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Experimental Observations of Thermal Spikes in Microwave Processing of Ceramic Oxide Fibers

Published online by Cambridge University Press:  15 February 2011

Gerald J. Vogt ,
Wesley P. Unruh  and
J. R. Thomas Jr
Gerald J. Vogt
Affiliation:
Los Alamos National Laboratory, Los Alamos, New Mexico 87545
Wesley P. Unruh
Affiliation:
Los Alamos National Laboratory, Los Alamos, New Mexico 87545
J. R. Thomas Jr
Affiliation:
On leave from Virginia Polytechnic Institute and State University, Blacksburg, VA 24061
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Abstract

Microwave heating of alumina/silica fiber tows in a single-mode microwave cavity at 2.45 GHz has produced a surprising thermal spike behavior on the fiber bundles. During a thermal spike, a “hot spot” on the tow brightens rapidly, persists for a few seconds, and then rapidly extinguishes. A hot spot can encompass the entire tow in the cavity or just a localized portion of the tow. Some local hot spots propagate along the fiber. Thermal spikes are triggered by relatively small (<15%) increases in power, thus having obvious implications for the development of practical microwave fiber processing systems. A tow can be heated through several successive thermal spikes, after which the tow is left substantially cooler than it was originally, although the applied microwave electric field is much larger. X-ray diffraction studies show that after each temperature spike there is a partial phase transformation of the tow material into mullite. After several excursions the tow has been largely transformed to the new, less lossy phase and is more difficult to heat. Heating experiments with Nextel 550 tows are examined for a plausible explanation of this microwave heating behavior.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 347: Symposium O – Microwave Processing of Materials IV , 1994 , 539
Copyright
Copyright © Materials Research Society 1994

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References

REFERENCES

1. Vogt, G.J. and Unruh, W.P., “Processing Aerosols and Filaments in a TM010 Microwave Cavity at 2.45 GHz” in Microwave Processing of Materials HI. edited by Beatty, R.L., Sutton, W.H., and Iskander, M.F. (Mater. Res. Soc. Proc. 269, Pittsburgh, PA, 1992) pp. 245–250.Google Scholar
2. Vogt, G.J. and Unruh, W.P., “Microwave Hybrid Heating of Alumina Filaments” in Microwaves: Theory and Application in Materials Processing II. edited by Clark, D.E., Tinga, W.R., and Laia, J.R. Jr, (Ceramic Transactions 36, The American Ceramic Society, Westerville, OH, 1993) pp. 297–306.Google Scholar
3. Thomas, J.R. Jr, Unruh, W.P., and Vogt, G.J., “Mathematical Model of Thermal Spikes in Microwave Heating of Ceramic Oxide Fibers,” published in this proceedings.Google Scholar