Also known as electrotransport electrical transmission method, a solid electrolytic method, which is based on the principle of dissolved solids (or liquid) conductors atoms can migrate under the influence of a DC electric field in a sequential manner, in particular in the case of gold having a high melting point near Mobility. The rare earth metal to be treated is placed between the two electrodes, and direct current is applied to cause the impurities to migrate toward one end, and the purity of the other end is correspondingly increased to achieve redistribution of the impurity concentration. The electromigration method has obvious effects on the removal of O, C, N, H and some metal impurities in rare earth metals. Generally, metal impurities migrate toward the anode, and interstitial impurities O, C, N, H, etc. migrate toward the cathode. After the end of one purification, the middle section is taken and a voltage is applied to both ends to migrate, and this is repeated until the purity requirement is reached. It can be seen that the equipment used in the purification method is relatively simple, and the disadvantages are long purification period, low yield and high energy consumption. At present, only a small amount of advanced pure metals are prepared in some research fields.
Atom dissolved in a solid or liquid migrates due to compositional gradients, temperature gradients, and electric field gradients. The change in metal purity during electromigration can be expressed by:
Â
Ln | c (x, ∞ ) | = | Ln | UEL | - | ( | UE | ) | x |  [5] |
Co | D | D |
Where co is the initial concentration of impurities;
c (x, ∞) - the depth of the impurity at the end x of the test bar along the length of the bar when the processing time t tends to ∞;
U - electromigration rate, cm 2 / (V · s);
E - electric field strength, A / cm 2 ;
D - diffusion coefficient, cm 2 ;
L - test rod length, cm.
Equation (6-5) is called electromigration equation, which shows that increasing the electric field strength E and prolonging the length L of the metal rod are all beneficial to improving the purity; the electromigration rate U and the diffusion coefficient D of an element are the basic parameters determining the purification effect. The greater the ratio of the two U/D values, the better the purification effect of the element. As shown in Table 1, the U/D values of carbon, nitrogen and oxygen in the metal lanthanum vary with temperature, and the elements vary, and the law of change is also different.
Table 1 U/D values of impurities in base metals at different temperatures
Electromigration temperature / °C | U/D | ||
C | N | O | |
1330 1450 1600 | 18.3 26.3 32.3 | 23.0 23.8 35.0 | 20.8 19.5 14.0 |
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