INDUSTRIAL AND ENGINEERING CHEMISTRY
1028
Conclusion
VOL. 32, NO. 8
Literature Cited
During the progressive improvement of the performance characteristics of paper, testing technique must necessarily undergo refinement in precision and reproducibility in order to define adequately paper which will function satisfactorily under increasingly exacting service conditions. I n doing so the chemist must continue to fulfill his obligations by furnishing better and more economical means t o the engineer and purchasing agent for selecting paper insulation of satisfachory quality.
Acknowledgment Assistance in the form of data and suggestions was received from R. R. Williams, G. T. Kohman, D. A. McLean, E. J. Murphy, J. H. Ingmanson, H. G. Walker, and W. J. Kiernan.
Brit. Plastics, 6 , No. 1, 364-70 (Jan., 1935). Burton, J. O., Bur. Standards J . Research, 7, 429-39 (1931). Electrician, 117, 657-8 (Nov. 27, 1936). Ingmanson. J. H., and Vacca, G: N., ISD. ESG. CHEY.,26, 1274-5 (1934). Jahn, E. C., Paper Trade J., 107, No. 9 , 38-40 (1938). Kohman, G. T., IKD. ENG.CHEY.,31, 807 (1939). Little. J . S.,Paper Trade J , 96, No. 8, 29-32 (1933). hlclean, D. A., unpublished rept. Peek, E. L., and Finch, J. M., Paper Trade J . , 88, No. 6, 56-62 (1929). Rasch, R. H., BUT.Standards J . Research, 7 , 465-75 (lcl31). Richter, G . A., IXD. ESG.CHEM.,26, 1154-7 (1934). Walker, H . G., and Ford, L. S., Bell System Tech. J . , 12, 1-21 (1933). Weil, C., Paper Ind., 16, N o . 12, 842 (1935). Williams, R. R., and Murphy, E. J., Trans. Am. Insl. Elec. Engrs., 48,No. 4, 568-75 (1929).
Materials for Vacuum Tube Manufacture A. J. MONACK RCA Manufacturing Company, Inc., Harrison, N. J.
The requirements of metals and alloys for cathodes, anodes, and grids are presented, and a brief treatment of thermionic emission in the cases of oxide-coated, thoriatedtungsten, and tungsten filaments is given. Metals and alloys for use in glass-metal seals are enumerated. The function of getters is explained in connection with the exhaust process. Types of photoelectric tubes and their uses are discussed. Silicates, sulfides, and tungstates are treated in terms of persistence and color of luminescence when used for fluorescent screens in cathode-ray tubes. Miscellaneous materials and parts are listed, and cleaning of vacuum tube parts is discussed in terms of methods.
HE term “vacuum tube” is used to identify a large variety of devices which have in common an outer en-
T
velope of glass or metal (occasionally quartz), into which one or more electrodes are sealed, and from which the air has been mostly exhausted or has been replaced by some other gas. Mercury rectifiers, gas-filled rectifiers, high-vacuum rectifiers, amplifiers, oscillators, modulators, detectors, photoelectric tubes, cold-cathode tubes, cathode-ray tubes, iconoscopes for television pickup, vacuum thermocouples used for measuring small alternating currents, electron multipliers, ionization gages for measuring the degree of vacuum are among the numerous types that are made. “Electron tubes” is a term considered synonymous with “vacuum tubes”; and the ex-
pression “thermonic tubes” is applicable in all cases where the primary electrons are produced thermally.
