Magnetic cooling is an emerging refrigeration technology with potential to surpass the performance of vapour compression devices. It has been successfully applied in the cryogenic temperature ranges, where magnetic cooling gas liquefiers surpass the performance of conventional liquefaction systems. Magnetic refrigeration technology is based on the magnetocaloric effect, a characteristic present in all magnetic materials and alloys. In magnetic thermodynamic cycles, magnetization of a magnetocaloric material is equivalent to the compression of a gas, while demagnetization is equivalent to expansion of a gas, with a subsequent diminution of the entropy. In this thesis, the applicability of this technology to the domestic environment is reviewed. First, the thermodynamics of magnetic refrigeration are explored. Then, a comprehensive review of magnetocaloric materials suitable for use at room temperature is presented. To ascertain the state of the art, the most relevant prototypes and their performances have been described. Concluding the documentation, a survey on the existing mathematic models has been performed, that provided the foundation to create a Matlab model of a magnetic refrigeration device. To gain greater insight on the internal working of these devices, a representative room temperature cooling device has been modelled, and used to simulate a magnetic refrigerator and room air conditioner. Its performance has been analysed and compared with that of vapour compression devices. Also, the influence of parameters such as magnetic field applied, temperature span, refrigerant fluid and different regenerator configurations has been investigated.

Caloric materials show prospect in replacing the function of vaporcompression systems in todays cooling devices, resulting in more energy efficient cooling and eliminating the need for refrigerents which contribute to climate change. This project has focused on magnetocaloric materials, which experience changes in temperature when exposed to magnetic fields. A step to finding viable materials is developing realistic simulations. To this end, this project has investigated if the calculated magnetocaloric effect is impacted by the choice of statistic. Three systems have been studied, bcc Fe, FeRh and Fe2P, using Monte Carlo simulations. The results have shown differences in the calculated entropy change depending on the statistic of choice. The quantum statistics have shown a ∆S = 0 below the phase transition, unlike the classical statistics. At the phase tranisitions quantum statistics resulted in either similar or smaller values for the calculated change in entropy. / Kaloriska material har potential att i framtiden ersätta funktionen hos ångkomprimeringssystem i dagens kylapparater, vilket i sin tur kan leda till mer energieffektiv kylning samt eliminerar behovet av kylmedier som bidrar till klimatförändringen. I detta projekt ligger fokus på magnetokaloriska material, vilka erfar temperaturförändringar då de utsätts för magnetfält. Ett steg mot att hitta gångbara material är att utveckla realistiska simulationer. För detta ändamål undersöktes huruvida den beräknade magnetokaloriska effekten påverkas av valet av statistik. Tre system studerades, bcc Fe, FeRh samt Fe2P, med hjälp av Monte Carlo simulationer. Resultaten visade skillnader i den beräknade entropiförändringen beroende på valet av statistik. För kvantstatistiken var  ∆S = 0 för temperaturer under fasövergångerna, vilket skiljde sig från de klassiska resultaten. Vid fasövergångarna gav kvantstatistiken liknande eller mindre värden för den beräknade entropiförändringen.

