Modelling of crystalline systems exhibiting anomalous properties

Abstract

Auxetic materials are materials with negative Poisson's ratios, i.e. they exhibit the unusual property of expanding when stretched and contracting when compressed. There have been numerous hypothetical and natural auxetic material that have attracted considerable attention in the recent years especially; hypothetical organic networked polymers and both natural occurring and hypothetical natrolitc-type silicates. This work discusses three different designs of hypothetical two-dimensional organic networked polymers (the reflexynes, the polytriangles and the polycalixes), for which it was concluded that they all possess different mechanical properties, with polytriangles being quasi isotropic in the plane of the networks, while the other networks especially reflexyne are highly anisotropic. This study suggests that polytrianglc and polycalix systems are effectively more auxetic than those based on the re-entrant model due to its weak shear, which result in highly positive off-axis Poisson's ratio. The study includes a preliminary study on the effect of external hydrostatic pressure on the elastic constants, of the Si02 equivalent of the zeolite natrolite (chapter 4) and real system of natrolite, Na2[AbSi301ol2H20 (chapter 5), and in particular the way the external pressure affects the Poisson's ratio. For both systems it was found that the lowest values of the Poisson's ratio were not exhibited at normal atmospheric pressure. However for the all-silicon equivalent natrolite maximum auxeticity was found at a positive external hydrostatic pressure, while for the natrolite proper maximum auxeticity was found at a negative external hydrostatic pressure, suggesting that the intra-framework species in the latter play an important role in the deformation mechanism.