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Smart Cellular Systems with Pressure Dependent Poisson’s Ratios

Attard D., Cauchi R., Gatt R., Grima-Cornish J. N., Grima J. N.

Computational Methods in Science and Technology

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2020

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Vol. 26, No. 2

37--45

EN

The Poisson’s ratio behaviour of cellular systems which change their internal features when subjected to pressure change to become a “re-entrant” or “non-re-entrant” honeycomb was investigated. It was shown, through finite elements simulations, that these changes in geometry permit the systems to exhibit a wide range of Poisson’s ratios, the magnitude and sign of which can be controlled through the external pressure. Auxetic behaviour was also shown to be obtainable at specific pressures with the right combination of design and materials.

2

Mechanical Properties of 2D Flexyne and Reflexyne Polyphenylacetylene Networks: A Comparative Computer Studies with Various Force-Fields

Trapani L., Gatt R., Mizzi L., Grima J. N.

TASK Quarterly : scientific bulletin of Academic Computer Centre in Gdansk

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2015

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Vol. 19, No 3

237--296

EN

Auxetic materials exhibit the very unusual property of becoming wider when stretched and narrower when compressed, – they have a negative Poisson’s ratio. This unusual behaviour is the source of many desired effects in the materials’ properties and it is therefore, no wonder that auxetics are described as being superior to conventional materials in many practical applications. Here we make use of force-field based molecular modelling simulations in order to investigate the mechanical properties of polypehyleacetylene systems known as (n, m)-flexyne and (n, m)-reflexyne in an attempt to extend the existing knowledge there is regarding these systems. These systems have already attracted considerable consideration since negative on-axis Poisson’s ratios have been discovered for the reflexynes. We first developed a methodology for the modelling and property determination of flexyne and reflexyne network systems which we validated against existing published data. Then, extended the study to prove the simulated results were independent of the modelling methodology or the force-field used. In particular, we showed that on-axis auxeticity in the reflexynes is a force-field independent property, i.e. a property which is not an artefact of the simulations but a property which is likely to be present in the real materials if these were to be synthesised. We also studied and reported the shear behaviour of these systems were we show that the flexynes and reflexynes have very low shear moduli, a property which regrettably limits the prospects of these systems in many practical applications. Finally we examine the in-plane off-axis mechanical properties of the systems and we report that in general, these mechanical properties are highly dependent on the direction of loading. We also find that the auxeticity exhibited by the reflexynes on-axis is lost when these systems are loaded off axis since the Poisson’s ratios becomes positive very rapidly as the structure is stretched slightly off-axis (e.g. 15deg off-axis). This is once again of great practical significance as it highlights another major limitation of these systems in their use as auxetics.

3

Simulations of the properties of elongated hexagonal dodecahedron systems

Grima J. N., Caruana-Gauci R., Attard D., Gatt R.

TASK Quarterly : scientific bulletin of Academic Computer Centre in Gdansk

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2014

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Vol. 18, No 2

117--135

EN

This study considers a 3D basic unit-cell proposed for auxetic and non-au xetic foams namely the elongated hexagonal dodecahedron deforming through changes in angle b etween its ligaments (idealised hinging model). This structure was studied in detail for the potential of exhibiting negative Poisson’s ratio and/or negative compressibility b y means of a method based on standard force-field molecular modelling technique, termed as Empiri cal Modelling Using Dummy Atoms ( EMUDA ). The mechanical properties obtained from this method were then compared to a previously published analytical model of this structure [Grima J N, CaruanaGauci R, Attard D, and Gatt R 2012, Proc. Roy. Soc. A 468 3121], and found to be in good agreement with each other. The results showed that this system can ex hibit zero Poisson’s ratios in one of its planes and positive or negative Poisson’s ratios in other plan es, depending on the geometry of the model. It was also shown that under certain conditions, ne gative linear and/or area compressibility was also exhibited.

4

Modelling of the static and dynamic properties of tho-type silicates

Cauchi R., Grima J. N.

TASK Quarterly : scientific bulletin of Academic Computer Centre in Gdansk

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2014

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Vol. 18, No 1

5--65

EN

Auxetic materials are materials exhibiting a negative Poisson’s ratio in one of their planes. This phenomenon has been studied in various materials. Zeolites are crystalline substances whose structure is characterised by the framework of linked tetrahedra, each consisting of four oxygen atoms surrounding a cation. The resulting interstitial spaces make them efficient for use as adsorbents and molecular sieves, and many studies have been focused on this aspect. Some of these zeolites may exhibit auxeticity at least in one of their planes. THO (and similar systems, such as NAT and EDI) together with the all-silica equivalent of these have been studied extensively via static simulations for their negative Poisson’s ratio in the (001) plane. In this paper a study of the all-silica equivalent of THO has been carried out via both static and dynamic simulations using the same force-field, where the system was subjected to stress along the x direction. The hypothesised semi-rigid mechanism of deformation, proposed by Grima et al. was then projected over this framework. The results obtained confirmed auxeticity along this plane by means of the COMPASS force-field, in both static and dynamic studies and compared well with the proposed mechanism of semi-rigid rotating polygons. It also showed that as the Young’s modulus of this mechanism increases other mechanisms of deformation increase in importance.

5

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

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2014

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Vol. 18, No 2

137--165

EN

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).

6

Negative Poisson's ratios from rotating rectangles

Grima J. N., Alderson A., Evans K. E.

Computational Methods in Science and Technology

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2004

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Vol. 10, nr 2

137-145

EN

Materials with a negative Poisson's ratio exhibit the unexpected property of becoming fatter when stretched and narrower when compressed. This counter-intuitive behaviour is known as 'auxetic behaviour' and imparts many beneficial effects on the material’s macroscopic properties. This paper discusses the potential of systems composed of rigid rectangles connected together through flexib1e hinges at their vertices. It will be shown that, on application of uniaxial loads, these rigid rectangles will rotate with respect to each other to form, in some cases, a more open structure hence giving rise to a negative Poisson's ratio.

7

Molecular modelling of the deformation mechanisms acting in auxetic silica

Alderson A., Alderson K. L., Evans K. E., Grima J. N., Williams M. R., Davies P. J.

Computational Methods in Science and Technology

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2004

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Vol. 10, nr 2

117-126

EN

Molecular mechanics simulations have been performed to undertake a systematic investigation into the structure and mechanical properties of alfa-cristobalite undergoing uniaxial loading along each of the 3 mutually orthogonal principal directions and also hydrostatic pressure loading. Simulations were performed using both the BKS and Burchart force-fields. The simulations indicate that pressure loading and uniaxial loading along the x3 direction leads to uniform variation of the four independent Si-O-Si intertetrahedral angles, indicative of cooperative tetrahedral rotation about tetrahedral axes which transform the alfa-cristobalite structure into the 'idealised' beat-cristobalite structure. Uniaxial loading along either of the transverse directions (x1; and x2) leads to a divergence of the intertetrahedral angles, consistent with tetrahedral rotation about the tetrahedral axes which transform the idealised beat-cristobalite structure into the "ordered' beta-cristobalite structure. The data also indicate that a phase transition to one of the proposed beta phases may be induced by a negative hydrostatic pressure or tensile stress along x3. The phase transition is accompanied by a change in sign of some of the Poisson's ratios (i.e. from positive to negative). A negative hydrostatic pressure is also predicted to lead to conversion of initially positive to negative Poisson's ratio values (within the same phase).