Air Force

Bulk permanent magnetic nanocomposites having a low rare earth metal compositions

Materials and streamlined production to meet the growing demands for permanent magnets

Materials Energy

Wind turbines need approximately 100 kg of permanent magnets installed per megawatt of power generation. (Cynthia McIntyre/USMC)

Use of rare earth metals as the hard magnetic phase in permanent magnets (PM) has been essential in modern technologies, such as electric motors, electric generators, actuators, hard disk drives, traveling wave tubes, missile guidance systems, and communication systems.

Concerns over the supply chain of rare earth metals coupled with the projected increase in demand for clean energy technologies are expected to cause a considerable rise in rare earth prices and to further limit availability. At present, about 20% of the total annual rare earth production is consumed in the form of PMs, wherein typical PM motors and or generators for a hybrid electric vehicle may require approximately 1.5 kg and 1.0 kg of sintered neodymium-iron-boron alloy (Nd2Fe14B), respectively.

Electric power steering in such vehicles increases that requirement by about 100 g of Nd2Fe14B use per vehicle. In other examples, PMs in household air conditioner compressors use 100 g to 200 g of Nd2Fe14B per unit and wind turbines use about 100 kg of sintered Nd2Fe14B magnet per megawatt of power generation.

A long-standing manufacturing goal has been to develop PMs comprising less rare earth content, a hard magnetic phase of the permanent magnet which exhibits crystalline alignment, the hard magnetic phase being magnetostatically coupled to a magnetically soft phase, the PM comprising at least 50% of the magnetically soft phase, the two phases uniformly distributed throughout the PM, and a coupling between the two phases such that a single phase behavior is observed.

Air Force scientists have developed a method of preparing a permanent magnet nanocomposite that meets the above requirements. The process includes melting a precursor alloy having a hard magnetic phase (Nd—Iron—Boron, Sumarium—Cobalt, Sumarium—Fe—Nitrogen, Iron—Platinum, or Cobalt—Platinum) and a magnetically soft phase (α-Iron, Iron—Cobalt, Iron—Nitrogen, Cobalt, Nickel, or combinations thereof).

In combination, the hard magnetic phase has less than a stoichiometric amount of rare earth metal (Samarium, Praseodymium, Boron) or noble metal (Platinum). The melted precursor is cast into flakes and milled into a powder. The powder is then crystallized by pressurizing and heating. The powder is held at a crystallization temperature and pressure for a hold time to promote crystal growth. Crystal growth may then be rapidly quenched.

In example materials the permanent magnet nanocomposite is SM2CO17; SM2Fe17N3; Pr2Fe14B.

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