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)

Neste trabalho avaliou-se a ação positiva da substituição de lantânio por praseodímio e de lantânio por magnésio na performance eletroquímica de eletrodos de ligas de armazenamento de hidrogênio em estado bruto de fusão e com tratamento térmico. O La foi substituído por Mg nas ligas La0,7-xMgxPr0,3Al0,3Mn0,4Co0,5Ni3,8 (x=0,0-0,7) e por Pr nas ligas La0,7-yPryMg0,3Al0,3Mn0,4Co0,5Ni3,8 (y=0,0-0,7). Os parâmetros eletroquímicos analisados foram ativação, capacidade de descarga, retenção da capacidade de descarga, autodescarga e alta taxa de descarga. As ligas apresentaram comportamento passivo em relação à corrosão. As análises por MEV/EDS e por DRX com refinamento por Rietveld revelaram a presença majoritária de fases similares às fases LaNi5, PrNi5, LaMg2Ni9 e PrMg2Ni9 em função das composições das ligas estudadas. Os parâmetros de rede e os volumes da célula unitária das fases diminuíram com a substituição crescente de La por Mg e de La por Pr. As capacidades de descarga máxima decresceram com a substituição crescente de La por Mg e de La por Pr, acompanhando o decréscimo da abundância da fase similar à fase LaNi5 e o aumento da abundância da fase similar à fase LaMg2Ni9. Comparativamente, menores taxas de autodescarga e maior estabilidade cíclica foram observadas para o eletrodo da liga na condição x=0,1, ao passo que o eletrodo da liga na condição y=0,0 apresentou maiores valores de alta taxa de descarga, indicando melhor performance cinética. / In this work, the positive action of the substitution of lanthanum by praseodymium and lanthanum by magnesium in the electrochemical performance of the as-cast and annealed hydrogen storage alloys electrodes had been evaluated. La was replaced by Mg in La0.7-xMgxPr0.3Al0.3Mn0.4Co0.5Ni3.8 (x=0.0-0.7) alloys and by Pr in La0.7-yPryMg0.3Al0.3Mn0.4Co0.5Ni3.8 (y=0.0-0.7) alloys. The electrochemical parameters analyzed were activation, discharge capacity, discharge capacity retention, self-discharge rate and high-rate dischargeability. The alloys showed a passive corrosion behavior. The analyses by SEM/EDS and XRD with Rietveld refinement revealed the majority presence of LaNi5, PrNi5, LaMg2Ni9 and PrMg2Ni9 similar reference phases depending on the compositions of the studied alloys. The lattice parameters and cell volumes of the component phases decreased with increasing substitution of La for Mg and with La for Pr. The maximum discharge capacity decreased with increasing substitution of La for Mg and with La for Pr, following the decrease in the abundance of LaNi5 similar phase and the increase in the abundance of to the LaMg2Ni9 similar phase. Lower self-discharge rates were observed for the alloy electrode when x=0.1, while higher high-rate dischargeability for the alloy electrode when y=0.0 indicated better kinetic performance, comparatively.

