Core-shell modelling of auxetic inorganic materials

Abstract

Materials which a negative Poisson's ratio (auxetic) undergo a lateral expansion upon being subjected to a uniaxial load. This property may exits on various metrological scales ranging from the nanoscale (molecular level) to the macroscale. Auxetic behaviour results in various improved characteristics which means that these materials may be exploited in many applications. This dissertation investigates the suitability of the General Utility Lattice Program (GULP) for studying auxetic materials at molecular level. GULP is force-field based molecular modelling package which incorporates the 'core-shell' model for simulating polarisability. In this dissertation, a validation procedure was performed where the capability of GULP to reproduce the structural and mechanical properties of α-cristobalite (an auxetic silicate which has been thoroughly studied both experimentally and through modelling) and SOD (a zeolite for which the single crystalline elastic constants have been experimentally measured). It was found that not all GULP libraries (force-fields) could reproduce these properties, although the Catlow 1992 and Sauer 1997 libraries were found the produce good results. These libraries were then used to study the all-silica forms of various 'presumably auxetic' zeolites. The simulations generally confirmed the conclusions reported in earlier studies, and in particular, the fibrous zeolites THO, NAT and EDI where. once again shown to be auxetic in the (001) plane. A study was also performed aimed at assessing the effect of interstitial species on the mechanical properties of NAT where it was shown that these species reduce the auxetic effect. This is very significant as once again we have confirmed the potential of these materials as molecular level auxetics, and hopefully, these results will result in generating more interest into the fascinating materials which could be used in many practical applications (e.g. tuneable molecular sieves).