High purity metal preparation technology (2)

Where V is the ion drift velocity; U is the ion mobility; F is the external force acting on the ion, which consists of the electric field force and the force of the conductive electron scattering on the ion. These forces are related to the effective charge number of the ions. The separation is achieved according to the difference in the number of charges of the parent ion and the impurity ion and the difference in diffusion and drift speed. As shown in Figure 4.

The combination of electromigration and regional melting methods is better (Fig. 5). Taking gallium as an example, after purification by the above method, the residual resistivity of gallium reaches R _residual = 100,000. This method has been widely used in the purification of beryllium, tungsten, yttrium, lanthanum, cerium and other metals.

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5. Electromagnetic Field Purification Techniques for deep purification of high melting point metals under electromagnetic fields are increasingly being used. The electromagnetic field is not limited to the agitation of the molten metal, and more importantly, the electromagnetic field can obtain a uniform distribution of structural defects in the crystallization process and refine the grain structure. When a semiconductor material is drawn into a single crystal, there is temperature fluctuation in the melt during directional crystallization, which causes a layered distribution of impurities, and a small constant magnetic field is sufficient to eliminate such temperature fluctuations. When the multiphase system is crystallized, the second phase is precipitated by the electromagnetic field to form an anisotropic structure similar to that of the magnetic composite. Electromagnetic fields are also used for suspension smelting, in which the electromagnetic field acts as an energy support and agitation, using the evaporation of impurities and drifting away the second phase (oxides, carbides, etc.) to purify the metal. Since the contact vessel and contamination due to the purification of the metal is not a problem, it is commonly used in almost all purified refractory metal, such as tungsten, molybdenum, tantalum, niobium, vanadium, rhenium, osmium, ruthenium, and zirconium.
6. Comprehensive application of purification methods Each purification method utilizes a certain physical property or chemical property of a metal and a difference between impurity elements to perform purification purposes. For example, vacuum distillation utilizes saturated vapor pressure and volatilization speed of metals and impurities. difference. The regional smelting is carried out by purifying and separating the impurity difference between the solid phase and the liquid phase. Therefore, each method has certain advantages (good separation effect for some impurities) and shortcomings (poor separation effect for other impurities). Even with the same purification method, the purification effect varies greatly depending on the nature of the metal. For example, the regional smelting has a good purification effect on the high melting point metal, but the purification effect on some rare earth metals is not satisfactory. In order to obtain the effect of deep purification of metals, it is generally necessary to comprehensively apply various purification methods. In this respect, the reasonable combination of applications and sequential use of each method is very important, usually by combining electron beam smelting or distillation with regional melting or electromigration, ie first electron beam melting or distillation purification, then regional melting or electricity Migration purification as the ultimate purification method, taking 铍 as an example, in order to obtain ultra-high purity 铍, it is best to first distill and purify, then vacuum smelt, and finally carry out regional smelting or electromigration purification, and the purity of the ruthenium single crystal obtained by such purification is up to 99.999%, residual resistivity R _residual >1000. In the preparation of ultra-pure ruthenium , phosphorus , arsenic , aluminum , silicon, boron and other impurities are generally removed by chemical methods, and then purified by zone melting to obtain electronic grade pure ruthenium; finally, multiple crystal pulling and cutting can achieve 13N purity. Claim. Table 2 shows the effects of various methods in combination with the purification of metal ruthenium.

7. Purification of metals under cosmic space The development of space has created new opportunities for the purification of metals. The ultra-high vacuum (about 10 ^-10 Pa) in space, ultra-low temperature and basically no gravity (g = 10 ^-5 g ₀ ) provide superior conditions for metal purification. Under such conditions, there will be no problem of convection in the liquid metal. The distribution of impurities during crystallization will only have pure diffusion properties, and the molten metal will need to be enthalpy. Ultra-high vacuum is especially beneficial for the volatilization and degassing of impurities. These are ideal conditions for the use of smelting, evaporation, and regional melting methods to extract highly active metal and semiconductor materials. Taking purified helium as an example, the separation coefficient of impurity gallium is 0.1/0.15 when it is sag on the earth, and 0.23/0.17 when it is in space. The integrity of a crystal drawn in a gravity-free condition is much better than that under gravity. In indium antimonide, for example, the dislocation density than a dislocation density of only 1/6 position under gravity conditions. Since the surface tension coefficient of liquid metal in the universe is very large, it is inevitable that a single crystal of extremely high purity and integrity can be prepared by using a flawless region melting method in the universe. In addition, ultra-low "universe" temperatures also have good application prospects.

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