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We have studied the multicaloric properties of a Ni-Mn-Ga-Cu alloy. In this alloy, application of magnetic field and uniaxial stress shift its martensitic transition towards higher temperatures which results in synergic magnetocaloric and elastocaloric effects. By a proper numerical treatment of the calorimetric curves obtained under applied magnetic field and uniaxial stress we have obtained the entropy S(T,μ0H,σ) as a function of the magnetic field, uniaxial stress, and temperature over the whole phase space under study. We have determined the different entropy contributions to the multicaloric effect in this alloy, and noticeably we have evidenced the role played by the interplay between magnetic and vibrational degrees of freedom. A comparison between single caloric and multicaloric effects shows that appropriate combinations of magnetic field and stress reduce the magnitude of the specific field required to obtain a given value of the isothermal entropy and adiabatic temperature changes. For example, at 299 K, to achieve an entropy change (ΔS) of −14 J kg−1K−1, a magnetic field of ∼2.5 T or a uniaxial stress of 19 MPa are required, while a combination of dual fields of (1 T, 12 MPa) yields to the same value of ΔS. Moreover, the maximum adiabatic temperature change is enlarged up to 9.4 K by the dual fields, higher than the value obtained by a single field (∼7 K). The advantage of multicaloric effect is particularly relevant at low magnetic fields which are achievable by permanent magnets. Our findings open new avenues for using multicaloric materials in novel refrigeration technologies.

Multiferroic materials with strong coupling between different degrees of freedom are prone to exhibit giant multicaloric effects resulting from the application or removal of diverse external fields. These materials exhibit a synergic response to the combined action of two fields when the monocaloric effects are both conventional (or both inverse), while a non-synergic response occurs when one of the monocaloric effects is conventional and the other is inverse. In all cases, the multicaloric properties (isothermal entropy and adiabatic temperature changes) do not result from the simple addition of the corresponding monocaloric quantities because there is a contribution from the interplay between degrees of freedom (cross-coupling term). In this paper, we analyse in detail the contribution of the cross-coupling term to the multicaloric entropy values obtained for both synergic and non-synergic multicaloric materials. We first introduce basic thermodynamic concepts accounting for the multicaloric effects, and next the contribution from the cross-coupling term is illustrated via several model examples. We finally analyse the realistic situation for two prototype materials with synergic and non-synergic multicaloric effects.

Solid-state cooling methods based on field-driven first-order phase transitions are often limited by significant hysteresis and small temperature span, which increase the input work required to drive the cooling cycle reversibly and reduce the temperature range of operation. Here we show that giant reversible caloric effects can be driven using low hydrostatic pressures in the molecular crystal of fullerene C60 across its order–disorder first-order phase transition due to a small transition hysteresis and a high sensitivity of the transition to pressure. In particular, we obtain isothermal entropy changes ¿S = 25 J K-1 kg-1 under reversible application and removal of a pressure as low as p = 0.05 GPa. We also demonstrate that these features allow us to obtain these giant effects in a wide temperature span around room temperature which, furthermore, is desirable for single-component regenerative coolers. For a pressure change of p = 0.41 GPa, we obtain giant reversible values of ¿S = 31 J K-1 kg-1 and ¿T = 11 K, in a temperature interval larger than 50 K. This very good barocaloric performance postulates C60 as one of the best candidates known so far to be considered by engineers for the development of barocaloric devices. The physics underlying these caloric effects is also analyzed.

High-sensitivity detection of acoustic emission from granite under uniaxial stress, together with advanced statistical analysis, shows changing collapse mechanisms when a sample is pre-heated. Massive microstructural changes occur at temperatures >500 °C while low-temperature (<

A Ginzburg-Landau model embedded into a vibrational model is used to study the flexocaloric effect in a beam near a ferroelastic transition. The caloric response upon bending is characterized by the isothermal entropy change and the adiabatic temperature change of the beam. We obtain a larger response relative to the strength of the applied forces at temperatures slightly above the transition temperature. It is also obtained that the maximum caloric response is almost linear with the bending angle of the beam, whereas the relation between the bending angle and the applied forces is highly nonlinear. Small hysteresis associated with the phase transition is obtained for sufficiently large bending forces due to the existence of a critical point in the temperature-stress phase diagram of the ferroelastic material. Finally, the microstructure changes with bending in the beam are consistent with previous experimental observations.

International audience; Plastic crystals undergo phase transitions with unusually large volume and entropy changes related to strong molecular orientational disordering. These features have led to a resurgent interest in these materials because recently they have shown great potential in solid-state cooling applications driven by pressure. Here we demonstrate that two plastic crystals derived from adamantane-1-Br-adamantane and 1-Cl-adamantane-undergo colossal reversible barocaloric effects under moderate pressure changes in a wide temperature span near room temperature thanks to a relatively small hysteresis and very high sensitivity of the transition temperature to pressure. In particular, 1-Cl-adamantane displays an optimal operational temperature range covering from ~40 K below and up to room temperature, and under a pressure change of 1 kbar this compound outperforms any other barocaloric material known so far. Our work gives strong support to plastic crystals as best candidates for barocaloric cooling. We also provide insight into the physical origin of the entropy changes through the analysis of the disorder on the involved phases.

