US2641555A - Nonemissive electrode and method of making - Google Patents

Description

emissive during use under ordinary operating conditions.

To facilitate a better understanding of the subject matter of this invention reference is made to the accompanying drawing which is a iiow sheet correlating the steps in the manufacture of a vacuum tube embodying the principles of this invention.

yIt will be noted that the first step in the manufacture of a vacuum tube as represented in this flow sheet is the formation of the basic structure of the electrode from a refractory metal. This step in the procedure is in accordance with common practice in the vacuum tube manufacturing art and need not be described in detail except to mention that the refractory metal used is one that retains its physical integrity at high temperatures without melting, softening or bending, and which does not so persistently retain occluded gases as to present problems during the outgasing and evacuation of the tube. Among the refractory metals that have been found satisfactory for this purpose are tantalum, platinum, molybdenum and nickel, the last mentioned metal, because of its low melting point being useful primarily in types of tubes that are operated at elevated but not abnormally high temperatures.

After the refractory metal has been shaped into the form desired for the finished electrode, it is provided with a coating comprising at least one alkaline earth oxide selected from the class consisting of barium oxide, strontium oxide and calcium oxide. This coating may be applied by spraying, dipping, electrophoresis or by any of the other known methods that are commonly employed in vacuum tube manufacture to produce adherent, uniform and pit-free coatings of this type. It is also to be recognized that the invention may be practiced by producing the oxide coating as a result of heat treatment of an alkaline earth carbonate coating in well known manner after it has been applied to the electrode in any of the ways above mentioned.

In accordance with the present invention the next step in producing the non-emissive electrode is a brief heat treatment of the alkaline earth oxide coated refractory metal in a carbonaceous atmosphere, as hereinafter defined, under reduced i. e. sub-atmospheric, pressure and With the electrode at a temperature at least approximating and preferably substantially exceeding the normal operating temperature of an alkaline earth oxide coated filament in a vacuum tube, whereby the character of thecoating is altered so that the coated electrode acquires its characteristic non-emlissivity.

The term carbonaceous atmosphere as herein employed means an atmosphere containing vapor of a liquid hydrocarbon of the type described below which has low viscosity and surface tension such that it will readily penetrate the minute pores of the oxide coated electrode and further characterized in decomposing at the temperatures used in this process to yield carbon or a residue predominantly comprised of carbon.

It will be understood that the term hydrocarbon oil known under the trade name Octoil which is dioctyl phthalate, an oily material having approximately the same viscosity as ordinary salad oil and a boiling point of about 122.5o C. at 10-2 mm. Pennzoil SAE 30, which is an automotive lubricant oil derived from parainic base Pennsylvania crude oil, may be used. It will be understood that these oils usually if not invariably contain oxygen either in dissolved elemental or in combined state.

The concentration of the hydrocarbon in the carbonaceous atmosphere is not critical, althoughof course it will be obvious that the time of treatment in order to produce the desired result is related to the hydrocarbon concentration. However, satisfactory effects are produced if the hydrocarbon-concentration is merely. that resulting from the normal vapor pressure of the hydrocarbon i. e. that quantity of hydrocarbon in the atmosphere supernatant the liquid hydrocarbon under the operating condition aforesaid.

A procedure that has been found satisfactory according to the present invention for processing the alkaline earth oxide coated electrodes to convert the same to non-emissiveelectrodes involves the mounting of the oxide coated electrode in a vacuum chamberconnected din rectly with an oil diffusion pump, using no means such as cold traps or charcoal for preventing normal back diffusion, and provided with means for heating the electrode to a high temperature. The oil in the diffusion pump is one of those above mentioned. Initially the electrode is out-gasscd at about 900 C. for about 10 ininutes at a pressure of 10-FI mm. mercury, then the pressure is adjusted to 10-4 to 10-3 mm. mercury and the electrode is flashed at about 1300" C. for a period of 10 minutes or thereabouts, after which the emission of the electrode is measured at about 900 C. and 10'1 mm. mercury pressure. If the electrode is found to be emissive, the preceeding stepsare repeated, although usually this is unnecessary. The foregoing procedure describes the treatment of a nickel base electrode but may be equally well applied to electrodes having a different higher melting point refractory metal base in which instance the flashing temperature used may be higher, for instance about 1600 C. for a platinum base electrode.

