Modelling of the mechanical and thermal properties in zeolites and related frameworks

Abstract

The Poisson's ratios, isothermal compressibilities and thermal expansion coefficients of common materials normally have positive values but, in some unusual cases, they may also have negative values. This work reports a number of studies relating to systems that can exhibit these counterintuitive negative thermo-mechanical properties. Static simulations of the mechanical properties of frameworks of APC and APD, two closely related aluminophosphates (AlPOs), suggests that these systems exhibit negative Poisson's ratio (NPR), auxetic, behaviour in the (100) plane. More interestingly, results suggest that APD also exhibits auxeticity in the (001) plane, while APC exhibits a small, negative linear compressibility (NC) in the [100] direction, where both effects can be explained in terms of rotating quasi-rigid units. Molecular dynamics simulations have also been performed to study in more detail the behaviour of APC under different pressure and temperature conditions. The results not only confirm the anomalous pressure behaviour of APC, but also suggest that it may also exhibit negative thermal expansion (NTE), and that it can undergo a pressure as well as a temperature induced phase transformation. A number of novel systems that, like APC and APD, can also exhibit combined negative thermo-mechanical properties but on a macroscale are also proposed and discussed. In particular, using finite element methods, it is shown that a new composite made from a non re-entrant hexagonal honeycomb embedded in a soft rubbery matrix can exhibit negative out-of-plane Poisson's ratios due to bulging out of the matrix when the honeycomb deforms in-plane. Another novel system which is shown to exhibit tuneable negative/positive in-plane Poisson's ratio consists of a honeycomb with T-shaped junctions. It is also shown that if composite ribs are used to construct these and other similar honeycombs then, through the correct choice of thermal and mechanical properties of the materials, it is possible to design honeycombs which exhibit negative thermal expansion and/or negative compressibility making them truly negative and multi-functional systems. It is also shown that such smart honeycombs can switch from negative to positive Poisson's ratio by changes in temperature and/or pressure.