Potential Replacements for Rare Earths
Toshiharu TERANISHI, Kyoto University

Rare earth elements (rare earths) are a series of seventeen chemical elements, the fifteen lanthanides plus Sc and Y, in which the two last are considered rare earths since they exhibit similar chemical properties to the lanthanides. At present, rare earths and their compounds have many practical applications including catalysts, phosphors, magnets, and so on, by making use of their characteristic f-electrons. From the perspective of value, applications in magnets and phosphors are more important. Especially, the development of highly efficient motors using high-performance permanent magnets is indispensible to huge energy saving, because the motor-loaded devices consume approximately 60% of electric energy (in Japan). Also the next-generation vehicles, hybrid and electric vehicles, are badly in need of high-performance permanent magnets containing rare earths.

In spite of their name, rare earths are relatively abundant in the Earth's crust, e.g., Nd being in the top 30 most abundant elements at 33 ppm (a half of copper). However, rare earths are geologically localized and not often found in concentrated and economically exploitable forms, which led to the term "rare earth". As a result of considering profitability, a specific country became the predominant supplier and thus the rare earth pricing has been increasing (Dy metal ≥ 99%: from $250 per kilogram in early 2010 to $2300 per kilogram in March 2012).

Under these circumstances, what we have to do is to replace rare earths with other more abundant and cheaper elements or to use only light rare earths (Sc, Y, La–Eu) without using less abundant heavy counterparts (Gd–Lu), retaining or enhancing the original properties. A similar strategy is taken for platinum catalyst of the fuel cell. For example of permanent magnets, the Nd-Fe-B magnet, which has the highest maximum energy product, retains high coercivity and magnetic flux density at a high-temperature range by adding a small amount of Dy. A drastic replacement of both Nd and Dy metals with, for instance, other 3d-transition metals or a non-use of less abundant Dy metal is desired in this case. However, a drastic replacement of rare earths is quite challenging, because atoms of rare earths with high magnetic moments are ascribed to incomplete filling of the inner f-orbital, which can contain up to 7 unpaired electrons with aligned spins. Moreover, the advantage of the rare earth magnets is their crystalline structures with very high magnetic anisotropy. For a potential replacement for rare earths, we should have a deep insight about principles underlying the unique properties of rare earth-containing materials.
In this session, we will discuss how rare earths can be replaced or effectively used through a perspective of materials science.

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