Zone melting
Zone melting, also known as zone refining, fractional crystallization, or zone recrystallization, is a purification technique used in material processing, particularly for the purification of metals and semiconductors. It was developed by William Gardner Pfann in the 1950s. Here are some key aspects of zone melting:
- Basic Principle: Zone melting involves melting a small region (zone) of a solid material and then moving this molten zone along the length of the solid. As the molten zone moves, it carries impurities with it, leaving behind a more pure material.
- Process: The process typically involves a rod of the material to be purified, which is slowly passed through a heating coil or a similar heating mechanism. The coil heats a short segment of the rod to a temperature just above its melting point, creating a molten zone. As the coil is slowly moved along the rod, the molten zone also moves, and impurities concentrate in the molten region.
- Impurity Segregation: The effectiveness of zone melting depends on the segregation coefficient of the impurities, which is the ratio of the concentration of impurities in the solid phase to that in the liquid phase. Impurities with a segregation coefficient less than one will tend to concentrate in the molten zone and be carried along with it.
- Applications:
- Semiconductor Fabrication: Zone melting is crucial in the semiconductor industry for purifying materials like silicon and germanium. High-purity semiconductors are essential for electronic device performance.
- Metal Purification: It is also used for purifying metals, including titanium, indium, and certain special alloys.
- Advantages: The main advantage of zone melting is its ability to achieve very high levels of purity. It is particularly useful for materials that need to have very low impurity concentrations.
- Limitations: The process can be slow and is typically more expensive than other purification methods. It is generally reserved for materials where high purity is essential and cannot be achieved by other means.
In summary, zone melting is a purification technique used to produce high-purity materials by exploiting the differences in impurity solubility in solid and liquid phases. It is particularly important in the semiconductor industry and for the purification of certain metals and alloys.