US2844433A - Method of manufacture for electric discharge devices - Google Patents

Description

United States Patent F r 2,844,433 I "METHOD OF MANUFACTURE FOR ELECTRIC DISCHARGE DEVICES Ralph L.- Meyer, Elgin, .and Serge Pakswer ,Elmhurst, 111., assignors'to The Rauland Corporation, a corporation of Illinois .No'Drawing. Application. November 21, 1952 SerialNo. 321,933

6Claims. (Cl. 31622) mixture of compounds which maybe broken down or decomposed by .heat treatment to .formlan electron-emissive coating. Typical examplesof .such compounds are bari- 'um nitrate or bariumcarbonate, used alone orin mix- .;tures with strontium and/or calcium carbonate, which may be decomposed byheat treatment to.form .a matrix of barium oxide and barium. Thedecomposition is'normally carried out after the cathode structureuhas been .mounted within the tube envelopeaud may cause evaporation anddeposition.of barium or other electron-emissive materials upon-the envelope-or other parts-"of the device other than the cathode. Furthermore, it iscommonpracticeto break-in ionic discharge devices during processing by establishing an arc between the cathode and the anode, which occasionally results in some sputtering when the arc is initially established; this alsolmay undesirably deposit some electron-emissive material on electrodes of the device other than the cathode. :In=some high-vacuum discharge devices, the operative effects .of .these,.extraneous electron-emissivedeposits are too minute-to .be of any great'concern. However, -in highcurrent .vacuum devices and in:most ionic rectifiers, .such deposits present a considerable problem by causing the how of1appreciable currents "between the control electrode and the anode, especially where the devices are :operatedat relativelyhigh ambient temperatures. These :deposits emit electrons whenever the control electrode 'is :negative so that an :opposite-polarity voltage is developed in' the grid resistor, changing the control characteristics of the device.

It is an object of this invention to .provide ;a novel 'method of neutralizing undesirable electron-emissive :deposits in electric discharge'devices.

It isa particular object of this inventionto provide a novel method of neutralizing undesirable :ele'ctron-Temissive deposits formed onthe control electrodes ofionic rectifiers during "the final stages of manufacture.

Tt-is a corollary object of this'invention toprovide a method of neutralizing undesirableelectron-emissive deposits in electric discharge devices which is simple and expedient to perform and which does not require unduly complicated or expensive machinery.

Accordingly, the invention is directed to a method of neutralizing undesirable electron-emissive deposits incurred in'the manufacture of an electric. discharge .device 2, 44,433 Patented July 22, 1958 ice 7 2 including an envelope and an anode, a cathode having 'a surface formed from a heat-decomposablecompound, and a control electrode, all mounted withinthe'envelope. The inventive method comprises the step of heatingthe cathode to decompose the compoundand form-onthe above-mentioned surface an-electron-emissive substance. The envelope of-the device is then filled-with an oxidizingatmospherein order to oxidizeany'undesirable electron-emissive deposits present therewithin. Following this,the envelopeis exhaustedand is subsequently sealed.

The features of the present 'inventionwhich are believedtobe novelare set forth with-particularityin'the appended claims. The organizationand sequence ofperformance of the steps of the inventive method, together with furtherobjects and advantages thereof, may'best be understood by reference to'the following 'detailed description.

In its preliminary stages, the processing schedule of the invention is identical with that normally employed in the manufacture of 'ionic discharge devices such .as thyratrons. The electrode system of the device is assembled and mounted within'the envelope after which the tube is sealed to a vacuum pump and exhausted to a relatively low pressure to avoid any undesirable and uncontrolled oxidation of-the tube components during subse- .quentgprocessing steps. The device is thenbaked inan .ovento drive off any volatilizable contaminants which might be present, and, while .still inthe oven, the cathodeis heated to decomposeits surfacecoating and'form an electron-emissive substance. The .device is .then-removed .fronrthe oven-and subjected'to a high-frequency magnetic field; cooling prior to the induction'heating is normally required with indirectly :heated cathodes to .preventoxidationof the filaments. The high-frequency ffield inductively heats the electrodesof .the device and effectively degases these elements. Following this, the envelope of .the device is filled withaselected .ioniz-able -gas, usuallythe same asthat to beemployed as an atmosphere when the device is completed, at a preselected pressure corresponding generally to the normal operating pressure .of the tube. An arc discharge is then :estab- .lished'between the anode and thecathode of .the .device and is-maintained for a considerableinterval, after which the arc isextinguished and the envelope is again evacuated. This step is included in the processing to ascertain whetherornot the devicewill later be operable; it should be noted that some cathode sputtering often occurs during initiation of the discharge, which may resultin deposition of undesirable electron-emissive deposits on the anode, control electrode, or envelope.- In the normal course of processing, the device is then refilled with an atmosphere corresponding to its operating atmosphere and is operated at normal average current "for a "short period as 'a check on its performanceunder rated conditions. The envelope is then again evacuated and cooled to room temperature to perniit'filling with a'desired .operating atmosphere under standardized conditionsp'it .is then sealed and is removed from the pump readyfor operation.

