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Scientists from the Technion and Germany have demonstrated the “shape memory” effect in gold particles for the first time. Researchers from the Technion and Germany have demonstrated for the first time the phenomena of shape memory and self-healing in gold microparticles. This is achieved through defects-mediated diffusion in the particle. The discovery may one day lead to development of micro- and nano-robots capable of self-repair; mechanically stable and damage-tolerant components and devices; and targeted drug delivery. The study, published in Advanced Science, was conducted by doctoral student Oleg Kovalenko and Dr. Leonid Klinger, led by Prof. Eugen Rabkin of the Technion Department of Materials Science and Engineering, together with Dr. Christian Brandl of KIT (Karlsruhe Institute of Technology, Germany). Shape-memory materials are characterized by the ability to repair the damage caused to them (such as plastic deformation) and to recover their original shape. These materials can exist in two stable crystalline forms, or phases: austenite, which is the more symmetrical primary form stable at elevated temperatures; and martensite, which is a phase characterized by lower symmetry, but also by greater strength. A well-known example of transition between the two phases is the quenching of steel. 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 re-orienting 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 will happen if the material is heated up, causing the reverse martensite-austenite phase transformation and transforming the thermal energy into mechanical energy that will restore the material to its original shape. 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 sub-micrometer 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% 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 Prof. 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.””

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