![Getter diagram](https://upload.wikimedia.org/wikipedia/commons/4/4a/Getter_diagram.png)
A getter is a deposit of reactive material that is placed inside a
vacuum
A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or " void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often di ...
system to complete and maintain the vacuum. When gas molecules strike the getter material, they combine with it chemically or by
absorption. 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 container to a
vacuum pump
A vacuum pump is a device that draws gas molecules from a sealed volume in order to leave behind a partial vacuum. The job of a vacuum pump is to generate a relative vacuum within a capacity. The first vacuum pump was invented in 1650 by Otto ...
. After achieving a sufficient vacuum, the container can be sealed, or the vacuum pump can be left running. Getters are especially important in sealed systems, such as
vacuum tube
A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied.
The type known as ...
s, including
cathode ray tubes (CRTs), Vacuum Insulating Glass (or Vacuum Glass) 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 residues of a reactive gas, such as oxygen, as long as it is desorbed from a surface, or continuously penetrating in the system (tiny leaks or diffusion through a permeable material). 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 is often present in minute amounts and has no moving parts, a getter behaves in itself as a vacuum pump. It is an ultimate chemical sink for reactive gases.
Getters cannot react with
inert gas
An inert gas is a gas that does not readily undergo chemical reactions with other chemical substances and therefore does not readily form chemical compounds. The noble gases often do not react with many substances and were historically referred to ...
es, though some getters will adsorb them in a reversible way. Also,
hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
is usually handled by
adsorption
Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which ...
rather than by reaction.
Types
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.
[ on Pete Miller']
Tubebooks
website Different types of getter use different ways of doing this:
; Flashed getter: The getter material is held inactive in a reservoir during assembly and initial evacuation, and then heated and evaporated, usually by
induction heating
Induction heating is the process of heating electrically conductive materials, namely metals or semi-conductors, by electromagnetic induction, through heat transfer passing through an induction coil that creates an electromagnetic field within th ...
. The vaporized getter, usually a volatile metal, instantly reacts with any residual gas, and 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 tube
A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied.
The type known as ...
s.
; Non-evaporable getter (NEG)
: The getter remains in solid form.
Flashed getters
![Opachki dead vacuum luminescent display bednyaga da](https://upload.wikimedia.org/wikipedia/commons/1/19/Opachki_dead_vacuum_luminescent_display_bednyaga_da.JPG)
Flashed getters are prepared by arranging a reservoir of volatile and reactive material inside the vacuum system. After the system has been evacuated and sealed under rough vacuum, the material is heated (usually by radio frequency
induction heating
Induction heating is the process of heating electrically conductive materials, namely metals or semi-conductors, by electromagnetic induction, through heat transfer passing through an induction coil that creates an electromagnetic field within th ...
). After evaporating, it deposits as a coating on the interior surfaces of the system. Flashed getters (typically made with
barium) are commonly used in
vacuum tube
A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied.
The type known as ...
s. Most getters can be seen as a silvery metallic spot on the inside of the tube's glass envelope. Large transmission tubes and specialty systems often use more exotic getters, including
aluminium
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...
,
magnesium
Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ta ...
,
calcium
Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar t ...
,
sodium
Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...
,
strontium,
caesium, and
phosphorus
Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...
.
If the getter is exposed to atmospheric air (for example, if the tube breaks or develops a leak), it turns white and becomes useless. For this reason, flashed getters are only used in
sealed systems. A functioning phosphorus getter looks very much like an oxidised metal getter, although 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
A titanium sublimation pump (TSP) is a type of vacuum pump used to remove residual gas in ultra-high vacuum systems, maintaining the vacuum.
Principle of operation
Its construction and principle of operation is simple. It consists of a titanium ...
provides similar functionality to flashed getters, but can be flashed repeatedly. Alternatively, nonevaporable getters may be used.
Those unfamiliar with sealed vacuum devices, such as
vacuum tubes
A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied.
The type known as a ...
/thermionic valves,
high-pressure sodium lamps or some types of
metal-halide lamp
A metal-halide lamp is an electrical lamp that produces light by an electric arc through a gaseous mixture of vaporized mercury and metal halides (compounds of metals with bromine or iodine). It is a type of high-intensity discharge (HID) g ...
s, often notice the shiny flash getter deposit and mistakenly think it is a sign of failure or degradation of the device. Contemporary
high-intensity discharge lamp
High-intensity discharge lamps (HID lamps) are a type of electrical gas-discharge lamp which produces light by means of an electric arc between tungsten electrodes housed inside a translucent or transparent fused quartz or fused alumina arc tu ...
s 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, with a shiny deposit indicating a good vacuum. As the getter is used up, the deposit often becomes thin and translucent, particularly at the edges. It can take on a brownish-red semi-translucent appearance, which indicates poor seals or extensive use of the device at elevated temperatures. A white deposit, usually
barium oxide
Barium oxide, also known as baria, is a white hygroscopic non-flammable compound with the formula BaO. It has a cubic structure and is used in cathode ray tubes, crown glass, and catalysts. It is harmful to human skin and if swallowed in larg ...
