Crumple zones form a critical safety measure to protect passengers during vehicular incidents. In a recent Matter article, researchers at the Center for Molecular Modeling (CMM) translated this safety concept to the atomic level. By designing atomic crumple zones in an otherwise robust material following a new strain engineering approach, they introduced locally flexible regions. These regions focus the strain caused by external impacts. This way, the strain-engineered material partially retains its favourable adsorptive and catalytic properties under extreme conditions.
Vehicular crumple zones absorb mechanical impacts by deforming. In this way, they protect the remainder of the vehicle, including its passengers, from the most severe effects of accidents. A similar approach, but designed from the atomic level, would also benefit expensive materials used for high-end applications. For instance, metal-organic frameworks are highly porous solid-state materials. Most of their potential for applications, including as catalysts or as absorbents for molecules, arises from this porosity. However, it also makes these materials vulnerable to impact. Many become irreversibly amorphous under too high temperatures or pressures. This work overcomes this challenge by designing atomic crumple zones in these materials using a new in silico strain engineering approach. Just like their vehicular counterpart, the crumple zones focus the strain upon an external impact, and safeguard the remainder of the material. As a result, the attractive macroscopic properties of the strain-engineered structures are also partially retained.
How do strain-engineered crumple zones focus the strain from the atomic level?
The key to creating such crumple zones is the strain engineering approach. This computational tool predicts how structural changes in a material – from the atomic to the macroscopic level – deform the material’s structure. It does so by calculating so-called strain fields and considering their distribution in space and time. This strain field is a tensorial quantity measuring the deformation of a structure from its equilibrium, and this for each volume in the material. As the figure shows, these strain fields accurately capture small-scale structural changes. By removing only a handful of atoms, the strain-engineered structure focuses the strain in well-defined regions of the material. This behaviour contrasts with the homogeneous strain distribution in the original material. Hence, while the original material collapses entirely upon too high impacts, the strain-engineered material absorbs the strain in the flexible crumples zones. This partially preserves its adsorptive and catalytic properties. This phenomenon is reversible: upon removing the impact, the material can revert to its original state.
Besides exploring how different strain fields interact, this Matter article also hints towards the existence of a mechanical analogy to the Braess’s paradox. The Braess’s paradox was originally formulated in the frame of traffic planning. It states that adding roads to an existing road network may, counterintuitively, slow down overall traffic through the network. In a similar vein, by removing atoms in strain-engineered material and creating more porosity, one of the materials becomes more instead of less stable. Also here, the strain engineering protocol provides a straightforward interpretation.
Towards other applications
The potential for strain engineering is not limited to introducing crumple zones but can be easily generalised to other materials. For instance, earlier work showed that, in some of these framework materials, multiple phases can coexist simultaneously. This is a rare phenomenon in solid-state materials. Also in these framework materials, it only occurs under specific temperatures and pressures, or when adsorbing particular molecules. The Matter article explains through strain engineering that phase coexistence is characterised by a large strain gradient across the interface. In contrast, increasing the temperature or adsorbing molecules redistributes and homogenises the strain over the material. As a result, varying these parameters allows one to tune the occurrence of phase coexistence in these materials easily. Both examples demonstrate that strain engineering forms a new and indispensable tool in the computational material design toolbox.
These results were published in Matter, a Cell Press journal:
- Absorbing stress via molecular crumple zones: Strain engineering flexibility into the rigid UiO-66 materials
- Sven M.J. Rogge, Sander Borgmans, and Veronique Van Speybroeck
- Matter, http://doi.org/10.1016/j.matt.2023.02.009
- dr. ir. Sven M. J. Rogge, ir. Sander Borgmans, prof. dr. ir. Veronique Van Speybroeck
- Center for Molecular Modeling
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