Neste trabalho, avaliou-se o efeito da substituição parcial do Ni pelos elementos Co, Cu e Sn, e do La e Mg pelo Pr em ligas do tipo AB5 para as ligas nominais La0,7Mg0,3Al0,3Mn0,4Cu0,5Ni3,8, La0,7Mg0,3Al0,3Mn0,4Co0,5Ni3,8 e La0,7Mg0,3Al0,3Mn0,4Sn0,5Ni3,8, La0,7Pr0,3Al0,3Mn0,4Co0,5Ni3,8, Pr0,7Mg0,3Al0,3Mn0,4Co0,5Ni3,8 sem e com tratamento térmico de 750° e 850°C por 9 e 16 horas. Estas ligas absorvedoras de hidrogênio foram utilizadas como material ativo em eletrodos negativos de baterias de Ni-HM. Avaliou-se a influência destas substituições e do tratamento térmico na microestrutura e nas propriedades eletroquímicas nas ligas. A substituição parcial do Ni pelo Co com o tratamento térmico de 750°C por 16 horas apresentou duas novas fases Al6Mn e PrCo13. Na substituição parcial do Ni pelo Sn formou a fase LaNi2Sn2 na liga. Utilizou-se caracterização de raios-X com refinamento de Rietveld para quantificar as fases em cada composição. Mediu-se a absorção de hidrogênio utilizando um Aparato Sieverts para obtenção das curvas PCT. Observou-se que as ligas La0,7Mg0,3Al0,3Mn0,4Co0,5Ni3,8 e La0,7Pr0,3Al0,3Mn0,4Co0,5Ni3,8 apresentaram as melhores capacidades de absorção de hidrogênio. Verificou-se o comportamento destas ligas na capacidade de descarga, estabilidade cíclica das baterias de Ni-HM. Comparando as ligas, a maior capacidade de descarga medida foi para a substituição parcial do Ni pelo Co, alcançando 406,1 mAh após o tratamento térmico de 850° C por 16 horas. A melhor capacidade de absorção obtida na analise de PCT, foi para a liga La0,7Pr0,3Al0,3Mn0,4Co0,5Ni3,8 com valor de H/M de 0,980. Também, foi avaliada uma correlação das propriedades eletroquímicas com a capacidade de absorção obtida na analise da curva PCT. / In this work, it was evaluated the effect of the partial substitution of Ni by the elements Co, Cu and Sn and of the La and Mg by the Pr in type AB5 alloys to the nominal alloys La0,7Mg0,3Al0,3Mn0,4Cu0,5Ni3,8, La0,7Mg0,3Al0,3Mn0,4Co0,5Ni3,8, La0,7Mg0,3Al0,3Mn0,4Sn0,5Ni3,8, La0,7Pr0,3Al0,3Mn0,4Co0,5Ni3,8 and Pr0,7Mg0,3Al0,3Mn0,4Co0,5Ni3,8, as castting and with annealing treatment of 750 °C and 850 °C for 9 and 16 hours. These hydrogen-absorbing alloys were used as active material on negative electrodes of Ni-HM batteries. The influence of these substitutions and the annealing treatment on the microstructure and on the electrochemical properties in the alloys was evaluated. The partial substitution of Ni by Co with the annealing treatment of 750 °C for 16 hours presented two new phases Al6Mn and PrCo13. In the partial substitution of Ni by Sn formed the LaNi2Sn2 phase in the alloy. It was characterized by X-ray diffraction using Rietveld\'s refinement to quantify the phases in each composition. Hydrogen absorption was measured using the Sieverts apparatus to obtain the PCT curves. It was observed that the alloys La0,7Mg0,3Al0,3Mn0,4Co0,5Ni3,8 and La0,7Pr0,3Al0,3Mn0,4Co0,5Ni3,8, presented the best capacities of hydrogen absorption. The behavior of these alloys in the discharge capacity, the cyclic stability of the Ni-HM batteries, was verified. Comparing the alloys, the biggest discharge capacity measured was for the partial substitution of Ni by Co alloy, reaching 406.1 mAh after the annealing treatment of 850 °C for 16 hours. The finest absorption capacity obtained in the PCT analysis was for the La0,7Pr0,3Al0,3Mn0,4Co0,5Ni3,8 alloy with an H/M value of 0.980. Also, a correlation of the electrochemical properties with the absorption capacity obtained in the analysis of the PCT curve was evaluated.

Neste trabalho avaliou-se a ação positiva da substituição de lantânio por praseodímio e de lantânio por magnésio na performance eletroquímica de eletrodos de ligas de armazenamento de hidrogênio em estado bruto de fusão e com tratamento térmico. O La foi substituído por Mg nas ligas La0,7-xMgxPr0,3Al0,3Mn0,4Co0,5Ni3,8 (x=0,0-0,7) e por Pr nas ligas La0,7-yPryMg0,3Al0,3Mn0,4Co0,5Ni3,8 (y=0,0-0,7). Os parâmetros eletroquímicos analisados foram ativação, capacidade de descarga, retenção da capacidade de descarga, autodescarga e alta taxa de descarga. As ligas apresentaram comportamento passivo em relação à corrosão. As análises por MEV/EDS e por DRX com refinamento por Rietveld revelaram a presença majoritária de fases similares às fases LaNi5, PrNi5, LaMg2Ni9 e PrMg2Ni9 em função das composições das ligas estudadas. Os parâmetros de rede e os volumes da célula unitária das fases diminuíram com a substituição crescente de La por Mg e de La por Pr. As capacidades de descarga máxima decresceram com a substituição crescente de La por Mg e de La por Pr, acompanhando o decréscimo da abundância da fase similar à fase LaNi5 e o aumento da abundância da fase similar à fase LaMg2Ni9. Comparativamente, menores taxas de autodescarga e maior estabilidade cíclica foram observadas para o eletrodo da liga na condição x=0,1, ao passo que o eletrodo da liga na condição y=0,0 apresentou maiores valores de alta taxa de descarga, indicando melhor performance cinética. / In this work, the positive action of the substitution of lanthanum by praseodymium and lanthanum by magnesium in the electrochemical performance of the as-cast and annealed hydrogen storage alloys electrodes had been evaluated. La was replaced by Mg in La0.7-xMgxPr0.3Al0.3Mn0.4Co0.5Ni3.8 (x=0.0-0.7) alloys and by Pr in La0.7-yPryMg0.3Al0.3Mn0.4Co0.5Ni3.8 (y=0.0-0.7) alloys. The electrochemical parameters analyzed were activation, discharge capacity, discharge capacity retention, self-discharge rate and high-rate dischargeability. The alloys showed a passive corrosion behavior. The analyses by SEM/EDS and XRD with Rietveld refinement revealed the majority presence of LaNi5, PrNi5, LaMg2Ni9 and PrMg2Ni9 similar reference phases depending on the compositions of the studied alloys. The lattice parameters and cell volumes of the component phases decreased with increasing substitution of La for Mg and with La for Pr. The maximum discharge capacity decreased with increasing substitution of La for Mg and with La for Pr, following the decrease in the abundance of LaNi5 similar phase and the increase in the abundance of to the LaMg2Ni9 similar phase. Lower self-discharge rates were observed for the alloy electrode when x=0.1, while higher high-rate dischargeability for the alloy electrode when y=0.0 indicated better kinetic performance, comparatively.