https://tedebc.ufma.br/jspui/handle/tede/1267 2026-08-27T00:04:41Z https://tedebc.ufma.br/jspui/handle/tede/7186 Título: Sais de tutton triplos CM2CD(SO4)2(H2O)6 (CM = K⁺, Rb⁺, NH4⁺; CD = Mg2+, Co2+ e Ni2+) para aplicações em dispositivos termoquímicos: síntese, estrutura e propriedades térmicas. Autor: ALVES , Eduardo Sousa Primeiro orientador: OLIVEIRA NETO, João Gomes de Abstract: The development of new materials for thermochemical heat storage (TCHS) is essential for harnessing intermittent renewable energy. Although hydrated salts offer high energy densities, their performance is often limited by issues such as poor cyclability, necessitating the production and study of new compositions. This work introduces a new subclass of Tutton's salts, in which three distinct divalent cations (Mg2+, Co2+ e Ni2+) simultaneously occupy a single crystallographic site, with the general formula CM2CD(SO4)2(H2O)6 (CM = K⁺, Rb⁺, NH4⁺; CD = Mg2+, Co2+ e Ni2+). These materials were synthesized via slow solvent evaporation, and their structures were determined using powder X-ray diffraction data, confirming crystallization in monoclinic symmetry with space group P21/a or P21/c. X-ray fluorescence spectroscopy revealed non-equimolar incorporation of the divalent cations, indicating a significant influence of the monovalent cation on site occupancy. Comprehensive structural analysis, including Hirshfeld surfaces and crystal voids, demonstrated that the NH₄⁺-based salt possesses a more porous structure with elongated void channels, contrasting with the compact structures of the K+ and Rb+ analogues. Raman spectroscopy provided insights into the local coordination environment and the hydrogen-bonding network. Thermal analyses revealed tunable dehydration temperatures (onset between 69 and 97 °C) and the release of all six water molecules in a single step. Dehydration enthalpies (ΔH) ranged from 299.64 to 426.83 kJ/mol, resulting in promising volumetric energy storage densities of up to 1.88 GJ/m3. The Rb+-based compound emerged as the most promising candidate for a thermochemical material (TCM), exhibiting the highest ΔH and a theoretical thermal efficiency (η) of 42.83%. Thus, this study establishes the first triple Tutton salts as versatile compounds for tuning thermochemical properties through cationic composition, paving the way for tunable, high-performance TCHS materials. Instituição: Universidade Federal do Maranhão Tipo do documento: Dissertação 2026-06-24T00:00:00Z https://tedebc.ufma.br/jspui/handle/tede/7179 Título: SÍNTESE E CARACTERIZAÇÃO DE VIDROS CÁLCIO BOROTELURETOS DOPADOS COM Eu2O3 SINTETIZADOS EM ATMOSFERA DE ARGÔNIO Autor: COSTA, Deckson Lacerda de Sousa Primeiro orientador: BARBOZA, Marcio José Abstract: In this study, glasses of Calcium Borotellurite (CaBTe) doped with Eu2O3 were synthesized in an argon atmosphere with the aim of examining the influence of TeO2 concentration on structural, thermal, optical, and spectroscopic properties. Additionally, the impact of the argon atmosphere on the valence change of the europium ion was investigated. The results were discussed in relation to the TeO2 content. The adopted composition was 10CaF2 – (29.5 – 0.4x)CaO – (60 – 0.6x)B2O3 – xTeO2 – 0.5Eu2O3, with x = 10, 16, 22, and 31% in mol; the glasses were prepared using the melt-quenching method. X-ray diffraction results indicate that the samples studied in this work have an amorphous nature. Fourier-transform infrared spectroscopy results show that the vitreous network is formed by the TeO3, TeO4, BO3, and BO4 structures. Differential thermal analysis results showed a decrease in the glass transition temperature with increasing TeO2, and