If a metal is heated to melting point in air, not much happens because it is being heavily bombarded with air molecules. If the same metal is melted in a high vacuum, something rather interesting happens; evaporation occurs. The vapor travels until it encounters a cold surface, where it condenses. For this to happen effectively, the pressure has to be in the region of free molecular flow. This is when the molecules in the residual gas are so few that although they strike the walls of the vacuum chamber, they rarely collide with each other. This occurs at pressures below about 10-7 bar. This process can be used to produce thin metal films, optical filters, mirrors and beam splitters.

Evaporation Using Boats

The source metal can be evaporated using a thin tray made from a high melting point metal, such as molybdenum which has a melting point of 2622C. The molybdenum tray is known as a boat. A high current is passed through the boat. The resistance of the boat causes it to heat up. When the melting point of the metal is reached, evaporation occurs. This method is simple, but it does have its drawbacks.

Aluminum for example, is quite inert in air because its surface oxidizes almost instantly. When it is molten in a vacuum it is highly reactive and alloys with molybdenum. Very low vapor pressure elements such as molybdenum, tungsten and carbon can not be evaporated using this method because the boat would melt before the source.

Other ohmic heating methods include the use of tungsten baskets and filaments. Tungsten has a melting point of 3382C. This allows it to be used for higher melting point materials.

Besides supplying almost all shapes of evaporation boats and filaments. T&D also supply tungsten, molybdenum and tantalum foils and wires.

http://www.tdmfginc.com/