The partially overlapped ferroelastic/martensitic and para-ferromagnetic phase transitions of a Ni$_{50.53}$Mn${33.65}$In$_{15.82}$ metamagnetic shape memory alloy have been studied from calorimetric, magnetic and acoustic emission measurement. We have taken advantage of the existence of thermal hysteresis of the first order ferroelastic/martensitic phase transition ($\sim2.5$K) to discriminate the latent heat contribution $\Delta$Ht = 7.21(15) kJ/kg and the specific heat contribution $\Delta$Hc = 216(1) J/kg to the total excess enthalpy of the phase transition. The specific heat was found to follow a step-like behavior at this phase transition. The intermittent dynamics of the ferroelastic/martensitic transition has been characterized as a series of avalanches detected both from acoustic emission and calorimetric measurements. The energy distribution of these avalanche events was found to follow a power law with a characteristic energy exponent $\epsilon\sim2$ which is in agreement with the expected value for a system undergoing a symmetry change from cubic to monoclinic. Finally, the critical behavior of the para-ferromagnetic austenite phase transition that takes place at $\sim 311$K has been studied from the behavior of the specific heat. A critical exponent $\alpha\sim0.09$ has been obtained, which has been shown to be in agreement with previous values reported for Ni-Mn-Ga alloys but different from the critical divergence reported for pure Ni., Comment: 18 pages, 11 figures

In the region of a martensitic transformation, conventional elastocaloric effect causes cooling when the forward parent to martensite transition is induced by loading under adiabatic conditions and by heating when the load is removed and the reverse transition occurs. An inverse elastocaloric effect might be expected if the reverse transition could be induced upon loading. However, this behavior is quite uncommon and inverse elastocaloric effect rarely occurs. Herein, we have successfully manufactured a Ti-50.8 at.% Ni crystal in a high proportion of favorable $R$ phase (martensite) variant containing single-variant ${\mathrm{Ti}}_{3}{\mathrm{Ni}}_{4}$ nanoprecipitates aligned along the ${\ensuremath{\langle}111\ensuremath{\rangle}}_{B2}$ direction of the $B2$ parent phase, by means of a compressive stress associated aging along the ${\ensuremath{\langle}111\ensuremath{\rangle}}_{B2}$ direction. The material displays inverse elastocaloric effect induced by compressive stress along the ${[111]}_{B2}$ direction in the temperature range of stability of the $R$ phase. This inverse elastocaloric effect is caused by the stress-induced reverse transformation of the high proportion of favorable $R$-phase variant that occurs when the external compressive stress is applied along the direction of the internal tensile stress induced by the aligned ${\mathrm{Ti}}_{3}{\mathrm{Ni}}_{4}$ nanoprecipitates. The mechanism giving rise to this inverse elastocaloric effect is explained by the transformation strain in the ${[111]}_{B2}$ and ${[1\overline{1}0]}_{B2}$ directions for different $R$-phase variants based on crystallography. Results indicate that the stress-associated aging process is an effective way of patterning materials to display inverse elastocaloric effect through controlling the alignment of the ${\mathrm{Ti}}_{3}{\mathrm{Ni}}_{4}$ nanoprecipitate in the Ni-rich Ti-Ni shape memory alloy.

Magnetic shape memory alloys (MSMAs) have attracted great interests due to their multifunctional properties associated with external-field-induced martensitic transition, such as magnetostrain, magnetoresistance, magnetocaloric effect and elastocaloric effect. Search for MSMAs displaying a unique combination of strong metamagnetic transition and stable, large caloric effect is nowadays very active. Here we report the simultaneous achievement of a strong metamagnetic transition as well as a large magnetic-field-induced entropy change of 17.4 J kg−1 K−1 under 2 T and a stable, large elastocaloric effect with adiabatic temperature change of ∼5.0 K during more than 2100 mechanical cycles, in a Ni-Fe-Co-Mn-Sn MSMA. Such combination in a single material has never been reported in the literature. Furthermore, we demonstrate that fully reversible metamagnetic transition and consequently large, reversible multicaloric effect can be achieved by increasing magnetic field from 0 T to 2 T at zero stress and decreasing field from 2 T to 0 T at a uniaxial stress of 23.5 MPa. This dual-stimulus magnetic-mechanical multicaloric cooling cycle also provides an opportunity for achieving reversible magnetoresistance and magnetostrain under the coupling of a low magnetic field and small stress, which is beneficial for practical applications. The high cyclic stability of large caloric effects is ascribed to the good geometric compatibility at the interface between austenite and martensite as revealed by employing multiscale characterization approaches, including in-situ synchrotron high-energy X-ray diffraction and transmission electron microscopy. This study has great implications on developing high-performance multifunctional alloys with magnetostructural transition for actuation, magnetic recording and solid-state refrigeration applications.