O efeito magnetocalórico, i.e., o aquecimento e/ou resfriamento de um material magnético sob variação do campo magnético aplicado é a base da refrigeração magnética.O efeito magnetocalórico é caracterizado pela variação da entropia em um processo isotérmico (O efeito magnetocalórico, i.e., o aquecimento e/ou resfriamento de um material magnético sob variação do campo magnético aplicado é a base da refrigeração magnética. O efeito magnetocalórico é caracterizado pela variação da entropia em um processo isotérmico (ΔSiso) e pela variação da temperatura em um processo adiabático ΔTad.Apesar dos inúmeros trabalhos experimentais e teóricos publicados nessa área, muitos aspectos desse efeito ainda não são bem compreendidos.Nesse trabalho discutimos os efeitos da anisotropia sobre as propriedades magnetocalóricas de um sistema de momentos magnéticos localizados. Para essa finalidade, utilizamos um modelo de spins interagentes com um termo de anisotropia uniaxial do tipo DS2 z , onde D é um parâmetro. Nesse modelo, em que o eixo z é a direção de fácil magnetização, a magnitude do parâmetro de anisotropia e a direção do campo magnético aplicado têm um papel fundamental no comportamento das grandezas magnetocalóricas ΔSiso e ΔTad. Realizamos um estudo sistemático para um sistema com J = 1 aplicando o campo magnético em diferentes direções. Os resultados mostram que, quando o campo magnético é aplicado ao longo da direção z, as grandezas magnetocalóricas apresentam o comportamento normal (valores positivos de ΔTad e valores negativos de ΔSiso para ΔB > 0). Quando o campo magnético é aplicado em uma direção diferente do eixo z, as grandezas magnetocalóricas podem apresentar o comportamento inverso (valores negativos de ΔTad e valores positivos de ΔSiso para ΔB > 0) ou o comportamento anômalo (troca de sinal nas curvas de ΔTad e ΔSiso). Resultados equivalentes também foram obtidos para um sistema com J = 7=2. / The magnetocaloric effect, i.e., heating and/or cooling of a magnetic material subjected to magnetic field variation is the basis of magnetic refrigeration. The magnetocaloric effect is caracterized by the entropy change in an isothermic process (ΔSiso) and by the temperature change in an adiabatic process (ΔTad). Despite the large number of experimental and theoretical works published in this area, there are many aspects of the magnetoccaloric effect which are not yet completely understood.In this work we discuss the effects of anisotropy on the magnetocaloric properties of a system of localized magnetic moments. In order to do that, we used a model of interacting spins with a uniaxial anisotropy term DS2 z , where D is a parameter. In this model, where the z axis is the easy magnetization direction, the magnitude of the anisotropy parameter and the direction of the applied magnetic field have an important role in the behavior of the magnetocaloric quantities ΔSiso and ΔTad. We perform a systematic study for a system with J = 1 by applying the magnetic field in different directions. The results show that, when the magnetic field is applied in the z direction, the magnetocaloric quantities have the normal behavior (positive values of ΔTad and negative values of ΔSiso with ΔB > 0). When the magnetic field is applied in a direction different from the z axis, the magnetocaloric quantities can show the inverse behavior (negative values of ΔTad and positive values of ΔSiso with ΔB > 0) or the anomalous behavior (change of sign in the curves of ΔTad and ΔSiso). Similar results have also been obtained for a system with J = 7=2.

O efeito magnetocalórico, i.e., o aquecimento e/ou resfriamento de um material magnético sob variação do campo magnético aplicado é a base da refrigeração magnética.O efeito magnetocalórico é caracterizado pela variação da entropia em um processo isotérmico (O efeito magnetocalórico, i.e., o aquecimento e/ou resfriamento de um material magnético sob variação do campo magnético aplicado é a base da refrigeração magnética. O efeito magnetocalórico é caracterizado pela variação da entropia em um processo isotérmico (ΔSiso) e pela variação da temperatura em um processo adiabático ΔTad.Apesar dos inúmeros trabalhos experimentais e teóricos publicados nessa área, muitos aspectos desse efeito ainda não são bem compreendidos.Nesse trabalho discutimos os efeitos da anisotropia sobre as propriedades magnetocalóricas de um sistema de momentos magnéticos localizados. Para essa finalidade, utilizamos um modelo de spins interagentes com um termo de anisotropia uniaxial do tipo