GB2028576A - Electron guns and resistors for cathode ray tubes - Google Patents

Abstract

An electron gun 1 in a c.r.t. (eg colour picture tube) has focusing and accelerating electrodes G1 to G5, and divider resistance element 15 comprising a ceramic substrate coated with layer 17 of resistive material, one end being electrically connected to the anode potential, and the other to a pin 4 at a low enough potential to avoid electric discharge between that pin and other stem lead pins. Potential for the electrodes is derived from intermediate taps a, b of the resistor. The resistive material is composed of a mixture of RuO2 and glass frit and is overcoated with a glass layer (32) (Fig 7A not shown), the coefficients of thermal expansion of substrate and glass layer being similar. The frit glass may be borosilicate, and the oxide may include Ti or Al2O3 additions and specified organic binder and solvent. Component concentrations and resistor dimensions are exemplified. Electrodes (30a-d) (Fig 5A, not shown) are also preferably RuO2/glass. Overcoating glass layer or layers (32) (Fig 7B not shown) may be Al2O3/borosilicate Pb glass in specified ratios. Characteristics are discussed wrt Figs 8,9 (not shown). The arrangement requires only one anode button in the c.r.t. and counters vapourization and sputtering during the knocking process to remove burrs. Guard patterns (31a-f) (Fig 5A) further counter arc formation on the electrodes and prohibit sputtering. <IMAGE>

Description

SPECIFICATION Electron guns and resistors for cathode ray tubes The present invention reiates to electron guns for television picture tubes or other cathode ray tubes, and resistors for cathode ray tubes.

In a conventional colour television picture tube, a high voltage such as 25 to 30 KV is applied to the last accelerating electrode of an electron gun unit and a picture screen through an anode button mounted at a funnel portion of the picture tube. At the same time, a voltage of 0 to 5 KV is applied to a focussing electrode forming a focussing electron lens positioned near the last accelerating electrode, through a terminal pin provided at the end of a neck portion of the picture tube.

In order to make a small beam spot on the picture screen, which results in a more precise and clear picture, it is desirable to reduce the aberration of the focussing lens as much as possible. To reduce the aberration of the focussing lens, it is necessary to relax or reduce the voltage gradient between the electrodes. To achieve this, such methods as increasing the distance between the electrodes, applying close voltage to the electrodes, or combination of the above, can be employed.

In the case of applying a similar voltage to the electrodes, it is necessary to apply a high voltage of more than 10 KV to the focussing electrode next to the last accelerating electrode. Such high voltage cannot be applied through a terminal pin provided at the end of the neck portion of the picture tube, because this would give rise to an electric discharge (spark) between the terminal pin and other terminal pins which supply voltage to other electrodes of the electron gun unit, for example, heats. It could be supplied through another button provided at the funnel portion.

However, this causes complicated assembly and a substantial increase in cost.

In the case of a picture tube widely known as the "Trinitron" (Registered Trade Mark of Sony Corporation), three electron beams are focussed by a single electron lens, each beam passing through the centre of a single electron lens of large diameter. The three focussed electron beams are deflected to hit the same position of an apertured grille provided in front of the picture screen by four convergence electrodes provided at the top end of the electron gun unit which makes three passages therebetween for each of the electron beams. Two inner electrodes of the convergence electrodes have applied thereto the same potential as the anode potential.Two outer electrodes of the convergence electrodes have applied thereto a voltage which is lower than the anode potential by 0.4 to 1.5 KV, so that the electron beams which pass through the convergence electrodes are deflected to the side of the central beam.

At one time, the voltages were applied through another button provided at the funnel portion and an electrically shielded cable connected to the button and the outer electrodes.

Now, a coaxial anode button, which has two cylindrical electrodes electrically insulated from each other, is used to provide an anode voltage through an outer electrode of the anode button, and a convergence voltage through an inner electrode of the anode button and an electrically shielded cable connecting the inner electrode and the convergence electrodes. By virtue of the above coaxial anode button, it is not necessary to provide two buttons at the funnel portion of the picture tube. However, it is still troublesome to connect the inner electrode of the anode button and the outer convergence electrodes by the electrically shielded cable.

Other disclosures of possible interest are our Japanese Publication No. 40987/72 and our US Patent No. 3 514 663, and US Patent No.

3 932 786.

