US3582702A

US3582702A - Thermionic electron-emissive electrode with a gas-binding material

Info

Publication number: US3582702A
Application number: US811846A
Authority: US
Inventors: Friedrich Hermann Raymun Almer
Original assignee: US Philips Corp
Current assignee: US Philips Corp
Priority date: 1968-04-04
Filing date: 1969-04-01
Publication date: 1971-06-01
Anticipated expiration: 1988-06-01
Also published as: DE1913071A1; CH495055A; ES365598A1; SE339962B; BE730961A; FR2007388A1; GB1265881A; OA03035A; NL6804720A

Abstract

A thermionic cathode including a support in the form of a metal sheet with an electron-emissive coating on one side and a coating of gas-binding material on the opposite side. The sheet may be formed into a tube and the electron-emissive coating may be on the outside or inside and the gas-binding material on the opposite side. Such cathodes may be employed in electric discharge tubes, such as cathode-ray tubes and low-pressure gasdischarge lamps.

Description

United States, Patent Inventor Friedrich Hermann Raymund Almer Emmasingel, Netherlands Appl. No. 811,846 Filed Apr. 1, 1969 Patented June 1,1971 Assignee U.S. Philips Corporation New York, N.Y. Priority Apr. 4, 1968 Netherlands 6804720 THERMIONIC ELECTRON-EMISSIVE ELECTRODE WITH A GAS-BINDING MATERIAL 15 Claims, 7 Drawing figs.

0.8. CI 313/174, 313/l76,3l3/178,313/346 Int. Cl ..H0lj 19/68, H01] 19/70 Field of Search 313/174,

[56] References Cited UNITED STATES PATENTS 2,741,717 4/1956 Katz 2,843,781 7/1958 Kerstetter et a1. 3 ,078,387 7/1968 Dorgelo 3,110,081 11/1968 Hendriks.... 3,495,116 2/1970 Horn et a1 Primary Ex aminer.lohn W. Huckert Assistant ExaminerAndrew .1. James Attorney-Frank R. Trifari 313/178 313/178 313/179X 313/346X 3l3/179X ABSTRACT: A thermionic cathode including a support in the form of a metal sheet with an electron-emissive coating on one side and a coating of gas-binding material on the opposite side. The sheet may be formed into a tube and the electron-emissive coating may be on the outside or inside and the gas-binding material on the opposite side. Such cathodes may be employed in electric discharge tubes, such as cathode-ray tubes and low-pressure gas-discharge lamps.

PATENTEUJUN 1m 3,582,702

INVENTOR. FRIEIIRICH H.R.ALMER .BY 2%., 4; K. L

AG NT THERMTONTC ELECTRON-EMHSSIVE ELECTRODE WHTH A GAS-BINDING MATERIAL The invention relates to a thermionic electrode which includes a metal support one side of which is provided with an electron emissive coating. Furthermore the invention relates to an electric discharge tube provided with such an electrode.

Many electrode constructions for electric discharge tubes starting from a supporting body, for example, from metal strip or sheet are known from literature. This starting material is preferably chosen because it is advantageous to have a support having a large surface and a small volume. Mostly at least one side of the supporting body is provided with an electronemissive coating. To this end many methods are known, for example, spraying, immersing, compressing or combinations thereof.

Prior to or after providing the emissive coating the electrode is formed into its ultimate shape.

in general the discharge space of an electric discharge tube contains different gaseous impurities. These are partly residual gases which have remained after exhausting of the tube and partly these are gases which are bound to or in the wall of the discharge tube, the electrodes or the current supply wires and the like, and which evolve during operation of the tube. These gaseous impurities are mostly disturbing for the satisfactory operation of the tube, because they may give rise to a poisoning of the emissive coating. in the case of low-pressure gas discharge lamps the gaseous impurities may also influence the efficiency of the lamp in an unfavorable sense. ln addition they may considerably increase the ignition voltage of the lamp.

To bind these impurities so that they can no longer exert their disturbing influence in the discharge space it is known to provide gas-binding materials sometimes also called getters, in the discharge space. It is, for example, known to form a gasbinding surface on a portion of the wall of the tube, or to place a sheet or strip of a gas-binding material in the vicinity of an electrode.

