WO1996039709A1

WO1996039709A1 - Directly heated dispenser cathode and method of manufacture therefor

Info

Publication number: WO1996039709A1
Application number: PCT/US1996/007846
Authority: WO
Inventors: Joel P. Moskowitz; Louis R. Falce; Glenn S. Breeze
Original assignee: Ceradyne, Inc.
Priority date: 1995-06-05
Filing date: 1996-05-28
Publication date: 1996-12-12
Also published as: AU5937196A

Abstract

A pellet-type dispenser cathode has an integral filament (18) for directly heating electron-emitting material and employs conductive legs (20) which serve a dual function of supplying heater current and structurally supporting the pellet. The shape of the legs (20) may be selected to provide heater power at a variety of voltages depending upon the electrical resistance of the legs (20). A photo-etching process may be used to fabricate the filament (18).

Description

DIRECTLY HEATED DISPENSER CATHODE AND METHOD OF MANUFACTURE THEREFOR

FIELD OF THE INVENTION

This invention pertains generally to thermionic cathodes and more particularly to dispenser cathodes which find particular advantageous application in cathode ray tubes that require relatively high current density.

BACKGROUND ART

The most relevant prior art known to the applicant are co-inventors' previous patents, U.S. Patent No. 4,823,044 issued April 18, 1989 for A DISPENSER CATHODE AND METHOD OF MANUFACTURE THEREFORE and U.S. Patent No. 5,218,263 issued June 8, 1993 for A HIGH THERMAL EFFICIENCY DISPENSER CATHODE AND METHOD OF MANUFACTURE THEREFORE. The 4,823,044 patent discloses a dispenser cathode which employs a novel structure, permitting a significant reduction in cost for a cathode capable of achieving extremely high current densities, such as for use in cathode ray tubes. The structure of that dispenser cathode is conducive to a uniform level of performance throughout the life of the cathode, namely uniformity of current density. The configuration of that prior invention produces a uniform flow of barium from a reservoir enclosed pellet. The barium passes through a pure tungsten enclosing pellet which has a porous configuration. The porous, pure tungsten pellet needs no impregnation because an activating barium is derived entirely from an underlying enclosed pellet. The pure tungsten overlying pellet and the underlying barium source pellet configuration, prevents clogging of pores in the tungsten pellet and also prevents current density changes or patchiness, both instantaneously and over the life of the cathode. The prior art dispenser cathode of Patent 4,823,044, comprises four separate pieces, namely 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 molybdenum and rhenium, tantalum, or other refractory metal and a support cylinder in the form of an extrusion or similarly processed structure formed of molybdenum, molybdenum/rhenium or tantalum. The resulting cathode is designed to operate at approximately 850-1,150 degrees centigrade, depending upon the current density objectives. The pellet contained within the reservoir provides a constant low level of barium evaporation to activate the tungsten in the overlying pellet.

The need for a high current density, relatively inexpensive cathode is driven by the demand for higher resolution cathode ray tubes for high definition television (HDTV) , automotive displays, large screen televisions, computer graphic displays, projection television and avionic applications. These new applications for cathode ray tubes require the employment of cathodes capable of producing higher current densities than those presently obtainable from the triple carbonate oxide cathode. In other than short pulse applications, the triple carbonate oxide cathode system, which has been the industry standard for decades, produces emission densities of less than an ampere per centimeter squared and therefore cannot be used in applications where higher densities are required. A cathode system which will meet the market demand for higher resolution must be capable of achieving two design criteria. First, a smaller diameter electron beam bundle which produces a smaller spot size at the viewing surface is required. This smaller electron beam is produced by using smaller apertures in the beam forming region (BFR) of the electron gun. Secondly, because brightness levels for these high resolution applications must be maintained, the currents of these smaller diameter electron beams must be the same as those of the conventional larger beam diameter systems. To achieve this goal, a cathode system must operate at a higher current density. The characteristic behavior of an oxide cathode is related to the fact that it is essentially a dielectric material and will "charge up". It can only achieve high current densities in short pulse length applications. Oxide cathodes are also susceptible to poisoning, requiring exacting and lengthy tube processing to obtain the best performance characteristics. The life of oxide cathodes and cathode ray tube guns is relatively short, particularly in applications where the current density is in excess of a few hundred milliAmperes per square centimeter. Because the dielectric nature of an oxide cathode limits the current density, a metal emitter as used in dispenser cathodes must be considered for cathode ray tubes.

