Daniel Enrique Soto-Parra

Solid-state refrigeration has emerged as the most promising alternative to conventional refrigeration technology. However, for this technology to be applicable, the caloric effects produced in the alloys must be highly reversible. In this context, we compare the elastocaloric effect of two Ni–Mn–Ga alloys with different types of modulated martensite. The elastocaloric effect, quantified as the isothermal entropy change (ΔSela), was investigated in Ni50Mn28Ga22 and Ni50Mn30Ga20 alloys with 5M and 7M modulated martensite, respectively. Maximum ΔSela values obtained were 1.91 J kg−1 K−1 during cooling and 1.83 J kg−1 K−1 during heating in martensite 5M and 0.19 J kg−1 K−1 during cooling and 0.26 J kg−1 K−1 during heating in martensite 7M, for a constant applied stress of 10 MPa. However, although the 7M modulated martensite exhibited a lower ΔSela, its reversibility was higher. Therefore, our results could be useful for selecting a good material to be used in solid-state refrigeration.

In Ti–Ni alloys, the low hysteresis emanating from the B2 ↔ R transition enables a fine control of reversibility in elastocaloric effects when there are lower values of applied stress. In this paper, we reported a Ni_50.5Ti_49.5 alloy synthesized by arc melting promoting the R phase. Using energy dispersive x-ray analysis, differential scanning calorimetry, x-ray diffraction, and elongation measures as a function of temperature, we have studied properties in an isolated R phase. We observe that there are similarities between the isothermal entropy change curves and the reversible adiabatic temperature changes, where a maximum value of reversible isothermal entropy of 14.6 J kg⁻¹ K⁻¹ at 310 K is obtained when a stress of 222 MPa is applied. We show that precipitates formed on the R phase play a role in the elastic behavior affecting the values of adiabatic temperature change.

We have studied acoustic emission (AE) during compression of La_0.8Ca_0.28Sr_0.08MnO_2.7 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 the elastocaloric properties of Ti-Ni shape-memory wires subjected to specific heat treatments to decouple the B2 ↔ R transition from the B2 ↔ B19' one. The entropy values at moderate stresses (∼170 MPa) for the B2 ↔ B19' transition are remarkably high (in the range 60–80 J/kg K). However, in spite of the B2 ↔ R transition exhibits significantly lower entropy changes (∼12 J/kg K), the much smaller hysteresis of this transition gives rise to a larger reversible elastocaloric effect for low applied stresses. Therefore, the reversible elastocaloric strength associated with the B2 ↔ R is larger than the elastocaloric strength associated with the B2 ↔ B19' transition.

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.

Compression experiments of goethite samples from an iron ore mine in New Caledonia revealed the collapse of the porous samples to follow a power law behavior. The porosity varies between 54 and 84%. The collapse under compression occurs in a series of individual events (avalanches). Each avalanche leads to a jerk in sample compression and an equivalent acoustic emission signal. The probability to find an acoustic emission signal with an energy within E and E + dE is p(E)dE, which has a power law distribution p(E) ~ E^−ɛ, and reveals avalanche criticality. The energy exponent ɛ varies systematically with the porosity of the sample between 1.6 and 2. The results are compared with previous measurements of porous silica (Vycor), which showed a slightly smaller exponent of 1.4. We observe fore- and after-shocks of the largest events. Significant correlations between the largest avalanches and fore-shocks were found in samples with high porosity. These correlations open the possibility for the prediction of a major collapse by acoustic detection of noise.