US1942080A

US1942080A - Heater for indirectly heated cathodes

Info

Publication number: US1942080A
Application number: US559753A
Authority: US
Inventors: Andrew H Young
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1931-08-27
Filing date: 1931-08-27
Publication date: 1934-01-02
Anticipated expiration: 1951-01-02
Also published as: FR42178E; GB390405A

Description

Jan. 2, 1934. A. H. YOUNG HEATER FOR INDIRECTLY HEATED CATHODES Filed Au 27, 1931 Fig. l

. Inventor: Andrew H.Youn2,

by Hus Attorney.

Patented Jan. 2.1934

PATENT OFFICE mm mm mmnncrnr HEATED cs'rnonss Andrew H. Young Schenectady, N. Y., assignor a.

to General metm of New York Company, a corporation Application All!!! 27, 1931. Serial No. 559,758

3 Claims. (01. etc-27.5)

The present invention relates to thermionic apparatus and more particularly to the cathodes thereof.

There has arisen the necessity for a cathode,

6 structure which may be energized directly on commercial voltages such as 110 volts, without requiring transformers, resistances and the like. Attempts in the past to design a satisfactory heater with this voltage rating for indirectly l heated cathodes have metwith partial failure,

not only from the standpoint of life and performance but also from that of production on a quantity basis. The requirements for a member of this sort are extremely critical on account of 'the limited space available in standard tube design which ordinarily precludes a filament of such length as is required at these high voltages and also on account of failing to allow sumcient space for the proper amount of insulation. The

combination of high voltage with the resulting high temperature of operation and the closeness of the filament turns made necessary by the smallness of the containing member have often caused short circuit between the turns even through intermediate insulation, and in practically all cases in which ordinary insulation material has been employed, severe electrolysis has taken place and destroyed the material. These failures are particularly common inthe 80 case of gas-filled devices in which local .dis-

charges tend to form about various portions of,

15 employed heater members inseparable from the cathode and their construction is such as not.

to allow ready substitution or renewal of the heater.

An object of the present invention is to provide a heater which may be energized directly from commercial voltages and which does not have the foregoing disadvantages, but on the contrary, has a long and satisfactory operating life, and is also adapted to quantity production methods. Other objects are to provide an indirectly heated cathode with a readily replaceable heater, also to provide a heater which may be manufactured as a member, complete in itself and separate from the cathode proper. These objects are attained in brief, by coiling the heater wire in a fine helix and winding the latter about an insulator designed to withstand electrolysis. The heater wire is held rigidly in place by the application of one or more coats of an insulating paste containing a material highly resistant to electrolysis at high temperatures and voltages, and which when dry, forms a hard, solid mass. The heater in the finished form is an article complete in itself. self-supporting, and is embedded in an insulating material which effectively precludes arcing between the turns and protects the enclosed metal member from injury. The invention will be betterunderstood when reference is made to the following specification and accompanying drawing in which Fig. 1 illustrates an exemplary tube which employs the improved heater to advantage; Fig. 2 is an enlarged view showing a preliminary step in the manufacture of the heater; Fig. 3 is a view of the heater infinal form and mounted in place, ready to be inserted into the cathode casing; Fig. 4 is a view partly in section of the heater shown in Fig. 3, while Fig. 5 is a sectional view of an improved cathode taken along the line 5-5 of Fig. 1.

In Fig. l, numeral 1 designates the envelope of an electrostatically controlled arc discharge device embodying a tipless seal so-called, and containing an indirectly heated cathode 2, an electrostatic control or grid member 3 of mesh material and an inverted dish-shaped anode 4. A device of this sort ordinarily contains an ionizabie medium such as mercury or an inert gas which in operation, reduces the space charge effect to such an extent as to allow large amounts of current to flow, controllable by the charge on the grid. Alternating voltage is usually impressed between the cathode and anode and is rectified within the tube.

