WO2004066412A2

WO2004066412A2 - Resistive high-voltage divider, electron gun incorporating a resistive divider and cathode ray tube

Info

Publication number: WO2004066412A2
Application number: PCT/EP2004/000400
Authority: WO
Inventors: Ronald Van Der Wilk; Herman Schreuders; Tom Van Zutphen; Petrus Jacobus Antonius Derks; Gosse De Vries
Original assignee: Lg. Philips Displays
Priority date: 2003-01-20
Filing date: 2004-01-13
Publication date: 2004-08-05
Also published as: WO2004066412A3

Abstract

A resistive high voltage divider (30) is provided at an outer surface with an insulating material having a relatively limited conductance. Furthermore, the material has a low secondary electron emission coefficient. These features reduce secondary electron emission processes that occur on the outer surface of such resistive dividers when used for an electron gun of a Cathode Ray Tube (CRT). As a result, flashovers in the CRT are less likely to take place, and the lifetime of the CRT is increased. A preferred material system for application on the resistive divider surface is a glass compound including at least 20 weight percent chromium oxide (Cr2O3).

Description

Resistive high- voltage divider, electron gun incorporating a resistive divider and cathode ray tube

The invention relates to a cathode ray tube having an electron gun with a resistive high- voltage divider, a resistive high-voltage divider and an electron gun.

Cathode ray tubes (CRTs) are widely applied in display devices such as television sets and computer monitor. A CRT is a vacuum glass envelope, in which an image is generated by accelerating an electron beam towards a screen, which illuminates at a location where the electron beam lands.

The operation of a CRT relies on modulating the electron beam in accordance with a video signal supplied to the display device. A screen of the CRT is provided with luminescent material such as a phosphor. A picture element where the electron beam hits the luminescent material illuminates at a brightness proportionate to a beam current of the electron beam. For displaying an image, the electron beam is scanned over the entire area of the display screen.

The CRT glass envelope comprises a screen portion, a funnel portion and a neck portion. In the neck portion, an electron gun is arranged for generating and accelerating the electron beam. The electron gun includes a number of electrodes for accelerating the electron beam, and focusing it such that a spot size of the electron beam hitting the screen is relatively small. The electron beam is mainly focused onto the screen by a main lens. The main lens is conventionally generated between two electrodes, namely a focus electrode at a potential of 6-8 kV and an anode at a potential of 25-32 kV.

Recent developments in CRTs, such as large screen sizes, a substantially flat front face of the screen portion, and large beam deflection angles, have led to higher requirements for the spot size. To decrease the spot size, the so-called bleeder electron gun has been developed. In such a gun, the main lens is formed by at least three electrodes. A resistive high- voltage divider (bleeder) is used to generate an intermediate voltage to be supplied to an intermediate electrode in between the focus electrode and the anode. The intermediate voltage is usually about 50% of the anode potential.

During the operation of a cathode ray tube, the electric potential of insulating surfaces, in particular insulating surfaces adjacent high-voltage portions of the CRT, such as the anode, may be disrupted due to charging. An example of such a surface is the inner surface of the neck portion of the glass envelope. The charging is, amongst others, caused by a secondary electron emission process at the insulating surface.

When an initial electron, usually a stray electron, impinges on the insulating surface, secondary electrons are generated. If a secondary electrons falls back onto the insulating surface, further secondary electrons are generated. The secondary electrons are accelerated towards the high- voltage portion. Thus, the secondary emission process leads to a so-called 'hopping' transport of electrons, along the insulating surface, in the high-voltage direction. As a result, the insulating surface itself is charged positively, particularly at the lower voltage side.

A particular instance of this charging process occurs between the multiform rod to which the electrodes are attached, and the inner surface of the neck portion of the CRT. Both elements are usually formed from glass, which has a particularly high secondary electron emission coefficient. As a result, the neck portion and multiform rod define a channel in which secondary electrons define the charge distribution on the surfaces. This can lead to an increase in electron current due to secondary electron emission.

In this case, particularly high charging is observed, and large voltage differences develop between the electrodes of the electron gun and the nearby insulating surfaces of multiform rod and neck portion. This can trigger cold electron emission (field emission) from the electrodes, which in turn provides additional electrons for the secondary electron emission process.

