SGER: High Energy "Exchange-Spring" Permanent Magnets via Spark Erosion

University Of California-San Diego, La Jolla CA

Investigators

Abstract

0077588 Berkowitz The goal of this SGER program is to determine the feasibility of using a novel technique, spark erosion, to provide the particles for a material with strong technological potential, high-energy product 'exchange-spring' permanent magnets (ESPMs). ESPMs are duplex alloys of magnetically 'hard' and 'soft' phases in which the required length scales for the two phases are in the several tens of nm range. These length scales must be well controlled for optimum performance and this has been a principal obstacle to achieving a satisfactory production method. The spark erosion approach controls the sizes of the phases by precipitating them within particles of suitable diameters and microstructure. The idea is to produce amorphous particles of the appropriate composition and diameter such that, after annealing to the equilibrium duplex crystalline state, the length scales of the two phases are constrained by the particle size and initial microstructure as optimum for ESPM. Spark erosion is an under-appreciated, but extremely versatile and economical particle production method. The particle synthesis method is on firmer ground than the control of particle size/microstructure constraining equilibrium phase dimensions, since a large variety of powders have been produced by this method. Hot pressing is the primary method for preparing the final product, high-density bulk magnets with dimensions of several cm. A second approach for preparing the bulk magnets is thermal spraying, and this technique will be pursued if warranted by the results of the hot pressing. Both of these methods should benefit from the initial amorphous state expected for the spark-eroded particles since the equilibrium crystalline duplex structure can be developed during or after consolidation. The conceptual approach, although reasonable, is currently unproven. Particle production by spark erosion without contaminants seems feasible, but must be demonstrated. In addition, the particle synthesis method is a novel and economical one, which deserves attention. It is a technique that has great potential for a wide range of applications. Finally, there is new materials science to be clarified by examining the influence of particle size on precipitate dimensions. %%% The concept for exchange-spring permanent magnets (ESPM) has been available since 1991, and it has been intensively pursued because ESPM offer the promise of permanent magnets with higher energy products than currently available, at a much-reduced cost for the rare-earth materials used. The permanent magnet market is huge, and this has inspired a great deal of very useful processing and modeling work on ESPM. However, no bulk processing technique is available at present. ***