Superelastic Ti-Ni-based shape memory alloys are promising elastocaloric materials for solid-state refrigeration. In this paper, we studied the mechanical properties and elastocaloric effect of a severely cold rolled Ti-44Ni-5Cu-1Al (at.%) alloy composed of nanograins $(\ensuremath{\sim}5\text{--}16\phantom{\rule{0.16em}{0ex}}\mathrm{nm})$. The alloy exhibits partial stress-induced martensitic transformation with a large quasilinear reversible strain of $\ensuremath{\sim}3.4\phantom{\rule{0.16em}{0ex}}%$ and a small energy dissipation of $\ensuremath{\sim}1.6\phantom{\rule{0.16em}{0ex}}\mathrm{MJ}/{\mathrm{m}}^{3}$ when a maximum stress of 1 GPa is applied. The temperature decrease induced by adiabatic removal of a stress of 1 GPa is of about \ensuremath{-}5 K, which is mainly due to the reverse transition occurring during unloading. The effective working temperature window of elastocaloric effect larger than 200 K. The coefficient of performance of this alloy reaches \ensuremath{\sim}13, which is higher than values reported so far for Ti-Ni alloys.

A giant barocaloric effect (BCE) in a molecular material Fe3 (bntrz)6 (tcnset)6 (FBT) is reported, where bntrz = 4-(benzyl)-1,2,4-triazole and tcnset = 1,1,3,3-tetracyano-2-thioethylepropenide. The crystal structure of FBT contains a trinuclear transition metal complex that undergoes an abrupt spin-state switching between the state in which all three FeII centers are in the high-spin (S = 2) electronic configuration and the state in which all of them are in the low-spin (S = 0) configuration. Despite the strongly cooperative nature of the spin transition, it proceeds with a negligible hysteresis and a large volumetric change, suggesting that FBT should be a good candidate for producing a large BCE. Powder X-ray diffraction and calorimetry reveal that the material is highly susceptible to applied pressure, as the transition temperature spans the range from 318 at ambient pressure to 383 K at 2.6 kbar. Despite the large shift in the spin-transition temperature, its nonhysteretic character is maintained under applied pressure. Such behavior leads to a remarkably large and reversible BCE, characterized by an isothermal entropy change of 120 J kg-1 K-1 and an adiabatic temperature change of 35 K, which are among the highest reversible values reported for any caloric material thus far.

Novel multicaloric cooling utilizing the giant caloric response of Ni-Mn-based metamagnetic shape-memory alloys to different external stimuli such as magnetic field, uniaxial load and hydrostatic pressure is a promising candidate for energy-efficient and environmentally-friendly refrigeration. However, the role of microstructure when several external fields are applied simultaneously or sequentially has been scarcely discussed in literature. Here, we synthesized ternary Ni-Mn-In alloys by suction casting and analyzed the microstructural influence on the response to magnetic fields and uniaxial load. SEM-EBSD reveals a distinct core-shell microstructure with a radially symmetric solidification texture resulting in a two-step martensitic transformation. In correlation with temperature-dependent XRD a significant effect of grain orientation on the stress-induced martensitic transformation is demonstrated. The influence of microstructure on the magnetic-field-induced transformation dynamics is studied by strain measurements in static and pulsed fields. Temperature-stress and temperature-magnetic field phase diagrams are established and single caloric performances are characterized in terms of ${\Delta}s_{T}$ and ${\Delta}T_{ad}$. The cyclic ${\Delta}T_{ad}$ values are compared to the ones achieved in the multicaloric exploiting-hysteresis cycle. It turns out that a tailored microstructure and the combination of both stimuli enable outstanding caloric effects in moderate external fields which can significantly exceed the single caloric performances. In particular for Ni-Mn-In, the maximum cyclic effect in magnetic fields of 1.9 T is increased by more than 200 % to -4.1 K when a moderate sequential stress of 59 MPa is applied. Our results illustrate the crucial role of microstructure for multicaloric cooling using Ni-Mn-based metamagnetic shape-memory alloys.

The world's growing hunger for artificial cold on the one hand, and the ever more stringent climate targets on the other, pose an enormous challenge to mankind. Novel, efficient and environmentally friendly refrigeration technologies based on solid-state refrigerants can offer a way out of the problems arising from climate-damaging substances used in conventional vapor-compressors. Multicaloric materials stand out because of their large temperature changes which can be induced by the application of different external stimuli such as a magnetic, electric, or a mechanical field. Despite the high potential for applications and the interesting physics of this group of materials, only few studies focus on their investigation by direct methods. In this paper, we report on the advanced characterization of all relevant physical quantities that determine the multicaloric effect of a Ni-Mn-In Heusler compound. We have used a purpose-designed calorimeter to determine the isothermal entropy and adiabatic temperature changes resulting from the combined action of magnetic field and uniaxial stress on this metamagnetic shape-memory alloy. From these results, we can conclude that the multicaloric response of this alloy by appropriate changes of uniaxial stress and magnetic field largely outperforms the caloric response of the alloy when subjected to only a single stimulus. We anticipate that our findings can be applied to other multicaloric materials, thus inspiring the development of refrigeration devices based on the multicaloric effect., To be published in Applied Physic Reviews

