Metals molten zone

In zone melting, a narrow molten zone is passed several times along a thin metal rod. Metallic impurities and carbon dissolve in the liquid and move with the molten zone to the end of the rod, whereas oxygen and nitrogen move to the opposite end. 

Zone melting causes impurities to migrate to one end of a cylindrical metal sample by generating a narrow molten zone and moving it repeatedly in one direction along the cylinder axis. Impurities more soluble in the liquid phase (metals, some halides, and carbon) move in the direction of zone travel, while those more soluble in the solid (interstitials) move in the opposite direction. This technique produces pure rare earth metals . ... 

Top to bottom An impure metal rod is moved slowly through a heating coil. As the metal rod moves forward, the impurities dissolve in the molten portion of the metal while pure metal crystallizes out in front of the molten zone. Eventually the end portion of the rod, which contains most of the impurities, is allowed to cool and is cut off from the rest of the rod. 

Zone melting techniques are slow, very slow. Rates of zone movement vary from 1 to 20 cm/h. Figure 2-8 shows a more efficient arrangement. Once the heated zone has refrozen there is no reason why it can t be remelted. In this case, three heaters are used, and the charge is moved. Figure 2-9 shows a helical type where you can get many molten zones. This is more appropriate for organic compounds, which are lower melting than metals. 

Zone melting (8) relies on selective distribution of impurity solutes between a liquid and solid phase to achieve a separation. Literally hundreds of metals have been refined by this technique, which, in its simplest form, involves nothing more than moving a molten zone slowly through an ingot by moving the heater or drawing the input past the heater, as in Fig. 1.10. 

Gill et al. (14) have shown by numerical computation that Equation 29e is a good approximation to a constant heat flux for fluids with finite values of Pr which are typical of liquid metals. The following discussion applies to all Pr fluids, but low Pr is the category that includes essentially all fluids of interest in semiconductor technology as well as all liquid metals. On the other hand, the most complete data on thermocapillary flows in molten zones has been reported by Preisser, Scharmann and Schwabe (15) and Schwabe and... 

Molten metal poured above its melting point is chilled at the walls and bottom of the ingot mold and begins to solidify. The molten metal in the center cools more slowly and contracts as it cools. As the metal solidifies at the exterior of the molten zone and contraction continues a depression or in its worst condition a pipe develops in the center of the ingot. 

The passage of a narrow molten zone along a solid rod has been successfully used to purify semiconductor materials and less reactive metals for over 25 years. Just as for the electrotransport purification method, zoning was unsuccessful until vacua of 10 Torr or better became standard practice in the laboratory. In this technique a molten zone is repeatedly passed in one direction, usually 20 passes are required to reach steady-state conditions. Impurities which raise the melting point of the solvent are left behind during solidification as the liquid zone moves forward. Thus with continued passes these impurities are moved to the beginning end of the rod. Those impurities which lower the melting point tend to remain in the molten zone... 

The application of zone melting to the purification of semiconductor materials has been well established. In this technique a molten zone is passed along the length of a solid rod in one direction several times. Impurities more soluble in the molten metal i ill move in the direction in which the molten zone is moved while impurities less soluble in the liquid metal will be deposited in the solid metal and will tend to move in the opposite direction. Since the degree of purification depends on the solubility of the impurities in the solid metal, those impurities which are soluble in the solid cannot be removed to below the equilibrium concentration. Thus the interstitial impurities which are quite soluble in the solid rare earths just below their melting point cannot be completely removed. 

Revel et al. (1974) studied the zone refining of Ce in an induction heated cold crucible. They showed that 12 passes of a molten zone reduced a number of metallic impurities to the part per billion range. They did not determine the concentration of interstitial impurities in their samples. However, it is not likely that purification with respect to interstitials was attained due to the inadequate vacuum used and the high solubility of these impurities in solid Ce. Thus when only impure Ce is available, many metallic impurities can be reduced by zone refining. 

Hukin and Jones (1976) zone refined Tb and observed that O, N and C moved in the direction opposite to the molten zone. They obtained significant purification with respect to these interstitial impurities as well as the metallic impurities. The zone melting was done under an atmosphere of pure argon in a horizontal cold crucible with induction heating. 

As shown in Fig. 6, the weld decay zone which contains chromium carbide precipitate is not adjacent to the cast metal, but at some distance from it, in austenitic stainless steels. The reason for this is that the temperature of the metal in the zone adjacent to the molten zones has been... 

Repetitive passage of a molten zone along a bar of the impure actinide metal (zone melting), particularly when followed by the induced migration of impurities in the solid by an impressed electric field (electrotransport), can be used to reduce the impurity level in Pu from 200 ppm to 20 ppm. 

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