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The influence of catalytic activity on the phase transition governed binary switch point of MISiC-FET lambda sensors
Linköping University, Department of Physics, Chemistry and Biology, Applied Physics . Linköping University, The Institute of Technology.
Linköping University, Department of Physics, Chemistry and Biology, Applied Physics . Linköping University, The Institute of Technology.ORCID iD: 0000-0002-2817-3574
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.

Place, publisher, year, edition, pages
2006. Vol. 252, no 20, 7473-7486 p.
Keyword [en]
MISiC-FET; Catalytic activity; Binary switch point; Phase transition; Lambda
National Category
Natural Sciences
URN: urn:nbn:se:liu:diva-13409DOI: 10.1016/j.apsusc.2005.09.003OAI: diva2:20681
Available from: 2005-11-10 Created: 2005-11-10 Last updated: 2014-01-09
In thesis
1. Studies of MISiC-FET sensors for car exhaust gas monitoring
Open this publication in new window or tab >>Studies of MISiC-FET sensors for car exhaust gas monitoring
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
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 931
field effect sensor, gas detection, selective catalytic reduction, lambda, cold start, ammonia, silicon carbide, engine exhaust
National Category
Physical Sciences
urn:nbn:se:liu:diva-4674 (URN)91-85297-61-5 (ISBN)
Public defence
2005-04-22, Hörsal Planck, Fysikhuset, Campus Valla, Linköpings univeristet, Linköping, 10:15 (English)
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

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