DS2 z , onde D é um parâmetro. Nesse modelo, em que o eixo z é a direção de fácil magnetização, a magnitude do parâmetro de anisotropia e a direção do campo magnético aplicado têm um papel fundamental no comportamento das grandezas magnetocalóricas ΔSiso e ΔTad. Realizamos um estudo sistemático para um sistema com J = 1 aplicando o campo magnético em diferentes direções. Os resultados mostram que, quando o campo magnético é aplicado ao longo da direção z, as grandezas magnetocalóricas apresentam o comportamento normal (valores positivos de ΔTad e valores negativos de ΔSiso para ΔB > 0). Quando o campo magnético é aplicado em uma direção diferente do eixo z, as grandezas magnetocalóricas podem apresentar o comportamento inverso (valores negativos de ΔTad e valores positivos de ΔSiso para ΔB > 0) ou o comportamento anômalo (troca de sinal nas curvas de ΔTad e ΔSiso). Resultados equivalentes também foram obtidos para um sistema com J = 7=2. / The magnetocaloric effect, i.e., heating and/or cooling of a magnetic material subjected to magnetic field variation is the basis of magnetic refrigeration. The magnetocaloric effect is caracterized by the entropy change in an isothermic process (ΔSiso) and by the temperature change in an adiabatic process (ΔTad). Despite the large number of experimental and theoretical works published in this area, there are many aspects of the magnetoccaloric effect which are not yet completely understood.In this work we discuss the effects of anisotropy on the magnetocaloric properties of a system of localized magnetic moments. In order to do that, we used a model of interacting spins with a uniaxial anisotropy term DS2 z , where D is a parameter. In this model, where the z axis is the easy magnetization direction, the magnitude of the anisotropy parameter and the direction of the applied magnetic field have an important role in the behavior of the magnetocaloric quantities ΔSiso and ΔTad. We perform a systematic study for a system with J = 1 by applying the magnetic field in different directions. The results show that, when the magnetic field is applied in the z direction, the magnetocaloric quantities have the normal behavior (positive values of ΔTad and negative values of ΔSiso with ΔB > 0). When the magnetic field is applied in a direction different from the z axis, the magnetocaloric quantities can show the inverse behavior (negative values of ΔTad and positive values of ΔSiso with ΔB > 0) or the anomalous behavior (change of sign in the curves of ΔTad and ΔSiso). Similar results have also been obtained for a system with J = 7=2.

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Die magnetische Kühlung ist eine etablierte Technologie im Bereich der Tieftemperaturphysik. Allerdings bieten die Skalierbarkeit des magnetokalorischen Effektes und die Möglichkeit zur kompakten Bauweise auch ein breites Anwendungsspektrum für den Einsatz bei Raumtemperatur. Besonders hervorzuheben ist die Möglichkeit zur Anpassung der magnetostrukturellen Umwandlungstemperatur in La(Fe, Si)13-basierten Materialien an die Arbeitstemperatur einer Kühleinheit. Die Herstellung von Ausgangsmaterial über das Schmelzspinnen, ist von hoher technologischer Relevanz, da im Vergleich zu konventionell gegossenem Massivmaterial die anschließende Glühdauer drastisch reduziert werden kann [1]. In der vorliegenden Arbeit wird zunächst auf die optimalen Glühbedingungen in rasch-erstarrtem Bandmaterial für die Bildung der relevanten magnetokalorischen Phase eingegangen. Durch Variation der Glühtemperatur wird der Einfluss von Sekundärphasen auf den magnetokalorischen Effekt bewertet. Darüber hinaus können bei optimaler Wahl der Legierungszusammensetzung ein großer magnetokalorischer Effekt und der gewünschte Arbeitstemperaturbereich eingestellt werden. Besonderes Augenmerk wird auf die Verknüpfung des Substitutionseffektes (hier: Si für Fe) und der Aufweitung des Gitters durch Hydrierung mit dem resultierenden magnetokalorischen Effekt gelegt. Ein weiterer Punkt, sind die Untersuchungen zur Langzeitstabilität der Eigenschaften von hydriertem Band- und Massivmaterial. Grundlegende und umfassende Untersuchungen zur Substitution von Eisen durch Mangan und zum daraus folgenden Einfluss auf Phasenbildung, Umwandlungstemperatur sowie auf den magnetokalorischen Effekt, insbesondere nach der Hydrierung, werden ebenfalls dargestellt. Die Ergebnisse der vorliegenden Arbeit erlauben damit die Bewertung verschiedener Strategien zur Optimierung der magnetokalorischen Eigenschaften von La(Fe, Si)13.

