A ferroelastic film at the edge of chaos

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Advances and obstacles in pressure-driven solid-state cooling: A review of barocaloric materials

Solid-state caloric effects promise since decades a disruptive cooling technology that should be more efficient and cleaner than current vapor compression. However, despite relevant achievements have been made, it is still difficult to foresee the time left for the development and wide implementation of competitive devices. Recent progress in the response of materials under hydrostatic pressure offers hope for overcoming some of the shortcomings posed by other solid-state methods and augurs a good outlook for barocaloric cooling, but there are still many struggles ahead to address in order to demonstrate its viability as a commercial cooling technique. Here we briefly review the milestones achieved in terms of barocaloric materials and discuss the pending challenges and expectations for the oncoming years.Peer ReviewedPostprint (author's final draft

Melting of orientational degrees of freedom

We use calorimetry and dilatometry under hydrostatic pressure, X-ray powder diffraction and available literature data in a series of composition-related orientationally disordered (plastic) crystals to characterize both the plastic and melting transitions and investigate relationships between associated thermodynamic properties. First, general common trends are identified: (i) The temperature range of stability of the plastic phase Tm-Tt (where Tt and Tm are the plastic and melting transition temperatures, respectively) increases with increasing pressure and (ii) both the rate of this increase, d(Tm-Tt)/dp, and the entropy change across the plastic transition analyzed as function of the ratio Tt/Tm are quite independent of the particular compound. However, the dependence of the entropy change at the melting transition on Tt/Tm at high pressures deviates from the behavior observed at normal pressure for these and other plastic crystals. Second, we find that the usual errors associated with the estimations of second-order contributions in the Clausius-Clapeyron equation are high and thus these terms can be disregarded in practice. Instead, we successfully test the validity of the Clausius-Clapeyron equation at high pressure from direct measurements. ReferencesPeer ReviewedPostprint (published version

Multiferroic and related hysteretic behavior in ferromagnetic shape memory alloys

We combine a Ginzburg–Landau model for a ferroelastic transition with the theory of micromagnetism to study the magnetostructural behavior leading to multicaloric effects in ferromagnetic shape memory alloys. We analyze the ferroelastic transition under different conditions of temperature, stress and magnetic field and establish the corresponding phase diagram. On the one hand, our results show that the proper combination of both fields may be used to reduce the transition hysteresis and thus improve the reversibility of the related elastocaloric effects, superelasticity and stress-mediated magnetocaloric effects. On the other hand, the stress-free magnetic field-driven and thermally driven magnetostructural evolution provides physical insight into the low-temperature field-induced domain reorientation, from which we derive strategies to modify the operational temperature ranges and thus the corresponding (magnetic) shape-memory effect.Peer ReviewedPostprint (published version

Giant reversible barocaloric response of (MnNiSi)(1-x)(FeCoGe)(x) (x=0.39, 0.40, 0.41)

MnNiSi-based alloys and isostructural systems have traditionally demonstrated impressive magnetocaloric properties near room temperature associated with a highly tunable first-order magnetostructural transition that involves large latent heat. However, these materials are limited by a small field-sensitivity of the transition, preventing significant reversible effects usable for cooling applications. Instead, the concomitant large transition volume changes prompt a high pressure-sensitivity, and therefore, promise substantial barocaloric performances, but they have been sparsely studied in these materials. Here, we study the barocaloric response in a series of composition-related (MnNiSi)1-x(FeCoGe)x (x = 0.39, 0.40, 0.41) alloys that span continuously over a wide temperature range around ambient. We report on giant reversible effects of ~40 J K-1 kg-1 and up to ~4 K upon application of ~2 kbar and find a degradation of the first-order transition properties with pressure that limits the barocaloric effects at high pressures. Our results confirm the potential of this type of alloys for barocaloric applications, where multicaloric and composite possibilities, along with the high density and relatively high thermal conductivity, constructively add to the magnitude of the caloric effects.Peer ReviewedPostprint (published version

Polymorphism of spironolactone: An unprecedented case of monotropy turning to enantiotropy with a huge difference in the melting temperatures

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Ultrastable glasses portray similar behaviour to ordinary glasses at high pressure