The cathode structure 2, shown more clearly in Fig. 5, consists of quadrant pieces of sheet metal bent to a shape so that when fitted together, two concentric cylinders 8 are formed, separated by four radially extending vanes 7. These vanes, as well as the two cylinders, are coated with an electronically active substance, such as barium carbonate (311003). There are several heat-shielding cylinders 8 surrounding the outer cylinder 6 and separated therefrom by depressions 9 formed in the material of the shields.

The improved heater which forms the subject of the present invention fits snugly within the inner cylinder 6 and serves to heat the latter, also the outer cylinder and vanes, to an electronemitting temperature in the well understood manner. The heater is constituted preferably of tungsten wire 10 of relatively small gauge, coiled as a fine helix, as shown more clearly in Fig. 2. The coil is wound in a cold state, as a coarse helix 11o aboutaroundrodorcore 11 of highlyfired alumina (AhOa) but which may also comprise zirconia (ZrOz) beryllia (BeO), or other insulating refractory which resists electrolysis to a high degree. There is a stiff molybdenum wire 12 driven through a central opening in the rod to which wire the upper end of the heater is attached so as to serve as a combined lead and tie wire. The lower end of the heater coil conveniently is bound to the alumina rod by a wire thong 13 which may be extended to constitute the other lead. The combined coil and its rod support are first dipped in water and then in a coating material consisting of a suspended solution of starch paste and fairly coarse, highly fired, pure alumina powder. While I prefer alumina for this purpose, other refractory insulating materials such as beryllia and zirconia, may be employed, provided they are inert with respect to the material of the embedded filament at high temperature and are highly resistant to electrolysis, also are capable of being thoroughly degassed; The slightest trace of gas in the material causes a severe shortening of the heater life. The coating is dried in the open air or in a closed oven operated at about .100 C. or less, and preferably is accomplished in such a manner as to insure uniformity of drying. The dipping process allows a very even coat to be picked up from the solution and as the water evaporates, the particles are drawn together by surface tension and the adhesive effect of the starch to form a relatively hard and compact mass. When the first coating is dry, the heater is again dipped in the suspension and dried in this manner until the proper thickness of material has been built up. A complete coat may be obtained with only two dips by regulating the condition of the suspension. It is desirable to have an extra heavy coat at the top of the heater which ordinarily is the point of maximum voltage between the heater and the cathode, and for this reason the upper portion of the coated member is given one or more extra dips.

The next step is to fire the heater, which is carried out at a temperature between 1575 C. and 1600 C. in an atmosphere of moist hydrogen. This firing step serves not only to get rid of the gas and volatile portion of the binder material and to decarbonize the less volatile portion of the binder, but also to sinter the coating. The filament heater is thus encased in a thick layer of pure aluminum oxide A10: which takes on an extremely hard, sintered character. The member as a whole is rugged, self-supporting, and is capable of being handled readily. It is also apparent that the described process of manufacture of the heater is relatively simple, and lends itself to quantity production methods.

The heater is now ready to be assembled within the cylinder or casing of the cathode, and in order to obtain a snug fit, it may be necessary to sandpaper the exterior of the coating. However, in practice, the constituency of the dipping solution and the number of dips are so regulated as to avoid the necessity of any paring of the coating material. The heater is supported in a vertical position by means of a combined cap and collar member 14 which has an opening in the top to receive the heater, and which fits snugly over a piece of glass tubing as a pant leg 15 sealed to the press 16 of the envelope 1. To ensure a rigid seat for the heater in the cap, the lower end of the heater member may be tapered slightly. The central wire 12 is passed through the glass tubing and is connected to one of the not only by the proper choice of insulating mabetween the filament and the cathode, so that external base contact 17. from the heater is secured in any nertoarigldconductorlafused and connected to another of the 17. The bottom of the cathode structure is closed by a metal cap (not shown) and the memberasawholesupportedfromthepreabya rigid metal rod 19, in addition to the conductor 18, which on account of being attached tothe cap serves as a lead for the cathode as well as for the heater. As shown in Fig. 1, the cathode preferably is mounted so as to extend a short distance within the grid member and is spaced concentrically therefrom by means of a surrounding disc of insulation material 20. In addition to the cathode, the press also supports the grid 3 conveniently by means of stay rods 21 secured to a clamp 22.

