Carlson Pereira de Souza

Niobium carbide synthesis by solid-gas reaction using a rotating cylinder reactor

A rotating cylinder reactor was designed for the synthesis of niobium carbide powders at 1173 K. Niobium carbide, NbC, was prepared by carbothermal reduction starting from commercial niobium pentoxide powders. The reactor was heated using a custom-made, two-part, hinged, electric furnace with programmable temperature control. The design and operational details of the reactor are presented. The longitudinal temperature gradient inside the reactor was determined. Total reaction time was monitored by a gas chromatograph equipped with an FID detector for determination of methane concentrations. The results show that time of reaction depended on rotation speed. NbC was also prepared in a static-bed alumina reactor using the same conditions as in the previous case. The niobium carbide powders were characterized by X-ray diffraction and compared with commercially available products. Morphological, particle size distribution and surface area analyses were obtained using SEM, LDPS and BET, respectively. Therefore, the present study offers a significant technological contribution to the synthesis of NbC powders in a rotating cylinder reactor.

Efeito da variação de pH na síntese e nas propriedades de BaCe0,2Pr0,8O3 obtido pelo método de complexação EDTA-citrato

Barium cerate (BaCeO3) is a type ABO3 perovskite, with A and B metal cations. Such materials, single or doped, have been studied as promising for the application in solid oxide fuel cell, and hydrogen and oxygen permeation; however, as these materials have been produced by different synthesis methods, some conditions directly influence the final properties, with a major doping site B, which may have direct influence on the crystallite size and, consequently, in the ionic and electronic conduction. This work aimed at the partial substitution of cerium for 80% praseodymium by the EDTA-citrate complexing method to verify the influence of the pH of the synthesis solution on obtaining the phase, the crystallographic and morphological properties. It was found that the material has orthorhombic crystal structure with space group Pmcn and the phase was obtained irrespective of change of the pH of the synthesis, indicating possibly that the metal cations in the perovskite were able to deprotonate the EDTA increasing the stability of the complexes formed. With the synthesis pH adjusted to 5 the crystallite size reached the highest value, 352.7 nm. This increase was probably due to insufficient time in which the sample was subjected to the action of synthesis temperature until the formation of the final gel, resulting in reduced breakdown of agglomerates formed in the complexation reaction , which can be seen in the results of scanning electron microscopy.

Synthesis and Characterization of CuNb2O6 from an Oxalic Precursor Via Solid State Reaction

The present papers objective was to synthesize and characterize both the oxalic niobium precursor (NH4)3NbO(C2O4)3.H2O, and the product of its doping with Cu and calcining, CuNb2O6. In order to obtain the niobium precursor, commercially available niobium oxide (Nb2O5) was subject to fusion with potassium bisulfate (KHSO4). Once leached with water, the powder was complexed with oxalic acid and ammonium oxalate. The as produced material was manually mixed with copper nitrate Cu(NO3)2.2H2O (25% Cu molar) and calcined at 1000°C in a muffle furnace. CuNb2O6 was then obtained. The precursor was characterized by XRD, SEM, FT-IR, XRF, TG/DTG. The calcination product was characterized by XRD, XRF and SEM. Results show that single phase CuNb2O6 could be obtained by this method without Nb2O5 contamination.

Monoclinic superstructure in orthorhombic Ce10W22O81 from transmission electron microscopy.

A complex rare-earth tungstate structure, present in a two-phased powder, was solved by electron diffraction, precession and high-resolution electron microscopy. The orthorhombic space group Pbnm and the atomic positions deduced from X-ray diffraction experiments were confirmed for Ce10W22O81. A C2/c monoclinic superstructure, with cell parameters a = 7.8, b = 36.1, c = 22.2 Å and β = 100.2°, was shown and attributed to a partial oxidation of Ce(3+) leading to interstitial oxygen ions.

Monoclinic superstructure in the orthorhombic Ce10W22O81 from transmission electron microscopy

A complex rare earth tungstate structure, present in a two-phased powder, was solved by electron diffraction, precession and high resolution electron microscopy. The orthorhombic space group Pbnm and the atomic positions deduced from X-rays diffraction experiments were confirmed for Ce10W22O81. A C2/c monoclinic superstructure, with cell parameters a = 7.8 A, b = 36.1 A, c = 22.2 A and β = 100.2°, was evidenced and attributed to a partial oxidation of Ce3+ leading to interstitial oxygen ions.

