GB2196786A - Cathode assembly - Google Patents

Abstract

In a cathode assembly, precise positioning of a cathode 10 relative to a grid plate 24 is achieved by mounting the cathode in a retaining ring 34 which is mounted in a housing 36. Both the ring 34 and housing 36 may be of ceramic material. In the preferred embodiment, three cathodes are similarly mounted within a single housing (Fig. 3, not shown), each being a dispenser cathode (10) capable of current densities up to and exceeding 10 Amperes per square centimeter and comprising a refractory material reservoir (14) containing a pellet of tungsten and barium calcium aluminate (16) sealed by a pellet (18) of porous tungsten or tungsten mixture. The reservoir/pellet assembly may be contained in a support cylinder to which the porous tungsten pellet may be welded. <IMAGE>

Description

SPECIFICATION Dispenser cathode for a cathode ray tube This invention pertains generally to thermionic cathodes and more particularly, to reservoirtype dispenser cathodes that find particular advantageous application in devices such as cathode ray tubes requiring very high current density, that is, current densities greater than 10 amps per square centimeter of cathode surface area. The cathode assembly of the present invention also finds advantageous application for use in color CRTs.

The most relevant prior art known to the applicant is Patent No. 4,165,473 which discloses an improved cathode invented by the inventor of the present invention and which is assigned to Varian Associates, Inc. of Paio Alto, California. That patent discloses a dispenser cathode comprising a porous metal matrix consisting of a compacted mixture of tungsten and iridium particles impregnated with a molten barium aluminate and other alkaline earth oxides which may be added to the matrix. The cathode structure disclosed in Patent No. 4,164,473 is apparently primarily intended for use in microwave electron tubes designed for continuous wave operation such as a Klystron amplifier.

The prior art section of that patent adequately describes the previous attempts to provide cathodes capable of generating high current densities and indicates that generally the prior art limit of current density from such prior attempts was about 3 amperes per square centimeter of cathode surface area.

Furthermore, that patent reveals a structure which is capable of generating at least 10 amperes per square centimeter of cathode surface area thus providing a significant increase in power particularly at very high frequencies for use in microwave devices.

The dispenser cathode of the aforementioned patent was primarily intended for specialized microwave tubes which are generally very costly. Therefore, the high cost of manufacturing such cathodes was not at the time considered a major disadvantage. Dispenser cathodes costing as much as ten to twenty dollars to manufacture were not considered too expensive for their application in microwave tubes costing as much as thousands of dollars. On the other hand, thermionic cathodes designed for use in cathode ray tubes such as those used in computer terminals and displays and in certain TV monitors, have always been considered very cost sensitive because of the high volume and competitive nature of the ultimate product into which those cathodes are installed.Consequently, cathodes used in the prior art for such cost sensitive applications in cathode ray tubes have generally been of the type comprising an insulator semiconductor oxide cathode combination which is not capable of current densities greater than about 1 amp per square centimeter of cathode surface area, but which was still adequate for the relatively low current density applications of such prior art CRT devices.

Unfortunately, significant improvements in the computer art specifically related to display applications as well as other advances in cathode ray tube applications, have created a demand for a cathode for use in cathode ray tubes which are capable of achieving the high current densities of 10 amps per square centimeter or greater thereby making the dispenser-type cathode a highly desirable electron beam source for more recent cathode ray tube applications. However, the manufacturing costs of such dispenser cathodes continues to be about an order of magnitude higher than that which would be feasible in the highly competitive, cost sensitive cathode ray tube industry.

Thus there is now a need for a dispensertype cathode which is capable of the aforementioned higher current densities but which may be manufactured for approximately 1/10 of the manufacturing costs of previously known high current density dispenser cathodes. Thus there are, in effect, two types of prior art to which the present invention may be compared. On the one hand there is the costly dispenser cathode prior art which is substantially unsuitable for application in cathode ray tubes because of the cost sensitivity of the ultimate product. On the other hand, there are the conventional cathodes that have previously found application in cathode ray tubes because of their relatively low cost but which are incapable of providing the high current densities that the more demanding applications required of today's cathode ray tubes.