During the above manufacturing procedure, undesira- .ble -electron-emissive deposits may beformed on various portions of the device other than its cathode; these deposits are particularly objectionable when formed on the control electrode. 'It has beendeterniined that most such deposits occur during the'heat'treatment or activation of the cathode or during that stage of the'processing in which an .arc discharge. is first establishedbetween'the anode and cathode. In order to neutralizethese deposits, in accordance with the invention, the above-outlined pro- 'cedure is modified following the evacuation o f the envelope subsequent to initiation of the oi'iginall'arc .fdisrequires approximately 30 seconds.

charge. At this point in the process, an oxidizing atmosphere is introduced into the envelope and remains therein for a short period of time; this step is taken to oxidize and thus neutralize any undesirable electron emissive deposits remaining in the tube after the original cleaning and degassing steps as well as to neutralize those deposits which may be formed during the cathode breakdown and original arc discharge stages of processing. The oxidizing atmosphere is then pumped out, after which the usual steps in the processing are again resumed.

In order to provide a complete and concrete illustration of the operation of the invention, the detailed steps followed in processing a type CSF 14 xenon thyratron are vset forth below. The thyratron comprises an anode, a

control electrode, and a cathode all mounted in a glass envelope; the decomposable compound applied to the cathode prior to mounting in the envelope is barium carbonate. The cathode is of the indirectly heated type. The general operating characteristics of the tube are shown in Table A.

Table A Filament voltage 14 volts. Atmosphere Xenon. Pressure 62 microns Hg. Normal average current amperes. Normal anode-cathode arc drop 8 volts.

Prior to final processing, as above, the electrodes of the device are mounted in the envelope which is then sealed to a vacuum pump and exhausted to a relatively low vacuum of the order of 20500 milli-microns of mercury. The device is then baked in an oven for approximately one hour at approximately 470 C. to volatilize and drive off any contaminants which might be present on the internal surface of the envelope or on jected to a high-frequency magnetic field for approximately five to ten minutes to heat and degas its metallic parts. At this point, the envelope of the device is filled with a xenon atmosphere at approximately 100 microns pressure and an arc discharge is established between the anode and the cathode with the grid connected to the anode through a 10,000ohm resistor. The are current is progressively increased to approximately 150% of the design maximum average current until the tube are drop is of the order of approximately 8 volts.

Following the initial activation of the cathode, the xenon atmosphere is evacuated, the entire tube is allowed to cool, and pure oxygen at several millimeters pressure is admitted into the envelope. The oxygen remains in the envelope for approximately two minutes, after which it is exhausted and the filament voltage is raised progressively to 30 volts and maintained until there is no indication of the presence of gas. This exhaustion usually The device is again filled with an atmosphere of Xenon at approximately 100 microns and the initial arc discharge procedure outlined above is repeated. Following this, the xenon is exhausted, the tubulations leading to the pump are flamed out, the device is refilled with xenon and is operated at the normal average current of five amperes for approximately 15 minutes. The device is then exhausted, cooled to room temperature, and filled with xenon at the operating pressure of 62 microns, after which the envelope is sealed and the thyratron is removed from the pump ready for operation.

It will be apparent to those skilled in the art that conall) siderable variation in the above-outlined processing sched ule may be made without departing from the basic teaching of the invention. For example, it has been determined that it is not necessary to use pure oxygen for neutralizing the unwanted electron-emissive deposits and that this may be carried out by the use of ordinary air or any other mixture of gases containing an oxidizing gas. Furthermore, for some devices, certain of the steps may be elim inated entirely, as, for example, the prescribed cooling prior to introduction of the oxidizing atmosphere or the break-in" period of operation at normal tube current. However, it is essential that the oxidation of the undesirable emissive deposits be done after the cathode is activated and preferably after the original arc discharge stage. It should be noted that the time interval during which the oxidizing atmosphere remains within the envelope must be restricted so that only a superficial layer of the cathode coating is oxidized-and the remainder of the emissive matrix is not affected. This time varies with the type of decomposable coating used and its thickness as well as the cathode temperature and the pressure of the oxidizing atmosphere; however, it may be readily determined for each individual cathode type. Although the paramount effect of the oxidation treatment is the neutralization of emissive deposits, the superficial oxidation of the cathode coating may also have a beneficial effect upon the operation of the device since it tends to reduce the rate of evaporation of emissive material from the cathode.

While a particular embodiment of the present invention has been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention in its broader aspects. Accordingly, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of the invention.

We claim:

1. In the manufacture of an electric discharge device having an envelope enclosing an anode, a cathode having a surface of a heat-decomposable compound, and a con trol electrode, the method of neutralizing undesirable electron-emissive deposits in said device comprising the following steps: evacuating said envelope; heating said cathode to decompose said compound and form on said surface an electron-emissive substance, which may concurrently cause formation of undesirable electron-emissive deposits on said envelope, said anode, or said control electrode; filling said envelope with an oxidizing atmosphere and maintaining said atmosphere for a time suflicient to oxidize any undesirable electron-emissive deposits present within said device; exhausting said envelope before oxidation of more than a superficial portion of said electron-emissive substance occurs; and sealing said envelope.