The following requirements are, inter alia, imposed on a getter material for a discharge tube. In the first place the material must be able to bind the unwanted gases to a sufficient extend; in the second place the gases must be bound sufficiently quickly. in general the getter materials have, however, a selective action, that is to say, they bind only very specific gases in sufficient quantities and at a sufficient velocity. in addition the operation of most getters is dependent on the temperature. Thus, for example, a barium coating will operate at a comparatively low temperature (lower than 100 C.) whereas a solid zirconium or titanium plate operates satisfactorily only at considerably higher temperatures (higher than b C.)

A thermionic electrode according to the invention is provided with a support consisting of metal sheet, one surface of which has an electron-emissive coating and is characterized in that the other surface of the support has a gas-binding coating.

in an electrode according to the invention two functions are combined in a simple manner, namely the supply of a thermally emitted electron current and the binding of the gaseous impurities. ln electrodes according to the invention the getter coating and the emissive coating do not disturb each other and the two mentioned functions are fulfilled in an efficient manner. it has surprisingly been found that even an improvement of the emissive qualities may occur in case of suitable choice of the gas-binding material, because the gas-binding material has an activating effect on the emitter material. ln fact, at the high temperature at which the electrode is operated part of the gas-binding material will diffuse through the support of the electrode according to the invention to the emissive coating, where it forms, for example, free barium due to reduction of barium oxide. Due to the permanent supply of activator material during the lifetime of the discharge tube, the emissive qualities retain their optimum values particularly when the gas-binding action is accompanied by an activating action it is possible to manufacture discharge tubes having a long lifetime.

The result of providing a getter coating on a thermionic electrode is that the coating is brought to a high temperature during the operation of the discharge tube. This is very advantageous if gas-binding materials are used, whose gas-binding properties enhance at comparatively high temperatures.

One or more of the elements zirconium, titanium, lanthanum, cerium or thorium are preferably used as gas-binding materials. All mentioned elements have excellent gasbinding properties at a high temperature. Particularly oxygen, water vapor, carbon dioxide and carbon monoxide are bound to a sufficient extent and at a sufficient velocity by these elements. Especially these gases have been found to be disturbing during the operation of the discharge tube. An advantage of the said gas-binding elements is that they have a great affinity for hydrogen at a comparatively low temperature. Prior to the discharge tube being ignited, substantially all hydrogen in the discharge space is thus bound to the gas-binding coating. As is known, too high a hydrogen pressure in the discharge space gives rise to an increase of the ignition voltage.

The form of the gas-binding coating is important for the gasbinding properties. Thus a foil, of, for example, zirconium provided on the support, or a zirconium layer having a small pore volume will have a smaller gas-binding power than a layer of zirconium granules which are only weakly sintered together. In fact, a weakly sintered layer has a large active surface.

A method of manufacturing an anode for electric discharge tubes provided with a gas-binding coating is known from British Pat. Specification No. 995,82l. The gas-binding coating is preferably provided on an electrode according to the invention in accordance with this known method. To this end, metal particles are provided on a metal strip from a holder, whereafter the strip provided with the particles is introduced into a furnace where the particles are sintered to the strip. One or more of the above-mentioned gas-binding materials are provided in the interstices between the metal particles, mostly in the form of hydrides. The coating thus formed is then compressed, for example, between steel rollers so that the metal particles are deformed and this to such an extent that they largely surround the gas-binding material and therefore retains it. An electrode according to the invention may now be formed, for example, by bending, welding etc. from pieces of the metal strip having a gas-binding coating and obtained in the manner as described above. The emissive coating may be provided on the other side of the strip prior to or after the ultimate design of the electrode.

Upon so-called formation and activation of the electrode in the discharge tube the getter material is only weakly sintered so that a large active surface is maintained. If the getter material is provided in the coating as a hydride, for example, zirconium hydride, the hydrogen gas evolves during formation which enhances the activation and formation of the emissive coating.

It has been found that addition of tungsten and/or nickel powder to the gas-binding material has a favorable influence on the gas-binding properties. The tungsten powder prevents too strong a sintering of the material and due to the addition of the nickel powder a larger quantity of getter material is incorporated between the metal particles. The overall quantity of tungsten and/or nickel is 20-60 percent by weight.

ln one preferred embodiment of an electrode according to the invention the emissive coating is provided in the same manner as described above for the gas-binding coating. Such a method of providing a thermionic coating on electrodes is known from British Pat. Specification No. 940,063. An electrode is then obtained whose emissive coating has a low work function as a result of the weak sintering of the emitting material. The electrode may then be operated at a temperature which need not be higher than approximately 900 C. in addition this embodiment of the electrode has the advantages of a small electric resistance and of an excellent resistance to sputtering by an ion bombardment possibly occurring.

if the two coatings are provided in the manner described above it is recommended to form initially the gas-binding coating on one side of the strip and thereafter the emissive coating on the other side, in which the sintering of the metal particles for the formation of the last-mentioned coating must be effected in a reducing atmosphere, for example, in hydrogen or in a reducing mixture of hydrogen and an inert gas in order to protect the hydrides in the gas-binding coating. It is alternatively possible to provide the two coatings simultaneously.

