US2800383A - Process for making gaseous discharge tube - Google Patents

Description

July 23, 1957 R. H. MITCHEL pnocsss FOR MAKING GASEOUS DISCHARGE TUBE Filed Dec. 30. 1954 32% ZEk/ wl w ATTORNEY Unite States ate'nt 2,800,383 Patented July 23, 1 957 Ralph H. Mitchel, Flint, Micln, assignor to General Motors Corporation,Detroialviicln, a corporation of Delaware Application December 30, 1954, Serial No. 478,556

11 Claims. (Cl. 31622) This invention relates to a process for manufacturing gaseous discharge tubes generally of the type shown in United States Patent 2,478,119, granted August 2, 1949, to Ralph H. Mitchel. Such gaseous discharge tubes, which find utility in high frequency ignition systems as well as in spark plug testing devices and the like, consist essentially of two electrodes having surfaces spacially separated to form a spark discharge gap and supported within a glass or similar envelope containing an inert gas atmosphere. The tubes are designed to discharge at a specific voltage, the features of the tube significant in this respect being the spacing and composition of the electrodes and the composition and pressure of the inert gas.

It has been found by experience that the presence of even minute impurities such as oxides within the envelope have a substantial effect not only upon the initial electrical characteristics of the tube, such as the voltage required to cause discharge, but also upon the rate at which these characteristics vary with continued operation. Thus, it is essential that such impurities be completely removed during manufacture. Also, it has been found highly advantageous, if not essential, that the electrode surfaces be quite dense and smooth as well as of very accurate dimensions. With tubes lacking these important features, the electrical characteristics vary substantially over the life of the tube. For example, with even very small amounts of oxide impurity present on the electrodes and with electrode surfaces which are relatively porous, the voltage required to cause discharge will gradually increase with operation, thus rendering the tube useless.

if is an object of the present invention to provide a process for manufacturing tubes, of the type referred to above, which will greatly increase their uniformity of performance and durability. More specifically, it is an object of the present invention to provide a process for manufacturing such tubes which will assure the removal of impurities from the surfaces of the metal parts such as the electrodes and electrode supports and which will further assure dense, smoothe electrode surfaces of accurate dimensions.

Other objects and advantages of the invention will appear more clearly from the following description of preferred embodiments and from the drawings in which:

Figure 1 is a view partially in vertical section and partially in elevation of one form of discharge tube manufactured in accordance with the present invention;

Figure 2 is an exploded view of the tube showing the various component parts; and

Figure 3 is an end view of the tube shown in Figure 1 during one step in its manufacture and prior to scaling the exhaust stems. I

Referring now to Figure 1, there is shown a tube illustrative of the type to which this invention relates and comprising two cupped end members 4 and 6, provided with out-turned annular flanges 8 and 10 respectively whlch are bonded to glass member 12 to form a gas sealed envelope therewith. Electrical contact members 9 and 13 are welded or otherwise secured to the concave exteriors of the end members as shown. The end members 4 and 6 are made of an alloy having substantially the same coefiicient of expansion as the glass to which it is bonded so that the structure is durable and resistant to cracking over a wide temperature range. Typical of such alloys is that consisting of 29% nickel, 17% cobalt, 3% manganese, and the balance iron, and referred to in the trade as Kovar. Other metals with matching glass may, of course, be used. Welded or otherwise secured to the inner convex surfaces of end members 4 and 6 are discshaped electrodes 14 and 16 respectively, the opposing fiat surfaces lift and 20 of these electrodes being in spaced relation with respect to each other to provide a spark discharge gap. The disc-shaped electrodes are preferably made of a fused or sintered mixtured of finely pulverized metal such as iron and a salt or oxide of a metal of low work function such as barium oxide. To provide uniform discharge characteristics irrespective of the amount of light to which the tube is exposed, the interior surfaces of the tube and especially the surface of the glass envelope may be coated with a small amount of radioactive substance such as radium bromide. The envelope is filled with a inert gas of predetermined composition and under predetermined pressure to impart the desired discharge characteristics. For further and more detailed description of such tubes and various embodiments and modifications thereof, reference may be made to the above-mentioned U. S. Letters Patent No. 2,478,119.