provides a sufficient quantity of carbonaceous component in the atmosphere to effect thel subtle change in the oxide coating whereby it loses its normal emissivity and becomes permanently non-emissive. The precise manner in which this change occurs and the changes, if such occur, in the chemical composition of the oxide surface during the treatment in the carbonaceous atmosphere, are presently little understood, hence the necessity of defining the article in terms of the process for its manufacture. In connection with the reaction temperatures above set forth, it will be recognized that the temperatures stated are approximations within the experimental limitations imposed by diiculties in measuring such high temperatures. Also to be considered is the fact that the reaction between the carbonaceous atmosphere and the oxide coating on the metal is an exothermic reaction and thus there is at least a tendency for the initiation of the reaction to be reflected ina tempelure rise independent4 of the temperature to which the electrode is heated. When reference is made herein to the temperature at which an alkaline earth oxide coated filament is operated while in a vacuum tube, it will be understood that this type of iilarnent is usually operated at about 850 to 900 C. In practicing the process according to this invention, however, it is desirable to use temperatures considerably higher than this, say about 1300" C., or even higher with those refractory metals of very high melting point.

When the oxide coated electrode has been processed as above described it is removed from the carbonaceous atmosphere of the chamber and it is then mounted in the usual manner within an envelope, together with the other tube elements and the alkaline earth oxide coated cathode, in the relationship that the various elements will occupy in the iinished vacuum tube and, if desired, the assembly is then further processed according to known and usual techniques in vacuum tube manufacture. It is found that under conditions of use the control electrode so prepared permanently retains its characteristic non-emissivity under ordinary operating conditions.

Having thus described the subject matter of the present invention what it is desired to secure by Letters Patent is:

1. A method for making a non-emissive electrode for use in electron discharge devices comprising the steps of forming a base from a refractory metal element selected from the group of non-ferrous metallic elements having a boiling point of 2900 degrees C. or over, a cubic lattice crystalline structure, and appearing in the periodic table in group V or a higher group, consisting of tantalum, platinum, nickel and molybdenum; coating said base with an alkaline earth oxide, and heating the oxide electrode in an atmosphere of a hydrocarbon oil vapor that readily penetrates the pores of the oxide electrode at a temperature sumcient to decompose the hydrocarbon and leave a carbon residue.

2. A non-emissive electrode for use in an electron discharge device comprising a base of a metallic element selected from the group of refractory non-ferrous metallic elements having a boiling point of 2900 degrees C. or over, a cubic lattice crystalline structure, and appearing in the periodic table in group V or a higher numbered group, consisting of tantalum, platinum, nickel and molybdenum; an initial coating of alkaline earth oxide which has :been heated in an atmosphere of a hydrocarbon oil vapor that readily penetrates the pores of the oxide electrode at a temperature sufcient to decompose the hydrocarbon and .leave a carbon residue.

CHARLES V. LITTON.

References Cited in the le 0f this patent UNITED STATES PATENTS FOREIGN PATENTS Country Date Australia July 16, 1935 Number Number

Claims (1)

1. A METHOD FOR MAKING A NON-EMISSIVE ELECTRODE FOR USE ING ELECTRON DISCHARGE DEVICES COMPRISING THE STEPS OF FORMING A BASE FROM A REFRACTORY METAL ELEMENT SELECTED FROM THE GROUP OF NON-FERROUS METALLIC ELEMENTS HAVING A BOILING POINT OF 2900 DEGREES C. OR OVER, A CUBIC LATTICE CRYSTALLINE STRUCTURE, AND APPEARING IN THE PERIODIC TABLE IN GROUP V OR A HIGHER GROUP, CONSISTING OF TANTALUM, PLATIMUM, NICKEL AND MOLYBDENUM; COATING SAID BASE WITH AN ALKALINE EARTH OXIDE, AND HEATING THE OXIDE ELECTRODE IN AN ATMOSPHERE OF A HYDROCARBON OIL VAPOR THAT READILY PENETRATES THE PORES OF THE OXIDE ELECTRODE AT A TEMPERATURE SUFFICIENT TO DECOMPOSE THE HYDROCARBON AND LEAVE A CARBON RESIDUE.

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