Orientador: Flávio César Guimarães Gandra / Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-17T22:36:59Z (GMT). No. of bitstreams: 1 Reis_RicardoDonizethdos_M.pdf: 3698782 bytes, checksum: 685ad61061d7b02d4c3347f86a4822eb (MD5) Previous issue date: 2011 / Resumo: O efeito magnetocalórico (EMC) é a base da refrigeração magnética. O potencial magnetocalórico é caracterizado por duas quantidades termodinâmicas: a variação isotérmica da entropia (?S) e a variação adiabática da temperatura (?T), as quais são calculadas sob uma variação na intensidade do campo magnético aplicado ao sistema. Em sistemas que apresentam anisotropia magnética é observada uma mudança no efeito magnetocalórico porque este potencial torna-se fortemente dependente da direção de aplicação do campo magnético. A anisotropia em sistemas magnéticos pode levar à definição de um efeito magnetocalórico anisotrópico, o qual, por definição, é obtido para um campo cuja intensidade é mantida constante e cuja orientação variamos de uma direção difícil de magnetização para a direção fácil de magnetização. Neste trabalho apresentaremos os resultados obtidos para o efeito magnetocalórico anisotrópico nos compostos monocristalinos de DyAl2, RBi(R=Dy,Ho) e RGa2 (R=Er,Ho). Para o composto DyAl2 , utilizando o hamiltoniano de campo cristalino (CC) e a aproximação de campo médio, foi possível simular as curvas de magnetização e calor específico obtendo boa concordância com os resultados experimentais. Neste composto a variação isotérmica da entropia ?Sanisotrópico obtida pela variação da direção do campo H (EMC anisotrópico) é maior do que ?Siso convencional que, entretanto, ocorre na temperatura de reorientação de spin (T=42K). A forte anisotropia do ErGa2 e do HoGa2 contribui para uma expressiva diferença no ?Smag (~12 e 23J/kgK@5T, respectivamente, para T~10K) quando o campo é aplicado paralela ou perpendicularmente ao eixo fácil. Em ambos os casos a variação anisotrópica de entropia com a temperatura é semelhante ao ?S convencional com o campo magnético aplicado paralelamente ao eixo fácil de magnetização (eixo c para o ErGa2 e plano ab para o HoGa2). Observamos ainda que o EMC do ErGa2 é fortemente afetado pelo campo cristalino. Medidas de calor específico mostraram um acentuado pico tipo Schottky centrado em 40K e, conseqüentemente, somente parte da entropia magnética total se apresenta na temperatura de ordenamento antiferromagnética. Nos compostos de DyBi e HoBi o valor obtido para o EMC anisotrópico foi maior do que o EMC convencional ( cerca de 15% para o DyBi e 45% para o HoBi). Para os dois compostos foi obtido o EMC anisotrópico para os campos magnéticos de 5T, 6T e 7T. Para o HoBi obtivemos um resultado bastante interessante, no qual o EMC anisotrópico encontrado para µ0H= 5T, 24.7J/KgK, é aproximadamente o dobro do obtido para µ0H =7T / Abstract: The magnetic refrigeration is based on the magnetocaloric effect. The magnetocaloric potential is characterized by two thermodynamic quantities: the isothermal entropy change (?S) and the adiabatic temperature change (?Tad), which are calculated upon under a change in the intensity of the applied magnetic field. In anisotropic magnetic systems it is observed a change in the magnetocaloric effect, since this potential becomes strongly dependent on the direction in which the external magnetic field is applied. The anisotropy in such magnetic systems can lead to an inverse magnetocaloric effect, as well as to the definition of an anisotropic magnetocaloric effect, that by definition is calculated upon a magnetic field which intensity is kept fixed and which orientation is changed from a hard direction of magnetization to the easy direction of magnetization. For DyAl2 compound, using crystal field and mean field approximations, it was possible to simulate the magnetization curves and specific heat obtaining a good agreement with experimental results. In this compound the isothermal entropy change ?Sanisotrópico obtained by varying the direction of the field H (anisotropic EMC) is higher than conventional ?Siso, however, occurs in spin reorientation temperature (T = 42K). The strong anisotropy of ErGa2 and HoGa2 contribute to a expressive difference in the ?Smag (~12 and 23J/kgK@50kOe, respectively at T=10K) when the magnetic field is applied parallel or perpendicular to the easy axes. In both cases the anisotropic variation of entropy with temperature is similar to conventional Ds with the applied magnetic field parallel to the easy axis of magnetization (c