https://repositorio.ufu.br/handle/123456789/19114 Sun, 30 Aug 2026 04:41:05 GMT 2026-08-30T04:41:05Z https://repositorio.ufu.br/handle/123456789/49581 Title: Estudo de geração de vapor de água mediado por folhas de grafeno oxidado e grafeno oxidado reduzido Abstract: This work investigates photothermal conversion and steam generation in nanofluids based on graphene oxide (GO) and reduced graphene oxide (rGO), obtained via reduction with L-ascorbic acid (ALA) and palladium chloride (PdCl2). The study focused on the correlation between modifications in the material's electronic and morphological structure and its thermal transport and optical absorption properties. Characterization via electron microscopy, Raman spectroscopy, and UV–Vis indicated that PdCl2 promotes the restoration of the graphitic lattice and sheet stacking, whereas ALA tends to keep the sheets more separated, resulting in a less compact morphology. Thermal Lens measurements revealed non-monotonic behavior of thermal diffusivity regarding the dispersed phase concentration: a reduction in diffusivity was observed at low concentrations relative to the base fluid, followed by an increase at higher concentrations. Although the exact causes of this behavior require further investigation, the data suggest a complex interaction between heat transport in the solvent and the presence of the nanostructures. In evaporation tests, the highest efficiencies (up to ∼70%) were not associated solely with the highest degree of structural ordering, but rather with hybrid samples combining high optical absorption with moderate thermal diffusivity, which favors the localization of thermal energy at the liquid-vapor interface. The results demonstrate that controlling aggregation and optical properties is crucial for optimizing solar-thermal energy conversion systems. Thu, 19 Feb 2026 00:00:00 GMT https://repositorio.ufu.br/handle/123456789/49581 2026-02-19T00:00:00Z https://repositorio.ufu.br/handle/123456789/49576 Title: Permeabilidade seletiva de água e metanol através de filmes ultrafinos de rGO/Polímero: uma investigação computacional Abstract: The search for clean and sustainable energy sources, driven by the growing energy demand and the problems caused by the use of fossil fuels, poses the challenge of developing technologies capable of replacing the polluting energy matrix (1). In this context, the Direct Methanol Fuel Cell (DMFC) stands out as an alternative, non-intermittent, and portable energy source, with low 𝐶𝑂2 emissions, no release of other polluting gases, and the ability to operate under conditions close to room temperature (2). Reduced graphene oxide (rGO) wrapped with poly(styrene sulfonate) (PSS) leads to the formation of a stable hybrid material (GPSS). This composite enables the fabrication of ultrathin films with promising barrier properties for DMFC applications, aiming to prevent methanol crossover poisoning, one of the major challenges of this technology (3). Experimental evidence shows that polymer electrolyte membranes of DMFCs coated with ultrathin GPSS-based films exhibit a significant reduction in methanol permeation and an increase in the cell’s power density. In this context, the objective of this work was to study the mechanism behind the methanol barrier provided by GPSS, which still allows the permeation of water and protons (4). To that end, classical Molecular Dynamics simulations were employed to investigate the structural and dynamic properties of GPSS at different polymer ionization fractions in a solvent composed of water, methanol, and hydronium. Regarding structural properties, the influence of the sulfonation fraction on the self-assembly of the layered film was discussed, including its effect on the PSS conformation and the distribution of rGO sheets, as well as the nature of the PSS–rGO interactions. As for the dynamic properties, the impact of GPSS structure on solvent diffusion was analyzed, and the mechanisms leading to the selective diffusion of methanol were elucidated. Wed, 01 Oct 2025 00:00:00 GMT https://repositorio.ufu.br/handle/123456789/49576 2025-10-01T00:00:00Z https://repositorio.ufu.br/handle/123456789/49359 Title: Caracterização fotofísica dos derivados de Carbazol e Tetrazina para sensoriamento de radiação ultravioleta Abstract: This work investigates the photophysical properties and charge transport mechanisms in donor–acceptor conjugated polymers derived from carbazole and tetrazine. The ma terials analyzed were synthesized at the Paulo Scarpa Polymer Laboratory (LaPPS), Federal University of Paraná (UFPR). The polymer LaPPS80 is based on the carba zole–tetrazine combination (2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2yl)-9-(tridecan 7-yl)-9H-carbazole- alt-3,6-bis(4-octylthiophen-2-yl)-1,2,4,5-tetrazine), whereas LaPPS81 is derived from condensed carbazole–tetrazine (5,11-bis(2-ethylhexyl)dihydroindolo[3,2 b]-carbazole-alt- 3,6-bis(4-octylthiophen-2-yl)-1,2,4,5-tetrazine). It is worth noting that LaPPS81 can be obtained through different synthetic routes and is therefore denoted throughout this work as LaPPS81-C and LaPPS81-S. This study proposes a novel application of these materials as selective