Filaments and Cathodes The passage of electric currents between the electrodes of vacuum tubes produces effects which make these tubes useful in various ways; and the cathode is the heart of the tube, since electron emission from this electrode forms the current which determines the characteristics of the tube. The cathode must satisfy three requirements: The required rate of electron emission must be available; the cathode must have a satisfactory life; and the structural strength a t high temperatures must be sufficient for the cathode to retain its shape. Since the first two requirements are in opposition, careful choice of materials and design is necessary. Metals for cathodes must emit electrons easily (low work functions) ; must have low vapor pressures so that evaporation does not cause early failure; must have high melting points, low thermal conductivities to prevent rapid heat conduction away from the cathode, high tensile strength, and stiffness; and must not fail by creep. Some compensation for failure to meet these requirements completely is possible by changes of cathode shape-. g., the use of ribbon cathodes. A heated tungsten filament was one of the early commercial sources of electrons, and tungsten is still the filament material in nearly all large power tubes. The wire must be pure and uniform in properties and dimensions. A local reduction in diameter, for instance, would cause overheating and subsequent burnout at that point. The electron emission efficiency (emission current per watt of heating power) increases as the temperature increases, but the evaporation rate of tungsten sets an upper limit. I n order to obtain greater emission efficiency without exceeding the safe operating temperature of the filament, tungsten wire to which has been added 1 to 2 per cent of thoria is often used. The emission efficiency a t the
AUGUST, 1940
INDUSTRIAL AND ENGINEERING CHEMISTRY
operating temperature is increased about tenfold by this method. These filaments may be heated in an atmosphere of hydrocarbons, such as acetylene, benzene, etc., in order to form a surface layer of tungsten carbide. Activation, by heating briefly a t 2300-2500" C., results in the production of some thorium by reaction between thorium oxide and tungsten; and subsequent "aging" a t 1750-2000" C. permits the
1029
amount of the active metals is produced continuously from the oxides by electrolysis resulting from the flow of electron current (1). With tubes having cathodes heated by alternating current, it is usually necessary to separate the cathode circuit from the heating circuit, which is done by inserting a heater wire into a tubular cathode, usually of nickel, coated with barium and strontium carbonates. An insulator of alumina, beryllia, or magnesia separates the heater from the cathode. I n this arrangement the cathode temperature is, as before, 700-900" C., but in order to attain this temperature, the heater must reach 1000-1400" C. Tungsten is one of the most used heater materials. Requirements for the purity of cathode metal, ceramic insulation, heater wire, and the carbonates are specially strict. A cathode is readily "poisoned" by numerous impurities. The resultant poor emission does not become apparent until the tube is practically completed. Oil and grease must be removed from all parts used in a tube, gases must be removed as completely as possible, and the tube parts must not be permitted to stand for any great length of time exposed to the atmosphere. I n addition to maintaining the purity requirements, it is essential that the physical and chemical characteristics of the carbonates be controlled with extreme care.
The Anode The anode (plate) is the second most important electrode in a vacuum tube, since the electrons emitted from the cathode must be collected by the anode in order to furnish output current. To keep the dimensions of the tube as small as possible, the thermal loading of the anode must be as high as possible. This stipulation places definite restrictions on the
GLASS-TYPE VACUUMTUBE
thorium to diffuse to the cathode surface (4),the active surface layer apparently being about one atom thick. Under operating conditions (1500-1700" C.) a small amount of metallic thorium is always diffusing toward the surface. The layer of tungsten carbide, having larger grain size than the tungsten, prevents too rapid diffusion and reduces thorium evaporation from the surface. Thoriated tungsten filaments need not be carbonized and are sometimes used without that treatment. The use of very high potentials, as in power tubes, results in bombardment of the filament by gaseous ions. These ions remove thorium from the cathode surface a t a relatively rapid rate, and for this reason pure tungsten filaments are generally used in high power tubes, despite the higher cathode temperature required and the poorer efficiency obtained. I n many cases i t is necessary to obtain good emission a t relatively low temperatures, as in the case of receiving-type radio tubes and similar types. Unfortunately, the best emitters have melting points which are too low for use even in such tubes. Cesium is an excellent emitter, as are barium and strontium; but the melting points are too low. However, i t has been found that if such metals are applied to the surface of other metals, the evaporation is not so great as when the second metal is not present. Cesium may be deposited upon silver or tungsten, and barium upon nickel, platinum, or their alloys. A mixture of barium and strontium carbonates is usually applied to a wire of nickel or nickel alloyed with silicon, cobalt, iron, or titanium, or to a wire of platinum alloys containing nickel, cobalt, rhodium, or iridium. At the operating temperature (700-900" C.) some metallic barium and strontium are produced ( 3 ) ,and these metals, in a layer pro& ably about one atom thick, act as the emitter. A small
STRUCTURE OF A METAL RADIOTUBE 1. Metal envelope
2. 3. 4. 5. 6.
7. 8. 9, 10. 11. 12. 13.
14. 1% 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 20.
27.
