Direct measurement of the adiabatic temperature change and study of the reversibility in the magnetocaloric e ect of Fe0.49Rh0.51

Treballs Finals de Grau de Física, Facultat de Física, Universitat de Barcelona, Any: 2014, Tutor: Lluís Maños

Giant caloric and multicaloric effects in magnetic alloys

[eng] The urgent need to reduce our footprint on the earth environment is leading to ever more stringent commitments to decrease greenhouse gases emissions, which entails one of the greatest challenges that mankind has to tackle. As a direct consequence, it is of utmost importance to develop novel, energy-efficient and environmentally-friendly refrigeration technologies that do not require the use of climate-damaging substances. In this regard, solid-state refrigerants based on the large thermal response exhibited by a variety of materials when field-inducing a ferroic phase transition are among the best alternatives. Specifically, materials undergoing a first-order phase transition are of particular interest as the latent heat associated with the phase transition contributes on enhancing the magnitude of the thermal response. Depending on the nature of the external field that drives the phase transition one distinguishes between magnetocaloric, electrocaloric, elastocaloric or barocaloric effects. In spite of all the intensive research devoted to the study of the diverse caloric effects, there are still a series of bottlenecks to overcome. Firstly, they require the application of strong external fields in order to induce a large thermal response. Secondly, the hysteresis associated with the phase transition can drastically reduce the efficiency and compromises its reversibility. A way out of such issues can be provided by materials exhibiting a strong coupling between the structural, magnetic or electronic degrees of freedom, denoted as multicaloric materials, which allow to drive their phase transition by the combination of diverse external fields, giving rise to multicaloric effects. Despite the high potential they exhibit, the research on multicaloric materials is germinal as it requires the use of non-commercial experimental systems. In this dissertation, we have focused on the study of materials displaying a magnetostructural first- order phase transition with a strong coupling between the structural and magnetic degrees of freedom. For such purpose, we have used distinct purpose-built calorimetric and adiabatic thermometry systems to investigate their caloric and multicaloric effects by direct methods. We have concentrated on two distinct families of multicaloric materials: Fe-Rh and Ni-Mn-based Heusler alloys. Our research is aimed at thoroughly characterizing the diverse advantages of multicaloric effects: showing that lower driving fields are required, that the operating temperature windows of the materials can be enlarged and discussing how their inherent hysteresis can be mastered or even exploited.[cat] La necessitat urgent de reduir la nostra empremta en el clima s’està materialitzant en compromisos globals per disminuir l’emissió de gasos d’efecte hivernacle que cada cop son més estrictes, representat avui en dia un dels majors reptes que la humanitat ha d’afrontar. Com a conseqüència directa, és de la màxima importància desenvolupar noves tecnologies de refrigeració que siguin eficients i respectuoses amb el medi ambient. En aquest sentit, entre les millors alternatives es troben els refrigerants en estat sòlid basats en materials que presenten una resposta tèrmica gran quan s’indueix una transició de fase ferroica mitjançant un camp extern. En concret, són d’especial interès els materials que presenten una transició de fase de primer ordre, ja que la calor latent associada a la transició de fase incrementa la magnitud de la resposta tèrmica. Depenent de la naturalesa del camp extern que s’utilitza per induir la transició de fase, es distingeix entre els efectes magnetocalòric, electrocalòric, elastocalòric o barocalòric. Malgrat els grans esforços dedicats en l’estudi dels diversos efectes calòrics, hi ha una sèrie d’obstacles que cal superar. En primer lloc, es necessiten camps intensos per induir una resposta tèrmica gran. En segon lloc, la histèresi associada a la transició de fase pot reduir dràsticament la seva eficiència i comprometre la reversibilitat de l’efecte calòric. Una possible sortida a aquests problemes pot venir donada pels materials que presenten un fort acoblament entre els graus de llibertat estructural, magnètic o electrònic, anomenats materials multicalòrics, ja que permeten que la seva transició de fase s’indueixi mitjançant la combinació de diversos camps externs, donant lloc als anomenats efectes multicalòrics. Malgrat l’alt potencial que presenten a l’hora d’abordar algunes de les mancances que s’han posat de manifest ens els diversos efectes calòrics, l’interès en la seva recerca és molt recent ja que requereix l’ús de sistemes experimentals no comercials. En aquesta tesi, ens hem centrat en l’estudi de materials que presenten transicions magnetoestructurals de primer ordre amb un fort acoblament entre els graus de llibertat magnètic i estructural. Amb aquest propòsit, hem utilitzat diversos dispositius experimentals dissenyats ad hoc que permeten realitzar mesures calorimètriques o termomètriques sota la influència de camp magnètic i esforç uniaxial per tal de caracteritzar-ne els efecte calòrics i multicalòrics mitjançant mètodes directes. Ens hem centrat en dues famílies de materials multicalòrics: Fe-Rh i aliatges tipus Heusler de base Ni-Mn. La recerca duta a terme s’ha centrat en caracteritzar a fons els diversos avantatges que presenten els efectes multicalòrics: demostrant que requereixen camps de menor intensitat per induir una resposta tèrmica gran, que permeten ampliar el rang de temperatura de treball dels materials o que proporcionen estratègies per controlar o fins i tot aprofitar la histèresi associada a la transició de fase

Caloric response of Fe49Rh51 subjected to uniaxial load and magnetic field.

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 Fe49Rh51 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 effects in metamagnetic Heusler Ni-Mn-In under uniaxial stress and magnetic field

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

Magnetic and structural entropy contributions to the multicaloric effects in Ni-Mn-Ga-Cu

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

A multicaloric cooling cycle that exploits thermal hysteresis

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 l

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

We report on the adiabatic temperature changes (DT) 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 DT for the barocaloric effect, which are indirectly determined from entropy curves. Large DT 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

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

We report on the adiabatic temperature changes (Delta T) 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 Delta T for the barocaloric effect, which are indirectly determined from entropy curves. Large Delta T 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. (C) 2015 AIP Publishing LLCPostprint (published version

Giant and Reversible Barocaloric Effect in Trinuclear Spin-Crossover Complex Fe3(bntrz)6(tcnset)6

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

Cross-coupling contribution to the isothermal entropy change in multicaloric materials

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