First Reactant Melts And Spreads

Powder with the lower melting point (blue) coats other particles. Increased contact between the two reactants and higher temperatures ignite the chemical reaction.

because the heat from the chemical reaction originates at every point inside the original sample. These reactions also attain temperatures anywhere from 1,500 to 4,000 degrees Celsius, and the material heats at astounding rates of up to one million degrees C per second. That means combustion synthesis takes just a few seconds to turn a mixture of reac-tants into a new solid, as opposed to the minutes or even hours that furnaces, which rarely reach 2,000 degrees C, require to do the same job. Intense and quick heating creates compounds with extremely uniform microscopic structures. In contrast, conventional furnaces heat material unevenly and can introduce flaws in the material's structure—a potentially fatal problem for an object that will experience tremendous stress. A microscopic crack in the metallic skin of an airplane, for example, can sometimes grow large enough to break catastrophically [see "Why Things Break," by Mark E. Eberhart; Scientific American, October 1999].

Combustion-synthesis reactions can involve gases, but in the most common method, called solid flame, all starting components and resulting products remain in a solid or liquid state. This type of reaction can combine metals such as nickel and aluminum to produce the lightweight and heat-resistant compounds needed for aircraft turbines and other engine parts. Starting powders are pressed into a pellet, typical ly in the shape of a cylinder. For some purposes, the entire pellet is heated uniformly until the reaction occurs simultaneously throughout the sample. In most cases, however, the sample is ignited only at one location, and the heat wave passes through from one end to the other.

Combustion synthesis is more than a quick and energy-efficient way to make valuable materials. The high temperatures and short reaction times can convert a compound rapidly enough to "fool" its molecules into solidifying as structures that would be unstable under normal conditions. This capability makes it possible to invent materials that are otherwise impossible to make, such as ceramics peppered with synthetic diamonds—an ideal combination for high-quality cutting tools. In slow-heating furnaces, diamond turns to soft graphite, but during rapid combustion, diamond particles retain their shape and hardness. Combustion synthesis can also yield so-called functionally graded materials, in which compounds and properties are distributed as desired. The materi-

HEAT WAVE surges back along a 10-centimeter-long rod of compressed powders (below) in only a few seconds. Researchers have now pinpointed exactly how the heat wave ignites chemical reactions and transforms the molecular structure to leave a new solid in its wake (enlargement at bottom).

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