We investigate acoustic emission (AE) that arises during the martensitic transition in a polycrystalline specimen of the prototypical superelastic/elastocaloric alloy ${\mathrm{Ni}}_{50.4}{\mathrm{Ti}}_{49.6}$ (at. %) driven using tensile strain. We use two independent AE sensors in order to locate AE events, and focus on contributions to the AE that arise away from the grips of the mechanical testing machine. Significant AE activity is present during the first mechanical loading primarily due to nucleation and growth of wide L\"uders-like bands during the forward martensitic transition (imaged using visible light and infrared radiation) that lead to persistent changes in intergranular interactions. AE activity is suppressed during the subsequent reverse martensitic transition on unloading, and in successive loading/unloading cycles, for which the L\"uders-like bands narrow and modify intergranular interactions to a much smaller extent. After the first loading, we find that the AE activity associated with the martensitic transition is weak, and we suggest that this is because the elastic anisotropy and strain incompatibility in Ni-Ti are low. We also find that the AE activity becomes weaker on mechanical cycling due to increased retained martensite.

The multicaloric effect is described by a temperature or entropy change of a material triggered by external stimuli applied or removed simultaneously or sequentially. The prerequisite for this is a material exhibiting multiple ferroic states. However, direct measurements of the effect are rarely reported. Now, for this reason, we built a measurement device allowing to determine the adiabatic temperature change in pulsed magnetic fields and, simultaneously, under the influence of a uniaxial load. We selected the all-𝑑-metal Heusler alloy Ni-Mn-Ti-Co for our first test because of its enhanced mechanical properties and enormous magneto- and elastocaloric effects. Ni-Mn-Ti-Co was exposed to pulsed magnetic fields up to 10 T and uniaxial stresses up to 80 MPa, and the corresponding adiabatic temperature changes were measured. With our new experimental tool, we are able to better understand multicaloric materials and determine their cross-coupling responses to different stimuli.

We have tracked the dynamics of the martensitic transformation in a Cu–Al–Ni single crystal by means of acoustic emission and infrared imaging techniques. A Fourier equation-based post-processing of temperature maps has enabled us to reveal the inhomogeneous and discontinuous character of heat power sources and sinks during the transition. A good correlation between the dynamics of thermal and mechanical energy release has been evidenced. It has also been shown that the merging of martensitic interfaces results in an enhanced heat absorption.

The limit of elastic deformation in conventional metals and alloys is usually within 0.1%. When deformed further, these materials show plastic deformation effects, which are a consequence of dislocation movements. Here, we report that a much larger elastic-like strain of more than 4% can be achieved in a tweed textured Fe-Pd single crystal under a compressive stress of 300 MPa. We show that this giant elastic response gives rise to significant and homogenous elastocaloric effect (∼4.1 K) that can sustain 106 or even more mechanical cycles without degradation. The large and stable elastic-like strain in Fe-Pd originates from the continuous and reversible structural change associated with the tweed texture under stress as confirmed by the in-situ high energy X-ray diffraction tests. Phase field simulation also confirms this giant elastic response and ultra-stable elastocaloric effect under a compressive stress in the Fe-Pd alloy.

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.

Avalanche events occur during mechanical loading of many material systems and are characterized by stress drops and acoustic emission (AE). Stress drops are directly related to the macroscopic response of the investigated material, but their detection capability is restricted to relatively large and slow events. AE measurements can detect events with smaller amplitude and shorter duration, but their energy and duration are not directly related to the change of the system. In this paper, we present simultaneous measurements of stress drops and AE during mechanically induced twin boundary motion in Ni-Mn-Ga. We found that the probability of finding an AE event during a stress drop is $\ensuremath{\sim}100$ times higher than between stress drops. Analysis of the relations between mechanical energy drops $\mathrm{\ensuremath{\Delta}}{U}_{m}$ and acoustic emitted energy ${E}_{\mathrm{AE}}$, on the level of individual events, reveals the existence of a lower bound for ${E}_{\mathrm{AE}}$, which is approximately proportional to $\mathrm{\ensuremath{\Delta}}{U}_{m}$. These results imply that the macroscopic stress changes generate acoustic waves, which contribute a well-defined amount of energy that is equal to the lower bound function. Furthermore, smaller scale events that are related to microscopic subprocesses by which the twin boundary moves generate additional AE energy. The latter contribution displays a power-law distribution, which implies that these processes are close to criticality.

