getter

(center) A vacuum tube with a "flashed getter" coating on the inner surface of the top of the tube. (left) The inside of a similar tube, showing the reservoir that holds the material that is evaporated to create the getter coating.

A getter is a reactive material used for removing traces of gas from vacuum systems, such as vacuum tubes. Residual gas can be left in vacuums by inadequate vacuum pumps, or adsorbed gasses can be released after evacuation by the inner surfaces of the container. The getter is usually a coating applied to a surface within the evacuated chamber. When molecules of residual gas strike the getter surface they chemically combine with the material, removing them from the evacuated space.

Flashed getters

"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 aluminum, magnesium, calcium, sodium, strontium, caesium and phosphorus.

If the tube is broken, the getter reacts with incoming air leaving a white deposit on the tube, and 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 has an iridescent appearance which oxidised metal getters lack.

Nonevaporable getters

For systems which need to be opened to air for maintenance, nonevaporable getters which work at high temperature are used instead. Generally, these consist of a film of a special alloy, often consisting primarily of 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 ^[1].

(St 707, St 787, St 101 are registered trademarks of SAES Getters S.p.A.) 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.

Even slight amounts of gas within a vacuum insulation panel can greatly compromise the insulation value of the wall, and the materials used to support the walls can outgas, so getters are used there to maintain the vacuum.

Getters obviously cannot react permanently with inert gases, though some of them will adsorb them in a reversible fashion; usually hydrogen is also handled by adsorption rather than reaction.

See also

• Ion pump (physics)

References

Look up getter in Wiktionary, the free dictionary.

• 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).