According to a first aspect of the invention there is provided an electron gun for a television picture tube or other cathode ray tube having an evacuated bulb including a funnel portion, a neck portion and a screen portion, comprising a plurality of electrodes for focussing and accelerating an electron beam generated by a cathode, the electrodes being aligned along the axis of said neck portion, and a resistor formed of an insulating substrate on which a resistive path is formed, said substrate being mounted along said plurality of electrodes and sealed in said neck portion, said resistive path having one end tap, another end tape and at least one intermediate tap between said end taps, said one end tap being connected to be supplied with the same voltage as a voltage supplied, in use, to said screen portion, said other end tap being connected to a terminal pin provided at one end of said neck portion for connection to a voltage low enough to avoid an electric discharge between the electrodes and said terminal pin, an operating voltage for the electrodes being obtained, in use, from said intermediate tap by dividing the voltage between both of said end taps, and said resistive path comprising a mixture of ruthenium oxide and glass, said substrate and said resistive path being coated with at least one layer of glass.

According to a second aspect of the invention there is provided a resistor for a cathode ray tube which is to be subjected, in use, to high voltages, the resistor comprising a substrate of insulating material, a resistive path formed on said substrate and comprising a mixture of borosilicate glass and ruthenium oxide, and electrodes formed on said substrate to engage said resistive path, the electrodes comprising a mixture of glass and ruthenium oxide.

The invention will now be further described, by way of illustrative and non-limiting example, with reference to the accompanying drawings, in which: Figure 1 is a perspective view of an electrnn gun unit embodying the present invention; Figure 2 is a schematic drawing showing the connection between electrodes and a resistor in the electron gun unit of Figure 1; Figure 3 is a schematic side elevational sectional view showing the electron gun unit of Figure 1 sealed in a neck portion of a cathode ray tube; Figure 4 graphically illustrates the characteristic relation between gas evaporation and temperature of a resistor embodying the invention and a prior art resistor, respectively; Figures 5A and SB are plane and side elevational views showing a first resistor embodying the invention;; Figure 6 is a side elevational view showing a second resistor embodying the invention; Figures 7A and 7B are plane and side elevational views, respectively, of a third resistor embodying the present invention; Figure 8 graphically illustrates the characteristic relation between the thickness of an overcoating glass layer and the resistivity variation of resistors embodying the invention, and; Figure 9 graphically illustrates the characteristic relation between gas evaporation and temperature of an electrode used in embodiments of the present invention and the prior art, respectively.

A first embodiment of the invention will now be described with reference to the drawings, in which an electron gun unit with a uni-potential electron lens is applied to a "Trinitron" television picture tube.

As seen in Figures 1 2 and 3, an electron gun or electron gun unit 1 (Figure 1) is mounted in the neck 23 (Figure 3) of the tube. The gun 1 includes three cathodes KR, KG and KB (Figure 3) aligned in a horizontal plane. The three cathodes are positioned behind a control grid G1, which is followed in turn by prefocussing grids G2 and G3.

Next in line is a main focussing lens which is formed by a grid Gd. The grids G3, G4 and a grid G5 are accelerating grids. Thereafter, convergence plate electrodes 8, 9 and 11, 12 are provided. In passing to the screen, an electron beam from the cathode KR passes through associated openings in the grid G, and the grid G2, respectively, then through the grids G3, G4 and G5, and finally between the plate electrodes 9 and 12. An electron beam from the cathode KG passes straight through the electron gun 1 and out between the convergence plates 8 and 9 before reaching an apertured grille AG.An electron beam from the cathode KB passes through associated apertures in the grids G1 and G2, then through the grids3, Grand Gs, and finally between the convergence electrodes 8 and 11 before reaching the apertured grille AG.

A conductive carbon coating 24 (Figure 3) is formed over the inner surface of the funnel or funnel portion (not shown) of the picture tube, and this coating also extends over the inner surface of the neck or neck portion of the tube back to the area of the convergence electrodes 8, 9, 11 and 12.

The gun 1 includes a stem 2 made of glass, an evacuation pipe 3 formed integrally with the stem 2, and terminal pins 4 mounted on the stem 2. The terminal pins 4 are connected to various electrodes, for example, heaters of the cathodes in the picture tube. The grids (electrodes) G1, G2, G3, G4, G5 are arranged coaxially, each having a cylindrical shape and being supported integrally by a pair of supporters 5, 6 made of bead glass. The convergence electrodes 8, 9 are attached to a flange portion 10 of the fifth grid G5, and the convergence electrodes 11, 1 2 are supported by the bead glass supporters 5, 6 through a supporting piece 13. Connecting pieces 14 4 are also provided integrally with the flange portion 10.

