 | ||
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 absorption. Thus the getter removes small amounts of gas from the evacuated space.
Contents
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 absorbed 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.
Getters cannot react permanently with inert gases, though some getters will adsorb them in a reversible fashion. Also, hydrogen is usually handled by adsorption rather than reaction.
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.
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. Different types of getter use different ways of doing this:
Flashed getters
Flashed getters are prepared by arranging a reservoir of volatile and reactive material inside the vacuum system. Once the system is evacuated and sealed, the material is heated (usually by radio frequency induction heating). After evaporating, it is deposited as a coating on the interior surfaces of the system. Flashed getters (typically made with barium) are commonly used in vacuum tubes. The getter will usually 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, magnesium, calcium, sodium, strontium, caesium and phosphorus.
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 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/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 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, 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, an R.F. induction heating coil connected to a powerful R.F. oscillator operating in the 27 MHz or 40.68 MHz 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. Large R. F. 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. Once 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—but 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; 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:
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.