH3 sensitivity of the materials was found to be in accordance with the earlier investigations on Si-based samples with cosputtered gate materials. Several metal-to-insulator ratios for each of the metals Pt and Ir were studied. The sensitivity of the layers as well as their selectivity to different concentrations of NH3 at temperatures ranging from 150 degC to 450 degC was investigated. The films containing 60%-70% Pt or Ir were found to give a high sensitivity toward NH3. These samples were shown to be sensitive also to propylene and H2 but were rather insensitive to NO and CO.

Keyword
Ammonia, cosputtered films, iridium, metal insulator silicon carbide (MISiC), platinum, selective catalytic reduction (SCR), silicon dioxide (SiO2), thick film
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-13410 (URN)10.1109/JSEN.2006.877973 (DOI)
Available from: 2005-11-10 Created: 2005-11-10 Last updated: 2015-03-24
7. Modifications of the Gate Material to Increase the Long-Term Stability of Field Effect Transistor Lambda Sensors
Open this publication in new window or tab >>Modifications of the Gate Material to Increase the Long-Term Stability of Field Effect Transistor Lambda Sensors
2005 (English)In: Sensor Letters, ISSN 1546-198X, Vol. 3, no 3, 225-230 p.Article in journal (Refereed) Published
Abstract [en]

The MISiC-FET Pt/TaSix sensor has been shown previously to work well for measuring the lambda value, or the air-to-fuel ratio, of gasoline engine exhausts. However, the catalytic gate metal that is crucial for the sensing properties tends to restructure in harsh exhaust gases, which affects the behavior of the device. To increase the long-term stability of the sensor several modifications of the sensing layer, such as depositing a protective film of SiO2 on top of the metal or co-sputtering the catalytic metal with SiO2, have been studied and are presented here. The sensitivity and long-term stability of these films compared to the Pt/TaSix layers traditionally used for MISiC-FET lambda sensors have been investigated through laboratory measurements and SEM (scanning electron microscope) analyses. It was found that the catalytic metal films with a protective layer restructured much less than those without, but that the protective layer decreased the sensitivity of the sensors. On the other hand, the sensor gained linear characteristics when exposing it to lambda stairs. Mixing Ir and SiO2 also gave less restructuring compared with the reference samples, while maintaining a high sensitivity to lambda.

Keyword
Cold start sensors; Sic-fet; Lambda; Long-term stability; Linear
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-13411 (URN)10.1166/sl.2005.034 (DOI)
Available from: 2005-11-10 Created: 2005-11-10 Last updated: 2014-01-09

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