Low-power silicon planar micro-calorimeter employing nanostructured catalyst
Lee, Siu Man (2002) Low-power silicon planar micro-calorimeter employing nanostructured catalyst. PhD thesis, University of Warwick.
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Official URL: http://webcat.warwick.ac.uk/record=b1677870~S15
This thesis describes the development of silicon planar micro-calorimetric gas sensors
employing a nanostructured palladium (Pd) catalyst. Present commercial, bead-type
calorimetric sensors have been manufactured for nearly forty years and are used in many
applications, such as mining, water treatment and emergency services, with an estimated
European market value of €221M by 2004. However, recent advances in both silicon
micro-machining and nano materials have created the technologies necessary to transform
the present labour-intensive fabrication process in to a new low-cost batch production. In
addition, a reduction in power consumption, improved sensitivity and increased
poisoning resistance of the sensor can also be achieved.
Here, two generations of micro-calorimeter have been designed and fabricated
comprising a silicon membrane structured micro-hotplate that can reach up to a
temperature of 870'C without failure and an ultra-high surface area nanoporous Pd
catalyst (about 20 m2/g), typically 25 run thick, deposited electrochemically on top of a
gold electrode above the micro-heater. The exothermic reaction caused by the target gas
(e.g. methane) interacting with the Pd catalyst results in an increase in the temperature
and so resistance of the micro-heater. A Wheatstone bridge interface circuit is normally
used to detect and measure the fractional resistance change.
Full 3-D thermo-mechanical simulations have been performed employing
experimental data in order to establish a simulation database for future developments.
The differences between simulated and experimental results were found to be as low as
4.6%. The response of the sensors has been characterised in both continuous powering
mode and pulse modulation powering mode. Device power consumption is only 50mW
at 500'C in continuous mode, which is up to 100mW lower than that for commercial
sensors. Typical response times of 2ms have been measured and so further power saving
can be achieved when the sensors are operated in a pulse mode, e.g. 50% duty-cycle at 10Hz. Hence, an overall power saving of 75% could be achieved compared to commercial
product. Infrared thermography revealed that a centre hot spot, commonly found with
meander style micro-heaters, has been eliminated by the new drive-wheel micro-heater
design. The sensitivity of the sensors has also been improved, up to a factor of 4 at 500'C
((60 mV/mm2)/%CH4), by the nanoporous catalyst and by heating it more isothermally.
Furthermore, improvements have also been found on the poisoning resistance. Therefore,
the potential commercialisation of the micro-calorimeter is very promising.
|Item Type:||Thesis or Dissertation (PhD)|
|Subjects:||T Technology > TP Chemical technology|
|Library of Congress Subject Headings (LCSH):||Gas detectors, Calorimeters, Palladium catalysts, Silicon|
|Official Date:||December 2002|
|Institution:||University of Warwick|
|Theses Department:||School of Engineering|
|Supervisor(s)/Advisor:||Gardner, J. W. (Julian W.), 1958-|
|Extent:||xvii, 271 p.|
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