What is the principle of zone refining ?

Explanation

### Step-by-Step Text Solution:

1. **Definition of Zone Refining**: Zone refining is a method used to purify crystalline materials, particularly metals and semiconductors. It is based on the principle of fractional crystallization.

2. **Basic Principle**: The fundamental principle of zone refining is that impurities tend to concentrate in the liquid phase when a solid is melted. As a result, when a small molten zone is moved along the length of a solid sample, the impurities are carried along with the molten zone.

3. **Process Explanation**:
- A rod of impure material is placed in a heating device, which creates a small molten zone at one end of the rod.
...