As pointed out above, it is highly desirable for the durability and uniformity of operation of such a tube that the interior of the envelope be freed of impurities such as oxides and the like and that the opposing flat electrode surfaces 18 and 20 be smooth, dense, and of accurate dimensions. Heretofore it has been the practice to remove impurities during manufacture of the tube by flowing hydrogen through the envelope while the metal and glass parts are heated. This method has been found to be unsatisfactory for the reason that in removing the oxygen impurities from the electrode surfaces 18 and 20, a somewhat porous, spongy surface results, thereby greatly reducing the desirable properties such as uniformity of electrical characteristics. Presumably, such a spongy surface is subject to considerable wear and sputtering from the force of the discharges and since the required discharge voltage is a function of the closest electrode spacing, such wear results in non-uniformity and eventually an increase in the breakdown voltage. Also, the sputtered material collects on the interior surfaces of the tube, especially the glass envelope, resulting in a reduction of the electrical resistance across the gap to an unsatisfactory low value.

In accordance with the present invention, the oxides and other impurities are removed by first treating the interior of the envelope chemically, that is, with an acid or other liquid solvent material which dissolves or loosens the metal oxides that may be present on the surfaces of the electrodes and other metal parts. Subsequently the interior surfaces are treated with hydrogen to remove the very last traces of impurity. Thus, in practicing the invention, there is first admitted to the interior of the envelope a quantity of acid or the like which is capable of dissolving or loosening the metal oxide and other impurities which may be present, removing the acid along with the loosened and dissolved oxides and other impurities, and then introducing hydrogen into the envelope and heating the electrodes in the hydrogen to thereby remove the lesser amounts of impurity. To assure the removal of any impurity which may be introduced during other processing steps, it is highly advantageous to treat with the hydrogen only after such other processing steps are completed and just prior to filling the envelope with inert gas and sealing.

By the term chemical solvent as used herein is meant a liquid which is capable of dissolving the oxides and other impurities at least to the extent necessary to loosen them from the interior surfaces. Thus, the invention contemplates the use of liquids which will loosen the coating of oxide and other impurities on the interior surfaces to cause them to flake off and drain out of the tube when the liquid is removed.

The following is a description of a preferred process embodying the invention, reference being made to Figures 2 and 3.

Figure 2 shows the various component parts used in manufacturing the tube. The glass member 12 which is substantially cylindrical in shape is preferably provided with two exhaust tubes 22 and 24 (shown after scaling in Figure 2) in order to facilitate the hereinafter described processing steps and in particular the step for chemically removing impurities. End members 4 and 6 may be formed in any well-known manner such, for example, as by progressive drawing of blanks cut from strip stock. If these end members are of 29% nickel, 17% cobalt, .3% manganese, and the balance iron or a similar alloy, it is desirable to anneal them in hydrogen at a temperature of about 1650 F. for about one hour before sealing the glass member 12 thereto. Flanges 8 and 10 may be machined to accurate dimensions in a lathe collet and should be thoroughly cleaned prior to the hydrogen annealing at 1650* F.

In a particular tube shown in the drawings, the electrodes 14 and 16 are of a sintered mixture of about 91% ion and 9% barium oxide. The processes for making such sintered structures are, of course, well-known in the art, and the precise method used forms no part of the present invention.

Electrodes 14 and 16 are bonded as by welding to the convex interior surfaces of end members 4 and 6 respectively and electrical contact members 9 and 13 welded or otherwise bonded to the concave exterior surfaces. Then flanges 8 and 10 are cleaned, hydrogen annealed, and joined to the edges of the glass member 12, all as shown in Figure 3. The latter joining operation may be accomplished by heating the flanges and the glass member until the edge portions of the glass sofetn and then bringing the metal and glass into contact to form the bond. The composition of the glass is, of course, such as to provide a coefficient of expansion which substantially matches that of the flanges so as to prevent strains and cracking over a wide temperature range and also such as to provide a good bond with the flange metal.