Energy-efficient and environment-friendly elastocaloric refrigeration, which is a promising replacement of the conventional vapor-compression refrigeration, requires extraordinary elastocaloric properties. Hitherto the largest elastocaloric effect is obtained in small-size films and wires of the prototype NiTi system. Here, we report a colossal elastocaloric effect, well exceeding that of NiTi alloys, in a class of bulk polycrystalline NiMn-based materials designed with the criterion of simultaneously having large volume change across phase transition and good mechanical properties. The reversible adiabatic temperature change reaches a strikingly high value of 31.5 K and the isothermal entropy change is as large as 45 J kg^{-1} K^{-1}. The achievement of such a colossal elastocaloric effect in bulk polycrystalline materials should push a significant step forward towards large-scale elastocaloric refrigeration applications. Moreover, our design strategy may inspire the discovery of giant caloric effects in a broad range of ferroelastic materials.

A systematic study of acoustic emission avalanches in coal and charcoal samples under slow uniaxial compression is presented. The samples exhibit a range of organic composition in terms of chemical elements as well as different degrees of heterogeneity in the microstructure. The experimental analysis focuses on the energies E of the individual acoustic emission events as well as on the time correlations between successive events. The studied samples can be classified into three groups. The more homogeneous samples (group I) with pores in the micro and nanoscales, with signatures of hardening effects in the stress-strain curves, exhibit the cleanest critical power-law behavior for the energy distributions g(E)dE∼E-ϵdE with a critical exponent ϵ=1.4. The more heterogeneous samples with voids, macropores, and granular microstructures (group III), show signatures of weakening effects and a larger effective exponent close to the value ϵ=1.66, but in some cases truncated by exponential damping factors. The rest of the samples (group II) exhibit a mixed crossover behavior still compatible with an effective exponent ϵ=1.4 but clearly truncated by exponential factors. These results suggest the existence of two possible universality classes in the failure of porous materials under compression: one for homogeneous samples and another for highly heterogeneous samples. Concerning time correlations between avalanches, all samples exhibit very similar waiting time distributions although some differences for the Omori aftershock distributions cannot be discarded., Y.X. and X.D. acknowledge financial support from National Natural Science Foundation of China (Grants No. 51571156, No. 51671157, No. 51621063, and No. 51320105014). E.V. and A.P. acknowledge financial support from the Spanish Ministry of Economy and Competitiveness (MAT2016-75823-R).

Materials with both giant and colossal mechanocaloric effects together with giant electrocaloric and magnetocaloric materials are expected to replace harmful fluids in more efficient and environmentally friendly refrigeration and heat pumping technologies. While mechanocaloric materials have only received attention in the last decade, they have already shown better caloric performances than their magnetic and polar counterparts. In particular, the recent discovery of colossal barocaloric and elastocaloric materials opens up bright perspectives for this class of materials. We envisage great promise in the use of mechanocaloric materials for future energy applications.

Avalanches in coal and sandstone samples under common uniaxial stress serve as a model for mixing of avalanche exponents in ceramics, multiferroics, and alloys. The two media are sandwiched together and subjected to common uniaxial stress using high- and low-stress compression. Each medium collapses individually through avalanches that often coincide with secondary avalanches into the other medium. The total avalanche time sequence allows a detailed investigation of the mixing by superposition and delayed coincidence. Correlations can be described by an inter-media Bath's law.

We have used differential scanning calorimetry and thermometry techniques under applied magnetic field and compressive uniaxial stress to determine isothermal entropy and adiabatic temperature changes that quantify the caloric effects associated with the magnetostructural transition of an ${\mathrm{Fe}}_{49}{\mathrm{Rh}}_{51}$ alloy. It is found that the transition temperature increases with increasing compressive stress while it decreases for increasing tensile stress. This behavior gives rise to a conventional elastocaloric effect for compressive stresses in contrast to the reported inverse elastocaloric effect for tensile stresses. The combined effect of stress and magnetic field does not lead to a significant increase of the maximum temperature and entropy changes associated with magnetocaloric and elastocaloric effects, but there is a modification of the temperature window where the sample exhibits giant caloric responses.

Multicaloric materials show thermal changes that can be driven simultaneously or sequentially by more than one type of external field, and the resulting multicaloric effects can be large in multiferroic materials. The use of more than one driving field can permit access to larger thermal changes, with smaller field magnitudes, over wider ranges of operating temperature, while manipulating hysteresis. The thermodynamics behind multicaloric effects is well established, but only a small number of multicaloric effects have been experimentally studied to date. Here we describe the fundamentals of multicaloric effects, and discuss the performance of representative multicaloric materials. Exploiting multicaloric effects could aid the future development of cooling devices, where key challenges include energy efficiency and operating temperature span.

