Context
High-entropy (HE) materials are a new class of materials where configurational entropy is used to stabilize the systems. This is achieved by combining similar amounts of five or more components. High-entropy phases of bulk materials have been created out of metals, metal oxides, and metal carbides. However, these complex materials are not yet well understood at the nanoscale. With this project, we focus on high entropy nanoalloys that we want to analyze thanks to numerical simulation. The ordering effect of a surface, which is important at the nanoscale, can be expected to counteract the entropic stabilization due to the large configurational space challenging the understanding of experimentally observed high-entropy phases of nanoalloys.
The extreme structural and chemical diversity in combination with the stability of high-entropy phases at high temperature makes this class of materials relevant for practical use in the broader energy applications domain. Particular use cases include hydrogen evolution, carbon dioxide conversion, and rechargeable batteries. Because, in contrast to traditional materials, the systems would be entropically stabilized at elevated temperatures that are unavoidable in the mentioned applications leading to anti-fragile behaviour. In such a material, the free energy G = H-TS is dominated by the entropic term, that provides only a very minor contribution in traditional "low-entropy" materials.
Research Methodology
Theoretical understanding is currently mostly limited to phase diagram calculations (CALPHAD), and electronic structure calculations such as density functional theory (DFT). CALPHAD, however, relies on extrapolations from experimental data of "low entropy" systems, and it is thus difficult to make predictions concerning this new class of materials. DFT, on the other hand, is general and can address multi-component systems, but has the difficulty of prohibitive computational cost when the effect of temperature is to be considered which is however crucial for the understanding of entropy-stabilized phases. For the modelling part of this project, we opt therefore for semi-empirical interatomic potentials of the TB-SMA (tight-binding second moment approximation) type. These models have been extensively used for metal and few-component alloy nanoparticles. Yet modelling alloys with a high number of components, five or more, poses additional challenges, as parametrizations for each of the binary alloy components need to be established.
We want to address this challenge with a new machine learning approach for the parametrization of semi-empirical interatomic potentials. Using extensive computer simulation, a large number of parametrization-property relationships can be obtained. This database can then be used to train a relatively simple neural net that allows to do the inverse: finding a parametrization matching certain experimentally observed material properties.
Once such interatomic models are available, powerful global optimization techniques, such as basin hopping, can be used to explore the potential energy landscape of multi-component nanoalloy systems. In particular, we aim to evaluate to what extent the random mixing in the entropy-stabilized phase can be perturbed by a local ordering of atoms. It will also be possible to calculate atomic stress to identify regions where local swaps of atoms may occur naturally on experimental timescales.
Verification Criteria
The following criteria will have to be verified in order to confirm the presence of entropy-stabilized phases with the help of Monte Carlo and/or molecular dynamics simulations:
- There should be a temperature-driven reversible transition in and out of the entropy-stabilized phase with the high-entropy phase present at higher temperature.
- Removing one element from the alloy system decreases the configurational entropy and should thus either increase the transition temperature to the high-entropy phase or suppress it entirely.
- The configurational entropy term is maximized at equi-composition, therefore the transition temperature to the high-entropy phase should be lowest close to equi-composition.
- If the transition to the high-entropy is endothermic, the only reason for the stabilization at high temperature can be the entropic term. In case the transition is exothermic, entropy may nonetheless stabilize the high-temperature phase, but this must be determined by other factors.
The entropy-stabilized phase should also exclude elemental clustering and show no chemical ordering of the particles. Furthermore, thanks to the interatomic model, we will be in the position to establish structure–property relationships. We expect interesting new properties, such as reduced self-diffusion, lattice distortion due to different atomic radii, and new synergistic effects due to the combination of a high number of different atom types.