〈Original Papers〉Hydrogenation Properties of CaMg_2 Alloy Produced by Mechanical Grinding

publisher[注記]NOBUKI, Toru[synopsis] Recently, nano-structured materials receive special attention as a novel hydrogen storage medium because of potentially large storage capacity, safety, and fast filling and delivering from the fuel tank of fuel cell vehicles. Magnesium (Mg) hydride is considered as one of the most interesting alternatives for the reversible storage of hydrogen due to several unique advantages such as element abundance, economic production cost, easy handling as well as a high hydrogen storage capacity of 7.6 mass％. However the major problem of magnesium as a rechargeable hydrogen carrier system is its slow reaction rate and high sorption temperature. A breakthrough in hydrogen storage technology has been achieved by preparing nano-crystalline hydrides using high-energy ball milling. These new materials show very fast absorption and desorption kinetics at 300 degrees Celsius. However, desorption and absorption at low temperatures is still too slow, limiting technical applications. Overseas researchers have been extensively studied from the viewpoint of developing advanced Mg-based hydrogen storage materials. For example, binary compounds such as calcium dimagnesium (CaMg2) with C14 Laves phase alloy is expected to absorb hydrogen more than 6 mass％, when the H/M ratio reaches 2. This study aims to clarify and discuss the mechanism of hydriding process of nano-crystallized CaMg_2 alloy conducted by mechanical grinding (MG) process. The mechanically milled CaMg_2 alloy was characterized by microstructure observation and XRD analysis, TEM analysis and hydrogen absorbing capacity. The long term of mechanical grinding process has lead to amorphous phase of CaMg_2 alloy. The 2 hours of MG treated CaMg_2 alloy showed not only absorbing 1 mass％ of hydrogen at low temperature such as 200 degree Celsius, but also decomposed to Ca and Mg at temperature from 240 degree Celsius. The long term mechanical grinding process has gave CaMg_2 a lloy homogeneous amorphous morphology and could not absorb hydrogen reversibly