US2275886A - Process of activating cathodes - Google Patents

Description

March 10, 1942. J BQND'LEY AL 2,275,886

PROCESS OF ACTIVATING CATHODES Filed April 50, 1941 OX/OE 60A TED CA THODE FAB/W624 T/O/V CA THODE EOUNDA T/ON 60A TE 0 W/ TH I ALKAL/NE EARTH HYD/POX/DE UNDER NON "10X lD/Z ING COND/T/ONS HYDROX/DE CONVERTED 7'0 02005 [650 -a00 "0/ NON-OX/D/Z/N6 ENVIRONMENT PREFERA BLY HYDROGEN CA THODE MOUNTED /N 0/5 CHARGE TUBE Inventors: Ralph J. Bondley, Herrn an A.Liebha1sky, Arthur F Winslow,

Then" Attorney.

Patented Mar. 10, 1942 PROCESS OF ACTIVATIN CATHODES Ralph J. Bondley, Scotia, Herman A. Liebhafsky, Schenectady, and Arthur F. Winslow, Scotia, N. Y., assignors to General Electric Company,

. a corporation of New York Application April 30, 1941, Serial No. 391,090

7 Claims.

The present invention concerns the fabrication of electrical discharge devices and is particularly directed to the preparation of coated cathodes for such devices.

It is the object of our invention to improve the performance and reduce the exhaust schedule of discharge devices containing coated or Wehnelt-type cathodes.

As is well known, the electron emissivity of cathodes is greatly enhanced by the presence thereon of alkaline earth oxide such, for example, as the oxide of barium. In accordance with a procedure described in Schumacher United States Patent 1,467,398, cathodes for vacuum tubes are coated by dipping them into alkaline earth hydroxide which has been melted in the presence of water of crystallization. Partial oxidation of the cathode to be coated is'brought about by either preliminary oxidation of the cathode before coating or by oxidation of the cathode in the coating bath which is operated at about 600 C., at which temperature it becomes an oxidizing medium.

We have discovered that marked improvements in the life and functioning of cathodes are obtained if the process of preparing the coating of such cathodes is carried out under conditions whereby oxidation of the foundation surface is avoided. We havealso discovered that marked advantages are obtained by carrying out or completing the conversion of the hydroxide to the oxide as a separate step prior to the mounting of the coated cathodes in vacuum tube or other discharge device.

The accompanying drawing is a diagram of the main steps of the process constituting our invention.

In accordance with our invention cathodes are coated initially with a fusion of hydroxide of alkaline earth metal containing little or no water of crystallization, preferably maintaining the temperature of such fusion below 500 C. and causing or completing conversion of the hydroxide coating on the cathode to the oxide by heat treatment in a non-oxidizing environment, such as a protective atmosphere. A hydrogen atmosphere is preferred as it not only is protective in the sense of being non-oxidizing but may have a positive reducing effect. The hydrogen should be dry, that is, substantially free from water vapor.

Other features of novelty .will be pointed out in connection with the following more detailed description.

In carrying out our invention, cathodes of clean, unoxidized tungsten, nickel, molybdenum, tantalum, or other suitable foundation material are coated initially with alkaline earth hydroxide free from, or in any event containing no more than, a minimum amount of water of crystallization. Conveniently the form of hydroxide containing one molecule of Water of crystallization in the molecule rather than the form containing 8H2O is employed. Hydroxide of barium, strontium, calcium or a desired mixture of hydroxides may be applied on a cathode by dipping the cathode to be coated into a bath of hydroxide which is maintained at a fusion temperature which is preferably below 500 C. At temperatures of 475 to 500 C. we have found that little or no oxidation of the foundation metal occurs. At these temperatures some conversion of hydroxide to oxide occurs, in the bath. As a result of dipping the cathode into such melt, a coating of the hydroxide (with or without oxide admixture) is caused to adhere to the foundation metal. The coating step may be carried out in the open air although the presence of an enveloping atmosphere of protective gas, for example nitrogen or hydrogen, is beneficial.

Alternatively, in some cases when cathode configuration permits, the initial coating of the foundation metal may be carried out by fusing the hydroxide in situ on the cathode. For example, a pellet of desired hydroxide, or mixture of hydroxides, may be placed on a cathode having a surface capable of holding a pellet. Alternatively, hydroxide in powdered form may be caused to fall on a heated cathode. When heat required for fusion is applied from any convenient source, that is, either from a separate heater or by causing the cathode itself to become heated by passage of current, the hydroxide melts and coats the surface of the cathode.

