Auxetic

Fig. 4. Effective metal charge, Z, in chromium(III) complexes as a function of the nephel-auxetic parameter, P, and the Racah interelectronic repulsion parameter, B...

There is no elaborate theory that can account properly for the nephel-auxetic parameter and the same is likely to be true for the AOM parameters. Still, nephelauxetism has proved a useful concept in inorganic chemistry and we believe the same will be true of the AOM parameters. 

In this section, three illustrative examples of the impact of scale on mechanical properties are outlined. First, solid lubricants are discussed, underlining the connection between crystal structure and the observed mechanical properties. Second, auxetic materials, in which crystal structure and microsttuc-ture combine to produce materials with negative values of Poisson s ratio, are discussed. Last, thin films, in which mechanical properties are measured by methods similar to that used in the bulk, are considered. 

Figure 10.32 An auxetic network. The bonds (drawn as lines) between elements of the stmcture are of fixed length, but the links (circles) are flexible, (a) Initial state, (b) Configuration under an applied force, F the material expands both parallel and perpendicular to the direction of the force...

Surprisingly, many cubic metals behave in a similar fashion, although the effect has been masked by the fact that the mechanical properties are most often measured on poly crystalline solids. For example, the commonplace metallic alloy /3-brass, CuZn, which has the CsCl structure, is noticeably auxetic. A tensile stress applied along the [001] direction... 

Figure 10.33 A solid composed of rigid blocks of crystal linked by bonds of constant length, but with flexible links to the crystallites (a) A normal solid becomes thinner under tension (right-hand side) (c) A material with re-entrant bonds (left-hand side) is auxetic and becomes thicker under tension (right-hand side)...

It has been found that many cubic metals with stmctures related to the body-centred cubic stmcture are, in fact, auxetic. A number of silicates also show this unusual property. In each case, the relationship between the bonding and stmcture controls the response to the tensile stress. 

In the families of cordierite, yS-eucryptite, /3-cordierite and NZP, a mechanism similar to that giving rise to auxetic (negative Poisson s ratio) materials seems to occur (Section 10.3.2). The stmcture is built from inflexible layers, similar to those found in clay minerals (see Sub-section... 

The negative sign is to ensure that the numerical value of Poisson s ratio is positive for a normal material that becomes thinner as it is stressed. Auxetic materials have a negative value of is, and become fatter when stressed. 

Auxetic materials have a unique property— when they are stretched, they actually become thicker in the directions perpendicular to the applied force Think about this in comparison to anything else you stretch it is really quite a stremge phenomenon. This occurs as a result of hinged arrangements within the materied that flex apart when a... 

Lastly, it has been predicted that auxetic properties (i.e. negative Poisson ratio orthogonal contraction upon axial compression, and vice versa), which are closely related to NTE, may also arise in MOFs, but this highly sought after behaviour is yet to be reported. 

In a realistic situation the adhesive filament will not act as a perfect elastic body uniformly stressed up to fracture. Uneven stress distributions and plastic yielding would be expected to increase the energy dissipation observed beyond that calculated for the ideal elastic model. It will be very interesting to see whether in the future auxetic materials can be developed to an extent that they can be used as coatings for such porous substrates. Even greater increases in fracture energy can then be anticipated. 

R H Baughman, Auxetic materials Avoiding the shrink. Nature 425, 667 (16 October 2003) doi 10.1038/425667a... 

The table is ordered within a certain symmetry type according to the spectrochemical series of the ligands (3). The molar extinction coefficients, assignments, crystal field parameters A, RAC AH parameters B and the nephel-auxetic ratio S (4) are given in so far as they could be calculated from the position of the ligand field bands. For non cubic cases A and B have of course only approximate values. 

Actuators Fabrics able to actuate or move some aspect of their environment, such as electroactive and auxetic fabrics. 

Although the possibility of materials with a negative Poisson s ratio had been recognized theoretically it is only comparatively recently, since the mid 1980s, that examples of such materials have become available. These materials expand laterally when stretched and contract laterally when compressed, and are called auxetic from the classical Greek word auxetos, meaning to increase. 

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