http://repositorio.ufc.br/handle/riufc/500 2026-06-11T09:31:07Z http://repositorio.ufc.br/handle/riufc/86596 Título: Synthesis and characterization of multi-spectral luminescent nanocrystals based on NaYF4 and NaGdF4 Autor(es): Resquin, Lucas Santiago Medina Abstract: Multispectral luminescent nanoparticles based on hexagonal-phase (β-) NaYF4 and NaGdF4 ma- trices, doped with rare-earth (RE) ions (Yb3+ /Er3+ /Tm3+ /Eu3+ ) and transition metals (Fe3+ /Mn2+ ), were successfully synthesized via the thermal decomposition method. The structural characteri- zation by X-ray diffraction (XRD) confirmed the formation of highly crystalline, single-phase hexagonal structures with lattice parameters consistent with the reference ICSD patterns. Fourier- transform infrared (FTIR) spectroscopy verified the effective surface capping of the nanoparticles with oleic acid (OA) ligands. The optical properties were evaluated using ultraviolet-visible (UV-Vis) absorption and photoluminescence (PL) spectroscopy. Under near-infrared (976 nm) excitation, efficient upconversion (UC) emission was observed, mediated by Yb3+ sensitization and energy transfer to Er3+ /Tm3+ activators. Core@shell architectures exhibited a significant enhancement in UC intensity compared to core-only structures, demonstrating the effectiveness of the inert shell in suppressing surface-related quenching. Under ultraviolet (365 nm) excitation, downconversion (DC) luminescence was dominated by characteristic emissions from Eu3+ , Er3+ , and Tm3+ , with OA acting simultaneously as a surface ligand, sensitizer, and blue emitter. The introduction of transition metals (5% Fe3+ or Mn2+ ) into the core of NaGdF4-based nanoparticles induced distinct modifications: Mn2+ doping enhanced the relative intensity of the (110) diffrac- tion peak and promoted resonant energy transfer that intensified red emission from Er3+ . The core@shell design effectively mitigated the non-radiative quenching typically associated with Mn2+ doping in core-only systems. These findings demonstrate that transition-metal codoping, combined with core@shell engineering, provides a viable route to tune energy-transfer pathways and emission profiles in RE-doped nanostructures. Tipo: Dissertação 2026-01-01T00:00:00Z http://repositorio.ufc.br/handle/riufc/86409 Título: Previsão computacional do teor de silício no ferro-gusa Autor(es): Xavier, João Victor Barroso Abstract: The silicon content of pig iron is one of the main indicators of its quality and the thermal state of the blast furnace. Furthermore, a high silicon content can damage industrial equipment, leading to the need for maintenance and, consequently, a loss of process efficiency. For these reasons, several studies have been developed over decades to predict and monitor the silicon content in pig iron, suggesting the use of data-driven models. In this context, this work tested models such as the logistic perceptron, multilayer perceptron artificial neural networks with up to two hidden layers, and different versions of support vector machines adapted for regression (SVR, TSVR, LSSVR) to predict the silicon content in pig iron. A technique for estimating the number of hidden neurons in neural networks based on singular value decomposition (SVD) was also investigated to reduce tuning time and computational cost. Among the neuron-based models, the neural network with one hidden layer presented the best balance between performance and computational cost, while the SVD-based technique provided a smaller hyperparameter testing window, therefore, it was used in a sensitivity analysis to study the influence of each input variable on the silicon content in pig iron. Tipo: Dissertação 2026-01-01T00:00:00Z http://repositorio.ufc.br/handle/riufc/85985 Título: Desenvolvimento do fio-máquina com o aço 1010 para produção de vergalhão CA60 em laminadores a frio Autor(es): Gondim Júnior, Irannildo Walter Abstract: Today's steel market is increasingly competitive, where every extra minute of production is a huge gain in financial results. In 2022, a steel mill in Ceará used 5.5mm 1008 steel wire rod to produce 3.40mm CA60 rebars and 5.5mm 1012 steel wire rod to produce 4.20mm CA60 rebars, two different raw materials for two different products, However, these materials presented opportunities, such as difficulties in meeting the property requirements of the Brazilian standard ABNT NBR 7480, which sets out the minimum requirements for rebar, so there was a need to develop a new technology for producing this material. The aim of this work is to develop a wire rod in 1010 steel so that it is suitable for the hot rolling process and then the cold rolling process, so that during the hardening process it achieves the desired properties, significantly reducing the generation of non-conforming products, as well as having a single steel that would produce both 3.40mm and 4.20mm CA60 rebar. This development was carried out by producing a pilot batch of 30 tonnes of SAE 1010 steel, where three temperature values were tested during hot rolling and then subjected to the hardening process (cold rolling), reducing its section to 4.20mm and 3.40mm. This material was validated by means of tensile tests carried out on a calibrated machine using an extensometer, as well as characterising the material for each temperature parameter by measuring its grain size, which is a determining factor for characteristics such as the yield strength. The results showed that the higher the hot rolling temperature, the larger the grain size, which consequently improves the values of the material's elastic ratio, which is the breaking limit divided by the yield limit, a parameter that is normative and which is very difficult to achieve using different steels, especially 1012, thus showing that the hot rolling temperature is a determining factor for the cold rolling process, favouring the achievement of the appropriate elastic ratio for the product Tipo: Dissertação 2025-01-01T00:00:00Z http://repositorio.ufc.br/handle/riufc/85303 Título: Thermomechanical modeling and simulation of an electric arc welding process: a computacional analysis of the influence of the element birth and death techinique Autor(es): Costa, Mateus Andrade de Sousa Abstract: Arc welding plays a vital role in modern industry, particularly in the fabrication of complex structures and components. While the process is widely employed and relatively straightforward, it is not without challenges. Welding can introduce residual stresses, distortions, and defects into the material, potentially compromising structural integrity, increasing costs, and necessitating rework. To address these challenges, this study conducts a detailed thermomechanical analysis of the welding process, exploring computational modeling as a viable alternative to traditional experimental methods. The research employs ANSYS® software and the Finite Element Method (FEM) to simulate the thermal and mechanical behavior of materials under specific welding conditions. The study focuses on single-pass butt welding of plates and pipes, incorporating filler metals and utilizing the Element Birth and Death Technique (EBDT) to simulate material addition during welding. By examining different mesh element orders (linear and quadratic), the analysis evaluates temperature distributions, residual stresses, and deformations while also considering computational efficiency. The results revealed that EBDT increased test duration by 16.5%, and a significantly temperature difference (665ºC with EBDT vs. 348ºC without) during heat source passage. Mesh element order notably influenced outcomes: quadratic elements reduced the element count by 85%, balancing accuracy and computational efficiency, while refined linear elements increased execution time by up to 90%. For the heat-affected zone (HAZ), simulations without EBDT may suffice, but weld bead analysis requires further validation, particularly in multi-pass welding, where reheating alters stress distributions. The methodology was validated through comparisons with analytical, numerical, and experimental data available in the literature, confirming its robustness for structural analysis and process optimization. This research contributes to the theoretical understanding of arc welding, offering a computational framework that effectively balances precision and cost. The study lays a strong foundation for future investigations, especially in exploring the complexities of multi-pass welding. Tipo: Dissertação 2025-01-01T00:00:00Z