New shape-shifting metals discovered

BBC News, Simon Redfern, Science writer, 4 October 2013

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.

The chevron pattern at the rear of a Boeing
 Dreamliner engine is made of shape-
changing smart metal

Some call this a "military transformation" because the rows of atoms that make up the metal crystal click into their new shape in an orderly manner.

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.