Abstract
Zone refining, as the currently most common industrial process to attain ultrapure metals, is influenced by a variety of factors. One of these parameters, the so-called “zone length”, affects not only the ultimate concentration distribution of impurities, but also the rate at which this distribution is approached. This important parameter has however neither been investigated experimentally, nor ever varied for the purpose of optimization. This lack of detections may be due to the difficult temperature measurement of a moving molten area in a vacuum system, of which the zone refining methodology is comprised. Up to now, numerical simulation as a combination of complex mathematical calculations, as well as many assumptions has been the only way to reveal it. This paper aims to propose an experimental method to accurately measure the molten zone length and to extract helpful information on the thermal gradient, temperature profile and real growth rate in the zone refining of an exemplary metal, in this case aluminum. This thermographic method is based on the measurement of the molten surface temperature via an infrared camera, as well as further data analysis through the mathematical software MATLAB. The obtained results show great correlation with the visual observations of zone length and provide helpful information to determine the thermal gradient and real growth rate during the whole process. The investigations in this paper approved the application of an infrared camera for this purpose as a promising technique to automatically control the zone length during a zone refining process.
Highlights
IntroductionIt works by repeatedly moving one or a series of molten zones very slowly along a solid bulk
Zone refining is an effective way to produce high purity metals by controlling the impurities’distribution in crystalline materials
When properly calibrated, infrared thermographic analysis has been proven to be a viable method for the detailed evaluation of the zone refining process
Summary
It works by repeatedly moving one or a series of molten zones very slowly along a solid bulk. This process, like any other methodologies based on the principle of fractional crystallization, can be applied to refine those metals, in which the distribution coefficient (k) of the impurities is smaller or bigger than unity. The smaller the k, the more probable it is to remove that impurity from the base metal through zone refining even up to ultra-purity levels This process was firstly introduced by W.G.Pfann for the purification of Germanium in 1952 [1], and since a large amount of investigation has been conducted with the focus on the optimization of experimental
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