https://tedebc.ufma.br/jspui/handle/tede/1267 Wed, 15 Jul 2026 06:08:35 GMT 2026-07-15T06:08:35Z http://tede2.ufma.br:8080/jspui/retrieve/1337/1.png https://tedebc.ufma.br/jspui/handle/tede/1267 https://tedebc.ufma.br/jspui/handle/tede/7080 Título: ESTUDO DAS PROPRIEDADES MAGNÉTICAS E MAGNETOCALÓRICAS DOS COMPÓSITOS METÁLICOS Dy-Pt-In e Gd-Pt-In Autor: FREITAS, Aline Alves de Primeiro orientador: SILVA, Luzeli Moreira da Abstract: This study presents the synthesis of metallic alloy composites based on Dy-Pt-In and Gd-Pt-In, followed by characterization through X-ray diffraction (XRD), Scanning Electron microscopy (SEM) magnetization, and specific heat measurements. XRD and SEM analysis revealed that the Dy-Pt-In composite consists of DyPtIn, DyPt and DyPt2In phases. Similarly, the Gd-Pt-In composite comprises GdPtIn and GdPt phases. Magnetic properties analysis indicates that both composites exhibit the magnetic transitions corresponding to the crystalline phases present in each material. The Dy composite displays two consecutive ferromagnetic transitions at 32.5 K and 23 K, and an antiferromagnetic transition at 7.5 K, which undergoes a metamagnetic phase transition to the ferromagnetic state upon applying a magnetic field, as demonstrated by specific heat data. Magnetocaloric properties analysis revealed that the three consecutive magnetic transitions extend the magnetocaloric effect (MCE) over a wide temperature range between 2.5 and 57 K for the Dy-Pt-In composite. Although the two ferromagnetic transitions in the Gd-Pt-In composite are relatively close, they are sufficiently broad to promote a significant MCE between 6.0 and 89 K. The maximum values obtained for magnetic entropy change are similar for both composites (6.3 and 6 J/kg. K at ΔH = 50 kOe). However, the relative cooling power is 47% higher in the Gd composite (~327 J/kg) compared to the Dy composite (~221 J/kg). The synthesis and characterization of the metallic composites in this study present promising results for applications in magnetic systems, especially in the context of the magnetocaloric effect. These results highlight the importance of developing composites with magnetic systems, since a material with optimized properties for technological application can be obtained by combining the individual magnetic properties of two or more metallic compounds. Instituição: Universidade Federal do Maranhão Tipo do documento: Dissertação Thu, 27 Jun 2024 00:00:00 GMT https://tedebc.ufma.br/jspui/handle/tede/7080 2024-06-27T00:00:00Z https://tedebc.ufma.br/jspui/handle/tede/7034 Título: Investigação teórica e experimental das propriedades estruturais, eletrônicas e térmicas do cristal de malonato de L-fenilalanina L-fenilalaninium Autor: COSTA, Alexandre Saraiva Primeiro orientador: FAÇANHA FILHO, Pedro de Freitas Abstract: In this work, the structural, electronic and vibrational properties of the crystalline environment of the L-phenylalanine L-phenylalaninium malonate salt were investigated. The synthesis of the compound occurred by the method of slow evaporation of the solvent with a 2: 1 ratio; the pH value measured was 2.61 under ambient conditions of temperature and pressure. The structural characterization by powder X-ray diffraction, combined with the Rietveld refinement method, confirmed that the material crystallized in a monoclinic system with space group P21, containing two molecules per unit cell Z = 2 and with the following network parameters : a = 14.08 Å, b = 5.53 Å, c = 14.65 Å and angle β = 107.39 º. The volume of the unit cell obtained after refinement was 1089.5 Å3 . The structural parameters, lengths and bond angles, analyzed via DFT were described satisfactorily with the LDA exchange and correlation functional. The vibrational properties were analyzed using the density functional theory (DFT) and FTIR and Raman spectroscopy techniques. In addition, to assign the vibrational bands we use the computational approach of calculations with the quantum_ESPRESSO package. Characterizations by thermal analysis Thermogravimetry (TGA), Derivative thermogravimetry (DTG), Differential Thermal Analysis (DTA) and Differential Scanning Calorimetry (DSC) indicate decomposition of the molecules from 120 ° C and a large endothermic peak around 185 ° C regarding the decomposition of the material, and suggest that the material has five stages of decomposition. The main objective of this work is to investigate the structural and vibrational properties via DFT with those based on the functional B3LYP, B3LYP / PCM and LDA using the quantum-ESPRESSO package. The vibrational study at high temperatures, indicated that the material does not present a phase transition until the melting point, showing only small modifications for smaller wave numbers and a decrease in the relative intensity, showing the decomposition of the NH3 and CO2 groups. Instituição: Universidade Federal do Maranhão Tipo do documento: Dissertação Wed, 16 Sep 2020 00:00:00 GMT https://tedebc.ufma.br/jspui/handle/tede/7034 2020-09-16T00:00:00Z https://tedebc.ufma.br/jspui/handle/tede/6788 Título: Propriedades estruturais, espectroscópicas e térmicas do sal de Tutton Rb2Co(SO4)2(H2O)6: uma abordagem experimental e teórica visando aplicações em dispositivos ópticos Autor: GOMES, Letícia Fonseca Primeiro orientador: OLIVEIRA NETO, João Gomes de Abstract: This work investigates the structural, spectroscopic, thermal, and optical properties of the Rb2Co(SO4)2(H2O)6 crystal, a