The impregnated dispenser cathode, a typical use of which is in microwave tubes, is made from porous tungsten which is impregnated with barium compounds. When heated, the barium compounds react with the tungsten matrix, allowing the barium to migrate to the surface of the cathode. Throughout its use, the cathode surface is constantly covered with barium and the emitter surface work function drops from 4.5 electron volts to as low as 2.0 electron volts. While the impregnated dispenser cathode is capable of producing high current densities and long lifetime use, it must be operated at about 200 degrees centigrade higher than the oxide cathode. In addition to requiring a higher operating temperature to produce the higher current density, this cathode also requires a longer activation cycle. These two performance characteristics result in excessive evaporation of the barium, which can cause unwanted grid emission and high voltage instability. Because of this and because the conventional impregnated dispenser cathode is more expensive to manufacture than the oxide cathode, the reservoir dispenser cathode was considered superior for use in cathode ray tube applications.

The reservoir cathode was the original type of dispenser cathode. With this design, barium compounds are held in a cavity or reservoir behind a porous disk, such as that disclosed in the aforementioned prior art patent of the applicant, namely Patent No. 4,823,044. When heated, the compounds decompose or react with a reducing agent. The barium is then dispensed through the porous disk to the surface. While this novel reservoir cathode is a significant improvement over the previous art in terms of life and cost to manufacture. the porous disk through which the barium is dispensed, such as the tungsten overlying disk described in Patent No. 4,823,044, has a porosity which is dependent upon the pressure, temperature and starting materials used in its fabrication. Furthermore, the number, size and location of the "pores" that are produced, such as for example by pressing and sintering pure tungsten, are random and relatively difficult to control.

Prior Art Patent No. 5,218,263 discloses a controlled porosity, reservoir cathode which produces the higher current densities and brightness levels that are required in high-resolution cathode ray tube guns. Instead of using a porous tungsten disk for the emitter, a refractory alloy metal sheet is substituted therefor. The porosity of the metal sheet is not random as it is in the porous tungsten disk. Instead, the metal sheet is provided with a precise array of holes or pores that are preferably laser-drilled in the refractory alloy emitter, directly behind the Gl aperture of the beam forming region of the electron gun. The size and spacing of the holes determine the dispensing rate of the barium from the underlying reservoir. Thus, evaporation rate is more precisely controlled. This results in a minimized work function which leads to a cathode which ope rates at a lower temperature thereby increas ing lifetime and consistently producing higher cur rent density. The structure of the cathode is simil ar to that disclosed in the prior Falce patent except that the porous tungsten disk thereof which forms the overlying pellet through which the barium passes , is replaced by a 50 micrometer thickness sheet of a tungsten-rhenium alloy in which the rhenium constitutes about 10-50 percent of the total volume .

Prior Art Patent No. 5 ,218 ,263 al so discloses a thermally high-eff iciency structure to fully exploit the perf ormance of the cathode in a long-life conf iguration in which the ratio of heater power to current density is very low . This long-life conf iguration compr ises two subassemblies , namely , an inner subassembly and an outer subassembly . The inner subasse bly comprises a laser-drilled tungsten-rhenium alloy cap which is laser seam-welded to a molybdenum heater cup. The welding of the cap to the cup assures excellent heat transfer from the heater cup to the electron emissive surface of the cathode . A pellet containing barium compound is captured between the cap and the heater cup to provide a long-life supply of barium to the cathode surface . The inverted tab structure disclosed in the 5 ,218 ,263 patent provides a rigid mechanical support for the inner cathode assembly and thermally isolates the inner subassembly from the outer subassembly .

