Física

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http://repositorio.ufes.br/handle/10/17492
Programa de Pós-Graduação em Física

Centro: CCE
Telefone: (27) 4009 2488
URL do programa: http://www.fisica.ufes.br/pos-graduacao/PPGFis

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Navegando Física por Autor "Alves, André Luíz"

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    A Influência da mecanossíntese na estabilização da fase tipo NaZn13 nos compostos [La0,95(RE)0,05]Fe11,4Si1,6 (RE = Y ou Gd)
    (Universidade Federal do Espírito Santo, 2007-10-30) Alves, André Luíz; Proveti, José Rafael Cápua; Caetano, Edson Passamani; Nunes Filho, Evaristo; Sommer, Rubem Luis; Larica, Carlos
    We have studied the stabilization of NaZn13 phase in the intermetallic compound of generic composition LaFe11,4sp1,6 (sp = Si, Al). This stabilization was investigated through the partial substitution of Fe by atoms of Al and Si, and trough the substitution of La by Y or Gd, which are rare earths elements with no orbital contribution to the magnetic moments. We have chosen to study the compounds with stoichiometry LaFe11,4sp1,6 (sp = Al, Si), with y = 0,88, for which compounds are metamagnetic. We firstly studied NaZn13 phase formation in some mixed compounds of composition LaFe11,4(AlxSi1-x)1,6 (0 less or equals x less or equals 1), prepared using an electric arc furnace. The compounds were characterized by Mössbauer Spectroscopy, X Rays Diffraction and Energy Dispersive X Ray Spectroscopy. Some compositions have a predominant NaZn13 phase, while in other regions the fraction of the NaZn13 phase is relative small due to the presence of other phases formed by segregation. We have also investigated the partial substitution of La atoms by atoms of Y and Gd, in the following compounds La1-x(RE)xFe11,4Si1,6 (RE = Gd or Y), with x= 5%. This substitution doesn t modify significantly the stabilization of the NaZn13 phase compared to the non-substituted compound and the fraction of the segregated a-Fe phase is maintained around 50%. We have used a new procedure using the mechanical alloying to prepare LaFe11,4Si1,6 compounds. It consists in submitting the melted sample to a 0.5 to 1 hour long high energy ball milling process and then in annealing the sample. The process induced a significant reduction of the rich-Fe phase formation compared to a conventional process. Therefore, the process allows to obtain samples with a higher fraction of the desired NaZn13 phase. Therefore, the process allows obtaining samples with a higher fraction of the desired NaZn13 phase
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    Influência da redução do grão cristalino e fatores de desordem química na transição de fase de primeira ordem da liga tipo Heusler Ni2Mn1,44Sn0,56
    (Universidade Federal do Espírito Santo, 2011-01-25) Alves, André Luíz; Caetano, Edson Passamani; Carvalho, Alexandre Magnus Gomes; Takeuchi, Armando Yoshihaki; Baggio-Saitovitch, Elisa Maria; Nascimento, Valberto Pedruzzi; Scopel, Wanderlã Luis
    In this work, we investigated the influence of the grain refinement and the effect of chemical disorder induced by high energy milling on the martensitic transformation of the Ni2Mn1,44Sn0,56 Heuler-type alloy with and without Fe doping in the different crystallographic sites of the L21-type structure. We used X-ray diffraction to estimate the grain size as a function of milling time and DC magnetization to study the magnetic properties and the martensitic transformation of this alloy. We determined that the increasing of milling time induces a grain size reduction, enhancing substantially the grain boundary regions, which have high chemical and topological disorder degrees and favoring magnetically frustrated spin configurations. We applied Mössbauer spectroscopy of 119Sn and of 57Fe to characterize the different magnetic fractions and to follow locally the magnetism of this alloy. The main results caused by the disorders induced by milling are the increasing of the Exchange Bias Field and a significant reduction of the magnetocaloric properties of the Ni2Mn1,44Sn0,56 Heusler-type alloy when the milling time is increased. The enhancement of the Exchange Bias Field, we have explained based on the increasing of the fractions of the antiferromagnetic and/or frustrated spin of the grain boundary regions relatively to the ferromagnetic grain core fraction when the milling time is increased. On the other hand, we conclude that the decrease of the magnetic entropy change (magnetocaloric effect) is due to the magnetization changes during the martensitic phase transformation, which depends on the fractions of the grain cores. Therefore, this reductions is correlated to the increase of the grain boundary regions, which have low magnetization and has not martensitic phase transformation. From the Fe doping effect on different crystallographic sites of the L21- B2 - type structure of the Ni2Mn1,44Sn0,56 Heusler-type alloy, we demonstrated X that the distint shapes and features/properties of the magnetization curves are due to the different fractions of Fe ferromagnetically coupled with Mn at room temperature. We also shown that a heat treatment on the milled alloy restores the L21 – B2 -type structure and improves the mains magnetic properties related to the structural phase transformation called as martensitic