US2400635A - Method of making tubes - Google Patents

Description

May 21, 1946. w. w, rrsg, ET'AL 2,400,635

METHOD OF MAKING TUBES Filed July 13, 1942 INVENTORS WILLIAM W. EITEL 'JACZ A. M5241 6H THEIR ATTORNEY Patented May 21, 1946 METHOD OF MAKING TUBES William W. Eitei, San Bruno, McCullough, Millbrae, Calif., McCullough. Inc., San Bruno,

tion of California and Jack A. on to Eitelf., a eorpora Application July 13, 1942, Serial No. 450,752

3 Claims.

Our invention relates to the manufacture of electronic devices, such as tubeshaving cathode. grid and plate electrodes enclosed within an evacuated envelope.

It is among the obiectsof our invention to provide an improved method of making high vacuum tubes.

Another object is to provide an improved methd of coating a plate electrode with a material having desired electrode surfacing characteristics, such as zirconium.

A further object includes the provision of methods for treating and cleaning the electrodes and envelope, whereby a high vacuum tube having stable and dependable characteristics is achieved.

The invention possesses other objects and features of advantage, some of which, with the foregoing, will be set forth in the following description of our invention. It is to be understood that we do not limit ourselves to this disclosure of species of our invention as we may adopt variant embodiments thereof within the scope of the claims.

Referring to the drawing:

The single figure is a perspective view of a tube embodying th improvements of our invent on.

In terms of broad inclusion, our method of making tubes comprises forming electrodes including plate and grid electrodes; cleaning the electrodes and parts by a special procedure before and after mounting within an envelope: evacuating the envelope; an heating the electrodes in situ. The

outer surface of the hollow metallic plate is preferably coated with a suitable surfacing material, such as zirconium powder and a binder; and the plate is heated within the evacuated envelope'by electronically bombarding the inner plate surface to bake the coating onto the outer surface thereof.

In greater detail, and referring to the drawing, our method is illustrated in connection with a tube comprising an envelope of glass or the like having an upper blank portion 2 and a lower stem portion I sealed together at the base of the tube. The triode shown in the figure comprises a plate or anode 4, grid 8 and posed within the envelope.

Cathode I is of the filamentary type, preferably of thorlated tungsten, comprising a pair of concentric coils welded at top and bottom to a pair of tungsten leads a sealed to stem 3. The stem is protected against heat and electron bombardment by a cup-shaped nickel shield I inserted cathode l coaxially' dissides of envelope blank 2 at horns ll.

over the stem and welded to one of the cathode In our preferred construction. shown in Figure 1, plate 4 comprises a hollow cylindrical body of molybdenum welded along a seam It. To improve the getter properties of the plate for absorbing residual gases in the tube, the outer surface of the plate is provided with a coating it of suitable electrode surfacing material, preferably zir-r conium. The latter also presents a dark surface, approaching the characteristics of a black body, a 't e ebv mp oving the heat dissipating properties of the plate. A cup-shaped cap 2|, also of molybdenum, is secured over the plate by depending tabs 22 welded to the rim. Arched recesses 23 between tabs 22 provide vent openings at the upper end of the plate. 'I'his cap protects the glass at the dome against heat and electron bombardment, and also increases the heat radiating surface of the plate structure.

The plate is supported by molybdenum brackets 28 welded to the rim and at the outer ends to tungsten leads 28 extending thru the sides of envelope blank 2 at horns 21. Coating I! preferably extends over the outer surfaces of the plate brackets.

New going to the method of making the tube. and taking up the plate structure first, we form the cylindrical body I of molybdenum in a suitable die. It is then degreased by dipping in carbon tetrachloride or treating with trichlorethylene. Seam III is then spot welded. Brackets 24 and cap 2| are formed in dies, degreased. and

welded to the plate, after which the outer surface of the plate is roughened.

Roughening is preferably accomplished by blasting the plate with hard metallic particles, such as steel. During blasting the plate is mounted on a core or mandrel for protecting the inner plate surface to keep it smooth. Roughening of theouter surface is to give a better bond for coat ing II. This roughening may be accomplished in other ways, as by etching the metal, although blasting is prefe After blasting. the molybdenum plate structure is acid cleaned by boiling for V4 to 1 hour in 25% sulphuric acid. or, alternatively. by soaking for about onehour in a cold bath of concentrated C. for outgassing the envelope.

nitric acid. This removes contamination and foreign matter accumulated during welding and blasting. The advantage of using steel particles for blasting is that these are completely removed by the acid; a decided advantage over sand blasting where the line silica particles embedded in the plate are practically impossible to remove.

Next, the plate structure is washed acid free by several rinses in tap water; a dip in 5% ammonium hydroxide; another rinse in tap water; and then a dip in distilled water. The base serves to neutralize the acid; ammonium hydroxide being preferred because it leaves no deposit on the metal, it being understood that our purpose is to remove from the electrodes all foreign matter which, if present, gives trouble during exhaust and subsequent operation of the tube.

With molybdenum we preferably additionally treat the plate by heating it in an atmosphere of pure hydrogen, preferably bringing the plate up to' about 1500 C. for about 5; min. and then allowing it to cool in the hydrogen atmosphere.