The total energy of acoustic emission (AE) events in externally stressed materials diverges when approaching macroscopic failure. Numerical and conceptual models explain this accelerated seismic release (ASR) as the approach to a critical point that coincides with ultimate failure. Here, we report ASR during soft uniaxial compression of three silica-based (SiO$_2$) nanoporous materials. Instead of a singular critical point, the distribution of AE energies is stationary and variations in the activity rate are sufficient to explain the presence of multiple periods of ASR leading to distinct brittle failure events. We propose that critical failure is suppressed in the AE statistics by dissipation and transient hardening. Some of the critical exponents estimated from the experiments are compatible with mean field models, while others are still open to interpretation in terms of the solution of frictional and fracture avalanche models., preprint, Main article: 7 pages, 3 figures. Supplementary material included in \anc folder: 6 pages, 3 figures

The giant magnetocaloric effect, in which large thermal changes are induced in a material on the application of a magnetic field, can be used for refrigeration applications, such as the cooling of systems from a small to a relatively large scale. However, commercial uptake is limited. We propose an approach to magnetic cooling that rejects the conventional idea that the hysteresis inherent in magnetostructural phase-change materials must be minimized to maximize the reversible magnetocaloric effect. Instead, we introduce a second stimulus, uniaxial stress, so that we can exploit the hysteresis. This allows us to lock-in the ferromagnetic phase as the magnetizing field is removed, which drastically removes the volume of the magnetic field source and so reduces the amount of expensive Nd–Fe–B permanent magnets needed for a magnetic refrigerator. In addition, the mass ratio between the magnetocaloric material and the permanent magnet can be increased, which allows scaling of the cooling power of a device simply by increasing the refrigerant body. The technical feasibility of this hysteresis-positive approach is demonstrated using Ni–Mn–In Heusler alloys. Our study could lead to an enhanced usage of the giant magnetocaloric effect in commercial applications. Magnetocaloric effects can be used for refrigeration, but application uptake is limited due to large amounts of magnetic material used. Here, a cooling cycle is shown that uses thermal hysteresis, significantly reducing magnetic material quantity.

Glassiness is ubiquitous in nature but it still keeps many fascinating phenomena hidden. The discovery about a decade ago of glassy behavior in strain nanoclusters (the strain glass) has extended ferroic glasses to include the ferroelastic property. Here, by means of numerical modelling and comparison with experimental data in the literature, we identify disorder and anisotropy as key parameters whose interplay determines the ferroelastic behavior in alloys: While anisotropy-driven systems exhibit a normal ferroelastic transition, disorder-driven systems may result in the strain glass state. Interestingly, strain glass preserves functional properties such as the shape memory effect (SME) and superelasticity. Moreover, it exhibits hysteresis reduction and widening of operational temperature-stress range, which enhances its technological appeal. Precisely based on the occurrence of the SME, the relevance of geometrical frustration in strain glass is called into question as it might play a minor role in the freezing process. In magnetostructural systems, the multiferroic coupling could yield strain-mediated magnetic glass.

Current interest in barocaloric effects has been stimulated by the discovery that these pressure-driven thermal changes can be giant near ferroic phase transitions in materials that display magnetic or electrical order. Here we demonstrate giant inverse barocaloric effects in the solid electrolyte AgI, near its superionic phase transition at ~420 K. Over a wide range of temperatures, hydrostatic pressure changes of 2.5 kbar yield large and reversible barocaloric effects, resulting in large values of refrigerant capacity. Moreover, the peak values of isothermal entropy change (60 J K−1 kg−1 or 0.34 J K−1 cm−3) and adiabatic temperature changes (18 K), which we identify for a starting temperature of 390 K, exceed all values previously recorded for barocaloric materials. Our work should therefore inspire the study of barocaloric effects in a wide range of solid electrolytes, as well as the parallel development of cooling devices., Barocaloric materials offer promise in solid-state cooling devices, but few materials have been show to display giant barocaloric effects near room temperature. Here, the authors demonstrate that solid electrolyte AgI displays giant inverse barocaloric effects near its superionic phase transition at ~420 K.

Caloric properties rely on the reversible thermal response of a given material to changes induced by an applied field. In magnetic shape-memory materials, thanks to a strong interplay between magnetism and structure, caloric effects can be induced by application of both magnetic and mechanical (stress) fields. The study of such effects is an expanding field of research opening novel opportunities in solid state refrigeration and energy harvesting applications. In this paper we present an overview of the present state of the art of the subject. After a brief discussion of the thermodynamics of systems with magneto-structural interplay, we survey the caloric and multicaloric properties of NiMn-based Heusler materials. The conditions required for optimal caloric efficiency in cycling processes are considered. Finally, we outline some present challenges and future perspectives in this topic.

Martensitic transitions take place intermittently as a sequence of avalanches which are accompanied by the emission of acoustic waves. The study of this acoustic emission (AE) reveals the scale-free nature of the avalanches. In a number of shape memory materials undergoing a martensitic transition it has been found that, in spite of relatively low hysteresis, the dynamics of forward and reverse transitions are different, which may explain the fact that the AE activity is different in both forward and reverse transitions. The asymmetry could be a consequence of the fact that, while nucleation is required for the transition from the parent to martensitic phase to take place, reverse transition occurs by fast shrinkage of martensitic domains. We have analysed in detail the distribution of avalanches in cooling and heating runs in Fe-Pd and Cu-Zn-Al shape-memory alloys. In the former, the martensitic transition is weakly first order while it shows a significant first order character in the latter. We have found that in Fe-Pd the distributions are power law for the forward and reverse transitions characterized by the same critical exponents. For Cu-Zn-Al the distribution of avalanches is critical in forward transitions but exponentially damped in the reverse transition. It is suggested that this different behaviour could originate from the different dynamic mechanisms in forward and reverse transitions.

