The material is deposited on the wire by melting or from a suitable salt solution. When sufficient current passes through the wire, the material evaporates. This type of source is used only for single materials and is often used only once. Thermal evaporation of larger amounts of material is handled by using a ceramic crucible that sits in a wire filament coiled to fit the crucible's outside dimensions. Tungsten, tantalum or molybdenum foil is formed into shapes that, again, meet mounting or experimental requirements.
Boats can be fully open at the top or partially closed to produce a more directed vapor stream.
Boats with a thin coating of aluminum oxide covering the 'dimple' where the material is placed are recommended for applications where the molten material will react with a refractory metal. For very specialized thermal source applications, we provide thick tungsten rods and fine meshes or screens. The electrons are made by thermionic emission from a hot filament placed below the hearth and focused on the material surface by permanent or electromagnets. The energy in the electron beam causes local Grids melting, and a pool of liquid forms from which material evaporates. Since the melt is surrounded by solid material of the same composition, the e-beam source is less prone to contamination than other types of thermal sources.