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Metal With a Memory
| Article
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11301 |
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Section : |
NATURAL SCIENCE
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| Issue
Date : |
9 / 1993 |
2,876 Words |
| Author
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George B. Kauffman and Isaac Mayo
George B. Kauffman, professor of chemistry at California
State University, Fresno, has authored 15 books and more than
1,200 publications on chemistry, chemical education, and the
history of science and technology. Isaac Mayo, a former
student of Kauffman's as well as a frequent collaborator, is
a student at the New York State College of Veterinary
Medicine at Cornell University. "I keep looking for some
problem where someone has made an observation that doesn't
fit into my picture of the universe. If it doesn't fit in,
then I find some way of fitting it in. " |
Driving down the street, you realize the expensive eyeglasses in your back pocket must be crushed and, forgetting all else, reach for them. All of a sudden, you've hit a telephone pole. Are you out countless dollars for new glasses and an inevitably expensive trip to the body shop? Not if your frames and fender are constructed of Nitinol--the metal with a memory. Within minutes, both your glasses and your car will return to their predisaster shape.
Nitinol is a sophisticated nickel-titanium alloy that remembers shapes. It has been around for about 30 years, but until recently it was used almost exclusively by the military. During the past several years Nitinol usage has significantly increased, and today it can be found in the worlds of engineering and medicine as well as in everyday consumer goods.
In 1958 William Buehler, a metallurgist at the U.S. Naval Ordnance Laboratory, began work on a project to find a metal for missile nose cones that could better withstand reentry. His main objective was to attempt to develop a low-density alloy with a high melting point and a high heat capacity. While testing various alloys, he noticed that a nickel-titanium alloy was markedly different from the others--it appeared to resist fatigue better and could easily be formed into a wire. Buehler named his discovery Nitinol (Nickel Titanium Naval Ordnance Laboratory).
At a laboratory management meeting in 1961 to demonstrate Nitinol's resistance to fatigue, Buehler's assistant pulled out one of their Nitinol "props"--an accordion-folded strip of the alloy. It was passed around a conference table and flexed repeatedly by all present. Then one of the laboratory's technical directors, the late David S. Muzzey, a pipe smoker, applied heat from his pipe lighter to the compressed Nitinol strip. To everyone's amazement, it stretched out to its original form. This was the first demonstration of Nitinol's shape-memory capabilities.
How Nitinol works
Everyone is familiar with phase changes (melting, vaporization, etc.) between solids, liquids, and gases. What is less well known is that such changes can occur when both materials are solid. Solid-to-solid phase changes involve rearrangement of the position of atoms, molecules, or ions within a crystal lattice. Many materials undergo such transformations; the unique property of Nitinol is that when the temperature is changed, a distorted object reverts to its original shape.For its shape to be established, a Nitinol
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