Nanoscale Shape-Memory Oxide

Another story for Materials Today…

A team from Berkeley Lab have demonstrated a nanoscale shape-memory effect in an oxide that surpasses the best performance measured in any metal to date.

Move over shape-memory alloys, there is a new material in town – the shape-memory oxide – and its performance surpasses any shape-memory effect ever measured in a metal. Shape-memory alloys have been used in everything from medicine to the automotive industry since the 1960s because of their unusual property – they “remember” their original form. If the alloy is deformed by stress, it can return to its original shape just by being heated, and can do so repeatedly over time.

But there are some limitations – the best alloys can endure a maximum strain of just 8%, the thermal recovery process can be slow, and as the size of the alloys shrink toward the nanoscale, they become unstable due to oxidation and fatigue-led micro-cracking. To investigate nanoscale shape-memory effects, a team at Berkeley Lab moved away from alloys and instead focussed on bismuth ferrite (BFO), a multiferroic oxide comprised of bismuth, iron and oxygen. Their results on the oxide, published in the November 2013 issue of Nature Communications, open the door for a new generation of small-scale devices. Led by Ramamoorthy Ramesh, the team firstly developed a crystal growth routine that could produce a material with high mismatch strain. Then, by reducing its lateral stress, they deformed the BFO and analysed changes in the crystal structure under a scanning probe and in-situ transmission electron microscope. They found that bismuth ferrite could withstand strains of up to 14%, surpassing the previous highest value for an alloy of just 8%.

In addition, because of its multiferroic nature, Ramesh and his team found that an elastic-like phase transition could be induced in the bismuth ferrite using only an electric field – a much faster recovery route than thermal-mediation as used in alloys. This is the first time such a large shape-memory effect has been measured in an oxide, and its implications may be profound for small-scale systems. Although this material has yet to be used in a working device, it has been shown to be fully compatible with silicon, so could be used in existing microelectromechanical systems (MEMS). And with further characterisation and development, BFO may play a leading role in the development of the next generation of nanoelectromechanical systems and other state-of-the-art nanodevices.

Nature Communications (2013) doi:10.1038/ncomms3768