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01 December 2004

Electron transfer, thermal stability

Chemical sensors for detecting hydrogen and hydrocarbons at high temperatures have applications in emission monitoring, space launch safety, and fire detection.

NASA's new work by Weijie Lu and W. Eugene Collins of Fisk University for Glenn Research Center debuted this past month (November 2004), reported Nanotech Briefs magazine.

The incorporation of nanostructured interfacial layers of CeO₂ , the scientists suppose, enhances the performances of Pd/SiC Schottky diodes used to sense hydrogen and hydrocarbons at high temperatures.

If successful, this development could prove beneficial in numerous applications that have requirements to sense hydrogen and hydrocarbons at high temperatures; examples include monitoring of exhaust gases from engines and detecting fires.

Sensitivity and thermal stability are major considerations affecting the development of high-temperature chemical sensors.

In the case of a metal/SiC Schottky diode for several metals, the SiC becomes more chemically active in the presence of the thin metal film on the SiC surface at high temperature.

This increase in chemical reactivity causes changes in chemical composition and structure of the metal/SiC interface. The practical effect of the changes is to alter the electronic and other properties of the device in such a manner as to degrade its performance as a chemical sensor.

To delay or prevent these changes, it is necessary to limit operation to a temperature <450°C for these sensor structures.

The present proposal to incorporate interfacial CeO₂ films comes partly from the observation that nanostructured materials in general have potentially useful electrical properties, including an ability to enhance the transfer of electrons.

Silicon-tetramers on top of SiC with a Si wetting layer.

In particular, nanostructured CeO₂—CeO₂  with nanosized grains—has shown promise for incorporation into high-temperature electronic devices.

Nanostructured CeO₂  films can be formed on SiC and have been shown to exhibit high thermal stability on SiC, characterized by the ability to withstand temperatures somewhat greater than 700°C for limited times.

The exchanges of oxygen between CeO₂  and SiC prevent the formation of carbon and other chemical species that are unfavorable for operation of a SiC-based Schottky diode as a chemical sensor.

Consequently, the two researchers think that in a Pd/CeO₂/SiC Schottky diode, the nanostructured interfacial CeO₂ layer would contribute to thermal stability and, by contributing to transfer of electrons, would also contribute to sensitivity.

Nicholas Sheble (nsheble@isa.org ) edits the Sensors department. Questions about the commercial use of this invention should go to NASA Glenn Research Center, Commercial Technology Office, Steve Fedor, Mail Stop 4-8, 21000 Brookpark Road, Cleveland, OH 44135.