GB1583585A - Dispenser cathode - Google Patents

Description

(54) DISPENSER CATHODE (71) We N.V. PHILIPS' GLOEILAM PENFABRIEKEN. a limited liability Company, organised and established under the laws of the Kingdom of the Netherlands, of Emmasingel 29, Eindhoven, the Netherlands do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to a method of manufacturing a dispenser cathode in which an impregnated and sintered moulding is embedded in a holder which encloses the surfaces of the moulding which are not to be used for electron emission.

The invention also relates to a dispenser cathode manufactured by said method, and to electric discharge tubes comprising such cathodes.

Such dispenser cathodes are used in electron guns for television display tubes. picture pick-up tubes, travelling wave tubes, klystrons. and the like.

Such a dispenser cathode and method are known from Netherlands Patent Application 6602973. laid open to public inspection, which describes a sintered porous tungsten moulding. impregnated with barium aluminate. which is compressed in a holder which is clamped to a cathode shank. The moulding in the form of a pill. is laid in the holder and is compressed bv means of a cylindrical die.

The holder has been obtained bv deep drawing a molybdenum sheet having a thickness of 100 m. The impregnated and sintered moulding is clamped in the holder bv deformation of the edge of said holder so that a gap remains between the moulding and the holder.

The use of semiconductors in television cameras and television sets has resulted in the need for rapidly warming-up cathodes.

These are cathodes which emit a sufficient amount of electrons after less thin 5 seconds from switching on to be able to produce a picture on the display screen of the display tube. It is known inter afia from the Philips Product Note 67, Quick Vision CTV picture tube A66-410 X, that the warming-up time (th) of a cathode is proportional to the ratio of the thermal capacity (Ctl,) to the effective heating power (Pc) of the heating body of the cathode: th < Ct / P.

In order to be able to shorten the warmingup time of the cathode. it should be composed of smaller components than conventional cathodes so as to reduce its thermal capacity. The drawback of this is. however, that the life of such a cathode is considerably reduced. The life is determined inter alia by the ratio between the quantity of introduced impregnate, which is proportional to the volume of the moulding, and the quantity of reaction products of the impregnate which leave the moulding by evaporation from the surface of the moulding. which quantity is proportional to the surface area of the moulding. It is thus obvious that reducing the size of the moulding results in shortening of the life. It has been found that the reduction in life is in fact even larger than follows from the above consideration.

This is because the exhaustion of the cathode is diffusion-limited. It has therefore been found that a cathode of the known described construction and of the rapidly warming-type (less than 5 seconds) which is operated at a voltage II I'k above the usual voltage has a life of only 5()()() hours.

An advantage of I cathode of the abovedescribed known construction is that the impregnated and sintered moulding, prior to being placed in the holder, can be tested for its composition, for example by accurate weigghing. German published Patent Application 1.764.260 discloses a method which does not have this advantage: the material from which the moulding is manufactured is in powder form and is compressed by means of a compression tool in the end face of a cathode shank to form the moulding, and is then sintered. This method has been found to be unsatisfactory in manufacturing small cathodes of the rapidly warming-up type, since the deformation of the end face of the very small cathode shank necessary to make it hollow is substantially impossible.

It is an object of the invention to provide a method of manufacturing a dispenser cathode which may have a relatively rapid warming-up time and/or a long life.

According to the invention in a method of manufacturing a dispenser cathode of the kind described in the opening paragraph, the holder comprises a metal foil which is formed around the moulding so that any gap remaining between the moulding and a side wall of the holder adjacent an emissive surface of the cathode is less than 10 m wide, the foil being from 10 m to 100 m thick immediately before it is formed around the moulding.

