Determination of the diffusion coefficient of foreign atoms in metals via surface segregation

Higher Temperatures Yield Smaller Grains in a Thermally Stable Phase-Transforming Nanocrystalline Alloy

Grains in crystalline materials usually grow with increased thermal exposure. Classical phenomena such as recrystallization may lead to a purely temporary decrease in the grain size, while recent advances in alloy design can yield thermally stable nanocrystalline materials in which grain growth stagnates. But grains never shrink, since there is a lack of interface-generating mechanisms at high temperatures, which are required to decrease the grain size if such was the system's thermodynamic tendency. Here we sidestep this paradigm by designing a nanocrystalline alloy having an allotropic phase transformation-an interface-generating mechanism-such that only the high-temperature phase is stabilized against grain growth. We demonstrate that for an Fe-Au alloy cycled through the α↔γ transformation, the high-temperature phase (γ-Fe) has a stable fine grain size, smaller than its low-temperature counterpart (α-Fe). The result is an unusual material in which an increase in temperature leads to finer grains that are stable in size.

Core(Fe)-shell(Au) nanoparticles obtained from thin Fe/Au bilayers employing surface segregation

Core(Fe)-shell(Au) nanoparticles are obtained by solid-state dewetting of thin Fe/Au bilayer films deposited on a sapphire substrate. The core-shell morphology is achieved by employing the equilibrium segregation phenomenon, where Au atoms form a homogeneous thin shell on the surfaces of an Fe nanoparticle and at its interface with the substrate, reducing the total interfacial energy of the system. The obtained nanoparticles are single crystalline (structurally perfect), thermally stable, and of high purity. Their size may be tuned by changing the initial film thickness. We demonstrate that the nanoparticles can subsequently be stripped from the substrate, and/or be modified by attaching thiol-containing organic molecules for use in various nanotechnology-related applications. The method presented herein may easily be extended to other metal combinations, especially those relevant for catalysis, thus helping to reduce precious-metal (e.g., Au, Pt, Rh) content in the catalyst.