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Lec 4

Zone refining is a technique used to purify materials by exploiting the differences in solubility of impurities in solid and liquid states. The process involves melting a small section of a long rod of the material and moving the molten zone to continuously solidify and remelt, allowing for the separation of purer material. This method can achieve extremely low impurity levels, making it essential for producing high-purity semiconductor materials.

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15 views10 pages

Lec 4

Zone refining is a technique used to purify materials by exploiting the differences in solubility of impurities in solid and liquid states. The process involves melting a small section of a long rod of the material and moving the molten zone to continuously solidify and remelt, allowing for the separation of purer material. This method can achieve extremely low impurity levels, making it essential for producing high-purity semiconductor materials.

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mohima.ghosal
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© © All Rights Reserved
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zone refining

• One of the important uses of a phase diagram is in the zone refining of materials.
• Impurities are more soluble in liquid state compared to solid state
• Consider the schematic phase diagram shown in Fig. 7.15. We need to purify (or
refine) A, which has B as the impurity component. Let the starting composition be co.
This composition starts to solidify at temperature T1.
• Just below this temperature, a small quantity of solid of composition cs’, purer in A
than co, separates from the liquid. Imagine that we stop cooling further, throw away
the liquid part and remelt the left-over solid.
• This small quantity of liquid is now purer, as its composition is cs’. This overall
composition will now start to solidify at a higher temperature T2. Again, the first solid
to separate will have a composition cs’’purer in A than cs’.
• By repeating this sequence of operations a few times, we can get very pure A, even
though the quantity of the of the purified material will be extremely small compared
to the starting material.
• Purification of Ge, Si, In
Principle
• In zone refining, this principle of phase separation is used. The material to be purified
is in the form of a long rod. At any time, only a small length of this rod is melted with
the aid of an induction coil or an electron beam. The coil or beam is moved slowly
from one end to the other end of the rod, continuously solidifying the molten zone
and remelting fresh material ahead.
• Surface tension forces are usually strong enough to hold the molten zone in place
without the need for a container, which may contaminate the melt. If the zone is
passed across many times, each time in the same direction, the material at the
starting end becomes much purer than the rest of the rod.
• In a typical case, ten passes of the molten zone can reduce the impurity level to as
low as 10–6 times the initial value. This technique to produce ultra high purity
materials has enabled us to achieve the sophistication required in the control of
impurity concentration in semiconductor crystals.
• The depression of the melting point of a pure solid, when another component goes into
solution, has its advantages and disadvantages.
• The problem of softening and melting of refractories due to impurities was referred. The
eutectic reaction in the iron-carbon alloys facilitates the melting of cast irons at a lower
temperature than is required for steels.
• So, the making of cast iron is easier and more economical. In cold countries, the
depression of the freezing point of water by the addition of salt is used to melt the ice on
roads.
• Pb–Sn eutectic alloys are useful as soldering material. Low melting eutectic alloys are
used as safety devices in fire fighting equipment and petroleum storage tanks.
• Gold–silicon eutectic finds uses in the manufacture of semiconductor devices.
Diffusion: Introduction
Diffusion phenomena

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