US2558491A - Method of manufacturing electric discharge tubes - Google Patents

Description

June 26, 1951 J. G. KRONOUER METHOD OF MANUFACTURING ELECTRIC DISCHARGE TUBES Filed July 29, 1948 I m w z JOHAN GEORGE KRONOUER A GENT Patented June 26, 19 51 METHOD. OF MANUFACTURING ELECTRIC v DISCHARGE TUBES Johan George Kronouer, ,Elndhoven, Netherlands, assignor to Hartford National Bank and Trust Company, Hartford, Conn., as trustee Application July 29, 1948, Serial No. 41,362

In the Netherlands August 19, 1947 The invention relates to a method of manufacturing electric discharge tubes, according to which method the cathode has applied to it compounds which are adapted to be dissociated by heat and thus to produce readily emitting substances, The invention relates in addition to electric discharge tubes manufactured by such a method and to an apparatus for carrying out such a method.

In electric discharge tubes of widely differing kinds, for example radio valves, gas discharge tubes, X-ray tubes and the like use is frequently made of cathodes the emissive substance of which is constituted by compounds of given elements of the periodic table and is obtainable by decomposition of other compounds. The most important representatives of this kind of cathodes are the alkaline earth oxide cathodes, the oxides constituting the emissive substance being preferably obtained as a'result of heating oarbonates or nitrates. A cathode of this kind is generally manufactured by applying a supply of the compound to be decomposed, for example, carbonate or nitrate, to a support, which may be a coiled filament, for example by immersion in asuspension, the cathode then being mounted the 'tube and heated during manufacture of the discharge tube, for example prior to, during or subsequent to exhaustion and expulsion of gases out of the various components of the tube.

It is known that the process of developing readily emitting substances comprises at least two stages. At the first stage the compounds are decomposed, a large amount of gas being evolved and compounds, for example, oxides, being left which at the second stage, owing to subsequent heating, the so-called,forming operation, exhibit a low work function.

The invention is based on recognition of the fact that during the forming operation the atmosphere in the discharge tube must fulfill particular requirements.

This gaseous atmosphere is formed by gases emanating from various sources. Firstly there is the gas which is normally contained in the tube, generally therefore air. Secondly, as a result of heating the tube, gas is evolved from all the components thereof, for example the wall and the electrodes. Thirdly, gas is evolved by decomposition of the compounds on the cathode.

4 Claims. (01. s s-24y Finally there is the possibility of introduction of a so-called rinsing gas into the tube. This will be referred to later.

It has been found that all these gases do not substantially affect the cathode at the stage of decomposition of the compounds. However, at the forming stage it is necessary that the pressure of the said gases should not exceed a given value, which is low enough to say briefly that during the forming operation a complete vacuum should prevail in the tube. This is related to the fact that during the forming operation substances (probably metals) are released from the available compounds, for example oxides and are liable to react with the gases and to form non-emissive compounds. This would consequently have the effect of converting part of the substances available on the cathode into non-emissive compounds. continue until substantially all the gas would be vinculated or pumped away; not until then a cathode of really satisfactory work function could be formed. However, in the case of the forming operation being effected in a high vacuum, there is only small probability that the developed active substances (metals) of the oathode may combine with gases from the atmosphere in the tube. As a rule, it may therefore be said that at the beginning of. the cathode forming operation the pressure must be low enough for the development of active substances on the cathode to exceed the loss of these substances after production due to the fixing of gas residues from the atmosphere in the tube.

In the methods hitherto adopted the gases available in the tube from thestart and those evolved from the wall, the electrodes, and so forth, have been allowed for. Exhaustion was therefore continued until a high vacuum prevailed in the tube to be manufactured. The cathode was then heated. However, it has been insufilciently noted that this heating operation had the effect of decomposing the compounds on the cathode, the quantity of gas thus released being such that the pressure in the tube again rises to a value, at which forming is prevented. So far as this increase in pressure has been ascertained, the exhausting operation was simply effected for a longer period, so that finally the forming pressure was again reached. As ex- This process would= the gases released from wall, electrodes and. other components of the discharge tube, by the said rinsing gas constitutes a, small improvement but it has not the effect of preventing evolution .of gases upon decomposition of the compounds on the cathode or of counteracting their harmful effect, particularly so, since these gases emanate from the cathode itself, that is to say precisely at the area at which any active substances (metals) already formed areliable to fix them immediately The aforesaid disadvantages may be greatly reduced by the use of a method according to the invention, which furthermore ensures' additional advantages.

