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Which container materials can be used for zone refining?
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zone melting
Figure 1: Schematic representation of (A) normal freezing, (B) zone refining
In zone refining, the primary requirement for a container is that it must not contaminate the material being processed. Several materials are commonly used for this purpose, including:
- Glass and Vycor (a heat- and chemical-resistant glass)
- Fused silica
- Molybdenum
- Tantalum
- Graphite
- Stainless steel (often used when an opaque container is acceptable)
When selecting a container, it is important to consider potential issues such as cracking caused by volume changes during melting or freezing, or differential thermal contraction if the charge adheres to the container walls.
To avoid the risk of contamination from containers entirely, a technique known as float zoning is often employed. In this method, the material is held by end clamps, and the molten zone is maintained by its own surface tension, eliminating the need for a physical container wall.1