As will be explained, the connecting pieces 1 4 contact the carbon layer on the inner wall of the funnel portion of the picture tube, through which a desired high voltage Eb which is the same voltage as applied to the picture screen (i.e. the anode voltages, is supplied to the fifth grig G5. A resistor 15 is provided along the grids G,Po G5, the resistor being supported at one end by a metal supporting piece 16 and at the other end by a lead 22. The resistor 1 5 is formed with a printed resistive layer or path 1 7 on one surface of a substrate made of an insulating material, for example, a ceramic substrate. The printed resistive path 1 7 is covered with a glass layer.The size of the resistor 15 is, for example, 10 mm wide, 50 mm long and 1.5 mm thick. An edge of the resistive path 17 and the fifth grid G5 are electrically connected by the supporting piece 1 6, and the fifth grid G5 and the third grid G3 are electrically connected by a lead 1 9. A predetermined position b, which is spaced a predetermined distance from one end of the resistive path 17, and the fourth grid G4 are electrically connected by a lead 20, and another position a which is spaced a predetermined distance from one end of the resistive path 1 7 is electrically connected to the convergence electrodes 11 and 12 by a lead 21. The other end of the resistive path 1 7 is electrically connected to a terminal pin 4a by a lead 22.The convergence electrodes 11 and 1 2 are electrically connected to each other.

The above-described electron gun unit 1 is sealed in the neck portion 23 of the picture tube, as shown in Figure 3. The carbon coating or layer 24 on the inner wall of the neck portion 23 and on the funnel portion of the picture tube is engaged by the connecting pieces 14. The carbon coating or layer 24 is electrically connected to a button' provided on the funnel portion of the picture tube, through which a high voltage of, for example 30KV is applied from the outside of the picture tube. With the above construction, the high voltage applied to the carbon coating or layer 24 is applied to the convergence electrodes 8, 9 and the fifth grid G5 through the connecting piece 14, and the same voltage is applied to the third grid G3 through the connecting lead 19 and one end of the resistive path 1 7 through the supporting piece 1 6. Thus, the convergence electrodes 8, 9 and the grids3, G5 are supplied with the same potential.

The high voltage supplied from the anode button is also applied to the picture screen.

The high voltage applied to the end of the resistive path 17 is divided at the position (intermediate tap) a by the voltage drop caused by the resistive path between the high voltage end and the intermediate tap a, and the derived voltage is applied to the convergence electrodes 11, 12 through the lead 21. It is also divided at the position (intermediate tap) b to derive a lower voltage than the anode voltage by the voltage drop between the high voltage end and the tap b, and the derived voltage is applied to the fourth grid G4 through the lead 20. Claws which can be attached to the intermediate taps are provided on the leads 21 and 20.Thus, the potential applied to the convergence electrodes 11 and 12 its a little lower than the potential applied to the convergence electrodes 8 and 9, for example 29 KV, and the potential of the fourth grid G4 is still lower than that or about 12 KV. The other end of the resistive path 1 7 is electrically connected by the lead 22 to the terminal pin 4a mounted in the stem 2. The terminal pin 4a is connected to ground potential through a variable resistor 25.

The variable resistor 25 provides fine control of the potential applied to the convergence electrodes 11 and 12 and the fourth grid G4. The first grid G, and the second grid G2 are supplied with a predetermined voltage through the terminal pins 4 from the outside of the picture tube.

Current for a heater of the cathode is also supplied through predetermined terminal pins. Thus, each of the electrodes is suppled with a desired voltage which is derived from an intermediate tap of the resistor 15 based on the anode voltage obtained by the connecting piece 1 4.

In the above exemplary embodiment, both the convergence voltage and the focussing voltage are obtained by dividing the anode voltage using the resistor 1 5. It is instead possible to obtain only the convergence voltage or the focussing voltage in this way. In the case when only the convergence voltage is obtained. by dividing the anode voltage, a low convergence voltage of O to 5 KV can be supplied through the terminal pin 4.

In conventional picture tubes other than the "Trinitron" (TM) picture tube, only the focussing voltage is obtained by dividing the anode voltage.

According to the above-mentioned structure, it is sufficient to provide only one anode button without any special structure, such as a coaxial button. Further, the cable which connects the anode button and the convergence electrodes is not necessary, so the assembly is simplified.