The invention is based on the recognition of the fact that the life may be considerably extended when a holder is made from a metal foil which is from 10 m to 100 m thick and which effectively seals the surfaces of the moulding which are not to be used for emission. The width of any gap between the holder and the moulding adjacent an emissive surface of the cathode should for that purpose he of the same order of magnitude as the pores in the moulding, and in any case smaller than 10 m. Thus substantially no migration of reaction products takes place from the moulding via any said gap. This is possible by forming the holder around the compressed and sintered moulding.With a thickness smaller than 10 m, the possibility of pores in the metal foil is too large, and with a thickness exceeding 100 m the manufacture of the holder around the moulding becomes difficult. The metal foil may consist of W. Mo. Ta. Zr. or alloys thereof.

Since the mass of the holder is also an important factor in obtaining a cathode of the rapidly warming-up type, the holder suitably has a thickness of from 10 m to 50 m thick.

The moulding may be placed against a flat said metal foil and be pressed through an apertured die by means of a pressingg tool.

the moulding acting as a plunger to draw the metal foil over the moulding.

As an alternative, the moulding may be pressed into a partly pre-shaped holder by means of a pressing tool and a pressure member be urged against a portion of the holder which is to form said side wall, the dimensions of the holder and the moulding being such that the partly pre-shaped holder is at least partly expanded.

A further alternative is to provide the holder at an elevated temperature around the cold moulding and then to shrink the holder around the moulding by cooling.

Embodiments of the invention will now be described, by way of example. with reference to the accompanying diagrammatic drawings, in which: Figure 1 is a partly sectional view of a dispenser cathode embodying the invention; Figures 2 to 4 and Figure 5 illustrate respectively two methods of manufacturing a dispenser cathode. the methods and the cathodes embodying the invention; Figures 6 and 7 show two other forms ol cathode embodying the invention, and Figure 8 shows graphically some lifetime measurements.

Referring to Figure 1 an annular cathode shank 1 consists of molybdenum foil 40 m thick, and is blackened on its internal surface 2 so as to be able to readily absorb the thermal energy irradiated by a heater filament 3. The length of the cathode shank 1 is approximately 2.3 mm and its outer diameter is 1.75 mm. 'Ihe filament 3 consists of a metal core 4 which is covered with a coating 5 at least the surface of which is black so that it readily radiates thermal energy. The cathode shank 1 has an end face 6 100 m thick to which a holder 7 30 m thick is secured by resistance welding. The holder 7 has been drawn around a sintered moulding 8 impregnated with barium aluminate.As a result of the small dimensions of the moulding 8 (diameter approximately 1.2 mm. height 0.4 mm) and the thin holder 7 (30 m thick) in combination with the thin blackened cathode shank 1 and the black filament 3. the cathode can reach its re quired operating temperature in less than 5 seconds after being switched on. An annular gap 9 between the holder 7 and the moulding 8 has a width of the same order of magnitude as the dimensions of the pores in the moulding, namely less than 10 m. As a result of this, evaporation takes place substantially only via the free end surface 10, so that the life of such a cathode is 2 to 3 times longer than that of comparable cathodes manufactured by known methods.

In display tubes for displaying coloured pictures, usually three clectron beams are generated. In such tubes, it is very annoying if the life of one of the cathodes is shorter than that of the other cathodes. causing colour shirts. It is therefore advantageous to make the life of all the cathodes so long that the electron beams generated by the three cathodes maintain substantially the same intcnsitv during the life of the display tube.

This may be done with cathodes embodying the invention.

As a result of their relatively long life, such dispenser cathodes are also very suitable for use in other beam tubes, such as television camera tubes, e.g. Plumbicons (Trade Mark), in travelling-wave tubes, klystrons and the like. Cathodes for such tubes are not necessarily of the rapidly warming-up type.

Figures 2, 3 and 4 illustrate three steps in a method of manufacturing a dispenser cathode, the method and cathode embodying the invention. The previously impregnated and sintered moulding 8 is placed on a metal foil 11 approximately 30 m thick, the foil being laid over an aperture 13 in a die 12 (Figure 2). The aperture is adapted to the desired shape of the holder 7, and has a minimum diameter slightly smaller than the diameter of the moulding 8 plus two times the thickness of the foil 11, not only to give the metal foil a deepdraw operation but also to produce a reduction in wall thickness by 5 m to 15 m so as to ensure resistance to deformation (because the metal is workhardened) and to make the gap 9 small (less than 10 m).By means of the pressing tool 14, the moulding 8 is forced through the aperture 13 (Figure 3), said moulding serving as a plunger to draw the metal foil 11 into the form of the holder 7. The supporting member 15 also serves to eject the holder with the moulding (see Figure 4).

