Unite States Meat [19] 1111 3,922,428 Cronin ' [45] Nov

Unite States Meat [19] 1111 3,922,428 Cronin ' [45] Nov. 25, 1975

[54] THERMIONIC CATHODE COMPRISING 1,921,066 8/1933 Bedford ...... 252/521

MIXTURE OF BARIUM OXIDE, CALCIUM 2,744,073 5/1956 Todd et a1...... 1 1 . . . .. 252/521 OXIDE AND SAMARIUM OXIDE 2,813,8073,076,916 11/19572/1963 LeviK0ppius...... 117/2241,17/224 [75] Inventor: Leo J. Cronin, Watsonville, Calif. 3,118,080 H1964 Koppiusnu 117/223 _ _ 3,155,864 11/1964 Coppola...... 117/224 [73] Asslgneer Spectra-Mat, Inc” Watsonv?le, 3,719,856 3/1973 Koppius...... 313/346 R Calif. I [22] Filed: Feb‘ 4’ 1972 Primary Examiner-Charles E. Van Horn [21] APPL NO; 223,539 Assistant Examiner—Michael W. Ball Attorney, Agent, or Firm—Limbach, Limbach & Sutton [52] US. Cl ...... 428/312; 313/311; 313/346 R; 252/521; 428/472 [51] Int. Cl ...... H01j l/l4;1-101j 12/06 [58] 'Field of Search ...... 117/224, 223; 252/521; [57] ABSTRACT 313/346 R’ 311 A cathode is disclosed utilizing an emission material of barium oxide, calcium oxide and samarium oxide. [56] References Cited . UNITED STATES PATENTS 8 Claims, 4 Drawing Figures 1,794,298 2/1931 Just ...... 117/220 US. Patent Nov. 25, 1975 3,922,2

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I000 2000 5000 LIFE (H0003) 3,922,428 1 2 ium oxide. The Just patent states that amount to which THERMIONIC CATI-IODE COMPRISING MIXTURE the rare earth oxide should be present for optimum re OF BARIUM OXIDE, CALCIUM OXIDE AND sults is determined to be approximately 0.1% of the SAMARIUM OXIDE amount of alkaline earth oxide or similar ingredient. The present invention is directed in general to cath 5 Broadly stated, the present invention is directed to a odes and. more particularly, to thermionic dispenser cathode having a metallic body and an emission mate cathodes. . rial comprising a mixture of barium oxide, calcium Over the years various different cathode structures oxide and samarium oxide and wherein the samarium have been proposed with selected different materials to oxide is present in‘ more than trace amounts. achieve high current densities, low evaporation and It has been‘found that cathodes having the preferred long life. Cathodeswhave been made in many forms. In compositions of the present invention produce en one form, the emission material is sprayed or painted hanced emission over cathodes similarly constructed on the surface of a support member, typically nickel, but having the most popular prior art emission material tungsten or molybdenum. Ribbons, wires or screens made up of barium oxide, calcium oxide and aluminum have been added to the surface to aid in holding the oxide. The cathode of the present invention can pro emission material on the surface. Additionally, porous duce the same emission as the most popular prior art body structures have been utilized to slowly dispense cathode but up to about 50°-75°C cooler. This permits the emission material through the pores to the emission a lower heater wire temperature and faster tube warm surface. U.S. Pat. No. 2,543,728 to Lemmens et al. de up time to achieve emission and requires less heater scribes the use of a cavity to contain the emission mate power. rial and a porous body portion providing communica It has also been discovered that the cathode of the tion between the cavity and the electron emission sur present invention produces up to 10 times less barium face for migration of the emission material. U.S. Pat. evaporation. No. 2,700,000 to Levi et a1. describes a cathode Additionally, the cathode of the present invention wherein the emission material is melted into the pores ‘can be activated at a temperature approximately 100°C of the porous-sintered body. lower than the activation temperature of the popular Barium oxide (BaO) has been the principal preferred prior art cathodes. emission material for many cathodes. However, barium Other objects and advantages of this invention will oxide is extremely hygroscopic and in normal atmo~ become apparent when reading the following descrip sphere readily converts to barium hydroxide. Conse 30 tion and referring to the accompanying drawing in quently, many cathodes include barium oxide in a com which similar characters of reference represent corre position with another material whereby the barium sponding parts in each of the several views. oxide and/or free barium can be released for desired In the drawings: electron emission. Barium carbonate has frequently FIG. 1 is a side sectional view of a cathode in‘accor been used as one composition wherein the carbonate 35 dance with one embodiment of the present invention.‘ will convert to barium oxide at elevated temperatures. FIG. 2 is a graph of emission in milliamps plotted ver~ In thermionic dispenser cathodes operating at higher sus life for a cathode in accordance with the present in temperatures and utilizing refractory metal bodies such vention as contrasted with two cathodes in accordance as tungsten, molybdenum and the like as the porous with the prior art. sintered main body portion of the cathode, barium car 40 ' FIG. 3 is a graph of current in milliamps plotted ver bonate reacts with tungsten in a manner unproductive sus cathode temperature for two cathodes in accor of free barium. Consequently, a number of alkaline dance with the present invention as contrasted with two earth metal compositions are suggested wherein bar cathodes in accordance with the prior art. ium oxide will be held in a stable form. Materials sug FIG. 4 is a graph of temperature plotted