US2478841A - Cathode for electric discharge tubes - Google Patents

Description

Aug. 9, 1949. G. SCHMlDT 2,478,341

1 CAT HQDE FOR ELECTRIC DISCHARGE TUBES Original Filed March. 22, 1943 ELEc'TRoN EMITTING COAT ING ALLOY CONSISTING OF MOLYBDENUM NICKEL AND lRON GERRIT 504M10 INVENTOR AGENT Patented Aug. 9, 1949 1 UNITED STATES PATENTv OFFICE CATHODE FOR ELECTRIC DISCHARGE TUBES Original application March 22, 1943, Serial No. 480,099. Divided and this application July 17,

1947, Serial No. 761,539. In the Netherlands June 5, 1940 Section 1, Public Law 690, August 8, 1946 Patent expires June 5, 1960 3 Claims. (01. 25027.5)

1 This invention relates to cathodes for electric discharge tubes, particularly to alloys for cathodes heated either directly or indirectly. Thi application is a division of my copending United Now, applicant has carried out a number of investigations on this subject and has found, that very special materials are fit to be used for cathodes, with which the dissipation of heat at the States application No. 480,099, filed March 22, 5 extremities has to be very small. According to 1943,now Patent No. 2,428,042. the invention, for this purpose a cathode to be One of the most important problems which heated directly, or a cathode sleeve to be heated arise in the construction of cathodes for elecindirectly, and having a fastening or current suptric discharge tubes is the loss of heat to the supply tab, consists of an allo of tungsten or moports at the extremities of the cathode. Due to lybdenum on the one hand, and nickel, cobalt or this heat loss, the temperature of the cathode at iron on the other hand with, if desired, the addithe extremities is always materially lower than tion of one or more other components, the perthat in the center. If the cathode temperature is centage of tungsten 0r molybdenum atoms being plotted graphically as a function of length, a at least 10 if alloys consisting of two components curve is obtained which shows a maximum in the are concerned, such as tungsten or molybdenum center and which gradually decreases to a higher with iron, nickel or cobalt, and at least 5 if use or lower extent at the extremities. Thus, the cenis made of alloys comprising more than two comter receives an excessively high temperature, or ponents. The upper limit to be maintained in if, on the other hand, the center is maintained at both cases is for tungsten 25 per cent of atoms approximately the correct temperature, the ends and for molybdenum 35 per cent of atoms. remain below emission temperature. The component to be added to these alloys pref- It has already been suggested to reduce the diferably consists of one or more of the elements ficulties involved by the use of particular conaluminum, silicon, manganese, magnesium, zirstructions. Thus it is known to surround the exconium or thorium. As has been found by applitremities of an incandescent cathode to be heated cant. an alloy which is very suitable for the purdirectly by an independent heating coil, in order pose of the invention consists of 16 per cent of to compensate for the loss of heat due to dlssitungsten atoms and 84 per cent of nickel atoms pation by means of an additional supply of heat. whilst very good results can also be obtained with Such a measure has been used With cathodes to a thr eecomponent alloy consisting of 9 per cent be heated directly by arranging for the extremiof aluminium'atoms, 13 per cent of molybdenum ties to comprise per centimeter cathode length atoms and 78 p o nickel atoms, or With more wire material of this heating element than an alloy composed of 16 per cent of iron atoms, 14 in the center. Consequently, the temperature at P c of y um atoms and P c t of the surface of the cathode body is rendered more nickel at since the ys a rd n t th inuniform. These constructions, however, have the vention may contain not only tungsten or molybdisadvantage that the supply of energy from the denum and one of the components iron, nickel or incandescent body of the cathode has been rencobalt, but also two of the last mentioned eledered unequal and, in addition, in the case of ments. cathodes to be heated indirectly, the undesirable As has been set out above, the conduction of condition arises that additional energy is sup- 40 heat of the material which enter into considerae plied especially to the extremities which have retion is the most important factor; in connection mained uncovered with emissive material and do therewith attention should also be paid to the not assist in the emission, thus materially inwall thickness of the body made from this matecreasing the losses at those points. rial, since with the use of the same material the Furthermore, it has been suggested that the conduction of heat may be reduced by choosing cathode body of a cathode to be heated indirectly thinner material. The thickness is limited by the should be made from two layers, 1. e. from an unrigidity or hardness of the material while these derlayer of a material having a small heat conproperties in turn are limited by the more or less ductive capacity and an upper layer of a material good workability. Especially the latter property having a low heat emissive capacity, which layer makes it impossible to utilize alloys in which the does not cover the extremities of the under layer, to which the support for the cathode is fastened. For the under layer use is made in this case of an iron-nickel alloy which may have chromium added to it, if desired; for the upper layer it is suggested to utilize copper,

percentage of atoms for tungsten or molybdenum are higherthan 25 and 35 respectively.