Spacer shield Insulating spacer Mount support Control grid Coated cathode Screen Heater Suppressor Plate Batalum getter Conical stem shield Header Glass seal Header insert Glass-button stem seal Cylindrical base shield Header skirt Lead wire Crimped lock Phenolic base Exhaust tube Base pin Exhaust tip Aligning key Solder Aligning plug
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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serious, and in such cases molybdenum and tantalum are used. The radiation may be increased by blasting with silicon carbide or by coating with finely powdered metal, such as tungsten. Anodes of graphite are used extensively, but they generally require special treatment to decrease the large gas content. When the heat generated becomes too great to be dissipated by radiation alone, water-cooled anodes are necessary. Such anodesIare part of the envelope, and must not only be ductile enough to be drawn into shape but must give a vacuumtight seal t o glass. Copper and Fernico meet both requirements.
important, since soft glasses do not match the thermal expansion as closely as could be desired. For hard glasses the sealing materials are tungsten, molybdenum, and an iron-nickel-cobalt alloy called “Kovar” or “Fernico” ( 7 ) which consists of approximately 54 per cent iron, 28 nickel, and 18 cobalt. Special borosilicate glasses are used with these metals, and while the thermal expansions in no case match exactly, good seals can be made. The expansion curve of Fernico is unusual in that it has an inflection point which coincides roughly with the inflection point of the glass used.
Grids
Getters
I n a three-electrode tube (triode) the grid is useful because jmall changes in grid voltage cause relatively large variations in plate current. This action is the basis of the amplification process in a vacuum tube. Additional grids are used to modify the electrical characteristics of a tube. The shape and position of-the grid relative to the other electrodes in the tube is extremely important. Consequently, there are definite requirements to be met. Material for grids should, wherever possible, have a relatively low thermal expansion in order that serious buckling a t elevated temperatures may be avoided. The drop in elastic limit at 1000” C. should be small enough so that the grid is not permanently deformed by the magnetic or thermal forces set up by high-frequency inductive heating during exhaust (8). The cold ductility must be sufficient for proper forming of the grids. The grid material must be one with a high “work function”; that is, it should be a poor emitter of primary and secondary electrons. This requirement may be made less severe by plating or coating the grid with metals or other substances that decrease electron emission. Molybdenum has most of the necessary properties but is rather expensive. When cost is a consideration it is often practicable, by changes in design and construction, to use other metals, such as alloys of chromium, iron, or manganese with nickel, or nickel alone.
Glass vacuum tubes are exhausted by the use of mechanical and diffusion (oil or mercury) pumps or by mechanical pumps alone. During the exhaust process the entire tube is heated to a temperature considerably above the operating temperature of the tube in order to outgas the metal and glass. The metal parts in the tube are further heated by high-frequency induction. During the exhausting of metal tubes the shells are usually heated to bright red heat by gas flames in order to outgas the metal shell and internal metal parts. To supplement these processes, it is customary to reduce the gas to an even lower level by the use of “getters”, which are small quantities of metals vaporized onto the surface of the envelope by means of heat a t the end of the exhaust cycle. The complete explanation of getter action is not known, but gas pickup by chemical combination, absorption, and adsorption seems a reasonable, even if only a partial answer. Magnesium and barium (or mixtures of the two) are most commonly used. Calcium, strontium, phosphorus, aluminum, arsenic, iodine, cerium, zirconium, and other materials have been tried with varying degrees of success. The high vapor pressures of the alkalies preclude their use, since a vapor pressure greater mm. a t 100” to 200” C. would interfere with than 10-6 to tube operation. d barium beryllate (“Batalum”) getter is finding increasing use (6).
Glass and Glass-Metal Seals
Unless a gas serves a definite function in a vacuum tube, i t must be kept to a minimum. The vacuum may be replaced by a gaseous atmosphere only when i t is desired to impart certain characteristics to the tube. The gases generally used are hydrogen, nitrogen, argon, helium, and neon. Mercury vapor forms the atmosphere in mercury rectifiers. Acetylene, benzene vapor, and naphthalene vapor are admitted for the purpose of carbonizing filaments of thoriated tungsten, but must be removed from the tube after they have accomplished their purpose. Illuminating gas or other hydrocarbons may be used to give a fine carbon deposit on grids or other tube parts, but these operations are done on the parts before assembly.