During these various processing steps, most of which require the application of heat, a considerable layer of metal oxide is formed on the metal surfaces of the electrodes and end members. In accordance with the present invention, the great bulk of this oxide is first removed by a chemical cleaning operation. The following procedure is illustrative:

First, one of the two exhaust stems (shown prior to sealing in Fig. 3) is plugged and there is admitted to the envelope through the other exhaust stem, a quantity of acid or similar oxide solvent at least sufiicient to dissolve or loosen the oxide and other impurity present. The solvent is left in the tube and agitated continuously until the oxide layer is removed and then is allowed to drain by removing the plug from the exhaust stem. For manufacturing on a small scale, a hypodermic syringe and needle may be used to introduce the chemical solvent into the envelope, agitation being produced by alternately squirting and sucking the solvent with said syringe. Alternatively, suflicient solvent to only about half fill the envelope may be added and then mechanically shaken to cause the agitation required to thoroughly remove all the oxide impurity. For large scale production operations, it may be desirable to flow the chemical solvent more or less rapidly through the envelope by using one of the exhaust stems as an entrance and the other as an exit,

2,soo,asa A 4.

the necessary agitation being provided by the movement of the solvent past the metal surfaces.

For chemically cleaning the type tube described herein, i. e., one having sintered iron electrodes and other metal parts of iron alloys, a preferred oxide solvent consists of a solution of hydrochloric acid containing about 450 cc. hydrochloric acid (saturated), 10 cc. Rhodine acid inhibitor, and 1350 cc. distilled water. It has been found preferable to leave this particular solution in the envelope with agitation for about 1 /2 to 2 minutes, and preferably for about 1% minutes. In this amount of time, said solution dissolves all the oxide but, at the same time, does not etch the metal to any considerable extent. It will be obvious that the exact composition and concentration of the chemical solvent and the period for which it is left in the envelope will depend on the composition of the electrodes and other metal parts, on the precise structure of the tube, and on the amounts and types of oxides and other impurities present. An essential feature is that the solvent be capable of removing the oxides present by dissolving or loosening and it is highly desirable that it be such as to not rapidly attack the metal surfaces. As indicated above, where the metal parts are iron or iron alloys, a dilute solution of hydrochloric acid containing a small portion of a suitable acid inhibitor such as is specified above is desirable.

After the chemical solvent containing the oxides and other impurities is drained, the envelope is thoroughly rinsed with distilled wter to remove the last traces of the acid and rinsed with acetone, alcohol, or the like to remove the water. Then one of the exhaust stems is again stoppered and vacuum applied through the other to thoroughly dry the interior surfaces.

After drying, a wire feeler gauge may be inserted through one of the exhaust stems to accurately measure the gap. between the electrode surfaces 18 and 20. In this regard, it is advantageous to locate the exhaust stems in alignment with the gap so as to facilitate such measurement. It will be noted that this accurate measurement of the spark discharge gap is taken only after the chemical cleaning operation. for the reason that the gap may significantly increase in size by the removal of the metal oxide layer which may be of considerable thickness, depending upon the nature of the metals and previous processing steps used. The importance of this accurate spark gap measurement at this stage of the process is that tubes having a gap size outside the allowed tolerances may be discarded prior to further processing steps. Another advantage is that such measurement indicates the amount of inert gas pressure which will be required for the tube to impartthe desired electrical characteristics.

After the gap measurement is completed, one of the two exhaust stems is sealed off with a high temperature gas flame or the like, and the envelope is again evacuated to remove any water vapor which might have condensed on the interior surfaces during the exhaust stem sealing operation. Then, a small quantity of a solution of radioactive material is introduced into the tube through the stillopen exhaust stem and is allowed to flow over interior surfaces, and particularly the interior surface of the glass member 12. About three drops of a solution of radium bromide containing about 500 micrograms of radium bromide in 50 cc. of alcohol may advantageously be used. Introduction of this solution into the tube may be by means of a medicine dropper, for example. After such solution has coated the interior surfaces, the tube is again evacuated, this time to remove the alcohol which is associated with the radium bromide as well as any water vapor which may be present. Then to further remove occluded gases, vapors and the like, the electrodes are heated to approximately 750 C. in an induction furnace or the like for about 30 seconds while vacuum is applied. The tube is allowed to cool while the vacuum pumping continues and when cold may be transferred to a high vacuum system (diffusion pump) where it is first tested for leaks. For

' discharge occurs.

all vacuum operations up to this point, a mechanical vacuum pump is adequate; however, for this last-mentioned testing step as well as for all subsequent vacuum operations, it is preferable to use a difiusion pump high vacuum system.

