As an innovative concept in materials science and engineering, the inspiration of self-healing materials comes from living organisms that have the innate ability to self-heal. Along with this line, the search for self-healing materials has been generally focused on "soft" materials like polymers and hydrogels. For solid-state metals instead, one may intuitively imagine that any form of self-healing will be much more difficult to achieve.
While a few past studies showcased the self-healing behavior in metals that more or less requires the assistance of external triggers (e.g. by heating, mechanical stimulus, or electron beam irradiation), whether the autonomous self-healing can occur in metal solids without any external intervention remains a scientific curiosity.
Now in a new study published in Matter, researchers from Institute of Physics (IOP) of the Chinese Academy of Sciences discovered that such an intrinsic and autonomous self-healing phenomenon can occur in nanoscale silver (Ag).
This study, which combines advanced in-situ transmission electron microscopy (TEM) with molecular dynamics (MD) simulations, reveals that nanoscale Ag can autonomously repair itself from structural damages such as nanocracks and nanopores without external intervention.
This remarkable ability is observed not only at room temperature but also at frigid temperatures as low as 173 K.
Notably, over the same damaging area, the repeated reversible self-healing cycles can also be achieved with the same level of efficiency.
The experiments were performed inside an atomic-resolution TEM by utilizing single-crystalline Ag nanosheets as testing specimens. Both nanopores and nanocracks were purposefully fabricated through in-situ drilling by TEM electron beam.
To avoid any possible intervention to the healing process, Ag nanosheet specimen was afterwards kept at a "beam-off" state all through until each moment for interval TEM imaging.
As an interesting and perhaps surprising result, the two representative kinds of structural damages were observed to undergo rapid self-healing autonomously within several to dozens of minutes, with the healed regions perfectly restoring the crystal lattice of Ag with atomically precise ordering.
Unlike Ag, gold (Au) did not show similar self-healing behavior at room temperature, despite the fact that Au is the most relevant element to Ag in the Periodic Table and they share many similarities in physical and chemical properties.
Further MD simulation results well reproduce the experimental observations, especially regarding the difference in the healing behavior between Ag and Au. What sets Ag apart from Au is its high mobility of surface diffusion, a trait not commonly found in other metal solids.
By employing TEM, the researchers were able to in-situ track the trajectories of healing process in Ag at the atomic level. With a combination of atomistic imaging and theoretical simulation results, the research highlights that self-healing is enabled by the surface-mediated self-diffusion of Ag atoms as driven by chemical potential imbalance due to Gibbs-Thomson effect.
When a nascent damage structure (either nanopore or nanocrack) begins its existence in Ag nanosheet, there will create a concave site with the negative local curvature. Due to the general curvature-dependence of chemical potential, the concave damage site will thereby have smaller chemical potential relative to the undamaged areas of the nanosheet. This built-in imbalance of chemical potential drives Ag atoms to migrate and repair the damage autonomously, showcasing a sophisticated form of material self-maintenance in negative feedback way.
The ability of Ag to autonomously self-heal nanoscale damages at room temperature and below shows promising possibility for developing damage-tolerant components and devices at the sub-micrometer length scale.
Perhaps more importantly, in a broader term, this unusual finding at a mechanistic level may provide a guiding framework for deeper understanding of the self-healing phenomena and concepts in metal solids in general.
