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Bounds on the critical parameter for the frog model on trees
(Universidade Federal de São Carlos, 2024-07-18) Pena, Caio Augusto de Carvalho; Gallo, Alexsandro Giacomo Grimbert; https://lattes.cnpq.br/4037274656833325; https://lattes.cnpq.br/8701983533033982
We consider the frog model with a lifetime on infinite trees. In this model, frogs (particles), when awake, perform a symmetric random walk on the tree, waking up all dormant frogs at the sites visited until they die. We consider variations of the model by changing the structure of the tree (oriented or not, random or not) and the survival distribution. In these models, the survival probability of a frog is controlled by a parameter $ p \in [0,1] $, and there is a critical value $ p_c $ such that if $ p < p_c $, then only finitely many frogs are awakened with probability 1, while if $ p > p_c $, infinitely many frogs are awakened with positive probability. The thesis is dedicated to obtaining lower and/or upper bounds for this critical parameter as a function of structural constants of the considered models.
Soldagem de tubos centrifugados brutos de fundição e carburizados da liga 35Cr45NiNb: microestrutura e propriedades mecânica em tração e fluência
(Universidade Federal de São Carlos, 2024-07-18) Corsi, Waleska Laisa; Zepon, Guilherme; https://lattes.cnpq.br/7924187202036614; Koga, Guilherme Yuuki; https://lattes.cnpq.br/0933095081175012; https://lattes.cnpq.br/4837694063559557; Rovere, Carlos Alberto Della; Higa, Silvia Midori; https://lattes.cnpq.br/8141224513975606; https://lattes.cnpq.br/7245298285401456
The 35Cr45NiNb alloy is widely used in hydrocarbon cracking furnaces due to its excellent resistance to oxidation and creep. However, this material is susceptible to carburization, which causes deterioration of mechanical properties and makes it necessary to replace the carburized sections. The remaining tubes in the equipment are eventually in an aged and/or carburized condition, therefore, they have low weldability, making it difficult to join new tubes, and compromising the mechanical performance of the weld. The objective of this work was to evaluate the effects of the welding process on the mechanical properties in tensile and creep of as cast and carburized 35Cr45NiNb tubes. The joining of the as cast segments (B) and tubes with approximately six years of use (C) was carried out using the Gas Tungsten Arc Welding (GTAW) technique, resulting in the joints B-B, B-C and C-C. The micrographs of the weld joint obtained by scanning electron microscopy (SEM) showed the growth of precipitates in the heat affected zones (HAZs), mainly in those located between segments C and the weld metal (WM), which acquired sizes relatively larger than those of the base metal (BM). These microstructural heterogeneities, combined with those pre-existing in the BM due to aging and carburization, caused a gradual reduction in the tensile strength and elongation at room temperature, as well as a decrease in the creep failure time (CFT) at 1100 ºC and 16 MPa.
Minimalidade de coincidências do tipo Borsuk-Ulam em superfícies utilizando tranças
(Universidade Federal de São Carlos, 2024-07-18) Silva, Caio Lima; Laass, Vinicius Casteluber; https://lattes.cnpq.br/8470339884809920; https://orcid.org/0000-0002-8796-9599; Vendrúscolo, Daniel; https://lattes.cnpq.br/8602232587914830; https://lattes.cnpq.br/3781924226187088; https://orcid.org/0009-0001-8697-9808
Given topological spaces $M$ and $N$, with $M$ equipped with a free involution $\tau$, we define the set of Borsuk-Ulam coincidences as $$ \operatorname{BUCoin}(f; \tau) = \{ \{ x, \tau(x) \} \ | \ f(x) = f(\tau(x)) \}. $$ In this work, we want to exhibit $g \colon M \to N$ homotopic to $f$ such that it minimizes the cardinality of set of Borsuk-Ulam coincidences. In the cases where spaces are surfaces, we will show an algebraic technique involving braid groups of surface that is useful to study such a problem. This technique also displays the index of each Borsuk-Ulam coincidence, seen as coincidences classes. We will give an explicit answer to the problem when both spaces are Klein bottles. Note: Due to special Character restrictions, please check the abstract in the full text available for download.
