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1 TASK Quarterly : scientific bulletin of Academic Computer Centre in Gdansk

1 2014

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Core-shell modelling of auxetic inorganic materials

Zammit V., Gatt R., Attard D., Grima J. N.

TASK Quarterly : scientific bulletin of Academic Computer Centre in Gdansk

2014

Vol. 18, No 2

137--165

This paper investigates the suitability of the General Utility Latti ce Program ( GULP ) for studying auxetic materials at the molecular level. GULP is a force-field based molecular modelling package which incorporates the ‘core-shell’ model for si mulating polarisability. A validation procedure was performed where the capability of GULP to reproduce the structural and mechanical properties of 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 t o study the all-silica forms of various ‘presumably auxetic’ zeolites. The simulations generally confi rmed 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, t hese results will result in generating more interest into the fascinating materials which could be used in many practical applications ( e.g. tuneable molecular sieves).