Adopted from Materials Today, 5 December 2013

Nanoscale Shape-Memory Oxide

Listen up nickel-titanium and all you othershape-memory alloys, there’s a new kid on the block that just claimed thechampionship for elasticity and is primed to take over the shape memory apps marketat the nanoscale. A research team at Berkeley Lab has discovered a way tointroduce a recoverable strain into bismuth ferrite of up to 14-percent on thenanoscale, larger than any shape-memory effect observed in a metal. Thisdiscovery opens the door to applications in a wide range of fields, includingmedical, energy and electronics.

“Our bismuth ferrite not only displayed the championshape-memory value, it was also far more stable when reduced to nanometer sizethan shape-memory alloys,” says Jinxing Zhang. “Also because our bismuthferrite can be activated with only an electrical field rather the thermalfields needed to activate shape-memory alloys, the response time is muchfaster.”

The shape-memory effect is the metallic equivalent ofelasticity, in which a solid material “remembers” and recovers its originalshape after being deformed by an applied stress. In the past, this has alwaysinvolved heating. Shape-memory alloys have had a big impact in the medicalfield with the most prominent being nickel-titanium or “nitinol,” which is usedin stents for angioplasty, and in mechanical joints. The shape-memory effect isalso expected to have a major impact in non-medical applications, such asactuators in smart materials and in Microelectro-Mechanical Systems (MEMS).However, as the size of current shape-memory alloys shrink towards thenano-scale, numerous problems and instabilities arise, including fatigue,micro-cracking and oxidation.

“By achieving the shape-memory effect in an oxidematerial rather than a metal alloy, we eliminate the surface issues and enableintegration with microelectronics,” says Zhang. “Our bismuth ferrite alsodisplays an ultra-high work function density during actuation that is almosttwo orders of magnitude higher than what a metal alloy can generate.”

Bismuth ferrite is multiferroic compound comprised ofbismuth, iron and oxygen that has been studied extensively in recent years byRamesh and his research group. As a multiferroic, bismuth ferrite displays bothferroelectric and ferromagnetic properties, meaning it will respond to theapplication of external electric or magnetic fields. In this latest study, inaddition to the conventional thermal activation, an elastic-like phasetransition was introduced into bismuth ferrite using only an electric field.

“The application of the electric field allowed us toachieve a phase transformation that was reversible without the assistanceof external recovery stress,” Ramesh says. “Although aspects such ashysteresis, micro-cracking and so on have to be taken into consideration forreal devices, the large shape-memory effect we demonstrated in bismuth ferriteshows it to be an extraordinary material with potential use in futurenanoelectromechanical devices and other state-of-art nanosystems.”

This story is reprinted from material from Berkeley Lab, with editorial changes made by MaterialsToday. The views expressed in this article do not necessarily represent thoseof Elsevier. Link to original source.