Electric lamp and discharge devices – Cathode ray tube – Envelope
Reexamination Certificate
1998-09-15
2001-07-17
Patel, Ashok (Department: 2879)
Electric lamp and discharge devices
Cathode ray tube
Envelope
C313S547000
Reexamination Certificate
active
06262527
ABSTRACT:
The invention relates to a cathode ray tube provided with an electron gun which comprises at least a first and a second grid, and at least one cathode which, during operation, emits electrons by way of semiconductor action.
A cathode ray tube is suitable as a pick-up or display tube, but may be alternatively used in apparatus for Auger spectroscopy, electron microscopy and electron lithography.
A cathode ray tube for a monochrome display device, for example a television or monitor, has a glass envelope which is composed of a screen and a cone. The widest end of the cone is secured to the screen. Its narrowest end terminates in a tubular end having a substantially circular cross-section, which end is referred to as the neck. A phosphor screen consisting of a phosphor layer is present on the screen. The tubular end accommodates an electron gun which emits an electron beam during operation. This beam can be sent to a given spot on the display screen by means of deflection coils which generate a given magnetic field.
The display screen is activated by scanning the electron beam along the screen, which beam is modulated by a video signal. This video signal ensures that the phosphors are excited in accordance with such a pattern that their luminescence produces an image. When many electrons land on the pixel during its excitation time, this pixel luminesces more brightly. The video signal is applied to the cathode via electric current conductors.
There are many pixels per unit of surface area. Moreover, the pixels are excited one after the other within a very short time. The viewer thus experiences a moving image from a normal viewing distance.
In a color display device, for example a color television or a color monitor, each pixel has three phosphor elements each luminescing in a different primary color. As it were, there are three uniform regular patterns on the display screen, each pattern having a different luminescence color. Instead of one electron beam, three electron beams emitted by three different cathodes in the color electron gun are scanned along the screen during operation. Each of these three beams excites the pixels with a given luminescence color. Since the phosphor elements of a pixel are located close together, the viewer experiences them as a single element, not as separate elements. The color which is experienced is a mixed color of the three elements. By exciting each element with a given intensity, the viewer experiences a given color. For example, if the red element and the blue element are excited to a large extent and the green element is excited to a small extent, the viewer will experience the mixed color purple. Furthermore, similarly as for a monochrome cathode ray tube, it holds that the pixels are situated so close together that the viewer does not see them as separate pixels from a normal viewing distance. This produces a color image.
During production, the envelope of the cathode ray tube must be vacuum-exhausted before it is sealed. This is essential for its operation because an electron beam can only propagate substantially undisturbed through vacuum.
The electron beams are generated in, and emitted by an electron gun. This electron gun comprises a plurality of electrostatic grids which, in their sequence of increasing distance to the neck, are referred to as G
1
, G
2
, G
3
and so forth. The different electrostatic grids have different electric potentials during operation and must therefore not be in contact with each other. To achieve this, they are fixed relative to each other by means of glass rods in which they are secured by means of brackets. The first grid G
1
(grid
1
) has a skirt accommodating one or more cathodes. These cathodes have a surface which emits electrons during operation. An electron emitted by such a cathode passes through an aperture in the G
1
and subsequently through apertures in the G
2
, G
3
, and so forth. Finally, the electron leaves the electron gun so as to move towards the display screen.
Hitherto, thermionic cathodes emitting electrons by thermal radiation have mainly been used in electron guns for cathode ray tubes. Such a cathode has an envelope accommodating a filament and a cap from which the electrons are emitted. The cap is made of a sintered material. The surface of this cap is provided with barium which has the effect of decreasing the work function for the thermal emission. However, this barium is oxidized on the surface by residual gases, particularly oxygen, which are still in the tube after it has been vacuum-exhausted and sealed or which are released from the wall of the envelope or the materials from which the other parts of the cathode ray tube are made. Due to diffusion, barium is supplemented from the sintered material. When the concentration of oxidizing gases in the vicinity of the cathode exceeds a given value, the dispense is too slow to maintain the barium layer. It has been found that the gas may have a maximal pressure of 10
−10
to 10
−9
Pa to ensure a satisfactory electron emission. This pressure range is maintained as a standard in the production of cathode ray tubes.
There is a problem when, instead of thermionic cathodes, cathodes are used which operate by way of semiconductor action (referred to as “semiconductor cathodes”). These may be, for example field emitters that particularly reverse-biased junction cathodes (such as the avalanche cold cathode). A cathode of this type is described in U.S. Pat. No. 5,243,197. The surface of a semiconductor cathode also bears a material decreasing the work function. This is preferably cesium. Here, too, the material decreasing the work function is attacked by residual gases. Particularly the oxidation by oxygen-containing gases is harmful. Dispensing cesium from within a semiconductor cathode is, however, impossible because this cathode does not have a thick cap of sintered material, which is porous, but has a smooth surface instead. Cesium can neither be dispensed from the bulk of the cathode because the cathode has such a low temperature that the cesium has a negligible diffusion rate. The standard gas pressure in a cathode ray tube which is allowed for a thermionic cathode will rapidly render a semiconductor cathode inactive. In a standard CRT, semiconductor cathodes will thus rapidly get out of order.
It is an object of the invention to provide a cathode ray tube comprising an electron gun in which a semiconductor cathode can function at a standard pressure.
To this end, the cathode ray tube according to the invention is characterized in that the electron gun comprises means for making the partial gas pressure of oxidizing residual gases near the cathode lower than in other parts of the tube.
This means may be a getter, positioned near the cathode, in the electron gun, which getter removes oxidizing gas molecules. The relevant space near the cathode is very small with respect to the other parts of the tube. When the tube is put into operation, gases can be removed from the cathode space with a small amount of getter. Subsequently, gas still enters the cathode space from the other parts of the tube, but this can be limited by means of a getter provided on the walls of the electron gun. This may be done in a very efficient way if the apertures in the cathode space comply with at least one of the following conditions:
The aperture is “out of sight” of the cathode. This means that no straight line can be drawn between the aperture and the cathode. A gas molecule thus has to collide first with a wall if it is to reach the cathode. If this wall is provided with a getter, the molecule will certainly be captured.
The means comprise means for reducing the distance between the first and the second grid, forming an aperture (
40
) between the first and the second grid having a length (
1
) which is at least more than twice its distance (d). There is a small risk that a gas molecule passes through such aperture without colliding with a wall. As a result, only a small quantity of gas diffuses through it per unit of time. When the walls a
Gehring Frederik C.
Van Zutphen Tom
Gerike Matthew J.
Patel Ashok
U.S. Philips Corporation
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