The envelope of vacuum tubes is either glass or metal I fused quartz in rare cases). The shell of metal tubes is nearly always cold-rolled steel, but since glass is used to make the vacuum-tight seal, the stem or header must be made of a metal which matches reasonably well the thermal expansion of the glass. Glasses for vacuum tubes are either soft, such as ordinary lead and lime glasses, or hard, such as certain borosilicate glasses. The electrical resistance of the glass must be high in every case where a possible electric circuit can be completed between tube elements through the glass. The problem of making electrical connection between the internal elements of a vacuum tube and the external wires and studs is often extremely troublesome because of the difference in thermal expansion between glass and metal. I n the early days platinum was widely used as the sealing metal; but this process was costly, and one not entirely free of difficulties. I n present practice with soft glass the most common sealing metal is a composite lead called “Dumet” ( 2 ) . I n manufacturing this material, a core of nickel-iron is sheathed with copper, and the two are brazed together and then drawn through a die to the desired diameter. The thermal expansion of such a combination does not exactly match that of lead glass, but the copper sheath flows sufficiently under stress to prevent cracking of the glass. Another type of alloy is finding increasing use with soft glass-namely, chromium-iron alloy which contains 26-30 per cent chromium. Proper design is
Gases
Photoelectric Materials When a photoelectric tube is to be used with visible light, the choice of materials is closely limited, since sodium, potassium, rubidium, cesium, lithium, strontium, and barium are the only metals sensitive to such light. The first four can be sublimed with glass vessels; the last three cannot. Photoelectric emission is a surface and not a bulk effect; and consequently, modern photoelectric tubes employ a thin alkali metal coating on a suitable metal base rather than the older type mass-layer cathode. Thin-film cathodes generally have peak responses nearer the red end of the spectrum than the mass-layer cathodes, and their maximum sensitivity more nearly approaches the peak of the energy emission curve of a tungsten filament in a lamp.
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INDUSTRIA I , .A 1;1) I+;N(iI U1313131NG CHII,MISTRY
Among thin-film types of tubes are those having cathodes consisting of a film of cesium on oxidized dver, a film of rubidium on oxidized silver, and a film of potassium on oxidized silver or oxidized copper. The commonest type used today consists of a layer of cesium (probably some cesium oxide in addition) deposited on a layer of oxidized silver. The cathode, about 0,010inch thick, is either pure silver or silvercoated metal. Because of its constancy and the linear relation between photoelectric current and incident radiation, the highvacuum type of photoelectric tube is used where reproducibility of measurements is important. Where detection of light only is the cKief requirement, tho gas-filled tube (usually with helium or argon) is preferable because of its higher emission current per unit of light. Television cameras employ a mosaic made up oi a large number of photoelectric s w t s on a suitalA! support material. Fluorescent Screens
Pluorescent materials are used in cathode-ray tubes-+. g., television receivers- to convert the electrical energy of the elcctrm beam into light. Three classes of materials
VOI.. 32, NO. 8
yellow zinwcathnium sulfide (or a yellow zinc-beryllium silicate) are used in producing white pictures on television screens. The stability of the sulfides is not so high as that of the silicates, and the iorrner are very susceptible to changes of characteristics resulting from minute traces of heavy metal irnpuritics. TUIYGSTATES. The tungstates are nonactivated fluorescent. materials. Their colors vary irom a deep blue for calcium tungstate to a light blue-white for cadmium zinc or magnesium tungstates. Tlicir very short persistence makes screens of these materials useful in the photography of high-speed transients. Ceramics
Ceramics serve two purposes in vacuum tubes-(a) electrical insulation and ( b ) support and spacing of the tube eleotrades. Electrical insulation is necessary both inside and outside of the tube (separation of lead wires in the base). Ceramics for use within tube must have good strength, high purity (especially freedom from alkalies), good resistance to. thermal shock, good refractoriness, and satisfactory electrical resistance, iiiclectric constant, arid phase &e; ceramics used
have low p o r ~ s i and sa1,isSi:ctory clcctrical Extcrirnl ceramics it i.sually steatite iioilies, and itities of t i m e ciiniprrsi high temjmature, high voltage, inbination, insulators , or mixtures of them with stcatitc mrist~be c n Fused quarta is surnetiiiics iiscd. iriir-t