Satisfactory results have been obtained with molybdenum, nickel-plated iron, nickel and so-called cathode-nickel, (nickel which contains one or more activating elements such as magnesium, aluminum, silicon or zirconium in very small quantities) as materials for the support of the electrode. Nickel and particularly cathode-nickel are, however, preferred because these materials are cheap and do not contain unwanted impurities. In addition these materials can easily be processed.

The metal particles sintered to the support may consist, for example, of vanadium, molybdenum, iron, cobalt or nickel, because these metals can satisfactorily be sintered to a metal substratum. Here again nickel is preferred, inter alia because it is cheap and because it can easily be provided on the support in the desired shape. In fact, when using nickel particles they can be fed to the strip from a magnetic container so that the particles are directed in such a manner that agglomerations are formed in a direction perpendicular to the strip, with mutual interstices between the agglomerations. Upon sintering along columns of nickel particles are then formed on the strip which may surround much gas-binding or electronemitting material.

For the supports in electrodes according to the invention, it is advantageous to use metal strip which has a slight thickness, for example, between 20 and 100 m. The electrode then has a slight heat capacity so that the time required to bring electrodes to the temperature of emission is short.

To obtain a satisfactory lifetime of the discharge tube the electrode according to the invention is preferably provided with at least 1 mg. of emitting material per cm. ofthe emitting surface. A quantity of emitting material larger than l mg./cm. is mostly not necessary. One or more of the alkaline earth oxides may be used as emitter material in the manufacture of the tube. These oxides are mostly formed carbonates, for example, a mixture of barium carbonate, strontium carbonate and calcium carbonate. The emitting material may advantageously be mixed with l l 0 percent by weight of one or more of the elements titanium, zirconium, hafnium or thorium. The said elements enhance the process of activation of the emitting material, particularly during the initial operating hours of the tube. When the admixed activating elements have been consumed the diffusion process of the gas-binding material has advanced so far that sufficient activators are post supplied.

An electrode according to the invention in an electric vacuum discharge tube is preferably formed as an indirectly heated cathode having a hollow cylindrical supporting body. The emissive coating is then provided on the outer side of the cylinder opposite the anode. The getter coating is provided on the inner side of the cylinder and binds the gases evolved from the filament so that these cannot penetrate the discharge space. At least one end of the cylinder has an aperture so that also the gases evolved elsewhere in the discharge space may be bound by the getter coating. In this use the permanent supply of activators from the getter coating to the emissive coating plays a great part so that tubes having a very long lifetime can be manufactured.

When using an electrode according to the invention in cathode-ray tubes, for example, for television purposes, the electrode is preferably formed as an indirectly heated cathode having a hollow cylindrical supporting body one end of which is closed by a metal sheet whose outer side is provided with an emissive coating. The getter coating is provided on the inner side of the sheet and possibly on the cylindrical portion of the support.

An electrode according to the invention may very advantageously be used, in a low-pressure gas discharge tube, for example, a low-pressure mercury vapor discharge lamp. Here again the electrode preferably has the shape of a hollow cylinder at least one end of which has an aperture In this case the emissive coating is provided on the inner side of the cylinder. The electrode material possibly sputtered by an ion bombardment then will not escape easily from the electrode. The getter coating is now provided on the outer side of the cylinder which is advantageous for the gas-binding action because the getter coating is then easily accessible to the impurities.

The invention with reference to the accompanying drawing, in which:

FIG. 1 is a cross-sectional view of an electrode according to the invention,

FIG. 2 is a cross-sectional view of a further embodiment of an electrode according to the invention,

FIG. 3 is a sectional view of nickel strip provided with a gas binding and an emissive coating which strip can be used in the manufacture of an electrode according to the invention,

FIG. 4 shows a further embodiment of an electrode according to the invention for use in cathode-ray tubes,

FIG. 5 is a sectional view of a vacuum discharge tube according to the invention, and

FIG. 6 is a sectional view of a cathode-ray tube according to the invention,

FIG. 7 finally shows a low-pressure mercury vapor discharge lamp according to the invention.

