Technion-Israel Institute of Technology,
Haifa, Israel

Researchers from the Technion-Israel Institute of Technology and Germany have demonstrated for the first time the phenomena of shape memory and self-healing in gold micro-particles. Achieved through defects-mediated diffusion in the particle, the discovery could one day lead to the development of micro- and nanorobots capable of self-repair, mechanically stable and damage-tolerant components and devices, and targeted drug delivery.

The study, published in the journal Advanced Science, was conducted by doctoral student Oleg Kovalenko and Dr. Leonid Klinger and led by Prof. Eugen Rabkin of the Technion Department of Materials Science and Engineering, together with Dr. Christian Brandl of Karlsruhe Institute of Technology, Germany (KIT).

This is the first time the phenomenon of shape memory has been demonstrated in submicrometer particles of gold. (Credit: Technion-Israel Institute of Technology)

The transformation of the austenite phase to the martensite can be activated by applying mechanical load to the material, or by cooling it down. The low-symmetry structure of the martensite allows the material to absorb considerable plastic strain by reorienting the distorted crystals of martensite according to the direction of the stress applied to it. Even after plastic deformation, the martensite crystals remember their parent austenite phase and are capable of restoring it in its original configuration. This happens if the material is heated up, causing the reverse martensite-austenite phase transformation and transforming the thermal energy into mechanical energy that restores the material to its original shape.

Prof. Rabkin’s research group (from left): Ehud Almog, Nimrod Gazit, Oleg Kovalenko, Prof. Eugen Rabkin, and Dr. Leonid Klinger. (Credit: Technion-Israel Institute of Technology)

Until now, this shape memory effect has only been observed in very few metal alloys such as nitinol (Ni-Ti). These alloys are characterized by polymorphism — multiplicity of possible stable crystalline phases. This is the first time the phenomenon of shape memory has been demonstrated in submicrometer particles of gold. The researchers indented the gold particles with a sharp diamond tip controlled by an atomic force microscope (AFM). Annealing of the indented particles at a temperature of 600 °C (about 65 percent of the absolute melting temperature of gold) resulted in full healing of the damage and recovery of the particles’ original shape prior to deformation.

According to Rabkin, the discovery of the shape memory effect in these particles is surprising for two reasons: “First, the particles’ original shape was not perfect in terms of energy and thermodynamic equilibrium. Second, gold in its solid state is not characterized by polymorphism.”

To understand the process in depth, the researchers investigated the atomic motion during indentation and heating, using atomistic molecular dynamic computer simulations. They demonstrated that the plastic deformation during the indentation process is mediated by nucleation and glide of dislocation half-loops (the dislocations are linear, one-dimensional defects in the crystal through which it undergoes plastic deformation). The loops that egress at the free surfaces form terraces and ledges on the flat facets of the particle, and these serve as guide rails, directing the diffusion of gold atoms back to the indented site during high-temperature anneal. Thus, the particle recovers its original shape.

Irreversible Process

Both plastic deformation and capillary-driven diffusion are classical examples of thermodynamically irreversible processes. It is remarkable that a combination of two irreversible processes can lead to damage recovery and reversible restoration of a particle shape.

Rabkin says that the self-healing and shape memory effect in metallic nano- and microparticles could be utilized for the design of mechanically stable and damage-tolerant components and devices at the submicrometer length scale.