good thermal stability within the margin of error. Refractive index results increased with the rise in TeO2 content. In the results of the optical absorption coefficient, strong absorption in the ultraviolet region was observed; the argon atmosphere was not efficient in reducing Eu3+ to Eu2+, and a shift in the absorption edge to longer wavelengths occurred with the increase of TeO2. Luminescence results showed that the Eu3+ doped CaBTe glass is a good luminescent source. In the chromaticity diagram, a predominance of the color red was observed for all samples. In the lifetime results, a decrease was observed with the increase of TeO2. Instituição: Universidade Federal do Maranhão Tipo do documento: Dissertação 2002-12-20T00:00:00Z https://tedebc.ufma.br/jspui/handle/tede/7124 Título: PROPRIEDADES MAGNÉTICAS E EFEITO MAGNETOCALÓRICO DOS SISTEMAS RMAl3 (R = La, Ce, Pr E Nd; M = Ag E Cu) E R2AgCuAl6 (R = Pr E Nd) Autor: LOPES, Maurício Silva Primeiro orientador: SILVA, Luzeli Moreira da Abstract: ABSTRACT: This study investigated the structural, magnetic, and magnetocaloric properties of intermetallic compounds in the RMAl3 (R = La, Ce, Pr, and Nd; M = Ag and Cu) and R2CuAgAl6 (R = Pr and Nd) systems. The samples were synthesized in an arc furnace under an argon atmosphere, subjected to heat treatment for 7 days at 700 °C, and structurally characterized via X-ray diffraction (XRD) and Rietveld refinement. Specific heat (Cp) measurements as a function of temperature and magnetic field revealed that CeAgAl3, PrAgAl3, and NdCuAl3 order magnetically at 4.8 K, 5.3 K, and 2.21 K, respectively, whereas NdAgAl3, PrCuAl3, and Nd2AgCuAl6 showed no detectable magnetic ordering above 1.8 K. Temperature-normalized Cp data indicate a Schottky anomaly in all analyzed samples, with the associated peak being most intense in the NdAgAl3, PrCuAl3, NdCuAl3, and Nd2AgCuAl6 compounds. The 4f-electron contribution to Cp in the CeAgAl3, PrAgAl3, PrCuAl3, NdAgAl3, and NdCuAl3 compounds was determined by subtracting the contributions of the non-magnetic analogs LaAgAl3 or LaCuAl3. This enabled the derivation of magnetic entropy (S4f) curves. These curves allowed for the estimation of the number of microstates contributing to the system's S4f within the 1.8–100 K temperature range. Parameters quantifying the Magnetocaloric Effect (MCE)—namely magnetic entropy change (-ΔS), adiabatic temperature change (ΔTad), and Relative Cooling Power (RCP)—were determined from the entropy (S) curves. For a magnetic field change (ΔH) of 5 T, the CeAgAl3 compound exhibited maximum values ​​of -ΔS (-ΔSmax) and ΔTad (ΔTad,max), as well as an RCP of approximately 10.32 J/kg·K, 3.15 K, and 56.87 J/kg, respectively. PrAgAl3 exhibits lower values ​​of -ΔSmax (~4.68 J/kg·K) and ΔTad,max (~2.61 K) but maintains a relatively high RCP (~62.49 J/kg). In this case, a significant portion of the contribution to the MCE is associated with crystal field effects, as evidenced by the results obtained for the non-magnetic compound PrCuAl3 within the measured range (-ΔSmax = 2.71 J/kg·K, ΔTad,max = 1.14 K, and RCP = 41.69 J/kg). On the other hand, NdCuAl3 demonstrates the best magnetocaloric performance (-ΔSmax = 17.08 J/kg·K, ΔTad,max = 3.80 K, and RCP = 159.66 J/kg). The results obtained for these materials are comparable to—and in the case of CeAgAl3, even superior to—intermetallics reported in the literature, indicating their potential for application in MCE-based magnetic refrigeration (MR) systems at cryogenic temperatures. Instituição: Universidade Federal do Maranhão Tipo do documento: Dissertação 2026-06-02T00:00:00Z 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 2024-06-27T00:00:00Z