Product Code Database
A getter is a deposit of reactive material that is placed inside a vacuum system to complete and maintain 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 container to a vacuum pump. 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 , including (CRTs), vacuum insulating glass (or vacuum glass)IGMA (FGIA) TB-2600; Vacuum Insulating 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 , though some getters will adsorb them in a reversible way. Also, hydrogen is usually handled by adsorption rather than by reaction.
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 hermetic seal. 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, 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 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, 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, indicates total failure of the seal on the vacuum system, as shown in the fluorescent display module depicted above.
During activation, while the bulb is still connected to the pump, an RF induction heating coil connected to a powerful RF 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 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.
It is, of course, important not to heat the getter when the system is not already in a good vacuum.
|
|
