Eduard Vives

Elastocaloric effect associated with the martensitic transition in shape-memory alloys.

The elastocaloric effect in the vicinity of the martensitic transition of a Cu-Zn-Al single crystal has been studied by inducing the transition by strain or stress measurements. While transition trajectories show significant differences, the entropy change associated with the whole transformation (DeltaS_(t)) is coincident in both kinds of experiments since entropy production is small compared to DeltaS_(t). The values agree with estimations based on the Clausius-Clapeyron equation. The possibility of using these materials for mechanical refrigeration is also discussed.

Large temperature span and giant refrigerant capacity in elastocaloric Cu-Zn-Al shape memory alloys

We report on the elastocaloric effect in a Cu-Zn-Al shape memory alloy. We show that both the isothermal entropy and adiabatic temperature changes are large and reproducible upon field cycling over a very broad temperature span of ∼130 K. The combination of large entropy and such a broad temperature span results in an outstanding refrigerant capacity of ∼2300 J/kg.

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.

Athermal Character of Structural Phase Transitions

The significance of thermal fluctuations in nucleation in structural first-order phase transitions has been examined. The prototypical case of martensitic transitions has been experimentally investigated by means of acoustic emission techniques. We propose a model based on the mean first-passage time to account for the experimental observations. Our study provides a unified framework to establish the conditions for isothermal and athermal transitions to be observed.

Modeling premartensitic effects in Ni 2 MnGa: A mean-field and Monte Carlo simulation study

The degenerate Blume-Emery-Griffiths model for martensitic transformations is extended by including both structural and magnetic degrees of freedom in order to elucidate premartensitic effects. Special attention is paid to the effect of the magnetoelastic coupling in

Large entropy change associated with the elastocaloric effect in polycrystalline Ni-Mn-Sb-Co magnetic shape memory alloys

{\mathrm{Ni}}_{2}\mathrm{MnGa}.

Search for charmless B → VV decays

The microscopic model is constructed and justified based on the analysis of the experimentally observed strain variables and precursor phenomena. The description includes the (local) tetragonal distortion, the amplitude of the plane-modulating strain, and the magnetization. The model is solved by means of mean-field theory and Monte Carlo simulations. This last technique reveals the crucial importance of fluctuations in pretransitional effects. The results show that a variety of premartensitic effects may appear due to the magnetoelastic coupling. In the mean-field formulation this coupling is quadratic in both the modulation amplitude and the magnetization. For large values of the magnetoelastic coupling parameter we find a premartensitic first-order transition line ending in a critical point. This critical point is responsible for the existence of large premartensitic fluctuations which manifest as broad peaks in the specific heat, not always associated with a true phase transition. The main conclusion is that premartensitic effects result from the interplay between the softness of the anomalous phonon driving the modulation and the magnetoelastic coupling. In particular, the premartensitic transition occurs when such coupling is strong enough to freeze the involved mode phonon. The implication of the results in relation to the available experimental data is discussed.

Direct observation of the magnetic-field-induced entropy change in Gd5(SixGe1−x)4 giant magnetocaloric alloys

We report on compressive strain measurements in polycrystalline magnetic shape memory alloys aimed at determining the entropy change associated with their elastocaloric effect. It is shown that for a maximum applied stress of 100 MPa, the stress-induced entropy change amounts to ΔS=21 J/kg K. This value compares well to the values reported for nonmagnetic shape memory alloys, and it is of the same order as those reported for the best giant magnetocaloric materials at moderate magnetic fields.

Driving rate effects in avalanche-mediated first-order phase transitions

Within the Tsallis thermodynamics framework, and using scaling properties of the entropy, we derive a generalization of the Gibbs-Duhem equation. The analysis suggests a transformation of variables that allows standard thermodynamics to be recovered. Moreover, we also generalize Einsteins formula for the probability of a fluctuation to occur by means of the maximum statistical entropy method. The use of the proposed transformation of variables also shows that fluctuations within Tsallis statistics can be mapped to those of standard statistical mechanics.

THE USE OF SHAPE-MEMORY ALLOYS FOR MECHANICAL REFRIGERATION

Direct observation of the entropy change in a first-order phase transition is obtained by using a differential scanning calorimeter in which the transition is field-induced under the application of an external magnetic field. This procedure enables direct evaluation of the magnetocaloric effect in materials showing first-order magnetostructural phase transitions. Results for Gd5(SixGe1−x)4 giant magnetocaloric alloys are reported. Calorimetric curves sweeping the field through the transition reveal a unusual increase of the entropy change with cycling. This increase is accounted for by considering both the structural and magnetic contributions to the total entropy change.