In FIG. 1, the reference numeral 1 indicates the supporting body from nickel strip of a hollow cylindrical electrode which may be used in vacuum discharge tubes. The dimensions of the support are dependent on the purpose for which the electrode is used. In a rectifier diode, for example, the length of the cylinder may be 60 mm. and the diameter may be 5 mm. The thickness of the strip is 75 am. in this case. The gas-binding coating 2 is provided on the inner side and the emissive coating 3 is provided on the the outer side of the electrode. A lock seam is indicated by 4 which in this embodiment of the electrode is located on the inner side.

FIG. 2 shows the cross section of a hollow cylindrical electrode for use in a low-pressure gas discharge lamp, the supporting body 1 of nickel is 15 mm. long and has a diameter of approximately 2.5 mm. while the thickness of the nickel strip is approximately 50 m. In this case the gas-binding coating 2 is provided on the outer side and the emissive coating 3 is provided on the inner side of the support. The lock seam 4 now extend along the outer side of the electrode.

FIG. 3 shows on an enlarged scale a cross section of part of an electrode according to the invention. Reference numeral 1 indicates a strip of cathode-nickel which contains, for example, 0.030.09 percent by weight of magnesium, and which serves as a support for the gas-binding

coating

2,5 and the

emissive coating

3,6. In this case the strip has a thickness of approximately 50 pm. The two coatings are approximately 10 m. thick. The gas-binding material 2 consists of zirconium to which approximately 30 percent by weight of nickel is added and it is largely surrounded and retained by the columns 5 of nickel particles and the nickel strip 1. The emitting material consists of a mixture of alkaline earth oxides to which 1-10 percent by weight of titanium and/or zirconium is added and it is largely surrounded by the columns 6 of nickel particles and the nickel strip 1. After the electrode has operated for some time, the nickel support will contain a small quantity of zirconium. The concentration of the zirconium in the support decreases, while going from the gas-binding surface to the emitting surface. After some time a stationary state sets in, the supply of zirconium to the emitting coating being exactly sufficient for an optimum action of this coating.

FIG. 4 shows the support 1 from nickel strip of an electrode which is used in a cathode-ray tube, for example, for the display of television pictures. The support consists of a hollow cylinder closed at one end and formed from one one piece of the nickel strip by deep drawing. The gas-binding coating 2 is provided across the entire inner surface of the electrode. The emissive coating 3 is provided on the outer side of the electrode and extends substantially throughout the surface of the portion closing the cylinder. The electrode is mostly used in combination with a metal shaft which surrounds the filament and which is not shown in the drawing.

FIG. 5 shows the glass envelope 7 of a triode according to the invention. The cathode l, 2, 3 has a shape as shown in FIG. 1 and is indirectly heated by a filament spiral 8. A grid is indicated by 9 and an anode is indicated by 10.

F IG. 6 shows a cathode-ray tube according to the invention which is used for the display of pictures. The indirectly heated electrode 1 provided with an emitting surface 3 and a gasbinding surface 2 is of the type as shown in FIG. 4. The filament is not shown. The electrode is secured with the aid of ceramic material 14 in a so-called Wehnelt cylinder 15. This cylinder and the remaining electrodes not shown in the Figure are supported by supporting columns 16. The envelope, for example, of glass of the discharge space is indicated by 7.

ln HO. 7 reference numeral 7 indicates the wall, for example, of glass of a low-pressure mercury vapor discharge lamp according to the invention which lamp consumes a power of 40 watt during operation. A pinch 11 is formed at either end of the lamp through which current supply wires 12 are passed. The current supply wires are connected in the discharge space to

electrodes

1, 2, 3 by means of spot-welding. A cross section of these electrodes is shown in FlG. 2. A luminescent layer is indicated by 13. The electrode construction is cheap and does not require means to preheat the electrode upon ignition of the lamp.

What 1 claim is:

l. A thermionic electron-emissive electrode comprising a support consisting of metal sheet, an electron-emissive coating on one surface of said support, and a coating of a gas-binding material on the opposite surface of said support.

2. A thermionic electron-emissive electrode as claimed in claim 1, wherein the support has a thickness of between and 100 **m.

3. A thermionic electron-emissive electrode as claimed in claim ll wherein the gas-binding material is also an activator for the emissive coating.

