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Duijn, Henricus Gerardus Maria. "Magnetotransport and magnetocaloric effects in intermetallic compounds." [S.l. : Amsterdam : s.n.] ; Universiteit van Amsterdam [Host], 2000. http://dare.uva.nl/document/83091.
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Amaral, João Cunha de Sequeira. "Studies on magnetocaloric and magnetic coupling effects." Doctoral thesis, Universidade de Aveiro, 2009. http://hdl.handle.net/10773/2685.
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Doutoramento em FísicaO presente trabalho apresenta novas metodologias desenvolvidas para a análise das propriedades magnéticas e magnetocalóricas de materiais, sustentadas em considerações teóricas a partir de modelos, nomeadamente a teoria de transições de fase de Landau, o modelo de campo médio molecular e a teoria de fenómeno crítico. São propostos novos métodos de escala, permitindo a interpretação de dados de magnetização de materiais numa perspectiva de campo médio molecular ou teoria de fenómeno crítico. É apresentado um método de estimar a magnetização espontânea de um material ferromagnético a partir de relações entropia/magnetização estabelecidas pelo modelo de campo médio molecular. A termodinâmica das transições de fase magnéticas de primeira ordem é estudada usando a teoria de Landau e de campo médio molecular (modelo de Bean-Rodbell), avaliando os efeitos de fenómenos fora de equilíbrio e de condições de mistura de fase em estimativas do efeito magnetocalórico a partir de medidas magnéticas. Efeitos de desordem, interpretados como uma distribuição na interacção magnética entre iões, estabelecem os efeitos de distribuições químicas/estruturais nas propriedades magnéticas e magnetocalóricas de materiais com transições de fase de segunda e de primeira ordem. O uso das metodologias apresentadas na interpretação das propriedades magnéticas de variados materiais ferromagnéticos permitiu obter: 1) uma análise quantitativa da variação de spin por ião Gadolínio devido à transição estrutural do composto Gd5Si2Ge2, 2) a descrição da configuração de cluster magnético de iões Mn na fase ferromagnética em manganites da família La-Sr e La-Ca, 3) a determinação dos expoentes críticos β e δ do Níquel por métodos de escala, 4) a descrição do efeito da pressão nas propriedades magnéticas e magnetocalóricas do composto LaFe11.5Si1.5 através do modelo de Bean-Rodbell, 5) uma estimativa da desordem em manganites ferromagnéticas com transições de segunda e primeira ordem, 6) uma descrição de campo médio das propriedades magnéticas da liga Fe23Cu77, 7) o estudo de efeitos de separação de fase na família de compostos La0.70-xErxSr0.30MnO3 e 8) a determinação realista da variação de entropia magnética na família de compostos de efeito magnetocalórico colossal Mn1-x-yFexCryAs.
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Álvarez, Alonso Pablo. "Magnetocaloric and magnetovolume effects in Fe-based alloys." Doctoral thesis, Universidad de Oviedo, 2011. http://hdl.handle.net/10803/51881.
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En esta memoria de Tesis Doctoral se presentan los resultados del estudio del efecto magnetocalórico y magnetovolúmico que se ha llevado a cabo en dos familias de compuestos ricos en Fe: aleaciones R2Fe17, sintetizadas en forma policristalina, y cintas amorfas de composición FeZrBCu. Estas aleaciones presentan transiciones magnéticas de segundo orden con temperaturas críticas en torno a temperatura ambiente.
La serie de aleaciones R2Fe17 (con R = Y, Ce, Pr,…) ha sido sintetizada mediante la fusión de los diferentes elementos por horno de arco. Se ha determinado la estructura cristalina de estos compuestos mediante difracción de rayos x y de neutrones de alta resolución. Los compuestos de esta familia pueden cristalizar en dos tipos de estructuras cristalinas dependiendo de la tierra rara que se emplee: para las tierras raras ligeras (Ce, Pr, Nd, Sm, Gd, Tb y Dy) los compuestos que se han sintetizado son romboédricos tipo Th2Zn17, para las pesadas (Ho, Er, Tm y Lu) son hexagonales tipo Th2Ni17, mientras que el compuesto Y2Fe17 presenta ambas estructuras cristalinas. A partir de la termodifracción de neutrones se ha determinado la dependencia con la temperatura (T) tanto de los parámetros de malla como de los momentos magnéticos de cada sitio cristalogáficico. Existe una magnetostricción espontánea anisótropa, más pronunciada a lo largo del eje uniáxico, con independencia de la estructura cristalina que presente el compuesto.
Más aún, se ha observado que la magnetostricción de volumen depende cuadráticamente con el momento total de la subred del Fe hasta temperaturas cercanas a la de Curie, TC. Experimentos de rayos x bajo alta presión han mostrado que existe una pequeña anisotropía en los compuestos romboédricos al comprimir la celda cristalográfica, puesto que es más fácil comprimirla en la dirección uniáxica. Ajustando la dependencia del volumen con la presión con una ecuación de estado de Birch-Murnaghan se han estimado los módulos de compresibilidad.
También se ha estudiado el efecto magnetocalórico a partir de la dependencia de la imanación con el campo magnético a diferentes temperaturas, obteniéndose la variación de la entropía magnética (∆SM) con T y el campo magnético aplicado (H). En los compuestos ferrimagnéticos ∆SM (T,H) presenta un máximo a bajas temperaturas, asociado con un efecto magnetocalórico inverso, y un mínimo, asociado con un efecto directo, el cual ocurre a T ≈TC. En cambio, en los ferromagnéticos sólo existe el efecto directo. Para el caso especial del Ce2Fe17 aparecen dos mínimos, estando el de más alta temperatura asociado a la transición de segundo orden del estado ferromagnético al paramagnético, mientras que el de más baja temperatura es debido a una transición metamagnética.
Se ha investigado la influencia que tiene la molienda mecánica en la microestructura de las aleaciones Pr2Fe17 y Nd2Fe17, así como los efectos de estas modificaciones en sus propiedades magnéticas. Mediante técnicas de difracción se ha determinado su estructura cristalina, la cual no se ve modificada tras el proceso de molienda. Sin embargo, sí se produce un cambio drástico en la microestructura debido a la rotura progresiva de granos cristalinos y la formación de partículas de tamaño nanoscópico, lo cual ha sido corroborado mediante la microscopía electrónica de barrido y de transmisión conjuntamente con la difracción. Conforme se incrementa el tiempo de molienda se produce una disminución del tamaño de partícula, y los granos presentan una dispersión de tamaños menor. Estas modificaciones en la microestructura tienen como resultado la aparición de una distribución de temperaturas de Curie, con el consiguiente ensanchamiento de la curva ∆SM(T), así como una disminución del valor máximo del cambio en la entropía magnética.
Asimismo se han sintetizado diferentes compuestos pseudobinarios tipo AxB2-xFe17 (siendo A y B tierras raras y/o Itrio). En este caso, mezclando diversas tierras raras se puede controlar el valor de la temperatura de orden magnético alrededor de la temperatura ambiente. Dependiendo de las tierras raras empleadas, estos compuestos pueden presentar cualquiera de las dos estructuras cristalinas en las que cristaliza la familia R2Fe17. En los compuestos sintetizados la estructura cristalina es romboédrica (R3m), no habiéndose detectado la existencia de fase hexagonal mediante la difracción de neutrones ni de rayos x, dentro de los límites de detección.
En el caso de las aleaciones amorfas tipo Nanoperm, FeZrBCu, las muestras se han obtenido en forma de cinta mediante la técnica de enfriamiento ultrarrápido. En estos compuestos, TC depende de la cantidad de Fe, por lo que se puede seleccionar la temperatura a la que se obtiene el máximo de l∆SM (T,H)l. Además, presentan una transición ferro-paramagnética que se extiende en un amplio rango de temperaturas, lo cual lleva asociado una curva l∆SM(T)l muy ancha. Definiendo la capacidad relativa de refrigeración de un material magnético como el producto de la anchura a mitad de altura de l∆SM (T,H)l por el valor máximo de l∆SM(T)l, se obtiene un valor alto, aún cuando el valor de │∆Speak M│es moderado, en comparación con los materiales que presentan una transición magnética de primer orden. Asimismo, se ha determinado ∆ SM en diferentes aleaciones de FeZrBCu para campos magnéticos entre 0 y 8 T, lo que permite discutir la existencia de un comportamiento común de la variación de la entropía magnética para los elementos esta familia.
Por último, se han estudiado las propiedades magnetocalóricas resultantes de la combinación de dos cintas de distintas composiciones. El resultado más destacado es que se puede producir un incremento de la capacidad de refrigeración y, además, la aparición de un aplanamiento de la curva ∆SM(T) en un amplio rango de temperaturas.
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Belliveau, Hillary Faith. "Reduced Dimensionality Effects in Gd-based Magnetocaloric Materials." Scholar Commons, 2016. http://scholarcommons.usf.edu/etd/6465.
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Magnetic refrigeration based on the magnetocaloric effect (MCE) is a promising alternative to conventional gas compression based cooling techniques. Understanding impacts of reduced dimensionality on the magnetocaloric response of a material such as Gadolinium (Gd) or its alloys is essential in optimizing the performance of cooling devices, which is also the overall goal of this thesis. We have determined, in the first part of the thesis, that laminate structures of pure Gd produced by magnetron sputtering have several disadvantages. The target material (pure Gd), ultra-high vacuum components, and the electrical energy it takes to run the manufacturing process are all very costly. To produce quality films requires a time and energy consuming chamber preparation (gettering) to produce films with a relative cooling power (RCP) of an order of magnitude smaller (~70 J/kg) than can be obtained with Gd-alloy microwires (~800 J/kg). The increased surface area for an array of wires as compared to a laminate structure allows for more efficient heat transfer. For all of these reasons, we turned the focus onto Gd-alloy microwires. In the latter part of this thesis, we have discussed the Gd-alloy microwires as a function of magnetocaloric parameters of magnetic entropy change, adiabatic temperature change, and refrigerant capacity (RC). We have demonstrated two effective methods for improving the RC of the microwires through creating novel biphase nanocrystalline/amorphous structures via thermal annealing and directly from adjusted melt-extraction. Through studying the effects of chemical doping, as well as studying arrays of microwires with a range of Curie temperature (TC) values, we have designed a new magnetic bed structure that has potential applications as a cooling device for micro-electro-mechanical systems and energy-conversion devices.
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Duffield, Toby. "A study of magnetocaloric effects in two spin glass alloys." Thesis, Imperial College London, 1985. http://hdl.handle.net/10044/1/37678.
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Aryal, Anil. "PHASE TRANSITIONS AND MAGNETOCALORIC EFFECTS IN Ni1−xCrxMnGe1.05 AND GdNi2Mnx." OpenSIUC, 2015. https://opensiuc.lib.siu.edu/theses/1755.
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The magnetocaloric and thermomagnetic properties of the Ni1-xCrxMnGe1.05 (for x = 0, 0.035, 0.070, 0.105, 0.110, 0.115, and 0.120) system have been studied by X-ray diffraction, differential scanning calorimetry (DSC), resistivity and magnetization measurements. A change in crystal structure from orthorhombic to hexagonal was observed in the XRD data with an increase in chromium concentrations. The values of the cell parameters and volume of the unit cell for hexagonal phase were determined. It was found that the partial substitution of Cr for Ni in Ni1-xCrxMnGe1.05 results in a first order magnetostructural transition from antiferromagnetic to ferromagnetic (FM) at TM of about132 K, 100 K, and 110 K for x= 0.105, 0.115, and 0.120, respectively. A FM to paramagnetic second order transition has been observed at TC around 200K. A magnetic entropy change of = 4.5 J/kg K, 5.6 J/Kg K, and 5.06 J/Kg K was observed in the vicinity of TC for x = 0.105, 0.115, and 0.120 respectively at ΔH = 5T. The values of the latent heat and corresponding total entropy changes have been determined from Differential Scanning Calorimetry (DSC) measurements. Magnetoresistance values of about -5% were measured near TC for x =0.105. The maximum value of refrigeration capacity (RC) and relative cooling power (RCP) was found to be 155 J/Kg and 175 J/Kg respectively for x = 0.120. A concentration-dependent (T-x) phase diagram of transition temperatures has been constructed using the magnetic and DSC data. The structural, magnetic and magnetocaloric properties of GdNi2Mnx system (for x = 0.5, 0.6, 0.8, 1.0, 1.2, 1.4, 1.5) have been studied by x-ray diffraction and magnetization measurements. A mixture of the Laves phase C15 and a phase with rhombohedral structure PuNi3- type (space group R m) was observed in the XRD data. A second order magnetic phase transition from ferromagnetic (FM) to paramagnetic (PM) was found, characterized by a long-range exchange interaction as predicted by mean field theory. The maximum value of magnetic entropy changes, -∆SM, near TC for ∆H = 5T, was found to be 3.1 J/KgK, 2.8 J/KgK, 2.9 J/KgK, and 2.5 J/Kg K for x = 0.8, 1.2, 1.4, and 1.5 respectively. In spite of the low values of ΔSM, the RC and RCP value was found to be 176 J/Kg and 220 J/Kg for the GdNi2Mn0.8 compound, respectively.
