Daniel Soto-Parra

Statistical similarity between the compression of a porous material and earthquakes

It has long been stated that there are profound analogies between fracture experiments and earthquakes; however, few works attempt a complete characterization of the parallels between these so separate phenomena. We study the acoustic emission events produced during the compression of Vycor (SiO(2)). The Gutenberg-Richter law, the modified Omoris law, and the law of aftershock productivity hold for a minimum of 5 decades, are independent of the compression rate, and keep stationary for all the duration of the experiments. The waiting-time distribution fulfills a unified scaling law with a power-law exponent close to 2.45 for long times, which is explained in terms of the temporal variations of the activity rate.

Elastocaloric and magnetocaloric effects in Ni-Mn-Sn(Cu) shape-memory alloy

We have studied magnetocaloric and elastocaloric properties of a Ni-Mn-Sn(Cu) metamagnetic shape-memory alloy undergoing a magneto-structural transition (martensitic type) close to room temperature. Changes of entropy have been induced by isothermally applying both mechanical (uniaxial stress) and magnetic fields. These entropy changes have been, respectively, estimated from dilatometric measurements giving the length of the sample as a function of temperature at selected applied forces and magnetic fields and from magnetization measurements as a function of temperature at selected applied magnetic fields. Our results indicate that the elastocaloric effect is conventional and occurs in two steps which reflect the interplay between the martensitic and the incipient magnetic transitions. By contrast, the magnetocaloric effect is inverse and occurs in a single step that encompasses the effect arising from both transitions.

Fe and Co selective substitution in Ni2MnGa: Effect of magnetism on relative phase stability

We studied the effect of selective substitution in Fe and Co 3d elements on the structural and magnetic phase transitions in the prototypical magnetic shape memory alloy Ni–Mn–Ga. We determined the phase diagram for each elemental doping substitution by means of calorimetry and ac susceptibility measurements. An effort was made to substitute each constituent element by the same amounts of doping and to study the role of parameters other than electronic concentration in controlling phase stability. Specifically, selective doping with atoms of similar atomic radii but different magnetic properties allowed us to investigate the role of magnetic interactions on the relative phase stability of the ternary compound. We determined the entropy change associated with the martensitic transition for each quaternary alloy to obtain further information on the effect of magnetism on the relative stability of the involved phases exhibited by these compounds.

Failure mechanism in porous materials under compression: crackling noise in mesoporous SiO2

The failure mechanism of porous materials under uniaxial stress has been investigated experimentally. Microporous SiO2, Vycor, has been subjected to slowly increasing compressive uniaxial stress with stress rates between 0.2 and 12.2 kPa/s. With increasing stress the strain changes stepwise with acoustic emission correlated with each volume collapse. The acoustic emission followed the characteristics of ‘crackling noise’ with a power law distribution over an exceptionally large interval of 6 decades at the slowest stress rate. The power law exponent is −1.39. Possible applications in mining industry and others are discussed.

Stress- and magnetic field-induced entropy changes in Fe-doped Ni–Mn–Ga shape-memory alloys

Isothermal stress- and magnetic field-induced entropy changes in a Fe-doped Ni–Mn–Ga alloy have been measured in the limits of low applied stress and magnetic field. We have obtained that in this limit while elastocaloric is conventional, giving rise to an increase of entropy when a stress is applied, magnetocaloric effect is inverse, which means that entropy decreases by application of an applied magnetic field. This inverse effect is a consequence of the magnetostructural coupling driven by the martensitic transition.

Recrystallization of a Ti-45Ni-5Cu cold-worked shape memory alloy characterized by thermoelectric power and electrical properties

The recrystallization of a cold-worked shape memory alloy (SMA) without R-phase transition, i.e. the Ti-45.0Ni-5.0Cu (% at.), has been studied by thermoelectric power (TEP) technique and electrical resistivity change (Dρ) on several heat treated samples after cold working. This study was also supported by differential scanning calorimetry (DSC), transmission electron microscopy (TEM) and hardness Vickers (HV) measurements. Transformation temperatures show an increase with heat treatment temperature, while hysteresis decreases. Hardness values decrease with aging temperature for the same heat treatment time. No important changes were observed in electrical resistivity with heat treatment temperature, except at 300oC. TEP results show an important increase with heat treatment temperature, and a linear relationship was found between hardness and thermoelectric power. With this correlation between TEP and microhardness it is possible to predict its mechanical response by knowing its TEP, so this correlation could be important for applications.

Acoustic emission avalanches during compression of granular manganites

We have studied acoustic emission (AE) during compression of La0.8Ca0.28Sr0.08MnO2.7 manganites with different microstructures obtained by selected synthesis techniques. In ceramic samples with large grains obtained by a solid-state method, avalanche criticality is confirmed when grain fracture is the AE dominant mechanism. In samples synthesized by means of micro-wave and sol-gel techniques, grains are much smaller and the AE is mainly originated from friction effects associated with relative displacement of grains during deformation. In this case, significant deviations from avalanche criticality have been detected.We have studied acoustic emission (AE) during compression of La0.8Ca0.28Sr0.08MnO2.7 manganites with different microstructures obtained by selected synthesis techniques. In ceramic samples with large grains obtained by a solid-state method, avalanche criticality is confirmed when grain fracture is the AE dominant mechanism. In samples synthesized by means of micro-wave and sol-gel techniques, grains are much smaller and the AE is mainly originated from friction effects associated with relative displacement of grains during deformation. In this case, significant deviations from avalanche criticality have been detected.