As shown in Figures 1 and 2, the resistor comprises a thick layer of resistive material and is constructed of an insulating substrate, the resistive layer 1 7 and electrodes 30a to 30d formed on the substrate.

There are some conditions required for the resistive material so it can be used in the resistor assembled into a cathode ray tube; Firstly, the temperature characteristic must not change at high temperatures. Secondly, it should not vapourise. Thirdly, it should resist a sputtering reaction. Fourthly, there should be only small resistance variations.

In the manufacturing process for making a cathode ray tube there can be used, for example, a knocking process, and it is very undesirable for the resistive material to have a tendency to vapourise at the temperatures of the knocking process.

Generally, decrease in vacuum is one of the factors which determines the lifetime of vacuum apparatus such as cathode ray tubes.

Thus, since the vapourising of material used within a vacuum apparatus is very harmful to such apparatus, the selection of materials and previous treatments must be carefully considered.

After assembly of the electron gun 1, during the knocking process, a high voltage of two times the rated voltage, for example, 50 to 60 KV is applied between the convergence electrode and terminal pin to cause discharge among the cylindrical grid electrodes such as G, to G6, which causes fine scraps of material which occur at rough cut edges of the cylindrical grid electrodes to be removed.

Since the high voltage is also applied to the resistive layer 17, Joule heating will be produced in the resistive layer 1 7 based on 12R, as the product of the resistance R and current I passing therethrough. Accordingly, it is necessary to prevent the resistivity (the resistance P) of the resistive layer 1 7 from changing and the resistive material from vapourising due to the heat produced by Joule's Law.

The resistance R is selected to be between 300 and 1000 Megohms, but the resistance variation should be as small as possible. Referring to Figure 2, the resistance of the resistive path 17 is rut between the electrode 30a and the position a and is R2 between the position a and the electrode 30d. The value of R/R1 +R2 must stay within +0.3 percent of the predetermined value to stabilise the resistance or resistivity.

Another serious problem is the surface discharge produced by the high voltage electric field during the knocking process, which causes a sputtering reaction along. the pattern of the resistive path 1 7. The resistivity changes and the sputtered material is harmful to the electron gun 1 due to the sputtering. Therefore, a sputtering reaction should be prevented.

Ruthenium oxide-glass mixture is used for the material of the resistive path 1 7. Such a material is made from a mixture of a binder, for example, borosilicate glass, ruthenium oxide powder with additions such as Ti or Al203, an organic binder such as ethylcellulose, and a solvent such as butyl carbitol acetate to obtain the desired characteristics.

A paste for making the resistive layer 1 7 is obtained by stirring up the above materials, then the paste is printed in a zig-zag pattern, as shown in Figures 1 and 2, on a ceramic substrate having a composition of, for example, of 90- to 97 percent alumina.

The printed substrate is then baked in the temperature range of 7500C to 8500C for 40 to 60 minutes, and a coating glass is applied over the resistive path 1 7 and electrodes. In the paste of ruthenium oxide and glass, as the ratio of RuOziglass (weight) is increased the surface resistivity decreases. As the grain size of ruthenium oxide increases the surface resistivity increases The ratio of RuOz/glass is selected to be about 20/80.

After baking, the thickness of the resistive layer 1 7 is 10 to 1 5 ,um. Even although the resistor produced is treated under high temperature and high pressure in the knocking process, the variation of resistivity will be less than 10 percent and almost no vaporisation occurs. Moreover, since ruthenium oxide has a small sputtering coefficient, damage to the electron gun by sputtering material can be reduced relative to prior art systems.

The electrodes 30a to 30dcan be constructed in the following manner.

Generally, Ag or Ag-Pd is used for the electrode material of resistor elements of this type and is formed of a thicker layer. When the resistor element is installed within a vacuum apparatus such as a cathode ray tube, the aforementioned four conditions are applicable to the electrodes as well as to the resistive path 17.

The most serious problem is vapourisation from the electrode material and a sputtering reaction to the electrode material under the high temperature and high electric field applied during the knocking process. Experiments during knocking on the resistor element comprising electrodes of Ag or Ag Pd and with the resistive path 17 therebetween and formed with RuO > /glass formed on the alumina substrate, respectively, as shown in Figure 4, results in more vapourising from the electrodes than in the case of electrodes of RuOz/glass and the arc discharge tends to concentrate on the surface of the electrodes during the knocking process.

