David Rios-Jara

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.

Calorimetric and ultrasonic investigation of the R-phase formation in a TiNi:Fe alloy

The authors report a systematic calorimetric study of the formation of the R-phase in a polycrystalline TiNi:Fe alloy, conducted using a high-sensitivity scanning device. Thermograms performed at very slow heating/cooling rates always appear as an overlap of two peaks. The high-temperature peak does not present any hysteresis and is associated with a continuous transition to the intermediate incommensurate phase. Extrapolation at zero heating/cooling rates gives a value of 1.8 K for the intrinsic thermal hysteresis of the second peak at lower temperatures, ascribed to the first-order transition from the incommensurate to the R-phase. The paper also reports ultrasonic measurements on this alloy. Elastic anomalies in both longitudinal and shear elastic constants and in the ultrasonic attenuation have been detected in the same temperature region as the overall thermal effect.

Expanding the magnetocaloric operation range in Ni–Mn–In Heusler alloys by Cu-doping

We study the magnetocaloric properties of a set of Cu-doped Ni–Mn–In metamagnetic shape memory alloys with the aim of designing composites with large operation range constituted of components of selected Cu-fractions. The studied Cu-doped alloys are characterized by a low hysteresis (~5 K) and by a strong sensitivity of their transition temperatures to an applied magnetic field which ensures excellent reproducibility of the magnetocaloric entropy change ( J K−1 kg−1) upon cyclic application of 6 T. Within the studied composition region, fine-tuning of the Cu-fraction enables shifting the transition temperature by desired amounts. By means of calorimetric measurements under applied magnetic field we show that by assembling samples of tailored compositions it is possible to build a composite showing reversible large magnetocaloric effect (~7.5 J kg−1 K−1) over a broad temperature range of ~30 K.

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.

Hybridization vs. bond stretching isomerism in Ru(II) cyclometalated complexes of 2-phenylpyridine.

The phenomenon of formation of two isomers, yellow and orange, of the cyclometalated Ru(II) complex, [Ru(o-C6H4-py)(MeCN)4]+, was investigated by EELS spectroscopy and theoretical calculations. Both forms show very similar structures and spectroscopic properties, but slight differences in X-ray data and absorption between them were noted. No double minimum on the potential energy surface was found and thus these two forms cannot be considered as bond stretching isomers. However, the DFT study revealed the change in the hybridization of the carbon in trans-position of one of acetonitrile ligands. This effect can be responsible for the difference in colour. The results of the theoretical modelling coincide well with the experimental EELS data.

Large and reversible elastocaloric effect near room temperature in a Ga-doped Ni–Mn–In metamagnetic shape-memory alloy

We report a giant elastocaloric effect near room temperature in a polycrystalline Ga-doped Ni–Mn–In ferromagnetic shape-memory alloy. The elastocaloric effect has been quantified by measuring both isothermal stress-induced entropy changes and adiabatic stress-induced temperature changes. A reproducible maximum entropy change, ΔSrev≃ 25 J⋅K−1⋅kg−1, upon cycling across the martensitic transition was obtained by application of a compressive stress of 100MPa. The corresponding maximum amount of cooling, ΔTadi≃−4.9K, was measured when this stress was rapidly removed. These values are comparable with those reported for giant magnetocaloric materials, which are induced by application and release of a high magnetic field. Therefore, the studied material is a good candidate to be used in solid-state refrigeration devices based on the elastocaloric effect.