Papers by Author: Francisca de Fatima P. Medeiros

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.

Abstract: The reaction of oxalic precursor {(NH4)3[NbO(C2O4)3]H2O}, was prepared from the Nb2O5, to niobium carbide (NbC) were performed in a rotating cylinder reactor scale-lab designed to niobium carbide powders synthesis at low temperature (1173 K). The NbC was prepared by a oxalic precursor and as well of commercial niobium pentoxide under flowing CH4- H2 mixtures. The reactor was heated through a bi-partied electric furnace with programmable temperature. The reaction overall time was determined from the curve of methane absorption evolution by gas chromatograph (FID) analysis. The preliminary results showed that the conversion function depends on the rotation, temperature, mixture flow, CH4/H2 ratio and heating rate. The niobium carbide was characterized through X-ray diffraction and compared to the commercial products. The reaction of oxalic precursor {(NH4)3[NbO(C2O4)3]H2O} to niobium carbide (NbC) in 3% (v/v) CH4/H2 yielded smallest grain size as well smaller overall time when compared with obtained direct by commercial Nb2O5, however it had small mass conversion due the solid carryover not controlled.