New glasses arise from liquid's slow flow.
Seemingly a translucent solid, glass is in fact a fluid unable to flow. Yet a careful look at the molecular properties of this amorphous, highly viscous liquid has spawned a diversity of new glass materials, improved manufacturing and coating techniques, and a better understanding of protein folding, researchers report in the March 31 Science.
"Structurally, a glass is barely distinguishable from the fluid substance it was before it passed, quite abruptly in some cases, into the glassy state," says Charles A. Angell, a chemist at Arizona State University in Tempe. "Why did this particular substance or solution suddenly undergo a dramatic slowing down in the diffusive motions of its particles?"
"Why do glasses not form a precisely ordered cystalline material at some precisely defined freezing point, like so many other, more 'normal' substances?" Angell wonders.
Investigating these questions, researchers have found alternative methods for forming glass, beyond the traditional techniques of rapidly cooling a liquid, Angell says.
Types of glass forged from metal alloys are giving rise to new kinds of high-strength, corrosion-resistant materials, reports A. Lindsay Greer, a materials scientist at Cambridge University in England. Scientists make the so-called amorphous metallic alloys by rapidly cooling a thick metal liquid, a technique that prevents the molecules from crystallizing.
Metallurgists, for example, can form glass from many types of metal, most easily from iron and palladium. Aluminum, on the other hand, resists forming a glass. But Greer points to recent successes in fashioning glasses from aluminum-based alloys that previously would not produce a glass. Researchers in France and Japan, for instance, have successfully forged light, ductile glass alloys containing more than 80 percent aluminum.
"This opens up all sorts of possibilities for making metal machine parts that are completely glassy," says Greer. "Amorphous metal alloys have no fixed molecular structure, so it's much easier to mold them into fine shapes. These alloys could be used for devices implanted in the human body."
Such new materials promise applications for strong, lightweight, corrosion-resistant engine parts often needed in the aerospace and automotive industries, as well as in microelectronics. Micrometer- sized machines, including small motors and gears, would also find improved performance when fashioned from metallic glass alloys.
New methods for producing metallic glasses efficiently in large quantities may make them more accessible to industry, says Greer. "It has always been thought that, to form a metallic glass, you must cool liquid metal extremely fast, nearly 1 million degrees a second. But some new alloys show that glasses can form by cooling much more slowly."
Wear-resistant coatings for more mundane items, such as kitchen appliances (even frying pans), have an allure for industry, says Greer. Manufacturers could deposit thin metallic glass films from a vapor onto items or try glazing a metal surface with a laser or an electron beam.
"A laser would melt a thin surface layer," says Greer. "It's the sort of thing one might do to the lining of a cylinder in a car engine."
Researchers are gleaning insights into glass formation by observing natural processes, Angell says. Scientists have realized that most of the water in the universe exists in a "glassy state," condensing onto comets from gas in frigid interstellar space without forming the crystals found in ice.
In addition, studies of food preservation and methods that insects use to protect themselves from droughts are showing how some biological molecules change into a glasslike state, Angell says.
Such observations are leading researchers to see similarities between the chemical transitions of some proteins and polymers and that of glasses, Angell says. Proteins contain so many moving components that materials scientists now liken them to a "many- particle glass-forming system."
Both in computer simulations and laboratory experiments, certain proteins appear to fold into their three-dimensional structures by moving through a glass transition state.
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|Title Annotation:||amorphous metal alloys|
|Date:||Apr 1, 1995|
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