The electrodes are desirably formed from the same material as the resistive path 17, that is from RuOiglass. Also, material with a high ratio of RuOz/glass and a lower sheet resistivity than that used for resistor 1 7 is suitable for use as the electrodes.

Afirst resistor 15 embodying the present invention is shown in Figures 5A and 5B. The method of manufacturing of the resistor is as follows: The electrodes 30a, 30b, 30c and 30d and the resistive path 1 7 are formed on the alumina substrate in the pattern shown. After baking, the thickness of the electrodes 30a to 30d is about 10 rn Experimental analysis of the resistor element shown in Figure 4 shows that the vapourisation from the RuO2 electrodes was less than that from electrodes of Ag or Ag-Pd. The composition of the gas vapourised from Ag or Ag-Pd electrodes is mostly oxygen. When Ag paste is baked at high temperature, it is oxidized to produce the mixture of a stable oxide, for example, AgzO.

Firstly, the electrodes 30a to 30d are formed in predetermined shapes on the surface of the alumina substrate by coating, for example by screen printing. Glass paste with a ratio of RuO2/glass greater than 35/65 is used for the electrodes.

The resistive path 1 7 is formed in a zig-zag pattern between the electrodes by coating RuO2/glass paste having high sheet resistivity as shown in Figure 5A. In this case, guard patterns 31 a to 31 fare formed to cover the opposite edges of the electrodes. The resistor element as shown in Figures 5A and 5B is manufactured by baking the alumina substrate with an unstable oxide, for example AgO or At292. The unstable oxide will be decomposed into Ag20 and O2 to form a stable oxide. The guard patterns 31 a to 31 f have high resistivity and cover the opposite edges-of the electrodes, which have low resistivity.

Therefore, during the knocking process, it is difficult for arc discharges to concentrate on the electrodes and a sputtering reaction is effectively prohibited.

In a second embodiment of the resistor 1 5 shown in Figure 6, each electrode 30a to 30d can be completely covered with the resistive layer 1 7.

In this case, although the contact resistivity increases by a small amount, no problem is caused because of the thin layer of the resistive material coated over the electrodes.

In a third embodiment of the resistor 15 shown in Figures 7A and 7B, the rssistive path 1 7 and electrodes 30a to 30d are formed on the substrate with a layer or layers of glass 32 coated over the whole surface thereof. Such a coating layer of glass prevents the electrodes and the resistive path from vapourising at high temperatures and the resistivity from changing due to a sputtering reaction.

A paste containing borosilicate lead glass and 10 to 40 weight percent Awl203 grained powder is used for the glass layer 32. The ratio of borosilicate lead glass to alumina (glass/AI203) is selected in ratios, for example of 90/10, 80/20 and 75/25 and all ratios between these examples.

The mixture of borosilicate lead glass and alumina of the predetermined mixing ratio and 10 to 20 percent organic binder and solvent is coated on the resistor element by screen printing. In this case, in order to make the layer thick, double or triple layers are formed by printing using 50 to 100-mesh screen (200 to 300 ,um thickness). A layer of glass 32 having a thickness of 200 to 400 ,um is obtained by baking in a temperature range of 550 to 6500C for 20 to 30 minutes.

The purpose of mixing Awl203 powder into the glass material is to improve the mechanical strength of the glass layer 32. Generally, when the glass layer 32 is thick, it is subject to cracks due to incidental forces. However, the mixture of Awl203 into the glass material prevents the glass layer from cracking. Moreover, it is possible for the expansion coefficient of the glass layer 32 to match that of the alumina substrate. The variation of resistivity of the resistive path 1 7 coated by glass containing Al2O3 after the process of knocking is shown in Figure 8.The glass paste containing Al203 is used and the mixing ratio of Al203 to glass is varied as shown by the upper curve with 0 percent Awl203,20 percent Awl203 by the middle curve and 10 percent Awl203 in the lower curve.

The resistor is overcoated by the glass layer and the thickness of the layer is varied as shown.

The electron gun 1 is processed by knocking.

The variation of resistivity after the knocking process is adjusted by means of the variable resistor 25 shown in Figure 2, and the adjusted resistance of the variable resistor 25 is shown on the ordinate axis of Figure 8. According to Figure 8, when the thickness of the glass layer 32 containing 10 to 20 weight percent Al203 is selected to be in the range of 200 to 400 ym, the variation of resistivity is very small because the curve is almost flat and is less than the other illustrated examples. On the other hand, if the glass layer does not contain any Awl203, the glass layer cannot be more than 80 to 100,um in thickness because of the mechanical strength and the stability of resistivity.In the case of the thickness of the glass layer without any Awl203 being under 80 to 100,*4m, the variation of resistivity is so large due to the sputtering process and the high temperature treatment that a glass of that composition cannot in practice be used.

