Zone smelting ( zone recrystallization ) is a method of purification of solids , based on the different solubility of impurities in solid and liquid phases. The method is a type of directional crystallization , from which it differs in that at each instant of time some small part of the sample is molten. Such a molten zone moves along the sample, which leads to the redistribution of impurities. If the impurity dissolves better in the liquid phase, then it gradually accumulates in the molten zone, moving with it. As a result, the impurity accumulates in one part of the initial sample. Compared with directional crystallization, this method is more efficient. The method was proposed by William Gardner Pfann in 1952 and has since gained great popularity. Currently, the method is used to purify more than 1,500 substances.
The scheme of the device for zone melting in the boat is shown in Fig. one.
The substance to be cleaned is placed in a boat of refractory material. Basic requirements for boat material:
- high melting point;
- boat material should not dissolve in or react with the substance being cleaned.
The boat is placed in a horizontal pipe, in which one end can be sealed or inert gas is supplied through it. If it is sealed, then the other end of the pipe is connected to a vacuum unit.
One end of the sample melts, then the molten zone begins to move along the ingot. The length of the molten zone depends on the length of the ingot and amounts to several centimeters. The substance is melted either by induction currents or by heat transfer in a resistance furnace. The speed of movement is, as a rule, from a few millimeters to several centimeters per hour. The movement can be carried out either by pulling the boat through a stationary furnace, or by shifting the heating zone. Sometimes to increase efficiency, increase the number of passes of a zone or the number of zones. The distribution of the impurity is characterized by a distribution coefficient , which is equal to
where С S is the concentration of the impurity in the solid phase, С L is the concentration of the impurity in the liquid phase.
Sometimes instead of the distribution coefficient K , a separation coefficient α is used, which is equal to
.
Impurities for which the distribution coefficient K <1 are concentrated in the molten zone and together with it move to the end of the ingot. On the other hand, layers of a substance purer relative to impurities for which K <1 are formed from the molten zone. Those impurities for which K > 1, on the contrary, are concentrated at the beginning of the ingot. If multiple passage of the molten zone is carried out, then impurities with K <1 will be collected at the end of the ingot. For impurities with K > 1, the method is not very effective. The cleanest parts of the ingot (from the middle) are used for the manufacture of devices. Using this method, germanium can be purified to samples with a resistivity of about 70 Ohm · cm, in which approximately one impurity atom per 10 10 germanium atoms remains.
If the melt reacts with the material of the crucible (boat), or the substance being purified has a high melting point (> 1500 ° C), use a crucible-free zone melting .
The method has several disadvantages. The main disadvantage is the impossibility of scaling, since the speed of the process is determined by the diffusion rate of the impurity. Therefore, the method is used for the final stage of purification upon receipt of highly pure substances. The maximum dimensions of the boat are 50 cm in length, 2–3 cm in thickness, and 5 cm in length of the molten zone.
See also
- Principles and methods of deep purification of substances
- Crucible zone melting
- Zone alignment
Literature
- Kharin A.N., Kataeva N.A., Kharina A.T. Chemistry course - M.: Higher School, 1975. - 416 p.
- Müller G. Crystal growth from melt. Convection and heterogeneity - M .: MIR, 1991. - 149 p. - ISBN 5-03-002101-9
- Herrington E. Zone melting of organic substances - M .: Mir, 1965. - 260 p.