Electric lamp and discharge devices – With getter – Electrode includes getter – supports getter – or is connected...
Patent
1995-11-15
1998-01-27
Horabik, Michael
Electric lamp and discharge devices
With getter
Electrode includes getter, supports getter, or is connected...
313481, 313553, H01J 1970
Patent
active
057125295
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to vacuum electron tubes. It is applicable particularly to cathode-ray visual-display tubes and will be described more precisely in the context of this type of tube.
DESCRIPTION OF THE BACKGROUND
Vacuum electron tubes, as their name indicates, require a high vacuum in an enclosure in which the electrons move. The tube is therefore pumped out during manufacture before it is finally closed. However, this pumping does not make it possible to form and conserve an absolute vacuum: particularly because a phenomenon of "outgassing" occurs during the latter stages of manufacture and during the subsequent use of the tube. Outgassing is the freeing of gaseous molecules present in certain components within the tube.
Vacuum tubes, and particularly cathode-ray visual-display tubes, therefore always include a material for absorbing the residual gases; this material is called "getter". It is placed within the tube and has strongly absorbing properties for the residual gas molecules which are found in the tube. Barium is a material very widely used as getter.
To be effective, the getter has to exhibit as large a surface area as possible; this is because absorption takes place in proportion to surface area. Hence a very fine layer of getter is deposited over part of the internal walls of the vacuum tube in order to take advantage of as large a surface area as possible. Moreover, this deposition is obviously not carried out until the vacuum has been formed within the tube, as the getter would immediately be saturated and would no longer fulfil its purpose of eliminating the residual gases after pumping.
For this reason, in the prior art, the getter is usually installed in the following way, for a cathode-ray tube including a bulb and a neck with an electron gun mounted in the neck: a trough containing the material of the getter is mounted on a support, which is itself fixed to the electron gun. The gun and getter support assembly is inserted into the neck. The tube is then closed. The vacuum is formed by pumping through a stem. Then the stem is closed, finally sealing the tube.
Only then is the getter evaporated onto the walls of the tube. Evaporation is carried out by heating the material at high frequency by induction, through the walls of the neck, up to a temperature of about 800.degree. to 1000.degree. C. At this temperature an exothermic reaction is triggered between the components present in the trough (generally a sintered powder of aluminium and of barium, Al.sub.4 Ba), and the evaporation process continues until the getter material is exhausted. The evaporation is directional, in all the directions allowed by the shape of the walls of the trough containing the starting material. These walls are shaped and oriented so that the barium comes to be deposited on the walls of the tube.
The constraints on installing and producing the getter support are many, particularly: tube, must not interfere with the optics for electrostatic focusing of the electron beam emitted by the gun; deflection of the electron beam; gun (insulating parts which would risk being short-circuited, or conducting parts which would come to emit spurious electrons since barium is a material of very high electron emissivity); be too expensive; to strict constraints on resistance to impacts and to acceleration.
In the prior art, the getter support is fixed, generally by welding, onto an electrode of the electron gun. This electrode is, in principle, the final electrode at the front end of the gun (the final electrode forwards, that is to say in the direction of the electron beam emitted by the gun); this end electrode is linked to the highest operating voltage of the tube, called screen voltage; there is generally no difficulty then: the trough containing the getter can be turned towards the front for evaporation onto a large surface area in the neck in front of the gun or even as far as the bulb. There is little risk of the barium returning towards the components of the electron gun; the getter support
REFERENCES:
patent: 3334258 (1967-08-01), Kaplan et al.
patent: 4721882 (1988-01-01), Van Daelen
patent: 5210460 (1993-05-01), Utsumi et al.
Gallet Philippe
Ricaud Jean-Luc
Day Michael
Horabik Michael
Thomson Tubes Electroniques
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