Scientists have increased the hydrogen capacity of mischmetal (German: mischmetall “mixed metal”) to 2.1 wt% with the development of a new lanthanum-nickel based alloy. It was manufactured using a process of arc melting followed by ball milling, inducing a nanostructure that fostered more efficient reactions than similar rare-earth alloys. While this material currently lags behind other hydrogen storage methods (>9 wt% for cryo-compressed H2, 7.6 wt% for Mg based hydrides), mischmetal and other metal hydrides are worth exploring due to their high theoretical hydrogen capacity and ease of storage and transport.
Meena et al., a group of scientists at University of Rajasthan and Malaviya National Institute of Technology (MNIT) in Jaipur, India, reported results in the recent issue of Journal of Materials Research and Technology. They synthesized an alloy of 6 rare-earth metals (La23Nd7.8Ti1.1Ni33.9Co32.9Al0.65) via arc melting and then ball milled the ingot to produce a 52 nm average particle size. After several cycles of heating and cooling, with and without hydrogen environments, the metal hydride had fully formed at an equilibrium pressure of 2 bar. The resulting alloy was found to have a higher volume and lower density, with a 31 nm average particle size confirming hydrogenation had occurred.
Hydrogen absorption/desorption process
Results showed that the hydrogen activation process caused “pulverization” of the alloy, mostly occurring during the first few heat-cool cycles and then decreasing in effect. This damage presented as micro cracks in the sample, which could provide clues about the material’s durability with more tests.
A key step in manufacturing mischmetal as a hydrogen storage material is the ball milling process. The sample material was obtained by melting and re-melting the 6 rare-earth metals together in an arc melting furnace. The resulting alloy ingot was annealed for one week at 900 ºC to produce a uniform microstructure; it was transformed into a nanostructure via a ball mill that bombarded it with grinding balls for 10 hours. This produced the 52 nm average particle size which was the starting point for the hydrogen absorption tests.
Historically, mischmetal production is merely one step in a tedious process to extract rare-earth metals from minerals. After mining the ore, a series of chemical reactions filters out impurities, producing a mischmetal that can then be further processed to separate the individual rare-earth metals. Isolating the elements is slow and incremental; the process depends upon slight variations in solubility, but it inspired a similar procedure used later by Marie Curie in her discovery and isolation of radioactive elements. Now, mischmetal has another use as a key component of promising clean fuel technologies.
Original Citation:
Meena et al., Synthesis and Hydrogen Storage of La23Nd7.8Ti1.1Ni33.9Co32.9Al0.65 Alloys. Journal of Materials Research and Technology, 2018; DOI: https://doi.org/10.1016/j.jmrt.2018.01.009.