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Lampen, Kelley Paula J. "Low Dimensionality Effects in Complex Magnetic Oxides." Scholar Commons, 2015. http://scholarcommons.usf.edu/etd/5874.
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Complex magnetic oxides represent a unique intersection of immense technological importance and fascinating physical phenomena originating from interwoven structural, electronic and magnetic degrees of freedom. The resulting energetically close competing orders can be controllably selected through external fields. Competing interactions and disorder represent an additional opportunity to systematically manipulate the properties of pure magnetic systems, leading to frustration, glassiness, and other novel phenomena while finite sample dimension plays a similar role in systems with long-range cooperative effects or large correlation lengths. A rigorous understanding of these effects in strongly correlated oxides is key to manipulating their functionality and device performance, but remains a challenging task. In this dissertation, we examine a number of problems related to intrinsic and extrinsic low dimensionality, disorder, and competing interactions in magnetic oxides by applying a unique combination of standard magnetometry techniques and unconventional magnetocaloric effect and transverse susceptibility measurements.
The influence of dimensionality and disorder on the nature and critical properties of phase transitions in manganites is illustrated in La0.7Ca0.3MnO3, in which both size reduction to the nanoscale and chemically-controlled quenched disorder are observed to induce a progressive weakening of the first-order nature of the transition, despite acting through the distinct mechanisms of surface effects and site dilution. In the second-order material La0.8Ca0.2MnO3, a strong magnetic field is found to drive the system toward its tricritical point as competition between exchange interactions in the inhomogeneous ground state is suppressed. In the presence of large phase separation stabilized by chemical disorder and long-range strain, dimensionality has a profound effect. With the systematic reduction of particle size in microscale-phase-separated (La, Pr, Ca)MnO3 we observe a disruption of the long-range glassy strains associated with the charge-ordered phase in the bulk, lowering the field and pressure threshold for charge-order melting and increasing the ferromagnetic volume fraction as particle size is decreased. The long-range charge-ordered phase becomes completely suppressed when the particle size falls below 100 nm. In contrast, low dimensionality in the geometrically frustrated pseudo-1D spin chain compound Ca3Co2O6 is intrinsic, arising from the crystal lattice. We establish a comprehensive phase diagram for this exotic system consistent with recent reports of an incommensurate ground state and identify new sub-features of the ferrimagnetic phase. When defects in the form of grain boundaries are incorporated into the system the low-temperature slow-dynamic state is weakened, and new crossover phenomena emerge in the spin relaxation behavior along with an increased distribution of relaxation times. The presence of both disorder and randomness leads to a spin-glass-like state, as observed in γFe2O3 hollow nanoparticles, where freezing of surface spins at low temperature generates an irreversible magnetization component and an associated exchange-biasing effect. Our results point to distinct dynamic behaviors on the inner and outer surfaces of the hollow structures.
Overall, these studies yield new physical insights into the role of dimensionality and disorder in these complex oxide systems and highlight the sensitivity of their manifested magnetic ground states to extrinsic factors, leading in many cases to crossover behaviors where the balance between competing phases is altered, or to the emergence of entirely new magnetic phenomena.
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Brock, Jeffrey Adams. "AN EXPERIMENTAL STUDY OF MAGNETIC AND STRUCTURAL PHASE TRANSITIONS AND ASSOCIATED PHENOMENA IN SELECTED NI-MN-DERIVATIVE HEUSLER ALLOYS." Miami University / OhioLINK, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=miami1500906786979139.
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Gottschall, Tino [Verfasser], Oliver [Akademischer Betreuer] Gutfleisch, and Heiko [Akademischer Betreuer] Wende. "On the magnetocaloric properties of Heusler compounds: Reversible, time- and size-dependent effects of the martensitic phase transition / Tino Gottschall. Betreuer: Oliver Gutfleisch ; Heiko Wende." Darmstadt : Universitäts- und Landesbibliothek Darmstadt, 2016. http://d-nb.info/1112333010/34.
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Guglielmo, Francesco. "Studio di un gruppo frigorifero basato sull'effetto magnetocalorico." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2017. http://amslaurea.unibo.it/12602/.
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La tecnologia di refrigerazione magnetocalorica si propone come una valida alternativa ai sistemi di refrigerazione convenzionale. Le problematiche relative all'impatto ambientale dei fluidi refrigeranti unitamente alle basse efficienze delle macchine refrigeranti a compressione tradizionali, hanno spinto un numero cospicuo di ricercatori alla costruzione di nuove macchine con tecnologia innovativa. All'interno dell'elaborato si può trovare una breve descrizione dei cicli frigoriferi tradizionali (a compressione e ad assorbimento), inoltre è stata effettuata una ricerca bibliografica sullo stato dell'arte dei dispositivi che sfruttano la tecnologia magnetocalorica. Infine si è proceduto con lo studio di alcune soluzioni per la realizzazione dell'unità di scambio termico in Gadolinio. Durante lo studio, sono state sviluppate sei ipotesi realizzative che possono essere in futuro utilizzate in un prototipo di refrigeratore magnetico rotativo a magneti permanenti.
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Sandberg, Anna. "Quantum statistics and the magnetocaloric effect." Thesis, Uppsala universitet, Materialteori, 2020. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-415830.
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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.
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Ghorbani-Zavareh, Mahdiyeh. "Direct Measurements of the Magnetocaloric Effect in Pulsed Magnetic Fields." Doctoral thesis, Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden, 2017. http://nbn-resolving.de/urn:nbn:de:bsz:14-qucosa-207504.
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The present thesis is devoted to the investigation of the magnetocaloric effect (MCE) by direct measurements in pulsed and quasi-static magnetic fields as well as by analyzing specific-heat data taken in static magnetic fields. The emphasis is on the direct measurement of the adiabatic temperature change Tad in pulsed magnetic fields, because the pulsed-field data allow for an analysis of the sample-temperature response to the magnetic field on a time scale of 10 to 100 ms, which is on the order of typical operation frequencies (10 - 100 Hz) of magnetocaloric cooling devices. Besides extending the accessible magneticfield range to beyond 70 T, the short pulse duration provides nearly adiabatic conditions during the measurement.
In this work, the magnetocaloric properties of various types of solids are investigated: Gadolinium (Gd) with a second-order transition, Ni50Mn35In15 with multiple magnetic transitions, and La(Fe,Si,Co)13 compounds with first and second-order transitions, depending on the Co concentration.
The adiabatic temperature change of a polycrystalline Gd sample has been measured in pulsed magnetic fields up to 70 T and also in quasi-static fields up to 2 T. A very large adiabatic temperature change of Tad 60 K is observed near the Curie temperature (TC = 294 K) for a field change of 70 T. In addition, we find that this maximum temperature change grows with H2=3.
We have studied the MCE in the shape-memory Heusler alloy Ni50Mn35In15 by direct measurements in pulsed magnetic fields up to 6 and 20 T. The results obtained for 6 T pulses are compared with data extracted from specific-heat experiments. We find a saturation of the inverse MCE, related to the firstorder martensitic transition, with a maximum adiabatic temperature change of Tad = 7 K at 250 K and a conventional field-dependent MCE near the second-order ferromagnetic transition in the austenitic phase. Our results disclose that in shape-memory alloys the different contributions to the MCE and hysteresis effects around the martensitic transition have to be carefully considered for future cooling applications.
Finally, a comparative study of the magnetic and magnetocaloric properties of La(Fe,Si,Co)13 alloys is presented by discussing magnetization, Tad, specificheat, and magnetostriction measurements. The nature of the transition can be changed from first to second order as well as the temperature of the transition can be tuned by varying the Co concentration. The MCE of two samples with nominal compositions of LaFe11:74Co0:13Si1:13 and LaFe11:21Co0:65Si1:11 have been measured in pulsed magnetic fields up to 50 T. We find that LaFe11:74Co0:13Si1:13 with a first-order transition (TC = 198 K) shows half of the net MCE already at low fields (2-10 T). Whereas the MCE of LaFe11:21Co0:65Si1:11 with secondorder transition (TC = 257 K) grows gradually.
The MCE in both compounds reaches almost similar values at a field of 50 T. The MCE results obtained in pulsed magnetic fields of 2 T are in good agreement with data from quasistatic field measurements. The pulsed-field magnetization of both compounds has been measured in fields up to 60 T under nearly adiabatic conditions and compared to steady-field isothermal measurements. The differences between the magnetization curves obtained under isothermal and adiabatic conditions give the MCE via the crossing points of the adiabatic curve with the set of isothermal curves. For LaFe11:74Co0:13Si1:13, a S - T diagram has been constructed from specific-heat measurements in static fields, which is used to extract the MCE indirectly. Magnetostriction measurements are carried out for two compounds in both static and pulsed magnetic fields. For LaFe11:74Co0:13Si1:13, the strain shows a sharp increase.
However, due to cracks appearing in the sample an irreversible magneto-volume effect of about 1% is observed in pulsed magnetic fields. Whereas for LaFe11:21Co0:65Si1:11 the data show a smooth increase of the sample length up to 60 T, and a 1.3% volume increase is obtained. We also find that magnetizing the latter sample in the paramagnetic state is tightly bound to the volume increase and this, likewise for the former sample, gives the main contribution to the entropy change.
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Bayer, Daniel Nicholas. "The Magnetocaloric Effect & Performance of Magnetocaloric Materials in a 1D Active Magnetic Regenerator Simulation." Wright State University / OhioLINK, 2019. http://rave.ohiolink.edu/etdc/view?acc_num=wright1578587695272946.
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Rebar, Drew. "Magnetocaloric effect in nanoparticles and bulk clathrates." [Tampa, Fla] : University of South Florida, 2006. http://purl.fcla.edu/usf/dc/et/SFE0001630.
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Bauer, Christopher. "Magnetocaloric Effect in Thin Films and Heterostructures." Scholar Commons, 2011. http://scholarcommons.usf.edu/etd/3003.
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The goals of this work are the optimization of the magnetocaloric effect in Gadolinium thin film structures. We approach this issue from two directions, that of process optimization and of interface effects. Past results showed Gd2O3 in our Gadolinium thin films, and the presence of such oxide seemed to grow with the temperature at which the film was grown or annealed. Comparison of samples grown without chamber gettering to those that were gettered show differences in their structural and magnetic properties, and we conclude that gettering is an effective step in enhancing the quality of Gd thin film samples.
Early work with Gd/W heterostructures showed a diminished magnetization of the interfacial gadolinium, which reduces the magnetocaloric response as magnetic entropy is proportional to m2/3. It is known that Fe interfaces can boost the Gd moments per atom to above that seen in bulk. As such, we fabricated a series of Fe/Gd heterostructures to study the effects on the structural and magnetic properties of Gd thin films. The use of Fe as a base layer shows increased high frequency oscillations in X-ray reflectivity measurements, indicating sharp interfaces between Gd and Fe. The magnetocaloric measurements produce a magnetic entropy curve with a novel tail extending leftward, making this an improved material over Gd for applications around 240K. All the same, vector magnetometry is needed to ensure that such tail is not due to rotations within the plane and is a direction for further study.
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Casanova, i. Fernàndez Fèlix. "Magnetocaloric Effect In Gd5(SixGe1-x)4 Alloys." Doctoral thesis, Universitat de Barcelona, 2004. http://hdl.handle.net/10803/1789.
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This Ph.D. Thesis has been devoted to the preparation and characterisation of bulk Gd5(SixGe1-x)4 alloys and to the study of the magnetocaloric effect at the first-order magnetostructural transition appearing in these compounds. We summarise the most relevant results from this research:- Bulk Gd5(SixGe1-x)4 samples with 0¡Âx ¡Â0.5 have been prepared by using our home-made arc-melting furnace. Characterisation techniques (SEM, microprobe, XRD, DSC, magnetisation, ac susceptibility) show that the 5:4 phase with the desired x is obtained. Some spread around the nominal value and secondary 5:3 and 1:1 phases are detected. Heat treatment favour the segregation of these secondary phases, but also reduce the spread in the x value. A treatment at 920 ¨¬C for 4 hours in a 10-5 mb vacuum furnace enables a trade-off between phase segregation and removal of x spread.
- A new differential scanning calorimeter (DSC), which operates under applied magnetic fields of up to 5 T and within the temperature range 10-300 K, has been developed. This calorimeter enables an accurate determination of the entropy change associated with a magnetostructural phase transition. The transition can be induced by sweeping either T or H.
- It has been shown that the Clausius-Clapeyron equation and DSC measurements yield the entropy change associated with the first-order magnetostructural transition, ∆S. If the Maxwell relation is evaluated only within the field range over which the transition takes place, the same value is obtained. When the Maxwell relation is evaluated over the whole field range, the T and H dependences of the magnetisation in each phase outside the transition region yield an additional entropy change to that associated with that of the actual first-order transition. The transition temperature Tt must significantly shift with the applied field, in order to achieve a large MCE taking advantage of ∆S.