The latter prior art, that is, prior art cathodes that have conventionally been used in cathode ray tubes, employ a nickel substrate with an impurity of magnesium or silicon as activators and which is coated with barium oxide, calcium oxide or strontium oxide applied as carbonates and which decompose to oxide during manufacture. Unfortunately, electron emission from such conventional cathode ray tube cathodes is far too limited for today's applications because the electron emission is induced from a semiconductor material and in order to increase the current density such materials require an extremely high voltage. Such high voltages applied for longer than a short pulse can cause arcing which is destructive to the cathode as a result of the charging effect of the material.The limit current density therefore has usually been less than one amp per square centimeter for cathodes in CRT applications.

Previous attempts to substitute a metal cathode for the semiconductor cathode of the CRT art such as the attempt described in the disclosure of Patent No. 4,165,473, has been limited to metals that can survive a hydrogen atmosphere used during the impregnation step such as where tungsten is impregnated with barium aluminate or barium calcium aluminate or other earth metal additives.

No prior art known to the applicant, teaches or even suggests the concept of an assembly comprising an integrated dispenser cathode, insulator and control grid nor such an assembly comprising three such dispenser cathodes for color CRT applications.

The present invention comprises a novel dispenser cathode which is capable of achieving the current densities of such prior art as disclosed in Patent No. 4,165,473, but which employs a novel structure permitting a significant reduction in cost on the order of onetenth of the cost to manufacture prior art dispenser cathodes. Consequently, the present invention consists of a cathode which is cost competitive with the semiconductor-type cathodes of the CRT art but which provides an order of magnitude improvement in current density to meet the more modern demands of cathode ray tubes.

The cathode of the present invention utilizes a reservoirtype dispenser cathode structure that can be produced in four separate pieces and readily assembled at a relatively low cost.

It permits inexpensive Production methods using automated equipment of long proven use such as pill presses and punch presses. Furthermore, the structure of the present invention is more conducive to a uniform level of performance throughout the life of the cathode. This contrasts with prior art dispenser cathodes which generally have a significant degradation in performance over the life of the cathode because of the changes in the extent of evaporation of the alkali earth metal through the pores of the emissive metal.

The aforementioned four separate pieces of the present invention comprise a pressed and sintered porous tungsten pellet; a pressed pellet made of barium calcium aluminate and tungsten; a punched pressed reservoir formed of molybdenum, rhenium, a combination of molybdenum and rhenium, tantalum or other refractory metal; and a support cylinder in the form of an extrusion or similar processed structure formed of molybdenum, molybdenum-rhenium or tantalum.

The process of manufacture of such a cathode comprises the steps of pressing and sintering the tungsten pellet using tungsten powder of selected characteristics, punch pressing the reservoir form and forming the support cylinder, pressing a pellet of barium calcium aluminate and tungsten, assembling the reservoir and support cylinder, inserting the aluminate tungsten pellet into the reservoir, then sealing the porous tungsten pellet to the top of the reservoir and cylinder assembly by either welding or brazing. The resultant cathode is designed to operate at approximately 850 to 1150 degrees Centigrade depending upon current density objectives. The pellet contained within the reservoir provides a constant low level of barium evaporation to activate the tungsten.More importantly, the cathode of the present invention provides the high current density of dispenser cathodes in a structural configuration which permits simple automated manufacture thereby significantly reducing the cost rendering the invention compatible in cost with prior art current-density limited CRT cathodes.

The assembly of the present invention comprises an integrated cathode/insulator/control grid structure particularly suited to use in cathode ray tubes. A preferred embodiment comprises three such dispenser cathodes mounted in a unitary support structure and integral precision spaced control grids commonly referred to as the G 1. The three cathode assembly is especially adapted for use in color CRTs.

It is therefore a principal object of the present invention to provide an improved dispenser cathode having a structure and manufacturing process associated with it that are conducive to costs comparable to conventional cathode ray tube cathode devices but that is capable of a current density of at least 10 amps per square centimeter of cathode emission surface area.

It is still an additional object of the present invention to provide an improved cathode in which a reservoir-type dispenser cathode can be produced in four separate pieces and readily assembled using automated equipment of long proven use.