The cathode which has been coated with hydroxide under conditions avoiding oxidation next is heated in accordance with the second step of our process to a temperature at which conversion from the hydroxide to the oxide occurs. This step preferably should be' carried out preliminary to mounting the cathode in the discharge tube in which it is desired for use and in a non-oxidizing environment, preferably in the presence of a hydrogen gas at atmospheric pressure. For example, the hydroxide-coated cathode may be heated during the conversion step for about one minute at a temperature of about 650 to 800 C. It is then preferably, but not necessarily, heated for a shorter time to a higher temperature, say to about 900 to 1100 C.

for During this heat treatment step v the hydroxide is dissociated into oxide and water vapor, the latter being carried away by convection currents in the hydrogen. Other protective gases such as argon or helium may be usedbut-dry hydrogen is preferred; and his desirable that a flowing stream of current of the electrical discharge device in which it is intended to function; The cathode coating-produced by the heat treatment step in the presence of a protective gas is surprisingly stable even in the presence of air under ordinary conditions. However, if a lengthy interval intervenes between its production and mounting in a vacuum tube it should be preserved in a vacuum or pro-. tective atmosphere, or be protected by'a suitable lacquer.

Although it is not necessary to convert the hydroxide completely to the oxide during the conversion step, the residue of unconverted hydroxide shouldnot be great enough to require significant physical change of the coating during the final heating of the cathode which accompanies the exhaust and sealing-oil of the device in which the coated cathode is mounted. As a result, little or no heat treatment of the cathode is required in the tube assembly to put the cathode into active electron-emitting condition.

As a consequence of our invention, the time required to prepare electrical discharge tubes. (commonly called "vacuum tubes" even when 2 W V 2,m,aae 1. The method of providing a cathode with alkaline earth oxide which consists in applying to the surface thereof a coating including alkaline earth hydroxide and thereupon dissof ciating. such hydroxide intooxide and water vaper in the presence of a non-oxidizing gas. 7

2. The methodof providinga cathode with a coating of alkaline earth oxide whiclrconsistsin applying thereto alkaline earth hydroxide. and thereupondissociating. such hydroxide into oxide and water vapor in the presence of hydrogen gas.

3. The metho of providing afoundation metal for a thermionic athode with an alkaline earth oxide which consists in applying thereto-a coating including alkaline earth hydroxide which contains insufiicient oxidizing ingredients to oxidize the surface of said metal and heating said coating to the dissociation temperature of said sivity of a cathode for an electrical discharge containing gas), that is, to properly degasify ing of disintegrated coating material on other parts, such as the grid, is practically eliminated. The initial emission of oxidecoated cathodes is uniformly high, irregularities in composition being eliminated as a consequence of our invention. It is more readily degasified than cathodes coated with activating material by other methods and hence is particularly useful in television tubes where gases tend to produce ion spo Howe er. such cathodes can be advantageously used in rectifier-s, amplifiers, vapor lamps, including fluorescent lamps and other forms of discharge devices, including both vacuum devices and gas-filled devices.

The preparation of oxide coating in accordance with our invention extends to both thermionic, or incandescent, and non-thermionic or cold" cathodes.

What we claim as new and desire to secure by Letters Patent of the United States is:

device which consists in applying thereto a fusion containing alkaline earth hydroxide and thermally dissociating said hydroxide into oxide and water vapor in the presence of a non-oxidizing gas at substantial pressure.

5. The method of coating a cathode of oxidizable metal with barium oxide which consists in bringing said cathode into contact with a fusion of barium hydroxide and oxide at a temperature of about 475 to 500 C., and heating said cathode to a temperature of about 650 to 800 C. in a current of hydrogen gas.

6. The method of providing an electrical discharge device with a cathode coated with an oxide of an alkaline earth metal which consists in fusing a pellet comprising the hydroxide of said metal in contact with a metallic cathode foundation, thermally decomposing said hydroxide while enveloped in hydrogen gas at substantially atmospheric pressure and finally mounting said coated foundation metal in operative position in said device.

7. The method of coating a cathode foundation of oxidizable metal with barium oxide which consists in dipping said foundation into a fusion comprising barium hydroxide and oxide maintained at about 475 to 500 C., removing said foundation coated with a layer of said fusion and heating to a temperature suillciently high to decompose said hydroxide while enveloped in hydrogen at substantially atmospheric pressure.

RALPH J. BONDLEY. HERMAN A. LIEBI-IAFSKY. ARTHUR F. WIN-SLOW.

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