A research team at Lawrence Berkeley National Laboratory, using a chemical compound called bismuth ferrite, has reported a large shape-memory breakthrough that is larger than any shape-memory effect observed in metal.
The shape-memory effect is considered a promising emerging technology for use as actuators in smart materials and MEMS.
The shape-memory effect is when a solid material "remembers" and recovers its original shape after being deformed by an applied stress. In the past, this has always involved heating.
But the Berkeley Labs team reported recovering a 14 percent strain into bismuth ferrite, using an electrical field instead of heat.
"The response time is much faster [than with metal and heat]," said Jinxing Zhang, a researcher at Berkeley Lab's Materials Sciences Division. "Our bismuth ferrite was also far more stable when reduced to nanometer size than shape-memory alloys."
Shape-memory alloys have been used in stents for angioplasty, and in mechanical joints. However, on the nano-scale, numerous problems and instabilities arise, including fatigue, micro-cracking and oxidation.
"By achieving the shape-memory effect in an oxide material rather than a metal alloy, we eliminate the surface issues and enable integration with microelectronics," Zhang said. And during actuation the tolerated stress level on the material is "almost two orders of magnitude higher than what a metal alloy can generate," he said.
Bismuth ferrite is a multiferroic compound comprised of bismuth, iron and oxygen that has been studied extensively in recent years by the Berkeley Lab research group. As a multiferroic, bismuth ferrite displays both ferroelectric and ferromagnetic properties, meaning it will respond to the application of external electric or magnetic fields.
The bismuth ferrite shape-memory effect was characterized at the National Center for Electron Microscopy (NCEM), a U.S. Department of Energy national user facility housed at Berkeley Lab.
Results were published in the journal Nature Communications.
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