It is still an additional object of the present invention to provide a dispenser cathode especially adapted for use as a high current density cathode ray tube cathode capable of generating a minimum of 10 amperes per square centimeter of cathode emission surface area by utilizing an emissive metal material but which is comparable in cost to semiconductortype CRT cathodes of the prior art.

It is still an additional object of the present invention to provide a cathode assembly having one or more of the aforementioned dispenser cathodes, an integral insulating structure and one or more precision spaced control grids for CRT applications.

The invention is not to be construed as limited to or by the objects of the invention stated above.

The aforementioned objects and advantages of the present invention as well as additional objects and advantages thereof will be more fully understood hereinafter as a result of a detailed description of a preferred embodiment of the invention when taken in conjunction with the following drawings in which: FIG. 1 is a block diagram representation of the manufacturing process of the present invention; FIG. 2 is a cross-sectional view of the apparatus of the present invention; FIG. 3 is a three-dimensional exterior view of a color CRT cathode assembly of the present invention; FIG. 4 is a cross-sectional view of a portion of the assembly of FIG. 3 taken along lines 4 ,4 thereof; and FIG. 5 is a top view of an insulator structure of the assembly of FIG. 3.

Referring simultaneously to FIGs. 1 and 2 it will be seen that the present invention comprises an improved dispenser cathode 10 having a support cylinder 12 and a reservoir 14.

The reservoir is substantially filled with a first pellet 16 comprising a mixture of tungsten and barium calcium aluminate. A second pellet 18 of pressed and sintered tungsten powdered is brazed or welded to the support cylinder 12 thereby in effect sealing the reservoir 14 and the pellet 16 contained therein. Support cylinder 12 provides access to the sealed reservoir for a conventional heater such as that disclosed in Patent. 4,165,473. The reservoir 14 is received and supported by the interior wall surface of the support cylinder 12.

The manufacturing process of the dispenser cathode may be carried out as follows: 1. Pressing and sintering a porous tungsten pellet of 70-80% density using powder from 4-7 microns in diameter. The tungsten powder may optionally include 20-50% by weight of iridium, osmium, ruthenium or rhenium; 2. forming a reservoir by punch pressing using either molybdenum, rhenium, molybdenum rhenium, tantalum, tungsten, tungsten rhenium or other refractory metal; 3. forming a support cylinder of molybdenum, rhenium, molybdenum-rhenium, tungsten, tungsten rhenium or tantalum by extrusion or similar process; 4. pressing pellets of barium calcium aluminate and tungsten wherein the tungsten constitutes between 20-50% of the mixture; 5. assembling the reservoir and support cylinder; 6. inserting the pellet of barium calcium aluminate and tungsten into the reservoir; and 7. sealing the porous tungsten pellet to the reservoir/cylinder assembly by welding or brazing.

In one preferred embodiment of the process of manufacture, step No. 1 comprises first applying a uniaxial pressure of between 10,000 and 20,000 psi. to the tungsten to achieve a density of between 50-55% and then sintering the pressed tungsten at between 2,000 to 2,500 degrees Centigrade for between 30 and 60 minutes to achieve the 70-80% density.

Furthermore, the reservoir forming process of step No. 2 was accomplished by using a simple die press. It should also be noted that although sealing step No. 7 of the process may use either welding or brazing, in the preferred embodiment of the process herein disclosed, welding appears to be a preferred form of sealing as compared to brazing.

The resultant dispenser cathode produced by the process hereinabove described and configured as shown in FIG. 2, is particularly advantageous as compared to the dispenser cathode of Patent No. 4,165,473 for a number of reasons. Perhaps the most important such reason is the simplicity of the manufacturing process which greatly reduces the cost of manufacture as previously described. Furthermore, the porous tungsten pellet produced in step No. 1 has no clogged pores, that is, it has open pores that are not clogged by an extraneous material thereby making the metal portion of the cathode more efficient in its response to activation by the barium evaporation emanating from the emissive material contained within the reservoir.In fact, the only thing passing through the pores of the porous tungsten material in the upper pellet is barium or barium oxide emitted at a constant low level of barium evaporation, thereby assuring a substantially constant performance level throughout the life of the cathode.

