Study of the evolution of the properties of a MmNi5-Ni mixture obtained by mechanical milling

M.R. ESQUIVEL; G. MEYER

Santa Bárbara-California-USA

Conferencia; International Symposium on Materials Issues on Hydrogen Production and Storage; 2006

International Center for Materials Research University of California in Santa Barbara

Mechanical milling has become one of the most successful methods for synthesis of alloys. Easy scaling up and low processing cost are mentioned as main advantages over both full equilibrium and chemical synthesis methods. This technique is specially appropriated to obtain alloys that will be used to interact with hydrogen because it produces defects and strain in the alloy microstructure that enhance the reaction. Nevertheless, the effects of mechanical milling on both the microstructure and hydrogen interaction properties on most hydride forming materials are not fully understood. A mixture of MmNi5-Ni obtained by a combination of mechanical milling and heating process was treated in a Uni-Ball-II apparatus. Chamber was opened at fixed milling times and samples were withdrawn in a glove box under Ar atmosphere. O2 level was maintained below a 5 ppm level to avoid sample oxidation. In this work, the effect of mechanical milling on both the structural properties and first hydrogen absorption – desorption curves of a MmNi5-Ni mixture is analyzed. Hydrogen-alloy interaction was characterized using a Sievert´s type equipment designed in our laboratory. Absorption – desorption curves were obtained at room temperature, 50 ºC and 90 ºC. From their analysis the main processes governing hydrogen absorption – desorption were obtained. Scanning electron microscopy (SEM) was used to study the particle size and morphology changes during the evolution of mechanical milling. Brittle fracture and cold welding were the processes governing at times shorter than 20 h and times longer than 30 h of milling, respectively. Equilibrium is reached between both processes between 20 and 30 h, respectively. Phase identity, crystallite size and strain of both MmNi5 and Ni were obtained using X-ray diffraction (XRD). These changes on structural parameters were correlated to the effect of milling time at short and long times and governing mechanisms during mechanical milling. Results from hydriding – dehydriding curves were also correlated to changes on microstructure during sample preparation.5-Ni obtained by a combination of mechanical milling and heating process was treated in a Uni-Ball-II apparatus. Chamber was opened at fixed milling times and samples were withdrawn in a glove box under Ar atmosphere. O2 level was maintained below a 5 ppm level to avoid sample oxidation. In this work, the effect of mechanical milling on both the structural properties and first hydrogen absorption – desorption curves of a MmNi5-Ni mixture is analyzed. Hydrogen-alloy interaction was characterized using a Sievert´s type equipment designed in our laboratory. Absorption – desorption curves were obtained at room temperature, 50 ºC and 90 ºC. From their analysis the main processes governing hydrogen absorption – desorption were obtained. Scanning electron microscopy (SEM) was used to study the particle size and morphology changes during the evolution of mechanical milling. Brittle fracture and cold welding were the processes governing at times shorter than 20 h and times longer than 30 h of milling, respectively. Equilibrium is reached between both processes between 20 and 30 h, respectively. Phase identity, crystallite size and strain of both MmNi5 and Ni were obtained using X-ray diffraction (XRD). These changes on structural parameters were correlated to the effect of milling time at short and long times and governing mechanisms during mechanical milling. Results from hydriding – dehydriding curves were also correlated to changes on microstructure during sample preparation.5-Ni mixture is analyzed. Hydrogen-alloy interaction was characterized using a Sievert´s type equipment designed in our laboratory. Absorption – desorption curves were obtained at room temperature, 50 ºC and 90 ºC. From their analysis the main processes governing hydrogen absorption – desorption were obtained. Scanning electron microscopy (SEM) was used to study the particle size and morphology changes during the evolution of mechanical milling. Brittle fracture and cold welding were the processes governing at times shorter than 20 h and times longer than 30 h of milling, respectively. Equilibrium is reached between both processes between 20 and 30 h, respectively. Phase identity, crystallite size and strain of both MmNi5 and Ni were obtained using X-ray diffraction (XRD). These changes on structural parameters were correlated to the effect of milling time at short and long times and governing mechanisms during mechanical milling. Results from hydriding – dehydriding curves were also correlated to changes on microstructure during sample preparation.5 and Ni were obtained using X-ray diffraction (XRD). These changes on structural parameters were correlated to the effect of milling time at short and long times and governing mechanisms during mechanical milling. Results from hydriding – dehydriding curves were also correlated to changes on microstructure during sample preparation.