Other Materials 111 ailditiun to the ririlt,erials wliich furiii the euvelope a i d t h e wbich actiially go into the tube, inany otlier materials
i t r ~o m 1
in im:parat.ion of parts, cleaning, etc. Some, while cxtcrnally, :rctually form part of the completed tobe. are nii~iieoi plrenolic plastics with plated brass studs or of iiietal with ceramic bottom disks to carry the studs. The leads from the internal members of the tube are soldered "-type bases find occasiona.1 use. I n innst be cemented to t,hc tube, and a typicdl c::ment coli~ailisrosin, marble dust, shellac, mahcllite green, alcohol, plicmilic resins, and paraffin. This cement is cured by heat. Large quantities of nickel and copper wire enter into the assembly of vacuum tubes. Sickel wire, i n addition to its wes for filaments arid grids, is a conimon support material for carrying tube elements, getters, and iitlrer parts. Kickel sheet is rised for platcs, getter tabs, and shields. Kickel mesh is employed for cathodes in mercury rectifiers and shields, and as a cushioning material where metal collars aurr(iund glass parts. ?Gekeel tubing is the usual cathode in inilirectly klWdted types Of tUbCs. Mica is a common separation arid support, matel.ia1: it is used in nearly all small and lower-power tulies i n place of iisml
ceramic insulators and spacers. Good grades of muscovite (n,llite or India mica) are required. Alloys find extensive application. Axnurig them are coppersili con-manganese, ni ckel-manganese, II i ck el - e11 r om i u ID, nickel-co~alt-irontitanium,iron-nickel-cob a1 t -e h r o m i u m, nickel-copper, copper-nickel-zinc, and others, most of them made and marketed under trade names. Many other materids are employed in iiroccssing part.s, etcling, clcctroplating, elcctropolishing, and other operations. Among them are solders and soldering materials, such as silver solder, tin-lead solder, borax and other fluxes: salts of cesium, aluminum, barium, strontium, nickel, sodium, manganese, cobalt, uranium; acids, such as nitric, hydrocliluric, aulfnric, chromic; shellac, lacquers; suspension media, such as amyl acetate; ceramic colors, sodium dicatc; liquid air fur exhaust pumps: bronze powder for decorative purposes on large tubes; paper for packing.
Cleaning There is probably no inore important operation in vacuinn tube manufacture than cleaning. Five methods art in ' common use: solvents, firing, eieotrolysis, sandblasting, and molten salts. Solvents find their greatest application in the removal of grease, oils, and contaminations of all kinds caused by handling. Large quantities are used, the most usual being ethyl alcohol, methyl alcohol, acetone, trichloroethylcne, and carbon tetrachloride. The quantities used are so large that at least one manufacturer is considering the installation of solvent recovery equipment. Firing in hydrogen at appropriate temperatures varying from 200' to 1700" C. follows solvent cleaning, and serves a triple purpose in that it not only cleans but anneals metal parts (after welding, for instance) and removes gases which would otherwise be objectionable. Electrolytic cleaning employs solutions such as sodium hydroxide, sulfuric acid, chromic acid, perchloric acid-acetic acid. Sandblasting is done with sand, with silicon carbide or steel as the abrasive material. The fuuction of molten salts in cleaning is exemplified by the use of molten sodium nitrite to remove oxide from tungsten wire.
Conclusion The rapidly increasing use of numerous electronic devices will create an even larger market for the many materials discussed here, and new materials, not now in use, will undoubtedly find prominence. Requirements regarding purity and properties are strict, but modern methods of production and manufacture should find no insurmountable problems here. Literatnre Cited
1.:xnausnxvc AIIL-COOLED TIL~MXITTLXG TUBE
The interior iiietnl parts are heated by iiigli-frcqueney induction while the tube is being pumped. In order to rolease the occluded gases, the temperature of tila plate is raised to about 2500" F. and is increa.srd above this vehie for brief periods. More than 15 kw. of power are uuod in heating the metal parts.
(1) Beaker, J. A., Phzls. Re%, 34, 1323-51 (1929). (2) Eldrod. B. E., U. S. Patent 1,140,136 (May IS. 1915). (3) Koller, L. R., P h s . Reo., 25, 671-6 (1926). (4) Langmuir, Irving, Ibid., 22, 387-98 (1923); Dushman, Saul, Ilen. Modern Phus.. 2, 381476 (1930). (5) Lrdcrer. E. A,, and Wamsley, D. H., RCA Roo., 2. 117-23 (1937): ,. Lederer. E. A,. Ibid.. . 4.. 3i0-18 (i94oj. Leremnu. €I. W., arid Seitz, Frederick, J . A p plied Phzls., 10, 479-93 (1939). Scott. 11.. Trona. Am. Inst. Mining Met. E w s . , 89, 506-38 (1930); Hull, A. W.. and Burger. E. E., Physios. 5, 384405 (1934). Umbrcit. Stenton. Metala & Allovs, 6 , 273-9 (1935).
