Research| Computational Mechanics of Molucular Crystals

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Molecular crystalline materials have molecules, instead of atoms, at their lattice points. Unlike ionic or covalent crystals, molecular crystals are held together by weak van der Waals forces, dipole-dipole interactions or hydrogen bounding. A variety of materials with a wide rage of physical, chemical, and mechanical properties are molecular crystalline in nature. For a long time molecular crystals had been ignored as engineering or structural materials, therefore, our understanding of the physical and chemical properties of such materials is not well developed. Recent interest in these materials has been spurred by their use in electronic, photonic and bio-nanoporous applications. We are developing multiscale modeling methods to explore the mechanical properties of these crystals.

Recent projects

Energetic molecular crystals: Molecular crystalline materials such as HMX, RDX, PETN, and Fox7 are widely used as energetic materials in solid propellants and explosives. We have developed a crystal plasticity-based model that is capable of predicting the highly anisotropic, orientation and size dependent response of the monoclinic β-HMX (Octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine) crystals (P2₁ /c or, equivalently, P2₁ /n space group, Z=2 molecules per unit cell, symmetry axis=b; 13 independent elastic coefficients). Through so-called deformation distribution maps we have identified directions in the crystals which have the highest propensity for plastic deformation, and therefore damage.

Figure: The deformation distribution maps (DDMs) for β-HMX single crystal at different φ₂ sections.

Bionanoporous molecular crystals: Bionanoporous materials such as proteins crystals are highly ordered three-dimensional structures, in which the protein molecules bind to each other with specific intermolecular interactions. We have developed a nonlinear anisotropic crystal plasticity model for the tetragonal lysozyme crystal and found that the yield stress and critical resolved shear stress are highly sensitive to temperature and the amount of intracrystalline water.

Photovoltaic bi-molecular crystals: Bulk heterojunctions (BHJs) are bi-molecular crystals that have two distinct chemical species: an electron donor and an electron acceptor, coexisting in a thermodynamically stable arrangement. Research in this area is centered about designing the structures of these bi-molecular crystals to maximize efficiency.

Molecular electronic crystals: Organic semiconductors, such as thin film transistors, are appealing in low-cost flexible electronics as they afford design flexibilities, compatibility with a wide range of substrates and cost effectiveness. However, organic molecules are not usually good conductors. Hence there is significant interest in improving the conductivity by inserting impurity atoms or mix two different kinds of molecules in the crystal. Strain effects on the characteristics and damage of thin film inorganic transistors are well-known, which need to be quantified for these organic molecular semiconductors.

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Sponsors: ONR

Investigators:

Suvranu De, DSc.

Amir Zamiri, PhD

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