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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

PHASE TRANSITIONS AND MAGNETOCALORIC EFFECT IN MnNiGe<sub>1−x</sub>Al<sub>x</sub>, Ni<sub>50</sub>Mn<sub>35</sub>(In<sub>1−x</sub>Cr<sub>x</sub>)<sub>15</sub> AND (Mn<sub>1−x</sub>Cr<sub>x</sub>)NiGe<sub>1.05</sub>

Quetz, Abdiel 01 August 2014 (has links)
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.
2

Cálculo teórico do efeito magnetocalórico do composto La(FexSi1-x)13. / Theoretical calculus of magnetocaloric effects of La(FexSi1-x)13.

Luciano Gomes de Medeiros Junior 14 March 2006 (has links)
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 (&#916;S) e para a variação adiabática da temperatura (&#916;Tad), 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 (&#916; S) and the adiabatic temperature change(&#916;Tad), 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.
3

Efeito magnetocalórico nos compostos Gd(Zn1-xCdx) e Gd(Pd1-xRhx) / Magnetocaloric effect in the compounds Gd(Zn1-xCdx) and Gd(Pd1-xRhx)

Mônica Borges Gomes 16 March 2006 (has links)
Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Nesta 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&#8722;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&#8722;xCdx) and Gd(Pd1&#8722;xRhx). The calculated adiabatic temperature changes and isothermal entropy changes upon magnetic field variations are in good agreement with the available experimental data.
4

Cálculo teórico do efeito magnetocalórico do composto La(FexSi1-x)13. / Theoretical calculus of magnetocaloric effects of La(FexSi1-x)13.

Luciano Gomes de Medeiros Junior 14 March 2006 (has links)
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 (&#916;S) e para a variação adiabática da temperatura (&#916;Tad), 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 (&#916; S) and the adiabatic temperature change(&#916;Tad), 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.
5

Efeito magnetocalórico nos compostos Gd(Zn1-xCdx) e Gd(Pd1-xRhx) / Magnetocaloric effect in the compounds Gd(Zn1-xCdx) and Gd(Pd1-xRhx)

Mônica Borges Gomes 16 March 2006 (has links)
Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Nesta 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&#8722;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&#8722;xCdx) and Gd(Pd1&#8722;xRhx). The calculated adiabatic temperature changes and isothermal entropy changes upon magnetic field variations are in good agreement with the available experimental data.
6

The Effect of Partial Substitution of Ni by Co and Cu on the Magnetic and Magnetocaloric Properties of the Intermetallic System Mn0.5Fe0.5Ni1-x(CuCo)xSi0.94Al0.06

Bhattacharjee, Sharmistha 26 July 2023 (has links)
No description available.
7

SYNTHESIS AND CHARACTERIZATION OF RARE EARTH-BASED MAGNETOCALORIC PHASES

Yuan, Fang January 2017 (has links)
In search of novel magnetocaloric materials, a number of rare earth-based phases were designed, synthesized and investigated. These compounds were prepared by arc-melting or sintering, followed by annealing at high temperature to obtain phase-pure materials. Single crystal and powder X-ray diffraction were employed for phase identification, purity assessment, structure solution and refinement. Energy dispersive X-ray spectroscopy (EDS) was used to determine sample compositions. A Quantum Design SQUID magnetometer equipped with an alternating current (ac) transport controller (model 7100) was employed to measure magnetic data and evaluate magnetocaloric properties. The crystal structure and physical properties were analyzed via electronic structure calculations. In this thesis work, the RE5Ga3 and RECo2 (RE = rare earth) materials were chosen as a starting point for structural modifications. Specifically, substitution of Co for Ga (and vice versa) or rare earth replacement was used to design new materials. In total, four families were investigated: Ho5Ga3-xCox (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 1), Er5Ga3-x(Fe/Co)x (x = 0, 0.4), RE(Co0.667Ga0.333)2 (RE = Gd, Tb, Dy, Ho, and Er), and Gd(Co1-xGax)2 (x = 0, 1/6, 1/3, 1/2, 2/3, 5/6, and 1). The samples were prepared by arc-melting, wrapped in Ta foil, sealed in evacuated silica tubes and annealed at specific temperatures. The Ho5Ga3-xCox and Er5Ga3-x(Fe/Co)x systems features a Mn5Si3-type-to-Cr5B3-type structural transformation, driven by geometric factors. On the other hand, the structural transformation in the RE(Co0.667Ga0.333)2 and Gd(Co1-xGax)2 systems appears to be controlled by the valence electron count (VEC). The RE(Co0.667Ga0.333)2 (RE = Gd, Tb, Dy, Ho, and Er) phases adopt a hexagonal MgZn2-type structure (P63/mmc). Structural and magnetic properties of the MgZn2-type RE(Co0.667Ga0.333)2 materials were investigated via single crystal and powder X-ray diffraction, powder neutron diffraction (PND), and magnetic measurements. In addition to the hexagonal MgZn2-type structure, four other structures were discovered in the Gd(Co1-xGax)2 system: cubic MgCu2- (Fd3 ̅m), orthorhombic MgSrSi- (Pnma), orthorhombic CeCu2- (Imma), and hexagonal AlB2-type structure (P6/mmm). When Ga content increases, the structure moves from a “condensed cluster-based arrangement” to a “3D Network” to a “2D Network”. Meanwhile, coordination number (CN) of Co or Ga atoms changes from 6 to 4, and then to 3. Magnetic properties of many of the RE-based phases were evaluated via temperature- and field-dependent magnetization measurement. Materials exhibited a sharp ferromagnetic transition and their MCE in terms of the isothermal magnetic entropy change, was explored. / Thesis / Doctor of Philosophy (PhD)
8

Synthesis and characterization of AlM2B2 (M = Cr, Mn, Fe, Co, Ni) : inorganic chemistry

Dottor, Maxime January 2015 (has links)
No description available.
9

Characterization of the Structural and Magnetic Properties of Gd Thin Films

Williams, Daryl V., Jr. 15 June 2010 (has links)
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.
10

Development and validation of an active magnetic regenerator refrigeration cycle simulation

Dikeos, John 10 August 2006 (has links)
An alternative cycle proposed for refrigeration and gas liquefaction is active magnetic regenerator (AMR) refrigeration. This technology relies on solid materials exhibiting the magnetocaloric effect, a nearly reversible temperature change induced by a magnetic field change. AMR refrigeration devices have the potential to be more efficient than those using conventional refrigeration techniques but, for this to be realized, optimum materials, regenerator design, and cycle parameters must be determined. This work focuses on the development and validation of a transient one-dimensional finite element model of an AMR test apparatus. The results of the model are validated by comparison to room temperature experiments for varying hot heat sink temperature, system pressure, and applied heat load. To demonstrate its applicability, the model is then used to predict the performance of AMRs in situations that are either time-consuming to test experimentally or not physically possible with the current test apparatus.

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