Pressure experiments provide a unique opportunity to unravel new insights into glass-forming liquids by exploring its effect on the dynamics of viscous liquids and on the evolution of the glass transition temperature. Here we compare the pressure dependence of the onset of devitrification, Ton, between two molecular glasses prepared from the same material but with extremely different ambient-pressure kinetic and thermodynamic stabilities. Our data clearly reveal that, while both glasses exhibit different dTon/dP values at low pressures, they evolve towards closer calorimetric devitrification temperature and pressure dependence as pressure increases. We tentatively interpret these results from the different densities of the starting materials at room temperature and pressure. Our data shows that at the probed pressures, the relaxation time of the glass into the supercooled liquid is determined by temperature and pressure similarly to the behaviour of liquids, but using stability-dependent parametersPostprint (published version

Colossal barocaloric effects in adamantane derivatives for thermal management

Plastic crystals are currently attracting interest because their solid-state caloric functionality could be used to tackle climate change in two critical areas: (i) more environmentally friendly cooling and heating driven by pressure and (ii) passive waste heat management. Here, we suggest that plastic crystals could also be used for active pressure-assisted (i.e., barocaloric) waste heat management. In contrast to the barocaloric cooling/heating cycle, for active barocaloric waste heat management, the hysteresis may not be a constraint and transition temperatures above ambient are usually desired. In contrast to passive waste heat management, the application of pressure can be an advantage to actively control the absorption and delivery of heat by the plastic crystal. Here, we have investigated the pressure-induced caloric response at the first-order phase transitions occurring above room temperature of three plastic crystals derived from adamantane: 1-adamantanol, 2-adamantanol, and 2-methyl-2-adamantanol. Colossal barocaloric effects have been found for two of them under small pressure changes of 50 MPa. This behavior occurs thanks to a colossal transition entropy change and a large transition sensitivity to pressure, which can simultaneously take place due to enormous transition volume changes. The balance between configurational and volumic entropy changes at the transition has also been discussed. For 2-adamantanol, in addition to the transition to the plastic phase, the less energetic triclinic-to-monoclinic transition at lower temperatures has also been analyzed. The transition temperatures above ambient make these compounds suitable for waste heat management and, thanks to a small hysteresis, also for industrial cooling and heat pumping.Peer ReviewedPostprint (published version

Giant and reversible inverse barocaloric effects near room temperature in ferromagnetic MnCoGeB0.03

Hydrostatic pressure represents an inexpensive and practical method of driving caloric effects in brittle magnetocaloric materials, which display first-order magnetostructural phase transitions whose large latent heats are traditionally accessed using applied magnetic fields. Here, moderate changes of hydrostatic pressure are used to drive giant and reversible inverse barocaloric effects near room temperature in the notoriously brittle magnetocaloric material MnCoGeB0.03. The barocaloric effects compare favorably with those observed in barocaloric materials that are magnetic. The inevitable fragmentation provides a large surface for heat exchange with pressure-transmitting media, permitting good access to barocaloric effects in cooling devices.Peer ReviewedPostprint (author's final draft

Barocaloric response of plastic crystal 2-methyl-2-nitro-1-propanol across and far from the solid-solid phase transition

Plastic crystals have emerged as benchmark barocaloric (BC) materials for potential solid-state cooling and heating applications due to huge isothermal entropy changes and adiabatic temperature changes driven by pressure. In this work we investigate the BC response of the neopentane derivative 2-methyl-2-nitro-1-propanol (NO2C(CH3)2CH2OH) in a wide temperature range using x-ray diffraction, dilatometry and pressure-dependent differential thermal analysis. Near the ordered-to-plastic transition, we find colossal BC effects of $\simeq$400¿J¿K-1¿kg-1 and $\simeq$5¿K upon pressure changes of 100¿MPa. Although reversible effects at the transition are obtained only from higher pressure changes due to hysteretic effects, we do obtain fully reversible BC effects from any pressure change in individual phases, that become giant at moderate pressures due to very large thermal expansion, especially in the plastic phase. From our measurements, we also determine the crystal structure of the low-temperature phase and estimate the contribution of the configurational disorder and the volume change to the total transition entropy change.Peer ReviewedPostprint (author's final draft