After testing for leaks, the tube is placed, while high vacuum pumping continues, in a baking furnace maintained at about 400 C. for about one-quarter hour to degas the envelope and to lessen the thermal shock on the tube in preparation for the next processing step. This next processing step consists of admitting into the tube While it is still hot about 35 mm. of spectroscopically pure hydrogen and applying about 1600 volts A. C. across the electrodes with 4000 ohms in series. The tube electrodes are thus heated slowly and held at a temperature of 750 C. for about 2 minutes. At the end of the 2 minutes, the tube is pumped hard to remove the last traces of hydrogen plus any water which might have formed. During this step, the last traces of oxide which might have been present in the tube are reduced, the oxygen combining with the hydrogen and being removed as water. Not only are the oxides which might have remained after the chemical treatment removed in this manner, but also any oxides which might have formed during operations subsequent to the chemical treatment such, for example, as during the sealing off of the first exhaust stem and during the addition of the radium bromide.

To further assure the complete removal of the last minute quantities of oxides in the tube, it is desirable to repeat the last-mentioned process step at least once and preferably twice. Then the electrodes are heated to about 75 C. by an induction furnace for about 1%. minutes, the tube being pumped hard to remove any absorbed or adsorbed gas from the electrodes. Then the tube is baked for about 15 minutes in a furnace at 400 C., hard pumping continuing at all times during this entire period. This step further assures the removal of any gases which might have remained in or on the glass envelope. Then the tube is allowed to cool, all the while under high vacuum, and when cold, the degree of vacuum is measured with a McCleod gauge or the like. When the tube shows a vacuum of .000005 mm. of mercury or better, the tube is filled with a rare gas mixture to a pressure necessary to attain the desired electrical characteristics and then the remaining exhaust stem is sealed closed by a gas flame or the like. For the type of tube hereinbefore described and shown in more detail in U. S. Letters Patent No. 2,478,119, I prefer to use a gas mixture consisting of about 18% neon, 54% argon, and 28% helium, all spectroscopically pure. To facilitate sealing off the exhaust stem, it is highly desirable that the inert gas pressure not substantially exceed about mm. below atmospheric pressure.

Since it is necessary that the finished tube discharge at a certain predetermined voltage, it is highly advantageous to fix the voltage required for discharge by adjusting the pressure of the inert gas prior to sealing off the exhaust stem. In this manner a great number of tubes which would otherwise have to be rejected because of an inaccurate gap size may be used just as advantageously as other tubes having a more accurate gap size. For example, in one type of tube heretofore described specifica tions call for a gap of .041" and a discharge voltage of from 680 to 730 peak volts. With a gap of this exact size, 73 centimeters mercury pressure of the above-listed inert gas composition is required to provide the specified discharge voltage. However, in practice it is diflicult if not impossible to control the gap size to any closer than from .036" to .043". In accordance with the present invention, a predetermined voltage the same or close to that for which the tube is designed is applied across the electrodes and the gas pressure is varied up or down until At this point, the exhaust stem is sealed off by heating it to its softening temperature and then pinching. Such sealing is aided by the fact that the internal pressure is lower than atmospheric and thus, when the glass is softened, the outside pressure itself exerts a sealing action.

As mentioned above, it is difficult to seal a tube in which the inert gas pressure is greater than 5 mm. less than atmospheric. When the gap size is such as to require a gas pressure which exceeds this, the tube may be cooled a calculated number of degrees by packing with dry ice or the like and sealing while so cooled to substantially below room temperature. In this manner, it is possible to obtain the required gas pressure. The pressure and voltage required at the reduced temperature to impart the required discharge voltage at room temperature may, of course, be predetermined by calculation. Because the pressure Within any closed system varies directly with temperature, it is possible, in this manner, to utilize tubes which might otherwise have to be discarded because of difficulties in sealing the exhaust stem.

