Impurities in the melt

FLINAK is purified by treatment with the HF released by ammonium bifluoride (NH4HF2) the HF converts oxide impurities in the melt to H20 [7]. In this purification procedure, the fluoride salt mixture is combined with 15 wt% NH4HF2 and heated to about 500°C in a graphite crucible. The molten mixture is poured into a platinum container and heated to 750°C. Hydrogen is passed through the molten mixture for approximately 2 days. Further purification can be achieved by con-trolled-potential electrolysis at an applied potential of about 3 V between a tungsten cathode and glassy carbon anode. 

Purification of Solution. An approximate model for the purification of the solution can be developed by assuming that a stagnant melt initially contains an impurity at a uniform concentration, Cf°, and loss of dopant occurs by evaporation at the top surface. The rate of evaporation is assumed to be directly proportional to the difference in concentrations at the top surface and at equilibrium. If the proportionality constant is z, the diffusion coefficient of the impurity in the melt is D v and the depth of the melt is Z, then the following expression for the impurity concentration in the melt,... 

The initial conditions for both impurities in the melt are defined as follows ... 

Second, in the fast-cooling solidification process, the m-c interface is concave to the crystal and the melt flows from the periphery to the center along the solidification front surface. Thus, the impurities in the melt and the SiC particles generated at the interface are transported to and accumulated in the... 

Here, we show how the Impurities change as a function of the number of passes, given a specific distribution coefficient for the impurities in the melt relative to those in the growing cr3rstal. Note that as the number of passes reaches 10, we obtain a change of some 90 times in the impurity content at the front of the rod. But, at the back, l.e.- L/Z = 8, we approach an ultimate distribution with 20 passes (the dotted line). Here, we have used ... 

Baraboshkin A.N. 1916) Electrocrystallization of metals from molten salt, Nauka Publishers, Moscow 2. Esina N.O., Valeev Z.I., Pankratov A. A. (1996) Structure of molybdenum electrodeposited from chloride melt with various contents of oxygen impurity in the melt. Extend Abstracts, Baltic Conference of Interface... 

A melt filter (Figure 4.83d) is used with small gates which are easily blocked as a result of mechanical impurities in the melt. A filter must be used when recycled material is being injected. The filter may be located in an open sprue bushing (see also Figure 4.60) or in the injection machine nozzle. A drawback to the filter is the substantial pressure loss, as much as 30%, depending on the type of filter. For this reason, filters with a filter area of at least 125 mm are recommended. 

For light scattering experiments at the solid-liquid interface it is very important to reduce background scattering from impurities in the melt or from grain boundaries in the crystal. 

We will work with the mass fractions u,f and U/s of the impurity in the melt and the solid phase, respectively. The impurity distribution coefficient >4 is given by... 

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