US1830825A - Cathode - Google Patents

Description

no Drawing.

Patented Nov. 10, 1931 UNITED STATES PATENT QF HUGH s. COOPER AND MAURICE nsARBEY, or CLEVELAND, oHIo, ASSIGNORS; TO KEMET LABORATORIES COMPANY, ma, A CORPORATION or NEW YORK CATHODE oxides of the alkaline earth metals to cathode structures made of such metals as platinum or nickel, usually in filament form, for increasing the electron-emitting power to a useful value. The coatings of alkaline earthmetal oxides have customarily been applied by repeatedly coating the structure with thin layers of solutions, or suspensions, of appropriate materials in a carrying medium and then driving ofi the medium.

Our invention provides a new process for the application of the alkaline earth metal oxides to appropriate cathode structures, which may be filaments of metals such as platinum or nickel or may be structures of more refractory metals, by a simple immersion of the structure in an appropriate solution and a heating therein.

Other objects and details of our invention will be apparent from the following description, 7

In practicing our invention as a means for applying to a metal cathode a suitable coating of the mixed oxides of barium, strontium and calcium we may take an appropriate mixture of the desired oxides and agitate them with cold water, thereby obtaining a solution of the mixed hydroxides containing 1 further portions in suspension. It'may be noted that while the solubilities of the hydroxides of all three metals are relatively low, they are suflicient for our purpose. We

then bubble carbon dioxide gas through the solution to convert the mixed hydroxides to the bicarbonates, which also show a small, but sufficient, solubility in water. We find Application filed November 20, 1928." Serial No. 320,743.

it convenient toprovide a small excess of the material which is kept suspended in the solution by the agitation of the gas bubbles, in order to maintain a saturated conditionin the solution. Continuing the bubbling of gas, we then immerse within the solution they desired cathode structure, which may conveniently be a filament,'and may be formed of the desired metal such as platinum, nickel, molybdenum, tantalum or tungsten or other suitable metal. We then pass an-electric current through the cathodestructuresuificient to raise it to a high temperature while immersed. The precipitation ot-the mixed oxides upon the cathode structure is in part,

at least, produced by the heating of the solution of bicarbonates and the breaking down thereby of the bicarbonates into the normal carbonates, which are less soluble than the bicarbonates and are accordingly thrown out of the solution. In order to maintain as high a concentration of the bicarbonates as possible in the solution, we findit desirable to utilize cooling means for keeping the bulk of the solution at a 'lowtemp'erature.

The high temperature throws substantial portions of the dissolved bicarbonates of the alkali earth metals out of the solution upon the cathode structure.- These materials ap 'parently precipitate in'pa'rt, at least, in the form of normal carbonatesand are at once baked upon the wire in a homogeneously adherent form. The thlckness of the coating 'formed depends upon the length of time that the heating is continued within the solution and upon the temperature to which the wire isheatedfl l/Vh-en a sufficiently heavy coating is obtained, the heating maybe discontinued and the filament removed from the solution. As so prepared,the coating is ex tremely resistant to atmospheric effects which might tend to injure its electron-emitting properties, and the filament may be kept in this form without particular precautions for storage until it is needed for use.

The prepared filament may then be assembled with other electrode members in the usuabway for the production of anelectronic dev1ce, and mounted within a .tubesuitable for evacuation: During the evacuatlon, the

cathode is heated in the usual way to a sulficiently high temperature to break down the carbonates or hydroxides into oxides, and drive off the carbon dioxide and moisture.

A particularly advantageous feature of the process above described lies in the fact that the bicarbonat'es are precipitated by an elevation in temperature of the solution, being partially decomposed into the normal carbonates which are of lower solubilitythan the bicarbonates. This fact results in aparticularly advantageous precipitation of. the normal carbonates from the solution in the neighborhood of theheated wire, with an evolution of carbon dioxide. The precipitated materials collect upon the'h'eatedfilamerits.

The material thus thrown down is baked upon the wire in a homogeneously adherent form, which appears to be composed largely of the" normal carbonates depending upon the temperature of the filamentupon which they are precipitatedyIf the temperature is sufficiently high, they may, of course, be partially dissociated into the oxides, or be cause of theipresence of water vapor, into hydroxides. However, the major portion-ofthe deposit appears, to remain in the form of the normal carbonates.

When so conducted the process of our invention produces a cathodic member composed of the desired metal, coated with oxides of one or'more of the a kaline earth metals in 'suchform as to have an excellent electron emission.

In general, our method of heating the cathode body while immersed ina bulk solution of the materials to be used as a coating, works particularly well with all those materials which are more soluble cold than" hot, and which are therefore thrown out. of solution by the local heating of the cathode and deposited upon the cathode.

Another good example of this class of materials is calcium hydroxide. WVe find that a. solution of this material with an excess in suspension is excellently adapted to the practice of our process. In this case it is of course unnecessary to use carbon dioxide but it isdesi-rable to keep the solution cool by suitable means so that the amount of hydroxide in solution may be a maximum, and so that the suspended material may pass into solution as the dissolved material is precipitated on the cathode body. The coated cathode is subsequently heated in vacuo as previously process which is simpler, more'certain and less expensive than any previous process. The

process is furthermore not limited to the common metals previously used, as has been found to be the case with the previous coatings, but is adapted to the production of excellent electron-emitting coatings upon other metals such as tungsten, molybdenum, etc. which have not previously been coated with satisfactory electron emissive coatings.

.Vhile we have disclosed several embodiments of the process of our invention as well as the product obtained thereby, it is capable of still other modifications therefrom without departure from the spirit thereof, and it is desired therefore that only such limitations shall be imposed upon the appended claims as arestatedtherein or required. by the prior art.

Vi e claim as our invention:

1. The process of producing an electronemitting cathode which comprises preparing a solution of the bicarbonate of the alkaline earth metals, immersing a cathode structure therein and heating while therein to decompose the bicarbonate and precipitate on said cathode from the decomposedsolution less soluble compounds of the alkaline earth metals. V v v I 2. The. process of producing an electronemitting cathode structure which comprises preparing a suspensionof the mixed bicarbonates of the al za'line earthv metals in a water solution thereof,-bubbling carbon dioxide therethrough, cooling said solution, immersing therein a cathode structure and heat ing the cathode structure while therein.

3. The process of producing an electronemitting cathode structure which comprises preparing a suspension of the mixedbicarbonates of the alkaline earth metals in a water solution thereof, bubbling carbon dioxide therethrough, cooling said solution, immersing. therein a cathode structure, heating the cathode structure while therein and precipitating upon the cathode structure decomposition productsof the dissolved and suspended materials. Y

l. The process of making an electronemitting cathode. which comprises immersing a metallic conductor in an aqueous solution containing the bicarbonate of an alkalineearth metal, and passing. electric current through the conductor to heat the same and deposit'an alkaline earth metal-compound on the conductor. V

v 5. The process of making an electron-emitting cathode which comprises immersing a metallic. conductor in an aqueous solutionof an alkaline-earth metal bicarbonate, and passing electric current through the conductor to heat thesame an ddeposit solid alkaline-earth metal compound on thejconductoiz. V In testimony whereof,,we afiix our signatures. r I r HUGH' S. COOPER. p

MAURICE D. ,SARBEY.