- DSC under H has been used to measure ∆S for Gd5(SixGe1-x)4, x ¡Â0.5. ∆S scales with Tt, which is a direct consequence of the fact that Tt is tuned by x and H and it is thus expected to be universal for any material showing a field-induced transition. The specific shape of ∆S vs. Tt will depend on the details of the phase diagram, Tt(x). The scaling of ∆S shows the equivalence of magnetovolume and substitution-related effects in Gd5(SixGe1-x)4 alloys.
- The variation of the transition field with the transition temperature, dHt/dTt, has been studied in Gd5(SixGe1-x)4 for 0¡Âx ¡Â0.5. It is shown that dHt/dTt governs the scaling of ∆S with Tt. Two distinct behaviours for dHt/dTt have been found on the two compositional ranges where the magnetostructural transition occurs, showing the difference in the strength of the magnetoelastic coupling in this system.
- It has been shown that an unreported field-induced magnetic phase transition exists from the AFM phase to a phase which presents short-range correlations (SRAFM). The results suggest that the transition results from the breaking of the long-range AFM correlations when a magnetic field is applied, which leads to competing FM and AFM short-range correlations. FM correlations are also relevant in the whole long-range AFM phase. The expected transition from the SRAFM to the PM phase takes place at ~240 K at zero field, being widened and smoothed under applied field. This finding in the Ge-rich Gd5(SixGe1-x)4 alloys arises from the competition between the intraslab FM interactions and the interslab AFM interactions.
- The dynamics of the first-order transition in Gd5(SixGe1-x)4 alloys has been studied by cycling virgin samples. The field-induced entropy change increases during the first cycles, then reaching a stationary value. This behaviour is related to the avalanche distribution, which also evolves with cycling. The structure of avalanches becomes repetitive after a few cycles tending towards a power-law distribution, unveiling the athermal character of the transition.
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Quetz, Abdiel. "EXPLORATION OF NEW MAGNETOCALORIC AND MULTIFUNCTIONAL MAGNETIC MATERIALS." OpenSIUC, 2017. https://opensiuc.lib.siu.edu/dissertations/1378.
Full textAbstract:
The magnetic properties of NiMnGe1−xAlx, Ni50Mn35(In1−xBx)15, Ni50Mn35In14.5B0.5 (Bulk, As-Solidified and Annealed melt-spun ribbon) and RE-Infuse Carbon Nanotubes, have been studied by x-ray diffraction, differential scanning calorimetry (DSC), and magnetization measurements. Partial substitution of Al for Ge in NiMnGe1−xAlx results in a first-order magnetostructural transition (MST) from a hexagonal ferromagnetic to an orthorhombic antiferromagnetic phase at 186 K (for x = 0.09). A large magnetic entropy change of ∆SM = -17.6 J/kg K for ∆H = 5 T was observed in the vicinity of TM = 186 K for x = 0.09. This value is comparable to those of well-known giant magnetocaloric materials, such as Gd5Si2Ge2, MnFeP0.45As0.55, and Ni50Mn37Sn13. The values of the latent heat (L = 6.6 J/g) and corresponding total entropy changes (∆ST = 35 J/kg K) have been evaluated for the MST using DSC measurements. Large negative values of ∆SM of -5.8 and -4.8 J/kg K for ∆H = 5 T and up to 9T in the vicinity of TC were observed for x = 0.09 and 0.085, respectively. The impact of B substitution in Ni50Mn35In15-xBx Heusler alloys on the structural, magnetic, transport, and parameters of the magnetocaloric effect (MCE) has been studied by means of room-temperature X-ray diffraction and thermomagnetic measurements (in magnetic fields (H) up to 5 T, and in the temperature interval 5-400 K ). Direct adiabatic temperature change (ΔTAD) measurements have been carried out for an applied magnetic field change of 1.8 T. The transition temperatures (T-x) phase diagram has been constructed for H = 0.005 T. The MCE parameters were found to be comparable to those observed in other MCE materials such as Ni50Mn34.8In14.2B and Ni50Mn35In14X (X=In, Al, and Ge) Heusler alloys. The maximum absolute value of ΔTAD = 2.5 K was observed at the magnetostructural transition for Ni50Mn35In14.5B0.5. The structural phase transition temperatures, phase structure, and parameters of the magnetocaloric effect (MCE) of Ni50Mn35In14.5B0.5 as Bulk, As-Solidified and Annealed melt-spun ribbon has been studied by means of room-temperature X-ray diffraction and thermomagnetic measurements (in magnetic fields (oH) up to 5 T, and in the temperature interval 5–400 K). Magnetic and structural transitions in Ni50Mn35In14.5B0.5 as ribbons were found to coincide in Ni50Mn35In14.5B0.5 bulk sample, leading to a large magnetocaloric effects associated with the first-order magnetostructural phase transition. In comparison to the bulk Ni50Mn35In14.5B0.5 alloys, both the martensitic transition temperature (TM) and Curie temperature (TC) shifted to lower temperatures. Magnetic measurements revealed that the ribbons undergo a structure transformation similar to the bulk material at the martensitic transformation. The temperature of the transformation depends strongly on lattice parameters of the ribbons. MST shows a weak broad magnetic transition at TCM∼ 160 K, while the Curie temperature of AST TCA is ∼ 297 K. The MCE parameters were found to be comparable to those observed in other MCE materials such as Ni50Mn34.8In14.2B and Ni50Mn35In14X (X = In, Al, and Ge) Heusler alloys. These results suggest the possibility to control the martensitic transition in Ni50Mn35In14.5B0.5 through rapid solidification process. A comparison of magnetic properties and magnetocaloric effects in Ni50Mn35In14.5B0.5 alloys as Bulk, As-Solidified and Annealed ribbons is discussed. Carbon nanotube (CNT)/metal-cluster-based composites are envisioned as new materials that possess unique electronic properties which may be utilized in a variety of future applications. Super paramagnetic behavior was reported for CNTs with Gd ions introduced into the CNT openings by internal loading with an aqueous GdCl3 chemical process. In the current work, the magnetic properties of the CNT/Gd composites were obtained by the joining and annealing of Gd metal and CNTs at 850 °C for 48 h. Energy dispersive X-ray analysis shows the presence of Gd intermingled with the CNT walls with maximum and average Gd concentrations of about 20% and 4% (by weight), respectively. The Gd clusters have a non-uniform distribution and are mostly concentrated at the ends of the CNTs. A ferromagnetic-type transition at TC ∼ 320 K, accompanied by jump like change in magnetization and temperature hysteresis typical for the temperature induced first order phase transitions has been observed by magnetization measurements. It was found that Gd infused into the CNTs by the annealing results in a first order paramagnetic-ferromagnetic transition at TC = 320 K.
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Bratko, Milan. "The magnetocaloric effect at a first order phase transition." Thesis, Imperial College London, 2013. http://hdl.handle.net/10044/1/23653.
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The magnetocaloric effect (MCE) can be defined as the isothermal entropy change (or adiabatic temperature change) of a material upon application/removal of an external magnetic field and is the key physics for a magnetic cooling device. A discontinuity of entropy at a first order phase transition (FOPT) allows for a large entropy change to be induced by a relatively small field. However, a hysteresis is necessarily associated with a FOPT. The effects of hysteresis, as measured in a sensitive microcalorimeter, are the focus of the thesis. The calorimetric setup used is unique in allowing a separate measurement of heat capacity and latent heat and thereby the possibility to clearly distinguish the first and higher order contributions to MCE. Due to the high measurement fidelity required, the experimental chapter is a core component of the thesis and includes a thorough analysis of the measurement errors associated with the microcalorimeter. Several improvements are proposed to improve precision and accuracy of the measurement in future studies. The first of the hysteresis effects is a spurious 'colossal' MCE. Its indirect observation was claimed in 2004 from magnetisation measurements analysed using a Maxwell relation and was widely disputed thereafter. It was shown that a different measurement protocol leads to non 'colossal' MCE. This thesis investigates whether the 'colossal' MCE can be achieved by a particular magnetisation history by reproducing the original measurement protocol in a more direct calorimetric measurement. It is shown that the 'colossal' MCE is just an artefact of the use of Maxwell relation in a non-equilibrium process. The final chapter discusses a second effect of hysteresis: a subtle difference between the indirect and calorimetric measurements of MCE that can be clearly observed when comparing measurements on field application and removal. Maxwell relation leads to an artefact related to temperature dependence of the hysteresis. In the calorimetric measurement the dissipation of magnetic work in a hysteretic magnetisation cycle is observed.
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Turcaud, Jeremy. "Magnetocaloric effect and thermal transport management in lanthanum manganites." Thesis, Imperial College London, 2014. http://hdl.handle.net/10044/1/40889.
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This thesis investigates two challenges associated with the use of manganites for magnetocaloric applications. The first challenge is associated with methods to engineer the thermal conductivity, K. The second challenge is to understand the limits of the entropy change achievable in magnetocaloric manganites. Thermal management has been achieved via different microstructuring routes and their influence on thermal transport properties such as K, resistivity and thermopower, have been studied. A factor of two increase in K is demonstrated by using density and grain size optimization, while three-fold and six-fold increases are seen by employing the introduction of a second highly conductive phase via: (1) silver impregnation and silver particle coating and (2) copper electroplating, respectively. Understanding the magnetocaloric effect (MCE) characteristics in manganites has been achieved by bringing together magnetisation, magneto-structural, magneto-Seebeck, and neutron diffraction independent measurements. We first show that the temperature T* up to which a spontaneous magnetisation is observed in the inverse magnetic susceptibility of La0.7Ca0.3MnO3 and La0.7Ba0.3MnO3 above Tc, is related to the transition temperature of the low temperature (high-magnetic field and high-magnetisation) magnetic phase. In the most widely studied La(1-x)CaxMnO3 (x = 0.2, 0.25, 0.3), we then conclude that unlike between the degree of static Jahn-Teller distortion and the interval [T*-Tc]/Tc where we show that there exists a close relationship, there is no apparent correlation between the magnitude of the MCE and [T*-Tc]/Tc . We then unravel the competing strength of the various degrees of freedom and show that the inhibition of a large magnetocaloric response is due to the strong correlations that underpin the collosal magnetoresistance effect: both clustering of magnetic Mn atoms due to polaron formation and the insulator to metal transition. Finally we discuss prospects to improve material properties for application in light of these findings.
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Barcza, Alexander. "The magnetocaloric effect and magnetoelastic interactions in CoMnSi-based alloys." Thesis, University of Cambridge, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.608462.
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Pandey, Sudip. "MAGNETIC, TRANSPORT, AND MAGNETOCALORIC PROPERTIES OF BORON DOPED Ni-Mn-In ALLOYS." OpenSIUC, 2015. https://opensiuc.lib.siu.edu/theses/1754.
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The impact of B substitution in Ni50Mn35In15-xBx Heusler alloys with x = (0, 0.5, 0.75, 1, 1.1 1.5, and 2) on the structural, magnetic, transport, and parameters of magnetocaloric effect has been studied by means of room temperature XRD-diffraction, differential scanning calorimetry (DSC), and thermomagnetic measurements (in a magnetic field up to 5 T and temperature interval 5-400 K). Direct adiabatic temperature (ΔTAD) measurements have been carried out for an applied magnetic field change (ΔH) of 1.8 T. The partial substitution of In by B in Ni50Mn35In15-xBx Heusler alloys induced a non-linear temperature shift of the magnetostructural transition while Curie temperature (TC) was found to be nearly constant (TC ~ 320 K) for all compounds. The transition temperatures (T-x) phase diagram has been constructed for H = 0.005 T. The MCE parameters were found to be larger or comparable to parameters observed in other MCE materials, such as Ni50Mn34.8In14.2B and Ni50Mn35In14X (X=In, Al, and Ge) Heusler alloys. It has been demonstrated that the martensitic transformation temperature and the corresponding ∆SM can be tuned through a slight variation in composition of B in NiMnInB alloys. A magnetoresistance associated with martensitic transformation was found to be -60% for x = 0.75 at T = 240 K for a magnetic field change of 5 T. The maximum absolute value of ΔTAD = 2.5 K was observed at the magnetostructural transition for Ni50Mn35In14.5B0.5. The roles of the magnetic and structural changes on the transition temperatures are discussed.
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Marconi, Davide. "Progettazione del rigeneratore di un sistema magnetocalorico." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2020.
Find full textAbstract:
Questa tesi tratta l’argomento della Refrigerazione Magnetica, una nuova tecnologia in ambito refrigerativo che mira a ridurre l’inquinamento derivante dai sistemi frigoriferi tradizionali ad uso domestico. La parte principale dell’elaborato tratta la progettazione di un nuovo rigeneratore per il prototipo FRIMAG dell’Università di Bologna in quanto quello attualmente installato presenta delle perdite di fluido che compromettono il funzionamento dell’intero prototipo.
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Posva, Ferdinand. "Setup Implementation for a Direct Measurement Technique of the Magnetocaloric Effect." Thesis, KTH, Materialvetenskap, 2020. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-277910.