photodetectors in the ultraviolet–visible region. Spectroscopic characterization by optical absorption, photoluminescence (PL), and photoluminescence excitation (PLE) revealed that the transition from solution to the solid state (thin films) induces strong intermolecular interactions. These interactions are evidenced by Stokes shifts on the order of approximately 59 nm, 101 nm, and 139 nm, as well as by pronounced red-shifts in the emission spectra. These results indicate the formation of molecular aggregates and delocalized excited states, which play a key role in charge transport in these polymeric systems. Photoresistor devices with planar architecture and interdigitated fluorine-doped tin oxide (FTO) electrodes were fabricated and characterized. Morphological and electrical analyses showed that the oligomer LaPPS81-C, due to its low molar mass, does not form efficient percolation networks. In contrast, the polymers LaPPS80 and LaPPS81-S yielded func tional devices with distinct and complementary physical behaviors. LaPPS80 exhibited maximum sensitivity at 450 nm but showed the anomalous phenomenon of negative photoconductivity (NPC) under UV excitation (280–370 nm), attributed to the activation of deep recombination centers by high-energy photons. LaPPS81-S demonstrated positive photoconductivity, with high selectivity at 370 nm, and notable anisotropy in the response to light polarization, indicating a macroscopic molecular alignment that favors directional charge transport. Electrical stability tests (20 voltage sweep cycles) indicated the robustness of the devices, which maintained ohmic behavior without significant degradation or hysteresis. Analysis of the I–V curves suggested the predominance of direct tunneling injection mechanisms at low electric fields, with a transition to regimes consistent with field emission at higher voltages. Impedance spectroscopy corroborated the modulation of bulk resistance under illumination, indicating that LaPPS81 exhibits higher intrinsic conductivity compared to LaPPS80. Taken together, these results suggest that the LaPPS family holds potential for application in selective optical sensors, with responses that can be tuned through molecular engineering and morphological control. Mon, 23 Feb 2026 00:00:00 GMT https://repositorio.ufu.br/handle/123456789/49359 2026-02-23T00:00:00Z https://repositorio.ufu.br/handle/123456789/48507 Title: Análise da deflexão gravitacional da luz em ocultações estelares Abstract: This dissertation investigates the astrometric effects of the gravitational deflection of light during stellar occultations, with emphasis on the simultaneous contribution of multiple bodies in the Solar System. The main objective is to evaluate how these effects contribute to the apparent position of the occulting body under different occultation geometries and to what extent they may affect high-precision astrometric measurements. To this end, the relativistic model of Klioner, with microarcsecond-level accuracy, is employed, from which numerical routines are implemented to compute the deflection vectors associated with different massive bodies. The methodology combines theoretical analysis of the gravitational effect, computational implementation supported by the SORA package, and real case studies. Initially, the fundamental concepts of stellar occultations are presented, including how astrometric positions are determined and which factors constrain their accuracy, such as observation quality, number of chords, and temporal coverage of the event. The Klioner light-deflection model is then described, with emphasis on the specific expressions and approximations used in this work. Based on this, a computational approach is developed to adapt the model to the context of stellar occultations, allowing the evaluation of angular separations, deflection angles, and offsets in right ascension and declination, as well as the combined contribution of each massive body. Validation is carried out through a case study of the stellar occultation by Ganymede on 21 December 2020, which is used as a reference to analyze the behavior of the corrections arising from the proximity of the Sun, Jupiter, and Saturn at that moment. The results show that, although the gravitational deflection due to multiple bodies often remains below the positional uncertainties of many events, its magnitude is comparable to the precision achieved in the best recorded occultations, becoming significant when angular separations are small or when the event has a high signal-to-noise ratio. The analysis demonstrates that the event geometry, the alignment between the star, observer, and deflecting bodies, and the uncertainties in the ephemerides are decisive in determining the final magnitude of the correction. It is concluded that the systematic consideration of gravitational light deflection by multiple bodies improves the astrometric consistency of stellar occultations and may contribute to the refinement of reference frames, especially in future observational scenarios with higher instrumental precision. Wed, 18 Feb 2026 00:00:00 GMT https://repositorio.ufu.br/handle/123456789/48507 2026-02-18T00:00:00Z