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The recent decades have seen the discovery of very exciting elastic and thermo-elastic phenomena challenging the common view of how a material should behave. Materials with such counterintuitive properties are often called metamaterials.Some materials get fatter when stretched (and thinner when compressed). You can picture it by imagining a honeycomb, where the hexagons are replaced by bow-ties (re-entrant hexagons really). Tension along the length of the bow-ties turns them into fatter shapes, up to rectangles. Materials with this property were once thought to be very rare, but in fact they are quite common (if you know in which direction to apply the deformation that is). The property does not depend on the scale, that is why it is shown by crystals (at the scale of atoms) or by plastic foams (the kind found in mattresses).Some other materials shrink when heated. This property is also independent of the scale and can be found in crystals or in quite complex assembly of bimetallic strips. These two properties (negative poisson's ratio and negative thermal expansion are their technical names) have received some attention, some mechanism are understood, and applications are starting to be developed: blast curtains that open small holes to let the air through but catch flying shards of glass, dental fillings that expand with the tooth or beams that do not change shape at all with temperature (ideal for geostationary satellites) to cite a few.Another property is Negative Linear Compressibility (NLC in short). A material with NLC expands in one or two directions under pressure. As a consequence, it contracts a lot in the other directions. Because the equations that control linear expansion under pressure and change of volume under stress are the same, it results that materials with NLC become denser when stretched (and are called stretch-densified). Materials with NLC (or stretch-densified) are very rare, only 16 are known to have the property. The property is so rare hat many scientist are not even aware of its existence, and only three research publications have looked at it. Furthermore, amongst the 16 NLC materials, there is not a lot of obvious similarities, and preliminary studies hint that there are more than one reasons why materials would behave this way, some quite subtle.In this project, we study this bizarre property in depth, interpret the mechanisms responsible for it, fabricate real prototypes and explore the potential applications.At first, we will re-examine experimental data of elastic properties to see whether we can identify NLC in already characterised crystals. The equations are not especially complicated, but they are quite awkward. We have developed a special software (name ElAM, for Elastic Anisotropy Measures) and have already identified two materials that had been previously missed (they have the triclinic symmetry, the most complex, and show NLC in a strange direction).We will also use quantum base simulations (taking into account the atoms and electrons) to understand what happens when these crystals are compressed, and identify generic mechanisms for NLC.Nature provides us with a set of example structures and we will create large scale frameworks where the beams replace the bonds, first with very simple models of essentially springs and torsion springs, then with Finite Elements and finally by making the frameworks for real. For this, we will use rapid prototyping technologies, which can build complex shapes (and these frameworks certainly are) layer by layer.We will also interact with chemists to synthesise the more extreme crystals, and with industry, to see if we can develop applications, such as improved pressure sensors, pressure driven actuators (artificial muscles), coatings for deep ocean cables, or ultra-light, strong frameworks for aerospace.
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