We report on the multicaloric response of the ${\mathrm{Fe}}_{49}{\mathrm{Rh}}_{51}$ alloy under the combined application of hydrostatic pressure and magnetic field. Experimental data are complemented by a mean field model that takes into account the interplay between structural and magnetic degrees of freedom. A large multicaloric strength has been found for this alloy, and it is shown that a suitable combination of pressure and magnetic field enables the sign of the entropy change to be reversed and thus the multicaloric effect can be tuned from conventional to inverse. It is also shown that an extended temperature window for the multicaloric effect can be achieved by taking advantage of the coupling between structure and magnetism which enables a cross response of the alloy under the application of different external fields. Mean field calculations remarkably reproduce experimental results.

We provide a general thermodynamic framework to study multicaloric effects in multiferroic materials. This is applied to the case of a magnetoelectric multiferroic such as BiFeO, which is described by means of a Landau free energy with a biquadratic coupling between polarization and magnetization. We obtain a phase diagram, the isothermal entropy change and the adiabatic temperature change across different continuous and first-order phase transitions as the applied electric and magnetic fields are varied. The multicaloric effects are suitably decomposed into the corresponding electrocaloric and magnetocaloric contributions.

In this paper, we propose a model that combines a Landau free energy part and a micromagnetic free energy part aimed at describing a magnetostructural multiferroic. We show that in the limit of high-elastic anisotropy and strong magnetostructural interplay, the model is able to reproduce characteristic strain and magnetization configurations typical of magnetic shapememory materials. For low-elastic anisotropy, in the presence of disorder arising from compositional fluctuations giving rise to a distribution of local transition temperatures, the model is able to reproduce strain glass behaviour. In this case, the temperature dependence of the magnetization measured after zero magnetic field cooling deviates from the magnetization measured after magnetic field cooling protocol, which proves that the strain glass behaviour induces non-ergodicity in the magnetic degrees of freedom as well. (C) 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We report on calorimetric measurements under hydrostatic pressure in a series of composition related metamagnetic shape memory alloys. We show that metamagnetic shape memory alloys exhibit a barocaloric effect whose magnitude compares well to the magnetocaloric effect exibited by this kind of alloys. While in metamagnetic alloys the magnetocaloric effect is inverse, the barocaloric effect has been found to be conventional. The values obtained for the pressure-induced entropy changes at moderate pressures are in the range of those reported for giant caloric materials.

Acoustic emission originates during martensitic transformations from sudden local changes of the displacement field across the propagating interfaces. Acoustic emission reveals the jerky character of the dissipation of the elastic energy excess, which proves that the transition occurs through avalanches. In this paper we show that the analysis of acoustic emission within an appropriate framework is a very useful technique, which provides quantitative dynamical information complementary to that obtained from more standard techniques. In particular, we will show that the probability density function of amplitudes, energies and durations of the detected acoustic signals follows power-law distributions related to avalanche criticality. This is illustrated with examples for non-magnetic and magnetic shape-memory alloys under selected external conditions.

Magnetic shape-memory properties refer to the ability of certain materials to show strong response in strain to an applied magnetic field. This strain is caused by either inducing the martensitic transition or rearranging martensitic variants. In the first, case a superelastic effect is possible, while in the second, the system is able to show the shape-memory effect. The complex behaviour displayed by these materials is mainly a consequence of a strong interplay between magnetism and structure which is driven by a martensitic transition. This interplay is the source of many other observed effects such as giant magneto-resistance, exchange bias and magnetocaloric effects. In this paper, we will overview the present state of the art, discuss present challenges and outline some future perspectives in the field.

The first-order diffusionless structural phase transition in Cu67.64Zn16.71Al15.65 is characterized by jerky propagation of phase front related to the appearance of avalanches. In this work we describe a full analysis of this avalanche behaviour using calorimetric heat-flux measurements and the results are compared with acoustic emission (AE) measurements.

We study the effect of anisotropy and disorder on the morphology of precursor microstructures within a Ginzburg-Landau free energy framework. In addition, with increasing disorder at low anisotropy we find a crossover from a twinned state to a non-transforming frozen state. Results are compared with available experimental data.

A review of recent Acoustic Emission experiments during the compression of synthetic porous SiO\(_2\) (Vycor© and Gelsil) under controlled force rates is presented. The statistical analysis of the recorded signals allows comparison with the statistics of earthquakes from available public seismological catalogues. We discuss different methods to characterize the properties of the individual acoustic emission avalanches and the time correlations between them.

