A getter is a deposit of reactive material that is placed inside a vacuum system, for the purpose of completing and maintaining the vacuum. When gas molecules strike the getter material, they combine with it chemically or by adsorption. Thus the getter removes small amounts of gas from the evacuated space.
The getter is usually a coating applied to a surface within the evacuated chamber.
A vacuum is initially created by connecting a closed container to a vacuum pump. After achieving a vacuum, the container can be sealed, or the vacuum pump can be left running. Getters are especially important in sealed systems, such as vacuum tubes, including cathode ray tubes (CRTs), and vacuum insulated panels, which must maintain a vacuum for a long time. This is because the inner surfaces of the container release adsorbed gases for a long time after the vacuum is established. The getter continually removes this residual gas as it is produced. Even in systems which are continually evacuated by a vacuum pump, getters are also used to remove residual gas, often to achieve a higher vacuum than the pump could achieve alone. Although it weighs almost nothing and has no moving parts, a getter is itself a vacuum pump.
Small amounts of gas within a vacuum tube will ionize, causing undesired conduction leading to major malfunction. Small amounts of gas within a vacuum insulated panel can greatly compromise its insulation value. Getters help to maintain the vacuum.
To avoid being contaminated by the atmosphere, the getter must be introduced into the vacuum system in an inactive form during assembly, and activated after evacuation. This is usually done by heat. Different types of getter use different ways of doing this:
- Flashed getter - The getter material is held inactive in a reservoir during assembly, then heated and evaporated after initial evacuation, usually by induction heating. The vaporized getter, usually a volatile metal, instantly reacts with any residual gas, then condenses on the cool walls of the tube in a thin coating, the getter spot or getter mirror, which continues to absorb gas. This is the most common type, used in low power vacuum tubes.
- Non-evaporable getter (NEG) The getter remains in solid form.
- Coating getter - a coating applied to metal parts of the vacuum system that will be heated during use. Usually a nonvolatile metal powder sintered in a porous coating to the surface of the electrodes of power vacuum tubes, maintained at temperatures of 200° to 1200°C during operation.
- Bulk getter - sheets, strips, wires or sintered pellets of gas absorbing metals which are heated, either by mounting them on hot components or by a separate heating element. These can often be renewed or replaced
- Getter pump or sorption pump - In laboratory vacuum systems the bulk NEG getter is often held in a separate vessel with its own heater, attached to the vacuum system by a valve, so that it can be replaced or renewed when saturated.
"Flashed getters" are prepared by arranging a reservoir of a volatile and reactive material inside the vacuum system. Once the system is evacuated and sealed, the material is heated, usually by RF induction heating, and evaporates, depositing itself on the walls to leave a coating. Flashed getters are commonly used in vacuum tubes, and the standard flashed getter material is barium. It can usually be seen as a silvery metallic spot on the inside of the tube's glass envelope. Large transmitting and specialized tubes often use more exotic getters, including aluminium, magnesium, calcium, sodium, strontium, caesium and phosphorus.
If the tube breaks, the getter reacts with incoming air leaving a white deposit inside the tube, and it becomes useless; for this reason, flashed getters are not used in systems which are intended to be opened. A functioning phosphorus getter looks very much like an oxidised metal getter, though it has an iridescent pink or orange appearance which oxidised metal getters lack. Phosphorus was frequently used before metallic getters were developed.
In systems which need to be opened to air for maintenance, a titanium sublimation pump provides similar functionality to flashed getters, but can be flashed repeatedly. Alternatively, nonevaporable getters may be used.
Non-evaporable getters which work at high temperature generally consist of a film of a special alloy, often primarily zirconium; the requirement is that the alloy materials must form a passivation layer at room temperature which disappears when heated. Common alloys have names of the form St (Stabil) followed by a number:
- St 707 is 70% zirconium, 24.6% vanadium and the balance iron,
- St 787 is 80.8% zirconium, 14.2% cobalt and balance mischmetal,
- St 101 is 84% zirconium and 16% aluminium.
In tubes used in electronics, the getter material coats plates within the tube which are heated in normal operation; when getters are used within more general vacuum systems, such as in semiconductor manufacturing, they are introduced as separate pieces of equipment in the vacuum chamber, and turned on when needed.
It is of course important not to heat the getter when the system is not already in a good vacuum.
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- Espe, Werner; Max Knoll, Marshall P. Wilder (October 1950). "Getter materials for electron tubes". Electronics (McGraw-Hill): 80–86. ISSN 0883-4989. Retrieved 21 October, 2013. on Pete Miller's Tubebooks website
- Jousten, Karl (2008). Handbook of Vacuum Technology. John Wiley & Sons. pp. 463–474. ISBN 3527407235.
- Nonevaporable getter alloys - US Patent 5961750
- Stokes, John W. 70 Years of Radio Tubes and Valves: A Guide for Engineers, Historians, and Collectors. Vestal Press, 1982.
- Reich, Herbert J. Principles of Electron Tubes. Understanding and Designing Simple Circuits. Audio Amateur Radio Publication, May 1995. (Reprint of 1941 original).