Tutton salt with potential application in selective optical devices. The material was synthesized by slow solvent evaporation and characterized by powder X-ray diffraction (PXRD) with Rietveld refinement, Fourier- transform infrared (FT-IR) and Raman spectroscopies, thermogravimetric and differential scanning calorimetry (TG-DSC) analyses, and ultraviolet–visible–near infrared (UV-Vis-NIR) spectroscopy. The monoclinic crystal structure, belonging to the P21/a space group, was confirmed, with lattice parameters consistent with literature data. The analysis of intermolecular interactions, via Hirshfeld surfaces and calculations of crystal voids, revealed a dense lattice packing, predominantly governed by O···H/H···O hydrogen bonds, O···Co/Co···O coordination interactions, and Rb···O/O···Rb electrostatic interactions. Periodic calculations based on Density Functional Theory (DFT) indicated an electronic band gap of approximately 3.0 eV, mainly associated with the contributions of the Co2+ ion's d orbitals. Vibrational characterization allowed for the suitable assignment of Raman and FT-IR modes based on the structural units [Co(H2O)6] 2+, [SO4] 2- , and Rb+ . TG-DSC analyses revealed a dehydration event at approximately 111 °C, with the associated enthalpy for the dehydration process. The optical study showed absorption bands in the UV region associated with ligand-to-metal charge transfer transitions and bands in the Vis region attributed to d-d electronic transitions of the Co2+ ion, evidencing the Jahn-Teller effect. The observed absorption and transmittance windows indicate the material's intrinsic spectral selectivity, reinforcing its potential for applications in optical filters and selective photonic systems. Instituição: Universidade Federal do Maranhão Tipo do documento: Dissertação Mon, 22 Dec 2025 00:00:00 GMT https://tedebc.ufma.br/jspui/handle/tede/6788 2025-12-22T00:00:00Z https://tedebc.ufma.br/jspui/handle/tede/6704 Título: Efeitos da pressão nas propriedades estruturais e magnéticas de compostos multifuncionais de composição nominal Ni50Mn35In15 Autor: COSTA, Andira Nurrielli de Oliveira Primeiro orientador: REIS, Ricardo Donizeth dos Abstract: Here we report the use hydrostatic pressure to tune the structural properties of the Heusler compounds and then to verify the effects of the lattice contraction on their magnetic properties. Two polycrystalline compounds with nominal composition of Ni50Mn35In15, but with different crystalline structures at room temperature, were studied. X-ray diffraction (XRD) analysis by Rietveld method showed that the first Ni50Mn35In15 samples, A133, present two crystalline phases at ambient pressure and temperature conditions, cubic (space group Fm3തm) and monoclinic modulated (space group I2/m) structural phase, while the second sample, A139, present only the cubic (space group Fm3തm) phase. XRD measurements as a function of temperature have shown that the second phase of sample A133 (monoclinic modulated phase) is responsible for the martensitic phase of the sample which stabilizes when the temperature is lower than 250 K. In contrast, for the sample A139 no structural change was observed with temperature variation. Magnetization measurements as a function of temperature and magnetic field showed that the compound A133 shows a magnetic transition from paramagnetic (PM) to ferromagnetic (FM) in the austenitic phase at 311 K, followed by a first order magnetic-structural transition of the FM austenite phase for an antiferromagnetic (AFM) martensitic phase between 290 K and 276 K, and the new magnetic transition from the AFM state to FM in the martensitic phase at 194 K. While for the A139 compound only a second-order magnetic transition was observed, from the PM to FM state around 316 K. With the increase of the magnetic field the magnetostructural transition temperatures and the thermal hysteresis decrease in the sample A133, while for the A139 sample the magnetization increases considerably while the magnetic transition temperature is shifted to higher temperatures. The application of pressure in these two materials showed different effects, while for A133 compound the increase of pressure causes the cubic phase to stabilize in 10 GPa, for the A139 sample we observe the appearance of the martensitic phase for pressures higher than 6 GPa. Measurements performed during decompression showed that A133 had structural shape memory, while for A139 sample the process was completely reversible. In order to further understanding the impact of the structural changes on magnetic properties of the studied materials, we developed a new diamond pressure cell to perform magnetization experiments on the commercial equipment MPMS. The cell was them commissioned and showed the efficiency for magnetization experiment under high pressure (up to 10 GPa). This new instrumentation will certainly help on the understanding of the magnetic properties of these materials as well of the other similar compounds. In general, the results obtained indicates that the crystalline structure of the Heusler Ni50Mn35In15 compounds can be altered by applying pressure, which should also induces changes in the macroscopic properties of these materials and may increase their potential for technological applications. Instituição: Universidade Federal do Maranhão Tipo do documento: Dissertação Tue, 15 Jan 2019 00:00:00 GMT https://tedebc.ufma.br/jspui/handle/tede/6704 2019-01-15T00:00:00Z