Reference will now be made to FIGs. 3-5 which illustrate a novel cathode assembly of the present invention. More specifically, referring first to FIG. 3 it will be seen that cathode assembly 20 comprises a housing 22 of generally open rectangular configuration and having a metaliic grid plate 24 substantially enclosing the housing and forming the upper surface thereof. Grid plate 24 comprises three grid apertures 26 (G1 holes) through which three respective electron beams flow upon activation of three respective dispenser cathodes contained within housing 22 in a manner to be described hereinafter. Grid plate 24 may be provided with a plurality of laterally extending claws 28 which can be used for positioning the assembly 20 within a CRT.

Referring now to FIG. 4, it will be seen that housing 22 comprises a ceramic insulating structure 36 which supports the grid plate 24 in precise spaced relation to a dispenser cathode 10, the latter comprising a reservoir 14 containing a first pellet 16 and enclosed by a second pellet 18. In this particular embodiment of dispenser cathode 10, the support cylinder 12 of FIG. 2 has been replaced by a retaining ring 34 which serves the dual function of supporting the cathode in a precisely centered position relative to grid plate 24 and grid aperture 26 and thermally isolating the cathode from the housing 22 and the remaining two cathodes. A heater 30 provides the requisite cathode activating temperature when heater leads 32 are connected to a suitable current source.Cathode lead 38 permits the application of a voltage to the cathode to provide a voltage differential between the cathode 10 and grid plate 24 in a well-known manner.

FIG. 5 illustrates the general geometric confi guration of retaining ring 34 which comprises an inner annular portion 40 for receiving reservoir 14 in the manner illustrated in FIG. 4 and wherein the annular portion is supported by a plurality of radially projecting members 41 separated from one another by air gaps 42.

In order to obtain higher resolution, the CRT designer is being forced to consider the use of cathodes capable of higher current densities. The cathode type used commonly in CRTs is the barium oxide cathode which is limited in current density to approximately 1 amp per square centimeter. This limit then places a restriction on how small the G1 hole can be made. The primary way to obtain higher resolution is to generate a smaller electron beam which commences with the hole size of the G1. Since the hole size is now limited, the demand changes to the optics design which appears to have been already fully exploited. Therefore, the only answer is to use a cathode capable of higher current densities so that the G1 hole size can be reduced.

A dispenser cathode is the logical choice.

Some work has been done previously to evaluate and produce limited quantities of CRTs using impregnated dispenser cathodes.

The cost of these cathodes has not been in the range that would lead to much commercial success for products using these cathodes.

Additionally, some problems have been encountered with power and warm-up time requirements, storage, activation and aging and grid emission.

The assembly of FIGs. 3-5 is thermally efficient using a pancake-type heater 30 that is placed directly behind the emitter. The emitter/heater structure is thermally isolated from the support ceramic, allowing for the cathode to be heated rapidly to dispenser cathode 1000 degrees C operating temperature with approximately the same input power as is currently used with conventional oxide cathodes.

The emitter is a cavity reservoir-type dispenser cathode instead of an impregnated porous tungsten cathode. A thin porous structure pellet 18 made primarily of tungsten is placed over a reservoir of barium oxide ceramic material pellet 16 which decomposes when heated, releasing barium which migrates to the surface, through the porous tungsten dispenser, where it activates the surface and reduces its work function allowing electrons to be extracted at a lower temperature compared to an impregnated dispenser. The pre-processed barium ceramic pellet in the reservoir 14 is produced in such a way that storage of the cathode is not a problem as it is with an impregnated cathode. Also, the evaporation rate of barium is greatly reduced because only enough barium is dispensed to activate the surface.In an impregnated cathode at the beginning of life and for some time thereafter, excess barium is dispensed because the impregnant is at or close to the surface. The excess evaporation can contribute to the problem of grid emission. The disclosed structure can also help eliminate grid emission because the GI plate 24 is brazed to the insulator ceramic 36 which acts as a heat sink and the GI will be at a lower temperature. Activation and aging can be performed more easily with this structure. There is no necessity for carbonate conversion as is required with an oxide cathode. The cathode needs only to be taken to a temperature of 1150 degrees C for 10 to 15 minutes and it will emit at full current. The current density capability of these cathodes is up to 10 amps per square centimeter at temperatures as low as 1050 degrees C.

