Zone refining is a process that was used to purify early semiconductor materials. Consider a long rod of solid polycrystalline siUcon, containing uniformly distributed small amounts of impurities in...

Zone refining is a process that was used to purify early semiconductor materials. Consider a long rod of solid polycrystalline siUcon, containing uniformly distributed small amounts of impurities in soUd solution. A narrow zone of the rod is melted at one end and the molten zone is slowly translated to the other end of the rod. At the advancing zone edge new impure solid is melted, and at the trailing edge liquid sohdifies.

a. By considering that impurities typically lower the melting point of Si, what is true of the purities of solid and liquid phases in equilibrium?

b. Why is the resulting rod purer at the initially melted end and less pure at the far end?