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Author Teixeira da Silva, V. L. S. ♦ Schmal, M. ♦ Oyama, S. T.
Source United States Department of Energy Office of Scientific and Technical Information
Content type Text
Language English
Subject Keyword MATERIALS SCIENCE ♦ NIOBIUM CARBIDES ♦ CHEMICAL PREPARATION ♦ CHEMICAL REACTION KINETICS ♦ REACTION INTERMEDIATES ♦ REDUCTION ♦ ACTIVATION ENERGY ♦ SCANNING ELECTRON MICROSCOPY ♦ MASS SPECTROSCOPY ♦ X-RAY DIFFRACTION ♦ X-RAY SPECTROSCOPY
Abstract The carburization of B-niobium oxide (B-Nb{sub 2}O{sub 5}) to niobium carbide (NbC) in 20% (v/v)CH{sub 4}/H{sub 2} was studied at temperature-programmed conditions, the reaction required high temperatures, greater than 1370K, and variations of heating rate (0.04-0.17 K s{sup {minus}1}) and molar space velocity (400-1600 h{sup {minus}1}) had only a minor effect on the product specific surface are (S{sub g}). In the course of the transformation S{sub g} increased from 1 m{sup 2}g{sup {minus}1} to about 20 m{sup 2}g{sup {minus}1}, and scanning electron microscopy showed the development of macropores of about 100 nm. The progress of the reaction was followed by mass spectroscopic analysis of the gaseous products, which identified two distinct stages. X-ray diffraction analysis of reaction intermediates showed that in the first stage B-Nb{sub 2}O{sub 5} was reduced to NbO{sub 2}, and in the second stage NbO{sub 2} was simultaneously reduced and carburized to NbC. The first reduction occurred by a nucleation mechanism with an activation energy of 100 kJ mol{sup {minus}1}. Independent experiments with NbO indicated that it was not involved in the reaction pathway. However, X-ray photoelectron spectroscopy revealed the presence of an oxycarbide phase which was probably the intermediate in the final transformation. The oxycarbide phase transformed rapidly to the product NbC and was not observable as a bulk phase by XRD.
ISSN 00224596
Educational Use Research
Learning Resource Type Article
Publisher Date 1996-04-01
Publisher Place United States
Journal Journal of Solid State Chemistry
Volume Number 123
Issue Number 1


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