Papers by Author: Uilame Umbelino Gomes

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Authors: A.C. Buriti, A.G.P. Silva, Uilame Umbelino Gomes
Authors: Uilame Umbelino Gomes, C. de Souza, Francisca de Fatima P. Medeiros, A.G.P. Silva, C. Ciaravino, M. Roubin
Authors: Marcio W.D. Mendes, António Carlos P. Santos, Francisca de Fatima P. Medeiros, Clodomiro Alves Jr., A.G.P. Silva, Uilame Umbelino Gomes
Abstract: The aluminothermic reduction is a highly exothermal reaction between a metal oxide and aluminium. Conventionally this reaction is ignited by an electric resistance and the reaction products after cooling are in the form of a rigid block of mixed metal and aluminium oxide. In this work a new process of aluminothermic reduction is presented, in which the reaction is ignited by a hydrogen plasma. The niobium oxide and aluminium powders are high energy milled for six hours to form particles constituted of oxide and aluminum. Stoichiometric, substoichiometric and superstoichiometric mixtures were prepared. The mixture was placed in a stainless steel tube (the hollow cathode) inside the reactor chamber. The chamber was firstly evacuated. Then hydrogen at low pressure was introduced. In the following an electric discharge between the cathode and the anode localized just above the cathode ignites the plasma. The plasma heats the particles on the surface of the powder layer and starts the reaction that proceeds in each particle since the reactants are intimately mixed. The heat generated by the reaction propagates deeper in the layer until the whole mixture reacts. Substoichiometric mixtures can be used because hydrogen takes part of the reduction. The Nb2O5 – Al starting powder mixture and the products of the reaction are characterized by laser grain size measurement and X-Ray diffraction (XRD). The products are in form of powder or agglomerates of particles. Phases of reaction products was determined by XRD analysis and the particle size trough SEM.
Authors: Uilame Umbelino Gomes, Wilson Acchar, E.C.S. Tavares, N.F. Silva
Authors: J.B. Manuel, J.H. de Araújo, Franciné Alves Costa, Harim Revoredo de Macedo, Uilame Umbelino Gomes, A.G.P. Silva
Abstract: Cemented tungsten carbides were produced by liquid-phase sintering. In these work high energy milling (HEM) was used to produce homogeneous and finely grained powder mixtures. The milling effect on the magnetic properties of sintered samples is studied. Different mixtures in same composition (WC-10wt.%Co) were prepared by conventional mixture technique and wet HEM up to 300 hs in Planetary Mill. Magnetic hysteresis measurements on the sintered samples detected a significant increase in the coercitive field and a decrease on the saturation magnetization with milling time increasing. X-ray diffractogram show phase transformations with milling time. The Magnetic properties are correlated with phase relations and microstructural properties of the sintered samples.
Authors: Uilame Umbelino Gomes, L.A. Oliveira, S.R.S. Soares, M. Furukava, C.P. Souza
Abstract: Sintered stainless steel has a wide range of applications mainly in the automotive industry. Properties such as wear resistance, density and hardness can be improved by addition of nanosized particles of refractory carbides. The present study compares the behavior of the sintering and hardness of stainless steel samples reinforced with NbC or TaC (particles size less than 20 nm) synthesized at UFRN. The main aim of this work was to identify the effect of the particle size and dispersion of different refractory carbides in the hardness and sintered microstructure. The samples were sintered in a vacuum furnace. The heating rate, sintering temperature and times were 20°C/min, 1290°C and 30, 60 min respectively. We have been able to produce compacts with a relative density among 95.0%. The hardness values obtained were 140 HV for the reinforced sample and 76 HV for the sample without reinforcement.
Authors: G.B. Pinto, S.R.S. Soares, Uilame Umbelino Gomes, Rubens Maribondo Nascimento, Antonio Eduardo Martinelli, José F. Silva Jr
Abstract: High-energy milling has been used for production of nano-structured WC-Co powders. During the High-Energy Milling, the powders suffer severe high-energy impacts in the process of ball-to-ball and ball-to-vial wall collisions of the grinding media. Hard metal produced from nanostructured powders have better mechanical properties after appropriate sintering process. During the milling the particles size of WC and Co can be reduced and plastic deformed. In the present work, a mixture of WC-10%Co was produced by high energy milling. The starting powders of the WC (0.87 μm - Wolfran Bergau) and Co (0.93 μm - H.C.Starck) were used to produce the hard metal. The influence of the milling time on the particle size distributions and in the lattice strain was investigated. Milling time of the 2, 10, 20, 50, 70, 100 and 150 hours were used. The powders after milling were characterized by X-ray diffraction (XRD) and Scanning Electronic Microscopy (SEM). The results show that 10 h milling were enough to reduce the crystallite size of WC and the increase of the milling time reduces the crystallite size.
Authors: C.M.F. Gomes, L.C.O. Santos, A.G.P. Silva, Uilame Umbelino Gomes, J.N.F. Holanda
Abstract: This work presents the results of a study concerning the influence of the addition of rareearth elements (La2O3 and CeO2) on the sintering of the WC/10Co cemented carbide. Several WC/10Co mixtures containing up to 3 wt.% rare-earth of the cobalt phase were prepared. Specimens were uniaxially pressed at 200 MPa, and sintered in a vacuum furnace at 1400 °C during 60 minutes. The sintering behaviour was accompanied by the linear shrinkage, density, and mechanical strength. The development of the microstructure was followed by XRD and SEM. The results showed that the sintering behaviour of the WC/10Co cemented carbide was influenced by adding of rare-earth element. In addition, the lanthanium oxide addition was more effective on the improvement of the physical-mechanical properties of the studied carbide.
Authors: José Cosme Cunha Gomes, C.P. Souza, Uilame Umbelino Gomes, Jean R. Gavarri, Jean P. Dallas, C. Leroux
Abstract: Rare earth oxides have been widely investigated in catalysis as structured and electronic promoters to improve the activity and thermal stability of catalysts. Cerium has an important role in three-way catalysis and fluid catalytic cracking, two significant catalytic processes by their economic relevance and tonnage. Cerium and other rare earths have been studied as possible heterogeneous catalysts at selective oxidation of hydrocarbons. Cerite and monazite are minerals with high concentration of cerium element. Extraction of cerium metal using conventional leaching processes has shown low yields or high costs. The main purpose of this research work is to optimize the parameters in cerium purification stage from this mineral using leaching process. To separate particles with different granulometries, the mineral is ground and fractioned with sieves of 80, 200, 250 and 400 mesh. In order to put off organic components and oxidize cerium(III) to cerium(IV), samples were roasted at 1073K by twenty-four hours. The roasted samples were solubilized by acid attack (leaching) for approximately twenty-four hours; according to the acid used hydrochloric or sulfuric), cerium and other trivalent elements are solubilized as chloride or sulfate solution. Cerium was extracted by selective precipitation at pH~3,4 using ammonium or natrium hydroxide as pH changer. After filtration and drying, the precipitated product was characterized by XRD (x-ray diffraction), and then process efficiency was determined (cerium percentage and the different phases in the powder). Particles granulometry, roasting process (time and temperature), as well as leaching parameters (acid used, time, temperature and concentration of reagents) were the main variables studied.
Authors: E.C.S. Tavares, Franciné Alves Costa, Uilame Umbelino Gomes, Wilson Acchar
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