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This project presents an attempt to construct a setup and implement a reliable technique for measuring the magnetocaloric effect (MCE) on various materials via a direct method for the acquisition of the data. The main objective of the latter is to produce a ∆Tadiabatic vs T graph over a reasonable temperature span (-100◦C up to 220◦C) by thermal monitoring of a magnetic material exposed to an oscillating magnetic field with a maximum strength of 1.2T. The setup consists of a vacuum-insulated glass tube containing the sample placed between two electromagnets of a vibrating-sample magnetometer (VSM) and increasingly heated by a resistance wire, while the temperature is recorded directly by a thermocouple. The first experiments are performed on Gadolinium (Gd) samples as a reference material in order to verify the overall reliability of the system. The obtained results on Gadolinium show that meaningful data can be acquired with this direct method, although the initially-extracted ∆Tadiabatic near room temperature stands at the accuracy limit (25%) generally accepted with this method. Unexpected interference signals from the thermocouple are encountered for high temperatures and are shown to be due to magnetic dependence from one of its constituents. Data from high temperatures can however be reliably corrected with respect to a baseline signal from a neutral nonmagnetic material. As such magnetocaloric properties of two Manganese-rich high entropy alloys are investigated with one exhibiting at most ∆Tadiabatic = 0.2◦C at its Curie temperature TC = 60◦C. Suggestions regarding the possibility of operating the setup at sub-zero temperatures are put forward and promising results from a new spot- welded thermocouple show a significantenhancement of the initial setup accuracy.Detta projekt presenterar ett försök att konstruera en installation och implementera en pålitlig teknik för att mäta den magnetokolorisk effekten (MCE) på olika material via en direkt metod för insamling av data. Det sistnämnda syftet är att producera en ∆Tadiabatisk vs T över ett rimligt temperaturintervall (-100◦C up to 220◦C). Detta genom en termisk övervakning av ett magnetiskt material utsatt för ett oscillerande magnetfält med en maximal magnitud på 1.2 T. Utrustningen utgörs av ett vakuumisolerade glasrör som innehåller provet, vilket är placerat mellan två elektromagneter från en vibrating-sample magnetometer (VSM) och som stegvis värms upp av en resistanstråd, medan temperaturen registreras direkt av ett termoelement. De inledande experimenten utförs på prover av Gadolinium (Gd) som referensmaterial för att verifiera systemets totala tillförlitlighet. De erhållna resultaten från Gadolinium proverna visar att meningsfulla data kan produceras med denna direkta metod. Även om de extraherade ∆Tadiabatisk vid rumstemperatur befinner sig inom precisions gränsen (25%), vilken är allmänt accepterad med avseende på den direkta metoden. Oväntade missvisande signaler från termoelementet uppträder vid höga temperaturer och visar sig bero på magnetiskt beroende från instumentet. Data från höga temperaturer kan emellertid pålitligt korrigeras med en baslinjesignal från ett neutralt icke-magnetiskt material. Därmed undersöks de magnetokoloriska egenskaper hos två Mangan-rika hög entropi legeringar, där en uppvisar som högst ∆Tadiabatisk = 0.2◦C vid dess Curie-temperatur TC = 60◦C. Förslag beträffande möjligheten att använda installationen vid temperaturer under noll läggs fram. Lovande resultat från ett nytt punktsvetsat termoelement visar en betydande förbättringav den inledande installationens noggrannhet.
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Aryal, Anil. "EXPLORATION OF NOVEL MAGNETOCALORIC MATERIALS FOR APPLICATIONS IN MAGNETIC COOLING TECHNOLOGY." OpenSIUC, 2020. https://opensiuc.lib.siu.edu/dissertations/1813.
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The effect of doping on the crystal structure, magnetic, magnetocaloric and transport properties of MnM′Ge (M′ = Ni, Co) intermetallic compounds and NiMnX (X = Sn, In) Heusler alloys have been studied by room temperature X-ray diffraction (XRD), differential scanning calorimetry (DSC), and magnetization measurements. The studied magnetic systems include Ni1-xCrxMnGe1.05 (0 ≤ x ≤ 0.120), Mn1-xAlxCoGe (0 ≤ x ≤ 0.05), MnCo1-xZrxGe (0.01 ≤ x ≤ 0.04), Mn1-xAgxCoGe (0.01 ≤ x ≤ 0.10), Ni50-xRxMn35Sn15 (x = 0, 1 and R = La, Pr, Sm), Ni43-xRxMn46Sn11 (x = 0, 1 and R = Pr, Gd, Ho, Er), and Ni50Mn35In15-xBix (0 ≤ x ≤ 1.5).A temperature induced first-order structural transition characterized by a change in crystal structure from high temperature austenite phase (AP) with Ni2In-type Hexagonal structure to low temperature martensite phase (MP) with TiNiSi-type orthorhombic structure was observed at T = TM (martensitic transition temperature) in some of the MnM′Ge-based compounds. The partial substitution of doping elements such as Cr, Al, Zr, and Ag resulted in a decrease in TM and at certain concentration, TM was found to decrease below / coincide with the ferromagnetic transition temperature (TC) of AP. Therefore, such system show a first-order magnetostructural transition (MST).In Ni1-xCrxMnGe1.05, a MST from antiferromagnetic (AFM) orthorhombic to ferromagnetic (FM) hexagonal phase was observed for 0.105 ≤ x ≤ 0.120. Both direct and inverse MCE were observed in this compound. The peak values of the magnetic entropy change (ΔSMpeak ) in the vicinity of TC for ΔH = 5T were found to be 4.5 J/kg K, 5.6 J/Kg K, and 5.1 J/Kg K for x = 0.105, 0.115, and 0.120 respectively. A magnetic field-induced transition from an AFM to a FM state in the martensite structure was observed in annealed Ni0.895Cr0.105MnGe1.05 melt-spun ribbons, which led to a coupled MST from a FM martensite to a PM austenite phase with a large change in magnetization. As a result of the field-induced MST, a large ΔSMpeak value of 16.1 J kg-1 K-1 (which is about a four times larger than the bulk) and Refrigeration capacity (RC-1) =144 J kg-1 at μ0∆H = 5 T was found. It was also found that the ribbon samples showed excellent magnetic reversibility that is important for application. MCE parameters, adiabatic temperature change (∆Tad) and |〖∆S〗_M |, with maximum value of ~ 2.6 K (µoH = 10 T) and 4.4 J kg-1 K-1(µo∆H = 5 T), respectively, were observed in the vicinity of TC. The ∆Tad (T) curves obtained for µoΔH = 10 T and magnetization isotherms were found to be completely reversible, which indicates the reversibility of the MCE in this system. A large temperature span (of about 61 K) and a non-saturating behavior of ∆Tad were observed at magnetic fields up to 10 T. The adiabatic temperature change was found to be a linear function of (µoH)2/3 near TC in accordance with Landau’s theory of phase transitions.In MnCoGe compounds doped with Al, Zr, and Ag, a tunable MST from the paramagnetic hexagonal to ferromagnetic orthorhombic phase was observed. The maximum ΔSM values of about 18, 7.2, and 22 J kg-1 K-1for ∆H = 5T at TM was observed for Al, Zr, and Ag doped compounds, respectively. The corresponding maximum value of RC was found to be (303, 266, and 308) JKg-1.The new compounds containing low concentration of rare earth (R) metals: Ni50-xRxMn35Sn15, Ni43-xRxMn46Sn11, with R = La, Pr, Sm, Gd, Ho, Er and Ni50Mn35In15-xBix were synthesized. The compounds crystallized in the cubic L21 austenite phase (AP) or a mixture of AP and low temperature martensitic phase (MP) at room temperature. For Ni50-xRxMn35Sn15 and Ni43-xRxMn46Sn11 alloys, TM shifted towards higher temperature with rare-earth doping, thus stabilizing the MP at higher temperature. A maximum shift in TM by ~ 60-62 K relative to the parent compound (TM = 190-195 K) was observed for the Ni49LaMn35Sn15 and Ni42PrMn46Sn11. TM shifted towards lower temperature if Bi is placed in In position in Ni50Mn35In15-xBix. A maximum shift of ~ 36 K was detected for x = 1.5. Abnormal shifts in TC and TM to higher temperatures were observed at high field for Bi concentration ≥ 0.5.The ground state magnetization decreased with the rare-earth doping and increasing Bi content. The compounds exhibit both inverse and normal magnetocaloric effects. Large values of ∆SM = 12 (Ni49PrMn35Sn15), 32 Jkg-1K-1(Ni42PrMn46Sn11), 28 Jkg-1K-1 (Ni42GdMn46Sn11), 25 Jkg-1K-1 (Ni42HoMn46Sn11), 40 J/kg K (Ni50Mn35In15) and 34 J/kg K (Ni50Mn35In15-xBix, x = 0.25) were found at TM for ∆H = 5T that can be tuned in a wide temperature range. RC values ranging from 267-336 Jkg-1 at TC, 182 -250 Jkg-1 at TM and 144-165 Jkg-1 at TC were found with ∆H = 5T for Ni50-xRxMn35Sn15, Ni43-xRxMn46Sn11, and Ni50Mn35In15-xBix, respectively. Significant magnetoresistance (MR) values of -30%, -20 % and -30% were observed in Ni49LaMn35Sn15, Ni42GdMn46Sn11, and Ni50Mn35In14.5Bi0.5 compounds, respectively, at TM and ∆H = 5T. A large exchange bias effect with HEB ~ 1.1 kOe at 10 K was observed for the Ni42PrMn46Sn11 compound in its MP. Thus, the pronounced multifunctional properties such as shape memory effects, MCE, EB, and MR make these new systems promising for the ongoing development of magnetocaloric and multifunctional technologies.
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Ghirlanda, Simone L. "Prototype and Testing of a MEMS Microcooler Based on Magnetocaloric Effect." Scholar Commons, 2006. http://scholarcommons.usf.edu/etd/3890.
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This thesis documents the work and research effort on the design, fabrication and testing of a magnetocaloric MEMS microcooler, focusing on the testing of the microcooler at low magnetic fields. The phenomenon of magnetocaloric effect (MCE), or adiabatic temperature change, which is obtained by heating or cooling magnetic materials due to a varying magnetic field, can be exploited in the area of magnetic refrigeration as a reliable, energy-efficient cooling system. In particular, its applications are being explored primarily in cryogenic technologies as a viable process for the liquefaction of hydrogen. The challenge for magnetic refrigeration is that the necessary MCE is most easily achieved with high magnetic fields (5-6 Tesla) provided by superconducting magnets. However, a significant magnetocaloric effect can be exhibited at lower magnetic fields (1-2 Tesla) by carefully controlling initial temperature conditions as well as by selecting, preparing and synthesizing the optimal fabrication process of Silicon (Si) wafers. A microcooler was integrated based on previous works of others and tested. Finally, testing of the magnetocaloric effect was conducted and results analyzed. Experimental results in these domains demonstrate that magnetic refrigeration can be part of the best current cooling technology, without having to use volatile, environmentally hazardous fluids. The MEMS magnetocaloric refrigerator demonstrated a ~ -12°C change in the temperature of cooling fluid at a magnetic field of 1.2 T.
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Bingham, Nicholas Steven. "Magnetism in Complex Oxides Probed by Magnetocaloric Effect and Transverse Susceptibility." Scholar Commons, 2013. http://scholarcommons.usf.edu/etd/4440.
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Magnetic oxides exhibit rich complexity in their fundamental physical properties determined by the intricate interplay between structural, electronic and magnetic degrees of freedom. The common themes that are often present in these systems are the phase coexistence, strong magnetostructural coupling, and possible spin frustration induced by lattice geometry. While a complete understanding of the ground state magnetic properties and cooperative phenomena in this class of compounds is key to manipulating their functionality for applications, it remains among the most challenging problems facing condensed-matter physics today. To address these outstanding issues, it is essential to employ experimental methods that allow for detailed investigations of the temperature and magnetic field response of the different phases.
In this PhD dissertation, I will demonstrate the relatively unconventional experimental methods of magnetocaloric effect (MCE) and radio-frequency transverse susceptibility (TS) as powerful probes of multiple magnetic transitions, glassy phenomena, and ground state magnetic properties in a large class of complex magnetic oxides, including La0.7Ca0.3-xSrxMnO3 (x = 0, 0.05, 0.1, 0.2 and 0.25), Pr0.5Sr0.5MnO3, Pr1-xSrxCoO3 (x = 0.3, 0.35, 0.4 and 0.5), La5/8−xPrxCa3/8MnO3 (x = 0.275 and 0.375), and Ca3Co2O6.
First, the influences of strain and grain boundaries, via chemical substitution and reduced dimensionality, were studied via MCE in La0.7Ca0.3-xSrxMnO3. Polycrystalline, single crystalline, and thin-film La0.7Ca0.3-xSrxMnO3 samples show a paramagnetic to ferromagnetic transition at a wide variety of temperatures as well as an observed change in the fundamental nature of the transition (i.e. first-order magnetic transition to second order magnetic transition) that is dependent on the chemical concentration and dimensionality.
Systematic TS and MCE experiments on Pr0.5Sr0.5MnO3 and Pr0.5Sr0.5CoO3 have uncovered the different nature of low-temperature magnetic phases and demonstrate the importance of coupled structural/magnetocrystalline anisotropy in these half-doped perovskite systems. These findings point to the existence of a distinct class of phenomena in transition-metal oxide materials due to the unique interplay between structure and magnetic anisotropy, and provide evidence for the interplay of spin and orbital order as the origin of intrinsic phase separation in manganites.
