25 research outputs found
Colossal magnetocaloric effect in magneto-auxetic systems
We show that a mechanically driven magnetocaloric effect (MCE) in magneto-auxetic systems (MASs) in the vicinity of room temperature is possible and the effect can be colossal. Even at zero external magnetic field, the magnetic entropy change in this reversible process can be a few times larger in magnitude than in the case of the giant MCE discovered by Pecharsky and Gschneidner in Gd5(Si2Ge2). MAS represent a novel class of metamaterials having magnetic insertions embedded within a non-magnetic matrix which exhibits a negative Poisson's ratio. The auxetic behaviour of the non-magnetic matrix may either enhance the magnetic ordering process or it may result in a transition to the disordered phase. In the MAS under consideration, a spin 1/2 system is chosen for the magnetic component and the well-known Onsager solution for the two-dimensional square lattice Ising model at zero external magnetic field is used to show that the isothermal change in magnetic entropy accompanying the auxetic behaviour can take a large value at room temperature. The practical importance of our findings is that MCE materials used in present engineering applications may be further enhanced by changing their geometry such that they exhibit auxetic behaviour.peer-reviewe
Smart hexagonal truss systems exhibiting negative compressibility through constrained angle stretching
The support of the Malta Council of Science and Technology through their national R&I programme as well as the support of the University of Malta is gratefully acknowledged.Negative compressibility is the ability to expand in at least one dimension rather than shrinking upon the application of an externally applied hydrostatic pressure. It is shown that, contrary to current perception, negative linear compressibility may be obtained from re-entrant hexagonal truss systems of specific geometric features which deform through non-equal changes in the lengths of the cell walls when deforming through a constrained angle stretching rather than other modes of deformation (such as flexure or hinging, modes of deformation that also lead to auxetic behaviour in honeycombs). Negative compressibility is predicted in the vertical direction for particular re-entrant geometries of this smart hexagonal truss system when the vertical ribs are much stiffer than the inclined ribs.peer-reviewe
On non-measurable sets and invariant tori
The question: "How many different trajectories are there on a single
invariant torus within the phase space of an integrable Hamiltonian system?" is
posed. A rigorous answer to the question is found both for the rational and the
irrational tori. The relevant notion of non-measurable sets is discussed.Comment: 8 pages, 1 figur
Poisson’s Ratio of the f.c.c. Hard Sphere Crystals with Periodically Stacked (001)-Nanolayers of Hard Spheres of Another Diameter
The results of studies on the influence of periodically stacked nanolayer inclusions, introduced into the face-centered cubic (f.c.c.) hard sphere crystal, on Poisson’s ratio of the obtained nanocomposite system are presented. The monolayers are orthogonal to the [ 001 ] -direction. They are formed by hard spheres with diameter different from the spheres forming the matrix of the system. The Monte Carlo computer simulations show that in such a case the symmetry of the system changes from the cubic to tetragonal one. When the diameter of the inclusion spheres increases at certain range, a decrease of the negative Poisson’s ratio in the [ 101 ] [ 1 ¯ 01 ] -directions is observed, i.e., the system enhances its partial auxeticity. The dependence of the maximal, average, and negative parts of the minimal Poisson’s ratio on the direction of the applied load are shown in a form of surfaces in spherical coordinates, plotted for selected values of nanolayer particle diameters. The most negative value of the Poisson’s ratio found among all studied systems was − 0.11 (at pressure p * = 100 , which is about ten times higher than the melting pressure) what is almost twice more negative than in the f.c.c. crystal of identical hard spheres. The observed effect weakens along with the decrease of pressure and becomes hardly noticeable near melting. This study indicates that modifying only the size of the inclusion particles one can change Poisson’s ratio of nanocomposites at high pressures
Extreme Poisson’s Ratios of Honeycomb, Re-Entrant, and Zig-Zag Crystals of Binary Hard Discs
Two-dimensional (2D) crystalline structures based on a honeycomb geometry are analyzed by computer simulations using the Monte Carlo method in the isobaric-isothermal ensemble. The considered crystals are formed by hard discs (HD) of two different diameters which are very close to each other. In contrast to equidiameter HD, which crystallize into a homogeneous solid which is elastically isotropic due to its six-fold symmetry axis, the systems studied in this work contain artificial patterns and can be either isotropic or anisotropic. It turns out that the symmetry of the patterns obtained by the appropriate arrangement of two types of discs strongly influences their elastic properties. The Poisson’s ratio (PR) of each of the considered structures was studied in two aspects: (a) its dependence on the external isotropic pressure and (b) in the function of the direction angle, in which the deformation of the system takes place, since some of the structures are anisotropic. In order to accomplish the latter, the general analytic formula for the orientational dependence of PR in 2D systems was used. The PR analysis at extremely high pressures has shown that for the vast majority of the considered structures it is approximately direction independent (isotropic) and tends to the upper limit for isotropic 2D systems, which is equal to +1. This is in contrast to systems of equidiameter discs for which it tends to 0.13, i.e., a value almost eight times smaller
Valores δO<sup>18</sup> de los basaltos alcalinos de la región volcánica de Olot, La Garrotxa, Cataluña
The basaitic lavas from the Olot volcanic region exhibit δO<sup>18</sup> values which range between +5.6 and +6.5 ‰. These values indicate that the magmas derived from a mantle source with a oxygen isotopic signature similar to other continental settings where this type of volcanism is presento Furthermore, since these data allow to discard any kind of crustal contamination, they confirm that partial melting and fractional crystallization are the only processes which led to the generation of these rocks.<br><br>Las lavas basálticas de la región volcánica de Olot presentan unos valores δO<sup>18</sup> comprendidos entre +5,6 y +6,5 ‰. Estos valores indican que los magmas han derivado de un manto que tiene una composición isotópica de oxígeno semejante a la de otras áreas continentales, en las que se ha generado un volcanismo de esta naturaleza. Asimismo, confirman que los fenómenos de fusión parcial y cristalización fraccionada son los únicos que han intervenido en la generación de estas rocas, ya que se puede descartar cualquier tipo de interacción cortical
Computational Modelling of Structures with Non-Intuitive Behaviour
This paper presents a finite-element analysis of honeycomb and re-entrant honeycomb structures made of a two-phase composite material which is optimized with respect to selected parameters. It is shown that some distributions of each phase in the composite material result in the counter-intuitive mechanical behaviour of the structures. In particular, negative values of effective Poisson’s ratio, i.e., effective auxeticity, can be obtained for a hexagonal honeycomb, whereas re-entrant geometry can be characterized by positive values. Topology optimization by means of the method of moving asymptotes (MMA) and solid isotropic material with penalization (SIMP) was used to determine the materials’ distributions