Manufacturing Considerations for Rare Earth Powders Used in Cryocooler and Magnetic Refrigerator Applications

Abstract

The high chemical reactivity and oxygen affinity of the rare earth metals make them especially difficult to produce and maintain as high purity powders. From the initial oxide reduction through bulk melt preparation and finally powder production the sensitivity of the metals and alloys to contamination must be attended to at all times. Production of these powders has raised new challenges for the powder manufacturer. Process adaptation to the unique requirements of the rare earth materials to insure a quality product will be described. The physics of the current powder production technique will be reviewed and related to the actual product made. Individual processing steps with their influence on final product quality will be reviewed. Finally, the knowledge gained during the pre-production learning curve will be used to define potential manufacturing process that could be used for the economic production of large quantities (>100 kg) of these powders when needed. The volumetric heat capacities of powders of Gd and Er metals, and of an Er-Pr alloy, which were prepared by the Plasma Rotating Electrode Process (PREP) have been measured and com-pared to the bulk starting materials. The heat capacities are essentially the same within experimen-tal error. The three materials have been used as regenerator materials: Gd for an active magnetic refrigerator, and Er and Er-Pr as passive regenerator materials for Gifford-McMahon and pulse tube cryocoolers, INTRODUCTION The rare earth powders used in this study were made by PREP