US3535757A

US3535757A - Method for making cathode assembly for electron tube

Info

Publication number: US3535757A
Application number: US715200A
Authority: US
Inventors: Paul L Nestleroth; William H Silvers
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1968-03-22
Filing date: 1968-03-22
Publication date: 1970-10-27
Anticipated expiration: 1987-10-27

Description

Oct. 27, 1970 P. L.NESTLEROTH ETAL 3,535,757

METHOD FOR MAKING CATHODE ASSEMBLY FOR ELECTRON TUBE Filed March 22, 1968 WELD NICKEL SUBSTRATE TO NICKEL-CHROMIUM SLEEVE HEAT SUBSTRATE AND SLEEVE IN ATMOSPHERE CONSISTING ESSENTIALLY OFHYDROGEN ANQ WATER VAPOR, THE DEW POINT OF SAID ATMOSPHERE BEING IN THE RANGE OF 35C. T0 25C.

COAT A PORTION OF THE SURFACE OF THE SUBSTRATE WITH A COMPOSITION WHICH PRODUCES AN ELECTRON-EMISSIVE COATING- UPON SUBSEQUENT HEATING AT ELEVATED TEMPERATURES WELD SLEEVE T0 CATHODE SUPPORT I N VEN TORS Paul L. Nestlerotlz and Willi m H. Silvers.

A TTORNE Y United States Patent O U.S. Cl. 29-2514 7 Claims ABSTRACT OF THE DISCLOSURE A two-piece cathode for electron tubes is prepared by first welding a cathode substrate of cathode nickel to a Nichrome sleeve. The sleeve and substrate assembly is heated at elevated temperatures in an atmosphere consisting essentially of hydrogen and water vapor which atmosphere has a dew point in the range of 35 C. to 25 C. The substrate is then coated with a composition -which produces an electron-emissive coating upon subsequent heating. Finally, the sleeve is welded to a cathode support to produce the cathode assembly. The cathode assembly is then ready for mounting into an electron tube.

BACKGROUND OF THE INVENTION This invention relates to a method of making cathode assemblies for electron tubes, particularly cathode assemblies in which a metal substrate having an electron emissive coating is carried on a sleeve of a dissimilar metal and the sleeve in turn is welded to a cathode support. Such a structure is sometimes referred to in the art as a two-piece coated cathode, the two pieces referred to being the metal substrate and the sleeve.

Two-piece coated cathodes for electron tubes have been described in the literature, for example in US. Pat. Nos. 2,507,812 to D. C. Phipps and 3,055,123 to L. E. Griffiths. In the particular structure of interest, the metal substrate is composed of cathode nickel and is welded to a sleeve of nichrome alloy. Cathode nickel may be substantially pure nickel. It is usually an alloy which is predominantly of nickel and contains minor amounts of tungsten, silicon, magnesium and manganese. Nichrome, an alloy of nickel, chromium and iron, is a relatively poor heat conductor. Hence, the flow of heat from the metal substrate by conduction is materially reduced as compared with those structures in which the sleeve is composed of a relatively good heat conductor.

In the preferred manufacturing procedure, the substrate, usually in the form of a cup, is welded to one end of the sleeve. Then, the cup with the sleeve attached is cleaned, and the cleaned substrate is coated with a combination of heat-decomposable compounds which upon heating produces the electron-emissive coating. The sleeve is then welded to a cathode support. This subassembly is assembled into the electron tube structure and subsequently, as during or after the exhaust and sealing step for makin the tube, the substrate and coating are heated to produce the desired electron-emissive coating.

The step of cleaning the metal substrate prior to coating was previously conducted by degreasing the surface of the substrate and then heating the substrate in wet hydrogen gas at about 1100 C. By wet hydrogen is meant a mixture of water vapor and hydrogen gas which has a dew point at about room temperature; that is, +20 C. or more. This step cannot be conducted with a Nichrome sleeve attached to the metal substrate because the surface of the sleeve becomes so oxidized as to interfere with the subsequent welding of the sleeve to the cathode support.

When dry hydrogen was substituted for wet hydrogen, it was found that, the Nichrome sleeve could be welded properly but the cathode coating tended to peel during its operational life so that the useful life of the tube was materially shorter on the average. By dry hydrogen is meant a mixture of water vapor and hydrogen gas which has a dew point of about 60 C. and lower.

SUMMARY OF THE INVENTION According to the invention, a metal substrate composed predominantly of nickel is first welded to a Nichrome sleeve. The welded combination is then heated in an atmosphere consisting essentially of hydrogen and water vapor, which atmosphere has a dew point in the range of 35 C. to 25 C. The metal substrate is then coated and the sleeve is welded to a cathode support.