Effectively, the secondary emission process can be self-amplifying. Eventually, the resulting potential differences become so large that an electrical discharge occurs in the tube, this is the so-called flashover. A flashover causes the picture displayed on the tube to be briefly disturbed, but moreover has detrimental effects on the lifetime of the cathode ray tube, and, in a display device incorporating the CRT, on the driving electronics supplying the video signals to thέ CRT.

In conventional CRTs generally a so-called tape loop is applied. This is a metal wire enclosing the multiform rod and being supplied with a voltage of for instance 6 kV. During manufacturing, the tape loop is processed using a high frequency generator, whereby a metal reflector is deposited on the parts of the multiform rod and the neck portion of the CRT adjacent the tape loop. These metal reflectors operate as a damper for the secondary electron emission process between the multiform rod and the insulating surface of the neck portion. However, in a CRT employing an electron gun with a high- voltage resistive divider, the beneficial effect of the tape loop is insufficient. Flashovers still occur, so that the lifetime of the cathode ray tube, and a television or monitor incorporating such a cathode ray tube, are relatively limited.

It is an object of the invention to increase the lifetime of a cathode ray tube incorporating an electron gun with a resistive divider.

The above-mentioned object has been achieved by means of a cathode ray tube according to the invention, as specified in independent Claim 1. Further advantageous embodiment are defined in dependent Claims 2-4.

The invention is based on the insight that by providing at least part of the outer surface of the resistive divider with a substantially insulating material, that still has a relatively limited conductance (i.e. a material having a sheet resistance at 50 °C between 10¹⁰ and 10¹⁶ Ohms, preferably between 10¹⁰ and 10¹⁴ Ohms) and a relatively low secondary electron emission coefficient (i.e. a secondary electron emission coefficient between 0.1 and 2 as measured for electrons striking the outer surface at a substantially right angle), charging due to secondary electron emission can be reduced, and the occurrence of flashovers may be decreased or even eliminated. The effect of the material is twofold:

Firstly, due to the conductivity of the material, electron transport takes place inside the material, so that voltage differences developing due to charging may be leveled. Thus, the resulting electric fields are generally smaller, and field emission is less likely to occur. The sheet resistance should not be larger than 10¹⁶ Ohms, because the electron transport is then insufficient. The range between 10¹⁴and 10¹⁶ Ohms forms a less preferred range in which the electron transport is often, but not always, sufficient, due to amongst others, the temperature dependence of the sheet resistance. On the other hand, the sheet resistance should not be less than 10¹⁰ Ohms, because then the resistive track of the bleeder resistor may be effectively short-circuited and/or the required divider ratio is too heavily influenced. Preferably, the sheet resistance of the material is between 10¹¹ and 10¹³ Ohms, preferably about 10¹² Ohms. The operation of the high-voltage resistive divider is then good, while electron transport in the material is sufficient to counteract charging effects. Secondly, because of the low secondary electron emission coefficient, the number of secondary electrons that is generated upon impact of an electron is relatively low. Both effects weaken the self-amplification of the secondary emission process, so that such a process is less likely to result in an electron avalanche inducing a flashover in the cathode ray tube.

The secondary electron emission coefficient is preferably between 0.1 and 2, and most preferably be smaller or equal to 1. In the latter case, on any impact of an electron with the insulating surface, only a single secondary electron is released, so that no amplification of the secondary electron current takes place and local charging of the insulating surface is prevented.

A secondary electron emission coefficient smaller than 1 will charge the surface negative which also stops the self-amplification of the secondary emission current.

In practice it is difficult to have an effective secondary electron emission coefficient independent of the primary energy of an incident electron. A practical material instead has a yield distribution of generated secondary electrons preferably within the above- mentioned range, for essentially the incident electron energy range practically occurring during CRT operation.

In a cathode ray tube with an electron gun having a high-voltage resistive divider, the inventors have identified that secondary emission is strong as compared to an electron gun without a resistive divider.

It has been found that three interactions between the resistive divider and its surroundings play a role:

1) Secondary emission at a top surface of the resistive divider facing the inner surface of the neck portion of the CRT. Field emission may not only occur from the electrodes of the gun, but here also from conductive contact pads on the top surface. These contact pads are electrically connected to the resistive track of the resistive divider, and have a well-defined electric potential.