As stated hereinbefore, a tube of this sort contains an ionizable medium such as mercury v r, or an inert gas at a pressure sufiicient to su port an arc-like discharge at the impressed voltages. It has been found that notwithstanding the presence of the medium in this condition, also the high voltages employed, any portion of the heater which is accessible to' the arc successfully withstands the positive ion bombardment. This may be due to the hard and dense texture of the fused coating. The heater may be energized directly from a 110 volt alternating current or direct current supply without the use of voltage reducing accessories, as the size and length ofthewirearesuchastolimitthe currentat these voltages to the proper amount. It has been found that even when heated to a tempera ture as high as 1300 C., the improved heater is not subject to electrolysis which ordinarily attacks all other heaters at temperatures of this order. This desirable result is brought about terlals in which the wire is embedded and 011' which it is wound, but also by the improved process of manufacture and design of the heater. It is known that the degree with which the phenomenon of electrolysis takes place in a given ma-- terial depends not only upon the electrical stress to which the material is subjected but also on the temperature of the material. The improved heater is so designed as to minimize this temperature, the design being an embodiment in which the heater is wound as a double helix on a rod of suitable insulating material. Moreover. it is apparent that this electrical stress is relatively high due to the presence of 110 volts between the adjacent legs of the filament and also the problem of eliminatingelectrolysis of which the slightest amount is fatal to the life of the heater, is a matter of no small importance when the thinness of the encasing material is considered. I have made 110-volt heaters of this sort with a coating of approximately 25 mils thick, at the thinnest part which operated as long as 4450 hours in a mercuryvapor discharge device of standard design. The maximum diameter of this 140 heater was 170 mils, approximately the same size as shown in Fig. 4 of the drawing. The member 14, which is completely insulated by reason of its glass support and also by virtue of the fact that only the insulating cover of the heater con- 5 tacts with it, serves to prevent arcing between the lower end of the wire 12 and of the helix 10 between which full heater voltage is applied and also precludes positive ion bombardment of the wire which tends to take place in the gas.

While I have described my invention with particular reference to a gas or vapor-filled rectifier, it is to be understood that the features described hereinbefore are applicable to all forms of gaseous discharge devices, such as glow lamps and the like. Indeed, the invention is not limited even to gas discharges, but may be extended into the high vacuum field because obviously, an in directly heated cathode which is able to operate satisfactorily under the severe conditions imposed by cumulative ionization and high voltage, is readily adapted to the less severe conditions occurring in a high or partial vacuum. Consequently, the invention applies to all types of electric discharge devices, gas-filled or highly evacuated, which employ indirectly heated cathodes and which advantageously may use a heater connected directly to a commercial voltage supply.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. An indirectly heated cathode for a gasfllled tube and adapted to be energized directly by current of commercial voltages, said cathode comprising a heater wire wound on a rod of refractory insulating material and completely embedded in a mass of aluminum oxide, a heater lead extending through the rod, said mass or aluminum oxide resting on a metal cap which is insulatingly supported within the tube. said lead passing through the interior of said cap.

2. An electron discharge device comprising an envelope terminating in a stem, cooperating electrodes in said envelope including an indirectly heated cathode adapted to be energized by current of commercial voltages, said cathode comprising a heater wound on a rod 01' refractory insulating material and completely embedded in a mass of refractory insulating material, a heater lead extending through the rod, a collar or refractory material for supporting said mass of insulating material from said stem, said lead passing through said collar to the exterior of the envelope.

3. An electron discharge device comprising an envelope terminating in a stem provided with a cylindrical projection, cooperating electrodes in said envelope including an indirectly heated cathode adapted to be energized by current of commercial voltages, said cathode comprising a heater wound on a rod of refractory insulating material and completely embedded in a mass of refractory insulating material, a heater lead extending through the rod, a metal collar fitting snugly over said projection and over said mass of insulating material for supporting the latter from said stem, said lead passing through the collar to the exterior of the envelope.

ANDREW H. YOUNG.