INIFTA   05425
INSTITUTO DE INVESTIGACIONES FISICO-QUIMICAS TEORICAS Y APLICADAS
Unidad Ejecutora - UE
artículos
Título:
Autocatalytic effects in the mechanically induced hydriding of refractory metals
Autor/es:
M. A. BAB; L. A. BAUM; L. A. MENDOZA-ZELIS
Revista:
PHYSICA B - CONDENSED MATTER
Editorial:
Elsevier Science Ltd.
Referencias:
Año: 2007 vol. 389 p. 193 - 197
ISSN:
0921-4526
Resumen:
Mechanical milling of powders in a gaseous ambient is an ef.cient tool to induce gas–solid reactions and several nanocrystalline metallic oxides, nitrides and hydrides may be obtained at room temperature and moderated pressures by this route. We present here a study of the mechanically induced hydriding of Ti, Zr and Hf elemental powders, ground in an oscillatory mill under hydrogen gas at constant volume. The .nal formation of nanocrystalline cubic d-MH2, together with a varying fraction of tetragonal e-MH2, was veri.ed by X-ray diffraction. From the measured pressure drop during the milling process the hydriding kinetics was determined and two distinct regimes were observed. For Ti and—at low milling frequencies—for Zr and Hf, a normal regime, characterized by a sigmoidal trend and a linear dependence of the rate constant on the milling intensity, was found. Otherwise, a sudden increase in the reacted fraction was observed in the absorption curve, typical of a self-sustained regime. The magnitude of the observed jump increases with the milling intensity and afterwards the reaction proceeds normally until it is completed. This critical behavior is discussed in terms of the reaction rate, the degree of transformation and the e-phase content. by X-ray diffraction. From the measured pressure drop during the milling process the hydriding kinetics was determined and two distinct regimes were observed. For Ti and—at low milling frequencies—for Zr and Hf, a normal regime, characterized by a sigmoidal trend and a linear dependence of the rate constant on the milling intensity, was found. Otherwise, a sudden increase in the reacted fraction was observed in the absorption curve, typical of a self-sustained regime. The magnitude of the observed jump increases with the milling intensity and afterwards the reaction proceeds normally until it is completed. This critical behavior is discussed in terms of the reaction rate, the degree of transformation and the e-phase content.