US2209708A - Cathode manufacture - Google Patents

Description

Patented July 30, 1940 UNITED STATES CATHODE lWIANUFACTURE Stanton Umbreit, West Orange, N. J., assignor to Radio Corporation of America, a corporation of Delaware No Drawing. Application October 31, 1939, Serial No. 302,264

Claims. (01. 250-275) My invention relates to cathodes for electron discharge devices, particularly to cathodes of the type in which the base metal carries an oxide coating usually of the alkaline earth metal 5 oxides.

An object of my invention is to provide an oxide coated cathode having a wrought core consisting predominantly of nickel and free of strong reducing agents for the oxide coating.

A more specific object of my invention is to provide an oxide coated nickel cathode that liberates only enough active metal to produce good primary emission with only negligible reverse current, and that is strong yet easy and inexlo pensive to manufacture.

The usual oxide coated cathodes is a sleeve or wire of commercial nickel coated with active metal oxides such as barium-strontium oxides. The usual commercial nickels contain some carbon, and also other elements such as silicon, magnesium and manganese, which are added to the molten nickel during the metallurgical treatment of the nickel to bring about deoxidation of the nickel. These deoxidizers are such strong reducing agents that the slight amount present in commercial nickel causes excessive liberation of the active metal of the oxides at the usual operating temperature of an oxide coated cathode made of commercial nickel. This excess active 'metal evaporates and deposits on the grid and.

anode electrodes in the tube and emits electrons that cause objectionable noise, reverse currents and low efficiency.

The commercial nickel used for filament wire and cathode sleeves is usually made by melting nickel to which some strong reducing agents have been added to eliminate the oxygen and oxides formed during the melting operation and facilitate working of the metal. Considerable effort has been made to add just the right amount of deoxidizer to eliminate the occludedoxygen or oxides in the melt yet leave a slight residue to insure complete deoxidation of the melt. Since the amount of oxides in the molten metal is 45 never known exactly, it is impossible to add'just the right amount of deoxidizing agent. Therefore, it is customary to add a slight excess of the deoxidizing agent to insure complete removal of oxides.

50 According to my invention cathode cores or the strip for making such cores may be made of nickel which has been melted and sufiicient amounts of deoxidizers such as carbon, silicon, magnesium, manganese and aluminum added to 55 insure easy working of the nickel into thin strips and fine cathode wires without regard to the amount of residual deoxidizers in the final strip or wire. According to my invention the wrought nickel cathode core containing the impurities which reduce the oxide coatings of the core is I5 fired in a slightly oxidizing atmosphere at a temperature and for a sufficient time to render the impurities harmless probably by oxidation. I have found that nickel strip wire or sleeves fired in a hydrogen atmosphere saturated with water 10 vapor is well suited for the purpose of oxidizing the impurities in the nickel. Wet hydrogen firing has been found to be easily controlled so that the common elementary impurities in the nickel, such as silicon, magnesium and carbon, may be 15 oxidized and rendered harmless without oxidizing the nickel. Use is made, according to my invention, of the lesser afiinity of nickel for oxygen than the affinity of silicon, magnesium and carbon for oxygen. Nickel strip .002" in thickness, 20 for example, fired at 1100 C. for one hour in wet hydrogen containing 20 to 30% moisture conditions the strip for cathode sleeves which will not liberate excessive amounts of active metal from the coating. Firing temperature and moisture 25 content of the atmosphere of course determines the length of firing. Hydrogen may conveniently be wet by bubbling it through water before it is introduced to the firing furnace, the temperature of the water being raised above room tempera- .30 ture if it is desired to increase the percent of moisture in the hydrogen. The wet hydrogen firing may of course follow the final filament wire drawing operation or the cathode sleeve wrapping operation. 35

' Reducing agents, such as silicon manganese or magnesium, may be rendered inactive by this treatment of mild oxidation of the wrought metal if the reducing agents are elementary and soluble in the nickel so that they are capable of 40 difiusing to the surface where they can be oxidized. This process is of course best carried out on thin strips or cathodes heated at a high temperature so that diffusion to the surface is rapid. Alternatively the metal may be heated in a bath which will oxidize the nickel less rapidly than its included impurities.

The cathode core prepared according to my novel method is substantially free from strong reducing agents, yet is active and cooperates with the usual alkaline earth metal oxide coating, such as barium-strontium oxide coating to produce good electron emission.

I claim:

1. The method of making oxide coated cathodes having a core of wrought metal, consisting predominantly of nickel, which comprises firing the core in a slightly oxidizing atmosphere at a temperature and for a time sufiicient to render inactive any reducing agents in said metal and insufiicient to oxidize said metal, and then covering said core with said oxide coating.

2. The method of making oxide coated cathodes which comprises firing in wet hydrogen a Wrought nickel core containing reducing agents until the reducing agents are rendered inactive to reduce alkaline earth metal oxides, and coating the core with alkaline earth metal oxides.

3. The method of making cathodes with a wrought core predominantly of nickel coated with alkaline earth metal oxides and containing strong reducing agents for the coating comprising firing the core at an elevated temperature in a slightly oxidizing atmosphere to oxidize said agents, the temperature being high enough to rapidly diffuse the reducing agents toward the surface of the core, and then coating the core with said alkaline earth metal oxides.

4. The method of making oxide coated cathodes comprising firing a wrought core of said cathode in wet hydrogen to oxidize the strong reducing agents of the core, and then applying the oxide coating to the core.

5. The method of making cathodes with a core predominantly of nickel coated with an alkaline earth metal oxide and containing in the core reducing agents for the oxide coating comprising heating the core in an oxidizing atmosphere only until most of said agents are oxidized, and then coating said core with said alkaline earth metal oxide.

STANTON UMBREIT.