4. A thermionic electron-emmissive electrode as claimed in claim 1 wherein the gas-binding material consists of an element selected from the group consisting of one zirconium, titanium, lanthanum, cerium and thorium.

5. A thermionic electron-emissive electrode as claimed in claim 1 wherein the gas-binding material is mixed with 2060 percent by weight of at least one of the powders of tungsten and nickel.

6. A thermionic electron-emissive electrode as claimed in claim 1 wherein the gas-binding coating consists of metal particles sintered to the support between which the gas-binding material is provided, the metal particles substantially surrounding the gas-binding material.

7. A thermionic electron-emissive electrode as claimed in claim I wherein the electron-emissive coating consists of metal particles sintered to the support between which emitting material having a work function smaller than 2 ev. is provided, the metal particles having a shape such that they largely surround the emitting material.

8. A thermionic electron-emissive electrode as claimed in claim 1 wherein the metal support consists at least in part of nickel.

9. A thermionic electron-emissive electrode as claimed in claim 7 wherein the metal particles consist of nickel.

10. A thermionic electron-emissive electrode as claimed in claim 9 wherein the electrode contains at least of 1 mg. of emitting material per cm. of emitting surface.

11. A thermionic electron-emissive electrode as claimed in claim 9 wherein the emitting material consists of at least one alkaline earth oxide.

12. A thermionic electron-emissive electrode as claimed in claim 11, wherein the emitting material is mixed with 1-10 percent by weight of at least one of the elements selected from the group consisting of titanium, zirconium, hafnium and thorium.

13. An electric discharge tube including a thermionic electron-emissive electrode comprising a hollow cylinder at least one end of which has an aperture, an emissive coating provided on the outer surface of the cylinder, and a getter material on the inner surface of the cylinder.

M. A cathode-ray tube including a thermionic electron emissive electrode comprising a hollow cylinder one end of which is closed by a metal sheet, the outer side of said cylinder being provided with an emissive coating and the inner side of the cylinder being provided with a gas'binding coating.

15. A low-pressure gas-discharge lamp comprising a thermionic electron-emissive electrode comprising a hollow cylinder at least one end of which has an aperture, an emissive coating provided on the inner side of the cylinder, and a gasbinding coating on the outer surface of said cylinder.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Dated June 1,

Patent No. 3 y 58 2 702 Friedrich Hermann Raymund Almer Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 42, "extend" should read extent Column 4, line 13, after "invention" insert will now be "**m" should read um described Column 5, claim 2,

Signed and sealed this 18th day of January 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Acting Commissioner of Patents Attesting Officer FORM po'wso USCOMM-DC 60376-P69 Q U 5 GOVERNMENT PRINYINE OFFICE 959 0-365'334

Claims

1. A thermionic electron-emissive electrode comprising a support consisting of metal sheet, an electron-emissive coating on one surface of said support, and a coating of a gas-binding material on the opposite surface of said support.

2. A thermionic electron-emissive electrode as claimed in claim 1, wherein the support has a thickness of between 20 and 100 m.

3. A thermionic electron-emissive electrode as claimed in claim 1 wherein the gas-binding material is also an activator for the emissive coating.

5. A thermionic electron-emissive electrode as claimed in claim 1 wherein the gas-binding material is mixed with 20- 60 percent by weight of at least one of the powders of tungsten and nickel.

7. A thermionic electron-emissive electrode as claimed in claim 1 wherein the electron-emissive coating consists of metal particles sintered to the support between which emitting material having a work function smaller than 2 ev. is provided, the metal particles having a shape such that they largely surround the emitting material.

10. A thermionic electron-emissive electrode as claimed in claim 9 wherein the electrode contains at least of 1 mg. of emitting material per cm.2 of emitting surface.

12. A thermionic electron-emissive electrode as claimed in claim 11, wherein the emitting material is mixed with 1-l0 percent by weight of at least one of the elements selected from the group consisting of titanium, zirconium, hafnium and thorium.

14. A cathode-ray tube including a thermionic electron-emissive electrode comprising a hollow cylinder one end of which is closed by a metal sheet, the outer side of said cylinder being provided with an emissive coating and the inner side of the cylinder being provided with a gas-binding coating.

15. A low-pressure gas-discharge lamp comprising a thermionic electron-emissive electrode comprising a hollow cylinder at least one end of which has an aperture, an emissive coating provided on the inner side of the cylinder, and a gas-binding coating on the outer surface of said cylinder.