However, the insulating top surface adjacent the contact pads, or the inner surface of the neck portion of the CRT opposing the contact pad, may charge, so that locally a relatively strong electric field is formed in the vicinity of the contact pad. This easily leads to increased field emission originating from the contact pads.

Furthermore, the local electric field is particularly strong in the low-voltage part of the electron gun, so that a discharge may be generated from, for example, one of the pre-focus electrodes with relative ease. In US patent application 2002/0053866 Al, it is disclosed to provide the top surface with an insulating coating having a low secondary electron emission coefficient. However, due to the insulating character of the coating, electron transport is very small and voltage differences resulting from charging of the coating are not leveled. Field emission from the contact pads may still occur, also because the coating does not cover the contact pads.

2) Secondary emission at an opposite surface of the resistive divider facing the multiform rod. This area is especially problematic because the distance between the resistive divider and the multiform rod is generally small. Field emission may easily occur from contact pads connected to the resistive track and from the wiring connecting the contact pads to the respective electrodes, which wiring is generally provided at this surface. Because of the small distance, small differences in local charge lead to relatively large electric fields.

3) Secondary emission at the side surfaces of the resistive divider. Generally, the substrate of a resistive divider is formed from a ceramic material having an even higher secondary electron emission coefficient than the glass cover layer at the bottom and top surfaces, which material comprises for example aluminum oxide. At the side surfaces, the substrate material is exposed. Thus, secondary emission processes are strongest along the side surface, and a preferred electrical discharge path is formed there due to strong local charging. By providing at least one of the top surface, the bottom surface and the side surface with the material showing a limited conductivity and a relatively low secondary electron emission coefficient, charging of the respective insulating surface and field emission from nearby conductors are reduced. Flashovers are less likely to occur, and lifetime of the cathode ray tube, and of a display device incorporating the cathode ray tube, is prolonged. Preferably, one or a combination of the above-mentioned surfaces is provided with the material over substantially its entire surface area.

More preferably, each of the above-mentioned surfaces is provided with the material over substantially its entire surface area. Thus, essentially, the material encloses the resistive divider. By applying the invention, furthermore, the use of a prior art tape loop may prove to be superfluous on the side of the electron gun where the resistive divider is provided, because a metal reflector on the inner surface of the CRT neck adjacent the improved resistive divider may no longer be required to limit the secondary electron emission process. A preferred material for carrying out the invention is a glass compound comprising at least 20 weight percent of chromium oxide (Cr₂O₃). Such a material has a sheet resistance at 50 °C of about 10¹² Ohms, and a secondary electron emission coefficient of approximately 1.2 (maximum peak value of the secondary electron emission coefficient-yield distribution). A relatively high layer porosity is observed, and it is assumed that this contributes to the low secondary electron emission coefficient.

Alternatively, a glass compound may be applied which is coated with chromium oxide, for instance by means of a spraying process.

The invention will now be elucidated with reference to the enclosed drawings.

In the drawings:

Fig. 1 shows a cathode ray tube;

Fig. 2 shows part of an electron gun incorporating a resistive high- voltage divider; Figs. 3 A and 3B show a resistive high- voltage divider according to the invention and

Fig. 4 is a SEM photo of an outer surface of the resistive divider according to the invention.

