Introducció a l'experimentació

Normalment les coses que s'aprenen en el laboratori requereixen el guiatge d'experts. Aquest llibre presenta una visió sumària i actualitzada dels procediments bàsics de l'experimentació cientificotècnica: assenyala i descriu els aspectes més importants de la vida en el laboratori i el món que l'envolta, i proporciona una introducció a les tècniques de tractament de dades i d'anàlisi d'errors més comunament utilitzades

Magnetocaloric effect in the low hysteresis Ni-Mn-In metamagnetic shape-memory Heusler alloy

"We have studied magnetocaloric properties of a Ni-Mn-In metamagnetic shape-memory alloy especially designed in order to display low thermal hysteresis. Magnetization and calorimetric measurements under a magnetic field have been used in order to determine isothermal magnetic field-induced entropy changes. Results obtained indirectly from magnetization data, quasi-directly from isofield calorimetric measurements, and directly from isothermal calorimetric runs are systematic and agree well with each other. We have analyzed the reproducibility of magnetocaloric properties with cycling from direct isothermal calorimetric measurements. Due to low thermal hysteresis, we have found that about 80% of the transition entropy change, ΔSt ≃ 25 J/kg K, can be reversibly induced under successive application and removal of a field of 6 T.

Elastocaloric effect in Ti-Ni shape-memory wires associated with the B2 ↔ B19' and B2 ↔ R structural transitions

"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 (similar to 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 (similar to 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.

Giant barocaloric effect in all-d-metal Heusler shape memory alloys

We have studied the barocaloric properties associated with the martensitic transition of a shape memory Heulser alloy Ni50Mn31.5Ti18.5 which is composed of all-d-metal elements. The composition of the sample has been tailored to avoid long range ferromagnetic order in both ausenite and martensite. The lack of ferromagnetism results in a weak magnetic contribution to the total entropy change thereby leading to a large transition entropy change. The combination of such a large entropy change and a relatively large volume change at the martensitic transition gives rise to giant barocaloric properties in this alloy. When compared to other shape memory Heusler alloys, our material exhibits values for adiabatic temperature and isothermal entropy changes significantly larger than values reported so far for this class of materials. Furthermore, our Ni50Mn31.5Ti18.5 also compares favourably to the best state-of-the-art magnetic barocaloric materials.Peer ReviewedPostprint (author's final draft

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.

Caloric effects induced by magnetic and mechanical fields in a Ni50Mn25-xGa25Cox magnetic shape memory alloy

"We have studied the elastocaloric effect in a Co-doped Ni-Mn-Ga magnetic Heusler shape memory alloy in the vicinity of its martensitic transition. Measurements of the length change as a function of temperature have been carried out across the transition under applied compression stresses and magnetic fields. The isothermal stress-induced entropy changes have been computed from the experimental data. Results evidence a significant elastocaloric effect associated with the large entropy change of the structural phase transition. The alloy also exhibits a magnetocaloric effect at low applied magnetic fields. It is shown that application of a magnetic field below 1 T increases the estimated elastocaloric relative cooling power by about 20%. A comparison of elasto-and magnetocaloric properties indicates that a similar relative cooling power is reached under application of 10 MPa or 0.8 T.

Inverse magnetocaloric effect in ferromagnetic Ni-Mn-Sn alloys

The magnetocaloric effect (MCE) in paramagnetic materials has been widely used for attaining very low temperatures by applying a magnetic field isothermally and removing it adiabatically. The effect can be exploited also for room temperature refrigeration by using recently discovered giant MCE materials. In this letter, we report on an inverse situation in Ni-Mn-Sn alloys, whereby applying a magnetic field adiabatically, rather than removing it, causes the sample to cool. This has been known to occur in some intermetallic compounds, for which a moderate entropy increase can be induced when a field is applied, thus giving rise to an inverse magnetocaloric effect. However, the entropy change found for some ferromagnetic Ni-Mn-Sn alloys is just as large as that reported for giant MCE materials, but with opposite sign. The giant inverse MCE has its origin in a martensitic phase transformation that modifies the magnetic exchange interactions due to the change in the lattice parameters.Comment: 12 pages, 4 figures, to appear in Nature Materials (online published, 15 May 2005

Tailoring barocaloric and magnetocaloric properties in low-hysteresis magnetic shape memory alloys

We report on the barocaloric and magnetocaloric effects in a series of low-hysteresis Ni-Mn-In magnetic shape memory alloys. We show that the behaviour exhibited by several quantities that characterise these caloric effects (isothermal entropy change, adiabatic temperature change and refrigerant capacity) can be rationalised in terms of the relative distance between the Curie point of the austenite and the martensitic transition temperature. It is found that the two caloric effects exhibit opposite trends. The behaviour of the barocaloric effect parallels that exhibited by the transition entropy change, thereby showing larger values for weakly magnetic samples. Regarding the magnetocaloric effect, the isothermal entropy change is maximum for those samples transforming martensitically close to the Curie point of the austenite. Such a maximum value does not correspond to the maximum adiabatic temperature change, and samples with martensitic transition slightly below the Curie point do have larger temperature changes as a result of the strongest sensitivity of the transition to the magnetic field. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.Postprint (author's final draft

Coexisting ferro- and antiferromagnetism in Ni2MnAl Heusler alloys

The structural and magnetic properties of stoichiometric Ni2MnAl are studied to clarify the conditions for ferromagnetic and antiferromagnetic ordering claimed to occur in this compound. X-ray and magnetization measurements show that although a single phase B2 structure can be stabilized at room temperature, a single L21 phase is not readily stabilized, but rather a mixed L21+B2 state occurs. The mixed state incorporates ferromagnetic and antiferromagnetic parts for which close-lying Curie and a Néel temperatures can be identified from magnetization measurements

Understanding the Thermodynamic Properties of the Elastocaloric Effect Through Experimentation and Modelling

This paper presents direct and indirect methods for studying the elastocaloric effect (eCE) in shape memory materials and its comparison. The eCE can be characterized by the adiabatic temperature change or the isothermal entropy change (both as a function of applied stress/strain). To get these quantities, the evaluation of the eCE can be done using either direct methods, where one measures (adiabatic) temperature changes or indirect methods where one can measure the stress-strain-temperature characteristics of the materials and from these deduce the adiabatic temperature and isothermal entropy changes. The former can be done using the basic thermodynamic relations, i.e. Maxwell relation and Clausius-Clapeyron equation. This paper further presents basic thermodynamic properties of shape memory materials, such as the adiabatic temperature change, isothermal entropy change and total entropy-temperature diagrams (all as a function of temperature and applied stress/strain) of two groups of materials (NiTi and CuZnAl alloys) obtained using indirect methods through phenomenological modelling and Maxwell relation. In the last part of the paper, the basic definition of the efficiency of the elastocaloric thermodynamic cycle (coefficient of performance) is defined and discussed