US2043720A - Thermionic cathode heater and method of making it - Google Patents

Description

Patented June 9, 1936 UNITED STATES THERMIONIG CATHODE HEATER, AND

' METHOD OF MAKING IT Edward R. Wagenhals, West Orange, N. J., assignor to Radio Corporation of America, a corporation of Delaware No Drawing. Application November 25, 1933, Serial N0. 699,708

,2 Claims. (01. 250--27.5)

My invention relates to heaters for use in vacuum electric devices and more particularly to an insulated heater element for use in an indirectly heated thermionic cathode for electron discharge devices.

A common type of heater for oxide coated cathodes is a non-inductive reverse wound helical coil of tungsten wire insulated with a coating consisting of a mixture of aluminum oxide and about 2% of talc or silica. Such heaters are in general satisfactory, but some times the insulation becomes somewhat leaky at high temperatures and allows an appreciable amount of stray and uncontrollable current to fiow between the cathode sleeve and the heater wire. Tubes containing cathode heaters with leaky insulation are not desirable and may be noisy.

In one method commonly used for making cathode heaters a mixture of finely ground aluminum oxide and about 2% of talc or silica is stirred into a solution of nitrocellulose to form a syrupy suspension, with which a coil of ductile tungsten wire is coated by spraying the suspension on the coil with an air pressure spray gun. The coating is dried and the coated coil heated from five to ten minutes in a hydrogen furnace to about 1400 C. to 1600 C. At this temperature the coating hardens on the tungsten wife, which remains ductile.

It has heretofore seemed necessary, in the commercial manufacture of such cathode heaters, to mix with the aluminum oxide insulation some talc or silica or similar material which softens at about 1400" C. in order to obtain a hard firm coating of insulation at a firing temperature at which the tungstenwire remains ductile. Aluminum oxide free from talc or silica sinters and forms a coherent layer when fired at temperatures of 1800 C. or higher, but when the heaters were fired in the usual hydrogen furnace in the usual way to the sintering temperature of aluminum oxide the tungsten heater wire became very brittle and practically useless.

One of the objects of myinvention is to provide a cathode heater element consisting of a ductile tungsten wire insulated by a coherent sintered coating of alumina of the kind usually designated in commerce as chemically pure aluminum oxide.

Another object is to provide a cathode heater of ductile tungsten with refractory insulation such that even at high temperature the electrical leakage between the heater and cathode is practically negligible.

A further object is to provide a method by which a. ductile tungsten heater wire can be coated with a dense coherent coating of chemically pure aluminum oxide without impairing the ductility of the wire.

In accordance with my invention the heater element consists of a tungsten wire which, although covered with a coherent coating of sintered chemically pure aluminum oxide, is practically as ductile as the tungsten heater wires of the heaters insulated with a mixture of aluminum oxide and talc. The heater element is made by coating a ductile tungsten wire with finely powdered chemically pure aluminum oxide suspended in a binder, such as a solution of nitrocellulose, and then fired in an atmosphere of exceedingly wet hydrogen at about 1800 C., a temperature at which the chemically pure aluminum oxide sinters into a coherent layer. I have found that under these conditions the ductile tungsten wire retains its ductility, although it is covered with chemically pure aluminum oxide sintered into a dense adherent coating. Prior to my invention, heaters with ductile tungsten wire insulated with aluminum ox de could be had only with aluminum oxide mixed with talc or silica, as attempts to insulate ductile tungsten wire by sintering chemi- 'cally pure aluminum oxide on it resulted in heaters with wire too brittle to be useful.

, One procedure by which, in accordance with my invention, I have obtained commercially useful heater elements involves the use of aluminum oxide containing about .2% or less of impurities. This oxide, generally made by drying precipitated aluminum hydroxide, can be obtained commercially in the form of a powder which is usually sold as chemically pure aluminum oxide. I find it advantageous to heat this powdered aluminum oxide to a temperature of about 1500 C. until it shrinks slightly and increases somewhat in density, and then to, grind it in a ball mill into a fine powder, of which the average particle size isabout two thousandths of an inch and the maximum particle size is not more than three times the average size. I find it desirable to ball-mill the oxide in a rubber lined mill with pebbles of pure flint or of mullite, a commercial form of alumina for about 18 hours. This fine powder of dense chemically pure aluminum oxide is suspended in a solution of nitrocellulose or similar decomposable carbonaceous binder, preferably of about the consistency and viscosity of a thin syrup. I prefer to make the suspension with the smallest amount of solution that will give a smooth even coating of oxide when the suspension is sprayed on the tungsten heater wire. A common and useful form of cathode heater is a ductile tungsten wire, wound into a reverse helix as shown in the United States Patent to Fredenburgh, 1,980,675,

November 13, 1934, and assigned to the assignee of my application. The sprayed coating on such a helix may be from 5 mils. to 10 mils. thick, de-

pending on the degree of insulation required.

