Enric Stern-Taulats

Giant Electrocaloric Strength in Single‐Crystal BaTiO3

Over the last fi fteen years, the discovery of giant magnetocaloric effects near room-temperature phase transitions in various magnetic materials [ 1 , 2 ] has led to suggestions of energy-effi cient and environmentally friendly household and industrial refrigeration. However, these large changes in isothermal entropy Δ S and adiabatic temperature Δ T require large changes in magnetic fi eld Δ H , which are challenging to generate economically. In contrast, it is straightforward to generate changes in electric fi eld Δ E in order to drive electrocaloric (EC) effects near ferroelectric phase transitions. Recently, giant EC effects near nominally second-order transitions have been reported in ferroelectric thin fi lms, [ 3 , 4 ] as thin fi lms can support large driving fi elds. However, two issues arise as follows. Firstly, measurements of heat Q and temperature change Δ T are typically indirect [ 3 , 4 ] as the direct measurement of fi lms is challenging. There is thus scope for error (e.g., because the possible role of thermal and electrical hysteresis is typically ignored). Secondly, the EC effects in fi lms are disproportionately small with respect to the large driving fi elds, and so EC strengths |Q |/| E | and | T |/| E | tend to be relatively small. Here we address both of these issues by presenting direct measurements of both Q and Δ T in single-crystal BaTiO 3 (BTO) near the ferroelectric phase transition at Curie temperature T C . We fi nd EC strengths |Q |/| E | and | T |/ | E | that are giant because the fi rst-order ferroelectric phase transition is very sharp. The observed EC effects are reversible at any temperature above the hysteretic transition regime. Giant EC strengths near sharp fi rst-order phase transitions with a large latent heat could therefore contribute to the future development of cooling devices with a high frequency of operation.

Barocaloric effect in the magnetocaloric prototype Gd5Si2Ge2

We report on calorimetric measurements under hydrostatic pressure that enabled us to determine the barocaloric effect in Gd5Si2Ge2. The values for the entropy change for moderate pressures compare favourably to those corresponding to the magnetocaloric effect in this compound. Entropy data are complemented with direct measurements of the adiabatic pressure-induced temperature change.

Giant barocaloric effects at low pressure in ferrielectric ammonium sulphate

Caloric effects are currently under intense study due to the prospect of environment-friendly cooling applications. Most of the research is centred on large magnetocaloric effects and large electrocaloric effects, but the former require large magnetic fields that are challenging to generate economically and the latter require large electric fields that can only be applied without breakdown in thin samples. Here we use small changes in hydrostatic pressure to drive giant inverse barocaloric effects near the ferrielectric phase transition in ammonium sulphate. We find barocaloric effects and strengths that exceed those previously observed near magnetostructural phase transitions in magnetic materials. Our findings should therefore inspire the discovery of giant barocaloric effects in a wide range of unexplored ferroelectric materials, ultimately leading to barocaloric cooling devices.

Barocaloric and magnetocaloric effects in Fe49Rh51

We report on calorimetry under applied hydrostatic pressure and magnetic field at the antiferromagnetic-ferromagnetic (AFM/FM) transition of Fe49Rh51. Results demonstrate the existence of a giant barocaloric effect in this alloy, a functional property that adds to the magnetocaloric and elastocaloric effects previously reported for this alloy. All caloric effects originate from the AFM/FM transition which encompasses changes in volume, magnetization, and entropy. The strong sensitivity of the transition temperatures to both hydrostatic pressure and magnetic field confers to this alloy outstanding values for the barocaloric and magnetocaloric strengths (|?S|/?p ~ 12 J kg-1K-1kbar-1 and |?S|/µ0?H~ 12 J kg-1K-1T-1). Both barocaloric and magnetocaloric effects have been found to be reproducible upon pressure and magnetic field cycling. Such a good reproducibility and the large caloric strengths make Fe-Rh alloys particularly appealing for solid-state cooling technologies at weak external stimuli.

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.

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

Large reversible entropy change at the inverse magnetocaloric effect in Ni-Co-Mn-Ga-In magnetic shape memory alloys

Calorimetry under magnetic field has been used to study the inverse magnetocaloric effect in Ni-Co-Mn-Ga-In magnetic shape memory alloys. It is shown that the energy dissipated during a complete transformation loop only represents a small fraction (5% to 7%) of the latent heat of the martensitic transition. It is found that the entropy values obtained from isofield temperature scans agree well with those obtained from isothermal magnetic field scans. The reproducibility of the magnetocaloric effect has been studied from isothermal measurements. Reproducible entropy values under field cycling have been found within a temperature interval bounded by the start temperature of the forward transition at zero field and the start temperature of the reverse transition under applied field. Large reversible entropy changes around 11 J/kg K have been found for fields up to 6 T.

Reversible adiabatic temperature changes at the magnetocaloric and barocaloric effects in Fe49Rh51

We report on the adiabatic temperature changes (ΔT) associated with the magnetocaloric and barocaloric effects in a Fe49Rh51 alloy. For the magnetocaloric effect, data derived from entropy curves are compared to direct thermometry measurements. The agreement between the two sets of data provides support to the estimation of ΔT for the barocaloric effect, which are indirectly determined from entropy curves. Large ΔT values are obtained at relatively low values of magnetic field (2 T) and hydrostatic pressure (2.5 kbar). It is also shown that both magnetocaloric and barocaloric effects exhibit good reproducibility upon magnetic field and hydrostatic pressure cycling, over a considerable temperature range.

Inverse barocaloric effects in ferroelectric BaTiO3 ceramics

We use calorimetry to identify pressure-driven isothermal entropy changes in ceramic samples of the prototypical ferroelectric BaTiO3. Near the structural phase transitions at ∼400 K (cubic-tetragonal) and ∼280 K (tetragonal-orthorhombic), the inverse barocaloric response differs in sign and magnitude from the corresponding conventional electrocaloric response. The differences in sign arise due to the decrease in unit-cell volume on heating through the transitions, whereas the differences in magnitude arise due to the large volumetric thermal expansion on either side of the transitions.

Reversibility of minor hysteresis loops in magnetocaloric Heusler alloys

The unavoidable existence of thermal hysteresis in magnetocaloric materials with a first-order phase transition is one of the central problems limiting their implementation in cooling devices. Using minor loops, however, allows achieving significant cyclic effects even in materials with relatively large hysteresis. Here, we compare thermometric measurements of the adiabatic temperature change Δ T ad and calorimetric measurements of the isothermal entropy change Δ S T when moving in minor hysteresis loops driven by magnetic fields. Under cycling in 2 T, the Ni-Mn-In-Co Heusler material provides a reversible magnetocaloric effect of Δ S T rev = 10.5 J kg–1 K−1 and Δ T ad rev = 3.0 K. Even though the thermodynamic conditions and time scales are very different in adiabatic and isothermal minor loops, it turns out that after a suitable scaling, a self-consistent reversibility region in the entropy diagram is found. This region is larger than expected from basic thermodynamic considerations based on isofield me...