Tubes manufactured in accordance with the invention may require a break-in period during which the voltage required to cause discharge rapidly increases, then decreases and finally becomes constant. However, such changes in the voltage required are relatively constant for any given type of tube and thus, it is only necessary to seal at a predetermined voltage and run the tubes through a break-in period to impart the desired constant discharge voltage.

It is to be understood that, although the invention has been described with specific reference to particular embodiments thereof, it is not to be so limited since changes and alterations therein may be made which are within the full intended scope of this invention as defined by the appended claims.

What is claimed is:

1. A process for manufacturing a gaseous discharge tube, including the steps of sealing a pair of electrodes consisting of a mixture of iron and barium oxide within a glass envelope to form the tube, introducing into said tube an aqueous solution of hydrochloric acid, agitating said acid within said tube for a suflicient period to remove oxide and other impurities from the interior surfaces of said tube, removing said acid, drying the interior of said tube, introducing radioactive material dissolved in a liquid to the interior of said tube, evacuating said tube to remove said liquid, again evacuating said tube while heated to about 750 C. to remove vapors and occluded gases, introducing hydrogen to the interior of said tube, heating said tube to cause said hydrogen to react with any oxide present on the interior surfaces of said tube, again evacuating said tube, introducing rare gas into said tube until a pressure required to permit discharge at a predetermined voltage is attained and then sealing said tube.

2. A process for manufacturing a gaseous discharge tube, including the steps of introducing into said tube a liquid chemical solvent for oxide and other impurities, agitating said solvent Within said tube for a sufiicient period to remove the oxide and other impurities from the interior surfaces of said tube, removing said solvent, drying the interior of said tube, introducing radioactive material dis solved in a liquid to the interior of said tube, evacuating said tube to remove said liquid, again evacuating said tube While heated to about 750 C. to remove occluded gases, introducing hydrogen to the interior of said tube, heating said tube to cause said hydrogen to react with any oxide present on the interior surfaces of said tube, again evacuating said tube, introducing rare gas into said tube until a pressure required to permit discharge at a predetermined voltage it attained and then sealing said tube.

3. A process for manufacturing a gaseous discharge tube including the steps of introducing into said tube a liquid chemical solvent for oxide and other impurities, agitating said solvent within said tube for a sufiicient period to remove the oxide and other impurities from the interior surfaces of said tube, removing said solvent, in-

troducing radioactive material to the interior of said tube, introducing hydrogen to the interior of said tube, 'heating said tube to cause said hydrogen to react with any oxide present on the interior surfaces of said tube, again evacuating said tube, introducing rare gas into said tube and then sealing said tube.

4. A process for manufacturing a gaseous discharge tube, including the steps of introducing into said tube a liquid chemical solvent for oxide and other impurities, allowing said solvent to remain in said tube for a sufiicient period to remove the oxide and other impurities from the interior surfaces of said tube, removing said solvent, evacuating said tube to remove vapors and occluded gases, introducing hydrogen to the interior of said tube, heating said tube to cause said hydrogen to react with any oxide present on the interior surfaces of said tube, again evacuating said tube, introducing rare gas into said tube until a pressure required to permit discharge at a predetermined voltage is attained and then sealing said tube.

5. A proces for manufacturing a gaseous discharge tube, including the steps of introducing into said tube a liquid chemical solvent for oxide and other impurities, allowing said solvent to remain in said tube for a sutficient period to remove the oxide and other impurities from the interior surfaces of said tube, removing said solvent, introducing radioactive material dissolved in a liquid to the interior of said tube, removing said liquid, introducing hydrogen to the interior of said tube, heating said tube to cause said hydrogen to react with any oxide present on the interior surfaces of said tube, evacuating said tube, introducing rare gas into said tube and then sealing said tube.

