Electrum is a gold-silver alloy that has been around since antiquity, and which was even mentioned in the Iliad poems by Homer. The Lydians first minted coins from the alloy during Croesus’ reign, and it was then used in jewellery. Nowadays bimetallic alloys like Electrum, which have very different physical and chemical properties to their individual constitutive elements, are again creating a flurry of interest, this time in nanotechnology, especially in catalyst and medical applications.

To fully understand the behaviour of such bimetallic nanomaterials, researchers need to determine their phase diagrams, which are not at all like the phase diagrams of bulk alloys containing the same metals. The techniques employed to obtain bulk phase diagrams cannot be applied to nanoscale materials because of the thermal effects that come into play at small dimensions.

Classical thermodynamics no longer applies

We can describe bulk alloys in the framework of classical thermodynamics and statistical physics because of the large number of atoms and large volumes involved, and small changes at the surface do not really affect the bulk properties in any way. Nanoscale systems, on the other hand, are dramatically affected by any changes on the surface and classical thermodynamics no longer applies.

A team led by Miguel José-Yacaman and Robert Whetten have now devised two segregation rules that predict how the size and shape of various gold-silver polyhedral nanoparticles (ranging from 4–10 nm in diameter) in Electrum affect its phase diagram (that is, its melting temperature and melting enthalpy among other parameters). The researchers also looked at how these nanoparticles (which were shaped like tetrahedrons, cubes, octahedrons, cuboctahedrons and icosahedrons) separate out in the alloy and what affect this segregation has on the phase diagram.

Metal surface energy is important too

“Our first segregation rule says that if the bulk melting temperature of element A is larger than that of element B, then element A will diffuse to the surface,” explains team member and lead author of the study, Gregory Guisbiers. “If the bulk melting temperature of both elements is more or less the same, then the element will segregate according to how much surface energy it has – and this is our second rule.”

In fact, the second rule says that if the solid surface energy of element A is smaller than that of element B, then element A will separate out to the surface, he adds. “To be completely precise, we have to determine the miscibility of the alloy using the so-called Hume-Rothery rules before applying our two segregation rules,” says Guisbiers. “When the alloy is completely or partially miscible, the first rule applies. And when it is completely miscible, then only the second rule applies.”

Silver preferentially segregates to the surface

The calculations by the researchers revealed that silver preferentially segregates to the surface of the alloy for all the polyhedral shapes studied, a result that agrees well with the latest transmission electron microscopy observations and energy dispersive spectroscopy analyses, but which, surprisingly goes against previous theories that predicted gold to predominate at the surface.

“Our work will be especially important in nanometallurgy because we now have the tools to predict which element will segregate to the surface in a bimetallic nanoalloy,” Guisbiers tells nanotechweb.org. “Being able to do this will be tremendously important for the automotive industry, for example.”

The team, reporting its work in ACS Nano DOI: 10.1021/acsnano.5b05755, says that it would now like to extend its segregation rules to ternary nanoalloys.