Mischmetal, cerium-free

A third class of hydrides, the AB5 type (Latki Hfc), also function at ordinary temperatures and are easily activated, but are costly due to the presence of lanthanum. Investigations seeking lower cost substitutes for lanthanum have resulted in the development of two useful alloys. One is based on the substitution of cerium-free Mischmetal (M), an unrefined rare-earth alloy the second is based on the substitution of calcium (Ca) and Mischmetal. These compositional changes have reduced the raw materials costs to about 30% of that for LaNi5 alloy, but further cost reductions are required. 

The above method has been successfully used to prepare the first four lanthanides (La, Ce, Pr and Nd) and mixtures of these metals, specifically mischmetal and a cerium-free mischmetal. It has not been successfully applied to prepare the other rare earths because of their higher melting points. Henrie and Morrice (1966) have, however, described a modification of the above... 

Two types of metal hydrides electrodes, comprising the AB, and AB2 classes of intermetallic compounds, are currently of interest. The AB, alloys have the hexagonal CaCu, structure where the A component comprises one or more rare earth elements and B consists of Ni, or another transition metal, or a transition metal combined with other metals, The paradigm compound of this class is LaNi, which has been well investigated because of its utility in conventional hydrogen-storage applications. Unfortunately LaNi, is too costly, too unstable, and too corrosion— sensitive for use as a battery electrode. Thus commercial AB, electrodes use mischmetal, a low-cost combination of rare earth elements, as a substitute for La. The B, component remains primarily Ni but is substituted in part with Co, Mn, Al, etc. The partial substitution of Ni increases the thermodynamic stability of the hydride phase fl2] and corrosion resistance. Such an alloy is commonly written as MmB, where Mm represents the mischmetal component. The compositions of normal and cerium-free mischmetal are given in Table 2. 

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