X-ray snapshots of 'solid flame' events.
Put a match to the tip of a Fourth of July sparkler. A dazzling display of light immediately begins inching down the shaft as the searing heat sparks successively lower regions into combustive action. Researchers have now recorded with unpecedented detail the rapid material changes that occur during related "solid flame" reactions lasting mere seconds or minutes.
For more than 20 years, materials scientists -- primarily in the Soviet Union -- have explored the chemistry, physics and technological promise of such reactions, also known as self-propagating, high-temperatur synthesis (SHS) reactions. Already, researchers have harnessed these "solid flames" to process solid ingredients directly into metallic alloys, composite materials and even superconducting ceramics.
Using one of the world's most intense synchrotron radiation sources, at Brookhaven National Laboratory in Upton, N.Y., a team led by Joe Wong of Lawrence Livermore (Calif.) National Laboratory has assembled crystallographic SHS snapshots. Scientists routinely compare starting ingredients against products formed after a reaction occurs. "But before synchrotron radiation, we never had the possibility of observing what happens during these [SHS] reactions," Wong says.
The researchers first compress combustible mixes -- say, titanium, carbon and nickel powders -- into blocks the size of ice cubes, which they place inside a crystallography chamber. Then they ignite the reaction as an intense beam of synchrotron X-rays bathes the sample. Detectors record the resulting diffraction patterns at intervals as brief as a tenth of a second. In a synchrotron, highly accelerated electrons copiously emit X-rays as they travel in bending paths.
In the Sept. 21 SCIENCE, the Livermore team displays several series of rapid X-ray diffraction scans taken both while and after the fast-moving reaction fronts of SHS reactions pass through samples. Viewed in sequence, these images indicate exactly when specific components undergo physical and chemical changes such as melting, crystallizing and forming alloys or ceramics.
SHS reactions can yield solid products requiring little postproduction machining and generate less waste material than conventional furnace processes, which take hours or days. Materials scientist James W. McCauley, dean of the New York State College of Ceramics at Alfred University, says he expects the more detailed pictures of these reactions to help researchers design better SHS recipes.
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|Date:||Sep 22, 1990|
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