6. A process for manufacturing a gaseous discharge tube, including the steps of introducing into said :tube a liquid chemical solvent for oxide and other impurities, allowing said solvent to remain in said tube for a sufiicient period to remove the oxide and other impurities from the interior surfaces of said tube, removing said solvent, introducing hydrogen to the .interior of said tube, heating said tube to cause said hydrogen to react with any oxide remaining on the interior surfaces of said tube, evacuating said tube, introducing rare gas into said tube until a pressure required to permit discharge at a predetermined voltage is attained and then sealing said tube.

7. A process for removing oxide and other impurities from the interior surfaces of a gaseous discharge tube during the manufacture thereof, including the steps of introducing into said tube a liquid chemical solvent for the oxide and other impurities, allowing said solvent to remain in said tube for a sufficient period to remove the oxide and other impurities from the interior surfaces of said tube, removing said solvent, introducing hydrogen to the interior of said tube, heating said tube to cause said .hydrogen to react with any oxide remaining on the interior surfaces of said tube and evacuating said tube.

8. A process for removing oxide and other impurities from the interior surfaces of a gaseous discharge tube during the manufacture thereof including the steps of introducing into said tube an acid solvent from the oxide and other impurities, agitating said solvent within said tube for a sufficient period to remove the oxide and other impurities from the interior surfaces of said tube, removing said solvent, evacuating said tube while heated to remove vapors and occluded gases, introducing hydrogen to the interior of said tube, heating said tube to cause said hydrogen to react with any oxide remaining on the interior surfaceso'f said tube, and again evacuating said tube.

9. A process for manufacturing a gaseous discharge tube, including the steps of sealing a plurality of metal electrodes within a glass envelope to form the tube, introducing into said tube a liquid chemical solvent for oxide and .other impurities, allowing said solvent to remain in said tube for a sufficient period to remove the oxide and other impurities from the interior surfaces of said tube, removing said solvent and subsequently treating the interior of said tube with hydrogen to reduce the remaining oxide impurities in said tube.

10. A process for manufacturing a gaseous discharge tube, including the steps of sealing a plurality of ferrous metal electrodes within a glass envelope to form the tube, introducing into said tube an aqueous solution of hydrochloric acid, agitating said acid within said tube for a sufiicient period to remove oxide and other impurities from the interior surfaces of said tube, removing said acid, drying the interior of said tube, introducing hydrogen to the interior of said tube, heating said tube to cause said hydrogen to'react with any oxide remaining on the interior surfaces of said tube, evacuating said tube, introducing rare gas into said .tube until a pressure required to permit discharge at a predetermined voltage is attained and then sealing said tube.

11. In a process for manufacturing a gas filled electronic device of the type having a glass envelope provided with an exhaust stem, the steps of evacuating said envelope, cooling said envelope to substantially below room temperature, filling said envelope with a gas to a pressure not greater than 5 mm. mercury below atmospheric pressure, sealing the exhaust stem by the application of heat thereto to soften the glass and then allowing said envelope to warm to room temperature.

References Cited in the file of this patent UNITED STATES PATENTS 1,165,338 Moench Dec. 21, 1915 2,362,510 Stutsman Nov. 14, 1944 2,560,273 Brigant et a1. July 10, 1951 FOREIGN PATENTS 608,268 Great Britain Sept. 13, 1948 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 2,800,383 July 23, 1957 Ralph H. Mitchel It is hereby certified that error appears in the printed specification f the above numbered patent requiring correction and that the said Let oers atent should read as corrected below.

Column 2, line 15, for "mixtured" read mixture column 3, inc 31, for "ion" read iron line 43, for "sofetn" read soften; olumn 4, line 25, for "portion" read proportion line 29, for wiser" read water column 6, line 69, for "voltage it" read voltage is column 7, line 21, for 'proces" read process olumn 8, line 53, list of references cited, under "UNITED STATES PATENTS", or "Brigant" read Briganti Signed and sealed this 1st day of October 1:957

SEAL) :test:

LARL H, AXLINE ROBERT C. WATSON testing Officer Conmisaioner of Patents

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