While Pr0.5Sr0.5MnO3 provides important insights into the influence of first- and second-order transitions on the MCE and refrigerant capacity (RC) in a single material, giving a good guidance on the development of magnetocaloric materials for active magnetic refrigeration, Pr1-xSrxCoO3 provides an excellent system for determining the structural entropy change and its contribution to the MCE in magnetocaloric materials. We have demonstrated that the structural entropy contributes significantly to the total entropy change and the structurally coupled magnetocrystalline anisotropy plays a crucial role in tailoring the magnetocaloric properties for active magnetic refrigeration technology.
In the case of La5/8−xPrxCa3/8MnO3, whose bulk form is comprised of micron-sized regions of ferromagnetic (FM), paramagnetic (PM), and charge-ordered (CO) phases, TS and MCE experiments have evidenced the dominance of low-temperature FM and high-temperature CO phases. The "dynamic" strain liquid state is strongly dependent on magnetic field, while the "frozen" strain-glass state is almost magnetic field independent. The sharp changes in the magnetization, electrical resistivity, and magnetic entropy just below the Curie temperature occur via the growth of FM domains already present in the material, even in zero magnetic field. The subtle balance of coexisting phases and kinetic arrest are also probed by MCE and TS experiments, leading to a new and more comprehensive magnetic phase diagram.
A geometrically frustrated spin chain compound Ca3Co2O6 provides an interesting case study for understanding the cooperative phenomena of low-dimensional magnetism and topological magnetic frustration in a single material. Our MCE studies have yielded new insights into the nature of switching between multi-states and competing interactions within spin chains and between them, leading to a more comprehensive magnetic phase diagram.
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Mbulunge, Masevhe Hamisi. "Giant Magnetocaloric effect and Magnetic Properties of selected Rare-Earth compounds." University of the Western Cape, 2021. http://hdl.handle.net/11394/7926.
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Masters of ScienceRare-earth (RE) compounds have been an attractive subject, based on the unique electronic structures of the rare-earth elements. In particular, the RETX (RE = rare-earth, T = 3d/4d/5d, transition metals, and X = p – block elements) series is a large family of intermetallic compounds which crystallizes in different crystal structure depending on the constituents. Most of these compounds crystalize in the hexagonal, orthorhombic, and tetragonal crystal structure. On the other hand, the family of compounds RET2X2 adopted the tetragonal crystal structure of the ThCr2Si2 or the CaBe2Be2 with different space groups. Owing to the different crystal structure, these compounds show versatile magnetic and electrical properties such as Kondo effect, complex magnetic behaviour, valence fluctuation, unconventional and conventional superconductivity, heavy fermion behaviour, Fermi and non – Fermi liquid behaviour, metamagnetism, spin – glass, memory effect, crystal electric field (CEF), magnetoresistance and magnetocaloric effect. The history of magnetism reveals that it is closely related to practical applications and magnetic materials from the most vital components in many applications. These are memory devices, permanent magnets, transformer cores, magneto-mechanical devices and magneto-electronic devices. Recent additions to this list include magnetic refrigeration through the studies of magnetocaloric effect as well as spintronics. Magnetic refrigeration (MR) is an emerging technology and shows real potential to enter conventional markets and the principles of MR obeys the magnetocaloric effect (MCE), which is based on the effect caused by a magnetic field on the materials that accept the property of varying the magnetic entropy, as well as its temperature when varying the magnetic field. In this thesis, we report giant magnetocaloric effect and magnetic properties of NdPd2Al2 and RECuGa (RE = Nd, Dy, and Ho) compounds. These investigations were done through measurements of X – ray diffraction (XRD), magnetic susceptibility, ((T)), magnetization, (M(H)), isothermal magnetization, (M(H, T)), heat capacity, (Cp(T)) and electrical resistivity, ((T)). MCE has been studied from the isothermal magnetization and heat capacity measurements.The first chapter of the thesis describes the theoretical background from which the experimental results have been analyzed and interpreted. This is followed by the chapter which presents experimental details and methodology carried out in this thesis. Chapter three presents the results and discussion of the transport, magnetic and magnetocaloric properties of NdPd2Al2 compounds. XRD studies confirm the tetragonal CaBe2Ge2 – type structure with space group P4/nmm (No. 129). The results of (T), (T) and Cp(T) indicate a putative antiferromagnetic (AFM) phase transition at low temperature at, TN = 3 K. On the other hand, (T) data at high temperatures follow the Curie – Weiss relationship giving an effective magnetic moment close to that expected for the trivalent Nd3+ ion. The magnetization results indicate metamagnetic – like transition at a low field that bears a first-order character which corroborates with the Below – Arrott plots. Giant MCE was obtained for the NdPd2Al2 compound similar to those reported for potential magnetic refrigerant materials. Chapter four discusses the magnetic and thermodynamic properties of the series of compounds RECuGa where RE = Nd, Dy, and Ho. XRD studies indicate the orthorhombic CeCu2 – type crystal structure with space group Imma (No. 74) for all three compounds. Magnetic measurements indicate a putative AFM phase transition below 𝑇𝑁 = 7.1, 8.5, and 3.7 K for Nd, Dy, and Ho compounds, respectively. The high-temperature (T) data for all three compounds follow the Curie – Weiss relationship giving an effective magnetic moment close to that expected for the trivalent rare-earth ion. Again, large MCE were obtained for all three compounds similar to those reported for materials that can be used as magnetic refrigerant materials.
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Cenacchi, Marco. "Modellazione del rigeneratore di un sistema frigorifero magnetocalorico." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2018. http://amslaurea.unibo.it/15040/.
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La tesi riguarda la modellazione del rigeneratore di un sistema frigorifero magnetocalorico e fa parte di uno studio condotto dal Dipartimento di Ingegneria Industriale (DIN) della Scuola di Ingegneria e Architettura dell’Università di Bologna. L’obiettivo del progetto è la realizzazione di un refrigeratore magnetico con potenzialità tali da poter entrare nel mercato in un prossimo futuro. Il lavoro di tesi è parte di questo progetto. Nella prima parte è stato studiato l’effetto magnetocalorico e i principali cicli termodinamici per l’applicazione alla refrigerazione magnetica. Nella seconda parte si è studiato il rigeneratore a partire da un modello geometrico preesistente, ipotizzando condizioni di scambio termico quanto più realistiche con riferimento ad una realizzazione pratica. La modellazione determina i profili di temperatura del materiale magnetocalorico e del fluido termovettore. Questa fase è stata condotta con l’ausilio del linguaggio di progettazione VBA in Excel grazie alla scrittura di cicli iterativi atti alla minimizzazione degli errori.Sulla base della geometria iniziale è stato sviluppato un nuovo modello di rigeneratore che consentisse una progressione termo-strutturale del sistema di scambio.Nella terza parte della tesi sono stati calcolati i profili di temperatura per il nuovo modello geometrico. Sono stati considerati in totale 5 casi. Ogni caso rappresenta la diretta evoluzione delle fasi del ciclo AMR fino all’ultima, con lo svolgimento del ciclo per intero. Nell'ultima parte della tesi è stata condotta un’analisi di sensibilità del modello, confrontando graficamente i risultati ottenuti dalle precedenti fasi e traendo opportune considerazioni di scambio termico.
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Himel, Md Sakhawat Hossain. "The Magnetic and Magnetocaloric Properties of Selected Al1.2Fe2B2 Derivative Intermetallic Systems." Miami University / OhioLINK, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=miami1595949827794125.
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Sanglé-Ferrière, Marie. "Tuning the Curie temperature and phase fraction of FeNi25-based alloys with Mn and Co for magnetocaloric applications." Thesis, KTH, Materialvetenskap, 2020. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-277927.
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This paper discusses the search of an FeNi25-based alloy with a face-centered cubic crystal structure exhibiting a Curie point around room temperature, for magnetocaloric applications. Fe was substituted in various amounts with FCC-stabilising elements Mn and Co as these elements respectively decrease and increase the Curie Temperature, thus enabling to tune the Curie point. Three characterization methods were carried out on the samples: Magneto- thermo-gravimetry (MTG), X-ray diffraction (XRD) and finally, vibrating sample magnetometer (VSM) measurements were performed. All samples displayed several Curie points, each corresponding to various FCC phases. Also, the last sample, FeNi25Mn6Co2, had an FCC phase fraction of almost 99% and presented two Curie points in the continuity of one another one at -35°C and another at 91°C. Hence, at room temperature, the sample underwent a magnetic phase transition passing from its ferromagnetic state to a paramagnetic one.Detta arbete består i att utröna möjligheterna att med utgångspunkt från den binära sammansättningen FeNi25 erhålla en ytcentrerad kubisk fas (fcc) med en Curie punkt vid rumstemperatur. Syftet är att använda dessa legeringar i magnetokaloriska tillämpningar. Strategin är att både Mn och Co är fcc stabliliserande grundämnen, och att Mn sänker och Co ökar Curie temperaturen. Tre olika karakteriseringsmetoder användes; röntgendiffraktometri (struktur), Magneto-Termo-Gravimetri (magnetisering vs temperatur) och konventionell magnetometri vid rumstemperatur (magnetisering vs magnetiskt fält, Vibrating Sample Magnetometry VSM). Resultaten visar att även om kristallstrukturen i det närmaste är fullständigt fcc, så ger de magnetiska mätningarna vid handen att flera olika faser är vid handen med avesvärt olika Curie temperaturer. Som en illustration av detta förhållande kan nämnas att sammansättningen FeNi25Mn6Co2 uppvisar en fcc-fraktion på i det närmaste 99%, men har vid en M(T) mätning ett förlopp som enklast förklaras med en Curie punkt vid ca -35C och en ytterligare vid ca 90°C. Denna observation signalerar att de magnetiska egenskaperna torde vara mer beroende av exakt distribution av de ingående atomslagen i fcc strukturen än vad de röntgendiffraktometriska undersökningarna kan detektera.
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Bylin, Johan. "Best practice of extracting magnetocaloric properties in magnetic simulations." Thesis, Uppsala universitet, Materialteori, 2019. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-388356.
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In this thesis, a numerical study of simulating and computing the magnetocaloric properties of magnetic materials is presented. The main objective was to deduce the optimal procedure to obtain the isothermal change in entropy of magnetic systems, by evaluating two different formulas of entropy extraction, one relying on the magnetization of the material and the other on the magnet's heat capacity. The magnetic systems were simulated using two different Monte Carlo algorithms, the Metropolis and Wang-Landau procedures. The two entropy methods proved to be comparably similar to one another. Both approaches produced reliable and consistent results, though finite size effects could occur if the simulated system became too small. Erroneous fluctuations that invalidated the results did not seem stem from discrepancies between the entropy methods but mainly from the computation of the heat capacity itself. Accurate determination of the heat capacity via an internal energy derivative generated excellent results, while a heat capacity obtained from a variance formula of the internal energy rendered the extracted entropy unusable. The results acquired from the Metropolis algorithm were consistent, accurate and dependable, while all of those produced via the Wang-Landau method exhibited intrinsic fluctuations of varying severity. The Wang-Landau method also proved to be computationally ineffective compared to the Metropolis algorithm, rendering the method not suitable for magnetic simulations of this type.
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Quetz, Abdiel. "PHASE TRANSITIONS AND MAGNETOCALORIC EFFECT IN MnNiGe1−xAlx, Ni50Mn35(In1−xCrx)15 AND (Mn1−xCrx)NiGe1.05." OpenSIUC, 2014. https://opensiuc.lib.siu.edu/theses/1514.
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The magnetocaloric and thermomagnetic properties of the MnNiGe1-xAlx, Ni50Mn35(In1−xCrx)15 and (Mn1−xCrx)NiGe1.05 systems have been studied by x-ray diffraction, differential scanning calorimetry (DSC), and magnetization measurements. Partial substitution of Al for Ge in MnNiGe1−xAlx results in a first-order magnetostructural transition (MST) from a hexagonal ferromagnetic to an orthorhombic antiferromagnetic phase at 186 K (for x = 0.09). A large magnetic entropy change of ∆SM = -17.6 J/kg K for ∆H = 5 T was observed in the vicinity of TM = 186 K for x = 0.09. This value is comparable to those of well-known giant magnetocaloric materials, such as Gd5Si2Ge2, MnFeP0.45As0.55, and Ni50Mn37Sn13 [1]. The values of the latent heat (L = 6.6 J/g) and corresponding total entropy changes (∆ST = 35 J/kg K) have been evaluated for the MST using DSC measurements. Large negative values of ∆SM of -5.8 and -4.8 J/kg K for ∆H = 5 T in the vicinity of TC were observed for x = 0.09 and 0.085, respectively. Partial substitution of Cr for Mn in(Mn1−xCrx)NiGe1.05 results in a MST from a hexagonal paramagnetic to an orthorhombic paramagnetic phase near TM ~ 380 K (for x = 0.07). Partial substitution of Cr for In in Ni50Mn35(In1−xCrx)15 shifts the magnetostructural transition to a higher temperature (TM ~ 450 K) for x = 0.1. Large magnetic entropy changes of ∆SM = -12 (J/kgK) and ∆S = -11 (J/kgK), both for a magnetic field change of 5 T, were observed in the vicinity of TM for (Mn1−xCrx)NiGe1.05 and Ni50Mn35(In1−xCrx)15, respectively. The concentration-dependent (T-x) phase diagram of transition temperatures (magnetic, structural, and magnetostructural) has been generated using magnetic, XRD, and DSC data. The role of magnetic and structural changes on transition temperatures are discussed.