For reasons which are not yet understood, and quite surprisingly, heating with this narrow dew point range cleans the metal substrate in a manner which produces a relatively long-lived cathode on the average, which is believed to be the result of reduced peeling of the cathode coating. After heating, the sleeve is in such condition that it can be welded easily to the cathode support.

BRIEF DESCRIPTION OF THE DRAWING The sole figure is a flow chart illustrating the novel method.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Example 1 A cathode substrate is prepared in the normal fashion using a cathode nickel alloy composition consisting essentially of nickel containing about 4.0 weight percent tungsten, about 0.025 weight percent magnesium and 0.035 weight percent silicon. The substrate in this example is in the shape of a cylindrical cup about 0.080 inch outside diameter and about 0.110 inch high. The substrate is degreased, washed in water and then dried. The substrate is then slipped over the end of a Nichrome sleeve (also degreased, washed in water and dried) and welded into place. The welded cup and sleeve is then passed through a continuous furnace having a peak temperature of about 1100 C., where it is exposed to temperature between 1000 and 1100 C. for about 7 minutes (and temperatures between 800 C. and 1100 C. for about 10 minutes). The furnace atmosphere consists essentially of water vapor and hydrogen gas and has a dew point that is carefully controlled to be in the range of 29 C. and 35 C. The welded cup and sleeve cools on the way out of the furnace. The cooled cup is then coated, as by spraying with a triple carbonate composition (containing carbonates of barium, strontium and calcium and a binder therefor) on the external end portion of the cup. The coating thickness is about 3.5 mils and the coating weight is about 0.35 milligram. Up on drying in air, the sleeve is positioned in the hole of a circular cathode support (called an eyelet) and welded thereto to produce the cathode assembly. The assembly is ready for assembly with other parts into an electron tube.

Example 2 Follow the procedure of Example 1 except substitute dissociated amonia for hydrogen gas. Such an atmosphere contains about 25 volume percent nitrogen, volume percent hydrogen and a dew point of about 30 C. for the water vapor present.

Example 3 Follow the procedure of Example 1 except substitute a cathode substrate of a cathode nickel composition con taining about 99 weight percent nickel, 0.06 weight percent magnesium, and 0.025 weight percent silicon.

Example 4 Follow the procedure of Example 1 except substitute a cathode substrate of a cathode nickel composition containing about 99.2 weight percent nickel, about 0.04 weight percent magnesium, and 0.2 weight percent manganese, and about 0.3 weight percent silicon.

The foregoing examples are illustrative of the invention. There are many variations possible within the scope of the invention. The cathode assembly may be of any size and geometry and still obtain improvements in weldability and longer tube life through reduced peeling of the cathode coating. The magnitude of the improvements, will, of course, differ with differences in size, geometry and other factors mentioned below.

The cathode nickel alloy composition consists essentially of nickel, 0.0 to 10.5 weight percent tungsten, 0.005 to 0.3 weight percent magnesium, and 0.005 to 0.3 weight percent silicon. The magnesium and silicon are known to be reducing for the oxide coating. The preferred compositional range is nickel containing 1.0 to 5.0 weight percent tungsten, 0.01 to 0.10 weight percent magnesium and 0.01 to 0.10 weight percent silicon. The alloy may contain (in weight percent) up to about 0.1% A1, 0.04% C, 1.0% Co, 0.20% Cu, 0.1% Fe, 0.20% Mn, and 0.008% S as nonessential ingredients. Titanium may be substituted for magnesium. The preferred composition is given in Example 1 wherein the maximum content of non-essential ingredients is preferably (in weight percent): 0.008% Al, 0.02% C, 0.06% Co, 0.10% Cu, 0.10% Fe, 0.05% Mn, and 0.008% S. The steps of degreasing and washing the substrate may be carried out by any of the techniques known in the art.

The alloy of the sleeve may be any of the Nichrome alloys. A typical alloy is composed of about 60 weight percent nickel, 24 weight percent iron and 16 weight percent chromium and contains about 0.1 percent carbon. The Nichrome may be in the range of 50 to 70 weight percent nickel, to 30 weight percent iron, 12 to 20 weight percent chromium. The Nichrome may contain small amounts of non-essential ingredients.

The atmosphere for heating the washed substrate is in the range referred to as damp hydrogen. By damp hydrogen is meant an atmosphere which consists essentially of water vapor and hydrogen, wherein the dew point for the water vapor is in the range of 40 C. to 0 C. In this invention, the cathode assembly is damp fired in the much narrower range of C. to C. in order to produce a desired combination of characteristics in the assembly. Part of the hydrogen may be replaced with one or more neutral gases, such as argon, neon and nitrogen. Example 2 which uses 25% nitrogen in the furnace atsmophere is illustrative.

The heating step may be conducted in a periodic furnace or a continuous furnace. It is preferred that the atmosphere be continuously flowing through the furnace, as by continuously introducing fresh atmosphere into the furnace and removing a corresponding amount of gas from the furnace chamber.

