Spark erosion: A method for producing rapidly quenched fine powders
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Spark erosion: A method for producing rapidly quenched fine powders A. E. Berkowitza) and J. L. Walter General Electric Corporate Research and Development, P. O. Box 8, Schenectady, New York 12301 (Received 29 September 1986; accepted 29 January 1987) Spark erosion is a method for producing fine powders of metals, alloys, semiconductors, and compounds. The technique involves maintaining repetitive spark discharges among chunks of material immersed in a dielectric liquid. As a result of the spark discharge there is highly localized melting or vaporization of the material. The powders are produced by the freezing of the molten droplets or the condensation and freezing of the vapor in the dielectric liquid. Since the powders are quenched in situ, they may be extremely rapidly cooled. Particles can be produced in sizes ranging from 5 nm to 75 //m. The average powder size and production rate depend on the power parameters, material used, and the dielectric liquid.
I. INTRODUCTION Spark erosion has been used as a technique for producing research quantities of very rapidly solidified micropowders1"7 as a general micropowder production method8"10 and for applications such as ferrofluids with metallic particles" and MnAlC permanent magnet powders. ' 2 Current investigations have the objectives of extending the range of metals, alloys, and compounds prepared by spark erosion; determining the influence of electrical, mechanical, and chemical operating parameters on particle production rate, size distribution, composition, and microstructure; and increasing production rate and power efficiency. Thus far the work has demonstrated that spark erosion is a versatile and economical method for producing powders of a large variety of materials. The more noteworthy features of the process are the wide range of applicable materials and the small sizes of the particles obtained. This review presents the phenomenology of the process as well as material properties, process developments, and production rates obtained by utilization of the process developments.
II. THE SPARK EROSION PROCESS When two electrodes are in close proximity in a dielectric liquid, application of a voltage pulse can produce a spark discharge between them. Some of the energy in the spark discharge is transferred to the electrodes and results in the heating of highly localized regions of the electrodes. If the regions are heated above their melting temperature, molten droplets or vaporized material may be ejected from the electrodes. Particles are produced by the freezing of the droplets or the conden" Present address: Center for Magnetic Recording Research, R-001, University of California, San Diego, La Jolla, California 92093.
J. Mater. Res. 2 (2), Mar/Apr 1987
sation and freezing of the vaporized material in the dielectric field. This phenomenon was investigated many years ago, particularly by Svedberg,13 who used it for the preparation of colloidal suspensions. A number of more recent applications are described by Rudorff.14 During the 1940's spark erosion was
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