In a method according to the invention of manufacturing electric discharge tubes, according to which the cathode has incorporated in it compounds which are adapted to decompose by heat into gaseous decomposition products and substances which, as a result of a subsequent heating operation, the so-called forming operation, in a high vacuum, without any appreciable production of gas, exhibit a low work function, decomposition takes place at an instant of the exhausting and de-gassing process to which the discharge tube is subjected, at which the pressure in the tube exceeds the forming pressure, whereupon heating of the cathode is stopped until the forming pressure is reached.

The term forming pressure is to be understood to mean in this specification that pressure in the discharge tube, measured subsequent to the decomposition of the compounds available on the cathode, at which, as a result of'heating the cathode, the development of substances of low work function exceeds the loss of these substances due to the fixing of gases from the atmosphere in the tube.

In a method according to the invention, decomposition takes place at such an instant of the cycle of manufacturing the discharge tube that the gases evolved upon decomposition have disappeared from the tube, at least in part, be-

As a matter of course, decomposition will be started preferably early in the cycle so that at the instant at which heating is started, the forming pressure again prevails in the tube, the probability of production of non-emissive compounds being thus slightest.

In addition to this advantage, there is a further material advantage in that in the use of the standard exhausting pumps, the decrease in pressure per unit time is reduced according as the pressure in the vessel to be exhausted diminishes so that the exhaustion curve exhibits a substantially exponential variation. Since, according to the invention, decomposition takes place before the forming pressure and hence a high vacuum is reached the gases evolved upon this-decomposition will be conducted away considerably more rapidly than in the methods hitherto adopted, in which a high-vacuum was started with. The use of the method according 4 to the invention consequently permits of the entire process being materially shortened. Decomposition of the compounds is brought about preferably at an early stage of the exhausting operation or at least before the pressure is reduced to less than 20 times the forming pressure. The cathode may be heated. for example, practically immediately at the start of the exhaustion, since at this instant the decrease in pressure per unit time is considerable and it has been found that the gases evolved from the other components of the discharge tube, for example water vapour, practically do not affect the com-' pounds thus formed on the cathode.

After the compounds on the cathode are decomposed the cathode is not heated until the forming pressure has been reached, since heating, when continued, has the effect of produc-. ing non-emissive compounds by which the cathode is rendered brittle.

When heating the cathode is referred to, this is to be understood to mean the particular heating of the cathode.

In the method according to the invention use may be made of a rinsing gas, for example argon. However, compared with the methods hitherto adopted, this does not offer my particular advantages.

Upon the cathode being formed, the discharge tube is treated in the usual manner in the method according to the invention.

Owing to the differences in construction between the various kinds of discharge tubes to which the invention may be applied, the effect will not always be identical, that is to say the gain of time achieved will not always be a like part of the total time involved for the manufacture of the discharge tube. With radio valves, for example, the capacity of the vessel is small, but the number of components to be de-gassed is great. With gas discharge tubes, operating at low pressures, the capacity is material, whereas the number of components to be de-gassed is comparatively small. These components, however, are to be understood to include the tube wall, and, if any, the luminescent layer applied to it, whichis particularly large.

In order that the invention may be more clearly understood and readily carried into effect, it will now be described more fully with reference to the accompanying drawings.

Figure 1 shows a graph of the pumping time for tube evacuation plotted against the pressure in the tube, and Fig. 2 illustrates the sequence of operations of the apparatus used in the method of invention.

In the graph shown in Fig. 1 the pumping time T is plotted on the horizontal axis, the pressure P on the vertical axis. With a given discharge tube and a given pump the exhaustion curve which would occur if the cathode were not heated is designated by the curve a. This curve has a substantially exponential variation but for two kinking points. The kinking point A is due to the heating and de-gassing operations to which the tube and its components are subjected being started; the kinking points on the curve L-l are produced upon exit of the tube from the heating zone (furnace). The dotted line parallel to the horizontal axis designates the forming pressure. Curve a intersects this dotted line at point Q and according to the methods hitherto adopted, heating of the cathode has invariably been started at this point. Owing to this heating a quantity of gas is evolved, so that the pressure evolved is substantially equal.

creases to the value V. From this instant the pressure decreases as shown by curve c. How:

ever, the decrease in pressure in the zone between U and Q is quite material, since the pumping rate is high. The curve intersects the line of the forming pressure at a point W. The use of the method according to the invention therefore results in a gain in time corresponding to the distance W-S.

In order that the invention may be clearly understood an apparatus will be briefly described which permits of carrying out the method accord- The pressure ining to the invention. This apparatus will be described as convenient for the exhaustion of lowpressure mercury-vapour discharge lamps but without departing from the scope of the invention, it may be arranged to be convenient for the manufacture of other kinds of discharge tubes.