A very important advantage of the alloys according to the inventionis that, in addition to being properly workable, they exhibit great hardness so that an improvement is obtained not only due to the conduction. a: heat itself; being smaller than that of the materials so far used, but also because these materials, due to their great hard ness which appears to be greatly heatproof under the influence of the high melting tungsten or molybdenum, may be thinner than the known materials when used for thesame purpose. Also with different alloys according to the invention this means an important advantage over the alloys stated above as known, viz. chrome-nickel and nickel-iron. Thus ituapp'eara, for example, that the hardness of a tungsten-nickel alloy of which the percentage of tungsten atoms is 17 is heatproof and is approximately twice as great as that of an iron-nickel or chromenickel alloy, Whilst the heat conductive capacity is of the same order of magnitude. Due to the materially greater heat-proof hardness, connected with good workability, this material may, according to the invention, be chosen considerably thinner than the nickel-iron or chrome-nickel, resulting moreover in an improvement of the heat conductive capacity. With an alloy containing '7 per cent of aluminium atoms, 16 per cent of tungsten atoms and 77 per cent of nickel atoms the hardness is almost thrice as great as that of an iron-nickel alloy of equal atom percentages whilst, in addition, the heat conductive capacity of the material itself is already smaller than that of ironnickel and chrome-nickel. During working with this aluminium tungsten nickel alloy it appears, however, that for definite purposes the limit of good workability is almost attained. For the alloys aluminium-Inolybdenum-nickel and ironmolybdenum-nickel which are mentioned above, the hardness is approximately one and a half times as great as that of iron-n1cke1 or chromenickel alloys whilst the heat conductive capacity is smaller. With regard to the iro'n nickel and chrome-nickel alloys here'mentioned, the materials according to the present invention have in addition the advantage that they give otff less gas than the known alloys, such as iron-nickel and chrome-nickel.

The fact that literature does refer to the lastmentioned alloys and does not mention those according to the present invention, though tungsten and molybdenum themselves are used for cathodes to be heated directly and for heating elements of cathodes to be heated indirectlyras in all probability due to the circumstance that on the one hand the opinion prevailed that these alloys, due to their great hardness, would be Workable with too great difficulty, and on the other hand it had been expected that the conduction of heat would be comparatively great since the heat conductive capacity of tungsten is at least ten times as great as that of iron-nickel and chromenickelwhilst the hardness oi tungsten is about 20 to 30 times greater than that of these known alloys. Consequently, it is evident that one will not rapidly decide to substitute these known al loys for alloys containing tungsten or molybdenum, if materials are wanted which exhibit good workability and have at the same time a low heat conductive capacity.

Now by means of the investigations made by applicant it has been found that the heat conductive capacity can be rendered very small and that, on the other hand, these materials are properly workable especially because they can be used as bodies of very small thickness. The work ability particularly plays a part in the manufacture of cathode bodies for cathodes to be heated indirectly which may be obtained from the material according to the present invention, for example by rolling. Thus, it is possible for indirectly heated cathodes to be advantageously made from a sleeve which is made from an alloy according to the present invention and covered with a layer of a material, for example nickel or copper, having a smaller capacity of radiation than the. underlying body. The outer layer preferably does not extend the full length of the sleeve. The manufacture of such a cathode may be efiected in a simple manner by using two thin plates whichare laid on one another and subsequently rolled together until a tubular cathode body is obtained.

The combination of rigidity and small heat conductive capacity renders the above described materials also highly adapted to be used for bodies which serve at the same time as fastening and current supply members for a cathode to be heated indirectly. Most generally also the construction of this cathode is simplified by utilizing the materials according to the present invention, since under certain conditions the centering of the cathode may be effected without special means by introducing the cathode body itself through an aperture in a plate consisting of mica or similar material and supporting the cathode body in this mica without excessive loss of heat at the contact surface between the cathode and the mica. Also other modes of fastening are possible with the use of the cathode according to the invention.

What I claim is: t

1. A thermionic cathode body comprising a heater element, a tubular body surrounding said element, said tubular body consisting of an alloy of molybdenum, nickel and iron, the molybdenum constituting from 5% to 35% of the atoms of the total alloy and the nickel constituting the major part of the balance, anda coating of alkaline earth metal oxides on and in contact with said body.

2. A thermionic cathode body comprising a heater element, a tubular body surrounding said element, and an electron emis'sive oxide forming a coatin on said tubular body, said tubular body consisting of an alloy of molybdenum, nickel, and iron, the molybdenum constituting from 5 to 35 per cent of the atoms ot the total alloy, the nickel constituting substantially 81 per cent of the atoms of the balance, and the iron constituting substantially 19 per cent of the atoms of the balance.

3. A thermionic cathode body comprising a heater element, a tubular body surrounding said element, and an electron emissive oxide forming a coating on said tubular body, said tubular body consisting of an alloy of molybdenum, nickel, and iron, th molybdenum constituting substantially 14 per cent of the atoms of the total alloy, the nickel constituting substantially 70 per cent of the atoms of the total allo'y, and the iron constituting substantially 16 per cent of the atoms of the total alloy. V

GERRIT SCHMIDT.

REFERENCES CITED The following references are of record in the file of this patent:

OTHER REFERENCES Number Name Date 1,963,844 Holladay June 19, 1934

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