axis for ErGa2 and plane ab for HoGa2). We also observed that the EMC ErGa2 is strongly affected by crystal field. Specific heat measurements show a sharp peak Schottky type centered at 40K and, therefore, only part of the total magnetic entropy is presented in the antiferromagnetic ordering temperature. In the compounds of DyBi and HoBi the value obtained for the anisotropic EMC was higher than the conventional EMC (~ 15% to DyBi and 45% for HoBi). For the two compounds was obtained the EMC anisotropic for magnetic fields of 5T, 6T and 7T. HoBi obtained for a very interesting result, in which the anisotropic found for EMC µ0H = 5T, 24.7J/KgK is approximately double that obtained for µ0H = 7T / Mestrado / Física da Matéria Condensada / Mestre em Física

Die magnetische Kühlung ist eine etablierte Technologie im Bereich der Tieftemperaturphysik. Allerdings bieten die Skalierbarkeit des magnetokalorischen Effektes und die Möglichkeit zur kompakten Bauweise auch ein breites Anwendungsspektrum für den Einsatz bei Raumtemperatur. Besonders hervorzuheben ist die Möglichkeit zur Anpassung der magnetostrukturellen Umwandlungstemperatur in La(Fe, Si)13-basierten Materialien an die Arbeitstemperatur einer Kühleinheit. Die Herstellung von Ausgangsmaterial über das Schmelzspinnen, ist von hoher technologischer Relevanz, da im Vergleich zu konventionell gegossenem Massivmaterial die anschließende Glühdauer drastisch reduziert werden kann [1]. In der vorliegenden Arbeit wird zunächst auf die optimalen Glühbedingungen in rasch-erstarrtem Bandmaterial für die Bildung der relevanten magnetokalorischen Phase eingegangen. Durch Variation der Glühtemperatur wird der Einfluss von Sekundärphasen auf den magnetokalorischen Effekt bewertet. Darüber hinaus können bei optimaler Wahl der Legierungszusammensetzung ein großer magnetokalorischer Effekt und der gewünschte Arbeitstemperaturbereich eingestellt werden. Besonderes Augenmerk wird auf die Verknüpfung des Substitutionseffektes (hier: Si für Fe) und der Aufweitung des Gitters durch Hydrierung mit dem resultierenden magnetokalorischen Effekt gelegt. Ein weiterer Punkt, sind die Untersuchungen zur Langzeitstabilität der Eigenschaften von hydriertem Band- und Massivmaterial. Grundlegende und umfassende Untersuchungen zur Substitution von Eisen durch Mangan und zum daraus folgenden Einfluss auf Phasenbildung, Umwandlungstemperatur sowie auf den magnetokalorischen Effekt, insbesondere nach der Hydrierung, werden ebenfalls dargestellt. Die Ergebnisse der vorliegenden Arbeit erlauben damit die Bewertung verschiedener Strategien zur Optimierung der magnetokalorischen Eigenschaften von La(Fe, Si)13.

This dissertation deals with two prospectives of rare earth technology. It’s application in magnetic cooling as well as its harvesting and recycling phase. The emphasis is on mapping and manipulating the transport processes of energy and mass, during magnetic cooling and rare earth magnetic separation, under the influence of magnetic field. Distinguished by the driving force of flow field, they belong to the context of magnetohydrodynamics and ferrohydrodynamics, respectively. Multiple aspects are investigated with respect to magnetic cooling. First, the transient dynamics of heat transfer from two periodically magnetized gadolinium (Gd) plates into a heat transfer fluid (n-decane) is studied. It demonstrates that the propagation of the thermal fronts emanating from the Gd plates after magnetization or demagnetization obeys a √t-dependence. A finite time required for magnetization and demagnetization causes a spatially delayed propagation of the thermal fronts. The diffusive heat flux, derived from the temperature profiles, experiences a drop down by about 80% after first 3 seconds while the percentage of thermal energy transferred into n-decane experiences a maximum there. With a stagnant fluid, this work provides reasons for lower bounds of geometry and operation frequency of a simplified parallel-plate structure in the diffusive limit. Furthermore, the potential of magnetohydrodynamic (MHD) convection to increase heat transfer during magnetic cooling is tested. To do this, a section of an active magnetic regenerator is considered, namely a flat gadolinium plate, immersed in an initially stagnant heat transfer fluid (NaOH) which is placed in a cuboid glass cell. To create the MHD flow, a small electric current is injected by means of two electrodes and interacts with the already present magnetic field. As a result, a Lorentz force is generated, which