versus life gested in U.S. Pat. No. 2,700,118 to Hughes et al. are 45 for a cathode in accordance with the present invention alkaline earth metal silicates, aluminates, thorates, as contrasted with a cathode in accordance with the berylliates, and borates. Of these materials, the barium prior art. ' aluminates were preferred and have been used most ex Referring now to FIG. 1 there is shown one embodi tensively. ment of the present invention. In this embodiment of Since it was known, as in “Therminoic Emission from 50 the present invention the cathode 10 includes a porous the BaO-CaO System ” by L. E. Grey, NATURE, Vol metallic body 1 l, of a refractory material, such as tung ume 165, pp. 773-774, May 13, 1950, that enhanced sten, provided with an emission material 12 substan emission can be achieved by the substitution of calcium tially uniformly dispersed throughout the pores of the oxide for part of the barium oxide, barium calcium alu body for migration through the pores to an emission minate dispenser cathodes have been used. U.S. Pat. 55 surface 13. The emission material 12 can be placed in No. 3,076,916 to Koppius describes one such dispenser the pores by mixing, pressing and sintering material 12 cathode having a porous-sintered tungsten body with with metal particles such as tungsten particles when the an impregnate material formed of barium oxide, cal body is of tungsten. Alternately, the cathode 10 can be cium oxide and aluminum oxide in the mole ratio of formed by ?rst forming the porous sintered body 11 by 4:1:1, respectively. U.S. Pat No. 3,201,639 describes a 60 pressing and sintering tungsten particles and then melt similar thermionic dispenser cathode wherein the im~ ing the emission material into the pores of the body. An pregnate material is barium oxide, calcium oxide and example of such a cathode is described below._ aluminum oxide in the mole ratio of 5:312, respectively. The body 11 is supported for use in a vacuum tube by U.S. Pat. 1,794,298 to Just describes a thermionic means of a support cylinder 14, such as molybdenum. cathode using a mixture of an oxide of the alkaline 65 A heater ?lament 15 such as a metal wire wound in a earth group of metals with a very small addition of one spiral is positioned within the cylinder 14 behind the of the oxides of the rare earth group of metals espe body 11 for heating the body 11 and emission material cially thorium oxide, praseodymium oxide and samar 12 for thermionic emission of electrons from the emis 3,922,428 3 4 sion surface 13 when the cathode is positioned within a While it is believed that the invention has been de vacuum envelope (not shown). Cathodes in accor scribed thus far in sufficient detail to enable one skilled dance with this invention are suitable for use in most in the art to manufacture cathodes in accordance with vacuum tubes or gas tubes. The cathode can be used as the present invention, a detailed operative example ofa a “cold cathode" wherein the heater 14 is utilized only 5 cathode of the type illustrated in FIG. 1 will be given. to initiate emission or dispensed with entirely and emis First of all, the body 11 is formed by pressing tung sion derived in other ways such as by electron bom sten powder, approximately 5 microns average diame bardment to produce “secondary” emission. ter, to form an ingot which is heated in hydrogen at The heater ?lament 15 is placed in location within about 2350°C for approximately 20 minutes to form a the cylinder 14 and “potted” in placed with an electri porous-sintered body having a density in the range of cal insulating material 16 such as alumina ceramic. 78—84%. Next, the porous body is impregnated with a The emission material 12 is a mixture of barium ox filler material such as a metal or plastic and the filled ide, samarium oxide and another alkaline earth metal ingot machined to form individual buttons or bodies oxide, other than barium oxide, preferably calcium ox having the dimensions of the desired cathodes. In the ide. I prefer as an emission material one comprising case of metal, such as copper, the filler material can be barium oxide, calcium oxide and samarium oxide, Spe melted into the pores, while in the case ofa plastic such ci?c compositions of the emission material in accor as methyl methacrylate, the porous body can be im dance with the present invention are set forth in greater pregnated with the liquid plastic which then sets up and detail below and with reference to the method of for hardens in the pores. After machining, the filler mate mation of these materials. rial is removed by heating to elevated temperature. However, the amount of samarium oxide must be For the emobidment having an emission material of greater than trace amounts. For a cathode in accor barium oxide, calcium oxide and samarium oxide in the dance with the present invention the mol percent of sa mole ratio of 4:1:1, 4 moles barium carbonate, 1 mole marium oxide should not be less than about 12% and calcium carbonate and 1 mole samarium oxide are the samarium oxide should not be less than about 25% heated to a temperature of about 1 100°C in an air at by weight. The preferred range for the samarium is 16 mosphere. At this elevated temperature, the carbon to 20 mol percent or 34 to 43% by weight. ates convert to the respective oxides and these oxides I have discovered that a cathode made generally in form compounds. The resultant material is allowed to accordance with the present invention produces cool. greater emission and has less barium evaporation than 30 