Moreover, if the glass layer contains Al203 over 40 weight percent, it becomes porous, and therefore it cannot protect the resistive layer 1 7 and the electrodes 30a to 30dfrom the influence of the sputtering reaction and arc discharge concentration. Although the electrodes 30a to 30d are not covered with the guard pattern in the embodiment of Figure 7, they are effectively protected from the sputtering reaction as well as in the case where the glass layer 32 is coated over the resistor shown in Figure 5 or Figure 6, and even if an uppermost layer portion of the glass layer 32 comprises a glass layer without Awl203 with thickness in the range of 50 to 100 4m, it can in practice be used.Generally, when the glass layer contains Awl203 in the mixture, the threshold voltage is slightly decreased. But according to the above-mentioned structure, the variations of resistivity can be reduced and the threshold voltage will be high.

In Figure 9, a dashed curve is plotted for a resistor with electrodes consisting of Ag and without a glass layer coated over it, and a solid line curve is plotted for a resistor with electrodes comprising RuO2. The graph illustrates the quantity of vapourising O2 gas from the electrode material at various temperatures. The quantity of vapourising O2 gas as indicated by the ionised current is converted by mass spectrometer analysis of O2 gas vapourising velocity as shown in the ordinate axis of Figure 9. The thick layer resistor with highly accurate resistivity that is obtained is stable in electric characteristics under high temperatures and high pressures required in the manufacturing process of cathode ray tubes.

The glass insulating layer coated over the entire surface of the resistor keeps it from sputtering when the electron gun is subjected to the knocking process which utilizes a double voltage that is applied to the high voltage terminal. The knocking process removes burrs due to the discharges.

If a glass insulating coating layer was not used, the resistor is likely to be damaged due to arcing between portions of the resistor during the knocking process. The layer thus provides protection of the resistor. Also, if resistors are constructed of conventional materials such as silver or silver compounds the resistivity variation will be large after the knocking process. Also, when silver material is used, oxygen gas will be released during the knocking process and when the temperatures of the resistive material increases some of the oxygen gas will be evaporated, which is injurious to the evacuated apparatus.

The use of ruthenium oxide does not result in a resistor which evaporates oxygen during the knocking process and the addition of a glass layer over the resistive layer protects the resistor. Such structure is illustrated in Figures 7A and 7B, for example. By coating the resistive paths with glass of predetermined thicknesses the resistor is completely protected from damage. Usually, when thick layers of glass are coated, they are apt to be porous and a porous layer is not effective for arc discharge. Also, it is difficult to coat glass thicker than 100 ,am. However, the overcoating glass layer has mixed therein alumina powder Awl203 so that the mixture makes a coating glass layer which is very strong and which has a substantially increased voltage breakdown characteristic. Also, the glass is not porous.

The resistor is formed of ruthenium oxide and glass and the terminal at top has a lower resistivity than the main part of the resistor.

The temperature thermal expansion coefficient of the glass layer is about the same as that of the substrate. The substrate is made of a ceramic such as Awl203 and the glass layer contains Awl203 powder, binder, solvent and glass so that the ratio of the Al203 to glass is selected so that the temperature coefficient of thermal expansion of the coating in the ceramic substrate will be very similar.

As shown in Figure 8, if the glass layer contains no Al203 the resistance characteristic change is very high as shown by the top curve. Also, if 100 percent glass layer with no Awl203 is used, it can easily be cracked by being hit accidentally.

By adding Al203 as shown by the curves labelled 10 percent and 20 percent, respectively, the resistance to cracking will be improved.

The glass should not contain more than 40 percent of Awl203 because the glass layer will become porous.

When the Awl203 is mixed with glass with the Awl203 being in the range of 10 to 40 percent by weight, the mechanical strength and the sputtering characteristics will be good and the thickness of the layer can be in the range of 100 to 400 ym, which gives very good characteristics.

Thus, as shown in Figure 8 in the thickness range between 200 to 400 um, the change in resistance is very low after knocking and is less than 10 Megohms. The resistivity can be adjusted with the resistor 25, but if the resistivity variation is high it cannot be effectively adjusted.