It would be highly advantageous to further reduce cost and further improve thermal ef f iciency in a dispense r cathode whil e maintaining high perf ormance features . One way of accomplishing cost reduction is to simplify structure and one way of improving thermal efficiency is to improve the thermal transfer between the heater and the electron-emitting material in the cathode .

SUMMARY OF THE INVENTION

The present invention comprises a dispenser cathode assembly for use in cathode ray tubes (CRTs) , that is directly heated and results in the cathode emitting electrons in less than 2 seconds from the time it is turned on. Existing designs which utilize indirectly heated cathodes take 9 seconds or more to come to a temperature sufficient to emit electrons.

The aforementioned prior art dispenser cathode pellets are heated indirectly by means of a filament through which electric current is passed. As a result of the electrical resistance of the filament (normally a coiled wire) the temperature of the filament, generally referred to as the heater, increases to temperatures in excess of 1200 degrees centigrade. By means of radiation and thermal conductance, the heat is transferred to the cathode pellet which emits electrons upon reaching temperatures in excess of 900 degrees centigrade. In the present invention the filament and cathode pellet are an integral unit consisting of two components, a barium-calcium-aluminate-impregnated porous tungsten cathode pellet and a filament which is preferably produced by chemical/photo lithographic etching techniques. The filament can be made from a variety of materials including molybdenum, tungsten, alloys of these elements with rhenium, tantalum or any other refractory metal having a melting point in excess of 1800 degrees centigrade. The process of photo etching allows mass production of the filaments which is considerably less expensive than wound filament wires used in the prior art. An additional unique feature of the chemical/photo lithographic etching process is that the legs of the filament can be produced as flat meander lines which provide increased resistance in the same confined space as well as providing a means of adjusting the voltages applied to the filament. Furthermore, the pellet may be structurally supported by the filament legs, thus obviating the far more complex and costly support structure of pellet-type dispenser cathode assemblies of the prior art. STATEMENT OF THE INVENTION

It is therefore a principal object of the present invention to provide a low-cost, highly thermal- efficient pellet-type dispenser cathode with quick response to heater current.

It is an additional object of the invention to provide a directly heated dispenser cathode wherein a cathode pellet is made integral with a heater filament.

It is still another object of the invention to provide a dispenser cathode having a directly heated electron emitting pellet wherein filament legs may be employed to structurally support the pellet, such as in a CRT.

It is still an additional object of the invention to provide a dispenser cathode having an integral heater and electron emitting material, wherein the heater is welded directly to the material and wherein the heater may be made by a chemical/photo lithographic etching process and includes heating current leads that may be made in virtually any desired shape. 13

BRIEF DESCRIPTION OF THE DRAWINGS

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 when taken in conjunction with the following drawings in which:

FIG. 1 is a three-dimensional view of a cathode assembly configured in accordance with a preferred embodiment of the present invention; and

FIG. 2 is a top view of the heater of an alternative embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to the accompanying figures it will be seen that a filament and cathode pellet form an integral unit 10 consisting of two components, a barium calcium aluminate impregnated porous tungsten cathode pellet 12 and a filament 14 which has been produced by chemical/photo lithographic etching techniques. The filament 14 can be made from a variety of materials including molybdenum, tungsten, alloys of these elements with rhenium, tantalum or any other refractory metal whose melting point is in excess of 1800 degrees centigrade. The process of photo etching allows mass production of the filaments which is considerably less expensive than separately fabricating wound filament wires used in the prior art. An additional and unique feature of the photo etching process is that the legs 16 of the filament 14 can be produced, instead of straight as shown in FIG. 1, as flat meander lines shown in FIG. 2 and which provide increased resistance in the same confined space as well as providing a means of adjusting the voltages applied to the filament. The preferred techniques used to join the pellet 12 to the filament 14 may be carried out as follows:

1) Prior to impregnation the tungsten pellet 12 is coated on one side with a material such as molybdenum ruthenium alloy, rhodium or any material that has a melting point in excess of 1800 degrees centigrade. This material is melted and flowed onto one side of the tungsten pellet. The remaining side of the tungsten pellet is coated with a layer of osmium- ruthenium alloy to reduce the work function of the emitting surface. After impregnation this coated pellet is joined to the filament by either resistance welding or laser welding.