Figure 5 shows another method of manufacturing a dispenser cathode. the method and cathode embodying the invention. In this method, the moulding 8 is pressed into a partly preshaped holder 18 which has an outwardly sloping circumferential wall and which is placed on a supporting block 17.

During the pressing operation. pressure members 16 are moved inwardlv and pressed against the holder. As a result. the holder 18 is partly expanded bv the moulding 8, the wall being forced into a vertical position and its thickness reduced.

The dimensions of the foil 11 may be chosen to be such that the moulding 8 projects above the holder 7, as shown in Figure 6, or that the edge 19 of the holder 7 projects just slightly above the moulding 8.

as shown in Figure 7. It has been found that as a result of the drawing process said edge 19 projects slightly inwardly.

Figure 8 shows three graphs I to Ill of measured saturation emission current against time during the life of three cathodes. Graph I relates to a known cathode manufactured bv the method described in the afore-mentioned Netherlands Patent Application 66()2()73. The diameter of the mouldingg was 1.8mm and its height was 0.6mm. The operating temperature was approximately 1317 K and the warming-up time to reach an emission of 0.1 A/cm- was 12 seconds. By making the diameter of the moulding 1.2 mm and reducing its height to 0.4 mm, a cathode was obtained having a warming-up time of 5 seconds to reach the same current density. A life measurement (graph II) has shown that the reduction in size of the moulding has considerably reduced its life, to less than 5000 hours.A cathode embodying the invention had a short warming-up time (less than 5 seconds) and a long life (graph III). All the life measurements were performed with a heater voltage which was I 1'S, higher than the usual heater voltage.

WHAT WE CLAIM IS: 1. A method of manufacturing a dispenser cathode in which an impregnated and sintered moulding is embedded in a holder which encloses the surfaces of the moulding which are not to be used for electron emission, wherein the holder comprises a metal foil which is formed around the moulding so that any gap remaining between the mouldng and a side wall of the holder adjacent an emissive surface of the cathode is less than it) tim wide, the foil being from 10 tim to 1()() yIm thick immediately before it is formed around the moulding.

2. A method as claimed in Claim 1, wherein the moulding is placed against a flat said metal foil and is pressed through an aperture die by means of a pressing tool, the moulding acting as a plunger to draw the metal foil over the moulding.

3. A method as claimed in Claim I wherein the moulding is pressed into a partly pre-shaped holder bv means of a pressing tool and a pressureemeniber is urged against a portion of the holder which is to form said side wall. the dimensions of the holder and the moulding being such that the partly pre-shaped holder is at least partly expanded.

4. A method of manufacturing a dispenser cathode, substantially as herein described with reference to Figures 2 to 4 or to any of Figures 5 to 7 of the accompanying drawings.

5. A dispenser cathode manufactured by a method as claimed in any preceding

Claims (1)

1. Claim.

   6. A dispenser cathode as claimed in Claim 5 wherein the moulding has a diameter smaller than 1.44 mm and a length smaller than 0.5 mm, and the holder has a thickness of 10 m to 50 m.

   7. A dispenser cathode as claimed in Claim 6 wherein the diameter of the moulding is substantially 1.2 mm. the length of the moulding is substanti'ilIy ().4 mm. and the holder is subst'inti'illy 3(1 tim thick.

   8. A dispenser cathode substantially as herein described with reference to Figure 1, 6 or 7 of the acconipanying drawings.

   9. An electric discharge tube comprising a cathode as claimed in any of Claims 5 to 8.

   10. A colour display tube comprising three electron guns each having a cathode as claimed in any of Claims 5 to 8.