.Thls cleans of! remaining contamination in the metal, which molybdenum appears to hold with particular tenacity. The treatment also prepares the surface for better bonding with the zirconium coating.

Coating I9 is next applied by means of a brush inserted thru the open end of envelope blank 1, care being taken not to get the coating material on the inside surface of plate. The coating composition comprises about 1 part by volume of pure zirconium dust to 5 parts of binder. Our preferred binder comprises about 1 part by volume of nitrocellulose to 5 parts amyl acetate thinner. This composition is of good consistency for brushing and spreads nicely. The wet binder temporarily holds the metallic particles to the plate with suillcient firmness to permit handling the tube prior to exhaust, at which latter time the coating is permanently bonded to the plate by baking.

Nickel shield l is formed in a die, degreased, and then cleaned by heating to a cherry red for {about 10 minutes in hydrogen and allowed to cool in the hydrogen atmosphere. The shield is then inserted over the stem and welded to a lead 8, after which filament coils 1 are mounted and welded in place. If the thorlated tungsten filament is to be carbonized outside the envelope, it is preferably accomplished by inserting the stem in a bell jar and heating the filament in a carbonizing atmosphere at reduced pressure.

Stem 3 is next joined to blank portion 2 by sealing the stem flare to the lower rim of the blank, care being taken to accurately center the filament within the other electrodes. If the diament: was not previously carbonized it may be done so at this stage by burning the filament in a carbonizlng atmosphere admitted thru tubulation ll.

Our tube is now ready for exhaust. In this procedure the envelope is connected to a suitable vacuum pump, preferably of the diffusion type, capable of reducing the pressure to a low value, say about 10- mm. Hg. While the pump is operating, the tube is heated in an oven to bring the glass temperature up to about 450 Thereafter the filament is energized and positive potentials are selectively applied to the plate or grid to beat them by electron bombardment. As will be readily understood, the negative electrons from the filament are attracted to any other electrode,

such as the anode or grid, which is positive with respect to the cathode. The electrode being bombarded with electrons becomes hot because of the kinetic energy given up to the electrode when the fast moving electrons are suddenly stopped. Our Patent No. 2,134,710 describes one type of circuit for heating electrodes by electron bombardment during exhaust oi a-tube. With a tube of the character described, embodying a platinum grid, the plate is first separately brought up gradually over a period of about 20 minutes to a maximum temperature of say about 1100 (3.; then the grid is separately elevated gradually over a period of about 10 minutes to a maximum temperature of say about 1400 0.; after which both plate and grid are allowed to run simultaneously at the above temperatures for about 2 hours, the heating of course being controlled by the amount of potential applied individually or simultaneously to the electrodes. This heating during exhaust outgasses the electrodes, renders the platinum grid non-emissive, and permanently bonds the zirconium coating to the plate.

A hereinbefore specified, the binder for the zirconium powder preferably comprises nitrocellulose and amyl acetate. The acetate being quite volatile is quickly driven off. Under the high plate temperature (1100" C.) the nitrocellulose disintegrates, leaving a residue which is princimlly carbon. The coating is thus baked onto the plate, the residual carbon serving as the permanent binder for holding the zirconium particles to the underlying metallic body. It is to be particularly noted that the hollow plate is heated by electron bombardment of its uncoated inner surface. The coating on the outer surface receives the necessary heating to bake it on, but is protected by the interposed plate against disrupting effects of the impinging electrons.

As a modification, we have used a zirconium coated tantalum plate with good results. In this case cap 2| and brackets 24 as well as the plate per se are made of tantalum instead of molybdenum. Tantalum inherently has good getter properties, and, while not on a par with airconium, supplements the coating as a getter for residual gases.

The process of making a tube having a airconium coated tantalum plate is the same as that described for molybdenum, except that the acid cleaning of the plate is somewhat different and the step of heating the plate in hydrogen is omitted. With a tantalum plate, the unfinished plate (without cap 2| or brackets 24 welded on) is boiled for about 1 hr. in agua regla. It

is then washed by rinsing in tap water; dipv ping in 5% ammonium hydroxide; and then rinsing in distilled water. After the cap and brackets are applied, the finished plate structure is again cleaned the same as 1 the unfinished tantalum plate plus two boilings of about /2 hour each in distilled water.

We have found the combination of a zirconium coated molybdenum anode with a carbonized thoriated cathode to be particularly important because tubes having this combination of electrodes exhibit much longerlife due to a slower rate of loss of carbon from the cathode.

We claim:

l. The method of making a tube which comprises forming an electrode of molybdenum, heat treating it in an atmosphere of hydrogen, coating it with zirconium, sealing the electrode within an ent'elc'me, and evacuating the envelope and simultaneously heating the electrode.

2. The method of making a tube which comprises forming a hollow electrode, applying a metallic coating to the outer surface only 0! the electrode, sealing the electrode within an envelope, and exhausting the envelope and simultaneously heating the electrode by bombarding its uncoated inner surface with electrons.

8. The method oi making a tube which oompriees forming a hollow electrode of molybdenum, 9.9m a coating of zirconium to the outer suriace only of the electrode, sealing the electrode within an envelope, and exhausting the envelope and simultaneously heating the electrode by bombar-ding its unconted inner surface with electrons.

' WILLIAM W. m'rnn.

JACK A. McCULlZOUGH.

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