It will now be understood that what has been disclosed herein comprises a novel improved dispenser cathode capable of generating high current densities which equal or exceed 10 amperes per square centimeter of cathode emission surface area. The novel cathode configuration disclosed results in a significant reduction in cost of manufacture as compared to prior art dispenser cathodes.

The present invention comprises an all metal dispenser cathode which improves the current density of the prior art cathodes normally used in cathode ray tubes by a factor of about 10 while at the same time providing a cathode which is cost comparable to the semiconductor cathodes of the prior art normally used in cathode ray tubes. The substantial reduction in manufacturing costs is obtained by utilizing a four piece assembly which may be readily produced by automated equipment thus providing the performance advantages of prior art dispenser cathodes but the cost advantages of lower current density semiconductor prior art cathodes normally used in cathode ray tubes.

It will also be understood that a unique new cathode assembly, Particularly suited for use in CRTs and specifically color CRTs, has heen disclosed herein. The assembly significantly simplifies CRT cathode and GI grid structure while providing relatively inexpensive high current density capability. A novel three-cathode/grid structure, comprising an integral ceramic housing and apertured grid plate, has been disclosed. Each cathode is thermally isolated by a ceramic retaining ring for rapid cathode heating. The cathode assembly housing provides precise cathode-to-cathode and cathode-to-G1 separation that facilitates lower cost CRT assembly and higher production yields.

Those having skill in the art to which the present invention pertains will now, as a result of the applicant's teaching herein, perceive various modifications and additions which may be made to he invention. By way of example, various modifications may be made to specific structure defined herein as well as to the specific steps of the process defined herein including the use of other ingredient compo nents in the porous tungsten pellet as well as in the underlying emissive tungsten pellet with which barium calcium aluminate is combined in the reservoir of the present invention. However, it will be understood that all such modifications and additions are deemed to be within the scope of the invention which is to be limited only by the claims appended hereto.

Claims (11)

1. A cathode assembly comprising: a cathode; a retaining ring having a portion for receiving said cathode; a housing to which said retaining ring is affixed, the region of affixation between said ring and said housing being spaced from said cathode receiving portion; and a grid having an aperture, the grid being supported by said housing in precise spaced relation to said cathode whereby activation of said cathode causes the emission of a beam of electrons which passes through said aperture.

2. The assembly recited in claim 1 wherein said cathode is a dispenser cathode.

3. The assembly recited in claim 1 wherein there are three of said cathodes and three of said grid apertures for use of said assembly in color CRTs.

4. The assembly recited in claim 1 wherein said housing and said retaining ring are each made of a ceramic material.

5. The assembly recited in claim 1 wherein said cathode comprises: a reservoir formed of a refractory metal; a first pellet contained within said reservoir and comprising barium calcium aluminate and tungsten; a second pellet overlying and sealing said reservoir and comprising pressed and sintered porous tungsten; and means for applying heat to said reservoir and pellets for emitting current therefrom.

6. The assembly recited in claim 5 wherein said reservoir is formed from a metal from the group consisting of molybdenum, rhenium, molybdenum and rhenium in combination, tungsten, tungsten and rhenium in combination and tantalum.

7. The assembly recited in claim 5 wherein said second pellet also comprises at least one metal of the group consisting of iridium, osmium, ruthenium and rhenium.

8. The assembly recited in claim 1 wherein said cathode receiving portion of said retaining ring is of an annular configuration.

9. A cathode assembly for a color CRT, the assembly comprising: at least three cathodes for generating respective electron beams; means for retaining said cathodes in relative spaced relation to one another; a grid plate; means for supporting said grid plate in fixed spaced relation to all of said cathodes, said grid plate having at least three apertures through which said electron beams may pass, respectively; and means for heating each of said cathodes for initiating said electron beams.

10. The assembly recited in claim 9 wherein said cathode retaining means and said grid plate supporting means are each made of a ceramic material.

11. A cathode assembly substantially as hereinbefore described with reference to the accompanying drawings.