In Figures 5A and 5B, the terminal top is covered with the resistive pattern and the top is protected from arc discharge by the resistive pattern. One portion must remain uncoated to allow electrical contact to be made to the electrode.

Claims (24)

1. An electron gun for a television picture tube or other cathode ray tube having an evacuated bulb including a funnel portion, a neck portion and a screen portion, comprising a plurality of electrodes for focussing and accelerating an electron beam generated by a cathode, the electrodes being aligned along the axis of said neck portion, and a resistor formed of an insulating substrate on which a resistive path is formed, said substrate being mounted along said plurality of electrodes and sealed in said neck portion, said resistive path having one end tap, another end tap and at least one intermediate tap between said end taps, said one end tap being connected to be supplied with the same voltage as a voltage supplied, in use, to said screen portion, said other end tap being connected to a terminal pin provided at one end of said neck portion for connection to a voltage low enough to avoid an electric discharge between the electrodes and said terminal pin, an operating voltage for the electrodes being obtained, in use, from said intermediate tap by dividing the voltage between both of said end taps, and said resistive path comprising a mixture of ruthenium oxide and glass, said substrate and said resistive path being coated with at least one layer of glass.

2. An electron gun according to claim 1 wherein said taps comprise a mixture of ruthenium oxide and glass.

3. An electron gun according to claim 2 wherein the sheet resistivity of said taps is lower than that of said resistive path.

4. An electron gun according to claim 2 or claim 3 wherein the ratio of ruthenium oxide to glass of said taps is higher than that of said resistive path.

5. An electron gun according to any one of the preceding claims wherein said layer of glass includes alumina.

6. An electron gun according to claim 5, wherein said layer of glass comprises borosilicate glass and alumina with the ratio of alumina to borosilicate glass being in the range of from 5 to 40 weight percent.

7. An electron gun according to claim 5 or claim 6 wherein said layer of glass contains 10 to 40 weight percent of alumina powder.

8. An electron gun according to claim 5, claim 6 or claim 7 wherein the thickness of said layer of glass is in the range of from 1 00 to 400 ym.

9. An electron gun according to any one of the preceding claims, including guard patterns of the same material as said resistive path formed on the substrate to cover opposite edges of said taps.

1 0. An electron gun according to claim 9 wherein the sheet resistivity of said guard patterns is the same as that of said resistive path.

11. An electron gun according to any one of the preceding claims wherein there are a plurality of said layers of glass and the uppermost layer thereof does not contain alumina powder.

12. An electron gun according to any one of the preceding claims wherein the thermal expansion coefficient of said glass layer is substantially the same as that of said insulating substrate.

13. An electron gun according to any one of the preceding claims wherein said insulating substrate is of alumina.

14. An electron gun substantially as herelnbefore described with reference to Figures 1 to 4, Figures 1 to 5B, Figures 1 to 4 and 6, or Figures 1 to 4, 7A and 7B of the accompanying drawings.

1 5. A resistor for a cathode ray tube which is to be subjected, in use, to high voltages, the resistor comprising a substrate of insulating material, a resistive path formed on said substrate and comprising a mixture of borosilicate glass and ruthenium oxide, and electrodes formed on said substrate to engage said resistive path, the electrodes comprising a mixture of glass and ruthenium oxide.

16. A resistor according to claim 1 5 wherein the weight percent of ruthenium oxide is greater in said electrodes than it is in said resistive path.

1 7. A resistor according to claim 16 wherein said resistive path at least partially overlays said electrodes.

18. A resistor according to claim 1 6 wherein said resistive path overlays said electrodes.

19. A resistor according to any one of claims 1 5 to 1 8, including a protective layer formed over said resistive path and at least a portion of said electrodes, said protective layer comprising a mixture of glass and alumina.

20. A resistor according-to claim 1 9 wherein the weight percent of alumina to glass is in the range of from 5 to 40.

21. A resistor according to claim 20 wherein the weight percentofalumina to glass is in the range of from 10 to 25.

22. A resistor according to claim 19, claim 20 or claim 21 wherein said protective layer has a thickness of between 100 to 400 ym.

23. A resistor according to claim 21 wherein said protective layer has a thickness of between 200and400,um.

24. A resistor for a cathode ray tube which is to be subjected, in use, to high voltages, the resistor being substantially as hereinbefore described with reference to Figures 5A and 5B, Figure 6 or Figures 7A and 7B of the accompanying drawings.