Análise de meios filtrantes em filtros de mangas do processo de dessulfuração da siderurgia
(Universidade Federal de São Carlos, 2024-07-18) Vieira, Ana Cristina Coelho; Sartim, Rafael; https://lattes.cnpq.br/0380809249444032; Aguiar, Mônica Lopes; https://lattes.cnpq.br/0431688649128529; https://orcid.org/0000-0003-4540-5776; https://lattes.cnpq.br/7910753759516014; https://orcid.org/0009-0000-1569-0196
The emissions of ultrafine particles in the air are considered a global problem. The increase is linked to economic development and extensive urban expansion. These particles are inhalable, and exposure to them can cause various diseases or even lead to death. As a result, there has been a need to develop and optimize methods and equipment to control their emissions. In this context, bag filters stand out as one of the most commonly used filters due to their high filtration efficiency and stability. However, the filter medium used directly affects filtration efficiency, and how the filter medium is manufactured—such as needle felt and hydroentangled felt—also impacts efficiency and lifespan. Given the existing gap in the literature regarding the influence of filter medium manufacturing techniques on filtration efficiency and their impact in industrial settings, the objective of this research is to analyze and compare the filtration performance of filter media manufactured using needle felt and hydroentangled felt techniques, both virgin and with 3- and 4-year usage, in the bag filter system of the desulfurization process in the steel industry. The particulate material and filter media used were characterized to understand their behavior during filtration tests. Filtration tests were conducted using a test filter based on the VDI 3926 standard, with pulse-jet cleaning, evaluating collection efficiency and pressure drop evolution over 30 filtration cycles. The results of particulate material characterization showed that it is predominantly composed of calcium and iron, with a median aerodynamic diameter of 2.64 µm. The hydroentangled filter medium, along with a larger fiber diameter, exhibited lower thickness and permeability compared to the needle felt. Consequently, the penetration of particulate material into the hydroentangled medium was lower, facilitating the cleaning process of the filter bags and increasing filtration efficiency. After 30 filtration cycles, the used hydroentangled filter medium achieved an efficiency of approximately 99.9%, while the needle-felt filter medium reached approximately 99.7%. Regarding lifespan over the years, the 4-year-old filter bags showed greater residual pressure drop than the 3-year-old bags, as expected due to particle accumulation in the filter medium over time. However, the increase in residual pressure drop was smaller for the hydroentangled bags than for the needle-felt bags, resulting in a longer lifespan. Therefore, under the analyzed conditions, the hydroentangled filter medium demonstrated greater effectiveness, with a longer lifespan and lower residual pressure drop during filtration.
Redução de CO2 a partir de reações de catálise homogênea com complexos bimetálicos de cobalto
(Universidade Federal de São Carlos, 2024-07-18) Santos, Mayara Fontanelli dos; Marques Netto, Caterina Gruenwaldt Cunha; https://lattes.cnpq.br/8042236152815373; https://orcid.org/0000-0002-3285-4129; https://lattes.cnpq.br/9422151542979888
This work addresses a topic of significant contemporary relevance: the growing concern about pollution caused by greenhouse gases, which exacerbate global warming and consequently result in extreme weather events in various regions worldwide. This study presents important data on homogeneous catalysis reactions and CO2 electroreduction, aiming to produce formaldehyde and methanol—compounds that are highly useful in the industry—using a bimetallic cobalt (II) complex as a catalyst. The research delves into the analysis of metallic complexes, providing detailed information on the coordination between the metal and the ligands, and justifying the choice of cobalt as the catalytic metal. In the context of CO2 reduction, different types of catalysis are explored, including photocatalysis and electrocatalysis, as well as the three main forms of CO2 capture: pre-combustion, post combustion, and direct air capture. By comparing with complexes reported in the literature, it is concluded that the CO2 reduction process using a bimetallic cobalt (II) complex is feasible. The presence of two metals in the complex offers significant advantages, enhancing the selectivity and efficiency of the catalytic process and the presence of the ligand can affect the stability.