For impregnating the emission material into the cath~ the conventional barium oxide, calcium oxide, and alu ode body the cooled, hardened mass is pulverized to minum oxide cathode. Also, an identical cathode body form a powder and heated in a hydrogen atmosphere to impregnated with an emission material of barium ox a temperature of about l550°C. and brought to contact ide, calcium oxide and samarium oxide produces satis with the previously machined cathode body whereby factory emission at a lower temperature than the popu 35 the emission material will migrate into the pores of the lar barium oxide, calcium oxide and aluminum oxide body by capillary attraction. Excess emission material type cathodes. is removed from the emission surface. ' I have discovered that cathodes with emission materi The heater may be “potted" (before impregnation) als formed of 4 moles of barium oxide, l mole of cal within the cylindrical support behind the cathode body cium oxide and 1 mole of samarium oxide as well as 5 by surrounding the heater with powdered alumina ce moles of barium oxide, 3 moles of calcium oxide and 2 ramic and heating the assembly to a temperature of moles of samarium oxide produce greater emission and about l900°C. to sinter the alumina. less barium evaporation than similar cathodes with The completed cathode can be mounted and ac emission materials of barium oxide, calcium oxide and tiviated in an electron discharge device by heating the aluminum oxide in these same mole ratios. 45 cathode to 1100°C. while exhausting the device. This FIGS. 2 and 3 show graphs of emission versus life and activation temperature is at least 50—100°C. below the emission versus operating temperature for cathodes in activation temperature required for the popular barium accordance with the present invention designated “S” oxide, calcium oxide and aluminum oxide (513:2 mole as contrasted with the more popular barium oxide, cal ratio) cathodes. cium oxide and aluminum oxide cathodes designated While calcium oxide is preferred as the other alkaline type “B”. The subscript “1” (i.e., B-1 and S-l) refer to earth metal to be utilized with barium oxide and samar cathodes where the respective materials are in the mole ium oxide, other alkaline earth metal oxides such as ratio of 4:1:1, and the subscript “2” (i.e., B-2 and 8-2) strontium oxide and magnesium oxide can be used. refer to cathodes where the respective materials are in The emission material can be applied to other sur the mole ratio of 5:3:2. 55 faces such as by evaporation onto a metal surface from FIG. 4 shows a graph of the apparent cathode tem which emission is to be drawn. perature measured with an optical pyrometer through Additionally the cathode can be made in the form of the glass of a test diode versus cathode life. A decrease a directly heated cathode body as described in US. Pat. in apparent temperature is a measure of the amount of No. 3,514,66l to R. T. Reaves assigned to the assignee barium evaporated from the cathode onto the glass 60 of this invention. bulb. FIG. 4 shows a lower evaporation rate for cath What is claimed is: odes of the present invention contrasted with popular 1. A cathode comprising: - prior art cathodes. In FIG. 4 curves are shown only for a metallic body structure of a high melting point B-1 and S-2; a B-2 cathode has 3 or 4 times the evapo metal and ration rate of a B-1 and the curve therefore would start 65 an emission material supported by said body struc higher at the left and decrease with a greater slope to ture, the right; and 5-] has a slightly lower evaporation rate said emission material comprising a mixture of bar than 5-2. ium oxide, calcium oxide and samarium oxide with 3,922,428 5 6 the amount of samarium oxide present in the mix 5. The dispenser cathode in accordance with claim 3 ture in excess of about 12 mol percent. wherein said barium oxide. calcium oxide and samar 2. A dispenser cathode comprising: ium oxide are in the mole ratio of about 513:2 respec a refractory metal body having an tively. electron emission surface and 5 6. A dispenser cathode comprising: a porous portion for migration of an emission mate a porous high-density tungsten body with intercon rial to said surface and necting pores communicating with an electron emission material supported by and in contact with emission surface and an emission material located within said pores for mi said body for migration through said porous por~ H) tion to said surface. gration through said pores to said surface. said emission material comprising a mixture of bar said emission material comprising a mixture of bar ium oxide, samarium oxide and a second alkaline ium oxide. samarium oxide and a second alkaline earth metal oxide other than barium oxide with earth metal oxide other than barium oxide with the amount of samarium oxide present in the the amount of samarium oxide present in the mixture in excess of about l2 mol percent. mixture in excess of about 12 mol percent. 7. The dispenser cathode in accordance with claim 6 3. The dispenser cathode in accordance with claim 2 wherein said second alkaline earth metal oxide is cal wherein said second alkaline earth metal oxide is cal cium oxide. cium oxide. 8. The dispenser cathode in accordance with claim 7 4. The dispenser cathode in accordance with claim 3 20 wherein said barium oxide. calcium oxide and samar wherein said barium oxide, calcium oxide and samar ium oxide are in the mole ratio of about 4:111, respec ium oxide are in the mole ratio of about 411:1, respec tively. tively.