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.
The specific flash getter seen above inside the 12AX7 vacuum tube/thermionic valve consists of a ring shaped structure made from a long strip of nickel, bent up into a long, narrow trough and then folded into the ring shape with the trough opening facing upwards in the specific case depicted above. The trough is filled with a mixture of barium azide and powdered glass.
During activation, whilst the bulb is still on the pump, and R.F. induction coil connected to a powerful R.F. oscillator operating on the 27MHz or 40.68Mz ISM band is positioned around the bulb in the plane of the ring. The coil acts as the primary of a transformer and the ring as a single shorted turn. The coil is moved along the axis of the bulb so as not to over couple and melt the ring. Once the ring is heated the barium azide decomposes into barium and nitrogen. The nitrogen is pumped out and the barium condenses on the bulb above the plane of the ring forming a mirror like deposit with a large surface area. The powdered glass in the ring melts and entraps any particles which could otherwise escape loose inside the bulb causing later problems. This getter acts when activated and continues to act after the bulb is sealed off from the pump. When these small, sealed vacuum systems are used and get hot, any adsorbed gasses released from metallic parts, such as anodes/plates or grids or non metallic, but porous parts, such as sintered ceramic parts are eventually trapped on the large area of reactive barium or other metal deposited on the bulb wall.
To those unfamiliar with sealed vacuum devices, such as vacuum tubes/thermionic valves, High pressure sodium lamps or some types of metal-halide lamps, are often mistaken into thinking the flash getter deposit is caused as a result of the failure of the device. Note that contemporary high intensity discharge lamps tend to use non-evaporable getters rather than flash getters.
Those familiar with such devices can often make qualitative assessments as to the hardness or quality of the vacuum within by the appearance of the flash getter deposit. A shiny deposit indicating a good vacuum. As the getter is used, the deposit often becomes thin and translucent particularly at the edges. It can take on a brownish-red semi translucent appearance and this indicates poor seals or extensive use of the device at elevated temperatures. A white deposit, usually of barium oxide indicates total failure of the seal on the vacuum system. as depicted in the fluorescent display module depicted below.
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).