The heating is conducted for such times and at such temperatures as will produce a surface with a desired character and quantity of nickel oxide on the substrate. This surface is best produced when the atmosphere is very slightly oxidizing to the substrate surface. This can be achieved in the range of 950 C. to 1150 C. and in a time of several minutes to about an hour. At temperatures near 950 C., it is preferred to use longer periods of time. At temperatures near 1150 C., it is preferred to use shorter periods of time.

The cooled substrate may be coated by any convenient coating technique to provide a coating of the desired thickness, weight and texture. In some techniques, as with some spraying methods, the substrate is warmed prior to spraying to aid in producing the desired coating. It is preferred to produce the coating on the substrate within four days after the substrate has been damp fired. Furthermore, no other heating at elevated temperatures should be conducted between the damp firing step and the coating step.

The coating composition may be any composition which upon heating will produce an electron-emissive coating. Preferably, such coatings are composed of at least one alkaline earth metal compound which is heat decomposable to an oxide. Combinations of two or more such compounds may be used and may be preferred.

Subsequent to coating the substrate, the sleeve is welded to the cathode support to produce the cathode assembly. The cathode support may be of any geometry and composition which is consistent with the welding technique and the tube structure involved. A typical structure of this type is shown in FIG. 4 of U.S. Pat. N0. 3,254,251 to Richard H. Hughes.

Then, the assembly is assembled with other parts into a complete electron tube structure including means for heating the cathode substrate during the operation of the tube. This complete structure is baked at elevated temperatures (typically about 400 C.), the tube exhausted and then sealed. During the baking and exhaust, the substrate is heated up to about 940 C. During this step, the constituents of the coating composition are decomposed, the volatile components are removed, and the remaining oxides are sintered into a firmly adherent coating on the substrate. Also, during this same step the metal structures of the tube including the cathode substrate are outgassed. Then, the tube is cooled to room temperature and aged with voltages applied to the electron gun. During aging, the substrate is heated to about 1100 C. to 1200 C. to activate the cathode coating. The completed tube is then tested.

We claim:

1. A method of preparing a cathode assembly for an electron tube, said cathode assembly being comprised of (a) a metal substrate composed principally of nickel welded to (b) a metal sleeve composed principally of nickel and chromium, (c) an electron-emissive coating on a surface of said substrate, said sleeve being welded to (d) a cathode support, said method including the steps of:

(1) welding said substrate to said sleeve,

(2) heating said substrate and sleeve in an atmosphere consisting essentially of hydrogen and water vapor, the dew point of said atmosphere being in the range of 35 C. to 25 C,

(3) coating a portion of the surface of said substrate with a composition which produces said electronemissive coating upon subsequent heating at elevated temperatures, 1

(4) and then welding said sleeve to said cathode support.

2. The method defined in claim 1 wherein said metal substrate is composed principally of nickel with about 0.0 to 10.5 weight percent tungsten, 0.005 to 0.3 weight percent magnesium and 0.005 to 0.3 weight percent silicon.

3. The method defined in claim 1 wherein said sleeve is composed principally of Nichrome alloy.

4. A method of preparing a cathode assembly for an electron tube, said cathode assembly being comprised of (a) a metal substrate of an alloy composed principally of nickel welded to (b) a metal sleeve of an alloy composed principally of nickel and chromium, (c) an electron-emissive coating on the surface of said substrate, said sleeve being welded to (d) a cathode support, said method including the steps of:

(1) welding said substrate to said sleeve,

(2) heating said substrate and sleeve in an atmosphere consisting essentially of hydrogen and water vapor, the dew point of said atmosphere being in the range of 35 C. and 29 C.,

(3) coating at least a portion of the surface of said substrate with a composition containing at least one alkaline earth compound which is heat-decomposable to an oxide and a binder therefor, there being no heating step between said heating in step (2) and said coating in step (3),

(4) and then Welding said sleeve to said cathode support.

5. The method defined in claim 4 wherein said substrate is composed principally of nickel with 1.0 to 5.0 weight percent tungsten, 0.01 to 0.05 Weight percent magnesium and 0.01 to 0.06 weight percent silicon.

6. The method defined in claim 4 wherein said sleeve is composed principally of nickel with about 16 Weight percent chromium and about 24 weight percent iron therein.

References Cited UNITED STATES PATENTS 2,413,689 1/1947 Clark et a1. 29-25.14 XR 2,551,871 5/1951 Briggs 2925.l5 XR 2,611,676 9/1952 Pohle 29,25.15 XR JOHN F. CAMPBELL, Primary Examiner R. B. LAZARUS, Assistant Examiner U.S. Cl. X.R. 29-25.13, 25.15