The equipment used for electromigration purification is shown in Figures 1 and 2. Electrical migration purified rare earth metal bars secured between the positive and negative electrodes, under high vacuum or inert atmosphere with the direct current test bar heated to a temperature and for a time, impurities move to the ends of the metal to give Purification. After the electrophoretic treatment of the test bar, the portion with high impurity content at both ends is cut off, and then the second and third purification treatments are performed as follows, which can significantly reduce the impurity content in the metal.
Figure 1 Schematic diagram of electromigration equipment
1-bar; 2-inch chuck; 3-insulation pad; 4-stainless steel cathode; 5-stainless steel anode;
6-observation hole; 7-vacuum line; 8- getter heating wire; 9-seal ring; 10-flange
Figure 2 Electrode structure of electromigration device
The electromigration method for the purification of rare earth metals was first proposed by Huffine and Williams [14] in 1961. At that time, the metal Y (O3800, N510, Si55, Ni250, Fe 150μg/g) was purified and continuously treated under argon gas protection at 1230 °C for 200 hours. The contents of O and N were reduced to 650 μg/g and 250 μg/g, respectively, and the contents of Ni, Si and Fe were also reduced by 3 to 5 times. In 1990, the Institute of Metal Materials of the Japan Science and Technology Agency treated the metal ruthenium by electromigration to reduce the oxygen content from 0.1% to 0.005%, which significantly reduced the gas impurities in the metal. Verhoeven theoretically explored the extent of electromigration purification of metals. He believes that the main limitation of solid-state electromigration is environmental pollution, unless ultra-high vacuum equipment and high-purity inert gas are used, in addition to increasing processing time and temperature. The maximum processing temperature is again limited by the melting point of the metal and its vapor pressure. Another major parameter is the current density, which increases the mobility of the solute. At the same time, the shape of the sample to be treated is changed so that the sample has the largest surface area to volume ratio, so that a higher current density can be obtained at the same processing temperature.
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