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Gottschlich, Michael [Verfasser]. "Structure, magnetism and excitations in some Mn-based magnetocaloric effect compounds / Michael Gottschlich." Aachen : Hochschulbibliothek der Rheinisch-Westfälischen Technischen Hochschule Aachen, 2013. http://d-nb.info/1037835301/34.
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Patiño, Julieth Caro. "Efeito da anisotropia sobre as propriedades magnetocalóricas de compostos metálicos: um estudo sistemático." Universidade do Estado do Rio de Janeiro, 2014. http://www.bdtd.uerj.br/tde_busca/arquivo.php?codArquivo=8464.
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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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Strandqvist, Nanny. "Magnetic Properites in Alloy Systems." Thesis, Luleå tekniska universitet, Materialvetenskap, 2017. http://urn.kb.se/resolve?urn=urn:nbn:se:ltu:diva-62614.
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The attention for materials displaying high magnetocaloric effect (MCE) has grown during the past 30 years. One of the most important properties of MCE is the adiabatic temperature change ( ). The main aim of this work was to develop a method to measure the temperature change ( ) for magnetocaloric materials in a changing magnetic field. A technique was developed where maximum reached for Gadolinium was 1.19 K in a changing magnetic field of 1.3 T, however, this is lower value in comparison with previous studies (3.3 K in a changing magnetic field of 1 T, Bjørk, et al., 2010) which makes the developed method not sufficient enough to measure . Furthermore, finding novel materials displaying high MCE is of great interest. MnFePSiB alloys display promising MCE properties but processing method is expensive and time consuming. Therefore, a MnFePSiB compound was simply remelted several times and heat treated to enhance its properties. The MnFePSiB alloy was remelted 1, 2 and 3 times after initial casting. Melting the material 3 times resulted improvement in both the magnetic and magnetocaloric properties due to enhanced homogeneity. The material melted 3 times was further heat treated to improve its magnetic magnetocaloric properties. Heat treating the material for 5 hours at 1373K improved the magnetic entropy change more than 10 times compared to the as cast sample, was moved closer to room temperature and maximum of 0.71 K was obtained.
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Pandey, Sudip. "EXPLORING THE STRUCTURAL, ELECTRONIC, AND MAGNETORESPONSIVE PROPERTIES OF NOVEL MAGNETIC MATERIALS IN BULK, RIBBONS, AND THIN FILMS." OpenSIUC, 2019. https://opensiuc.lib.siu.edu/dissertations/1682.
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The structural, electronic, magnetic, magnetocaloric, and transport properties of doped Ni-Mn-(In, Sn) based Heusler alloys were studied using neutron diffraction, x-ray diffraction (XRD), differential scanning calorimetry (DSC), high field magnetization, specific heat, x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism (XMCD), and hydrostatic pressure measurements. The adiabatic temperature change (∆Tad) by a direct method and through thermomagnetic measurements in magnetic fields up to 14 T has been performed for these alloys. Also the mixed effect of pressure and magnetic field on the transition temperature of these alloys are discussed. In order to develop new magnetocaloric and multifunctional materials, the synthesis and characterization of Heusler alloys in reduced dimensions, i.e., ribbons and thin films has been performed. In addition, the structural, magnetic, and magnetocaloric properties of Ni-based binary alloys were investigated, including saturation magnetization and Curie temperature (TC) for the possible applications in self controlled magnetic hyperthermia applications.
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Campos, Ariana de. "Estudo do efeito magnetocalórico em compostos de MnAs1-xAx, A = P, Sb, Te e Mn1-xFexAs." [s.n.], 2006. http://repositorio.unicamp.br/jspui/handle/REPOSIP/277161.
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Orientadores: Sergio Gama, Nilson Antunes de OliveiraTese (doutorado) - Universidade Estadual de Campinas, Instituto de Fisica Gleb Wataghin
Made available in DSpace on 2018-08-08T01:08:08Z (GMT). No. of bitstreams: 1 Campos_Arianade_D.pdf: 9214751 bytes, checksum: 227e7e0b1cc697ea73f810066346ff6d (MD5) Previous issue date: 2006
Resumo: Neste trabalho descreveremos a obtenção dos compostos da família MnA s1-xAx (A= Te,P, Sb) e Mn1-xFe xAs para várias concentrações. Dividimos este trabalho em duas etapas, a primeira via obtenção em forno de alta pressão e a segunda via obtenção em forno tubular em tubos de quartzos. A primeira etapa, ainda se dividiu em obtenção indireta e direta dos materiais. Na obtenção indireta dos materiais, focamos nosso trabalho nos compostos de MnAs e MnSb para a produção da série MnAs1-xSbx. Na obtenção direta, partimos dos elementos para sintetizar os materiais, utilizando o mesmo método adotado na obtenção indireta. Na segunda etapa do trabalho, obtemos os compostos diretamente em tubos de quartzo. As amostras produzidas foram caracterizadas por difração de raios-X, microscopia óptica, microscopia eletrônica de varredura utilizando a técnica de WDS e, finalmente, análises magnéticas para a obtenção do efeito magnetocalórico de cada material, e assim a avaliação destes materiais como possíveis candidatos a materiais refrigerantes. Após o cálculo do efeito magnetocalórico, utilizamos um modelo fenomenológico que considera a dependência da temperatura crítica da fase magnética na mudança de volume, o modelo utilizado parte das descrições propostas por Bean e Rodbell que correlaciona fortes interações magnetoelásticos com a transição de fase de primeira ordem
Abstract: In this work we describe the obtaining processes of the MnAs1-x Ax (A= Te, P, Sb) and Mn1-xFexAs series for several concentrations. We divided this work in two stages: in the first one the samples were obtained using a high pressure furnace and in the second one using a resistive furnaces with the samples sealed in quartz tubes. The first stage, can be split in direct and indirect obtaining of the materials. In the indirect obtaining of the materials, our work was focused on the MnAs and MnSb compounds for the production of the series MnAs1-xSbx. In the direct obtaining, we synthesized the materials directly from the elements, using the same method adopted in the indirect obtaining. In the second stage of the work, we obtained the samples directly from the elements in quartz tubes. The produced samples were characterized by ray-X diffraction, optical microscopy, electron microscopy using the WDS technique and finally magnetic analysis for the calculation of the magnetocaloric effect of each material and, in this way evaluate these materials as possible candidates to refrigerant materials. After the calculation of the magnetocaloric effect, we used a phenomenological model that considers the dependence of the critical temperature of the magnetic phase in the volume change, the model used part of the descriptions proposed by Bean and Rodbell [1] that correlates strong magnetoelastic interaction with the first order phase transition
Doutorado
Física da Matéria Condensada
Doutor em Ciências
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Junior, Luciano Gomes de Medeiros. "Cálculo teórico do efeito magnetocalórico do composto La(FexSi1-x)13." Universidade do Estado do Rio de Janeiro, 2006. http://www.bdtd.uerj.br/tde_busca/arquivo.php?codArquivo=563.
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O estudo teórico do efeito magnetocalórico no composto La(FexSi1-x )13 tornouse
muito importante, tendo em vista que experimentos recentes revelaram que este
composto apresenta grandes valores para a variação isotérmica da entropia (ΔS) e para a
variação adiabática da temperatura (ΔTad), que são as grandezas utilizadas para
caracterizarem o poder de refrigeração magnética de um composto magnético. Estudamos o
efeito magnetocalórico do composto La(FexSi1-x)13, propondo um modelo teórico simples,
a uma única banda e a uma única subrede. Tratamos a desordem do sistema com uma
aproximação do potencial coerente (CPA)de interesse para obter a função de Greene, com
isso, determinar as grandezas termodinâmicas relevantes. Conseguimos uma boa
concordância entre os resultados teóricos e os dados experimentais. Nesta dissertação,
também estudamos o efeito da adição de hidrogênio nas propriedades magnetocalóricas do
composto La(FexSi1-x)13. Os resultados teóricos obtidos para o composto La(FexSi1-x)13Hy ,
também estão em acordo com os dados experimentais.The study of the magnetocaloric effect of the compound La(FexSi1-x)13 became very important, once recent experiments revealed that this compound exhibits great values of the isothermal entropy change (Δ S) and the adiabatic temperature change(ΔTad), which are the quantities used to characterize the power of refrigeration of any compound. We studied the magnetocaloric effect of the compound La(FexSi1-x)13 , using a simple theoretical model, in which only one band and only one sublattice are considered. We treated the disorder of the system with then on diagonal Coherent Potential Approximation (CPA). We got a good agreement between our theoretical calculations and experimental data. We also studied the effect of the addition of hydrogen on the magnetocaloric properties of the compound La(FexSi1-x)13. We also found very satisfactory theoretical results for the composition La(FexSi1-x)13Hy , compared with the experimental data.
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Gomes, Mônica Borges. "Efeito magnetocalórico nos compostos Gd(Zn1-xCdx) e Gd(Pd1-xRhx)." Universidade do Estado do Rio de Janeiro, 2006. http://www.bdtd.uerj.br/tde_busca/arquivo.php?codArquivo=684.
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Coordenação de Aperfeiçoamento de Pessoal de Nível SuperiorNesta dissertação, calculamos as propriedades magnéticas e termodinâmicas e o efeito magnetocalórico em compostos do tipo Gd(A1-xBBx), onde A e B são elementos não magnéticos. Para tal finalidade, usamos um modelo hamiltoniano de spins localizados, incluindo o acoplamento com um campo magnético externo. A interação spin-spin é tratada na aproximação de campo molecular. O parâmetro de interação de troca indireta entre os spins localizados é calculado como uma função da concentração de impurezas. Para esse fim, usamos um modelo no qual a desordem química é tratada na aproximação do potencial coerente. Aplicamos o modelo para estudar o efeito magnetocalórico nos compostos Gd(Zn1-x Cdx) e Gd (Pd1-xRHx). As variações adiabáticas da temperatura e as variações isotérmicas da entropia calculadas para variações de campo magnético estão em bom acordo com os dados experimentais.
In this work we calculated the magnetic and thermodynamic properties as well as the magnetocaloric effect in the compounds Gd(A1−xBBx), where A and B are non-magnetic impurities. For this purpose, we use a model Hamiltonian of interacting spin including the coupling with an external magnetic field. The spin-spin interaction is treated in the molecular field approximation. The indirect exchange interaction parameter between localized spins is calculated as a function of the impurity concentration. To this end we use a model in which the disorder is treated in the coherent potential approximation. We apply the model to study the magnetocaloric effect in the compounds Gd(Zn1−xCdx) and Gd(Pd1−xRhx). The calculated adiabatic temperature changes and isothermal entropy changes upon magnetic field variations are in good agreement with the available experimental data.
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Yan, Haile. "Crystal structure, martensitic transformation crystallography, mechanical and magnetocaloric performance of Ni(Co)MnIn multifunctional alloys." Thesis, Université de Lorraine, 2016. http://www.theses.fr/2016LORR0105/document.
