as shown in Fig. 6.35. The zone at the end of the sample is the impurity dump: when
the heater has gone by, it cools to a dirty solid which can be discarded.
The technique makes use of the non-equilibrium properties of the system. It relies
on the impurities being more soluble in the molten sample than in the solid, and
sweeps them up by passing a molten zone repeatedly from one end to the other along
a sample. The phase diagram in Fig. 6.36 gives some insight into the process. Consider
a liquid (this represents the molten zone) on the isopleth through a1, and let it cool
without the entire sample coming to overall equilibrium. If the temperature falls to a2
a solid of composition b2 is deposited and the remaining liquid (the zone where the
heater has moved on) is at a2 ′. Cooling that liquid down an isopleth passing through a2 ′
deposits solid of composition b3 and leaves liquid at a3 ′. The process continues until
the last drop of liquid to solidify is heavily contaminated with B. There is plenty of
everyday evidence that impure liquids freeze in this way. For example, an ice cube is
clear near the surface but misty in the core: the water used to make ice normally contains
dissolved air; freezing proceeds from the outside, and air is accumulated in the
retreating liquid phase. It cannot escape from the interior of the cube, and so when
that freezes it occludes the air in a mist of tiny bubbles.
A modification of zone refining is zone levelling. It is used to introduce controlled
amounts of impurity (for example, of indium into germanium). A sample rich in the
required dopant is put at the head of the main sample, and made molten. The zone is
then dragged repeatedly in alternate directions through the sample, where it deposits
a uniform distribution of the impurity.