Papers by Author: Marcelo R. Esquivel

Abstract: Mechanical milling (MM) and a combination of MM and temperature heating were used to synthesize the intermetallic MmNi5 and a MmNi5-Ni mixture, respectively. The process evolution with time was analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD). SEM was used to characterize size and morphology of particles in order to reveal the governing mechanisms during the milling of each system. X-ray diffraction (XRD) was utilized to determine strain and crystallite size changes introduced by milling. The effects of integrated milling time on the sorption properties of the MmNi5-Ni mixture were analyzed. Results were correlated to defects introduced into microstructure. A comparison of the final characteristics of alloys obtained in each case is also shown.

Abstract: Structure, microstructure and hydriding properties of mechanically alloyed 2Mg-Ni mixture were investigated. Two different nanocomposites were synthesized by mechanical alloying (MA) in a low-energy planetary mill, namely MN100 (100 h of milling) and MN200 (200 h of milling). The formation of nanocrystalline Mg2Ni was detected as a function of the milling time. An appropriate combination of MA plus annealing under mild conditions accomplishes the complete formation of Mg2Ni phase. The pressure-composition isotherms of the two samples reveal different hydrogen storage capacities and plateau slopes. In addition, the low temperature Mg2NiH4 (LT) formed by hydriding/cooling of MN100 h decomposes at 190 °C, whereas this hydride produced from MN200 first transforms to the high temperature Mg2NiH4 and then decomposes near 245 °C. The differences in the hydriding/dehydriding properties of MN100 and MN200 were associated with the microstructure and structure of the phases formed during MA followed by heating under argon/hydrogen.