2. In the manufacture of an ionic discharge device including an envelope and an anode, a cathode having a surface formed from a heat-decomposable compound, and a control electrode all mounted within said envelope, the method of neutralizing undesirable electron-emissive deposits in' said device comprising the following steps: evacuating said envelope; heating said cathode to decompose said compound and form on said surface an electron-emissive substance, which may concurrently cause formation of undesirable electron-emissive deposits on said envelope, said anode, or said control electrode; filling said envelope with an oxidizing atmosphere and maintaining said atmosphere for a time sufficient to oxidize any undesirable electron-emissive deposits present within said device; exhausting said envelope before oxidation of more than a superficial portion of said electron-emissive substance occurs; filling said envelope with a selected ionizable gas at a preselected operating pressure; and sealing said envelope.

3. In the manufacture of an ionic discharge device including an envelope and an anode, a cathode having an oxidizing atmosphere and maintaining said atmosphere for a time sufiicient to oxidize any undesirable electronemissive deposits present Within said device; exhausting said envelope before oxidation of more than a superficial portion of said electron-emissive substance occurs; filling said envelope with a selected ionizable gas at a preselected operating pressure; and sealing said envelope.

4. In the manufacture of an ionic discharge device including an envelope and an anode, a cathode having a surface formed from a heat-decomposable compound, and a control electrode all mounted within said envelope,

the method of neutralizing undesirable electron-emissive deposits in said device comprising the following steps: evacuating said envelope; heating said cathode to decompose said compound and form on said surface an electron-emissive substance, which may concurrently cause formation of undesirable electron-emissive deposits on said envelope, said anode, or said control electrode; filling said envelope with a selected ionizable gas at a preselected pressure; establishing an arc discharge between said anode and said cathode and maintaining said discharge for a predetermined interval, which may concurrently cause formation of undesirable electronemissive deposits on said envelope, said anode, or said control electrode; extinguishing said are discharge and evacuating said envelope; filling said envelope with an oxidizing atmosphere and maintaining said atmosphere for a time sufficient to oxidize any undesirable electronemissive deposits present within said device; exhausting said envelope before oxidation of more than a superficial portion of said electron-emissive substance occurs; filling said envelope with a selected ionizable gas at a preselected operating pressure; and sealing said envelope.

5. In the manufacture of an ionic discharge device including an envelope and an anode, a cathode having a surface formed from a heat-decomposable compound, and a control electrode all mounted within said envelope, the method of neutralizing undesirable electron-emissive deposits in said device comprising the following steps: evacuating said envelope; heating said cathode to decompose said compound and form on said surface an electron-emissive substance, which may concurrently cause formation of undesirable electron-emissive deposits on said envelope, said anode, or said control electrode; subjecting said device to a high-frequency magnetic field to heat and degas all of said electrodes; filling said envelope .with a selected ionizable gas at a preselected pressure; generating an arc discharge between said anode and said cathode and maintaining said discharge for a predetermined interval, which may concurrently cause formation of undesirable electron-emissive deposits on said envelope, said anode, or said control electrode; extinguishing said are discharge and evacuating said envelope; filling said envelope with an oxidizing atmosphere and maintaining that atmosphere for a time sufficient to oxidize any undesirable electron-emissive deposits present within said device; exhausting said envelope before oxidation of more than a superficial portion of said electronemissive substance occurs; filling said envelope with a selected ionizable gas at a preselected operating pressure; and sealing said envelope.

6. In the manufacture of an ionic discharge device including an envelope and an anode, a cathode having a surface formed from a heat-decomposable compound, and a control electrode all mounted within said envelope, the method of neutralizing undesirable electron-emissive deposits in said device comprising the following steps: evacuating said envelope; heating said cathode to decompose said compound and form on said surface an electronemissive substance, which may concurrently cause formation of undesirable electron-emissive deposits on said envelope, said anode, or said control electrode; filling said envelope with a selected ionizable gas at a preselected pressure; generating an arc discharge between said anode and said cathode and maintaining said discharge for a predetermined interval, which may concurrently cause formation of undesirable electron-emissive deposits on said envelope, said anode, or said control electrode; extinguishing said are discharge and evacuating said envelope; filling said envelope with an oxidizing atmosphere and maintaining said atmosphere for a time sufiicient to oxidize any undesirable electron-emissive deposits present within said device; exhausting said envelope before oxidation of more than a superficial portion of said electron-emissive substance occurs; filling said envelope with a selected ionizable gas at a preselected pressure; again establishing an arc discharge between said anode and said cathode and operating said device at normal average current for a predetermined interval; extinguishing said are discharge and evacuating said envelope; filling said envelope with a selected ionizable gas at a preselected operating pressure; and sealing said envelope.

References Cited in the file of this patent UNITED STATES PATENTS 1,831,314 McMaster et a1. Nov. 10, 1931 1,965,585 Foulke July 10, 1934 2,533,387 McLean Dec. 12, 1950