In accordance with my invention the tungsten wire helix, coated with a smooth uniform coating of chemically pure aluminum oxide and nitrocellulose binder, is dried, preferably by heating it for a short time to about 100 C. to drive off the solvent. The helix with the dried coating can be handled with ordinary care without injury to the coating. After the coated helix is dried, it is fired in an electric furnace in a hydrogen atmosphere to about 1800 C. Ordinarily a firing of 5 to 10 minutes is sufficient. I prefer to fire the coated heater coils by putting them in molybdenum boats which are then passed through the hydrogen furnace in the usual way. In accordance with my invention-the hydrogen atmosphere which surrounds the coated heater during the firing in the furnace is kept very wet, practically as wet as possible while the heaters are being fired. The carbon in the binder makes the ductile tungsten wire brittle when fired at 1800 C. in the usual hydrogen atmosphere, but the hydrogen atmosphere saturated with water vapor renders this carbon innocuous to the ductile tungsten. A convenient Way of Wetting the hydrogen to the desired extent is'to bubble it through water kept at a temperature of about to 0., whereby the hydrogen is saturated with water vapor and wetted much more than when it is bubbled through Water at room temperature. I have found that if the heaters of ductile tungsten wire coated with a suspension of chemically pure aluminum oxide are fired in this very wet or saturated hydrogen atmosphere at about 1800 C., the ductile tungsten wire of the heaters is not affected or rendered brittle by the carbon in the binder, and remains ductile, and the particles of chemically pure aluminum oxide sinter together into a dense coherent coating which adheres to the wire. To the naked eye this coating seems smooth and uniform, with a dull finish. Under the microscope the particles of aluminum oxide appear to be sintered together into a coherent layer. In sintering the particles do not melt and flow together, but soften slightly on the surface and stick together. The sintered aluminum oxide forms on the ductile heater wire an insulating coating which consists of chemically pure aluminum oxide and'is of very low electrical conductivity.

Although ductile tungsten is the preferred metal for the heater coils, molybdenum or other ductile metal of high melting point may be used. The pure aluminum oxide may, instead of being mixed with a nitrocellulose binder and sprayed on the heater wire, be mixed with flour paste and extruded as a rod suitable for use as a core rod for the reverse coiled helix. The extruded rod fired at 1800 C. or higher in very wet hydrogen is strong and is well adapted for use as a core rod for heater coils with chemically pure aluminum oxide insulation. A heater made of a ductile tungsten wire coil mounted on such a core rod and insulated in accordance with my invention is sturdy, and has the advantage that both the core and the coating are of chemically pure aluminum oxide.

The ductile tungsten wire of a heater coil coated with chemically pure sintered aluminum oxide in accordance with my invention retains its pliability to such an extent that a 5 mil. tungsten Wire coil of the pitch and diameter of the usual cathode heater coils can be pulled out substan tially straight without breaking the wire, although the sintered coating may break. Prior to my invention a heater with a tungsten coil of such pliability could not be made with insulation of chemically pure aluminum oxide.

While I have described a few specific embodiments of my invention, it is to be understood that various changes and modifications are contemplated and that the invention is limited in its scope only by the appended claims.

What is claimed as new is 1. The method of making an electrically insulated heater for electron discharge tubes which comprises coating a ductile tungsten heater wire wi h chemically pure aluminum oxide powder of particies of average size of about 2 microns and of maximum size not greater than 6 microns in suspension in a carbonaceous binder and heating said coated ductile tungsten wire to a temperature of approximately 1800 C. in an atmosphere of hydrogen saturated with water vapor until said coating is sintered into a hard dense coherent layer adherent to said wire.

2. The method of making an electrically insulated heater for electron discharge tubes which comprises spraying a ductile tungsten wire with a suspension in a nitrocellulose binder of aluminum oxide containing less than 0.2% of impurities and powdered to particles of maximum size of about 6 microns and average size of about 2 microns to form a coating on said wire, heating said coated wire in an atmosphere of hydrogen substantially saturated with water vapor to about 1800 C. to remove the nitrocellulose binder, and maintain said coated wire ductile and to sinter the aluminum oxide on said wire into a hard dense coherent coating adherent to said wire.

EDWARD R. WAGENHALS.