2) A preformed sheet made of a material such as platinum, tantalum or other similar and appropriate metal, is welded to the pellet 12 prior to impregnation and is joined to the filament by the same means as described above.

As shown in FIG. 2, a filament 18 can be configured with higher resistance legs 20 of a meandering shape. Such shapes elongate the legs and permit use of higher voltages to be applied to the legs 20 to reach the desired temperature. It will, of course, be understood that other shapes of the legs 20 may be readily provided depending upon the desired resistance. In this manner, both the heater voltage and structural characteristics of the filament legs may be selected for electrical and mechanical accommodation of the surrounding system such as the CRT.

It will now be understood that what has been disclosed herein is a low-cost, high performance, directly heated dispenser cathode having a simplified structure and high thermal efficiency. Those having skill in the relevant art, will perceive various modifications which may be made to the invention. By way of example, the illustrated shape and disclosed method of manufacture may be readily altered to accommodate different cathode material configurations and other structural support requirements. Accordingly, all such modifications are deemed to be 17

within the scope of the invention, which is to be limited only by the appended claims and their equivalents.

We claim:

Claims

1. A dispenser cathode comprising: a pellet formed of an electron emissive material; an electrical heater conf igured to have a substantially planar portion and having a plurality of radially extending legs through which heater current may flow to heat said planar portion; said heater planar portion being integral to said pellet for directly heating said pellet to a temperatu re sufficient to emit electrons from said electron emissive material .

2 . The dispenser cathode recited in claim 1 wherein said material comprises barium-calcium- aluminate-impregnated porous tungsten having a surface overcoated with osmium ruthenium alloy.

3. The dispenser cathode recited in claim 1 wherein said heater comprises a metal having a melting point higher than said temperature sufficient to emit electrons.

4. The dispenser cathode recited in claim 1 wherein said pellet is structurally supported only by said heater legs.

5. The dispenser cathode recited in claim 1 wherein each of said heater legs is shaped in a meandering configuration.

6. A method for fabricating a dispenser cathode comprising the steps of: a) providing a porous tungsten pellet; b) welding a heater material directly to said pellet; and c) impregnating said pellet with barium- calcium aluminate; d) overcoating the impregnated pellet with osmium ruthenium alloy. 20

7. The method recited in claim 6 wherein step b) comprises the preparatory step of forming a heater out of an electrically conductive material shaped as a substantially planar portion having a plurality of radially extending legs.

8. The method recited in claim 7 wherein step b) further comprises the step of making said heater out of a preformed sheet of refractory metal.

9. The method recited in claim 7 wherein step b) further comprises the step of etching said conductive material to form a heater having a flat central portion and a plurality of radially extending legs.

10. The method recited in claim 7 wherein step b) further comprises the step of configuring said radially extending legs in a meandering shape.

11. A dispenser cathode comprising: a heater in direct contact with an electron- emitting material , said heater having a plurality of extending legs ; said heater legs providing structural support of said cathode and electrical connection to said heater .

12. The cathode recited in claim 11 wherein said material is in the shape of a cylindrical pellet and said heater is af fixed to said pellet.

13 . The cathode recited in claim 11 wherein said material comprises barium-calcium-aluminate- impregnated porous tungsten .

14 . The cathode recited in claim 12 wherein said pellet comprises barium-calcium-aluminate- impregnated porous tungsten .

15. The cathode recited in claim 14 wherein said pellet comprises an electron emitting surface coated with an alloy of osmium and ruthenium.