For the purpose of explanation, a conventional, prior art, three color CRT

(cathode ray tube) is briefly described in the following, with reference to Figures 1 and 2. Thus, figure 1 is a schematic illustration of a three-color CRT 1. The cathode ray tube 1 comprises an evacuated glass envelope 2 with a neck portion 5, a funnel-portion 4 and a screen portion 3. During operation of the tube, an electron gun 6 arranged in the tube neck portion 5 sends electron beams 7,8,9, each one associated with a color, through a shadow mask 12 to the screen 3, so that phosphors at the screen 3 emits light. A deflection device 11 ensures that the electron beams systematically scan the display screen 3. The shadow mask 12 ensures that the screen phosphors of each color only receive the electron beam 7,8,9 associated with that particular color. Fig 2 shows the electron gun 6 in more detail. This particular gun comprises a beaded unit, including a pair of insulating glass beads 20, a plurality of electrodes Gl, G2, G3, G4, G5 attached to the glass beads 20 and a cathode structure K attached to the glass beads. The beaded unit is attached to a base plate (not shown) provided with pins for providing the required voltages for operating the electron gun. The cathode structure emits three electron beams 7, 8, 9 (see Figure 1) which are focused and accelerated by the grid electrodes G1-G5 and then strike red, green and blue phosphors coated on the screen portion 3 of the CRT envelope 2. The main lens is formed, in operation, between electrodes G3 and G4, and between electrodes G4 and G5. In order to obtain an intermediate voltage for the intermediate electrode G4, a resistive high- voltage divider (bleeder) 30 is mounted on or close to one of the glass beads 20 of the electron gun 6.

Figs. 3 A and 3B show a bleeder 30 according to the invention in more detail. Fig. 3 A is a top view of the surface of the insulating bleeder substrate 36, on which the first and second resistive tracks 35, 37 are laid out. The resistive tracks are coupled in series between a first, feeder terminal 31 and a second, anode terminal 32, and interconnected at a third, intermediate terminal 33. The first terminal 31 is connected to an electrical contact 41 wired to ground, the second terminal 32 is coupled to an electrical contact 42 wired to the anode G5 of the electron gun and the third terminal 33 is coupled to an electrical contact 43 wired to the intermediate main lens electrode G4 of the CRT. The length LI of a bleeder resistor is typically 50-70 mm and the width Wl is typically 5.7 mm. The first and second resistive tracks 35, 37 have for example a meandering or zigzag shape and are, for example, formed by a high-resistive ruthenate lead system comprising 56.1% PbO and 6.4% Ru, of which a typical sheet resistivity is 1 to 10 Mohm. The terminals 31,32,33 are, for example, formed by a low-resistive ruthenate lead system comprising 57.8% PbO and 16.3% Ru. The insulating substrate 36 is typically made out of aluminum oxide (Al₂O₃). The ruthenate containing resistive tracks 35,37 are usually laid out by a printing process and subsequently fired at 800 °C.

Fig. 3B is a side view of the bleeder along the long side (length LI). After the substrate 36 is provided with the resistive tracks 35, 37, both surfaces of the substrate are covered with a protective glass layer 38. This glass layer is usually a lead-borosilicate glass which is fired at 500-600 °C.

After this, an electrical contact 41,42,43 is inserted into the bleeder (This contact is formed by insertion of a pin through the substrate. This pin is laser welded to a contact lead, a portion of which is equipped with a spring that supplies a spring-load to the actual contact point that presses on the low resistive contact patch that provides the contact to the high resistive bleeder track. Each contact 41,42,43 is connected to a respective terminal 31,32,33 on the substrate. Finally, the bleeder 30 is enclosed with a layer 40 having a limited conductance and a low secondary electron emission coefficient. The layer 40 is preferably also provided on the long side surface of the bleeder 30 (not shown).

A suitable material for the layer 40, and a preferred method of applying such a layer 40, are set out in the following example.

EXAMPLE

A paste is prepared by mixing the following components:

35,7 weight percent chromium oxide (Cr₂O₃) and 64,3 weight percent glass.

In this example, the applied glass is Philips 116 lead glass having a composition of

80 weight percent lead oxide (PbO);

16 weight percent boron oxide (B₂O₃) and 4 weight percent zinc oxide (ZnO).

The paste is subsequently grinded using a roller-bank, so that the chromium oxide is finely distributed within the paste. In this example, this grinding step was carried out for about one day.

The paste is then applied on at least one surface of a conventional resistive divider. Preferably, the resistive divider is fully covered with the paste, which is easily attainable for example by dipcoating. After the paste is applied on the resistive divider, the glass compound material is formed by a firing step carried out at a temperature between 400 and 550 °C, preferably 450 °C.

The final material appears as a green coating on the surface of the resistive divider. The sheet resistance of the final material, when measured at 50 °C, is approximately

10¹² Ohms, and the secondary electron emission coefficient is close to 1.2 for electrons hitting the surface at a substantially right angle.

