Electromagnetic Containerless Reaction of Samarium with Cobalt for the Formation of Samarium-Cobalt Alloys
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Electromagnetic Containerless Reaction of Samarium with Cobalt for the Formation of Samarium-Cobalt Alloys C. W. CHANG, D. K. DAS, R. T. FROST, and K. KUMAR Magnetization reversal in SmCo5 magnets has been found to result from nucleation and motion of reverse magnetic domains. This is believed to be associated with the presence of defect sites in the conventionally prepared SmCos magnets. Work at the Charles Stark Draper Laboratory and elsewhere ~'2'3 indicates that these defects may be compositional inhomogeneities resulting from precipitation of dissolved oxygen during cooling of the magnet from sintering and optimization temperatures. Because of the high chemical reactivity of the samarium, other contaminants also enter the alloy from the crucible materials in which the SmCo5 alloys are formed. These other contaminants are also suspected to be contributors to undesirable defects. Ideally, crucible contamination can be eliminated completely by the reaction of the pure elements while levitated by electromagnetic fields. There is a noticeable absence of C.W. CHANG, formerly with the General Electric Company, King of Prussia, PA 19406, is now Group Leader, KBI Division, with the Cabot Corporation, Boyertown, PA 19512. D.K. DAS and K. KUMAR are both Staff Scientists with Charles Stark Draper Laboratory, Inc., 555 Technology Square, Cambridge, MA 02138. R.T. FROST is Consulting Engineer, Space Systems Division, General Electric Company, P. O. Box 8555, Philadelphia, PA 19101. Manuscript submit'ted February 23, 1982. 1868--VOLUME 13A, OCTOBER 1982
papers that report detailed studies of formation of an alloy from solid materials by levitation technique except the work by Polonis et a l . 4 The work reported here describes the successful application of this technique to the formation of near stoichiometric SmCo5 by suitable charges of the elemental materials and by use of a gas jet to control temperature during the reaction. Although temperature control was rendered difficult by the requirement of field intensities capable of levitating the specimen against gravity, sufficient control was obtained to allow achievement of stoichiometry approximating SmCos composition by providing excess Sm in the charge to compensate for evaporation during temperature excursions. The General Electric levitation facility 5 (450 kHz, 25 kw RF power generator) was used for carrying out the reaction experiments. Elemental samarium from Cotronics Corporation and United Mineral and Chemical Corporation and cobalt from Materials Research Corporation were obtained for the experiments. A radiation pyrometer was assembled by using optical filters (0.65 and 0.85 microns) and a CID (Charge Injection Device) imager. Pyrometer calibration is mandatory to provide accurate temperature readings. Observation of melting gold (m.p. 1336 K) and Constantan (m.p. 1543 K) wires inserted into small cavities drilled into cobalt specimens was used for absolute calibration. The instrumentation was employed as described later for controlling reaction temperatures fo
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