, indicates total failure of the seal on the vacuum system, as shown in the fluorescent display module depicted above.
Activation
The typical flashed getter used in small vacuum tubes ''(seen in 12AX7 tube, top)'' consists of a ring-shaped structure made from a long strip of nickel, which is folded into a long, narrow trough, filled with a mixture of
barium azide and powdered glass, and then folded into the closed ring shape. The getter is attached with its trough opening facing upward toward the glass, in the specific case depicted above.
During activation, while the bulb is still connected to the pump, an RF
induction heating
Induction heating is the process of heating electrically conductive materials, namely metals or semi-conductors, by electromagnetic induction, through heat transfer passing through an induction coil that creates an electromagnetic field within th ...
coil connected to a powerful
RF oscillator operating in the 27 MHz or 40.68 MHz
ISM band
The ISM radio bands are portions of the radio spectrum reserved internationally for industrial, scientific and medical (ISM) purposes, excluding applications in telecommunications.
Examples of applications for the use of radio frequency (RF) ener ...
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. Large RF currents flow in the ring, heating it. The coil is moved along the axis of the bulb so as not to overheat and melt the ring. As the ring is heated, the barium azide decomposes into barium vapor 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. The barium combines with any free gas when activated and continues to act after the bulb is sealed off from the pump. During use, the internal electrodes and other parts of the tube get hot. This can cause adsorbed gases to be released from metallic parts, such as anodes (plates), grids, or non-metallic porous parts, such as sintered ceramic parts. The gas is trapped on the large area of reactive barium on the bulb wall and removed from the tube.
Non-evaporable getters
''Non-evaporable getters'', which work at high temperature, generally consist of a film of a special alloy, often primarily
zirconium
Zirconium is a chemical element with the symbol Zr and atomic number 40. The name ''zirconium'' is taken from the name of the mineral zircon, the most important source of zirconium. The word is related to Persian '' zargun'' (zircon; ''zar-gun'' ...
; 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
Zirconium is a chemical element with the symbol Zr and atomic number 40. The name ''zirconium'' is taken from the name of the mineral zircon, the most important source of zirconium. The word is related to Persian '' zargun'' (zircon; ''zar-gun'' ...
, 24.6%
vanadium, and the balance
iron
Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundanc ...
.
*St 787 is 80.8%
zirconium
Zirconium is a chemical element with the symbol Zr and atomic number 40. The name ''zirconium'' is taken from the name of the mineral zircon, the most important source of zirconium. The word is related to Persian '' zargun'' (zircon; ''zar-gun'' ...
, 14.2%
cobalt
Cobalt is a chemical element with the symbol Co and atomic number 27. As with nickel, cobalt is found in the Earth's crust only in a chemically combined form, save for small deposits found in alloys of natural meteoric iron. The free element, p ...
, and the balance
mischmetal
Mischmetal (from german: Mischmetall – "mixed metal") is an alloy of rare-earth elements. It is also called cerium mischmetal, or rare-earth mischmetal. A typical composition includes approximately 55% cerium, 25% lanthanum, and 15 ...
.
*St 101 is 84%
zirconium
Zirconium is a chemical element with the symbol Zr and atomic number 40. The name ''zirconium'' is taken from the name of the mineral zircon, the most important source of zirconium. The word is related to Persian '' zargun'' (zircon; ''zar-gun'' ...
and 16%
aluminium
Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...
.
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
Semiconductor device fabrication is the process used to manufacture semiconductor devices, typically integrated circuit (IC) chips such as modern computer processors, microcontrollers, and memory chips such as NAND flash and DRAM that are pres ...
, they are introduced as separate pieces of equipment in the vacuum chamber, and turned on when needed.
Deposited and patterned getter material is being used in microelectronics packaging to provide an ultra-high vacuum in a sealed cavity. To enhance the getter pumping capacity, the activation temperature must be maximized, considering the process limitations.
High-Q MEMS gyroscope
/ref>
It is, of course, important not to heat the getter when the system is not already in a good vacuum.
See also
*Ion pump (physics)
An ion pump (also referred to as a sputter ion pump) is a type of vacuum pump which operates by sputtering a metal getter. Under ideal conditions, ion pumps are capable of reaching pressures as low as 10−11 mbar. An ion pump first ionizes ga ...
References
*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).
External links
How to activate getter in GU74B / 4CX800A
An Ultrahigh Vacuum Packaging Process Demonstrating Over 2 Million Q-Factor in MEMS Vibratory Gyroscopes
IEEE Sensors Letters
{{Thermionic valves
Vacuum tubes