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Dependence of Pt gate restructuring on the linearity of SiC Field Effect Transistor lambda sensors
Linköpings universitet, Institutionen för fysik, kemi och biologi, Tillämpad Fysik. Linköpings universitet, Tekniska högskolan.
Linköpings universitet, Institutionen för fysik, kemi och biologi, Tillämpad Fysik. Linköpings universitet, Tekniska högskolan.ORCID-id: 0000-0002-2817-3574
2003 (Engelska)Ingår i: Sensor Letters, ISSN 1546-198X, Vol. 1, nr 1, s. 37-41Artikel i tidskrift (Refereegranskat) 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.

Ort, förlag, år, upplaga, sidor
2003. Vol. 1, nr 1, s. 37-41
Nationell ämneskategori
Naturvetenskap
Identifikatorer
URN: urn:nbn:se:liu:diva-13403DOI: 10.1166/sl.2003.011OAI: oai:DiVA.org:liu-13403DiVA, id: diva2:20674
Tillgänglig från: 2005-11-10 Skapad: 2005-11-10 Senast uppdaterad: 2014-01-09
Ingår i avhandling
1. Development of high temperature SiC based field effect sensors for internal combustion engine exhaust gas monitoring
Öppna denna publikation i ny flik eller fönster >>Development of high temperature SiC based field effect sensors for internal combustion engine exhaust gas monitoring
2003 (Engelska)Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
Abstract [en]

While the car fleet becomes increasingly larger it is important to lower the amounts of pollutants from each individual diesel or gasoline engine to almost zero levels. The pollutants from these engines predominantly originate from high NOx emissions and particulates, in the case when diesel is utilized, and emissions at cold start from gasoline engines. One way of treating the high NOx levels is to introduce ammonia in the diesel exhausts and let it react with the NOx 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. 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.

This thesis presents the efforts made to test the SiC-based field effect transistor (SiC-FET) sensor technology both as an ammonia sensor for SCR systems and as a cold start lambda sensor. The SiC-FET sensor has been shown to be highly sensitive to ammonia both in laboratory and engine measurements. As a lambda sensor it has proven to be both sensitive and selective, and its properties have been studied in lambda stairs both in engine exhausts and in the laboratory. The influence of metal gate restructuring on the linearity of the sensor has also been investigated. The speed of response for both sensor types has been found to be fast enough for closed loop control in each application.

Ort, förlag, år, upplaga, sidor
Institutionen för fysik, kemi och biologi, 2003. s. 74
Serie
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1051
Nyckelord
field effect sensor, gas detection, selective catalytic reduction, lambda, cold start, ammonia, silicon carbide, engine exhaust.
Nationell ämneskategori
Fysik
Identifikatorer
urn:nbn:se:liu:diva-4673 (URN)91-7373-767-4 (ISBN)
Presentation
2003-11-05, Hörsal Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (Engelska)
Opponent
Handledare
Anmärkning
On the day of the public defence of the doctoral thesis, the status of article III was: in press. Report code: LiU-Tek-Lic-2003:50.Tillgänglig från: 2005-11-10 Skapad: 2005-11-10 Senast uppdaterad: 2014-01-09
2. Studies of MISiC-FET sensors for car exhaust gas monitoring
Öppna denna publikation i ny flik eller fönster >>Studies of MISiC-FET sensors for car exhaust gas monitoring
2005 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
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.

Ort, förlag, år, upplaga, sidor
Institutionen för fysik, kemi och biologi, 2005
Serie
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 931
Serie
Nyckelord
field effect sensor, gas detection, selective catalytic reduction, lambda, cold start, ammonia, silicon carbide, engine exhaust
Nationell ämneskategori
Fysik
Identifikatorer
urn:nbn:se:liu:diva-4674 (URN)91-85297-61-5 (ISBN)
Disputation
2005-04-22, Hörsal Planck, Fysikhuset, Campus Valla, Linköpings univeristet, Linköping, 10:15 (Engelska)
Opponent
Handledare
Anmärkning
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.Tillgänglig från: 2005-11-10 Skapad: 2005-11-10 Senast uppdaterad: 2014-01-09

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