Síntesis y Refinado por el Método Rietveld de la fase perovsquita obtenida a partir del Método de Co-precipitación vía Oxalato

Powders of perovskite SrCo0.8Fe0.2O3-δ were synthesized using oxalate co-precipitation method. According to the thermal gravimetry and differential thermal analysis (TG/DTA), total mass loss was 40 % corresponding to three different steps and the precipitant agent as well the starting chemical elements have no influence on the process. Perovskite phase was identified by x-ray diffraction and the average crystallite size was 77.22 nm using iron nitrate (III) as starting material while 42.23 nm were obtained using iron oxalate (III). Ammonium oxalate and equimolar mixtures of ammonium oxalate and oxalic acid were used as precipitant agents and the results shown that these reactants have no influence on the average crystallite size (42.95 nm and 42.23 nm). The material presents isotherms of type II, with especific surface of approximately 4,41m 2 g -1 .

Leaching Process of Cerium Extraction from Mixture of Cerite-Monazite Mineral

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.

Structural, Morphological and Textural Features of EDTA-Citrate-Synthesized BaPrO3 Perovskite-Type Oxide

BaCeO3 has attracted great interest in research worldwide, whether doped with other elements on the site A and / or B of perovskite ABO3 or not, thus enabling its use in different applications. However, the properties exhibited by perovskite depend on partial or total replacement at the site, as well as the applied synthesis method. In this work we studied crystal structure, crystallographic parameters, morphology and textural properties of BaPrO3 perovskite synthesized by EDTA-Citrate complexation process. The adopted methodology makes it possible to obtain powders with orthorhombic crystal structure ( a=6.07 Å; b=6.3 Å e c=8.7 Å) and crystallite size of approximately 100 nm. That perovskite has dense and irregular agglomerates with surface area and pore volume of about 2.37 m2 g-1 and 0.7x10-8m3g-1, respectively. Finally, the methodology has demonstrated effectiveness in achieving crystalline phase with similar characteristics and properties obtained by different methods.

Synthesis of Nanostructured Tungsten Carbide (WC) from Ammonia Paratungstate-APT and its Characterization by XRD and Rietveld Refinement

The carbides of refractory metals like tungsten carbide (WC), tantalum carbide (TaC) and niobium carbide (NbC), has been extensively studied due to their applications in several areas of industry, because of their specific properties; such as high melting point, high hardness, wear resistance, oxidation resistance and good electrical conductivity. The tungsten carbide, particularly, is generally used at hardmetal industries due to its high hardness and wear resistance. New synthesis techniques have been developed to reduce the synthesis temperature of refractory metal carbides using more reactive precursors and gas-solid reactions for carbon reduction. The result is producing pure carbides suitable properties for production of high quality cemented carbides and more selective catalysts. In this work, pure and nanostructured WC was obtained from the ammonium paratungstate hydrate (APT), at low temperature and short reaction time. Hydrogen (H2) and methane (CH4) were used as a reducing gas and carbon source, respectively. The precursor and obtained product were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results obtained by diffraction of X-rays showed that complete reduction and carburization of APT have been took place resulted in pure WC formation. The average crystallite size was in nanometer order reaching values of approximately 20.8 nm and a surface area (BET) of 26.9 m2/g.

NANOMETRIC TANTALUM CARBIDE: PRODUCTION AT LOW TEMPERATURE AND CHARACTERIZATION

Tantalum carbide (TaC) is an extremely hard material with features such as high hardness, high melting point, high chemical stability, good resistance to chemical and thermal shock and excellent resistance to oxidation and corrosion. Thus, this study aimed to obtain nanostructured TaC from precursor tris (oxalate) hydrate ammonium oxitantalato through gas-solid reaction at low temperature (1000°C) and short reaction time. The materials obtained were characterized by X-ray diffraction (XRD), Rietveld refinement, Scanning Electronic Microscopy (SEM), Infrared Spectroscopy and Thermogravimetric analysis (TG). Through XRD analyses and Reitiveld refinement of TaC with S = 1.1584, the formation of pure tantalum carbide with cubic structure and average crystallite size in the order of 12.5 nanometers was observed.