Shape memory alloys are a class of ferroic materials which undergo a structural (martensitic) transition where the associated ferroic property is a lattice distortion (strain). The sensitiveness of the transition to the conjugated external field (stress), together with the latent heat of the transition gives rise to giant mechanocaloric effects. In non-magnetic shape memory alloys, the lattice distortion is mostly described by a pure shear and the martensitic transition in this family of alloys is strongly affected by uniaxial stress whereas it is basically insensitive to hydrostatic pressure. As a result, non-magnetic alloys exhibit giant elastocaloric effects but negligible barocaloric effects. By contrast, in a number of magnetic shape memory alloys, the lattice distortion at the martensitic transition involves a volume change in addition to the shear strain. Those alloys are affected by both uniaxial stress and hydrostatic pressure and they exhibit giant elastocaloric and barocaloric effects. The paper aims at providing a critical survey of available experimental data on elastocaloric and barocaloric effects in magnetic and non-magnetic shape memory alloys., Comment: To appear in Philosophical Transactions A

We studied the two-step martensitic transition of a Cu-Al-Ni shape-memory alloy by calorimetry, acoustic emission (AE), and resonant ultrasound spectroscopy (RUS) measurements. The transition occurs under cooling from the cubic (β, Fm3m) parent phase near 242 K to a mixture of orthorhombic 2H and monoclinic 18R phases. Heating leads first to the back transformation of small 18R domains to β and/or 2H near 255 K, and then to the transformation 2H to β near 280 K. The total transformation enthalpy is ΔHT=328±10 J/mol and is observed as one large latent heat peak under cooling. The back-transformation entropy under heating breaks down into a large component 18R to β at 255 K and a smaller, smeared component of the transformation 2H to β near 280 K. The proportions inside the phase mixture depend on the thermal history of the sample. The elastic response of the sample is dominated by large elastic softening during cooling. The weakening of the elastic shear modulus shows a peak at 242 K, which is typical for the formation of complex microstructures. Cooling the sample further leads to additional changes of the microstructure and domain wall freezing, which is seen by gradual elastic hardening and increasing damping of the RUS signal. Heating from 220 K to room temperature leads to elastic anomalies due to the initial transformation, which is now shifted to high temperatures. The transition is smeared over a wider temperature interval and shows strong elastic damping. The shear modulus of the cubic phase is recovered at 280 K. The phase transformation leads to avalanches, which were recorded by AE and by time-resolved calorimetry. The cooling transition shows very extended avalanche signals in calorimetry with power-law distributions. Cooling and heating runs show AE signals over a large temperature interval above 260 K. Splitting the transformation into two martensite phases leads to power-law exponents ɛ∼2 (β↔ 18R) and ɛ∼1.5 (β↔ 2H) while the phase mixture shows an effective AE exponent of 1.7.

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.

The failure mechanism of porous materials under uniaxial stress has been investigated experimentally. Microporous SiO2, Vycor, has been subjected to slowly increasing compressive uniaxial stress with stress rates between 0.2 and 12.2 kPa/s. With increasing stress the strain changes stepwise with acoustic emission correlated with each volume collapse. The acoustic emission followed the characteristics of ‘crackling noise’ with a power law distribution over an exceptionally large interval of 6 decades at the slowest stress rate. The power law exponent is −1.39. Possible applications in mining industry and others are discussed.

Different experimental procedures for the location of sources of Acoustic Emission (AE) avalanches during Martensitic Transformations are discussed. A first example corresponds to the 1D location of AE events during stress-induced martensitic transitions in a Cu-Zn-Al shape memory alloy (3.5 cm length). The obtained data allows monitoring of the interface advancement with a spatial resolution of less than 1 mm. Secondly, we discuss two different ideas that have significant potential for improving this resolution in the case of thermally induced transitions in small single crystalline samples (~1 cm): the use of elastodynamic simulations based on finite element methods and the simultaneous detection of AE and Barkhausen noise in ferromagnetic samples.

We study spatially inhomogeneous states that occur as precursors of marten-sitic/ferroelastic transitions. We will show that cross-hatched modulations (tweed patterns)arise at temperatures above the phase transition in the limit of high elastic anisotropy or lowdisorder while a nano-cluster phase-separated state occurs at low anisotropy or high disorder.In the latter case, nanoscale inhomogeneities give rise to glassy behaviour while the structuraltransition is inhibited. Interestingly, in this case the ferroelastic system also displays a largethermo-mechanical response so that the low symmetry structure can be easily induced by theapplication of relatively small stresses within a broad temperature range.

Reversible strains up to 14% driven by changes in temperature or electric field have been realized in a thin film of bismuth ferrite oxide.

We study the giant elastocaloric effect in a Cu-Zn-Al shape-memory alloy by means of simultaneous calorimetry and dilatometry measurements under applied uniaxial compression which have been carried out with a bespoke experimental setup. The output data allow a unique quasidirect and indirect characterization of the thermal response that provides accurate ΔS and ΔT values. We report a large thermal response of |ΔS| = 22 J K kg−1, |ΔT| = 12.3 K, and RC = 325 K kg−1 driven by moderate uniaxial compression changes of 32.2 MPa, which is comparable with the performance of the best magnetocaloric materials. In view of the hysteresis effects, the thermal response is predicted to exhibit reversible values for |Δσ| = 32 MPa.