drives a swirling flow in the present model configuration. By means of particle image velocimetry and Mach-Zehnder interferometry, the flow field and its impact on the heat transfer at the gadolinium plate is analyzed. For the magnetization stage, a heat transfer enhancement by about 40 % can be achieved even with low currents of 3 mA is found. In parallel to enhance the heat transfer by an actively stirring of the heat transfer fluid by means of MHD, alternative fluid candidate is also investigated. A room temperature eutectic liquid metal GaInSn, with superior Pr≈ 0.03, and comparable viscosity like that of water is tested in a segment of parallel plate AMR. Due to the high electric conductivity, velocity field of GaInSn contrasting to that of aqueous based ones is strongly influenced by magnetic field due to Lorentz force. Therefore, preliminary velocity measurements by means of ultrasound doppler velocimetry with a quasi homogeneous static magnetic field (220 mT) in a duct channel at the non-conducting Shercliff walls are conducted. The Hartmann walls are constituted of two parallel Gd plates. The second part of this dissertation, rare earth harvesting and recycling, aims to answer the question of why an enrichment of paramagnetic ions can be observed in a magnetic field gradient despite the presence of a counteracting Brownian motion. For that purpose, a rare-earth chloride (DyCl3) solution is studied in which weak evaporation is adjusted by means of small differences in the vapor pressure. The temporal evolution of the refractive index field of this solution, as a result of heat and mass transfer, is measured by means of a Mach–Zehnder interferometer. A numerical algorithm is developed that splits the refractive index field into two parts, one space-dependent and conservative and the other time-dependent and transient. By using this algorithm in conjunction with a numerical simulation of the temperature and concentration field, it is able to show that 90% of the refractive index in the evaporation-driven boundary layer is caused by an increase in the concentration of Dy(III) ions. A simplified analysis of the gravitational and magnetic forces, entering the Rayleigh number, leads to a diagram of the system’s instability. Accordingly, the enrichment layer of elevated Dy(III) concentration is placed in a spatial zone dominated by a field gradient force. This leads to the unconditional stability of this layer in the present configuration. The underlying mechanism is the levitation and reshaping of the evaporation-driven boundary layer by the magnetic field gradient.

Ever since the discovery of the giant magnetocaloric effect (GMCE) in the Gd5(Si,Ge)4 phases, magnetic cooling has gained significant interest because of its potential environmental benefits and increased efficiency compared to vapour-based refrigeration. This current work is focused on one of the most promising GMCE systems, the (Mn,Fe)2(Si,P) materials. An alternative synthetic route has been explored for the Mn2-xFexSi0.5P0.5 and MnFeSiyP1-y series which is capable of producing phase-pure samples. The new preparation technique eliminates common impurities that arise from established methods thus providing a more accurate description of the structural and physical properties. The low cost, non-toxicity, abundance of starting materials and easy tuning of the magnetic properties make these materials desirable for potential applications. Phase-pure magnetocaloric Mn2-xFexSi0.5P0.5 materials (x = 0.6, 0.7, 0.8, 0.9) were synthesized through arc-melting followed by high temperature sintering. Structural features of samples with x = 0.6, 0.9 were studied through temperature dependent synchrotron powder x-ray diffraction. Magnetic measurements established the Curie temperature, thermal hysteresis, and magnetic entropy change of this system. According to the diffraction and magnetization data, all of the samples were shown to have a first-order magnetostructural transition which becomes less pronounced for Mn-richer samples. The MnFeSixP1-x phases (x = 0.30, 0.35, 0.40, 0.48, 0.52, 0.54, 0.56) have also been synthesized by the same method. For the first time, single crystals of x = 0.30, 0.40 were successfully grown. Variable temperature x-ray diffraction experiments for x = 0.30 were completed which show the structural changes across the phase transition. This structural data was complemented with magnetization data providing Curie temperatures and thermal hysteresis. / Thesis / Master of Science (MSc)