Modelling of negative Poisson's ratio nanomaterials: Deformation mechanisms, structure-property relationships and applications

Analytical and Molecular Mechanics methods have been used to study the deformation mechanisms acting at the molecular level in the auxetic polymorph of crystalline silica (alpha-cristobalite). The analytical models indicate that alpha-cristobalite deforms by concurrent tetrahedral dilation and cooperative rotation when stretched along the x(3) axis, and that a second phase is predicted to exist for this loading scenario, having a geometry similar to that of 'idealised' beta-cristobalite. This is supported by preliminary Molecular Mechanics simulations, which also indicate that the cooperative rotation predicted for loading along x(3) is not sufficient to describe the deformation mechanism for loading along x(1). A negative hydrostatic pressure offset is observed to lead to a change in the sign of the predicted Poisson's ratio from positive to negative, leading to improved agreement of the Molecular Mechanics model with experiment.