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| Crystals seen on the fine scale: River-like patterns form and change as the metal morphs |
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A new shape-changing metal crystal is reported in the journal Nature, by scientists at University of Minnesota.
It is the prototype of a new family of smart materials that
could be used in applications ranging from space vehicles to electronics to jet
engines.
Called a "martensite", the crystal has two
different arrangements of atoms, switching seamlessly between them.
It can change shape tens of thousands of times when heated
and cooled without degrading, unlike existing technology.
Currently, martensite metals are made of an alloyed mixture
of nickel and titanium.
They have the remarkable ability to "remember"
their shape and even after being bent will return to their original form. For
this, they are called "shape memory" metals.
They have been used in spectacle frames and brassiere wires,
but also in surgery as frameworks for shaping healing bones, and as
"stents" for holding heart arteries open.
Martensite metals change shape when heated or cooled through
a certain temperature, when the atoms that make up their structure rearrange
themselves in a sudden transformation.
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| The chevron pattern at the rear of a Boeing Dreamliner engine is made of shape- changing smart metal |
The transformation means that martensite can be used in
smart mechanisms that respond to temperature change.
Examples include automatic windows-openers in glasshouses, a
means for automatically guiding solar panels to point at the Sun on the Hubble
Space Telescope, and, very recently, in the Boeing 787 Dreamliner to morph the trailing
edge of the engine cowling, making it quieter when it runs hot on take-off.
The pitfall of current martensites is that after repeated
shape changes, they build up stresses inside that degrade them and eventually
break them apart. The new alloy, made of a mixture of zinc, gold and copper,
changes back and forth almost indefinitely with little internal damage, opening
up a new range of applications for these types of "active materials".
The aim is now to apply the lessons learned from the new
metal to make a family of ceramic solids that can also be shape-switched back
and forth.
"The real advance is to make the transformations
reversible that could be applied in many situations" explains Prof Richard James, one of the authors of the study.
"You could make devices that convert heat to
electricity directly. They could use the waste heat from computers and cell
phones to recharge the battery and make them more efficient."
As the material cycles through its different atomic arrangements, crystals can be seen at its surface in ever-changing patterns,
looking like microscopic rivers.
The structures fit together without any stress layers
between them, and this seems to be the key to their longevity and potential.
The new materials could also be used in improved and
efficient microelectromechanical systems - energy harvesting devices, in which
small vibrations can be converted directly into power.
These sorts of gizmos are already used in tyre
pressure-monitoring systems in cars to power the electronics in the sensors.
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