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Les alliages à base de Ni-Mn-In ont attiré une attention considérable en raison de leurs propriétés multifonctionnelles depuis leur découverte en 2004, telles que l’effet de mémoire de forme métamagnétique (Metamagnetic shape memory effect MMSME), l'effet magnétocalorique (MCE) et l'effet de magnétorésistance (MR). Cependant, certaines connaissances fondamentales sur ces alliages manquent toujours jusqu'à présent, telles que la structure cristalline de la martensite, les caractéristiques cristallographiques de microstructure et de transition magnétostructurale. Dans cette thèse, les caractéristiques cristallographiques, les comportements mécaniques et les propriétés magnétiques des alliages Ni-Mn-In base ont été étudiés théoriquement et expérimentalement. Tout d'abord, les structures cristallines des alliages Ni-Mn-In ont été déterminées avec précision par la méthode de Rietveld dans le cadre de la théorie du superespace. Ensuite, la microstructure de la martensite, notamment l'organisation et l'interface des variantes, ainsi que les caractéristiques cristallographiques de la transformation martensitique, telles que les relations d'orientation (OR), le chemin de déformation de la transformation et la compatibilité géométrique entre l'austénite et la martensite, ont été systématiquement étudiés. Enfin, avec cette connaissance fondamentale sur les alliages Ni-Mn-In, les comportements et les mécanismes de sélection /réarrangement des variantes de martensite sous deux types de stratégies de chargement mécanique, à savoir le chargement à l'état martensitique et le chargement durant la transition structurelle, et les effets du recuit sur l'effet MCE et les pertes d'hystérésis associées ont été explorées. Les principaux résultats sont les suivants. La martensite modulé a une structure cristalline incommensurable avec la structure cristalline 6M et le groupe de superespace I2/m(α0γ)00 qui peut être approximée par un modèle de superstructure de multiplicité 3 dans l'espace à tridimensionnel. La microstructure de martensite est en forme de plaques et auto-organisée en colonies. Chaque colonie a quatre variantes d'orientations distinctes. Le maximum de 6 colonies distinctes et 24 variantes peut être généré à l'intérieur d'un grain austénitique. Bien que jusqu'à 14 types de relations de maclage sont proposées dans le cadre des théories cristallographiques de transformation martensitique, seuls trois types de relations de maclage sont généralement observés, à savoir des macles de type I, type II et composées. Les interfaces des variantes sont définies à l'échelle mésoscopique par leur plan de maclage K1 correspondant. Cependant, à l'échelle atomique, la macle de type I a une interface cohérente, alors que celles de type-II et les macles composées ont des interfaces étagées. Les deux relations d'orientations K-S et Pitsch sont appropriés pour décrire la correspondance de réseau entre austénite et martensite dans les alliages Ni-Mn-In. Cependant, le chemin de déformation lié à la relation de Pitsch est mis en évidence pour être efficace pour la déformation de la structure. Avec le chemin de transformation déterminé, le mécanisme sous-jacent de l'organisation des variantes est révélé. À travers la transformation martensitique, en dépit de l'existence d'une relativement large couche contrainte (de l'ordre de 20 nm), le plan d'habitat est bordé par une variante de martensite simple avec l'austénite plutôt que la structure généralement observée "en sandwich", ce qui suggère une relativement bonne compatibilité géométrique entre les phases correspondantes. Pour le chargement en compression à l'état martensitique, l'arrangement des variantes est réalisé par des processus de démaclage. Il est démontré que l'état de variante unique dans certaines colonies pourrait être obtenu lorsque l'orientation de chargement est située dans la zone de Facteur de Schmid (SF) positif commune pour les trois systèmes de démaclage. [...]Ni-Mn-In based alloys have attracted considerable attention due to their multifunctional properties since its discovery in 2004, such as metamagnetic shape memory effect (MMSME), magnetocaloric effect (MCE) and magnetoresistance (MR) effect. However, some fundenmental knowledge on these alloys is still missing until now, such as crystal structure of martensite, crystallographic features of microstructure and magnetostructural transition. In this dissertation, the crystallographic features, mechanical behaviors and magnetic properties of Ni-Mn-In based alloys were studied theoretically and experimentally. First, the crystal structures of Ni-Mn-In alloys were accurately determined by Rietveld method in the frame of superspace theory (Chapter 3). Then, the microstructure of martensite (Chapter 4), such as variant organization and interface structure, and the crystallographic features of martensitic transformation, such as orientation relationship (OR), transformation strain path and geometrical compatibility between austenite and martensite, were systematically studied (Chapter 5). Finally, with this fundamental knowledge on Ni-Mn-In alloys, the behaviors and mechanisms of martensite variant rearrangement/ selection under two kinds of mechanical loading strategies, i.e. loading at martensite state and loading across the structural transition, and the effects of annealing on MCE and its related hysteresis loss were explored (Chapter 6). The main results are as follows. The modulated martensite has an incommensurate 6M crystal structure with superspace group I2/m(α0γ)00 that can be approximated by a three-fold superstructure model in the three-dimensional space. The microstructure of martensite is in plate shape and self-organized in colonies. Each colony has four distinct orientation variants. The maximum of 6 distinct colonies and 24 variants can be generated within one austenite grain. Although as many as 14 kinds of twin relations are suggested in the frame of crystallographic theories of martensitic transformation, only three types of twin relations are generally detected, i.e. type-I, type-II and compound twin. Variant interfaces are defined by their corresponding twinning plane K1 at mesoscopic scale. However, at atomic scale, the type-I twin has a coherent interface, whereas type-II and compound twins have “stepped” interfaces. Both the K-S and Pitsch ORs are appropriate to describe the lattice correspondence between austenite and martensite in Ni-Mn-In alloys. However, the strain path related to the Pitsch relation is evidenced to be the effective for the structural distortion. With the determined transformation path, the underlying mechanism of variant organization is revealed. Across the martensitic transformation, despite the existence of a relative wide stressed layer (around 20 nm), the habit plane is bordered by single martensite variant with austenite rather than the generally observed “sandwich-like” structure, implying a relative good geometrical compatibility between the corresponding phases. For compressive loading at martensite, variant arrangement is realized by the detwinning process. It is evidenced that a single variant state in some colonies can be obtained when the loading orientation is located in the common positive Schmid factor (SF) zone of the three detwinning systems. For loading across the structural transition, the prestrain is obtained by variant selection in which the number of colonies is significantly reduced and the variant organization within colony is greatly changed. The SF for transformation strain path is introduced to evaluate the possible selection of variants. Heat treatment can significantly enhance the magnetic entropy change ΔSM but simultaneously increase the magnetic hysteresis loss. For ΔSM, the chemical ordered degree should play a prominent role [...]
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Mboukam, Jean Jules. "Magnetocaloric effect and critical behaviour near the magnetic phase transition temperature in rare-earth compounds." University of the Western Cape, 2018. http://hdl.handle.net/11394/6218.
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Magister Scientiae - MSc (Physics)Rare-earth intermetallic compounds continue to draw considerable attention, due to their fundamental importance in understanding physical properties and potential applications based on a variety of phenomena. The focus of this project is to employ two family of rare-earth intermetallic compounds: RE2Pt2In (RE = Pr, Nd) and RE8Pd24Ga (RE = Gd, Tb, Dy) ternary intermetallic systems as a model candidate to uncover the underlying ground state properties that result in a strong coupling between the conduction electron and the 4f-electron of the rare-earth ions.
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Das, Ranjit Chandra. "The Effect of Stoichiometric Variation on the Magnetocaloric Properties of Selected Mn-Fe-Ni-Si-Al Intermetallic Compounds." Miami University / OhioLINK, 2021. http://rave.ohiolink.edu/etdc/view?acc_num=miami1626959102771612.
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Othmani, Safa. "Elaboration et étude des propriétés physiques de nouveaux manganites à effet magnétocalorique : la1-xCexMnO3; La0,7(CaSr)0,3Mn1-xFexO3 ; La0,6Ca0,4Mn1-xFexO3." Thesis, Grenoble, 2011. http://www.theses.fr/2011GRENY018.
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Fin des années 1980, la découverte de l'effet magnétorésistif géant, qui se caractérise par une variation importante de la résistance électrique d'un matériau lorsqu'on le soumet à un champ magnétique, a eu un impact très important tant au niveau des études fondamentales qu'en vue d'applications industrielles telles que la réduction de la taille des disques durs des ordinateurs (Prix Nobel d'A. Fert en 2007). L'engouement ainsi suscité a permis de mettre en évidence cet effet, au début des années 1990, dans les couches minces d'oxyde de type pérovskite ABO3 et plus particulièrement dans les manganites de terres rares (Ln1-xAx)MnO3. Le but de ce travail s'inscrit dans ce cadre et concerne l'élaboration et l'étude des propriétés physiques (structurales, magnétiques, de transport et magnétocaloriques) de nouveaux manganites qui pourraient avoir des applications dans un domaine connexe qui est la réfrigération magnétique. En effet, cette dernière décennie, a vu les découvertes de nouveaux composés présentant des effets magnétocaloriques géants qui ont conduit aux premiers essais de laboratoire de la réfrigération magnétique. Celle-ci semble être l'une des alternatives très sérieuses pour le remplacement des systèmes de réfrigération classique basés sur la compression-détente des gaz. Cette nouvelle technique, comparée aux techniques traditionnelles, présente plusieurs avantages, elle est plus efficace sur le plan énergétique, plus compacte et surtout moins nuisible à l'environnement. La première partie de ce travail porte sur l'élaboration et la caractérisation des composés de formule La1-xCexMnO3. Nous avons étudié l'effet du recuit sur les propriétés morphologique, structurale, magnétique et magnétocalorique de ces composés. L'application du modèle de Landau, en bon accord avec les résultats expérimentaux de la mesure l'entropie magnétique SM, a montré que la nature de transition de phase dépend aussi de la température de recuit. La composition x=0.4 de ce composé présente la valeur la plus élevée du facteur de mérite RCP, ce qui en fait un bon candidat pour les applications à la réfrigération magnétique. Dans une deuxième partie une étude des propriétés morphologique, structurale, magnétique et magnétocalorique des manganites de formule La0,7Ca0,15Sr0,15Mn1-xFexO3 a été réalisée. Le fer n'influe pas sur les propriétés structurales mais entraîne une diminution de la température de Curie TC. Afin d'approfondir ces études, nous avons proposé un matériau composite basé sur deux composés La0,7Ca0,15Sr0,15Mn1-xFexO3 (x = 0,025 et 0,75). La variation d'entropie du composite reste approximativement constante entre 260 et 300 K. En conséquence, ce matériau composite peut être un très bon candidat pour la réfrigération magnétique au voisinage de l'ambiante. Dans une dernière partie, nous avons étudié l'effet du double échange, de la méthode de préparation, le rayons du site A et la nature magnétique du dopant au site B sur les propriétés magnétocaloriques en caractérisant la famille des composés La0,6A0,4Mn1-xFexO3 (A= Ca, Sr et 0≤x≤0,2) par diffraction des rayons X et par mesures magnétiques. D'une part, l'entropie magnétique maximale augmente avec le rayon du site A et est peu affecté par le rayon du site B et d'autre part, la méthode de préparation solide-solide est à privilégier puisqu'elle permet d'obtenir les plus grandes valeurs d'entropie magnétique maximaleSince the discovery of the giant magnetoresistance effect (end of 1980s), which is characterized by a large change in the electrical resistance of a material under the effect of a magnetic field, a major impact has been motivated both on fundamental and practical aspects (Nobel Prize of A. Fert in 2007). The intensive research activities in this field have leaded in the end of 1990 to point out the giant magnetoresitance in thin films of perovskite family, in particular the manganites (Ln1-xAx)MnO3. The aim of this work concern the study of the structural, magnetic, electrical and magnetocaloric properties of new manganites based materials in view of their application in the magnetic cooling. It is worth noting that in recent years, a giant magnetocaloric effect has been reported in several materials leading to the implementation of new efficient magnetic cooling systems. This technology is considered actually as the most alternative to replace the classical systems based on the compression-relaxation process. Compared with conventional refrigeration, magnetic cooling presents relevant advantages such as a decrease of energy consumption (high efficiency) and reduction of the acoustic and environmental pollution (elimination of the standard coolants: CFC, HCFC). The first part of this work concerns the elaboration as well as the characterization of the compound with La1-xCexMnO3 formula. We have studied the role of the annealing on the morphological, structural, magnetic and magnetocaloric properties of these materials. Using the Landau theory, we have calculated the magnetic entropy change ΔSM, which is found in good agreement with the measurements, and we have shown that the nature of the magnetic transition depends also on the annealing temperature. The compound with the composition x = 0.4, presents a large value of the figure of merit RCP, which make this material a good candidate for magnetic cooling application. In the second part, a detailed study of the morphological, structural, magnetic and magnetocaloric properties of the compounds with La0,7Ca0,15Sr0,15Mn1-xFexO3 formula has been performed. The iron Fe don't affect the structural properties, but induces a decrease of the Curie temperature. Based on the La0,7Ca0,15Sr0,15Mn1-xFexO3 (x = 0, 025 et 0,075) compositions, a composite material was proposed. The entropy change of the composite remains approximately constant in the temperature range between 260 and 300 K. Consequently, the proposed composite can be a good refrigerant for room temperature applications, in particular the magnetic cooling systems that use AMR or Ericsson thermodynamic cycles. In the last part, we have investigated the effect of the double exchange, preparation method and, ionic radius in A site and the magnetic nature on the doping in B site on the physical properties of La0,6A0,4Mn1-xFexO3 (A= Ca, Sr et 0≤x≤0,2) by using X-rays diffraction and magnetic measurements. The results demonstrate that the maximum entropy change increases with the ray of A site while it is slightly affected by the B site ray. On the other hand, it seems that the solid-solid preparation technique allows to obtain compounds with high magnetocaloric performances
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Pathak, Arjun Kumar. "EXPLORATION OF NEW MULTIFUNCTIONAL MAGNETIC MATERIALS BASED ON A VARIETY OF HEUSLER ALLOYS AND RARE-EARTH COMPOUNDS." OpenSIUC, 2011. https://opensiuc.lib.siu.edu/dissertations/353.
