Electron-Beam Atomic Spectroscopy for In Situ Measurements of Melt Composition for Refractory Metals: Analysis of Fundam
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I.
INTRODUCTION
THE recycling and recovery of high value and high temperature metals, known as refractory metals, is important, as refractory metals are near critical or critical materials. Refractory metals such as Tantalum, Niobium, Molybdenum, and Tungsten have been identified as being at risk by several academic and industry groups.[1–4] These materials also currently have low recycling rates.[5] So new methods for recycling and recovery of refractory metals need to be developed. To process these metals, which are used by the aerospace, semiconductor, nuclear industries, and others, electron beam (EB) melting is used. EB melting allows producers to meet the high purity requirements that these industries need. Currently though, there is no in situ melt cognition to help improve EB melting techniques. Melt cognition is especially important for recovery and recycling from secondary sources, as these introduce high levels of impurities into the melt. These impurities can include both light elements, such as carbon or oxygen, and refractory metal impurities that cannot be removed as easily by evaporation. Melt cognition can reduce energy use during melting by monitoring impurity levels and measuring dynamic processes, such as element specific evaporation rates. Melt cognition can also be used to improve alloy PAUL JOSEPH GASPER, M.S. Graduate, and DIRAN APELIAN, Director, are with the Metals Processing Institute, Worcester Polytechnic Institute, Worcester, MA. Contact e-mail: [email protected] Manuscript submitted October 2, 2014. Article published online November 5, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS B
production from secondary feedstock. Lastly, in situ composition measurement simplifies quality control, as ingot composition can be measured almost continuously throughout the length of the ingot. There exists an opportunity to develop an in situ technique to measure composition. In some experimental trials using laser-induced breakdown spectroscopy (LIBS), representatives from the Energy Research Company (ERCo) noted that very clear spectra were being generated even without laser excitation during EB melting. This phenomenon was termed electron-beam atomic spectroscopy (EBAS), as it is a novel environment for measuring composition using spectroscopy, and could be used during EB melting and other applications making use of EBs in vacuum. Initial trials showed good correlation of peak intensities to changes in composition. EBAS is a method to determine the elemental composition of a melt by measuring the optical emission spectra produced when a high power EB impinges on a target material. To develop this technology, a fundamental understanding of the physics is needed. This paper is a review on the physics of EB generated plasmas and the plasma models used for calculating plasma composition from optical emission spectra. A method for calibration-free composition measurement is then described, and the challenges for its implementation addressed.
II.
OVERVIEW OF EBAS
The basic phenomenology of the generat
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