Relaxation Mechanisms Under Dynamical Loading of Metal-Organic Frameworks
Metal-organic frameworks (MOFs) are a class of nano-porous crystalline solids consisting of inorganic units coordinated to organic linkers. The unique molecular structures with ultra-high porosity, outstanding properties and tunable chemical functionality by various choices of metal clusters and organic ligands make this class of materials attractive for many applications. The shock-wave energy absorption is one of possible applications of MOFs and various computational and experimental studies have been devoted to understand their responses to pressure and low deformation rate phenomena. However, the deformation mechanisms beyond the unit cell level and the dynamical effects of MOFs in response to shock loading have not been understood yet. Under shock compression, the materials can undergo various deformation processes including elastic deformation, plastic deformation, phase transition, and structural decomposition or fracture. Thus, understanding of mechanical properties is the key first step for shock investigation. In this research, we first investigate the mechanical properties of MOFs using different levels of theory depending on the properties of interest. We use density functional theory (DFT) to calculate the elastic properties of various types of MOFs. The effect of temperature on elastic properties is also studied using ab-initio molecular dynamics (AIMD) simulations. We characterize the direction-dependent elastic moduli for various types of MOFs with increasing unit cell size and at different temperatures to establish the relationships between the MOFs structure and elastic properties, including anisotropy and temperature dependences. We find that although increasing the porosity of MOFs structure via the longer organic linker could improve the energy storage performance, it can reduce the mechanical stability of the MOFs structure. We also find that temperature has a significant effect on reducing the calculated elastic moduli due to the dynamics of flexible organic ligands. The mechanical properties of MOFs beyond the elastic regime have been investigated using the large-scale classical MD simulations in order to capture the nucleation and propagation of plastic deformation under quasistatic deformation. MOFs undergo plastic deformation, which involves volume collapse to relax under applied stress, leading to strain localization and the interaction between the local plastic bands. Under low deformation rate, the structural collapse of MOFs involves the bending of the flexible bonds between metal-oxygen units and organic linkers to avoid bond breaking. Based on what is known about the mechanical properties and structural transition of MOFs under applied pressure, we investigate the dynamical evolution of MOFs under shock loading along various orientations using MD simulations. We find that MOFs exhibit two-wave structure, with an elastic precursor followed by the second wave that corresponds to the structural transition. The second wave compresses the MOFs structure from elastic deformation (by the first wave) to the pore collapse state, resulting in the transition between two pressure-volume, or Hugoniot, curves. This transition involves large volume reduction and it is beneficial for mechanical energy absorption. A combined experimental and computational study shows that the shock-wave energy generated from the impact of a flyer plate is attenuated via large volume collapse of MOFs structure and the lower energy (~50%) is transmitted to the object in front of the wave. Finally, the simulations with reactive ReaxFF force field show that MOF undergoes chemical decomposition into the small molecules or the high-energy chemical species at high piston speed, and this is the indication of a possible endothermic chemical reaction, which could also absorb the shock energy.
Strachan, Purdue University.
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