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Magnetic, magnetocaloric, magnetotransport and magnetoelastic properties of Ni-Mn-X (X = In, and Ga) Heusler alloys and La-Fe-Si based rare earth compounds have been synthesized and investigated by x-ray diffraction, magnetization, strain, and electrical resistivity measurements. The phase transitions, magnetic, magnetocaloric, magnetotransport and magnetoelastic properties strongly depend on the composition of these systems. In Ni50Mn50-xInx with x = 13.5, magnetocaloric and magnetotransport properties associated with the paramagnetic martensitic to paramagnetic austenitic transformation were studied. It was shown that magnetic entropy changes (SM) and magnetoresistance (MR) associated with this transformation are larger and the hysteresis effect is significantly lower when compared to that associated with paramagnetic-ferromagnetic transitions or ferromagnetic-antiferromagnetic/paramagnetic transitions in other systems. The Hall resistivity and the Hall angle shows unusual behavior in the vicinity of the martensitic phase transition for Ni50Mn50-xInx with x = 15.2. The observed Hall resistivity and Hall angle are 50 μ*cm and , respectively. It was observed that the presence of Ge, Al and Si atoms on the In sites strongly affects the crystal structure, and the electric and magnetic behaviors of Ni50Mn35In15. It was found that the partial substitution of In atoms by Si in Ni50Mn35In15 results in an increase in the magnetocaloric effect, exchange bias and shape memory effect. In Ni50Mn35In15-xSix, the peak values of positive SM for magnetic field changes H = 5 T were found to depend on composition and vary from 82 Jkg-1K-1 for x = 1 (at T = 275 K) to 124 Jkg-1K-1 for x = 3 (at T = 239 K). The partial substitution of Ni by Co in Ni50Mn35In15 significantly improves the magnetocaloric effect and MR in the vicinity of martensitic transition. In addition, significantly large inverse SM and MR were observed at the inverse martensitic phase transitions of the Ga-based magnetic shape memory Heusler alloys Ni50-xCoxMn32-yFeyGa18. The phase transition temperatures and magnetic properties were found to be correlated with the degree of tetragonal distortion in these samples. In LaFe11.57Si1.43Bx the crystal cell parameters and Curie temperatures were found to increase linearly with increasing B concentration up to ~ 0.1 % and 9 %, respectively. It was found that the characteristics of the magnetocaloric effect of LaFe11.57Si1.43 can be adjusted by a change in B concentration in the LaFe11.57Si1.43Bx system. A study of the influence of a small substitution of Ni, Cu, Cr, and V for Fe in LaFe11.4Si1.6 revealed that the magnetic, magnetocaloric, and magnetovolume coupling constant is related to an increase in the average Fe-Fe interatomic distances, leading to a change in the d-d exchange interaction.
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Ahmim, Smail. "Conversion de la chaleur fatale de bas niveau en énergie électrique par effet magnétocalorique." Thesis, université Paris-Saclay, 2020. http://www.theses.fr/2020UPAST015.
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Mes travaux de thèse visent à récupérer, grapiller, la chaleur fatale de bas à très bas niveau pour produire de l'énergie électrique et ainsi alimenter des petits systèmes autonomes (µW à mW). Le générateur développé convertit l'énergie en trois étapes. Tout d'abord l'énergie thermique est convertie en énergie magnétique au travers d'un cycle thermodynamique opéré à l'aide d'un matériau magnétocalorique. Cette première conversion est intimement liée à la seconde, conversion de l'énergie magnétique en énergie mécanique, car le déplacement du matériau magnétocalorique contrôle aussi le champ appliqué et les échanges thermiques avec les réservoirs. C'est l'imbrication de ces deux cycles, thermodynamique et dynamique, qui permet au système d'auto-osciller. L'énergie mécanique du système pseudo-oscillant est finalement convertie en énergie électrique via des éléments piézoélectriques. Mes travaux expérimentaux, théoriques et numériques ont cherché à maximiser l'énergie électrique récupérée tout en assurant l'auto-oscillation de la structure. Les dispositifs développés sont en mesure d'auto-osciller pour des écarts de température de 35 °C tout en produisant de l'énergie électrique. Notre prototype le plus performant présente une énergie de 10,6 μJ par cycle pour une fréquence de 0,41 Hz, soit une puissance de 4,2 μW (240 μW/cm3). Ces travaux mettent l'accent sur les cycles associés à la conversion d'énergieThe main objective of my thesis is the design and development of a device suitable to recover, and scavenge, low grade heat to produce electrical energy and thus supply small autonomous systems (μW to mW). The developed generator converts energy in three steps. First of all, thermal energy is converted into magnetic energy through a thermodynamic cycle operated by a magnetocaloric material. This first conversion is closely linked to the second, conversion of magnetic energy into mechanical energy, because the displacement of the magnetocaloric material also controls the applied field and the heat exchanges with the reservoirs. It is the interweaving of these two cycles, thermodynamic and dynamic, which allows the system to self-oscillate. The mechanical energy of the pseudo-oscillating system is converted into electrical energy via piezoelectric elements. My experimental, theoretical and numerical works aimed to maximize the electrical energy recovered while ensuring the self-oscillation of the structure. All devices developed are able to self-oscillate for temperature difference of 35 °C while producing electrical energy. Our most efficient prototype has an energy of 10.6 μJ per cycle for a frequency of 0.41 Hz, i.e. a power of 4.2 μW (240 μW/cm3). This work, especially, focuses on the cycles associated with energy conversion
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Alvarenga, Thiago da Silva Teixeira. "Investigação do efeito magnetocalórico convencional e anisotrópico no sistema Er(1-y)Ho(y)N." Universidade do Estado do Rio de Janeiro, 2012. http://www.bdtd.uerj.br/tde_busca/arquivo.php?codArquivo=6287.
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Coordenação de Aperfeiçoamento de Pessoal de Nível SuperiorO efeito magnetocalórico, base da refrigeração magnética, é caracterizado por duas quantidades: a variação isotérmica da entropia (ΔST) e a variação adiabática da temperatura (ΔTad) as quais podem ser obtidas sob variações na intensidade de um campo magnético aplicado. Em sistemas que apresentam anisotropia magnética, pode‐se definir o efeito magnetocalórico anisotrópico, o qual, por definição, é calculado através da variação na direção de aplicação de um campo magnético cuja intensidade se mantém fixa. Nos materiais de nosso interesse, o efeito magnetocalórico é estudado teoricamente partindo de um hamiltoniano modelo que leva em conta a rede magnética (que pode ser composta por diversas sub-redes magnéticas acopladas), rede cristalina e a dinâmica dos elétrons de condução. No hamiltoniano magnético são consideradas as interações de troca, Zeeman e campo cristalino (esta ultima responsável pela anisotropia magnética). Recentemente, estudamos o efeito magnetocalórico convencional e o efeito magnetocalórico anisotrópico nos compostos mononitretos com terras-raras, a saber: Ho(y)Er(1-y)N para as concentrações y= 0,1,0.5 e 0.75. Comparações entre nossos resultados teóricos e os dados experimentais para o EMC foram bastante satisfatórias [3,9]. Além disso, diversas predições teóricas como a existência de uma fase ferrimagnética no sistema Ho(y)Er(1-y)N (para a concentração y=0.5) e reorientações de spin nas sub-redes do Ho e Er foram feitas [25].
The magnetocaloric effect, magnetic refrigeration base, is characterized by two quantities: the isothermal entropy change (ΔST) and the adiabatic temperature change (ΔTad) which can be obtained through variations in the intensity of a magnetic field applied. In systems which present magnetic anisotropy, one can define anisotropic magnetocaloric effect, which, by definition, is calculated through the variation the direction of application of a magnetic field whose intensity remains fixed. In the materials of our interest, the magnetocaloric effect is studied theoretically starting from a model Hamiltonian which takes into account the magnetic lattice (that can be composed of several magnetic sublattices coupled), crystalline lattice and the dynamics of the conduction electrons. In the magnetic hamiltonian are considered the exchange interactions, Zeeman and crystalline electrical field (this latter responsible for the magnetic anisotropy). Recently, we studied the conventional magnetocaloric effect and anisotropic magnetocaloric effect in mononitrides compounds with rare earths, namely: o(Y)Er(1-Y)N for concentrations y= 0,1,0.5 e 0.75 . Comparisons between our theoretical results and experimental data for EMC were quite satisfactory [26].Furthermore, several theoretical predictions how to the existence of a phase ferrimagnetic in the system Ho(y)Er(1-y)N (for concentration ) and spin reorientations in the sublattices of Ho and Er were made [25].
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Almanza, Morgan. "La réfrigération magnétique : conceptualisation, caractérisation et simulation." Thesis, Grenoble, 2014. http://www.theses.fr/2014GRENT105/document.
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La réfrigération magnétique est une alternative pertinente dans un contexte où les gaz réfrigérants sont soumis à des restrictions environnementales. Ces restrictions nécessitent l'évolution de la technologie actuelle ou bien l'émergence d'une nouvelle, d'où l'opportunité pour la réfrigération magnétique de prouver son potentiel. En effet, elle pourrait s'avérer énergiquement plus efficace et avec des densités de puissance supérieure. Ces travaux de thèse apportent des réponses sur le potentiel de la réfrigération magnétique. Dans cette logique, la thermodynamique et le magnétisme, outils indispensables à notre étude, sont développés dans le cas des matériaux à effet magnétocalorique. Puis, nous verrons que les caractérisations de ces derniers sont en mesure de fournir des modèles matériaux cohérents et réalistes, si des précautions sont prises. L'effet magnétocalorique étant limité en termes de variation de température, nous allons étudier différentes structures de réfrigération. Enfin, des modèles numériques sont développés pour permettre d'optimiser les structures à régénérations actives, qui sont les plus utilisées. Ces modèles doivent permettre de dimensionner des systèmes proches de leurs optimumsMagnetic refrigeration is a relevant alternative in consideration of environmental restrictions of refrigerants gases. These restrictions require to improve the current technology or to pave the way for a new one, hence the opportunity for magnetic refrigeration to demonstrate its potential. Indeed, it could be energetically efficient and with higher power densities. This work aims to estimate the potential of magnetic refrigeration. Magnetism and thermodynamic, essential tools for our study, are developed in a case of magnetocaloric effect. With some care, we show that material characterizations are able to give consistence and relevant model. Magnetocaloric effect suffers of small temperature variations; therefore structures that increase the temperature span and give competitive system are studied. Finally numerical models are developed to optimize active magnetic regenerators, which are currently the most used. These models are used to calculate and design systems close to their optimum
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Williams, Daryl V. Jr. "Characterization of the Structural and Magnetic Properties of Gd Thin Films." Scholar Commons, 2010. http://scholarcommons.usf.edu/etd/3698.
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The standard material by which all materials exhibiting magnetocaloric effect are
measured is Gadnolinium. In this work we are attempting to understand how
nanostructuring can impact the magnetocaloric effect, to this end we have grown Gd in
various thin film structures. The samples made were grown via magnetron sputtering on
MgO(100) substrates. Samples of thick Gd (2000 A) were grown and sandwiched
between two layers of Cr or W and annealed at increasing temperatures to study how this
can perturb the magnetic and structural properties of the Gd. Another set of samples was
grown in which Gd (at various thicknesses) is in a multilayer system with W. Here the
purpose is to explore how changing the thickness of the Gd can change its magnetic
properties. Using the appropriate Maxwell relation, the magnetic entropy change was
observed to increase with increasing annealing temperature. In a 0-4T magnetic field
change, the peak entropy was found to go from approximately 1.5 J/kg-K for the
unannealed sample to 4.4 J/kg-K when annealed to 600°C. The multilayers were found
to all have a T C near 280 K, in contrast with what is predicted by finite size scaling. This
is likely due to pinholes in the W layers allowing the Gd to act as one magnetic material.
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Dottor, Maxime. "Synthesis and characterization of AlM2B2 (M = Cr, Mn, Fe, Co, Ni) : inorganic chemistry." Thesis, Uppsala universitet, Oorganisk kemi, 2015. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-255853.
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Santana, Rafael Pereira. "Efeitos magnetocalórico e barocalórico em sistemas físicos com dois níveis de energia." Universidade do Estado do Rio de Janeiro, 2008. http://www.bdtd.uerj.br/tde_busca/arquivo.php?codArquivo=888.
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Neste trabalho estudamos os aspectos teóricos dos efeitos magnetocalórico e barocalórico em sistemas físicos simples com dois e quatro níveis de energia. Para esta finalidade utilizamos um hamiltoniano que considera um sistema de momentos localizados interagindo entre si e com um campo magnético externo. No hamiltoniano também são incluídos a interação magnetoelástica, e um termo extra para simular anisotropia. O efeito de pressão externa é levado em consideração através da renormalização do parâmetro deinteração de troca. Fizemos um estudo sistemático das propriedades magnetocalóricas e barocalóricas para vários conjuntos de parâmetros do modelo. Os resultados obtidos mostram diversos tipos de comportamento dos potenciais magnetocalóricos, como o efeito mesa, o efeito inverso, o efeito gigante e uma estrutura com dois picos.In this work we study the theoretical aspects of the magnetocaloric and barocaloric effect in simple physical systems with two and four energy levels. In order to do that, we used a Hamiltonian that consider local magnetic moments interacting among them and with an external magnetic field.We include in the Hamiltonian the magnetoelastic interaction, and an extra term to simulate anisotropy. We consider the external pressure effect using a renormalization of the interaction exchange parameter. We performed systematical study about the magnetocaloric and barocaloric properties for a lot of sets of model parameters. The results show different types of behavior of the magnetocaloric potentials, such as the table-like effect, the inverse effect, the giant effect and a structure with two peaks.