The apparatus comprises a so-called rotary pump system having 36 positions as shown in Fig. 2. In each of these positions the gas dis charge tube may be subjected to a definite treatment. Positions I, 2 and 3 are required, for example, for the insertion of the tube. During the rotation of the pump system the tube is connected in the next following position 4 to a vacuum conduit and remains connected thereto as far as position 30, so that the pressure in the tube is reduced. Next, the tube passes through a number of positions, for example, I to 18, which are located in a furnace in which heating of the tube and its parts is effected. After this furnace a number of positions, for example I! to 30, are available for firing the cathode and after this firing a further number of positions, for example 30 to 35, are available for the introduction into the tube of a loading gas, for example argon, and of the mercury required for the discharge. In the method hitherto known heating of the cathode was started after the tube has passed through the furnace, that is to say, in position l9. In the apparatus for carrying out the method according to the invention firing is effected in position 5, i of the cycle. The switches required for this purpose are provided on the apparatus.

What I claim is:

1. The method of manufacturing an electric discharge tube having an envelope and within the said envelope a cathode having a surface of emissive substances of low work function formed from a heat-decomposable compound applied to the said cathode, comprising the steps of evacuating the gaseous filling of the said envelope to a given pressure greater than the pressure at which the development of the said emissive substances of low work function exceeds the loss of the developed emissive substances occurring from the interaction of the said emissive substances with the gases of the atmosphere of the tube, heating the said cathode at the said given pressure to decompose the said compound and increase the pressure within the said envelope, further evacuating the said envelope to a pressure at which the development of emissive substances of low work function exceeds the loss of the developed emissive substances occurring from the interaction of the emissive substances with the gas of the tube atmosphere, and thereafter activating the said cathode surface to produce the emissive substances of low work functionthereon while continuing the evacuation of the said envelope.

2. The method of manufacturing an electric discharge tube having an envelope and within the said envelope a cathode having a surface of emissive substances of low work function formed from a heat-decomposable compound applied to the said cathode, comprising the steps of evacuating the gaseous filling of the said envelope to a given pressure which is approximately twenty times the pressure at which the development of the said emissive substances of low work function exceeds the loss of the developed emissive substances occurring from the interaction of the said emissive substances with the gases of the atmosphere of the tube, heating the said cathode at the said given pressure to decompose the said compound and increase the pressure within the said envelope, further evacuating the said envelope to a pressure at which the development of emissive substances of low work function exceeds the loss of the developed emissive substances occurring from the interaction of the emissive substances" with the gas of the tube atmosphere, and thereafter activating the said cathode surface to produce the emissive substances of low work function thereon while continuing the evacuation of the said envelope.

3. The method of manufacturing an electric discharge tube having an envelope and within the said envelope a cathode having a surface of emissive substances of low work function formed from a heat-decomposable alkaline earth carbonate applied to the said cathode, comprising the steps of evacuating the gaseous filling of the said envelope to a given pressure greater than the pressure at which the development of the said emissive substances of low work function exceeds the loss of the developed emissive substances occurring from the interaction of the said emissive substances with the gases of the atmosphere of the tube, heating the said cathodeat the said given pressure to decompose the said alkaline earth carbonate and increase the pressure within the said envelope, further evacuating the said envelope to a pressure at which the development of emissive substances of low work function exceeds the loss of the developed emissive substances occurring from the interaction of the emissive substances with the gas of the tube atmosphere, and thereafter activating the said cathode surface to produce the emissive substances of low work function thereon while continuing the evacuation of the said envelope.

4. The method of manufacturing an electric discharge tube having an envelope and within the said envelope a cathode having a surface of emissive substances of low work function formed from a heat-decomposable alkaline earth nitrate applied to the said cathode, comprising the steps of evacuating the gaseous filling of the said envelope to a given pressure greater than the pressure at which the development of the said emissive substances of low work function exceeds the loss of the developed emissive substances occurring from the interaction of the said emissive substances with the gases of the atmosphere of the tube, heating the said cathode at the said given pressure to decompose the said alkaline earth nitrate and increase the pressure within the said envelope, further evacuating the said envelope to a pressure at which the. development of emissive substances of low work function exceeds the-loss of the developed emissive substances occurring from the interaction of the emissive substances with the gas of the tube atmosphere, and thereafter activating the said cathode surface to produce the emissive substances of low work function thereon while continuing the exacuation of the said envelope.

JOHAN GEORGE KRONOUER.

Qua-1.91

8 .HREFERENCES v01mm The following references'are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date Espe et'al. Jan. 24, 1933

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