A 10-micron scale scanning electron microscope (SEM) photo showing a surface of the finally formed glass compound material is shown in Fig. 4. Agglomerates of chromium oxide (Cr₂O₃) are observed which are essentially embedded within a lead oxide

(PbO) matrix. Close to the surface, the amount of lead glass material is relatively low, so that it is the chromium oxide that predominantly determines the secondary electron emission characteristics. Furthermore, it can be seen that the surface has considerable roughness. It is assumed that this roughness contributes to the observed low secondary electron emission coefficient. On a rough surface, an electron has a relatively large chance of falling in a surface pit. In this case, secondary electrons generated inside the pit have a low probability of exiting from the pit. Consequently, the secondary electron transport is disturbed, and local charging is reduced.

The above-described example should be construed as the currently preferred embodiment of the invention. The inventors have carried out preliminary experiments on two further materials, namely - a material as described in the example above, but with a Cr₂O₃ : lead glass ratio of about 1:3. Thus, the material contains approximately 25 weight percent Cr₂O₃. Preliminary results show a sufficient reduction of flashovers in the CRT. a chromium oxide coating sprayed directly onto lead glass. This results in a bleeder having a thin coating essentially consisting of Cr₂O₃. This material is theoretically expected to operate even more efficiently, however when using this sprayed coating the

Cr₂O₃ is not embedded into the glass. As a result, the adherence of the Cr₂O₃ containing layer to the bleeder could be lower. If this results in chromium oxide being released during the lifetime of a CRT, this sprayed coating would be less suitable for use on a bleeder.

The enclosed drawings are, with the exception of the SEM photo of Fig. 4, schematic and not to scale. The invention has been described with reference to a preferred embodiment and example, but should not be construed as being limited to this. Other materials may be equally suitable and readily applied within the concept of the invention, without limitation to the materials disclosed in the examples. For example, the described lead glass / chromium oxide materials system could be replaced by a system of a borosilicate glass combined with an oxide of a different transitional metal.

Summarizing, a resistive high- voltage divider (30) is provided at an outer surface with an insulating material having a relatively limited conductance. Furthermore, the material has a low secondary electron emission coefficient. These features reduce secondary electron emission processes that occur on the outer surface of such resistive dividers when used for the electron gun of a Cathode Ray Tube (CRT). As a result, flashovers in the CRT are less likely to take place, and the lifetime of the CRT is increased. A preferred material system for application on the resistive divider surface is a glass compound including at least 20 weight percent chromium oxide (Cr₂O₃).

Claims

CLAIMS:

1. A cathode ray tube (1) including an electron gun (6) in a neck portion (5) of the tube, said electron gun (6) comprising electrodes (G1-G5) attached to a multiform rod (20), a number of said electrodes (G3, G4, G5) defining a main electron lens section, and means for supplying voltages to said electrodes, which means include a resistive high- voltage divider (30) arranged between said multiform rod (20) and an inner surface of said neck portion (5), wherein an outer surface (40) of said resistive divider (30) comprises a material having a sheet resistance at 50 °C between 10¹⁰ and 10¹⁶ Ohms, and a secondary electron emission coefficient between 0.1 and 2 as measured for electrons striking the outer surface at a substantially right angle.

2. A cathode ray tube as claimed in claim 1, wherein the sheet resistance of the material is between 10¹⁰ and 10¹⁴ Ohms.

3. A cathode ray tube as claimed in claim 2, wherein the sheet resistance of the material is between 10¹¹ and 10¹³ Ohms.

4. A cathode ray tube as claimed in Claim 3, wherein the sheet resistance of the material is about 10¹² Ohms.

5. A cathode ray tube as claimed in Claim 1, wherein the material essentially encloses the resistive divider.

6. A cathode ray tube as claimed in Claim 1, wherein the material comprises a glass compound including at least 20 weight percent of chromium oxide (Cr₂O₃).

7. A resistive high- voltage divider for use in a cathode ray tube, having an outer surface comprising a material having a sheet resistance at 50 °C between 10¹⁰ and 10¹⁶ Ohms, and a secondary electron emission coefficient between 0.5 and 2 as measured for electrons striking the outer surface at a substantially right angle.

8. An electron gun including a resistive high- voltage divider as claimed in Claim 7.