SAES Getters' development of monochrome, color picture and display tube getters has been continuous for the last 60 years.
Getter development began with the mass production of the stable barium-aluminum alloy (BaAl4), followed by the
introduction of the ring-shaped exothermic and nitrogen-doped getters. The subsequent development of total-yield, ceramic and delay-nitrogen-doped getters enabled the production of the first rectangular color cathode ray tube (CCRT) in the Sixties.
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With the advent of soft-flash technology, SAES Getters Group’s R&D laboratories developed the high-wall, closed centre getter with wire supports for use in normal neck tubes, 20 mm in diameter, and in mini-neck tubes, 22.5 mm diameter.
In the 80s and 90s SAES Getters developed advanced products which are nowadays largely adopted in most of CCRTs, such as low-argon and low-particle getters for high definition and high resolution tubes using dispenser cathodes, and High Yield Getters that ensure a barium yield up to 400 mg for large size tubes. Frittable Getters which can be mounted inside the tube before the frit sealing process, are also available by SAES. The low-gas characteristics of the series 600 getter ensure a cleaner residual gas atmosphere, with the advantage of exploiting the cathode performances, and provide economical saving including a shorter time of residence for the tube in the production process. |
Exothermic Barium Ring Getters
When properly heated, barium getter devices produce a film of pure elemental barium deposited on the internal surfaces of evacuated tubes. The chemical activity of the barium film is extremely high, which makes it capable of permanently absorbing active gas molecules such as CO, CO2, N2, O2, H2O, H2 outgassed from the tube components during shelf life and operation.
The barium film, however, does not absorb rare gases such as argon, helium or krypton. These gases are not chemically active with barium or any other element. The basic components of barium getters are a powdered barium-aluminum alloy mixed with powdered nickel.
When this mixture is heated to 800 °C, an exothermic reaction between aluminum and nickel takes place. The getter temperature raises to approximately 1250 °C in a fraction of a second. The chemical formula describing this reaction is:
BaAl4 + 4Ni --> Ba + 4NiAl
The barium vapor pressure at this temperature is above 2 mbar, so the barium readily evaporates. Exothermic getters ensure a controlled, reliable barium evaporation, since the heat generated by the chemical reaction between aluminum and nickel can accommodate fluctuations of radio frequency (RF) power.
Getter Activation
The getter releases barium rapidly, due to the exothermic reaction which occurs above 800°C. This action prevents damage to the internal electronic parts of the tube and allows for maximum throughput in the production facility. The getter is usually heated by induced currents generated inside it by closely positioning an external coil connected to a radio frequency generator.
This getter activation process is known as “getter flash”.
Relevant Getter Parameters
Another important characteristic of a barium getter is the melting point. If too much power is applied to the coil, the getter will show a very short start time and the expected barium yield will be very high. When this occurs, the getter is not able to dissipate all of the energy generated by the external coil and locally melt, or the powder packet may lift upward. Melting of the getter container or lifting of the powder packet must of course be avoided, since in both cases particles may be lost from the getter, thus causing mask aperture blockage or arcing between the grids. It is very important to follow the recommended flashing conditions, for instance the start time and the total time, reported in the getter yield curves, to avoid insufficient barium yields or melting of the getter container.
Getters Parameters - Term Description
Total Time
The time that elapses between the switch-on and switch-off of the radio frequency generator connected to the coil: total time is, therefore, the duration of the external getter heating process performed by means of the RF coil. This is generally a fixed parameter, such as 30, 35 or 40 seconds.
Start Time
The time that elapses between switch-on of the RF generator and the beginning of the exothermic reaction. The start of the exothermic reaction can be observed when the getter suddenly becomes very bright. The getter color changes dramatically from dark red to bright red due to the intense heat generated by the exothermic reaction. This reaction increases the getter temperature from 800 to 1250 °C in a fraction of a second and initiates barium evaporation.
Barium Yield
The amount of barium, expressed in milligrams, evaporated from the getter. From the description of the getter heating process, this yield signifies the barium released between the start time and the R.F. generator switch-off.
Nitrogen-doped Getters
An inadequate barium film distribution in the past often caused a dark area in the screen center. This was first observed in monochrome picture tubes, where the getter was typically gun mounted. Several solutions were proposed, including the use of mechanical deflectors attached to the getter, in order to direct the barium away from the centre of the screen.
Nitrogen gas doping is the most successful way of controlling barium distribution. A small amount of Fe4N is added to exothermic alloy fill. The thermal stability of Fe4N ensures that nitrogen is not released during tube processing. During the getter flash, temperatures sufficient to decompose Fe4N are achieved just prior to barium evaporation. Nitrogen forms a "cloud" in front of the getter immediately before the barium evaporates and the first substantial amount of barium leaving the channel of the getter encounters localized high-nitrogen pressure. This pressure causes the barium to scatter in all directions, but particularly away from the screen.
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RECOMMENDED BARIUM YIELDS FOR GETTERS
AS A FUNCTION OF THE TUBE SIZE |
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TUBE SIZE (inches) |
BARIUM YELD RANGE (mg) |
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14 |
120 - 140 |
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16 |
140 - 160 |
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18 |
160 - 180 |
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20 |
180 - 200 |
|
22 |
200 - 220 |
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24 |
220 - 240 |
|
26 |
240 - 260 |
|
28 |
260 - 280 |
|
30 |
280 - 300 |
|
32 |
300 - 320 |
|
34 |
320 - 340 |
|
36 |
340 - 360 |
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38 |
360 - 380 |
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