&Bullet; physics 14, 87
Experiments in which cubes were compressed with gold nanowires suggest that a previously overlooked feature could explain the surprising strength of tiny objects.
Structures smaller than a micrometer can, paradoxically, become quite strong. New experiments suggest that a previously overlooked material characteristic known as Surface Triple Junction (STJ) could be an important factor in the strength of nanomaterials  . An STJ is a line on a surface where the crystalline grains of a material meet, analogous to the boundaries between planks in a hardwood floor. STJs are common in materials such as thin film semiconductors, so their strength properties could become an important consideration when designing nanoscale devices. However, some experts have raised questions about the results, suggesting that the STJ effect is not the only possible interpretation of the data.
Over the past few decades, precision measurements and advances in theory have enabled researchers to explain some of the mechanisms responsible for the surprising power of small things. A crystal can be weakened by dislocations – abrupt shifts of atoms in a lattice, similar to displaced blocks in a jenga tower. However, depending on their number and configuration, dislocations can also strengthen a crystal by preventing it from yielding when compressed.
Because dislocations can have an important impact on strength, researchers have focused on places where they cluster. For example, dislocations often form at grain boundaries, the 2D interfaces that separate the grains – regions with different crystal structure orientations – within a crystalline material. STJs are the lines formed by grain boundaries on the surface of a material, and no one has carefully studied the effects of STJs on material strength.
“It is very difficult to separate the effects of STJs from those of grain boundaries and non-STJ surface phenomena,” says Hai-Jun Jin of the Institute of Metal Research in China. But Jin and his colleagues have developed techniques that they say will enable them to differentiate the STJ effect.
The researchers used millimeter-sized cubes made of nanoporous gold – sponge-like structures made of around a trillion gold nanowires or “ribbons”. They examined two types of samples: coarse-grained with few grain boundaries and fine-grained with many grain boundaries. The team measured the strengths of the samples by compressing them and observing their deformation. “Strength” in these experiments is the force that is required to continuously deform the sample, in the so-called plastic regime, in which its shape does not spring back. They carried out the compression experiments in an electrolyte bath, which produced an oxide on the gold surface when a high voltage was applied.
The researchers measured the strength difference between the oxidized and the clean state in both coarse and fine-grained samples with different bandwidths. (Thinner tapes were expected to increase surface effects due to their higher surface-to-volume ratio.) For samples with the thinnest tapes, surface oxidation had a much greater impact on the strength of fine-grained samples than on the strength of coarser-grained samples. Fine grain samples had more grain boundaries and more STJs than coarse grains, but the grain boundaries are internal and are not affected by the surface treatment. So Jin and his colleagues came to the conclusion that only STJs could be responsible for the difference between the two sample types.
In fact, the data suggested that the STJ effect could be a dominant factor in material strength in the sub-100 nanometer range, based on the effect of tape width on strength differences. The team suggests that STJs could affect material strength because they create a place for dislocations to grow.
One of the challenges of the analysis was that the team could only measure the thickness of the sample, not the individual ligaments. Without a better understanding of the relationship between macroscopic measurements and band properties, they couldn’t determine whether the STJs made individual bands stronger or weaker.
Materials scientists Cynthia Volkert from the University of Göttingen, Germany, and David Srolovitz from the City University of Hong Kong both say that apart from the dislocation, there could be other mechanisms at play, so that more than just STJs may be involved. Srolovitz suggests that a process called grain boundary slip in these experiments could also affect material strength. In response, Jin says grain boundary shifting was the first mechanism that crossed his mind when he saw the data, but he and his colleagues couldn’t find any conclusive evidence for it.
Materials scientist Jörg Weismüller from the Technical University of Hamburg in Germany says the work is important. “The observation must have been astonishing for the authors that they saw such a strong and systematic effect,” he says. “I was particularly impressed that they found such an elegant explanation.”
Dan Garisto is a freelance science writer based in New York.
- Y.-Y. Zhang et al., “Surface triple junctions determine the strength of a nanoscale solid”, Phys. Rev. Lett.126, 235501 (2021).